Second Messenger oparated Ca+2 influx/SOCE

Second messenger-operated Ca2+ influx occurs through TRP and IP3 receptors and TMEM16Fe.g. activated rhodopsin activates PLC which produces DAG. DAG is converted to polyunsaturated fatty acids  like arachidonic acid (AA) by DAG lipase. These products can then activate TRP channels.TRP channels are 6TM ion channels. TRPC1-7 are candidate mediators for Ca2+ influx into platelets.

Application of 30µM ATP to a megakaryocyte results in an inward current with two phases: a large, transient influx through P2X and a smaller more sustained influx, mediated by P2Y. This is known as P2X -/- platelets still show the second component to the current.

Reverse transcription PCR of mRNA from megakaryocytes indicates that TRPC1 and TRPC6 are expressed in the cell. Only TRPC6 is activated by DAG so this is the only real candidate for mediating this current. Also, TRPC1 KO mice exhibit normal platelet responses.

Controversy surrounds this topic in the literature: some papers have suggested TRPC6 to play a role in thrombogenesis and haemostasis, whereas another paper has said TRPC6 -/- platelets are normal. A paper from last year suggested TRP channels on platelets function as coincidence detectors to mediate phosphatidylserine exposure. It has been proposed that Na+ influx through TRPC3 and 6 causes reversal of the sodium/calcium exchanger (NCX) to promote Ca2+ entry.

IP3 receptors are non-elective cation channels located in the ER  and are activated by cytosolic IP3/Ca2+, . In 2006, a paper provided evidence that there MAY be a PM-bound IP3R. This was thought as application of IP3 or an analog resulted in Ca2+ influx through the PM (this is not actually the case).

Yang et al. (2012) proposed TMEM16F to be cation-selective, allowing Ca2+ entry, which would pose another Ca2+ entry mechanism in these cells. However, Mahaut-Smith’s lab have been unable to replicate the Ca2+ current through TMEM16F. It is worth noting Yang et al.’s (2012) findings were under very high [Ca2+]o in excised patches. Addition to intracellular Ca2+ results in an outward current proportional to the amount inserted. Current recordings show strong outward rectification with reversal ~0mV - which does not correspond to TMEM16F -/-.

 The role of TMEM16F is still unclear. It may be that: Ca2+ entry through it may activate a scramblase, it may be a Ca2+-activated mediator of scrambles activity, or it may perform both functions of allowing Ca2+ entry and scrambling phospholipids. Yang et al.’s (2012) study suggests TMEM16F is important but not solely responsible for Ca2+-mediated scrambling for phosphatidyserine exposure (it still happens in TMEM16F KO).