Obesity-driven disruption of haematopoiesis and the bone marrow niche

Abstract 

Obesity markedly increases susceptibility to a range of diseases and simultaneously undermines the viability and fate selection of haematopoietic stem cells (HSCs), and thus the kinetics of leukocyte production that is critical to innate and adaptive immunity. Considering that blood cell production and the differentiation of HSCs and their progeny is orchestrated, in part, by complex interacting signals emanating from the bone marrow microenvironment, it is not surprising that conditions that disturb bone marrow structure inevitably disrupt both the numbers and lineage-fates of these key blood cell progenitors. In addition to the increased adipose burden in visceral and subcutaneous compartments, obesity causes a marked increase in the size and number of adipocytes encroaching into the bone marrow space, almost certainly disturbing HSC interactions with neighbouring cells, which include osteoblasts, osteoclasts, mesenchymal cells and endothelial cells. As the global obesity pandemic grows, the short-term and long-term consequences of increased bone marrow adiposity on HSC lineage selection and immune function remain uncertain. This Review discusses the differentiation and function of haematopoietic cell populations, the principal physicochemical components of the bone marrow niche, and how this environment influences HSCs and haematopoiesis in general. The effect of adipocytes and adiposity on HSC and progenitor cell populations is also discussed, with the goal of understanding how obesity might compromise the core haematopoietic system.

Key Points

• Obesity represents a substantial public health challenge, including an increased burden of infectious diseases—suggesting the involvement of a compromised immune syste

• Haematopoietic stem cells reside in (and are regulated by) a complex, heterogeneous and tightly controlled microenvironment within the bone marrow

• The bone marrow environment undergoes considerable changes during obesity, including adipocyte hyperplasia and a phenotypic shift of adipocytes towards a white adipose profile

• Methods of limiting the progression of obesity and controlling its systemic and bone-marrow-based sequelae, such as exercise, might represent suitable approaches to maintain haematopoiesis and immune function



Introduction

 

There are now 1.7 billion overweight adults worldwide,¹ 300 million of whom are categorized as obese.² In the USA, data from the National Health and Nutrition Examination Survey estimated the overall prevalence of obesity in adults to be 35.7%.³ Alarmingly, obesity rates have increased most rapidly among young individuals,⁴ predisposing them to type 2 diabetes mellitus (T2DM),⁵ an increased lifetime risk of cardiovascular disease,⁶ cancer⁷ and early mortality,⁸ culminating in health-care costs of >$200 billion in the USA alone.⁹ Attempts to control either obesity¹⁰ or T2DM¹¹ by pharmacological intervention have been disappointing and plagued by off-target complications. In addition to the metabolic comorbidities traditionally associated with increased adiposity, obesity also increases an individual's susceptibility to infectious diseases¹² and catalyses a decrease in general health¹³—implying that obesity either directly or indirectly compromises immune function. Indeed, the progression of obesity, like that of other metabolic disorders such as anorexia, osteoporosis and ageing,¹⁴ is accompanied by an increase in bone marrow adiposity and disruption to the haematopoietic microenvironment, which presages damage to the immune system.
This Review seeks to explore the interactions between obesity, and haematopoiesis and the immune system through systemic effects and changes to the primary haematopoietic environment—the bone marrow niche. Firstly, the structure and function of the haematopoietic system is described by introducing the haematopoietic stem cell, its development and differentiation. Secondly, the heterogeneous regulatory environment of the bone marrow is discussed, with several key components described in detail. Thirdly, adipocytes and adipose tissue are described in states of both health and obesity. Fourthly, the available information on the effect of obesity on the bone marrow niche, haematopoiesis and immunity is presented. The article concludes with a focus on methods to suppress bone marrow adiposity.
 

Hematopoiesis

 

Haematopoiesis is the group of processes by which all formed elements of blood are created. Differentiated haematopoietic cells include erythrocytes, platelets and nearly a dozen types of leukocytes (white blood cells), which are responsible for both innate and adaptive immune surveillance and performance. Traditionally, haematopoietic differentiation is divided into either the myeloid lineage, which includes erythrocytes, granulocytes, macrophages and monocytes, and platelets, or into the lymphoid lineage, which includes B lymphocytes, T lymphocytes and natural killer (NK) cells. Although this model of haematopoiesis crudely identifies functional differences between mature cells, the initial descriptions were based on the location of cellular maturation (bone marrow versus lymph nodes and spleen), rather than on knowledge of the hierarchy of differentiation.¹⁵ With some acknowledged exceptions, myeloid cells are critical to innate immunity whereas lymphoid cells comprise the adaptive immune response.

Innate immunity is evolutionarily conserved in many organisms and enables a rapid defence against pathogens introduced through disease or injury.¹⁶ Although innate immunity can distinguish between host and foreign elements at the time an immune response is initiated, that response is not specific, and prolonged activation can readily lead to damage to healthy tissues.¹⁶ A suitable example is found in the experimental overexpression of one of the haematopoietic growth factors, granulocyte-macrophage colony-stimulating factor (GM-CSF), which leads to increased numbers and activation of macrophages as well as other leukocytes, resulting in massive inflammation and destruction of normal tissues.¹⁷ By contrast, adaptive immunity has the advantage of being highly specific in its targets, because of genetic rearrangements within cell-surface immune receptors and immunoglobulin proteins, and can reliably combat a much wider range of infectious agents.¹⁸ This specificity is achieved at the cost of rapid action, however, as lymphocytes of the adaptive immune system must first be primed to respond to specific antigens, at least when encountering a novel agent. Subsequent infections with the same antigen are met more quickly and forcefully with each exposure, as a consequence of immune memory.^{16, 18}

Haematopoietic Stem Cells

Haematopoietic stem cells (HSCs) are the developmental origin of the haematopoietic system. These cells are primitive, pluripotent progenitors that differentiate, in part, in response to environmental signals that mediate homeostatic maintenance of the entire haematopoietic system, as well as when increased blood cell production is required in times of enhanced demand, such as in response to infection or enhanced blood cell destruction.^{19, 20} The balanced and timely regulation of HSC differentiation is critical for maintaining normal immune function. The notion that HSCs are the developmental source of the haematopoietic system originated in the early part of the 20^th century,²¹ but was not proven until the 1960s by lethal irradiation and bone marrow transplantation studies.²² To this day, HSCs are still functionally defined as being capable of rescuing a lethally irradiated animal and restoring stable, long-term and full-spectrum haematopoiesis.^{23, 24}

The hierarchy of haematopoietic differentiation, with regard to the functional and phenotypic identity of sequentially generated populations, and how differentiation 'fate' is achieved, is an active area of research. The transitions at the origin of this hierarchy are self-evidently important to the haematopoietic system as a whole. The existence of phenotypically distinct subpopulations that maintain haematopoietic reconstitution for differing durations within the progenitor population capable of producing the full spectrum of haematopoietic progeny cells has long been recognized.²⁵

 

The presence of long-term HSCs (LT-HSCs) and short-term HSCs (ST-HSCs) in human bone marrow is of clinical relevance when considering the efficacy of therapeutic bone marrow transplantation.²⁶ In most models of haematopoiesis, HSC multipotency is coupled with the ability to self-renew.²⁷ In such models, ST-HSCs are considered an intermediary state in which some 'stem-like' properties are retained until the cell commits to one of the principal (myeloid or lymphoid) lineages.²⁸ These lineage decisions, which are influenced by cytokine signalling,²⁹ metabolism³⁰ and stochastic processes,³¹ occur during maturation, after the loss of 'stemness'. This concept has changed lately, with the recognition that even primitive HSCs can be biased in their differentiation profiles.³² Interestingly, the clonal bias of certain HSC subpopulations has been identified in both ST-HSCs and LT-HSCs.²⁷ In this model, differentiation of LT-HSCs generates ST-HSCs and myeloid progenitors, whereas lymphoid (and continued myeloid) potential derives from ST-HSCs.²⁷ In this hypothesis, there are no distinct HSC populations dedicated to maintaining a certain lineage; rather, the lineage potential changes as HSCs mature.

 

Adult HSCs arise from the dorsal aortic section of the aorta–gonad–mesonephros region at embryonic day 10.5 in the mouse.^{24, 33} These cells then migrate to populate the fetal liver, from which they later migrate and home to the spleen and most importantly the bone marrow,³⁴ where adult HSCs reside and their progeny develop. Ultimately, the relationship between the bone marrow and the haematopoietic populations is dependent on the composition of the bone marrow. Consequently, changes to the bone marrow phenotype, such as those that occur with increased adiposity, inevitably affect haematopoietic maintenance and differentiation, ultimately putting immune function at risk.

Paracrine and endocrine regulation

A number of haematopoietic cytokines have been identified that contribute to regulation, survival, proliferation and differentiation of the bone marrow. Whilst this Review focuses on the bone marrow microenvironment and cell–cell interactions, a brief overview of the effects of soluble growth factors on HSCs is provided.

 

Although humoral regulation of erythropoiesis was first described in the early 1900s,³⁵ it was not until the 1950s that erythropoietin was confirmed as the primary regulator of red cell production.³⁶ In the 1960s and 1970s, the development of in vitro assays to measure growth factors secreted by bone marrow cells that produced leukocytes identified additional humoral regulators of leukocyte and megakaryocyte (platelet) production.³⁷ Even though >30 such factors have now been identified, only a few cytokines regulate HSCs and only two of these have essential and non-redundant roles: Kit ligand (also known as stem cell factor or SCF)³⁸ and thrombopoietin.³⁹ Both of these cytokines, which are produced in peripheral organs and by bone marrow stromal cells, act by binding to their cognate receptors on the cell surface of HSCs and stimulating intracellular kinase activity that leads to the activation of several signalling pathways involved in cell survival and differentiation. Not surprisingly, given these properties, the receptors for both SCF⁴⁰ and thrombopoietin⁴¹ are involved in neoplastic bone marrow diseases.

The bone marrow niche

In adult individuals, the bone marrow is the primary site of haematopoiesis. The bone marrow microenvironment, or niche, provides a critical regulatory milieu that is responsible, in part (along with the stochastic increased and decreased expression of lineage-specific transcription factors and haematopoietic cytokines generated both locally and from afar), for orchestrating the differentiation of HSCs into the different types of mature blood cells.⁴² Among all niche residents, HSCs are actually quite rare, comprising ~0.001% of total bone marrow cells.⁴³ The remaining cells are haematopoietic progenitors at various stages of maturity, nearly mature blood cells and nonhaematopoietic cells that provide key regulatory signals in the haematopoietic niche. The fact that heterogeneous populations develop in the niche denotes that changes in the composition of the niche can affect haematopoiesis at multiple developmental levels and via a variety of mechanisms. A brief overview of the inherent complexity of the niche, both in terms of its anatomy and signalling pathways, structured around several of the regulatory influences of resident cell types and their capacity to bias the activity and fate of different haematopoietic populations follows.

Complexity

Osteoblasts

The endosteal bone surface has long been recognized as the principal component of the haematopoietic niche, with osteoblasts, osteoclasts and mesenchymal stem cells (MSCs) all influencing HSC fate selection (Figure 1).⁴² Although the primary function of osteoblasts is to secrete nonmineralized bone matrix (osteoid) in the phase preceding bone mineralization, osteoblasts located on the endosteal surface also have a major role in the regulation of HSCs. Osteoblast lineage cells immobilize HSCs by, among other mechanisms, the binding of integrin α₄β₁ to VCAM1, which reduces apoptosis and induces HSC quiescence.⁴⁴ Similarly, soluble stromal-cell-derived factor 1 (also known as CXCL12) and angiopoietin-1 secreted by osteoblasts prevents HSC mobilization from the niche and promotes cell quiescence.^{45, 46} Osteoblasts are also closely coupled to HSC proliferation, as increases in the osteoblast population lead to concomitant increases in HSC numbers.^{47, 48} This expansion of the HSC compartment seems to be mediated by osteoblastic Notch signalling,⁴⁸ as osteoblastic expression of Notch ligands⁴⁹ leads to proliferation of both ST-HSCs and LT-HSCs. However this role might be limited to haematopoietic recovery and not homeostasis per se.⁵⁰ Other factors involved in the osteoblast lineage regulation of HSC numbers include osteopontin,⁵¹ Wnt, N-cadherin, thrombopoietin⁵² and angiopoietin.⁵³ Nevertheless, a change in the number or activity of osteoblasts, which invariably occurs with ageing,⁵⁴ diabetes⁵¹ or obesity,⁵⁵ will ultimately have an effect on HSCs. Interestingly, all these conditions are characterized by increased inflammation, which harms both osteoblast proliferation and function,^{54, 56} as well as the maintenance of HSCs^{57, 58}—further highlighting the delicate interaction between these cell populations

 

Figure 2: Haematopoietic stem cell differentiation.

HSC differentiation in bone marrow of a healthy (top right) and obese mouse (bottom right), as represented at the proximal tibial metaphysis. HSCs differentiate to produce all blood lineages under niche-based regulatory control. In homeostasis, differentiation is balanced in order to maintain immune function and to replenish erythrocytes. In states of obesity, the adipocyte phenotype shifts from a hybrid brown and white (beige) profile towards a classical white adipose profile. The bone marrow becomes entangled with these adipocytes, which produce excess concentrations of adipokines and cause inflammation. This inflammation results in a proinflammatory bias and a loss of leukocyte balance, in which some myeloid lineage populations are increased, seemingly at the expense of lymphoid populations. This loss of lymphopoiesis and the proinflammatory state of the immune system are likely to contribute to the immune deficits observed in states of obesity. Abbreviations: CLP, common lymphoid progenitor; CMP, common myeloid progenitor; HSC, haematopoietic stem cell; LT, long term; MPP, multipotent progenitor; NK, natural killer; ST, short term.

 

Ultimately, any conclusions regarding the role of adipocytes in the bone marrow niche must be approached cautiously, owing to issues in extrapolating data from studies of mice to humans. This statement is particularly true considering the different patterns of fat accretion between the two species. Specifically, whereas ageing accelerates bone marrow adiposity in both mice and humans, sites of red bone marrow (for example in the femur) become highly adipocytic long before senescence in humans, whereas comparable mouse bone marrow remains relatively adipocyte-free until advanced old age.^{149, 150} These different rates of adipocyte accumulation indicate the potential for differences in the physiological roles and activity of adipocytes between humans and mice. At the very least, these observations emphasize the conclusion reached earlier, which regards the presence of adipocytes as not necessarily disruptive, particularly in the absence of a phenotypic shift to bone marrow adipocytes (as has been observed in advanced ageing and metabolic disorders such as obesity and diabetes mellitus).^{106, 114}

Suppression of bone marrow adiposity

The potential negative effect of increased adipogenesis on bone marrow organization and haematopoietic function highlights a critical need for strategies that safely and effectively limit or reverse adipocyte invasion into the bone marrow space. Although reduced caloric intake remains a key mechanism for controlling adiposity in patients with obesity, exercise also fosters weight loss^{151, 152} and improves physical performance and physiological function.¹⁵¹ Indeed, exercise can directly address many of the health consequences of obesity, without first reducing adiposity. For example, exercise improves cardiac function in a dose-dependent manner,¹⁵³ reduces blood pressure in individuals with hypertension¹⁵⁴ and can improve serum markers of liver function and suppress hepatocyte apoptosis in patients with nonalcoholic fatty liver disease, before altering body composition.¹⁵⁵ Bone marrow adiposity, assessed as bone marrow density using MRI in female athletes, can also be proportionally reduced by high-impact loading regimes.^{156, 157} Similarly, in mice fed a high-fat diet, bone marrow adiposity (shown by osmium tetroxide staining) was greatly increased compared to that in lean control mice. However, voluntary use of a running wheel significantly decreased the level of bone marrow adiposity in both lean animals and those fed a high-fat diet.¹⁵⁸ Interestingly, protection of the bone marrow phenotype in obese animals was achieved without reducing systemic adiposity,¹⁵⁸ indicating that this process might be related more to the influence of mechanical stimulation on bone marrow adipocytes and the niche as a whole, rather than to changes in energy balance. Such a relationship is highlighted by the association of reduced bone marrow adiposity with concomitant increases in skeletal parameters in these studies.^{156, 158}

 

The mechanisms responsible for the reduction of bone marrow adiposity are complex but, considering the inverse relationship between bone marrow adiposity and skeletal mass, might involve a competitive balance between osteoblast and adipocyte differentiation. One key factor in this process is growth hormone. During ageing, the bone marrow becomes gradually filled with adipocytes and bone mass is lost, and levels of growth hormone also decrease. Impairment of growth hormone production in mice led to an accumulation of adipocytes in the bone marrow despite a reduction in body weight;¹⁵⁹ the increased levels of bone marrow adipocytes were normalized and osteoblast activity was increased by growth hormone replacement.¹⁵⁹ Importantly, growth hormone is produced in response to exercise,^{160, 161} providing some insight into how exercise influences the bone-marrow microenvironment. Similarly, whereas PPAR-γ is required for adipocyte differentiation, its absence leads to elevated osteoblastogenesis.¹⁶² Treadmill running in rats prevents ovariectomy-induced bone loss by limiting PPAR-γ expression, whilst simultaneously suppressing adipocyte encroachment into the bone marrow.¹⁶³ Supporting this interaction, MSCs subjected to very subtle mechanical signals in vivo are biased away from adipogenesis and towards osteoblastogenesis,¹⁶⁴ even in highly adipogenic environments.¹⁶⁵ Likewise, in vitro stretching of MSCs reduced PPAR-γ signalling and adipogenesis, even during PPAR-γ activation.¹⁶⁶

 

Another important and mechanosensitive regulatory mechanism in the stem cell niche is β-catenin signalling. Diminished levels of β-catenin are a hallmark of adipogenesis in MSCs, whereas mechanical stretching increases levels of β-catenin and inhibits adipogenesis in vitro.^{167, 168} Functional β-catenin signalling and its activation of the canonical Wnt pathway are also necessary for haematopoietic homing to the bone marrow niche¹⁶⁹ and engraftment of ex vivo expanded HSCs.¹⁷⁰

 

Furthermore, β-catenin is necessary for haematopoietic regeneration following injury.¹⁷¹ The level of β-catenin signalling within HSCs themselves is dependent on niche health, as HSCs isolated from diabetic mice have reduced β-catenin expression, which can be rescued through co-culture with stromal cells from nondiabetic mice.⁵¹ The importance of β-catenin signalling to haematopoiesis, its inverse relationship to adiposity and its sensitivity to mechanical interventions, suggest that the canonical Wnt pathway is a link between exercise and haematopoietic health. Cumulatively, these studies demonstrate the potential of exercise, or its surrogates, to bias MSC fate selection and thus restrict bone marrow adiposity and simultaneously stimulate osteoblastic activity, ultimately leading to the promotion of haematopoietic function.¹²⁴

Conclusions

The bone marrow niche regulates haematopoiesis through complex and sometimes compensating influences. The targets of this regulation include not only the most primitive HSCs, but also leukocytes transitioning through a range of developmental stages. These influences are delivered via cell–cell contact, soluble signalling factors and even changes in the local physicochemical and inflammatory environments. The delicate interactions of cellular, physical and chemical factors are, in aggregate, responsible for balancing haematopoietic quiescence, proliferation and differentiation. As such, changes to the niche composition can have profound effects on haematopoiesis and the proper functioning and robustness of the immune system.

 

Obesity is thought to increase bone marrow adiposity (in some cases rapidly)^{110, 111, 112, 139, 158}—a condition that can disrupt both chemical and physical connections, as well as the cellular and signalling content of this fragile but vital niche. Such disruption results in diminished lymphocyte populations,^{123, 124, 139} immune function,^{125, 126} and ultimately the suppression of HSC proliferation.^{114, 143} In addition to its effect on the bone marrow, obesity also increases susceptibility to a range of chronic comorbidities, including increased inflammation related to the increase in systemic adiposity. As yet, the relative consequences of obesity for the haematopoietic system, as caused by systemic changes versus local shifts in the niche, cannot be defined. Nevertheless, considering other conditions of elevated bone marrow adiposity, such as ageing and anorexia,^{14, 84} we can safely conclude that the increased adiposity observed in individuals with obesity can quickly lead to profound haematopoietic insult and a compromised immune system. In conclusion, these results, ranging from signal transduction studies to bone marrow phenotypes, emphasize the importance of the bone marrow composition on overall health of the immune system and the organism itself, and advocate strategies such as exercise to limit the consequences of excess bone marrow adiposity.

      





 



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Nature Jobs: The postdoc decision

Naturejobs | Naturejobs Blog

The postdoc decision

03 Oct 2014 | 06:00 BST | Posted by Julie Gould | Category: Career paths, Postdoc

Career ambitions are the most popular reasons for taking on a postdoc, followed closely by a love for science.

OK, no surprises there, but we at NatureJobs are always interested in our readers’ career choices. Knowing our audience is the first step in providing you with useful information.
Our latest poll, “Why did you decide to do a postdoc?” sought to uncover the reasons why our readers took on a postdoc position. On average, these are three year, fixed-term contract research positions; a chance for researchers to develop their skills and get some papers under their belts.
Readers were given the chance to choose three of the following reasons:

• Because it is the next step in your academic career after your PhD
• Because you love science and research
• To reach your dream of becoming a tenured academic
• Because you couldn’t find another job
• To discover something new
• To contribute to the body of scientific knowledge
• Don’t know what else to do
• You’re afraid of the unknown outside of academia
• To change location
• Because you couldn’t find a tenure-track position
• To contribute to society
• For convenience
• Because you entered on the promise that the lab was expanding and further opportunities would become available
• Other

And after 3 months, 482 people from all over the world have cast 817 votes, and these are the latest results, as of Tuesday September 16^th, ranked by popularity



Nearly a quarter of votes were for “Because it is the next step in your academic career”, indicating that, for better or worse, many scientists’ default option is to continue on their current career path.
However, it’s not all about the career conveyer belt: the more altruistic option of “Because you love science and your research” was second most popular with 16% of the votes.  Another career-focused answer, “To reach your dream of becoming an academic” came a close third with 15% of the votes.
We can group these answers into 3 (overlapping) categories:
Future ambitions

• Because it is the next step in your academic career after your PhD
• To reach your dream of becoming a tenured academic
• Because you couldn’t find another job
• To change location
• Because you entered on the promise that the lab was expanding and further opportunities would become available

Lack of other options

• For convenience
• Because it is the next step in your academic career after your PhD
• Because you couldn’t find a tenure-track position
• Don’t know what else to do
• Because you couldn’t find another job
• You’re afraid of the unknown outside of academia

Love of science

• Because you love science and research
• To discover something new
• To contribute to the body of scientific knowledge
• To contribute to society

These categories are not mutually exclusive. For example, the “Next step” option could fall under future ambitions, and lack of other options. Future ambitions seem to be the biggest driving force behind choosing to do a postdoc, with 45% of the votes, followed by “Love of science”, with 30%.

What does it mean?

Our small-scale survey is only for interest, but it does support other more robust investigations into the career paths of scientists.

In 2012, Sauerman and Roach (1) looked at US PhD students’ career preferences. They surveyed students from all life science and physics disciplines and found that a faculty research career was deemed “extremely attractive” for more than 50% of those surveyed.

Then in 2012, the Eurodoc Survey I published results from a survey that ran from 2008-2009 and found that more than three-quarters of PhD students consider an academic research career the most attractive.

So our “future ambitions” group of votes is representative of these broader findings.

However, as attractive as the academic career path is, it is not an easy one. In the UK, the Royal Society’s The Scientific Century, analysed HEFCE, the Research Base Funders Forum, HESA and DLHE surveys and found that only 47% of UK science PhD graduates remain in academia.

Scientific Century, Royal society

In contrast, for those outside with non-science PhDs, the prospects of becoming a professor is more promising. HEFCE recently released data that researchers in arts, humanities and social science disciplines make up 55% of all academics but only 34% work in STEM subjects.

Of course, as difficult as it is to stay in academia, it can seem harder to leave. The 23% of votes that were in the ‘lack of other options’ category tell a sad but all-too common story. Many a PhD graduate will struggle to find employment outside of academia, so jumping onto the postdoc treadmill is at least a temporary fix.

But let’s not forget our lovers of science, and we salute you! May your votes move up from the 30% share of our survey where they currently stand. And if you need inspiration for your chosen career, why not check out the #IAmAScientistBecause Twitter campaign Naturejobs started last month.


 

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NEWS: Tracking a Serial Killer: Could Ebola Mutate to Become More Deadly?

Why we need to terminate Ebola 2014 before the virus learns too much about us.

The Ebola virus, seen here through a scanning electron microscope, is mutating as cases increase, raising the odds that the virus could become more transmissible.

Photograph by NIAID, Alamy

 

By David Quammen

National Geographic News

 

Published October 15, 2014

 

Forty years ago, Ebola was just the name of a river. It was a small waterway of no particularly sinister character that flowed through northern Zaire, not far from the village hospital where the first known outbreak of a new viral disease had been centered. That river gave its name to the new virus, and now "Ebola" is a global byword for ugly death, misery, and fear of contagion.

 

The 2014 epidemic of Ebola virus disease in West Africa is unprecedented in scope, and much attention has been focused, rightly, on how it has gotten so badly out of control.

 

Behind that question are three others, less obvious, more complicated, and crucial to seeing Ebola in a broader context: Where did the virus come from? Where is it going? What's next? We do well to consider these questions even as we react to the daily headlines, urge our leaders to take more deeply committed action, and support the organizations (such as Doctors Without Borders) that are fighting the epidemic so courageously in West Africa.

 

Where Did It Come From?

The outbreak began in early December, in a village called Meliandou, southeastern Guinea, not far from the borders with both Liberia and Sierra Leone. The first known case was a two-year-old child who died, after fever and vomiting and passing black stool, on December 6. The child's mother died a week later, then a sister and a grandmother, all with symptoms that included fever, vomiting, and diarrhea. Then, by way of caregiving visits or attendance at funerals, the outbreak spread to other villages.

See graphic:
Ebola: Tracking the Outbreak

It wasn't until March, three months later, that local officials alerted the Guinean Ministry of Health about these clusters of a strange, lethal disease in the countryside. By then, human-to-human transmission had started to multiply the case count. But tracing linked cases raises the question of ultimate origin. How did that first child get sick?

 

Ebola virus is a zoonosis, meaning an animal infection transmissible to humans. The animal in which a zoonosis lives its customary existence, discreetly, over the long term, and without causing symptoms, is called a reservoir host. The reservoir host of Ebola virus is still unknown—even after 38 years of efforts to identify it, since the original 1976 outbreak—although one or more kinds of fruit bat, including the hammer-headed bat, are suspects. There are hammer-headed bats in southeastern Guinea. It's possible that somebody killed one for food and brought it to Meliandou, where the child became infected either by direct contact with the bat or by virus passed on the hands of an adult.

 

Why are these facts and suppositions significant? Because they remind us that Ebola virus abides endemically in the forests of equatorial Africa. It will never be eradicated as long as those forests exist, unless the reservoir host itself is eradicated (not recommended) or cured of the viral infection (not likely possible). The virus may retire into its hiding place for years at a time, but eventually it will return, as a result of some disruptive contact by humans with the reservoir host. Then it will spill over into us again. All thinking and planning about how to defend against Ebola virus disease in the future needs to take account of that reality.

 

Another puzzling fact about origins is that the West Africa epidemic involves a species of ebolavirus (that's the label for the group, which includes five species) previously known only from outbreaks in the Democratic Republic of the Congo and its close neighbors.

 

A different species has emerged in Ivory Coast, another West African country, just east of Guinea and Liberia. According to a study published in Science in late August by Stephen K. Gire of Harvard and a long list of co-authors, the virus in West Africa seems to have diverged from its lineage in Central Africa just within the past decade. It somehow leapfrogged over or around the Ivory Coast ebolavirus in order to situate itself in southeastern Guinea. That suggests the unnerving prospect that the Central African ebolavirus (the only one strictly known as Ebola virus) is expanding its range, either by infecting new populations of reservoir hosts or by migrations of those host animals.

 

One way or another, it has been on the move.

 

Fruit bats are sold at an outdoor market in Brazzaville, capital of the Republic of the Congo. The reservoir host of Ebola virus is still unknown, but one or more kinds of fruit bat are suspects.

 

Photograph by Jiro Ose/Redux

 

Where Is It Going?

 

The virus has also traveled within living human bodies. We know that it went from Liberia to Dallas within the late Thomas Eric Duncan, from Liberia to Nigeria by way of the late Patrick Sawyer, and from Sierra Leone to Spain by way of two Spanish missionary priests, both also now deceased, who were evacuated for treatment.

 

And it has been carried to Omaha, Atlanta, London, Paris, Hamburg, Frankfurt, and Oslo within infected people, mostly health and aid workers brought home to be treated.

 

But just as worrisome as the virus's geographic spread is its journey across the evolutionary landscape. Is it mutating in ways that could make it more dangerous to humans? Is there any chance that it might become transmissible through the air, like the flu, the SARS virus, or a common cold?

Although Ebola becoming airborne is the ultimate disease nightmare, that seems to be almost vanishingly improbable, for reasons well put in a recent article in the Washington Post by Laurie Garrett, a senior fellow for global health at the Council on Foreign Relations. What is now a fluid-borne virus attaching itself to cells lining the circulatory system can't easily change into one that targets the tiny air sacs in the lungs.

 

"That's a genetic leap in the realm of science fiction," Garrett wrote.

 

The virus probably will not go airborne, but it could conceivably increase its Darwinian fitness in other ways, becoming more subtle and elusive.

 

The genetic study by Gire and his colleagues (five of whom were dead of Ebola by the time their study appeared) found 341 mutations as of late August, some of which are significant enough to change the bug's functional identity. The higher the case count in West Africa goes, the more chances for further mutations, and therefore the greater possibility that the virus might adapt somehow to become more transmissible-perhaps by becoming less pathogenic, sickening or killing its victims more slowly and thereby leaving them more time to infect others.

 

That's why, the Gire group wrote, we need to stop this thing everywhere as soon as possible. Future spillovers of Ebola are bound to occur, but those freshly emerged strains of the virus, direct from the reservoir host, won't contain any adaptive mutations that the West Africa strain is acquiring now.

We need to terminate Ebola 2014 before the virus learns too much about us.

 

Kumba Conde cries after her sister Marie, 14, died from Ebola in Koundony, Guinea, in July 2014. The current outbreak began in December 2013 in southeastern Guinea, not far from the borders with both Liberia and Sierra Leone.

 

Photograph by Samuel Aranda, The New York Times/Redux

 

What's Next?

 

No one knows, of course, how much worse the epidemic in West Africa will get. The U.S. Centers for Disease Control and Prevention issued a report, in late September, projecting that under the worst-case scenario there could be 1.4 million cases by early next year. The World Health Organization said Tuesday that new cases could rise to 10,000 per week by December, ten times the rate of the previous month. And the World Bank has warned that costs of the epidemic could reach $32.6 billion, which would be an economic catastrophe for the three West African countries that would compound their health catastrophes.

 

Will the epidemic spread more widely, igniting outbreaks in other parts of the world? We hope not. Will it turn up as additional cases, here and there, among people who have traveled from West Africa unaware, as Thomas Eric Duncan was reportedly unaware, that they were infected before boarding the airplane? Probably.

 

What's the best way to limit such occurrences? Rigorous screening at airports, quarantine for travelers who test positive, travel restrictions, or perhaps total bans on commercial flights arriving from Liberia, Guinea, and Sierra Leone-these measures should help. The most important and effective thing we can do, though, is to provide all possible assistance toward ending the outbreak where it began, in West Africa.

 

The world won't be free of Ebola 2014 until West Africa is free of it. Even severe restrictions, barring entry to anyone traveling from West Africa, would not make it impossible for the virus to get into America, or Europe, or wherever. To understand why, consider what I call the Nairobi Tabletop Scenario.

Imagine a doctor who departs from Monrovia, the capital of Liberia, feeling fine, on a flight to Nairobi, Kenya's capital, in East Africa. In transit he begins suffering a headache-nothing terrible yet, just discomfort, but it's the first hint of Ebola. At the Nairobi airport, in a café, the Liberian doctor coughs onto a table. Five minutes later, an American businessman touches that table. He rubs his eye. He departs to Singapore and spends three days there, in good health, discussing finance for his project in Kenya. Then he flies home to Los Angeles. To the screeners at LAX, he is an American businessman arriving from Singapore, with no history of recent travel in West Africa. But he's now infected with Ebola, carrying it into the United States.

 

How do you defend against the Nairobi Tabletop Scenario? By doing everything possible to end the epidemic in West Africa, and thereby to ensure that the Liberian doctor is healthy when he visits Nairobi.

 

Our safety against the menace of killer viruses can never be an absolute safety. There are too many of them, lurking within reservoir hosts amid distant forests or closer to home-viruses such as Nipah in Bangladesh, Marburg in Uganda, Lassa in West Africa, Sin Nombre virus in the American West, all the new influenzas coming out of southeastern Asia, plus many others that haven't yet been identified and named.

 

And there are too many of us humans, sharing the landscape with the reservoir hosts and with one another. We are too interconnected by air travel and transport. Viruses are simple organisms but well-adapted to the modern world. This year it's Ebola, devastating and scary. Next year it will be something else.

 

David Quammen is a contributing writer for National Geographic and author of Spillover and The Song of the Dodo, among other works. His new book Ebola: The Natural and Human History of a Deadly Virus will be published by W.W. Norton next week.

 

 

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Cardiac valve cells and their microenvironment—insights from in vitro studies

Nature Reviews Cardiology | Review

 

In conditions of prolonged stress valvular cells can contribute to valve disease progression. In this Review, Wang and colleagues describe how the biophysical and biochemical properties of the extracellular matrix... can regulate valve cell function in the context of calcific valvular diseases. The authors also describe how new cell culture approaches can be employed to better understand the pathophysiology of valve diseases.

 

Abstract

 

During every heartbeat, cardiac valves open and close coordinately to control the unidirectional flow of blood. In this dynamically challenging environment, resident valve cells actively maintain homeostasis, but the signalling between cells and their microenvironment is complex. When homeostasis is disrupted and the valve opening obstructed, haemodynamic profiles can be altered and lead to impaired cardiac function. Currently, late stages of cardiac valve diseases are treated surgically, because no drug therapies exist to reverse or halt disease progression. Consequently, investigators have sought to understand the molecular and cellular mechanisms of valvular diseases using in vitro cell culture systems and biomaterial scaffolds that can mimic the extracellular microenvironment. In this Review, we describe how signals in the extracellular matrix regulate valve cell function. We propose that the cellular context is a critical factor when studying the molecular basis of valvular diseases in vitro, and one should consider how the surrounding matrix might influence cell signalling and functional outcomes in the valve. Investigators need to build a systems-level understanding of the complex signalling network involved in valve regulation, to facilitate drug target identification and promote in situ or ex vivo heart valve regeneration.

 

Key points

• The interaction between valve cells and their microenvironment signals determine the functional output of cardiac valve tissues; cardiac valve diseases might be caused by a disruption of these interactions
• In vitro cell culture studies can help understand cardiac valve disease pathobiology, which is enhanced by culturing cells on biomaterials that mimic the native valve cell matrix environment
• Many biochemical signals regulate pathogenic phenotypes of cells in the valve; however, their effects are dependent on the matrix microenvironment
• Synthetic biomaterials can be used to understand valve cell regulation by biochemical, biophysical, and other matrix-associated signals, especially related to the development of valvular diseases
• Understanding the dynamic interplay between microenvironmental signals and valvular cell phenotypes will enable the identification of new therapies and the design of ex vivo tissue engineered valves

Introduction

 

As the heart evolved from a single-chamber to a multiple-chamber structure, cardiac valves arose to control the unidirectional flow of blood during cardiac cycles. For example, aortic valves open in response to higher blood pressure in the left ventricle than in the aorta, and close when the pressure equilibrates. These valves function in a similar manner to valves in water dams or car engines, but cardiac valves are living tissue with the ability to repair and remodel in response to damage. During an average human life span, heart valves open and close approximately 3 billion times,¹ withstanding various mechanical stresses, including fluid shear stresses and bending stretch.^{2, 3}

 



Figure 1: Valve cells and their matrix regulate tissue homeostasis and disease progression.

a | Cardiac valves have three extracellular matrix layers: the ventricularis, spongiosa, and fibrosa. In this image, a cross-section of a porcine aortic valve was visualized by second-harmonic generation confocal imaging. Elastin in green, collagen in red. Scale bar: 100 μm. b | Two main cell populations comprise the cardiac valve. VECs, which are CD31⁺ (green), line the boundaries of the leaflets. VICs reside throughout the valve leaflet. Nuclear staining (DAPI; blue), Scale bar: 100 μm. c | In healthy valves (left), very few VICs and VECs are activated to repair local tissue damage during normal valve function. Collagen fibres are circumferentially aligned, and activated VICs undergo continual turn over into deactivated fibroblasts and other cell fates (such as apoptosis). In diseased valves (right), VICs and VECs are recruited to remodel the tissue, leading to excessive collagen accumulation, a degraded and disarrayed matrix, which in some cases becomes calcified. Valve cells lose the homeostatic equilibrium and remain in an activated state. Abbreviations: VEC, valvular endothelial cell; VIC, valvular interstitial cell.





The material composition and structure of cardiac valves confer their robustness and durability. In humans, cardiac valves are made of thin (~500 μm) pliable cusps, and only mitral and tricuspid valves have supporting chordae tendineae and papillary muscles.⁴ A close examination of the tissue architecture of an aortic valve reveals three distinct layers of extracellular matrix (ECM), rich in collagens, proteoglycans, or elastin (Figure 1a).⁴ These ECM proteins impart unique macroscopic mechanical properties to valves, enabling them to withstand tension when closed and flexure when open. For example, the elastin fibres on the flow side of the valves (known as ventricularis) are radially aligned and elastic, which extend when the valves open and recoil when valves close.⁵ Proteoglycans in the middle layer, or spongiosa, function as a cushion for absorbing tension and friction between the top and the bottom layers.⁴ Finally, the fibrosa layer contains circumferentially oriented collagen fibres, which confer stiffness and strength to the valves.⁴



Figure 2: Dynamic cellular phenotypes in cardiac valve diseases.

a | Mineralized spherical particles (red) have been observed in human calcified valves, but not in normal valves. Scale bars 2 μm. b | Cells in the valve adopt diseased phenotypes (myofibroblast and osteoblast-like cells) in response to environmental signals. Activated myofibroblasts can revert back to quiescence, senescence, or undergo apoptosis. Osteoblast-like cells might also undergo apoptosis. (1) Myofibroblasts are characterized by ACTA2-positive stress fibres (green). Scale bar 50 μm. (2) VICs can be induced to mineralize, indicated by Alizarin Red S staining. Scale bar: 50 μm. Abbreviations: ACTA2, actin, aortic smooth muscle; VEC, valvular endothelial cell; VIC, valvular interstitial cell. Permission for panel a obtained from NPG © Bertazzo, S. et al. Nano-analytical electron microscopy reveals fundamental insights into human cardiovascular tissue calcification.

 

 







Figure 3: Matrix signals regulate valve cell phenotypes.

a | Plastic culture plates have high stiffness and biochemical signals must be introduced as soluble factors in the media. The stiffness of biomaterial-based cultures can mimic native tissues, and enables cells to grow in 3D space, with biochemical signals presented throughout the matrix. b | Adhesive signals (yellow in schematic) are tethered to an inert matrix to study specific cellular phenotype effects. For example, an elastin-derived peptide (VGVAPG, green bars) or a collagen-derived peptide (P15, blue bars) covalently linked to PEG hydrogels has distinct effects on ACTA2 levels. c | Elasticity can elicit intracellular cytoskeletal changes. In a modulus gradient of 7–32 kPa, >80% of PAVICs activated to myofibroblasts near the stiffer region and <10 1="" 3="" abbreviations:="" acta2="" actin="" after="" aortic="" became="" blue="" culture="" day="" days="" differences="" end="" i="" line="" muscle="" myofibroblasts="" near="" not="" observed="" of="" on="" red="" smooth="" softer="" the="" these="" were="">E

, Young's modulus; PAVIC, porcine aortic valve interstitial cell; PEG, poly(ethylene glycol). Permission for panel b obtained from Gould, S. T. et al. Small peptide functionalized thiol-ene hydrogels as culture substrates for understanding valvular interstitial cell activation and de novo tissue deposition. Acta Biomaterialia, 8, 3201–3209 (2012). Permission for panel c obtained from Kloxin, A. M. et al. In situ elasticity modulation with dynamic substrates to direct cell phenotype.





Figure 4: Valve cells and the ECM microenvironment are dynamic and mutually regulated.

Valvular interstitial cells interact with the ECM directly through integrins, with biochemical signals via receptors, and with neighbouring cells via cadherins. In healthy tissue (left), the cells and ECM are in a homeostatic state. When microenvironmental signals of valvular disease are present (right), cells can be activated, and biological outputs are altered. Secretion of ECM proteins, matrix-remodelling enzymes, and biochemical signals is increased; more contractile stress fibres and cell-matrix adhesions form; apoptosis increases where effective matrix interactions are lost. These cellular functions alter the microenvironment and valve tissue structure in diseased valves. Abbreviations: ACTA2, actin, aortic smooth muscle; ECM, extracellular matrix; MMP, matrix metalloproteinase.



                                                                                                                                                                                                                                                                                                                                                                                                                                         Huan Wang,^1, Leslie A. Leinwand^2, & Kristi S. Anseth^1,

 

 

Published online 14 October 2014

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Novel antiplatelet agents in acute coronary syndrome

Nature Reviews Cardiology | Review   

Dual antiplatelet therapy (DAPT) with aspirin and the P2Y purinoceptor 12 (P2Y12)-receptor inhibitor clopidogrel has been considered the gold standard of care in patients with acute coronary syndrome (ACS). The FDA approval of the novel P2Y12-receptor antagonists prasugrel and ticagrelor, which are faster-acting and more potent than clopidogrel, in the past 5 years have improved clinical outcomes in patients with ACS. Even with these newer compounds, many patients continue to experience adverse ischaemic events. This Review will provide an overview of the antiplatelet agents currently used to treat patients with ACS. Furthermore, emerging antiplatelet therapies, including intravenous P2Y12 antagonists, oral PAR-1 antagonists and thromboxane-receptor inhibitors will be discussed.

Abstract

For more than 10 years, dual antiplatelet therapy with aspirin and clopidogrel has remained the cornerstone of treatment for patients with acute coronary syndrome (ACS). The novel oral P2Y purinoceptor 12 (P2Y₁₂)-receptor inhibitors prasugrel and ticagrelor were approved by the FDA for clinical use in 2009 and 2011, respectively. These agents have a faster-acting, more-potent, and more-predictable antiplatelet effect than clopidogrel, which translates into improved clinical outcomes in patients with ACS, albeit at the expense of an increased risk of bleeding. However, some patients continue to experience adverse ischaemic events despite treatment with aspirin and a P2Y₁₂-receptor antagonist, because platelets can remain activated via pathways not inhibited by these agents, such as the protease-activated receptor (PAR)-1 platelet activation pathway stimulated by thrombin. Emerging antiplatelet therapies that might address these limitations include intravenous P2Y₁₂ antagonists, oral PAR-1 antagonists, and thromboxane-receptor inhibitors. In this Review, we provide an overview of these novel antiplatelet drugs, including newly approved agents and emerging compounds currently under clinical development, and also discuss evolving concepts and unmet needs related to antiplatelet therapy for the treatment of ACS.

At a glance

• Platelet adhesion, activation, and aggregation after plaque rupture or erosion are the major determinants of arterial thrombosis leading to acute coronary syndrome (ACS)
• Antiplatelet therapy, which targets pathways of platelet activation and aggregation, is pivotal in both the acute treatment and long-term secondary prevention of ischaemic events in patients with ACS
• Dual antiplatelet therapy with a combination of aspirin and either prasugrel or ticagrelor should be the treatment of choice in patients with ACS
• Drug selection should be based on contraindications and the individual patient's characteristics; clopidogrel should be used when both prasugrel and ticagrelor are contraindicated or not available
• Patients might continue to experience adverse events despite the use of prasugrel or ticagrelor
• New intravenous P2Y purinoceptor 12 inhibitors and agents that target other platelet-activation pathways have been developed

Figure 1: Mechanism of thrombus formation.

Plaque rupture exposes subendothelial components. Platelet adhesion during the rolling phase is mediated by interactions between vWF and GP Ib/V/IX receptor complexes located on the platelet surface, and between platelet collagen recep…

Figure 2: Platelet activation pathways targeted by current and novel antiplatelet agents.

Platelet adherence to injured vascular endothelium is mediated by binding of GP receptors to exposed extracellular matrix proteins (collagen and vWF). Complex intracellular signalling processes result in the local production and releas…

Figure 3: Mechanisms of action of P2Y₁₂ inhibitors.

Activation of P2Y₁₂ inhibits AC, causing a decrease in cAMP and VASP-P levels, and activation of P2Y₁ causes an increase in intracellular Ca²⁺ levels. These changes promote platelet aggregation by altering the ligand-binding properties…

Introduction

Arterial thrombosis after atherosclerotic plaque rupture or erosion is the major determinant of acute coronary syndrome (ACS).^{1, 2} Platelet adhesion, activation, and aggregation have important roles in the development of arterial thrombi; antiplatelet therapy is, therefore, pivotal in the treatment of ACS.^{2, 3, 4} Multiple platelet-signalling pathways are involved in thrombus formation and are potential targets for therapies.^{2, 3, 4} At present, three main classes of antiplatelet drugs are clinically approved for the treatment and secondary prevention of thrombotic complications in patients with ACS: oral cyclooxygenase-1 (COX-1; also known as prostaglandin G/H synthase 1) inhibitors, oral P2Y purinoceptor 12 (P2Y₁₂)-receptor inhibitors, and intravenous glycoprotein (GP) IIb/IIIa (also known as integrin α_IIbβ₃) inhibitors.^{5, 6, 7, 8}

The synergistic platelet-inhibitory effects exerted by dual antiplatelet therapy (DAPT) consisting of aspirin-induced blockade of COX-1 in conjunction with P2Y₁₂-receptor inhibition led to more than 10 years of clinical trials in patients with manifestations of ACS, including unstable angina, non-ST-segment elevation (NSTE) myocardial infarction (MI) and ST-segment elevation MI (STEMI).^{5, 6, 7, 8, 9, 10} Clopidogrel is the most broadly investigated P2Y₁₂-receptor inhibitor, owing to its favourable safety profile compared with ticlopidine.¹¹

Therefore, clopidogrel in combination with aspirin is still the cornerstone of treatment in patients with ACS.^{5, 6, 7, 8} However, despite the undisputable benefits of DAPT, a considerable number of patients continue to experience adverse thrombotic events.¹² These adverse events have been in part attributed to the nonuniform platelet inhibitory effects induced by clopidogrel, also known as interindividual clopidogrel response variability, where individuals who persist with high platelet reactivity despite clopidogrel therapy are at an increased risk of atherothrombotic recurrences.^{12, 13, 14, 15} Moreover, despite adequate COX-1 and P2Y₁₂-receptor blockade,^{3, 4} other platelet signalling pathways continue to be activated, and can contribute to thrombus formation. These observations have prompted the development of novel antithrombotic agents.

In this Review, we provide an overview of advances in antiplatelet therapy in the setting of ACS, focusing on novel drugs that have already been approved for clinical use, in addition to emerging agents at different phases of clinical development. However, a detailed description of the role of novel oral anticoagulants is beyond the scope of this Review, and the use of such agents is only briefly mentioned.

Mechanisms of atherothrombosis

Plaque rupture, fissure, or erosion exposes the subendothelial layer and leads to the recruitment and activation of platelets, as well as the generation of excessive levels of thrombin, which ultimately results in the formation of a fibrin-rich thrombus.^{1, 2, 3, 4} NSTE-ACS is frequently characterized by a nonocclusive or transiently occlusive thrombus, whereas a more stable and occlusive thrombus is typical in STEMI.¹ Platelet-activated thrombosis involves three principal steps: platelet adhesion, activation and recruitment of additional platelets, and platelet aggregation (Figure 1).^{3, 4, 16} Platelet adhesion during the initial rolling phase is mediated by interactions between the GP Ib/V/IX receptor complex, located on the surface of the platelet, and von Willebrand factor (vWF) bound to collagen exposed at the site of vascular injury.¹⁷ Rolling also brings the collagen receptors GP VI and GP Ia on platelets into contact with collagen.¹⁷ Binding of collagen to these receptors initiates platelet activation and triggers intracellular mechanisms that shifts platelet integrins to a high-affinity state, which induces the release of activating factors, in turn promoting aggregation, further recruitment, and further activation of circulating platelets.^{3, 4, 16, 17} These factors include ADP, epinephrine, serotonin, thrombin, and thromboxane A₂.^{3, 4} Platelet activation by these and other mediators (such as collagen) induces changes in platelet shape, expression of proinflammatory molecules (such as P-selectin and soluble CD40 ligand) and platelet procoagulant activity.^{3, 4, 16} The final step in platelet aggregation and thrombus formation involves the conversion of platelet GP IIb/IIIa (the main receptor mediating platelet aggregation) into its active form, which subsequently binds to the extracellular ligands fibrinogen and vWF, leading to platelet aggregation and thrombus formation mediated by platelet–platelet interaction.^{17, 18} In addition, vascular injury also exposes subendothelial tissue factor, which forms a complex with factor VIIa, activating the clotting cascade and leading to thrombin generation.¹⁹ However, only a modest amount of thrombin is produced as a result of the coagulation cascade; its main source within a platelet plug is the surface of activated platelets.^{16, 20} Thrombin converts fibrinogen to fibrin, generating a fibrin-rich clot, and further activates platelets by binding to protease-activated receptor (PAR)-1 and PAR-4.^{3, 4, 21, 22} Pathogenic thrombosis, therefore, involves a complex interplay between platelets and plasma components (coagulation factors) that interact in an auto-amplified process (Figure 1).

Francesco Franchi^1, & Dominick J. Angiolillo^1,

Nature Reviews Cardiology Year published: (2014) DOI: doi:10.1038/nrcardio.2014.156

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