SHPing in different directions in platelet production

Uncoupling ITIM receptor G6b-B from tyrosine phosphatases Shp1 and Shp2 disrupts murine platelet homeostasis

The immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptor G6b-B has emerged as a key regulator of platelet homeostasis. However, it remains unclear how it mediates its effects. Tyrosine phosphorylation of ITIM and immunoreceptor tyrosine-based switch motif (ITSM) within the cytoplasmic tail of G6b-B provides a docking site for Src homology 2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2, which are also critical regulators of platelet production and function. In this study, we investigate the physiological consequences of uncoupling G6b-B from Shp1 and Shp2. To address this, we generated a transgenic mouse model expressing a mutant form of G6b-B in which tyrosine residues 212 and 238 within ITIM and ITSM were mutated to phenylalanine. Mice homozygous for the mutation (G6b-B diY/F) were macrothrombocytopenic, as a result of the reduction in platelet production, and had large clusters of megakaryocytes and myelofibrosis at sites of hematopoiesis, similar to those observed in G6b-deficient mice and patients. Platelets from G6b-B diY/F mice were hyporesponsive to collagen, as a result of the significant reduction in the expression of the immunoreceptor tyrosine-based activation motif (ITAM)-containing collagen receptor complex GPVI-FcR γ-chain, as well as thrombin, which could be partially rescued by costimulating the platelets with adenosine diphosphate. In contrast, platelets from G6b-B diY/F, G6b KO, and megakaryocyte-specific Shp2 KO mice were hyperresponsive to antibody-mediated cross-linking of the hemi-ITAM-containing podoplanin receptor CLEC-2, suggesting that G6b-B inhibits CLEC-2-mediated platelet activation through Shp2. Findings from this study demonstrate that G6b-B must engage with Shp1 and Shp2 to mediate its regulatory effects on platelet homeostasis.

HIV-1 Tat C phosphorylates VE-cadherin complex and increases human brain microvascular endothelial cell permeability

Background

Human brain microvascular endothelial cells (hBMVECs) are integral part of the blood brain barrier. Post-translational modifications of adherens junction proteins regulate the permeability of human brain microvascular endothelial cells. Pro-inflammatory signals can induce tyrosine phosphorylation of adherens junction proteins. The primary objective of this work is to provide a molecular model; how the HIV-1 Tat protein can compromise the BBB integrity and eventually lead to neurological consequences. We exposed hBMVECs to recombinant HIV-1 clade C Tat protein to study the effect of HIV-1 Tat C on permeability of hBMVECs. Trans-endothelial electrical resistance and fluorescent dye migration assay have been used to check the permeability of hBMVECs. DCFDA staining has been used for intracellular reactive oxygen species (ROS) detection. Western blotting has been used to study the expression levels and co-immunoprecipitation has been used to study the interactions among adherens junction proteins.

Results

HIV-1 Tat C protein induced NOX2 and NOX4 expression level and increased intracellular ROS level. Redox-sensitive kinase; PYK2 activation led to increased tyrosine phosphorylation of VE-cadherin and β-catenin, leading to disruption of junctional assembly. The dissociation of tyrosine phosphatases VE-PTP and SHP2 from cadherin complex resulted into increased tyrosine phosphorylation of VE-cadherin and β-catenin in HIV-1 Tat C treated hBMVECs.

Conclusion

Unrestricted phosphorylation of junctional proteins in hBMVECs, in response to HIV-1 Tat C protein; leads to the disruption of junctional complexes and increased endothelial permeability.