Lynn Alison Raymond

Background: Synaptic dysfunction underlies early sensorimotor and cognitive deficits in Huntington's disease (HD) and precedes the degeneration of striatal spiny projection neurons and cortical pyramidal neurons. Movement selection and motor learning, which are impaired early in HD, are regulated by connections between the motor cortex, basal ganglia and thalamus. In particular, plasticity at corticostriatal synapses, including endocannabinoid-mediated long-term depression (LTD), is critical for motor learning. Previously, we found impaired endocannabinoid-mediated LTD, induced by high frequency stimulation (HFS) at corticostriatal synapses in brain slice recordings from pre-manifest HD mouse models, which was corrected by JZL184, an inhibitor of endocannabinoid 2-arachidonoyl glycerol (2-AG) degradation. Objective: Determine the effects of in vivo JZL184 administration on YAC128 HD model and wild-type (WT) littermate mice. Methods: JZL184 was administered to mice orally over a 3-week period and their motor function was assessed using several behavioral tasks. In addition, brain tissue was collected from mice in order to quantify changes in endocannabinoid levels and measure HFS-induced plasticity at corticostriatal synapses. Results: Oral administration of JZL184 significantly increased levels of 2-AG in striatal tissue. While JZL184 treatment had no impact on open field behavior, the treatment eliminated the difference in motor learning on the rotarod task between YAC128 and WT mice. Moreover, HFS-induced striatal plasticity in YAC128 mice was normalized to WT levels after JZL184 treatment. Conclusions: These results suggest a novel target for mitigating early symptoms of HD and support the need for clinical trials of therapies that modulate the endocannabinoid system.

Huntington’s disease (HD) is a neurodegenerative disorder that affects numerous brain functions, yet how altered sensory processing contributes to behavioral and learning deficits remains poorly understood. Previous wide-field mesoscale imaging and electrophysiological recording from anesthetized HD mice revealed that sensory stimulation induced exaggerated, prolonged cortical activity across more brain regions compared to wildtype (WT) littermates. This suggests differences in sensory processing; as such, this study aimed to investigate the cortical activity in a cue-based sensory-guided learning task in a custom-built Raspberry Pi-controlled two-alternative forced choice (2AFC) rig. The rig was designed to enable head-fixed zQ175 knock-in HD mice and WT controls crossed with Thy1-GCaMP6s mice, to perform a cue-based visual discrimination task while mesoscale calcium imaging recorded activity across layer 2/3 of the cortex. Mice that successfully licked the reward spout displayed decreased global cortical activity before the reward presentation, with WT mice showing more spatially localized suppression than HD mice. HD mice exhibited exaggerated cortical responses to visual stimuli and prolonged cortical motor-related activity during licking. While this is an exploratory pilot study with limited sample size, preventing definitive genotype-based comparisons, the custom behavioral system lays the ground-work for future studies into how sensory processing deficits contribute to cognitive impairments in HD. This work provides an important step toward understanding the interplay between cortical circuit dysfunction and behavioral outcomes in HD, offering a novel platform to investigate early sensorimotor integration learning impairments.

Huntington disease (HD) is a genetic neurodegenerative disorder characterized by progressive motor dysfunction, cognitive decline, and neuropsychiatric symptoms. Assessing early motor skill deficits in HD mouse models is challenging with traditional behavioral tasks. This study uses a home cage-based lever-pulling task, PiPaw2.0, to evaluate motor learning in 6–7 months-old zQ175 knock-in HD mice in a more naturalistic environment. In this task, mice learn to pull a lever for a water reward, with the requirement to hold the lever within a specific goal range for a required hold time. As the mice improved, the required hold time increased, thereby gradually increasing the task demands. Both wild type (WT) and zQ175 mice initially showed similar task engagement, but zQ175 mice had significant deficits in adapting to increasing hold time. The WT mice refined their strategies over time, shifting from random to more precise lever pulls, while zQ175 mice failed to make this adjustment, maintaining erratic performance. Additionally, in group-housing WT mouse lever performance benefited from peer interactions, an effect absent in zQ175 mice. Post-task neural assessments revealed that WT mice developed experience-mediated synaptic plasticity in the left striatum (contralateral to lever-pulling paw), while zQ175 mice showed no significant changes, consistent with known corticostriatal plasticity impairments in HD mouse models. In conclusion, our findings demonstrate the effectiveness of group-housed, home cage-based assessments for evaluating motor learning and adaptation in HD mouse models. This study provides insights into the motor control and adaptive learning deficits in HD, emphasizing the value of automated home cage systems in advancing neurodegenerative disease research and highlighting the importance of peer influences on performance.

Synaptic dysfunction underlies early sensorimotor and cognitive deficits, and precedes neurodegeneration in a variety of disorders, including Alzheimer, Parkinson and Huntington disease (HD). A monogenic inherited disorder, HD manifests with cognitive, motor and mood disorders associated with progressive degeneration of striatal spiny projection neurons and cortical pyramidal neurons. Cortico-basal ganglia-thalamic loops regulate movement selection and motor learning, which are impaired early in HD. Skilled motor learning is mediated in part by plasticity at cortico-striatal synapses, including endocannabinoid-mediated, high-frequency stimulation induced long-term depression (HFS-LTD). Previously, we found impaired HFS-LTD in brain slice recordings from pre-manifest HD mouse models, which was corrected by JZL184, an inhibitor of endocannabinoid 2-arachidonoyl glycerol (2-AG) degradation. Here, we tested the effects of JZL184 administered in vivo to YAC128 HD model and wild-type (WT) littermate mice. JZL184, given orally daily over a 3-week period, significantly increased levels of 2-AG in striatal tissue. While JZL184 treatment had no impact on open field behavior which was similar for the two genotypes, the treatment improved motor learning on the rotarod task in YAC128 mice to the level observed in WT mice. Moreover, HFS-induced striatal plasticity measured by field potential recording in acute brain slice from YAC128 mice was normalized to WT levels after JZL184 treatment. These results suggest a novel target for mitigating early symptoms of HD, and support the need for clinical trials to test the efficacy of modulating the endocannabinoid system in treatment of HD.

Huntington's disease (HD) is a monogenic disorder with autosomal dominant inheritance. In HD patients, neurons in the striatum and cortex degenerate, leading to motor, psychiatric and cognitive disorders. Dysregulated synaptic function and calcium handling are common in many neurodegenerative diseases, including HD. N‐methyl‐D‐aspartate (NMDA) receptor function is enhanced in HD at extrasynaptic sites, altering the balance of calcium‐dependent neuronal survival versus death signalling pathways. Endoplasmic reticulum (ER) calcium handling is also abnormal in HD. The ER, which is continuous with the nuclear envelope, is purportedly involved in nuclear calcium signalling; based on this, we hypothesised that nuclear calcium signalling is altered in HD. We explored this hypothesis with calcium imaging techniques, including simultaneous epifluorescent imaging of cytosolic and nuclear calcium using jRCaMP1b and GCaMP3 sensors, respectively, in striatal spiny projection neurons in cortical–striatal co‐cultures from the YAC128 mouse model of HD. Our data show contributions from a variety of calcium channels to nuclear calcium signalling. NMDA receptors (NMDARs) play an essential role in initiating action potential‐dependent calcium signalling to the nucleus, and ryanodine receptors (RyR) contribute to both cytosolic and nuclear calcium signals. Unlike previous reports in glutamatergic hippocampal and cortical neurons, we found that in GABAergic striatal neurons, L‐type voltage‐gated calcium channels (CaV) contribute to cytosolic, but not nuclear calcium signalling. Calcium imaging also suggests impairments in nuclear calcium signalling in HD striatal neurons, where spontaneous action potential‐dependent calcium transients in the nucleus were smaller in YAC128 striatal neurons compared to those of wild‐type (WT). Our results elucidate mechanisms involved in action potential‐dependent nuclear calcium signalling in GABAergic striatal neurons, and have revealed a clear deficit in this signalling in HD. image

The neurodegenerative disorder, Huntington disease (HD), manifests as disorders of movement, cognition and mood. Although studies report abnormal corticostriatal synaptic function early in HD mouse models, less is known about cortical–cortical activity across brain regions and disease stages. Recently, we reported enhanced mesoscale spread of cortical responses to sensory stimulation in vivo at early-manifest stages of two HD mouse models. Here, we investigated cortical excitability of zQ175 HD-model mice compared to their wild-type littermates across different cell types, ages and/or cortical regions using ex vivo electrophysiology. Cortical pyramidal neurons (CPNs) in somatosensory cortex of zQ175 mice showed intrinsic hyper-excitability at 3–4 months, but hypo-excitability at early-manifest stage (8–9 months); reduced frequency of spontaneous excitatory postsynaptic currents (sEPSCs) was seen at both ages. In contrast, motor cortex CPNs in early-manifest zQ175 mice showed increased intrinsic excitability and sEPSC frequency. Large-amplitude excitatory discharges recorded from CPNs in early-manifest zQ175 mice showed increased frequency only in somatosensory cortex, suggesting the intrinsic hypo-excitability of these CPNs may be compensatory against cortical network hyper-excitability. Similarly, in early-manifest zQ175 mice, region-dependent differences were seen in fast-spiking interneurons (FSIs): somatosensory but not motor FSIs from early-manifest zQ175 mice had reduced intrinsic excitability. Moreover, CPNs showed decreased frequency of spontaneous inhibitory postsynaptic currents and increased excitatory-inhibitory (E-I) balance of evoked synaptic currents in somatosensory cortex. Aberrant large-amplitude discharges and reduced inhibitory drive may therefore underlie E-I imbalances that result in circuit changes and synaptic dysfunction in early-manifest HD.

Cortical-striatal synaptic dysfunction, including enhanced toxic signaling by extrasynaptic N-methyl-D-aspartate receptors (eNMDARs), precedes neurodegeneration in Huntington disease (HD). A previous study showed Activin A, whose transcription is upregulated by calcium influx via synaptic NMDARs, suppresses eNMDAR signaling. Therefore, we examined the role of Activin A in the YAC128 HD mouse model, comparing it to wild-type controls. We found decreased Activin A secretion in YAC128 cortical-striatal co-cultures, while Activin A overexpression in this model rescued altered eNMDAR expression. Striatal overexpression of Activin A in vivo improved motor learning on the rotarod task, and normalized striatal neuronal eNMDAR-mediated currents, membrane capacitance and spontaneous excitatory postsynaptic current frequency in the YAC128 mice. These results support the therapeutic potential of Activin A signaling and targeting eNMDARs to restore striatal neuronal health and ameliorate behavioral deficits in HD.

Action potential-independent (miniature) neurotransmission occurs at all chemical synapses, but remains poorly understood, particularly in pathological contexts. Axonal endoplasmic reticulum (ER) Ca²⁺stores are thought to influence miniature neurotransmission, and aberrant ER Ca²⁺handling is implicated in progression of Huntington disease (HD). Here, we report elevated miniature excitatory postsynaptic current (mEPSC) frequencies in recordings from YAC128 mouse (HD-model) neurons (from cortical cultures and striatum-containing brain slices – both from male and female animals). Pharmacological experiments suggest this is mediated indirectly by enhanced tonic ER Ca²⁺release. Calcium imaging, using an axon-localized sensor, revealed slow action potential (AP)-independent ER Ca²⁺release waves in both YAC128 and wild-type cultures. These Ca²⁺waves occurred at similar frequencies in both genotypes, but spread less extensively and were of lower amplitude in YAC128 axons, consistent with axonal ER Ca²⁺store depletion. Surprisingly, basal cytosolic Ca²⁺levels were lower in YAC128 boutons and YAC128 mEPSCs were less sensitive to intracellular Ca²⁺chelation. Together, these data suggest elevated miniature glutamate release in YAC128 cultures is associated with axonal ER Ca²⁺depletion but not directly mediated by ER Ca²⁺release into the cytoplasm. In contrast to increased mEPSC frequencies, cultured YAC128 cortical neurons showed less frequent AP-dependent (spontaneous) Ca²⁺events in soma and axons, although evoked glutamate release detected by iGluSnFR in brain slices was similar between genotypes. Our results indicate axonal ER dysfunction selectively elevates miniature glutamate release from cortical terminals in HD. This, together with reduced spontaneous cortical neuron firing, may cause a shift from activity-dependent to -independent glutamate release in HD, with potential implications for fidelity and plasticity of cortical excitatory signaling.Significance statementMiniature neurotransmitter release persists at all chemical neuronal synapses in the absence of action potential firing but remains poorly understood, particularly in disease states. We show enhanced miniature glutamate release from cortical neurons in the YAC128 mouse Huntington disease model. This effect is mediated by axonal ER Ca²⁺store depletion, but is not obviously due to elevated ER-to-cytosol Ca²⁺release. Conversely, YAC128 cortical pyramidal neurons fired fewer action potentials and evoked cortical glutamate release was similar between WT an YAC128 preparations, indicating axonal ER depletion selectively enhances miniature glutamate release in YAC128 mice. These results extend our understanding of action potential independent neurotransmission and highlight a potential involvement of elevated miniature glutamate release in Huntington disease pathology.