cAMP-dependent protein kinase A mediation of vasopressin gene expression in the hypothalamus of the osmotically challenged rat

Expression of a dominant negative PKA mutation in the kidney elicits a diabetes insipidus phenotype

PKA plays a critical role in water excretion through regulation of the production and action of the antidiuretic hormone arginine vasopressin (AVP). The AVP prohormone is produced in the hypothalamus, where its transcription is regulated by cAMP. Once released into the circulation, AVP stimulates antidiuresis through activation of vasopressin 2 receptors in renal principal cells. Vasopressin 2 receptor activation increases cAMP and activates PKA, which, in turn, phosphorylates aquaporin (AQP)2, triggering apical membrane accumulation, increased collecting duct permeability, and water reabsorption. We used single-minded homolog 1 (Sim1)-Cre recombinase-mediated expression of a dominant negative PKA regulatory subunit (RIαB) to disrupt kinase activity in vivo and assess the role of PKA in fluid homeostasis. RIαB expression gave rise to marked polydipsia and polyuria; however, neither hypothalamic Avp mRNA expression nor urinary AVP levels were attenuated, indicating a primary physiological effect on the kidney. RIαB mice displayed a marked deficit in urinary concentrating ability and greatly reduced levels of AQP2 and phospho-AQP2. Dehydration induced Aqp2 mRNA in the kidney of both control and RIαB-expressing mice, but AQP2 protein levels were still reduced in RIαB-expressing mutants, and mice were unable to fully concentrate their urine and conserve water. We conclude that partial PKA inhibition in the kidney leads to posttranslational effects that reduce AQP2 protein levels and interfere with apical membrane localization. These findings demonstrate a distinct physiological role for PKA signaling in both short- and long-term regulation of AQP2 and characterize a novel mouse model of diabetes insipidus.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

PKA and Epac synergistically inhibit smooth muscle cell proliferation

Cyclic AMP signalling promotes VSMC quiescence in healthy vessels and during vascular healing following injury. Cyclic AMP inhibits VSMC proliferation via mechanisms that are not fully understood. We investigated the role of PKA and Epac signalling on cAMP-induced inhibition of VSMC proliferation. cAMP-mediated growth arrest was PKA-dependent. However, selective PKA activation with 6-Benzoyl-cAMP did not inhibit VSMC proliferation, indicating a requirement for additional pathways. Epac activation using the selective cAMP analogue 8-CPT-2′-O-Me-cAMP, did not affect levels of hyperphosphorylated Retinoblastoma (Rb) protein, a marker of G1-S phase transition, or BrdU incorporation, despite activation of the Epac-effector Rap1. However, 6-Benzoyl-cAMP and 8-CPT-2′-O-Me-cAMP acted synergistically to inhibit Rb-hyperphosphorylation and BrdU incorporation, indicating that both pathways are required for growth inhibition. Consistent with this, constitutively active Epac increased Rap1 activity and synergised with 6-Benzoyl-cAMP to inhibit VSMC proliferation. PKA and Epac synergised to inhibit phosphorylation of ERK and JNK. Induction of stellate morphology, previously associated with cAMP-mediated growth arrest, was also dependent on activation of both PKA and Epac. Rap1 inhibition with Rap1GAP or siRNA silencing did not negate forskolin-induced inhibition of Rb-hyperphosphorylation, BrdU incorporation or stellate morphology. This data demonstrates for the first time that Epac synergises with PKA via a Rap1-independent mechanism to mediate cAMP-induced growth arrest in VSMC. This work highlights the role of Epac as a major player in cAMP-dependent growth arrest in VSMC.

Secretin as a neurohypophysial factor regulating body water homeostasis

Hypothalamic magnocellular neurons express either one of the neurohypophysial hormones, vasopressin or oxytocin, along with different neuropeptides or neuromodulators. Axonal terminals of these neurons are generally accepted to release solely the two hormones but not others into the circulation. Here, we show that secretin, originally isolated from upper intestinal mucosal extract, is present throughout the hypothalamo-neurohypophysial axis and that it is released from the posterior pituitary under plasma hyperosmolality conditions. In the hypothalamus, it stimulates vasopressin expression and release. Considering these findings together with our previous findings that show a direct effect of secretin on renal water reabsorption, we propose here that secretin works at multiple levels in the hypothalamus, pituitary, and kidney to regulate water homeostasis. Findings presented here challenge previous understanding regarding the neurohypophysis and could provide new concepts in treating disorders related to osmoregulation.

Differential effects of phosphorylation on DNA binding properties of N Oct-3 are dictated by protein/DNA complex structures

N Oct-3, a transcription factor member of the POU protein family, is implicated in normal central nervous system development but also in melanoma growth. Its DNA-binding domain (DBD) comprises two subdomains, POUs and POUh, joined by a linker peptide. We have previously shown that N Oct-3 can interact with the already described PORE and MORE DNA motifs, but also with a new structural element we have termed NORE. Having observed that both the PORE and NORE DNA-association modes depend on a strong anchoring of the POUh subdomain rigid arm into the DNA-target minor groove, in contrast to the MORE mode, we have formulated the hypothesis that phosphorylation of the conserved Ser101 residue located in the N Oct-3 POUh arm could lead to differential results in DNA binding according to the type of target. Here we demonstrate that, in vitro, Ser101 is phosphorylated by protein kinase A (PKA), either purified or contained in melanoma (624 mel) nuclear extract, and that this phosphorylation indeed significantly reduced N Oct-3 DBD binding to PORE and NORE motifs, most likely by hampering the POUh rigid arm insertion in the DNA minor groove. Conversely, no effect was observed on the binding of N Oct-3 DBD to MORE sequences. Finally, once bound to its DNA targets, N Oct-3 DBD is less susceptible to PKA activity. We conclude that transcription of genes exhibiting a MORE motif in their promoter should be less affected by N Oct-3 phosphorylation than that of genes switched on by PORE or NORE sequences.

NO-cGMP mediated galanin expression in NGF-deprived or axotomized sensory neurons

Leukaemia inhibitory factor (LIF) and nerve growth factor (NGF) are well characterized regulators of galanin expression. However, LIF knockout mice containing the rat galanin 5' proximal promoter fragment (- 2546 to + 15 bp) driving luciferase responded to axotomy in the same way as control mice. Also, LIF had no effect on reporter gene expression in vitro, neither in the presence or absence of NGF, suggesting that other factors mediate an axotomy response from the galanin promoter. We then addressed the role of nitric oxide (NO) using NGF-deprived rat dorsal root ganglion (DRG) neuron cultures infected with viral vectors containing the above-mentioned construct, and also studied endogenous galanin expression in axotomized DRG in vivo. Blocking endogenous NO in NGF-deprived DRG cultures suppressed galanin promoter activity. Consistent with this, axotomized/NGF-deprived DRG neurons expressed high levels of neuronal NO synthase (nNOS) and galanin. Further, using pharmacological NOS blockers, or adenoviral vectors expressing dominant-negative either for nNOS or soluble guanylate cyclase in vivo and in vitro, we show that the NO-cGMP pathway induces endogenous galanin in DRG neurons. We propose that both LIF and NO, acting at different promoter regions, are important for the up-regulation of galanin, and for DRG neuron survival and regeneration after axotomy.

Transcription factor expression in the hypothalamo-neurohypophyseal system of the dehydrated rat: upregulation of gonadotrophin inducible transcription factor 1 mRNA is mediated by cAMP-dependent protein kinase A

The supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamo-neurohypophyseal system (HNS) undergo a dramatic function-related plasticity during dehydration. We hypothesize that alterations in steady-state transcript levels might be partially responsible for this remodeling. In turn, regulation of transcript abundance might be mediated by transcription factors. We used microarrays to identify changes in the expression of mRNAs encoding transcription factors in response to water deprivation in the SON. We observed downregulation of 10 and upregulation of 28 transcription factor transcripts. For five of the upregulated mRNAs, namely gonadotropin inducible ovarian transcription factor 1 (Giot1), Giot2, cAMP-responsive element binding protein 3-like 1, CCAAT/enhancer binding protein beta, and activating transcription factor 4, in situ hybridization was used to confirm the array results, demonstrating a significant increase in expression in SON and PVN magnocellular neurons (MCNs) after 3 d of water deprivation and, in some cases, upregulation in parvocellular PVN neurons. Using a viral vector expressing a potent inhibitor of cAMP-dependent protein kinase A (PKA), we show that the osmotically induced increase in the abundance of transcripts encoding Giot1 is mediated in vivo by the PKA pathway. We thus suggest that signaling pathways activated by dehydration in MCNs mediate transcription factor gene activation, which, in turn, regulate target genes that mediate HNS remodeling.

Microarray analysis of gene expression in the supraoptic nucleus of normoosmotic and hypoosmotic rats

1. Hypoosmolality produces a dramatic inhibition of vasopressin (VP) and oxytocin (OT) gene expression in the supraoptic nucleus (SON). This study examines the effect of sustained hypoosmolality on global gene expression in the OT and VP magnocellular neurons (MCNs) of the hypothalamo-neurohypophysial system (HNS), in order to detect novel genes in this system that might be involved in osmoregulation in the MCNs. 2. For this purpose, we used Affymetrix oligonucleotide arrays to analyze the expression of specific genes in laser microdissected rat SONs, and their changes in expression during chronic hypoosmolality. We identified over 40 genes that had three-fold or more greater expression in the SON versus total hypothalamus, and that also changed more than two fold in expression as a result of the chronic hypoosmolar treatment. These genes contained both novel as well as genes previously known to be present in the SON. All of the raw data for the genes that are expressed in the SON and altered by hypoosmolality can be found on the following NINDS website URL address: http://data.ninds.nih.gov/Gainer/Publications.

Glial-mediated neuroprotection: evidence for the protective role of the NO-cGMP pathway via neuron-glial communication in the peripheral nervous system

The NO-cGMP pathway has emerged as a neuroprotective signaling system involved in communication between neurons and glia. We have previously shown that axotomy or nerve growth factor (NGF)-deprivation of dorsal root ganglion (DRG) neurons leads to increased production of NO and at the same time an increase in cGMP production in their satellite glia cells. Blockade of NO or its receptor, the cGMP synthesizing enzyme soluble guanylate cyclase (sGC), results in apoptosis of neurons and glia. We now show that co-culture of neonatal DRG neurons with either Schwann cells pre-treated with an NO donor or a membrane-permeant cGMP analogue; or neurons maintained in the medium from Schwann cell cultures treated in the same way, prevents neuronal apoptosis. Both NO donor and cGMP treatment of Schwann cells results in synthesis of NGF and NT3. Furthermore, if the Schwann cells are previously infected with adenoviral vectors expressing a dominant negative sGC mutant transgene, treatment of these Schwann cells with an NO donor now fails to prevent neuronal apoptosis. Schwann cells treated in this way also fail to express neither cGMP nor neurotrophins. These findings suggest NO-sGC-cGMP-mediated NGF and NT3 synthesis by Schwann cells protect neurons.

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Deciphering the mechanisms of homeostatic plasticity in the hypothalamo-neurohypophyseal system—genomic and gene transfer strategies

The hypothalamo-neurohypophyseal system (HNS) is the specialised brain neurosecretory apparatus responsible for the production of a peptide hormone, vasopressin, that maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of hormone from the HNS, and this is accompanied by a plethora of changes in morphology, electrical properties and biosynthetic and secretory activity, all of which are thought to facilitate hormone production and delivery, and hence the survival of the organism. We have adopted a functional genomic strategy to understand the activity dependent plasticity of the HNS in terms of the co-ordinated action of cellular and genetic networks. Firstly, using microarray gene-profiling technologies, we are elucidating which genes are expressed in the HNS, and how the pattern of expression changes following physiological challenge. The next step is to use transgenic rats to probe the functions of these genes in the context of the physiological integrity of the whole organism.