We investigated the expression of the nuclear-encoded genes controlling the mitochondrial properties in the mouse gastrocnemius muscle to gain insight into the mitochondrial biogenesis that occurs during the muscle degeneration/regeneration induced by freezing. In addition, we tested whether the muscle regeneration is affected by pharmacologically blocking the mitochondrial protein synthesis to elucidate the possible involvement of mitochondrial biogenesis in muscle regeneration. The activity of citrate synthase dramatically increased soon after the initial injury when the myoblasts began to differentiate into myotubes, indicating that mitochondrial biogenesis occurs early during the muscle regeneration. At the same time, the expression of mitochondrial biogenesis-related genes including PGC-1β, PRC, NRF-1, NRF-2, TFAM, mtSSB, fission 1, and Lon protease synchronized with that of the myogenic regulatory genes including MyoD and myogenin. The skeletal muscles forced to regenerate in the presence of chloramphenicol to block the mitochondrial protein synthesis were of poor repair with small myofibers and an increased amount of connective tissue. These results suggest that mitochondrial biogenesis activated early during the muscle regeneration and that mitochondrial biogenesis plays a role in muscle regeneration.

To gain insight into the regulation of mitochondrial adaptations to hindlimb unloading (HU), the activity of mitochondrial enzymes and the expression of nuclear-encoded genes which control mitochondrial properties in mouse gastrocnemius muscle were investigated. Biochemical and enzyme histochemical analysis showed that subsarcolemmal mitochondria were lost largely than intermyofibrillar mitochondria after HU. Gene expression analysis revealed disturbed or diminished gene expression patterns. The three main results of this analysis are as follows. First, in contrast to peroxisome proliferator-activated receptor γ coactivator 1 β (PGC-1β) and PGC-1-related coactivator, which were down-regulated by HU, PGC-1α was up-regulated concomitant with decreased expression of its DNA binding transcription factors, PPARα, and estrogen-related receptor α (ERRα). Moreover, there was no alteration in expression of nuclear respiratory factor 1, but its downstream target gene, mitochondrial transcription factor A, was down-regulated. Second, both mitofusin 2 and fission 1, which control mitochondrial morphology, were down-regulated. Third, ATP-dependent Lon protease, which participates in mitochondrial-protein degradation, was also down-regulated. These findings suggest that HU may induce uncoordinated expression of PGC-1 family coactivators and DNA binding transcription factors, resulting in reducing ability of mitochondrial biogenesis. Furthermore, down-regulation of mitochondrial morphology-related genes associated with HU may be also involved in alterations in intracellular mitochondrial distribution.

Heat-shock protein90 (HSP90) plays an essential role in maintaining stability and activity of its clients. HSP90 is involved in cell differentiation and survival in a variety of cell types. To elucidate the possible role of HSP90 in myogenic differentiation and cell survival, we examined the time course of changes in the expression of myogenic regulatory factors, intracellular signaling molecules, and anti-/pro-apoptotic factors when C2C12 cells were cultured in differentiation condition in the presence of a HSP90-specific inhibitor, geldanamycin. Furthermore, we examined the effects of geldanamycin on muscle regeneration in vivo. Our results showed that geldanamycin inhibited myogenic differentiation with decreased expression of MyoD, myogenin and reduced phosphorylation levels of Akt1. Geldanamycin had little effect on the phosphorylation levels of p38MAPK and ERK1/2 but reduced the phosphorylation levels of JNK. Along with myogenic differentiation, geldanamycin increased apoptotic nuclei with decreased expression of Bcl-2. The skeletal muscles forced to regenerate in the presence of geldanamycin were of poor repair with small regenerating myofibers and increased connective tissues. Together, our findings suggest that HSP90 may modulate myogenic differentiation and may be involved in cell survival.

The abundance, morphology, and functional properties of mitochondria become altered in response to denervation. To gain insight into the regulation of this process, mitochondrial enzyme activities and gene expression involved in mitochondrial biogenesis and dynamics in mouse gastrocnemius muscle was investigated. Sciatic nerve transactions were performed on mice, and then gastrocnemius muscles were isolated at days 5 and 30 after surgery. Muscle weight was decreased significantly by 15% and 62% at days 5 and 30 after surgery, respectively. The activity of citrate synthase, a marker of oxidative enzyme, was reduced significantly by 31% and 53% at days 5 and 30, respectively. Enzyme histochemical analysis revealed that subsarcolemmal mitochondria were largely lost than intermyofibrillar mitochondria at day 5, and this trend was further progressed at day 30 after surgery. Expression levels of peroxisome proliferator-activated receptor, γ coactivator 1 (PGC-1)α, estrogen-related receptor α (ERRα), and mitofusin 2 were down-regulated throughout the experimental period, whereas those of PGC-1β, PRC, nuclear respiratory factor (NRF)-1, NRF-2, TFAM, and Lon protease were down-regulated at day 30 after surgery. These results suggest that PGC-1α, ERRα, and mitofusin 2 may be important factors in the process of denervation-induced mitochondrial adaptation. In addition, other PGC-1 family of transcriptional coactivators and DNA binding transcription factors may also contribute to mitochondrial adaptation after early response to denervation.

The inferior temporal cortex (ITC) contains neurons selective to multiple levels of visual categories. However, the mechanisms by which these neurons collectively construct hierarchical category percepts remain unclear. By comparing decoding accuracy with simultaneously acquired electrocorticogram (ECoG), local field potentials (LFPs), and multi-unit activity in the macaque ITC, we show that low-frequency LFPs/ECoG in the early evoked visual response phase contain sufficient coarse category (e.g., face) information, which is homogeneous and enhanced by spatial summation of up to several millimeters. Late-induced high-frequency LFPs additionally carry spike-coupled finer category (e.g., species, view, and identity of the face) information, which is heterogeneous and reduced by spatial summation. Face-encoding neural activity forms a cluster in similar cortical locations regardless of whether it is defined by early evoked low-frequency signals or late-induced high-gamma signals. By contrast, facial subcategory-encoding activity is distributed, not confined to the face cluster, and dynamically increases its heterogeneity from the early evoked to late-induced phases. These findings support a view that, in contrast to the homogeneous and static coarse category-encoding neural cluster, finer category-encoding clusters are heterogeneously distributed even outside their parent category cluster and dynamically increase heterogeneity along with the local cortical processing in the ITC.