Improvement in reproductive parameters in hypogonadal female mice by regulated gene replacement therapy in the central nervous system

Hypothalamic reproductive neurons communicate through signal transduction to control reproduction

Gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus coordinate fertility and puberty. In order to achieve successful reproductive capacity, they receive signals from the periphery and from other hypothalamic neurons that coordinate energy homeostasis. Hormones, such as estradiol, insulin, leptin, and adiponectin, act directly or indirectly on GnRH and its associated reproductive neurons. Nutrients like glucose and fatty acids can also affect reproductive neurons to signal nutrient availability. Additionally, acute and chronic inflammation is reported to detrimentally affect GnRH and kisspeptin expression. All of these cues activate signal transduction pathways within neurons that lead to the changes in GnRH neuronal function. The signalling pathways can also be dysregulated by endocrine disrupting chemicals, which impair fertility by misappropriating common signalling pathways. The complex mechanisms controlling the levels of GnRH during the reproductive cycle rely on a carefully orchestrated set of signal transduction events to regulate the positive and negative feedback arms of the hypothalamic-pituitary-gonadal axis. If these signalling events are dysregulated, this will result is a downregulatory event leading to hypogonadal hypogonadism with decreased or absent fertility. Therefore, an understanding of the mechanisms involved in distinct neuronal signalling could provide an advantage to inform therapeutic interventions for infertility and reproductive disorders.

Palmitate differentially regulates Spexin, and its receptors Galr2 and Galr3, in GnRH neurons through mechanisms involving PKC, MAPKs, and TLR4

The function of the gonadotropin-releasing hormone (GnRH) neuron is critical to maintain reproductive function and a significant decrease in GnRH can lead to disorders affecting fertility, including hypogonadotropic hypogonadism. Spexin (SPX) is a novel hypothalamic neuropeptide that exerts inhibitory effects on reproduction and feeding by acting through galanin receptor 2 (GALR2) and galanin receptor 3 (GALR3). Fatty acids can act as nutritional signals that regulate the hypothalamic-pituitary-gonadal (HPG) axis, and elevated levels of circulating saturated fatty acids associated with high fat diet (HFD)-feeding have been shown to induce neuroinflammation, endoplasmic reticulum stress and hormonal resistance in the hypothalamus, as well as alter neuropeptide expression. We previously demonstrated that palmitate, the most common saturated fatty acid in a HFD, elevates the expression of Spx, Galr2 and Galr3 mRNA in a model of appetite-regulating neuropeptide Y hypothalamic neurons. Here, we found that Spx, Galr2 and Galr3 mRNA were also significantly induced by palmitate in a model of reproductive GnRH neurons, mHypoA-GnRH/GFP. As a follow-up to our previous report, we examined the molecular pathways by which Spx and galanin receptor mRNA was regulated in this cell line. Furthermore, we performed inhibitor studies, which revealed that the effect of palmitate on Spx and Galr3 mRNA involved activation of the innate immune receptor TLR4, and we detected differential regulation of the three genes by the protein kinases PKC, JNK, ERK, and p38. However, the intracellular metabolism of palmitate to ceramide did not appear to be involved in the palmitate-mediated gene regulation. Overall, this suggests that SPX may play a role in reproduction at the level of the hypothalamus and the pathways by which Spx, Galr2 and Galr3 are altered by fatty acids could provide insight into the mechanisms underlying reproductive dysfunction in obesity.

Mouse models to study the central regulation of puberty

At puberty, the mammalian reproductive axis is activated by neuroendocrine events within the hypothalamus that initiate pulsatile secretion of gonadotropin releasing hormone (GnRH) to activate the pituitary/gonadal axis. Thus, puberty is critically dependent on the integrity of GnRH neuronal activity. Defects in the migration of GnRH neurons into the forebrain during development or in GnRH synthesis or release prevent pubertal maturation of the reproductive axis. Both naturally occurring and genetically modified mutant mice have provided valuable information about the cellular and molecular events required for normal pubertal development. This review focuses specifically on the molecules that have been identified from studies in mutant mice that act centrally to control entry into puberty.

HSV-1-derived amplicon vectors: recent technological improvements and remaining difficulties--a review

Amplicons are defective and non-integrative vectors derived from herpes simplex virus type 1. As the vector genome carries no virus genes, amplicons are both non-toxic for the infected cells and non-pathogenic for the inoculated organisms. In addition, the large transgenic capacity of amplicons, which allow delivery of up to 150 Kbp of foreign DNA, makes these vectors one of the most powerful, interesting and versatile gene delivery platforms. We present here recent technological developments that have significantly improved and extended the use of amplicons, both in cultured cells and in living organisms. In addition, this review also discusses the many difficulties still pending to be solved, in order to achieve stable and physiologically regulated transgene expression.

Progress and prospects: biological properties and technological advances of herpes simplex virus type 1-based amplicon vectors

The last two years have seen significant advances in our understanding of the cellular innate responses elicited or activated by the entry of amplicon particles, which may, in part, explain the transient nature of transgene expression often observed in cells infected with helper-free amplicon stocks. At the technological level, the most consistent progress has been in strategies to enhance the stability of transgene cassettes, either through integration into host chromosomes or through the conversion of the amplicon genome into a replication-competent extrachromosomal element.

The gonadotropin-releasing hormone (GnRH) neuronal population is normal in size and distribution in GnRH-deficient and GnRH receptor-mutant hypogonadal mice

Hypothalamic GnRH neurons are essential for initiation and regulation of reproductive function. In addition to pituitary gonadotrope stimulation, activity of GnRH through its receptor (GnRHR) has been suggested to include autocrine regulation of the GnRH neuron. Two hypogonadal mouse strains, the Gnrh1 mutant (hpg) mice and Gnrhr mutant mice were used to investigate the potential role of GnRH signaling in the proper development and maintenance of GnRH neurons. Immunocytochemical analysis of heterozygous hpg mice revealed a GnRH neuron population that was normal in size and distribution, indicating no effect from reduced Gnrh1 gene dosage on the neurons themselves. To visualize GnRH neurons in homozygous GnRH-deficient hpg mice, heterozygous hpg mice were crossed with GnRH-green fluorescent protein (GFP) transgenic mice with targeted expression of the GFP reporter gene in GnRH neurons. Analysis of forebrains of homozygous hpg/GFP-positive mice immunostained for GFP revealed a normal population size and appropriate distribution of GnRH neurons in hpg mice, with immunoreactive neuronal processes present at the median eminence. Similarly, adult mice deficient in functional GnRHR possessed a full complement of GnRH neurons in the basal forebrain that was indistinguishable from the distribution of GnRH neurons in their wild-type counterparts. Moreover, hpg/GFP neurons retained the ability to generate spontaneous bursts of action potential firing activity, suggesting that GnRH peptide is not required for this function. These data establish that autocrine-paracrine GnRH-signaling is not a prerequisite for the developmental migration of GnRH neurons into the brain or for the projection of GnRH neurosecretory axons.

Hypothalamic control of anterior pituitary function: a history

The concept of neurohumoral control of anterior pituitary function championed by Geoffrey Harris was based upon clinical and biological observation backed by rigorous experimental testing. The areas of the brain involved in the control of gonadotrophic hormone synthesis and release were identified by electrical stimulation, lesioning and fibre tract cutting. The medial preoptic area (MPOA) proved to be a major integrating centre, with axon terminals from this region terminating at the median eminence releasing factors into the portal vessels to give a direct route from brain to pituitary. It took over a decade before the gonadotrophic hormone-releasing hormone (GnRH) was isolated, sequenced and synthesised. With antibodies raised against this peptide, the MPOA was identified as a site rich in GnRH neurones and the hormone was detected at high levels in portal blood extracts. A natural knockout of the GnRH gene was discovered in a hypogonadal (hpg) mouse. Hormone injections, gene replacement methods and neural grafting in these mutants all confirmed the central role of GnRH in reproduction. The modern techniques of molecular biology have allowed us to extend our knowledge base. In the last few years the role of kisspeptin and its receptor (GPR54) in the control of the GnRH neurone has added a further level of hypothalamic involvement in the modulation of reproduction.