Pathogenic Protein Aggregates Disrupt Clathrin-Mediated Endocytosis and Actin Dynamics, Altering Cellular Mechanics in Neurodegenerative Disorders

Core Concepts

Pathogenic protein aggregates, such as those found in Huntington's disease and amyotrophic lateral sclerosis, disrupt clathrin-mediated endocytosis and actin dynamics, leading to altered cellular mechanics that contribute to neurodegeneration.

Abstract

The content examines how pathogenic protein aggregates, particularly those associated with Huntington's disease and amyotrophic lateral sclerosis, affect clathrin-mediated endocytosis (CME) and the actin cytoskeleton in live cells. Key highlights: Cells expressing mutant Huntingtin (HTTQ138) show impaired movement and directionality of clathrin-coated structures (CCSs), indicating compromised CME. The defective CCS movement is accompanied by altered actin dynamics, including reduced filopodia formation and actin flow. The sequestration of actin and the Arp2/3 complex by HTTQ138 aggregates contributes to the impairment of actin organization and CCS movement. Overexpression of actin-binding proteins like Arp3, Hip1, and Mrj can partially rescue the defective CCS movement in the presence of HTTQ138 aggregates. Cells expressing HTTQ138 or TDP-43 aggregates exhibit increased stiffness and reduced fluidity, which can be rescued by modulating the actin cytoskeleton. The effects on CME, actin dynamics, and cellular mechanics are specific to certain pathogenic aggregates, such as HTTQ138 and TDP-43, and not observed with other aggregates like Aβ-42, FUS R521C, or α-Synuclein. The study highlights the intimate connection between functional CME, actin organization, and cellular mechanics in the context of neurodegeneration, and suggests that restoring the dynamic state of the actin cytoskeleton can help overcome the detrimental effects of pathogenic aggregates.

Stats

Cells expressing HTTQ138 show a severe compromise in the radial speed and directionality of clathrin-coated structures (CCSs) compared to wild-type cells. Internalization of mBSA, a cargo for clathrin-mediated endocytosis, is reduced in HTTQ138 expressing cells compared to wild-type. The number and length of filopodia are dramatically reduced in cells expressing HTTQ138 or TDP-43 aggregates compared to wild-type cells. Cells expressing HTTQ138 or TDP-43 aggregates exhibit increased stiffness (higher Young's modulus, E0) and reduced fluidity (lower α value) compared to wild-type cells.

Quotes

"Clathrin mediated endocytosis (CME) is a critical process involved in the cellular uptake of various molecules from the extracellular milieu and the plasma membrane." "Previous studies using cell lines and animal model systems have shown defects in the uptake of cargo through endocytic processes in neurodegenerative diseases such as Huntington's disease, Amyotrophic lateral sclerosis and Parkinson's disease." "Together our results indicate that Huntingtin aggregates remodel the cellular actin cytoskeleton in a manner rendering the cells stiffer, where it is unable to assist CCS movement."

Key Insights Distilled From

Pathogenic aggregates alter actin organization and cellular viscosity resulting in stalled clathrin mediated endocytosis

by Singh,S. B.,... at www.biorxiv.org 07-11-2023

https://www.biorxiv.org/content/10.1101/2023.07.10.548473v1

Deeper Inquiries

How do the specific interactions between pathogenic aggregates and proteins involved in actin organization and endocytosis vary across different neurodegenerative diseases?

In the context of neurodegenerative diseases, the specific interactions between pathogenic aggregates and proteins involved in actin organization and endocytosis can vary significantly across different conditions. For example, in Huntington's disease, the presence of mutant Huntingtin aggregates has been shown to sequester actin-binding proteins like Arp2/3, leading to altered actin dynamics and impaired clathrin-mediated endocytosis (CME). This interaction results in increased cellular stiffness and reduced fluidity, impacting the overall cellular mechanics. On the other hand, in diseases like amyotrophic lateral sclerosis (ALS) and Parkinson's disease, different pathogenic aggregates such as TDP-43 and α-synuclein may interact with distinct sets of proteins involved in actin organization and endocytosis. These interactions can lead to specific phenotypic changes in CME, actin dynamics, and cellular stiffness unique to each disease. For example, TDP-43 aggregates have been shown to disrupt actin dynamics and impair CME, resulting in altered cellular mechanics similar to the effects seen in Huntington's disease. Overall, the variations in these specific interactions across different neurodegenerative diseases contribute to the diverse molecular mechanisms underlying each condition and the distinct cellular phenotypes observed in each case.

What are the potential counter-arguments to the idea that restoring the dynamic state of the actin cytoskeleton can mitigate the detrimental effects of pathogenic aggregates in neurodegeneration?

While restoring the dynamic state of the actin cytoskeleton has shown promise in mitigating the detrimental effects of pathogenic aggregates in neurodegeneration, there are potential counter-arguments to consider: Complexity of Disease Mechanisms: Neurodegenerative diseases are multifactorial and involve intricate molecular pathways beyond actin dynamics. Restoring actin dynamics alone may not address all aspects of disease pathology, especially if other critical pathways are also dysregulated by the aggregates. Specificity of Therapeutic Targets: Targeting actin dynamics may not be universally effective across all types of neurodegenerative diseases. Different aggregates may interact with diverse sets of proteins, requiring tailored therapeutic approaches for each condition. Potential Side Effects: Modulating actin dynamics pharmacologically or genetically may have unintended consequences on other cellular processes. It is essential to consider the potential side effects of interventions aimed at restoring actin organization. Stage of Disease Progression: The effectiveness of restoring actin dynamics may vary depending on the stage of disease progression. In advanced stages, where extensive neuronal damage has occurred, targeting actin dynamics alone may not be sufficient to reverse the neurodegenerative process. Individual Variability: Patients with neurodegenerative diseases exhibit individual variability in disease presentation and response to treatments. The efficacy of restoring actin dynamics may vary among different individuals based on genetic, environmental, and other factors. Considering these counter-arguments is crucial in developing comprehensive therapeutic strategies that address the complex nature of neurodegenerative diseases beyond actin dynamics alone.

Given the links between cellular mechanics, endocytosis, and neurodegeneration, how might these insights inform the development of novel therapeutic strategies targeting the physical properties of cells in addition to the aggregation process?

The insights into the links between cellular mechanics, endocytosis, and neurodegeneration offer valuable opportunities for the development of novel therapeutic strategies targeting the physical properties of cells alongside the aggregation process: Multimodal Approaches: Combining therapies that target both the aggregation of pathogenic proteins and the restoration of cellular mechanics, such as actin dynamics and CME, could provide synergistic effects in mitigating neurodegeneration. Precision Medicine: Understanding the specific alterations in cellular mechanics associated with different neurodegenerative diseases can enable the development of personalized therapeutic interventions tailored to individual disease profiles. Drug Development: Identifying small molecules or biologics that modulate cellular mechanics, actin dynamics, and endocytosis pathways could lead to the discovery of novel drug targets for neurodegenerative diseases. Gene Therapy: Gene editing technologies can be utilized to target genes involved in actin organization and endocytosis, offering potential gene therapy approaches to restore cellular mechanics in neurodegenerative conditions. Biomarker Development: Monitoring changes in cellular mechanics as biomarkers for disease progression and treatment response could facilitate early detection and evaluation of therapeutic efficacy in neurodegenerative diseases. By integrating these insights into the development of therapeutic strategies, researchers and clinicians can explore innovative approaches to address the complex pathophysiology of neurodegenerative diseases and potentially improve patient outcomes.