The amount of mutant vasopressin precursor in the supraoptic and paraventricular nucleus of Brattleboro rats increases with age

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Processing of frameshifted vasopressin precursors

Biosynthesis of the vasopressin (VP) prohormone in magnocellular neurones of the hypothalamo-neurohypophysial system comprises endoplasmic reticulum (ER) transit, sorting into the regulated secretory pathway and subsequent processing in the individual proteins VP, neurophysin and a glycoprotein. These processes are severely disrupted in the homozygous diabetes insipidus (di/di) Brattleboro rat, which expresses a mutant VP precursor due to a single nucleotide deletion in the neurophysin region of the VP gene resulting in VP deficiency. Previous studies have shown the presence of additional frameshift mutations in VP transcripts, in solitary magnocellular neurones of the di/di rat due to a GA dinucleotide deletion resulting in two different mutant VP precursors with partly restored reading frame. Frameshifted VP precursors are also expressed in several magnocellular neurones in wild-type rats. In this study, we determined if the +1 frameshifted precursors from di/di and wild-type rats can lead to biosynthesis of the hormone VP. Therefore, eukaryotic expression plasmids containing the frameshifted VP cDNAs were transiently expressed in peptidergic tumour cell lines, and cells were analysed by reversed phase high-performance liquid chromatography and specific radioimmunoassays, and by immunofluoresence. Neuro2A neuroblastoma cells expressing the +1 frameshifted precursors of di/di rats retained products in the cell body. Only precursor or insignificant quantities of neurophysin-immunoreactive products were detected. In contrast, in AtT20 cells, frameshifted VP precursors were at least partly processed to yield the VP peptide, indicating that they have access to the regulated secretory pathway. Comparison between the two cell lines showed a very slow ER transit of the wild-type prohormone combined with inefficient processing in Neuro2A cells. The results show that mutant precursors can reach the regulated secretory pathway if ER transport is sufficiently rapid as in the case of AtT20 cells. This suggests that the di/di rat may regain the capacity to biosynthesize authentic VP through these +1 frameshifted precursors in magnocellular neurones.

Functions of the perikaryon and dendrites in magnocellular vasopressin-secreting neurons: new insights from ultrastructural studies

Magnocellular hypothalamic neurosecretory neurons secreting vasopressin or oxytocin provide a robust model system for the investigation and understanding of many aspects of peptidergic neuronal function. Many of their functions and the cellular organelles involved are well understood. However, recent ultrastructural studies have thrown new light on various aspects of magnocellular neurosecretory function which have not previously received much attention. This review concerns two of these: the effects of mutations in the vasopressin gene on the handling of the translated peptide by the rough endoplasmic reticulum; and the role of the magnocellular dendrites in the production, secretion and localisation of peptides. Investigation of the synthesis of proteins derived from vasopressin genes which have undergone various mutations has at the moment provided more answers than questions: Why do some abnormal products accumulate as masses of peptide in the rough endoplasmic reticulum while others do not? Why do accumulations in humans appear to be damaging to the neurons while those in the rat do not? Investigations of the role of dendrites in the production and release of peptides show that the dendrites have all the machinery needed for protein translation and appear to synthesize locally proteins required for dendritic function. Of particular interest is the possibility that various transmitter receptor proteins could be synthesized in the dendrites close to the synapses in which they become localized. Precisely how such membrane proteins are inserted into the synaptic complex is, however, unclear, because the most part of the dendrites lack any form of the Golgi packaging organelle that can be recognised as such either by immunocytochemistry or electron microscopy. Better established is the ability of magnocellular dendrites to secrete either vasopressin or oxytocin in response to a variety of stimuli including sex steroids. This local release of peptide into the magnocellular nuclei has important but as yet incompletely defined effects on the functioning of the neurons.

Specific expression of secretogranin II in magnocellular vasopressin neurons of the rat supraoptic and paraventricular nucleus in response to osmotic stimulation

As secretogranin II is considered to be a marker for the regulated secretory pathway, its distribution in the hypothalamo-neurohypophyseal system of salt-loaded Wistar rats was studied in detail by immunocytochemistry. Although after an osmotic challenge both vasopressin and oxytocin neurons are stimulated, secretogranin II was exclusively expressed in a subpopulation of vasopressinergic magnocellular neurons in the supraoptic and paraventricular nucleus of Wistar rats. Secretogranin II was only surely visualized after a combination of osmotic challenge and blockade of axonal transport by colchicine treatment. When these pre-treatments were not performed, only punctate fibers situated around the magnocellular neurons within the paraventricular and supraoptic nucleus were observed. Oxytocinergic magnocellular neurons never displayed any secretogranin II immunoreactivity, not even during lactation and after colchicine treatment. These findings suggest that secretogranin II is of functional importance during enhanced secretory activity within vasopressinergic neurons.

Immunocytochemical evidence for the presence of vasopressin in intermediate sized neurosecretory granules of solitary neurohypophyseal terminals in the homozygous Brattleboro rat

A single base deletion (delta G) in the vasopressin gene is the cause of diabetes insipidus in the homozygous Brattleboro rat (di/di). The resulting frameshift leads to the expression of an aberrant vasopressin precursor which is unable to enter the secretory pathway, thereby preventing vasopressin biosynthesis. In a small number of solitary magnocellular hypothalamic neurons within the supraoptic and paraventricular nuclei, the reading frame is restored by a dinucleotide (delta GA) frameshift mutation, at two separate GAGAG motifs downstream of the original G-deletion. This results in two + 1 di-vasopressin precursors that are still partially mutated within the neurophysin region. The present study provides immunocytochemical evidence which demonstrates that, within magnocellular solitary neurons of the supraoptic and paraventricular nuclei of the di/di rat, the + 1 di-vasopressin precursors can enter the secretory pathway followed by their enzymatic processing into vasopressin during axonal transport to the neural lobe. However, the cellular characteristics of biosynthesis are different from those of wild-type rats. Immunoelectron microscopical localization of vasopressin gene products in the neural lobe of did/di rats revealed their presence in neurosecretory granules, the diameter of which is intermediate (116 nm) between those of the neurosecretory granules in the di/di (80-100 nm) and wild-type (160 nm) rats.

Immunocytochemical localization of neuropeptide FF (FMRF amide-like peptide) in the hypothalamo-neurohypophyseal system of Wistar and Brattleboro rats by light and electron microscopy

Neuropeptide FF (F8Famide, FMRFamide-like, or morphine modulating peptide) immunoreactivity was localized by light and electron microscopy in the hypothalamo-neurohypophyseal system of Wistar and Brattleboro rats. In Wistar rats neuropeptide FF was present in part of the magnocellular neurones of the paraventricular and supraoptic nuclei in which it was coexpressed with vasopressin. Neuropeptide FF containing fibres were present in the paraventricular and the supraoptic nuclei, and in the central part of the neural lobe. At the electron microscopic level, neuropeptide FF containing nerve terminals in the neural lobe formed synaptoid contacts exclusively with pituicytes. No neuropeptide FF containing neurovascular contacts or contacts with other neuronal structures were observed. In contrast with Wistar rats, neuropeptide FF was almost completely absent in cell bodies of the paraventricular and supraoptic nuclei, and in fibres of the neural lobe in Brattleboro rats. Only a few solitary cells could be observed in these structures. The present results demonstrate that neuropeptide FF coexists with vasopressin within the hypothalamo-neurohypophyseal system. As we did not observe neuropeptide FF containing neurovascular contacts, neuropeptide FF containing nerve terminals probably have a local function within the neural lobe. Neuropeptide FF may be involved in the modulation of oxytocin and vasopressin release, with the pituicyte as an intermediate cell.

Animal models for osmoregulatory disturbances

For the various types of di in humans, animal models are available. However, their value for explaining human di is for the major part an indirect one; by studying cellular mechanisms in these animal models, fundamental aspects of the cellular processes become available, which will help to understand similar processes in human di and subsequently lead to the molecular cause(s) of the various types of human di. Finally, it is to be expected that in the very near future transgenic animals will be raised in which very specific genetic information is overexpressed (or knocked out by homologous recombination; McMahon and Bradley, 1990). Recently hypervasopressinemia could be shown in transgenic mice, providing an animal model for the syndrome of the inappropriate VP secretion (Bartter and Schwartz, 1967), which is often observed in patients with lung cancers that ectopically express the VP gene (Habener et al., 1989). Furthermore it will be possible to study the exact cause(s) of human di by performing in vitro mutagenesis and to express the RNA constructs within a cell-free translation system and in oocytes (e.g., Schmale et al., 1989) and subsequently study the pattern of precursor synthesis, packaging and processing.