Balance is the challenge--the impact of mitochondrial dynamics in Parkinson's disease

Beneficial effects of a new neuroprotective compound in neuronal cells and MPTP-administered mouse model of Parkinson's disease

A new compound, a derivative of 3,4,5-trimethoxy-N-phenyl benzamide bearing an 8''-methylimidazopyridine moiety, is found to demonstrate neuroprotective effects by preventing cell death caused by oxidative stress. The compound possesses high solubility and metabolic stability, and inhibits MPTP-induced effects in vivo, indicating high potential as a therapeutic drug for Parkinson's disease.

Squamosamide Derivative FLZ Diminishes Aberrant Mitochondrial Fission by Inhibiting Dynamin-Related Protein 1

Mitochondrial dysfunction is involved in the pathogenesis of Parkinson’s disease (PD). Mitochondrial morphology is dynamic and precisely regulated by mitochondrial fission and fusion machinery. Aberrant mitochondrial fragmentation, which can result in cell death, is controlled by the mitochondrial fission protein, dynamin-related protein 1 (Drp1). Our previous results demonstrated that FLZ could correct mitochondrial dysfunction, but the effect of FLZ on mitochondrial dynamics remain uncharacterized. In this study, we investigated the effect of FLZ and the role of Drp1 on 1-methyl-4-phenylpyridinium (MPP⁺)–induced mitochondrial fission in neurons. We observed that FLZ blocked Drp1, inhibited Drp1 enzyme activity, and reduced excessive mitochondrial fission in cultured neurons. Furthermore, by inhibiting mitochondrial fission and ROS production, FLZ improved mitochondrial integrity and membrane potential, resulting in neuroprotection. FLZ curtailed the reduction of synaptic branches of primary cultured dopaminergic neurons caused by MPP⁺ exposure, reduced abnormal fission, restored normal mitochondrial distribution in neurons, and exhibited protective effects on dopaminergic neurons. The in vitro research results were validated using an MPTP-induced PD mouse model. The in vivo results revealed that FLZ significantly reduced the mitochondrial translocation of Drp1 in the midbrain of PD mice, which, in turn, reduced the mitochondrial fragmentation in mouse substantia nigra neurons. FLZ also protected dopaminergic neurons in PD mice and increased the dopamine content in the striatum, which improved the motor coordination ability of the mice. These findings elucidate this newly discovered mechanism through which FLZ produces neuroprotection in PD.

Parkinson's Disease and Impairment in Mitochondrial Metabolism: A Pathognomic Signature

Mitochondrial bioenergetics is vital for the proper functioning of cellular compartments. Impairments in mitochondrial DNA encoding the respiratory chain complexes and other assisting proteins, accumulation of intracellular reactive oxygen species, an imbalance in cellular calcium transport, or the presence of organic pollutants, high fat-ketogenic diets or toxins, and advancing age can result in complex disorders, including cancer, metabolic disease, and neurodegenerative disorders. Such manifestations are distinctly exhibited in several age-related neurodegenerative diseases, such as in Parkinson's disease (PD). Defects in complex I along with perturbed signaling pathways is a common manifestation of PD. Impaired oxidative phosphorylation could increase the susceptibility to PD. Therefore, unraveling the mechanisms of mitochondrial complexes in clinical scenarios will assist in developing potential early biomarkers and standard tests for energy failure diagnosis and assist to pave a new path for targeted therapeutics against PD.

Mitochondria interaction networks show altered topological patterns in Parkinson's disease

Mitochondrial dysfunction is linked to pathogenesis of Parkinson’s disease (PD). However, individual mitochondria-based analyses do not show a uniform feature in PD patients. Since mitochondria interact with each other, we hypothesize that PD-related features might exist in topological patterns of mitochondria interaction networks (MINs). Here we show that MINs formed nonclassical scale-free supernetworks in colonic ganglia both from healthy controls and PD patients; however, altered network topological patterns were observed in PD patients. These patterns were highly correlated with PD clinical scores and a machine-learning approach based on the MIN features alone accurately distinguished between patients and controls with an area-under-curve value of 0.989. The MINs of midbrain dopaminergic neurons (mDANs) derived from several genetic PD patients also displayed specific changes. CRISPR/CAS9-based genome correction of alpha-synuclein point mutations reversed the changes in MINs of mDANs. Our organelle-interaction network analysis opens another critical dimension for a deeper characterization of various complex diseases with mitochondrial dysregulation.

New perspectives on the role of Drp1 isoforms in regulating mitochondrial pathophysiology

Mitochondria are dynamic organelles constantly undergoing fusion and fission. A concerted balance between the process of mitochondrial fusion and fission is required to maintain cellular health under different physiological conditions. Mutation and dysregulation of Drp1, the major driver of mitochondrial fission, has been associated with various neurological, oncological and cardiovascular disorders. Moreover, when subjected to pathological insults, mitochondria often undergo excessive fission, generating fragmented and dysfunctional mitochondria leading to cell death. Therefore, manipulating mitochondrial fission by targeting Drp1 has been an appealing therapeutic approach for cytoprotection. However, studies have been inconsistent. Studies employing Drp1 constructs representing alternate Drp1 isoforms, have demonstrated differing impacts of these isoforms on mitochondrial fission and cell death. Furthermore, there are distinct expression patterns of Drp1 isoforms in different tissues, suggesting idiosyncratic engagement in specific cellular functions. In this review, we will discuss these inherent variations among human Drp1 isoforms and how they could affect Drp1-mediated mitochondrial fission and cell death.

Biophysical Consequences of EVEN-PLUS Syndrome Mutations for the Function of Mortalin

HSPA9, the gene coding for the mitochondrial chaperone mortalin, is involved in various cellular roles such as mitochondrial protein import, folding, degradation, Fe-S cluster biogenesis, mitochondrial homeostasis, and regulation of the antiapoptotic protein p53. Mutations in the HSPA9 gene, particularly within the region coding for the nucleotide-binding domain (NBD), cause the autosomal disorder known as EVEN-PLUS syndrome. The resulting mutants R126W and Y128C are located on the surface of the mortalin-NBD near the binding interface with the interdomain linker (IDL). We used differential scanning fluorimetry (DSF), biolayer interferometry, X-ray crystallography, ATP hydrolysis assays, and Rosetta docking simulations to study the structural and functional consequences of the EVEN-PLUS syndrome-associated R126W and Y128C mutations within the mortalin-NBD. These results indicate that the surface mutations R126W and Y128C have far-reaching effects that disrupt ATP hydrolysis, interdomain linker binding, and thermostability and increase the propensity for aggregation. The structural differences observed provide insight into how the conformations of mortalin differ from other heat shock protein 70 (Hsp70) homologues. Combined, our biophysical and structural studies contribute to the understanding of the molecular basis for how disease-associated mortalin mutations affect mortalin functionality and the pathogenesis of EVEN-PLUS syndrome.

The genetic architecture of mitochondrial dysfunction in Parkinson's disease

Mitochondrial impairment is a well-established pathological pathway implicated in Parkinson's disease (PD). Defects of the complex I of the mitochondrial respiratory chain have been found in post-mortem brains from sporadic PD patients. Furthermore, several disease-related genes are linked to mitochondrial pathways, such as PRKN, PINK1, DJ-1 and HTRA2 and are associated with mitochondrial impairment. This phenotype can be caused by the dysfunction of mitochondrial quality control machinery at different levels: molecular, organellar or cellular. Mitochondrial unfolded protein response represents the molecular level and implicates various chaperones and proteases. If the molecular level of quality control is not sufficient, the organellar level is required and involves mitophagy and mitochondrial-derived vesicles to sequester whole dysfunctional organelle or parts of it. Only when the impairment is too severe, does it lead to cell death via apoptosis, which defines the cellular level of quality control. Here, we review how currently known PD-linked genetic variants interfere with different levels of mitochondrial quality control. We discuss the graded risk concept of the most recently identified PARK loci (PARK 17-23) and some susceptibility variants in GBA, LRRK2 and SNCA. Finally, the emerging concept of rare genetic variants in candidates genes for PD, such as HSPA9, TRAP1 and RHOT1, complete the picture of the complex genetic architecture of PD that will direct future precision medicine approaches.

5-HT1F receptor-mediated mitochondrial biogenesis for the treatment of Parkinson's disease

BACKGROUND AND PURPOSE

Parkinson's disease is characterized by progressive decline in motor function due to degeneration of nigrostriatal dopaminergic neurons, as well as other deficits including cognitive impairment and behavioural abnormalities. Mitochondrial dysfunction, leading to loss of ATP-dependent cellular functions, calcium overload, excitotoxicity and oxidative stress, is implicated in the pathophysiology of Parkinson's disease. Using the 5-HT1F receptor agonist LY344864, a known inducer of mitochondrial biogenesis (MB), we investigated the therapeutic efficacy of stimulating MB on dopaminergic neuron loss in a mouse model of Parkinson's disease.

EXPERIMENTAL APPROACH

Male C57BL/6 mice underwent bilateral intrastriatal 6-hydroxydopamine or saline injections and daily treatment with 2 mg·kg-1 LY344864 or vehicle for 14 days beginning 7 days post-lesion. Tyrosine hydroxylase immunoreactivity (TH-ir) and MB were assessed in the brains of all groups following treatment, and locomotor activity was evaluated prior to lesioning, 7 days post-lesion and after treatment.

KEY RESULTS

Increased mitochondrial DNA content and nuclear- and mitochondrial-encoded mRNA and protein expression was observed in specific brain regions of LY344864-treated naïve and lesioned mice, indicating augmented MB. LY344864 attenuated TH-ir loss in the striatum and substantia nigra compared to vehicle-treated lesioned animals. LY344864 treatment also increased locomotor activity in 6-hydroxydopamine lesioned mice, while vehicle treatment had no effect.

CONCLUSIONS AND IMPLICATIONS

These data revealed that LY344864-induced MB attenuates dopaminergic neuron loss and improves behavioural endpoints in this model. We suggest that stimulating MB may be beneficial for the treatment of Parkinson's disease and that the 5-HT1F receptor may be an effective therapeutic target.

Cypermethrin Activates Autophagosome Formation Albeit Inhibits Autophagy Owing to Poor Lysosome Quality: Relevance to Parkinson's Disease

Parkinson's disease (PD) is the second most familiar, progressive and movement-related neurodegenerative disorder after Alzheimer disease. This study aimed to decipher the role of autophagy in cypermethrin-induced Parkinsonism, an animal model of PD. Indicators of autophagy [expression of beclin 1, autophagy-related protein 12 (Atg 12), unc-51 like autophagy activating kinase 1 (Ulk 1), p62 and lysosome-associated membrane protein 2 (LAMP 2) and conversion of microtubule-associated protein 1A/1B-light chain 3 (LC3) I to II], signalling cascade [phosphorylated (p) 5' adenosine monophosphate-activated protein kinase (p-AMPK), sirtuin 1 (Sirt 1), phosphorylated-mammalian target of rapamycin (p-mTOR), tuberous sclerosis complex 2 (TSC 2), p₃₁₇Ulk 1 and p₇₅₇Ulk 1 levels] and lysosome morphology were assessed in control and cypermethrin-treated rat model of PD. Autophagy markers were also measured in cypermethrin-treated neuroblastoma cells in the presence of 3-methyl adenine, a phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) class III inhibitor; vinblastine, an autophagosome elongation inhibitor; bafilomycin A1, an autophagolysosome and lysosome fusion/abnormal acidification inhibitor or torin 1, a mechanistic target of rapamycin inhibitor. Cypermethrin reduced LAMP 2 and increased p-AMPK and Sirt 1 without causing any change in other signalling proteins. 3-Methyl adenine did not change LC3 conversion; vinblastine and bafilomycin A1 decreased LAMP 2 expression in controls. While cypermethrin increased LC3 conversion in the presence of 3-methyl adenine, LAMP 2 reduction was more pronounced in vinblastine and bafilomycin A1-treated cells. Torin 1 normalized the expression of LAMP 2 without any change in other autophagy markers. Results demonstrate that albeit cypermethrin activates autophagosome formation, it reduces LAMP 2 expression and lysosome quality leading to autophagy inhibition.

Neuroprotective Effects of Eexenatide in a Rotenone-Induced Rat Model of Parkinson's Disease

BACKROUND

Several studies suggest an association between Parkinson's disease (PD) and type 2 diabetes mellitus; these 2 diseases are both known to affect the common molecular pathways. As a synthetic agonist for the glucagon-like peptide 1 receptor, exenatide has been evaluated as a neuroprotective agent in multiple animal models. Rotenone models of PD have great potential for the investigation of PD pathology and motor and nonmotor symptoms, as well as the role of gene-environment interactions in PD causation and pathogenesis. Therefore, in this study, the neurochemical, behavioral and histologic effects of exenatide on a rotenone-induced rat model of PD were examined.

MATERIALS AND METHODS

Eighteen adult male rats were randomly divided into the following 3 groups (n = 6): 1 group received stereotaxical infusion of dimethyl sulfoxide (vehicle, group 1) and the others received stereotaxical infusion of rotenone (groups 2 and 3). Apomorphine-induced rotation test was applied to the rats after 10 days. Thereafter, group 2 was administered isotonic saline, whereas group 3 was administered exenatide for 28 days.

RESULTS

Malondialdehyde and tumor necrosis factor alpha levels increased in the rats with PD induced by rotenone, whereas malondialdehyde and tumor necrosis factor alpha levels markedly decreased in the rats treated with exenatide. The apomorphine-induced rotation test scores of exenatide-treated rats were determined to be lower compared with the untreated group. Additionally, treatment with exenatide significantly reduced the loss of dopaminergic neurons in striatum.

CONCLUSIONS

These results have shown that exenatide has neuroprotective, anti-inflammatory and antioxidant effects in a rotenone-induced rat model of PD.

Altered Mitochondrial Respiration and Other Features of Mitochondrial Function in Parkin-Mutant Fibroblasts from Parkinson's Disease Patients

Mutations in the parkin gene are the most common cause of early-onset Parkinson's disease (PD). Parkin, an E3 ubiquitin ligase, is involved in respiratory chain function, mitophagy, and mitochondrial dynamics. Human cellular models with parkin null mutations are particularly valuable for investigating the mitochondrial functions of parkin. However, published results reporting on patient-derived parkin-mutant fibroblasts have been inconsistent. This study aimed to functionally compare parkin-mutant fibroblasts from PD patients with wild-type control fibroblasts using a variety of assays to gain a better understanding of the role of mitochondrial dysfunction in PD. To this end, dermal fibroblasts were obtained from three PD patients with homozygous whole exon deletions in parkin and three unaffected controls. Assays of mitochondrial respiration, mitochondrial network integrity, mitochondrial membrane potential, and cell growth were performed as informative markers of mitochondrial function. Surprisingly, it was found that mitochondrial respiratory rates were markedly higher in the parkin-mutant fibroblasts compared to control fibroblasts (p = 0.0093), while exhibiting more fragmented mitochondrial networks (p = 0.0304). Moreover, cell growth of the parkin-mutant fibroblasts was significantly higher than that of controls (p = 0.0001). These unanticipated findings are suggestive of a compensatory mechanism to preserve mitochondrial function and quality control in the absence of parkin in fibroblasts, which warrants further investigation.

Parkin Regulation and Neurodegenerative Disorders

Parkin is a unique, multifunctional ubiquitin ligase whose various roles in the cell, particularly in neurons, are widely thought to be protective. The pivotal role that Parkin plays in maintaining neuronal survival is underscored by our current recognition that Parkin dysfunction represents not only a predominant cause of familial parkinsonism but also a formal risk factor for the more common, sporadic form of Parkinson’s disease (PD). Accordingly, keen research on Parkin over the past decade has led to an explosion of knowledge regarding its physiological roles and its relevance to PD. However, our understanding of Parkin is far from being complete. Indeed, surprises emerge from time to time that compel us to constantly update the paradigm of Parkin function. For example, we now know that Parkin’s function is not confined to mere housekeeping protein quality control (QC) roles but also includes mitochondrial homeostasis and stress-related signaling. Furthermore, emerging evidence also suggest a role for Parkin in several other major neurodegenerative diseases including Alzheimer’s disease (AD) and Amyotrophic Lateral Sclerosis (ALS). Yet, it remains truly amazing to note that a single enzyme could serve such multitude of functions and cellular roles. Clearly, its activity has to be tightly regulated. In this review, we shall discuss this and how dysregulated Parkin function may precipitate neuronal demise in various neurodegenerative disorders.

Computational prediction of a putative binding site on drp1: implications for antiparkinsonian therapy

Parkinson's disease is the second most common neurodegenerative disorder, for which no cure or disease-modifying therapies exist. It is evident that mechanisms impairing mitochondrial dynamics will damage cell signaling pathways, leading to neuronal death that manifests as Parkinson's disease. Dynamin related protein1, a highly conserved profission protein that catalyzes the process of mitochondrial fission, is also associated with the excessive fragmentation of mitochondria, impaired mitochondrial dynamics and cell death. Hence, Dynamin related protein1 has emerged as a key therapeutic target for diseases involving mitochondrial dysfunction. In this work, we employed a relatively novel and integrated computational strategy to identify a cryptic binding site of Dynamin related protein1 and exploited the predicted site in the rational drug designing process. This novel approach yielded three potential inhibitors, and all of them were evaluated for their neuroprotective efficacy in C. elegans model of Parkinson's disease.

Mitochondrial proteolytic stress induced by loss of mortalin function is rescued by Parkin and PINK1

The mitochondrial chaperone mortalin was implicated in Parkinson's disease (PD) because of its reduced levels in the brains of PD patients and disease-associated rare genetic variants that failed to rescue impaired mitochondrial integrity in cellular knockdown models. To uncover the molecular mechanisms underlying mortalin-related neurodegeneration, we dissected the cellular surveillance mechanisms related to mitochondrial quality control, defined the effects of reduced mortalin function at the molecular and cellular levels and investigated the functional interaction of mortalin with Parkin and PINK1, two PD-related proteins involved in mitochondrial homeostasis. We found that reduced mortalin function leads to: (1) activation of the mitochondrial unfolded protein response (UPR(mt)), (2) increased susceptibility towards intramitochondrial proteolytic stress, (3) increased autophagic degradation of fragmented mitochondria and (4) reduced mitochondrial mass in human cells in vitro and ex vivo. These alterations caused increased vulnerability toward apoptotic cell death. Proteotoxic perturbations induced by either partial loss of mortalin or chemical induction were rescued by complementation with native mortalin, but not disease-associated mortalin variants, and were independent of the integrity of autophagic pathways. However, Parkin and PINK1 rescued loss of mortalin phenotypes via increased lysosomal-mediated mitochondrial clearance and required intact autophagic machinery. Our results on loss of mortalin function reveal a direct link between impaired mitochondrial proteostasis, UPR(mt) and PD and show that effective removal of dysfunctional mitochondria via either genetic (PINK1 and Parkin overexpression) or pharmacological intervention (rapamycin) may compensate mitochondrial phenotypes.

Inactivation of Omi/HtrA2 protease leads to the deregulation of mitochondrial Mulan E3 ubiquitin ligase and increased mitophagy

Omi/HtrA2 is a nuclear encoded mitochondrial serine protease with dual and opposite functions that depend entirely on its subcellular localization. During apoptosis, Omi/HtrA2 is released into the cytoplasm where it participates in cell death. While confined in the inter-membrane space of the mitochondria, Omi/HtrA2 has a pro-survival function that may involve the regulation of protein quality control (PQC) and mitochondrial homeostasis. Loss of Omi/HtrA2's protease activity causes the neuromuscular disorder of the mnd2 (motor neuron degeneration 2) mutant mice. These mice develop multiple defects including neurodegeneration with parkinsonian features. Loss of Omi/HtrA2 in non-neuronal tissues has also been shown to cause premature aging. The normal function of Omi/HtrA2 in the mitochondria and how its deregulation causes neurodegeneration or premature aging are unknown. Here we report that the mitochondrial Mulan E3 ubiquitin ligase is a specific substrate of Omi/HtrA2. During exposure to H(2)O(2), Omi/HtrA2 degrades Mulan, and this regulation is lost in cells that carry the inactive protease. Furthermore, we show accumulation of Mulan protein in various tissues of mnd2 mice as well as in Omi/HtrA2(-/-) mouse embryonic fibroblasts (MEFs). This causes a significant decrease of mitofusin 2 (Mfn2) protein, and increased mitophagy. Our work describes a new stress-signaling pathway that is initiated in the mitochondria and involves the regulation of Mulan by Omi/HtrA2 protease. Deregulation of this pathway, as it occurs in mnd2 mutant mice, causes mitochondrial dysfunction and mitophagy, and could be responsible for the motor neuron disease and the premature aging phenotype observed in these animals.

Knockdown of Hsc70-5/mortalin induces loss of synaptic mitochondria in a Drosophila Parkinson's disease model

Mortalin is an essential component of the molecular machinery that imports nuclear-encoded proteins into mitochondria, assists in their folding, and protects against damage upon accumulation of dysfunctional, unfolded proteins in aging mitochondria. Mortalin dysfunction associated with Parkinson’s disease (PD) increases the vulnerability of cultured cells to proteolytic stress and leads to changes in mitochondrial function and morphology. To date, Drosophila melanogaster has been successfully used to investigate pathogenesis following the loss of several other PD-associated genes. We generated the first loss-of-Hsc70-5/mortalin-function Drosophila model. The reduction of Mortalin expression recapitulates some of the defects observed in the existing Drosophila PD-models, which include reduced ATP levels, abnormal wing posture, shortened life span, and reduced spontaneous locomotor and climbing ability. Dopaminergic neurons seem to be more sensitive to the loss of mortalin than other neuronal sub-types and non-neuronal tissues. The loss of synaptic mitochondria is an early pathological change that might cause later degenerative events. It precedes both behavioral abnormalities and structural changes at the neuromuscular junction (NMJ) of mortalin-knockdown larvae that exhibit increased mitochondrial fragmentation. Autophagy is concomitantly up-regulated, suggesting that mitochondria are degraded via mitophagy. Ex vivo data from human fibroblasts identifies increased mitophagy as an early pathological change that precedes apoptosis. Given the specificity of the observed defects, we are confident that the loss-of-mortalin model presented in this study will be useful for further dissection of the complex network of pathways that underlie the development of mitochondrial parkinsonism.

Collapsin response mediator protein 5 (CRMP5) induces mitophagy, thereby regulating mitochondrion numbers in dendrites

Degradation of damaged mitochondria by mitophagy is an essential process to ensure cell homeostasis. Because neurons, which have a high energy demand, are particularly dependent on the mitochondrial dynamics, mitophagy represents a key mechanism to ensure correct neuronal function. Collapsin response mediator proteins 5 (CRMP5) belongs to a family of cytosolic proteins involved in axon guidance and neurite outgrowth signaling during neural development. CRMP5, which is highly expressed during brain development, plays an important role in the regulation of neuronal polarity by inhibiting dendrite outgrowth at early developmental stages. Here, we demonstrated that CRMP5 was present in vivo in brain mitochondria and is targeted to the inner mitochondrial membrane. The mitochondrial localization of CRMP5 induced mitophagy. CRMP5 overexpression triggered a drastic change in mitochondrial morphology, increased the number of lysosomes and double membrane vesicles termed autophagosomes, and enhanced the occurrence of microtubule-associated protein 1 light chain 3 (LC3) at the mitochondrial level. Moreover, the lipidated form of LC3, LC3-II, which triggers autophagy by insertion into autophagosomes, enhanced mitophagy initiation. Lysosomal marker translocates at the mitochondrial level, suggesting autophagosome-lysosome fusion, and induced the reduction of mitochondrial content via lysosomal degradation. We show that during early developmental stages the strong expression of endogenous CRMP5, which inhibits dendrite growth, correlated with a decrease of mitochondrial content. In contrast, the knockdown or a decrease of CRMP5 expression at later stages enhanced mitochondrion numbers in cultured neurons, suggesting that CRMP5 modulated these numbers. Our study elucidates a novel regulatory mechanism that utilizes CRMP5-induced mitophagy to orchestrate proper dendrite outgrowth and neuronal function.

The optic nerve: a "mito-window" on mitochondrial neurodegeneration

Retinal ganglion cells (RGCs) project their long axons, composing the optic nerve, to the brain, transmitting the visual information gathered by the retina, ultimately leading to formed vision in the visual cortex. The RGC cellular system, representing the anterior part of the visual pathway, is vulnerable to mitochondrial dysfunction and optic atrophy is a very frequent feature of mitochondrial and neurodegenerative diseases. The start of the molecular era of mitochondrial medicine, the year 1988, was marked by the identification of a maternally inherited form of optic atrophy, Leber's hereditary optic neuropathy, as the first disease due to mitochondrial DNA point mutations. The field of mitochondrial medicine has expanded enormously over the last two decades and many neurodegenerative diseases are now known to have a primary mitochondrial etiology or mitochondrial dysfunction plays a relevant role in their pathogenic mechanism. Recent technical advancements in neuro-ophthalmology, such as optical coherence tomography, prompted a still ongoing systematic re-investigation of retinal and optic nerve involvement in neurodegenerative disorders. In addition to inherited optic neuropathies, such as Leber's hereditary optic neuropathy and dominant optic atrophy, and in addition to the syndromic mitochondrial encephalomyopathies or mitochondrial neurodegenerative disorders such as some spinocerebellar ataxias or familial spastic paraparesis and other disorders, we draw attention to the involvement of the optic nerve in classic age-related neurodegenerative disorders such as Parkinson and Alzheimer disease. We here provide an overview of optic nerve pathology in these different clinical settings, and we review the possible mechanisms involved in the pathogenesis of optic atrophy. This may be a model of general value for the field of neurodegeneration. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

The parkinson's disease-associated LRRK2 mutation R1441G inhibits neuronal differentiation of neural stem cells

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene cause familial as well as sporadic Parkinson's disease (PD) that is characterized by an age-dependent degeneration of dopaminergic neurons. LRRK2 is strongly expressed in neural stem cells (NSCs), but still the exact molecular function of LRRK2 in these cells remains unknown. By performing a systemic analysis of the gene expression profile of LRRK2-deficient NSCs, we found that the expression of several PD-associated genes, such as oxidation and reduction in mitochondria, are deregulated on LRRK2 absence. Our data, indeed, indicate that LRRK2 regulates the level of cellular oxidative stress and thereby influences the survival of NSCs. Furthermore, the lack of LRRK2 leads to an up-regulation of neuronal differentiation-inducing processes, including the Let-7a pathway. On the other hand, the constitutive mutant of LRRK2(R1441G), known to cause PD, leads to down-regulation of the same pathway. In agreement with the function of Let-7a during neuronal differentiation, LRRK2-deficient NSCs differentiate faster than wild-type cells, while LRRK2(R1441G)-expressing NSCs show impaired neuronal differentiation. These results might help better characterize the molecular mechanisms underlying the role of LRRK2 in NSCs and would further improve potential cell-replacement strategies as well as drug discovery approaches.

A multi-centre clinico-genetic analysis of the VPS35 gene in Parkinson disease indicates reduced penetrance for disease-associated variants

Background

Two recent studies identified a mutation (p.Asp620Asn) in the vacuolar protein sorting 35 gene as a cause for an autosomal dominant form of Parkinson disease . Although additional missense variants were described, their pathogenic role yet remains inconclusive.

Methods and results

We performed the largest multi-center study to ascertain the frequency and pathogenicity of the reported vacuolar protein sorting 35 gene variants in more than 15,000 individuals worldwide. p.Asp620Asn was detected in 5 familial and 2 sporadic PD cases and not in healthy controls, p.Leu774Met in 6 cases and 1 control, p.Gly51Ser in 3 cases and 2 controls. Overall analyses did not reveal any significant increased risk for p.Leu774Met and p.Gly51Ser in our cohort.

Conclusions

Our study apart from identifying the p.Asp620Asn variant in familial cases also identified it in idiopathic Parkinson disease cases, and thus provides genetic evidence for a role of p.Asp620Asn in Parkinson disease in different populations worldwide.

The use of primary human fibroblasts for monitoring mitochondrial phenotypes in the field of Parkinson's disease

Parkinson's disease (PD) is the second most common movement disorder and affects 1% of people over the age of 60 (1). Because ageing is the most important risk factor, cases of PD will increase during the next decades (2). Next to pathological protein folding and impaired protein degradation pathways, alterations of mitochondrial function and morphology were pointed out as further hallmark of neurodegeneration in PD (3-11). After years of research in murine and human cancer cells as in vitro models to dissect molecular pathways of Parkinsonism, the use of human fibroblasts from patients and appropriate controls as ex vivo models has become a valuable research tool, if potential caveats are considered. Other than immortalized, rather artificial cell models, primary fibroblasts from patients carrying disease-associated mutations apparently reflect important pathological features of the human disease. Here we delineate the procedure of taking skin biopsies, culturing human fibroblasts and using detailed protocols for essential microscopic techniques to define mitochondrial phenotypes. These were used to investigate different features associated with PD that are relevant to mitochondrial function and dynamics. Ex vivo, mitochondria can be analyzed in terms of their function, morphology, colocalization with lysosomes (the organelles degrading dysfunctional mitochondria) and degradation via the lysosomal pathway. These phenotypes are highly relevant for the identification of early signs of PD and may precede clinical motor symptoms in human disease-gene carriers. Hence, the assays presented here can be utilized as valuable tools to identify pathological features of neurodegeneration and help to define new therapeutic strategies in PD.

Mitochondrial dysfunction in Parkinson's disease: molecular mechanisms and pathophysiological consequences

Neurons are critically dependent on mitochondrial integrity based on specific morphological, biochemical, and physiological features. They are characterized by high rates of metabolic activity and need to respond promptly to activity-dependent fluctuations in bioenergetic demand. The dimensions and polarity of neurons require efficient transport of mitochondria to hot spots of energy consumption, such as presynaptic and postsynaptic sites. Moreover, the postmitotic state of neurons in combination with their exposure to intrinsic and extrinsic neuronal stress factors call for a high fidelity of mitochondrial quality control systems. Consequently, it is not surprising that mitochondrial alterations can promote neuronal dysfunction and degeneration. In particular, mitochondrial dysfunction has long been implicated in the etiopathogenesis of Parkinson's disease (PD), based on the observation that mitochondrial toxins can cause parkinsonism in humans and animal models. Substantial progress towards understanding the role of mitochondria in the disease process has been made by the identification and characterization of genes causing familial variants of PD. Studies on the function and dysfunction of these genes revealed that various aspects of mitochondrial biology appear to be affected in PD, comprising mitochondrial biogenesis, bioenergetics, dynamics, transport, and quality control.

A mitochondrial etiology of Alzheimer and Parkinson disease

BACKGROUND

The genetics and pathophysiology of Alzheimer Disease (AD) and Parkinson Disease (PD) appears complex. However, mitochondrial dysfunction is a common observation in these and other neurodegenerative diseases.

SCOPE OF REVIEW

We argue that the available data on AD and PD can be incorporated into a single integrated paradigm based on mitochondrial genetics and pathophysiology.

MAJOR CONCLUSIONS

Rare chromosomal cases of AD and PD can be interpreted as affecting mitochondrial function, quality control, and mitochondrial DNA (mtDNA) integrity. mtDNA lineages, haplogroups, such haplogroup H5a which harbors the mtDNA tRNA(Gln) A8336G variant, are important risk factors for AD and PD. Somatic mtDNA mutations are elevated in AD, PD, and Down Syndrome and Dementia (DSAD) both in brains and also systemically. AD, DS, and DSAD brains also have reduced mtDNA ND6 mRNA levels, altered mtDNA copy number, and perturbed Aβ metabolism. Classical AD genetic changes incorporated into the 3XTg-AD (APP, Tau, PS1) mouse result in reduced forebrain size, life-long reduced mitochondrial respiration in 3XTg-AD males, and initially elevated respiration and complex I and IV activities in 3XTg-AD females which markedly declines with age.

GENERAL SIGNIFICANCE

Therefore, mitochondrial dysfunction provides a unifying genetic and pathophysiology explanation for AD, PD, and other neurodegenerative diseases. This article is part of a Special Issue entitled Biochemistry of Mitochondria.

A progressive mouse model of Parkinson's disease: the Thy1-aSyn ("Line 61") mice

Identification of mutations that cause rare familial forms of Parkinson's disease (PD) and subsequent studies of genetic risk factors for sporadic PD have led to an improved understanding of the pathological mechanisms that may cause nonfamilial PD. In particular, genetic and pathological studies strongly suggest that alpha-synuclein, albeit very rarely mutated in PD patients, plays a critical role in the vast majority of individuals with the sporadic form of the disease. We have extensively characterized a mouse model over-expressing full-length, human, wild-type alpha-synuclein under the Thy-1 promoter. We have also shown that this model reproduces many features of sporadic PD, including progressive changes in dopamine release and striatal content, alpha-synuclein pathology, deficits in motor and nonmotor functions that are affected in pre-manifest and manifest phases of PD, inflammation, and biochemical and molecular changes similar to those observed in PD. Preclinical studies have already demonstrated improvement with promising new drugs in this model, which provides an opportunity to test novel neuroprotective strategies during different phases of the disorder using endpoint measures with high power to detect drug effects.

DJ-1 null dopaminergic neuronal cells exhibit defects in mitochondrial function and structure: involvement of mitochondrial complex I assembly

DJ-1 is a Parkinson's disease-associated gene whose protein product has a protective role in cellular homeostasis by removing cytosolic reactive oxygen species and maintaining mitochondrial function. However, it is not clear how DJ-1 regulates mitochondrial function and why mitochondrial dysfunction is induced by DJ-1 deficiency. In a previous study we showed that DJ-1 null dopaminergic neuronal cells exhibit defective mitochondrial respiratory chain complex I activity. In the present article we investigated the role of DJ-1 in complex I formation by using blue native-polyacrylamide gel electrophoresis and 2-dimensional gel analysis to assess native complex status. On the basis of these experiments, we concluded that DJ-1 null cells have a defect in the assembly of complex I. Concomitant with abnormal complex I formation, DJ-1 null cells show defective supercomplex formation. It is known that aberrant formation of the supercomplex impairs the flow of electrons through the channels between respiratory chain complexes, resulting in mitochondrial dysfunction. We took two approaches to study these mitochondrial defects. The first approach assessed the structural defect by using both confocal microscopy with MitoTracker staining and electron microscopy. The second approach assessed the functional defect by measuring ATP production, O₂ consumption, and mitochondrial membrane potential. Finally, we showed that the assembly defect as well as the structural and functional abnormalities in DJ-1 null cells could be reversed by adenovirus-mediated overexpression of DJ-1, demonstrating the specificity of DJ-1 on these mitochondrial properties. These mitochondrial defects induced by DJ-1mutation may be a pathological mechanism for the degeneration of dopaminergic neurons in Parkinson's disease.

Parkin, PINK1 and mitochondrial integrity: emerging concepts of mitochondrial dysfunction in Parkinson's disease

Mitochondria are dynamic organelles which are essential for many cellular processes, such as ATP production by oxidative phosphorylation, lipid metabolism, assembly of iron sulfur clusters, regulation of calcium homeostasis, and cell death pathways. The dynamic changes in mitochondrial morphology, connectivity, and subcellular distribution are critically dependent on a highly regulated fusion and fission machinery. Mitochondrial function, dynamics, and quality control are vital for the maintenance of neuronal integrity. Indeed, there is mounting evidence that mitochondrial dysfunction plays a central role in several neurodegenerative diseases. In particular, the identification of genes linked to rare familial variants of Parkinson's disease has fueled research on mitochondrial aspects of the disease etiopathogenesis. Studies on the function of parkin and PINK1, which are associated with autosomal recessive parkinsonism, provided compelling evidence that these proteins can functionally interact to maintain mitochondrial integrity and to promote clearance of damaged and dysfunctional mitochondria. In this review we will summarize current knowledge about the impact of parkin and PINK1 on mitochondria.

Bioenergetic origins of complexity and disease

The organizing power of energy flow is hypothesized to be the origin of biological complexity and its decline the basis of "complex" diseases and aging. Energy flow through organic systems creates nucleic acids, which store information, and the annual accumulation of information generates today's complexity. Energy flow through our bodies is mediated by the mitochondria, symbiotic bacteria whose genomes encompass the mitochondrial DNA (mtDNA) and more than 1000 nuclear genes. Inherited and/or epigenomic variation of the mitochondrial genome determines our initial energetic capacity, but the age-related accumulation of somatic cell mtDNA mutations further erodes energy flow, leading to disease. This bioenergetic perspective on disease provides a unifying pathophysiological and genetic mechanism for neuropsychiatric diseases such as Alzheimer and Parkinson Disease, metabolic diseases such as diabetes and obesity, autoimmune diseases, aging, and cancer.

A transcriptomic atlas of mouse neocortical layers

In the mammalian cortex, neurons and glia form a patterned structure across six layers whose complex cytoarchitectonic arrangement is likely to contribute to cognition. We sequenced transcriptomes from layers 1-6b of different areas (primary and secondary) of the adult (postnatal day 56) mouse somatosensory cortex to understand the transcriptional levels and functional repertoires of coding and noncoding loci for cells constituting these layers. A total of 5,835 protein-coding genes and 66 noncoding RNA loci are differentially expressed ("patterned") across the layers, on the basis of a machine-learning model (naive Bayes) approach. Layers 2-6b are each associated with specific functional and disease annotations that provide insights into their biological roles. This new resource (http://genserv.anat.ox.ac.uk/layers) greatly extends currently available resources, such as the Allen Mouse Brain Atlas and microarray data sets, by providing quantitative expression levels, by being genome-wide, by including novel loci, and by identifying candidate alternatively spliced transcripts that are differentially expressed across layers.

DJ-1 knock-down impairs astrocyte mitochondrial function

Mitochondrial dysfunction has long been implicated in the pathogenesis of Parkinson's disease (PD). PD brain tissues show evidence for mitochondrial respiratory chain Complex I deficiency. Pharmacological inhibitors of Complex I, such as rotenone, cause experimental parkinsonism. The cytoprotective protein DJ-1, whose deletion is sufficient to cause genetic PD, is also known to have mitochondria-stabilizing properties. We have previously shown that DJ-1 is over-expressed in PD astrocytes, and that DJ-1 deficiency impairs the capacity of astrocytes to protect co-cultured neurons against rotenone. Since DJ-1 modulated, astrocyte-mediated neuroprotection against rotenone may depend upon proper astrocytic mitochondrial functioning, we hypothesized that DJ-1 deficiency would impair astrocyte mitochondrial motility, fission/fusion dynamics, membrane potential maintenance, and respiration, both at baseline and as an enhancement of rotenone-induced mitochondrial dysfunction. In astrocyte-enriched cultures, we observed that DJ-1 knock-down reduced mitochondrial motility primarily in the cellular processes of both untreated and rotenone treated cells. In these same cultures, DJ-1 knock-down did not appreciably affect mitochondrial fission, fusion, or respiration, but did enhance rotenone-induced reductions in the mitochondrial membrane potential. In neuron-astrocyte co-cultures, astrocytic DJ-1 knock-down reduced astrocyte process mitochondrial motility in untreated cells, but this effect was not maintained in the presence of rotenone. In the same co-cultures, astrocytic DJ-1 knock-down significantly reduced mitochondrial fusion in the astrocyte cell bodies, but not the processes, under the same conditions of rotenone treatment in which DJ-1 deficiency is known to impair astrocyte-mediated neuroprotection. Our studies therefore demonstrated the following new findings: (i) DJ-1 deficiency can impair astrocyte mitochondrial physiology at multiple levels, (ii) astrocyte mitochondrial dynamics vary with sub-cellular region, and (iii) the physical presence of neurons can affect astrocyte mitochondrial behavior.

Converging environmental and genetic pathways in the pathogenesis of Parkinson's disease

As a prototypic neurodegenerative disorder Parkinson's disease (PD) is characterized by the progressive loss of specific neuronal subpopulations leading to a late-onset movement disorder. Based on familial forms of PD, to date a significant number of genes were identified that allowed first insight into the molecular pathogenesis of this common movement disorder. These pathways include impaired protein degradation and subsequent aggregation within neuronal cells and impaired mitochondrial function followed by energy depletion due to oxidative stress leading to cell death. The respective disease models were supported by pathoanatomical and biochemical findings in brains of sporadic PD patients without apparent genetic contribution to pathogenesis. Indeed recent genetic and epidemiological studies hint to a complex interplay of genetic susceptibility factors and environmental risk factors to converge to processes of pathological protein accumulation and mitochondrial damage that trigger neurodegeneration in PD. Therefore large-scale geneticoepidemiological studies combining genetic whole genome approaches with a detailed ascertainment of environmental exposures are expected to provide important clues to decipher the complexity of neurodegeneration of this most frequent neurodegenerative movement disorder.