Creatine kinase transcript accumulation: effect of nerve during muscle development

Myofiber integrity depends on desmin network targeting to Z-disks and costameres via distinct plectin isoforms

Dysfunction of plectin, a 500-kD cytolinker protein, leads to skin blistering and muscular dystrophy. Using conditional gene targeting in mice, we show that plectin deficiency results in progressive degenerative alterations in striated muscle, including aggregation and partial loss of intermediate filament (IF) networks, detachment of the contractile apparatus from the sarcolemma, profound changes in myofiber costameric cytoarchitecture, and decreased mitochondrial number and function. Analysis of newly generated plectin isoform-specific knockout mouse models revealed that IF aggregates accumulate in distinct cytoplasmic compartments, depending on which isoform is missing. Our data show that two major plectin isoforms expressed in muscle, plectin 1d and 1f, integrate fibers by specifically targeting and linking desmin IFs to Z-disks and costameres, whereas plectin 1b establishes a linkage to mitochondria. Furthermore, disruption of Z-disk and costamere linkages leads to the pathological condition of epidermolysis bullosa with muscular dystrophy. Our findings establish plectin as the major organizer of desmin IFs in myofibers and provide new insights into plectin- and desmin-related muscular dystrophies.

Neuronal control of myogenic regulatory factor accumulation in fetal muscle

The lumbosacral spinal cords of 14.5-day gestation mice (E14.5) were ablated. The number of molecules of each of the four myogenic regulatory factor (MRF) mRNAs per nanogram of total RNA were evaluated in innervated and aneural fetal crural muscles. Accumulation of all four MRF mRNAs was affected in aneural muscle, but was never more than threefold different than in innervated muscles, considerably less than after adult denervation. The effect of the nerve varied with the MRF, the fetal age, and with the muscle (extensor digitorum longus muscle [EDL] vs. soleus muscle), with the nerve having multiple effects including down-regulation of certain MRF genes at specific periods (e.g., myoD and myogenin [E16.5-E18.5] and MRF4 in the EDL only [E18.5-E19.5]); limiting the up-regulation of certain genes, which occurred in the absence of innervation (e.g., myf-5 [E18.5-E19.5] and myogenin [E14.5-E16.5]); and even enhancing the accumulation of MRF4 mRNA (E14.5-E16.5). We hypothesize that factors other than nerve contribute to the down-regulation of myf-5 and myogenin mRNAs to adult levels. Innervation was required for the emergence of the slow, but not the fast, MRF mRNA profile at birth. MyoD, found in both the nuclear and cytoplasmic protein extracts of innervated fetal muscle, increased by approximately 5-fold in the nuclear extracts (approximately 2.5-fold in the cytoplasmic) of E19.5 aneural muscles, significantly less than the 12-fold increase found in the nuclear extract of 4-day denervated adult muscle. This increase in aneural fetal muscle was due primarily to an increased concentration of myoD in muscle lineage nuclei, rather than to the presence of additional myoD(+) muscle lineage nuclei.

PGC-1alpha and PGC-1beta have both similar and distinct effects on myofiber switching toward an oxidative phenotype

Peroxisome proliferator-activated receptor-gamma coactivator-1alpha and -1beta (PGC-1alpha and PGC-1beta) were overexpressed by adenovirus-mediated gene transfer in cultures of primary rat skeletal muscle cells derived from neonatal myoblasts. Effects on muscle fiber type transition and metabolism were studied from days 5 to 22 of culture. PGC-1alpha and PGC-1beta overexpression caused a three- to fourfold increase in mRNA level, a doubling of enzymatic activity of citrate synthase, a slight increase in short-chain acyl-CoA dehydrogenase mRNA, a doubling of the mRNA level, and a 30-50% increase in enzymatic activity of glyceraldehyde-3-phosphate dehydrogenase. Lactate dehydrogenase or creatine kinase activity was unchanged. PGC-1alpha enhanced glycogen buildup twofold at 5 or 25 mM glucose, whereas PGC-1beta caused a decrease. Both PGC-1alpha and PGC-1beta overexpression caused a faster maturation of myotubes, as seen by mRNA downregulation of the immature embryonal and perinatal myosin heavy-chain (MHC) isoforms. PGC-1alpha or PGC-1beta overexpression enhanced mRNA of the slow oxidative-associated MHC isoform MHCIb and downregulated mRNA levels of the fast glycolytic-associated MHC isoforms MHCIIX and MHCIIB. Only PGC-1beta overexpression caused an increase in mRNA of the intermediary fast oxidative-associated MHC isoform MHCIIA. PGC-1alpha or PGC-1beta overexpression upregulated GLUT4 mRNA and downregulated myocyte enhancer factor 2C transcription factor mRNA; only PGC-1alpha overexpression caused an increase in the mRNA expression of TRB3, a negative regulator of insulin signaling. These results show that both PGC-1alpha and PGC-1beta are involved in the regulation of skeletal muscle fiber transition and metabolism and that they have both overlapping and differing effects.

Effect of chronic denervation and denervation-reinnervation on cytoplasmic creatine kinase transcript accumulation

The extensor digitorum longus (EDL) and soleus muscles of adult mice were chronically denervated or denervated and allowed to reinnervate. Muscles were evaluated 1, 5, 14, 21, and 52 days after sciaticectomy. In terms of weight loss, myofiber atrophy, degeneration, and fibrosis, the soleus muscle was more affected than the EDL by chronic denervation. Fifty-two days after chronic denervation, the number of molecules of MCK/ng total RNA in both muscles (determined with competitive PCR) decreased, with the soleus muscle being more affected. At that stage, BCK mRNA levels in the denervated soleus were unchanged, but they were increased (>50%) in the EDL. Reinnervation restored MCK transcript accumulation in the EDL, whereas, in the soleus MCK, transcripts exceeded control values by 57%, approaching levels in the reinnervated EDL. Despite restoration of MCK mRNA levels, the number of molecules of BCK mRNA/ng total RNA was four- to fivefold higher in reinnervated versus control muscles, suggesting that the genes encoding the CK mRNAs are not coordinately regulated in adult muscle. The role of denervation induced, fiber type changes in regulating CK mRNA accumulation has been evaluated. Electron microscopic analyses have established that fibrosis is not a factor that determines BCK mRNA levels in the chronically denervated or denervated-reinnervated muscles. CK isozyme analyses support the hypothesis that a greater proportion of BCK mRNA found in 52 day chronically denervated and denervated-reinnervated muscles is produced in myofibers vs. nonmuscle cells than in control muscles.