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Current focus ::
NF-E2 & platelet biogenesis
B1-tubulin & platelet structure
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Past projects ::
GATA1 in megakaryopoiesis
Developmental hematopoiesis
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P l a t e l e t D i f f e r e n t i a t i o n
 
Circulating blood platelets are the end-products of differentiation of megakaryocytes (MKs), large but rare cells that populate the bone marrow and produce thousands of platelets each. MKs derive from hematopoietic stem cells and are most closely related in lineage to erythroid (red blood) cells. Although our present understanding of the molecular basis of thrombopoiesis is limited, it is increasingly clear that terminally mature MKs produce platelets through intermediate microtubule-based cytoplasmic extensions known as proplatelets. As the mature platelet form is actively generated within proplatelets, an important aspect is the creation of the stable platelet cytoskeleton, including the marginal band, a peripheral bundle of microtubule coils that is found along the platelet long axis and maintains discoid cell shape.
Mice lacking two erythro-megakaryocytic transcription factors, GATA-1 and NF-E2, show severely perturbed thrombopoiesis and provide an important avenue to study the underlying mechanisms. The basic-leucine zipper protein NF-E2 is particularly interesting because knockout mice have a complete absence of platelets and the defects are restricted to terminal stages of MK differentiation. In a screen for candidate transcriptional targets of NF-E2, we identified a few genes that encode structural or modulatory components of the cytoskeleton. These include the most divergent isoform of b1 tubulin (b1), which is exquisitely restricted in expression to mature MKs and the marginal bands of blood platelets. We generated mice with targeted disruption of the b1 tubulin gene locus. These mice show thrombocytopenia, failure to produce proplatelets in vitro, prolonged bleeding times, and a spherical rather than discoid platelet shape; the structure and integrity of the marginal microtubule band is severely compromised. The specific functions of individual b-tubulin isotypes has been a longstanding topic of debate. Mice specifically lacking b1 tubulin point to an essential role for this divergent, MK-specific isoform in formation and function of mammalian blood platelets.
The lab is now characterizing proteins that bind b1 tubulin selectively and the associated signaling and morphogenetic pathways.
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