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In vivo microscopy reveals extensive embedding of capillaries within the sarcolemma of skeletal muscle fibers.

ABSTRACT: OBJECTIVE:To provide insight into mitochondrial function in vivo, we evaluated the 3D spatial relationship between capillaries, mitochondria, and muscle fibers in live mice. METHODS:3D volumes of in vivo murine TA muscles were imaged by MPM. Muscle fiber type, mitochondrial distribution, number of capillaries, and capillary-to-fiber contact were assessed. The role of Mb-facilitated diffusion was examined in Mb KO mice. Distribution of GLUT4 was also evaluated in the context of the capillary and mitochondrial network. RESULTS:MPM revealed that 43.6 ± 3.3% of oxidative fiber capillaries had ?50% of their circumference embedded in a groove in the sarcolemma, in vivo. Embedded capillaries were tightly associated with dense mitochondrial populations lateral to capillary grooves and nearly absent below the groove. Mitochondrial distribution, number of embedded capillaries, and capillary-to-fiber contact were proportional to fiber oxidative capacity and unaffected by Mb KO. GLUT4 did not preferentially localize to embedded capillaries. CONCLUSIONS:Embedding capillaries in the sarcolemma may provide a regulatory mechanism to optimize delivery of oxygen to heterogeneous groups of muscle fibers. We hypothesize that mitochondria locate to PV regions due to myofibril voids created by embedded capillaries, not to enhance the delivery of oxygen to the mitochondria.

SUBMITTER: Glancy B

PROVIDER: S-EPMC4185432 | biostudies-literature | 2014 Feb

REPOSITORIES: biostudies-literature

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In vivo microscopy reveals extensive embedding of capillaries within the sarcolemma of skeletal muscle fibers.

Glancy Brian B Hsu Li-Yueh LY Dao Lam L Bakalar Matthew M French Stephanie S Chess David J DJ Taylor Joni L JL Picard Martin M Aponte Angel A Daniels Mathew P MP Esfahani Shervin S Cushman Samuel S Balaban Robert S RS

Microcirculation (New York, N.Y. : 1994) 20140201 2

<h4>Objective</h4>To provide insight into mitochondrial function in vivo, we evaluated the 3D spatial relationship between capillaries, mitochondria, and muscle fibers in live mice.<h4>Methods</h4>3D volumes of in vivo murine TA muscles were imaged by MPM. Muscle fiber type, mitochondrial distribution, number of capillaries, and capillary-to-fiber contact were assessed. The role of Mb-facilitated diffusion was examined in Mb KO mice. Distribution of GLUT4 was also evaluated in the context of the ...[more]

PMID: 25279425

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Project description:In skeletal muscle, excitation-contraction (EC) coupling requires depolarization-induced conformational rearrangements in L-type Ca(2+) channel (Ca(V)1.1) to be communicated to the type 1 ryanodine-sensitive Ca(2+) release channel (RYR1) of the sarcoplasmic reticulum (SR) via transient protein-protein interactions. Although the molecular mechanism that underlies conformational coupling between Ca(V)1.1 and RYR1 has been investigated intensely for more than 25 years, the question of whether such signaling occurs via a direct interaction between the principal, voltage-sensing ?(1S) subunit of Ca(V)1.1 and RYR1 or through an intermediary protein persists. A substantial body of evidence supports the idea that the auxiliary ?(1a) subunit of Ca(V)1.1 is a conduit for this intermolecular communication. However, a direct role for ?(1a) has been difficult to test because ?(1a) serves two other functions that are prerequisite for conformational coupling between Ca(V)1.1 and RYR1. Specifically, ?(1a) promotes efficient membrane expression of Ca(V)1.1 and facilitates the tetradic ultrastructural arrangement of Ca(V)1.1 channels within plasma membrane-SR junctions. In this paper, we demonstrate that overexpression of the RGK protein Rem, an established ? subunit-interacting protein, in adult mouse flexor digitorum brevis fibers markedly reduces voltage-induced myoplasmic Ca(2+) transients without greatly affecting Ca(V)1.1 targeting, intramembrane gating charge movement, or releasable SR Ca(2+) store content. In contrast, a ?(1a)-binding-deficient Rem triple mutant (R200A/L227A/H229A) has little effect on myoplasmic Ca(2+) release in response to membrane depolarization. Thus, Rem effectively uncouples the voltage sensors of Ca(V)1.1 from RYR1-mediated SR Ca(2+) release via its ability to interact with ?(1a). Our findings reveal Rem-expressing adult muscle as an experimental system that may prove useful in the definition of the precise role of the ?(1a) subunit in skeletal-type EC coupling.

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In vivo microscopy reveals extensive embedding of capillaries within the sarcolemma of skeletal muscle fibers.

Publications

In vivo microscopy reveals extensive embedding of capillaries within the sarcolemma of skeletal muscle fibers.

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