Aberrant Microtubule Organization and Wiskott-Aldrich Syndrome-like Defects in Platelets and Megakaryocytes of Profilin1-Deficient Mice

Wiskott-Aldrich syndrome (WAS) is a rare, X-chromosomal recessive disorder which is caused by mutations in the WAS gene and characterized by eczema, immunodeficiency and microthrombocytopenia (Thrasher and Burns, Nat Rev Immunol 2010). Interestingly, WAS protein (WASp)-deficient mice have normal-sized platelets and thus the molecular link between WAS mutations and its central hallmark microthrombocytopenia remains elusive.

Profilin1 (Pfn1) is a key actin-regulating protein that, besides actin, interacts with phosphoinositides and multiple proline-rich proteins including the WAS protein (WASp)/WASp-interacting protein (WIP) complex (Witke, Trends Cell Biol 2004; Ramesh et al., Proc Natl Acad Sci USA, 1997). Interestingly, similar to WAS patients, mice with Pfn1-null megakaryocytes/platelets suffered from microthrombocytopenia. We identified accelerated platelet clearance by macrophages and pre-mature platelet release into the bone marrow compartment as the major cause of the reduced platelet count in Pfn1-deficient mice.

Both, platelets from Pfn1-null mice and WAS patients contained abnormally organized and hyperstable microtubules. We next tested, if increased microtubule stability could account for the reduced size of Pfn1-deficient platelets. Treatment of control platelets with microtubule stabilizing toxins, such as the histone-deacetylase inhibitor trichostatin A (TSA) or taxol resulted in a decreased platelet size. This finding indicates that increased microtubule stability could account for the reduced platelet size in Pfn1-deficient mice but also in WAS patients. Based on these results we speculate that WASp might modulate Pfn1 function and dysregulation of this interaction leads to increased stability and altered organization of microtubules. In support of this, the subcellular localization of Pfn1 was altered in platelets of three WAS patients.

Together, these results reveal an unexpected function of Pfn1 as a regulator of microtubule organization and point to a previously unrecognized mechanism underlying the platelet formation defect in WAS patients.