Transgenic and transcriptional studies on neurosecretory cell gene expression

The Biology of Vasopressin

Vasopressins are evolutionarily conserved peptide hormones. Mammalian vasopressin functions systemically as an antidiuretic and regulator of blood and cardiac flow essential for adapting to terrestrial environments. Moreover, vasopressin acts centrally as a neurohormone involved in social and parental behavior and stress response. Vasopressin synthesis in several cell types, storage in intracellular vesicles, and release in response to physiological stimuli are highly regulated and mediated by three distinct G protein coupled receptors. Other receptors may bind or cross-bind vasopressin. Vasopressin is regulated spatially and temporally through transcriptional and post-transcriptional mechanisms, sex, tissue, and cell-specific receptor expression. Anomalies of vasopressin signaling have been observed in polycystic kidney disease, chronic heart failure, and neuropsychiatric conditions. Growing knowledge of the central biological roles of vasopressin has enabled pharmacological advances to treat these conditions by targeting defective systemic or central pathways utilizing specific agonists and antagonists.

APeg3, a novel paternally expressed gene 3 antisense RNA transcript specifically expressed in vasopressinergic magnocellular neurons in the rat supraoptic nucleus

Vasopressin (VP) and oxytocin (OT) play critical roles in the regulation of salt and water balance, lactation, and various behaviors and are expressed at very high levels in specific magnocellular neurons (MCNs) in the hypothalamo-neurohypophysial system (HNS). In addition to the cell-specific expression of the VP and OT genes in these cells, there are other transcripts that are preferentially expressed in the VP or OT MCNs. One such gene, paternally expressed gene 3 (Peg3), is an imprinted gene expressed exclusively from the paternal allele that encodes a Kruppel-type zinc finger-containing protein involved in maternal behavior and is abundantly expressed in the VP-MCNs. We report here the robust expression in the VP-MCNs of an RNA, which we designate APeg3 that is transcribed in the antisense direction to the 3' untranslated region of the Peg3 gene. The APeg3 mRNA is about 1 kb in size, and the full-length sequence of APeg3, as determined by 5' and 3' RACE, contains an open reading frame that predicts a protein of 93 amino acids and is predominantly expressed in VP-MCNs. Both Peg3 and APeg3 gene expression in the VP-MCNs increase during systemic hyperosmolality in vivo, demonstrating that both of these genes are osmoregulated.

Selective gene expression in magnocellular neurons in rat supraoptic nucleus

Oxytocin- and vasopressin-producing magnocellular neurons (MCNs) of the hypothalamo-neurohypophysial system are the only neuronal phenotypes present in the rat supraoptic nucleus (SON). Laser microdissection of the SON, extraction and T7-based amplification of its RNAs, and analysis of the resulting cDNAs by hybridization on a 35, 319 element DNA microarray have provided a detailed composite view of the gene expression profile of the MCNs. The genes expressed in the SON were compared with those expressed in a reference tissue consisting of total hypothalamus, and this "expression ratio" indicated which genes were preferentially expressed in the SON. Of the 26,000 unique genes on the array, 1385 were found to be expressed in the SON at levels more than two times greater than in the hypothalamus as a whole. Of these, 123 were expressed > or =3.4-fold higher in the SON versus hypothalamus. Most of these preferentially expressed genes were not previously known to be expressed in the MCNs. Quantitative and double-label in situ hybridization histochemistry was used selectively to confirm a number of these microarray observations and to evaluate the osmotic regulation and cell-specific expression of these genes, respectively.

Transgenesis and the study of expression, cellular targeting and function of oxytocin, vasopressin and their receptors

The neuropeptides oxytocin and vasopressin and the neurons in the hypothalamus that synthesize them have been a rich source for the exploration and understanding of both the brain and the endocrine system. Because of their large size and compact nuclear organization the magnocellular neurons of the hypothalamoneurohypophysial system have traditionally attracted scientists using state-of-the-art techniques, including the subject of this review, transgenesis. We discuss the role of transgenics in deciphering gene elements necessary for the appropriate expression of oxytocin and vasopressin and to deliver exogenous genes, such as green fluorescent protein, selectively to secretory granules in the neurons in the hypothalamoneurohypophysial system. Finally, we review the studies of mice whose genes for oxytocin and, most recently, for the oxytocin and vasopressin receptors have been knocked out through homologous recombination.

Regulatory domains in the intergenic region of the oxytocin and vasopressin genes that control their hypothalamus-specific expression in vitro

Previous studies of oxytocin (OT) and vasopressin (VP) cell-specific gene expression in the hypothalamus using transgenic mouse and rat models focused attention on the intergenic region (IGR) as the site of critical enhancer elements. In this study, we used organotypic slice-explant cultures of rat hypothalamus as in vitro models, and particle-mediated gene transfer (biolistics) transfection methods to identify critical DNA sequences in the IGR between the OT and VP genes responsible for hypothalamic-specific gene expression. Reducing the 5' flanking region in the mouse VP gene from 3.5 kbp to 288 bp did not alter the efficacy of its expression in hypothalamic slices. All subsequent VP constructs were based on this 288 bp VP gene construct with changes made only to the IGR. These studies, which used various constructs with OT and VP promoters driving enhanced green fluorescent protein reporter gene expression, demonstrated that the IGR is necessary for OT and VP gene expression in hypothalamic slices in vitro. The DNA sequences in the IGR responsible for both OT and VP gene expression were located in a 178 bp domain immediately downstream of exon 3 of the VP gene. In addition, another domain in the IGR, 430 bp immediately downstream of exon 3 of the OT gene, contained a positive regulatory element for OT gene expression in the hypothalamus. Alignment of the DNA sequences in the 178 and 430 bp domains reveals four common sequences (motifs) that may be candidates for the putative enhancers in the IGR that regulate OT and VP gene hypothalamic-specific expression.

Transgenesis and neuroendocrine physiology: a transgenic rat model expressing growth hormone in vasopressin neurones

Human growth hormone (hGH) and bovine neurophysin (bNP) DNA reporter fragments were inserted into the rat vasopressin (VP) and oxytocin (OT) genes in a 44 kb cosmid construct used to generate two lines of transgenic rats, termed JP17 and JP59. Both lines showed specific hGH expression in magnocellular VP cells in the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON). hGH was also expressed in parvocellular neurones in suprachiasmatic nuclei (SCN), medial amygdala and habenular nuclei in JP17 rats; the rat OT-bNP (rOT-bNP) transgene was not expressed in either line. Immunohistochemistry and radioimmunoassay showed hGH protein in the hypothalamus from where it was transported in varicose fibres via the median eminence to the posterior pituitary gland. Immunogold electron microscopy showed hGH co-stored with VP-NP in the same granules. The VP-hGH transgene did not affect water balance, VP storage or release in vivo. Drinking 2 % saline for 72 h increased hypothalamic transgene hGH mRNA expression, and depleted posterior pituitary hGH and VP stores in parallel. In anaesthetised, water-loaded JP17 rats, hGH was released with VP in response to an acute hypovolumic stimulus (sodium nitrosopentacyano, 400 microg I.V.). JP17 rats had a reduced growth rate, lower anterior pituitary rGH contents, and a reduced amplitude of endogenous pulsatile rGH secretion assessed by automated blood microsampling in conscious rats, consistent with a short-loop feedback of the VP-hGH on the endogenous GH axis. This transgenic rat model enables us to study physiological regulation of hypothalamic transgene protein production, transport and secretion, as well as its effects on other neuroendocrine systems in vivo.

The magnocellular neuronal phenotype: cell-specific gene expression in the hypothalamo-neurohypophysial system

The magnocellular oxytocin (OT) and vasopressin (VP) neurons of the hypothalamo-neurohypophysial system are exceptional cell biological models to study mechanisms of cell-specific gene expression and neurosecretion of neuropeptides in the central nervous system. Single cell differential gene expression experiments have further defined these phenotypes by identifying novel and distinct regulatory molecules in these neurons. Transgenic mouse studies have led to the intergenic region (IGR) hypothesis, which states that the DNA sequences between the OT- and VP-genes contain critical enhancer sites for their cell-specific expression. The recent cloning and sequencing of the human IGR, and its comparison with the mouse IGR sequence has identified conserved sequences as putative, cell-specific enhancer sites which are now being evaluated by biolistic transfections of organotypic hypothalamic cultures. With these data, it is possible to target the gene expression of specific molecules to magnocellular neurons both in vivo and in vitro, in order to perturb and/or visualize neurosecretory and other processes.

Status and opportunities for genomics research with rainbow trout

The rainbow trout (Oncorhynchus mykiss) is one of the most widely studied of model fish species. Extensive basic biological information has been collected for this species, which because of their large size relative to other model fish species are particularly suitable for studies requiring ample quantities of specific cells and tissue types. Rainbow trout have been widely utilized for research in carcinogenesis, toxicology, comparative immunology, disease ecology, physiology and nutrition. They are distinctive in having evolved from a relatively recent tetraploid event, resulting in a high incidence of duplicated genes. Natural populations are available and have been well characterized for chromosomal, protein, molecular and quantitative genetic variation. Their ease of culture, and experimental and aquacultural significance has led to the development of clonal lines and the widespread application of transgenic technology to this species. Numerous microsatellites have been isolated and two relatively detailed genetic maps have been developed. Extensive sequencing of expressed sequence tags has begun and four BAC libraries have been developed. The development and analysis of additional genomic sequence data will provide distinctive opportunities to address problems in areas such as evolution of the immune system and duplicate genes.

Targeting of green fluorescent protein to secretory granules in oxytocin magnocellular neurons and its secretion from neurohypophysial nerve terminals in transgenic mice

Oxytocin (OT) is a hypothalamic nonapeptide that is synthesized as part of a larger precursor protein that also contains an approximately 10-kDa protein called neurophysin at its C-terminus. This precursor protein is trafficked through the regulated secretory pathway into secretory granules and then axonally transported to and secreted from nerve terminals in the neural lobe of the pituitary. In this paper, we show that the AI-03 transgene that contains enhanced green fluorescent protein (EGFP) fused to the end of the neurophysin at the C-terminus of the OT pre-prohormone, is expressed selectively in OT-magnocellular neurons and is trafficked to secretory granules in transgenic mice. The EGFP-containing secretory granules are then transported to OT-neurosecretory terminals in the neurohypophysis, where the EGFP fluorescence undergoes depolarization-induced calcium-dependent secretion. The endogenous fluorescence in the neural lobes is sufficiently intense to image secretory events in individual OT nerve terminals (neurosecretosomes) isolated from the posterior pituitaries in these transgenic mice.

Vasopressin gene expression: experimental models and strategies

The intergenic region (IGR) separating the genes for vasopressin (VP) and oxytocin (OT) has been shown to be critical for the cell-specific expression of these peptide genes in hypothalamic neurons. To date, the most relevant information about the putative cis-elements in the IGR that might determine cell-specific gene expression has come from studies in transgenic models. As a first step toward increasing the efficiency of the IGR sequence deletion studies in transgenic animals, a comparative genomics approach comparing the IGR sequence in humans versus mice was used to identify conserved sequences that might be candidate regulatory elements. The nucleotide sequence of the IGR between the human VP and OT genes was determined and compared to the mouse IGR, and 26 conserved sequences in three distinct clusters were found. These conserved sequences and motifs may be important for the cell-specific expression of the VP and OT genes. However, before further significant progress can be made, a "high-throughput" method for the analysis of deletion constructs in relevant cell types in vitro is needed. It is proposed here that organotypic culture models combined with the use of particle-mediated gene transfer methods may provide an effective, general strategy for the study of cell-specific expression in the central nervous system.

Cell-specific expression and subcellular localization of neurophysin-CAT-fusion proteins expressed from oxytocin and vasopressin gene promoter-driven constructs in transgenic mice

The cell-specific expression of both the oxytocin (OT) and vasopressin (VP) genes in magnocellular neurons (MCNs) of the hypothalamus has been proposed to be under the control of cis-elements in an intergenic region downstream of the VP gene. We examined this hypothesis using transgenic mice containing mouse genomic DNA-derived constructs linked to chloramphenicol acetyltransferase (CAT) reporters. VP gene expression was studied using constructs containing 3.8 kbp of the 5' flanking region and all the exons and introns in the mouse VP gene, which was fused at the end of exon 3 to a CAT reporter. The two VP-transgene constructs differed by the lengths of their VP gene 3' flanking regions (2.1 versus 3.6 kbp). A similar construct for the oxytocin CAT transgene was used which contained the full-length (3.6 kbp) downstream intergenic region between the mouse genes. All three transgenic constructs produced cell-specific expression of the CAT-reporter in the magnocellular neurons as determined by CAT-immunoreactivity. Oxytocin transgene expression was restricted to OT cells in two founders, and the two VP transgenes to VP cells in five founders. Electron microscopic immunocytochemistry showed that the CAT fusion proteins produced from the OT- and VP-transgenes were efficiently trafficked through the regulated secretory pathways in their respective magnocellular neurons, packaged into large dense core vesicles, and transported to nerve terminals in the posterior pituitary.

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.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

Transgenic animal models for neuropharmacology

The establishment of novel animal models using gene targeting and transgenic technology has opened a new area of neuropharmacological research. For the first time, it became possible to alter the expression of a gene in a specific cell type of an intact animal by either overexpression, inhibition or ablation. This review describes the technology and lists the relevant tools, such as reporter genes, suicide genes, immortalizing genes, and promoters, necessary for the targeted expression of these and other genes in specific cells of the central nervous system. In addition, the problem is discussed that the mouse is the species in which this technology is by far the most developed, while the rat has been used as the model species for neuropharmacology during the last century.

Transgenic expression of green fluorescent protein in mouse oxytocin neurones

Routine targeting of neurones for expression of exogenous genes would facilitate our ability to manipulate their internal milieu or functions, providing insight into physiology of neurones. The magnocellular neurones of the paraventricular and supraoptic nuclei of the hypothalamus have been the objects of limited success by this approach. Here we report on the placement of the enhanced green fluorescent protein (eGFP) coding sequence at various locations within an oxytocin transgene. Placement within the first exon yielded little to no expression, whereas placement in the third exon (as an in-frame fusion with the carboxyl terminus of the oxytocin preprohormone) resulted in cell-specific expression of eGFP in oxytocin neurones. Furthermore, placement of the eGFP sequence downstream of a picornavirus internal ribosomal entry site (IRES), also in the third exon, allowed expression of the eGFP as a separate protein. Other coding sequences should now be amenable to expression within oxytocin neurones to study their physiology.

Molecular diversity in neurosecretion: reflections on the hypothalamo-neurohypophysial system

1. The diversity of molecules involved in various aspects of neurosecretion, such as proprotein processing, axonal transport of large dense core vesicles (LDCVs), and regulated secretion, is discussed in the context of the hypothalamo-neurohypophysial system (HNS). 2. Recent studies have uncovered a family of at least seven processing enzymes known as proprotein convertases (PCs) which are involved in proteolytically cleaving protein precursors at paired basic amino acid motifs to yield biologically active peptides. Three of these, PC1(3), 2, and 5, are found in neurons and are involved in producing regulated secretory peptide products. 3. The axonal transport of LDCVs occurs on microtubule tracks by still unknown mechanisms. There are over 11 distinct kinesin-related molecules that have now been identified as possible microtubule motor candidates. 4. Calcium channels in the nervous system are known to be derived from at least five alpha-subunit and four beta-subunit genes with multiple alternatively spliced isoforms in each case. These could account, in part, for the varied calcium currents found in the HNS. 5. The large number of proteins and isoforms now demonstrated to be involved in regulated secretion are discussed, with a focus on LDCV compositions and the synaptotagmin gene family.