Rôle des caspases dans la formation des plaquettes

Platelets are born from the cytoplasmic fragmentation of megakaryocytes by a highly dynamic process called proplatelet formation. At the end of differentiation, megakaryocytes form long pseudopods (proplatelets) which contain all the future platelet organelles (granules, especially α-granules and mitochondrion). The microtubules and the actin cytoskeleton play a key role in this process. Whereas megakaryocyte differentiation takes place in the marrow, platelet formation occurs in blood circulation, either by extruding proplatelets into marrow sinusoids, or by the entire migration into blood of megakaryocytes. Platelets are formed, either with their definitive spherical form at the distal end of the proplatelet, or by rupture of the proplatelets at the levels of constriction along their axis. The mechanisms which regulate proplatelet formation are poorly known, but there is increasing evidence that thrombopoietin is not directly involved in this process. Based on morphological observations, it has been suggested that platelet formation may be a form of apoptotic process. This hypothesis has been reinforced by the fact that senescent megakaryocytes (nucleus surrounded by a cytoplasm ring), which remain after proplatelet formation, are eliminated by apoptosis. Recent evidence demonstrates that the apoptotic machinery is necessary for the proplatelet formation process. Indeed, Bcl-2 and Bcl-xL (anti-apoptotic genes) transgenic mice or Bim (apoptotic gene) knockout mice are slightly thrombocytopenic with an increased number of marrow megakaryocytes. In human, Bcl-2 overexpression in megakaryocytes or caspase inhibitor addition in culture leads to an inhibition of proplatelet formation. Activated caspases 3 and 9 and cleavage of some of their substrates are detected in mature megakaryocytes, as well as in megakaryocytes undergoing proplatelet formation. Activation of caspases is compartimentalized in the cytoplasm of these megakaryocytes, whereas it is diffuse in the cytosol of senescent megakaryocytes. These results indicate that cleavage of some cellular substrates are absolutely necessary for proplatelet formation and that caspase may have, first a non apoptotic function in mature megakaryocyte when their activation is moderate and compartimentalized, and then, an apoptotic function when they are massively and diffusely activated in senescent megakaryocytes.