Defective cerebellar ryanodine receptor type 1 and endoplasmic reticulum calcium 'leak' in tremor pathophysiology.

Essential Tremor (ET) is a prevalent neurological disease characterized by an 8-10 Hz action tremor. Molecular mechanisms of ET remain poorly understood. Clinical data suggest the importance of the cerebellum in disease pathophysiology, and pathological studies indicate Purkinje Cells (PCs) incur damage. Our recent cerebellar cortex and PC-specific transcriptome studies identified alterations in calcium (Ca2+) signaling pathways that included ryanodine receptor type 1 (RyR1) in ET. RyR1 is an intracellular Ca2+ release channel located on the Endoplasmic Reticulum (ER), and in cerebellum is predominantly expressed in PCs. Under stress conditions, RyR1 undergoes several post-translational modifications (protein kinase A [PKA] phosphorylation, oxidation, nitrosylation), coupled with depletion of the channel-stabilizing binding partner calstabin1, which collectively characterize a "leaky channel" biochemical signature. In this study, we found markedly increased PKA phosphorylation at the RyR1-S2844 site, increased RyR1 oxidation and nitrosylation, and calstabin1 depletion from the RyR1 complex in postmortem ET cerebellum. Decreased calstabin1-RyR1-binding affinity correlated with loss of PCs and climbing fiber-PC synapses in ET. This 'leaky' RyR1 signature was not seen in control or Parkinson's disease cerebellum. Microsomes from postmortem cerebellum demonstrated excessive ER Ca2+ leak in ET vs. controls, attenuated by channel stabilization. We further studied the role of RyR1 in tremor using a mouse model harboring a RyR1 point mutation that mimics constitutive site-specific PKA phosphorylation (RyR1-S2844D). RyR1-S2844D homozygous mice develop a 10 Hz action tremor and robust abnormal oscillatory activity in cerebellar physiological recordings. Intra-cerebellar microinfusion of RyR1 agonist or antagonist, respectively, increased or decreased tremor amplitude in RyR1-S2844D mice, supporting a direct role of cerebellar RyR1 leakiness for tremor generation. Treating RyR1-S2844D mice with a novel RyR1 channel-stabilizing compound, Rycal, effectively dampened cerebellar oscillatory activity, suppressed tremor, and normalized cerebellar RyR1-calstabin1 binding. These data collectively support that stress-associated ER Ca2+ leak via RyR1 may contribute to tremor pathophysiology.

Physiological Recordings of the Cerebellum in Movement Disorders.

The cerebellum plays an important role in movement disorders, specifically in symptoms of ataxia, tremor, and dystonia. Understanding the physiological signals of the cerebellum contributes to insights into the pathophysiology of these movement disorders and holds promise in advancing therapeutic development. Non-invasive techniques such as electroencephalogram and magnetoencephalogram can record neural signals with high temporal resolution at the millisecond level, which is uniquely suitable to interrogate cerebellar physiology. These techniques have recently been implemented to study cerebellar physiology in healthy subjects as well as individuals with movement disorders. In the present review, we focus on the current understanding of cerebellar physiology using these techniques to study movement disorders.

The Role of the Cerebellum in Learning to Predict Reward: Evidence from Cerebellar Ataxia.

Recent findings in animals have challenged the traditional view of the cerebellum solely as the site of motor control, suggesting that the cerebellum may also be important for learning to predict reward from trial-and-error feedback. Yet, evidence for the role of the cerebellum in reward learning in humans is lacking. Moreover, open questions remain about which specific aspects of reward learning the cerebellum may contribute to. Here we address this gap through an investigation of multiple forms of reward learning in individuals with cerebellum dysfunction, represented by cerebellar ataxia cases. Nineteen participants with cerebellar ataxia and 57 age- and sex-matched healthy controls completed two separate tasks that required learning about reward contingencies from trial-and-error. To probe the selectivity of reward learning processes, the tasks differed in their underlying structure: while one task measured incremental reward learning ability alone, the other allowed participants to use an alternative learning strategy based on episodic memory alongside incremental reward learning. We found that individuals with cerebellar ataxia were profoundly impaired at reward learning from trial-and-error feedback on both tasks, but retained the ability to learn to predict reward based on episodic memory. These findings provide evidence from humans for a specific and necessary role for the cerebellum in incremental learning of reward associations based on reinforcement. More broadly, the findings suggest that alongside its role in motor learning, the cerebellum likely operates in concert with the basal ganglia to support reinforcement learning from reward.

Cerebellar Oscillations in Familial and Sporadic Essential Tremor.

Enhanced cerebellar oscillations have recently been identified in essential tremor (ET) patients as a key pathophysiological change. Since ET is considered a heterogeneous group of diseases, we investigated whether cerebellar oscillations differ in ET subtypes (familial vs. sporadic). This study aims to determine cerebellar physiology in familial and sporadic ET. Using surface electroencephalogram, we studied cerebellar physiology in 40 ET cases (n = 22 familial and n = 18 sporadic) and 20 age-matched controls. Both familial and sporadic ET cases had an increase in the intensity of cerebellar oscillations when compared to controls. Interestingly, cerebellar oscillations correlated with tremor severity in familial ET but not in sporadic ET. Our study demonstrated that ET cases have enhanced cerebellar oscillations, and the different relationships between cerebellar oscillations and tremor severity in familial and sporadic ET suggest diverse cerebellar pathophysiology.

Increased Climbing Fiber Lateral Crossings on Purkinje Cell Dendrites in the Cerebellar Hemisphere in Essential Tremor.

BACKGROUND: Climbing fibers (CFs) innervate Purkinje cells (PCs) with 1:1 relationship to ensure proper cerebellar function. Although CFs abnormally extend into the parallel fiber domain of PC dendrites in essential tremor (ET), the architecture of CFs in relation to PCs has yet to be investigated in detail. OBJECTIVE: The aim of this work was to study the architecture of CFs in relation to PCs in ET. METHODS: The number of PC somas and PC dendrites that a single CF crossed was quantified in the postmortem cerebellum of 15 ET cases and 15 control cases. RESULTS: In ET, CFs crossed a greater number of PC somas and PC dendrites than in control cases, raising the possibility that there is abnormal CF wiring onto the PCs. Interestingly, the increase in CF-PC crossings positively correlated with tremor severity. CONCLUSIONS: Patients with ET have increased CF crossings on PC dendrites. This abnormal architectural arrangement may contribute to synchronous brain activity and tremor. © 2021 International Parkinson and Movement Disorder Society.

External light activates hair follicle stem cells through eyes via an ipRGC-SCN-sympathetic neural pathway.

Changes in external light patterns can alter cell activities in peripheral tissues through slow entrainment of the central clock in suprachiasmatic nucleus (SCN). It remains unclear whether cells in otherwise photo-insensitive tissues can achieve rapid responses to changes in external light. Here we show that light stimulation of animals' eyes results in rapid activation of hair follicle stem cells with prominent hair regeneration. Mechanistically, light signals are interpreted by M1-type intrinsically photosensitive retinal ganglion cells (ipRGCs), which signal to the SCN via melanopsin. Subsequently, efferent sympathetic nerves are immediately activated. Increased norepinephrine release in skin promotes hedgehog signaling to activate hair follicle stem cells. Thus, external light can directly regulate tissue stem cells via an ipRGC-SCN autonomic nervous system circuit. Since activation of sympathetic nerves is not limited to skin, this circuit can also facilitate rapid adaptive responses to external light in other homeostatic tissues.

Current Opinions and Consensus for Studying Tremor in Animal Models.

Tremor is the most common movement disorder; however, we are just beginning to understand the brain circuitry that generates tremor. Various neuroimaging, neuropathological, and physiological studies in human tremor disorders have been performed to further our knowledge of tremor. But, the causal relationship between these observations and tremor is usually difficult to establish and detailed mechanisms are not sufficiently studied. To overcome these obstacles, animal models can provide an important means to look into human tremor disorders. In this manuscript, we will discuss the use of different species of animals (mice, rats, fruit flies, pigs, and monkeys) to model human tremor disorders. Several ways to manipulate the brain circuitry and physiology in these animal models (pharmacology, genetics, and lesioning) will also be discussed. Finally, we will discuss how these animal models can help us to gain knowledge of the pathophysiology of human tremor disorders, which could serve as a platform towards developing novel therapies for tremor.

Deep brain stimulation and climbing fiber synaptic pathology in essential tremor.

Essential tremor (ET) patients have abnormal climbing fiber (CF) synapses in the parallel fiber territory in the cerebellum, and these abnormal CF synapses are inversely correlated with tremor severity. We therefore examined CF synaptic pathology in ET cases with and without thalamic deep brain stimulation (DBS) and assessed the association with tremor severity. We found that CF synaptic pathology was inversely correlated with tremor severity in ET cases without DBS, and this correlation disappeared in ET cases with DBS. Our data suggest that DBS might have effects in modulating excitatory synapses in ET cerebellum, in addition to its symptomatic effects on tremor. Ann Neurol 2016;80:461-465.

Climbing fiber-Purkinje cell synaptic pathology in tremor and cerebellar degenerative diseases.

Changes in climbing fiber-Purkinje cell (CF-PC) synaptic connections have been found in the essential tremor (ET) cerebellum, and these changes are correlated with tremor severity. Whether these postmortem changes are specific to ET remains to be investigated. We assessed CF-PC synaptic pathology in the postmortem cerebellum across a range of degenerative movement disorders [10 Parkinson's disease (PD) cases, 10 multiple system atrophy (MSA) cases, 10 spinocerebellar ataxia type 1 (SCA1) cases, and 20 ET cases] and 25 controls. We observed differences in terms of CF pathological features across these disorders. Specifically, PD cases and ET cases both had more CFs extending into the parallel fiber (PF) territory, but ET cases had more complex branching and increased length of CFs in the PF territory along with decreased CF synaptic density compared to PD cases. MSA cases and SCA1 cases had the most severely reduced CF synaptic density and a marked paucity of CFs extending into the PF territory. Furthermore, CFs in a subset of MSA cases formed collateral branches parallel to the PC layer, a feature not seen in other diagnostic groups. Using unsupervised cluster analysis, the cases and controls could all be categorized into four clusters based on the CF pathology and features of PC pathology, including counts of PCs and their axonal torpedoes. ET cases and PD cases co-segregated into two clusters, whereas SCA1 cases and MSA cases formed another cluster, separate from the control cluster. Interestingly, the presence of resting tremor seemed to be the clinical feature that separated the cases into the two ET-PD clusters. In conclusion, our study demonstrates that these degenerative movement disorders seem to differ with respect to the pattern of CF synaptic pathology they exhibit. It remains to be determined how these differences contribute to the clinical presentations of these diseases.

Cerebellar Pathology in Familial vs. Sporadic Essential Tremor.

Familial and sporadic essential tremor (ET) cases differ in several respects. Whether they differ with respect to cerebellar pathologic changes has yet to be studied. We quantified a broad range of postmortem features (Purkinje cell (PC) counts, PC axonal torpedoes, a host of associated axonal changes, heterotopic PCs, and hairy basket ratings) in 60 ET cases and 30 controls. Familial ET was defined using both liberal criteria (n = 27) and conservative criteria (n = 20). When compared with controls, ET cases had lower PC counts, more torpedoes, more heterotopic PCs, a higher hairy basket rating, an increase in PC axonal collaterals, an increase in PC thickened axonal profiles, and an increase in PC axonal branching. Familial and sporadic ET had similar postmortem changes, with few exceptions, regardless of the definition criteria. The PC counts were marginally lower in familial than sporadic ET (respective p values = 0.059 [using liberal criteria] and 0.047 [using conservative criteria]). The PC thickened axonal profile count was marginally lower in familial ET than sporadic ET (respective p values = 0.037 [using liberal criteria] and 0.17 [using conservative criteria]), and the PC axonal branching count was marginally lower in familial than sporadic ET (respective p values = 0.045 [using liberal criteria] and 0.079 [using conservative criteria]). After correction for multiple comparisons, however, there were no significant differences. Overall, familial and sporadic ET cases share very similar cerebellar postmortem features. These data indicate that pathological changes in the cerebellum are a part of the pathophysiological cascade of events in both forms of ET.

Cerebellar Pathology in Early Onset and Late Onset Essential Tremor.

Early onset and late onset essential tremor (ET) cases differ in several respects. Whether they differ with respect to cerebellar pathologic changes remains to be determined. We quantified a broad range of postmortem features (Purkinje cell (PC) counts, PC axonal torpedoes and associated axonal changes, heterotopic PCs, and hairy basket ratings) in 30 ET cases with age of tremor onset <50 years, 30 ET cases with age of tremor onset ≥50 years, and 30 controls (total n = 90). We also used two alternative age of onset cut-points (<40 vs. ≥40 years, and <60 vs. ≥60 years) to define early onset vs. late onset ET. We found that ET cases with tremor onset <50 years and tremor onset ≥50 years had similar PC counts (8.78 ± 1.70 vs. 8.86 ± 1.24, p = 0.839), PC axonal torpedo counts (17.87 ± 18.27 [median =13.00] vs. 12.90 ± 10.60 [median =9.0], p = 0.486) and associated axonal pathology (all p values >0.05), heterotopic PC counts (9.90 ± 11.55 [median =6.00] vs. 5.40 ± 5.10 [median =3.50], p = 0.092), and hairy basket ratings (1.95 ± 0.62 [median =2.00] vs. 2.05 ± 0.92 [median =2.00], p = 0.314). When using the age of onset cut-points of 40 or 60 years, results were similar. Early onset and late onset ET cases share similar cerebellar postmortem features. These data do not support the notion that these age-of-onset related forms of ET represent distinct clinical-pathological entities.

The T-type calcium channel as a new therapeutic target for Parkinson's disease.

Parkinson's disease (PD) is one of the most prevalent movement disorder caused by degeneration of the dopaminergic neurons in substantia nigra pars compacta. Deep brain stimulation (DBS) at the subthalamic nucleus (STN) has been a new and effective treatment of PD. It is interesting how a neurological disorder caused by the deficiency of a specific chemical substance (i.e., dopamine) from one site could be so successfully treated by a pure physical maneuver (i.e., DBS) at another site. STN neurons could discharge in the single-spike or the burst modes. A significant increase in STN burst discharges has been unequivocally observed in dopamine-deprived conditions such as PD, and was recently shown to have a direct causal relation with parkinsonian symptoms. The occurrence of burst discharges in STN requires enough available T-type Ca(2+) currents, which could bring the relatively negative membrane potential to the threshold of firing Na(+) spikes. DBS, by injection of negative currents into the extracellular space, most likely would depolarize the STN neuron and then inactivate the T-type Ca(2+) channel. Burst discharges are thus decreased and parkinsonian locomotor deficits ameliorated. Conversely, injection of positive currents into STN itself could induce parkinsonian locomotor deficits in animals without dopaminergic lesions. Local application of T-type Ca(2+) channel blockers into STN would also dramatically decrease the burst discharges and improve parkinsonian locomotor symptoms. Notably, zonisamide, which could inhibit T-type Ca(2+) currents in STN, has been shown to benefit PD patients in a clinical trial. From the pathophysiological perspectives, PD can be viewed as a prototypical disorder of "brain arrhythmias". Modulation of relevant ion channels by physical or chemical maneuvers may be important therapeutic considerations for PD and other diseases related to deranged neural rhythms.

Reduced sensitivity to future consequences underlies gambling decision in cerebellar ataxia.

INTRODUCTION: Previous research has identified that people with cerebellar ataxia (CA) showed impaired reward-related decision-making in the Iowa Gambling Task (IGT). To investigate the mechanisms underlying this impairment, we examined CA participants' combination of performance in the IGT, which predominantly tests reward seeking, and the modified IGT (mIGT), which mainly assesses punishment avoidance. METHODS: Fifty participants with CA and one hundred controls completed the IGT and mIGT. Task performance in each of the five twenty-trial blocks was compared between groups and the learning rates were assessed with simple linear regressions. Each participant's IGT score and mIGT score were compared. RESULTS: CA participants performed worse than controls in both the IGT and the mIGT, especially in the last block (IGT: -0.24 ± 10.05 vs. 3.88 ± 10.31, p = 0.041; mIGT: 2.72 ± 7.62 vs. 8.65 ± 8.64, p < 0.001). In contrast to the controls, those with CA did not significantly improve their scores over time in either task. Controls performed better in the mIGT than the IGT, while CA participants' scores in the two tasks showed no significant difference. IGT and mIGT performance did not correlate with ataxia severity or depressive symptoms. CONCLUSION: Individuals with CA showed impaired performance in both the IGT and mIGT, which indicates disruption in both short-term reward seeking and short-term punishment avoidance. Therefore, these results suggest that reduced sensitivity to long-term consequences drives the risky decision-making in CA.

Neuronal population activity in the olivocerebellum encodes the frequency of essential tremor in mice and patients.

Essential tremor (ET) is the most prevalent movement disorder, characterized primarily by action tremor, an involuntary rhythmic movement with a specific frequency. However, the neuronal mechanism underlying the coding of tremor frequency remains unexplored. Here, we used in vivo electrophysiology, optogenetics, and simultaneous motion tracking in the Grid2dupE3 mouse model to investigate whether and how neuronal activity in the olivocerebellum determines the frequency of essential tremor. We report that tremor frequency was encoded by the temporal coherence of population neuronal firing within the olivocerebellums of these mice, leading to frequency-dependent cerebellar oscillations and tremors. This mechanism was precise and generalizable, enabling us to use optogenetic stimulation of the deep cerebellar nuclei to induce frequency-specific tremors in wild-type mice or alter tremor frequencies in tremor mice. In patients with ET, we showed that deep brain stimulation of the thalamus suppressed tremor symptoms but did not eliminate cerebellar oscillations measured by electroencephalgraphy, indicating that tremor-related oscillations in the cerebellum do not require the reciprocal interactions with the thalamus. Frequency-disrupting transcranial alternating current stimulation of the cerebellum could suppress tremor amplitudes, confirming the frequency modulatory role of the cerebellum in patients with ET. These findings offer a neurodynamic basis for the frequency-dependent stimulation of the cerebellum to treat essential tremor.

Subthalamic discharges as a causal determinant of parkinsonian motor deficits.

OBJECTIVE: We have reported that intrinsic membrane properties, especially T-type Ca2+ channels, play a key role in the genesis of burst discharges in the subthalamic nucleus (STN) and parkinsonian locomotor symptoms. Whether deep brain stimulation (DBS) exerts its clinical benefits on Parkinson disease (PD) with changes in T currents or other conductances, however, remains elusive. METHODS: Different stimulation protocols, including constant currents of opposite polarity, were applied to STN in vivo or in vitro, and the electrophysiological and behavioral effects were documented in normal and parkinsonian rodents. The effect of correlatively adjusted DBS protocols was also explored in 3 PD patients. RESULTS: Delivery of negative constant current into STN dramatically ameliorated locomotor deficits in parkinsonian rats. It also depolarized STN neurons and decreased T-channel availability as well as burst discharges. In contrast, delivery of positive constant currents to STN induced PD-like locomotor deficits and increased STN burst discharges in normal rats. In addition, the therapeutic effect of DBS was greatly improved in 3 PD patients simply by increasing the pulse width from 60 to 240 microseconds, even at a lower stimulation frequency of 60 Hz. INTERPRETATION: The increased tendency of STN burst discharges may by itself serve as a direct cause of parkinsonian locomotor deficits, even in the absence of deranged dopaminergic innervation. Effective DBS therapy in PD very likely relies on adequate depolarization, and consequent modification of the relevant ionic currents and discharge patterns, of STN neurons.

T-type calcium channels as therapeutic targets in essential tremor and Parkinson's disease.

Neuronal action potential firing patterns are key components of healthy brain function. Importantly, restoring dysregulated neuronal firing patterns has the potential to be a promising strategy in the development of novel therapeutics for disorders of the central nervous system. Here, we review the pathophysiology of essential tremor and Parkinson's disease, the two most common movement disorders, with a focus on mechanisms underlying the genesis of abnormal firing patterns in the implicated neural circuits. Aberrant burst firing of neurons in the cerebello-thalamo-cortical and basal ganglia-thalamo-cortical circuits contribute to the clinical symptoms of essential tremor and Parkinson's disease, respectively, and T-type calcium channels play a key role in regulating this activity in both the disorders. Accordingly, modulating T-type calcium channel activity has received attention as a potentially promising therapeutic approach to normalize abnormal burst firing in these diseases. In this review, we explore the evidence supporting the theory that T-type calcium channel blockers can ameliorate the pathophysiologic mechanisms underlying essential tremor and Parkinson's disease, furthering the case for clinical investigation of these compounds. We conclude with key considerations for future investigational efforts, providing a critical framework for the development of much needed agents capable of targeting the dysfunctional circuitry underlying movement disorders such as essential tremor, Parkinson's disease, and beyond.

Neurophysiology of cerebellar ataxias and gait disorders.

There are numerous forms of cerebellar disorders from sporadic to genetic diseases. The aim of this chapter is to provide an overview of the advances and emerging techniques during these last 2 decades in the neurophysiological tests useful in cerebellar patients for clinical and research purposes. Clinically, patients exhibit various combinations of a vestibulocerebellar syndrome, a cerebellar cognitive affective syndrome and a cerebellar motor syndrome which will be discussed throughout this chapter. Cerebellar patients show abnormal Bereitschaftpotentials (BPs) and mismatch negativity. Cerebellar EEG is now being applied in cerebellar disorders to unravel impaired electrophysiological patterns associated within disorders of the cerebellar cortex. Eyeblink conditioning is significantly impaired in cerebellar disorders: the ability to acquire conditioned eyeblink responses is reduced in hereditary ataxias, in cerebellar stroke and after tumor surgery of the cerebellum. Furthermore, impaired eyeblink conditioning is an early marker of cerebellar degenerative disease. General rules of motor control suggest that optimal strategies are needed to execute voluntary movements in the complex environment of daily life. A high degree of adaptability is required for learning procedures underlying motor control as sensorimotor adaptation is essential to perform accurate goal-directed movements. Cerebellar patients show impairments during online visuomotor adaptation tasks. Cerebellum-motor cortex inhibition (CBI) is a neurophysiological biomarker showing an inverse association between cerebellothalamocortical tract integrity and ataxia severity. Ataxic gait is characterized by increased step width, reduced ankle joint range of motion, increased gait variability, lack of intra-limb inter-joint and inter-segmental coordination, impaired foot ground placement and loss of trunk control. Taken together, these techniques provide a neurophysiological framework for a better appraisal of cerebellar disorders.

Tracking motion kinematics and tremor with intrinsic oscillatory property of instrumental mechanics.

Tracking kinematic details of motor behaviors is a foundation to study the neuronal mechanism and biology of motor control. However, most of the physiological motor behaviors and movement disorders, such as gait, balance, tremor, dystonia, and myoclonus, are highly dependent on the overall momentum of the whole-body movements. Therefore, tracking the targeted movement and overall momentum simultaneously is critical for motor control research, but it remains an unmet need. Here, we introduce the intrinsic oscillatory property (IOP), a fundamental mechanical principle of physics, as a method for motion tracking in a force plate. The overall kinetic energy of animal motions can be transformed into the oscillatory amplitudes at the designed IOP frequency of the force plate, while the target movement has its own frequency features and can be tracked simultaneously. Using action tremor as an example, we reported that force plate-based IOP approach has superior performance and reliability in detecting both tremor severity and tremor frequency, showing a lower level of coefficient of variation (CV) compared with video- and accelerometer-based motion tracking methods and their combination. Under the locomotor suppression effect of medications, therapeutic effects on tremor severity can still be quantified by dynamically adjusting the overall locomotor activity detected by IOP. We further validated IOP method in optogenetic-induced movements and natural movements, confirming that IOP can represent the intensity of general rhythmic and nonrhythmic movements, thus it can be generalized as a common approach to study kinematics.