Zinc Deficiency Exacerbates Behavioral Impediments and Dopaminergic Neuron Degeneration in a Mouse Model of Parkinson Disease

BACKGROUND

Circulating zinc (Zn) concentrations are lower than normal in patients with Parkinson disease (PD). It is unknown whether Zn deficiency increases the susceptibility to PD.

OBJECTIVES

The study aimed to investigate the effect of dietary Zn deficiency on behaviors and dopaminergic neurons in a mouse model of PD and to explore potential mechanisms.

METHODS

Male C57BL/6J mice aged 8-10 wk were fed Zn adequate (ZnA; 30 μg/g) or Zn deficient (ZnD; <5 μg/g) diet throughout the experiments. Six weeks later 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was injected to generate the PD model. Controls were injected with saline. Thus, 4 groups (Saline-ZnA, Saline-ZnD, MPTP-ZnA, and MPTP-ZnD) were formed. The experiment lasted 13 wk. Open field test, rotarod test, immunohistochemistry, and RNA sequencing were performed. Data were analyzed with t-test, 2-factor ANOVA, or Kruskal-Wallis test.

RESULTS

Both MPTP and ZnD diet treatments led to a significant reduction in blood Zn concentrations (P_MPTP = 0.012, P_Zn = 0.014), reduced total distance traveled (P_MPTP < 0.001, P_Zn = 0.031), and affected the degeneration of dopaminergic neurons in the substantia nigra (P_MPTP < 0.001, P_Zn = 0.020). In the MPTP-treated mice, the ZnD diet significantly reduced total distance traveled by 22.4% (P = 0.026), decreased latency to fall by 49.9% (P = 0.026), and reduced dopaminergic neurons by 59.3% (P = 0.002) compared with the ZnA diet. RNA sequencing analysis revealed a total of 301 differentially expressed genes (156 upregulated; 145 downregulated) in the substantia nigra of ZnD mice compared with ZnA mice. The genes were involved in a number of processes, including protein degradation, mitochondria integrity, and α-synuclein aggregation.

CONCLUSIONS

Zn deficiency aggravates movement disorders in PD mice. Our results support previous clinical observations and suggest that appropriate Zn supplementation may be beneficial for PD.

Oxidative phosphorylation mediated pathogenesis of Parkinson's disease and its implication via Akt signaling

Substantia Nigra Pars-compacta (SNpc), in the basal ganglion region, is a primary source of dopamine release. These dopaminergic neurons require more energy than other neurons, as they are highly arborized and redundant. Neurons meet most of their energy demand (∼90%) from mitochondria. Oxidative phosphorylation (OxPhos) is the primary pathway for energy production. Many genes involved in Parkinson's disease (PD) have been associated with OxPhos, especially complex I. Abrogation in complex I leads to reduced ATP formation in these neurons, succumbing to death by inducing apoptosis. This review discusses the interconnection between complex I-associated PD genes and specific mitochondrial metabolic factors (MMFs) of OxPhos. Interestingly, all the complex I-associated PD genes discussed here have been linked to the Akt signaling pathway; thus, neuron survival is promoted and smooth mitochondrial function is ensured. Any changes in these genes disrupt the Akt pathway, which hampers the opening of the permeability transition pore (PTP) via GSK3β dephosphorylation; promotes destabilization of OxPhos; and triggers the release of pro-apoptotic factors.

Unraveling the Dichotomy of Enigmatic Serine Protease HtrA2

Mitochondrial high-temperature requirement protease A2 (HtrA2) is an integral member of the HtrA family of serine proteases that are evolutionarily conserved from prokaryotes to humans. Involvement in manifold intricate cellular networks and diverse pathophysiological functions make HtrA2 the most enigmatic moonlighting protease amongst the human HtrAs. Despite perpetuating the oligomeric architecture and overall structural fold of its homologs that comprises serine protease and regulatory PDZ domains, subtle conformational alterations and dynamic enzymatic regulation through the distinct allosteric mode of action lead to its functional diversity. This mitochondrial protease upon maturation, exposes its one-of-a-kind N-terminal tetrapeptide (AVPS) motif that binds and subsequently cleaves Inhibitor of Apoptosis Proteins (IAPs) thus promoting cell death, and posing as an important molecule for therapeutic intervention. Interestingly, unlike its other human counterparts, HtrA2 has also been implicated in maintaining the mitochondrial integrity through a bi-functional chaperone-protease activity, the on-off switch of which is yet to be identified. Furthermore, its ability to activate a wide repertoire of substrates through both its N- and C-terminal regions presumably has calibrated its association with several cellular pathways and hence diseases including neurodegenerative disorders and cancer. Therefore, the exclusive structural attributes of HtrA2 that involve multimodal activation, intermolecular PDZ-protease crosstalk, and an allosterically-modulated trimeric active-site ensemble have enabled the protease to evolve across species and partake functions that are fine-tuned for maintaining cellular homeostasis and mitochondrial proteome quality control in humans. These unique features along with its multitasking potential make HtrA2 a promising therapeutic target both in cancer and neurodegeneration.

TRAP1 in Oxidative Stress and Neurodegeneration

Tumor necrosis factor receptor-associated protein 1 (TRAP1), also known as heat shock protein 75 (HSP75), is a member of the heat shock protein 90 (HSP90) chaperone family that resides mainly in the mitochondria. As a mitochondrial molecular chaperone, TRAP1 supports protein folding and contributes to the maintenance of mitochondrial integrity even under cellular stress. TRAP1 is a cellular regulator of mitochondrial bioenergetics, redox homeostasis, oxidative stress-induced cell death, apoptosis, and unfolded protein response (UPR) in the endoplasmic reticulum (ER). TRAP1 has attracted increasing interest as a therapeutical target, with a special focus on the design of TRAP1 specific inhibitors. Although TRAP1 was extensively studied in the oncology field, its role in central nervous system cells, under physiological and pathological conditions, remains largely unknown. In this review, we will start by summarizing the biology of TRAP1, including its structure and related pathways. Thereafter, we will continue by debating the role of TRAP1 in the maintenance of redox homeostasis and protection against oxidative stress and apoptosis. The role of TRAP1 in neurodegenerative disorders will also be discussed. Finally, we will review the potential of TRAP1 inhibitors as neuroprotective drugs.

Oligomeric assembly regulating mitochondrial HtrA2 function as examined by methyl-TROSY NMR

Human High temperature requirement A2 (HtrA2) is a mitochondrial protease chaperone that plays an important role in cellular proteostasis and in regulating cell-signaling events, with aberrant HtrA2 function leading to neurodegeneration and parkinsonian phenotypes. Structural studies of the enzyme have established a trimeric architecture, comprising three identical protomers in which the active sites of each protease domain are sequestered to form a catalytically inactive complex. The mechanism by which enzyme function is regulated is not well understood. Using methyl transverse relaxation optimized spectroscopy (TROSY)-based solution NMR in concert with biochemical assays, a functional HtrA2 oligomerization/binding cycle has been established. In the absence of substrates, HtrA2 exchanges between a heretofore unobserved hexameric conformation and the canonical trimeric structure, with the hexamer showing much weaker affinity toward substrates. Both structures are substrate inaccessible, explaining their low basal activity in the absence of the binding of activator peptide. The binding of the activator peptide to each of the protomers of the trimer occurs with positive cooperativity and induces intrasubunit domain reorientations to expose the catalytic center, leading to increased proteolytic activity. Our data paint a picture of HtrA2 as a finely tuned, stress-protective enzyme whose activity can be modulated both by oligomerization and domain reorientation, with basal levels of catalysis kept low to avoid proteolysis of nontarget proteins.

Gut Microbial Metabolites in Parkinson's Disease: Implications of Mitochondrial Dysfunction in the Pathogenesis and Treatment

The search for therapeutic targets for Parkinson's disease (PD) is hindered by the incomplete understanding of the pathophysiology of the disease. Mitochondrial dysfunction is an area with high potential. The neurobiological signaling connections between the gut microbiome and the central nervous system are incompletely understood. Multiple lines of evidence suggest that the gut microbiota participates in the pathogenesis of PD. Gut microbial dysbiosis may contribute to the loss of dopaminergic neurons through mitochondrial dysfunction. The intervention of gut microbial metabolites via the microbiota-gut-brain axis may serve as a promising therapeutic strategy for PD. In this narrative review, we summarize the potential roles of gut microbial dysbiosis in PD, with emphasis on microbial metabolites and mitochondrial function. We then review the possible ways in which microbial metabolites affect the central nervous system, as well as the impact of microbial metabolites on mitochondrial dysfunction. We finally discuss the possibility of gut microbiota as a therapeutic target for PD.

Loss of GSK-3β mediated phosphorylation in HtrA2 contributes to uncontrolled cell death with Parkinsonian phenotype

HtrA2, a proapoptotic mitochondrial serine protease, promotes cellular protection against oxidative damage. Literature reports show positive correlation between loss of HtrA2 protease activity and Parkinson's Disease (PD) susceptibility. Homozygous loss-of-function mutations in murine-HtrA2, and when they rarely occur in humans result in severe neurodegeneration and infantile death. Here, we report a novel heterozygous pathogenic HTRA2 variant, c.725C > T (p.T242M) in Indian PD patients. Although, this mutation exhibits no significant conformational changes compared to the wild-type, functional studies with HtrA2-T242M transfected neurons reveal common features of PD pathogenesis such as dysfunction, altered morphology and mitochondrial membrane depolarization. Despite exhibiting two-fold decrease in enzyme activity, observation of excessive cell-death due to over-expression of the mutant has been correlated with it being constitutively active. This interesting behavioral anomaly has been attributed to the loss of phosphorylation-mediated regulatory checkpoint at the T242M mutation site that is otherwise controlled by glycogen synthase kinase-3β (GSK-3β). This study, with seamless amalgamation of biophysical and biomedical research unravels a mechanistic pathway of HtrA2 regulation and delineates its biological role in PD. Therefore, this investigation will not only prove beneficial toward devising therapeutic strategies against HtrA2-associated diseases mediated by GSK-3β but also suggest new avenues for treatment of Parkinsonian phenotype.

[Essential tremor: genetic update]

Essential tremor (ET) is a neurological movement disorder characterised by bilateral limb kinetic/postural tremor, with or without tremor in other body parts including head, voice and lower limbs. Since no causative genes for ET have been identified, it is likely that the disorder occurs as a result of complex genetic factors interacting with various cellular and environmental factors that can result in abnormal function of circuitry involving the cerebello-thalamo-cortical pathway. Genetic analyses have uncovered at least 14 loci and 11 genes that are related to ET, as well as various risk or protective genetic factors. Limitations in ET genetic analyses include inconsistent disease definition, small sample size, varied ethnic backgrounds and many other factors that may contribute to paucity of relevant genetic data in ET. Genetic analyses, coupled with functional and animal studies, have led to better insights into possible pathogenic mechanisms underlying ET. These genetic studies may guide the future development of genetic testing and counselling, and specific, pathogenesis-targeted, therapeutic strategies.

The HSP90 Family: Structure, Regulation, Function, and Implications in Health and Disease

The mammalian HSP90 family of proteins is a cluster of highly conserved molecules that are involved in myriad cellular processes. Their distribution in various cellular compartments underlines their essential roles in cellular homeostasis. HSP90 and its co-chaperones orchestrate crucial physiological processes such as cell survival, cell cycle control, hormone signaling, and apoptosis. Conversely, HSP90, and its secreted forms, contribute to the development and progress of serious pathologies, including cancer and neurodegenerative diseases. Therefore, targeting HSP90 is an attractive strategy for the treatment of neoplasms and other diseases. This manuscript will review the general structure, regulation and function of HSP90 family and their potential role in pathophysiology.

Mitochondrial HTRA2 Plays a Positive, Protective Role in Dictyostelium discoideum but Is Cytotoxic When Overexpressed

HTRA2 is a mitochondrial protein, mutations in which are associated with autosomal dominant late-onset Parkinson’s disease (PD). The mechanisms by which HTRA2 mutations result in PD are poorly understood. HTRA2 is proposed to play a proteolytic role in protein quality control and homeostasis in the mitochondrial intermembrane space. Its loss has been reported to result in accumulation of unfolded and misfolded proteins. However, in at least one case, PD-associated HTRA2 mutation can cause its hyperphosphorylation, possibly resulting in protease hyperactivity. The consequences of overactive mitochondrial HTRA2 are not clear. Dictyostelium discoideum provides a well-established model for studying mitochondrial dysfunction, such as has been implicated in the pathology of PD. We identified a single homologue of human HTRA2 encoded in the Dictyostelium discoideum genome and showed that it is localized to the mitochondria where it plays a cytoprotective role. Knockdown of HTRA2 expression caused defective morphogenesis in the multicellular phases of the Dictyostelium life cycle. In vegetative cells, it did not impair mitochondrial respiration but nonetheless caused slow growth (particularly when the cells were utilizing a bacterial food source), unaccompanied by significant defects in the requisite endocytic pathways. Despite its protective roles, we could not ectopically overexpress wild type HTRA2, suggesting that mitochondrial HTRA2 hyperactivity is lethal. This toxicity was abolished by replacing the essential catalytic serine S300 with alanine to ablate serine protease activity. Overexpression of protease-dead HTRA2 phenocopied the effects of knockdown, suggesting that the mutant protein competitively inhibits interactions between wild type HTRA2 and its binding partners. Our results show that cytopathological dysfunction can be caused either by too little or too much HTRA2 activity in the mitochondria and suggest that either could be a cause of PD.

Current perspective of mitochondrial biology in Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative movement disorder characterized by preferential loss of dopaminergic neurons of the substantia nigra pars compacta and the presence of Lewy bodies containing α-synuclein. Although the cause of PD remains elusive, remarkable advances have been made in understanding the possible causative mechanisms of PD pathogenesis. An explosion of discoveries during the past two decades has led to the identification of several autosomal dominant and recessive genes that cause familial forms of PD. The investigations of these familial PD gene products have shed considerable insights into the molecular pathogenesis of the more common sporadic PD. A growing body of evidence suggests that the etiology of PD is multifactorial and involves a complex interplay between genetic and environmental factors. Substantial evidence from human tissues, genetic and toxin-induced animal and cellular models indicates that mitochondrial dysfunction plays a central role in the pathophysiology of PD. Deficits in mitochondrial functions due to bioenergetics defects, alterations in the mitochondrial DNA, generation of reactive oxygen species, aberrant calcium homeostasis, and anomalies in mitochondrial dynamics and quality control are implicated in the underlying mechanisms of neuronal cell death in PD. In this review, we discuss how familial PD-linked genes and environmental factors interface the pathways regulating mitochondrial functions and thereby potentially converge both familial and sporadic PD at the level of mitochondrial integrity. We also provide an overview of the status of therapeutic strategies targeting mitochondrial dysfunction in PD. Unraveling potential pathways that influence mitochondrial homeostasis in PD may hold the key to therapeutic intervention for this debilitating neurodegenerative movement disorder.

The genetics of Parkinson disease

About 15% of patients with Parkinson disease (PD) have family history and 5-10% have a monogenic form of the disease with Mendelian inheritance. To date, at least 23 loci and 19 disease-causing genes for parkinsonism have been found, but many more genetic risk loci and variants for sporadic PD phenotype have been identified in various association studies. Investigating the mutated protein products has uncovered potential pathogenic pathways that provide insights into mechanisms of neurodegeneration in familial and sporadic PD. To commemorate the 200th anniversary of Parkinson's publication of An Essay on the Shaking Palsy, we provide a comprehensive and critical overview of the current clinical, neuropathological, and genetic understanding of genetic forms of PD. We also discuss advances in screening for genetic PD-related risk factors and how they impact genetic counseling and contribute to the development of potential disease-modifying therapies.

Metformin reverses TRAP1 mutation-associated alterations in mitochondrial function in Parkinson's disease

The mitochondrial proteins TRAP1 and HTRA2 have previously been shown to be phosphorylated in the presence of the Parkinson's disease kinase PINK1 but the downstream signalling is unknown. HTRA2 and PINK1 loss of function causes parkinsonism in humans and animals. Here, we identified TRAP1 as an interactor of HTRA2 using an unbiased mass spectrometry approach. In our human cell models, TRAP1 overexpression is protective, rescuing HTRA2 and PINK1-associated mitochondrial dysfunction and suggesting that TRAP1 acts downstream of HTRA2 and PINK1. HTRA2 regulates TRAP1 protein levels, but TRAP1 is not a direct target of HTRA2 protease activity. Following genetic screening of Parkinson's disease patients and healthy controls, we also report the first TRAP1 mutation leading to complete loss of functional protein in a patient with late onset Parkinson's disease. Analysis of fibroblasts derived from the patient reveal that oxygen consumption, ATP output and reactive oxygen species are increased compared to healthy individuals. This is coupled with an increased pool of free NADH, increased mitochondrial biogenesis, triggering of the mitochondrial unfolded protein response, loss of mitochondrial membrane potential and sensitivity to mitochondrial removal and apoptosis. These data highlight the role of TRAP1 in the regulation of energy metabolism and mitochondrial quality control. Interestingly, the diabetes drug metformin reverses mutation-associated alterations on energy metabolism, mitochondrial biogenesis and restores mitochondrial membrane potential. In summary, our data show that TRAP1 acts downstream of PINK1 and HTRA2 for mitochondrial fine tuning, whereas TRAP1 loss of function leads to reduced control of energy metabolism, ultimately impacting mitochondrial membrane potential. These findings offer new insight into mitochondrial pathologies in Parkinson's disease and provide new prospects for targeted therapies.

Mechanisms of Gene-Environment Interactions in Parkinson's Disease

PURPOSE OF REVIEW

The purpose of the study was to discuss the main mechanisms associated with environmental and genetic factors that contribute to the development of Parkinson's disease (PD).

RECENT FINDINGS

Novel genetic contributors to PD are being identified at a rapid pace in addition to novel environmental factors. The discovery of mutations in alpha-synuclein and leucine-rich repeat kinase 2 causing inherited forms of PD along with epidemiological, in vitro, and in vivo studies identifying herbicides, pesticides, and metals as risk factors have dramatically improved our understanding of mechanisms involved in the development of PD. However, at the same time, these discoveries have also added layers of complexity to the disease. Within the last several years, the genetics associated with PD has dominated the field in many ways; however, the majority of PD cases are likely due to different combinations of environmental exposures and genetic susceptibility. The most common toxicants used to model PD including rotenone, paraquat, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine have been shown to interact with many of the genes linked with PD such as alpha-synuclein. Therefore, an understanding of mechanisms common between genetic and environmental factors is essential for early detection and successful translation of potential therapies, which is the ultimate goal.

Mitochondrial Quality Control Proteases in Neuronal Welfare

The functional integrity of mitochondria is a critical determinant of neuronal health and compromised mitochondrial function is a commonly recognized factor that underlies a plethora of neurological and neurodegenerative diseases. Metabolic demands of neural cells require high bioenergetic outputs that are often associated with enhanced production of reactive oxygen species. Unopposed accumulation of these respiratory byproducts over time leads to oxidative damage and imbalanced protein homeostasis within mitochondrial subcompartments, which in turn may result in cellular demise. The post-mitotic nature of neurons and their vulnerability to these stress factors necessitate strict protein homeostatic control to prevent such scenarios. A series of evolutionarily conserved proteases is one of the central elements of mitochondrial quality control. These versatile proteolytic enzymes conduct a multitude of activities to preserve normal mitochondrial function during organelle biogenesis, metabolic remodeling and stress. In this review we discuss neuroprotective aspects of mitochondrial quality control proteases and neuropathological manifestations arising from defective proteolysis within the mitochondrion.