The brains of six African mole-rat species show divergent responses to hypoxia.

Mole-rats are champions of self-preservation, with increased longevity compared with other rodents their size, strong antioxidant capabilities and specialized defenses against endogenous oxidative stress. However, how the brains of these subterranean mammals handle acute in vivo hypoxia is poorly understood. This study is the first to examine the molecular response to low oxygen in six different species of hypoxia-tolerant mole-rats from sub-Saharan Africa. Protein carbonylation, a known marker of DNA damage (hydroxy-2'-deoxyguanosine), and antioxidant capacity did not change following hypoxia but HIF-1 protein levels increased significantly in the brains of two species. Nearly 30 miRNAs known to play roles in hypoxia tolerance were differentially regulated in a species-specific manner. The miRNAs exhibiting the strongest response to low oxygen stress inhibit apoptosis and regulate neuroinflammation, likely providing neuroprotection. A principal component analysis (PCA) using a subset of the molecular targets assessed herein revealed differences between control and hypoxic groups for two solitary species (Georychus capensis and Bathyergus suillus), which are ecologically adapted to a normoxic environment, suggesting a heightened sensitivity to hypoxia relative to species that may experience hypoxia more regularly in nature. By contrast, all molecular data were included in the PCA to detect a difference between control and hypoxic populations of eusocial Heterocephalus glaber, indicating they may require many lower-fold changes in signaling pathways to adapt to low oxygen settings. Finally, none of the Cryptomys hottentotus subspecies showed a statistical difference between control and hypoxic groups, presumably due to hypoxia tolerance derived from environmental pressures associated with a subterranean and social lifestyle.

Identification of a prosurvival neuroprotective mitochondrial peptide in a mammalian hibernator.

Hibernation requires the intricate regulation of physiological and biochemical adaptations to facilitate the decrease in metabolic rate and activation of prosurvival factors needed for winter survival. Mitochondria play important roles in eliciting these responses and in coordinating the required energy shifts. Herein, we report the presence of a novel mitochondrial peptide, s-humanin, in the hibernating 13-lined ground squirrel, Ictidomys tridecemlineatus. S-humanin was shown to have strong structural and sequence similarities to its human analogue, humanin-a powerful neuroprotective mitochondrial peptide. An assessment of the protein and gene expression levels of this peptide in ground squirrels revealed stark tissue-specific regulatory responses whereby transcript levels increased in brain cortex, skeletal muscle, and adipose tissues during hibernation, suggesting a protective torpor-induced activation. Accompanying peptide measurements found that s-humanin levels were suppressed in liver of torpid squirrels but enhanced in brain cortex. The enhanced transcript and protein levels of s-humanin in brain cortex suggest that it is actively involved in protecting delicate brain tissues and neuronal connections from hibernation-associated stresses. We propose that this squirrel-specific peptide is involved in modulating tissue-specific cytoprotective functions, expanding its role from human-specific neuroprotection to environmental stress protection. SIGNIFICANCE OF THE STUDY: Understanding the molecular mechanisms, which protect against oxidative stress in a model hibernator such as the ground squirrel, could be pivotal to the regulation of cytoprotection. This study expands on our knowledge of metabolic rate depression and could suggest a potential role for humanin therapy in neurodegenerative diseases.

Translational regulation in the anoxic turtle, Trachemys scripta elegans.

The red-eared slider turtle (Trachemys scripta elegans), has developed remarkable adaptive mechanisms for coping with decreased oxygen availability during winter when lakes and ponds become covered with ice. Strategies for enduring anoxia tolerance include an increase in fermentable fuel reserves to support anaerobic glycolysis, the buffering of end products to minimize acidosis, altered expression in crucial survival genes, and strong metabolic rate suppression to minimize ATP-expensive metabolic processes such as protein synthesis. The mammalian target of rapamycin (mTOR) is at the center of the insulin-signaling pathway that regulates protein translation. The present study analyzed the responses of the mTOR signaling pathway to 5 (5H) or 20 h (20H) of anoxic submergence in liver and skeletal muscle of T. scripta elegans with a particular focus on regulatory changes in the phosphorylation states of targets. The data showed that phosphorylation of multiple mTOR targets was suppressed in skeletal muscle, but activated in the liver. Phosphorylated mTORSer2448 showed no change in skeletal muscle but had increased by approximately 4.5-fold in the liver after 20H of anoxia. The phosphorylation states of upstream positive regulators of mTOR (p-PDK-1Ser241, p-AKTSer473, and protein levels of GßL), the relative levels of dephosphorylated active PTEN, as well as phosphorylation state of negative regulators (TSC2Thr1462, p-PRAS40Thr246) were generally found to be differentially regulated in skeletal muscle and in liver. Downstream targets of mTOR (p-p70 S6KThr389, p-S6Ser235, PABP, p-4E-BP1Thr37/46, and p-eIF4ESer209) were generally unchanged in skeletal muscle but upregulated in most targets in liver. These findings indicate that protein synthesis is enhanced in the liver and suggests an increase in the synthesis of crucial proteins required for anoxic survival.

Analysis of microRNA expression during the torpor-arousal cycle of a mammalian hibernator, the 13-lined ground squirrel.

Hibernation is a highly regulated stress response that is utilized by some mammals to survive harsh winter conditions and involves a complex metabolic reprogramming at the cellular level to maintain tissue protections at low temperature. In this study, we profiled the expression of 117 conserved microRNAs in the heart, muscle, and liver of the 13-lined ground squirrel (Ictidomys tridecemlineatus) across four stages of the torpor-arousal cycle (euthermia, early torpor, late torpor, and interbout arousal) by real-time PCR. We found significant differential regulation of numerous microRNAs that were both tissue specific and torpor stage specific. Among the most significant regulated microRNAs was miR-208b, a positive regulator of muscle development that was found to be upregulated by fivefold in the heart during late torpor (13-fold during arousal), while decreased by 3.7-fold in the skeletal muscle, implicating a potential regulatory role in the development of cardiac hypertrophy and skeletal muscle atrophy in the ground squirrels during torpor. In addition, the insulin resistance marker miR-181a was upregulated by 5.7-fold in the liver during early torpor, which supports previous suggestions of hyperinsulinemia in hibernators during the early stages of the hibernation cycle. Although microRNA expression profiles were largely unique between the three tissues, GO annotation analysis revealed that the putative targets of upregulated microRNAs tend to enrich toward suppression of progrowth-related processes in all three tissues. These findings implicate microRNAs in the regulation of both tissue-specific processes and general suppression of cell growth during hibernation.