Highly Similar Tetramerization Domains from the p53 Protein of Different Mammalian Species Possess Varying Biophysical, Functional and Structural Properties

The p53 protein is a transcriptional regulatory factor and many of its functions require that it forms a tetrameric structure. Although the tetramerization domain of mammalian p53 proteins (p53TD) share significant sequence similarities, it was recently shown that the tree shrew p53TD is considerably more thermostable than the human p53TD. To determine whether other mammalian species display differences in this domain, we used biophysical, functional, and structural studies to compare the properties of the p53TDs from six mammalian model organisms (human, tree shrew, guinea pig, Chinese hamster, sheep, and opossum). The results indicate that the p53TD from the opossum and tree shrew are significantly more stable than the human p53TD, and there is a correlation between the thermostability of the p53TDs and their ability to activate transcription. Structural analysis of the tree shrew and opossum p53TDs indicated that amino acid substitutions within two distinct regions of their p53TDs can dramatically alter hydrophobic packing of the tetramer, and in particular substitutions at positions corresponding to F341 and Q354 of the human p53TD. Together, the results suggest that subtle changes in the sequence of the p53TD can dramatically alter the stability, and potentially lead to important changes in the functional activity, of the p53 protein.

Conservation of Glutathione Transferase mRNA and Protein Sequences Similar to Human and Horse Alpha Class GST A3-3 across Dog, Goat, and Opossum Species

The glutathione transferase A3-3 (GST A3-3) homodimeric enzyme is the most efficient enzyme that catalyzes isomerization of the precursors of testosterone, estradiol, and progesterone in the gonads of humans and horses. However, the presence of GST A3-3 orthologs with equally high ketosteroid isomerase activity has not been verified in other mammalian species, even though pig and cattle homologs have been cloned and studied. Identifying GSTA3 genes is a challenge because of multiple GSTA gene duplications (e.g., 12 in the human genome); consequently, the GSTA3 gene is not annotated in most genomes. To improve our understanding of GSTA3 gene products and their functions across diverse mammalian species, we cloned homologs of the horse and human GSTA3 mRNAs from the testes of a dog, goat, and gray short-tailed opossum, the genomes of which all currently lack GSTA3 gene annotations. The resultant novel GSTA3 mRNA and inferred protein sequences had a high level of conservation with human GSTA3 mRNA and protein sequences (≥70% and ≥64% identities, respectively). Sequence conservation was also apparent for the 12 residues of the "H-site" in the 222 amino acid GSTA3 protein that is known to interact with the steroid substrates. Modeling predicted that the dog GSTA3-3 may be a more active ketosteroid isomerase than the corresponding goat or opossum enzymes. However, expression of the GSTA3 gene was higher in liver than in other dog tissue. Our results improve understanding of the active sites of mammalian GST A3-3 enzymes, inhibitors of which might be useful for reducing steroidogenesis for medical purposes, such as fertility control or treatment of steroid-dependent diseases.

Ang II acutely stimulates Na,K-pump in cells from proximal tubules by increasing its phosphorylation at S938 via a PI3K/AKT pathway

Angiotensin II (Ang II)-dependent stimulation of the AT1 receptor in proximal tubules increases sodium reabsorption and blood pressure. Reabsorption is driven by the Na,K-pump that is acutely stimulated by Ang II, which requires phosphorylation of serine-938 (S938). This site is present in humans and only known to phosphorylated by PKA. Yet, activation of AT1 decreases cAMP required to activate PKA and inhibiting PKA does not block Ang II-dependent phosphorylation of S938. We tested the hypothesis that Ang II-dependent activation is mediated via increased phosphorylation at S938 through a PI3K/AKT-dependent pathway. Experiments were conducted using opossum kidney cells, a proximal tubule cell line, stably co-expressing the AT1 receptor and either the wild-type (α-1.wild-type) or an alanine substituted (α-1.S938A) form of rat kidney Na,K-pump. A 5-min exposure to 10 pM Ang II significantly activated Na,K-pump activity (56%) measured as short-circuit current across polarized α-1.wild-type cells. Wortmannin, at a concentration that selectively inhibits PI3K, blocked that Ang II-dependent activation. Ang II did not stimulate Na,K-pump activity in α-1.S938A cells. Ang II at 10 and 100 pM increased phosphorylation at S938 in α-1.wild-type cells measured in whole cell lysates. The increase was inhibited by wortmannin plus H-89, an inhibitor of PKA, not by either alone. Ang II activated AKT inhibited by wortmannin, not H-89. These data support our hypothesis and show that Ang II-dependent phosphorylation at S938 stimulates Na,K-pump activity and transcellular sodium transport.

An Adaptable Physiological Model of Endocytic Megalin Trafficking in Opossum Kidney Cells and Mouse Kidney Proximal Tubule

The cells that comprise the proximal tubule (PT) are specialized for high-capacity apical endocytosis necessary to maintain a protein-free urine. Filtered proteins are reclaimed via receptor-mediated endocytosis facilitated by the multiligand receptors megalin and cubilin. Despite the importance of this pathway, we lack a detailed understanding of megalin trafficking kinetics and how they are regulated. Here, we utilized biochemical and quantitative imaging methods in a highly differentiated model of opossum kidney (OK) cells and in mouse kidney in vivo to develop mathematical models of megalin traffic. A preliminary model based on biochemically quantified kinetic parameters was refined by colocalization of megalin with individual apical endocytic compartment markers. Our model predicts that megalin is rapidly internalized, resulting in primarily intracellular distribution of the receptor at steady state. Moreover, our data show that early endosomes mature rapidly in PT cells and suggest that Rab11 is the primary mediator of apical recycling of megalin from maturing endocytic compartments. Apical recycling represents the rate-limiting component of endocytic traffic, suggesting that this step has the largest impact in determining the endocytic capacity of PT cells. Adaptation of our model to the S1 segment of mouse PT using colocalization data obtained in kidney sections confirms basic aspects of our model and suggests that our OK cell model largely recapitulates in vivo membrane trafficking kinetics. We provide a downloadable application that can be used to adapt our working parameters to further study how endocytic capacity of PT cells may be altered under normal and disease conditions.

Molecular Evolution of CatSper in Mammals and Function of Sperm Hyperactivation in Gray Short-Tailed Opossum

Males have evolved species-specifical sperm morphology and swimming patterns to adapt to different fertilization environments. In eutherians, only a small fraction of the sperm overcome the diverse obstacles in the female reproductive tract and successfully migrate to the fertilizing site. Sperm arriving at the fertilizing site show hyperactivated motility, a unique motility pattern displaying asymmetric beating of sperm flagella with increased amplitude. This motility change is triggered by Ca²⁺ influx through the sperm-specific ion channel, CatSper. However, the current understanding of the CatSper function and its molecular regulation is limited in eutherians. Here, we report molecular evolution and conservation of the CatSper channel in the genome throughout eutherians and marsupials. Sequence analyses reveal that CatSper proteins are slowly evolved in marsupials. Using an American marsupial, gray short-tailed opossum (Monodelphis domestica), we demonstrate the expression of CatSper in testes and its function in hyperactivation and unpairing of sperm. We demonstrate that a conserved IQ-like motif in CatSperζ is required for CatSperζ interaction with the pH-tuned Ca²⁺ sensor, EFCAB9, for regulating CatSper activity. Recombinant opossum EFCAB9 can interact with mouse CatSperζ despite high sequence divergence of CatSperζ among CatSper subunits in therians. Our finding suggests that molecular characteristics and functions of CatSper are evolutionarily conserved in gray short-tailed opossum, unraveling the significance of sperm hyperactivation and fertilization in marsupials for the first time.

Didelphis albiventris: an overview of unprecedented transcriptome sequencing of the white-eared opossum

BACKGROUND

The white-eared opossum (Didelphis albiventris) is widely distributed throughout Brazil and South America. It has been used as an animal model for studying different scientific questions ranging from the restoration of degraded green areas to medical aspects of Chagas disease, leishmaniasis and resistance against snake venom. As a marsupial, D. albiventris can also contribute to the understanding of the molecular mechanisms that govern the different stages of organogenesis. Opossum joeys are born after only 13 days, and the final stages of organogenesis occur when the neonates are inside the pouch, depending on lactation. As neither the genome of this opossum species nor its transcriptome has been completely sequenced, the use of D. albiventris as an animal model is limited. In this work, we sequenced the D. albiventris transcriptome by RNA-seq to obtain the first catalogue of differentially expressed (DE) genes and gene ontology (GO) annotations during the neonatal stages of marsupial development.

RESULTS

The D. albiventris transcriptome was obtained from whole neonates harvested at birth (P0), at 5 days of age (P5) and at 10 days of age (P10). The de novo assembly of these transcripts generated 85,338 transcripts. Approximately 30% of these transcripts could be mapped against the amino acid sequences of M. domestica, the evolutionarily closest relative of D. albiventris to be sequenced thus far. Among the expressed transcripts, 2077 were found to be DE between P0 and P5, 13,780 between P0 and P10, and 1453 between P5 and P10. The enriched GO terms were mainly related to the immune system, blood tissue development and differentiation, vision, hearing, digestion, the CNS and limb development.

CONCLUSIONS

The elucidation of opossum transcriptomes provides an out-group for better understanding the distinct characteristics associated with the evolution of mammalian species. This study provides the first transcriptome sequences and catalogue of genes for a marsupial species at different neonatal stages, allowing the study of the mechanisms involved in organogenesis.

Platypus and opossum calcitonins exhibit strong activities, even though they belong to mammals

In mammalian assay systems, calcitonin peptides of non-mammalian species exhibit stronger activity than those of mammals. Recently, comparative analyses of a wide-range of species revealed that platypus and opossum, which diverged early from other mammals, possess calcitonins that are more similar in amino acid sequence to those of non-mammals than mammals. We herein determined whether platypus and opossum calcitonins exhibit similar biological activities to those of non-mammalian calcitonins using an assay of actin ring formation in mouse osteoclasts. We also compared the dose-dependent effects of each calcitonin on cAMP production in osteoclasts. Consistent with the strong similarities in their primary amino acid sequences, platypus and opossum calcitonins disrupted actin rings with similar efficacies to that of salmon calcitonin. Human calcitonin exhibited the weakest inhibitory potency and required a 100-fold higher concentration (EC₅₀=3×10^-11M) than that of salmon calcitonin (EC₅₀=2×10^-13M). Platypus and opossum calcitonins also induced cAMP production in osteoclast cultures with the same efficacies as that of salmon calcitonin. Thus, platypus and opossum calcitonins exhibited strong biological activities, similar to those of the salmon. In addition, phylogenetic analysis revealed that platypus and opossum calcitonins clustered with the salmon-type group but not human- or porcine-type group. These results suggest that platypus and opossum calcitonins are classified into the salmon-type group, in terms of the biological activities and amino acid sequences.

Metal accumulation in wild-caught opossum

The Virginia opossum (Didelphis virginiana) is widespread in the USA, ranging south through Latin America. The ecology of opossums is such that they are in frequent contact with soils, suggesting that they may function as a valuable bioindicator for chemical contamination in terrestrial environments. Surprisingly, there have been virtually no toxicology studies on opossums. Here, we provide the first analysis of metal contaminants in opossum liver tissues. Liver samples were obtained from 471 opossums, collected from 2003 to 2006, at four sites in North Florida and South Georgia, USA, and concentrations of copper, lead, nickel, selenium, and zinc were measured. We found little evidence of age differences in the concentration of any of the metals. However, there were at least some significant differences between years, males and females, and between sites for each metal, although the pattern of these differences was not always consistent across metals. Concentrations of metals in liver tissue were positively correlated with one another, primarily of each metal (except Pb) with zinc. Reference levels of metal contaminants are not available for opossums, but concentrations of Cu, Ni, Pb, and Zn in our samples were for the most part significantly higher than those reported from liver tissues of nine-banded armadillos (Dasypus novemcinctus) collected at the same sites and in the same years. Data from other small mammals studied elsewhere further indicate that metal concentrations in opossums were high, but at this time, it is not possible to determine if these elevated levels generated toxicity. The substantial temporal and spatial variation we found in metal concentrations suggests that determination of baseline levels for opossums may not be straightforward. Nonetheless, this is the first study quantifying metal accumulation in the livers of Didelphis virginiana and, as such, provides an important starting point for future research.

Myosin 1b Regulates Amino Acid Transport by Associating Transporters with the Apical Plasma Membrane of Kidney Cells

Amino acid transporters (AATers) in the brush border of the apical plasma membrane (APM) of renal proximal tubule (PT) cells mediate amino acid transport (AAT). We found that the membrane-associated class I myosin myosin 1b (Myo1b) localized at the apical brush border membrane of PTs. In opossum kidney (OK) 3B/2 epithelial cells, which are derived from PTs, expressed rat Myo1b-GFP colocalized in patched microvilli with expressed mouse V5-tagged SIT1 (SIT1-V5), which mediates neutral amino acid transport in OK cells. Lentivirus-mediated delivery of opossum Myo1b-specific shRNA resulted in knockdown (kd) of Myo1b expression, less SIT1-V5 at the APM as determined by localization studies, and a decrease in neutral AAT as determined by radioactive uptake assays. Myo1b kd had no effect on Pi transport or noticeable change in microvilli structure as determined by rhodamine phalloidin staining. The studies are the first to define a physiological role for Myo1b, that of regulating renal AAT by modulating the association of AATers with the APM.

Anin situsingle-pass perfusion model for assessing absorption across the intestinal mucosa of the brushtail possum

AIM

To develop an in situ animal model for assessing absorption of molecules across the intestinal mucosa of possums.

METHODS

A surgical preparation was used to perfuse known concentrations of reference compounds (fluorescein and luteinising hormone-releasing hormone; LHRH) through measured sections of selected regions (jejunum, caecum, proximal colon) of the intestinal tract of 19 possums, over a 2-h period. Plasma concentrations of the compounds, which were perfused either with or without co-administration of a permeation enhancer (sodium deoxycholic acid; SDA), were determined in the perfusion effluent, peripheral and in some instances in the pre-hepatic circulation by spectrofluorometry (fluorescein) or radioimmunoassay (LHRH). Pharmacokinetic parameters of both compounds in the possum were determined over a period of up to 4 h in a further 30 animals (fluorescein, n = 6; LHRH n = 24), from their plasma profiles following intravenous (I/V) administration of a bolus dose.

RESULTS

In animals perfused with 25 mg/ml fluorescein (Perfusion Experiment (PE) 1), the mean plasma concentration was 2.8 (SE 0.12) microg/ml in post-hepatic blood samples. When possums were perfused with 2.5 mg/ml fluorescein and 7 microg/ml LHRH (PE 2), mean plasma concentrations were 0.3 (SE 0.01) and 7.8 (SE 1.64) microg/ml fluorescein and 0.1 (SE 0.02) and 6.3 (SE 0.45) ng/ml LHRH, in the absence and presence of permeation enhancer, respectively. There was a poor correlation between pre-hepatic and post-hepatic concentrations.

CONCLUSIONS

The single-pass in situ perfusion technique provided a useful model for investigating basic information on the absorption of biocontrol agents across the intestinal tract of possums, but had limitations that must be recognised.

Effect of ezetimibe on plasma cholesterol levels, cholesterol absorption, and secretion of biliary cholesterol in laboratory opossums with high and low responses to dietary cholesterol

Partially inbred lines of laboratory opossums differ in plasma low-density lipoprotein cholesterol concentration and cholesterol absorption on a high-cholesterol diet. The aim of the present studies was to determine whether ezetimibe inhibits cholesterol absorption and eliminates the differences in plasma cholesterol and hepatic cholesterol metabolism between high and low responders on a high-cholesterol diet. Initially, we determined that the optimum dose of ezetimibe was 5 mg/(kg d) and treated 6 high- and 6 low-responding opossums with this dose (with equal numbers of controls) for 3 weeks while the opossums consumed a high-cholesterol and low-fat diet. Plasma and low-density lipoprotein cholesterol concentrations decreased significantly (P < .05) in treated but not in untreated high-responding opossums. Plasma cholesterol concentrations increased slightly (P < .05) in untreated low responders but not in treated low responders. The percentage of cholesterol absorption was significantly higher in untreated high responders than in other groups. Livers from high responders with or without treatment were significantly (P < .01) heavier than livers from low responders with or without treatment. Hepatic cholesterol concentrations in untreated high responders were significantly (P < .05) higher than those in low responders with or without treatment (P < .001). The gall bladder bile cholesterol concentrations in untreated high responders were significantly (P < .05) lower than those in other groups. A decrease in biliary cholesterol in low responders treated with ezetimibe was associated with a decrease in hepatic expression of ABCG5 and ABCG8. These studies suggest that ezetimibe decreases plasma cholesterol levels in high responders mainly by decreasing cholesterol absorption and increasing biliary cholesterol concentrations. Because ezetimibe's target is NPC1L1 and NPC1L1 is expressed in the intestine of opossums, its effect on cholesterol absorption may be mediated by inhibiting NPC1L1 function in the intestine.

Behavioral and reproductive responses of female opossums to volatile and nonvolatile components of male suprasternal gland secretion

Female gray short-tailed opossums (Monodelphis domestica) lack an estrous cycle and are induced into estrus by exposure to a pheromone in male scent marks. Behavioral and physiological responses of females to the volatile and nonvolatile components of scent marks were examined in two experiments. Young females (n=9) were tested prior to and during their first estrus for behavioral responses to scent marks, collected on a 7-ml glass vial rubbed over the suprasternal gland of a mature male. The response to volatile components of the scent mark, recorded when marked and unmarked vials were covered with a perforated shield, was compared to the response to these vials when unshielded. Estrous females nuzzled the shields over marked vials (55.8+/-8.5 nuzzles/10 min) more than the shielded clean vial (10.9+/-2.4) (P<0.05); a similar response was observed in anestrous females. Nuzzling of unshielded, scent-marked vials was higher (P<0.05) during anestrus than in the same females when in estrus. The role of nonvolatile pheromones in reproductive activation was tested in adult females (n=11) exposed for up to 14 days to a shielded, marked vial or to an unshielded, marked vial in a crossover design. All females exposed to unshielded vials expressed estrus, and 10 copulated. Only 2 females expressed estrus (significantly fewer, P<0.05), when exposed to shielded marked vials, and neither copulated. These results demonstrate that females detect and respond behaviorally to both volatile and nonvolatile components of male suprasternal gland secretion, but the estrus-inducing pheromone in these secretions is nonvolatile.

Organization of brain extracellular matrix in the Chilean fat-tailed mouse opossum Thylamys elegans (Waterhouse, 1839)

We investigated the structural and molecular organization of the extracellular matrix in Thylamys elegans, a marsupial representative of the mammalian order Didelphimorphia. Perineuronal nets (PNs) associated with distinct types of neurons were visualized by detection of chondroitin sulfate proteoglycans and hyaluronan, and by labeling with Wisteria floribunda agglutinin (WFA), a marker for PNs in the mammalian brain. In the neocortex of Thylamys, these methods revealed PNs on pyramidal cells. In contrast, parvalbumin-immunoreactive interneurons in the neocortex and hippocampal formation (displaying robust, WFA-labeled PNs in placental mammals) were ensheathed only with a delicate rim of hyaluronan and proteoglycans not detectable with WFA. The absence of WFA staining was characteristic also of some subcortical regions which contained PNs intensely labeled for chondroitin sulfate proteoglycan and hyaluronan. However, corresponding to placental mammals, numerous subcortical nuclei showed clearly WFA-stained PNs. Similar as in placental mammals, cholinergic basal forebrain neurons and tyrosine hydroxylase-immunoreactive neurons of the substantia nigra and locus coeruleus were devoid of PNs. Together with our earlier study on Monodelphis, the present results reveal that South American opossums show either a particular "marsupial" or "Didelphid" type of extracellular matrix chemoarchitecture, supporting the view that these components may vary phylogenetically as integral parts of neuronal physiology at the systems and single cell level.

Interaction of MAP17 with NHERF3/4 induces translocation of the renal Na/Pi IIa transporter to the trans-Golgi

The function of the NaPiIIa renal sodium-phosphate transporter is regulated through a complex network of interacting proteins. Several PDZ domain-containing proteins interact with its COOH terminus while the small membrane protein MAP17 interacts with its NH(2) end. To elucidate the function of MAP17, we identified its interacting proteins using both bacterial and mammalian two-hybrid systems. Several PDZ domain-containing proteins, including the four NHERF proteins, as well as NaPiIIa and NHE3, were found to bind to MAP17. The interactions of MAP17 with the NHERF proteins and with NaPiIIa were further analyzed in opossum kidney (OK) cells. Expression of MAP17 alone had no effect on the NaPiIIa apical membrane distribution, but coexpression of MAP17 and NHERF3 or NHERF4 induced internalization of NaPiIIa, MAP17, and the PDZ protein to the trans-Golgi network (TGN). This effect was not observed when MAP17 was cotransfected with NHERF1/2 proteins. Inhibition of protein kinase C (PKC) prevented expression of the three proteins in the TGN. Activation of PKC in OK cells transfected only with MAP17 induced complete degradation of MAP17 and NaPiIIa. When lysosomal degradation was prevented, both proteins accumulated in the TGN. When the dopamine D1-like receptor was activated with fenoldopam, both NaPiIIa and MAP17 also accumulated in the TGN. Finally, cotransfection of MAP17 and NHERF3 prevented the adaptive upregulation of phosphate transport activity in OK cells in response to low extracellular phosphate. Therefore, the interaction between MAP17, NHERF3/4, and NaPiIIa in the TGN could be an important intermediate or alternate path in the internalization of NaPiIIa.

Immunohistochemical localization of gastrin-releasing peptide, neuronal nitric oxide synthase and neurone-specific enolase in the uterus of the North American opossum, Didelphis virginiana

The present study has demonstrated the immunohistochemical localization of gastrin-releasing peptide (GRP), neuronal nitric oxide synthase (nNOS) and neurone-specific enolase (NSE) in the uterus of the North American opossum. Although the presence of GRP, nNOS and NSE has been reported recently in the uterus of eutherian species this is the first description of these peptides in a metatherian species. Metatherian mammals are of interest because in these species it is the prolonged lactation phase of development that is the period of primary reproductive investment rather than intrauterine development as is true of eutherian mammals. The opossum, like other marsupial species, has a very abbreviated gestation period which in Didelphis lasts only 12.5 days. GRP was localized in the cytoplasm of cells forming the surface lining epithelium and the glandular epithelium of the opossum endometrium late in pregnancy, at 11.5 days of gestation. Likewise, immunoreactivities of nNOS and NSE were found primarily within the epithelial cells of the endometrium at 11.5 days of gestation. As these peptides and enzymes appear primarily at the time of establishment of the yolk sac placenta (between day 10 and day 12.5 gestation), the present results strongly suggest that these factors may play a fundamental role in the placentation of the opossum.

Prolactin in the brushtail possum (Trichosurus vulpecula): development of homologous radioimmunoassay using recombinant possum prolactin

We report the production of recombinant possum prolactin (posPrl), and its use in the development and validation of a highly specific homologous radioimmunoassay for the measurement of prolactin (Prl) in brushtail possums. This enabled the subsequent investigation of some basic mechanisms involved in the regulation of Prl secretion in this species. Recombinant posPrl spanning the entire coding region was expressed in Escherichia coli, resulting in a 199 amino acid protein with a molecular weight approximately 23 kDa. The potency of posPrl was 45.3 +/- 4.8% that of ovine Prl in a radioreceptor assay using possum mammary gland receptors and induced a 3.4 +/- 0.8-fold increase in progesterone secretion in primary possum granulosa cells. Antiserum (G27) was raised against recombinant posPrl and was highly specific for possum Prl (approximately 30% binding at 1:60,000 final dilution), and exhibited negligible cross-reactivity (<0.0001%) with possum growth hormone. Serial dilutions of pituitary gland extracts, and plasma samples from male and female possums gave parallel inhibition curves to recombinant posPrl standards in the assay. Biological validation of the RIA included treating possums with drugs known to alter Prl secretion in other mammals. In seasonally anoestrous female possums, administration of 20 microg thyrotropin-releasing hormone (TRH) resulted in a 15-fold increase (P < 0.01) in plasma Prl concentrations. In mid-late lactating female possums, a bolus of cabergoline (dopamine agonist; 75 microg) reduced (P < 0.05) plasma Prl levels to baseline for 24 h, while repeated administration (6 x 75 microg at 12 h intervals) suppressed (P < 0.01) plasma Prl concentrations until 24h after the last injection. Prolonged inhibition of Prl levels subsequently caused marked (P < 0.01) attenuation in rate of bodyweight increase of pouch young. The amplitude of the Prl surge in response to a bolus of TRH (15 microg) was 5-fold lower in cabergoline-treated, compared to control mid-late lactating possums. In conclusion, we report the development and validation of a robust and sensitive RIA for measuring Prl concentrations in the plasma of brushtail possums.

Glucocorticoids acutely increase cell surface Na+/H+ exchanger-3 (NHE3) by activation of NHE3 exocytosis

Glucocorticoids have important effects on renal function, including the modulation of renal acidification by the major proximal tubular Na(+)/H(+) exchanger, NHE3. While the chronic effect of glucocorticoids is considered to be primarily at the transcriptional level, with increases in NHE3 mRNA and protein expression driving increased transport activity, the mechanisms by which glucocorticoids activate NHE3 in an acute setting have not been investigated. Previous studies have shown that a glucocorticoid-stimulated increase in NHE3 activity can occur before any detectable change in NHE3 mRNA. The present study examines the acute effects of glucocorticoids on NHE3 using opossum kidney (OKP) cells as a cell model. In OKP cells, total NHE3 protein abundance was not changed by 3 h of treatment with dexamethasone (10(-6) M). However, the biotin-accessible fraction representing NHE3 at the apical membrane as well as Na(+)/H(+) exchange activity measured fluorimetrically using the pH-sensitive dye BCECF-AM were significantly increased. These effects were not prevented by the protein synthesis inhibitor cycloheximide. NHE3 insertion (biotinylatable NHE3 after sulfo-NHS-acetate blockade) was stimulated by dexamethasone incubation, with or without cycloheximide. The rate of NHE3 endocytic retrieval, assessed either by the avidin protection assay (early endocytosis) or by the sodium 2-mercaptoethane sulfonate (MesNa) cleavage assay (early and late endocytosis), was not affected by dexamethasone. These findings suggest that trafficking plays a key role in the acute stimulation of NHE3 by glucocorticoids, with exocytosis being the major contributor to the glucocorticoid-induced rapid increase in cell surface NHE3 protein abundance and Na(+)/H(+) exchange activity.

Seasonal Changes in Mesotocin and Localization of Its Receptor in the Prostate of the Brushtail Possum (Trichosurus vulpecula)

The prostate gland in the brushtail possum grows and regresses seasonally. It has similarities to the human prostate and may therefore provide a unique model for investigating prostatic hyperplasia. Oxytocin has been implicated in the regulation of prostate growth in eutherian mammals, and the initial aim of this study was to identify and localize the marsupial equivalent, mesotocin, and its receptor in the prostate of the brushtail possum. Seasonal changes in prostatic mesotocin concentrations and receptor localization were then assessed and related to prostate growth. Mesotocin and mesotocin receptor gene transcripts with high sequence homology to eutherian oxytocin/oxytocin receptors were demonstrated, and mesotocin, neurophysin, and the receptor were all localized predominantly in the epithelial cells of the glandular acini. Western blot analysis confirmed the presence of a single immunoreactive receptor protein of approximately 60 Mr-3. Prostatic mesotocin concentrations were highest immediately before the increases in prostate weight associated with the autumn and spring breeding periods. At this time, mesotocin receptors were also present in the prostatic capsule in addition to those present in the glandular tissue. Mesotocin concentrations proceeded to decrease in association with the regression of prostate size toward the end of the breeding periods. No significant differences were present in serum testosterone or dihydrotestosterone throughout the year. The identification of mesotocin and its receptor in the possum prostate and the demonstration of seasonal changes in local mesotocin concentrations preceding changes in prostate size suggests that mesotocin may play a physiological role in regulating prostate growth and regression.

Cholesterol absorption and hepatic acyl-coenzyme A:cholesterol acyltransferase activity play major roles in lipemic response to dietary cholesterol and fat in laboratory opossums

Partially inbred lines of laboratory opossums differ considerably in their low-density lipoprotein (LDL) cholesterol responses to dietary cholesterol and fat. Genetic analysis suggested that a single major gene is responsible for the variation in LDL cholesterol on the high cholesterol and high fat (HCHF) diet. We measured cholesterol absorption and acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in intestine and liver to narrow the search for the major gene. We measured plasma lipoproteins and percent cholesterol absorption by the fecal isotope ratio method in high and low responding lines of opossums on basal and HCHF diets. We also measured lipids in liver and ACAT activity in liver and intestine on the HCHF diet. High and low lines exhibited no differences in percent cholesterol absorption on the basal diet. However, high responding opossums had significantly higher percent cholesterol absorption, hepatic free and esterified cholesterol, and hepatic ACAT activity than low responding opossums on the HCHF diet. Hepatic ACAT activity but not the intestinal ACAT activity was associated with hepatic cholesterol concentration and percent cholesterol absorption. Cholesterol absorption is a major determinant of diet-induced hyperlipidemia in opossums. Hepatic ACAT activity but not the intestinal ACAT may also play a role in diet-induced hyperlipidemia in opossums.

OKP cells express the Na-dicarboxylate cotransporter NaDC-1

Urinary citrate concentration, a major factor in the formation of kidney stones, is primarily determined by its rate of reabsorption in the proximal tubule. Citrate reabsorption is mediated by the Na-dicarboxylate cotransporter-1 (NaDC-1). The opossum kidney (OKP) cell line possesses many characteristics of the renal proximal tubule. The OKP NaDC-1 (oNaDC-1) cDNA was cloned and encodes a 2.4-kb mRNA. When injected into Xenopus oocytes, the cotransporter is expressed and demonstrates Na-coupled citrate transport with a stoichiometry of >or=3 Na:1 citrate, specificity for di- and tricarboxylates, pH-dependent citrate transport, and pH-independent succinate transport, all characteristics of the other NaDC-1 orthologs. Chronic metabolic acidosis increases proximal tubule citrate reabsorption, leading to profound hypocitraturia and an increased risk for stone formation. Under the conditions studied, endogenous OKP NaDC-1 mRNA abundance is not regulated by changes in media pH. In OKP cells transfected with a green fluorescent protein-oNaDC-1 construct, however, media acidification increases Na-dependent citrate uptake, demonstrating posttranscriptional acid regulation of NaDC-1 activity.

Effect of steroid hormones on tissue remodelling and progesterone receptors in the uterus of seasonally anoestrous brushtail possums (Trichosurus vulpecula)

This study describes progesterone receptor (PR) location within uterine cells and associated morphological changes to the uterine luminal and glandular epithelium in seasonally anoestrous brushtail possums treated with oestradiol and/or progesterone.

Twenty-four adult female possums (n= 6/group) were treated with oestradiol, progesterone, oestradiol followed by progesterone or with the oil vehicle alone for 6-day periods. Uterine tissue was recovered, weighed and processed for light and transmission electron microscopy and for immunohistochemistry for PRs. Stereological techniques were used to quantify epithelial cell and constituent volumes for both luminal and glandular tissues. Plasma concentrations of oestradiol and progesterone were determined by radioimmunoassay.

Mean uterine wet weights were significantly heavier (P< 0.001) following oestradiol/progesterone treatment and maximum gland dilation, cellular and stromal growth, maximum cell height, and cell and constituent volumes were recorded after this treatment regimen. Cell nuclei and debris were commonly observed in gland lumina, and nuclear PRs were found predominantly in stromal cells following oestradiol-only treatment. Sequential treatment with oestradiol and then progesterone caused a decline in the number of positively stained cells. Epithelial cells contained extensive secretory organelles and degenerating cells were common within the glands.

Oestradiol treatment induced cell and cell constituent growth and promoted PR formation in anoestrous possum uterine tissue. Subsequent exposure to progesterone stimulated uterine tissues to reach maximum wet weights and led to the cellular maturation necessary to remodel the uterus in preparation for pregnancy.

Calbindin D28k, parvalbumin, and calretinin immunoreactivity in the main and accessory olfactory bulbs of the gray short-tailed opossum,Monodelphis domestica

The vertebrate main and accessory olfactory bulbs (MOB and AOB) are the first synaptic sites in the olfactory pathways. The MOB is a cortical structure phylogenetically well conserved in its laminar structure and overall synaptic organization, while the AOB has significant species variation in size. In order to better understand signal processing in the two olfactory systems and the species differences, immunocytochemical staining and analysis were done of the neuronal expression patterns of the calcium-binding proteins calbindin D28k (CB), parvalbumin (PV), and calretinin (CR) in the MOB and AOB in a marsupial species, the gray short-tailed opossum, Monodelphis domestica. In the MOB, antibody to CB labeled periglomerular cells, superficial short axon cells / Van Gehuchten cells; antibody to PV labeled Van Gehuchten cells; and antibody to CR immunostained periglomerular cells, superficial short axon cells / Van Gehuchten cells, and granule cells. In the AOB, CB immunoreactivity was detected in periglomerular cells and a subpopulation of granule cells; antibody to PV labeled the superficial short axon cells / Van Gehuchten cells and granule cells; and antibody to CR labeled a small number of periglomerular cells, superficial short axon cells / Van Gehuchten cells, and granule cells. These results showed that the patterns of CB, PV, and CR expression differ in the opossum main and accessory olfactory bulbs and differ from that in other animal species. These varying patterns of neuronal immunostaining may be related to the different functions of the main and accessory olfactory bulbs and to the differing signal processing features.

Tumor necrosis factor-alpha promotes survival of opossum kidney cells via Cdc42-induced phospholipase C-gamma1 activation and actin filament redistribution

Although the renal proximal tubular epithelial cells are targeted in a variety of inflammatory diseases of the kidney, the signaling mechanism by which tumor necrosis factor (TNF)-alpha exerts its effects in these cells remains unclear. Here, we report that TNF-alpha elicits antiapoptotic effects in opossum kidney cells and that this response is mediated via actin redistribution through a novel signaling mechanism. More specifically, we show that TNF-alpha prevents apoptosis by inhibiting the activity of caspase-3 and this effect depends on actin polymerization state and nuclear factor-kappaB activity. We also demonstrate that the signaling cascade triggered by TNF-alpha is governed by the phosphatidylinositol-3 kinase, Cdc42/Rac1, and phospholipase (PLC)-gamma1. In this signaling cascade, Cdc42 was found to be selectively essential for PLC-gamma1 activation, whereas phosphatidylinositol-3,4,5-triphosphate alone is not sufficient to activate the phospholipase. Moreover, PLC-gamma1 was found to associate in vivo with the small GTPase(s). Interestingly, PLC-gamma1 was observed to associate with constitutively active (CA) Cdc42V12, but not with CA Rac1V12, whereas no interaction was detected with Cdc42(T17N). The inactive Cdc42(T17N) and the PLC-gamma1 inhibitor U73122 prevented actin redistribution and depolymerization, confirming that both signaling molecules are responsible for the reorganization of actin. Additionally, the actin filament stabilizer phallacidin potently blocked the nuclear translocation of nuclear factor-kappaB and its binding activity, resulting in abrogation of the TNF-alpha-induced inhibition of caspase-3. To conclude, our findings suggest that actin may play a pivotal role in the response of opossum kidney cells to TNF-alpha and implicate Cdc42 in directly regulating PLC-gamma1 activity.

Free amino acids mimic the anabolic but not the proliferative effect of albumin in OK proximal tubular cells

When plasma proteins leak from circulation into the renal tubular lumen in the proteinuric renal diseases, nephrotoxicity of filtered albumin (and/or molecules bound to it) may be important in the subsequent development of tubulo-interstitial damage which contributes to the progression of the disease. When cultured opossum kidney (OK) proximal tubular cells were exposed to bovine serum albumin for 3 days in vitro, increased cell division ([3H]-thymidine incorporation) and cellular hypertrophy (increased protein/DNA ratio) were observed. Both effects were halved if defatted albumin was used. A trivial explanation for the growth responses is that free fatty acids carried on the albumin, and amino acids generated by intracellular degradation of the albumin, are exerting a non-specific growth effect as metabolic fuels which are oxidized to generate ATP. However, the water-soluble free fatty acid octanoate (1 mmol l(-1)) had no significant effect on protein/DNA ratio and a very variable stimulatory effect on [3H]-thymidine incorporation, whereas an essential amino acid mixture or 1 mmol/l(-1) l-Ala or l-Phe only increased the protein/DNA ratio. Furthermore no carnitine was added to the culture medium. This absence would have impaired mitochondrial transport (and hence oxidation) of long-chain free fatty acids derived from the albumin. l-Phe is also a poor substrate for mitochondrial oxidation in kidney. It is therefore concluded that the growth effects of albumin in OK proximal tubular cells are specific effects of the albumin protein and of the free fatty acids and amino acids derived from it, and not a non-specific effect on metabolic fuel supply.

Chronic cold exposure increases liver oxidative capacity in the marsupial Monodelphis domestica

Marsupials lack brown adipose tissue, and therefore rely exclusively on other tissues for thermogenesis. To determine the magnitude of phenotypic plasticity of the liver in response to changing metabolic demand, gray short-tailed opossums (M. domestica) were exposed to thermoneutral (28 degrees C) or cold (9-12 degrees C) conditions continuously for 6 weeks. Half of each group was also endurance trained with a treadmill program during their respective temperature exposure. Mass specific summit metabolism (VO(2)summit) increased 11% following cold acclimation, though there was no significant main effect by training on VO(2)summit. To estimate the contribution of the liver to whole animal oxidative activity, we determined liver mass, mitochondrial volume density, and total mitochondrial volume. Relative liver mass was 48% greater in cold-acclimated animals, whereas training had no effect on liver mass. The stereological analysis of hepatocyte ultrastructure suggests the percentage of intracellular volumes remained unchanged in response to either aerobic challenge. Thus, following cold-acclimation, there is a 20% increase in the total mitochondrial volume of the liver. This increase could account for nearly half (44%) of the observed increase in whole animal VO(2)summit following cold exposure.

Changes in the epithelium of the vaginal complex during the estrous cycle of the marsupial Monodelphis domestica. 1. Transmission electron microscopy study

The vaginal complex of marsupials differs from that of eutherians. Cervices open separately in a sinus vaginalis or cul-de-sac. Two lateral vaginae adjoin the sinus vaginalis and fuse at the level of the urethra opening and form the sinus urogenitalis. During the estrous cycle the vaginal epithelium undergoes a number of specified morphological changes. This paper is the first to describe these changes on an ultrastructural level in a marsupial. Investigations in Monodelphis vagina reveal that a cyclic switch exists between a keratinized and a stratified nonkeratinized epithelium. Keratinization starts during proestrus and reaches its maximum during estrus. In the postestrus, desquamation of the stratum corneum takes place, mostly in two steps. In metestrus one to two additional layers of the now nonkeratinized surface cells are shed into the vaginal lumen. Typical cell structures, such as keratin filaments, keratohyalin and membrane-coating granules, are involved in the keratinization process. Keratohyalin is found in the cytoplasm as well as in the nucleus of stratum granulosum cells, a phenomenon which is known from other parakeratinized epithelia of rapid turnover. Membrane-coating granules, responsible for the permeability barrier between the epithelial cells, are of the nonlamellated type in the nonkeratinized epithelium and produce an amorphous material in the intercellular spaces after extrusion. At periods, however, when the epithelium is keratinized, membrane-coating granules are of the lamellated type and form a lamellated barrier structure after extrusion in the intercellular space. The loss of the protective keratinized layers asks for an additional defense mechanism for the epithelium. The migration of leukocytes through the epithelium predominantly during post- and metestrus and their presence in the vaginal lumen may play a protective role together with the bacterial content.

Na(+)/H(+) exchanger activity and dopamine D(1)-like receptor function in two opossum kidney cell clonal sublines

BACKGROUND/AIMS

The enhanced renal reabsorption of Na(+) in hypertension is accompanied by a defective transduction of the renal dopamine D(1) receptor signal. The present study evaluated the response of the Na(+)/H(+) exchanger to dopamine D(1)-like receptor stimulation in two clonal subpopulations of opossum kidney (OK) cells (OK(LC) and OK(HC)) that are functionally different with respect to their ability to transport Na(+).

METHODS

Na(+)/H(+) exchanger activity was assayed as the initial rate of intracellular pH (pH(i)) recovery after an acid load. The presence of D(1)-like receptors was measured in saturation experiments with [(3)H]-Sch 23390 in cell membranes.

RESULTS

V(max) values (in pH units/s) for Na(+)-dependent pH(i) recovery in OK(HC) cells (0.00521+/-0.0004) were twice those in OK(LC) (0.00202+/-0.0001), with similar K(m) values. The selective D(1)-like receptor agonist SKF 38393 (30 to 3000 nM) attenuated the Na(+)/H(+) exchanger activity in OK(HC) cells more potently than in OK(LC) cells.GTPgammaS and forskolin were equipotent in inhibiting the Na(+)/H(+) exchanger in OK(HC) cells and OK(LC) cells. The SKF 38393-induced increase in cyclic AMP levels in OK(HC) cells was greater than in OK(LC) cells. B(max) values for the binding of [(3)H]-Sch 23390 in OK(HC) cells were twice that in OK(LC) cells, with similar K(D) values. The abundance of G(Salpha) protein in cell membranes of OK(HC) cells was similar to that in OK(LC) cells.

CONCLUSION

The enhanced sensitivity of the Na(+)/H(+) exchanger to inhibition by the D(1)-like receptor agonist in OK(HC) cells correlated positively with the high density of D(1)-like binding sites and the enhanced production of cyclic AMP during D(1)-like receptor stimulation in OK(HC) cells.

Cloning and expression of a novel cDNA encoding shell coat protein, cp4, from the brushtail possum (Trichosurus vulpecula)

The marsupial conceptus is enclosed by several egg coats of evolutionary significance and unknown composition, of which the shell coat in mammals occurs only in marsupials and monotremes. Intact coats are vital to marsupial embryonic development. Towards a better understanding of the marsupial coat proteins, a cDNA sequence (cp4) encoding a shell coat protein was cloned from the brushtail possum. A cDNA library of a zygote stage uterus was screened using a deduced oligonucleotide sequence based on a partial amino acid sequence of the coat protein. This study has confirmed a single copy cp4 gene encoding a unique protein of 306 amino acids, although its N-terminus shares high sequence identity with the C-terminal half of the enzyme alpha-enolase. Using Northern blots, the expression of cp4 in adult tissues showed that cp4 transcript is restricted predominantly to the uterus with two stages of expression occurring in the gravid uterus at early cleavage and bilaminar stages, suggesting an important developmental role for CP4. Using RT-PCR, cp4-specific expression as represented by the 3'-end 400 bp was present in heart, liver, oviduct, and uterus. Uterine expression reflected the increase found with Northern blot except that expression was low at unilaminar and bilaminar stages.

Whole-body mechanics and gaits in the gray short-tailed opossum Monodelphis domestica: integrating patterns of locomotion in a semi-erect mammal

Gaits (footfall patterns) and external mechanical energy patterns of the center of mass were quantified in a generalized, semi-erect mammal in order to address three general questions. First, do semi-erect mammals exhibit the walk/run gait transitions that have been proposed as the primitive condition for tetrapods? Second, do small, semi-erect mammals employ the energy-saving pendular and spring-based mechanics used by erect mammals? Third, how well do mechanical locomotor patterns of the center of mass correlate with gaits? Monodelphis domestica utilizes only fast walking and running trot gaits over a fivefold increase in speed, over which we could illicit constant velocity steps, although running trots were their preferred gait. In sustained level locomotion the opossums did not use other walking gaits presumed to be primitive for tetrapods. Across the full range of speeds their trotting gaits exhibited force patterns and in-phase mechanical energy fluctuations that are characteristic of spring-mass mechanics. Thus, opossums appear to prefer trotting gaits with bouncing mechanics for sustained locomotion. Integration of center-of-mass versus footfall perspectives reveals that spring-mass mechanics is associated with both walking trot and running trot gaits. Furthermore, the onset of an aerial phase was not clearly associated with either the walk/run gait transition (50% duty factor) or a change in center-of-mass mechanics. The assumption that energy-saving mechanisms are ubiquitous among mammals is tenuous because small non-cursorial mammals do not appear to use pendular-based mechanics for sustained locomotion and, although they prefer spring-based mechanics, they probably lack clear musculoskeletal spring elements that could store energy during running. Thus, it appears that simply paying for locomotion with muscular work may be the primitive condition for mammals.

Development of different mast cell types in the opossum Didelphis albiventris

Previous studies have disclosed three types of mast cell in opossums: connective tissue (CTMC), mucosal (MMC), and lymphatic sinus (LSMC). In contrast to most opossum lymph nodes, the mesenteric lymph node is virtually devoid of LSMC, displaying medullary cord CTMC. The present study aimed to describe the development of these mast cell populations. Toluidine blue staining and a histochemical method for demonstrating heparin allowed the identification of immature and mature mast cells. Immature CTMC devoid of detectable heparin were rare until postnatal day 10. Mature CTMC filled with heparin-containing granules became numerous by day 30 to day 40. In the ileum, despite the presence of mature CTMC in the submucosa and mucosa (villus base), immature mast cells first appeared in the villus core by day 65 and adult features were apparent by day 100. In LSMC-containing lymph nodes, immature mast cells were found in lymphatic sinuses by day 10. Clear signs of LSMC differentiation were observed from day 20. Compared with the 10-day value, the mean diameter of cytoplasmic granules at day 40 had doubled and that at day 110 had tripled. In the mesenteric lymph nodes, immature mast cells differentiated into lymphatic sinus CTMC-like cells. After day 80, most of them were located in medullary cords. Weaning and complete maturation of mucosa preceded the differentiation of MMC. In lymph nodes, LSMC differentiation occurred in parallel with the development of the medullary region and deep cortex units.

Expression in Escherichia coli and immunological characterization of three zona pellucida proteins (ZP1, ZP2, and ZP3) from a marsupial, the brushtail possum (Trichosurus vulpecula)

The brushtail possum (Trichosurus vulpecula) zona pellucida (ZP) is composed of three major glycoproteins, designated ZP1, ZP2, and ZP3 based on their size and homology with eutherian ZP proteins. These proteins are candidate antigens for the development of an immunocontraceptive vaccine to control the fertility of the brushtail possum in New Zealand, where it is an introduced pest. In order to further their immunological and functional characterization, recombinant possum ZP proteins were produced in Escherichia coli (E. coli) strain JM109, M15, SG13009, or BL21 codon plus. Each of the proteins produced possessed a N-terminal six histidine tag (His)(6) to facilitate purification and consisted of amino acid (aa) residues 18-471 of possum ZP1, aa residues 40-311 of ZP2 (ZP2-N), aa residues 305-634 of ZP2 (ZP2-C), and aa residues 23-342 of ZP3. Immunoblot using anti-RGS(His)(4) antibodies and polyclonal rabbit anti-porcine ZP antibodies detected major bands at 54 kDa for ZP1, 32 kDa for ZP2-N, 39 kDa for ZP2-C, and 40 kDa for ZP3. Immunization of male and female rabbits with ZP2-N, ZP2-C, and ZP3 purified on Ni-NTA resin under denaturing conditions generated antibodies reactive with recombinant ZP proteins on Western blot and with native ZP proteins in possum ovarian sections using immunofluorescence. Antibodies generated against ZP1 in the same way were reactive with recombinant ZP proteins on Western blot only. The recombinant possum ZP proteins and specific antibodies produced in this study give an indication of the antigenic relationship of the possum ZP proteins and are vital tools for future studies of sperm-ZP binding in marsupials and for the evaluation of ZP-based contraceptive vaccines in possums and other marsupials.

Species differences in the ovarian distribution of 3beta-hydroxysteroid dehydrogenase/delta5-->4 isomerase (3beta-HSD) in two marsupials: the brushtail possum Trichosurus vulpecula and the grey, short-tailed opossum Monodelphis domestica

The ovarian distribution of the steroidogenic enzyme 3beta-hydroxysteroid dehydrogenase/delta(5-->4) isomerase (3beta-HSD) was investigated by immunocytochemistry in two marsupial species throughout the reproductive cycle, using a rabbit polyclonal antibody raised against human placental 3beta-HSD. In the polyoestrous and polyovular South American opossum Monodelphis domestica, immunostaining was positive for 3beta-HSD in the adrenal cortex, the ovarian interstitial tissue, the corpus luteum and the granulosa cells of antral and atretic follicles. The theca interna was weakly positive for 3beta-HSD, but only in late preantral to early antral stages of follicular development. The adrenal medulla and smaller preantral follicles were completely negative for 3beta-HSD. In contrast, in the polyoestrous and monovular Australian brushtail possum Trichosurus vulpecula, immunostaining showed a strong positive reaction for 3beta-HSD in the theca, whereas the granulosa layer remained predominantly negative for 3beta-HSD except in the largest follicles. The atretic follicles were completely negative for 3beta-HSD. The ovaries of pregnant animals contained grossly enlarged, persistent, antral follicles, which reacted positively for 3beta-HSD. The function of these follicles in T. vulpecula and the 3beta-HSD-positive atretic follicles in M. domestica has not been determined. The differences between the two marsupials represent species variations. The situation in M. domestica does not represent a marsupial-eutherian dichotomy as previously conjectured.

The effect of excesses and deficiencies in amino acids on the feeding behaviour of the common brushtail possum (Trichosurus vulpecula)

In this study of the amino acid nutrition of a marsupial we tested three hypotheses: (a) that brushtail possums eat less when diets contain excesses or deficiencies in essential amino acids, (b) that brushtail possums choose diets that do not contain amino acid excesses, and (c) that amino acid consumption is mediated partly by the 5HT3 receptor. Possums ate less when 0.2-1.0% methionine (wet matter) was added to the diet, but similar concentrations of lysine and threonine had little effect. However, when given a choice, possums always selected the basal ration over one with added lysine, methionine or threonine at concentrations between 0.05% and 0.9%. In contrast to the experiments with excess amino acids, possums did not eat less of a diet almost devoid of an essential amino acid. Instead, the possums ate less when their diets contained synthetic amino acids rather than similar amounts and proportions of amino acids as casein. Contrary to the third hypothesis, the 5HT3 receptor antagonist, ondansetron, did not affect feeding by possums given a diet containing 0.8% methionine, suggesting that post-ingestive feedback, via the 5HT3 receptor, does not regulate amino acid intake when diets contain amino acid excesses.

Protein and peptide degradation in the intestine of the common brushtail possum (Trichosurus vulpecula)

A protein (bovine serum albumin: BSA) and a peptide (luteinizing hormone releasing hormone: LHRH) were used to evaluate proteolytic activity in the intestine of common brushtail possums (Marsupiala, Trichosurus vulpecula). Luminal and mucosal extracts were isolated from the duodenum, jejunum, ileum, caecum, proximal colon and distal colon, their protein content assessed and specific activities in metabolising LHRH and BSA determined in vitro. The degradation of LHRH by luminal extracts was compared with that by the pancreatic enzymes, chymotrypsin, trypsin, and elastase. The protein concentration (microg x mg-1) of mucosal extract in the duodenum was higher ( P<0.05) than in the proximal colon, but that of luminal extracts did not differ significantly between regions. Proteolytic activity of luminal extracts was greater ( P<0.01) in the jejunum and ileum than in the hindgut. In the small intestine, proteolytic activity of luminal enzymes far exceeded that of mucosal enzymes ( P<0.05). All three pancreatic enzymes hydrolysed LHRH, but chymotrypsin had the greatest activity. This study has demonstrated that, in possums, proteolysis occurs primarily in the small intestine through luminal enzymes, with chymotrypsin playing a major role. The possum hindgut contributes little to the metabolism of peptides and proteins, identifying it as a potential site to target for their absorption following oral delivery.

Interplay among energy metabolism, organ mass and digestive enzyme activity in the mouse-opossum Thylamys elegans: the role of thermal acclimation

The potential for thermal acclimation in marsupials is controversial. Initial studies suggest that the thermoregulatory maximum metabolic rate (MMR)in metatherians cannot be changed by thermal acclimation. Nevertheless, recent studies reported conspicuous seasonality in both MMR and in basal metabolic rate (BMR). We studied the role of thermal acclimation in the Chilean mouse-opossum, Thylamys elegans, by measuring MMR and BMR before and after acclimation to cold or warm conditions. Following acclimation we also measured the mass of metabolically active organs, and the activity of a key digestive enzyme, aminopeptidase-N. No significant effect of thermal acclimation (i.e. between cold- and warm-acclimated animals) was observed for body mass, MMR, body temperature or factorial aerobic scope. However, the BMR of cold-acclimated animals was 30 % higher than for warm-acclimated individuals. For organ mass, acclimation had a significant effect on the dry mass of caecum, liver and kidneys only. Stepwise multiple regression using pooled data showed that 71 % of the variation in BMR is explained by the digestive organs. Overall, these results suggest that MMR is a rather rigid variable, while BMR shows plasticity. It seems that T. elegans cannot respond to thermal acclimation by adjusting its processes of energy expenditure (i.e. thermogenic capacity and mass of metabolically active organs). The lack of any significant difference in aminopeptidase-N specific activity between warm- and cold-acclimated animals suggests that this response is mainly quantitative (i.e. cell proliferation) rather than qualitative (i.e. differential enzyme expression). Finally, as far as we know, this study is the first to report the effects of thermal acclimation on energy metabolism, organ mass and digestive enzyme activity in a marsupial.

Electrogenic ion transport in the intestine of the Australian common brushtail possum, Trichosurus vulpecula: indications of novel transport patterns in a marsupial

In this study, electrogenic ion transport in the intestine of the Australian common brushtail possum, Trichosurus vulpecula was investigated. In the ileum, a Na(+)-dependent, phloridzin- and amiloride-insensitive short-circuit current ( Isc) was present. Mucosal glucose stimulated a further phloridzin-sensitive, dose-dependent increase in Isc. A Na(+)-dependent, ouabain-sensitive Isc was also present in the caecum and colon. In the proximal and distal colon, amiloride (100 micro mol l(-1), mucosal) inhibited this Isc by 81+/-4% and 65+/-3%, respectively and the Ki for amiloride (approximately 1 micro mol l(-1)) was consistent with the inhibition of a classical epithelial Na(+) channel. In the caecum, 50% of the Isc was inhibited by amiloride (100 micro mol l(-1), mucosal). The amiloride-insensitive Isc in the colon was not due to electrogenic Cl(-) secretion, as serosal bumetanide (100 micro mol l(-1)) had no effect on the Isc. Furthermore, the secretagogues forskolin (10 micro mol l(-1)), carbachol (100 micro mol l(-1)) and dibutyryl-cAMP or dibutyryl-cGMP (100 micro mol l(-1)) did not stimulate electrogenic Cl(-) secretion by the colon. These results indicate that the transport properties of the hindgut of the possum differ significantly from those of eutherian mammals and may be associated with different functions of the hindgut of possums when compared to eutherian mammals.

Aldosterone does not regulate amiloride-sensitive Na(+) transport in the colon of the Australian common brushtail possum, Trichosurus vulpecula

Plasma aldosterone concentrations and the magnitude of amiloride-sensitive Na(+) transport in the proximal and distal colon of newly caught wild possums, and of possums maintained on either low or high Na(+) diets, were determined to evaluate their role in the regulation of salt content of faeces in common brushtail possums. Wild animals had a plasma aldosterone concentration of 439+/-115 pmol l(-1) and high levels of amiloride-sensitive Na(+) transport in both the proximal and distal colon. Animals maintained on a low Na(+) diet for 14 days excreted very little Na(+) in the urine and had an elevated aldosterone concentration (840+/-246 pmol l(-1)), but levels of amiloride-sensitive Na(+) transport in the proximal and distal colon were similar to those in wild animals. Conversely, while provision of a high Na(+) diet (7 days or 14 days) resulted in increased urinary Na(+) excretion and a fall in aldosterone (approximately 250 pmol l(-1)), there was no reduction in the amount of amiloride-sensitive Na(+) transport in either the proximal or distal colon. These data strongly indicate that aldosterone does not regulate amiloride-sensitive Na(+) transport in the colon of possums and that the colon is not involved in the regulation of salt content of the faeces in this species.

Retinofugal projections in the short-tailed opossum (Monodelphis domestica)

In the current investigation, retinofugal projections to midbrain and thalamic nuclei of Monodelphis domestica were investigated using wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Large intraocular injections of WGA-HRP were placed into the eye, and patterns of labeled axon terminals were related to nuclear boundaries in tissue that was stained for Nissl or reacted for cytochrome oxidase (CO). Our results demonstrate that the major projection from the retina is to the contralateral dorsal lateral geniculate nucleus (LGNd) and the superior colliculus (SC). Connections were also observed with the contralateral pretectal nucleus (PRT), the lateral posterior nucleus (LP), and the ventral division of the lateral geniculate nucleus (LGNv). Ipsilateral connections were with the LGNv and LGNd. These findings are consistent with reports in other marsupials as well as with studies in a number of eutherian mammals. Thus, there appears to be a common pattern of retinofugal projections that all mammals share, probably due to retention from a common ancestor. However, some features such as a lack of ipsilateral input to the SC (which are absent only in certain species like Monodelphis, platypus, and echidnas) may represent a primitive state retained from a common ancestor. When comparisons of retinofugal connections and LGNd organization are made across taxa, three types of organization are observed: a homogenous LGNd with a high degree of binocular overlap of projections; a partially differentiated LGNd with some segregation of eye-specific inputs; and a fully segregated structure with a large degree of segregation of eye-specific inputs. We discuss the factors that contribute to the organization observed in extant mammals and conclude that phylogeny and lifestyle appear to be the underlying factors contributing to the organization of the LGNd.

Microsomal metabolism and enzyme kinetics of the terpene p-cymene in the common brushtail possum (Trichosurus vulpecula), koala (Phascolarctos cinereus) and rat

1. p-Cymene is an aromatic monoterpene found in the leaves of Eucalyptus spp. and is ingested in the diet of two marsupial folivores, the brushtail possum (Trichosurus vulpecula) and koala (Phascolarctos cinereus). The metabolism of p-cymene by liver microsomes from the possum, koala and rat (an animal whose diet is not terpene-rich) was examined. 2. The major p-cymene metabolite in each species was cuminyl alcohol, with smaller amounts of other side-chain alcohols being formed. No phenolic metabolites were detected. Possum and koala microsomes further oxidized cuminyl alcohol to cumic acid. 3. Pretreatment with a terpene diet more than doubled the V(max) for cuminyl alcohol formation by possum microsomes. This is similar to a previous finding by our group with 1,8-cineole and indicates that a terpene-containing diet results in induction of the enzymes responsible for terpene metabolism. 4. The rank order of the ability to metabolize p-cymene, measured by intrinsic clearance (Cl(int) = V(max)/K(m) [microl mg protein(-1) min(-1)]) was: terpene-treated possum (128)>control possum (107)>koala (69)>rat (38). 5. The findings support the hypothesis that animals that have adapted to a diet of Eucalypus leaf have a greater capacity than generalist feeders to metabolize dietary terpenes.

Ovarian expression of messenger RNA encoding the receptors for luteinizing hormone and follicle-stimulating hormone in a marsupial, the brushtail possum (Trichosurus vulpecula)

Both LH and FSH play a central role in controlling ovarian function in mammals. However, little is known about the type of ovarian cells that are responsive to LH and FSH in marsupials. We determined, using in situ hybridization, the localization of mRNA encoding the receptors (R) for LH and FSH in ovaries of brushtail possums. The mRNA encoding FSH-R was observed in granulosa cells of healthy follicles containing at least two complete layers of cells. The mRNA encoding LH-R was first observed in granulosa cells at the time of antrum formation. Cells of the theca interna expressed LH-R mRNA but not FSH-R mRNA. Neither FSH-R nor LH-R mRNA was detected in atretic follicles. Both FSH-R and LH-R mRNAs were observed in luteal tissue, but only LH-R mRNA was observed in interstitial cells. Granulosa cells from follicles of various sizes (0.5 to >2 mm in diameter) responded to LH and FSH treatment with an increase in cAMP synthesis. In contrast, luteal tissue did not respond to either FSH or LH treatment. In conclusion, expression of FSH-R in the brushtail possum ovary was similar to that observed in many eutherian mammals. However, active LH-R was expressed in granulosa cells much earlier in follicular development than has been previously observed. In addition, although mRNAs for both FSH-R and LH-R were observed, neither FSH nor LH treatment stimulated cAMP synthesis in luteal tissue.

Role of internalization in AT(1A) receptor function in proximal tubule epithelium

Angiotensin II (ANG II), acting through angiotensin type I (AT(1)) receptors on apical and basolateral surfaces of proximal tubule epithelial cells, increases sodium reabsorption in proximal tubules. Apical and basolateral receptors internalize after exposure to ANG II, but the role of internalization in receptor signaling and transport is not well defined. To determine the role of receptor internalization in ANG II-mediated receptor signaling and sodium transport, we stably expressed full-length and truncated AT(1A) receptors in opossum kidney cells. After stimulation with ANG II, wild-type receptors on apical and basolateral surfaces rapidly internalized, inhibited adenylate cyclase, and increased transcellular sodium transport. Truncation of the cytoplasmic tail of the AT(1A) receptor (TL314) resulted in receptors that were expressed on apical and basolateral surfaces but did not internalize, inhibit adenylate cyclase, or increase sodium transport. Because the cytoplasmic tail contains putative G protein coupling sites, mutant receptors that leave G protein interaction sites intact were designed. Cells expressing the truncation (TK333) or deletion (Del 315-329) also failed to internalize. When ANG II was added to basolateral surfaces of TK333 or Del 315-329, adenylate cyclase activity was inhibited and sodium transport was increased. In contrast, apical addition of ANG II was not associated with decreases in adenylate cyclase or increases in sodium transport. In conclusion, internalization pathways are important for AT(1A) receptor function in polarized proximal tubule epithelial cells. Apical AT(1A) receptors internalize before they interact with G proteins and signal cAMP. In contrast, basolateral AT(1A) receptors interact with G proteins and signal cAMP without internalizing.

Characterization of circular muscle motor neurons of the duodenum and distal colon in the Australian brush-tailed possum

The motor innervation of the duodenum and distal colon remains uncharacterized within the same species. Our aim was to compare the projections and neurochemical properties of duodenal and distal colon circular muscle motor neurons. Circular muscle motor neurons were retrogradely traced by using a neural tracer in vitro, processed for choline acetyltransferase (ChAT) and nitric oxide synthase (NOS) immunoreactivity and then visualized by using indirect immunofluorescence. A mean of 372 +/- 64 and 156 +/- 23 neurons (mean +/- SEM) were tracer-labeled within the duodenum and colon, respectively. The ChAT+/NOS- neurons comprised 57.6 +/- 6.6% and 39.6 +/- 4.4% of all labeled cells in the duodenum and colon, respectively, and projected mainly in the oral direction. Of all labeled cells, the ChAT-/NOS+ neurons comprised 8.5 +/- 2.3% in the duodenum and 46.6 +/- 5.0% in the distal colon and projected mainly in the anal direction. Of the remainder, 20.6 +/- 5.0% and 8.2 +/- 2.4% were ChAT+/NOS+ and 13.2 +/- 0.9% and 5.6 +/- 1.4% were ChAT-/NOS- in the duodenum and distal colon, respectively. Within both regions, the distribution of the ChAT+/NOS- and ChAT-/NOS+ neurons are consistent with the ascending excitatory and descending inhibitory reflexes. The proportion of ChAT-/NOS+ neurons is greater within the colon in comparison with the duodenum. A considerable proportion of duodenal motor neurons were ChAT+/NOS+ and ChAT-/NOS-. These two classes may underlie nonperistaltic motor patterns, which predominate within the duodenum. These findings demonstrate regional differences in the innervation of intestinal circular muscle.

Cloning of a marsupial kappa-casein cDNA from the brushtail possum (Trichosurus vulpecula)

The main role of kappa-casein in milk is to stabilize the formation of casein micelles. Although marsupial milk contains casein micelles, kappa-casein had not been identified in any species. In these experiments, the first marsupial kappa-casein has been prepared as enriched casein fractions and the cDNA cloned from the brushtail possum (Trichosurus vulpecula). Possum kappa-casein is a 158 amino acid peptide that shares low amino acid sequence identity (20-30%) with that of eutherian kappa-caseins. In the gut of suckling young, casein micelles clot when kappa-casein is cleaved by chymosin at a specific site. Eutherian kappa-casein sequences are classified according to the sequence of the chymosin cleavage site: Phe-Met, Phe-Ile or Phe-Leu. Possum kappa-casein appears to form a separate class, with a putative chymosin cleavage site of Phe-Ala, which is different from that found in eutherian mammals. Other features of kappa-caseins, such as the location of the N-terminal cysteine, solubility in the presence of calcium, and the O-glycosylation sites on threonine residues in the C-terminus of the molecule, are conserved in the possum sequence. The kappa-casein gene was expressed throughout lactation in the mammary gland, and although mRNA levels of kappa-, alpha- and beta-casein varied between animals there appeared to be a correlation in the expression of these genes within an individual animal. This suggests that a common transcription regulatory region may be controlling expression of all three genes in the possum.

Extracts from tumors causing oncogenic osteomalacia inhibit phosphate uptake in opossum kidney cells

In oncogenic osteomalacia (OOM), a tumor produces an unknown substance that inhibits phosphate reabsorption in the proximal tubules. This causes urinary phosphate wasting and, as a consequence, hypophosphatemic osteomalacia. To characterize this poorly understood biological tumor activity we generated aqueous extracts from several OOM tumors. Extracts from three of four tumors inhibited, dose- and time-dependently, (32)P-orthophosphate uptake by opossum kidney (OK) cells; maximum inhibition was about 45% of untreated control. Further characterization revealed that the factor is resistant to heat and several proteases, and that it has a low molecular weight. The tumor extracts also stimulated cAMP accumulation in OK cells, but not in osteoblastic ROS 17/2.8 and UMR106 cells, or in LLC-PK1 kidney cells expressing the parathyroid hormone (PTH)/PTH-related peptide receptor or the PTH-2 receptor. HPLC separation of low molecular weight fractions of the tumor extracts revealed that the flow-through of all three positive tumor extracts inhibited (32)P uptake and stimulated cAMP accumulation in OK cells. Additionally, a second peak with inhibitory activity on phosphate transport, but without cAMP stimulatory activity, was identified in the most potent tumor extract. We have concluded that several low molecular weight molecules with the ability to inhibit phosphate transport in OK cells can be found in extracts from OOM tumors. It remains uncertain, however, whether these are related to the long-sought phosphaturic factor responsible for the phosphate wasting seen in OOM patients.

SFKs, Ras, and the classic MAPK pathway couple muscarinic receptor activation to increased Na-HCO(3) cotransport activity in renal epithelial cells

Cholinergic agents are known to affect the epithelial transport of H2O and electrolytes in the kidney. In proximal tubule cells, cholinergic agonists increase basolateral Na-HCO(3) cotransport activity via M(1) muscarinic receptor activation. The signaling intermediates that couple these G protein-coupled receptors to cotransporter activation, however, are not well defined. We therefore sought to identify distal effectors of muscarinic receptor activation that contribute to increased NBC activity in cultured proximal tubule cells. As demonstrated previously for acute CO2-regulated cotransport activity, we found that inhibitors of Src family kinases (SFKs) or the classic mitogen-activated protein kinase (MAPK) pathway prevented the stimulation of NBC activity by carbachol. The ability of carbachol to activate Src, as well as the proximal (Raf) and distal [extracellular signal-regulated kinases 1 and 2 (ERK1/2)] elements of the classic MAPK module, was compatible with these findings. Cholinergic stimulation of ERK1/2 activity was also completely prevented by overexpression of a dominant negative mutant of Ras (N17-Ras). Taken together, these findings suggest a requirement for the sequential activation of SFKs, Ras, and the classic MAPK pathway [Raf-->MAPK/ERK kinase (MEK)-->ERK]. These findings provide important insights into the molecular mechanisms underlying cholinergic regulation of NBC activity in renal epithelial cells. They also suggest a specific mechanism whereby cholinergic stimulation of the kidney can contribute to pH homeostasis.

Metabolic changes in the nucleus of the optic tract after monocular enucleation as revealed by cytochrome oxidase histochemistry

The histochemistry for the mitochondrial enzyme cytochrome oxidase (CO) was used to evaluate the levels of metabolic activity in neurons of the nucleus of the optic tract (NOT) and dorsal terminal nucleus (DTN) in the opossum (Didelphis aurita). The observations were performed in four groups: normal juveniles (4 months old), monocularly enucleated juveniles analysed when adults, normal adults (8 to 18 months old) and monocularly enucleated adults. CO labeled cells were observed to have a similar distribution along the NOT-DTN anteroposterior axis in both juvenile and adult normal animals. Monocular enucleation performed in adults produced a significant reduction of the reactive neuropil but not of the number of CO labeled cells in the deafferented NOT-DTN: the number of labeled neurons per section in the deafferented side matched those of the ipsilateral complex. In juveniles, however, this procedure caused a systematic reduction of the number of CO labeled cells in the contralateral NOT-DTN in comparison to the spared complex. The lack of reduction in the number of neurons found on the deafferented side of the NOT-DTN of monocularly enucleated adult opossums compared with the ipsilateral side might result from the presence of compensatory inputs to maintain their metabolic equivalence. However, when the monocular enucleation was performed in juvenile opossums, a statistically significant asymmetry of CO neurons in the NOT-DTN was observed. In other words, the compensatory mechanisms proposed for the adults were either absent or insufficient to achieve symmetry in juveniles, suggesting a more heavily reliance in the retinal input.

Differential localization of SNARE complex proteins SNAP-25, syntaxin, and VAMP during development of the mammalian retina

SNARE complex proteins have critical functions during regulated vesicular release of neurotransmitter. In addition, they play critical roles during neurite outgrowth and synaptogenesis. Although it is clear that the function of any one SNARE complex protein during release of neurotransmitter is dependent on its association with other members of the complex, it is less certain whether their function during development and differentiation is dependent on interaction with one another. Previously, we have observed transient high levels of SNARE complex protein SNAP-25 in developing cholinergic amacrine cells (West Greenlee et al. [1998] J Comp Neurol 394:374-385). In addition, we detected, high levels of SNAP-25 in developing and mature photoreceptors. To better understand the functional significance of these high levels of SNAP-25 expression, we used immunocytochemistry to examine the developmental expression of the three members of the SNARE complex, SNAP-25, Syntaxin, and vesicle associated membrane protein (VAMP/also Synaptobrevin). Our results demonstrate that the high levels of SNAP-25 in cholinergic amacrine cells and photoreceptors are not accompanied by the same relatively high levels of other SNARE complex proteins. These results suggest that high levels of SNAP-25 in specific cell types may function independently of association with Syntaxin and VAMP. In this analysis, we characterized the changing patterns of immunoreactivity for the three SNARE complex proteins during the development and differentiation of the mammalian retina. We have compared the pattern of expression of the core SNARE complex proteins in the Brazilian opossum, Monodelphis domestica, and in the rat and found common patterns of expression between these diverse mammalian species. We observed temporal differences in the onset of immunoreactivity between these three proteins, and differences in their localization within synaptic layers in the developing and mature mammalian retina. This study is the first to characterize the changing expression patterns of the three SNARE complex proteins in the developing central nervous system. The differential distribution of SNAP-25, Syntaxin, and VAMP may indicate additional roles for these proteins during vesicle trafficking events, which are independent of their association with one another.

Maintenance nitrogen requirement of an obligate nectarivore, the honey possum, Tarsipes rostratus

A nitrogen balance feeding trial was carried out with the marsupial honey possum, Tarsipes rostratus, using four pollen-honey diets varying in nitrogen content from 9.4 mg.g(-1) to 2.3 mg.g(-1) dry matter. The dietary maintenance nitrogen requirement (MNR) was determined by regression analysis as 89 +/- 21 mg N.kg(-0.75).day(-1) and the truly digestible MNR was 79 mg N.kg(-0.75).day(-1). Regressing nitrogen balance on daily nitrogen intake separately for ten males and seven females revealed that the slopes of the fitted lines did not vary significantly, but the difference in the intercepts approached significance. This suggests that the MNR for females may be lower than that of males. The nitrogen digestibility of the diet was 76% and the biological value (BV) was 58%. A comparison of the MNR of the honey possum with that of other marsupials shows that it is indeed much lower than that of herbivorous macropodid marsupials and is close to that of the sap- and gum-feeding sugar glider, Petarurus breviceps. The endogenous urinary nitrogen excretion (EUN) of the honey possum was 42 mg N.kg(-0.75.day(-1) and a regression analysis with other published data showed that the EUN per unit basal heat production is significantly lower than that of eutherian mammals. Measurements of the actual feeding rates of animals in the field, taken together with the low MNR, do not lend support to the hypothesis that the honey possum exhibits a reduced rate of reproduction due to a deficiency in dietary nitrogen. It is possible that the quality of nitrogen provided by pollen, as reflected in its composition of essential amino acids, may be a limiting factor.

Insulin-like growth factor 2 cDNA cloning and ontogeny of gene expression in the liver of the marsupial brushtail possum (Trichosurus vulpecula)

The cDNA sequence for insulin-like growth factor 2 (IGF-2) was determined from the liver of the marsupial brushtail possum (Trichosurus vulpecula) using reverse transcription followed by polymerase chain reaction (RT-PCR) with gene-specific primers. The 359 bp of possum sequence encompassed the mature peptide, 27 bp of the signal peptide, and 125 bp of the E-peptide. Alignment of the deduced amino acid sequence with those from other species indicated that the mature peptide was 71 amino acids in length, 4 amino acids longer than most other mammals. At both the nucleotide and amino acid levels there was a high degree of sequence identity with IGF-2 from other mammalian and nonmammalian species. Amino acid identity ranged from 94.4% with a variant form of human IGF-2 to 80.3% with zebrafinch IGF-2. Northern analysis revealed that radiolabeled possum IGF-2 cDNA hybridized to multiple transcripts in the liver of both adult possums and 150-day-old pouch young and that the overall level of expression was greater in pouch young. Semiquantitative RT-PCR with total RNA from liver samples of pouch young aged 12 to 150 days postpartum and adults confirmed that IGF-2 gene expression was two to three times more abundant in pouch young than in adults but there was no significant change in the level of expression during pouch life. Unlike other mammalian species, in which there is a decline in levels of liver IGF-2 gene expression around the time of birth, levels in the marsupial brushtail possum remain elevated for at least 150 days after birth. This suggests that the decline in liver IGF-2 expression in marsupials and eutherians occurs at a similar stage of development and may reflect a role for this growth factor during the postnatal growth and development of the marsupial.

Characterisation of tolbutamide hydroxylase activity in the common brushtail possum, (Trichosurus vulpecula) and koala (Phascolarctos cinereus): inhibition by the eucalyptus terpene 1,8-cineole

Plant constituents such as terpenes are major constituents of the essential oil in Eucalyptus sp. 1,8-Cineole and p-cymene (Terpenes present in high amounts in Eucalyptus leaves) are potential substrates for the CYP family of enzymes. We have investigated tolbutamide hydroxylase as a probe substrate reaction in both koala and terpene pretreated and control brushtail possum liver microsomes and examined inhibition of this reaction by Eucalyptus terpenes. The specific activity determined for tolbutamide hydroxylase in the terpene treated brushtails was significantly higher than that for the control animals (1865+/-334 nmol/mg microsomal protein per min versus 895+/-27 nmol/mg microsomal protein per min). The activity determined in koala microsomes was 8159+/-370 nmol/mg microsomal protein per min. Vmax values and Km values for the terpene treated possum, control, possum and koala were 1932-2225 nmol/mg microsomal protein per min and 0.80 0.81 mM; 1406-1484 nmol/mg microsomal protein per min and 0.87-0.92 mM and 5895-6403 nmol/mg microsomal protein per min and 0.067-0.071 mM, respectively. Terpenes were examined as potential inhibitors of tolbutamide hydroxylase activity. 1,8-Cineole was found to be a competitive inhibitor for the enzyme responsible for tolbutamide hydroxylation (Ki 15 microM) in the possum. In koala liver microsomes stimulation of tolbutamide hydroxylase activity was observed when concentrations of cineole were increased. Therefore, although inhibition was observed, the type of inhibition could not be determined.