An electrophysiological perspective on Parkinson’s disease: symptomatic pathogenesis and therapeutic approaches

Parkinson’s disease (PD), or paralysis agitans, is a common neurodegenerative disease characterized by dopaminergic deprivation in the basal ganglia because of neuronal loss in the substantia nigra pars compacta. Clinically, PD apparently involves both hypokinetic (e.g. akinetic rigidity) and hyperkinetic (e.g. tremor/propulsion) symptoms. The symptomatic pathogenesis, however, has remained elusive. The recent success of deep brain stimulation (DBS) therapy applied to the subthalamic nucleus (STN) or the globus pallidus pars internus indicates that there are essential electrophysiological abnormalities in PD. Consistently, dopamine-deprived STN shows excessive burst discharges. This proves to be a central pathophysiological element causally linked to the locomotor deficits in PD, as maneuvers (such as DBS of different polarities) decreasing and increasing STN burst discharges would decrease and increase the locomotor deficits, respectively. STN bursts are not so autonomous but show a “relay” feature, requiring glutamatergic synaptic inputs from the motor cortex (MC) to develop. In PD, there is an increase in overall MC activities and the corticosubthalamic input is enhanced and contributory to excessive burst discharges in STN. The increase in MC activities may be relevant to the enhanced beta power in local field potentials (LFP) as well as the deranged motor programming at the cortical level in PD. Moreover, MC could not only drive erroneous STN bursts, but also be driven by STN discharges at specific LFP frequencies (~ 4 to 6 Hz) to produce coherent tremulous muscle contractions. In essence, PD may be viewed as a disorder with deranged rhythms in the cortico-subcortical re-entrant loops, manifestly including STN, the major component of the oscillating core, and MC, the origin of the final common descending motor pathways. The configurations of the deranged rhythms may play a determinant role in the symptomatic pathogenesis of PD, and provide insight into the mechanism underlying normal motor control. Therapeutic brain stimulation for PD and relevant disorders should be adaptively exercised with in-depth pathophysiological considerations for each individual patient, and aim at a final normalization of cortical discharge patterns for the best ameliorating effect on the locomotor and even non-motor symptoms.

Synapse-associated astrocyte mitochondria stabilize motor circuits to prevent excitotoxicity

Early stages of the devastating neurodegenerative disease amyotrophic lateral sclerosis (ALS) are characterized by motor neuron hyperexcitability. During this phase, peri-synaptic astrocytes are neuroprotective. When reactive, loss of wild-type astrocyte functions results in excitotoxicity. How astrocytes stabilize motor circuit function in early-stage ALS is poorly understood. Here, we used Drosophila motor neurons to define the role of astrocyte-motor neuron metabolic coupling in a model of ALS: astrocyte knockdown of the ALS-causing gene tbph/TARDBP. In wild-type, astrocyte mitochondria were dynamically trafficked towards active motor dendrites/synapses to meet local metabolic demand. Knockdown of tbph in astrocytes resulted in motor neuron hyperexcitability, reminiscent of early-stage ALS, which was met with a compensatory accumulation of astrocyte mitochondria near motor dendrites/synapses. Finally, we blocked mitochondria-synapse association in tbph knockdown animals and observed locomotor deficits and synapse loss. Thus, synapse-associated astrocyte mitochondria stabilize motor circuits to prevent the transition from hyperexcitability to excitotoxicity.

A LRRK2/dLRRK-mediated lysosomal pathway that contributes to glial cell death and DA neuron survival

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial and sporadic Parkinson's disease (PD). A plethora of evidence has indicated a role for LRRK2 in endolysosomal trafficking in neurons, while LRRK2 function in glia, although highly expressed, remains largely unknown. Here we present evidence that LRRK2/dLRRK mediates a glial lysosomal pathway that contributes to the mechanism of PD. Independent of its kinase activity, glial LRRK2/dLRRK knockdown in the immortalized microglial cells or flies results in enlarged and swelling lysosomes fewer in number. These lysosomes are less mobile, wrongly acidified, and exhibit defective membrane permeability and reduced activity of the lysosome hydrolase cathespin B. In addition, microglial LRRK2 depletion causes increased Caspase 3 levels, leading to glial apoptosis, dopaminergic neurodegeneration, and locomotor deficits in an age-dependent manner. Taken together, these findings demonstrate a functional role of LRRK2/dLRRK in regulating the glial lysosomal pathway; deficits in lysosomal biogenesis and function linking to glial apoptosis potentially underlie the mechanism of DA neurodegeneration, contributing to the progression of PD.

VPS35 Protects Against TMEM230-Mutation-Induced Progressive Locomotor Deficits in Drosophila

BackgroundRecently, four Parkinson’s disease (PD)-linked mutations (Y92C, R141L, 184PGext*5 and 184Wext*5) in transmembrane protein 230 (TMEM230) were identified in PD patients, and these mutations have implications in protein trafficking and neurodegeneration. However, there is a lack of in vivo studies on the roles of PD-related variants of TMEM230 in PD pathogenesis.MethodsIn this study, we generated human wild-type (WT) and mutant TMEM230 (Y92C, R141L, 184PGext*5 and 184Wext*5) transgenic Drosophila using isoform Ⅱ cDNA. ResultsWe found that the expression of TMEM230 184PGext*5 in pan-neurons or dopaminergic neurons in Drosophila induced PD-like phenotypes, which included impaired locomotor ability, a shortened lifespan, reduced TH levels, and increased phosphorylated JNK and cleaved caspase-3 levels. Moreover, rotenone, a common pesticide, enhanced TMEM230-184PGext*5-induced PD-like phenotypes. In contrast, the overexpression of wild-type (WT) VPS35 rescued TMEM230-184PGext*5-induced PD-like phenotypes, while the knockdown of VPS35 by RNA interference (RNAi) or the expression of mutant VPS35 D620N worsened PD-like phenotypes. ConclusionThese results indicate that VPS35, as a downstream effector of TMEM230, plays a critical role in TMEM230-linked JNK/caspase-3 signalling pathways and that mutations in TMEM230 and VPS35 disrupt these pathways, resulting in dopaminergic neurodegeneration and PD-like phenotypes. These findings provide novel insight into the molecular mechanisms of mutant TME230- and VPS35-induced abnormalities underlying the pathogenesis of PD.

Optogenetic stimulation of glutamatergic neurons in the cuneiform nucleus controls locomotor movements in a mouse model of Parkinson’s disease

ABSTRACTIn Parkinson’s disease (PD), the loss of midbrain dopaminergic cells results in severe locomotor deficits such a gait freezing and akinesia. Growing evidence indicates that these deficits can be attributed to decreased activity in the Mesencephalic Locomotor Region (MLR), a brainstem region controlling locomotion. Clinicians are exploring deep brain stimulation of the MLR as a treatment option to improve locomotor function. The results are variable, from modest to promising. However, within the MLR, clinicians have targeted the pedunculopontine nucleus exclusively, while leaving the cuneiform nucleus unexplored. To our knowledge, the effects of cuneiform nucleus stimulation have never been determined in parkinsonian conditions in any animal model. Here, we addressed this issue in a mouse model of Parkinson’s disease based on bilateral striatal injection of 6-hydroxydopamine (6-OHDA), which damaged the nigrostriatal pathway and decreased locomotor activity. We show that selective optogenetic stimulation of glutamatergic neurons in the cuneiform nucleus in mice expressing channelrhodopsin in a Cre-dependent manner in Vglut2-positive neurons (Vglut2-ChR2-EYFP mice) increased the number of locomotor initiations, increased the time spent in locomotion, and controlled locomotor speed. Using deep learning-based movement analysis, we found that limb kinematics of optogenetic-evoked locomotion in pathological conditions were largely similar to those recorded in freely moving animals. Our work identifies the glutamatergic neurons of the cuneiform nucleus as a potentially clinically relevant target to improve locomotor activity in parkinsonian conditions. Our study should open new avenues to develop targeted stimulation of these neurons using deep brain stimulation, pharmacotherapy or optogenetics.SIGNIFICANCE STATEMENTIn Parkinson’s disease, alleviating locomotor deficits is a challenge. Clinicians are exploring deep brain stimulation of the Mesencephalic Locomotor Region, a brainstem region controlling locomotion, but results are mixed. However, the best target in this region in Parkinson’s disease remains unknown. Indeed, this region which comprises the pedunculopontine and cuneiform nuclei, contains different cell types with opposing effects on locomotor output. Here, using a mouse model where midbrain dopaminergic cells were damaged by a neurotoxin, we demonstrate that optogenetic activation of glutamatergic neurons in the cuneiform nucleus increases locomotion, controls speed, and evokes limb movements similar to those observed during spontaneous locomotion in intact animals. Our study identifies a potentially clinically relevant target to improve locomotor function in Parkinson’s disease.

Locomotor deficits in a mouse model of ALS are paralleled by loss of V1-interneuron connections onto fast motor neurons

ALS is characterized by progressive inability to execute movements. Motor neurons innervating fast-twitch muscle-fibers preferentially degenerate. The reason for this differential vulnerability and its consequences on motor output is not known. Here, we uncover that fast motor neurons receive stronger inhibitory synaptic inputs than slow motor neurons, and disease progression in the SOD1G93A mouse model leads to specific loss of inhibitory synapses onto fast motor neurons. Inhibitory V1 interneurons show similar innervation pattern and loss of synapses. Moreover, from postnatal day 63, there is a loss of V1 interneurons in the SOD1G93A mouse. The V1 interneuron degeneration appears before motor neuron death and is paralleled by the development of a specific locomotor deficit affecting speed and limb coordination. This distinct ALS-induced locomotor deficit is phenocopied in wild-type mice but not in SOD1G93A mice after appearing of the locomotor phenotype when V1 spinal interneurons are silenced. Our study identifies a potential source of non-autonomous motor neuronal vulnerability in ALS and links ALS-induced changes in locomotor phenotype to inhibitory V1-interneurons.

A deep-learning-based toolbox for Automated Limb Motion Analysis (ALMA) in murine models of neurological disorders

In neuroscience research, the refined analysis of rodent locomotion is complex and cumbersome, and access to the technique is limited because of the necessity for expensive equipment. In this study, we implemented a new deep-learning-based toolbox for Automated Limb Motion Analysis (ALMA) that requires only basic behavioral equipment and an inexpensive camera. The ALMA toolbox enables the unbiased and comprehensive analyses of locomotor kinematics and paw placement and can be applied to neurological conditions affecting the brain and spinal cord. We demonstrated that the ALMA toolbox can (1) robustly track the evolution of locomotor deficits after spinal cord injury, (2) sensitively detect locomotor abnormalities after traumatic brain injury, and (3) correctly predict disease onset in a multiple sclerosis model. We, therefore, established a broadly applicable automated and standardized approach that requires minimal financial and time commitments to facilitate the comprehensive analysis of locomotion in rodent disease models.

Equine Rehabilitation: A Scoping Review of the Literature

Injuries to the locomotor system are a common problem in athletic horses. Veterinarians address these injuries using appropriate medical, surgical, and pharmacological treatments. During or after recovery from the initial injury, horses may be treated for functional locomotor deficits using specific rehabilitation techniques aimed at restoring full athletic performance. This study reviews the literature to identify which rehabilitative techniques have been used most frequently in horses over the past 20 years, the protocols that were used, and the outcomes of the treatments in naturally occurring injuries and diseases. Publications were identified using keyword selection (Equine Athlete OR Equine OR Horse) AND (Rehabilitation OR Physiotherapy OR Physical Therapy). After removing duplicates and screening papers for suitability, 49 manuscripts were included in the study. The majority of publications that met the inclusion criteria were narrative reviews (49%) in which the authors cited the relatively small number of published evidence-based studies supplemented by personal experience. Observational/descriptive studies were also popular (35%). Randomized control trials accounted for only 10%. The most frequently reported rehabilitation techniques were exercise, electrotherapy, and hydrotherapy. The findings highlight the need for further information regarding type of intervention, parameterization, and outcomes of equine rehabilitation in clinical practice.

Assessing olfactory, memory, social and circadian phenotypes associated with schizophrenia in a genetic model based on Rim

Schizophrenia shows high heritability and several of the genes associated with this disorder are involved in calcium (Ca2+) signalling and synaptic function. One of these is the Rab-3 interacting molecule-1 (RIM1), which has recently been associated with schizophrenia by Genome Wide Association Studies (GWAS). However, its contribution to the pathophysiology of this disorder remains unexplored. In this work, we use Drosophila mutants of the orthologue of RIM1, Rim, to model some aspects of the classical and non-classical symptoms of schizophrenia. Rim mutants showed several behavioural features relevant to schizophrenia including social distancing and altered olfactory processing. These defects were accompanied by reduced evoked Ca2+ influx and structural changes in the presynaptic terminals sent by the primary olfactory neurons to higher processing centres. In contrast, expression of Rim-RNAi in the mushroom bodies (MBs), the main memory centre in flies, spared learning and memory suggesting a differential role of Rim in different synapses. Circadian deficits have been reported in schizophrenia. We observed circadian locomotor activity deficits in Rim mutants, revealing a role of Rim in the pacemaker ventral lateral clock neurons (LNvs). These changes were accompanied by impaired day/night remodelling of dorsal terminal synapses from a subpopulation of LNvs and impaired day/night release of the circadian neuropeptide pigment dispersing factor (PDF) from these terminals. Lastly, treatment with the commonly used antipsychotic haloperidol rescued Rim locomotor deficits to wildtype. This work characterises the role of Rim in synaptic functions underlying behaviours disrupted in schizophrenia.