Type 1 metabotropic glutamate receptor and its signaling molecules as therapeutic targets for the treatment of cerebellar disorders

Protein kinase Cγ negatively regulates the intrinsic excitability in zebrin-negative cerebellar Purkinje cells

Protein kinase C γ (PKCγ), a neuronal isoform present exclusively in the central nervous system, is most abundantly expressed in cerebellar Purkinje cells (PCs). Targeted deletion of PKCγ causes a climbing fiber synapse elimination in developing PCs and motor deficit. However, physiological roles of PKCγ in adult mouse PCs are little understood. In this study, we aimed to unravel the roles of PKCγ in mature mouse PCs by deleting PKCγ from adult mouse PCs of PKCγfl/fl mice via cerebellar injection of adeno-associated virus (AAV) vectors expressing Cre recombinase under the control of the PC-specific L7-6 promoter. Whole cell patch-clamp recording of PCs showed higher intrinsic excitability in PCs virally lacking PKCγ [PKCγ-conditional knockout (PKCγ-cKO) PCs] than in wild-type (WT) mouse PCs in the zebrin-negative module, but not in the zebrin-positive module. AAV-mediated PKCγ re-expression in PKCγ-deficient mouse PCs in the zebrin-negative module restored the enhanced intrinsic excitability to a level comparable to that of wild-type mouse PCs. In parallel with higher intrinsic excitability, we found larger hyperpolarization-activated cyclic nucleotide-gated (HCN) channel currents in PKCγ-cKO PCs located in the zebrin-negative module, compared with those in WT mouse PCs in the same region. However, pharmacological inhibition of the HCN currents did not restore the enhanced intrinsic excitability in PKCγ-cKO PCs in the zebrin-negative module. These results suggested that PKCγ suppresses the intrinsic excitability in zebrin-negative PCs, which is likely independent of the HCN current inhibition.

Ablation of KIF2C in Purkinje cells impairs metabotropic glutamate receptor trafficking and motor coordination in male mice

Kinesin family member 2C (KIF2C)/mitotic centromere-associated kinesin (MCAK), is thought to be oncogenic as it is involved in tumour progression and metastasis. Moreover, it also plays a part in neurodegenerative conditions like Alzheimer's disease and psychiatric disorders such as suicidal schizophrenia. Our previous study conducted on mice demonstrated that KIF2C is widely distributed in various regions of the brain, and is localized in synaptic spines. Additionally, it regulates microtubule dynamic properties through its own microtubule depolymerization activity, thereby affecting AMPA receptor transport and cognitive behaviour in mice. In this study, we show that KIF2C regulates the transport of mGlu1 receptors in Purkinje cells by binding to Rab8. KIF2C deficiency in Purkinje cells results in abnormal gait, reduced balance ability and motor incoordination in male mice. These data suggest that KIF2C is essential for maintaining normal transport and synaptic function of mGlu1 and motor coordination in mice. KEY POINTS: KIF2C is localized in synaptic spines of hippocampus neurons, and regulates excitatory transmission, synaptic plasticity and cognitive behaviour. KIF2C is extensively expressed in the cerebellum, and we investigated its functions in development and synaptic transmission of cerebellar Purkinje cells. KIF2C deficiency in Purkinje cells alters the expression of metabotropic glutamate receptor 1 (mGlu1) and the AMPA receptor GluA2 subunit at Purkinje cell synapses, and changes excitatory synaptic transmission, but not inhibitory transmission. KIF2C regulates the transport of mGlu1 receptors in Purkinje cells by binding to Rab8. KIF2C deficiency in Purkinje cells affects motor coordination, but not social behaviour in male mice.

Mitochondrial dysfunction and calcium dysregulation in COQ8A-ataxia Purkinje neurons are rescued by CoQ10 treatment

COQ8A-ataxia is a rare form of neurodegenerative disorder due to mutations in the COQ8A gene. The encoded mitochondrial protein is involved in the regulation of coenzyme Q10 biosynthesis. Previous studies on the constitutive Coq8a-/- mice indicated specific alterations of cerebellar Purkinje neurons involving altered electrophysiological function and dark cell degeneration. In the present manuscript, we extend our understanding of the contribution of Purkinje neuron dysfunction to the pathology. By generating a Purkinje-specific conditional COQ8A knockout, we demonstrate that loss of COQ8A in Purkinje neurons is the main cause of cerebellar ataxia. Furthermore, through in vivo and in vitro approaches, we show that COQ8A-depleted Purkinje neurons have abnormal dendritic arborizations, altered mitochondria function and intracellular calcium dysregulation. Furthermore, we demonstrate that oxidative phosphorylation, in particular Complex IV, is primarily altered at presymptomatic stages of the disease. Finally, the morphology of primary Purkinje neurons as well as the mitochondrial dysfunction and calcium dysregulation could be rescued by CoQ10 treatment, suggesting that CoQ10 could be a beneficial treatment for COQ8A-ataxia.

Pathophysiology of Cerebellar Degeneration in Mitochondrial Disorders: Insights from the Harlequin Mouse

By means of a proteomic approach, we assessed the pathways involved in cerebellar neurodegeneration in a mouse model (Harlequin, Hq) of mitochondrial disorder. A differential proteomic profile study (iTRAQ) was performed in cerebellum homogenates of male Hq and wild-type (WT) mice 8 weeks after the onset of clear symptoms of ataxia in the Hq mice (aged 5.2 ± 0.2 and 5.3 ± 0.1 months for WT and Hq, respectively), followed by a biochemical validation of the most relevant changes. Additional groups of 2-, 3- and 6-month-old WT and Hq mice were analyzed to assess the disease progression on the proteins altered in the proteomic study. The proteomic analysis showed that beyond the expected deregulation of oxidative phosphorylation, the cerebellum of Hq mice showed a marked astroglial activation together with alterations in Ca²⁺ homeostasis and neurotransmission, with an up- and downregulation of GABAergic and glutamatergic neurotransmission, respectively, and the downregulation of cerebellar "long-term depression", a synaptic plasticity phenomenon that is a major player in the error-driven learning that occurs in the cerebellar cortex. Our study provides novel insights into the mechanisms associated with cerebellar degeneration in the Hq mouse model, including a complex deregulation of neuroinflammation, oxidative phosphorylation and glutamate, GABA and amino acids' metabolism.

Ceftriaxone ameliorates hippocampal synapse loss by inhibiting microglial/macrophages activation in glial glutamate transporter-1 dependent manner in the APP/PS1 mouse model of Alzheimer's disease

Synapse loss is a major contributor to cognitive dysfunction in Alzheimer's disease (AD). Impairments in the expression and/or glutamate uptake activity of glia glutamate transporter-1 (GLT-1) contribute to synapse loss in AD. Hence, targeting the restoration of GLT-1 activity may have potential for alleviating synapse loss in AD. Ceftriaxone (Cef) can upregulate the expression and glutamate uptake activity of GLT-1 in many disease models, including those for AD. The present study investigated the effects of Cef on synapse loss and the role of GLT-1 using APP/PS1 transgenic and GLT-1 knockdown APP/PS1 AD mice. Furthermore, the involvement of microglia in the process was investigated due to its important role in synapse loss in AD. We found that Cef treatment significantly ameliorated synapse loss and dendritic degeneration in APP/PS1 AD mice, evidenced by an increased dendritic spine density, decreased dendritic beading density, and upregulated levels of postsynaptic density protein 95 (PSD95) and synaptophysin. The effects of Cef were suppressed by GLT-1 knockdown in GLT-1^+/-/APP/PS1 AD mice. Simultaneously, Cef treatment inhibited ionized calcium binding adapter molecule 1 (Iba1) expression, decreased the proportion of CD11b⁺CD45^hi cells, declined interleukin-6 (IL-6) content, and reduced the co-expression of Iba1 with PSD95 or synaptophysin in APP/PS1 AD mice. In conclusion, Cef treatment ameliorated synapse loss and dendritic degeneration in APP/PS1 AD mice in a GLT-1-dependent manner, and the inhibitory effect of Cef on the activation of microglia/macrophages and their phagocytosis for synaptic elements contributed to the mechanism.

Genetic Variant in GRM1 Underlies Congenital Cerebellar Ataxia with No Obvious Intellectual Disability

Metabotropic glutamate receptor 1 (mGluR1) plays a crucial role in slow excitatory postsynaptic conductance, synapse formation, synaptic plasticity, and motor control. The GRM1 gene is expressed mainly in the brain, with the highest expression in the cerebellum. Mutations in the GRM1 gene have previously been known to cause autosomal recessive and autosomal dominant spinocerebellar ataxias. In this study, whole-exome sequencing of a patient from a family of Azerbaijani origin with a diagnosis of congenital cerebellar ataxia was performed, and a new homozygous missense mutation in the GRM1 gene was identified. The mutation leads to the homozygous amino acid substitution of p.Thr824Arg in an evolutionarily highly conserved region encoding the transmembrane domain 7, which is critical for ligand binding and modulating of receptor activity. This is the first report in which a mutation has been identified in the last transmembrane domain of the mGluR1, causing a congenital autosomal recessive form of cerebellar ataxia with no obvious intellectual disability. Additionally, we summarized all known presumable pathogenic genetic variants in the GRM1 gene to date. We demonstrated that multiple rare variants in the GRM1 underlie a broad diversity of clinical neurological and behavioral phenotypes depending on the nature and protein topology of the mutation.

Consensus Paper: Strengths and Weaknesses of Animal Models of Spinocerebellar Ataxias and Their Clinical Implications

Spinocerebellar ataxias (SCAs) represent a large group of hereditary degenerative diseases of the nervous system, in particular the cerebellum, and other systems that manifest with a variety of progressive motor, cognitive, and behavioral deficits with the leading symptom of cerebellar ataxia. SCAs often lead to severe impairments of the patient's functioning, quality of life, and life expectancy. For SCAs, there are no proven effective pharmacotherapies that improve the symptoms or substantially delay disease progress, i.e., disease-modifying therapies. To study SCA pathogenesis and potential therapies, animal models have been widely used and are an essential part of pre-clinical research. They mainly include mice, but also other vertebrates and invertebrates. Each animal model has its strengths and weaknesses arising from model animal species, type of genetic manipulation, and similarity to human diseases. The types of murine and non-murine models of SCAs, their contribution to the investigation of SCA pathogenesis, pathological phenotype, and therapeutic approaches including their advantages and disadvantages are reviewed in this paper. There is a consensus among the panel of experts that (1) animal models represent valuable tools to improve our understanding of SCAs and discover and assess novel therapies for this group of neurological disorders characterized by diverse mechanisms and differential degenerative progressions, (2) thorough phenotypic assessment of individual animal models is required for studies addressing therapeutic approaches, (3) comparative studies are needed to bring pre-clinical research closer to clinical trials, and (4) mouse models complement cellular and invertebrate models which remain limited in terms of clinical translation for complex neurological disorders such as SCAs.

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.

Negative Allosteric Modulator of mGluR1 Improves Long-Term Neurologic Deficits after Experimental Subarachnoid Hemorrhage

Aneurysmal subarachnoid hemorrhage (SAH) causes permanent neurological sequelae, but the underlying mechanism needs to be further clarified. Here, we show that inhibition of metabotropic glutamate receptor 1 (mGluR1) with negative allosteric modulator JNJ16259685 improves long-term neurobehavioral outcomes in an endovascular perforation model of SAH. JNJ16259685 improves cerebrovascular dysfunction through attenuation of cerebral blood flow (CBF) reduction, cerebral vasoconstrictio, and microthrombosis formation in a rat SAH model. Moreover, JNJ16259685 reduces experimental SAH-induced long-term neuronal damage through alleviation of neuronal death and degeneration. Mechanically, JNJ16259685 maintains phosphorylation of endothelial NO synthase (eNOS) and vasodilator-stimulated phosphoprotein (VASP) and decreases apoptosis-related factors Bax, active caspase-9, and active caspase-3 following experimental SAH. Altogether, our results suggest JNJ16259685 improves long-term functional impairment through neurovascular protection.

Molecular Mechanisms and Therapeutics for Spinocerebellar Ataxia Type 2

The effective therapeutic treatment and the disease-modifying therapy for spinocerebellar ataxia type 2 (SCA2) (a progressive hereditary disease caused by an expansion of polyglutamine in the ataxin-2 protein) is not available yet. At present, only symptomatic treatment and methods of palliative care are prescribed to the patients. Many attempts were made to study the physiological, molecular, and biochemical changes in SCA2 patients and in a variety of the model systems to find new therapeutic targets for SCA2 treatment. A better understanding of the uncovered molecular mechanisms of the disease allowed the scientific community to develop strategies of potential therapy and helped to create some promising therapeutic approaches for SCA2 treatment. Recent progress in this field will be discussed in this review article.

Contribution of Thyrotropin-Releasing Hormone to Cerebellar Long-Term Depression and Motor Learning

Thyrotropin-releasing hormone (TRH) regulates various physiological activities through activation of receptors expressed in a broad range of cells in the central nervous system. The cerebellum expresses TRH receptors in granule cells and molecular layer interneurons. However, the function of TRH in the cerebellum remains to be clarified. Here, using TRH knockout (KO) mice we studied the role of TRH in the cerebellum. Immunohistochemistry showed no gross morphological differences between KO mice and wild-type (WT) littermates in the cerebellum. In the rotarod test, the initial performance of KO mice was comparable to that of WT littermates, but the learning speed of KO mice was significantly lower than that of WT littermates, suggesting impaired motor learning. The motor learning deficit in KO mice was rescued by intraperitoneal injection of TRH. Electrophysiology revealed absence of long-term depression (LTD) at parallel fiber-Purkinje cell synapses in KO mice, which was rescued by bath-application of TRH. TRH was shown to increase cyclic guanosine monophosphate (cGMP) content in the cerebellum. Since nitric oxide (NO) stimulates cGMP synthesis in the cerebellum, we examined whether NO-cGMP pathway was involved in TRH-mediated LTD rescue in KO mice. Pharmacological blockade of NO synthase and subsequent cGMP production prevented TRH-induced LTD expression in KO mice, whereas increase in cGMP signal in Purkinje cells by 8-bromoguanosine cyclic 3’,5’-monophosphate, a membrane-permeable cGMP analog, restored LTD without TRH application. These results suggest that TRH is involved in cerebellar LTD presumably by upregulating the basal cGMP level in Purkinje cells, and, consequently, in motor learning.