Distinct multilevel misregulations of Parkin and PINK1 revealed in cell and animal models of TDP-43 proteinopathy

Mitochondrial quality control in human health and disease

Mitochondria, the most crucial energy-generating organelles in eukaryotic cells, play a pivotal role in regulating energy metabolism. However, their significance extends beyond this, as they are also indispensable in vital life processes such as cell proliferation, differentiation, immune responses, and redox balance. In response to various physiological signals or external stimuli, a sophisticated mitochondrial quality control (MQC) mechanism has evolved, encompassing key processes like mitochondrial biogenesis, mitochondrial dynamics, and mitophagy, which have garnered increasing attention from researchers to unveil their specific molecular mechanisms. In this review, we present a comprehensive summary of the primary mechanisms and functions of key regulators involved in major components of MQC. Furthermore, the critical physiological functions regulated by MQC and its diverse roles in the progression of various systemic diseases have been described in detail. We also discuss agonists or antagonists targeting MQC, aiming to explore potential therapeutic and research prospects by enhancing MQC to stabilize mitochondrial function.

Spaced training activates Miro/Milton-dependent mitochondrial dynamics in neuronal axons to sustain long-term memory

Neurons have differential and fluctuating energy needs across distinct cellular compartments, shaped by brain electrochemical activity associated with cognition. In vitro studies show that mitochondria transport from soma to axons is key to maintaining neuronal energy homeostasis. Nevertheless, whether the spatial distribution of neuronal mitochondria is dynamically adjusted in vivo in an experience-dependent manner remains unknown. In Drosophila, associative long-term memory (LTM) formation is initiated by an early and persistent upregulation of mitochondrial pyruvate flux in the axonal compartment of neurons in the mushroom body (MB). Through behavior experiments, super-resolution analysis of mitochondria morphology in the neuronal soma and in vivo mitochondrial fluorescence recovery after photobleaching (FRAP) measurements in the axons, we show that LTM induction, contrary to shorter-lived memories, is sustained by the departure of some mitochondria from MB neuronal soma and increased mitochondrial dynamics in the axonal compartment. Accordingly, impairing mitochondrial dynamics abolished the increased pyruvate consumption, specifically after spaced training and in the MB axonal compartment, thereby preventing LTM formation. Our results thus promote reorganization of the mitochondrial network in neurons as an integral step in elaborating high-order cognitive processes.

Disruption of Mitophagy Flux through the PARL-PINK1 Pathway by CHCHD10 Mutations or CHCHD10 Depletion

Coiled-coil-helix-coiled-coil-helix domain-containing 10 (CHCHD10) is a nuclear-encoded mitochondrial protein which is primarily mutated in the spectrum of familial and sporadic amyotrophic lateral sclerosis (ALS)-frontotemporal dementia (FTD). Endogenous CHCHD10 levels decline in the brains of ALS-FTD patients, and the CHCHD10S59L mutation in Drosophila induces dominant toxicity together with PTEN-induced kinase 1 (PINK1), a protein critical for the induction of mitophagy. However, whether and how CHCHD10 variants regulate mitophagy flux in the mammalian brain is unknown. Here, we demonstrate through in vivo and in vitro models, as well as human FTD brain tissue, that ALS/FTD-linked CHCHD10 mutations (R15L and S59L) impair mitophagy flux and mitochondrial Parkin recruitment, whereas wild-type CHCHD10 (CHCHD10WT) normally enhances these measures. Specifically, we show that CHCHD10R15L and CHCHD10S59L mutations reduce PINK1 levels by increasing PARL activity, whereas CHCHD10WT produces the opposite results through its stronger interaction with PARL, suppressing its activity. Importantly, we also demonstrate that FTD brains with TAR DNA-binding protein-43 (TDP-43) pathology demonstrate disruption of the PARL-PINK1 pathway and that experimentally impairing mitophagy promotes TDP-43 aggregation. Thus, we provide herein new insights into the regulation of mitophagy and TDP-43 aggregation in the mammalian brain through the CHCHD10-PARL-PINK1 pathway.

Pathological insights from amyotrophic lateral sclerosis animal models: comparisons, limitations, and challenges

In order to dissect amyotrophic lateral sclerosis (ALS), a multigenic, multifactorial, and progressive neurodegenerative disease with heterogeneous clinical presentations, researchers have generated numerous animal models to mimic the genetic defects. Concurrent and comparative analysis of these various models allows identification of the causes and mechanisms of ALS in order to finally obtain effective therapeutics. However, most genetically modified rodent models lack overt pathological features, imposing challenges and limitations in utilizing them to rigorously test the potential mechanisms. Recent studies using large animals, including pigs and non-human primates, have uncovered important events that resemble neurodegeneration in patients' brains but could not be produced in small animals. Here we describe common features as well as discrepancies among these models, highlighting new insights from these models. Furthermore, we will discuss how to make rodent models more capable of recapitulating important pathological features based on the important pathogenic insights from large animal models.

Biochemical and Molecular Pathways in Neurodegenerative Diseases: An Integrated View

Neurodegenerative diseases (NDDs) like Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS) are defined by a myriad of complex aetiologies. Understanding the common biochemical molecular pathologies among NDDs gives an opportunity to decipher the overlapping and numerous cross-talk mechanisms of neurodegeneration. Numerous interrelated pathways lead to the progression of neurodegeneration. We present evidence from the past pieces of literature for the most usual global convergent hallmarks like ageing, oxidative stress, excitotoxicity-induced calcium butterfly effect, defective proteostasis including chaperones, autophagy, mitophagy, and proteosome networks, and neuroinflammation. Herein, we applied a holistic approach to identify and represent the shared mechanism across NDDs. Further, we believe that this approach could be helpful in identifying key modulators across NDDs, with a particular focus on AD, PD, and ALS. Moreover, these concepts could be applied to the development and diagnosis of novel strategies for diverse NDDs.

The Oxygen and Glucose Deprivation of Immature Cells of the Nervous System Exerts Distinct Effects on Mitochondria, Mitophagy, and Autophagy, Depending on the Cells' Differentiation Stage

Perinatal brain damage, one of the most common causes of lifelong impairment, is predominantly caused by a lack of oxygen and glucose during early development. These conditions, in turn, affect cells of the nervous tissue through various stages of their maturation. To quantify the influence of these factors on cell differentiation and mitochondrial parameters, we exposed neural cell precursors to oxygen and glucose deprivation (OGD) during three stages of their differentiation: day 1, day 7, and day 14 (D1, D7, and D14, respectively). The obtained results show that OGD slows down cellular differentiation and causes cell death. Regardless of the level of cell maturity, the overall area of the mitochondria, their length, and the branching of their filaments decreased uniformly when exposed to OGD-related stress. Moreover, the cells in all stages of differentiation exhibited an increase in ROS production, hyperpolarization of the mitochondrial membrane, and autophagy. Interestingly, day 7 was the only stage in which a significant increase in mitochondrial fission, along with measurable instances of mitophagy, were detected. Taken together, the results of this study suggest that, apart from common reactions to a sudden lack of oxygen and glucose, cells in specific stages of neural differentiation can also exhibit increased preferences for mitochondrial fission and mitophagy. Such findings could play a role in guiding the future development of novel therapeutic approaches targeting perinatal brain damage during specific stages of nervous system development.

PINK1/Parkin-mediated mitophagy in neurodegenerative diseases

Mitochondria play key roles in bioenergetics, metabolism, and signaling; therefore, stable mitochondrial function is essential for cell survival, particularly in energy-intensive neuronal cells. In neurodegenerative diseases, damaged mitochondria accumulate in neurons causing associated bioenergetics deficiency, impaired cell signaling, defective cytoplasmic calcium buffering, and other pathological changes. Mitochondrial quality control is an important mechanism to ensure the maintenance of mitochondrial health, homeostasis, and mitophagy, the latter of which is a pathway that delivers defective mitochondria to the lysosome for degradation. Defective mitophagy is thought to be responsible for the accumulation of damaged mitochondria, which leads to cellular dysfunction and/or death in neurodegenerative diseases. PINK1/Parkin mainly regulates ubiquitin-dependent mitophagy, which is crucial for many aspects of mitochondrial physiology, particularly the initiation of autophagic mechanisms. Therefore, in the present review, we summarize the current knowledge of the conventional mitophagy pathway, focusing on the molecular mechanisms underlying mitophagy dysregulation in prion disease and other age-related neurodegenerative diseases, especially in relation to the PINK1/Parkin pathway. Moreover, we list the inducers of mitophagy that possess neuroprotective effects, in addition to their mechanisms related to the PINK1/Parkin pathway. These mechanisms may provide potential interventions centered on the regulation of mitophagy and offer therapeutic strategies for the treatment of neurodegenerative diseases.

Interlinked role of ASN, TDP-43 and Miro1 with parkinopathy: Focus on targeted approach against neuropathy in parkinsonism

Parkinsonism is a complex neurodegenerative disease that is difficult to differentiate because of its idiopathic and unknown origins. The hereditary parkinsonism known as autosomal recessive-juvenile parkinsonism (AR-JP) is marked by tremors, dyskinesias, dystonic characteristics, and manifestations that improve sleep but do not include dementia. This was caused by deletions and point mutations in PARK2 (chromosome 6q25.2-27). Diminished or unusual sensations (paresthesias), loss of neuron strength both in the CNS and peripheral nerves, and lack of motor coordination are the hallmarks of neuropathy in parkinsonism. The incidence of parkinsonism during oxidative stress and ageing is associated with parkinopathy. Parkinopathy is hypothesized to be triggered by mutation of the parkin (PRKN) gene and loss of normal physiological functions of PRKN proteins, which triggers their pathogenic aggregation due to conformational changes. Two important genes that control mitochondrial health are PRKN and phosphatase and tensin homologue deleted on chromosome 10-induced putative kinase 1 (PINK1). Overexpression of TAR DNA-binding protein-43 (TDP-43) increases the aggregation of insoluble PRKN proteins in OMM. Foreign α-synuclein (ASN) promotes parkinopathy via S-nitrosylation and hence has a neurotoxic effect on dopaminergic nerves. Miro1 (Miro GTPase1), a member of the RAS superfamily, is expressed in nerve cells. Due to PINK1/PRKN and Miro1's functional relationship, an excess of mitochondrial calcium culminates in the destruction of dopaminergic neurons. An interlinked understanding of TDP-43, PINK1/PRKN, ASN, and Miro1 signalling in the communication among astrocytes, microglia, neurons, and immune cells within the brain explored the pathway of neuronal death and shed light on novel strategies for the diagnosis and treatment of parkinsonism.

Drug discovery and amyotrophic lateral sclerosis: Emerging challenges and therapeutic opportunities

Amyotrophic lateral sclerosis (ALS) is characterized by the degeneration of upper and lower motor neurons (MNs) leading to paralysis and, ultimately, death by respiratory failure 3-5 years after diagnosis. Edaravone and Riluzole, the only drugs currently approved for ALS treatment, only provide mild symptomatic relief to patients. Extraordinary progress in understanding the biology of ALS provided new grounds for drug discovery. Over the last two decades, mitochondria and oxidative stress (OS), iron metabolism and ferroptosis, and the major regulators of hypoxia and inflammation - HIF and NF-κB - emerged as promising targets for ALS therapeutic intervention. In this review, we focused our attention on these targets to outline and discuss current advances in ALS drug development. Based on the challenges and the roadblocks, we believe that the rational design of multi-target ligands able to modulate the complex network of events behind the disease can provide effective therapies in a foreseeable future.

Molecular Investigations of Protein Aggregation in the Pathogenesis of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating progressive neurodegenerative disorder characterized by selective loss of lower and upper motor neurons (MNs) in the brain and spinal cord, resulting in paralysis and eventually death due to respiratory insufficiency. Although the fundamental physiological mechanisms underlying ALS are not completely understood, the key neuropathological hallmarks of ALS pathology are the aggregation and accumulation of ubiquitinated protein inclusions within the cytoplasm of degenerating MNs. Herein, we discuss recent insights into the molecular mechanisms that lead to the accumulation of protein aggregates in ALS. This will contribute to a better understanding of the pathophysiology of the disease and may open novel avenues for the development of therapeutic strategies.

CHMP2B regulates TDP-43 phosphorylation and cytotoxicity independent of autophagy via CK1

The ESCRT protein CHMP2B and the RNA-binding protein TDP-43 are both associated with ALS and FTD. The pathogenicity of CHMP2B has mainly been considered a consequence of autophagy–endolysosomal dysfunction, whereas protein inclusions containing phosphorylated TDP-43 are a pathological hallmark of ALS and FTD. Intriguingly, TDP-43 pathology has not been associated with the FTD-causing CHMP2B^Intron5 mutation. In this study, we identify CHMP2B as a modifier of TDP-43–mediated neurodegeneration in a Drosophila screen. Down-regulation of CHMP2B reduces TDP-43 phosphorylation and toxicity in flies and mammalian cells. Surprisingly, although CHMP2B^Intron5 causes dramatic autophagy dysfunction, disturbance of autophagy does not alter TDP-43 phosphorylation levels. Instead, we find that inhibition of CK1, but not TTBK1/2 (all of which are kinases phosphorylating TDP-43), abolishes the modifying effect of CHMP2B on TDP-43 phosphorylation. Finally, we uncover that CHMP2B modulates CK1 protein levels by negatively regulating ubiquitination and the proteasome-mediated turnover of CK1. Together, our findings propose an autophagy-independent role and mechanism of CHMP2B in regulating CK1 abundance and TDP-43 phosphorylation.

Parkin beyond Parkinson’s Disease—A Functional Meaning of Parkin Downregulation in TDP-43 Proteinopathies

Parkin and PINK1 are key regulators of mitophagy, an autophagic pathway for selective elimination of dysfunctional mitochondria. To this date, parkin depletion has been associated with recessive early onset Parkinson’s disease (PD) caused by loss-of-function mutations in the PARK2 gene, while, in sporadic PD, the activity and abundance of this protein can be compromised by stress-related modifications. Intriguingly, research in recent years has shown that parkin depletion is not limited to PD but is also observed in other neurodegenerative diseases—especially those characterized by TDP-43 proteinopathies, such as amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Here, we discuss the evidence of parkin downregulation in these disease phenotypes, its emerging connections with TDP-43, and its possible functional implications.

New perspectives on cytoskeletal dysregulation and mitochondrial mislocalization in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by selective, early degeneration of motor neurons in the brain and spinal cord. Motor neurons have long axonal projections, which rely on the integrity of neuronal cytoskeleton and mitochondria to regulate energy requirements for maintaining axonal stability, anterograde and retrograde transport, and signaling between neurons. The formation of protein aggregates which contain cytoskeletal proteins, and mitochondrial dysfunction both have devastating effects on the function of neurons and are shared pathological features across several neurodegenerative conditions, including ALS, Alzheimer's disease, Parkinson's disease, Huntington's disease and Charcot-Marie-Tooth disease. Furthermore, it is becoming increasingly clear that cytoskeletal integrity and mitochondrial function are intricately linked. Therefore, dysregulations of the cytoskeletal network and mitochondrial homeostasis and localization, may be common pathways in the initial steps of neurodegeneration. Here we review and discuss known contributors, including variants in genetic loci and aberrant protein activities, which modify cytoskeletal integrity, axonal transport and mitochondrial localization in ALS and have overlapping features with other neurodegenerative diseases. Additionally, we explore some emerging pathways that may contribute to this disruption in ALS.

Molecular targets and approaches to restore autophagy and lysosomal capacity in neurodegenerative disorders

Autophagy is a catabolic process that promotes cellular fitness by clearing aggregated protein species, pathogens and damaged organelles through lysosomal degradation. The autophagic process is particularly important in the nervous system where post-mitotic neurons rely heavily on protein and organelle quality control in order to maintain cellular health throughout the lifetime of the organism. Alterations of autophagy and lysosomal function are hallmarks of various neurodegenerative disorders. In this review, we conceptualize some of the mechanistic and genetic evidence pointing towards autophagy and lysosomal dysfunction as a causal driver of neurodegeneration. Furthermore, we discuss rate-limiting pathway nodes and potential approaches to restore pathway activity, from autophagy initiation, cargo sequestration to lysosomal capacity.

Fly for ALS: Drosophila modeling on the route to amyotrophic lateral sclerosis modifiers

Amyotrophic lateral sclerosis (ALS) is a rare, devastating disease, causing movement impairment, respiratory failure and ultimate death. A plethora of genetic, cellular and molecular mechanisms are involved in ALS signature, although the initiating causes and progressive pathological events are far from being understood. Drosophila research has produced seminal discoveries for more than a century and has been successfully used in the past 25 years to untangle the process of ALS pathogenesis, and recognize potential markers and novel strategies for therapeutic solutions. This review will provide an updated view of several ALS modifiers validated in C9ORF72, SOD1, FUS, TDP-43 and Ataxin-2 Drosophila models. We will discuss basic and preclinical findings, illustrating recent developments and novel breakthroughs, also depicting unsettled challenges and limitations in the Drosophila-ALS field. We intend to stimulate a renewed debate on Drosophila as a screening route to identify more successful disease modifiers and neuroprotective agents.

Niclosamide affects intracellular TDP-43 distribution in motor neurons, activates mitophagy, and attenuates morphological changes under stress

Amyotrophic lateral sclerosis (ALS) is characterized by progressive motor neuron loss in the brain and spinal cord; however, its etiology is unknown, and no curative treatment exists. TAR DNA-binding protein 43 (TDP-43), encoded by TARDBP, is a genetic mutation observed in 2-5% of familial ALS, and TDP is known to be mislocalized in the cytoplasm. This study aimed to identify compounds that inhibited the nuclear to cytoplasmic localization of TDP-43 in human induced pluripotent stem (iPS) cells-derived neurons. TDP-43 transgenic human iPS cells were constructed, differentiated into motor neurons, and then treated with MG-132 and sodium arsenite (stressors) to induce nuclear to cytoplasmic localization of TDP-43. STAT3 inhibitors, such as niclosamide, prevented TDP-43 mislocalization and degraded TDP-43 aggregates, and attenuated morphological changes under stress. Furthermore, niclosamide activated mitophagy via the PINK1-parkin-ubiquitin pathway. These findings suggest niclosamide may be a therapeutic candidate for ALS.

TDP-43 and Inflammation: Implications for Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

The abnormal mislocalisation and ubiquitinated protein aggregation of the TAR DNA binding protein 43 (TDP-43) within the cytoplasm of neurons and glia in the central nervous system (CNS) is a pathological hallmark of early-onset neurodegenerative disorders amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The pathomechanisms underlying abnormal mislocalisation and aggregation of TDP-43 remain unknown. However, there is a growing body of evidence implicating neuroinflammation and immune-mediated mechanisms in the pathogenesis of neurodegeneration. Importantly, most of the evidence for an active role of immunity and inflammation in the pathogenesis of ALS and FTD relates specifically to TDP-43, posing the question as to whether immune-mediated mechanisms could hold the key to understanding TDP-43’s underlying role in neurodegeneration in both diseases. Therefore, this review aims to piece together key lines of evidence for the specific association of TDP-43 with key immune and inflammatory pathways to explore the nature of this relationship and the implications for potential pathomechanisms underlying neurodegeneration in ALS and FTD.

Parkin Levels Decrease in Fibroblasts With Progranulin (PGRN) Pathogenic Variants and in a Cellular Model of PGRN Deficiency

Frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative diseases with TDP-43 mislocalization and aggregation. Genetic forms of FTLD and ALS are caused by pathogenic variants in various genes, such as PGRN (progranulin). To date, depletion of parkin E3 ubiquitin protein ligase, a key mitophagy regulator, has been reported in sporadic ALS patients and ALS mice models with TDP-43 proteinopathy. In this work, we show parkin downregulation also in fibroblasts derived from FTLD patients with four different PGRN pathogenic variants. We corroborate this finding in control fibroblasts upon PGRN silencing, demonstrating additionally the decrease of parkin downstream targets, mitofusin 2 (MFN2) and voltage dependent anion channel 1 (VDAC1). Importantly, we show that TDP-43 overexpression rescues PRKN levels upon transient PGRN silencing, but not in FTLD fibroblasts with PGRN pathogenic variants, despite upregulating PGRN levels in both cases. Further observation of PRKN downregulation upon TDP-43 silencing, suggests that TDP-43 loss-of-function contributes to PRKN decrease. Our results provide further evidence that parkin downregulation might be a common and systemic phenomenon in neurodegenerative diseases with TDP- 43 loss-of-function.

Molecular Mechanisms Underlying TDP-43 Pathology in Cellular and Animal Models of ALS and FTLD

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are neurodegenerative disorders that exist on a disease spectrum due to pathological, clinical and genetic overlap. In up to 97% of ALS cases and ~50% of FTLD cases, the primary pathological protein observed in affected tissues is TDP-43, which is hyperphosphorylated, ubiquitinated and cleaved. The TDP-43 is observed in aggregates that are abnormally located in the cytoplasm. The pathogenicity of TDP-43 cytoplasmic aggregates may be linked with both a loss of nuclear function and a gain of toxic functions. The cellular processes involved in ALS and FTLD disease pathogenesis include changes to RNA splicing, abnormal stress granules, mitochondrial dysfunction, impairments to axonal transport and autophagy, abnormal neuromuscular junctions, endoplasmic reticulum stress and the subsequent induction of the unfolded protein response. Here, we review and discuss the evidence for alterations to these processes that have been reported in cellular and animal models of TDP-43 proteinopathy.

Where and Why Modeling Amyotrophic Lateral Sclerosis

Over the years, researchers have leveraged a host of different in vivo models in order to dissect amyotrophic lateral sclerosis (ALS), a neurodegenerative/neuroinflammatory disease that is heterogeneous in its clinical presentation and is multigenic, multifactorial and non-cell autonomous. These models include both vertebrates and invertebrates such as yeast, worms, flies, zebrafish, mice, rats, guinea pigs, dogs and, more recently, non-human primates. Despite their obvious differences and peculiarities, only the concurrent and comparative analysis of these various systems will allow the untangling of the causes and mechanisms of ALS for finally obtaining new efficacious therapeutics. However, harnessing these powerful organisms poses numerous challenges. In this context, we present here an updated and comprehensive review of how eukaryotic unicellular and multicellular organisms that reproduce a few of the main clinical features of the disease have helped in ALS research to dissect the pathological pathways of the disease insurgence and progression. We describe common features as well as discrepancies among these models, highlighting new insights and emerging roles for experimental organisms in ALS.

Connecting TDP-43 Pathology with Neuropathy

Transactive response DNA-binding protein 43 kDa (TDP-43), a multifunctional nucleic acid-binding protein, is a primary component of insoluble aggregates associated with several devastating nervous system disorders; mutations in TARDBP, its encoding gene, are a cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here, we review established and emerging roles of TDP-43 and consider how its dysfunction impinges on RNA homeostasis in the nervous system, thereby contributing to neural degeneration. Notably, improper splicing of the axonal growth-associated factor STMN2 has recently been connected to TDP-43 dysfunction, providing a mechanistic link between TDP-43 proteinopathies and neuropathy. This review highlights how a deep understanding of the function of TDP-43 in the brain might be leveraged to develop new targeted therapies for several neurological disorders.

TDP-43 and PINK1 mediate CHCHD10S59L mutation-induced defects in Drosophila and in vitro

Mutations in coiled-coil-helix-coiled-coil-helix domain containing 10 (CHCHD10) can cause amyotrophic lateral sclerosis and frontotemporal dementia (ALS-FTD). However, the underlying mechanisms are unclear. Here, we generate CHCH10^S59L-mutant Drosophila melanogaster and HeLa cell lines to model CHCHD10-associated ALS-FTD. The CHCHD10^S59L mutation results in cell toxicity in several tissues and mitochondrial defects. CHCHD10^S59L independently affects the TDP-43 and PINK1 pathways. CHCHD10^S59L expression increases TDP-43 insolubility and mitochondrial translocation. Blocking TDP-43 mitochondrial translocation with a peptide inhibitor reduced CHCHD10^S59L-mediated toxicity. While genetic and pharmacological modulation of PINK1 expression and activity of its substrates rescues and mitigates the CHCHD10^S59L-induced phenotypes and mitochondrial defects, respectively, in both Drosophila and HeLa cells. Our findings suggest that CHCHD10^S59L-induced TDP-43 mitochondrial translocation and chronic activation of PINK1-mediated pathways result in dominant toxicity, providing a mechanistic insight into the CHCHD10 mutations associated with ALS-FTD.

The multifaceted role of kinases in amyotrophic lateral sclerosis: genetic, pathological and therapeutic implications

Amyotrophic lateral sclerosis is the most common degenerative disorder of motor neurons in adults. As there is no cure, thousands of individuals who are alive at present will succumb to the disease. In recent years, numerous causative genes and risk factors for amyotrophic lateral sclerosis have been identified. Several of the recently identified genes encode kinases. In addition, the hypothesis that (de)phosphorylation processes drive the disease process resulting in selective motor neuron degeneration in different disease variants has been postulated. We re-evaluate the evidence for this hypothesis based on recent findings and discuss the multiple roles of kinases in amyotrophic lateral sclerosis pathogenesis. We propose that kinases could represent promising therapeutic targets. Mainly due to the comprehensive regulation of kinases, however, a better understanding of the disturbances in the kinome network in amyotrophic lateral sclerosis is needed to properly target specific kinases in the clinic.

Ubiquitin signalling in neurodegeneration: mechanisms and therapeutic opportunities

Neurodegenerative diseases are characterised by progressive damage to the nervous system including the selective loss of vulnerable populations of neurons leading to motor symptoms and cognitive decline. Despite millions of people being affected worldwide, there are still no drugs that block the neurodegenerative process to stop or slow disease progression. Neuronal death in these diseases is often linked to the misfolded proteins that aggregate within the brain (proteinopathies) as a result of disease-related gene mutations or abnormal protein homoeostasis. There are two major degradation pathways to rid a cell of unwanted or misfolded proteins to prevent their accumulation and to maintain the health of a cell: the ubiquitin–proteasome system and the autophagy–lysosomal pathway. Both of these degradative pathways depend on the modification of targets with ubiquitin. Aging is the primary risk factor of most neurodegenerative diseases including Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis. With aging there is a general reduction in proteasomal degradation and autophagy, and a consequent increase of potentially neurotoxic protein aggregates of β-amyloid, tau, α-synuclein, SOD1 and TDP-43. An often over-looked yet major component of these aggregates is ubiquitin, implicating these protein aggregates as either an adaptive response to toxic misfolded proteins or as evidence of dysregulated ubiquitin-mediated degradation driving toxic aggregation. In addition, non-degradative ubiquitin signalling is critical for homoeostatic mechanisms fundamental for neuronal function and survival, including mitochondrial homoeostasis, receptor trafficking and DNA damage responses, whilst also playing a role in inflammatory processes. This review will discuss the current understanding of the role of ubiquitin-dependent processes in the progressive loss of neurons and the emergence of ubiquitin signalling as a target for the development of much needed new drugs to treat neurodegenerative disease.

PINK1/PARKIN signalling in neurodegeneration and neuroinflammation

Mutations in the PTEN-induced kinase 1 (PINK1) and Parkin RBR E3 ubiquitin-protein ligase (PARKIN) genes are associated with familial forms of Parkinson’s disease (PD). PINK1, a protein kinase, and PARKIN, an E3 ubiquitin ligase, control the specific elimination of dysfunctional or superfluous mitochondria, thus fine-tuning mitochondrial network and preserving energy metabolism. PINK1 regulates PARKIN translocation in impaired mitochondria and drives their removal via selective autophagy, a process known as mitophagy. As knowledge obtained using different PINK1 and PARKIN transgenic animal models is being gathered, growing evidence supports the contribution of mitophagy impairment to several human pathologies, including PD and Alzheimer’s diseases (AD). Therefore, therapeutic interventions aiming to modulate PINK1/PARKIN signalling might have the potential to treat these diseases. In this review, we will start by discussing how the interplay of PINK1 and PARKIN signalling helps mediate mitochondrial physiology. We will continue by debating the role of mitochondrial dysfunction in disorders such as amyotrophic lateral sclerosis, Alzheimer’s, Huntington’s and Parkinson’s diseases, as well as eye diseases such as age-related macular degeneration and glaucoma, and the causative factors leading to PINK1/PARKIN-mediated neurodegeneration and neuroinflammation. Finally, we will discuss PINK1/PARKIN gene augmentation possibilities with a particular focus on AD, PD and glaucoma.

Stress Induces Dynamic, Cytotoxicity-Antagonizing TDP-43 Nuclear Bodies via Paraspeckle LncRNA NEAT1-Mediated Liquid-Liquid Phase Separation

Despite the prominent role of TDP-43 in neurodegeneration, its physiological and pathological functions are not fully understood. Here, we report an unexpected role of TDP-43 in the formation of dynamic, reversible, liquid droplet-like nuclear bodies (NBs) in response to stress. Formation of NBs alleviates TDP-43-mediated cytotoxicity in mammalian cells and fly neurons. Super-resolution microscopy reveals distinct functions of the two RRMs in TDP-43 NB formation. TDP-43 NBs are partially colocalized with nuclear paraspeckles, whose scaffolding lncRNA NEAT1 is dramatically upregulated in stressed neurons. Moreover, increase of NEAT1 promotes TDP-43 liquid-liquid phase separation (LLPS) in vitro. Finally, we discover that the ALS-associated mutation D169G impairs the NEAT1-mediated TDP-43 LLPS and NB assembly, causing excessive cytoplasmic translocation of TDP-43 to form stress granules, which become phosphorylated TDP-43 cytoplasmic foci upon prolonged stress. Together, our findings suggest a stress-mitigating role and mechanism of TDP-43 NBs, whose dysfunction may be involved in ALS pathogenesis.

RNA-Binding Proteins Implicated in Mitochondrial Damage and Mitophagy

The mitochondrial lifecycle comprises biogenesis, fusion and cristae remodeling, fission, and breakdown by the autophagosome. This cycle is essential for maintaining proper cellular function, and inhibition of any of these processes results in deterioration of bioenergetics and swift induction of apoptosis, particularly in energy-craving cells such as myocytes and neurons. Regulation of gene expression is a fundamental step in maintaining mitochondrial plasticity, mediated by (1) transcription factors that control the expression of mitochondrial mRNAs and (2) RNA-binding proteins (RBPs) that regulate mRNA splicing, stability, targeting to mitochondria, and translation. More recently, RBPs have been also shown to interact with proteins modulating the mitochondrial lifecycle. Importantly, misexpression or mutations in RBPs give rise to mitochondrial dysfunctions, and there is strong evidence to support that these mitochondrial impairments occur early in disease development, constituting leading causes of pathogenesis. This review presents key aspects of the molecular network of the disease-relevant RBPs, including transactive response DNA-binding protein 43 (TDP43), fused in sarcoma (FUS), T-cell intracellular antigen 1 (TIA1), TIA-related protein (TIAR), and pumilio (PUM) that drive mitochondrial dysfunction in the nervous system.

PARIS induced defects in mitochondrial biogenesis drive dopamine neuron loss under conditions of parkin or PINK1 deficiency

BACKGROUND

Mutations in PINK1 and parkin cause autosomal recessive Parkinson's disease (PD). Evidence placing PINK1 and parkin in common pathways regulating multiple aspects of mitochondrial quality control is burgeoning. However, compelling evidence to causatively link specific PINK1/parkin dependent mitochondrial pathways to dopamine neuron degeneration in PD is lacking. Although PINK1 and parkin are known to regulate mitophagy, emerging data suggest that defects in mitophagy are unlikely to be of pathological relevance. Mitochondrial functions of PINK1 and parkin are also tied to their proteasomal regulation of specific substrates. In this study, we examined how PINK1/parkin mediated regulation of the pathogenic substrate PARIS impacts dopaminergic mitochondrial network homeostasis and neuronal survival in Drosophila.

METHODS

The UAS-Gal4 system was employed for cell-type specific expression of the various transgenes. Effects on dopamine neuronal survival and function were assessed by anti-TH immunostaining and negative geotaxis assays. Mitochondrial effects were probed by quantitative analysis of mito-GFP labeled dopaminergic mitochondria, assessment of mitochondrial abundance in dopamine neurons isolated by Fluorescence Activated Cell Sorting (FACS) and qRT-PCR analysis of dopaminergic factors that promote mitochondrial biogenesis. Statistical analyses employed two-tailed Student's T-test, one-way or two-way ANOVA as required and data considered significant when P < 0.05.

RESULTS

We show that defects in mitochondrial biogenesis drive adult onset progressive loss of dopamine neurons and motor deficits in Drosophila models of PINK1 or parkin insufficiency. Such defects result from PARIS dependent repression of dopaminergic PGC-1α and its downstream transcription factors NRF1 and TFAM that cooperatively promote mitochondrial biogenesis. Dopaminergic accumulation of human or Drosophila PARIS recapitulates these neurodegenerative phenotypes that are effectively reversed by PINK1, parkin or PGC-1α overexpression in vivo. To our knowledge, PARIS is the only co-substrate of PINK1 and parkin to specifically accumulate in the DA neurons and cause neurodegeneration and locomotor defects stemming from disrupted dopamine signaling.

CONCLUSIONS

Our findings identify a highly conserved role for PINK1 and parkin in regulating mitochondrial biogenesis and promoting mitochondrial health via the PARIS/ PGC-1α axis. The Drosophila models described here effectively recapitulate the cardinal PD phenotypes and thus will facilitate identification of novel regulators of mitochondrial biogenesis for physiologically relevant therapeutic interventions.

New insights of poly(ADP-ribosylation) in neurodegenerative diseases: A focus on protein phase separation and pathologic aggregation

Abnormal protein aggregation is a common pathological feature of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). Protein posttranslational modifications (PTMs) play a crucial regulatory role in the formation of pathologic aggregation. Among the known PTMs involved in neurodegeneration, poly(ADP-ribosylation) (PARylation) has emerged with promising therapeutic potentials of the use of poly(ADP-ribose) (PAR) polymerase (PARP) inhibitors. In this review, we describe the mounting evidence that abnormal PARP activation is involved in various neurodegenerative diseases, and discuss the underpinning mechanisms with a focus on the recent findings that PARylation affects liquid-liquid phase separation and aggregation of amyloid proteins. We hope this review will stimulate further investigation of the unknown functions of PARylation and promote the development of more effective therapeutic agents in treating neurodegeneration.

TDP-43 proteinopathy and mitochondrial abnormalities in neurodegeneration

Genetic mutations in TAR DNA-binding protein 43 (TDP-43) cause amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Importantly, TDP-43 proteinopathy, characterized by aberrant phosphorylation, ubiquitination, cleavage or nuclear depletion of TDP-43 in neurons and glial cells, is a common prominent pathological feature of various major neurodegenerative diseases including ALS, FTD, and Alzheimer's disease (AD). Although the pathomechanisms underlying TDP-43 proteinopathy remain elusive, pathologically relevant TDP-43 has been repeatedly shown to be present in either the inside or outside of mitochondria, and functionally involved in the regulation of mitochondrial morphology, trafficking, and function, suggesting mitochondria as likely targets of TDP-43 proteinopathy. In this review, we first describe the current knowledge of the association of TDP-43 with mitochondria. We then review in detail multiple mitochondrial pathways perturbed by pathological TDP-43, including mitochondrial fission and fusion dynamics, mitochondrial trafficking, bioenergetics, and mitochondrial quality control. Lastly, we briefly discuss how the study of TDP-43 proteinopathy and mitochondrial abnormalities may provide new avenues for neurodegeneration therapeutics.

SESN2 protects against doxorubicin-induced cardiomyopathy via rescuing mitophagy and improving mitochondrial function

The clinical application of doxorubicin (Dox) in cancer therapy is limited by its serious cardiotoxicity. Our previous studies and others have recognized that mitochondrial dysfunction is the common feature of Dox-induced cardiotoxicity. However, mechanisms underlying mitochondrial disorders remained largely unknown. SESN2, a highly conserved and stress-inducible protein, is involved in mitochondrial function and autophagy in cardiovascular diseases. This study aimed to investigate whether SESN2 affects Dox-induced cardiotoxicity and the underlying mechanisms. Sprague-Dawley rats and neonatal rat cardiomyocytes were treated with Dox. SESN2 expression was assessed. The effects of SESN2 on Dox-induced cardiotoxicity were assessed by functional gain and loss experiments. Echocardiographic parameters, morphological and histological analyses, transmission electron microscope and immunofluorescence assays were used to assess cardiac and mitochondrial function. The protein expression of SESN2 was significantly reduced following Dox stimulation. Both knockout of SESN2 by sgRNA and Dox treatment resulted in the inhibition of Parkin-mediated mitophagy, marked cardiomyocytes apoptosis and mitochondria dysfunction. Ectopic expression of SESN2 effectively protected against Dox-induced cardiomyocyte apoptosis, mitochondrial injury and cardiac dysfunction. Mechanistically, SESN2 interacted with Parkin and p62, promoted accumulation of Parkin to mitochondria and then alleviated Dox-caused inhibition of Parkin mediated mitophagy. Ultimately, the clearance of damaged mitochondria and mitochondrial function were improved following SESN2 overexpression. SESN2 protected against Dox-induced cardiotoxicity through improving mitochondria function and mitophagy. These results established SESN2 as a key player in mitochondrial function and provided a potential therapeutic approach to Dox-induced cardiomyopathy.

PARPs and PAR as novel pharmacological targets for the treatment of stress granule-associated disorders

Among the post-translational modifications, ADP-ribosylation has been for long time the least integrated in the scheme of the structural protein modifications affecting physiological functions. In spite of the original findings on bacterial-dependent ADP-ribosylation catalysed by toxins such as cholera and pertussis toxin, only with the discovery of the poly-ADP-ribosyl polymerase (PARP) family the field has finally expanded and the role of ADP-ribosylation has been recognised in both physiological and pathological processes, including cancer, infectious and neurodegenerative diseases. This is now a rapidly expanding field of investigation, centred on the role of the different PARPs and their substrates in various diseases, and on the potential of PARP inhibitors as novel pharmacological tools to be employed in relevant pathological context. In this review we analyse the role that members of the PARP family and poly-ADP-ribose (PAR; the product of PARP1 and PARP5a activity) play in the processes following the exposure of cells to different stresses. The cell response that arises following conditions such as heat, osmotic, oxidative stresses or viral infection relies on the formation of stress granules, which are transient cytoplasmic membrane-less structures, that include untranslated mRNA, specific proteins and PAR, this last one serving as the "collector" of all components (that bind to it in a non-covalent manner). The resulting phenotypes are cells in which translation, intracellular transport or pro-apoptotic pathways are reversibly inhibited, for the time the given stress holds. Interestingly, the formation of defective stress granules has been detected in diverse pathological conditions including neurological disorders and cancer. Analysing the molecular details of stress granule formation under these conditions offers a novel view on the pathogenesis of these diseases and, as a consequence, the possibility of identifying novel drug targets for their treatment.