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Cytoskeletal disrupting agents prevent calmodulin kinase, IQ domain and voltage-dependent facilitation of L-type Ca²⁺ channels

Igor Dzhura *, Yuejin Wu *, Roger J. Colbran †, Jackie D. Corbin †, Jeffrey R. Balser ‡§ and Mark E. Anderson *§

A calmodulin (CaM) binding 'IQ' domain on the L-type Ca²⁺ channel (LTCC) C terminus and calmodulin kinase II (CaMK) both signal increases in LTCC opening probability (P_o) by shifting LTCCs into a gating mode (mode 2) with long openings through a process called facilitation. However, the mechanism whereby CaMK and the IQ domain are targeted to LTCCs is unknown. Endogenous CaMK is targeted to LTCCs in excised cell membrane patches because LTCC P_o increased significantly in CaM-enriched (20 µM) bath solution and this effect was prevented by a specific CaMK inhibitory peptide, but not by an inactive control peptide. Pre-exposure of myocytes to the cytoskeletal disrupting agents nocodazole (microtubule specific) or cytochalasin D (microfilament specific) prevented the effects of CaM-dependent increases in P_o of LTCCs in excised membrane patches. Neither cytochalasin D nor nocodazole altered the distribution of LTCC gating modes under basal conditions in on-cell mode or excised cell membrane patches, but each of these agents occluded the response of LTCCs to exogenous, constitutively active CaMK and to an IQ-mimetic peptide (IQmp). Cytochalasin D and nocodazole pretreatment also prevented LTCC facilitation that followed a cell membrane depolarizing prepulse. In contrast, cytochalasin D and nocodazole did not affect the increase in LTCC P_o or prevent the shift to mode 2 gating in response to protein kinase A, indicating that cytoskeletal disruption specifically prevents prepulse, CaMK and IQ-dependent LTCC facilitation.

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