MCM8- and MCM9-deficient mice reveal gametogenesis defects and genome instability due to impaired homologous recombination.

We generated knockout mice for MCM8 and MCM9 and show that deficiency for these genes impairs homologous recombination (HR)-mediated DNA repair during gametogenesis and somatic cells cycles. MCM8(-/-) mice are sterile because spermatocytes are blocked in meiotic prophase I, and females have only arrested primary follicles and frequently develop ovarian tumors. MCM9(-/-) females also are sterile as ovaries are completely devoid of oocytes. In contrast, MCM9(-/-) testes produce spermatozoa, albeit in much reduced quantity. Mcm8(-/-) and Mcm9(-/-) embryonic fibroblasts show growth defects and chromosomal damage and cannot overcome a transient inhibition of replication fork progression. In these cells, chromatin recruitment of HR factors like Rad51 and RPA is impaired and HR strongly reduced. We further demonstrate that MCM8 and MCM9 form a complex and that they coregulate their stability. Our work uncovers essential functions of MCM8 and MCM9 in HR-mediated DSB repair during gametogenesis, replication fork maintenance, and DNA repair.

Influence of estrogens on GH-cell network dynamics in females: a live in situ imaging approach.

The secretion of endocrine hormones from pituitary cells finely regulates a multitude of homeostatic processes. To dynamically adapt to changing physiological status and environmental stimuli, the pituitary gland must undergo marked structural and functional plasticity. Endocrine cell plasticity is thought to primarily rely on variations in cell proliferation and size. However, cell motility, a process commonly observed in a variety of tissues during development, may represent an additional mechanism to promote plasticity within the adult pituitary gland. To investigate this, we used multiphoton time-lapse imaging methods, GH-enhanced green fluorescent protein transgenic mice and sexual dimorphism of the GH axis as a model of divergent tissue demand. Using these methods to acutely (12 h) track cell dynamics, we report that ovariectomy induces a dramatic and dynamic increase in cell motility, which is associated with gross GH-cell network remodeling. These changes can be prevented by estradiol supplementation and are associated with enhanced network connectivity as evidenced by increased coordinated GH-cell activity during multicellular calcium recordings. Furthermore, cell motility appears to be sex-specific, because reciprocal alterations are not detected in males after castration. Therefore, GH-cell motility appears to play an important role in the structural and functional pituitary plasticity, which is evoked in response to changing estradiol concentrations in the female.

Pituitary growth hormone network responses are sexually dimorphic and regulated by gonadal steroids in adulthood.

There are well-recognized sex differences in many pituitary endocrine axes, usually thought to be generated by gonadal steroid imprinting of the neuroendocrine hypothalamus. However, the recognition that growth hormone (GH) cells are arranged in functionally organized networks raises the possibility that the responses of the network are different in males and females. We studied this by directly monitoring the calcium responses to an identical GH-releasing hormone (GHRH) stimulus in populations of individual GH cells in slices taken from male and female murine GH-eGFP pituitary glands. We found that the GH cell network responses are sexually dimorphic, with a higher proportion of responding cells in males than in females, correlated with greater GH release from male slices. Repetitive waves of calcium spiking activity were triggered by GHRH in some males, but were never observed in females. This was not due to a permanent difference in the network architecture between male and female mice; rather, the sex difference in the proportions of GH cells responding to GHRH were switched by postpubertal gonadectomy and reversed with hormone replacements, suggesting that the network responses are dynamically regulated in adulthood by gonadal steroids. Thus, the pituitary gland contributes to the sexually dimorphic patterns of GH secretion that play an important role in differences in growth and metabolism between the sexes.

Specific involvement of gonadal hormones in the functional maturation of growth hormone releasing hormone (GHRH) neurons.

Growth hormone (GH) is the key hormone involved in the regulation of growth and metabolism, two functions that are highly modulated during infancy. GH secretion, controlled mainly by GH releasing hormone (GHRH), has a characteristic pattern during postnatal development that results in peaks of blood concentration at birth and puberty. A detailed knowledge of the electrophysiology of the GHRH neurons is necessary to understand the mechanisms regulating postnatal GH secretion. Here, we describe the unique postnatal development of the electrophysiological properties of GHRH neurons and their regulation by gonadal hormones. Using GHRH-eGFP mice, we demonstrate that already at birth, GHRH neurons receive numerous synaptic inputs and fire large and fast action potentials (APs), consistent with effective GH secretion. Concomitant with the GH secretion peak occurring at puberty, these neurons display modifications of synaptic input properties, decrease in AP duration, and increase in a transient voltage-dependant potassium current. Furthermore, the modulation of both the AP duration and voltage-dependent potassium current are specifically controlled by gonadal hormones because gonadectomy prevented the maturation of these active properties and hormonal treatment restored it. Thus, GHRH neurons undergo specific developmental modulations of their electrical properties over the first six postnatal weeks, in accordance with hormonal demand. Our results highlight the importance of the interaction between the somatotrope and gonadotrope axes during the establishment of adapted neuroendocrine functions.