Formation of Cytoplasmic Actin-Cofilin Rods is Triggered by Metabolic Stress and Changes in Cellular pH

Hellen C Ishikawa-Ankerhold,Arzu S Kinali,Sophie Kurzbach,Annette Müller-Taubenberger

doi:10.3389/fcell.2021.742310

Hellen C Ishikawa-Ankerhold, Arzu S Kinali + Show 2 more

Open Access

https://doi.org/10.3389/fcell.2021.742310

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Abstract

Actin dynamics plays a crucial role in regulating essential cell functions and thereby is largely responsible to a considerable extent for cellular energy consumption. Certain pathological conditions in humans, like neurological disorders such as Alzheimer’s disease or amyotrophic lateral sclerosis (ALS) as well as variants of nemaline myopathy are associated with cytoskeletal abnormalities, so-called actin-cofilin rods. Actin-cofilin rods are aggregates consisting mainly of actin and cofilin, which are formed as a result of cellular stress and thereby help to ensure the survival of cells under unfavorable conditions. We have used Dictyostelium discoideum, an established model system for cytoskeletal research to study formation and principles of cytoplasmic actin rod assembly in response to energy depletion. Experimentally, depletion of ATP was provoked by addition of either sodium azide, dinitrophenol, or 2-deoxy-glucose, and the formation of rod assembly was recorded by live-cell imaging. Furthermore, we show that hyperosmotic shock induces actin-cofilin rods, and that a drop in the intracellular pH accompanies this condition. Our data reveal that acidification of the cytoplasm can induce the formation of actin-cofilin rods to varying degrees and suggest that a local reduction in cellular pH may be a cause for the formation of cytoplasmic rods. We hypothesize that local phase separation mechanistically triggers the assembly of actin-cofilin rods and thereby influences the material properties of actin structures.

Highlights

Actin is among the most abundant and most highly conserved proteins in eukaryotic cells, and as such essential for many processes including cell growth, differentiation, cell division, motility, intracellular movement, and mechanic stability
Earlier work conducted in our laboratory and data from other studies suggested that cytoplasmic actin-cofilin rods are induced by treatments that cause depletion of ATP (Ashworth et al, 2003; Bernstein and Bamburg, 2003; Bernstein et al, 2006; Huang et al, 2008)
First actin-cofilin rods become visible within 10 min of sodium azide treatment, and after 1 h, all cells are rounded up, and rods are detected in about 80% of cells

Summary

Introduction

Actin is among the most abundant and most highly conserved proteins in eukaryotic cells, and as such essential for many processes including cell growth, differentiation, cell division, motility, intracellular movement, and mechanic stability. To fulfil these different functions, actin assemblies form a variety of dynamically regulated membrane-associated or intracellular structures (Chhabra and Higgs, 2007), and after long debates on the existence of actin in the nucleus, the specific roles have been elucidated in recent years (Kelpsch and Tootle, 2018; Caridi et al, 2019; Hyrskyluoto and Vartiainen, 2020). The assembly of monomeric actin into filaments and the disassembly of filamentous actin involves a whole arsenal of actin-binding proteins that regulate actin functions including nucleation, sequestering, and crosslinking, and the highly energydependent dynamic turnover has been described as “actintreadmilling” (Pollard and Borisy, 2003)

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