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By Michalina Hanzel, PhD
In a series of three blog posts I will briefly explore the major cellular mechanisms responsible for many neurodegenerative disorders. The first, and perhaps the most apparent, is the accumulation of misfolded, aggregate-prone proteins, as a result of a failure of the autophagy system, which leads to the loss of neuronal structure and function and eventual neuronal loss.
Autophagy is a conserved cellular process used to eliminate obsolete proteins and cell components by degradation and recycling. Various types of autophagy have been described but the dysfunction of macroautophagy, a process where the autophagosome, a double-membraned vesicular engulfing organelle, delivers cytoplasmic protein to the lysosome, an organelle containing proteolytic and hydrolyzing enzymes, is most highly associated with neurodegenerative disease. Neurons, terminally differentiated cells of complex morphology, are especially sensitive to any perturbations in this process. Knockout mice for various autophagy-related genes clearly demonstrate the importance of autophagy in the maintenance of homeostasis and clearance of protein aggregates within neurons.
Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease and certain tauopathies, show impairments at various steps of the autophagy pathway. Many genes required for the formation of the autophagosome are implicated in neurodegeneration, as are genes responsible for cargo recognition and selection. Lysosomal function and the formation of autolysosomes are also frequently disrupted.
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A. Immunohistochemistry-Paraffin: alpha-Synuclein [Nitrate Tyr125, Nitrate Tyr133] Antibody (24.8) [NBP1-26380] - Analysis of FFPE human brain using alpha-Synuclein antibody (clone 24.8) at 1:200 on a Bond Rx autostainer (Leica Biosystems). The assay involved 20 minutes of heat induced antigen retrieval (HIER) using 10mM sodium citrate buffer (pH 6.0) and endogenous peroxidase quenching with peroxide block. The sections were incubated with primary antibody for 30 minutes and Bond Polymer Refine Detection (Leica Biosystems) with DAB was used for signal development followed by counterstaining with hematoxylin. Whole slide scanning and capturing of representative images was performed using Aperio AT2 (Leica Biosystems). Cytoplasmic puncta and varicosities staining were observed. Staining was performed by Histowiz. |
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For example, in Parkinson’s disease, increased levels of alpha-synuclein inhibit autophagy by mislocalizing ATG9, a protein with key functions in autophagosome formation. In Huntington’s disease, the mutant version of huntingtin with expanded polyQ repeats forms toxic protein aggregates that affect the autophagy pathway at various steps. For instance, they sequester and inactivate mTOR, a negative regulator of autophagy, leading to the aberrant induction of autophagy. On the other hand, sequestration of Beclin1, another key autophagy protein, has the opposite effect of inhibiting autophagy. Overall, aberrant proteins are involved in various pathways modulating autophagy and can be either toxic or protective, depending on specific cellular contexts.
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In Alzheimer’s disease, the interplay between autophagy and amyloid-beta (Aβ), a protein fragment produced by the cleavage of Amyloid Precursor Protein (APP) and found in the toxic extracellular plaques, is particularly complex. Some studies suggest that amyloid-beta may be degraded by autophagy, whilst others propose that amyloid-beta may be generated in autophagosomes and secreted into extracellular space using the autophagy pathway. Mutations linked to familial autosomal dominant AD often encode proteins with important functions in autophagy. For example, presenilins, which are involved in APP cleavage, also have roles in acidification of lysosomes. Therefore, autophagy processes are frequently disrupted at various stages, adding to the challenge of deciphering autophagy’s various roles in health and disease.
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A. Immunohistochemistry-Paraffin:beta Amyloid Antibody (MOAB-2) [NBP2-13075] - IHC analysis of a formalin fixed paraffin embedded tissue section of human brain (Alzheimer’s disease, hippocampus) using 1:200 dilution of anti-beta Amyloid antibody (clone MOAB-2). The staining was developed with HRP labeled anti-mouse secondary antibody and DAB reagent, and nuclei of cells were counter-stained with hematoxylin. This beta Amyloid antibody specifically stained the cells with Abeta 42/ Abeta protein aggregates while the normal cells were negative for abeta peptide. |
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Nevertheless, autophagy is a promising target for potential therapies for neurodegenerative diseases. Multiple studies provide proof of principle that the modulation of autophagy can increase clearance of protein aggregates and decrease neuronal loss through reduced apoptosis and necrosis. Currently, the mechanisms of action of various autophagy modulators and their effect on other systems, such as inflammatory or immune processes, are not defined in sufficient detail to realise their full therapeutic potential. Ongoing work is essential to find optimal strategies for each neurodegenerative disease.
Michalina Hanzel, PhD
Postdoctoral Associate at The Rockefeller University
Dr Hanzel is currently studying synaptic function in the cerebellum to understand neurodevelopmental disorders and has a background in developmental neurobiology, molecular and cell biology.
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