Phosphoinositide substrates of myotubularin affect voltage-activated Ca2+ release in skeletal muscle

Skeletal muscle excitation–contraction (E–C) coupling is altered in several models of phosphatidylinositol phosphate (PtdInsP) phosphatase deficiency and ryanodine receptor activity measured in vitro was reported to be affected by certain PtdInsPs, thus prompting investigation of the physiological role of PtdInsPs in E–C coupling. We measured intracellular Ca²⁺ transients in voltage-clamped mouse muscle fibres microinjected with a solution containing a PtdInsP substrate (PtdIns(3,5)P ₂ or PtdIns(3)P) or product (PtdIns(5)P or PtdIns) of the myotubularin phosphatase MTM1. No significant change was observed in the presence of either PtdIns(5)P or PtdIns but peak SR Ca²⁺ release was depressed by ~30% and 50% in fibres injected with PtdIns(3,5)P ₂ and PtdIns(3)P, respectively, with no concurrent alteration in the membrane current signals associated with the DHPR function as well as in the voltage dependence of Ca²⁺ release inactivation. In permeabilized muscle fibres, the frequency of spontaneous Ca²⁺ release events was depressed in the presence of the three tested phosphorylated forms of PtdInsP with PtdIns(3,5)P ₂ being the most effective, leading to an almost complete disappearance of Ca²⁺ release events. Results support the possibility that pathological accumulation of MTM1 substrates may acutely depress ryanodine receptor-mediated Ca²⁺ release. Overexpression of a mCherry-tagged form of MTM1 in muscle fibres revealed a striated pattern consistent with the triadic area. Ca²⁺ release remained although unaffected by MTM1 overexpression and was also unaffected by the PtdIns-3-kinase inhibitor LY2940002, suggesting that the 3-phosphorylated PtdIns lipids active on voltage-activated Ca²⁺ release are inherently maintained at a low level, inefficient on Ca²⁺ release in normal conditions.