Parkinson disease is a neurodegenerative disorder whose symptoms are caused by the loss of dopaminergic neurons innervating the striatum. As striatal dopamine levels fall, striatal acetylcholine release rises, exacerbating motor symptoms. This adaptation is commonly attributed to the loss of interneuronal regulation by inhibitory D(2) dopamine receptors. Our results point to a completely different, new mechanism. After striatal dopamine depletion, D(2) dopamine receptor modulation of calcium (Ca(2+)) channels controlling vesicular acetylcholine release in interneurons was unchanged, but M(4) muscarinic autoreceptor coupling to these same channels was markedly attenuated. This adaptation was attributable to the upregulation of RGS4-an autoreceptor-associated, GTPase-accelerating protein. This specific signaling adaptation extended to a broader loss of autoreceptor control of interneuron spiking. These observations suggest that RGS4-dependent attenuation of interneuronal autoreceptor signaling is a major factor in the elevation of striatal acetylcholine release in Parkinson disease.

Abstract-Activation of the ␣ 1A -adrenergic receptor (␣ 1A -AR)/Gq pathway has been implicated as a critical trigger for the development of cardiac hypertrophy. However, direct evidence from in vivo studies is still lacking. To address this issue, transgenic mice with cardiac-targeted overexpression of the ␣ 1A -AR (4-to 170-fold) were generated, using the rodent ␣-myosin heavy chain promoter. Heterozygous animals displayed marked enhancement of cardiac contractility, evident from increases in dP/dt max (80%, PϽ0.0001), dP/dt max /LVP inst (76%, PϽ0.001), dP/dt max :dP/dt min (104%, PϽ0.0001), and fractional shortening (33%, PϽ0.05). Moreover, changes in the dP/dt max -end-diastolic volume relationship provided load-independent evidence of a primary increase in contractility. Blood pressure and heart rate were largely unchanged, and there was a small increase in (Ϫ)norepinephrine-stimulated, but not basal, phospholipase C activity. Increased contractility was directly related to the level of receptor overexpression and could be completely reversed by acute ␣ 1A -but not ␤-AR blockade. Despite the robust changes in contractility, transgenic animals displayed no morphological, histological, or echocardiographic evidence of left ventricular hypertrophy. In addition, apart from an increase in atrial natriuretic factor mRNA, expression of other hypertrophy-associated genes was unchanged. To our knowledge, these data provide the first in vivo evidence for an inotropic action of the ␣ 1A -AR.

Gα12/13 have been implicated in numerous cellular processes, however, their roles in vertebrate gastrulation are largely unknown. Here, we show that during zebrafish gastrulation, suppression of both Gα12 and Gα13 signaling by overexpressing dominant negative proteins and application of antisense morpholino-modified oligonucleotide translation interference disrupted convergence and extension without changing embryonic patterning. Analyses of mesodermal cell behaviors revealed that Gα12/13 are required for cell elongation and efficient dorsalward migration during convergence independent of noncanonical Wnt signaling. Furthermore, Gα12/13 function cell-autonomously to mediate mediolateral cell elongation underlying intercalation during notochord extension, likely acting in parallel to noncanonical Wnt signaling. These findings provide the first evidence that Gα12 and Gα13 have overlapping and essential roles in distinct cell behaviors that drive vertebrate gastrulation.

A yeast two-hybrid approach was used to discern possible new effectors for the ␤␥ subunit of heterotrimeric G proteins. Three of the clones isolated are structurally similar to G␤, each exhibiting the WD40 repeat motif. Two of these proteins, the receptor for activated C kinase 1 (RACK1) and the dynein intermediate chain, coimmunoprecipitate with G␤␥ using an anti-G␤ antibody. The third protein, AAH20044, has no known function; however, sequence analysis indicates that it is a WD40 repeat protein. Further investigation with RACK1 shows that it not only interacts with G␤ 1 ␥ 1 but also unexpectedly with the transducin heterotrimer G␣ t ␤ 1 ␥ 1 . G␣ t alone does not interact, but it must contribute to the interaction because the apparent EC 50 value of RACK1 for G␣ t ␤ 1 ␥ 1 is 3-fold greater than that for G␤ 1 ␥ 1 (0.1 versus 0.3 M). RACK1 is a scaffold that interacts with several proteins, among which are activated ␤IIPKC and dynamin-1 (1). ␤IIPKC and dynamin-1 compete with G␤ 1 ␥ 1 and G␣ t ␤ 1 ␥ 1 for interaction with RACK1. These findings have several implications: 1) that WD40 repeat proteins may interact with each other; 2) that G␤␥ interacts differently with RACK1 than with its other known effectors; and/or 3) that the G protein-RACK1 complex may constitute a signaling scaffold important for intracellular responses.Heterotrimeric G proteins are a family of proteins that transduce an extracellular signal to an intracellular response via a seven helical transmembrane G protein-coupled receptor (GPCR).1 Upon activation, the receptor facilitates the exchange of GDP for GTP in the G␣ subunit. G␣ is then thought to dissociate from the G␤␥ heterodimer allowing both complexes to individually activate a number of effectors (2, 3). Free G␤␥ interacts with a large assortment of effector proteins, including phospholipases (4), adenylyl cyclases (5), ion channels (6), and G protein-coupled receptor kinases (7). There are, however, G protein-coupled receptor responses, such as MAP kinase activation (8 -10), receptor internalization (11, 12), and organelle transport (13-15) that are mediated through the G␤␥ subunit but that have not been definitively linked to known G␤␥ effectors.G␤ is the prototypical member of a family of proteins known as WD40 repeat proteins, which seem to function as adaptors and enzyme regulators (16,17). G␤ is the only WD40 repeat protein whose three-dimensional structure is known, and it exhibits a toroidal bladed ␤-propeller structure, with each blade consisting of 4 anti-parallel ␤-strands (18). Because the WD repeat motif is a structural element of the ␤-propeller, all of these proteins are thought to be ␤-propeller proteins with a variable number of blades. Furthermore, G␤ subunits are known to interact with G␥ subunits, proteins containing a G␥-like domain (19), a pleckstrin homology domain (20), a QXXER domain (found in adenylyl cyclases) (21), and a domain contained within phosducin and its relatives (22). In this work we propose that G␤␥ also interacts with certain other WD40 repeat prote...

Science 264, 1593-1596). Here, we evaluated the ability of G h as compared with G q to mediate receptor-stimulated inositol phosphate turnover by the three ␣ 1 -subtypes (␣ 1A , ␣ 1B , and ␣ 1D ). In addition, we questioned if the transglutaminase function of G h is involved in its receptor signaling activity. A mutant form of a human TGase II cDNA in which the codon for the active site cysteine (Cys 277 ) was replaced by serine was cloned into the mammalian expression vector pMT2. Compared with wild-type TGase II, no transglutaminase activity was observed with transient transfection of this Cys3 Ser mutant in COS-1 cells. However, like wild-type TGase, the Cys3 Ser mutant mediated receptor-stimulated inositol phosphate turnover when cotransfected with an ␣ 1B -AR cDNA. G ␣q supported ␣ 1 -AR-mediated inositol phosphate turnover by all three receptor subtypes. By contrast, although both the wild-type and Cys3 Ser construct mediated receptor signaling by the ␣ 1B AR and ␣ 1D AR, the ␣ 1A -AR was unable to interact with G h . However, a G h -dependent signaling phenotype could be rescued by a chimeric ␣ 1A construct in which the third intracellular loop of the ␣ 1A -AR was replaced by that of the ␣ 1B -AR. Thus, the signaling function of G h is independent of its transglutaminase activity and is ␣ 1 -AR subtype specific. This subtype specificity of the interaction between ␣ 1 ARs and G h involves important determinants in their third intracellular loops.