Insulin Does Not Target CamkIIα Neurones to Critically Regulate the Neuroendocrine Reproductive Axis in Mice

Abstract

Insulin signalling in the brain plays an important role in the central regulation of energy homeostasis and fertility, such that mice exhibiting widespread deletion of insulin receptors (InsR) throughout the brain and peripheral nervous system display diet sensitive obesity and hypothalamic hypogonadism. However, the specific cell types mediating the central effects of insulin on fertility remain largely unidentified. To date, the targeted deletion of InsR from individual neuronal populations implicated in the metabolic control of fertility has failed to recapitulate the hypogonadic and subfertile phenotype observed in brain-specific InsR knockout mice. Because insulin and leptin share similar roles as centrally-acting metabolic regulators of fertility, we used the Cre-loxP system to generate mice with a selective inactivation of the Insr gene from the same widespread neuronal population previously shown to mediate the central effects of leptin on fertility by crossing Insr-flox mice with calcium/calmodulin-dependent protein kinase type IIα (CamkIIα)-Cre mice. Multiple reproductive and metabolic parameters were then compared between male and female Insr-flox/Cre-positive (CamK-IRKO) and Insr-flox/Cre-negative control mice. Consistent with brain-specific InsR knockout mice, CamK-IRKO mice exhibited a mild but significant obesogenic phenotype. Unexpectedly, CamK-IRKO mice exhibited normal reproductive maturation and function compared to controls. No differences in the age of puberty onset, oestrous cyclicity or fecundity were observed between CamK-IRKO and control mice. We conclude that the central effects of insulin on the neuroendocrine reproductive axis are not critically mediated via the same neuronal populations targeted by leptin.

Ablating astrocyte insulin receptors leads to delayed puberty and hypogonadism in mice

“…Reproduction is essential for species survival. Because energy is required to locate a mate, maintain a pregnancy, and rear young, fertility is modulated by the status of energy stores [1–3]. Excessive energy expenditure or insufficient caloric intake in humans and rodents delays the pubertal transition and reduces fertility [4, 5].…”

“…A seminal paper by Brüning and colleagues [8] showed that 50% of mice lacking the IR in cells expressing nestin (NIRKO mice) displayed hypogonadotropic hypogonadism in adulthood. Targeted deletion of IRs in specific neuronal populations, however, has failed to induce the subfertile phenotype and GnRH network dysregulation of NIRKO mice [2, 3, 6, 22, 23]. For instance, Divall and colleagues found that mice with IR deletion in GnRH neurons experienced normal pubertal timing and fertility [6].…”

“…In another example, mice with IR deletion in gamma-amino butyric acid (GABA)-ergic or glutamatergic cells showed normal pubertal progression, estrous cyclicity, and fertility [23]. More widespread deletion of IR in Ca 2+ /calmodulin-dependent protein kinase-expressing neurons, located in the dentate gyrus, cortex, olfactory bulb, amygdala, striatum, thalamus, and hypothalamus [24], also produced mice with normal reproductive maturation and fertility [3]. These numerous negative results suggest that insulin action in neurons does not play an essential role in hypothalamic-pituitary-gonadal (HPG) axis function.…”

“…Alternatively, it has been suggested [3] that the hypothalamic hypogonadism observed in NIRKO mice results from the chronic absence of insulin signaling in glia rather than neurons. Indeed, the nestin-cre line drives deletion of IR in both neuronal and non-neuronal cells [8, 25–27].…”

Neuroendocrine integration of nutritional signals on reproduction

“…Although insulin can act both peripherally and centrally to modulate HPG axis function, we know that insulin's central actions play a role in mediating many of its reproductive effects as mice exhibiting brain-specific deletion of insulin receptors (InsR) exhibit subfertility due to mild hypothalamic hypogonadism (Bruning et al 2000). However, in contrast to these brain-specific InsR-knockout mice, our group recently demonstrated that mice exhibiting forebrain neuron-specific deletion of InsR displayed normal HPG axis function (Evans et al 2015), which challenges the hypothesis that neuronal insulin signaling is required. Nevertheless, many findings implicate abnormal insulin signaling in the pathogenesis of reproductive dysfunction.…”