The regulation of food intake in mammalian hibernators: a review

One of the most profound hallmarks of mammalian hibernation is the dramatic reduction in food intake during the winter months. Several species of hibernator completely cease food intake (aphagia) for nearly 7 months regardless of ambient temperature and in many cases, whether or not food is available to them. Food intake regulation has been studied in mammals that hibernate for over 50 years and still little is known about the physiological mechanisms that control this important behavior in hibernators. It is well known from lesion experiments in non-hibernators that the hypothalamus is the main brain region controlling food intake and therefore body mass. In hibernators, the regulation of food intake and body mass is presumably governed by a circannual rhythm since there is a clear seasonal rhythm to food intake: animals increase food intake in the summer and early autumn, food intake declines in autumn and actually ceases in winter in many species, and resumes again in spring as food becomes available in the environment. Changes in circulating hormones (e.g., leptin, insulin, and ghrelin), nutrients (glucose, and free fatty acids), and cellular enzymes such as AMP-activated protein kinase (AMPK) have been shown to determine the activity of neurons involved in the food intake pathway. Thus, it appears likely that the food intake pathway is controlled by a variety of inputs, but is also acted upon by upstream regulators that are presumably rhythmic in nature. Current research examining the molecular mechanisms and integration of environmental signals (e.g., temperature and light) with these molecular mechanisms will hopefully shed light on how animals can turn off food intake and survive without eating for months on end.     

Characteristics of ion transport in kidney cortex of mammalian hibernators

Slices of kidney cortex of two species of hibernating mammals (hamsters and ground squirrels) have been leached of K, and their subsequent ability to reaccumulate K in vitro has been determined at temperatures between 38° and 0°C. At 5°C (body temperature of a hibernating mammal) uptake is appreciable in kidney cortex of both species. In the kidney cortex of hamsters, for example, the tissue K of slices incubated at 5°C reaches the same steady-state concentration after 2 hours that is observed in slices at 38°C after 20 minutes. At 0°C there is also a measurable uptake. This K transport is blocked by metabolic inhibitors and, in ground squirrel kidneys, by ouabain. In kidney cortex slices from guinea pigs net K accumulation is slight at 5°C and absent at 0°C. The initial rapid uptake of K at 38°C occurs at the same rate in kidney cortex slices of hamsters as in those of rabbits. Lowering the temperature of incubation decreases this initial rate of uptake in hamster kidney slices with a Q ₁₀ of 1.8 between 38° and 15° and of 5.7 between 15° and 0°C. In hamsters this uptake of K has been shown to require the outward extrusion of Na. Conversely, about half of the outward extrusion of Na requires K in the medium, while the remainder appears to be independent of K. The conclusions warranted are that kidney cells of hibernators possess an unusual ability to transport ions at low temperature, that this ability does not depend upon a more rapid rate at higher temperatures, and that the characteristics of transport at low temperature are qualitatively similar to those at 38°C in cells of nonhibernators.     

Cohesin: a critical chromatin organizer in mammalian gene regulation

Cohesins are evolutionarily conserved essential multi-protein complexes that are important for higher-order chromatin organization. They play pivotal roles in the maintenance of genome integrity through mitotic chromosome regulation, DNA repair and replication, as well as gene regulation critical for proper development and cellular differentiation. In this review, we will discuss the multifaceted functions of mammalian cohesins and their apparent functional hierarchy in the cell, with particular focus on their actions in gene regulation and their relevance to human developmental disorders.     

A discriminant analysis of amino acid frequency in collagen-like proteins.

The amino acid composition of collagen-like proteins from seven vertebrate and invertebrate animal phyla was used to construct a set of multiple discriminant classification functions. A stepwise procedure based on conditional F ratios was employed to identify those amino acids whose frequencies varied most consistently among the phyla. Out of 19 possible amino acids, proline, aspartate, lysine, alanine, phenylalanine, threonine, tyrosine, serine and hydroxyproline were selected to become independent variables of the discriminant functions. The reduction in the number of independent variables from 19 to nine is one of the advantages of this method. Variation among phylum centroids on the derived set of five discriminant functions (axes) was statistically highly significant and these five axes accounted for 98·11% of the total variance. All of the possible pairwise comparisons of phylum centroids on these five axes revealed differences which would occur by chance less than 1% of the time. By these criteria, the seven phyla studied were successfully distinguished one from the other on the basis of the amino acid composition of their collagen. Using the classification functions, the most probable phylum affinities for 46 species were calculated and all of these species were identified with their correct phylum. This offers the possibility that this method might be useful in defining membership in more refined taxa.     

Inter-chromosomal gene regulation in the mammalian cell nucleus

Cellular phenotypes can critically rely on mono-allelic gene expression. Recent studies suggest that in mammalian cells inter-chromosomal DNA interactions may mediate the decision which allele to activate and which to silence. Here, these findings are discussed in the context of knowledge on gene competition, chromatin dynamics, and nuclear organization. We argue that data obtained by 4C technology strongly support the idea that chromatin folds according to self-organizing principles. In this concept, the nuclear positioning of a given locus is probabilistic as it also depends on the properties of neighbouring DNA segments and, by extrapolation, the whole chromosome. The linear distribution of repetitive DNA sequences and of active and inactive DNA regions is important for the folding and relative positioning of chromosomes. This stochastic concept of nuclear organization predicts that tissue-specific interactions between two selected loci present on different chromosomes will be rare.     

Role and regulation of prolyl hydroxylase domain proteins

Oxygen-dependent hydroxylation of hypoxia-inducible factor (HIF)-alpha subunits by prolyl hydroxylase domain (PHD) proteins signals their polyubiquitination and proteasomal degradation, and plays a critical role in regulating HIF abundance and oxygen homeostasis. While oxygen concentration plays a major role in determining the efficiency of PHD-catalyzed hydroxylation reactions, many other environmental and intracellular factors also significantly modulate PHD activities. In addition, PHDs may also employ hydroxylase-independent mechanisms to modify HIF activity. Interestingly, while PHDs regulate HIF-alpha protein stability, PHD2 and PHD3 themselves are subject to feedback upregulation by HIFs. Functionally, different PHD isoforms may differentially contribute to specific pathophysiological processes, including angiogenesis, erythropoiesis, tumorigenesis, and cell growth, differentiation and survival. Because of diverse roles of PHDs in many different processes, loss of PHD expression or function triggers multi-faceted pathophysiological changes as has been shown in mice lacking different PHD isoforms. Future investigations are needed to explore in vivo specificity of PHDs over different HIF-alpha subunits and differential roles of PHD isoforms in different biological processes.     

Streptogramin-based gene regulation systems for mammalian cells

Here we describe repressible (PipOFF) as well as inducible (PipON) systems for regulated gene expression in mammalian cells, based on the repressor Pip (pristinamycin-induced protein), which is encoded by the streptogramin resistance operon of Streptomyces coelicolor. Expression of genes placed under control of these systems was responsive to clinically approved antibiotics belonging to the streptogramin group (pristinamycin, virginiamycin, and Synercid). The versatility of these systems was demonstrated by streptogramin-regulated expression of mouse erythropoietin (EPO), human placental secreted alkaline phosphatase (SEAP), or green fluorescent protein (GFP) in diverse cell lines (BHK, CHO, HeLa, and mouse myoblasts). Analysis of isogenic constructs in CHO cells demonstrated the PipOFF system gave lower background and higher induction ratios than the widely used tetracycline-repressible (TetOFF) expression systems. The streptogramin-based expression technology was functionally compatible with the TetOFF system, thus enabling the selective use of different antibiotics to independently control two different gene activities in the same cell.