Improper Remodeling of Organelles Deputed to Ca2+ Handling and Aerobic ATP Production Underlies Muscle Dysfunction in Ageing

Abstract

Proper skeletal muscle function is controlled by intracellular Ca²⁺ concentration and by efficient production of energy (ATP), which, in turn, depend on: (a) the release and re-uptake of Ca²⁺ from sarcoplasmic-reticulum (SR) during excitation-contraction (EC) coupling, which controls the contraction and relaxation of sarcomeres; (b) the uptake of Ca²⁺ into the mitochondrial matrix, which stimulates aerobic ATP production; and finally (c) the entry of Ca²⁺ from the extracellular space via store-operated Ca²⁺ entry (SOCE), a mechanism that is important to limit/delay muscle fatigue. Abnormalities in Ca²⁺ handling underlie many physio-pathological conditions, including dysfunction in ageing. The specific focus of this review is to discuss the importance of the proper architecture of organelles and membrane systems involved in the mechanisms introduced above for the correct skeletal muscle function. We reviewed the existing literature about EC coupling, mitochondrial Ca²⁺ uptake, SOCE and about the structural membranes and organelles deputed to those functions and finally, we summarized the data collected in different, but complementary, projects studying changes caused by denervation and ageing to the structure and positioning of those organelles: a. denervation of muscle fibers-an event that contributes, to some degree, to muscle loss in ageing (known as sarcopenia)-causes misplacement and damage: (i) of membrane structures involved in EC coupling (calcium release units, CRUs) and (ii) of the mitochondrial network; b. sedentary ageing causes partial disarray/damage of CRUs and of calcium entry units (CEUs, structures involved in SOCE) and loss/misplacement of mitochondria; c. functional electrical stimulation (FES) and regular exercise promote the rescue/maintenance of the proper architecture of CRUs, CEUs, and of mitochondria in both denervation and ageing. All these structural changes were accompanied by related functional changes, i.e., loss/decay in function caused by denervation and ageing, and improved function following FES or exercise. These data suggest that the integrity and proper disposition of intracellular organelles deputed to Ca²⁺ handling and aerobic generation of ATP is challenged by inactivity (or reduced activity); modifications in the architecture of these intracellular membrane systems may contribute to muscle dysfunction in ageing and sarcopenia.

Keywords: Ca2+ entry unit (CEU); Ca2+ release unit (CRU); excitation–contraction (EC) coupling; mitochondria; sarcoplasmic-reticulum (SR); store-operated Ca2+ entry (SOCE); transverse tubule (TT).

Figure 1

Ca²⁺ handling in skeletal muscle fibers. Intracellular Ca²⁺ concentrations in muscle fibers depend on: (A) the release and re-uptake of Ca²⁺ from intracellular SR stores during EC coupling, which controls the contraction and relaxation of sarcomeres; (B) the uptake of Ca²⁺ into the mitochondrial matrix during excitation–metabolism coupling, which stimulates aerobic ATP production; and finally (C) the entry of Ca²⁺ from the extracellular space via SOCE, a mechanism that is important to limit/delay muscle fatigue.

Figure 2

Architecture of the membrane systems and organelles involved in Ca²⁺ handling and aerobic ATP production. (A–C) In adult mammalian fibers, triads (or CRUs) are placed approximately at the A–I band transition when sarcomeres are relaxed. The SR that does not participate in the formation of junctions with TT constitutes the longitudinal SR, placed either at the A or I band. Triads contain the molecular players of EC coupling (RYRs, DHPRs, and CASQ) shown in panel (B) and are tethered to mitochondria, which are preferentially placed at the I band. In panel C, the cartoon shows a mitochondrion-triad couple. (D–F) Following acute exercise, sarcotubular membranes at the I band (both SR and TTs), are capable of significant remodeling, which eventually leads to the assembly of CEUs. CEUs are formed by SR-stacks, coupled with a TT and contain the molecular players of SOCE (STIM1 and ORAI1) shown in panel (E). In panel (F), the cartoon shows a triad and a CEU, two intracellular junctions which are structurally and molecularly different. Scale bars: (D) 0.2 μm; (E) 0.1 μm.

Figure 3

Disarray of intracellular organelles in denervation and ageing. (A–D) Representative EM images of EDL muscle fibers from adult (A), aged (B,C) and denervated mice (D). Ageing and denervation cause a similar misplacement and damage of mitochondria (B,D, empty arrows). In panel (C), a TA in ageing muscle (black arrow) and a disordered TT network (dashed oval), labelled in black with ferrocyanide, are shown. (E) A cartoon summarizing the main structural changes in ageing and denervation: loss of proper position of triads and mitochondria, and mitochondrial damage. Scale bar: (A,B,D) 0.5 μm; (C), 1 μm; insets, 0.1 μm.