Leptin effects on testis and epididymis in the lizard Podarcis sicula, during summer regression

Seasonal change of circulating leptin associated with testicular activities of the wild ground squirrels (Citellus dauricus)

The purpose of this study was to explore the variations in the circulating leptin concentrations of the wild ground squirrels in relation to seasonal changes in testicular activities. Hematoxylin-eosin staining showed all types of elongated spermatids and spermatogenic cells existed in the testis in April, while the primary spermatocytes and spermatogonia were most advanced stages of germ cells in June. In addition, the primary spermatocytes, secondary spermatocytes, and spermatogonia were most advanced stages of germ cells in September. The highest circulating leptin concentration was consistent with the maximum body weight results from accumulation of adipose tissue in September. The mRNA expression level of leptin receptor (Ob-R) and STAT3 was lowest in June, raised in September, and remained increased in April. Ob-R and STAT3 were stronger staining in the Leydig cells in July. Moreover, the concentrations of testosterone (T) showed the maximum values in April, the minimum values in June, and significant increases in September. Furthermore, it is worth noting that the levels of T increased with the mRNA levels of Ob-R, STAT3, StAR, and testicular steroidogenic enzymes (3β-HSD, P450c17, and P450scc). Moreover, RNA-seq analyses of testis during the different periods showed that a total of 4209 genes were differentially expressed genes (DEGs); further analysis revealed that DEGs related with the Jak/STAT pathways and reproduction were altered. Taken together, the results suggested that the leptin regulated testicular function through the Jak/STAT pathways and testicular steroidogenic factor expressions.

Trajectory of leptin and leptin receptor in vertebrates: Structure, function and their regulation

The present review provides a comparative insight into structure, function and control of leptin system in fishes, herptiles, birds and mammals. In general, leptin acts as an anorexigenic hormone since its administration results in decrease of food intake in vertebrates. Nonetheless, functional paradox arises in fishes from contradictory observations on level of leptin during fasting and re-feeding. In addition, leptin is shown to modulate metabolic functions in fishes, reptiles, birds and mammals. Leptin also regulates reproductive and immune functions though more studies are warranted in non-mammalian vertebrates. The expression of leptin and its receptor is influenced by numerous factors including sex steroids, stress and stress-induced catecholamines and glucocorticoids though their effect in non-mammalian vertebrates is hard to be generalized due to limited studies.

Exogenous leptin promotes reproductive behavior during aphagia in red-sided garter snakes (Thamnophis sirtalis parietalis)

Despite the established dichotomy between investment in either reproduction or self-maintenance, a hormonal mechanism that influences an organism's decision to prioritize these behaviors remains elusive. The protein hormone leptin is a likely candidate because it is secreted from adipocytes in proportion to the amount of stored fat in numerous species. Although the majority of studies suggest that leptin stimulates reproduction, the actions of leptin can be context-dependent. Leptin increases sexual behavior in fed individuals, but inhibits sexual behavior in food-restricted individuals. We investigated if exogenous leptin influences sexual behavior in red-sided garter snakes (Thamnophis sirtalis parietalis) experiencing a predictable bout of aphagia during the mating season. We tested two doses of recombinant murine leptin injected for three days. Males were subjected to three mating trials, one on each day of injections, while females were subjected to one mating trial on the last day of injections. Leptin affects male and female snakes similarly by increasing both appetitive (i.e., mating behavior score) and consummatory (i.e., number of copulations, proportion of individuals copulated) sex behavior. We found no evidence to suggest that leptin influenced latency to copulate or duration of copulation. Because leptin promotes reproductive behavior in non-feeding garter snakes, these findings do not align with research on food-restricted mammals. Further investigations into how leptin affects sexual behavior in snakes exposed to food-restriction manipulations would clarify if the role of leptin is evolutionarily divergent.

Avian Leptin: Bird's-Eye View of the Evolution of Vertebrate Energy-Balance Control

Discovery of the satiety hormone leptin in 1994 and its characterization in mammals provided a key tool to deciphering the complex mechanism governing adipose tissue regulation of appetite and energy expenditure. Surprisingly, despite the perfectly logical notion of an energy-storing tissue announcing the amount of fat stores using leptin signaling, alternate mechanisms were chosen in bird evolution. This conclusion emerged based on the recent discovery and characterization of genuine avian leptin - after it had been assumed missing by some, and erroneously identified by others. Critical evaluation of the past and present indications of the role of leptin in Aves provides a new perspective on the evolution of energy-balance control in vertebrates; proposing a regulation strategy alternative to the adipostat mechanism.

On the Molecular Evolution of Leptin, Leptin Receptor, and Endospanin

Over a decade passed between Friedman's discovery of the mammalian leptin gene (1) and its cloning in fish (2) and amphibians (3). Since 2005, the concept of gene synteny conservation (vs. gene sequence homology) was instrumental in identifying leptin genes in dozens of species, and we now have leptin genes from all major classes of vertebrates. This database of LEP (leptin), LEPR (leptin receptor), and LEPROT (endospanin) genes has allowed protein structure modeling, stoichiometry predictions, and even functional predictions of leptin function for most vertebrate classes. Here, we apply functional genomics to model hundreds of LEP, LEPR, and LEPROT proteins from both vertebrates and invertebrates. We identify conserved structural motifs in each of the three leptin signaling proteins and demonstrate Drosophila Dome protein's conservation with vertebrate leptin receptors. We model endospanin structure for the first time and identify endospanin paralogs in invertebrate genomes. Finally, we argue that leptin is not an adipostat in fishes and discuss emerging knockout models in fishes.

Claudin 11 inter-sertoli tight junctions in the testis of the korean soft-shelled turtle (Pelodiscus maackii)

Expression of claudin 11 (CLDN11), a tight junction (TJ) protein, was examined in the Korean soft-shelled turtle (Pelodiscus maackii) testis. Spermatogenesis began during the breeding season and peaked at the end of the breeding season. Spermiation started in summer and peaked in autumn. The deduced amino acid sequence of P. maackii CLDN11 was similar to those of avian and mammalian species. During the nonbreeding season when spermatogenesis and testosterone production were active, testicular Cldn11 levels were high. In the seminiferous epithelium, strong, wavy CLDN11 strands parallel to the basement membrane delaminate the spermatogonia, and early spermatocytes are in the open compartment. Otherwise, CLDN11 was found beneath the early spermatocytes and in the Sertoli cell cytoplasm. Punctate zonula occludens 1 (ZO-1) immunoreactivity was found within the CLDN11 strands parallel to the basement membrane or at the outermost periphery of the seminiferous epithelium close to the basal lamina. During the breeding season, when circulating testosterone levels and spermatogenic activity was low, testicular CLDN11 level was lower than those during the nonbreeding season. CLDN11 was found at apicolateral contact sites between adjacent Sertoli cells devoid of the postmeiotic germ cells. At this time, lanthanum tracer diffused to the adluminal compartment of seminiferous epithelium. In cultured testis tissues, testosterone propionate significantly increased the level of Cldn11 mRNA. In P. maackii testis, CLDN11 participates in the development of the blood-testis barrier (BTB), where the CLDN11 expression was coupled with spermatogenic activity and circulating androgen levels, indicating the conserved nature of TJs expressing CLDN11 at the BTB in amniotes.

When do we eat? Ingestive behavior, survival, and reproductive success

The neuroendocrinology of ingestive behavior is a topic central to human health, particularly in light of the prevalence of obesity, eating disorders, and diabetes. The study of food intake in laboratory rats and mice has yielded some useful hypotheses, but there are still many gaps in our knowledge. Ingestive behavior is more complex than the consummatory act of eating, and decisions about when and how much to eat usually take place in the context of potential mating partners, competitors, predators, and environmental fluctuations that are not present in the laboratory. We emphasize appetitive behaviors, actions that bring animals in contact with a goal object, precede consummatory behaviors, and provide a window into motivation. Appetitive ingestive behaviors are under the control of neural circuits and neuropeptide systems that control appetitive sex behaviors and differ from those that control consummatory ingestive behaviors. Decreases in the availability of oxidizable metabolic fuels enhance the stimulatory effects of peripheral hormones on appetitive ingestive behavior and the inhibitory effects on appetitive sex behavior, putting a new twist on the notion of leptin, insulin, and ghrelin "resistance." The ratio of hormone concentrations to the availability of oxidizable metabolic fuels may generate a critical signal that schedules conflicting behaviors, e.g., mate searching vs. foraging, food hoarding vs. courtship, and fat accumulation vs. parental care. In species representing every vertebrate taxa and even in some invertebrates, many putative "satiety" or "hunger" hormones function to schedule ingestive behavior in order to optimize reproductive success in environments where energy availability fluctuates.

Does leptin signal adiposity in the egg-laying mammal, Tachyglossus aculeatus?

Leptin is a peptide hormone best known for its role in feedback regulation of adiposity in eutherian mammals. Normally an increase in adipose tissue mass leads to an increase in circulating leptin which increases energy expenditure and limits food intake, but in hibernating eutherian mammals this relationship may change to allow prehibernatory fattening. The echidna (Tachyglossus aculeatus) is a monotreme mammal which accumulates significant fat reserves before entering hibernation, and mates immediately at the end of hibernation. We hypothesised that echidnas would show a strong relationship between body mass and plasma leptin for most of the year which would change during the pre-hibernatory period. We measured plasma leptin and body mass in free-ranging echidnas over several reproductive and hibernation cycles. There were significant seasonal variations in plasma leptin in both sexes, with the highest levels occurring in hibernation and in mating females. The lowest levels were found in males when they were foraging maximally after the reproductive period. We used mass%, body mass at the time of sampling as a percentage of long term mean mass, as a proxy for adiposity. There was a weak negative relationship between mass% and plasma leptin, from which we infer a weak negative relationship between adiposity and plasma leptin as has been found in reptiles and birds, rather than the strong positive relationship found in other mammals.

Leptin, a neuroendocrine mediator of immune responses, inflammation, and sickness behaviors

Effective immune responses are coordinated by interactions among the nervous, endocrine, and immune systems. Mounting immune, inflammatory, and sickness responses requires substantial energetic investments, and as such, an organism may need to balance energy allocation to these processes with the energetic demands of other competing physiological systems. The metabolic hormone leptin appears to be mediating trade-offs between the immune system and other physiological systems through its actions on immune cells and the brain. Here we review the evidence in both mammalian and non-mammalian vertebrates that suggests leptin is involved in regulating immune responses, inflammation, and sickness behaviors. Leptin has also been implicated in the regulation of seasonal immune responses, including sickness; however, the precise physiological mechanisms remain unclear. Thus, we discuss recent data in support of leptin as a mediator of seasonal sickness responses and provide a theoretical model that outlines how seasonal cues, leptin, and proinflammatory cytokines may interact to coordinate seasonal immune and sickness responses.

Leptin as a physiological mediator of energetic trade-offs in ecoimmunology: implications for disease

Organisms must distribute sufficient energy among different and often competing physiological systems. This task can become challenging, however, as resources are often limiting, resulting in energetic trade-offs. For example, energetically based trade-offs between the reproductive and immune systems are common across taxa, yet the regulatory mechanisms underlying these trade-offs remain unclear. The adipose tissue hormone leptin is an ideal candidate for the modulation of energetic trade-offs between different physiological systems as this hormone serves as a gage of fat reserves and also modulates a range of physiological activities including the reproductive and immune processes. This article presents a review of the evidence for the role of leptin as a modulator of energetic trade-offs with the immune system and suggests its importance in disease ecology. In addition, we provide a case study of the ornate tree lizard (Urosaurus ornatus), testing whether leptin is involved in mediating a well-documented influence of energy state on the trade-off between reproductive activity and immune function. Overall, the combined results suggest that leptin serves as a proximate endocrine signal of available energy to the immune system, and therefore likely to affect susceptibility to diseases.

Ecological immunology: The organism in context

A major challenge in integrative biology is understanding the mechanisms by which organisms regulate trade-offs among various functions competing for limiting resources. Key among these competing processes is the maintenance of health and the production of offspring. Optimizing both, given limited resources, can prove challenging. The physiological and behavioral changes that occur during reproduction have been shown to greatly influence an organism's immune system, which can have consequences for susceptibility to disease. Likewise, investing in costly immunological defenses can impair reproductive function. However, the precise nature of these physiological and behavioral interactions appears to be greatly dependent upon the environmental context in which they occur. Here we take a comparative look at interactions between the reproductive and immune systems, including current immunological approaches, and discuss how similar studies can reveal vastly disparate results. Specifically, we highlight results from the ornate tree lizard (Urosuarus ornatus) and the Siberian hamster (Phodopus sungorus) model systems, which provide an example of current research in the field. Collectively, these results emphasize the importance of resource availability and an individual's energy stores for the existence of life-history trade-offs and the efficiency of physiological processes in general. Akin to Dobzhansky's famous line, like other aspects of biology, nothing in ecoimmunology seems to make sense except in the context of an organism's environment.