Protein phosphorylation in fast and slow chicken skeletal muscles: effect of denervation

Neurectomy preserves fast fibers when combined with tenotomy of infraspinatus muscle via upregulation of myogenesis

INTRODUCTION

We evaluated the contribution of denervation-related molecular processes to rotator cuff muscle degeneration after tendon release.

METHODS

We assessed the levels of myogenic (myogenin and myogenic differentiation factor [myoD]) and proadipogenic (peroxisome proliferator-activated receptor γ) transcription factors; the denervation-associated proteins tenascin-C, laminin-2, and calcium/calmodulin-dependent kinase II (CaMKII); and cellular alterations in sheep after infraspinatus tenotomy (TEN), suprascapular neurectomy (NEU), or both (TEN-NEU).

RESULTS

Extracellular ground substance increased at the expense of contractile tissue 16 weeks after surgery, correlating with CaMKII isoform levels. Sheep undergoing NEU and TEN-NEU had exaggerated infraspinatus atrophy and increased fast fibers compared with TEN sheep. The βMCaMKII isoform levels increased with TEN, and myoD levels tripled after denervation and were associated with slow fibers.

DISCUSSION

In sheep, denervation did not affect muscle-to-fat conversion after TEN of the infraspinatus. Furthermore, concurrent NEU mitigated the loss of fast fibers after TEN by inducing a fast-contractile phenotype. Muscle Nerve 59:100-107, 2019.

Early changes of type 2B fibers after denervation of rat EDL skeletal muscle

Skeletal muscle type 2B fibers normally receive a moderate level of motoneuron discharge. As a consequence, we hypothesize that type 2B fiber properties should be less sensitive to the absence of the nerve. Therefore, we have investigated the response of sarcoplasmic reticulum and myofibrillar proteins of type 2B fibers isolated from rat extensor digitorum longus muscle after denervation (2 and 7 days). Single fibers were identified by SDS-PAGE of myosin heavy chain isoforms. Electrophysiological and isometric contractile properties of the whole muscle were also analyzed. The pCa-tension relationship of type 2B single fibers was shifted to the left at 2 days and to right at 7 days after denervation, with significant differences in the Hill coefficients and pCa threshold values in 2- vs. 7-day-denervated fibers. The sarcoplasmic reticulum Ca2+ uptake capacity and rate significantly decreased after 2 days of denervation, whereas both increased at 7 days. Caffeine sensitivity of sarcoplasmic reticulum Ca2+ release was transitory and markedly increased in 2-day-denervated fibers. Our results indicate that type 2B fiber functional properties are highly sensitive to the interruption of nerve supply. Moreover, most of 2-day-denervated changes were reverted at 7 days.

Skeletal muscle CaMKII enriches in nuclei and phosphorylates myogenic factor SRF at multiple sites

We characterized the activity of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in homogenates and nuclear extracts of skeletal muscle and analyzed their capacity to phosphorylate the myogenic factor SRF. Isoforms of CaMKII enriched from skeletal muscle phosphorylated SRF in vitro to high stoichiometries and produced multiple forms on SDS-PAGE, suggesting that SRF was phosphorylated at multiple sites. Phosphopeptide-mapping experiments using truncated SRF proteins located the residues of SRF phosphorylated by recombinant CaMKII within amino acids 1-171, with at least one site residing in amino acids 142-171. Microsequencing of these phosphorylated peptides identified that both Ser-103 and a novel residue, Thr-160 in the MADS box of SRF, were sites of phosphorylation. CaMKII activity was enriched in nuclear extracts relative to crude homogenates from skeletal muscle and similarly phosphorylated the nuclear transcription factor SRF in vitro. The location of Thr-160 in the 3-D structure of SRF suggests that its phosphorylation by nuclear CaMKII may directly influence DNA binding of SRF and other MADS box factors.