Stable G protein-effector complexes in striatal neurons: mechanism of assembly and role in neurotransmitter signaling

Smooth operator(s): dialing up and down neurotransmitter responses by G-protein regulators

G-protein-coupled receptors (GPCRs) are essential mediators of neuromodulation and prominent pharmacological targets. While activation of heterotrimeric G-proteins (Gαβɣ) by GPCRs is essential in this process, much less is known about the postreceptor mechanisms that influence G-protein activity. Neurons express G-protein regulators that shape the amplitude and kinetics of GPCR-mediated synaptic responses. Although many of these operate by directly altering how G-proteins handle guanine-nucleotides enzymatically, recent discoveries have revealed alternative mechanisms by which GPCR-stimulated G-protein responses are modulated at the synapse. In this review, we cover the molecular basis for, and consequences of, the action of two G-protein regulators that do not affect the enzymatic activity of G-proteins directly: Gα inhibitory interacting protein (GINIP), which binds active Gα subunits, and potassium channel tetramerization domain-containing 12 (KCTD12), which binds active Gβγ subunits.

The G protein modifier KCTD5 tunes the decoding of neuromodulatory signals necessary for motor function in striatal neurons

G proteins (Gα and Gβγ subtypes) drive adenylyl cyclase type 5 (AC5) synthesis of cAMP in striatal neurons, which is essential for motor coordination. KCTD5 directly interacts with Gβγ to delimit signaling events, yet downstream impact of KCTD5 in striatal circuits is not known. Here, generation of a conditional Kctd5 knockout mouse identified that loss of striatal KCTD5 leads to a dystonic phenotype, coordination deficits, and skewed transitions between behavioral syllables. 2-photon imaging of a cAMP biosensor revealed electrically evoked dopaminergic responses were significantly augmented in the absence of KCTD5 in striatal circuits. cAMP sensitization was rescued in situ by expression of a Gβγ-scavenging nanobody and motor deficits were partially rescued in vivo by pharmacological antagonism of the indirect striatal cAMP pathway. Therefore, KCTD5 acts as a brake on cAMP signaling in striatal neurons important for tuning dopaminergic signaling and motor coordination.

Neuron specific quantitation of Gαolf expression and signaling in murine brain tissue

The heterotrimeric G-protein α subunit, Gαolf, acts to transduce extracellular signals through G-protein coupled receptors (GPCRs) and stimulates adenylyl cyclase mediated production of the second messenger cyclic adenosine monophosphate. Numerous mutations in the GNAL gene, which encodes Gαolf, have been identified as causative for an adult-onset dystonia. These mutations disrupt GPCR signaling cascades in in vitro assays through several mechanisms, and this disrupted signaling is hypothesized to lead to dystonic motor symptoms in patients. However, the cells and circuits that mutations in GNAL corrupt are not well understood. Published patterns of Gαolf expression outside the context of the striatum are sparse, conflicting, often lack cell type specificity, and may be confounded by expression of the close GNAL homolog of GNAS. Here, we use RNAScope in-situ hybridization to quantitatively characterize Gnal mRNA expression in brain tissue from wildtype C57BL/6J adult mice. We observed widespread expression of Gnal puncta throughout the brain, suggesting Gαolf is expressed in more brain structures and neuron types than previously accounted for. We quantify transcripts at a single cell level, and use neuron type specific markers to further classify and understand patterns of GNAL expression. Our data suggests that brain regions classically associated with motor control, initiation, and regulation show the highest expression of GNAL, with Purkinje Cells of the cerebellum showing the highest expression of any neuron type examined. Subsequent conditional Gnal knockout in Purkinje cells led to markedly decreased intracellular cAMP levels and downstream cAMP-dependent enzyme activation. Our work provides a detailed characterization of Gnal expression throughout the brain and the biochemical consequences of loss of Gαolf signaling in vivo in neurons that highly express Gnal.

KCTD1 regulation of Adenylyl cyclase type 5 adjusts striatal cAMP signaling

Dopamine transfers information to striatal neurons, and disrupted neurotransmission leads to motor deficits observed in movement disorders. Striatal dopamine converges downstream to Adenylyl Cyclase Type 5 (AC5)-mediated synthesis of cAMP, indicating the essential role of signal transduction in motor physiology. However, the relationship between dopamine decoding and AC5 regulation is unknown. Here, we utilized an unbiased global protein stability screen to identify Potassium Channel Tetramerization Domain 1 (KCTD1) as a key regulator of AC5 level that is mechanistically tied to N-linked glycosylation. We then implemented a CRISPR/SaCas9 approach to eliminate KCTD1 in striatal neurons expressing a Förster resonance energy transfer (FRET)-based cAMP biosensor. 2-photon imaging of striatal neurons in intact circuits uncovered that dopaminergic signaling was substantially compromised in the absence of KCTD1. Finally, knockdown of KCTD1 in genetically defined dorsal striatal neurons significantly altered motor behavior in mice. These results reveal that KCTD1 acts as an essential modifier of dopaminergic signaling by stabilizing striatal AC5.

Post-transcriptional regulation and subcellular localization of G-protein γ7 subunit: implications for striatal function and behavioral responses to cocaine

The striatal D1 dopamine receptor (D1R) and A2a adenosine receptor (A2aR) signaling pathways play important roles in drug-related behaviors. These receptors activate the Golf protein comprised of a specific combination of αolfβ2γ7 subunits. During assembly, the γ7 subunit sets the cellular level of the Golf protein. In turn, the amount of Golf protein determines the collective output from both D1R and A2aR signaling pathways. This study shows the Gng7 gene encodes multiple γ7 transcripts differing only in their non-coding regions. In striatum, Transcript 1 is the predominant isoform. Preferentially expressed in the neuropil, Transcript 1 is localized in dendrites where it undergoes post-transcriptional regulation mediated by regulatory elements in its 3' untranslated region that contribute to translational suppression of the γ7 protein. Earlier studies on gene-targeted mice demonstrated loss of γ7 protein disrupts assembly of the Golf protein. In the current study, morphological analysis reveals the loss of the Golf protein is associated with altered dendritic morphology of medium spiny neurons. Finally, behavioral analysis of conditional knockout mice with cell-specific deletion of the γ7 protein in distinct populations of medium spiny neurons reveals differential roles of the Golf protein in mediating behavioral responses to cocaine. Altogether, these findings provide a better understanding of the regulation of γ7 protein expression, its impact on Golf function, and point to a new potential target and mechanisms for treating addiction and related disorders.

Loss-of-function of GNAL dystonia gene impairs striatal dopamine receptors-mediated adenylyl cyclase/ cyclic AMP signaling pathway

Loss-of-function mutations in the GNAL gene are responsible for DYT-GNAL dystonia. However, how GNAL mutations contribute to synaptic dysfunction is still unclear. The GNAL gene encodes the Gαolf protein, an isoform of stimulatory Gαs enriched in the striatum, with a key role in the regulation of cAMP signaling. Here, we used a combined biochemical and electrophysiological approach to study GPCR-mediated AC-cAMP cascade in the striatum of the heterozygous GNAL (GNAL+/-) rat model. We first analyzed adenosine type 2 (A2AR), and dopamine type 1 (D1R) receptors, which are directly coupled to Gαolf, and observed that the total levels of A2AR were increased, whereas D1R level was unaltered in GNAL+/- rats. In addition, the striatal isoform of adenylyl cyclase (AC5) was reduced, despite unaltered basal cAMP levels. Notably, the protein expression level of dopamine type 2 receptor (D2R), that inhibits the AC5-cAMP signaling pathway, was also reduced, similar to what observed in different DYT-TOR1A dystonia models. Accordingly, in the GNAL+/- rat striatum we found altered levels of the D2R regulatory proteins, RGS9-2, spinophilin, Gβ5 and β-arrestin2, suggesting a downregulation of D2R signaling cascade. Additionally, by analyzing the responses of striatal cholinergic interneurons to D2R activation, we found that the receptor-mediated inhibitory effect is significantly attenuated in GNAL+/- interneurons. Altogether, our findings demonstrate a profound alteration in the A2AR/D2R-AC-cAMP cascade in the striatum of the rat DYT-GNAL dystonia model, and provide a plausible explanation for our previous findings on the loss of dopamine D2R-dependent corticostriatal long-term depression.

Visualizing the chaperone-mediated folding trajectory of the G protein β5 β-propeller

The Chaperonin Containing Tailless polypeptide 1 (CCT) complex is an essential protein folding machine with a diverse clientele of substrates, including many proteins with β-propeller domains. Here, we determine the structures of human CCT in complex with its accessory co-chaperone, phosducin-like protein 1 (PhLP1), in the process of folding Gβ5, a component of Regulator of G protein Signaling (RGS) complexes. Cryoelectron microscopy (cryo-EM) and image processing reveal an ensemble of distinct snapshots that represent the folding trajectory of Gβ5 from an unfolded molten globule to a fully folded β-propeller. These structures reveal the mechanism by which CCT directs Gβ5 folding through initiating specific intermolecular contacts that facilitate the sequential folding of individual β sheets until the propeller closes into its native structure. This work directly visualizes chaperone-mediated protein folding and establishes that CCT orchestrates folding by stabilizing intermediates through interactions with surface residues that permit the hydrophobic core to coalesce into its folded state.

Adenosine Receptors as Potential Therapeutic Analgesic Targets

Pain represents an international burden and a major socio-economic public health problem. New findings, detailed in this review, suggest that adenosine plays a significant role in neuropathic and inflammatory pain, by acting on its metabotropic adenosine receptors (A1AR, A2AAR, A2BAR, A3AR). Adenosine receptor ligands have a practical translational potential based on the favorable efficacy and safety profiles that emerged from clinical research on various agonists and antagonists for different pathologies. The present review collects the latest studies on selected adenosine receptor ligands in different pain models. Here, we also covered the many hypothesized pathways and the role of newly synthesized allosteric adenosine receptor modulators. This review aims to present a summary of recent research on adenosine receptors as prospective therapeutic targets for a range of pain-related disorders.

Visualizing the chaperone-mediated folding trajectory of the G protein β5 β-propeller

The cytosolic Chaperonin Containing Tailless polypeptide 1 (CCT) complex is an essential protein folding machine with a diverse clientele of substrates, including many proteins with β-propeller domains. Here, we determined structures of CCT in complex with its accessory co-chaperone, phosducin-like protein 1 (PhLP1), in the process of folding Gβ5, a component of Regulator of G protein Signaling (RGS) complexes. Cryo-EM and image processing revealed an ensemble of distinct snapshots that represent the folding trajectory of Gβ5 from an unfolded molten globule to a fully folded β-propeller. These structures reveal the mechanism by which CCT directs Gβ5 folding through initiating specific intermolecular contacts that facilitate the sequential folding of individual β-sheets until the propeller closes into its native structure. This work directly visualizes chaperone-mediated protein folding and establishes that CCT directs folding by stabilizing intermediates through interactions with surface residues that permit the hydrophobic core to coalesce into its folded state.

Adenosine A2A receptor as a potential target for improving cancer immunotherapy

The adenosine nucleoside performs a wide range of actions on various human tissues by activating four cell surface receptors. Adenosine A_2A receptors (A_2ARs) are widely expressed in the striatum, olfactory bulb, platelets, leukocytes, spleen, and thymus. They promote vasodilatation, platelet antiaggregatory effect, protection from ischemic damage, and regulation of sensorimotor neurons in basal ganglia. Adenosine signaling plays a vital part in modulating in vivo pathophysiological responses. A_2ARs are potent negative regulators of the antitumor and proinflammatory actions of activated T cells. This axis offers several therapeutic targets, the most important of which are A_2ARs, HIF-1α, and CD39/CD73. Downregulation of this axis increases the effectiveness of modern immunotherapeutic approaches against cancer, such as αCTLA-4/αPD-1. These discoveries have led to a promising novel role of antagonists of A_2AR in blocking angiogenesis in immunotherapy of cancer. A small molecule, AZD4635, strongly inhibits A_2AR, lowering cancer volume and increasing anticancer immunity. Deletion of A_2AR with CRISPR/Cas9 in both human and murine CAR T cells produces a substantial increase in the efficiency of these cells. This review asserts that inhibition of the adenosinergic pathway can boost antitumor immunity, and this axis should be a target for future immunotherapeutic strategies.

Functional abnormalities in the cerebello-thalamic pathways in a mouse model of DYT25 dystonia

Dystonia is often associated with functional alterations in the cerebello-thalamic pathways, which have been proposed to contribute to the disorder by propagating pathological firing patterns to the forebrain. Here, we examined the function of the cerebello-thalamic pathways in a model of DYT25 dystonia. DYT25 (Gnal^+/−) mice carry a heterozygous knockout mutation of the Gnal gene, which notably disrupts striatal function, and systemic or striatal administration of oxotremorine to these mice triggers dystonic symptoms. Our results reveal an increased cerebello-thalamic excitability in the presymptomatic state. Following the first dystonic episode, Gnal^+/- mice in the asymptomatic state exhibit a further increase of the cerebello-thalamo-cortical excitability, which is maintained after θ-burst stimulations of the cerebellum. When administered in the symptomatic state induced by a cholinergic activation, these stimulations decreased the cerebello-thalamic excitability and reduced dystonic symptoms. In agreement with dystonia being a multiregional circuit disorder, our results suggest that the increased cerebello-thalamic excitability constitutes an early endophenotype, and that the cerebellum is a gateway for corrective therapies via the depression of cerebello-thalamic pathways.

G protein-coupled receptor-effector macromolecular membrane assemblies (GEMMAs)

G protein-coupled receptors (GPCRs) are the largest group of receptors involved in cellular signaling across the plasma membrane and a major class of drug targets. The canonical model for GPCR signaling involves three components - the GPCR, a heterotrimeric G protein and a proximal plasma membrane effector - that have been generally thought to be freely mobile molecules able to interact by 'collision coupling'. Here, we synthesize evidence that supports the existence of GPCR-effector macromolecular membrane assemblies (GEMMAs) comprised of specific GPCRs, G proteins, plasma membrane effector molecules and other associated transmembrane proteins that are pre-assembled prior to receptor activation by agonists, which then leads to subsequent rearrangement of the GEMMA components. The GEMMA concept offers an alternative and complementary model to the canonical collision-coupling model, allowing more efficient interactions between specific signaling components, as well as the integration of the concept of GPCR oligomerization as well as GPCR interactions with orphan receptors, truncated GPCRs and other membrane-localized GPCR-associated proteins. Collision-coupling and pre-assembled mechanisms are not exclusive and likely both operate in the cell, providing a spectrum of signaling modalities which explains the differential properties of a multitude of GPCRs in their different cellular environments. Here, we explore the unique pharmacological characteristics of individual GEMMAs, which could provide new opportunities to therapeutically modulate GPCR signaling.

The Signaling and Pharmacology of the Dopamine D1 Receptor

The dopamine D1 receptor (D1R) is a Gα_s/olf-coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα_s or Gα_olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.

Selective Manipulation of G-Protein γ7 Subunit in Mice Provides New Insights into Striatal Control of Motor Behavior

Stimulatory coupling of dopamine D₁ (D₁R) and adenosine A_2A receptors (A_2AR) to adenylyl cyclase within the striatum is mediated through a specific Gα_olfβ₂γ₇ heterotrimer to ultimately modulate motor behaviors. To dissect the individual roles of the Gα_olfβ₂γ₇ heterotrimer in different populations of medium spiny neurons (MSNs), we produced and characterized conditional mouse models, in which the Gng7 gene was deleted in either the D₁R- or A_2AR/D₂R-expressing MSNs. We show that conditional loss of γ₇ disrupts the cell type-specific assembly of the Gα_olfβ₂γ₇ heterotrimer, thereby identifying its circumscribed roles acting downstream of either the D₁Rs or A_2ARs in coordinating motor behaviors, including in vivo responses to psychostimulants. We reveal that Gα_olfβ₂γ₇/cAMP signal in D₁R-MSNs does not impact spontaneous and amphetamine-induced locomotor behaviors in male and female mice, while its loss in A_2AR/D₂R-MSNs results in a hyperlocomotor phenotype and enhanced locomotor response to amphetamine. Additionally, Gα_olfβ₂γ₇/cAMP signal in either D₁R- or A_2AR/D₂R-expressing MSNs is not required for the activation of PKA signaling by amphetamine. Finally, we show that Gα_olfβ₂γ₇ signaling acting downstream of D₁Rs is selectively implicated in the acute locomotor-enhancing effects of morphine. Collectively, these results support the general notion that receptors use specific Gαβγ proteins to direct the fidelity of downstream signaling pathways and to elicit a diverse repertoire of cellular functions. Specifically, these findings highlight the critical role for the γ₇ protein in determining the cellular level, and hence, the function of the Gα_olfβ₂γ₇ heterotrimer in several disease states associated with dysfunctional striatal signaling.SIGNIFICANCE STATEMENT Dysfunction or imbalance of cAMP signaling in the striatum has been linked to several neurologic and neuropsychiatric disorders, including Parkinson's disease, dystonia, schizophrenia, and drug addiction. By genetically targeting the γ₇ subunit in distinct striatal neuronal subpopulations in mice, we demonstrate that the formation and function of the Gα_olfβ₂γ₇ heterotrimer, which represents the rate-limiting step for cAMP production in the striatum, is selectively disrupted. Furthermore, we reveal cell type-specific roles for Gα_olfβ₂γ₇-mediated cAMP production in the control of spontaneous locomotion as well as behavioral and molecular responses to psychostimulants. Our findings identify the γ₇ protein as a novel therapeutic target for disease states associated with dysfunctional striatal cAMP signaling.

The critical balance between dopamine D2 receptor and RGS for the sensitive detection of a transient decay in dopamine signal

In behavioral learning, reward-related events are encoded into phasic dopamine (DA) signals in the brain. In particular, unexpected reward omission leads to a phasic decrease in DA (DA dip) in the striatum, which triggers long-term potentiation (LTP) in DA D2 receptor (D2R)-expressing spiny-projection neurons (D2 SPNs). While this LTP is required for reward discrimination, it is unclear how such a short DA-dip signal (0.5-2 s) is transferred through intracellular signaling to the coincidence detector, adenylate cyclase (AC). In the present study, we built a computational model of D2 signaling to determine conditions for the DA-dip detection. The DA dip can be detected only if the basal DA signal sufficiently inhibits AC, and the DA-dip signal sufficiently disinhibits AC. We found that those two requirements were simultaneously satisfied only if two key molecules, D2R and regulators of G protein signaling (RGS) were balanced within a certain range; this balance has indeed been observed in experimental studies. We also found that high level of RGS was required for the detection of a 0.5-s short DA dip, and the analytical solutions for these requirements confirmed their universality. The imbalance between D2R and RGS is associated with schizophrenia and DYT1 dystonia, both of which are accompanied by abnormal striatal LTP. Our simulations suggest that D2 SPNs in patients with schizophrenia and DYT1 dystonia cannot detect short DA dips. We finally discussed that such psychiatric and movement disorders can be understood in terms of the imbalance between D2R and RGS.

The mini player with diverse functions: extracellular vesicles in cell biology, disease, and therapeutics

Extracellular vesicles (EVs) are tiny biological nanovesicles ranging from approximately 30-1000 nm in diameter that are released into the extracellular matrix of most cell types and in biofluids. The classification of EVs includes exosomes, microvesicles, and apoptotic bodies, dependent on various factors such as size, markers, and biogenesis pathways. The transition of EV relevance from that of being assumed as a trash bag to be a key player in critical physiological and pathological conditions has been revolutionary in many ways. EVs have been recently revealed to play a crucial role in stem cell biology and cancer progression via intercellular communication, contributing to organ development and the progression of cancer. This review focuses on the significant research progress made so far in the role of the crosstalk between EVs and stem cells and their niche, and cellular communication among different germ layers in developmental biology. In addition, it discusses the role of EVs in cancer progression and their application as therapeutic agents or drug delivery vehicles. All such discoveries have been facilitated by tremendous technological advancements in EV-associated research, especially the microfluidics systems. Their pros and cons in the context of characterization of EVs are also extensively discussed in this review. This review also deliberates the role of EVs in normal cell processes and disease conditions, and their application as a diagnostic and therapeutic tool. Finally, we propose future perspectives for EV-related research in stem cell and cancer biology.

Gαo is a major determinant of cAMP signaling in the pathophysiology of movement disorders

The G protein alpha subunit o (Gαo) is one of the most abundant proteins in the nervous system, and pathogenic mutations in its gene (GNAO1) cause movement disorder. However, the function of Gαo is ill defined mechanistically. Here, we show that Gαo dictates neuromodulatory responsiveness of striatal neurons and is required for movement control. Using in vivo optical sensors and enzymatic assays, we determine that Gαo provides a separate transduction channel that modulates coupling of both inhibitory and stimulatory dopamine receptors to the cyclic AMP (cAMP)-generating enzyme adenylyl cyclase. Through a combination of cell-based assays and rodent models, we demonstrate that GNAO1-associated mutations alter Gαo function in a neuron-type-specific fashion via a combination of a dominant-negative and loss-of-function mechanisms. Overall, our findings suggest that Gαo and its pathological variants function in specific circuits to regulate neuromodulatory signals essential for executing motor programs.

Muntean et al. describe biochemical, cellular, and physiological mechanisms by which the heterotrimeric G protein subunit Gαo controls neuromodulatory signaling in the striatum and elucidate mechanisms by which Gαo mutations compromise movements in GNAO1 disorder.

Signaling models for dopamine-dependent temporal contiguity in striatal synaptic plasticity

Animals remember temporal links between their actions and subsequent rewards. We previously discovered a synaptic mechanism underlying such reward learning in D1 receptor (D1R)-expressing spiny projection neurons (D1 SPN) of the striatum. Dopamine (DA) bursts promote dendritic spine enlargement in a time window of only a few seconds after paired pre- and post-synaptic spiking (pre-post pairing), which is termed as reinforcement plasticity (RP). The previous study has also identified underlying signaling pathways; however, it still remains unclear how the signaling dynamics results in RP. In the present study, we first developed a computational model of signaling dynamics of D1 SPNs. The D1 RP model successfully reproduced experimentally observed protein kinase A (PKA) activity, including its critical time window. In this model, adenylate cyclase type 1 (AC1) in the spines/thin dendrites played a pivotal role as a coincidence detector against pre-post pairing and DA burst. In particular, pre-post pairing (Ca2+ signal) stimulated AC1 with a delay, and the Ca2+-stimulated AC1 was activated by the DA burst for the asymmetric time window. Moreover, the smallness of the spines/thin dendrites is crucial to the short time window for the PKA activity. We then developed a RP model for D2 SPNs, which also predicted the critical time window for RP that depended on the timing of pre-post pairing and phasic DA dip. AC1 worked for the coincidence detector in the D2 RP model as well. We further simulated the signaling pathway leading to Ca2+/calmodulin-dependent protein kinase II (CaMKII) activation and clarified the role of the downstream molecules of AC1 as the integrators that turn transient input signals into persistent spine enlargement. Finally, we discuss how such timing windows guide animals' reward learning.

Impaired dopamine- and adenosine-mediated signaling and plasticity in a novel rodent model for DYT25 dystonia

Dystonia is a neurological movement disorder characterized by sustained or intermittent involuntary muscle contractions. Loss-of-function mutations in the GNAL gene have been identified to be the cause of "isolated" dystonia DYT25. The GNAL gene encodes for the guanine nucleotide-binding protein G(olf) subunit alpha (Gα_olf), which is mainly expressed in the olfactory bulb and the striatum and functions as a modulator during neurotransmission coupling with D1R and A2AR. Previously, heterozygous Gα_olf -deficient mice (Gnal^+/-) have been generated and showed a mild phenotype at basal condition. In contrast, homozygous deletion of Gnal in mice (Gnal^-/-) resulted in a significantly reduced survival rate. In this study, using the CRISPR-Cas9 system we generated and characterized heterozygous Gnal knockout rats (Gnal^+/-) with a 13 base pair deletion in the first exon of the rat Gnal splicing variant 2, a major isoform in both human and rat striatum. Gnal^+/- rats showed early-onset phenotypes associated with impaired dopamine transmission, including reduction in locomotor activity, deficits in rotarod performance and an abnormal motor skill learning ability. At cellular and molecular level, we found down-regulated Arc expression, increased cell surface distribution of AMPA receptors, and the loss of D2R-dependent corticostriatal long-term depression (LTD) in Gnal^+/- rats. Based on the evidence that D2R activity is normally inhibited by adenosine A2ARs, co-localized on the same population of striatal neurons, we show that blockade of A2ARs restores physiological LTD. This animal model may be a valuable tool for investigating Gα_olf function and finding a suitable treatment for dystonia associated with deficient dopamine transmission.

NF1-cAMP signaling dissociates cell type-specific contributions of striatal medium spiny neurons to reward valuation and motor control

The striatum plays a fundamental role in motor learning and reward-related behaviors that are synergistically shaped by populations of D1 dopamine receptor (D1R)- and D2 dopamine receptor (D2R)-expressing medium spiny neurons (MSNs). How various neurotransmitter inputs converging on common intracellular pathways are parsed out to regulate distinct behavioral outcomes in a neuron-specific manner is poorly understood. Here, we reveal that distinct contributions of D1R-MSNs and D2R-MSNs towards reward and motor behaviors are delineated by the multifaceted signaling protein neurofibromin 1 (NF1). Using genetic mouse models, we show that NF1 in D1R-MSN modulates opioid reward, whereas loss of NF1 in D2R-MSNs delays motor learning by impeding the formation and consolidation of repetitive motor sequences. We found that motor learning deficits upon NF1 loss were associated with the disruption in dopamine signaling to cAMP in D2R-MSN. Restoration of cAMP levels pharmacologically or chemogenetically rescued the motor learning deficits seen upon NF1 loss in D2R-MSN. Our findings illustrate that multiplex signaling capabilities of MSNs are deployed at the level of intracellular pathways to achieve cell-specific control over behavioral outcomes.

A mouse genetic study reveals that the multifaceted signaling protein neurofibromin (known for its role in the human genetic disease neurofibromatosis type 1) plays a key role in differential routing of motor and reward signals in populations of striatal medium spiny neurons.

Nuclear Receptor Nr4a1 Regulates Striatal Striosome Development and Dopamine D1 Receptor Signaling

The GABAergic medium-size spiny neuron (MSN), the striatal output neuron, may be classified into striosome, also known as patch, and matrix, based on neurochemical differences between the two compartments. At this time, little is known regarding the regulation of the development of the two compartments. Nr4a1, primarily described as a nuclear receptor/immediate early gene involved in the homeostasis of the dopaminergic system, is a striosomal marker. Using Nr4a1-overexpressing and Nr4a1-null mice, we sought to determine whether Nr4a1 is necessary and/or sufficient for striosome development. We report that in vivo and in vitro, Nr4a1 and Oprm1 mRNA levels are correlated. In the absence of Nr4a, there is a decrease in the percentage of striatal surface area occupied by striosomes. Alterations in Nr4a1 expression leads to dysregulation of multiple mRNAs of members of the dopamine receptor D₁ signal transduction system. Constitutive overexpression of Nr4a1 decreases both the induction of phosphorylation of ERK after a single cocaine exposure and locomotor sensitization following chronic cocaine exposure. Nr4a1 overexpression increases MSN excitability but reduces MSN long-term potentiation. In the resting state, type 5 adenylyl cyclase (AC5) activity is normal, but the ability of AC5 to be activated by Drd1 G-protein-coupled receptor inputs is decreased. Our results support a role for Nr4a1 in determination of striatal patch/matrix structure and in regulation of dopaminoceptive neuronal function.

Regulation of adenylyl cyclase 5 in striatal neurons confers the ability to detect coincident neuromodulatory signals

Long-term potentiation and depression of synaptic activity in response to stimuli is a key factor in reinforcement learning. Strengthening of the corticostriatal synapses depends on the second messenger cAMP, whose synthesis is catalysed by the enzyme adenylyl cyclase 5 (AC5), which is itself regulated by the stimulatory Gαolf and inhibitory Gαi proteins. AC isoforms have been suggested to act as coincidence detectors, promoting cellular responses only when convergent regulatory signals occur close in time. However, the mechanism for this is currently unclear, and seems to lie in their diverse regulation patterns. Despite attempts to isolate the ternary complex, it is not known if Gαolf and Gαi can bind to AC5 simultaneously, nor what activity the complex would have. Using protein structure-based molecular dynamics simulations, we show that this complex is stable and inactive. These simulations, along with Brownian dynamics simulations to estimate protein association rates constants, constrain a kinetic model that shows that the presence of this ternary inactive complex is crucial for AC5's ability to detect coincident signals, producing a synergistic increase in cAMP. These results reveal some of the prerequisites for corticostriatal synaptic plasticity, and explain recent experimental data on cAMP concentrations following receptor activation. Moreover, they provide insights into the regulatory mechanisms that control signal processing by different AC isoforms.

The high efficacy of muscarinic M4 receptor in D1 medium spiny neurons reverses striatal hyperdopaminergia

The opposing action of dopamine and acetylcholine has long been known to play an important role in basal ganglia physiology. However, the quantitative analysis of dopamine and acetylcholine signal interaction has been difficult to perform in the native context because the striatum comprises mainly two subtypes of medium-sized spiny neurons (MSNs) on which these neuromodulators exert different actions. We used biosensor imaging in live brain slices of dorsomedial striatum to monitor changes in intracellular cAMP at the level of individual MSNs. We observed that the muscarinic agonist oxotremorine decreases cAMP selectively in the MSN subpopulation that also expresses D₁ dopamine receptors, an action mediated by the M₄ muscarinic receptor. This receptor has a high efficacy on cAMP signaling and can shut down the positive cAMP response induced by dopamine, at acetylcholine concentrations which are consistent with physiological levels. This supports our prediction based on theoretical modeling that acetylcholine could exert a tonic inhibition on striatal cAMP signaling, thus supporting the possibility that a pause in acetylcholine release is required for phasic dopamine to transduce a cAMP signal in D1 MSNs. In vivo experiments with acetylcholinesterase inhibitors donepezil and tacrine, as well as with the positive allosteric modulators of M₄ receptor VU0152100 and VU0010010 show that this effect is sufficient to reverse the increased locomotor activity of DAT-knockout mice. This suggests that M₄ receptors could be a novel therapeutic target to treat hyperactivity disorders.

Evidence for functional pre-coupled complexes of receptor heteromers and adenylyl cyclase

G protein-coupled receptors (GPCRs), G proteins and adenylyl cyclase (AC) comprise one of the most studied transmembrane cell signaling pathways. However, it is unknown whether the ligand-dependent interactions between these signaling molecules are based on random collisions or the rearrangement of pre-coupled elements in a macromolecular complex. Furthermore, it remains controversial whether a GPCR homodimer coupled to a single heterotrimeric G protein constitutes a common functional unit. Using a peptide-based approach, we here report evidence for the existence of functional pre-coupled complexes of heteromers of adenosine A_2A receptor and dopamine D₂ receptor homodimers coupled to their cognate Gs and Gi proteins and to subtype 5 AC. We also demonstrate that this macromolecular complex provides the necessary frame for the canonical Gs-Gi interactions at the AC level, sustaining the ability of a Gi-coupled GPCR to counteract AC activation mediated by a Gs-coupled GPCR.

It is unclear whether GPCRs, G proteins and adenylyl cyclase (AC) associate through random collisions or defined pre-coupling mechanisms. Using a peptide-based approach, the authors show that heteromers of adenosine A_2A and dopamine D₂ receptors form pre-coupled complexes with their cognate G proteins and AC5.

Development of novel biosensors to study receptor-mediated activation of the G-protein α subunits Gs and Golf

Gαs (Gs) and Gαolf (Golf) are highly homologous G-protein α subunits that activate adenylate cyclase, thereby serving as crucial mediators of intracellular signaling. Because of their dramatically different brain expression patterns, we studied similarities and differences between their activation processes with the aim of comparing their receptor coupling mechanisms. We engineered novel luciferase- and Venus-fused Gα constructs that can be used in bioluminescence resonance energy transfer assays. In conjunction with molecular simulations, these novel biosensors were used to determine receptor activation-induced changes in conformation. Relative movements in Gs were consistent with the crystal structure of β2 adrenergic receptor in complex with Gs Conformational changes in Golf activation are shown to be similar to those in Gs Overall the current study reveals general similarities between Gs and Golf activation at the molecular level and provides a novel set of tools to search for Gs- and Golf-specific receptor pharmacology. In view of the wide functional and pharmacological roles of Gs- and Golf-coupled dopamine D1 receptor and adenosine A2A receptor in the brain and other organs, elucidating their differential structure-function relationships with Gs and Golf might provide new approaches for the treatment of a variety of neuropsychiatric disorders. In particular, these novel biosensors can be used to reveal potentially therapeutic dopamine D1 receptor and adenosine A2A receptor ligands with functionally selective properties between Gs and Golf signaling.

Conditional Deletion of Ric-8b in Olfactory Sensory Neurons Leads to Olfactory Impairment

The olfactory system can discriminate a vast number of odorants. This ability derives from the existence of a large family of odorant receptors expressed in the cilia of the olfactory sensory neurons. Odorant receptors signal through the olfactory-specific G-protein subunit, Gαolf. Ric-8b, a guanine nucleotide exchange factor, interacts with Gαolf and can amplify odorant receptor signal transduction in vitro To explore the function of Ric-8b in vivo, we generated a tissue specific knock-out mouse by crossing OMP-Cre transgenic mice to Ric-8b floxed mice. We found that olfactory-specific Ric-8b knock-out mice of mixed sex do not express the Gαolf protein in the olfactory epithelium. We also found that in these mice, the mature olfactory sensory neuron layer is reduced, and that olfactory sensory neurons show increased rate of cell death compared with wild-type mice. Finally, behavioral tests showed that the olfactory-specific Ric-8b knock-out mice show an impaired sense of smell, even though their motivation and mobility behaviors remain normal.SIGNIFICANCE STATEMENT Ric-8b is a guanine nucleotide exchange factor (GEF) expressed in the olfactory epithelium and in the striatum. Ric-8b interacts with the olfactory Gαolf subunit, and can amplify odorant signaling through odorant receptors in vitro However, the functional significance of this GEF in the olfactory neurons in vivo remains unknown. We report that deletion of Ric-8b in olfactory sensory neurons prevents stable expression of Gαolf. In addition, we demonstrate that olfactory neurons lacking Ric-8b (and consequently Gαolf) are more susceptible to cell death. Ric-8b conditional knock-out mice display impaired olfactory guided behavior. Our results reveal that Ric-8b is essential for olfactory function, and suggest that it may also be essential for Gαolf-dependent functions in the brain.

The BEACH Protein LRBA Promotes the Localization of the Heterotrimeric G-protein Golf to Olfactory Cilia

BEACH domain proteins are involved in membrane protein traffic and human diseases, but their molecular mechanisms are not understood. The BEACH protein LRBA has been implicated in immune response and cell proliferation, and human LRBA mutations cause severe immune deficiency. Here, we report a first functional and molecular phenotype outside the immune system of LRBA-knockout mice: compromised olfaction, manifesting in reduced electro-olfactogram response amplitude, impaired food-finding efficiency, and smaller olfactory bulbs. LRBA is prominently expressed in olfactory and vomeronasal chemosensory neurons of wild-type mice. Olfactory impairment in the LRBA-KO is explained by markedly reduced concentrations (20–40% of wild-type levels) of all three subunits α_olf, β₁ and γ₁₃ of the olfactory heterotrimeric G-protein, G_olf, in the sensory cilia of olfactory neurons. In contrast, cilia morphology and the concentrations of many other proteins of olfactory cilia are not or only slightly affected. LRBA is also highly expressed in photoreceptor cells, another cell type with a specialized sensory cilium and heterotrimeric G-protein-based signalling; however, visual function appeared unimpaired by the LRBA-KO. To our knowledge, this is the first observation that a BEACH protein is required for the efficient subcellular localization of a lipid-anchored protein, and of a ciliary protein.

Heterozygous Gnal Mice Are a Novel Animal Model with Which to Study Dystonia Pathophysiology

Dystonia is a movement disorder characterized by sustained or intermittent muscle contractions and its pathophysiological mechanisms are still poorly understood. Dominant mutations of the GNAL gene are a cause of isolated dystonia (DYT25) in patients. Some mutations result in a complete loss of function of the encoded protein, Gα_olf, an adenylyl-cyclase-stimulatory G-protein highly enriched in striatal projection neurons, where it mediates the actions of dopamine and adenosine. We used male and female heterozygous Gnal knock-out mice (Gnal^+/-) to study how GNAL haplodeficiency is implicated in dystonia. In basal conditions, no overt dystonic movements or postures or change in locomotor activity were observed. However, Gnal haploinsufficiency altered self-grooming, motor coordination, and apparent motivation in operant conditioning, as well as spine morphology and phospho-CaMKIIβ in the striatum. After systemic administration of oxotremorine, an unselective cholinergic agonist, Gnal^+/- mice developed more abnormal postures and movements than WT mice. These effects were not caused by seizures as indicated by EEG recordings. They were prevented by the M1-preferring muscarinic antagonists, telenzepine, pirenzepine, and trihexyphenidyl, which alleviate dystonic symptoms in patients. The motor defects were worsened by mecamylamine, a selective nicotinic antagonist. These oxotremorine-induced abnormalities in Gnal^+/- mice were replicated by oxotremorine infusion into the striatum, but not into the cerebellum, indicating that defects in striatal neurons favor the appearance of dystonia-like movement alterations after oxotremorine. Untreated and oxotremorine-treated Gnal^+/- mice provide a model of presymptomic and symptomatic stages of DYT25-associated dystonia, respectively, and clues about the mechanisms underlying dystonia pathogenesis.SIGNIFICANCE STATEMENT Adult-onset dystonia DYT25 is caused by dominant loss-of-function mutations of GNAL, a gene encoding the stimulatory G-protein Gαolf, which is critical for activation of the cAMP pathway in the striatal projection neurons. Here, we demonstrate that Gnal-haplodeficient mice have a mild neurological phenotype and display vulnerability to developing dystonic movements after systemic or intrastriatal injection of the cholinergic agonist oxotremorine. Therefore, impairment of the cAMP pathway in association with an increased cholinergic tone creates alterations in striatal neuron functions that can promote the onset of dystonia. Our results also provide evidence that untreated and oxotremorine-treated Gnal-haplodeficient mice are powerful models with which to study presymptomic and symptomatic stages of DYT25-associated dystonia, respectively.