engleski

The oculocerebrorenal syndrome of lowe protein controls actin and microtubule rearrangements during human platelet spreading

Lowe syndrome (LS) is a rare X-linked disorder characterized by symptoms affecting the brain, the eyes, and the kidneys leading to central hypotonia, mental retardation, glaucoma, congenital cataracts, and renal Fanconi syndrome. LS is caused by mutations in the oculocerebrorenal syndrome of Lowe protein (OCRL) which is a 5- phosphatase that dephosphorylates phosphatidylinositol-4, 5-bisphosphate [PI(4, 5)P2] to phosphatidylinositol-4-monophosphate (PI4P). Numerous roles of PI(4, 5)P2 in the cell include the regulation of actin nucleation and reorganization which is a crucial step during the activation of the smallest blood cells, platelets (PLTs). Upon vessel wall injury, PLTs adhere to the exposed molecules of the extracellular matrix, activate, aggregate, and with the help of fibrinogen form a clot. Furthermore, it has been shown that PLTs have impaired wound closure times in some LS patients. Therefore, we hypothesized that OCRL could have an important role during PLT spreading. We show by immunocytochemistry that inhibition of OCRL with the YU142670 inhibitor impairs PLT spreading on fibrinogen, resulting in the extensive formation of actin nodules as opposed to the actin stress fibres of control PLTs. These actin nodules colocalize with proteins implicated in actin dynamics (ARP2/3, vinculin, SNX9) and with phospho-tyrosines showing they are sites of active signalling. Although OCRL- inhibited PLTs have impaired actin reorganization, the flow cytometry analysis revealed no change in the net F-actin levels. However, Western Blot analysis showed that OCRL inhibition decreases the levels of myosin light chain (MLC) phosphorylation upon stimulation with thrombin and TRAP-6. Interestingly, OCRL inhibition also impairs the disassembly of the microtubular coil and increases the levels of acetylated tubulin. Impaired cytoskeletal rearrangements were confirmed by electron microscopy. The OCRL inhibition did not alter the release of PLT granules and activation of integrins after stimulation of thrombin or collagen receptors. We conclude that OCRL has an important role in the reorganization of actin and tubulin cytoskeleton during PLT spreading but does not affect PLT degranulation or integrin activation which could explain the mild bleeding problems of LS patients.

OCRL, PI(4, 5)P2, platelets, Lowe syndrome, microtubules

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