Akito Tanoue

The vasopressin V1b receptor critically regulates hypothalamic-pituitary-adrenal axis activity under both stress and resting conditions

The neurohypophyseal peptide [Arg(8)]-vasopressin (AVP) exerts major physiological actions through three distinct receptor isoforms designated V1a, V1b, and V2. Among these three subtypes, the vasopressin V1b receptor is specifically expressed in pituitary corticotrophs and mediates the stimulatory effect of vasopressin on ACTH release. To investigate the functional roles of V1b receptor subtypes in vivo, gene targeting was used to create a mouse model lacking the V1b receptor gene (V1bR-/-). Under resting conditions, circulating concentrations of ACTH and corticosterone were lower in V1bR-/- mice compared with WT mice (V1bR+/+). The normal increase in circulating ACTH levels in response to exogenous administration of AVP was impaired in V1bR-/- mice, while corticotropin-releasing hormone-stimulated ACTH release in the V1bR-/- mice was not significantly different from that in the V1bR+/+ mice. AVP-induced ACTH release from primary cultured pituitary cells in V1bR-/- mice was also blunted. Furthermore, the increase in ACTH after a forced swim stress was significantly suppressed in V1bR-/- mice. Our results clearly demonstrate that the V1b receptor plays a crucial role in regulating hypothalamic-pituitary-adrenal axis activity. It does this by maintaining ACTH and corticosterone levels, not only under stress but also under basal conditions.

The α1D-adrenergic receptor directly regulates arterial blood pressure via vasoconstriction

To investigate the physiological role of the α1D-adrenergic receptor (α1D-AR) subtype, we created mice lacking the α1D-AR (α1D ‐/‐) by gene targeting and characterized their cardiovascular function. In α1D ‐/‐ mice, the RT-PCR did not detect any transcript of the α1D-AR in any tissue examined, and there was no apparent upregulation of other α1-AR subtypes. Radioligand binding studies showed that α1-AR binding capacity in the aorta was lost, while that in the heart was unaltered in α1D ‐/‐ mice. Non-anesthetized α1D ‐/‐ mice maintained significantly lower basal systolic and mean arterial blood pressure conditions, relative to wild-type mice, and they showed no significant change in heart rate or in cardiac function, as assessed by echocardiogram. Besides hypotension, the pressor responses to phenylephrine and norepinephrine were decreased by 30‐40% in α1D ‐/‐ mice. Furthermore, the contractile response of the aorta and the pressor response of isolated perfused mesenteric arterial beds to α1-AR stimulation were markedly reduced in α1D ‐/‐ mice. We conclude that the α1D-AR participates directly in sympathetic regulation of systemic blood pressure by vasoconstriction.

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

V1a vasopressin receptors maintain normal blood pressure by regulating circulating blood volume and baroreflex sensitivity

Arginine-vasopressin (AVP) is a hormone that is essential for both osmotic and cardiovascular homeostasis, and exerts important physiological regulation through three distinct receptors, V1a, V1b, and V2. Although AVP is used clinically as a potent vasoconstrictor (V1a receptor-mediated) in patients with circulatory shock, the physiological role of vasopressin V1a receptors in blood pressure (BP) homeostasis is ill-defined. In this study, we investigated the functional roles of the V1a receptor in cardiovascular homeostasis using gene targeting. The basal BP of conscious mutant mice lacking the V1a receptor gene (V1a−/−) was significantly (P < 0.001) lower compared to the wild-type mice (V1a+/+) without a notable change in heart rate. There was no significant alteration in cardiac functions as assessed by echocardiogram in the mutant mice. AVP-induced vasopressor responses were abolished in the mutant mice; rather, AVP caused a decrease in BP, which occurred in part through V2 receptor-mediated release of nitric oxide from the vascular endothelium. Arterial baroreceptor reflexes were markedly impaired in mutant mice, consistent with a loss of V1a receptors in the central area of baroreflex control. Notably, mutant mice showed a significant 9% reduction in circulating blood volume. Furthermore, mutant mice had normal plasma AVP levels and a normal AVP secretory response, but had significantly lower adrenocortical responsiveness to adrenocorticotropic hormone. Taken together, these results indicate that the V1a receptor plays an important role in normal resting arterial BP regulation mainly by its regulation of circulating blood volume and baroreflex sensitivity.

Impaired social interaction and reduced anxiety-related behavior in vasopressin V1a receptor knockout mice

The arginine vasopressin (AVP) system plays an important role in social behavior. Autism, with its hallmark disturbances in social behavior, has been associated with the V1a receptor (V1aR) gene. Furthermore, impairments of social function are often observed in symptoms of schizophrenia. Subchronic phencyclidine (PCP) produces behaviors relating to certain aspects of schizophrenic symptoms such as impairing social interaction in animals and it reduces the density of V1aR binding sites in several brain regions. Here, we report that V1aR knockout (KO) mice exhibited impairment of social behavior in a social interaction test, and showed reduced anxiety-related behavior in elevated plus-maze and marble-burying behavior tests. Given the current findings, the V1aR may be involved in the regulation of social interaction, and V1aR KO mice could be used as an animal model of psychiatric disorders associated with social behavior deficits, such as autism and schizophrenia.

TrkB binds and tyrosine-phosphorylates Tiam1, leading to activation of Rac1 and induction of changes in cellular morphology.

Small GTPases of the Rho family play key roles in the formation of neuronal axons and dendrites by transducing signals from guidance cues, such as neurotrophins, to the actin cytoskeleton. However, there is little insight into the mechanism by which neurotrophins regulate Rho GTPases. Here, we show the crucial role of the ubiquitous Rac1-specific guanine nucleotide exchange factor, Tiam1 (T lymphoma invasion and metastasis 1), in transducing a neurotrophin-mediated change in cell shape. We demonstrate that BDNF, acting through TrkB, directly binds and specifically activates Tiam1 by phosphorylating Tyr-829, leading to Rac1 activation and lamellipodia formation in Cos-7 cells and increased neurite outgrowth from cortical neurons. A point mutation in Tiam1, Tyr-829 to Phe-829, blocked these BDNF-induced changes in cellular morphology. The findings are evidence of a previously uncharacterized mechanism for the activation of Tiam1 and of a role for this effector in neurotrophin-mediated signal transduction leading to changes in cellular morphology.

Recent advances in α1-adrenoceptor pharmacology

Abstract α 1 -Adrenergic receptors (ARs) mediate some of the main actions of the natural catecholamines, adrenaline and noradrenaline. They participate in many essential physiological processes, such as sympathetic neurotransmission, modulation of hepatic metabolism, control of vascular tone, cardiac contraction, and the regulation of smooth muscle activity in the genitourinary system. Here, we review recent progress on subtype-specific subcellular localization, participation in signaling cascades, and the pivotal function of α 1 -ARs, as delineated through studies on genetically engineered animals. Together, these findings will provide new insights into the physiological and pathophysiological roles of the α 1 -ARs.

MCM3AP, a novel acetyltransferase that acetylates replication protein MCM3

The MCM proteins are essential for the initiation of DNA replication. We have isolated an MCM3‐associated protein (MCM3AP) in a two‐hybrid screen using MCM3. Here we demonstrate that MCM3AP is an acetyltransferase which acetylates MCM3 and that chromatin‐bound MCM3 is acetylated in vivo. The MCM3 acetylase, MCM3AP, is also chromatin‐bound. This study also indicates that MCM3AP contains putative acetyl CoA binding motifs conserved within the GCN5‐related N‐acetyltransferase superfamily. Mutation of those motifs significantly inhibits the MCM3 acetylase activity. Over‐expression of MCM3AP inhibits DNA replication, whereas mutation of the acetylase motifs abolishes this effect, suggesting that acetylation plays a role in DNA replication. Taken together, we suggest that MCM3 acetylation is a novel pathway which might regulate DNA replication.

Protective Effect of Geranylgeranylacetone via Enhancement of HSPB8 Induction in Desmin-Related Cardiomyopathy

Background An arg120gly (R120G) missense mutation in HSPB5 (α-β-crystallin ), which belongs to the small heat shock protein (HSP) family, causes desmin-related cardiomyopathy (DRM), a muscle disease that is characterized by the formation of inclusion bodies, which can contain pre-amyloid oligomer intermediates (amyloid oligomer). While we have shown that small HSPs can directly interrupt amyloid oligomer formation, the in vivo protective effects of the small HSPs on the development of DRM is still uncertain. Methodology/Principal Findings In order to extend the previous in vitro findings to in vivo, we used geranylgeranylacetone (GGA), a potent HSP inducer. Oral administration of GGA resulted not only in up-regulation of the expression level of HSPB8 and HSPB1 in the heart of HSPB5 R120G transgenic (R120G TG) mice, but also reduced amyloid oligomer levels and aggregates. Furthermore, R120G TG mice treated with GGA exhibited decreased heart size and less interstitial fibrosis, as well as improved cardiac function and survival compared to untreated R120G TG mice. To address possible mechanism(s) for these beneficial effects, cardiac-specific transgenic mice expressing HSPB8 were generated. Overexpression of HSPB8 led to a reduction in amyloid oligomer and aggregate formation, resulting in improved cardiac function and survival. Treatment with GGA as well as the overexpression of HSPB8 also inhibited cytochrome c release from mitochondria, activation of caspase-3 and TUNEL-positive cardiomyocyte death in the R120G TG mice. Conclusions/Significance Expression of small HSPs such as HSPB8 and HSPB1 by GGA may be a new therapeutic strategy for patients with DRM.

V1a receptor knockout mice exhibit impairment of spatial memory in an eight-arm radial maze

In this study, we examined the performance of vasopressin V1a receptor (V1aR) and vasopressin V1b receptor (V1bR) knockout (KO) mice compared to wild-type (WT) mice in an eight-arm radial maze. V1aR KO mice exhibited an impairment of spatial memory in comparison to WT mice. By contrast, we did not observe any significant differences between the V1bR KO mice and the WT mice in the eight-arm radial maze. Moreover, OPC-21268, a selective V1aR antagonist, impaired spatial memory in the eight-arm radial maze in WT mice characterized by an increased number of errors. These results suggest that the V1aR controls spatial memory in mice.