Ultrastructure of the adrenal cortex of hibernating, arousing, and euthermic dormouse, Muscardinus avellanarius

Renal adaptation during hibernation

Hibernators periodically undergo profound physiological changes including dramatic reductions in metabolic, heart, and respiratory rates and core body temperature. This review discusses the effect of hypoperfusion and hypothermia observed during hibernation on glomerular filtration and renal plasma flow, as well as specific adaptations in renal architecture, vasculature, the renin-angiotensin system, and upregulation of possible protective mechanisms during the extreme conditions endured by hibernating mammals. Understanding the mechanisms of protection against organ injury during hibernation may provide insights into potential therapies for organ injury during cold storage and reimplantation during transplantation.

Subcellular distribution of key enzymes of lipid metabolism during the euthermia-hibernation-arousal cycle

Mammalian hibernation is a natural, fully reversible hypometabolic state characterized by a drastic reduction of body temperature and metabolic activity, which ensures survival to many species under adverse environmental conditions. During hibernation, many hibernators rely for energy supply almost exclusively on lipid reserves; the shift from carbohydrate to lipid metabolism implies profound rearrangement of the anabolic and catabolic pathways of energetic substrates. However, the structural counterpart of such adaptation is not known. In this study we investigated, by using immunoelectron microscopy, the fine intracellular distribution of two key enzymes involved in lipid metabolism, namely, the fatty acid synthase (FAS) and the long-chain fatty acyl-CoA synthetase (ACSL), in hepatocytes of euthermic, hibernating and arousing hazel dormice. Our results show that the two enzymes are differentially distributed in cellular compartments (cytoplasm, mitochondria and cell nuclei) of hepatocytes during euthermia. Quantitative redistribution of both enzymes among cellular compartments takes place during hibernation and arousal, in accordance with the physiological changes. Interestingly, this redistribution follows different seasonal patterns in cytoplasm, mitochondria and nuclei. In conclusion, our data represent the first quantitative morphological evidence of lipid enzyme distribution in a true hibernator throughout the year cycle, thus providing a structural framework to biochemical changes associated with the hypometabolism of hibernation.

The effect of the enkephalin DADLE on transcription does not depend on opioid receptors

[D-Ala(2),D-Leu(5)] enkephalin (DADLE) is a synthetic peptide capable of inducing a hibernation-like state in mammals in vivo and in cultured cells in vitro. The effects of DADLE seem to be due to its binding to opioid receptors; however, it inhibits the growth of LNCaP cells, devoid of opioid receptors. We have investigated the effects of DADLE on this cell line using transmission electron microscopy, immunocytochemistry and cytometry, in order to elucidate the general mechanism(s) by which this enkephalin affects cell metabolism. We demonstrated that, similar to cell lines provided with opioid receptors, in LNCaP cells DADLE induces structural modifications of cytoplasmic and nuclear constituents, as well as a decrease in transcription and proliferation. However, DADLE does not provoke an increase in apoptotic or necrotic cell fraction, and, after removing the enkephalin from the culture medium, all effects disappear. We also demonstrated that DADLE molecules enter the cytoplasm and the nucleus of LNCaP cells, mostly binding to perichromatin fibrils and dense fibrillar component, where transcription and early splicing of pre-mRNAs and pre-rRNAs occur. In conclusion, our data demonstrate that the effect of DADLE on transcription and on cultured cells does not depend on opioid receptors. DADLE can, therefore, be envisaged as an extremely promising molecule to be used for inducing a reversible hypometabolic state in various cultured cells, without provoking cell damage or death.

Ultrastructural and immunocytochemical analyses of opioid treatment effects on PC3 prostatic cancer cells

Some opioid peptides are able to inhibit the growth of human prostatic cancer cells; in particular, the [D-Ala(2),D-Leu(5)] enkephalin (DADLE) reduces PC3 cell growth. In order to understand how DADLE decreases cell proliferation, we investigated, by electron microscopy, its effects on PC3 cellular components. PC3 cells were incubated with DADLE and processed for both ultrastructural morphology and immunoelectron microscopy. Some cells were incubated with BrU to determine the transcriptional rate. BrU and DADLE molecules were detected by immunogold techniques and the labeling was quantitatively evaluated. Modifications of some cytoplasmic and nuclear components were observed in DADLE-treated cells. Moreover, treated cells incorporated lower amounts of BrU than control cells. DADLE molecules were located in the cytoplasm and in the nucleus, especially on mRNA transcription and early splicing sites. Our data suggest that DADLE is able to slow down the synthetic activity of PC3 cells, perhaps interfering with nuclear functions.

Quantitative ultrastructural changes of hepatocyte constituents in euthermic, hibernating and arousing dormice (Muscardinus avellanarius)

Hibernating animals represent a suitable model for investigating the structural effects of drastic changes in cell activity under physiological conditions. In this study we investigated by means of electron microscopy and morphometrical analysis the fine structural counterpart of functional rest in hepatocytes of the hibernating dormouse, Muscardinus avellanarius, in comparison with arousing and euthermic dormice. Our observations demonstrate that during hibernation several structural constituents of the hepatocyte undergo modifications. In particular, during deep hibernation, the total cell and cytoplasm area significantly reduced, as well as the total and percent glycogen and residual body area, and the Golgi apparatus almost disappeared. Upon arousal, the amount of glycogen was minimal, whereas total cell and cytoplasm area significantly increased towards the euthermic value as well as total and percent residual body area. In comparison with the euthermic condition, the total and percent cell lipid area significantly increased in early hibernation, reduced in deep hibernation and almost disappeared during arousal. Taken together, our findings give quantitative ultrastructural support to the marked reduction found in hepatocyte functional activities during hibernation. Such a reduced activity involves profound rearrangement of the euthermic cell structure, which is rapidly resumed upon arousal.

mRNA stability and polysome loss in hibernating Arctic ground squirrels (Spermophilus parryii)

All small mammalian hibernators periodically rewarm from torpor to high, euthermic body temperatures for brief intervals throughout the hibernating season. The functional significance of these arousal episodes is unknown, but one suggestion is that rewarming may be related to replacement of gene products lost during torpor due to degradation of mRNA. To assess the stability of mRNA as a function of the hibernation state, we examined the poly(A) tail lengths of liver mRNA from arctic ground squirrels sacrificed during four hibernation states (early and late during a torpor bout and early and late following arousal from torpor) and from active ground squirrels sacrificed in the summer. Poly(A) tail lengths were not altered during torpor, suggesting either that mRNA is stabilized or that transcription continues during torpor. In mRNA isolated from torpid ground squirrels, we observed a pattern of 12 poly(A) residues at greater densities approximately every 27 nucleotides along the poly(A) tail, which is a pattern consistent with binding of poly(A)-binding protein. The intensity of this pattern was significantly reduced following arousal from torpor and undetectable in mRNA obtained from summer ground squirrels. Analyses of polysome profiles revealed a significant reduction in polyribosomes in torpid animals, indicating that translation is depressed during torpor.

The kidney during hibernation and arousal from hibernation. A natural model of organ preservation during cold ischaemia and reperfusion

BACKGROUND

During hibernation the kidney is in a hypothermic condition where renal blood flow is minimal and urine production is much reduced. Periodical arousal from hibernation is associated with kidney reperfusion at increasing body temperature, and restored urine production rate.

METHODS

To assess the degree of structural preservation during such extreme conditions, the kidney cortex was investigated by means of electron microscopy in the dormouse Muscardinus avellanarius during winter hibernation, arousal from hibernation and the summer active period.

RESULTS

Results show that the fine structure of the kidney cortex is well preserved during hibernation. In the renal corpuscle, a sign of slight lesion was the focal presence of oedematous endothelial cells and/or podocytes. Proximal convoluted tubule cells showed fully preserved ultrastructure and polarity, and hypertrophic apical endocytic apparatus. Structural changes were associated with increased plasma electrolytes, creatinine and urea nitrogen, and proteinuria. During the process of arousal the fine structure of the kidney cortex was also well maintained.

CONCLUSION

These results demonstrate that dormice are able to fully preserve kidney cortex structure under extreme conditions resembling e.g. severe ischaemia or hypothermic organ storage for transplantation, and reperfusion. Elucidation of the mechanisms involved in such a natural model of organ preservation could be relevant to human medicine.