Muscle wasting is common in mammals during extended periods of immobility. However, many small hibernating mammals manage to avoid muscle atrophy despite remaining stationary for long periods during hibernation. Recent research has highlighted roles for short non-coding microRNAs (miRNAs) in the regulation of stress tolerance. We proposed that they could also play an important role in muscle maintenance during hibernation. To explore this possibility, a group of 10 miRNAs known to be normally expressed in skeletal muscle of non-hibernating mammals were analyzed by RT-PCR in hibernating little brown bats, Myotis lucifugus. We then compared the expression of these miRNAs in euthermic control bats and bats in torpor. Our results showed that compared to euthermic controls, significant, albeit modest (1.2–1.6 fold), increases in transcript expression were observed for eight mature miRNAs, including miR-1a-1, miR-29b, miR-181b, miR-15a, miR-20a, miR-206 and miR-128-1, in the pectoral muscle of torpid bats. Conversely, expression of miR-21 decreased by 80% during torpor, while expression of miR-107 remained unaffected. Interestingly, these miRNAs have been either validated or predicted to affect multiple muscle-specific factors, including myostatin, FoxO3a, HDAC4 and SMAD7, and are likely involved in the preservation of pectoral muscle mass and functionality during bat hibernation.

Several recent studies of vertebrate adaptation to environmental stress have suggested roles for microRNAs (miRNAs) in regulating global suppression of protein synthesis and/or restructuring protein expression patterns. The present study is the first to characterize stress-responsive alterations in the expression of miRNAs during natural freezing or anoxia exposures in an invertebrate species, the intertidal gastropod Littorina littorea. These snails are exposed to anoxia and freezing conditions as their environment constantly fluctuates on both a tidal and seasonal basis. The expression of selected miRNAs that are known to influence the cell cycle, cellular signaling pathways, carbohydrate metabolism and apoptosis was evaluated using RT-PCR. Compared to controls, significant changes in expression were observed for miR-1a-1, miR-34a and miR-29b in hepatopancreas and for miR-1a-1, miR-34a, miR-133a, miR-125b, miR-29b and miR-2a in foot muscle after freezing exposure at −6 °C for 24 h (P < 0.05). In addition, in response to anoxia stress for 24 h, significant changes in expression were also observed for miR-1a-1, miR-210 and miR-29b in hepatopancreas and for miR-1a-1, miR-34a, miR-133a, miR-29b and miR-2a in foot muscle (P < 0.05). Moreover, protein expression of Dicer, an enzyme responsible for mature microRNA processing, was increased in foot muscle during freezing and anoxia and in hepatopancreas during freezing. Alterations in expression of these miRNAs in L. littorea tissues may contribute to organismal survival under freezing and anoxia.

A newly identified lethal form of hereditary sensory and autonomic neuropathy (HSAN), designated HSAN-VI, is caused by a homozygous mutation in the bullous pemphigoid antigen 1 (BPAG1)/dystonin gene (DST). The HSAN-VI mutation impacts all major neuronal BPAG1/dystonin protein isoforms: dystonin-a1, -a2 and -a3. Homozygous mutations in the murine Dst gene cause a severe sensory neuropathy termed dystonia musculorum (dt). Phenotypically, dt mice are similar to HSAN-VI patients, manifesting progressive limb contractures, dystonia, dysautonomia and early postnatal death. To obtain a better molecular understanding of disease pathogenesis in HSAN-VI patients and the dt disorder, we generated transgenic mice expressing a myc-tagged dystonin-a2 protein under the regulation of the neuronal prion protein promoter on the dt(Tg4/Tg4) background, which is devoid of endogenous dystonin-a1 and -a2, but does express dystonin-a3. Restoring dystonin-a2 expression in the nervous system, particularly within sensory neurons, prevented the disorganization of organelle membranes and microtubule networks, attenuated the degeneration of sensory neuron subtypes and ameliorated the phenotype and increased life span in these mice. Despite these improvements, complete rescue was not observed likely because of inadequate expression of the transgene. Taken together, this study provides needed insight into the molecular basis of the dt disorder and other peripheral neuropathies including HSAN-VI.

No abstract

The roles of specific microRNAs (miRNA) in oligodendrocyte (OL) differentiation have been studied in depth. However, miRNAs in OL precursors and oligodendrocyte progenitor cells (OPCs) have been less extensively investigated. MiR-145-5p is highly expressed in OPCs relative to differentiating OLs, suggesting this miRNA may serve a function specifically in OPCs. Knockdown of miR-145-5p in primary OPCs led to spontaneous differentiation, as evidenced by an increased proportion of MAG + cells, increased cell ramification, and upregulation of multiple myelin genes including MYRF, TPPP, and MAG, and OL cell cycle exit marker Cdkn1c. Supporting this transition to a differentiating state, proliferation was reduced in miR-145-5p knockdown OPCs. Further, knockdown of miR-145-5p in differentiating OLs showed enhanced differentiation, with increased branching, myelin membrane production, and myelin gene expression. We identified several OL-specific genes targeted by miR-145-5p that exhibited upregulation with miR-145-5p knockdown, including myelin gene regulatory factor (MYRF), that could be regulating the prodifferentiation phenotype in both miR-145 knockdown OPCs and OLs. Indeed, spontaneous differentiation with knockdown of miR-145-5p was fully rescued by concurrent knockdown of MYRF. However, proliferation rate was only partially rescued with MYRF knockdown, and overexpression of miR-145-5p in OPCs increased proliferation rate without affecting expression of already lowly expressed differentiation genes. Taken together, these data suggest that in OPCs miR-145-5p both prevents differentiation at least in part by preventing expression of MYRF and promotes proliferation via as-yet-unidentified mechanisms. These findings clarify the need for differential regulation of miR-145-5p between OPCs and OLs and may have further implications in demyelinating diseases such as multiple sclerosis where miR-145-5p is dysregulated.