The role of α-synuclein in exocytosis.

The pathogenesis of degeneration in Parkinson's disease (PD) remains poorly understood but multiple lines of evidence have converged on the presynaptic protein α-synuclein (αsyn). αSyn has been shown to regulate several cellular processes, however, its normal function remains poorly understood. In this review, we will specifically focus on its role in exocytosis.

The Membrane Interactions of Synuclein: Physiology and Pathology.

Specific proteins accumulate in neurodegenerative disease, and human genetics has indicated a causative role for many. In most cases, however, the mechanisms remain poorly understood. Degeneration is thought to involve a gain of abnormal function, although we do not know the normal function of many proteins implicated. The protein α-synuclein accumulates in the Lewy pathology of Parkinson's disease and related disorders, and mutations in α-synuclein cause degeneration, but we have not known its normal function or how it triggers disease. α-Synuclein localizes to presynaptic boutons and interacts with membranes in vitro. Overexpression slows synaptic vesicle exocytosis, and recent data suggest a normal role for the endogenous synucleins in dilation of the exocytic fusion pore. Disrupted membranes also appear surprisingly prominent in Lewy pathology. Synuclein thus interacts with membranes under both physiological and pathological conditions, suggesting that the normal function of synuclein may illuminate its role in degeneration.

A DNM2 Centronuclear Myopathy Mutation Reveals a Link between Recycling Endosome Scission and Autophagy.

Autophagy involves engulfment of cytoplasmic contents by double-membraned autophagosomes, which ultimately fuse with lysosomes to enable degradation of their substrates. We recently proposed that the tubular-vesicular recycling endosome membranes were a core platform on which the critical early events of autophagosome formation occurred, including LC3-membrane conjugation to autophagic precursors. Here, we report that the release of autophagosome precursors from recycling endosomes is mediated by DNM2-dependent scission of these tubules. This process is regulated by DNM2 binding to LC3 and is increased by autophagy-inducing stimuli. This scission is defective in cells expressing a centronuclear-myopathy-causing DNM2 mutant. This mutant has an unusual mechanism as it depletes normal-functioning DNM2 from autophagosome formation sites on recycling endosomes by causing increased binding to an alternative plasma membrane partner, ITSN1. This "scission" step is, thus, critical for autophagosome formation, is defective in a human disease, and influences the way we consider how autophagosomes are formed.

LC3-positive structures are prominent in autophagy-deficient cells.

Autophagy is an evolutionarily conserved process across eukaryotes that degrades cargoes like aggregate-prone proteins, pathogens, damaged organelles and macromolecules via delivery to lysosomes. The process involves the formation of double-membraned autophagosomes that engulf the cargoes destined for degradation, sometimes with the help of autophagy receptors like p62, which are themselves autophagy substrates. LC3-II, a standard marker for autophagosomes, is generated by the conjugation of cytosolic LC3-I to phosphatidylethanolamine (PE) on the surface of nascent autophagosomes. As LC3-II is relatively specifically associated with autophagosomes and autolysosomes (in the absence of conditions stimulating LC3-associated phagocytosis), quantification of LC3-positive puncta is considered as a gold-standard assay for assessing the numbers of autophagosomes in cells. Here we find that the endogenous LC3-positive puncta become larger in cells where autophagosome formation is abrogated, and are prominent even when LC3-II is not formed. This occurs even with transient and incomplete inhibition of autophagosome biogenesis. This phenomenon is due to LC3-I sequestration to p62 aggregates, which accumulate when autophagy is impaired. This observation questions the reliability of LC3-immunofluorescence assays in cells with compromised autophagy.

Autophagy in childhood neurological disorders.

Autophagy is a tightly modulated lysosomal degradation pathway. Genetic disorders of autophagy during nervous system development may lead to developmental delay, neurodegeneration, and other neurological signs in children. Here we aimed to summarize single gene disorders that perturb various steps of autophagy pathway and their roles in the causation of childhood neurological diseases. Numerous childhood-onset disorders are caused by mutations that impact the autophagy pathway. These can manifest with a range of features including ataxia, spastic paraplegia, and intellectual disability. Defective proteins causing such diseases can interfere with autophagy flux at different stages of the itinerary. Defective autophagy may be an important contributor to the pathological features of various childhood neurodegenerative diseases and lead to the accumulation of aberrant protein and dysfunctional organelles. Insights into the relevant cell biological processes may help understand pathophysiological mechanisms and inspire autophagy-restoring therapeutic approaches. WHAT THIS PAPER ADDS: Numerous childhood-onset disorders are caused by mutations that impact the autophagy pathway. Defective autophagy is a feature of some mutations that cause ataxia, spastic paraplegia, and intellectual disability.

AUTOFAGIA EN TRASTORNOS NEUROLÓGICOS INFANTILES: La autofagia es una vía de degradación lisosomal estrechamente modulada. Los trastornos genéticos de la autofagia durante el desarrollo del sistema nervioso pueden llevar a retrasos en el desarrollo, neurodegeneración y otros signos neurológicos en los niños. Aquí nos propusimos resumir los trastornos de un solo gen que perturban varios pasos de la vía de autofagia y sus funciones en la causa de las enfermedades neurológicas infantiles. Numerosos trastornos de inicio en la infancia son causados ​​por mutaciones que afectan la vía de la autofagia. Estos pueden manifestarse con una variedad de características que incluyen ataxia, paraplejia espástica y discapacidad intelectual. Las proteínas defectuosas que causan tales enfermedades pueden interferir con el flujo de autofagia en diferentes etapas del itinerario. La autofagia defectuosa puede contribuir de manera importante a las características patológicas de diversas enfermedades neurodegenerativas infantiles y conducir a la acumulación de proteínas aberrantes y orgánulos disfuncionales. La información sobre los procesos biológicos celulares relevantes puede ayudar a comprender los mecanismos fisiopatológicos e inspirar enfoques terapéuticos de restauración de la autofagia.

AUTOFAGIA EM DESORDENS NEUROLÓGICAS DA INFÂNCIA: Autofagia é uma via de degradação lisossômica fortemente modulada. Desordens genéticas de autofagia durante o desenvolvimento do sistema nervoso central podem causar atraso do desenvolvimento, neurodegeneração, e outros sinais neurológicos em crianças. Aqui, visamos sintetizar desordens de genes únicos que perturbam várias etapas da via de autofagia, e seu papel na ocorrência de doenças neurológicas da infância. Várias desordens de início na infância são causadas por mutações que afetam a via da autofagia. Estas podem se manifestar com uma variedade de aspectos incluindo ataxia, paraplegia espástica, e deficiência intelectual. Proteínas defeituosas que causam tais doenças podem inteferir com o fluxo de autofagia em diferentes estágios do itinerário. A autofagia defeituosa pode ser um fator importante contrbuindo para aspectos patológicos de diversas doenças neurodegenerativas da infância e causar acúmulo de proteínas aberrantes e organelas disfuncionais. A compreensão dos processos biológicos celulares relevantes pode ajudar a compreender mecanismos patofisiológicos e inspirar abordagens terapêuticas que restaurem a autofagia.

Transcriptional regulation of Annexin A2 promotes starvation-induced autophagy.

Autophagy is an important degradation pathway, which is induced after starvation, where it buffers nutrient deprivation by recycling macromolecules in organisms from yeast to man. While the classical pathway mediating this response is via mTOR inhibition, there are likely to be additional pathways that support the process. Here, we identify Annexin A2 as an autophagy modulator that regulates autophagosome formation by enabling appropriate ATG9A trafficking from endosomes to autophagosomes via actin. This process is dependent on the Annexin A2 effectors ARP2 and Spire1. Annexin A2 expression increases after starvation in cells in an mTOR-independent fashion. This is mediated via Jun N-terminal kinase activation of c-Jun, which, in turn, enhances the trans-activation of the Annexin A2 promoter. Annexin A2 knockdown abrogates starvation-induced autophagy, while its overexpression induces autophagy. Hence, c-Jun-mediated transcriptional responses support starvation-induced autophagy by regulating Annexin A2 expression levels.