The amidated PACAP1-23 fragment is a potent reduced-size neuroprotective agent

BACKGROUND

Neurodegenerative disorders, such as Parkinson's disease (PD), are characterized by neuronal death involving, among other events, mitochondrial dysfunction and excitotoxicity. Along these lines, several attempts have been made to slow this pathology but none have been yet discovered. Based on its capacity to cross the blood-brain barrier and provide neuronal protection in vitro and in vivo, the pituitary adenylate cyclase-activating polypeptide (PACAP) represents a promising lead molecule. Pharmacological studies showed that PACAP interacts with three different G protein-coupled receptors, i.e. PAC1, VPAC1 and VPAC2. However, only PAC1 is associated with neuronal anti-apoptotic actions, whilst VPAC activation might cause adverse effects. In the context of the development of PAC1-selective agonists, PACAP(1-23) (PACAP23) appears as the shortest known PACAP bioactive fragment.

METHODS

Hence, the capacity of this peptide to bind PACAP receptors and protect neuroblastoma cells was evaluated under conditions of mitochondrial dysfunction and glutamate excitotoxicity. In addition, its ability to activate downstream signaling events involving G proteins (Gα_s and Gα_q), EPAC, and calcium was also assessed.

RESULTS

Compared to the endogenous peptide, PACAP23 showed a reduced affinity towards PAC1, although this fragment exerted potent neuroprotection. However, surprisingly, some disparities were observed for PACAP23 signaling compared to full length PACAP, suggesting that downstream signaling related to neuroprotection is distinctly regulated following subtle differences in their PAC1 interactions.

CONCLUSIONS

Altogether, this study demonstrates the potent neuroprotective action of amidated PACAP23.

GENERAL SIGNIFICANCE

PACAP23 represents an attractive template for development of shorter PACAP-derived neuroprotective molecules.

Functional selectivity in GPCR signaling: understanding the full spectrum of receptor conformations

The great versatility of G protein-coupled receptors (GPCRs), in terms of both their ability to bind different types of ligands and initiate a large number of distinct cellular signaling events, remains incompletely understood. In recent years, the classical view of the nature and consequences of ligand binding to GPCRs has dramatically changed. The notion of functional selectivity, achieved through both biased ligands and allosteric modulators, has brought substantial new insight into our comprehension of the pluridimensionality of signaling achieved by GPCRs. Moreover, receptor heterodimerization adds another important dimension to the diversity of cellular responses controlled by GPCRs. Here, we review these considerations and discuss how they will impact the design of improved therapeutics.

Cardiac-directed overexpression of wild-type alpha1B-adrenergic receptor induces dilated cardiomyopathy

Using transgenesis as a paradigm, we show here that alpha1-adrenergic receptors (alpha1AR) play an important role in cardiac homeostasis. Cardiomyocyte-specific overexpression of the alpha(1B)AR subtype resulted in the development of dilated cardiomyopathy and death at ~9 mo of age with typical signs of heart failure. Histological analyses showed the enlargement of all four cardiac chambers and cardiomyocyte disarray in the failing hearts. Transgenic animals showed increased left ventricular areas, as assessed by echocardiography. In addition, a progressive decrease in left ventricular systolic function was revealed. The abundance and activity of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2) were reduced, and the ratio of phospholamban to SERCA2 was increased. alpha-Myosin heavy chain (MHC) mRNA was less abundant in older transgenic ventricles, whereas beta-MHC was induced in the failing hearts. Titin mRNA abundance was decreased at 9 mo, whereas atrial natriuretic factor mRNA was elevated at all times. This model mimics structural and functional features of idiopathic dilated cardiomyopathy. The results of this study suggest that chronic alpha1AR activity is deleterious for cardiac function.

Gamma-aminobutyric acid type B receptors with specific heterodimer composition and postsynaptic actions in hippocampal neurons are targets of anticonvulsant gabapentin action

Gamma-aminobutyric acid (GABA) activates two qualitatively different inhibitory mechanisms through ionotropic GABA(A) multisubunit chloride channel receptors and metabotropic GABA(B) G protein-coupled receptors. Evidence suggests that pharmacologically distinct GABA(B) receptor subtypes mediate presynaptic inhibition of neurotransmitter release by reducing Ca2+ conductance, and postsynaptic inhibition of neuronal excitability by activating inwardly rectifying K+ (Kir) conductance. However, the cloning of GABA(B) gb1 and gb2 receptor genes and identification of the functional GABA(B) gb1-gb2 receptor heterodimer have so far failed to substantiate the existence of pharmacologically distinct receptor subtypes. The anticonvulsant, antihyperalgesic, and anxiolytic agent gabapentin (Neurontin) is a 3-alkylated GABA analog with an unknown mechanism of action. Here we report that gabapentin is an agonist at the GABA(B) gb1a-gb2 heterodimer coupled to Kir 3.1/3.2 inwardly rectifying K+ channels in Xenopus laevis oocytes. Gabapentin was practically inactive at the human gb1b-gb2 heterodimer, a novel human gb1c-gb2 heterodimer and did not block GABA agonism at these heterodimer subtypes. Gabapentin was not an agonist at recombinant GABA(A) receptors as well. In CA1 pyramidal neurons of rat hippocampal slices, gabapentin activated postsynaptic K+ currents, probably via the gb1a-gb2 heterodimer coupled to inward rectifiers, but did not presynaptically depress monosynaptic GABA(A) inhibitory postsynaptic currents. Gabapentin is the first GABA(B) receptor subtype-selective agonist identified providing proof of pharmacologically and physiologically distinct receptor subtypes. This selective agonism of postsynaptic GABA(B) receptor subtypes by gabapentin in hippocampal neurons may be its key therapeutic advantage as an anticonvulsant.

Gbetagamma subunit combinations differentially modulate receptor and effector coupling in vivo

In vitro, little specificity is seen for modulation of effectors by different combinations of Gbetagamma subunits from heterotrimeric G proteins. Here, we demonstrate that the coupling of specific combinations of Gbetagamma subunits to different receptors leads to a differential ability to modulate effectors in vivo. We have shown that the beta(1)AR and beta(2)AR can activate homomultimers of the human inwardly rectifying potassium channel Kir 3.2 when coexpressed in Xenopus oocytes, and that this requires a functional mammalian Gs heterotrimer. Modulation was independent of cAMP production, suggesting a membrane-delimited mechanism. To analyze further the importance of different Gbetagamma combinations, we have tested the facilitation of Kir 3.2 activation by betaAR mediated by different Gbetagamma subunits. The subunits tested were Gbeta(1,5) and Ggamma(1,2,7,11). These experiments demonstrated significant variation between the ability of the Gbetagamma combinations to activate the channels after receptor stimulation. This was in marked contrast to the situation in vitro where little specificity for binding of a Kir 3.1 C-terminal GST fusion protein by different Gbetagamma combinations was detected. More importantly, neither receptor, although homologous both structurally and functionally, shared the same preference for Gbetagamma subunits. In the presence of beta(1)AR, Gbeta(5)gamma(1) and Gbeta(5)gamma(11) activated Kir 3.2 to the greatest extent, while for the beta(2)AR, Gbeta(1)gamma(7), Gbeta(1)gamma(11,) and Gbeta(5)gamma(2) produced the greatest responses. Interestingly, no preference was seen in the ability of different Gbetagamma subunits to facilitate receptor-stimulated GTPase activity of the Gsalpha. These results suggest that it is not the receptor/G protein alpha subunit interaction or the Gbetagamma/effector interaction that is altered by Gbetagamma, but rather that the ability of the receptor to interact productively with the Gbetagamma subunit directly and/or the G protein/effector complex is dependent on the specific G protein heterotrimer associated with the receptor.

Stable association of G proteins with beta 2AR is independent of the state of receptor activation

beta 2-Adrenergic receptors expressed in Sf9 cells activate endogenous Gs and adenylyl cyclase [Mouillac B., Caron M., Bonin H., Dennis M. and Bouvier M. (1992) J. Biol. Chem. 267, 21733-21737]. However, high affinity agonist binding is not detectable under these conditions suggesting an improper stoichiometry between the receptor and the G protein and possibly the effector molecule as well. In this study we demonstrate that when beta 2-adrenergic receptors were co-expressed with various mammalian G protein subunits in Sf9 cells using recombinant baculoviruses signalling properties found in native receptor systems were reconstituted. For example, when beta 2AR was co-expressed with the Gs alpha subunit, maximal receptor-mediated adenylyl cyclase stimulation was greatly enhanced (60 +/- 9.0 versus 150 +/- 52 pmol cAMP/min/mg protein) and high affinity, GppNHp-sensitive, agonist binding was detected. When G beta gamma subunits were co-expressed with Gs alpha and the beta 2AR, receptor-stimulated GTPase activity was also demonstrated, in contrast to when the receptor was expressed alone, and this activity was higher than when beta 2AR was co-expressed with Gs alpha alone. Other properties of the receptor, including receptor desensitization and response to inverse agonists were unaltered. Using antisera against an epitope-tagged beta 2AR, both Gs alpha and beta gamma subunits could be co-immunoprecipitated with the beta 2AR under conditions where subunit dissociation would be expected given current models of G protein function. A desensitization-defective beta 2AR (S261, 262, 345, 346A) and a mutant which is constitutively desensitized (C341G) could also co-immunoprecipitate G protein subunits. These results will be discussed in terms of a revised view of G protein-mediated signalling which may help address issues of specificity in receptor/G protein coupling.

Structural and functional aspects of G protein-coupled receptor oligomerization

G protein-coupled receptors (GPCRs) represent the single largest family of cell surface receptors involved in signal transduction. It is estimated that several hundred distinct members of this receptor family in humans direct responses to a wide variety of chemical transmitters, including biogenic amines, amino acids, peptides, lipids, nucleosides, and large polypeptides. These transmembrane receptors are key controllers of such diverse physiological processes as neurotransmission, cellular metabolism, secretion, cellular differentiation, and growth as well as inflammatory and immune responses. GPCRs therefore represent major targets for the development of new drug candidates with potential application in all clinical fields. Many currently used therapeutics act by either activating (agonists) or blocking (antagonists) GPCRs. Studies over the past two decades have provided a wealth of information on the biochemical events underlying cellular signalling by GPCRs. However, our understanding of the molecular interactions between ligands and the receptor protein and, particularly, of the structural correlates of receptor activation or inhibition by agonists and inverse agonists, respectively, is still rudimentary. Most of the work in this area has focused on mapping regions of the receptor responsible for drug binding affinity. Although binding of ligand molecules to specific receptors represents the first event in the action of drugs, the efficacy with which this binding is translated into a physiological response remains the only determinant of therapeutic utility. In the last few years, increasing evidence suggested that receptor oligomerization and in particular dimerization may play an important role in the molecular events leading to GPCR activation. In this paper, we review the biochemical and functional evidence supporting this notion.