Climbing fiber-Purkinje cell synaptic pathology across essential tremor subtypes

Pathologically based criteria to distinguish essential tremor from controls: analyses of the human cerebellum

OBJECTIVE

Essential tremor is among the most prevalent neurological diseases. Diagnosis is based entirely on neurological evaluation. Historically, there were few postmortem brain studies, hindering attempts to develop pathologically based criteria to distinguish essential tremor from control brains. However, an intensive effort to bank essential tremor brains over recent years has resulted in postmortem studies involving >200 brains, which have identified numerous degenerative changes in the essential tremor cerebellar cortex. Although essential tremor and controls have been compared with respect to individual metrics of pathology, there has been no overarching analysis to derive a combination of metrics to distinguish essential tremor from controls. We asked whether there is a constellation of pathological findings that separates essential tremor from controls, and how well that constellation performs.

METHODS

Analyses included 100 essential tremor brains from the essential tremor centralized brain repository and 50 control brains. A standard tissue block from the cerebellar cortex was used to quantify 11 metrics of pathological change. Three supervised classification algorithms were investigated, with data divided into training and validation samples.

RESULTS

Using three different algorithms, we illustrate the ability to correctly predict a diagnosis of essential tremor, with sensitivity and specificity >87%, and in the majority of situations, >90%. We also provide a web-based application that uses these metric values, and based on specified cutoffs, determines the likely diagnosis.

INTERPRETATION

These analyses set the stage for use of pathologically based criteria to distinguish clinically diagnosed essential tremor cases from controls, at the time of postmortem.

Association Analysis of Essential Tremor-Associated Genetic Variants in Sporadic Late-Onset Parkinson's Disease

Background

Parkinson's disease (PD) and Essential tremor (ET) are the two most common tremor diseases with recognized genetic pathogenesis. The overlapping clinical features suggest they may share genetic predispositions. Our previous study systematically investigated the association between rare coding variants in ET-associated genes and early-onset PD (EOPD), and found the suggestive association between teneurin transmembrane protein 4 (TENM4) and EOPD. In the current research, we explored the potential genetic interplay between ET-associated genetic loci/genes and sporadic late-onset PD (LOPD).

Methods

We performed whole-genome sequencing in the 1962 sporadic LOPD cases and 1279 controls from mainland China. We first used logistic regression analysis to test the top 16 SNPs identified by the ET genome-wide association study for the association between ET and LOPD. Then we applied the optimized sequence kernel association testing to explore the rare variant burden of 33 ET-associated genes in this cohort.

Results

We did not observe a significant association between the included SNPs with LOPD. We also did not discover a significant burden of rare deleterious variants of ET-associated genes in association with LOPD risk.

Conclusion

Our results do not support the role of ET-associated genetic loci and variants in LOPD.

Highlights

1962 cases and 1279 controls were recruited to study the potential genetic interplay between ET-associated genetic loci/variants and sporadic LOPD.No significant association between the ET-associated SNPs and LOPD were observed.No significant burden of rare deleterious variants of ET-associated gene in LOPD risk were found.

Gene Expression Analysis of Laser-Captured Purkinje Cells in the Essential Tremor Cerebellum

Essential tremor (ET) is a common, progressive neurological disease characterized by an 8-12-Hz kinetic tremor. Despite its high prevalence, the patho-mechanisms of tremor in ET are not fully known. Through comprehensive studies in postmortem brains, we identified major morphological changes in the ET cerebellum that reflect cellular damage in Purkinje cells (PCs), suggesting that PC damage is central to ET pathogenesis. We previously performed a transcriptome analysis in ET cerebellar cortex, identifying candidate genes and several dysregulated pathways. To directly target PCs, we purified RNA from PCs isolated by laser capture microdissection and performed the first ever PC-specific RNA-sequencing analysis in ET versus controls. Frozen postmortem cerebellar cortex from 24 ETs and 16 controls underwent laser capture microdissection, obtaining ≥2000 PCs per sample. RNA transcriptome was analyzed via differential gene expression, principal component analysis (PCA), and gene set enrichment analyses (GSEA). We identified 36 differentially expressed genes, encompassing multiple cellular processes. Some ET (13/24) had greater dysregulation of these genes and segregated from most controls and remaining ETs in PCA. Characterization of genes/pathways enriched in this PCA and GSEA identified multiple pathway dysregulations in ET, including RNA processing/splicing, synapse organization/ion transport, and oxidative stress/inflammation. Furthermore, a different set of pathways characterized marked heterogeneity among ET patients. Our data indicate a range of possible mechanisms for the pathogenesis of ET. Significant heterogeneity among ET combined with dysregulation of multiple cellular processes supports the notion that ET is a family of disorders rather than one disease entity.

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.

Clinical Heterogeneity of Essential Tremor: Understanding Neural Substrates of Action Tremor Subtypes

Essential tremor (ET) is a common movement disorder affecting millions of people. Studies of ET patients and perturbations in animal models have provided a foundation for the neural networks involved in its pathophysiology. However, ET encompasses a wide variability of phenotypic expression, and this may be the consequence of dysfunction in distinct subcircuits in the brain. The cerebello-thalamo-cortical circuit is a common substrate for the multiple subtypes of action tremor. Within the cerebellum, three sets of cerebellar cortex-deep cerebellar nuclei connections are important for tremor. The lateral hemispheres and dentate nuclei may be involved in intention, postural and isometric tremor. The intermediate zone and interposed nuclei could be involved in intention tremor. The vermis and fastigial nuclei could be involved in head and proximal upper extremity tremor. Studying distinct cerebellar circuitry will provide important framework for understanding the clinical heterogeneity of ET.

Essential tremor

Essential tremor (ET) is a chronic and progressive neurologic disease. Its central and defining clinical feature is a 4-12Hz kinetic tremor, that is, tremor that occurs during voluntary movements such as drinking from a cup or writing. Patients may also exhibit a range of other tremors-postural, rest, intention, additional motor features (e.g., mild gait ataxia, mild dystonia), as well as nonmotor features. The disease itself seems to be a risk factor for other degenerative diseases such as Alzheimer's disease and Parkinson's disease. Both genetic and toxic environmental factors have been explored as etiologic factors. In addition to a growing appreciation of the presence of clinical, etiologic, and pathologic heterogeneity, there is some support for the notion that ET itself may not be a single disease, but may be a family of diseases whose central defining feature is kinetic tremor of the arms, and which might more accurately be referred to as "the essential tremors." Recent research has increasingly placed the seat of the disease in the cerebellum and cerebellar system and identified a host of neurodegenerative changes within the cerebellum, indicating that this progressive disorder is likely degenerative.

Differences in clinical manifestations of late-onset, compared to earlier-onset essential tremor: A scoping review

Across the world, Essential Tremor (ET) is the most common tremor diagnosis but up to half of these diagnoses are inaccurate. The misdiagnosis rate is particularly high in late-onset ET, when tremor begins after the age of 60 years. Currently, ET is reported to affect 5.5% of those over 65 years old and 21.7% aged over 95 but there is emerging evidence that late-onset ET has associations with dementia, mortality and more rapid progression. With ageing populations, and a range of new surgical treatments for ET, there is urgent need to clarify whether the clinical manifestations of late-onset ET are the same as for earlier-onset ET. This scoping review used MEDLINE, EMBASE and CINAHL as the information sources of published peer-reviewed research articles between 2011 and 2021. Analysis was done by narrative synthesis. 14 relevant papers were retrieved from studies conducted in Denmark, India, Italy, Germany, Spain and the US and, together, they comprised 7684 participants in total. Compared to older adults with earlier-onset ET, there is evidence that late-onset ET is associated with higher risk of cognitive impairment and dementia, higher mortality rate, faster rate of progression, lack of family history, altered cortical electrical activity, prolonged pupillary responses, and less propensity to demonstrate characteristic alcohol sensitivity. There is evidence that late-onset ET has different clinical manifestations to earlier-onset ET; in particular there is higher risk of dementia and mortality. The prognosis is important for clinicians to consider when selecting candidates for deep brain stimulation surgery and also for advanced care planning.

Is essential tremor a disorder of GABA dysfunction? No

Although essential tremor is common, its underlying pathophysiology remains uncertain, and several hypotheses seek to explain the tremor mechanism. The GABA hypothesis states that disinhibition of deep cerebellar neurons due to reduced GABAergic input from Purkinje cells results in increased pacemaker activity, leading to rhythmic output to the thalamo-cortical circuit and resulting in tremor. However, some neuroimaging, spectroscopy, and pathology studies have not shown a clear or consistent GABA deficiency in essential tremor, and animal models have indicated that large reductions of Purkinje cell inhibition may improve tremor. Instead, tremor is increasingly attributable to dysfunction in oscillating networks, where altered (but not necessarily reduced) inhibitory signaling can result in tremor. Hypersynchrony of Purkinje cell activity may account for excessive oscillatory cerebellar output, with potential contributions along multiple sites of the olivocerebellar loop. Although older animal tremor models, such as harmaline tremor, have explored contributions from the inferior olivary body, increasing evidence has pointed to the role of aberrant climbing fiber synaptic organization in oscillatory cerebellar activity and tremor generation. New animal models such as hotfoot17j mice, which exhibit abnormal climbing fiber organization due to mutations in Grid2, have recapitulated many features of ET. Similar abnormal climbing fiber architecture and excessive cerebellar oscillations as measured by EEG have been found in humans with essential tremor. Further understanding of hypersynchrony and excessive oscillatory activity in ET phenotypes may lead to more targeted and effective treatment options.

Essential tremor: Clinical perspectives and pathophysiology

Essential tremor (ET) is one of the most common neurological disorders and can be highly disabling. In recent years, studies on the clinical perspectives and pathophysiology have advanced our understanding of ET. Specifically, clinical heterogeneity of ET, with co-existence of tremor and other neurological features such as dystonia, ataxia, and cognitive dysfunction, has been identified. The cerebellum has been found to be the key brain region for tremor generation, and structural alterations of the cerebellum have been extensively studied in ET. Finally, four main ET pathophysiologies have been proposed: 1) environmental exposures to β-carboline alkaloids and the consequent olivocerebellar hyper-excitation, 2) cerebellar GABA deficiency, 3) climbing fiber synaptic pathology with related cerebellar oscillatory activity, 4) extra-cerebellar oscillatory activity. While these four theories are not mutually exclusive, they can represent distinctive ET subtypes, indicating multiple types of abnormal brain circuitry can lead to action tremor. This article is part of the Special Issue "Tremor" edited by Daniel D. Truong, Mark Hallett, and Aasef Shaikh.

Cerebello-Thalamo-Cortical Network Dynamics in the Harmaline Rodent Model of Essential Tremor

Essential Tremor (ET) is a common movement disorder, characterised by a posture or movement-related tremor of the upper limbs. Abnormalities within cerebellar circuits are thought to underlie the pathogenesis of ET, resulting in aberrant synchronous oscillatory activity within the thalamo-cortical network leading to tremors. Harmaline produces pathological oscillations within the cerebellum, and a tremor that phenotypically resembles ET. However, the neural network dynamics in cerebellar-thalamo-cortical circuits in harmaline-induced tremor remains unclear, including the way circuit interactions may be influenced by behavioural state. Here, we examined the effect of harmaline on cerebello-thalamo-cortical oscillations during rest and movement. EEG recordings from the sensorimotor cortex and local field potentials (LFP) from thalamic and medial cerebellar nuclei were simultaneously recorded in awake behaving rats, alongside measures of tremor using EMG and accelerometery. Analyses compared neural oscillations before and after systemic administration of harmaline (10 mg/kg, I.P), and coherence across periods when rats were resting vs. moving. During movement, harmaline increased the 9-15 Hz behavioural tremor amplitude and increased thalamic LFP coherence with tremor. Medial cerebellar nuclei and cerebellar vermis LFP coherence with tremor however remained unchanged from rest. These findings suggest harmaline-induced cerebellar oscillations are independent of behavioural state and associated changes in tremor amplitude. By contrast, thalamic oscillations are dependent on behavioural state and related changes in tremor amplitude. This study provides new insights into the role of cerebello-thalamo-cortical network interactions in tremor, whereby neural oscillations in thalamocortical, but not cerebellar circuits can be influenced by movement and/or behavioural tremor amplitude in the harmaline model.

Is essential tremor a degenerative disorder or an electric disorder? Degenerative disorder

Essential tremor (ET) is a highly prevalent neurologic disease and is the most common of the many tremor disorders. ET is a progressive condition with marked clinical heterogeneity, associated with a spectrum of both motor and non-motor features. However, its disease mechanisms remain poorly understood. Much debate has centered on whether ET should be considered a degenerative disorder, with underlying pathological changes in brain causing progressive disease manifestations, or an electric disorder, with overactivity of intrinsically oscillatory motor networks that occur without underlying structural brain abnormalities. Converging data from clinical, neuroimaging and pathological studies in ET now provide considerable evidence for the neurodegenerative hypothesis. A major turning point in this debate is that rigorous tissue-based studies have recently identified a series of structural changes in the ET cerebellum. Most of these pathological changes are centered on the Purkinje cell and connected neuronal populations, which can result in partial loss of Purkinje cells and circuitry reorganizations that would disturb cerebellar function. There is significant overlap in clinical and pathological features of ET with other disorders of cerebellar degeneration, and an increased risk of developing other degenerative diseases in ET. The combined implication of these studies is that ET could be degenerative. The evidence in support of the degenerative hypothesis is presented.

Is the inferior olive central to essential tremor? Yes

We consider the question whether the inferior olive (IO) is required for essential tremor (ET). Much evidence shows that the olivocerebellar system is the main system capable of generating the widespread synchronous oscillatory Purkinje cell (PC) complex spike (CS) activity across the cerebellar cortex that would be capable of generating the type of bursting cerebellar output from the deep cerebellar nuclei (DCN) that could underlie tremor. Normally, synchronous CS activity primarily reflects the effective electrical coupling of IO neurons by gap junctions, and traditionally, ET research has focused on the hypothesis of increased coupling of IO neurons as the cause of hypersynchronous CS activity underlying tremor. However, recent pathology studies of brains from humans with ET and evidence from mutant mice, particularly the hotfoot17 mouse, that largely replicate the pathology of ET, suggest that the abnormal innervation of multiple Purkinje cells (PCs) by climbing fibers (Cfs) is related to tremor. In addition, ET brains show partial PC loss and axon terminal sprouting by surviving PCs. This may provide another mechanism for tremor. It is proposed that in ET, these three mechanisms may promote tremor. They all involve hypersynchronous DCN activity and an intact IO, but the level at which excessive synchronization occurs may be at the IO level (from abnormal afferent activity to this nucleus), the PC level (via aberrant Cfs), or the DCN level (via terminal PC collateral innervation).

Essential tremor

Essential tremor (ET) is one of the most common movement disorders, with a reported >60 million affected individuals worldwide. The definition and underlying pathophysiology of ET are contentious. Patients present primarily with motor features such as postural and action tremors, but may also have other non-motor features, including cognitive impairment and neuropsychiatric symptoms. Genetics account for most of the ET risk but environmental factors may also be involved. However, the variable penetrance and challenges in validating data make gene-environment analysis difficult. Structural changes in cerebellar Purkinje cells and neighbouring neuronal populations have been observed in post-mortem studies, and other studies have found GABAergic dysfunction and dysregulation of the cerebellar-thalamic-cortical circuitry. Commonly prescribed medications include propranolol and primidone. Deep brain stimulation and ultrasound thalamotomy are surgical options in patients with medically intractable ET. Further research in post-mortem studies, and animal and cell-based models may help identify new pathophysiological clues and therapeutic targets and, together with advances in omics and machine learning, may facilitate the development of precision medicine for patients with ET.

Reducing Ataxic Side Effects from Ventral Intermediate Nucleus of the Thalamus Deep Brain Stimulation Implantation in Essential Tremor: Potential Advantages of Directional Stimulation

OBJECTIVE

The aim of the study was to retrospectively evaluate the effect of directional deep brain stimulation (DBS) on ataxia in an essential tremor patient population.

MATERIALS AND METHODS

A retrospective chart review of documented Scale for Assessment and Rating of Ataxia (SARA) scores were analyzed using a case-control design. All subjects we evaluated were treated at a single, tertiary care academic center. We reviewed 14 patients who underwent bilateral ventral intermediate nucleus of the thalamus (VIM) implantation with microelectrode recording, with electrodeposition and segmented contact orientation confirmed via postoperative computed tomography. The main outcome was to determine change in ataxia scores between directional versus monopolar circumferential stimulation.

RESULTS

Fourteen patients (9 males, median age at implantation 69 [range 63-82]) underwent surgery between October 2017 and July 2020 at the UNC Movement Disorders Center. SARA scores between directional stimulation and monopolar circumferential stimulation demonstrated a significant reduction in total scores with best possible segmented stimulation (n = 13, p < 0.0001, 95% confidence interval [CI] -3.496 to -6.789). This difference remained statistically significant even after removing the SARA tremor subscore (n = 13, p < 0.0001, 95% CI -3.155 to -6.274). In line with prior reports, SARA score changes from the preoperative state were generally worsened when applying monopolar circumferential stimulation bilaterally (n = 13, p = 0.655; 95% CI -2.836 to 4.359), but improved with directional stimulation (n = 13, p = 0.010; 95% CI -1.216 to -7.547).

CONCLUSION

This retrospective analysis appears to show evidence for improved outcomes through directional stimulation in bilateral VIM DBS implantation with reduction of ataxic side effects that have traditionally plagued postoperative results, all while providing optimized tremor reduction via stimulation.

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.

Cerebellar oscillations driven by synaptic pruning deficits of cerebellar climbing fibers contribute to tremor pathophysiology

Essential tremor (ET) is one of the most common movement disorders and the prototypical disorder for abnormal rhythmic movements. However, the pathophysiology of tremor generation in ET remains unclear. Here, we used autoptic cerebral tissue from patients with ET, clinical data, and mouse models to report that synaptic pruning deficits of climbing fiber (CF)-to-Purkinje cell (PC) synapses, which are related to glutamate receptor delta 2 (GluRδ2) protein insufficiency, cause excessive cerebellar oscillations and might be responsible for tremor. The CF-PC synaptic pruning deficits were correlated with the reduction in GluRδ2 expression in the postmortem ET cerebellum. Mice with GluRδ2 insufficiency and CF-PC synaptic pruning deficits develop ET-like tremor that can be suppressed with viral rescue of GluRδ2 protein. Step-by-step optogenetic or pharmacological inhibition of neuronal firing, axonal activity, or synaptic vesicle release confirmed that the activity of the excessive CF-to-PC synapses is required for tremor generation. In vivo electrophysiology in mice showed that excessive cerebellar oscillatory activity is CF dependent and necessary for tremor and optogenetic-driven PC synchronization was sufficient to generate tremor in wild-type animals. Human validation by cerebellar electroencephalography confirmed that excessive cerebellar oscillations also exist in patients with ET. Our findings identify a pathophysiologic contribution to tremor at molecular (GluRδ2), structural (CF-to-PC synapses), physiological (cerebellar oscillations), and behavioral levels (kinetic tremor) that might have clinical applications for treating ET.

Non-motor symptoms in essential tremor, akinetic rigid and tremor-dominant subtypes of Parkinson's disease

Objectives

To compare non-motor symptoms (NMSs) among patients with essential tremor (ET), Parkinson’s disease (PD) subtypes (akinetic-rigid type (ART) and tremor-dominant type (TDT)), and healthy controls.

Patients and methods

This retrospective study included 129 participants, 72 PD (33 PD-ART, 33 PD-TDT, and 6 Mixed), 29 ET patients, and 28 controls. PD patients were assessed by the unified Parkinson’s disease rating scale (UPDRS), Hoehn, and Yahr scale (H&Y), while ET patients were evaluated by the Fahn Tolosa Marin Tremor Rating Scale. All subjects were evaluated by non-motor symptoms scale (NMSS) for NMSs and Beck depression inventory (BDI) for depression.

Results

PD subtypes groups, ET, and controls were age and gender-matched. Compared to controls, all PD, PD subtypes, and ET showed significantly worse most of NMSs (p<0.001) and depression. Compared to ET, all PD and PD-ART had significantly worse gastrointestinal (p = 0.002), urinary symptoms (p = 0.001, p = 0.003) and depression (p = 0.002) and PD-TDT worse depression, while ET patients showed worse memory/attention than PD subtypes. Total NMSS of ET is highly correlated to depression and moderately to tremor severity and age of onset, while total of NMSS is highly correlated to depression, disease severity, and disability.

Conclusion

The current study demonstrated several comparable domains of NMSs of PD subtypes and ET, except worse gastrointestinal and urinary symptoms among PD-ART. Identifying different NMSs profiles is important for predicting, better assessing, and tailoring management of ET and PD subtypes.

Essential Tremor - A Cerebellar Driven Disorder?

Abnormal tremors are the most common of all movement disorders. In this review we focus on the role of the cerebellum in Essential Tremor, a highly debilitating but poorly treated movement disorder. We propose a variety of mechanisms driving abnormal burst firing of deep cerebellar nuclei neurons as a key initiator of tremorgenesis in Essential Tremor. Targetting these mechanisms may generate more effective treatments for Essential Tremor.

Increased Purkinje Cell Complex Spike and Deep Cerebellar Nucleus Synchrony as a Potential Basis for Syndromic Essential Tremor. A Review and Synthesis of the Literature

We review advances in understanding Purkinje cell (PC) complex spike (CS) physiology that suggest increased CS synchrony underlies syndromic essential tremor (ET). We searched PubMed for papers describing factors that affect CS synchrony or cerebellar circuits potentially related to tremor. Inferior olivary (IO) neurons are electrically coupled, with the degree of coupling controlled by excitatory and GABAergic inputs. Clusters of coupled IO neurons synchronize CSs within parasagittal bands via climbing fibers (Cfs). When motor cortex is stimulated in rats at varying frequencies, whisker movement occurs at ~10 Hz, correlated with synchronous CSs, indicating that the IO/CS oscillatory rhythm gates movement frequency. Intra-IO injection of the GABA_A receptor antagonist picrotoxin increases CS synchrony, increases whisker movement amplitude, and induces tremor. Harmaline and 5-HT_2a receptor activation also increase IO coupling and CS synchrony and induce tremor. The hotfoot17 mouse displays features found in ET brains, including cerebellar GluRδ2 deficiency and abnormal PC Cf innervation, with IO- and PC-dependent cerebellar oscillations and tremor likely due to enhanced CS synchrony. Heightened coupling within the IO oscillator leads, through its dynamic control of CS synchrony, to increased movement amplitude and, when sufficiently intense, action tremor. Increased CS synchrony secondary to aberrant Cf innervation of multiple PCs likely also underlies hotfoot17 tremor. Deep cerebellar nucleus (DCN) hypersynchrony may occur secondary to increased CS synchrony but might also occur from PC axonal terminal sprouting during partial PC loss. Through these combined mechanisms, increased CS/DCN synchrony may plausibly underlie syndromic ET.

A pilot study of essential tremor: cerebellar GABA+/Glx ratio is correlated with tremor severity

Objective

Essential tremor is a common movement disorder with an unclear origin. Emerging evidence suggests the role of the cerebellum and the thalamus in tremor pathophysiology. We examined the two main neurotransmitters acting inhibitory (GABA+) and excitatory (Glx) respectively, in the thalamus and cerebellum, in patients diagnosed with severe essential tremor. Furthermore, we also investigated the relationship between determined neurotransmitter concentrations and tremor severity in the essential tremor patients.

Methods

Ten essential tremor patients (prior to deep brain stimulation surgery) and six healthy controls, were scanned using a 3 T MR system. GABA+ and Glx concentrations were measured using magnetic resonance spectroscopy (MRS) performed using single voxel MEGA-PRESS. For the purpose of assessing the tremor severity, the essential tremor rating scale (ETRS) was used in accordance with Fahn, Tolosa, and Marin.

Results

We demonstrated that the cerebellar GABA+/Glx ratio was positively correlated to the ETRS (r = 0.70, p = 0.03) in essential tremor. Cerebellar and thalamic GABA+ and Glx concentrations did not show any significant difference when comparing essential tremor patients with healthy controls, at the group level.

Conclusion

We demonstrated a positive correlation between increasing tremor disability and the ratio of GABA+/ Glx in the cerebellum of essential tremor patients. This highlights the impact of an altered balance of the excitatory and inhibitory neurotransmitters in tremor severity. Rather than a change in GABA+, which was constant, we attribute this finding to an overall decrease of Glx.

Essential tremor pathology: neurodegeneration and reorganization of neuronal connections

Essential tremor (ET) is the most common tremor disorder globally and is characterized by kinetic tremor of the upper limbs, although other clinical features can also occur. Postmortem studies are a particularly important avenue for advancing our understanding of the pathogenesis of ET; however, until recently, the number of such studies has been limited. Several recent postmortem studies have made important contributions to our understanding of the pathological changes that take place in ET. These studies identified abnormalities in the cerebellum, which primarily affected Purkinje cells (PCs), basket cells and climbing fibres, in individuals with ET. We suggest that some of these pathological changes (for example, focal PC axonal swellings, swellings in and regression of the PC dendritic arbor and PC death) are likely to be primary and degenerative. By contrast, other changes, such as an increase in PC recurrent axonal collateral formation and hypertrophy of GABAergic basket cell axonal processes, could be compensatory responses to restore cerebellar GABAergic tone and cerebellar cortical inhibitory efficacy. Such compensatory responses are likely to be insufficient, enabling the disease to progress. Here, we review the results of recent postmortem studies of ET and attempt to place these findings into an anatomical-physiological disease model.

Oral propranolol for treatment of the subgroups of essential tremor: a systematic review and meta-analysis protocol

INTRODUCTION

Essential tremor (ET), a tremor disorder, is one of the most common movement disorders. Only oral drugs (propranolol, primidone, topiramate, etc）are still the first-line treatment recommended by the Food and Drug Administration. Propranolol is thought to potentially reduce upper limb action tremor. However, it has a poor effect on axial tremor symptoms, such as essential head tremor and voice tremor. Studies have shown that tremor severity develops over time, possibly producing other clinical tremors and neurological soft signs (such as memory loss, gait abnormalities, balance disorders, etc), which further increases the difficulty of treating tremors. However, some recent studies provide emerging evidence for oral propranolol on subgroups of ET, which is based on the anatomical distribution of ET (lower extremities, head, sound, tongue, etc). This systematic review aims to synthesise these new data to improve the efficacy of propranolol in ET subgroups.

METHODS AND ANALYSIS

We will search for randomised controlled trials from the PubMed, MEDLINE, EMBASE, Cochrane Library, UptoDate and PEDro databases from inception to June 2019. All data will be extracted independently by two reviewers and compared at the end of the review. The two reviewers will screen the study quality, and the Cochrane Collaboration's tool in Review Manager (RevMan) V.5.3.3 will be used to evaluate risk of bias. Our primary outcome will be the functional disability component related to tremors, as measured by the Fahn-Tolosa-Marin Tremor Rating Scale subscales B and C. Secondary outcomes will include severity of tremors and quality of life. Narrative and meta-analytical syntheses are planned.

ETHICS AND DISSEMINATION

Published aggregated data will be used in this review analysis and therefore no ethical approval is required. The results will be published in peer-reviewed journals, and proliferation activities will include diverse social stakeholders, non-academic groups and patients.

PROSPERO REGISTRATION NUMBER

CRD42018112580.

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.

Gene expression analysis of the cerebellar cortex in essential tremor

Essential tremor (ET) is one of the most common neurological diseases, with a central feature of an 8-12 Hz kinetic tremor. While previous postmortem studies have identified a cluster of morphological changes in the ET cerebellum centered in/around the Purkinje cell (PC) population, including a loss of PCs in some studies, the underlying molecular mechanisms for these changes are not clear. As genomic studies of ET patients have yet to identify major genetic contributors and animal models that fully recapitulate the human disease do not yet exist, the study of human tissue is currently the most applicable method to gain a mechanistic insight into ET disease pathogenesis. To begin exploration of an underlying molecular source of ET disease pathogenesis, we have performed the first transcriptomic analysis by direct sequencing of RNA from frozen cerebellar cortex tissue in 33 ET patients compared to 21 normal controls. Principal component analysis showed a heterogenous distribution of the expression data in ET patients that only partially overlapped with control patients. Differential expression analysis identified 231 differentially expressed gene transcripts ('top gene hits'), a subset of which has defined expression profiles in the cerebellum across neuronal and glial cell types but a largely unknown relationship to cerebellar function and/or ET pathogenesis. Gene set enrichment analysis (GSEA) identified dysregulated pathways of interest and stratified dysregulation among ET cases. By GSEA and mining curated databases, we compiled major categories of dysregulated processes and clustered string networks of known interacting proteins. Here we demonstrate that these 'top gene hits' contribute to regulation of four main biological processes, which are 1) axon guidance, 2) microtubule motor activity, 3) endoplasmic reticulum (ER) to Golgi transport and 4) calcium signaling/synaptic transmission. The results of our transcriptomic analysis suggest there is a range of different processes involved among ET cases, and draws attention to a particular set of genes and regulatory pathways that provide an initial platform to further explore the underlying biology of ET.

Pupillary response to light and tasks in early and late onset essential tremor patients

INTRODUCTION

Late-onset essential tremor is characterised by shorter life expectancy and more advanced aging parameters and may therefore be an 'aging-related' tremor. Brainstem functions involved in pupillary responses are hypothesized to reflect such earlier aging. The pupillary light response (PLR) and a task evoked pupillary response (TEPR) were used to test this hypothesis in same-aged patients with early onset (EOET) and late onset (LOET) essential tremor and healthy controls.

SUBJECTS AND METHODS

Age related changes of the PLR and TEPR during the paced auditory serial addition test (PASAT) were tested in 57 normal subjects. Subsequently, 13 patients with LOET and 16 patients with EOET were compared with 15 age matched healthy controls. Standard parameters of PLR were recorded, amongst others the time to maximum acceleration of the PLR (T1) and the time to maximum velocity (T2). The TEPR was determined during the PASAT as the percentage change in pupil size (PCPS). Data were analysed with ANOVA and post-hoc testing.

RESULTS

In normal subjects the pupil diameter, latency, maximum acceleration/velocity and percentage amplitude were correlated with age. Latency of the pupillary light response was significantly longer in LOET compared to controls and EOET while no differences were found between EOET and controls. The TEPR showed no significant differences between the three groups.

CONCLUSION

LOET showed a prolonged latency of the PLR compared to EOET possibly indicating premature aging or rather pathophysiological differences on brainstem level. This study further supports the hypothesis of abnormal aging in LOET.

Essential Tremor and Parkinson's Disease: Exploring the Relationship

Background

There is longstanding controversy surrounding the possible link between essential tremor (ET) and Parkinson's disease (PD). Inconsistent and unreliable diagnostic criteria may in part account for some of the difficulties in defining the relationship between these two common movement disorders.

Methods

References for this systematic review were identified using PubMed with the search terms "essential tremor" AND "Parkinson's disease" with articles published in English between 1960 and September 2018 included.

Results

In this review we provide evidence that some patients diagnosed with ET have an increased risk of developing PD years or decades after onset of action tremor. There are several still unresolved questions about the link between the two disorders including lack of verifiable diagnostic criteria for the two disorders and marked overlap in phenomenology. Here we review clinical, epidemiologic, imaging, pathologic, and genetic studies that address the ET-PD relationship. Several lines of evidence support the association between ET and PD, including overlapping motor and non-motor features, relatively high prevalence of rapid eye movement sleep behavior disorder (26-43%) in ET patients, increased prevalence of PD in patients with longstanding antecedent ET, increased prevalence of ET in family members of patients with PD, and the presence of Lewy bodies in the brains of some ET patients (15-24%).

Discussion

There is a substantial body of evidence supporting the association between ET and PD within at least a subset of patients, although the nature and possible pathogenic mechanisms of the relationship are not well understood.

Animal Models of Tremor: Relevance to Human Tremor Disorders

Background

Tremor is the most common movement disorder; however, the pathophysiology of tremor remains elusive. While several neuropathological alterations in tremor disorders have been observed in post-mortem studies of human brains, a full understanding of the relationship between brain circuitry alterations and tremor requires testing in animal models. Additionally, tremor animal models are critical for our understanding of tremor pathophysiology, and/or to serve as a platform for therapy development.

Methods

A PubMed search was conducted in May 2018 to identify published papers for review.

Results

The methodology used in most studies on animal models of tremor lacks standardized measurement of tremor frequency and amplitude; instead, these studies are based on the visual inspection of phenotypes, which may fail to delineate tremor from other movement disorders such as ataxia. Of the animal models with extensive tremor characterization, harmaline-induced rodent tremor models provide an important framework showing that rhythmic and synchronous neuronal activities within the olivocerebellar circuit can drive action tremor. In addition, dopamine-depleted monkey and mouse models may develop rest tremor, highlighting the role of dopamine in rest tremor generation. Finally, other animal models of tremor have involvement of the cerebellar circuitry, leading to altered Purkinje cell physiology.

Discussion

Both the cerebellum and the basal ganglia are likely to play a role in tremor generation. While the cerebellar circuitry can generate rhythmic movements, the nigrostriatal system is likely to modulate the tremor circuit. Tremor disorders are heterogeneous in nature. Therefore, each animal model may represent a subset of tremor disorders, which collectively can advance our understanding of tremor.