Pharmacological enhancement of mGlu1 metabotropic glutamate receptors causes a prolonged symptomatic benefit in a mouse model of spinocerebellar ataxia type 1

Pre-ataxic loss of intrinsic plasticity and motor learning in a mouse model of SCA1

Spinocerebellar ataxias are neurodegenerative diseases, the hallmark symptom of which is the development of ataxia due to cerebellar dysfunction. Purkinje cells, the principal neurons of the cerebellar cortex, are the main cells affected in these disorders, but the sequence of pathological events leading to their dysfunction is poorly understood. Understanding the origins of Purkinje cells dysfunction before it manifests is imperative to interpret the functional and behavioural consequences of cerebellar-related disorders, providing an optimal timeline for therapeutic interventions. Here, we report the cascade of events leading to Purkinje cells dysfunction before the onset of ataxia in a mouse model of spinocerebellar ataxia 1 (SCA1). Spatiotemporal characterization of the ATXN1[82Q] SCA1 mouse model revealed high levels of the mutant ATXN1[82Q] weeks before the onset of ataxia. The expression of the toxic protein first caused a reduction of Purkinje cells intrinsic excitability, which was followed by atrophy of Purkinje cells dendrite arborization and aberrant glutamatergic signalling, finally leading to disruption of Purkinje cells innervation of climbing fibres and loss of intrinsic plasticity of Purkinje cells. Functionally, we found that deficits in eyeblink conditioning, a form of cerebellum-dependent motor learning, precede the onset of ataxia, matching the timeline of climbing fibre degeneration and reduced intrinsic plasticity. Together, our results suggest that abnormal synaptic signalling and intrinsic plasticity during the pre-ataxia stage of spinocerebellar ataxias underlie an aberrant cerebellar circuitry that anticipates the full extent of the disease severity. Furthermore, our work indicates the potential for eyeblink conditioning to be used as a sensitive tool to detect early cerebellar dysfunction as a sign of future disease.

Spinophilin Limits Metabotropic Glutamate Receptor 5 Scaffolding to the Postsynaptic Density and Cell Type Specifically Mediates Excessive Grooming

BACKGROUND

Grooming dysfunction is a hallmark of the obsessive-compulsive spectrum disorder trichotillomania. Numerous preclinical studies have utilized SAPAP3-deficient mice for understanding the neurobiology of repetitive grooming, suggesting that excessive grooming is caused by increased metabotropic glutamate receptor 5 (mGluR5) activity in striatal direct- and indirect-pathway medium spiny neurons (MSNs). However, the MSN subtype-specific signaling mechanisms that mediate mGluR5-dependent adaptations underlying excessive grooming are not fully understood. Here, we investigated the MSN subtype-specific roles of the striatal signaling hub protein spinophilin in mediating repetitive motor dysfunction associated with mGluR5 function.

METHODS

Quantitative proteomics and immunoblotting were utilized to identify how spinophilin impacts mGluR5 phosphorylation and protein interaction changes. Plasticity and repetitive motor dysfunction associated with mGluR5 action were measured using our novel conditional spinophilin mouse model in which spinophilin was knocked out from striatal direct-pathway MSNs and/or indirect-pathway MSNs.

RESULTS

Loss of spinophilin only in indirect-pathway MSNs decreased performance of a novel motor repertoire, but loss of spinophilin in either MSN subtype abrogated striatal plasticity associated with mGluR5 function and prevented excessive grooming caused by SAPAP3 knockout mice or treatment with the mGluR5-specific positive allosteric modulator VU0360172 without impacting locomotion-relevant behavior. Biochemically, we determined that the spinophilin-mGluR5 interaction correlates with grooming behavior and that loss of spinophilin shifts mGluR5 interactions from lipid raft-associated proteins toward postsynaptic density proteins implicated in psychiatric disorders.

CONCLUSIONS

These results identify spinophilin as a novel striatal signaling hub molecule in MSNs that cell subtype specifically mediates behavioral, functional, and molecular adaptations associated with repetitive motor dysfunction in psychiatric disorders.

GPCR interactions involving metabotropic glutamate receptors and their relevance to the pathophysiology and treatment of CNS disorders

Cellular responses to metabotropic glutamate (mGlu) receptor activation are shaped by mechanisms of receptor-receptor interaction. mGlu receptor subtypes form homodimers, intra- or inter-group heterodimers, and heteromeric complexes with other G protein-coupled receptors (GPCRs). In addition, mGlu receptors may functionally interact with other receptors through the βγ subunits released from G proteins in response to receptor activation or other mechanisms. Here, we discuss the interactions between (i) mGlu₁ and GABA_B receptors in cerebellar Purkinje cells; (ii) mGlu₂ and 5-HT_2Aserotonergic receptors in the prefrontal cortex; (iii) mGlu₅ and A_2A receptors or mGlu₅ and D₁ dopamine receptors in medium spiny projection neurons of the indirect and direct pathways of the basal ganglia motor circuit; (iv) mGlu₅ and A_2A receptors in relation to the pathophysiology of Alzheimer's disease; and (v) mGlu₇ and A₁ adenosine or α- or β₁ adrenergic receptors. In addition, we describe in detail a novel form of non-heterodimeric interaction between mGlu₃ and mGlu₅ receptors, which appears to be critically involved in mechanisms of activity-dependent synaptic plasticity in the prefrontal cortex and hippocampus. Finally, we highlight the potential implication of these interactions in the pathophysiology and treatment of cerebellar disorders, schizophrenia, Alzheimer's disease, Parkinson's disease, l-DOPA-induced dyskinesias, stress-related disorders, and cognitive dysfunctions.

Therapeutic Strategies for Spinocerebellar Ataxia Type 1

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder that affects one or two individuals per 100,000. The disease is caused by an extended CAG repeat in exon 8 of the ATXN1 gene and is characterized mostly by a profound loss of cerebellar Purkinje cells, leading to disturbances in coordination, balance, and gait. At present, no curative treatment is available for SCA1. However, increasing knowledge on the cellular and molecular mechanisms of SCA1 has led the way towards several therapeutic strategies that can potentially slow disease progression. SCA1 therapeutics can be classified as genetic, pharmacological, and cell replacement therapies. These different therapeutic strategies target either the (mutant) ATXN1 RNA or the ataxin-1 protein, pathways that play an important role in downstream SCA1 disease mechanisms or which help restore cells that are lost due to SCA1 pathology. In this review, we will provide a summary of the different therapeutic strategies that are currently being investigated for SCA1.

Targeting mGlu1 Receptors in the Treatment of Motor and Cognitive Dysfunctions in Mice Modeling Type 1 Spinocerebellar Ataxia

Type 1 spinocerebellar ataxia (SCA1) is a progressive neurodegenerative disorder with no effective treatment to date. Using mice modeling SCA1, it has been demonstrated that a drug that amplifies mGlu1 receptor activation (mGlu1 receptor PAM, Ro0711401) improves motor coordination without the development of tolerance when cerebellar dysfunction manifests (i.e., in 30-week-old heterozygous ataxin-1 [154Q/2Q] transgenic mice). SCA1 is also associated with cognitive dysfunction, which may precede cerebellar motor signs. Here, we report that otherwise healthy, 8-week-old SCA1 mice showed a defect in spatial learning and memory associated with reduced protein levels of mGlu1α receptors, the GluN2B subunit of NMDA receptors, and cannabinoid CB1 receptors in the hippocampus. Systemic treatment with Ro0711401 (10 mg/kg, s.c.) partially corrected the learning deficit in the Morris water maze and restored memory retention in the SCA1 mice model. This treatment also enhanced hippocampal levels of the endocannabinoid, anandamide, without changing the levels of 2-arachidonylglycerol. These findings suggest that mGlu1 receptor PAMs may be beneficial in the treatment of motor and nonmotor signs associated with SCA1 and encourage further studies in animal models of SCA1 and other types of SCAs.

mGluR5 Is Substitutable for mGluR1 in Cerebellar Purkinje Cells for Motor Coordination, Developmental Synapse Elimination, and Motor Learning

Group I metabotropic glutamate receptors (mGluRs) include mGluR1 and mGluR5, which are coupled to the Gq family of heterotrimeric G-proteins and readily activated by their selective agonist 3,5-dihydroxyphenilglycine (DHPG). mGluR1 and mGluR5 exhibit nearly complementary distributions spatially or temporally in the central nervous system (CNS). In adult cerebellar Purkinje cells (PCs), mGluR1 is a dominant group I mGluR and mGluR5 is undetectable. mGluR1 expression increases substantially during the first three weeks of postnatal development and remains high throughout adulthood. On the other hand, mGluR5 expression is observed during the first two postnatal weeks and then decreases. However, functional differences between mGluR1 and mGluR5 in the CNS remains to be elucidated. To address this issue, we generated “mGluR5-rescue” mice in which mGluR5 is specifically expressed in PCs in global mGluR1-knockout (KO) mice. mGluR5-rescue mice exhibited apparently normal motor coordination, developmental elimination of redundant climbing fiber (CF)-PC synapses, and delay eyeblink conditioning, which were severely impaired in mGluR1-KO mice. We concluded that mGluR5 is functionally comparable with mGluR1 in cerebellar PCs.

Coordination chemogenetics for activation of GPCR-type glutamate receptors in brain tissue

Direct activation of cell-surface receptors is highly desirable for elucidating their physiological roles. A potential approach for cell-type-specific activation of a receptor subtype is chemogenetics, in which both point mutagenesis of the receptors and designed ligands are used. However, ligand-binding properties are affected in most cases. Here, we developed a chemogenetic method for direct activation of metabotropic glutamate receptor 1 (mGlu1), which plays essential roles in cerebellar functions in the brain. Our screening identified a mGlu1 mutant, mGlu1(N264H), that was activated directly by palladium complexes. A palladium complex showing low cytotoxicity successfully activated mGlu1 in mGlu1(N264H) knock-in mice, revealing that activation of endogenous mGlu1 is sufficient to evoke the critical cellular mechanism of synaptic plasticity, a basis of motor learning in the cerebellum. Moreover, cell-type-specific activation of mGlu1 was demonstrated successfully using adeno-associated viruses in mice, which shows the potential utility of this chemogenetics for clarifying the physiological roles of mGlu1 in a cell-type-specific manner.

Cell-type-specific activation of receptors is desirable for elucidating their roles in tissues or animals. Here, the authors developed a chemogenetic method for direct activation of mGlu1, a GPCR-type glutamate receptor subtype, and demonstrate its use in mouse brain tissue.

The Clinical Concept of LTDpathy: Is Dysregulated LTD Responsible for Prodromal Cerebellar Symptoms?

Long-term depression at parallel fibers-Purkinje cells (PF-PC LTD) is essential for cerebellar motor learning and motor control. Recent progress in ataxiology has identified dysregulation of PF-PC LTD in the pathophysiology of certain types of immune-mediated cerebellar ataxias (IMCAs). Auto-antibodies towards voltage-gated Ca channel (VGCC), metabotropic glutamate receptor type 1 (mGluR1), and glutamate receptor delta (GluR delta) induce dysfunction of PF-PC LTD, resulting in the development of cerebellar ataxias (CAs). These disorders show a good response to immunotherapies in non-paraneoplastic conditions but are sometimes followed by cell death in paraneoplastic conditions. On the other hand, in some types of spinocerebellar ataxia (SCA), dysfunction in PF-PC LTD, and impairments of PF-PC LTD-related adaptive behaviors (including vestibulo-ocular reflex (VOR) and prism adaptation) appear during the prodromal stage, well before the manifestations of obvious CAs and cerebellar atrophy. Based on these findings and taking into account the findings of animal studies, we re-assessed the clinical concept of LTDpathy. LTDpathy can be defined as a clinical spectrum comprising etiologies associated with a functional disturbance of PF-PC LTD with concomitant impairment of related adaptative behaviors, including VOR, blink reflex, and prism adaptation. In IMCAs or degenerative CAs characterized by persistent impairment of a wide range of molecular mechanisms, these disorders are initially functional and are followed subsequently by degenerative cell processes. In such cases, adaptive disorders associated with PF-PC LTD manifest clinically with subtle symptoms and can be prodromal. Our hypothesis underlines for the first time a potential role of LTD dysfunction in the pathogenesis of the prodromal symptoms of CAs. This hypothesis opens perspectives to block the course of CAs at a very early stage.

TRPC3 Channel Activity and Viability of Purkinje Neurons can be Regulated by a Local Signalosome

Canonical transient receptor potential channels (TRPC3) may play a pivotal role in the development and viability of dendritic arbor in Purkinje neurons. This is a novel postsynaptic channel for glutamatergic synaptic transmission. In the cerebellum, TRPC3 appears to regulate functions relating to motor coordination in a highly specific manner. Gain of TRPC3 function is linked to significant alterations in the density and connectivity of dendritic arbor in Purkinje neurons. TRPC3 signals downstream of class I metabotropic glutamate receptors (mGluR1). Moreover, diacylglycerol (DAG) can directly bind and activate TRPC3 molecules. Here, we investigate a key question: How can the activity of the TRPC3 channel be regulated in Purkinje neurons? We also explore how mGluR1 activation, Ca²⁺ influx, and DAG homeostasis in Purkinje neurons can be linked to TRPC3 activity modulation. Through systems biology approach, we show that TRPC3 activity can be modulated by a Purkinje cell (PC)–specific local signalosome. The assembly of this signalosome is coordinated by DAG generation after mGluR1 activation. Our results also suggest that purinergic receptor activation leads to the spatial and temporal organization of the TRPC3 signaling module and integration of its key effector molecules such as DAG, PKCγ, DGKγ, and Ca²⁺ into an organized local signalosome. This signaling machine can regulate the TRPC3 cycling between active, inactive, and desensitized states. Precise activity of the TRPC3 channel is essential for tightly regulating the Ca²⁺ entry into PCs and thus the balance of lipid and Ca²⁺ signaling in Purkinje neurons and hence their viability. Cell-type–specific understanding of mechanisms regulating TRPC3 channel activity could be key in identifying therapeutic targeting opportunities.

BACE1 controls synaptic function through modulating release of synaptic vesicles

BACE1 initiates production of β-amyloid peptides (Aβ), which is associated with cognitive dysfunction in Alzheimer's disease (AD) due to abnormal oligomerization and aggregation. While BACE1 inhibitors show strong reduction in Aβ deposition, they fail to improve cognitive function in patients, largely due to its role in synaptic function. We show that BACE1 is required for optimal release of synaptic vesicles. BACE1 deficiency or inhibition decreases synaptic vesicle docking in the synaptic active zones. Consistently, BACE1-null mice or mice treated with clinically tested BACE1 inhibitors Verubecestat and Lanabecestat exhibit severe reduction in hippocampal LTP and learning behaviors. To counterbalance this synaptic deficit, we discovered that BACE1-null mice treated with positive allosteric modulators (PAMs) of metabotropic glutamate receptor 1 (mGluR1), whose levels were reduced in BACE1-null mice and significantly improved long-term potentiation and cognitive behaviors. Similarly, mice treated with mGluR1 PAM showed significantly mitigated synaptic deficits caused by BACE1 inhibitors. Together, our data suggest that a therapy combining BACE1 inhibitors for reducing amyloid deposition and an mGluR1 PAM for counteracting BACE1-mediated synaptic deficits appears to be an effective approach for treating AD patients.

Therapeutic potential of d-cysteine against in vitro and in vivo models of spinocerebellar ataxia

Spinocerebellar ataxia (SCA) is a group of autosomal-dominantly inherited ataxia and is classified into SCA1-48 by the difference of causal genes. Several SCA-causing proteins commonly impair dendritic development in primary cultured Purkinje cells (PCs). We assume that primary cultured PCs expressing SCA-causing proteins are available as in vitro SCA models and that chemicals that improve the impaired dendritic development would be effective for various SCAs. We have recently revealed that D-cysteine enhances the dendritic growth of primary cultured PCs via hydrogen sulfide production. In the present study, we first investigated whether D-cysteine is effective for in vitro SCA models. We expressed SCA1-, SCA3-, and SCA21-causing mutant proteins to primary cultured PCs using adeno-associated viral serotype 9 (AAV9) vectors. D-Cysteine (0.2 mM) significantly ameliorated the impaired dendritic development commonly observed in primary cultured PCs expressing these three SCA-causing proteins. Next, we investigated the therapeutic effect of long-term treatment with D-cysteine on an in vivo SCA model. SCA1 model mice were established by the cerebellar injection of AAV9 vectors, which express SCA1-causing mutant ataxin-1, to ICR mice. Long-term treatment with D-cysteine (100 mg/kg/day) significantly inhibited the progression of motor dysfunction in SCA1 model mice. Immunostaining experiments revealed that D-cysteine prevented the reduction of mGluR1 and glial activation at the early stage after the onset of motor dysfunction in SCA1 model mice. These findings strongly suggest that D-cysteine has therapeutic potential against in vitro and in vivo SCA models and may be a novel therapeutic agent for various SCAs.

Brain 11 C-ITMM PET to longitudinally assess type 1 metabotropic glutamate receptor availability in Alzheimer's disease

BACKGROUND AND PURPOSE

Little evidence exists on the role of type 1 metabotropic glutamate receptor (mGluR1) in the pathophysiology of Alzheimer's disease (AD), although mGluR1 may be involved in the regulation of neuronal excitability and synaptic plasticity. We have recently reported that mGluR1 availability in the early stage of AD is equivalent to that in healthy subjects. This study aimed to address whether mGluR1 availability changes with the progression of AD.

METHODS

Eight patients with AD (79.1 ± 4.6 years) underwent a total of two positron emission tomography (PET) examinations using the mGluR1 radioligand during the early-to-middle stages of AD. The mean interval was 2.8 years. Volumes-of-interest were placed on the frontal, parietal, and temporal cortices, hippocampus, anterior and posterior lobes, and vermis in the cerebellum. The binding potential (BP_ND ) was calculated to estimate mGluR1 availability, applying partial volume correction to the BP_ND values.

RESULTS

No significant difference was observed in BP_ND values between the first and second PET examinations in the frontal cortex (p = 0.94), parietal cortex (p = 0.67), temporal cortex (p = 0.20), hippocampus (p = 0.17), anterior lobe (p = 0.73), posterior lobe (p = 0.21), and vermis (p = 0.22).

CONCLUSION

This study suggests that mGluR1 availability is unchanged in the follow-up period of a few years during the early-to-middle stages of AD.

mGluR1 signaling in cerebellar Purkinje cells: Subcellular organization and involvement in cerebellar function and disease

The cerebellum is essential for the control, coordination, and learning of movements, and for certain aspects of cognitive function. Purkinje cells are the sole output neurons in the cerebellar cortex and therefore play crucial roles in the diverse functions of the cerebellum. The type 1 metabotropic glutamate receptor (mGluR1) is prominently enriched in Purkinje cells and triggers downstream signaling pathways that are required for functional and structural plasticity, and for synaptic responses. To understand how mGluR1 contributes to cerebellar functions, it is important to consider not only the operational properties of this receptor, but also its spatial organization and the molecular interactions that enable its proper functioning. In this review, we highlight how mGluR1 and its related signaling molecules are organized into tightly coupled microdomains to fulfill physiological functions. We also describe emerging evidence that altered mGluR1 signaling in Purkinje cells underlies cerebellar dysfunction in ataxias of human patients and mouse models.

Modulation of Increased mGluR1 Signaling by RGS8 Protects Purkinje Cells From Dendritic Reduction and Could Be a Common Mechanism in Diverse Forms of Spinocerebellar Ataxia

Spinocerebellar ataxias (SCAs) are a group of hereditary neurodegenerative diseases which are caused by diverse genetic mutations in a variety of different genes. We have identified RGS8, a regulator of G-protein signaling, as one of the genes which are dysregulated in different mouse models of SCA (e.g., SCA1, SCA2, SCA7, and SCA14). In the moment, little is known about the role of RGS8 for pathogenesis of spinocerebellar ataxia. We have studied the expression of RGS8 in the cerebellum in more detail and show that it is specifically expressed in mouse cerebellar Purkinje cells. In a mouse model of SCA14 with increased PKCγ activity, RGS8 expression was also increased. RGS8 overexpression could partially counteract the negative effects of DHPG-induced mGluR1 signaling for the expansion of Purkinje cell dendrites. Our results suggest that the increased expression of RGS8 is an important mediator of mGluR1 pathway dysregulation in Purkinje cells. These findings provide new insights in the role of RGS8 and mGluR1 signaling in Purkinje cells and for the pathology of SCAs.

Functional gene networks reveal distinct mechanisms segregating in migraine families

Migraine is the most common neurological disorder worldwide and it has been shown to have complex polygenic origins with a heritability of estimated 40-70%. Both common and rare genetic variants are believed to underlie the pathophysiology of the prevalent types of migraine, migraine with typical aura and migraine without aura. However, only common variants have been identified so far. Here we identify for the first time a gene module with rare mutations through a systems genetics approach integrating RNA sequencing data from brain and vascular tissues likely to be involved in migraine pathology in combination with whole genome sequencing of 117 migraine families. We found a gene module in the visual cortex, based on single nuclei RNA sequencing data, that had increased rare mutations in the migraine families and replicated this in a second independent cohort of 1930 patients. This module was mainly expressed by interneurons, pyramidal CA1, and pyramidal SS cells, and pathway analysis showed association with hormonal signalling (thyrotropin-releasing hormone receptor and oxytocin receptor signalling pathways), Alzheimer's disease pathway, serotonin receptor pathway and general heterotrimeric G-protein signalling pathways. Our results demonstrate that rare functional gene variants are strongly implicated in the pathophysiology of migraine. Furthermore, we anticipate that the results can be used to explain the critical mechanisms behind migraine and potentially improving the treatment regime for migraine patients.

Aberrant Cerebellar Circuitry in the Spinocerebellar Ataxias

The spinocerebellar ataxias (SCAs) are a heterogeneous group of neurodegenerative diseases that share convergent disease features. A common symptom of these diseases is development of ataxia, involving impaired balance and motor coordination, usually stemming from cerebellar dysfunction and neurodegeneration. For most spinocerebellar ataxias, pathology can be attributed to an underlying gene mutation and the impaired function of the encoded protein through loss or gain-of-function effects. Strikingly, despite vast heterogeneity in the structure and function of disease-causing genes across the SCAs and the cellular processes affected, the downstream effects have considerable overlap, including alterations in cerebellar circuitry. Interestingly, aberrant function and degeneration of Purkinje cells, the major output neuronal population present within the cerebellum, precedes abnormalities in other neuronal populations within many SCAs, suggesting that Purkinje cells have increased vulnerability to cellular perturbations. Factors that are known to contribute to perturbed Purkinje cell function in spinocerebellar ataxias include altered gene expression resulting in altered expression or functionality of proteins and channels that modulate membrane potential, downstream impairments in intracellular calcium homeostasis and changes in glutamatergic input received from synapsing climbing or parallel fibers. This review will explore this enhanced vulnerability and the aberrant cerebellar circuitry linked with it in many forms of SCA. It is critical to understand why Purkinje cells are vulnerable to such insults and what overlapping pathogenic mechanisms are occurring across multiple SCAs, despite different underlying genetic mutations. Enhanced understanding of disease mechanisms will facilitate the development of treatments to prevent or slow progression of the underlying neurodegenerative processes, cerebellar atrophy and ataxic symptoms.

Cerebellar Development and Circuit Maturation: A Common Framework for Spinocerebellar Ataxias

Spinocerebellar ataxias (SCAs) affect the cerebellum and its afferent and efferent systems that degenerate during disease progression. In the cerebellum, Purkinje cells (PCs) are the most vulnerable and their prominent loss in the late phase of the pathology is the main characteristic of these neurodegenerative diseases. Despite the constant advancement in the discovery of affected molecules and cellular pathways, a comprehensive description of the events leading to the development of motor impairment and degeneration is still lacking. However, in the last years the possible causal role for altered cerebellar development and neuronal circuit wiring in SCAs has been emerging. Not only wiring and synaptic transmission deficits are a common trait of SCAs, but also preventing the expression of the mutant protein during cerebellar development seems to exert a protective role. By discussing this tight relationship between cerebellar development and SCAs, in this review, we aim to highlight the importance of cerebellar circuitry for the investigation of SCAs.

G-Protein-Coupled Receptors in CNS: A Potential Therapeutic Target for Intervention in Neurodegenerative Disorders and Associated Cognitive Deficits

Neurodegenerative diseases are a large group of neurological disorders with diverse etiological and pathological phenomena. However, current therapeutics rely mostly on symptomatic relief while failing to target the underlying disease pathobiology. G-protein-coupled receptors (GPCRs) are one of the most frequently targeted receptors for developing novel therapeutics for central nervous system (CNS) disorders. Many currently available antipsychotic therapeutics also act as either antagonists or agonists of different GPCRs. Therefore, GPCR-based drug development is spreading widely to regulate neurodegeneration and associated cognitive deficits through the modulation of canonical and noncanonical signals. Here, GPCRs’ role in the pathophysiology of different neurodegenerative disease progressions and cognitive deficits has been highlighted, and an emphasis has been placed on the current pharmacological developments with GPCRs to provide an insight into a potential therapeutic target in the treatment of neurodegeneration.

Recent Advances in the Treatment of Cerebellar Disorders

Various etiopathologies affect the cerebellum, resulting in the development of cerebellar ataxias (CAs), a heterogeneous group of disorders characterized clinically by movement incoordination, affective dysregulation, and cognitive dysmetria. Recent progress in clinical and basic research has opened the door of the ‘‘era of therapy” of CAs. The therapeutic rationale of cerebellar diseases takes into account the capacity of the cerebellum to compensate for pathology and restoration, which is collectively termed cerebellar reserve. In general, treatments of CAs are classified into two categories: cause-cure treatments, aimed at arresting disease progression, and neuromodulation therapies, aimed at potentiating cerebellar reserve. Both forms of therapies should be introduced as soon as possible, at a time where cerebellar reserve is still preserved. Clinical studies have established evidence-based cause-cure treatments for metabolic and immune-mediated CAs. Elaborate protocols of rehabilitation and non-invasive cerebellar stimulation facilitate cerebellar reserve, leading to recovery in the case of controllable pathologies (metabolic and immune-mediated CAs) and delay of disease progression in the case of uncontrollable pathologies (degenerative CAs). Furthermore, recent advances in molecular biology have encouraged the development of new forms of therapies: the molecular targeting therapy, which manipulates impaired RNA or proteins, and the neurotransplantation therapy, which delays cell degeneration and facilitates compensatory functions. The present review focuses on the therapeutic rationales of these recently developed therapeutic modalities, highlighting the underlying pathogenesis.

Moving Towards Therapy in SCA1: Insights from Molecular Mechanisms, Identification of Novel Targets, and Planning for Human Trials

The spinocerebellar ataxias (SCAs) are a group of neurodegenerative disorders inherited in an autosomal dominant fashion. The SCAs result in progressive gait imbalance, incoordination of the limbs, speech changes, and oculomotor dysfunction, among other symptoms. Over the past few decades, significant strides have been made in understanding the pathogenic mechanisms underlying these diseases. Although multiple efforts using a combination of genetics and pharmacology with small molecules have been made towards developing new therapeutics, no FDA approved treatment currently exists. In this review, we focus on SCA1, a common SCA subtype, in which some of the greatest advances have been made in understanding disease biology, and consequently potential therapeutic targets. Understanding of the underlying basic biology and targets of therapy in SCA1 is likely to give insight into treatment strategies in other SCAs. The diversity of the biology in the SCAs, and insight from SCA1 suggests, however, that both shared treatment strategies and specific approaches tailored to treat distinct genetic causes of SCA are likely needed for this group of devastating neurological disorders.

Spinocerebellar ataxias: prospects and challenges for therapy development

The spinocerebellar ataxias (SCAs) comprise more than 40 autosomal dominant neurodegenerative disorders that present principally with progressive ataxia. Within the past few years, studies of pathogenic mechanisms in the SCAs have led to the development of promising therapeutic strategies, especially for SCAs caused by polyglutamine-coding CAG repeats. Nucleotide-based gene-silencing approaches that target the first steps in the pathogenic cascade are one promising approach not only for polyglutamine SCAs but also for the many other SCAs caused by toxic mutant proteins or RNA. For these and other emerging therapeutic strategies, well-coordinated preparation is needed for fruitful clinical trials. To accomplish this goal, investigators from the United States and Europe are now collaborating to share data from their respective SCA cohorts. Increased knowledge of the natural history of SCAs, including of the premanifest and early symptomatic stages of disease, will improve the prospects for success in clinical trials of disease-modifying drugs. In addition, investigators are seeking validated clinical outcome measures that demonstrate responsiveness to changes in SCA populations. Findings suggest that MRI and magnetic resonance spectroscopy biomarkers will provide objective biological readouts of disease activity and progression, but more work is needed to establish disease-specific biomarkers that track target engagement in therapeutic trials. Together, these efforts suggest that the development of successful therapies for one or more SCAs is not far away.

Unchanged type 1 metabotropic glutamate receptor availability in patients with Alzheimer's disease: A study using 11C-ITMM positron emission tomography

Imaging of type 1 metabotropic glutamate receptor (mGluR1) has recently become possible using positron emission tomography (PET). To date, little evidence exists on the role of mGluR1 in the pathophysiology of Alzheimer's disease (AD). We aimed to examine mGluR1 availability in patients with AD. Ten patients with AD (78.9 ± 5.9 years) and 12 age-matched volunteers (74.6 ± 2.6 years) underwent PET using an mGluR1 radiotracer. All patients were anti-dementia drug-naive. Volumes-of-interest were placed on the anterior and posterior lobes and vermis in the cerebellum and frontal, parietal, and temporal cortices. The binding potential (BP_ND) was calculated to estimate mGluR1 availability, and partial volume correction was applied to the BP_ND values. Mini Mental State Examination (MMSE) scores were also obtained (22.0 ± 4.8). No significant difference was observed in BP_ND between the AD and control groups in the anterior lobe (p = .30), posterior lobe (p = .95), vermis (p = .96), frontal cortex (p = .61), parietal cortex (p = .59), or temporal cortex (p = .27). No significant correlation was observed between BP_ND and MMSE scores in the anterior lobe (p = .59), posterior lobe (p = .35), vermis (p = .92), frontal cortex (p = .78), parietal cortex (p = .83), or temporal cortex (p = .82). In conclusions, this study suggests that mGluR1 availability is unchanged in the relatively early stage of AD. However, because regional mGluR1 availability may change with the progression of AD, further longitudinal follow-up is necessary.

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Metabotropic glutamate receptors (mGluR) can be affected in AD.

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Little evidence exists on the role of type 1 mGluR (mGluR1) in AD.

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We examined mGluR1 availability in patients with AD.

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mGluR1 availability was unchanged in the relatively early stage of AD.

mGlu1 Receptors Monopolize the Synaptic Control of Cerebellar Purkinje Cells by Epigenetically Down-Regulating mGlu5 Receptors

In cerebellar Purkinje cells (PCs) type-1 metabotropic glutamate (mGlu1) receptors play a key role in motor learning and drive the refinement of synaptic innervation during postnatal development. The cognate mGlu5 receptor is absent in mature PCs and shows low expression levels in the adult cerebellar cortex. Here we found that mGlu5 receptors were heavily expressed by PCs in the early postnatal life, when mGlu1α receptors were barely detectable. The developmental decline of mGlu5 receptors coincided with the appearance of mGlu1α receptors in PCs, and both processes were associated with specular changes in CpG methylation in the corresponding gene promoters. It was the mGlu1 receptor that drove the elimination of mGlu5 receptors from PCs, as shown by data obtained with conditional mGlu1α receptor knockout mice and with targeted pharmacological treatments during critical developmental time windows. The suppressing activity of mGlu1 receptors on mGlu5 receptor was maintained in mature PCs, suggesting that expression of mGlu1α and mGlu5 receptors is mutually exclusive in PCs. These findings add complexity to the the finely tuned mechanisms that regulate PC biology during development and in the adult life and lay the groundwork for an in-depth analysis of the role played by mGlu5 receptors in PC maturation.

Circular RNAs profiling in the cystathionine-β-synthase mutant mouse reveals novel gene targets for hyperhomocysteinemia induced ocular disorders

Cystathionine-β-synthase (CBS) gene encodes L-serine hydrolyase which catalyzes β-reaction to condense serine with homocysteine (Hcy) by pyridoxal-5'-phosphate helps to form cystathionine which in turn is converted to cysteine. CBS resides at the intersection of transmethylation, transsulfuration, and remethylation pathways, thus lack of CBS fundamentally blocks Hcy degradation; an essential step in glutathione synthesis. Redox homeostasis, free-radical detoxification and one-carbon metabolism (Methionine-Hcy-Folate cycle) require CBS and its deficiency leads to hyperhomocysteinemia (HHcy) causing retinovascular thromboembolism and eye-lens dislocation along with vascular cognitive impairment and dementia. HHcy results in retinovascular, coronary, cerebral and peripheral vessels' dysfunction and how it causes metabolic dysregulation predisposing patients to serious eye conditions remains unknown. HHcy orchestrates inflammation and redox imbalance via epigenetic remodeling leading to neurovascular pathologies. Although circular RNAs (circRNAs) are dominant players regulating their parental genes' expression dynamics, their importance in ocular biology has not been appreciated. Progress in gene-centered analytics via improved microarray and bioinformatics are enabling dissection of genomic pathways however there is an acute under-representation of circular RNAs in ocular disorders. This study undertook circRNAs' analysis in the eyes of CBS deficient mice identifying a pool of 12532 circRNAs, 74 exhibited differential expression profile, _∼27% were down-regulated while most were up-regulated (_∼73%). Findings also revealed several microRNAs that are specific to each circRNA suggesting their roles in HHcy induced ocular disorders. Further analysis of circRNAs helped identify novel parental genes that seem to influence certain eye disease phenotypes.

Loss of cerebellar glutamate transporters EAAT4 and GLAST differentially affects the spontaneous firing pattern and survival of Purkinje cells

Loss of excitatory amino acid transporters (EAATs) has been implicated in a number of human diseases including spinocerebellar ataxias, Alzhiemer's disease and motor neuron disease. EAAT4 and GLAST/EAAT1 are the two predominant EAATs responsible for maintaining low extracellular glutamate levels and preventing neurotoxicity in the cerebellum, the brain region essential for motor control. Here using genetically modified mice we identify new critical roles for EAAT4 and GLAST/EAAT1 as modulators of Purkinje cell (PC) spontaneous firing patterns. We show high EAAT4 levels, by limiting mGluR1 signalling, are essential in constraining inherently heterogeneous firing of zebrin-positive PCs. Moreover mGluR1 antagonists were found to restore regular spontaneous PC activity and motor behaviour in EAAT4 knockout mice. In contrast, GLAST/EAAT1 expression is required to sustain normal spontaneous simple spike activity in low EAAT4 expressing (zebrin-negative) PCs by restricting NMDA receptor activation. Blockade of NMDA receptor activity restores spontaneous activity in zebrin-negative PCs of GLAST knockout mice and furthermore alleviates motor deficits. In addition both transporters have differential effects on PC survival, with zebrin-negative PCs more vulnerable to loss of GLAST/EAAT1 and zebrin-positive PCs more vulnerable to loss of EAAT4. These findings reveal that glutamate transporter dysfunction through elevated extracellular glutamate and the aberrant activation of extrasynaptic receptors can disrupt cerebellar output by altering spontaneous PC firing. This expands our understanding of disease mechanisms in cerebellar ataxias and establishes EAATs as targets for restoring homeostasis in a variety of neurological diseases where altered cerebellar output is now thought to play a key role in pathogenesis.

Ca2+ signaling and spinocerebellar ataxia

Spinocerebellar ataxia (SCA) is a neural disorder, which is caused by degenerative changes in the cerebellum. SCA is primarily characterized by gait ataxia, and additional clinical features include nystagmus, dysarthria, tremors and cerebellar atrophy. Forty-four hereditary SCAs have been identified to date, along with >35 SCA-associated genes. Despite the great diversity and distinct functionalities of the SCA-related genes, accumulating evidence supports the occurrence of a common pathophysiological event among several hereditary SCAs. Altered calcium (Ca²⁺) homeostasis in the Purkinje cells (PCs) of the cerebellum has been proposed as a possible pathological SCA trigger. In support of this, signaling events that are initiated from or lead to aberrant Ca²⁺ release from the type 1 inositol 1,4,5-trisphosphate receptor (IP₃R1), which is highly expressed in cerebellar PCs, seem to be closely associated with the pathogenesis of several SCA types. In this review, we summarize the current research on pathological hereditary SCA events, which involve altered Ca²⁺ homeostasis in PCs, through IP₃R1 signaling.

Purkinje Cell Signaling Deficits in Animal Models of Ataxia

Purkinje cell (PC) dysfunction or degeneration is the most frequent finding in animal models with ataxic symptoms. Mutations affecting intrinsic membrane properties can lead to ataxia by altering the firing rate of PCs or their firing pattern. However, the relationship between specific firing alterations and motor symptoms is not yet clear, and in some cases PC dysfunction precedes the onset of ataxic signs. Moreover, a great variety of ionic and synaptic mechanisms can affect PC signaling, resulting in different features of motor dysfunction. Mutations affecting Na⁺ channels (Na_V1.1, Na_V1.6, Na_Vβ4, Fgf14 or Rer1) reduce the firing rate of PCs, mainly via an impairment of the Na⁺ resurgent current. Mutations that reduce Kv3 currents limit the firing rate frequency range. Mutations of Kv1 channels act mainly on inhibitory interneurons, generating excessive GABAergic signaling onto PCs, resulting in episodic ataxia. Kv4.3 mutations are responsible for a complex syndrome with several neurologic dysfunctions including ataxia. Mutations of either Cav or BK channels have similar consequences, consisting in a disruption of the firing pattern of PCs, with loss of precision, leading to ataxia. Another category of pathogenic mechanisms of ataxia regards alterations of synaptic signals arriving at the PC. At the parallel fiber (PF)-PC synapse, mutations of glutamate delta-2 (GluD2) or its ligand Crbl1 are responsible for the loss of synaptic contacts, abolishment of long-term depression (LTD) and motor deficits. At the same synapse, a correct function of metabotropic glutamate receptor 1 (mGlu1) receptors is necessary to avoid ataxia. Failure of climbing fiber (CF) maturation and establishment of PC mono-innervation occurs in a great number of mutant mice, including mGlu1 and its transduction pathway, GluD2, semaphorins and their receptors. All these models have in common the alteration of PC output signals, due to a variety of mechanisms affecting incoming synaptic signals or the way they are processed by the repertoire of ionic channels responsible for intrinsic membrane properties. Although the PC is a final common pathway of ataxia, the link between specific firing alterations and neurologic symptoms has not yet been systematically studied and the alterations of the cerebellar contribution to motor signals are still unknown.

Genetic inactivation of mGlu5 receptor improves motor coordination in the Grm1crv4 mouse model of SCAR13 ataxia

Deleterious mutations in the glutamate receptor metabotropic 1 gene (GRM1) cause a recessive form of cerebellar ataxia, SCAR13. GRM1 and GRM5 code for the metabotropic glutamate type 1 (mGlu1) and type 5 (mGlu5) receptors, respectively. Their different expression profiles suggest they could have distinct functional roles. In a previous study, homozygous mice lacking mGlu1 receptors (Grm1^crv4/crv4) and exhibiting ataxia presented cerebellar overexpression of mGlu5 receptors, that was proposed to contribute to the mouse phenotype. To test this hypothesis, we here crossed Grm1^crv4 and Grm5^ko mice to generate double mutants (Grm1^crv4/crv4Grm5^ko/ko) lacking both mGlu1 and mGlu5 receptors. Double mutants and control mice were analyzed for spontaneous behavior and for motor activity by rotarod and footprint analyses. In the same mice, the release of glutamate from cerebellar nerve endings (synaptosomes) elicited by 12mM KCl or by α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) was also evaluated. Motor coordination resulted improved in double mutants when compared to Grm1^crv4/crv4 mice. Furthermore, in in vitro studies, glutamate release elicited by both KCl depolarization and activation of AMPA autoreceptors resulted reduced in Grm1^crv4/crv4 mice compared to wild type mice, while it presented normal levels in double mutants. Moreover, we found that Grm1^crv4/crv4 mice showed reduced expression of GluA2/3 AMPA receptor subunits in cerebellar synaptosomes, while it resulted restored to wild type level in double mutants. To conclude, blocking of mGlu5 receptor reduced the dysregulation of glutamate transmission and improved motor coordination in the Grm1^crv4 mouse model of SCAR13, thus suggesting the possible usefulness of pharmacological therapies based on modulation of mGlu5 receptor activity for the treatment of this type of ataxia.

Prolonged Type 1 Metabotropic Glutamate Receptor Dependent Synaptic Signaling Contributes to Spino-Cerebellar Ataxia Type 1

Type 1 metabotropic glutamate receptor (mGluR1)-dependent signaling at parallel fiber to Purkinje neuron synapses is critical for cerebellar function. In a mouse model of human spino-cerebellar ataxia type 1 (early SCA1, 12 weeks) we find prolonged parallel fiber mGluR1-dependent synaptic currents and calcium signaling. Acute treatment with a low dose of the potent and specific activity-dependent mGluR1-negative allosteric modulator JNJ16259685 shortened the prolonged mGluR1 currents and rescued the moderate ataxia. Our results provide exciting new momentum for developing mGluR1-based pharmacology to treat ataxia.

SIGNIFICANCE STATEMENT

Ataxia is a progressive and devastating degenerative movement disorder commonly associated with loss of cerebellar function and with no known cure. In the early stages of a mouse model of human spinocerebellar ataxia type 1, SCA1, where mice exhibit only moderate motor impairment, we detect excess "gain of function" of metabotropic glutamate receptor signaling at an important cerebellar synapse. Because careful control of this type of signaling is critical for cerebellar function in mice and humans, we sought to remove the excess signaling with a powerful, readily available pharmacological modulator. Remarkably, this pharmacological treatment acutely restored normal motor function in the ataxic mice. Our results pave the way for exploring a new avenue for early treatment of human ataxias.

A positive feedback loop linking enhanced mGluR function and basal calcium in spinocerebellar ataxia type 2

Metabotropic glutamate receptor 1 (mGluR1) function in Purkinje neurons (PNs) is essential for cerebellar development and for motor learning and altered mGluR1 signaling causes ataxia. Downstream of mGluR1, dysregulation of calcium homeostasis has been hypothesized as a key pathological event in genetic forms of ataxia but the underlying mechanisms remain unclear. We find in a spinocerebellar ataxia type 2 (SCA2) mouse model that calcium homeostasis in PNs is disturbed across a broad range of physiological conditions. At parallel fiber synapses, mGluR1-mediated excitatory postsynaptic currents (EPSCs) and associated calcium transients are increased and prolonged in SCA2 PNs. In SCA2 PNs, enhanced mGluR1 function is prevented by buffering [Ca²⁺] at normal resting levels while in wildtype PNs mGluR1 EPSCs are enhanced by elevated [Ca²⁺]. These findings demonstrate a deleterious positive feedback loop involving elevated intracellular calcium and enhanced mGluR1 function, a mechanism likely to contribute to PN dysfunction and loss in SCA2.

DOI:http://dx.doi.org/10.7554/eLife.26377.001

Type-1 metabotropic glutamate receptor signaling in cerebellar Purkinje cells in health and disease

The cerebellum is a brain structure involved in coordination, control, and learning of movements, as well as certain aspects of cognitive function. Purkinje cells are the sole output neurons from the cerebellar cortex and therefore play crucial roles in the overall function of the cerebellum. The type-1 metabotropic glutamate receptor (mGluR1) is a key “hub” molecule that is critically involved in the regulation of synaptic wiring, excitability, synaptic response, and synaptic plasticity of Purkinje cells. In this review, we aim to highlight how mGluR1 controls these events in Purkinje cells. We also describe emerging evidence that altered mGluR1 signaling in Purkinje cells underlies cerebellar dysfunctions in several clinically relevant mouse models of human ataxias.

Expression of the K+/Cl- cotransporter, KCC2, in cerebellar Purkinje cells is regulated by group-I metabotropic glutamate receptors

The neuronal K+/Cl- symporter, KCC2, shapes synaptic responses mediated by Cl--permeant GABAA receptors. Moving from the evidence that excitatory neurotransmission drives changes in KCC2 expression in cerebellar neurons, we studied the regulation of KCC2 expression by group-I metabotropic glutamate (mGlu) receptors in the cerebellum of adult mice. Mice lacking mGlu5 receptors showed a large reduction in cerebellar KCC2 protein levels and a loss of KCC2 immunoreactivity in Purkinje cells. Similar changes were seen in mice treated with the mGlu5 receptor antagonist, MPEP, whereas treatment with the mGlu5 receptor positive allosteric modulator (PAM), VU0360172, increased KCC2 expression. In contrast, pharmacological inhibition of mGlu1 receptors with JNJ16259685 enhanced cerebellar KCC2 protein levels and KCC2 immunoreactivity in Purkinje cells, whereas treatment with the mGlu1 receptor PAM, RO0711401, reduced KCC2 expression. To examine whether the reduction in KCC2 expression caused by the absence or the inhibition of mGlu5 receptors could affect GABAergic transmission, we performed electrophysiological and behavioral studies. Recording of extracellular action potentials in Purkinje cells showed that the inhibitory effect of the GABAA receptor agonist, muscimol, was lost in cerebellar slices prepared from mGlu5-/- mice or from mice treated systemically with MPEP, in line with the reduction in KCC2 expression. Similarly, motor impairment caused by the GABAA receptor PAM, diazepam, was attenuated in mice pre-treated with MPEP. These findings disclose a novel function of mGlu5 receptors in the cerebellum and suggest that mGlu5 receptor ligands might influence GABAergic transmission in the cerebellum and affect motor responses to GABA-mimetic drugs. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Recent advances in drug discovery of GPCR allosteric modulators for neurodegenerative disorders

The activation or the inhibition of G-protein coupled receptors (GPCRs) implicated in the pathophysiology of neurodegenerative disorders is considered as a relevant approach for the treatment of these diseases. The modulation of the relevant GPCRs targets by positive or by negative allosteric modulators appears to be promising, the major challenge remaining the discovery of these molecules. In this review, we highlight the recent development in this field and the therapeutic potential of selected GPCRs allosteric modulators.

Comparison of imaging using 11C-ITMM and 18F-FDG for the detection of cerebellar ataxia

Objective Newly developed methods for imaging type 1 metabotropic glutamate receptor (mGluR1) have the potential use for estimating cerebellar function. We aimed to compare mGluR1 imaging using N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-4-¹¹C-methoxy-N-methylbenzamide (¹¹C-ITMM) with the existing marker, fluorine-18-labeled fluorodeoxyglucose (¹⁸F-FDG) imaging, in the cerebellum.

METHODS

Fourteen subjects consisting of 12 patients with cerebellar ataxia and two healthy subjects underwent ¹¹C-ITMM and ¹⁸F-FDG positron emission tomography. The degree of ataxia was scored with the Scale for the Assessment and Rating of Ataxia (SARA). Volumes-of-interest were placed on the anterior and posterior lobes and vermis. The binding potential (BP_ND) was calculated to estimate mGluR1 availability using the white matter as a reference region. ¹⁸F-FDG uptake was normalized using the white matter (FU_wm).

RESULTS

There were significant positive correlations between the BP_ND and FU_wm values in the anterior lobe (r=0.83, P<0.001), posterior lobe (r=0.69, P=0.009), and vermis (r=0.58, P=0.042). Regarding the relationship of SARA scores with the BP_ND and FU_wm values, a significant negative correlation was found only in the anterior lobe between the SARA scores and BP_ND values (r=-0.64, P=0.029).

CONCLUSION

This study showed that mGluR1 imaging was comparable to ¹⁸F-FDG imaging in the cerebellum. However, mGluR1 imaging was more strongly associated with the SARA scores than ¹⁸F-FDG imaging was, suggesting that mGluR1 imaging can be a more specific technique than ¹⁸F-FDG imaging for evaluating cerebellar ataxia.

Transduction of group I mGluR-mediated synaptic plasticity by β-arrestin2 signalling

Conventional signalling by the group I metabotropic glutamate receptors, mGluR1 and mGluR5, occurs through G-protein coupling, but evidence suggests they might also utilize other, non-canonical effector pathways. Here we test whether group I mGluRs require β-arrestin signalling during specific forms of plasticity at hippocampal excitatory synapses. We find that genetic ablation of β-arrestin2, but not β-arrestin1, results in deficits in plasticity mediated by mGlu1 receptors in CA3 pyramidal neurons and by mGlu5 receptors in CA1 pyramidal neurons. Pharmacological studies additionally support roles for Src kinases and MAPK/ERK downstream of β-arrestin2 in CA3 neurons. mGluR1 modulation of intrinsic conductances is otherwise preserved in β-arrestin2^−/− mice with the exception of a rebound depolarization, and non-mGluR-mediated long-term potentiation is unaltered. These results reveal a signalling pathway engaged by group I mGluRs to effect changes in synaptic and cell intrinsic physiology dependent upon β-arrestin rather than G proteins. Pharmacological manipulation of mGluRs with effector-biased ligands could lead to novel therapies to treat neurological disease.

mGluRs are known to undergo non-canonical signalling regulation, although the underlying mechanisms are unclear. Here, the authors identify a role for β-arrestin2, but not β-arrestin1, in group I mGluR-mediated plasticity at hippocampal synapses.

Modulation, Plasticity and Pathophysiology of the Parallel Fiber-Purkinje Cell Synapse

The parallel fiber-Purkinje cell (PF-PC) synapse represents the point of maximal signal divergence in the cerebellar cortex with an estimated number of about 60 billion synaptic contacts in the rat and 100,000 billions in humans. At the same time, the Purkinje cell dendritic tree is a site of remarkable convergence of more than 100,000 parallel fiber synapses. Parallel fiber activity generates fast postsynaptic currents via α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and slower signals, mediated by mGlu1 receptors, resulting in Purkinje cell depolarization accompanied by sharp calcium elevation within dendritic regions. Long-term depression (LTD) and long-term potentiation (LTP) have been widely described for the PF-PC synapse and have been proposed as mechanisms for motor learning. The mechanisms of induction for LTP and LTD involve different signaling mechanisms within the presynaptic terminal and/or at the postsynaptic site, promoting enduring modification in the neurotransmitter release and change in responsiveness to the neurotransmitter. The PF-PC synapse is finely modulated by several neurotransmitters, including serotonin, noradrenaline and acetylcholine. The ability of these neuromodulators to gate LTP and LTD at the PF-PC synapse could, at least in part, explain their effect on cerebellar-dependent learning and memory paradigms. Overall, these findings have important implications for understanding the cerebellar involvement in a series of pathological conditions, ranging from ataxia to autism. For example, PF-PC synapse dysfunctions have been identified in several murine models of spino-cerebellar ataxia (SCA) types 1, 3, 5 and 27. In some cases, the defect is specific for the AMPA receptor signaling (SCA27), while in others the mGlu1 pathway is affected (SCA1, 3, 5). Interestingly, the PF-PC synapse has been shown to be hyper-functional in a mutant mouse model of autism spectrum disorder, with a selective deletion of Pten in Purkinje cells. However, the full range of methodological approaches, that allowed the discovery of the physiological principles of PF-PC synapse function, has not yet been completely exploited to investigate the pathophysiological mechanisms of diseases involving the cerebellum. We, therefore, propose to extend the spectrum of experimental investigations to tackle this problem.

Progressive impairment of cerebellar mGluR signalling and its therapeutic potential for cerebellar ataxia in spinocerebellar ataxia type 1 model mice

KEY POINTS

Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disease caused by a gene defect, leading to movement disorder such as cerebellar ataxia. It remains largely unknown which functional defect contributes to the cerebellar ataxic phenotype in SCA1. In this study, we report progressive dysfunction of metabotropic glutamate receptor (mGluR) signalling, which leads to smaller slow synaptic responses, reduced dendritic Ca²⁺ signals and impaired synaptic plasticity at cerebellar synapses, in the early disease stage of SCA1 model mice. We also show that enhancement of mGluR signalling by a clinically available drug, baclofen, leads to improvement of motor performance in SCA1 mice. SCA1 is an incurable disease with no effective treatment, and our results may provide mechanistic grounds for targeting mGluRs and a novel drug therapy with baclofen to treat SCA1 patients in the future.

ABSTRACT

Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disease that presents with cerebellar ataxia and motor learning defects. Previous studies have indicated that the pathology of SCA1, as well as other ataxic diseases, is related to signalling pathways mediated by the metabotropic glutamate receptor type 1 (mGluR1), which is indispensable for proper motor coordination and learning. However, the functional contribution of mGluR signalling to SCA1 pathology is unclear. In the present study, we show that SCA1 model mice develop a functional impairment of mGluR signalling which mediates slow synaptic responses, dendritic Ca²⁺ signals, and short- and long-term synaptic plasticity at parallel fibre (PF)-Purkinje cell (PC) synapses in a progressive manner from the early disease stage (5 postnatal weeks) prior to PC death. Notably, impairment of mGluR-mediated dendritic Ca²⁺ signals linearly correlated with a reduction of PC capacitance (cell surface area) in disease progression. Enhancement of mGluR signalling by baclofen, a clinically available GABA_B receptor agonist, led to an improvement of motor performance in SCA1 mice and the improvement lasted ∼1 week after a single application of baclofen. Moreover, the restoration of motor performance in baclofen-treated SCA1 mice matched the functional recovery of mGluR-mediated slow synaptic currents and mGluR-dependent short- and long-term synaptic plasticity. These results suggest that impairment of synaptic mGluR cascades is one of the important contributing factors to cerebellar ataxia in early and middle stages of SCA1 pathology, and that modulation of mGluR signalling by baclofen or other clinical interventions may be therapeutic targets to treat SCA1.

Relationship between type 1 metabotropic glutamate receptors and cerebellar ataxia

Imaging of type 1 metabotropic glutamate receptor (mGluR1) has recently become possible using positron emission tomography (PET). We aimed to examine the relationship between mGluR1 and cerebellar ataxia. Families with spinocerebellar ataxia type 19/22 (SCA19/22) and SCA6, six patients with sporadic SCA, and 26 healthy subjects underwent PET using an mGluR1 radiotracer. Volumes-of-interest were placed on the anterior and posterior lobes and vermis. The binding potential (BP_ND) was calculated to estimate mGluR1 availability. A partial volume correction was applied to the BP_ND values. The Scale for the Assessment and Rating of Ataxia (SARA) score were measured. In each patient with SCA19/22 and SCA6, the anterior lobe showed the highest decrease rates in the BP_ND values, compared with healthy subjects. In the families with SCA19/22 and SCA6, the disease durations and SARA scores were shorter and lower, respectively, in the offspring, compared with the parents. However, the offspring paradoxically showed lower BP_ND values, especially in the anterior lobe, compared with the parents. The patients with sporadic SCA showed significantly lower BP_ND values in all subregions than healthy subjects. The BP_ND values significantly correlated with the SARA scores in all participants. In conclusion, these results showed a decrease in mGluR1 availability in patients with hereditary and sporadic SCA, a correlation between mGluR1 availability and degree of cerebellar ataxia, and paradoxical findings in two families. These results suggest the potential use of mGluR1 imaging as a specific biomarker of cerebellar ataxia.

Cellular and circuit mechanisms underlying spinocerebellar ataxias

Degenerative ataxias are a common form of neurodegenerative disease that affect about 20 individuals per 100,000. The autosomal dominant spinocerebellar ataxias (SCAs) are caused by a variety of protein coding mutations (single nucleotide changes, deletions and expansions) in single genes. Affected genes encode plasma membrane and intracellular ion channels, membrane receptors, protein kinases, protein phosphatases and proteins of unknown function. Although SCA-linked genes are quite diverse they share two key features: first, they are highly, although not exclusively, expressed in cerebellar Purkinje neurons (PNs), and second, when mutated they lead ultimately to the degeneration of PNs. In this review we summarize ataxia-related changes in PN neurophysiology that have been observed in various mouse knockout lines and in transgenic models of human SCA. We also highlight emerging evidence that altered metabotropic glutamate receptor signalling and disrupted calcium homeostasis in PNs form a common, early pathophysiological mechanism in SCAs. Together these findings indicate that aberrant calcium signalling and profound changes in PN neurophysiology precede PN cell loss and are likely to lead to cerebellar circuit dysfunction that explains behavioural signs of ataxia characteristic of the disease.

Practical Strategies and Concepts in GPCR Allosteric Modulator Discovery: Recent Advances with Metabotropic Glutamate Receptors

Allosteric modulation of GPCRs has initiated a new era of basic and translational discovery, filled with therapeutic promise yet fraught with caveats. Allosteric ligands stabilize unique conformations of the GPCR that afford fundamentally new receptors, capable of novel pharmacology, unprecedented subtype selectivity, and unique signal bias. This review provides a comprehensive overview of the basics of GPCR allosteric pharmacology, medicinal chemistry, drug metabolism, and validated approaches to address each of the major challenges and caveats. Then, the review narrows focus to highlight recent advances in the discovery of allosteric ligands for metabotropic glutamate receptor subtypes 1-5 and 7 (mGlu1-5,7) highlighting key concepts ("molecular switches", signal bias, heterodimers) and practical solutions to enable the development of tool compounds and clinical candidates. The review closes with a section on late-breaking new advances with allosteric ligands for other GPCRs and emerging data for endogenous allosteric modulators.

Re-exploration of the mGlu₁ PAM Ro 07-11401 scaffold: Discovery of analogs with improved CNS penetration despite steep SAR

This letter describes the re-exploration of the mGlu1 PAM Ro 07-11401 scaffold through a multi-dimensional, iterative parallel synthesis approach. Unlike recent series of mGlu1 PAMs with robust SAR, the SAR around the Ro 07-11401 structure was incredibly steep (only ∼6 of 200 analogs displayed mGlu1 PAM activity), and reminiscent of the CPPHA mGlu5 PAM scaffold. Despite the steep SAR, two new thiazole derivatives were discovered with improved in vitro DMPK profiles and ∼3- to 4-fold improvement in CNS exposure (Kps 1.01-1.19); albeit, with a ∼3-fold diminution in mGlu1 PAM potency, yet comparable efficacy (∼5-fold leftward shift of the glutamate concentration-response curve at 10μM). Thus, this effort has provided additional CNS penetrant mGlu1 PAM tools in a different chemotype than the VU0486321 scaffold. These compounds will permit a better understanding of the pharmacology and therapeutic potential of selective mGlu1 activation, while highlighting the steep SAR challenges that can often be encountered in GPCR allosteric modulator discovery.

Are Type 1 metabotropic glutamate receptors a viable therapeutic target for the treatment of cerebellar ataxia?

The cerebellum is a key brain structure for accurate coordination of sensory and motor function. Compared with other brain regions, the cerebellum expresses a particularly high level of Type 1 metabotropic glutamate receptors (mGluR1). In this review we aim to explore the significance of these receptors for cerebellar synapse function and their potential for treating cerebellar ataxia, a poorly treated degenerative motor disorder that is often hereditary. We find a significant and historical literature showing pivotal mechanisms linking mGluR1 activity with healthy cerebellar synaptic function and motor coordination. This is best illustrated by the impaired motor behaviour in mGluR1 knockout mice that bears strong resemblance to human ataxias. More recent literature also indicates that an imbalance of mGluR1 signalling is as critical as its removal. Too much, as well as too little, mGluR1 activity contributes to ataxia in several clinically relevant mouse models, and perhaps also in humans. Given the availability and ongoing refinement of selective pharmacological tools to either reduce (negative allosteric modulation) or boost (positive allosteric modulation) mGluR1 activity, our findings suggest that pharmacological manipulation of these receptors should be explored as an exciting new approach for the treatment of a variety of human cerebellar ataxias.

Development of Novel, CNS Penetrant Positive Allosteric Modulators for the Metabotropic Glutamate Receptor Subtype 1 (mGlu1), Based on an N-(3-Chloro-4-(1,3-dioxoisoindolin-2-yl)phenyl)-3-methylfuran-2-carboxamide Scaffold, That Potentiate Wild Type and Mutant mGlu1 Receptors Found in Schizophrenics

The therapeutic potential of selective mGlu1 activation is vastly unexplored relative to the other group I mGlu receptor, mGlu5; therefore, our lab has focused considerable effort toward developing mGlu1 positive allosteric modulators (PAMs) suitable as in vivo proof of concept tool compounds. Optimization of a series of mGlu1 PAMs based on an N-(3-chloro-4-(1,3-dioxoisoindolin-2-yl)phenyl)-3-methylfuran-2-carboxamide scaffold provided 17e, a potent (mGlu1 EC50 = 31.8 nM) and highly CNS penetrant (brain to plasma ratio (Kp) of 1.02) mGlu1 PAM tool compound, that potentiated not only wild-type human mGlu1 but also mutant mGlu1 receptors derived from deleterious GRM1 mutations found in schizophrenic patients. Moreover, both electrophysiological and in vivo studies indicate the mGlu1 ago-PAMs/PAMs do not possess the same epileptiform adverse effect liability as mGlu5 ago-PAMs/PAMs and maintain temporal activity suggesting a broader therapeutic window.

Parallel fiber to Purkinje cell synaptic impairment in a mouse model of spinocerebellar ataxia type 27

Genetically inherited mutations in the fibroblast growth factor 14 (FGF14) gene lead to spinocerebellar ataxia type 27 (SCA27), an autosomal dominant disorder characterized by heterogeneous motor and cognitive impairments. Consistently, genetic deletion of Fgf14 in Fgf14 (-/-) mice recapitulates salient features of the SCA27 human disease. In vitro molecular studies in cultured neurons indicate that the FGF14 (F145S) SCA27 allele acts as a dominant negative mutant suppressing the FGF14 wild type function and resulting in inhibition of voltage-gated Na(+) and Ca(2+) channels. To gain insights in the cerebellar deficits in the animal model of the human disease, we applied whole-cell voltage-clamp in the acute cerebellar slice preparation to examine the properties of parallel fibers (PF) to Purkinje neuron synapses in Fgf14 (-/-) mice and wild type littermates. We found that the AMPA receptor-mediated excitatory postsynaptic currents evoked by PF stimulation (PF-EPSCs) were significantly reduced in Fgf14 (-/-) animals, while short-term plasticity, measured as paired-pulse facilitation (PPF), was enhanced. Measuring Sr(2+)-induced release of quanta from stimulated synapses, we found that the size of the PF-EPSCs was unchanged, ruling out a postsynaptic deficit. This phenotype was corroborated by decreased expression of VGLUT1, a specific presynaptic marker at PF-Purkinje neuron synapses. We next examined the mGluR1 receptor-induced response (mGluR1-EPSC) that under normal conditions requires a gradual build-up of glutamate concentration in the synaptic cleft, and found no changes in these responses in Fgf14 (-/-) mice. These results provide evidence of a critical role of FGF14 in maintaining presynaptic function at PF-Purkinje neuron synapses highlighting critical target mechanisms to recapitulate the complexity of the SCA27 disease.

Decreased metabotropic glutamate receptor type 1 availability in a patient with spinocerebellar ataxia type 6: A (11)C-ITMM PET study

OBJECTIVE

Imaging of metabotropic glutamate receptor type 1 (mGluR1), localized exclusively in the cerebellar Purkinje cells and related to cerebellar function, has recently become possible using positron emission tomography (PET). We report the initial mGluR1 imaging in a 74-year-old woman with spinocerebellar ataxia type 6 (SCA6).

METHODS

The patient and 9 age-matched healthy controls underwent PET scanning with a mGluR1 radiotracer, N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl] -4-(11)C-methoxy-N-methylbenzamide. Volumes-of-interest were placed on the anterior and posterior lobes, vermis, and flocculus. Binding potential (BPND) was calculated to estimate mGluR1 availability using the simplified reference tissue model. A partial volume correction was applied to the BPND values. Additionally, the volume of the whole cerebellum was measured using MRI.

RESULTS

The corrected BPND values of the cerebellar subregions and the volume of the whole cerebellum in the patient were 51.0% to 68.3% and 72.6%, respectively, of the controls. Thus, the magnitude of reduced BPND values was relatively larger than the magnitude of cerebellar atrophy in the patient.

CONCLUSION

These findings suggest that the measurement of mGluR1 availability is more sensitive than morphological measurements by MRI to detect reduced cerebellar function. Thus, imaging of mGluR1, probably reflecting the number and distribution of Purkinje cells, can be a specific and sensitive marker for estimation of cerebellar function.