Autophagy Function and Benefits of Autophagy Induction in Models of Spinocerebellar Ataxia Type 3

Genetic modeling of degenerative diseases and mechanisms of neuronal regeneration in the zebrafish cerebellum

The cerebellum is a highly conserved brain compartment of vertebrates. Genetic diseases of the human cerebellum often lead to degeneration of the principal neuron, the Purkinje cell, resulting in locomotive deficits and socio-emotional impairments. Due to its relatively simple but highly conserved neuroanatomy and circuitry, these human diseases can be modeled well in vertebrates amenable for genetic manipulation. In the recent years, cerebellar research in zebrafish has contributed to understanding cerebellum development and function, since zebrafish larvae are not only molecularly tractable, but also accessible for high resolution in vivo imaging due to the transparency of the larvae and the ease of access to the zebrafish cerebellar cortex for microscopy approaches. Therefore, zebrafish is increasingly used for genetic modeling of human cerebellar neurodegenerative diseases and in particular of different types of Spinocerebellar Ataxias (SCAs). These models are well suited to address the underlying pathogenic mechanisms by means of in vivo cell biological studies. Furthermore, accompanying circuitry characterizations, physiological studies and behavioral analysis allow for unraveling molecular, structural and functional relationships. Moreover, unlike in mammals, zebrafish possess an astonishing ability to regenerate neuronal populations and their functional circuitry in the central nervous system including the cerebellum. Understanding the cellular and molecular processes of these regenerative processes could well serve to counteract acute and chronic loss of neurons in humans. Based on the high evolutionary conservation of the cerebellum these regeneration studies in zebrafish promise to open therapeutic avenues for counteracting cerebellar neuronal degeneration. The current review aims to provide an overview over currently existing genetic models of human cerebellar neurodegenerative diseases in zebrafish as well as neuroregeneration studies using the zebrafish cerebellum. Due to this solid foundation in cerebellar disease modeling and neuronal regeneration analysis, the zebrafish promises to become a popular model organism for both unraveling pathogenic mechanisms of human cerebellar diseases and providing entry points for therapeutic neuronal regeneration approaches.

Trehalose prevents the formation of aggregates of mutant ataxin-3 and reduces soluble ataxin-3 protein levels in an SCA3 cell model

Spinocerebellar ataxia type 3 (SCA3) is a neurodegenerative disorder caused by mutant ataxin-3 with an abnormally expanded polyQ tract and is the most common dominantly inherited ataxia worldwide. There are no suitable therapeutic options for this disease. Autophagy, a defense mechanism against the toxic effects of aggregation-prone misfolded proteins, has been shown to have beneficial effects on neurodegenerative diseases. Thus, trehalose, which is an autophagy inducer, may have beneficial effects on SCA3. In the present study, we examined the effects of trehalose on an SCA3 cell model. After trehalose treatment, aggregate formation, soluble ataxin-3 protein levels and cell viability were evaluated in HEK293T cells overexpressing ataxin-3-15Q or ataxin-3-77Q. We also explored the mechanism by which trehalose affects autophagy and stress pathways. A filter trap assay showed that trehalose decreased the number of aggregates formed by mutant ataxin-3 containing an expanded polyQ tract. Western blot and Cell Counting Kit-8 (CCK-8) results demonstrated that trehalose also reduced the ataxin-3 protein levels and was safe for ataxin-3-expressing cells, respectively. Western blot and total antioxidant capacity assays suggested that trehalose had great therapeutic potential for treating SCA3, likely through its antioxidant activity. Our data indicate that trehalose plays a neuroprotective role in SCA3 by inhibiting the aggregation and reducing the protein level of ataxin-3, which is also known to protect against oxidative stress. These findings provide a new insight into the possibility of treating SCA3 with trehalose and highlight the importance of inducing autophagy in SCA3.

Autophagy in neural stem cells and glia for brain health and diseases

Autophagy is a multifaceted cellular process that not only maintains the homeostatic and adaptive responses of the brain but is also dynamically involved in the regulation of neural cell generation, maturation, and survival. Autophagy facilities the utilization of energy and the microenvironment for developing neural stem cells. Autophagy arbitrates structural and functional remodeling during the cell differentiation process. Autophagy also plays an indispensable role in the maintenance of stemness and homeostasis in neural stem cells during essential brain physiology and also in the instigation and progression of diseases. Only recently, studies have begun to shed light on autophagy regulation in glia (microglia, astrocyte, and oligodendrocyte) in the brain. Glial cells have attained relatively less consideration despite their unquestioned influence on various aspects of neural development, synaptic function, brain metabolism, cellular debris clearing, and restoration of damaged or injured tissues. Thus, this review composes pertinent information regarding the involvement of autophagy in neural stem cells and glial regulation and the role of this connexion in normal brain functions, neurodevelopmental disorders, and neurodegenerative diseases. This review will provide insight into establishing a concrete strategic approach for investigating pathological mechanisms and developing therapies for brain diseases.

Spermidine treatment: induction of autophagy but also apoptosis?

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3, is a fatal neurodegenerative disease that causes loss of balance and motor co-ordination, eventually leading to paralysis. It is caused by the autosomal dominant inheritance of a long CAG trinucleotide repeat sequence within the ATXN3 gene, encoding for an expanded polyglutamine (polyQ) repeat sequence within the ataxin-3 protein. Ataxin-3 containing an expanded polyQ repeat is known to be highly prone to intraneuronal aggregation, and previous studies have demonstrated that protein quality control pathways, such as autophagy, are impaired in MJD patients and animal models of the disease. In this study, we tested the therapeutic potential of spermidine on zebrafish and rodent models of MJD to determine its capacity to induce autophagy and improve functional output. Spermidine treatment of transgenic MJD zebrafish induced autophagy and resulted in increased distances swum by the MJD zebrafish. Interestingly, treatment of the CMVMJD135 mouse model of MJD with spermidine added to drinking water did not produce any improvement in motor behaviour assays, neurological testing or neuropathology. In fact, wild type mice treated with spermidine were found to have decreased rotarod performance when compared to control animals. Immunoblot analysis of protein lysates extracted from mouse cerebellar tissue found little differences between the groups, except for an increased level of phospho-ULK1 in spermidine treated animals, suggesting that autophagy was indeed induced. As we detected decreased motor performance in wild type mice following treatment with spermidine, we conducted follow up studies into the effects of spermidine treatment in zebrafish. Interestingly, we found that in addition to inducing autophagy, spermidine treatment also induced apoptosis, particularly in wild type zebrafish. These findings suggest that spermidine treatment may not be therapeutically beneficial for the treatment of MJD, and in fact warrants caution due to the potential negative side effects caused by induction of apoptosis.

Treatment with sodium butyrate induces autophagy resulting in therapeutic benefits for spinocerebellar ataxia type 3

Spinocerebellar ataxia type 3 (SCA3, also known as Machado Joseph disease) is a fatal neurodegenerative disease caused by the expansion of the trinucleotide repeat region within the ATXN3/MJD gene. Mutation of ATXN3 causes formation of ataxin-3 protein aggregates, neurodegeneration, and motor deficits. Here we investigated the therapeutic potential and mechanistic activity of sodium butyrate (SB), the sodium salt of butyric acid, a metabolite naturally produced by gut microbiota, on cultured SH-SY5Y cells and transgenic zebrafish expressing human ataxin-3 containing 84 glutamine (Q) residues to model SCA3. SCA3 SH-SY5Y cells were found to contain high molecular weight ataxin-3 species and detergent-insoluble protein aggregates. Treatment with SB increased the activity of the autophagy protein quality control pathway in the SCA3 cells, decreased the presence of ataxin-3 aggregates and presence of high molecular weight ataxin-3 in an autophagy-dependent manner. Treatment with SB was also beneficial in vivo, improving swimming performance, increasing activity of the autophagy pathway, and decreasing the presence of insoluble ataxin-3 protein species in the transgenic SCA3 zebrafish. Co-treating the SCA3 zebrafish with SB and chloroquine, an autophagy inhibitor, prevented the beneficial effects of SB on zebrafish swimming, indicating that the improved swimming performance was autophagy-dependent. To understand the mechanism by which SB induces autophagy we performed proteomic analysis of protein lysates from the SB-treated and untreated SCA3 SH-SY5Y cells. We found that SB treatment had increased activity of Protein Kinase A and AMPK signaling, with immunoblot analysis confirming that SB treatment had increased levels of AMPK protein and its substrates. Together our findings indicate that treatment with SB can increase activity of the autophagy pathway process and that this has beneficial effects in vitro and in vivo. While our results suggested that this activity may involve activity of a PKA/AMPK-dependent process, this requires further confirmation. We propose that treatment with sodium butyrate warrants further investigation as a potential treatment for neurodegenerative diseases underpinned by mechanisms relating to protein aggregation including SCA3.

Interplay of Impaired Cellular Bioenergetics and Autophagy in PMM2-CDG

Congenital disorders of glycosylation (CDG) and mitochondrial disorders are multisystem disorders with overlapping symptomatology. Pathogenic variants in the PMM2 gene lead to abnormal N-linked glycosylation. This disruption in glycosylation can induce endoplasmic reticulum stress, contributing to the disease pathology. Although impaired mitochondrial dysfunction has been reported in some CDG, cellular bioenergetics has never been evaluated in detail in PMM2-CDG. This prompted us to evaluate mitochondrial function and autophagy/mitophagy in vitro in PMM2 patient-derived fibroblast lines of differing genotypes from our natural history study. We found secondary mitochondrial dysfunction in PMM2-CDG. This dysfunction was evidenced by decreased mitochondrial maximal and ATP-linked respiration, as well as decreased complex I function of the mitochondrial electron transport chain. Our study also revealed altered autophagy in PMM2-CDG patient-derived fibroblast lines. This was marked by an increased abundance of the autophagosome marker LC3-II. Additionally, changes in the abundance and glycosylation of proteins in the autophagy and mitophagy pathways further indicated dysregulation of these cellular processes. Interestingly, serum sorbitol levels (a biomarker of disease severity) and the CDG severity score showed an inverse correlation with the abundance of the autophagosome marker LC3-II. This suggests that autophagy may act as a modulator of biochemical and clinical markers of disease severity in PMM2-CDG. Overall, our research sheds light on the complex interplay between glycosylation, mitochondrial function, and autophagy/mitophagy in PMM2-CDG. Manipulating mitochondrial dysfunction and alterations in autophagy/mitophagy pathways could offer therapeutic benefits when combined with existing treatments for PMM2-CDG.

Advances in cerebellar disorders: pre-clinical models, therapeutic targets, and challenges

INTRODUCTION

Cerebellar ataxias (CAs) represent neurological disorders with multiple etiologies and a high phenotypic variability. Despite progress in the understanding of pathogenesis, few therapies are available so far. Closing the loop between preclinical studies and therapeutic trials is important, given the impact of CAs upon patients' health and the roles of the cerebellum in multiple domains. Because of a rapid advance in research on CAs, it is necessary to summarize the main findings and discuss future directions.

AREAS COVERED

We focus our discussion on preclinical models, cerebellar reserve, the therapeutic management of CAs, and suitable surrogate markers. We searched Web of Science and PubMed using keywords relevant to cerebellar diseases, therapy, and preclinical models.

EXPERT OPINION

There are many symptomatic and/or disease-modifying therapeutic approaches under investigation. For therapy development, preclinical studies, standardization of disease evaluation, safety assessment, and demonstration of clinical improvements are essential. Stage of the disease and the level of the cerebellar reserve determine the goals of the therapy. Deficits in multiple categories and heterogeneity of CAs may require disease-, stage-, and symptom-specific therapies. More research is needed to clarify how therapies targeting the cerebellum influence both basal ganglia and the cerebral cortex, poorly explored domains in CAs.