CD148: a positive regulator of GPVI and \(\alpha\)II\(\beta\)3 proximal signalling in platelets

Platelets are small anucleate blood cells that plug holes in damage blood vessels. They do so by adhering to exposed extracellular matrix proteins at sites of injury and aggregating together. Platelet responsiveness to injury is controlled by a diverse repertoire of surface receptors that can be divided into two broad categories based on how they signal; the tyrosine kinased-linked receptors and the G protein-coupled receptors (GPCRs). There has been much work on elucidating the functions of tyrosine kinases in platelets, whereas protein tyrosine phosphatases (PTPs) have been under-investigated. To date, six non-transmembrane PTPs (NTPTPs), PTP-1B, Shp1, Shp2, MEG2-PTP, LMW-PTP and HePTP and a single receptor-like PTP (RPTP), CD148, have been identified in platelets. The main objective of this thesis was to determine the functional role of CD148 in platelets, which had never been studied in platelets. Using a mouse model, I demonstrate that CD148 is a critical positive regulator of signalling from the main collagen activation receptor GPVI and the fibrinogen integrin \(\alpha\)II\(\beta\)3, and also plays a minor role in regulating thrombin and thromboxane A\(_\\) (TxA\(_2\) mediated aggregation and secretion via the PAR-4 and TP receptors, respectively. The molecular mechanism of how CD148 regulates signalling from so many receptors is by maintaining a pool of active Src family kinases (SFKs) in platelets, which it does by dephosphorylating a tyrosine residue in the C-terminal of all SFKs. In an attempt to identify other PTPs that perform a similar function to CD148 in platelets, I analyzed platelets from PTP-1B- and TC-PTP-deficient mouse models for functional and phosphorylation defects. PTP-1B-deficient platelets exhibited minor aggregation/secretion and phosphorylation defects relative to CD148-deficient platelets; and TC-PTP, which I show to be expressed in human and mouse platelets for the first time, is involved in platelet development. My conclusion is that CD148, PTP-1B and TC-PTP have distinct functional roles in platelets