Computational analysis of calcium signaling and membrane electrophysiology in cerebellar Purkinje neurons associated with ataxia

Toward a cerebello-thalamo-cortical computational model of spinocerebellar ataxia

Computational neural network modelling is an emerging approach for optimization of drug treatment of neurological disorders and fine-tuning of rehabilitation strategies. In the current study, we constructed a cerebello-thalamo-cortical computational neural network model to simulate a mouse model of cerebellar ataxia (pcd^5J mice) by manipulating cerebellar bursts through reduction of GABAergic inhibitory input. Cerebellar output neurons were projected to the thalamus and bidirectionally connected with the cortical network. Our results showed that reduction of inhibitory input in the cerebellum orchestrated the cortical local field potential (LFP) dynamics to generate specific motor outputs of oscillations of the theta, alpha, and beta bands in the computational model as well as in mouse motor cortical neurons. The therapeutic potential of deep brain stimulation (DBS) was tested in the computational model by increasing the sensory input to restore cortical output. Ataxia mice showed normalization of the motor cortex LFP after cerebellum DBS. We provide a novel approach to computational modelling to investigate the effect of DBS by mimicking cerebellar ataxia involving degeneration of Purkinje cells. Simulated neural activity coincides with findings from neural recordings of ataxia mice. Our computational model could thus represent cerebellar pathologies and provide insight into how to improve disease symptoms by restoring neuronal electrophysiological properties using DBS.

Computational models of neurotransmission at cerebellar synapses unveil the impact on network computation

The neuroscientific field benefits from the conjoint evolution of experimental and computational techniques, allowing for the reconstruction and simulation of complex models of neurons and synapses. Chemical synapses are characterized by presynaptic vesicle cycling, neurotransmitter diffusion, and postsynaptic receptor activation, which eventually lead to postsynaptic currents and subsequent membrane potential changes. These mechanisms have been accurately modeled for different synapses and receptor types (AMPA, NMDA, and GABA) of the cerebellar cortical network, allowing simulation of their impact on computation. Of special relevance is short-term synaptic plasticity, which generates spatiotemporal filtering in local microcircuits and controls burst transmission and information flow through the network. Here, we present how data-driven computational models recapitulate the properties of neurotransmission at cerebellar synapses. The simulation of microcircuit models is starting to reveal how diverse synaptic mechanisms shape the spatiotemporal profiles of circuit activity and computation.

Poetic Science: Bidirectional Reflection in Science and Medicine

The arts are making their mark in science, technology, engineering, arts, and mathematics/medicine (STEAM). Integrating creative expression-poetry and other visual and performing arts-can help clinicians, scientists, and others use familiar social constructs to embody science and medicine, in what may be termed poetic science. Poetic science imbues bidirectional reflections of science and medicine on the clinician or scientist or other inquisitor, creatively engaging the learner's brain cells as mirrors. This ultimately leads to a subjective perspective on the understanding or the proposition of underlying principles. Such an approach is encouraged here with poignant examples that can be accessed publicly online and used widely by readers, teachers, learners, clinicians, scientists, students, and others.

Identification of a potential exosomal biomarker in spinocerebellar ataxia Type 3/Machado-Joseph disease

Aim: To identify spinocerebellar ataxia Type 3 (SCA3)-related exosomal biomarkers and the underlying mechanisms. Materials & methods: Exosomal RNAs from plasma and cerebrospinal fluid (CSF) were extracted from 24 SCA3 patients and 22 controls, respectively. Small RNA sequencing and quantitative PCR verification were performed. Gene ontology annotation and Kyoto Encyclopedia of Genes and Genomes pathway analyses of the results were carried out. Results: One novel miRNA is notably downregulated in plasma-derived exosomes, while upregulated in CSF-derived exosomes of SCA3 patients. Besides, it is successively upregulated in CSF-derived exosomes from Type 1, Type 2 and Type 3 groups. The downstream target genes were enriched in protein processing in endoplasmic reticulum and axon guidance. Conclusion: One exosomal biomarker was identified in SCA3, and this is the first time to report an exosomal miRNA as a biomarker in SCA3 internationally.

Pathophysiological consequences of isoform-specific IP3 receptor mutations

Ca²⁺ signaling governs a diverse range of cellular processes and, as such, is subject to tight regulation. A main component of the complex intracellular Ca²⁺-signaling network is the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R), a tetrameric channel that mediates Ca²⁺ release from the endoplasmic reticulum (ER) in response to IP₃. IP₃R function is controlled by a myriad of factors, such as Ca²⁺, ATP, kinases and phosphatases and a plethora of accessory and regulatory proteins. Further complexity in IP₃R-mediated Ca²⁺ signaling is the result of the existence of three main isoforms (IP₃R1, IP₃R2 and IP₃R3) that display distinct functional characteristics and properties. Despite their abundant and overlapping expression profiles, IP₃R1 is highly expressed in neurons, IP₃R2 in cardiomyocytes and hepatocytes and IP₃R3 in rapidly proliferating cells as e.g. epithelial cells. As a consequence, dysfunction and/or dysregulation of IP₃R isoforms will have distinct pathophysiological outcomes, ranging from neurological disorders for IP₃R1 to dysfunctional exocrine tissues and autoimmune diseases for IP₃R2 and -3. Over the past years, several IP₃R mutations have surfaced in the sequence analysis of patient-derived samples. Here, we aimed to provide an integrative overview of the clinically most relevant mutations for each IP₃R isoform and the subsequent molecular mechanisms underlying the etiology of the disease.

Inositol 1,4,5-trisphosphate receptors and neurodegenerative disorders

The inositol 1,4,5-trisphosphate receptor (IP₃ R) is an intracellular ion channel that mediates the release of calcium ions from the endoplasmic reticulum. It plays a role in basic biological functions, such as cell division, differentiation, fertilization and cell death, and is involved in developmental processes including learning, memory and behavior. Deregulation of neuronal calcium signaling results in disturbance of cell homeostasis, synaptic loss and dysfunction, eventually leading to cell death. Three IP₃ R subtypes have been identified in mammalian cells and the predominant isoform in neurons is IP₃ R type 1. Dysfunction of IP₃ R type 1 may play a role in the pathogenesis of certain neurodegenerative diseases as enhanced activity of the IP₃ R was observed in models of Huntington's disease, spinocerebellar ataxias and Alzheimer's disease. These results suggest that IP₃ R-mediated signaling is a potential target for treatment of these disorders. In this review we discuss the structure, functions and regulation of the IP₃ R in healthy neurons and in conditions of neurodegeneration.

Determining the Roles of Inositol Trisphosphate Receptors in Neurodegeneration: Interdisciplinary Perspectives on a Complex Topic

It is well known that calcium (Ca²⁺) is involved in the triggering of neuronal death. Ca²⁺ cytosolic levels are regulated by Ca²⁺ release from internal stores located in organelles, such as the endoplasmic reticulum. Indeed, Ca²⁺ transit from distinct cell compartments follows complex dynamics that are mediated by specific receptors, notably inositol trisphosphate receptors (IP3Rs). Ca²⁺ release by IP3Rs plays essential roles in several neurological disorders; however, details of these processes are poorly understood. Moreover, recent studies have shown that subcellular location, molecular identity, and density of IP3Rs profoundly affect Ca²⁺ transit in neurons. Therefore, regulation of IP3R gene products in specific cellular vicinities seems to be crucial in a wide range of cellular processes from neuroprotection to neurodegeneration. In this regard, microRNAs seem to govern not only IP3Rs translation levels but also subcellular accumulation. Combining new data from molecular cell biology with mathematical modelling, we were able to summarize the state of the art on this topic. In addition to presenting how Ca²⁺ dynamics mediated by IP3R activation follow a stochastic regimen, we integrated a theoretical approach in an easy-to-apply, cell biology-coherent fashion. Following the presented premises and in contrast to previously tested hypotheses, Ca²⁺ released by IP3Rs may play different roles in specific neurological diseases, including Alzheimer's disease and Parkinson's disease.

Progressive impairment of cerebellar mGluR signalling and its therapeutic potential for cerebellar ataxia in spinocerebellar ataxia type 1 model mice

KEY POINTS

Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disease caused by a gene defect, leading to movement disorder such as cerebellar ataxia. It remains largely unknown which functional defect contributes to the cerebellar ataxic phenotype in SCA1. In this study, we report progressive dysfunction of metabotropic glutamate receptor (mGluR) signalling, which leads to smaller slow synaptic responses, reduced dendritic Ca²⁺ signals and impaired synaptic plasticity at cerebellar synapses, in the early disease stage of SCA1 model mice. We also show that enhancement of mGluR signalling by a clinically available drug, baclofen, leads to improvement of motor performance in SCA1 mice. SCA1 is an incurable disease with no effective treatment, and our results may provide mechanistic grounds for targeting mGluRs and a novel drug therapy with baclofen to treat SCA1 patients in the future.

ABSTRACT

Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disease that presents with cerebellar ataxia and motor learning defects. Previous studies have indicated that the pathology of SCA1, as well as other ataxic diseases, is related to signalling pathways mediated by the metabotropic glutamate receptor type 1 (mGluR1), which is indispensable for proper motor coordination and learning. However, the functional contribution of mGluR signalling to SCA1 pathology is unclear. In the present study, we show that SCA1 model mice develop a functional impairment of mGluR signalling which mediates slow synaptic responses, dendritic Ca²⁺ signals, and short- and long-term synaptic plasticity at parallel fibre (PF)-Purkinje cell (PC) synapses in a progressive manner from the early disease stage (5 postnatal weeks) prior to PC death. Notably, impairment of mGluR-mediated dendritic Ca²⁺ signals linearly correlated with a reduction of PC capacitance (cell surface area) in disease progression. Enhancement of mGluR signalling by baclofen, a clinically available GABA_B receptor agonist, led to an improvement of motor performance in SCA1 mice and the improvement lasted ∼1 week after a single application of baclofen. Moreover, the restoration of motor performance in baclofen-treated SCA1 mice matched the functional recovery of mGluR-mediated slow synaptic currents and mGluR-dependent short- and long-term synaptic plasticity. These results suggest that impairment of synaptic mGluR cascades is one of the important contributing factors to cerebellar ataxia in early and middle stages of SCA1 pathology, and that modulation of mGluR signalling by baclofen or other clinical interventions may be therapeutic targets to treat SCA1.

Mitochondrial impairments contribute to Spinocerebellar ataxia type 1 progression and can be ameliorated by the mitochondria-targeted antioxidant MitoQ

Spinocerebellar ataxia type 1 (SCA1), due to an unstable polyglutamine expansion within the ubiquitously expressed Ataxin-1 protein, leads to the premature degeneration of Purkinje cells (PCs), decreasing motor coordination and causing death within 10-15 years of diagnosis. Currently, there are no therapies available to slow down disease progression. As secondary cellular impairments contributing to SCA1 progression are poorly understood, here, we focused on identifying those processes by performing a PC specific proteome profiling of Sca1(154Q/2Q) mice at a symptomatic stage. Mass spectrometry analysis revealed prominent alterations in mitochondrial proteins. Immunohistochemical and serial block-face scanning electron microscopy analyses confirmed that PCs underwent age-dependent alterations in mitochondrial morphology. Moreover, colorimetric assays demonstrated impairment of the electron transport chain complexes (ETC) and decrease in ATPase activity. Subsequently, we examined whether the mitochondria-targeted antioxidant MitoQ could restore mitochondrial dysfunction and prevent SCA1-associated pathology in Sca1(154Q/2Q) mice. MitoQ treatment both presymptomatically and when symptoms were evident ameliorated mitochondrial morphology and restored the activities of the ETC complexes. Notably, MitoQ slowed down the appearance of SCA1-linked neuropathology such as lack of motor coordination as well as prevented oxidative stress-induced DNA damage and PC loss. Our work identifies a central role for mitochondria in PC degeneration in SCA1 and provides evidence for the supportive use of mitochondria-targeted therapeutics in slowing down disease progression.

Toll-Like Receptor 4 Deficiency Impairs Motor Coordination

The cerebellum plays an essential role in balance and motor coordination. Purkinje cells (PCs) are the sole output neurons of the cerebellar cortex and are critical for the execution of its functions, including motor coordination. Toll-like receptor (TLR) 4 is involved in the innate immune response and is abundantly expressed in the central nervous system; however, little is known about its role in cerebellum-related motor functions. To address this question, we evaluated motor behavior in TLR4 deficient mice. We found that TLR4(-∕-) mice showed impaired motor coordination. Morphological analyses revealed that TLR4 deficiency was associated with a reduction in the thickness of the molecular layer of the cerebellum. TLR4 was highly expressed in PCs but not in Bergmann glia or cerebellar granule cells; however, loss of TLR4 decreased the number of PCs. These findings suggest a novel role for TLR4 in cerebellum-related motor coordination through maintenance of the PC population.

The 40-year history of modeling active dendrites in cerebellar Purkinje cells: emergence of the first single cell "community model"

The subject of the effects of the active properties of the Purkinje cell dendrite on neuronal function has been an active subject of study for more than 40 years. Somewhat unusually, some of these investigations, from the outset have involved an interacting combination of experimental and model-based techniques. This article recounts that 40-year history, and the view of the functional significance of the active properties of the Purkinje cell dendrite that has emerged. It specifically considers the emergence from these efforts of what is arguably the first single cell "community" model in neuroscience. The article also considers the implications of the development of this model for future studies of the complex properties of neuronal dendrites.

Disturbed calcium signaling in spinocerebellar ataxias and Alzheimer's disease

Neurodegenerative disorders, such as spinocerebellar ataxias (SCAs) and Alzheimer's disease (AD) represent a huge scientific and medical question, but the molecular mechanisms of these diseases are still not clear. There is increasing evidence that neuronal calcium signaling is abnormal in many neurodegenerative disorders. Abnormal neuronal calcium release from the endoplasmic reticulum may result in disturbances of cell homeostasis, synaptic dysfunction, and eventual cell death. Neuronal loss is observed in most cases of neurodegenerative diseases. Recent experimental evidence supporting the role of neuronal calcium signaling in the pathogenesis of SCAs and AD is discussed in this review.

Computational neurobiology is a useful tool in translational neurology: the example of ataxia

Hereditary ataxia, or motor incoordination, affects approximately 150,000 Americans and hundreds of thousands of individuals worldwide with onset from as early as mid-childhood. Affected individuals exhibit dysarthria, dysmetria, action tremor, and diadochokinesia. In this review, we consider an array of computational studies derived from experimental observations relevant to human neuropathology. A survey of related studies illustrates the impact of integrating clinical evidence with data from mouse models and computational simulations. Results from these studies may help explain findings in mice, and after extensive laboratory study, may ultimately be translated to ataxic individuals. This inquiry lays a foundation for using computation to understand neurobiochemical and electrophysiological pathophysiology of spinocerebellar ataxias and may contribute to development of therapeutics. The interdisciplinary analysis suggests that computational neurobiology can be an important tool for translational neurology.

Integration of modeling with experimental and clinical findings synthesizes and refines the central role of inositol 1,4,5-trisphosphate receptor 1 in spinocerebellar ataxia

A suite of models was developed to study the role of inositol 1,4,5-trisphosphate receptor 1 (IP3R1) in spinocerebellar ataxias (SCAs). Several SCAs are linked to reduced abundance of IP3R1 or to supranormal sensitivity of the receptor to activation by its ligand inositol 1,4,5-trisphosphate (IP3). Detailed multidimensional models have been created to simulate biochemical calcium signaling and membrane electrophysiology in cerebellar Purkinje neurons. In these models, IP3R1-mediated calcium release is allowed to interact with ion channel response on the cell membrane. Experimental findings in mice and clinical observations in humans provide data input for the models. The SCA modeling suite helps interpret experimental results and provides suggestions to guide experiments. The models predict IP3R1 supersensitivity in SCA1 and compensatory mechanisms in SCA1, SCA2, and SCA3. Simulations explain the impact of calcium buffer proteins. Results show that IP3R1-mediated calcium release activates voltage-gated calcium-activated potassium channels in the plasma membrane. The SCA modeling suite unifies observations from experiments in a number of SCAs. The cadre of simulations demonstrates the central role of IP3R1.