title: Phosphoinositide substrates of myotubularin affect voltage-activated Ca²⁺ release in skeletal muscle.
abstract: 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)P2 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)P2 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)P2 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.
authors.forename: Estela González
authors.initials: EG
authors.lastname: Rodríguez
authors.forename: Romain
authors.initials: R
authors.lastname: Lefebvre
authors.forename: Dóra
authors.initials: D
authors.lastname: Bodnár
authors.forename: Claude
authors.initials: C
authors.lastname: Legrand
authors.forename: Peter
authors.initials: P
authors.lastname: Szentesi
authors.forename: János
authors.initials: J
authors.lastname: Vincze
authors.forename: Karine
authors.initials: K
authors.lastname: Poulard
authors.forename: Justine
authors.initials: J
authors.lastname: Bertrand-Michel
authors.forename: Laszlo
authors.initials: L
authors.lastname: Csernoch
authors.forename: Anna
authors.initials: A
authors.lastname: Buj-Bello
authors.forename: Vincent
authors.initials: V
authors.lastname: Jacquemond
mesh_terms: D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D056966:Excitation Contraction Coupling; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000378:metabolism* / Q000502:physiology; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism*; D054558:Protein Tyrosine Phosphatases, Non-Receptor / Q000235:genetics / Q000378:metabolism*
chemical_list: D015220:Calcium Channels; D018129:Phosphatidylinositol Phosphates; D054558:Protein Tyrosine Phosphatases, Non-Receptor; C099588:myotubularin; D002118:Calcium
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