Delivering new insight into the biology of megakaryopoiesis and thrombopoiesis

Expression profiling of apoptosis-related genes in megakaryocytes: BNIP3 is downregulated in primary myelofibrosis

OBJECTIVE

In order to identify factors involved in the aberrantly regulated apoptosis of megakaryocytes in primary myelofibrosis (PMF), the mRNA expression of human megakaryocytes in situ was quantified by real-time polymerase chain reaction low-density arrays.

MATERIALS AND METHODS

The mRNA from 200 to 300 laser-microdissected megakaryocytes per case from PMF (n=22) and control (n=10) bone marrow was reverse-transcribed into cDNA by random priming and subsequently amplified by primer-specific cDNA amplification. The mRNA of corresponding total bone marrow cells was reverse-transcribed into cDNA without the following amplification. For relative mRNA quantification, custom-made TaqMan low-density arrays with a setup of 48 different genes were applied. In addition, methylation analysis and immunohistochemistry of a selected candidate gene were accomplished.

RESULTS

A trend toward an overall downregulation of apoptosis-associated genes could be observed in megakaryocytes, whereas the total bone marrow cellularity exhibited an overall upregulation of these factors. Among several candidates with statistically significant deregulation BCL2/adenovirus E1B 19 kDa interacting protein 3 (BNIP3) and protein kinase C beta1 were shown to be the most aberrantly expressed genes.

CONCLUSION

Apoptosis-related gene expression profiling of human megakaryocytes reveals a set of candidates, most notably BNIP3, indicating that the increase of megakaryocytes in myeloproliferative neoplasia might not only be the result of increased proliferation but also of disturbed apoptosis.

The role of vasoactive intestinal peptide (VIP) in megakaryocyte proliferation

Megakaryocytopoiesis is a multistage process that involves differentiation of hematopoietic stem cells through the myeloid lineage, ultimately producing megakaryocytes and platelets. Vasoactive intestinal peptide (VIP) stimulates adenylate cyclase and induces differentiation in multiple cell types; VIP is expressed in hematopoietic stem cells and in megakaryocytes, but its function in these cells has not yet been delineated. The present study was designed to investigate whether the type 1 VIP receptor, VPAC1, mediates VIP effects on megakaryocytopoiesis. The human megakaryoblastic leukemia cell line (CMK) was transfected with VPAC1 and the transgene expression was confirmed by qualitative polymerase chain reaction and immunohistochemistry. The rate of proliferation and the patterns of differentiation were then compared for CMK and CMK/VPAC1 through multiple growth cycles. Upregulation of VPAC1 expression resulted in a decreased proliferation rate (p = 0.0003) and enhanced differentiation with CMK/VPAC1 cells having twice the cell surface area of control CMK cells (p = 0.001), thus increasing potential for proplatelet formation. These results suggest that VIP acts in an autocrine fashion via VPAC1 to inhibit megakaryocyte proliferation and induce proplatelet formation.

Downregulation of signal transducer and activator of transcription 5 (STAT5) in CD34+ cells promotes megakaryocytic development, whereas activation of STAT5 drives erythropoiesis

Although it has been proposed that the common myeloid progenitor gives rise to granulocyte/monocyte progenitors and megakaryocyte/erythroid progenitors (MEP), little is known about molecular switches that determine whether MEPs develop into either erythrocytes or megakaryocytes. We used the thrombopoietin receptor c-Mpl, as well as the megakaryocytic marker CD41, to optimize progenitor sorting procedures to further subfractionate the MEP (CD34(+)CD110(+)CD45RA(-)) into erythroid progenitors (CD34(+)CD110(+)CD45RA(-)CD41(-)) and megakaryocytic progenitors (CD34(+)CD110(+)CD45RA(-)CD41(+)) from peripheral blood. We have identified signal transducer and activator of transcription 5 (STAT5) as a critical denominator that determined lineage commitment between erythroid and megakaryocytic cell fates. Depletion of STAT5 from CD34(+) cells by a lentiviral RNAi approach in the presence of thrombopoietin and stem cell factor resulted in an increase in megakaryocytic progenitors (CFU-Mk), whereas erythroid progenitors (BFU-E) were decreased. Furthermore, an increase in cells expressing megakaryocytic markers CD41 and CD42b was observed in STAT5 RNAi cells, as was an increase in the percentage of polyploid cells. Reversely, overexpression of activated STAT5A(1*6) mutants severely impaired megakaryocyte development and induced a robust erythroid differentiation. Microarray and quantitative reverse transcription-polymerase chain reaction analysis revealed changes in expression of a number of genes, including GATA1, which was downmodulated by STAT5 RNAi and upregulated by activated STAT5.