Amplification and sequencing of mature microRNAs in uncharacterized animal models using stem-loop reverse transcription-polymerase chain reaction

Loss of miR-145 promotes remyelination and functional recovery in a model of chronic central demyelination

Strategies for treating progressive multiple sclerosis (MS) remain limited. Here, we found that miR-145-5p is overabundant uniquely in chronic lesion tissues from secondary progressive MS patients. We induced both acute and chronic demyelination in miR-145 knockout mice to determine its contributions to remyelination failure. Following acute demyelination, no advantage to miR-145 loss could be detected. However, after chronic demyelination, animals with miR-145 loss demonstrated increased remyelination and functional recovery, coincident with altered presence of astrocytes and microglia within the corpus callosum relative to wild-type animals. This improved response in miR-145 knockout animals coincided with a pathological upregulation of miR-145-5p in wild-type animals with chronic cuprizone exposure, paralleling human chronic lesions. Furthermore, miR-145 overexpression specifically in oligodendrocytes (OLs) severely stunted differentiation and negatively impacted survival. RNAseq analysis showed altered transcriptome in these cells with downregulated major pathways involved in myelination. Our data suggest that pathological accumulation of miR-145-5p is a distinctive feature of chronic demyelination and is strongly implicated in the failure of remyelination, possibly due to the inhibition of OL differentiation together with alterations in other glial cells. This is mirrored in chronic MS lesions, and thus miR-145-5p serves as a potential relevant therapeutic target in progressive forms of MS.

Gold Nanoparticle-Based Enzyme-Assisted Cyclic Amplification for the Highly-Sensitive Detection of miRNA-21

Because microRNAs (miRNAs) are biological indicators for the diagnosis, treatment, and monitoring of tumors, cancers, and other diseases, it is significant to develop a rapid, sensitive, and reliable miRNA detection platform. In this study, based on miRNA-21 detection, DNA-a with a 3′ end overhang and Texas Red fluorophore-labeled 5′ end was designed, which reacts with miRNA-21 and hybridizes with exonuclease III (Exo III), where the part connected to miRNA-21 is hydrolyzed, leaving a-DNA. At the same time, miRNA-21 is released to participate in the following reaction, to achieve cyclic amplification. a-DNA reacts with DNA-b conjugated to gold nanoparticles to achieve fluorescence quenching, with the quenching value denoted as F; additionally, after adding DNA-d and linked streptavidin immunomagnetic beads (SIBs), fluorescence recovery was achieved using DNA-c, with the recovered fluorescence recorded as F0. By comparing the difference in the fluorescence (F0 − F) between the two experiments, the amount of DNA-a hydrolyzed to produce a-DNA was established to determine the target miRNA-21 content. Under optimized conditions, by comparing the changes in the fluorescence signal, the developed strategy shows good sensitivity and repeatability, with a detection limit of 18 pM, good discriminative ability and selectivity, and promise for the early diagnosis of breast and intestinal cancers.

Dynamic regulation of histone H3 lysine (K) acetylation and deacetylation during prolonged oxygen deprivation in a champion anaerobe

Trachemys scripta elegans can survive up to three months of absolute anoxia at 3 °C and recover with minimal cellular damage. Red-eared sliders employ various physiological and biochemical adaptations to survive anoxia with metabolic rate depression (MRD) being the most prominent adaptation. MRD is mediated by epigenetic, transcriptional, post-transcriptional, and post-translational mechanisms aimed at shutting down cellular processes that are not needed for anoxia survival, while reprioritizing ATP towards cell processes that are vital for anaerobiosis. Histone acetylation/deacetylation are epigenetic modifications that maintain a proper balance between permissive chromatin and restricted chromatin, yet very little is known about protein regulation and enzymatic activity of the writers and erasers of acetylation during natural anoxia tolerance. As such, this study explored the interplay between transcriptional activators, histone acetyltransferases (HATs), and transcriptional repressors, sirtuins (SIRTs), along with three prominent acetyl-lysine (K) moieties of histone H3 in the liver of red-eared sliders. Western immunoblotting was used to measure acetylation levels of H3-K14, H3-K18, and H3-K56, as well as protein levels of histone H3-total, HATs, and nuclear SIRTs in the liver in response to 5 h and 20 h anoxia. Global and nuclear enzymatic activity of HATs and enzymatic activity of nuclear SIRTs were also measured. Overall, a strong suppression of HATs-mediated H3 acetylation and SIRT-mediated deacetylation was evident in the liver of red-eared sliders that could play an important role in ATP conservation as part of the overall reduction in metabolic rate.

MiRNA-145-5p prevents differentiation of oligodendrocyte progenitor cells by regulating expression of myelin gene regulatory factor

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.

Functional impact of microRNA regulation in models of extreme stress adaptation

When confronted with severe environmental stress, some animals are able to undergo a substantial reorganization of their cellular environment that enables long-term survival. One molecular mechanism of adaptation that has received considerable attention in recent years has been the action of reversible transcriptome regulation by microRNA. The implementation of new computational and high-throughput experimental approaches has started to uncover the vital contributions of microRNA towards stress adaptation. Indeed, recent studies have suggested that microRNA may have a major regulatory influence over a number of cellular processes that are essential to prolonged environmental stress survival. To date, a number of studies have highlighted the role of microRNA in the regulation of a metabolically depressed state, documenting stress-responsive microRNA expression during mammalian hibernation, frog and insect freeze tolerance, and turtle and marine snail anoxia tolerance. These studies collectively indicate a conserved principle of microRNA stress response across phylogeny. As we are on the verge of dissecting the role of microRNA in environmental stress adaptation, this review summarizes recent research advances and the hallmark expression patterns that facilitate stress survival.

Effects of fasting and refeeding on protein and glucose metabolism in Arctic charr

Refeeding, following a period of food deprivation will often lead to compensatory growth. Although many studies have focused on molecular mechanisms behind this accelerated growth response in fish, little is known on the roles of protein and metabolism. We also assessed, for the first time, the potential roles of miRNAs in regulating compensatory growth. Artcic charr, Salvelinus alpinus, a northern freshwater species, was deprived of food for 101 days and then fed to satiety for 126 days. The refeeding period resulted in compensatory growth, with a partial compensation of body mass. The feed deprivation period lead to a decrease in hepatosomatic index (HSI) and intestinal somatic index (ISI). HSI and ISI were then gradually replenished during early refeeding, following a lag phase prior to the compensatory growth response. mRNA transcripts regulating protein degradation via the autophagy pathway (Cathepsin D and Cathepsin L) in muscle were upregulated during feed restriction and downregulated after refeeding, which could allow for greater protein accretion in muscle, facilitating compensatory growth. Transcript levels from the ubiquitin proteasome pathway (Mafbx and Murf1) and the calpain system (Calpain 7 and Calpastatin) suggested that these pathways were not involved in regulating compensatory growth. Furthermore, we've shown that miRNAs (miR-29a and miR-223) could be involved in fish glycogen homeostasis during the early stages of refeeding. These findings provide a deeper understanding of the molecular mechanisms regulating growth in fish.

Dynamic regulation of six histone H3 lysine (K) methyltransferases in response to prolonged anoxia exposure in a freshwater turtle

The importance of histone lysine methylation is well established in health, disease, early development, aging, and cancer. However, the potential role of histone H3 methylation in regulating gene expression in response to extended periods of oxygen deprivation (anoxia) in a natural, anoxia-tolerant model system is underexplored. Red-eared sliders (Trachemys scripta elegans) can tolerate and survive three months of absolute anoxia and recover without incurring detrimental cellular damage, mainly by reducing the overall metabolic rate by 90% when compared to normoxia. Stringent regulation of gene expression is a vital aspect of metabolic rate depression in red-eared sliders, and as such we examined the anoxia-responsive regulation of histone lysine methylation in the liver during 5 h and 20 h anoxia exposure. Interestingly, this is the first study to illustrate the existence of histone lysine methyltransferases (HKMTs) and corresponding histone H3 lysine methylation levels in the liver of anoxia-tolerant red-eared sliders. In brief, H3K4me1, a histone mark associated with active transcription, and two corresponding histone lysine methyltransferases that modify H3K4me1 site, significantly increased in response to anoxia. On the contrary, H3K27me1, another transcriptionally active histone mark, significantly decreased during 20 h anoxia, and a transcriptionally repressive histone mark, H3K9me3, and the corresponding KMTs, similarly increased during 20 h anoxia. Overall, the results suggest a dynamic regulation of histone H3 lysine methylation in the liver of red-eared sliders that could theoretically aid in the selective upregulation of genes that are necessary for anoxia survival, while globally suppressing others to conserve energy.

Identification of Differentially Expressed miRNAs in Colorado Potato Beetles (Leptinotarsa decemlineata (Say)) Exposed to Imidacloprid

The Colorado potato beetle (Leptinotarsa decemlineata (Say)) is a significant pest of potato plants that has been controlled for more than two decades by neonicotinoid imidacloprid. L. decemlineata can develop resistance to this agent even though the molecular mechanisms underlying this resistance are not well characterized. MicroRNAs (miRNAs) are short ribonucleic acids that have been linked to response to various insecticides in several insect models. Unfortunately, the information is lacking regarding differentially expressed miRNAs following imidacloprid treatment in L. decemlineata. In this study, next-generation sequencing and quantitative real-time polymerase chain reaction (qRT-PCR) were used to identify modulated miRNAs in imidacloprid-treated versus untreated L. decemlineata. This approach identified 33 differentially expressed miRNAs between the two experimental conditions. Of interest, miR-282 and miR-989, miRNAs previously shown to be modulated by imidacloprid in other insects, and miR-100, a miRNA associated with regulation of cytochrome P450 expression, were significantly modulated in imidacloprid-treated beetles. Overall, this work presents the first report of a miRNA signature associated with imidacloprid exposure in L. decemlineata using a high-throughput approach. It also reveals interesting miRNA candidates that potentially underly imidacloprid response in this insect pest.

miR-122 promotes metastasis of clear-cell renal cell carcinoma by downregulating Dicer

Although overall downregulation of microRNAs (miRNAs) is a general feature of clear-cell renal cell carcinoma (ccRCC), several miRNAs are consistently upregulated, among which miR-122 was markedly increased in ccRCC tissues. Our study aims to determine the functional importance and underlying mechanism of miR-122 in ccRCC metastasis. Here, we demonstrate that the expression of miR-122 increased in ccRCC tissues, and higher miR-122 expression was found in ccRCC tissues with metastatic disease than in those without metastasis. The increased miR-122 levels were associated with poor metastasis-free survival in ccRCC patients with localized disease. Dicer was validated as a direct functional target of miR-122. Overexpression of miR-122 promoted migration and invasion of ccRCC cells in vitro and metastatic behavior of ccRCC cells in vivo. Inhibition of miR-122 attenuated this metastatic phenotype in vitro. Importantly, miR-122 exerted its pro-metastatic properties in ccRCC cells by downregulating Dicer and its downstream effector, the miR-200 family, thereby inducing epithelial-mesenchymal transition (EMT). Our results suggest an important role of the miR-122/Dicer/miR-200s/EMT pathway in ccRCC metastasis. Furthermore, miR-122 may serve as a biomarker for discriminating ccRCC with metastatic potential.

Amplification and quantification of cold-associated microRNAs in the Colorado potato beetle (Leptinotarsa decemlineata) agricultural pest

The Colorado potato beetle [Leptinotarsa decemlineata (Say)] is an important insect pest that can inflict considerable damage to potato plants. This insect can survive extended periods of cold exposure, and yet the molecular switches underlying this phenomenon have not been fully elucidated. A better characterization of this process would highlight novel vulnerabilities associated with L. decemlineata that could serve as targets for the management of this devastating pest. Using high-throughput sequencing, the current work reveals a cold-associated signature group of microRNAs (miRNAs) in control (15 °C) and -5 °C-exposed L. decemlineata. The results show 42 differentially expressed miRNAs following cold exposure including miR-9a-3p, miR-210-3p, miR-276-5p and miR-277-3p. Functional analysis of predicted targets associated with these cold-responsive miRNAs notably linked these changes with vital metabolic and cellular processes. Overall, this study highlights the miRNAs probably responsible for facilitating cold adaptation in L. decemlineata and implicates miRNAs as a key molecular target to consider in the development of novel pest management strategies against these insects.

Dicer suppresses the malignant phenotype in VHL-deficient clear cell renal cell carcinoma by inhibiting HIF-2α

Both the von Hippel-Lindau (VHL)/hypoxia-inducible factor (HIF) pathway and microRNA (miRNA) regulation are important mechanisms underlying the development and progression of clear cell renal cell carcinoma (ccRCC). Here we demonstrate that VHL deficiency leads to downregulation of Dicer and, in turn, defects in the miRNA biogenesis machinery in ccRCCs. Dicer inhibited expression of HIF-2α, which was a direct target of Dicer-dependent miR-182-5p in VHL-deficient ccRCCs. Ectopic Dicer expression in VHL-deficient ccRCCs suppressed tumor growth and angiogenesis by inhibiting HIF-2α both in vitro and in vivo. Reduced Dicer mRNA levels served as an independent prognostic factor for poor survival in patients with VHL-deficient ccRCC. Our results indicate that downregulation of Dicer in VHL-deficient ccRCCs contributes to high levels of HIF-2α and a malignant phenotype, which suggests Dicer could be a useful therapeutic target for managing this disease.

Antioxidants inhibit advanced glycosylation end-product-induced apoptosis by downregulation of miR-223 in human adipose tissue-derived stem cells

Advanced glycosylation end products (AGEs) are endogenous inflammatory mediators that induce apoptosis of mesenchymal stem cells. A potential mechanism includes increased generation of reactive oxygen species (ROS). MicroRNA-223 (miR-223) is implicated in the regulation of cell growth and apoptosis in several cell types. Here, we tested the hypothesis that antioxidants N-acetylcysteine (NAC) and ascorbic acid 2-phosphate (AAP) inhibit AGE-induced apoptosis via a microRNA-dependent mechanism in human adipose tissue-derived stem cells (ADSCs). Results showed that AGE-HSA enhanced apoptosis and caspase-3 activity in ADSCs. AGE-HSA also increased ROS generation and upregulated the expression of miR-223. Interestingly, reductions in ROS generation and apoptosis, and upregulation of miR-223 were found in ADSCs treated with antioxidants NAC and AAP. Furthermore, miR-223 mimics blocked antioxidant inhibition of AGE-induced apoptosis and ROS generation. Knockdown of miR-223 amplified the protective effects of antioxidants on apoptosis induced by AGE-HSA. miR-223 acted by targeting fibroblast growth factor receptor 2. These results indicate that NAC and AAP suppress AGE-HSA-induced apoptosis of ADSCs, possibly through downregulation of miR-223.

Identification and profiling of miRNAs in the freeze-avoiding gall moth Epiblema scudderiana via next-generation sequencing

The rapid development of high-throughput next-generation sequencing approaches in recent years has facilitated large-scale discovery and expression analysis of non-coding RNAs, including miRNAs, in traditional and non-traditional animal models. Such an approach has been leveraged to amplify, identify, and quantify miRNAs in several models of cold adaptation. The present study is the first to investigate the status of these small RNAs in an insect species that uses the freeze avoidance strategy of cold hardiness, the gall moth Epiblema scudderiana. To characterize the overall miRNA expression profile and to identify cold-modulated miRNAs in control (5 °C) and cold-exposed (-15 °C) E. scudderiana larvae, a next-generation sequencing-based approach was undertaken. A total of 44 differentially expressed miRNAs were identified between the two conditions; 21 up-regulated miRNAs and 23 down-regulated miRNAs in -15 °C-exposed larvae as compared with controls. Among the most significant changes observed in miRNAs with potential relevance to cold adaptation were elevated miR-1-3p, miR-92b-3p, and miR-133-3p levels as well as reduced miR-13a-3p and miR-13b-3p levels in E. scudderiana larvae exposed to cold temperatures. Expression values obtained from next-generation sequencing were also validated by a quantitative PCR approach for five miRNAs; miR-34-5p, miR-274-5p, miR-275-3p, miR-307a-3p, and miR-316-5p. Overall, this work provides the first description of a miRNA signature for subzero survival by a freeze-avoiding insect using a high-throughput approach and positions a new group of miRNAs at the forefront of the molecular changes underlying cold adaptation.

Down but Not Out: The Role of MicroRNAs in Hibernating Bats

MicroRNAs (miRNAs) regulate many physiological processes through post-transcriptional control of gene expression and are a major part of the small noncoding RNAs (snRNA). As hibernators can survive at low body temperatures (Tb) for many months without suffering tissue damage, understanding the mechanisms that enable them to do so are of medical interest. Because the brain integrates peripheral physiology and white adipose tissue (WAT) is the primary energy source during hibernation, we hypothesized that both of these organs play a crucial role in hibernation, and thus, their activity would be relatively increased during hibernation. We carried out the first genomic analysis of small RNAs, specifically miRNAs, in the brain and WAT of a hibernating bat (Myotis ricketti) by comparing deeply torpid with euthermic individual bats using high-throughput sequencing (Solexa) and qPCR validation of expression levels. A total of 196 miRNAs (including 77 novel bat-specific miRNAs) were identified, and of these, 49 miRNAs showed significant differences in expression during hibernation, including 33 in the brain and 25 in WAT (P≤0.01 &│logFC│≥1). Stem-loop qPCR confirmed the miRNA expression patterns identified by Solexa sequencing. Moreover, 31 miRNAs showed tissue- or state-specific expression, and six miRNAs with counts >100 were specifically expressed in the brain. Putative target gene prediction combined with KEGG pathway and GO annotation showed that many essential processes of both organs are significantly correlated with differentially expressed miRNAs during bat hibernation. This is especially evident with down-regulated miRNAs, indicating that many physiological pathways are altered during hibernation. Thus, our novel findings of miRNAs and Interspersed Elements in a hibernating bat suggest that brain and WAT are active with respect to the miRNA expression activity during hibernation.

Expression of miRNAs in response to freezing and anoxia stresses in the freeze tolerant fly Eurosta solidaginis

Insect cold hardiness is associated with substantial metabolic rate suppression, often including developmental diapause as well as metabolic suppression imposed by freezing and freeze-associated oxygen limitation. MicroRNAs, small non-coding transcripts that bind to mRNA, are known modulators of hypometabolism in freeze tolerant insects. To further contribute to the growing signature of stress-responsive miRNAs, this study amplified and quantified changes in the expression levels of four microRNA species, miR-8, miR-9, miR-92b and miR-277, in response to freezing or anoxia exposures of freeze tolerant gall fly larvae, Eurosta solidaginis. MiR-92b levels were significantly elevated by 1.57-fold in frozen E. solidaginis at -15°C as compared with 5°C controls, whereas miR-92b levels were significantly reduced in anoxic E. solidaginis to levels that were 0.77-fold as compared with larvae held under normoxic conditions. The other miRNAs investigated showed no significant changes in stressed larvae. These data demonstrate differential miR-92b expression in frozen/anoxic versus control insect larvae and position this miRNA as a stress responsive marker in this model insect.

Insight into post-transcriptional gene regulation: stress-responsive microRNAs and their role in the environmental stress survival of tolerant animals

Living animals are constantly faced with various environmental stresses that challenge normal life, including: oxygen limitation, very low or high temperature, as well as restriction of water and food. It has been well established that in response to these stresses, tolerant organisms regularly respond with a distinct suite of cellular modifications that involve transcriptional, translational and post-translational modification. In recent years, a new mechanism of rapid and reversible transcriptome regulation, via the action of non-coding RNA molecules, has emerged into post-transcriptional regulation and has since been shown to be part of the survival response. However, these RNA-based mechanisms by which tolerant organisms respond to stressed conditions are not well understood. Recent studies have begun to show that non-coding RNAs control gene expression and translation of mRNA to protein, and can also have regulatory influence over major cellular processes. For example, select microRNAs have been shown to have regulatory influence over the cell cycle, apoptosis, signal transduction, muscle atrophy and fatty acid metabolism during periods of environmental stress. As we are on the verge of dissecting the roles of non-coding RNA in environmental stress adaptation, this Commentary summarizes the hallmark alterations in microRNA expression that facilitate stress survival.

MicroRNA-185 inhibits cell proliferation and induces cell apoptosis by targeting VEGFA directly in von Hippel-Lindau-inactivated clear cell renal cell carcinoma

OBJECTIVES

The von Hippel-Lindau (VHL) gene acts as a tumor suppressor in most clear cell renal cell carcinomas (ccRCCs). Tumor growth in ccRCCs relies on many factors that result from the loss of VHL. This study aims to identify new microRNAs with therapeutic potential for VHL-inactivated ccRCCs.

MATERIALS AND METHODS

We used 786O, A498 (VHL inactivated), and Caki-1 (VHL intact) ccRCC cell lines and 40 ccRCC samples and their adjacent nontumor tissues to measure the expression of microRNA-185 (miR-185) by real-time quantitative polymerase chain reaction. Overexpression or knockdown of VEGFA expression in renal cancer cells was fulfilled by transfecting expression plasmids or small interfering RNAs. Overexpression of miR-185 in ccRCC cell lines was fulfilled by transfecting chemically synthesized miR-185 mimics. The effects of miR-185 on ccRCC cell lines were detected by MTS assay, colony formation assay, and flow cytometric analysis.

RESULTS

Compared with adjacent nontumor renal tissues, miR-185 expression levels decreased significantly in ccRCC tissues. The expression of miR-185 had a negative correlation with tumor size, Fuhrman grade, and TNM staging. Luciferase assay showed that VEGFA was a direct target gene of miR-185. The overexpression of miR-185 significantly inhibited cell proliferation and induced cell apoptosis by down-regulating VEGFA expression in VHL-inactivated ccRCC cells.

CONCLUSIONS

Our results suggest that the miR-185, as a tumor suppressor, plays a pivotal role by inhibiting VEGFA in VHL-inactivated ccRCC.

High-throughput amplification of mature microRNAs in uncharacterized animal models using polyadenylated RNA and stem-loop reverse transcription polymerase chain reaction

This study makes a significant advancement on a microRNA amplification technique previously used for expression analysis and sequencing in animal models without annotated mature microRNA sequences. As research progresses into the post-genomic era of microRNA prediction and analysis, the need for a rapid and cost-effective method for microRNA amplification is critical to facilitate wide-scale analysis of microRNA expression. To facilitate this requirement, we have reoptimized the design of amplification primers and introduced a polyadenylation step to allow amplification of all mature microRNAs from a single RNA sample. Importantly, this method retains the ability to sequence reverse transcription polymerase chain reaction (RT-PCR) products, validating microRNA-specific amplification.

Akt signaling and freezing survival in the wood frog, Rana sylvatica

BACKGROUND

The wood frog (Rana sylvatica) exhibits well-developed natural freeze tolerance supported by multiple mechanisms of biochemical adaptation. The present study investigated the role and regulation of the Akt signaling pathway in wood frog tissues (with a focus on liver) responding to freezing stress.

METHODS

Immunoblotting was used to assess total and phospho-Akt levels, total and phospho-PDK1, PTEN protein level, as well as total and phospho-FOXO1 levels. RT-PCR was used to investigate transcript levels of PTEN and microRNAs.

RESULTS

Akt was inhibited in skeletal muscle, kidney and heart after 24h freezing exposure with a reversal after thawing. The responses of the main kinase (PDK-1) and phosphatase (PTEN) that regulate Akt were consistent with freeze activation of Akt in liver; freezing exposure activated PDK-1 via enhanced Ser-241 phosphorylation whereas PTEN protein levels were reduced. Levels of three microRNAs (miR-26a, miR-126 and miR-217) that regulate pten expression were elevated in liver during freezing. One well-known role of Akt is in anti-apoptosis, mediated in part by Akt phosphorylation of Ser-256 on FOXO1. Freezing triggered an increase in liver phospho-FOXO1 Ser-256 content, suggesting that an important action of Akt may be apoptosis inhibition.

CONCLUSIONS

Akt activation in wood frog is stress and tissue specific, with multi-facet regulations (posttranslational and posttranscriptional) involved in supporting this specific signal transduction response.

GENERAL SIGNIFICANCE

This study implicates the Akt pathway in the metabolic reorganization of cellular metabolism in support of freezing survival.

Metabolic mechanisms for anoxia tolerance and freezing survival in the intertidal gastropod, Littorina littorea

The gastropod mollusk, Littorina littorea L., is a common inhabitant of the intertidal zone along rocky coastlines of the north Atlantic. This species has well-developed anoxia tolerance and freeze tolerance and is extensively used as a model for exploring the biochemical adaptations that support these tolerances as well as for toxicological studies aimed at identifying effective biomarkers of aquatic pollution. This article highlights our current understanding of the molecular mechanisms involved in anaerobiosis and freezing survival of periwinkles, particularly with respect to anoxia-induced metabolic rate depression. Analysis of foot muscle and hepatopancreas metabolism includes anoxia-responsive changes in enzyme regulation, signal transduction, gene expression, post-transcriptional regulation of mRNA, control of translation, and cytoprotective strategies including chaperones and antioxidant defenses. New studies describe the regulation of glucose-6-phosphate dehydrogenase by reversible protein phosphorylation, the role of microRNAs in suppressing mRNA translation in the hypometabolic state, modulation of glutathione S-transferase isozyme patterns, and the regulation of the unfolded protein response.

Regulation of p53 by reversible post-transcriptional and post-translational mechanisms in liver and skeletal muscle of an anoxia tolerant turtle, Trachemys scripta elegans

The red-eared slider turtle (Trachemys scripta elegans) exhibits well-developed natural anoxia tolerance that depends on multiple biochemical adaptations, including anoxia-induced hypometabolism. We hypothesized that signaling by the p53 protein could aid in establishing the hypometabolic state by arresting the cell cycle, protecting against DNA damage as well as altering pathways of energy metabolism. Immunoblotting was used to evaluate the regulation and post-transcriptional modifications of p53 in liver and skeletal muscle of red-eared slider turtles subjected to 5h or 20h of anoxic submergence. Tissue specific regulation of p53 was observed with the liver showing a more rapid activation of p53 in response to anoxia as well as differential expression of seven serine phosphorylation and two lysine acetylation sites when compared with skeletal muscle. Protein expression of MDM2, a major p53 inhibitor, was also examined but did not change during anoxia. Reverse-transcriptase PCR was used to assess transcript levels of selected p53 target genes (14-3-3σ, Gadd45α and Pgm) and one microRNA (miR-34a); results showed down-regulation of Pgm and up-regulation of the other three. These findings show an activation of p53 in response to anoxia exposure and suggest an important role for the p53 stress response pathway in regulating natural anoxia tolerance and hypometabolism in a vertebrate facultative anaerobe.

Biochemical adaptations of mammalian hibernation: exploring squirrels as a perspective model for naturally induced reversible insulin resistance

An important disease among human metabolic disorders is type 2 diabetes mellitus. This disorder involves multiple physiological defects that result from high blood glucose content and eventually lead to the onset of insulin resistance. The combination of insulin resistance, increased glucose production, and decreased insulin secretion creates a diabetic metabolic environment that leads to a lifetime of management. Appropriate models are critical for the success of research. As such, a unique model providing insight into the mechanisms of reversible insulin resistance is mammalian hibernation. Hibernators, such as ground squirrels and bats, are excellent examples of animals exhibiting reversible insulin resistance, for which a rapid increase in body weight is required prior to entry into dormancy. Hibernator studies have shown differential regulation of specific molecular pathways involved in reversible resistance to insulin. The present review focuses on this growing area of research and the molecular mechanisms that regulate glucose homeostasis, and explores the roles of the Akt signaling pathway during hibernation. Here, we propose a link between hibernation, a well-documented response to periods of environmental stress, and reversible insulin resistance, potentially facilitated by key alterations in the Akt signaling network, PPAR-γ/PGC-1α regulation, and non-coding RNA expression. Coincidentally, many of the same pathways are frequently found to be dysregulated during insulin resistance in human type 2 diabetes. Hence, the molecular networks that may regulate reversible insulin resistance in hibernating mammals represent a novel approach by providing insight into medical treatment of insulin resistance in humans.

MicroRNA regulation in extreme environments: differential expression of microRNAs in the intertidal snail Littorina littorea during extended periods of freezing and anoxia

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.

Differential expression of mature microRNAs involved in muscle maintenance of hibernating little brown bats, Myotis lucifugus: a model of muscle atrophy resistance

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.

HIF-1α regulation in mammalian hibernators: role of non-coding RNA in HIF-1α control during torpor in ground squirrels and bats

A potential role for non-coding RNAs, miR-106b and antisense hypoxia inducible transcription factor-1 (HIF-1α), in HIF-1α regulation during mammalian hibernation was investigated in two species, the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) and the little brown bat (Myotis lucifugus). Both species showed differential regulation of HIF-1α during hibernation. HIF-1α protein levels increased significantly in skeletal muscle of both species when animals entered torpor, as well as in bat liver. HIF-1α mRNA levels correlated with the protein increase in bat skeletal muscle and liver but not in squirrel skeletal muscle. Antisense HIF-1α transcripts were identified in skeletal muscle of both hibernators. The expression of antisense HIF-1α was reduced in skeletal muscle of torpid bats compared with euthermic controls, suggesting that release of inhibition by the antisense RNA contributes to regulating HIF-1α translation in this tissue during torpor. The expression of miR-106b, a microRNA associated with HIF-1α regulation, also decreased during torpor in both skeletal muscle and liver of bats and in ground squirrel skeletal muscle. These data present the first evidence that non-coding RNA provides novel post-transcriptional mechanisms of HIF-1α regulation when hibernators descend into deep cold torpor, and also demonstrate that these mechanisms are conserved in two divergent mammalian orders (Rodentia and Chiroptera).

Biogenesis of mammalian microRNAs by a non-canonical processing pathway

Canonical microRNA biogenesis requires the Microprocessor components, Drosha and DGCR8, to generate precursor-miRNA, and Dicer to form mature miRNA. The Microprocessor is not required for processing of some miRNAs, including mirtrons, in which spliceosome-excised introns are direct Dicer substrates. In this study, we examine the processing of putative human mirtrons and demonstrate that although some are splicing-dependent, as expected, the predicted mirtrons, miR-1225 and miR-1228, are produced in the absence of splicing. Remarkably, knockout cell lines and knockdown experiments demonstrated that biogenesis of these splicing-independent mirtron-like miRNAs, termed 'simtrons', does not require the canonical miRNA biogenesis components, DGCR8, Dicer, Exportin-5 or Argonaute 2. However, simtron biogenesis was reduced by expression of a dominant negative form of Drosha. Simtrons are bound by Drosha and processed in vitro in a Drosha-dependent manner. Both simtrons and mirtrons function in silencing of target transcripts and are found in the RISC complex as demonstrated by their interaction with Argonaute proteins. These findings reveal a non-canonical miRNA biogenesis pathway that can produce functional regulatory RNAs.