A M1 muscarinic acetylcholine receptor-specific positive allosteric modulator VU0486846 reduces neurogliosis in female Alzheimer's mice.

Alzheimer's disease (AD) is the most prevalent type of dementia, disproportionately affecting females, who make up nearly 60% of diagnosed cases. In AD patients, the accumulation of beta-amyloid (Aß) in the brain triggers a neuroinflammatory response driven by neuroglia, worsening the condition. We have previously demonstrated that VU0486846, an orally available positive allosteric modulator (PAM) targeting M1 muscarinic acetylcholine receptors, enhances cognitive function and reduces Aß pathology in female APPswe/PSEN1ΔE9 (APP/PS1) mice. However, it remained unclear whether these improvements were linked to a decrease in neuroglial activation. To investigate, we treated nine-month-old APP/PS1 and wildtype mice with VU0486846 for 8 weeks and analyzed brain slices for markers of microglial activation (ionized calcium binding adaptor molecule 1, Iba1) and astrocyte activation (Glial fibrillary acidic protein, GFAP). We find that VU0486846 reduces the presence of Iba1-positive microglia and GFAP-positive astrocytes in the hippocampus of female APP/PS1 mice and limits the recruitment of these cells to remaining Aß plaques. This study sheds light on an additional mechanism through which novel M1 mAChR PAMs exhibit disease-modifying effects by reducing neuroglial activation and underscore the potential of these ligands for the treatment of AD, especially in females.

In vitro modulation of mTOR and mGlur5 influence α-synuclein accumulation.

One of the main hallmarks of Parkinson's disease (PD) is abnormal alpha-synuclein (α-syn) aggregation which forms the main component of intracellular Lewy body inclusions. This short report used preformed α-syn fibrils, as well as an A53T mutant α-syn adenovirus to mimic conditions of pathological protein aggregation in dopaminergic human derived SH-SY5Y neural cells. Since there is evidence that the mTOR pathway and glutamatergic signaling each influence protein aggregation, we also assessed the impact of the mTOR inhibitor, rapamycin and the mGluR5 allosteric modulator, CTEP. We found that both rapamycin and CTEP induced a significant reduction of α-syn fibrils in SH-SY5Y cells and this effect was associated with a reduction in mTOR signaling and enhancement in autophagic pathway factors. These data support the possibility that CTEP (or rapamycin) might be a useful pharmacological approach to target abnormal α-syn accumulation by promoting intracellular degradation or enhanced clearance.

Comparison of Huntington's disease phenotype progression in male and female heterozygous FDNQ175 mice.

Huntington's Disease (HD) is an inherited autosomal dominant neurodegenerative disorder that leads to progressive motor and cognitive impairment due to the expansion of a polyglutamine (CAG) repeat in the N-terminal region of the huntingtin (Htt) protein. The creation of HD mouse models represents a critical step in the research for HD treatment. Among the currently available HD mouse models, the zQ175 knock-in mouse line is the first to display robust disease phenotype on a heterozygous background. The newer FDNQ175 mouse model is derived from the zQ175 mouse line and presents a more aggressive phenotype. Moreover, increasing evidence has implicated sex as a contributing factor in the progression of HD symptoms. Here, we compared the progression of HD phenotypes in male and female heterozygous FDNQ175 mice. We found that both male and female heterozygous mice showed deficits in forelimb grip strength and cognition as early as 6 months of age. However, female FDNQ175 mice were less vulnerable to HD-associated decline in limb coordination and movement. Neither male nor female FDNQ175 mice exhibited reduced locomotor activity in the open field or exhibit consistent differences in anxiety at 6-12 months of age. Both male and female FDNQ175 mice exhibited increased numbers of huntingtin aggregates with age and 8-month-old female FDNQ175 mice had significantly more aggregates than their male counterparts. Taken together, our results provide further evidence that sex can influence the progression of HD phenotype in preclinical animal models and must be taken into consideration for future HD research.

VGLUT3 Deletion Rescues Motor Deficits and Neuronal Loss in the zQ175 Mouse Model of Huntington's Disease.

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disease characterized by progressive motor and cognitive impairments, with no disease-modifying therapies yet available. HD pathophysiology involves evident impairment in glutamatergic neurotransmission leading to severe striatal neurodegeneration. The vesicular glutamate transporter-3 (VGLUT3) regulates the striatal network that is centrally affected by HD. Nevertheless, current evidence on the role of VGLUT3 in HD pathophysiology is lacking. Here, we crossed mice lacking Slc17a8 gene (VGLUT3 -/-) with heterozygous zQ175 knock-in mouse model of HD (zQ175:VGLUT3 -/-). Longitudinal assessment of motor and cognitive functions from 6 to 15 months of age reveals that VGLUT3 deletion rescues motor coordination and short-term memory deficits in both male and female zQ175 mice. VGLUT3 deletion also rescues neuronal loss likely via the activation of Akt and ERK1/2 in the striatum of zQ175 mice of both sexes. Interestingly, the rescue in neuronal survival in zQ175:VGLUT3 -/- mice is accompanied by a reduction in the number of nuclear mutant huntingtin (mHTT) aggregates with no change in the total aggregate levels or microgliosis. Collectively, these findings provide novel evidence that VGLUT3, despite its limited expression, can be a vital contributor to HD pathophysiology and a viable target for HD therapeutics.SIGNIFICANCE STATEMENT Dysregulation of the striatal network centrally contributes to the pathophysiology of Huntington's disease (HD). The atypical vesicular glutamate transporter-3 (VGLUT3) has been shown to regulate several major striatal pathologies, such as addiction, eating disorders, or L-DOPA-induced dyskinesia. Yet, our understanding of VGLUT3's role in HD remains unclear. We report here that deletion of the Slc17a8 (Vglut3) gene rescues the deficits in both motor and cognitive functions in HD mice of both sexes. We also find that VGLUT3 deletion activates neuronal survival signaling and reduces nuclear aggregation of abnormal huntingtin proteins and striatal neuron loss in HD mice. Our novel findings highlight the vital contribution of VGLUT3 in HD pathophysiology that can be exploited for HD therapeutic management.

The Role of Neuroglial Metabotropic Glutamate Receptors in Alzheimer's Disease.

Glutamate, the major excitatory neurotransmitter in the brain exerts its effects via both ionotropic glutamate receptors and metabotropic glutamate receptors (mGluRs). There are three subgroups of mGluRs, pre-synaptic Group II and Group III mGluRs and post-synaptic Group I mGluRs. mGluRs are ubiquitously expressed in the brain and their activation is poised upstream of a myriad of signaling pathways, resulting in their implication in the pathogenesis of various neurodegenerative diseases including, Alzheimer's Disease (AD). While the exact mechanism of AD etiology remains elusive, ß-amyloid (Aß) plaques and hyperphosphorylated tau tangles remain the histopathological hallmarks of AD. Though less electrically excitable, neuroglia are a major non-neuronal cell type in the brain and are composed of astrocytes, microglia, and oligodendrocytes. Astrocytes, microglia, and oligodendrocytes provide structural and metabolic support, active immune defence, and axonal support and sheathing, respectively. Interestingly, Aß and hyperphosphorylated tau are known to disrupt the neuroglial homeostasis in the brain, pushing them towards a more neurotoxic state. In this review, we discuss what is currently known regarding the expression patterns of various mGluRs in neuroglia and how Aß and tau alter the normal mGluR function in the neuroglia and contribute to the pathophysiology of AD.

Targeting mGluR2/3 for treatment of neurodegenerative and neuropsychiatric diseases.

Glutamate is the primary excitatory neurotransmitter in the brain and plays critical roles in all aspects of neuronal function. Disruption of normal glutamate transmission has been implicated in a variety of neurodegenerative and neuropsychiatric diseases. Glutamate exerts its effect through ionotropic and metabotropic glutamate receptors (mGluRs). mGluR2 and mGluR3 are members of the Group II mGluR family and their activation leads to the inhibition of glutamate release from presynaptic nerve terminals and is also poised upstream of a myriad of signaling pathways in postsynaptic nerve terminals and neuroglia. Therefore, mGluR2 and mGluR3 have been considered as potential drug targets for the treatment of many neurological conditions and several compounds targeting these receptors have been developed. In this review, we discuss what is currently known regarding the contribution of mGluR2 and mGluR3 to the pathophysiology of some neurodegenerative and neuropsychiatric diseases including Amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease, Parkinson's diseases, schizophrenia and depression as well as drug addiction. We then highlight the evidence supporting the use of various drugs including orthosteric and allosteric ligands acting on either mGluR2, mGluR3 or both for the management of these brain disorders.

VGLUT3 Ablation Differentially Modulates Glutamate Receptor Densities in Mouse Brain.

Type 3 vesicular glutamate transporter (VGLUT3) represents a unique modulator of glutamate release from both nonglutamatergic and glutamatergic varicosities within the brain. Despite its limited abundance, VGLUT3 is vital for the regulation of glutamate signaling and, therefore, modulates the activity of various brain microcircuits. However, little is known about how glutamate receptors are regulated by VGLUT3 across different brain regions. Here, we used VGLUT3 constitutive knock-out (VGLUT3-/-) mice and explored how VGLUT3 deletion influences total and cell surface expression of different ionotropic and metabotropic glutamate receptors. VGLUT3 deletion upregulated the overall expression of metabotropic glutamate receptors mGluR5 and mGluR2/3 in the cerebral cortex. In contrast, no change in the total expression of ionotropic NMDAR glutamate receptors were observed in the cerebral cortex of VGLUT3-/- mice. We noted significant reduction in cell surface levels of mGluR5, NMDAR2A, NMDAR2B, as well as reductions in dopaminergic D1 receptors and muscarinic M1 acetylcholine receptors in the hippocampus of VGLUT3-/- mice. Furthermore, mGluR2/3 total expression and mGluR5 cell surface levels were elevated in the striatum of VGLUT3-/- mice. Last, AMPAR subunit GluA1 was significantly upregulated throughout cortical, hippocampal, and striatal brain regions of VGLUT3-/- mice. Together, these findings complement and further support the evidence that VGLUT3 dynamically regulates glutamate receptor densities in several brain regions. These results suggest that VGLUT3 may play an intricate role in shaping glutamatergic signaling and plasticity in several brain areas.

Metabotropic Glutamate Receptor 5 Antagonism Reduces Pathology and Differentially Improves Symptoms in Male and Female Heterozygous zQ175 Huntington's Mice.

Huntington's disease (HD) is an inherited autosomal dominant neurodegenerative disorder that leads to progressive motor and cognitive impairment. There are currently no available disease modifying treatments for HD patients. We have previously shown that pharmacological blockade of metabotropic glutamate receptor 5 (mGluR5) signaling rescues motor deficits, improves cognitive impairments and mitigates HD neuropathology in male zQ175 HD mice. Mounting evidence indicates that sex may influence HD progression and we have recently reported a sex-specific pathological mGluR5 signaling in Alzheimer's disease (AD) mice. Here, we compared the outcomes of treatment with the mGluR5 negative allosteric modulator CTEP (2-chloro-4-[2-[2,5-dimethyl-1-[4-(trifluoromethoxy)phenyl]imidazol-4-yl]ethynyl]pyridine) in both male and female symptomatic zQ175 mice. We found that female zQ175 mice required a longer treatment duration with CTEP than male mice to show improvement in their rotarod performance. Unlike males, chronic CTEP treatment did not improve the grip strength nor reverse the cognitive decline of female zQ175 mice. However, CTEP reduced the number of huntingtin aggregates, improved neuronal survival and decreased microglia activation in the striatum of both male and female zQ175 mice. Together, our results indicate that mGluR5 antagonism can reduce HD neuropathology in both male and female zQ175 HD mice, but sex has a clear impact on the efficacy of the treatment and must be taken into consideration for future HD drug development.

Noncanonical Metabotropic Glutamate Receptor 5 Signaling in Alzheimer's Disease.

Metabotropic glutamate receptor 5 (mGluR5) is ubiquitously expressed in brain regions responsible for memory and learning. It plays a key role in modulating rapid changes in synaptic transmission and plasticity. mGluR5 supports long-term changes in synaptic strength by regulating the transcription and translation of essential synaptic proteins. ß-Amyloid 42 (Aß42) oligomers interact with a mGluR5/cellular prion protein (PrPC) complex to disrupt physiological mGluR5 signal transduction. Aberrant mGluR5 signaling and associated synaptic failure are considered an emerging pathophysiological mechanism of Alzheimer's disease (AD). Therefore, mGluR5 represents an attractive therapeutic target for AD, and recent studies continue to validate the efficacy of various mGluR5 allosteric modulators in improving memory deficits and mitigating disease pathology. However, sex-specific differences in the pharmacology of mGluR5 and activation of noncanonical signaling downstream of the receptor suggest that its utility as a therapeutic target in female AD patients needs to be reconsidered.

A positive allosteric modulator for the muscarinic receptor (M1 mAChR) improves pathology and cognitive deficits in female APPswe/PSEN1ΔE9 mice.

BACKGROUND AND PURPOSE: Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive cognitive decline, and women account for 60% of diagnosed cases. ß-Amyloid (Aß) oligomers are considered the principal neurotoxic species in AD brains. The M1 muscarinic ACh receptor (M1 mAChR) plays a key role in memory and learning. M1 mAChR agonists show pro-cognitive activity but cause many adverse off-target effects. A new orally bioavailable M1 mAChR positive allosteric modulator (PAM), VU0486846, is devoid of direct agonist activity or adverse effects but was not tested for disease-modifying efficacy in female AD mice. EXPERIMENTAL APPROACH: Nine-month-old female APPswe/PSEN1ΔE9 (APPswe) and wildtype mice were treated with VU0486846 in drinking water (10 mg·kg-1 ·day-1 ) for 4 or 8 weeks. Cognitive function of mice was assessed after treatment, and brains were harvested for biochemical and immunohistochemical assessment. KEY RESULTS: VU0486846 improved cognitive function of APPswe mice when tested in novel object recognition and Morris water maze. This was paralleled by a significant reduction in Aß oligomers and plaques and neuronal loss in the hippocampus. VU0486846 reduced Aß oligomer production in APPswe mice by increasing M1 mAChR expression and shifting the processing of amyloid precursor protein from amyloidogenic cleavage to non-amyloidogenic cleavage. Specifically, VU0486846 reduced the expression of ß-secretase 1 (BACE1), whereas it enhanced the expression of the α-secretase ADAM10 in APPswe hippocampus. CONCLUSION AND IMPLICATIONS: Using M1 mAChR PAMs can be a viable disease-modifying approach that should be exploited clinically to slow AD in women.

Metabotropic Glutamate Receptor 2/3 Activation Improves Motor Performance and Reduces Pathology in Heterozygous zQ175 Huntington Disease Mice.

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease that leads to progressive motor impairments with no available disease-modifying treatment. Current evidence indicates that exacerbated postsynaptic glutamate signaling in the striatum plays a key role in the pathophysiology of HD. However, it remains unclear whether reducing glutamate release can be an effective approach to slow the progression of HD. Here, we show that the activation of metabotropic glutamate receptors 2 and 3 (mGluR2/3), which inhibit presynaptic glutamate release, improves HD symptoms and pathology in heterozygous zQ175 knockin mice. Treatment of both male and female zQ175 mice with the potent and selective mGluR2/3 agonist LY379268 for either 4 or 8 weeks improves both limb coordination and locomotor function in all mice. LY379268 also reduces mutant huntingtin aggregate formation, neuronal cell death, and microglial activation in the striatum of both male and female zQ175 mice. The reduction in mutant huntingtin aggregates correlates with the activation of a glycogen synthase kinase 3ß-dependent autophagy pathway in male, but not female, zQ175 mice. Furthermore, LY379268 reduces both Akt and ERK1/2 phosphorylation in male zQ175 mice but increases both Akt and ERK1/2 phosphorylation in female zQ175 mice. Taken together, our results indicate that mGluR2/3 activation mitigates HD neuropathology in both male and female mice but is associated with the differential activation and inactivation of cell signaling pathways in heterozygous male and female zQ175 mice. This further highlights the need to take sex into consideration when developing future HD therapeutics. SIGNIFICANCE STATEMENT: The mGluR2/3 agonist LY379268 improves motor impairments and reduces pathology in male and female zQ175 Huntington's disease mice. The beneficial outcomes of LY379268 treatment in Huntington's disease mice were mediated by divergent cell signaling pathways in both sexes. We provide evidence that mGluR2/3 agonists can be repurposed for the treatment of Huntington's disease, and we emphasize the importance of investigating sex as a biological variable in preclinical disease-modifying studies.

Optineurin deletion disrupts metabotropic glutamate receptor 5-mediated regulation of ERK1/2, GSK3ß/ZBTB16, mTOR/ULK1 signaling in autophagy.

Optineurin (OPTN) is a multifunctional protein that mediates a network of cellular processes regulating membrane trafficking, inflammatory responses and autophagy. The OPTN-rich interactome includes Group I metabotropic glutamate receptors (mGluR1 and 5), members of the Gαq/11 protein receptor family. Recent evidence has shown that mGluR5, in addition to its canonical Gαq/11 protein-coupled signaling, regulates autophagic machinery via mTOR/ULK1 and GSK3ß/ZBTB16 pathways in both Alzheimer's and Huntington's disease mouse models. Despite its potential involvement, the role of OPTN in mediating mGluR5 downstream signaling cascades remains largely unknown. Here, we employed a CRISPR/Cas9 OPTN-deficient STHdhQ7/Q7 striatal cell line and global OPTN knockout mice to investigate whether Optn gene deletion alters both mGluR5 canonical and noncanonical signaling. We find that OPTN is required for mGluR5-activated Ca2+ flux and ERK1/2 signaling following receptor activation in STHdhQ7/Q7 cells and acute hippocampal slices. Deletion of OPTN impairs both GSK3ß/ZBTB16 and mTOR/ULK1 autophagic signaling in STHdhQ7/Q7 cells. Furthermore, mGluR5-dependent regulation of GSK3ß/ZBTB16 and mTOR/ULK1 autophagic signaling is impaired in hippocampal slices of OPTN knockout mice. Overall, we show that the crosstalk between OPTN and mGluR5 can have major implication on receptor signaling and therefore potentially contribute to the pathophysiology of neurodegenerative diseases.

Metabotropic glutamate type 5 receptor binding availability during dextroamphetamine sensitization in mice and humans.

Background: Glutamate transmission is implicated in drug-induced behavioural sensitization and the associated long-lasting increases in mesolimbic output. Metabotropic glutamate type 5 (mGlu5) receptors might be particularly important, but most details are poorly understood. Methods: We first assessed in mice (n = 51, all male) the effects of repeated dextroamphetamine administration (2.0 mg/kg, i.p.) on locomotor activity and binding of the mGlu5 ligand [3H]ABP688. In a parallel study, in 19 stimulant-drug-naïve healthy human volunteers (14 female) we administered 3 doses of dextroamphetamine (0.3 mg/kg, p.o.) or placebo, followed by a fourth dose 2 weeks later. We measured [11C]ABP688 binding using positron emission tomography before and after the induction phase. We assessed psychomotor and behavioural sensitization using speech rate, eye blink rate and self-report. We measured the localization of mGlu5 relative to synaptic markers in mouse striatum using immunofluorescence. Results: We observed amphetamine-induced psychomotor sensitization in mice and humans. We did not see group differences in mGlu5 availability following 3 pre-challenge amphetamine doses, but group differences did develop in mice administered 5 doses. In mice and humans, individual differences in mGlu5 binding after repeated amphetamine administration were negatively correlated with the extent of behavioural sensitization. In drug-naïve mice, mGlu5 was expressed at 67% of excitatory synapses on dendrites of striatal medium spiny neur. Limitations: Correlational results should be interpreted as suggestive because of the limited sample size. We did not assess sex differences. Conclusion: Together, these results suggest that changes in mGlu5 availability are not part of the earliest neural adaptations in stimulant-induced behavioural sensitization, but low mGlu5 binding might identify a higher propensity for sensitization.

Aß oligomers induce pathophysiological mGluR5 signaling in Alzheimer's disease model mice in a sex-selective manner.

The prevalence, presentation, and progression of Alzheimer's disease (AD) differ between men and women, although ß-amyloid (Aß) deposition is a pathological hallmark of AD in both sexes. Aß-induced activation of the neuronal glutamate receptor mGluR5 is linked to AD progression. However, we found that mGluR5 exhibits distinct sex-dependent profiles. Specifically, mGluR5 isolated from male mouse cortical and hippocampal tissues bound with high affinity to Aß oligomers, whereas mGluR5 from female mice exhibited no such affinity. This sex-selective Aß-mGluR5 interaction did not appear to depend on estrogen, but rather Aß interaction with cellular prion protein (PrPC), which was detected only in male mouse brain homogenates. The ternary complex between mGluR5, Aß oligomers, and PrPC was essential to elicit mGluR5-dependent pathological suppression of autophagy in primary neuronal cultures. Pharmacological inhibition of mGluR5 reactivated autophagy, mitigated Aß pathology, and reversed cognitive decline in male APPswe/PS1ΔE9 mice, but not in their female counterparts. Aß oligomers also bound with high affinity to human mGluR5 isolated from postmortem donor male cortical brain tissue, but not that from female samples, suggesting that this mechanism may be relevant to patients. Our findings indicate that mGluR5 does not contribute to Aß pathology in females, highlighting the complexity of mGluR5 pharmacology and Aß signaling that supports the need for sex-specific stratification in clinical trials assessing AD therapeutics.

Targeting Vesicular Glutamate Transporter Machinery: Implications on Metabotropic Glutamate Receptor 5 Signaling and Behavior.

Cross talk between both pre- and postsynaptic components of glutamatergic neurotransmission plays a crucial role in orchestrating a multitude of brain functions, including synaptic plasticity and motor planning. Metabotropic glutamate receptor (mGluR) 5 exhibits promising therapeutic potential for many neurodevelopmental and neurodegenerative disorders as a consequence of its modulatory control over diverse neuronal networks required for memory, motor coordination, neuronal survival, and differentiation. Given these crucial roles, mGluR5 signaling is under the tight control of glutamate release machinery mediated through vesicular glutamate transporters (VGLUTs) that ultimately dictate glutamatergic output. A particular VGLUT isoform, VGLUT3, exhibits an overlapping, but unique, distribution with mGluR5, and the dynamic cross talk between mGluR5 and VGLUT3 is key for the function of specific neuronal networks involved in motor coordination, emotions, and cognition. Thus, aberrant signaling of the VGLUT3-mGluR5 axis is linked to various pathologies including, but not limited to, Parkinson disease, anxiety disorders, and drug addiction. We argue that a comprehensive profiling of how coordinated VGLUT3-mGluR5 signaling influences overall glutamatergic neurotransmission is warranted. SIGNIFICANCE STATEMENT: Vesicular glutamate receptor (VGLUT) 3 machinery orchestrates glutamate release, and its distribution overlaps with metabotropic glutamate receptor (mGluR) 5 in regional brain circuitries, including striatum, hippocampus, and raphe nucleus. Therefore, VGLUT3-mGluR5 cross talk can significantly influence both physiologic and pathophysiologic glutamatergic neurotransmission. Pathological signaling of the VGLUT3-mGluR5 axis is linked to Parkinson disease, anxiety disorders, and drug addiction. However, it is also predicted to contribute to other motor and cognitive disorders.

mGluR5 regulates REST/NRSF signaling through N-cadherin/ß-catenin complex in Huntington's disease.

Repressor element 1-silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) is a transcription repressor and its expression is regulated by the Wnt pathway through ß-catenin. Metabotropic glutamate receptor 5 (mGluR5) signaling plays a key role in controlling neuronal gene expression. Interestingly, REST/NRSF nuclear translocation and signaling, as well as mGluR5 signaling are altered in the presence of mutant huntingtin. It remains unclear whether mGluR5 can modulate Wnt and REST/NRSF signaling under physiological conditions and whether this modulation is altered in Huntington's disease (HD). Using primary corticostriatal neurons derived from wild type mouse embryos, we find that targeting mGluR5 using the agonist, DHPG, or the negative allosteric modulator, CTEP, modulates REST/NRSF expression by regulating the assembly of N-cadherin/ ß-catenin complex in a Src kinase-dependent manner. We have validated our in vitro findings in vivo using two HD mouse models. Specifically, we show that pharmacological inhibition of mGluR5 in zQ175 mice and genetic ablation of mGluR5 in BACHD mice corrected the pathological activation of Src and rescued REST/NRSF-dependent signaling. Together, our data provide evidence that mGluR5 regulates REST/NRSF expression via the Wnt pathway and highlight the contribution of impaired REST/ NRSF signaling to HD pathology.

mGluR5 Contribution to Neuropathology in Alzheimer Mice Is Disease Stage-Dependent.

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease and is characterized by a progressive cognitive decline in affected individuals. Current therapeutic strategies are limited in their efficacy and some have proven to be even less effective at later disease stages or after extended use. We previously demonstrated that chronic inhibition of mGluR5 signaling using the selective negative allosteric modulator (NAM) CTEP in APPswe/PS1ΔE9 mice can rescue cognitive function, activating the ZBTB16-mediated autophagy pathway to reduce Aß, the principal neurotoxic species in AD brains. Here, we evaluated the efficacy of long-term treatment with CTEP in 6 month old APPswe/PS1ΔE9 mice for either 24 or 36 weeks. CTEP maintained its efficacy in reversing working and spatial memory deficits and mitigating neurogliosis in APPswe/PS1ΔE9 mice when administered for 24 weeks. This was paralleled by a significant reduction in Aß oligomer and plaque load as a result of autophagy activation via ZBTB16 and mTOR-dependent pathways. However, further extension of CTEP treatment for 36 weeks was found ineffective in reversing memory deficit, neurogliosis, or Aß-related pathology. We found that this loss in CTEP efficacy in 15 month old APPswe/PS1ΔE9 mice was due to the abolished contribution of ZBTB16 and mTOR-mediated signaling to AD neuropathology at this advanced disease stage. Our findings indicate that the contribution of pathological mGluR5-signaling to AD may shift as the disease progresses. Thus, we provide the first evidence that the underlying pathophysiological mechanism(s) of AD may unfold along the course of the disease and treatment strategies should be modified accordingly to ensure maximal therapeutic outcomes.

mGluR5 Allosteric Modulation Promotes Neurorecovery in a 6-OHDA-Toxicant Model of Parkinson's Disease.

Parkinson's disease is a neurodegenerative disease characterized by a loss of dopaminergic substantia nigra neurons and depletion of dopamine. To date, current therapeutic approaches focus on managing motor symptoms and trying to slow neurodegeneration, with minimal capacity to promote neurorecovery. mGluR5 plays a key role in neuroplasticity, and altered mGluR5 signaling contributes to synucleinopathy and dyskinesia in patients with Parkinson's disease. Here, we tested whether the mGluR5-negative allosteric modulator, (2-chloro-4-[2[2,5-dimethyl-1-[4-(trifluoromethoxy) phenyl] imidazol-4-yl] ethynyl] pyridine (CTEP), would be effective in improving motor deficits and promoting neural recovery in a 6-hydroxydopamine (6-OHDA) mouse model. Lesions were induced by 6-ODHA striatal infusion, and 30 days later treatment with CTEP (2 mg/kg) or vehicle commenced for either 1 or 12 weeks. Animals were subjected to behavioral, pathological, and molecular analyses. We also assessed how long the effects of CTEP persisted, and finally, using rapamycin, determined the role of the mTOR pathway. CTEP treatment induced a duration-dependent improvement in apomorphine-induced rotation and performance on rotarod in lesioned mice. Moreover, CTEP promoted a recovery of striatal tyrosine hydroxylase-positive fibers and normalized FosB levels in lesioned mice. The beneficial effects of CTEP were paralleled by an activation of mammalian target of rapamycin (mTOR) pathway and elevated brain-derived neurotrophic factor levels in the striatum of lesioned mice. The mTOR inhibitor, rapamycin (sirolimus), abolished CTEP-induced neurorecovery and rescue of motor deficits. Our findings indicate that mTOR pathway is a useful target to promote recovery and that mGluR5 allosteric regulators may potentially be repurposed to selectively target this pathway to enhance neuroplasticity in patients with Parkinson's disease.

Structural determinants governing ß-arrestin2 interaction with PDZ proteins and recruitment to CRFR1.

ß-Arrestins are multifunctional adaptor proteins best know for their vital role in regulating G protein coupled receptor (GPCR) trafficking and signaling. ß-arrestin2 recruitment and receptor internalization of corticotropin-releasing factor receptor 1 (CRFR1), a GPCR whose antagonists have been shown to demonstrate both anxiolytic- and antidepressant-like effects, have previously been shown to be modulated by PDZ proteins. Thus, a structural characterization of the interaction between ß-arrestins and PDZ proteins can delineate potential mechanism of PDZ-dependent regulation of GPCR trafficking. Here, we find that the PDZ proteins PSD-95, MAGI1, and PDZK1 interact with ß-arrestin2 in a PDZ domain-dependent manner. Further investigation of such interaction using mutational analyses revealed that mutating the alanine residue at 175 residue of ß-arrestin2 to phenylalanine impairs interaction with PSD-95. Additionally, A175F mutant of ß-arrestin2 shows decreased CRF-stimulated recruitment to CRFR1 and reduced receptor internalization. Thus, our findings show that the interaction between ß-arrestins and PDZ proteins is key for CRFR1 trafficking and may be targeted to mitigate impaired CRFR1 signaling in mental and psychiatric disorders.