Transgenic expression of green fluorescent protein in mouse oxytocin neurones

Central and peripheral release of oxytocin: Relevance of neuroendocrine and neurotransmitter actions for physiology and behavior

The hypothalamic neuropeptide oxytocin (OT) is critically involved in the modulation of socio-emotional behavior, sexual competence, and pain perception and anticipation. While intracellular signaling of OT and its receptor (OTR), as well as the functional connectivity of hypothalamic and extra-hypothalamic OT projections, have been recently explored, it remains elusive how one single molecule has pleotropic effects from cell proliferation all the way to modulation of complex cognitive processes. Moreover, there are astonishing species-dependent differences in the way OT regulates various sensory modalities such as touch, olfaction, and vision, which can be explained by differences in OTR expression in brain regions processing sensory information. Recent research highlights a small subpopulation of OT-synthesizing cells, namely, parvocellular cells, which merely constitute 1% of the total number of OT cells but act as "master cells' that regulate the activity of the entire OT system. In this chapter, we summarize the latest advances in the field of OT research with a particular focus on differences between rodents, monkeys and humans and highlight the main differences between OT and its "sister" peptide arginine-vasopressin, which often exerts opposite effects on physiology and behavior.

Relaxin-3/RXFP3 signalling in mouse hypothalamus: no effect of RXFP3 activation on corticosterone, despite reduced presynaptic excitatory input onto paraventricular CRH neurons in vitro

Relaxin-3/RXFP3 signalling is proposed to be involved in the neuromodulatory control of arousal- and stress-related neural circuits. Furthermore, previous studies in rats have led to the proposal that relaxin-3/RXFP3 signalling is associated with activation of the hypothalamic-pituitary-adrenal axis, but direct evidence for RXFP3-related actions on the activity of hypothalamic corticotropin-releasing hormone (CRH) neurons is lacking. In this study, we investigated characteristics of the relaxin-3/RXFP3 system in mouse hypothalamus. Administration of an RXFP3 agonist (RXFP3-A2) intra-cerebroventricularly or directly into the paraventricular nucleus of hypothalamus (PVN) of C57BL/6J mice did not alter corticosterone levels. Similarly, there were no differences between serum corticosterone levels in Rxfp3 knockout (C57BL/6J^RXFP3TM1) and wild-type mice at baseline and after stress, despite detection of the predicted stress-induced increases in serum corticosterone. We examined the nature of the relaxin-3 innervation of PVN in wild-type mice and in Crh-IRES-Cre;Ai14 mice that co-express the tdTomato fluorophore in CRH neurons, identifying abundant relaxin-3 fibres in the peri-PVN region, but only sparse fibres associated with densely packed CRH neurons. In whole-cell voltage-clamp recordings of tdTomato-positive CRH neurons in these mice, we observed a reduction in sEPSC frequency following local application of RXFP3-A2, consistent with an activation of RXFP3 on presynaptic glutamatergic afferents in the PVN region. These studies clarify the relationship between relaxin-3/RXFP3 inputs and CRH neurons in mouse PVN, with implications for the interpretation of current and previous in vivo studies and future investigations of this stress-related signalling network in normal and transgenic mice, under normal and pathological conditions.

60 YEARS OF NEUROENDOCRINOLOGY: The structure of the neuroendocrine hypothalamus: the neuroanatomical legacy of Geoffrey Harris

In November 1955, Geoffrey Harris published a paper based on the Christian A Herter Lecture he had given earlier that year at Johns Hopkins University in Baltimore, MD, USA. The paper reviewed the contemporary research that was starting to explain how the hypothalamus controlled the pituitary gland. In the process of doing so, Harris introduced a set of properties that helped define the neuroendocrine hypothalamus. They included: i) three criteria that putative releasing factors for adenohypophysial hormones would have to fulfill; ii) an analogy between the representation of body parts in the sensory and motor cortices and the spatial localization of neuroendocrine function in the hypothalamus; and iii) the idea that neuroendocrine neurons are motor neurons and the pituitary stalk functions as a Sherringtonian final common pathway through which the impact of sensory and emotional events on neuroendocrine neurons must pass in order to control pituitary hormone release. Were these properties a sign that the major neuroscientific discoveries that were being made in the early 1950s were beginning to influence neuroendocrinology? This Thematic Review discusses two main points: the context and significance of Harris's Herter Lecture for how our understanding of neuroendocrine anatomy (particularly as it relates to the control of the adenohypophysis) has developed since 1955; and, within this framework, how novel and powerful techniques are currently taking our understanding of the structure of the neuroendocrine hypothalamus to new levels.

Fluorescent visualization of oxytocin in the hypothalamo-neurohypophysial system

Oxytocin (OXT) is well known for its ability to the milk ejection reflex and uterine contraction. It is also involved in several other behaviors, such as anti-nociception, anxiety, feeding, social recognition, and stress responses. OXT is synthesized in the magnocellular neurosecretory cells (MNCs) in the hypothalamic paraventricular (PVN) and the supraoptic nuclei (SON) that terminate their axons in the posterior pituitary (PP). We generated transgenic rats that express the OXT and fluorescent protein fusion gene in order to visualize OXT in the hypothalamo-neurohypophysial system (HNS). In these transgenic rats, fluorescent proteins were observed in the MNCs and axon terminals in the PP. This transgenic rat is a new tool to study the physiological role of OXT in the HNS.

[The use of genetic modification techniques in the fluorescent visualization of oxytocin neurons]

Oxytocin, a neurohypophyseal hormone, is synthesized in the magnocellular neurosecretory cells located in the paraventricular and supraoptic nuclei of the hypothalamus, and is secreted into the systemic blood flow from the axon terminals. It is well known that plasma oxytocin is involved in contraction of the uterus during parturition and milk ejection reflex during lactation. It has recently come to the attention of researchers that oxytocin receptors are abundant in the brain and oxytocin is involved in higher brain functions such as bonding between parent and child and trust. Since it was difficult to identify neurohypophyseal hormones, oxytocin- and vasopressin-producing neurons in a living cell, we tried to generate transgenic animals that express fluorescent proteins as a tag protein to visualize neurohypophyseal hormones. In this paper we review the use of genetic modification techniques in the fluorescent visualization of oxytocin neurons and its application.

Deconstruction of a neural circuit for hunger

Hunger is a complex behavioural state that elicits intense food seeking and consumption. These behaviours are rapidly recapitulated by activation of starvation-sensitive AGRP neurons, which present an entry point for reverse-engineering neural circuits for hunger. We mapped synaptic interactions of AGRP neurons with multiple cell populations and probed the contribution of these distinct circuits to feeding behaviour using optogenetic and pharmacogenetic techniques. An inhibitory circuit with paraventricular hypothalamus (PVH) neurons substantially accounted for acute AGRP neuron-evoked eating, whereas two other prominent circuits were insufficient. Within the PVH, we found that AGRP neurons target and inhibit oxytocin neurons, a small population that is selectively lost in Prader-Willi syndrome, a condition involving insatiable hunger. By developing strategies for evaluating molecularly-defined circuits, we show that AGRP neuron suppression of oxytocin neurons is critical for evoked feeding. These experiments reveal a new neural circuit that regulates hunger state and pathways associated with overeating disorders.

Cell-type specific expression of oxytocin and vasopressin genes: an experimental odyssey

The supraoptic nucleus (SON) is a particularly good model for the study of cell-type specific gene expression because it contains two distinct neuronal phenotypes, the oxytocin (OT) and vasopressin (AVP) synthesising magnocellular neurones (MCNs). The MCNs are found in approximately equal numbers and selectively express either the OT or the AVP gene in approximately 97% of the MCN population in the SON. An unresolved issue has been to determine what mechanisms are responsible for the highly selective regulation of the cell-type specific expression of OT and AVP genes in the MCNs. Previous attempts to address this question have used various bioinformatic and molecular approaches, which included using heterologous cell lines to study the putative cis-elements in the OT and AVP genes, and the use of OT and/or AVP transgenes in transgenic rodents. The data from all of the above studies identified a region < 0.6 kbp upstream of OT exon I and approximately 3 kb upstream of AVP exon I as being sufficient to produce cell-specific expression of the OT and AVP genes, respectively, although they failed to identify the specific cis-domains responsible for the MCN-specific gene expression. An alternative experimental approach to perform promoter deletion analysis in vivo (i.e. to use stereotaxic viral vector gene transfer into the SON to further dissect the cis-elements in the OT and AVP genes) will be described here. This in vivo method uses adeno-associated viral (AAV) vectors expressing OT-promoter deletion constructs and utilises the enhanced green fluorescent protein (EGFP) as the reporter. The AAV constructs are stereotaxically injected into the rat brain above the SON and, 2 weeks post injection, the rats are sacrificed and assayed for EGFP expression. Using this method, it has been possible to identify specific regions upstream of the transcription start site in the OT and AVP gene promoters that are responsible for conferring the cell-type specificity of the OT and AVP gene expression in the SON.

Highly visible expression of an oxytocin-monomeric red fluorescent protein 1 fusion gene in the hypothalamus and posterior pituitary of transgenic rats

We have generated rats bearing an oxytocin (OXT)-monomeric red fluorescent protein 1 (mRFP1) fusion transgene. The mRFP1 fluorescence was highly visible in ventral part of the supraoptic nucleus (SON) and the posterior pituitary in a whole mount. mRFP1 fluorescence in hypothalamic sections was also observed in the SON, the paraventricular nucleus (PVN), and the internal layer of the median eminence. Salt loading for 5 d caused a marked increase in mRFP1 fluorescence in the SON, the PVN, the median eminence, and the posterior pituitary. In situ hybridization histochemistry revealed that the expression of the mRNA encoding the OXT-mRFP1 fusion gene was observed in the SON and the PVN of euhydrated rats and increased dramatically after chronic salt loading. The expression of the endogenous OXT and the arginine vasopressin (AVP) genes were significantly increased in the SON and the PVN after chronic salt loading in both nontransgenic and transgenic rats. These responses were not different between male and female rats. Compared with nontransgenic rats, euhydrated and salt-loaded male and female transgenic rats showed no significant differences in plasma osmolality, sodium concentration, OXT, and AVP levels. Finally, we succeeded in generating a double-transgenic rat that expresses both the OXT-mRFP1 fusion gene and the AVP-enhanced green fluorescent protein fusion gene. Our new transgenic rats are valuable new tools to study the physiology of the hypothalamo-neurohypophysial system.

REVIEW: Oxytocin: Crossing the bridge between basic science and pharmacotherapy

Is oxytocin the hormone of happiness? Probably not. However, this small nine amino acid peptide is involved in a wide variety of physiological and pathological functions such as sexual activity, penile erection, ejaculation, pregnancy, uterus contraction, milk ejection, maternal behavior, osteoporosis, diabetes, cancer, social bonding, and stress, which makes oxytocin and its receptor potential candidates as targets for drug therapy. In this review, we address the issues of drug design and specificity and focus our discussion on recent findings on oxytocin and its heterotrimeric G protein‐coupled receptor OTR. In this regard, we will highlight the following topics: (i) the role of oxytocin in behavior and affectivity, (ii) the relationship between oxytocin and stress with emphasis on the hypothalamo–pituitary–adrenal axis, (iii) the involvement of oxytocin in pain regulation and nociception, (iv) the specific action mechanisms of oxytocin on intracellular Ca²⁺ in the hypothalamo neurohypophysial system (HNS) cell bodies, (v) newly generated transgenic rats tagged by a visible fluorescent protein to study the physiology of vasopressin and oxytocin, and (vi) the action of the neurohypophysial hormone outside the central nervous system, including the myometrium, heart and peripheral nervous system. As a short nine amino acid peptide, closely related to its partner peptide vasopressin, oxytocin appears to be ideal for the design of agonists and antagonists of its receptor. In addition, not only the hormone itself and its binding to OTR, but also its synthesis, storage and release can be endogenously and exogenously regulated to counteract pathophysiological states. Understanding the fundamental physiopharmacology of the effects of oxytocin is an important and necessary approach for developing a potential pharmacotherapy.

Specific expression of an oxytocin-enhanced cyan fluorescent protein fusion transgene in the rat hypothalamus and posterior pituitary

We have generated rats bearing an oxytocin (OXT)-enhanced cyan fluorescent protein (eCFP) fusion transgene designed from a murine construct previously shown to be faithfully expressed in transgenic mice. In situ hybridisation histochemistry revealed that the Oxt-eCfp fusion gene was expressed in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) in these rats. The fluorescence emanating from eCFP was observed only in the SON, the PVN, the internal layer of the median eminence and the posterior pituitary (PP). In in vitro preparations, freshly dissociated cells from the SON and axon terminals showed clear eCFP fluorescence. Immunohistochemistry for OXT and arginine vasopressin (AVP) revealed that the eCFP fluorescence co-localises with OXT immunofluorescence, but not with AVP immunofluorescence in the SON and the PVN. Although the expression levels of the Oxt-eCfp fusion gene in the SON and the PVN showed a wide range of variations in transgenic rats, eCFP fluorescence was markedly increased in the SON and the PVN, but decreased in the PP after chronic salt loading. The expression of the Oxt gene was significantly increased in the SON and the PVN after chronic salt loading in both non-transgenic and transgenic rats. Compared with wild-type animals, euhydrated and salt-loaded male and female transgenic rats showed no significant differences in plasma osmolality, sodium concentration and OXT and AVP levels, suggesting that the fusion gene expression did not disturb any physiological processes. These results suggest that our new transgenic rats are a valuable new tool to identify OXT-producing neurones and their terminals.

Robust up-regulation of nuclear red fluorescent-tagged fos marks neuronal activation in green fluorescent vasopressin neurons after osmotic stimulation in a double-transgenic rat

The up-regulation in the expression of mRNA or protein encoded by the c-fos gene is widely used as a marker of neuronal activation elicited by various stimuli. To facilitate the detection of activated neurons, we generated transgenic rats expressing a fusion gene consisting of c-fos coding sequences in frame with monomeric red fluorescent protein 1 (mRFP1) under the control of c-fos gene regulatory sequences (c-fos-mRFP1 rats). In c-fos-mRFP1 transgenic rats, 90 min after hypertonic saline ip administration, nuclear mRFP1 fluorescence was observed abundantly in brain regions known to be osmosensitive, namely the median preoptic nucleus, organum vasculosum lamina terminalis, supraoptic nucleus, paraventricular nucleus, and subfornical organ. Immunohistochemistry for Fos protein confirmed that the distribution of Fos-like immunoreactivity in nontransgenic rats was similar to those of mRFP1 fluorescence after ip administration of hypertonic saline in the transgenic rats. Several double-transgenic rats were obtained from matings between transgenic rats expressing an arginine vasopressin-enhanced green fluorescent protein fusion gene (AVP-eGFP rats) and c-fos-mRFP1 rats. In these double-transgenic rats, almost all eGFP neurons in the supraoptic nucleus and PVN expressed nuclear mRFP1 fluorescence 90 min after hypertonic saline administration. The c-fos-mRFP1 rats are a powerful tool that enables the facile identification of activated neurons in the nervous system. Furthermore, when combined with transgenes expressing another fluorophore under the control of cell-specific regulatory sequences, activation of specific neuronal cell types in response to physiological cues can be readily detected.

Impaired episodic LH secretion in female mice with GFP in GnRH neurons

The ability to assess the activity of gonadotropin-releasing hormone (GnRH) neurons has been greatly enhanced by transgenic animal models with targeted expression of green fluorescent protein (GFP). However, it has yet to be demonstrated that the GnRH system continues to exhibit a full range of normal physiological functions in the presence of such genetic manipulation. Accordingly, we have used repetitive blood sampling via indwelling venous catheters to define LH secretory patterns in normal and transgenic mice. Transgenic females proved to be reproductively competent as defined by fecundity, appropriate cyclic changes in vaginal cytology in intact adult females, and spontaneous LH surges as well as surges in response to steroid or mating stimuli. The expression of c-fos following such steroid treatment and mating in ovariectomized transgenics was similar to the expression previously reported in nontransgenic mice. Likewise, the percentage of retrogradely labeled GnRH neurons was similar to that reported in nontransgenic mice. However, episodic LH secretion, an index of GnRH pulse generator activity, was dramatically compromised in ovariectomized female transgenics compared with C57BL6 controls of both sexes and castrated transgenic males. Taken together, these findings suggest that the GnRH pulse generator is selectively impaired in ovariectomized females in which GnRH neurons express GFP.

Specific expression of optically active reporter gene in arginine vasopressin-secreting neurosecretory cells in the hypothalamic-neurohypophyseal system

The anti-diuretic hormone arginine vasopressin (AVP) is synthesised in the magnocellular neurosecretory cells (MNCs) in the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) of the hypothalamus. AVP-containing MNCs that project their axon terminals to the posterior pituitary can be identified using immunohistochemical techniques with specific antibodies recognising AVP and neurophysin II, and by virtue of their electrophysiological properties. Recently, we generated transgenic rats expressing an AVP-enhanced green fluorescent protein (eGFP) fusion gene in AVP-containing MNCs. In this transgenic rat, eGFP mRNA was observed in the PVN and the SON, and eGFP fluorescence was seen in the PVN and the SON, and also in the posterior pituitary, indicating transport of transgene protein down MNC axons to storage in nerve terminals. The expression of the AVP-eGFP transgene and eGFP fluorescence in the PVN and the SON was markedly increased after dehydration and chronic salt-loading. On the other hand, AVP-containing parvocellular neurosecretory cells in the PVN that are involved in the activation of the hypothalamic-pituitary adrenal axis exhibit robust AVP-eGFP fluorescence after bilateral adrenalectomy and intraperitoneal administration of lipopolysaccharide. In the median eminence, the internal and external layer showed strong fluorescence for eGFP after osmotic stimuli and stressful conditions, respectively, again indicating appropriate transport of transgene traslation products. Brain slices and acutely-dissociated MNCs and axon terminals also exhibited strong fluorescence, as observed under fluorescence microscopy. The AVP-eGFP transgenic animals are thus unique and provide a useful tool to study AVP-secreting cells in vivo for electrophysiology, imaging analysis such as intracellular Ca(2+) imaging, organ culture and in vivo monitoring of dynamic change in AVP secretion.

EGFP-tagged vasopressin precursor protein sorting into large dense core vesicles and secretion from PC12 cells

1. Hypothalamic magnocellular neurons synthesize, store, and secrete large quantities of the neuropeptides, vasopressin (VP) and oxytocin (OT), which are synthesized as protein precursors also containing proteins called neurophysins. These protein precursors are sorted through the regulated secretory pathway (RSP), packaged into large dense core vesicles LDCVs, and their peptide products are secreted from nerve terminals in the posterior pituitary. 2. It has been hypothesized that this efficient packaging is dependent on the interaction of the peptide with neurophysin in a complex that forms the granule core. To test this, PC12 cells were transfected with vasopressin precursor DNA constructs that either contained or deleted the neurophysin moiety and tagged with enhanced green fluorescent protein (EGFP) as reporters. The intracellular routing and secretion of the EGFP-tagged VP precursor proteins were studied by in differentiated PC12 cells by fluorescence microscopy, electron microscopic immunocytochemistry, and fluorescent imaging techniques. 3. The data showed that only when the neurophysin was present in the VP precursor construct did the fluorescent fusion protein become routed to the RSP and get efficiently packaged into LDCVs and secreted. These data are consistent with the view that routing of the precursor to LDCVs requires the amino acids that encode the intravesicular chaperone, neurophysin.

Analysis of green fluorescent protein expression in transgenic rats for tracking transplanted neural stem/progenitor cells

Green fluorescent protein (GFP) expression was evaluated in tissues of different transgenic rodents--Sprague-Dawley (SD) rat strain [SD-Tg(GFP)Bal], W rat strain [Wistar-TgN(CAG-GFP)184ys], and M mouse strain [Tg(GFPU)5Nagy/J]--by direct fluorescence of native GFP expression and by immunohistochemistry. The constitutively expressing GFP transgenic strains showed tissue-specific differences in GFP expression, and GFP immunohistochemistry amplified the fluorescent signal. The fluorescence of stem/progenitor cells cultured as neurospheres from the ependymal region of the adult spinal cord from the GFP SD and W rat strains was assessed in vitro. After transplantation of the cells into wild-type spinal cord, the ability to track the grafted cells was evaluated in vivo. Cultured stem/progenitor cells from the SD strain required GFP immunostaining to be visualized. Likewise, after transplantation of SD cells into the spinal cord, immunohistochemical amplification of the GFP signal was required for detection. In contrast, GFP expression of stem/progenitor cells generated from the W strain was readily detected by direct fluorescence both in vitro and in vivo without the need for immunohistochemical amplification. The cultured stem/progenitor cells transplanted into the spinal cord survived for at least 49 days after transplantation, and continued to express GFP, demonstrating stable expression of the GFP transgene in vivo.

Transgenic expression of enhanced green fluorescent protein enables direct visualization for physiological studies of vasopressin neurons and isolated nerve terminals of the rat

We have generated transgenic rats expressing an arginine vasopressin (AVP)-enhanced green fluorescent protein (eGFP) fusion gene. The expression of the eGFP gene and strong fluorescence were observed in the supraoptic nucleus (SON), the paraventricular nucleus (PVN), and the suprachiasmatic nucleus (SCN) in transgenic rats. The hypothalamo-neurohypophyseal tract, isolated SON neurons, and isolated axon terminals in the neurohypophysis also showed robust eGFP fluorescence. Water deprivation for 2 d increased the fluorescence of the eGFP in the SON and the PVN but not the SCN. The whole-cell patch-clamp technique was then used to record the electrical activities specifically identifying eGFP-expressing SON, PVN, and SCN AVP neurons in in vitro brain slice preparations. The AVP-eGFP transgenic rats are a unique new tool with which to study the physiological role of AVP-secreting neurons in the central nervous system and the dynamics of the regulation of AVP secretion in the living neurons and their axon terminals.

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.

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.

Expression of conditional cre recombinase in epithelial tissues of transgenic mice

SUMMARY

Keratin 18 (K18) expression is a defining characteristic of internal epithelial cells of mammals. Here, we used the K18 gene and an internal ribosome entry site (IRES) to express green fluorescent protein, human placental alkaline phosphatase, and a modified Cre recombinase in an epithelial specific pattern in transgenic mice. The K18-driven alkaline phosphatase was expressed in liver, kidney, uterine endometrium, and other internal epithelia. The enzymatic activity of the Cre recombinase-mutant estrogen receptor fusion protein was dependent on tamoxifen administration and resulted in a mosaic pattern in internal epithelia, including bladder, uterus, liver, and kidney. This conditional Cre activity in internal epithelial organs should be valuable for strategies utilizing Cre for activation of gene expression. This study demonstrates that the tissue-specific, position-independent transcriptional activity of the K18 gene is not compromised by the use of an IRES element for the expression of a second protein from a bicistronic mRNA.

In vivo gene transfer studies on the regulation and function of the vasopressin and oxytocin genes

Novel genes can be introduced into the germline of rats and mice by microinjecting fertilized one-cell eggs with fragments of cloned DNA. A gene sequence can thus be studied within the physiological integrity of the resulting transgenic animals, without any prior knowledge of its regulation and function. These technologies have been used to elucidate the mechanisms by which the expression of the two genes in the locus that codes for the neuropeptides vasopressin and oxytocin is confined to, and regulated physiologically within, specific groups of neurones in the hypothalamus. A number of groups have described transgenes, derived from racine, murine and bovine sources, in both rat and mouse hosts, that mimic the appropriate expression of the endogenous vasopressin and genes in magnocellular neurones (MCNs) of the supraoptic and paraventricular nuclei. However, despite considerable effort, a full description of the cis-acting sequences mediating the regulation of the vasopressin-oxytocin locus remains elusive. Two general conclusions have nonetheless been reached. First, that the proximal promoters of both genes are unable to confer any cell-specific regulatory controls. Second, that sequences downstream of the promoter, within the structural gene and/or the intergenic region that separates the two genes, are crucial for appropriate expression. Despite these limitations, sufficient knowledge has been garnered to specifically direct the expression of reporter genes to vasopressin and oxytocin MCNs. Further, it has been shown that reporter proteins can be directed to the regulated secretory pathway, from where they are subject to appropriate physiological release. The use of MCN expression vectors will thus enable the study of the physiology of these neurones through the targeted expression of biologically active molecules. However, the germline transgenic approach has a number of limitations involving the interpretation of phenotypes, as well as the large cost, labour and time demands. High-throughput somatic gene transfer techniques, principally involving the stereotaxic injection of hypothalamic neuronal groups with replication-deficient adenoviral vectors, are now being developed that obviate these difficulties, and which enable the robust, long-lasting expression of biologically active proteins in vasopressin and oxytocin MCNs.

Expression of the gene of interest fused to the EGFP-expressing gene in transgenic mice derived from selected transgenic embryos

The present paper describes the expression of a target fusion gene, WAP/hGH fused to the EGFP-expressing gene in transgenic mice derived from the transfer of transgenic embryos selected because of their expression of enhanced green fluorescent protein (EGFP). The 6.7-kb fusion gene was microinjected as a single cassette gene construct into the pronuclei of mouse zygotes. The surviving embryos were cultured and were classified according to the EGFP expression patterns at the morula or blastocyst stage. After the transfer of embryos with uniform-expression or mosaic-expression of EGFP, transgenesis occurred in 85.7% to 86% or 44.1% to 44% of the pups, respectively. No transgenic pups were derived from EGFP negative embryos. In the transgenic females, EGFP was ubiquitously expressed under the control of the CAG promoter, and hGH was expressed under the control of the WAP promoter in an appropriate fashion: hGH was secreted into the milk of lactating transgenic females. The presence or absence of the expression of EGFP coincided with that of the hGH gene in the transgenic mice. The present cassette gene construct is a useful example for circumventing the routine analyses of DNA and RNA required for the generation and maintenance of transgenic lines.

Gene expression studies using microarrays: principles, problems, and prospects

A number of mammalian genomes having been sequenced, an important next step is to catalog the expression patterns of all transcription units in health and disease by use of microarrays. Such discovery programs are crucial to our understanding of the gene networks that control developmental, physiological, and pathological processes. However, despite the excitement, the full promise of microarray technology has yet to be realized, as the superficial simplicity of the concept belies considerable problems. Microarray technology is very new; methodologies are still evolving, common standards have yet to be established, and many problems with experimental design and variability have still to be fully understood and overcome. This review will describe the time course of a microarray experiment-RNA isolation from sample, target preparation, hybridization to the microarray probe, data capture, and bioinformatic analysis. For each stage, the advantages and disadvantages of competing techniques are compared, and inherent sources of error are identified and discussed.

Cloning of rat ARHGAP4/C1, a RhoGAP family member expressed in the nervous system that colocalizes with the Golgi complex and microtubules

The Rho GTPase family of intracellular molecular switches control multiple cellular functions via the regulation of the actin cytoskeleton. Increasing evidence implicates a critical involvement of these molecules in the nervous system, particularly during neuronal migration and polarity, axon and growth cone guidance, dendritic arborization and synaptic formation. However, the molecules regulating Rho GTPase activities in the nervous system are less known. Here, we present the cloning of rat ARHGAP4, a member of the Rho GTPase activating protein family, and also demonstrate its close linkage to the vasopressin 2 receptor gene. In vitro, recombinant ARHGAP4 stimulated the GTPase activity of three members of Rho GTPases, Rac1, Cdc42 and RhoA. ARHGAP4 mRNA expression was observed in multiple tissues with marked expression throughout the developing and adult nervous systems. On closer analysis of protein levels, ARHGAP4 was significantly restricted to specific regions in the nervous system. These included the stratum lucidem in the CA3 area of the hippocampus, neuronal fibers in the ventral region of the brainstem and striatum, and in the cerebellar granule cells. Subcellularly, endogenous ARHGAP4 expression localized to the Golgi complex and could redistribute to the microtubules, for example during mitosis. In addition, distinct protein expression was observed in the tips of differentiating neurites of PC12 cells. Collectively, these results demonstrate that ARHGAP4 is more widely expressed than previously thought but potentially possesses specialized activity in regulating members of the Rho GTPase family in specific cellular compartments of the nervous system.

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.

Genetic engineering of neural function in transgenic rodents: towards a comprehensive strategy?

As mammalian genome projects move towards completion, the attention of molecular neuroscientists is currently moving away from gene identification towards both cell-specific gene expression patterns (neuronal transcriptions) and protein expression/interactions (neuronal proteomics). In the long term, attention will increasingly be directed towards experimental interventions which are able to question neuronal function in a sophisticated manner that is cognisant of both transcriptomic and proteomic organization. Central to this effort will be the application of a new generation of transgenic approaches which are now evolving towards an appropriate level of molecular, temporal and spatial resolution. In this review, we summarize recent developments in transgenesis, and show how they have been applied in the principal model species for neuroscience, namely rats and mice. Current concepts of transgene design are also considered together with an overview of new genetically-encoded tools including both cellular indicators such as fluorescent activity reporters, and cellular regulators such as dominant negative signalling factors. Application of these tools in a whole animal context can be used to question both basic concepts of brain function, and also current concepts of underlying dysfuction in neurological diseases.

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.

Using reporter genes to label selected neuronal populations in transgenic mice for gene promoter, anatomical, and physiological studies

This review summarizes recent work on the use of reporter genes to label selected neuronal populations in transgenic mice, with particular emphasis on gonadotropin-releasing hormone (GnRH) neurons. Reporter genes discussed are the lacZ, green fluorescent protein (GFP), luc, and bla genes, which encode the reporter proteins beta-galactosidase, GFP, luciferase, and beta-lactamase, respectively. Targeted transgenic expression of these reporter proteins is obtained by fusing the corresponding reporter gene, with or without a subcellular localization signal, to a cell type- or brain region-specific gene promoter. Mice carrying GnRH promoter-driven reporter genes have proven useful for revealing the promoter elements required for cell type-specific expression of GnRH, the full anatomical profile of the GnRH neuronal network, and its electrophysiological activity, suggesting that similar approaches will assist in elucidating the properties of other neuronal populations as well.

Species- and tissue-specific physiological regulation of vasopressin mRNA poly(A) tail length

Transgenic experiments can be used to test the extent to which genes from different species can be swapped around, but still retain function, and be appropriately regulated. A vector has been developed that directs the expression of foreign genes to specific groups of vasopressin (VP) hypothalamic neurons in transgenic rats. Using this vector, we have expressed the bovine VP (bVP) RNA in the rat brain. In contrast to the situation in a mouse host, but like its endogenous rat counterpart, the mRNA encoded by the bVP transgene is subject to posttranscriptional physiological regulation in the hypothalamus; its poly(A) tail dramatically lengthens as a consequence of 3 days of dehydration. Transgene expression is also seen in the adrenal cortex, but here, despite a marked increase in transgene RNA levels with dehydration, there is no change in poly(A) tail length. These data suggest that the mouse hypothalamus and the rat adrenal gland do not have the transcript recognition or enzymatic machinery required for the physiologically responsive poly(A) tail length modulation seen in the rat brain.

A secreted fluorescent reporter targeted to pituitary growth hormone cells in transgenic mice

In stable transfection experiments in the GH-producing GC cell line, a construct containing the entire signal peptide and the first 22 residues of human GH linked in frame with enhanced green fluorescent protein (eGFP), produced brightly fluorescent cells with a granular distribution of eGFP. This eGFP reporter was then inserted into a 40-kb cosmid transgene containing the locus control region for the hGH gene and used to generate transgenic mice. Anterior pituitaries from these GH-eGFP transgenic mice showed numerous clusters of strongly fluorescent cells, which were also immunopositive for GH, and which could be isolated and enriched by fluorescence-activated cell sorting. Confocal scanning microscopy of pituitary GH cells from GH-eGFP transgenic mice showed a markedly granular appearance of fluorescence. Immunogold electron microscopy and RIA confirmed that the eGFP product was packaged in the dense cored secretory vesicles of somatotrophs and was secreted in parallel with GH in response to stimulation by GRF. Using eGFP fluorescence, it was possible to identify clusters of GH cells in acute pituitary slices and to observe spontaneous transient rises in their intracellular Ca2+ concentrations after loading with Ca2+ sensitive dyes. This transgenic approach opens the way to direct visualization of spontaneous and secretagogue-induced secretory mechanisms in identified GH cells.