Dynamic Use of cDNA Arrays: Heterologous Probing for Gene Discovery and Exploration of Organismal Adaptation to Environmental Stress

Primate Torpor: Regulation of Stress-activated Protein Kinases During Daily Torpor in the Gray Mouse Lemur, Microcebus murinus

Very few selected species of primates are known to be capable of entering torpor. This exciting discovery means that the ability to enter a natural state of dormancy is an ancestral trait among primates and, in phylogenetic terms, is very close to the human lineage. To explore the regulatory mechanisms that underlie primate torpor, we analyzed signal transduction cascades to discover those involved in coordinating tissue responses during torpor. The responses of mitogen-activated protein kinase (MAPK) family members to primate torpor were compared in six organs of control (aroused) versus torpid gray mouse lemurs, Microcebus murinus. The proteins examined include extracellular signal-regulated kinases (ERKs), c-jun NH₂-terminal kinases (JNKs), MAPK kinase (MEK), and p38, in addition to stress-related proteins p53 and heat shock protein 27 (HSP27). The activation of specific MAPK signal transduction pathways may provide a mechanism to regulate the expression of torpor-responsive genes or the regulation of selected downstream cellular processes. In response to torpor, each MAPK subfamily responded differently during torpor and each showed organ-specific patterns of response. For example, skeletal muscle displayed elevated relative phosphorylation of ERK1/2 during torpor. Interestingly, adipose tissues showed the highest degree of MAPK activation. Brown adipose tissue displayed an activation of ERK1/2 and p38, whereas white adipose tissue showed activation of ERK1/2, p38, MEK, and JNK during torpor. Importantly, both adipose tissues possess specialized functions that are critical for torpor, with brown adipose required for non-shivering thermogenesis and white adipose utilized as the primary source of lipid fuel for torpor. Overall, these data indicate crucial roles of MAPKs in the regulation of primate organs during torpor.

Impact of high predation risk on genome-wide hippocampal gene expression in snowshoe hares

The population dynamics of snowshoe hares (Lepus americanus) are fundamental to the ecosystem dynamics of Canada's boreal forest. During the 8- to 11-year population cycle, hare densities can fluctuate up to 40-fold. Predators in this system (lynx, coyotes, great-horned owls) affect population numbers not only through direct mortality but also through sublethal effects. The chronic stress hypothesis posits that high predation risk during the decline severely stresses hares, leading to greater stress responses, heightened ability to mobilize cortisol and energy, and a poorer body condition. These effects may result in, or be mediated by, differential gene expression. We used an oligonucleotide microarray designed for a closely-related species, the European rabbit (Oryctolagus cuniculus), to characterize differences in genome-wide hippocampal RNA transcript abundance in wild hares from the Yukon during peak and decline phases of a single cycle. A total of 106 genes were differentially regulated between phases. Array results were validated with quantitative real-time PCR, and mammalian protein sequence similarity was used to infer gene function. In comparison to hares from the peak, decline phase hares showed increased expression of genes involved in metabolic processes and hormone response, and decreased expression of immune response and blood cell formation genes. We found evidence for predation risk effects on the expression of genes whose putative functions correspond with physiological impacts known to be induced by predation risk in snowshoe hares. This study shows, for the first time, a link between changes in demography and alterations in neural RNA transcript abundance in a natural population.

Tribute to P. L. Lutz: putting life on 'pause'--molecular regulation of hypometabolism

Entry into a hypometabolic state is an important survival strategy for many organisms when challenged by environmental stress, including low oxygen, cold temperatures and lack of food or water. The molecular mechanisms that regulate transitions to and from hypometabolic states, and stabilize long-term viability during dormancy, are proving to be highly conserved across phylogenic lines. A number of these mechanisms were identified and explored using anoxia-tolerant turtles as the model system, particularly from the research contributions made by Dr Peter L. Lutz in his explorations of the mechanisms of neuronal suppression in anoxic brain. Here we review some recent advances in understanding the biochemical mechanisms of metabolic arrest with a focus on ideas such as the strategies used to reorganize metabolic priorities for ATP expenditure, molecular controls that suppress cell functions (e.g. ion pumping, transcription, translation, cell cycle arrest), changes in gene expression that support hypometabolism, and enhancement of defense mechanisms (e.g. antioxidants, chaperone proteins, protease inhibitors) that stabilize macromolecules and promote long-term viability in the hypometabolic state.

Cold Ischemic Organ Preservation: Lessons from Natural Systems

Mammalian hibernators offer natural models for investigating solutions to the metabolic injuries that accrue during cold ischemic storage of human organs removed for transplant. Knowledge of the biochemical mechanisms that regulate and stabilize metabolism to ensure long-term viability in the hypometabolic, hypothermic state of hibernation could lead to applied treatments that could increase the time that excised organs can be maintained in cold storage and/or improve recovery of function after implantation. New research has documented the widespread role of reversible protein phosphorylation control of metabolism in achieving the coordinated suppression of metabolic rate that greatly extends viability during torpor. Analysis of hibernation-induced gene expression is proving to be of crucial importance for identifying the genes and proteins that are up-regulated to address organ-specific concerns during torpor. In particular, the power of complementary deoxyribonucleic acid (cDNA) array screening is identifying families of proteins that are up-regulated during hibernation (eg, serpins, heat shock proteins, antioxidants, membrane transporters) and highlighting previously unrecognized areas of cellular metabolism as contributing to the hibernation phenotype. These offer new targets for innovative applied treatments that could enhance cyto-protection and cold ischemia survival of organ explants.

Life in the slow lane: molecular mechanisms of estivation

Estivation is a state of aerobic hypometabolism used by organisms to endure seasonally arid conditions, often in desert environments. Estivating species are often active for only a few weeks each year to feed and breed and then retreat to estivate in sheltered sites, often underground. In general, estivation includes a strong reduction in metabolic rate, a primary reliance on lipid oxidation to fuel metabolism, and methods of water retention, both physical (e.g. cocoons) and metabolic (e.g. urea accumulation). The present review focuses on several aspects of metabolic adaptation during estivation including changes in the activities of enzymes of intermediary metabolism and antioxidant defenses, the effects of urea on estivator enzymes, enzyme regulation by reversible protein phosphorylation, protein kinases and phosphatases involved in signal transduction mechanisms, and the role of gene expression in estivation. The focus is on two species: the spadefoot toad, Scaphiopus couchii, from the Arizona desert; and the land snail, Otala lactea, a native of the Mediterranean region. The mechanisms of metabolic depression in estivators are similar to those seen in hibernation and anaerobiosis, and contribute to the development of a unified set of biochemical principles for the control of metabolic arrest in nature.