editorial comment
The coming of age of the megakaryocyte-platelet Nichola Cooper,1 James B. Bussel2 and Finbarr Cotter3 1
Department of Haematology, Hammersmith Hospital, London, UK, 2Pediatric Hematology-Oncology, Weill-Cornell Medical College, New York, NY, USA and 3Haemato-Oncology Centre, Barts Cancer Institute, Queen Mary University of London, London, UK
Until recently, the megakaryocyte-platelet system has been relatively unexplored. However, over the last 10 years, with the advent of modern biological techniques, there has been an exponential increase in the understanding of megakaryocytes and platelets. Nearly 20 years after the discovery of thrombopoietin (TPO, also termed THPO) and with the recent license of the TPO receptor agonists (TPO-RAs), this supplement brings together some of the world leaders in the biology of megakaryocytes and platelets. The reviews broadly fall in to four sections: what we have learnt from congenital platelet disorders; functional aspects of platelets; current understanding of megakaryocyte development and platelet production and finally, two reviews of the milestones of our understanding of megakaryocytes, platelets and TPO. The first three reviews focus on how congenital platelet disorders have helped the understanding of both platelet production and function. Although rare, many mutations result in disease without being lethal. Identification of these defects has helped to uncover not only the cause of disease, but have also been critical in helping to clarify aspects of the fundamental biology of the megakaryocyte and platelet. Nurden and Nurden provide a comprehensive overview of the known genetic causes of platelet function disorders. These mutations cause defects in surface constituents as well as intracellular components, including both signalling and organelle biosynthesis. In addition, the authors explain how the fundamental roles in biology of some of the genes that result in platelet function disorders, such as the MUNC and BEACH proteins, can explain other phenotypic changes associated with the thrombocytopenic syndromes. In the second review, Pecci and Balduini describe how congenital thrombocytopenic disorders have helped to identify genes critical for megakaryocyte development. The review covers 20 genes with a role in platelet production including mutations of the TPO receptor (MPL) in congenital amegakaryocytic thrombocytopenia mutations in signalling pathways
Correspondence: Dr Nichola Cooper, Department of Haematology, Hammersmith Hospital, Du Cane Road, London W12 0HS, UK. E-mail: [email protected]
ª 2014 John Wiley & Sons Ltd British Journal of Haematology, 2014, 165, 161–162
such as RUNX1 and GATA1, and mutations in genes involved in cytoskeletal development including MYH9, FLNA and WAS. These genetic defects have helped to piece together the complex movement of megakaryocytes along the bone marrow niche with the final delivery of platelets to the blood vessel and begin to identify some of the sophisticated processes required to generate platelets. In the third review, Daly et al describe how next generation sequencing (NGS) is likely to provide further information on the genetic basis of disease. Up to half of the patients with platelet function defects or thrombocytopenia remain undiagnosed. Diagnosis of platelet disorders has been limited by the lack of sensitivity of functional tests, especially in thrombocytopenic patients. This review explains the use of genetics for the diagnosis of platelet function disorders, from candidate gene analysis and the identification of diseases, such as Bernard Soulier, to genome-wide associations using linkage analysis in large families with platelet disorders. Combining linkage analysis with NGS and associating phenotype with genotype in projects such as in the GAPP (Genotyping And Platelet Phenotyping) study has helped to unravel novel genes involved in platelet function. This review also describes some of the pitfalls of genomic approaches and explains the potential future diagnostic approaches in patients with platelet defects. The next two reviews describe some of the additional functional activity of platelets (those not necessarily related to haemostasis/thrombosis): Golebiewska and Poole, in a review on platelet secretions, concentrate on novel findings in the production and secretion of granules from platelets, and what that tells us about the function of the platelet. They describe the different platelet granules, which secrete over 300 factors between them, and how both the granules and the factors themselves provide insight in to the many roles of the platelet: from thrombus formation to inflammation and immunity, to the potential role of platelets in tumour propagation. In addition to understanding the role of the granules, they also describe the importance of the tight regulation and orchestration of granule secretion – a process that is not yet fully understood. They explain the complex mechanism of protein transport, doi:10.1111/bjh.12799
Editorial comment membrane fusion and calcium-mediated granule release. Finally, they hypothesize on possible mechanisms of how careful orchestration of granule release could be achieved. In the second review of this section, Kile explains the process of apoptosis and its importance in the platelet. He defines the different aspects of apoptosis, including the principle regulators of the intrinsic and extrinsic pathways and the process of activation of the caspase cascade leading to cell death. Using evidence from the thrombocytopenia caused by cancer drugs and mouse models, the author explains how apoptotic pathways regulate the life span of the platelet in the circulation. He also observes some of the pitfalls of investigating programmed cell death, explaining the differences between apoptosis, pyroptosis and necroptosis as well as exploring the problems of differentiating activation from apoptosis in the context of the platelet (such as exposure of phosphatidylserine in both). The role of apoptosis in megakaryocyte development and platelet production remains a conundrum; this review discusses the controversial evidence of how these pathways may (or may not) influence platelet production in both health and disease. The subsequent two reviews focus on platelet production: Machlus et al explains the complexity of platelet production in ‘interpreting the megakaryocyte dance’. This review outlines the process of megakaryocyte development and platelet production. Complexities of this system include the role of TPO, the unique process of endomytosis as well as the importance of granule formation. It also describes new understanding of the development of the invaginated membrane system (also called the demarcation membrane system). It explores what is currently understood about proplatelet development, the importance of the microtubules and cytoskeleton and considers the controversial role of apoptosis. Despite major advances over the last 10–15 years, Machlus et al describe the limited understanding of the final process of release of platelets in the blood vessels. Utilizing the understanding of megakaryocyte development from the studies outlined in the megakaryocyte dance review, Avanzi and Mitchel describe current methods to produce platelets ex vivo. They explore the challenges of stem cell source, with marked differences between cord blood- and adult-derived stem cells. They discuss reprogramming of induced pluripotent stem cells; alteration of gene expression, and the importance of the stem cell niche in platelet production. They describe attempts to recreate the bone marrow niche, with use of NOTCH expression and the development of matrix systems to enhance platelet release. Overall, major hurdles still remain, with only a few platelets produced per
162
megakaryocyte using current protocols. Finally, they discuss the importance of ensuring that the in vitro-derived platelets are able to maintain their vital function. The last two reviews provide an overview of the critical developments and milestones in the understanding of platelet production. David Kuter and Ken Kaushansky were instrumental in identifying TPO and, in 2013, were recognized by the American Society of Haematology with the dual Beutler awards for their achievements. They each describe the cloning of TPO, which took place in their laboratories, together with others at around the same time. Kuter provides a comprehensive account of the last 100 years of megakaryocyte/platelet developments, starting with the first description of ‘blood dust’ in the 1880s. This review describes the milestones in the understanding of megakaryocytes and platelet production (including important figures), and the ideas and processes that lead to the cloning of TPO. It also describes the process behind the development of the TPO-RAs, from the first use of pegalated TPO and the subsequent antibody-associated aplastic anaemia in healthy individuals, through the remarkable planning surrounding the development of the two agents currently licensed for use in patients with immune thrombocytopenia. The review ends with a summary of the clinical outcome of these exciting new agents. Hitchcock and Kaushansky further explore the story of TPO; explaining its production and regulation, its vital role for not only the production of platelets, but also for the maintenance of haematopoietic stem cells. They explain the structure of TPO and its receptor (c-MPL, now termed MPL) and the downstream pathways activated through the receptor, focusing also on the contribution of alterations in this system in disease. They explain how the identification of the c-MPL receptor and the cloning of TPO nearly 20 years ago have provided the tools necessary for unlocking many of the findings described in earlier reviews. Despite the comprehensive, novel and topical reviews in this supplement, many aspects remain uncovered. These include the role of the megakaryocyte in the stem cell niche, the role of platelets in cancer and metastases, the role of the platelet in immunity, and the difference in megakaryocytes and platelets in the fetus and newborn. We are only in the infancy of our understanding of the role of both the megakaryocyte and the platelet, and the holy grail of in vitro production of platelets has yet to be realized. All of the reviews are interesting reading. We highly recommend reading this selection of megakaryocyte-platelet papers.
ª 2014 John Wiley & Sons Ltd British Journal of Haematology, 2014, 165, 161–162
The coming of age of the megakaryocyte-platelet Nichola Cooper,1 James B. Bussel2 and Finbarr Cotter3 1
Department of Haematology, Hammersmith Hospital, London, UK, 2Pediatric Hematology-Oncology, Weill-Cornell Medical College, New York, NY, USA and 3Haemato-Oncology Centre, Barts Cancer Institute, Queen Mary University of London, London, UK
Until recently, the megakaryocyte-platelet system has been relatively unexplored. However, over the last 10 years, with the advent of modern biological techniques, there has been an exponential increase in the understanding of megakaryocytes and platelets. Nearly 20 years after the discovery of thrombopoietin (TPO, also termed THPO) and with the recent license of the TPO receptor agonists (TPO-RAs), this supplement brings together some of the world leaders in the biology of megakaryocytes and platelets. The reviews broadly fall in to four sections: what we have learnt from congenital platelet disorders; functional aspects of platelets; current understanding of megakaryocyte development and platelet production and finally, two reviews of the milestones of our understanding of megakaryocytes, platelets and TPO. The first three reviews focus on how congenital platelet disorders have helped the understanding of both platelet production and function. Although rare, many mutations result in disease without being lethal. Identification of these defects has helped to uncover not only the cause of disease, but have also been critical in helping to clarify aspects of the fundamental biology of the megakaryocyte and platelet. Nurden and Nurden provide a comprehensive overview of the known genetic causes of platelet function disorders. These mutations cause defects in surface constituents as well as intracellular components, including both signalling and organelle biosynthesis. In addition, the authors explain how the fundamental roles in biology of some of the genes that result in platelet function disorders, such as the MUNC and BEACH proteins, can explain other phenotypic changes associated with the thrombocytopenic syndromes. In the second review, Pecci and Balduini describe how congenital thrombocytopenic disorders have helped to identify genes critical for megakaryocyte development. The review covers 20 genes with a role in platelet production including mutations of the TPO receptor (MPL) in congenital amegakaryocytic thrombocytopenia mutations in signalling pathways
Correspondence: Dr Nichola Cooper, Department of Haematology, Hammersmith Hospital, Du Cane Road, London W12 0HS, UK. E-mail: [email protected]
ª 2014 John Wiley & Sons Ltd British Journal of Haematology, 2014, 165, 161–162
such as RUNX1 and GATA1, and mutations in genes involved in cytoskeletal development including MYH9, FLNA and WAS. These genetic defects have helped to piece together the complex movement of megakaryocytes along the bone marrow niche with the final delivery of platelets to the blood vessel and begin to identify some of the sophisticated processes required to generate platelets. In the third review, Daly et al describe how next generation sequencing (NGS) is likely to provide further information on the genetic basis of disease. Up to half of the patients with platelet function defects or thrombocytopenia remain undiagnosed. Diagnosis of platelet disorders has been limited by the lack of sensitivity of functional tests, especially in thrombocytopenic patients. This review explains the use of genetics for the diagnosis of platelet function disorders, from candidate gene analysis and the identification of diseases, such as Bernard Soulier, to genome-wide associations using linkage analysis in large families with platelet disorders. Combining linkage analysis with NGS and associating phenotype with genotype in projects such as in the GAPP (Genotyping And Platelet Phenotyping) study has helped to unravel novel genes involved in platelet function. This review also describes some of the pitfalls of genomic approaches and explains the potential future diagnostic approaches in patients with platelet defects. The next two reviews describe some of the additional functional activity of platelets (those not necessarily related to haemostasis/thrombosis): Golebiewska and Poole, in a review on platelet secretions, concentrate on novel findings in the production and secretion of granules from platelets, and what that tells us about the function of the platelet. They describe the different platelet granules, which secrete over 300 factors between them, and how both the granules and the factors themselves provide insight in to the many roles of the platelet: from thrombus formation to inflammation and immunity, to the potential role of platelets in tumour propagation. In addition to understanding the role of the granules, they also describe the importance of the tight regulation and orchestration of granule secretion – a process that is not yet fully understood. They explain the complex mechanism of protein transport, doi:10.1111/bjh.12799
Editorial comment membrane fusion and calcium-mediated granule release. Finally, they hypothesize on possible mechanisms of how careful orchestration of granule release could be achieved. In the second review of this section, Kile explains the process of apoptosis and its importance in the platelet. He defines the different aspects of apoptosis, including the principle regulators of the intrinsic and extrinsic pathways and the process of activation of the caspase cascade leading to cell death. Using evidence from the thrombocytopenia caused by cancer drugs and mouse models, the author explains how apoptotic pathways regulate the life span of the platelet in the circulation. He also observes some of the pitfalls of investigating programmed cell death, explaining the differences between apoptosis, pyroptosis and necroptosis as well as exploring the problems of differentiating activation from apoptosis in the context of the platelet (such as exposure of phosphatidylserine in both). The role of apoptosis in megakaryocyte development and platelet production remains a conundrum; this review discusses the controversial evidence of how these pathways may (or may not) influence platelet production in both health and disease. The subsequent two reviews focus on platelet production: Machlus et al explains the complexity of platelet production in ‘interpreting the megakaryocyte dance’. This review outlines the process of megakaryocyte development and platelet production. Complexities of this system include the role of TPO, the unique process of endomytosis as well as the importance of granule formation. It also describes new understanding of the development of the invaginated membrane system (also called the demarcation membrane system). It explores what is currently understood about proplatelet development, the importance of the microtubules and cytoskeleton and considers the controversial role of apoptosis. Despite major advances over the last 10–15 years, Machlus et al describe the limited understanding of the final process of release of platelets in the blood vessels. Utilizing the understanding of megakaryocyte development from the studies outlined in the megakaryocyte dance review, Avanzi and Mitchel describe current methods to produce platelets ex vivo. They explore the challenges of stem cell source, with marked differences between cord blood- and adult-derived stem cells. They discuss reprogramming of induced pluripotent stem cells; alteration of gene expression, and the importance of the stem cell niche in platelet production. They describe attempts to recreate the bone marrow niche, with use of NOTCH expression and the development of matrix systems to enhance platelet release. Overall, major hurdles still remain, with only a few platelets produced per
162
megakaryocyte using current protocols. Finally, they discuss the importance of ensuring that the in vitro-derived platelets are able to maintain their vital function. The last two reviews provide an overview of the critical developments and milestones in the understanding of platelet production. David Kuter and Ken Kaushansky were instrumental in identifying TPO and, in 2013, were recognized by the American Society of Haematology with the dual Beutler awards for their achievements. They each describe the cloning of TPO, which took place in their laboratories, together with others at around the same time. Kuter provides a comprehensive account of the last 100 years of megakaryocyte/platelet developments, starting with the first description of ‘blood dust’ in the 1880s. This review describes the milestones in the understanding of megakaryocytes and platelet production (including important figures), and the ideas and processes that lead to the cloning of TPO. It also describes the process behind the development of the TPO-RAs, from the first use of pegalated TPO and the subsequent antibody-associated aplastic anaemia in healthy individuals, through the remarkable planning surrounding the development of the two agents currently licensed for use in patients with immune thrombocytopenia. The review ends with a summary of the clinical outcome of these exciting new agents. Hitchcock and Kaushansky further explore the story of TPO; explaining its production and regulation, its vital role for not only the production of platelets, but also for the maintenance of haematopoietic stem cells. They explain the structure of TPO and its receptor (c-MPL, now termed MPL) and the downstream pathways activated through the receptor, focusing also on the contribution of alterations in this system in disease. They explain how the identification of the c-MPL receptor and the cloning of TPO nearly 20 years ago have provided the tools necessary for unlocking many of the findings described in earlier reviews. Despite the comprehensive, novel and topical reviews in this supplement, many aspects remain uncovered. These include the role of the megakaryocyte in the stem cell niche, the role of platelets in cancer and metastases, the role of the platelet in immunity, and the difference in megakaryocytes and platelets in the fetus and newborn. We are only in the infancy of our understanding of the role of both the megakaryocyte and the platelet, and the holy grail of in vitro production of platelets has yet to be realized. All of the reviews are interesting reading. We highly recommend reading this selection of megakaryocyte-platelet papers.
ª 2014 John Wiley & Sons Ltd British Journal of Haematology, 2014, 165, 161–162
