Ca²⁺ microdomains organized by junctophilins

Abstract

Excitable cells typically possess junctional membrane complexes (JMCs) constructed by the plasma membrane and the endo/sarcoplasmic reticulum (ER/SR) for channel crosstalk. These JMCs are termed triads in skeletal muscle, dyads in cardiac muscle, peripheral couplings in smooth and developing striated muscles, and subsurface cisterns in neurons. Junctophilin subtypes contribute to the formation and maintenance of JMCs by serving as a physical bridge between the plasma membrane and ER/SR membrane in different cell types. In muscle cells, junctophilin deficiency prevents JMC formation and functional crosstalk between cell-surface Ca(2+) channels and ER/SR Ca(2+) release channels. Human genetic mutations in junctophilin subtypes are linked to congenital hypertrophic cardiomyopathy and neurodegenerative diseases. Furthermore, growing evidence suggests that dysregulation of junctophilins induces pathological alterations in skeletal and cardiac muscle.

Keywords: Calcium channel; Excitation–contraction coupling; Muscle.

Figure 1. Junctional membrane complexes (JMCs) in striated muscles

Two types of three-dimensional (3D) electron microscopy (EM) technologies (EM tomography and serial block-face scanning EM) are used to demonstrate JMC nano 3D architectures in tibia anterior muscle (A, B) and in ventricular muscle (C, D) obtained from adult mice. Triads consisting of the terminal cisternae of the SR and T-tubules are located at the junction of A-band and I-band in fast skeletal muscles (A). The triad structure is depicted as a 3D geometric model reconstructed in EM tomography in B. In contrast, cardiac dyads formed by T-tubules and SR, are located near Z-lines (C, EM tomography) [8] and are widely distributed in the cytoplasm, associating with the T-tubule network (D, serial block-face scanning EM) [24]. E schematically shows the structure of JMCs and distribution of Ca²⁺ channel molecules in JMCs in cardiac muscle. DHPR, dihydropyridine receptor; RyR, ryanodine receptor; JP2, junctophilin 2; ER/SR, endo/sarcoplasmic reticulum; jSR, junctional SR; PM, plasma membrane; Nuc, nucleus.

Figure 2. Structure and function of junctophilin

(A) Hydropathicity profile of rabbit JP1 [48]. Repeated MORN motifs and an SR membrane-spanning segment are indicated. (B) Proposed JP structure in junctional membrane complex. (C) Phylogeny tree of junctophilin proteins. Invertebrates contain a single junctophilin gene, while there are four genes encoding tissue-specific junctophilin subtypes (JP1~JP4) in vertebrates. The simplified tree is adapted from a previous publication [87]. (D) Functional expression of rabbit JP1 in amphibian embryonic cells [48]. Exogenous JP1 proteins are localized to the cell periphery and form ER-PM complexes (upper left panel). Mutant JP1 lacking the C-terminal transmembrane segment remains tethered to the PM (upper right panel), but no junctional membrane complex is formed (lower panel).

Figure 3. Channel signalosomes in JMCs organized by JP subtypes

The hypothetical mechanisms are based on the data from knockout mice lacking ryanodine receptor or junctophilin subtypes. Channel communication in junctophilin-mediated membrane complexes may have gained functional diversity during the development of excitable cells. VDCC, voltage-dependent Ca²⁺ channel; RyR, ryanodine receptor channel; BK Ch, big-conductance Ca²⁺-dependent K⁺ channel; SK Ch, small-conductance Ca²⁺-dependent K⁺ channel; NMDAR, N-methyl-D-aspartate receptor channel; CaM, calmodulin; TnC, troponin C.

Figure 4. Mechanisms underlying JP2 dysregulation in heart disease

Depicted are miRNA-mediated gene silencing by miR-24, calpain-mediated proteolysis, and redistribution to the cell periphery in response to microtubule polymerization.