Role of oculocerebrorenal syndrome of Lowe (OCRL) protein in megakaryocyte maturation, platelet production and functions: a study in patients with Lowe syndrome

Roles of G proteins and their GTPase-activating proteins in platelets

Platelets are small anucleate blood cells supporting vascular function. They circulate in a quiescent state monitoring the vasculature for injuries. Platelets adhere to injury sites and can be rapidly activated to secrete granules and to form platelet/platelet aggregates. These responses are controlled by signalling networks that include G proteins and their regulatory guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Recent proteomics studies have revealed the complete spectrum of G proteins, GEFs, and GAPs present in platelets. Some of these proteins are specific for platelets and very few have been characterised in detail. GEFs and GAPs play a major role in setting local levels of active GTP-bound G proteins in response to activating and inhibitory signals encountered by platelets. Thus, GEFs and GAPs are highly regulated themselves and appear to integrate G protein regulation with other cellular processes. This review focuses on GAPs of small G proteins of the Arf, Rab, Ras, and Rho families, as well as of heterotrimeric G proteins found in platelets.

What can we learn from the platelet lipidome?

Besides their proteome, platelets use, in all responses to the environmental cues, a huge and diverse family of hydrophobic and amphipathic small molecules involved in structural, metabolic and signaling functions; the lipids. Studying how platelet lipidome changes modulate platelet function is an old story constantly renewed through the impressive technical advances allowing the discovery of new lipids, functions and metabolic pathways. Technical progress in analytical lipidomic profiling by top-of-the-line approaches such as nuclear magnetic resonance and gas chromatography or liquid chromatography coupled to mass spectrometry enables either large-scale analysis of lipids or targeted lipidomics. With the support of bioinformatics tools and databases, it is now possible to investigate thousands of lipids over a concentration range of several orders of magnitude. The lipidomic landscape of platelets is considered a treasure trove, not only able to expand our knowledge of platelet biology and pathologies but also to bring diagnostic and therapeutic opportunities. The aim of this commentary article is to summarize the advances in the field and to highlight what lipidomics can tell us about platelet biology and pathophysiology.

Soluble endoglin reduces thrombus formation and platelet aggregation via interaction with αIIbβ3 integrin

BACKGROUND

The circulating form of human endoglin (sEng) is a cleavage product of membrane-bound endoglin present on endothelial cells. Because sEng encompasses an RGD motif involved in integrin binding, we hypothesized that sEng would be able to bind integrin αIIbβ3, thereby compromising platelet binding to fibrinogen and thrombus stability.

METHODS

In vitro human platelet aggregation, thrombus retraction, and secretion-competition assays were performed in the presence of sEng. Surface plasmon resonance (SPR) binding and computational (docking) analyses were carried out to evaluate protein-protein interactions. A transgenic mouse overexpressing human sEng (hsEng+) was used to measure bleeding/rebleeding, prothrombin time (PT), blood stream, and embolus formation after FeCl3-induced injury of the carotid artery.

RESULTS

Under flow conditions, supplementation of human whole blood with sEng led to a smaller thrombus size. sEng inhibited platelet aggregation and thrombus retraction, interfering with fibrinogen binding, but did not affect platelet activation. SPR binding studies demonstrated that the specific interaction between αIIbβ3 and sEng and molecular modeling showed a good fitting between αIIbβ3 and sEng structures involving the endoglin RGD motif, suggesting the possible formation of a highly stable αIIbβ3/sEng. hsEng+ mice showed increased bleeding time and number of rebleedings compared to wild-type mice. No differences in PT were denoted between genotypes. After FeCl3 injury, the number of released emboli in hsEng+ mice was higher and the occlusion was slower compared to controls.

CONCLUSIONS

Our results demonstrate that sEng interferes with thrombus formation and stabilization, likely via its binding to platelet αIIbβ3, suggesting its involvement in primary hemostasis control.

Oculocerebrorenal syndrome of Lowe protein controls cytoskeletal reorganisation during human platelet spreading

Lowe syndrome (LS) is a rare, X-linked disorder characterised by numerous symptoms affecting the brain, the eyes, and the kidneys. It is caused by mutations in the oculocerebrorenal syndrome of Lowe (OCRL) protein, a 5-phosphatase localised in different cellular compartments that dephosphorylates phosphatidylinositol-4,5-bisphosphate into phosphatidylinositol-4-monophosphate. Some patients with LS also have bleeding disorders, with normal to low platelet (PLT) count and impaired PLT function. However, the mechanism of PLT dysfunction in patients with LS is not completely understood. The main function of PLTs is to activate upon vessel wall injury and stop the bleeding by clot formation. PLT activation is accompanied by a shape change that is a result of massive cytoskeletal rearrangements. Here, we show that OCRL-inhibited human PLTs do not fully spread, form mostly filopodia, and accumulate actin nodules. These nodules co-localise with ARP2/3 subunit p34, vinculin, and sorting nexin 9. Furthermore, OCRL-inhibited PLTs have a retained microtubular coil with high levels of acetylated tubulin. Also, myosin light chain phosphorylation is decreased upon OCRL inhibition, without impaired degranulation or integrin activation. Taken together, these results suggest that OCRL contributes to cytoskeletal rearrangements during PLT activation that could explain mild bleeding problems in patients with LS.

The inositol 5-phosphatase INPP5B regulates B cell receptor clustering and signaling

Upon antigen binding, the B cell receptor (BCR) undergoes clustering to form a signalosome that propagates downstream signaling required for normal B cell development and physiology. BCR clustering is dependent on remodeling of the cortical actin network, but the mechanisms that regulate actin remodeling in this context remain poorly defined. In this study, we identify the inositol 5-phosphatase INPP5B as a key regulator of actin remodeling, BCR clustering, and downstream signaling in antigen-stimulated B cells. INPP5B acts via dephosphorylation of the inositol lipid PI(4,5)P2 that in turn is necessary for actin disassembly, BCR mobilization, and cell spreading on immobilized surface antigen. These effects can be explained by increased actin severing by cofilin and loss of actin linking to the plasma membrane by ezrin, both of which are sensitive to INPP5B-dependent PI(4,5)P2 hydrolysis. INPP5B is therefore a new player in BCR signaling and may represent an attractive target for treatment of B cell malignancies caused by aberrant BCR signaling.

Quantification and optimization of clot retraction in washed human platelets by Sonoclot coagulation analysis

INTRODUCTION

Clot retraction is a pivotal process for haemostasis, where platelets develop a contractile force in fibrin meshwork and lead to the increased rigidity of clot. The pathophysiological alteration in contractile forces generated by the platelet-fibrin meshwork can lead to haemostatic disorders. Regardless of its utter significance, clot retraction remains a limited understood process owing to lack of quantification methodology. Sonoclot analysis is a point-of-care technique used in clinical laboratories for whole blood analysis that provides in vitro qualitative as well as quantitative assessment of coagulation process from initial fibrin formation to clot retraction.

METHODS

Human washed platelets were isolated by differential centrifugation method and analysed via optical imaging, microscopy and Sonoclot analysis using 1-2 × 10⁸ /mL of washed platelets, 1 U/mL of thrombin, 1 mg/mL of fibrinogen and 1 mM of calcium chloride.

RESULTS

In this study, we demonstrate the novelty of this instrument in the quantitative evaluation of clot retraction in washed platelets and attempted to optimize the reference range of Sonoclot parameters including ACT - 87.3 ± 20.997, CR - 16.23 ± 3.538 and PF - 3.57 ± 0.629, (n = 10).

DISCUSSION

Sonoclot analysis provides a simple and quantitative method to better understand in vitro clot retraction and its modulation by retraction components including platelet count, fibrinogen and platelet-fibrin interaction compared with existing conventional methods. Sonoclot may prove to be a valuable tool in thrombus biology research to understand fundamental basis of blood clot retraction.