Showing papers by "Darius Ebrahimi-Fakhari published in 2014"

Emerging role of autophagy in pediatric neurodegenerative and neurometabolic diseases

Abstract: Pediatric neurodegenerative diseases are a heterogeneous group of diseases that result from specific genetic and biochemical defects. In recent years, studies have revealed a wide spectrum of abnormal cellular functions that include impaired proteolysis, abnormal lipid trafficking, accumulation of lysosomal content, and mitochondrial dysfunction. Within neurons, elaborated degradation pathways such as the ubiquitin-proteasome system and the autophagy-lysosomal pathway are critical for maintaining homeostasis and normal cell function. Recent evidence suggests a pivotal role for autophagy in major adult and pediatric neurodegenerative diseases. We herein review genetic, pathological, and molecular evidence for the emerging link between autophagy dysfunction and lysosomal storage disorders such as Niemann-Pick type C, progressive myoclonic epilepsies such as Lafora disease, and leukodystrophies such as Alexander disease. We also discuss the recent discovery of genetically deranged autophagy in Vici syndrome, a multisystem disorder, and the implications for the role of autophagy in development and disease. Deciphering the exact mechanism by which autophagy contributes to disease pathology may open novel therapeutic avenues to treat neurodegeneration. To this end, an outlook on novel therapeutic approaches targeting autophagy concludes this review.

Chronic Treatment with Novel Small Molecule Hsp90 Inhibitors Rescues Striatal Dopamine Levels but Not α-Synuclein-Induced Neuronal Cell Loss

Abstract: Hsp90 inhibitors such as geldanamycin potently induce Hsp70 and reduce cytotoxicity due to α-synuclein expression, although their use has been limited due to toxicity, brain permeability, and drug design. We recently described the effects of a novel class of potent, small molecule Hsp90 inhibitors in cells overexpressing α-synuclein. Screening yielded several candidate compounds that significantly reduced α-synuclein oligomer formation and cytotoxicity associated with Hsp70 induction. In this study we examined whether chronic treatment with candidate Hsp90 inhibitors could protect against α-synuclein toxicity in a rat model of parkinsonism. Rats were injected unilaterally in the substantia nigra with AAV8 expressing human α-synuclein and then treated with drug for approximately 8 weeks by oral gavage. Chronic treatment with SNX-0723 or the more potent, SNX-9114 failed to reduce dopaminergic toxicity in the substantia nigra compared to vehicle. However, SNX-9114 significantly increased striatal dopamine content suggesting a positive neuromodulatory effect on striatal terminals. Treatment was generally well tolerated, but higher dose SNX-0723 (6–10 mg/kg) resulted in systemic toxicity, weight loss, and early death. Although still limited by potential toxicity, Hsp90 inhibitors tested herein demonstrate oral efficacy and possible beneficial effects on dopamine production in a vertebrate model of parkinsonism that warrant further study.

Child Neurology: PRRT2-associated movement disorders and differential diagnoses

Abstract: Paroxysmal kinesigenic dyskinesia (PKD) (MIM 128200) is a rare paroxysmal movement disorder that occurs at an estimated prevalence of 1:150,000 individuals. 1 Onset is most commonly in childhood or adolescence, with sporadic and familial cases being reported. 2,3 PKD is characterized by short and frequent episodes of dystonic or choreiform movements that are precipitated by sudden voluntary movements or startle. Classic clinical criteria for PKD therefore include an identifiable kinesigenic trigger, short duration of attacks, no loss of consciousness or pain during attacks, normal interictal neurologic examination results, the exclusion of other organic diseases, onset between 1 and 20 years of age (if no family history), and a response to treatment with anticonvulsants (sodium channel blockers). 3 Genetically, most cases

International electives in the final year of German medical school education--a student's perspective.

Abstract: The final year of medical school has a unique role for introducing students to their future responsibilities and challenges. At many medical schools, electives at an accredited institution abroad are a common part of the student’s final year experience. International electives provide an opportunity for a personal and academic experience that will often create new perspectives on clinical medicine and research, medical education and healthcare policy. In this article the authors reflect on their experience as elective students abroad and discuss the contribution of international electives to the constant development and progress of local final year rotations. They identify key areas for improving final year electives and outline essential features for a valuable and successful final year elective.

Parkinson's disease: A disorder of axonal mitophagy?

Abstract: Neurons, perhaps more than many other cell types, depend on mitochondria to maintain structure and function, but their postmitotic character and exceptional cellular architecture pose tremendous challenges. Mitochondrial homeostasis is preserved by the effective clearance of damaged mitochondria through mitophagy, a selective subtype of the autophagy‐lysosomal pathway. A large proportion of neuronal mitochondria reside in distal dendritic and axonal processes, where the local demand for adenosine triphosphate production and calcium buffering is high, but lysosomes, the organelles necessary for degradation of dysfunctional or superfluous mitochondria, are thought to be scarce. The latter has led to the widespread notion that mitochondria have to be delivered back to the soma through retrograde transportation to be degraded, although this process is clearly time and energy intensive, would not allow for rapid clearance of damaged mitochondria, and thus might expose axons to significant levels of oxidative stress. Mitochondrial dysfunction, oxidative stress, and defective mitophagy have been implicated in the pathogenesis of Parkinson’s disease (PD) through several lines of evidence. One such line of evidence stems from the sequential involvement of the familial PD associated proteins PINK1 (PARK6), a serine/threonine kinase, and Parkin (PARK2), an E3 ubiquitin ligase, in the initiation and regulation of mitophagy. PINK1 acts as a sensor for mitochondrial damage and accumulates on the outer mitochondrial membrane on loss of mitochondrial membrane potential. This leads to the subsequent recruitment of Parkin to the mitochondrial membrane, where it initiates the autophagic machinery to engulf and degrade dysfunctional mitochondria. In an elegant study, recently published in the Journal of Cell Biology, Ashrafi and colleagues set out to investigate whether local mitophagy might occur in regions far from the soma, such as in distal axons, and thus might provide a more rapid and dynamic means to maintain mitochondrial homeostasis locally. 1 To study mitophagy in the axonal compartment and to induce mitochondrial damage at a level that might approximate the extent of mitochondrial dysfunction normally found in neurons, the authors employed a novel methodological approach by combining time-lapse confocal microscopy in cultured living, intact neurons with the application of a genetically encoded mitochondriatargeted photosensitizer, mito-KillerRed, to induce mitochondrial damage in a spatiotemporally controlled manner. Using this approach or pharmacological depolarization of axonal mitochondria in specific cell culture chambers, socalled microfluidic devices, they were able to show that Parkin is rapidly recruited to damaged axonal mitochondria followed by the local formation and engulfment by autophagic vesicles, so-called autophagosomes. These fuse with local, mobile lysosomes, leading to an effective clearance of mitochondrial cargo. Having studied and established local axonal mitophagy in an unprecedented way, the authors next asked whether this process required the presence and function of PINK1 and Parkin, respectively, a question that had been previously addressed in immortalized non-neuronal cell lines but has remained controversial for neurons. Using primary neuronal cultures derived from PINK1 mutant rats and Parkin knockout mice, they found that both proteins are indeed required for the effective removal of axonal mitochondria through mitophagy, therefore highlighting the importance of the PINK1/Parkin‐mediated pathway for neuronal mitophagy. The seminal findings by Ashrafi et al. are of great importance because they help to clarify previous controversies in the field and establish PINK1- and Parkin-mediated local axonal mitophagy as a novel pathway that abrogates the need for retrograde organelle transportation and ensures a rapid and local control of dysfunctional mitochondria in axons. The contribution of defective axonal mitophagy to mitochondrial dysfunction and axonal degeneration in PD and other neurodegenerative diseases will have to explored in future studies.

Modeling Parkinson's disease in a dish — A story of yeast and men

Abstract: In vitro disease modeling of neurodegenerative diseases holds the promise to generate valuable platforms for the identification of disease mechanisms and testing of therapeutic strategies in proof-of-principle or large-scale screening approaches. However, given the complexity of the combined impact of genes and environment on disease onset and progression, none of the currently available models fully recapitulates the slow and progressive nature of neurodegeneration in diseases such as Parkinson’s disease (PD). Induced pluripotent stem cells (iPSCs) offer an unparalleled opportunity to study physiologicallyand disease-relevant cell types and to identify innate cellular pathologies. In the case of PD, several iPSC lines have been generated from patient primary cells carrying several known PD-related mutations (reviewed in Badger et al.). Although these models display key features of PD pathology, a neurodegenerative phenotype, based solely on the presence of disease-causing mutations, has been notoriously difficult to establish. In an unprecedented way, Chung and Khurana et al. now provide novel insights into early pathogenic phenotypes in neurons derived from PD patients using a cross-species cellular discovery platform. Turning to previously established a-synuclein yeast models and unbiased yeast genetic screens, robust modifiers of a-synuclein toxicity were identified and subjected to investigations in iPSC-derived cortical neurons from a PD patient harboring the A53T a-synuclein mutation. In a first set of experiments, nitrosative stress, similar to previous findings in yeast and rat primary cortical neurons, was found to accumulate in mutant iPSC-derived neurons when compared to genetically corrected neurons that control for genetic background effects. Interestingly, markers of nitrosative stress also colocalized with endoplasmic reticulum (ER) markers, leading the researches to look closer into ER stress and altered ER-associated degradation (ERAD), a feature previously described in synucleinopathies. In A53T mutant a-synuclein neurons, hallmark substrates of ER stress increased and altered trafficking of ERAD substrates, namely glucocerebrosidase and nicastrin, was noted. These findings led to the conclusion that ERAD dysfunction is part of an early and progressive phenotype in response to mutant a-synuclein in PD patient-derived neurons. Moving on to modifiers of a-synuclein toxicity that were previously identified in yeast-based screens, lentiviral expression of Hrd1 (Syvn1 in humans), a highly-conserved E3 ubiquitin ligase, was found to rescue a-synuclein toxicity in primary rat cortical neurons and to reduce the accumulation of ERAD substrates in patient-derived neurons. Again taking advantage of compounds identified through previous small-molecule screens in yeast, NAB2, a small molecule that salvages a-synuclein toxicity through activation of Rsp5/Nedd4, an ubiquitin ligase critically involved in vesicletrafficking, was tested in iPSC-derived patient neurons. Application of this compound, similar to overexpression of Nedd4, effectively rescued the accumulation of ERAD substrates and decreased nitric oxide levels in patient-derived neurons. Collectively the findings by Chung and Khurana et al. highlight the advantages of using a cross-species in vitro discovery approach to decipher innate pathological phenotypes. By bridging data from yeast to iPSC-derived neurons this approach allows the application and validation of previously identified genes and small molecules capable of reverting pathological phenotypes in a-synuclein models. The existence of established yeast models for other neurodegenerative diseases and the rapidly evolving techniques for generating and differentiating patient-derived iPSC will allow researchers to pursue similar approaches for a great spectrum of diseases and as such highlights the value of novel in vitro models for the study of neurodegeneration.