Molecular Deconvolution Platform to Establish Disease Mechanisms by Surveying GPCR Signaling

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.

Implications of TRPM3 and TRPM8 for sensory neuron sensitisation

Sensory neurons serve to receive and transmit a wide range of information about the conditions of the world around us as well as the external and internal state of our body. Sensitisation of these nerve cells, i.e. becoming more sensitive to stimuli or the emergence or intensification of spontaneous activity, for example in the context of inflammation or nerve injury, can lead to chronic diseases such as neuropathic pain. For many of these disorders there are only very limited treatment options and in order to find and establish new therapeutic approaches, research into the exact causes of sensitisation with the elucidation of the underlying mechanisms and the identification of the molecular components is therefore essential. These components include plasma membrane receptors and ion channels that are involved in signal reception and transmission. Members of the transient receptor potential (TRP) channel family are also expressed in sensory neurons and some of them play a crucial role in temperature perception. This review article focuses on the heat-sensitive TRPM3 and the cold-sensitive TRPM8 (and TRPA1) channels and their importance in sensitisation of dorsal root ganglion sensory neurons is discussed based on studies related to inflammation and injury- as well as chemotherapy-induced neuropathy.

Conditional Knockout of Striatal Gnal Produces Dystonia-like Motor Phenotypes

Loss-of-function mutations in GNAL have been linked to an adult-onset, isolated dystonia that is largely indistinguishable from idiopathic dystonia. GNAL encodes Gα olf , a heterotrimeric G-protein α subunit with a defined molecular function to increase the production of the second messenger cAMP. Gα olf is abundant in the striatum, and is the only stimulatory G-protein in many cell types of the striatum. Due to the defined molecular signaling pathway and expression pattern of Gα olf , the clear genetic link to dystonia makes GNAL an exciting target to understand the pathological mechanisms of not only this genetic dystonia, but also the larger idiopathic disease. To better understand GNAL -linked dystonia, we generated a novel genetic mouse model that allows us to conditionally knock out Gnal in a site and time-specific manner. In the current study we used genetic or AAV based approaches to express Cre to knockout striatal Gnal in our novel Gnal fl/fl model. We then performed motor behavioral testing and ex vivo whole-cell patch clamp electrophysiology of striatal spiny projection neurons to interrogate how loss of Gnal leads to dystonia. Mice with conditional striatal knockout of Gnal show hindlimb clasping, other dystonia-like postures, less motor coordination, slowness, and torticollis as compared to age-matched controls. Furthermore, striatal spiny projection neurons show increased excitability in Gnal knockout animals. These exciting data are the first to report uninduced, overt dystonia in a mouse model of GNAL- linked dystonia, and directly correlate these with changes in spiny projection neuron electrophysiological properties. Our results show that adult loss of Gnal in the striatum leads to the development of dystonia, through homeostatic, paradoxical increases in spiny projection neuron excitability, and suggest that therapeutic strategies aimed at decreasing this hyperexcitable phenotype may provide symptomatic relief for patients with disease. One Sentence Summary: When Gnal is knocked out in the striatum of mice we observe overt behavioral symptoms and hyperexcitability in striatal spiny projection neurons.

G α olf Regulates Biochemical Signaling in Neurons Associated with Movement Control and Initiation

The heterotrimeric G-protein α subunit, Gα olf , 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 Gα olf , 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 Gα olf 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 Gα olf 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 Gα olf signaling in vivo in neurons that highly express Gnal .

Severity of GNAO1-Related Disorder Correlates with Changes in G-Protein Function

OBJECTIVE

GNAO1-related disorders (OMIM #615473 and #617493), caused by variants in the GNAO1 gene, are characterized by developmental delay or intellectual disability, hypotonia, movement disorders, and epilepsy. Neither a genotype-phenotype correlation nor a clear severity score have been established for this disorder. The objective of this prospective and retrospective observational study was to develop a severity score for GNAO1-related disorders, and to delineate the correlation between the underlying molecular mechanisms and clinical severity.

METHODS

A total of 16 individuals with GNAO1-related disorders harboring 12 distinct missense variants, including four novel variants (p.K46R, p.T48I, p.R209P, and p.L235P), were examined with repeated clinical assessments, video-electroencephalogram monitoring, and brain magnetic resonance imaging. The molecular pathology of each variant was delineated using a molecular deconvoluting platform.

RESULTS

The patients displayed a wide variability in the severity of their symptoms. This heterogeneity was well represented in the GNAO1-related disorders severity score, with a broad range of results. Patients with the same variant had comparable severity scores, indicating that differences in disease profiles are not due to interpatient variability, but rather, to unique disease mechanisms. Moreover, we found a significant correlation between clinical severity scores and molecular mechanisms.

INTERPRETATION

The clinical score proposed here provides further insight into the correlation between pathophysiology and phenotypic severity in GNAO1-related disorders. We found that each variant has a unique profile of clinical phenotypes and pathological molecular mechanisms. These findings will contribute to better understanding GNAO1-related disorders. Additionally, the severity score will facilitate standardization of patients categorization and assessment of response to therapies in development. ANN NEUROL 2023;94:987-1004.

Multiple potassium channel tetramerization domain (KCTD) family members interact with Gβγ, with effects on cAMP signaling

G protein-coupled receptors (GPCRs) initiate an array of intracellular signaling programs by activating heterotrimeric G proteins (Gα and Gβγ subunits). Therefore, G protein modifiers are well positioned to shape GPCR pharmacology. A few members of the potassium channel tetramerization domain (KCTD) protein family have been found to adjust G protein signaling through interaction with Gβγ. However, comprehensive details on the KCTD interaction with Gβγ remain unresolved. Here, we report that nearly all the 25 KCTD proteins interact with Gβγ. In this study, we screened Gβγ interaction capacity across the entire KCTD family using two parallel approaches. In a live cell bioluminescence resonance energy transfer-based assay, we find that roughly half of KCTD proteins interact with Gβγ in an agonist-induced fashion, whereas all KCTD proteins except two were found to interact through coimmunoprecipitation. We observed that the interaction was dependent on an amino acid hot spot in the C terminus of KCTD2, KCTD5, and KCTD17. While KCTD2 and KCTD5 require both the Bric-à-brac, Tramtrack, Broad complex domain and C-terminal regions for Gβγ interaction, we uncovered that the KCTD17 C terminus is sufficient for Gβγ interaction. Finally, we demonstrated the functional consequence of the KCTD-Gβγ interaction by examining sensitization of the adenylyl cyclase-cAMP pathway in live cells. We found that Gβγ-mediated sensitization of adenylyl cyclase 5 was blunted by KCTD. We conclude that the KCTD family broadly engages Gβγ to shape GPCR signal transmission.

A Naturally Occurring Membrane-Anchored Gαs Variant, XLαs, Activates Phospholipase Cβ4

Extra-large stimulatory Gα (XLα_s) is a large variant of G protein α_s subunit (Gα_s) that uses an alternative promoter and thus differs from Gα_s at the first exon. XLα_s activation by G protein–coupled receptors mediates cAMP generation, similarly to Gα_s; however, Gα_s and XLα_s have been shown to have distinct cellular and physiological functions. For example, previous work suggests that XLα_s can stimulate inositol phosphate production in renal proximal tubules and thereby regulate serum phosphate levels. In this study, we show that XLα_s directly and specifically stimulates a specific isoform of phospholipase Cβ (PLCβ), PLCβ4, both in transfected cells and with purified protein components. We demonstrate that neither the ability of XLα_s to activate cAMP generation nor the canonical G protein switch II regions are required for PLCβ stimulation. Furthermore, this activation is nucleotide independent but is inhibited by Gβγ, suggesting a mechanism of activation that relies on Gβγ subunit dissociation. Surprisingly, our results indicate that enhanced membrane targeting of XLα_s relative to Gα_s confers the ability to activate PLCβ4. We also show that PLCβ4 is required for isoproterenol-induced inositol phosphate accumulation in osteocyte-like Ocy454 cells. Taken together, we demonstrate a novel mechanism for activation of phosphoinositide turnover downstream of G_s-coupled receptors that may have a critical role in endocrine physiology.

Bradykinin-Induced Sensitization of Transient Receptor Potential Channel Melastatin 3 Calcium Responses in Mouse Nociceptive Neurons

TRPM3 is a calcium-permeable cation channel expressed in a range of sensory neurons that can be activated by heat and the endogenous steroid pregnenolone sulfate (PS). During inflammation, the expression and function of TRPM3 are both augmented in somatosensory nociceptors. However, in isolated dorsal root ganglion (DRG) neurons application of inflammatory mediators like prostaglandins and bradykinin (BK) inhibit TRPM3. Therefore, the aim of this study was to examine the effect of preceding activation of cultured 1 day old mouse DRG neurons by the inflammatory mediator BK on TRPM3-mediated calcium responses. Calcium signals were recorded using the intensity-based dye Fluo-8. We found that TRPM3-mediated calcium responses to PS were enhanced by preceding application of BK in cells that responded to BK with a calcium signal, indicating BK receptor (BKR) expression. The majority of cells that co-expressed TRPM3 and BKRs also expressed TRPV1, however, only a small fraction co-expressed TRPA1, identified by calcium responses to capsaicin and supercinnamaldehyde, respectively. Signaling and trafficking pathways responsible for sensitization of TRPM3 following BK were characterized using inhibitors of second messenger signaling cascades and exocytosis. Pharmacological blockade of protein kinase C, calcium–calmodulin-dependent protein kinase II and diacylglycerol (DAG) lipase did not affect BK-induced sensitization, but inhibition of DAG kinase did. In addition, release of calcium from intracellular stores using thapsigargin also resulted in TRPM3 sensitization. Finally, BK did not sensitize TRPM3 in the presence of exocytosis inhibitors. Collectively, we show that preceding activation of DRG neurons by BK sensitized TRPM3-mediated calcium responses to PS. Our results indicate that BKR-mediated activation of intracellular signaling pathways comprising DAG kinase, calcium and exocytosis may contribute to TRPM3 sensitization during inflammation.

Extended Phenotyping and Functional Validation Facilitate Diagnosis of a Complex Patient Harboring Genetic Variants in MCCC1 and GNB5 Causing Overlapping Phenotypes

Identifying multiple ultra-rare genetic syndromes with overlapping phenotypes is a diagnostic conundrum in clinical genetics. This study investigated the pathogenicity of a homozygous missense variant in GNB5 (GNB5L; NM_016194.4: c.920T > G (p. Leu307Arg); GNB5S; NM_006578.4: c.794T > G (p. Leu265Arg)) identified through exome sequencing in a female child who also had 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency (newborn screening positive) and hemoglobin E trait. The proband presented with early-onset intellectual disability, the severity of which was more in keeping with GNB5-related disorder than 3-MCC deficiency. She later developed bradycardia and cardiac arrest, and upon re-phenotyping showed cone photo-transduction recovery deficit, all known only to GNB5-related disorders. Patient-derived fibroblast assays showed preserved GNB5S expression, but bioluminescence resonance energy transfer assay showed abolished function of the variant reconstituted Gβ5S containing RGS complexes for deactivation of D2 dopamine receptor activity, confirming variant pathogenicity. This study highlights the need for precise phenotyping and functional assays to facilitate variant classification and clinical diagnosis in patients with complex medical conditions.

Constitutive activity of GPR26 regulated by ubiquitin-dependent degradation and its antitumor role

G protein-coupled receptors (GPCRs) play important roles in many physiological functions and numerous diseases. In addition to the classic ligand-stimulated receptor activity, an increasing number of studies have established that many GPCRs function constitutively in a receptor dose-dependent manner. Previous observations showed that following gene transfection, little or no protein was detectable for certain GPCRs (designated apparent state A), such as GPR26, GPR39, GPR78, GPR133, GPR139, BRS3, and LGR5, which showed strong constitutive activities. When we lysed cells in the immediate presence of western blot loading buffer, a significant increase of protein levels was detected (actual state B), which was much closer to the true expression levels under physiological conditions. GPR26 was chosen for further functional experiments as the actual state B. We identified an important ubiquitination site, K286, as well as the ubiquitin ligase E3 homologous to the E6-associated protein carboxyl terminus domain containing 3 interacting with GPR26. The pronounced differences in the protein expression and constitutive activity of GPR26 were a consequence of the ubiquitin-mediated rapid degradation mechanism. Furthermore, we identified in vitro and in vivo antitumor activity associated with high expression levels and constitutive activity of GPR26 in liver cancer cells. Hence, GPR26 could act as an antitumor gene for hepatocellular carcinoma. This study also represents the actual state B of a batch of GPCRs that actually play potentially important roles in physiological functions by their constitutive activity, which is controlled by rapid ubiquitin-dependent degradation.

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.

Striatal Dopamine Induced ERK Phosphorylation Is Altered in Mouse Models of Monogenic Dystonia

BACKGROUND

Similar to some monogenic forms of dystonia, levodopa-induced dyskinesia is a hyperkinetic movement disorder with abnormal nigrostriatal dopaminergic neurotransmission. Molecularly, it is characterized by hyper-induction of phosphorylation of extracellular signal-related kinase in response to dopamine in medium spiny neurons of the direct pathway.

OBJECTIVES

The objective of this study was to determine if mouse models of monogenic dystonia exhibit molecular features of levodopa-induced dyskinesia.

METHODS

Western blotting and quantitative immunofluorescence was used to assay baseline and/or dopamine-induced levels of the phosphorylated kinase in the striatum in mouse models of DYT1, DYT6, and DYT25 expressing a reporter in dopamine D1 receptor-expressing projection neurons. Cyclic adenosine monophosphate (cAMP) immunoassay and adenylyl cyclase activity assays were also performed.

RESULTS

In DYT1 and DYT6 models, blocking dopamine reuptake with cocaine leads to enhanced extracellular signal-related kinase phosphorylation in dorsomedial striatal medium spiny neurons in the direct pathway, which is abolished by pretreatment with the N-methyl-d-aspartate antagonist MK-801. Phosphorylation is decreased in a model of DYT25. Levels of basal and stimulated cAMP and adenylyl cyclase activity were normal in the DYT1 and DYT6 mice and decreased in the DYT25 mice. Oxotremorine induced increased abnormal movements in the DYT1 knock-in mice.

CONCLUSIONS

The increased dopamine induction of extracellular signal-related kinase phosphorylation in 2 genetic types of dystonia, similar to what occurs in levodopa-induced dyskinesia, and its decrease in a third, suggests that abnormal signal transduction in response to dopamine in the postsynaptic nigrostriatal pathway might be a point of convergence for dystonia and other hyperkinetic movement disorders, potentially offering common therapeutic targets. © 2021 International Parkinson and Movement Disorder Society.

Isolated dystonia: clinical and genetic updates

Four genes associated with isolated dystonia are currently well replicated and validated. DYT-THAP1 manifests as young-onset generalized dystonia with predominant craniocervical symptoms; and is associated with mostly deleterious missense variation in the THAP1 gene. De novo and inherited missense and protein truncating variation in GNAL as well as primarily missense variation in ANO3 cause isolated focal and/or segmental dystonia with preference for the upper half of the body and older ages at onset. The GAG deletion in TOR1A is associated with generalized dystonia with onset in childhood in the lower limbs. Rare variation in these genes causes monogenic sporadic and inherited forms of isolated dystonia; common variation may confer risk and imply that dystonia is a polygenic trait in a subset of cases. Although candidate gene screens have been successful in the past in detecting gene-disease associations, recent application of whole-genome and whole-exome sequencing methods enable unbiased capture of all genetic variation that may explain the phenotype. However, careful variant-level evaluation is necessary in every case, even in genes that have previously been associated with disease. We review the genetic architecture and phenotype of DYT-THAP1, DYT-GNAL, DYT-ANO3, and DYT-TOR1A by collecting case reports from the literature and performing variant classification using pathogenicity criteria.

The structural basis for an on-off switch controlling Gβγ-mediated inhibition of TRPM3 channels

TRPM3 channels play important roles in the detection of noxious heat and in inflammatory thermal hyperalgesia. The activity of these ion channels in somatosensory neurons is tightly regulated by µ-opioid receptors through the signaling of Gβγ proteins, thereby reducing TRPM3-mediated pain. We show here that Gβγ directly binds to a domain of 10 amino acids in TRPM3 and solve a cocrystal structure of this domain together with Gβγ. Using these data and mutational analysis of full-length proteins, we pinpoint three amino acids in TRPM3 and their interacting partners in Gβ1 that are individually necessary for TRPM3 inhibition by Gβγ. The 10-amino-acid Gβγ-interacting domain in TRPM3 is subject to alternative splicing. Its inclusion in or exclusion from TRPM3 channel proteins therefore provides a mechanism for switching on or off the inhibitory action that Gβγ proteins exert on TRPM3 channels.

The orphan receptor GPR139 signals via Gq/11 to oppose opioid effects

The interplay between G protein-coupled receptors (GPCRs) is critical for controlling neuronal activity that shapes neuromodulatory outcomes. Recent evidence indicates that the orphan receptor GPR139 influences opioid modulation of key brain circuits by opposing the actions of the µ-opioid receptor (MOR). However, the function of GPR139 and its signaling mechanisms are poorly understood. In this study, we report that GPR139 activates multiple heterotrimeric G proteins, including members of the G_q/11 and G_i/o families. Using a panel of reporter assays in reconstituted HEK293T/17 cells, we found that GPR139 functions via the G_q/11 pathway and thereby distinctly regulates cellular effector systems, including stimulation of cAMP production and inhibition of G protein inward rectifying potassium (GIRK) channels. Electrophysiological recordings from medial habenular neurons revealed that GPR139 signaling via G_q/11 is necessary and sufficient for counteracting MOR-mediated inhibition of neuronal firing. These results uncover a mechanistic interplay between GPCRs involved in controlling opioidergic neuromodulation in the brain.