The physiological mechanisms of endothermy in marsupials : identification of thermogenic abilities and expression of uncoupling proteins

This thesis examines the extent of and the physiological mechanisms behind the endothermic ability in two marsupials, Bettongia gaimardi and Sarcophilus harrisii. It also investigates the physiological development of endothermy in the pouch young B. gaimardi, as well as the expression of uncoupling proteins in several other marsupials and a monotreme, including Sminthopsis crassicaudata, Trichosurus vulpecula, Tachyglossus aculeatus, and Perameles gunnii. This thesis is divided into three main sections. The first section is dedicated to investigating the thermal mechanisms used by S. harrisii. This chapter identifies the ability of this species to perform nonshivering thermogenesis by measuring oxygen consumption prior to and post cold-exposure, and with and without ˜í‚â§-agonist induction. This ability was correlated to the possible expression of uncoupling protein 1 using molecular techniques at both the protein and genetic levels. However, as uncoupling protein 1 was not identified, the expression of two other uncoupling proteins was investigated to determine any possible role in thermogenesis. This study was able to show that S. harrisii expresses uncoupling protein 2 but not uncoupling protein 3, although this study was unable to shed any new light on the role these proteins may play. This chapter also reveals that S. harrisii increases intermuscular temperature in response to long-term cold exposure. The second section of this thesis deals with the thermoregulatory mechanisms employed by Bettongia gaimardi. The identification of nonshivering thermogenesis in the absence of uncoupling protein 1 has been previously documented in the literature, although the mechanisms behind this ability remain unknown. This chapter investigates the expression of uncoupling proteins 2 and 3. This study was able to identify the expression of both proteins in adult B. gaimardi. It was subsequently determined to use the `ectothermic to endothermic' shift seen in the pouch young B. gaimardi to identify a possible role for these proteins, as well as identify the physiologidal developments that occur allowing endothermy to be achieved. These data established that the pouch young B. gaimardi use a previously undocumented mechanism of increasing muscle tone in response to the cold. It was also shown that the pouch young B. gaimardi express uncoupling protein 3 at an early age, and presumably from birth. However, the pouch young only began to express uncoupling protein 2 concurrently with the onset of endothermy. This may suggest a role for uncoupling protein 2 in the development of endothermy. The identification of the novel mechanism of increasing muscular tone in response to - the cold in the pouch young B. gaimardi prompted a subsequent investigation of this ability in adults. This was primarily conducted to rule out the possibility that this mechanism, seen in the pouch young, is not a temporary device only used during the transition from ectothermy to endothermy. The data, however, suggest that this ability is maintained throughout the animal's life, although promoted by acclimation to the cold in the adult B. gaimardi. This may be one possible mechanism of thermogenesis in the absence of uncoupling protein 1 and/or brown adipose tissue. The final section of this thesis explores the proposal that there is a phylogenetic difference in the nonshivering thermogenic ability of marsupials and monotremes. This chapter attempts to correlate the expression of uncoupling proteins 1, 2, and 3 with the reported differences in thermogenic abilities to glean an understanding of a possible role for these proteins in marsupials and a monotreme. This study looked at several species of marsupial and one monotreme, that have has their thermogenic abilities documented, including Bettongia gaimardi, Sarcophilus harrisii, Sminthopsis crassicaudata, Trichosurus vulpecula, Perameles gunnii, and Tachyglossus aculeatus, and investigated the expression of uncoupling proteins. It was concluded that no species used in this investigation expressed uncoupling protein 1, although uncoupling protein 2 was ubiquitously expressed. There was, however, a phylo genetic difference in uncoupling protein 3 expression, although there was no positive or negative correlation with thermogenic ability. This thesis, as a whole, concludes that they may use many of the same thermoregulatory mechanisms that eutherians use, but may also have evolved some novel means. It suggests that no marsupials and the echidna use uncoupling protein 1 or BAT in a thermogenic manner, regardless of expression. The expression of uncoupling protein 3 in marsupials and the echidna is not determined by nonshivering thermogenic ability, and the protein has no obvious role in thermogenesis. Lastly, this thesis shows that uncoupling protein 2 is expressed in all species investigated within this study, and it appears that this ubiquitous protein has a relationship with the development of endothermy in marsupials.