The hypothalamic–neurohypophysial system: genomes, genes and genetics

Abstract

Steve Lolait reports on the meeting from July 2013 at The University of Bristol, UK here This issue of Experimental Physiology contains symposium papers on Genomes, genes and genetics featured at the 10th World Congress on Neurohypophysial Hormones, held on the 15-19 July 2013 in Bristol, which covered the broader theme of Old hormones – new insights. The hypothalamic-neurohypophysial system integrates information about hydromineral balance and cardiovascular homeostasis and plays a vital role in lactation and parturition. It comprises the large, vasopressin (VP) and oxytocin (OT) magnocellular neurones of the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei that project through the median eminence to terminate in the posterior pituitary gland, from where the peptides are released into the bloodstream. These neurones are a cornerstone of neuroendocrine research, providing excellent physiological models that have yielded important insights into how neuropeptides/neurotransmitters (and their receptors) function at the molecular and cellular level. In the first report, Laurence Amar and co-workers (Baroin-Tourancheau et al. 2014) describe the use of high-throughput (Illumina) sequencing to identify short, non-coding microRNAs (miRNAs) in hypothalamic arcuate nuclei. In general terms, miRNAs interact with the 3' untranslated regions of target mRNAs in a sequence-specific manner, resulting in mRNA degradation and/or repression of protein translation. The arcuate nucleus is an attractive target for miRNA-led post-transcriptional regulation because it is a hypothalamic hotspot for the control of whole-body energy homeostasis via a complex interplay between a number of orexigenic and anorexigenic genes and by providing integrated signals to the hypothalamic PVN. MicroRNA profiling by Illumina sequencing represents the ‘deepest’ miRNA sampling to date and has the potential to reveal novel miRNAs. Amar and co-workers (Baroin-Tourancheau et al. 2014) argue the merits of sampling individual (microdissected) brain regions for miRNA expression profiling, rather than pooled nuclei from a number of animals, to facilitate the detection of possible genetic or epigenetic differences between individuals. They describe the basic steps in cDNA library construction for establishing miRNA expression profiles using Illumina sequencing. The sequences from seven individual arcuate nuclei were then analysed with the miRanalyzer Web server to identify known and/or unannotated miRNAs. The challenge will be to identify the major targets of these miRNAs; that some targets are undoubtedly important is evidenced by the fact that deletion of an enzyme, Dicer, essential for producing the mature form of miRNAs, in arcuate nuclei pro-opiomelanocortin neurones leads to obesity (Schneeberger et al. 2013). It will also be interesting to see how hypothalamic nuclei miRNA expression profiles alter in response to changes in water or nutrient balance, which can be performed in parallel to compiling the expression profiles of coding mRNAs from the same cDNA library. Hiroshi Arima et al. (2014) shift our attention to their findings on a mouse model of familial neurohypophysial diabetes insipidus (FNDI) and pose the question, how do these mice cope with an accumulation of unfolded or misfolded VP precursor in neurones? Familial neurohypophysial diabetes insipidus is characterized by poluria and polydipsia caused by a deficiency in VP production. Vasopressin is synthesized primarily in the hypothalamic PVN, SON and suprachiasmatic nuclei. Besides maintaining water homeostasis, VP also regulates vascular tone and hypothalamic-pituitary-adrenal (HPA) activity and has many central behavioural effects. The authors established a mouse model of FNDI [in this case, mutating the VP ‘carrier’ neurophysin (NP) II (Cys98stop; one of more than 60 VP gene (Avp) mutations that cause FNDI in humans)], which led to progressive polyuria. In FNDI mice, there are aggregates in the endoplasmic reticulum of VP neurones, and reduced VP expression associated with decreased Avp mRNA poly(A) tail length in the SON. There is also diminished axonal expression of NPII, suggesting that mutant NPII may reduce the trafficking of normal NPII in FNDI mice by a ‘dominant-negative effect’. Interestingly, there was no loss of VP neurones in male FNDI mice up to 12 months of age, but there was VP neuronal loss in females at 12 months; however, there was no evidence of apoptosis. Thus, progressive polyuria in FNDI mice could proceed in the absence of VP neuronal cell death, in contrast to the autophagy described in a number of previous studies cited in this report. Arima et al. (2014) suggest that the mechanism underlying decreased VP production in their FNDI mice is akin to processes involved in endoplasmic reticulum stress. They conclude that the shortening of Avp poly(A) length in FNDI mice may result in decreased Avp mRNA stability and decreased VP in FNDI mice or that the decreased VP poly(A) tail may be a cellular protective mechanism to diminish accumulated mutant VP. It would be interesting, in future studies, to examine the effects of other FNDI Avp mutations in the context of endoplasmic reticulum stress, perhaps by recapitulating the mutants in cell lines. Next, Christian Gruber (2014) provides an overview of efforts to discover OT- and VP-like peptides in invertebrates using genome-mining approaches. Genome mining is typically performed using publicly available sequence similarity search tools, such as tBLASTn from the National Center for Biotechnology Information (NCBI database at http://blast.ncbi.nlm.nih.gov/Blast.cgi), and other methods that give structure prediction and sequence alignments. The evolution of OT/VP-like substances and their cognate G protein-coupled receptors, beginning some 600 million years or so ago, has long been regarded as a fine example of the co-evolution of two lineages of peptides (ligands and receptors) and provides the basis for sequence comparisons across invertebrate and vertebrate animals. Gruber (2014) relates that in the ant genomes the sequences and functional domains of the precursors for OT/VP-like peptides, called inotocins, are remarkably well conserved. In addition, the corresponding receptors for these peptides show high similarity. The author also presents new sequences for the OT/VP-like equivalents in other arthropods, such as mites and a centipede; again, these peptides show high similarity to homologuesinnematodes, molluscs, annelids and vertebrates.Auseful summary provided onthe function of OT/VP-like peptides in invertebrates shows that OT/VP-like peptides are involved in neurotransmitter/hormone-like functions throughout invertebrates. Gruber mentions in passing that not all species appear to have these OT/VP-like peptides (or their receptors). This interesting point was elaborated on previously (Stafflinger et al. 2008), where it was noted that the genes for the OT/VP-like peptides and their receptors appear to have been ‘lost’ at least twice during evolution (e.g. dropped from members of the honeybee, mosquito, fruit fly and silkworm genomes). The reason(s) for these gross deletions are unclear, but it is possible that another hormonal–neurotransmitter system took over the function of OT/VP-like peptides. Finally, Gruber highlights that the genome-mining approach applied to invertebrates could reveal new OT/VP-likelead compounds, with their ‘natural’amino acid substitutions that may have enhanced specificity for members of the family of human VP (V1a, V1b and V2) and OT receptors. This approach appears to be bearing fruit, because they have already found one peptide, conopressin-T from cone snails, which appears to act as a specific antagonist for the human OT receptor. Chris Murgatroyd (2014) discusses the effects of early life adversity on the vulnerability to stress in later life in the context of epigenetic changes. Epigenetic modifications of the genome, such as those involving alterations in DNA methylation, provide a mechanism by which early life events and/or the environment can have lasting effects on overall health and can influence future generations. As an example, Murgatroyd describes the epigenetic changes to Avp that ensue following a model of maternal separation during early postnatal life in rats. Numerous stressors increase VP synthesis in parvocellular PVN neurones, from where it is secreted into the hypophysial portal blood to act on pituitary VP V1b receptors to release adrenocorticotrophic hormone (ACTH) into the bloodstream. Adrenocorticotrophic hormone triggers glucocorticoid release from the adrenal cortex as a key mediator of the HPA neuroendocrine response to stress. Studies from the author's laboratory showed that maternal separation reduces DNA methylation at a downstream promoter of Avp, leading to a persistent increase in Avp mRNA expression in the parvocellular PVN neurones and, consequently, sustained HPA hyperactivity to stress (i.e. hypersecetion of corticosterone) and behavioural changes that lasted for at least 1 year. The underlying mechanism for the change in Avp expression appears to involve hypomethylation of specific cytosine–guanine dinucleotide residues in the Avp enhancer that impair the ability of another protein, methyl-CpG binding protein 2, to bind to this methylated DNA, with the subsequent relief of repression of Avp. Some questions remain. How stable are these and other epigenetic changes? Are they permanent or are they reversible/plastic? Are Avp methylation patterns transmitted across generations? Are they gender specific? Are they applicable to other stressors in early life? A significant impact of these findings is that, in some cases, it may help to explain the altered HPA activity associated with some mood disorders, such as depression. Establishing the mechanisms of how early life experiences can alter the expression of critical genes involved in the HPA response to stress should ultimately enhance our understanding of how we cope with stress throughout life. Together, these symposium reports provide concise, timely, up-to-date and thought-provoking summaries of our current knowledge of aspects of ‘neuroendocrine’ miRNA profiling, Avp regulation, synthesis and secretion, and the evolution of OT- and VP-like peptides. They point to future studies that have possible translational significance. Readers are invited to give their opinion on this article. To submit a comment, go to: http://ep.physoc.org/letters/submit/expphysiol;99/1/52.