Differential regulation of pituitary adenylate cyclase-activating peptide receptor variants in the rat suprachiasmatic nucleus

A role for alternative splicing in circadian control of exocytosis and glucose homeostasis

The circadian clock is encoded by a negative transcriptional feedback loop that coordinates physiology and behavior through molecular programs that remain incompletely understood. Here, we reveal rhythmic genome-wide alternative splicing (AS) of pre-mRNAs encoding regulators of peptidergic secretion within pancreatic β cells that are perturbed in Clock-/- and Bmal1-/- β-cell lines. We show that the RNA-binding protein THRAP3 (thyroid hormone receptor-associated protein 3) regulates circadian clock-dependent AS by binding to exons at coding sequences flanking exons that are more frequently skipped in clock mutant β cells, including transcripts encoding Cask (calcium/calmodulin-dependent serine protein kinase) and Madd (MAP kinase-activating death domain). Depletion of THRAP3 restores expression of the long isoforms of Cask and Madd, and mimicking exon skipping in these transcripts through antisense oligonucleotide delivery in wild-type islets reduces glucose-stimulated insulin secretion. Finally, we identify shared networks of alternatively spliced exocytic genes from islets of rodent models of diet-induced obesity that significantly overlap with clock mutants. Our results establish a role for pre-mRNA alternative splicing in β-cell function across the sleep/wake cycle.

Pleiotropic pituitary adenylate cyclase-activating polypeptide (PACAP): Novel insights into the role of PACAP in eating and drug intake

Pituitary adenylate cyclase-activating polypeptide (PACAP) was discovered thirty years ago, but its role in eating and drug use disorders has only recently begun to be investigated. The present review develops the hypothesis that, although PACAP normally functions to tightly regulate intake, inhibiting it through negative feedback, this relationship can become dysregulated with the development of dependence, such that PACAP instead acts through positive feedback to promote excessive intake. We propose that repeated exposure to palatable food and drugs of abuse can alter the downstream responses of specific populations of neurons to stimulation by PACAP, leading to the perpetuation of the addiction cycle. Thus, this review will first describe published literature on homeostatic food intake, which shows that PACAP suppresses food intake, while its levels are themselves increased by overfeeding. Next, it will present literature on palatable food, cocaine, alcohol, and nicotine, which overall demonstrates that PACAP in specific limbic brain regions can promote their seeking and intake and itself is stimulated by their intake. Then, it will present literature on affective behavior, which shows that chronic stress increases levels of PACAP, which then promotes anxiety and depression, factors that can trigger substance seeking. Finally, the review will address mechanisms through which chronic substance exposure may dysregulate the PACAP system, proposing that it alters expression of PACAP receptor splice variants. While many questions remain to be addressed, the current evidence suggests that PACAP could be a viable medication target for the treatment of binge eating and drug and alcohol use disorders.

PACAP38 and PAC1 receptor blockade: a new target for headache?

Pituitary adenylate cyclase activating polypeptide-38 (PACAP38) is a widely distributed neuropeptide involved in neuroprotection, neurodevelopment, nociception and inflammation. Moreover, PACAP38 is a potent inducer of migraine-like attacks, but the mechanism behind this has not been fully elucidated.

Migraine is a neurovascular disorder, recognized as the second most disabling disease. Nevertheless, the antibodies targeting calcitonin gene-related peptide (CGRP) or its receptor are the only prophylactic treatment developed specifically for migraine. These antibodies have displayed positive results in clinical trials, but are not effective for all patients; therefore, new pharmacological targets need to be identified.

Due to the ability of PACAP38 to induce migraine-like attacks, its location in structures previously associated with migraine pathophysiology and the 100-fold selectivity for the PAC₁ receptor when compared to VIP, new attention has been drawn to this pathway and its potential role as a novel target for migraine treatment. In accordance with this, antibodies against PACAP38 (ALD 1910) and PAC₁ receptor (AMG 301) are being developed, with AMG 301 already in Phase II clinical trials. No results have been published so far, but in preclinical studies, AMG 301 has shown responses comparable to those observed with triptans. If these antibodies prove to be effective for the treatment of migraine, several considerations should be addressed, for instance, the potential side effects of long-term blockade of the PACAP (receptor) pathway. Moreover, it is important to investigate whether these antibodies will indeed represent a therapeutic advantage for the patients that do not respond the CGRP (receptor)-antibodies.

In conclusion, the data presented in this review indicate that PACAP38 and PAC₁ receptor blockade are promising antimigraine therapies, but results from clinical trials are needed in order to confirm their efficacy and side effect profile.

Novel Rbfox2 isoforms associated with alternative exon usage in rat cortex and suprachiasmatic nucleus

Transcriptome diversity in adult neurons is partly mediated by RNA binding proteins (RBPs), including the RBFOX factors. RBFOX3/NeuN, a neuronal maturity marker, is strangely depleted in suprachiasmatic nucleus (SCN) neurons, and may be compensated by a change in Rbfox2 expression. In this study, we found no superficial changes in Rbfox2 expression in the SCN, but mRNA population analysis revealed a distinct SCN transcript profile that includes multiple novel Rbfox2 isoforms. Of eleven isoforms in SCN and cerebral cortex that exhibit exon variation across two protein domains, we found a 3-fold higher abundance of a novel (‘−12–40’) C-terminal domain (CTD)-variant in the SCN. This isoform embraces an alternative reading frame that imparts a 50% change in CTD protein sequence, and functional impairment of exon 7 exclusion activity in a RBFOX2-target, the L-type calcium channel gene, Cacna1c. We have also demonstrated functional correlates in SCN gene transcripts; inclusion of Cacna1c exon 7, and also exclusion of both NMDA receptor gene Grin1 exon 4, and Enah exon 12, all consistent with a change in SCN RBFOX activity. The demonstrated regional diversity of Rbfox2 in adult brain highlights the functional adaptability of this RBP, enabling neuronal specialization, and potentially responding to disease-related neuronal dysfunction.

Expression of VIP and/or PACAP receptor mRNA in peptide synthesizing cells within the suprachiasmatic nucleus of the rat and in its efferent target sites

The suprachiasmatic nucleus (SCN) contains the predominant circadian pacemaker in mammals. Considerable evidence indicates that VPAC(2) and PAC(1), receptors for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP), play critical roles in maintaining and entraining circadian rhythms. Retinal projections to the rat SCN contain PACAP and terminate mostly in the ventral SCN, the site of VIP neurons. The incidence of VPAC(2) and PAC(1) mRNAs within distinct neuronal populations of the rat SCN has been determined using double-label in situ hybridization. VPAC(2) mRNA was detected in almost all arginine-vasopressin (AVP) neurons of the dorsomedial SCN and in 41% of the VIP neurons; somatostatin (SST) neurons, predominantly in dorsomedial and intermediate regions, showed a decreased incidence (23%). PAC(1) mRNA was present in nearly half of the VIP and SST neurons (45% and 40%, respectively) and in one-third of the AVP neurons (32%). Cells expressing VPAC(2) mRNA also were detected in diencephalic areas that receive VIP-immunoreactive SCN efferents, such as the peri-suprachiasmatic region, lateral subparaventricular zone, parvocellular hypothalamic paraventricular subdivisions, dorsomedial hypothalamic nucleus, and anterior thalamic paraventricular and paratenial nuclei. The extensive distribution of PAC(1) mRNA within the SCN suggests that actions of PACAP are not restricted to the predominantly retinorecipient region. The presence of VPAC(2) mRNA in nearly half the VIP neurons, in almost all the AVP neurons, and at sites receiving VIP-immunoreactive SCN efferents suggests that the SCN VIP neurons are coupled and/or autoregulated and also influence the AVP-containing dorsomedial SCN and distal sites via VPAC(2).

An essential role for peptidergic signalling in the control of circadian rhythms in the suprachiasmatic nuclei

Two structurally related neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP), colocalized with glutamate in neurones of the retinohypothalamic tract, and vasoactive intestinal peptide (VIP), present in light-responsive cells of the suprachiasmatic nuclei (SCN), appear to play distinct and important roles in the control of mammalian circadian rhythms. Mice deficient in the PACAP-selective PAC1 receptor exhibit altered responsiveness of the SCN clock to light-induced phase-shifts, but display robust circadian patterns of wheel-running behaviour. By contrast, our studies of mice lacking the VPAC2 receptor, which responds to both PACAP and VIP, indicate that this receptor plays a critical role in rhythm generation in the SCN. The predominant factor determining wheel-running activity in VPAC2 receptor null (Vipr2-/-) mice is "masking" by light. Mutant animals re-entrain immediately to advances or delays in the light/dark cycle and do not exhibit robust circadian rhythms of behaviour when in constant darkness. The mice do not exhibit circadian expression of core clock genes (mPer1, mPer2, mCry1), or of the clock-controlled gene arginine vasopressin (AVP), in the SCN. We propose that VIP signalling between SCN neurones provides a paracrine reinforcing signal that is essential for sustained rhythm generation. The presence of VIP signalling in the SCN may explain why SCN neurones are capable of generating long-lasting self-sustained oscillations, whereas rhythmic clock gene expression in other tissues is dependent on periodic reinforcement by neural or hormonal signals.

The mouse VPAC2 receptor confers suprachiasmatic nuclei cellular rhythmicity and responsiveness to vasoactive intestinal polypeptide in vitro

Expression of coherent and rhythmic circadian (approximately 24 h) variation of behaviour, metabolism and other physiological processes in mammals is governed by a dominant biological clock located in the hypothalamic suprachiasmatic nuclei (SCN). Photic entrainment of the SCN circadian clock is mediated, in part, by vasoactive intestinal polypeptide (VIP) acting through the VPAC2 receptor. Here we used mice lacking the VPAC2 receptor (Vipr2-/-) to examine the contribution of this receptor to the electrophysiological actions of VIP on SCN neurons, and to the generation of SCN electrical firing rate rhythms SCN in vitro. Compared with wild-type controls, fewer SCN cells from Vipr2-/- mice responded to VIP and the VPAC2 receptor-selective agonist Ro 25-1553. By contrast, similar proportions of Vipr2-/- and wild-type SCN cells responded to gastrin-releasing peptide, arginine vasopressin or N-methyl-D-aspartate. Moreover, VIP-evoked responses from control SCN neurons were attenuated by the selective VPAC2 receptor antagonist PG 99-465. In firing rate rhythm experiments, the midday peak in activity observed in control SCN cells was lost in Vipr2-/- mice. The loss of electrical activity rhythm in Vipr2-/- mice was mimicked in control SCN slices by chronic treatment with PG 99-465. These results demonstrate that the VPAC2 receptor is necessary for the major part of the electrophysiological actions of VIP on SCN cells in vitro, and is of fundamental importance for the rhythmic and coherent expression of circadian rhythms governed by the SCN clock. These findings suggest a novel role of VPAC2 receptor signalling, and of cell-to-cell communication in general, in the maintenance of core clock function in mammals, impacting on the cellular physiology of SCN neurons.