Actin-Interacting Protein 1 Contributes to Intranuclear Rod Assembly in Dictyostelium discoideum

Prostaglandins limit nuclear actin rod formation during Drosophila oogenesis

Expression of GFP-Actin results in nuclear actin rod formation during specific stages of Drosophila melanogaster oogenesis. Loss of prostaglandin (PG) synthesis and signaling results in an increased frequency of cells with nuclear actin rods; there are less rods per cell, but the rods are longer. These findings suggest that loss of PGs results in increased nuclear actin and are consistent with prior findings assessing the roles of PGs in modulating endogenous nuclear actin. Thus, GFP-Actin rod formation can be used as a tool to screen for new regulators of nuclear actin.

Cofilin Acts as a Booster for Progression of Malignant Tumors Represented by Glioma

Cofilin, as a depolymerization factor of actin filaments, has been widely studied. Evidences show that cofilin has a role in actin structural reorganization and dynamic regulation. In recent years, several studies have demonstrated a regulatory role for cofilin in the migration and invasion mediated by cell dynamics and epithelial to mesenchymal transition (EMT)/EMT-like process, apoptosis, radiotherapy resistance, immune escape, and transcriptional dysregulation of malignant tumor cells, particularly glioma cells. On this basis, it is practical to evaluate cofilin as a biomarker for predicting tumor metastasis and prognosis. Targeting cofilin regulating kinases, Lin11, Isl-1 and Mec-3 kinases (LIM kinases/LIMKs) and their major upstream molecules inhibits tumor cell migration and invasion and targeting cofilin-mediated mitochondrial pathway induces apoptosis of tumor cells represent effective options for the development of novel anti-malignant tumor drug, especially anti-glioma drugs. This review explores the structure, general biological function, and regulation of cofilin, with an emphasis on the critical functions and prospects for clinical therapeutic applications of cofilin in malignant tumors represented by glioma.

Cytoskeletal dysregulation and neurodegenerative disease: Formation, monitoring, and inhibition of cofilin-actin rods

The presence of atypical cytoskeletal dynamics, structures, and associated morphologies is a common theme uniting numerous diseases and developmental disorders. In particular, cytoskeletal dysregulation is a common cellular feature of Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. While the numerous activators and inhibitors of dysregulation present complexities for characterizing these elements as byproducts or initiators of the disease state, it is increasingly clear that a better understanding of these anomalies is critical for advancing the state of knowledge and plan of therapeutic attack. In this review, we focus on the hallmarks of cytoskeletal dysregulation that are associated with cofilin-linked actin regulation, with a particular emphasis on the formation, monitoring, and inhibition of cofilin-actin rods. We also review actin-associated proteins other than cofilin with links to cytoskeleton-associated neurodegenerative processes, recognizing that cofilin-actin rods comprise one strand of a vast web of interactions that occur as a result of cytoskeletal dysregulation. Our aim is to present a current perspective on cytoskeletal dysregulation, connecting recent developments in our understanding with emerging strategies for biosensing and biomimicry that will help shape future directions of the field.

Formation of actin-cofilin rods by depletion forces

Various stress conditions induce the formation of actin-cofilin rods in either the nucleus or the cytoplasm, although the mechanism of rod formation is unclear. In this study, we constituted actin-cofilin rods using purified actin, cofilin and actin interacting protein 1 (AIP1) in the presence of a physiological buffer containing a crowding agent, 0.8% methylcellulose (MC), which led to bundled actin filaments formed by depletion forces. Most of the F-actin bundles formed with methylcellulose were linear, whereas cofilin-bound F-actin bundles often had bent, looped, and often ring-like shapes. Increasing the amount of AIP1 shortened actin-cofilin bundles into rod-like bundles with tapering at both ends. As much shorter actin-cofilin filaments were formed in the presence of AIP1 before MC was added to the mixture, the rod-like bundle might be a mass of those short filaments. Furthermore, the small rods fused with each other to become larger rods, indicating that these rods were anisotropic liquid droplets. Several minutes after the addition of MC to the F-actin-cofilin-AIP1 mixture, we observed some long bundles in which the thick and thin parts appear alternately, reminiscent of a Plateau-Rayleigh instability observed in fluid columns. Simultaneously, we found images in which thin parts were interrupted, but the thick parts were arranged in a row in the longitudinal direction. These structures were also observed in cytoplasmic actin-cofilin rods in cells overexpressing cofilin-GFP, suggesting that cytoplasmic actin-cofilin rods have the same structure formation process as the rods reconstituted in vitro.

Dictyostelium spastin is involved in nuclear envelope dynamics during semi-closed mitosis

Dictyostelium amoebae perform a semi-closed mitosis, in which the nuclear envelope is fenestrated at the insertion sites of the mitotic centrosomes and around the central spindle during karyokinesis. During late telophase the centrosome relocates to the cytoplasmic side of the nucleus, the central spindle disassembles and the nuclear fenestrae become closed. Our data indicate that Dictyostelium spastin (DdSpastin) is a microtubule-binding and severing type I membrane protein that plays a role in this process. Its mitotic localization is in agreement with a requirement for the removal of microtubules that would hinder closure of the fenestrae. Furthermore, DdSpastin interacts with the HeH/ LEM-family protein Src1 in BioID analyses as well as the inner nuclear membrane protein Sun1, and shows subcellular co-localizations with Src1, Sun1, the ESCRT component CHMP7 and the IST1-like protein filactin, suggesting that the principal pathway of mitotic nuclear envelope remodeling is conserved between animals and Dictyostelium amoebae.

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

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.

Cofilin and Actin Dynamics: Multiple Modes of Regulation and Their Impacts in Neuronal Development and Degeneration

Proteins of the actin depolymerizing factor (ADF)/cofilin family are ubiquitous among eukaryotes and are essential regulators of actin dynamics and function. Mammalian neurons express cofilin-1 as the major isoform, but ADF and cofilin-2 are also expressed. All isoforms bind preferentially and cooperatively along ADP-subunits in F-actin, affecting the filament helical rotation, and when either alone or when enhanced by other proteins, promotes filament severing and subunit turnover. Although self-regulating cofilin-mediated actin dynamics can drive motility without post-translational regulation, cells utilize many mechanisms to locally control cofilin, including cooperation/competition with other proteins. Newly identified post-translational modifications function with or are independent from the well-established phosphorylation of serine 3 and provide unexplored avenues for isoform specific regulation. Cofilin modulates actin transport and function in the nucleus as well as actin organization associated with mitochondrial fission and mitophagy. Under neuronal stress conditions, cofilin-saturated F-actin fragments can undergo oxidative cross-linking and bundle together to form cofilin-actin rods. Rods form in abundance within neurons around brain ischemic lesions and can be rapidly induced in neurites of most hippocampal and cortical neurons through energy depletion or glutamate-induced excitotoxicity. In ~20% of rodent hippocampal neurons, rods form more slowly in a receptor-mediated process triggered by factors intimately connected to disease-related dementias, e.g., amyloid-β in Alzheimer’s disease. This rod-inducing pathway requires a cellular prion protein, NADPH oxidase, and G-protein coupled receptors, e.g., CXCR4 and CCR5. Here, we will review many aspects of cofilin regulation and its contribution to synaptic loss and pathology of neurodegenerative diseases.

Profilin2 regulates actin rod assembly in neuronal cells

Nuclear and cytoplasmic actin-cofilin rods are formed transiently under stress conditions to reduce actin filament turnover and ATP hydrolysis. The persistence of these structures has been implicated in disease pathology of several neurological disorders. Recently, the presence of actin rods has been discovered in Spinal Muscular Atrophy (SMA), a neurodegenerative disease affecting predominantly motoneurons leading to muscle weakness and atrophy. This finding underlined the importance of dysregulated actin dynamics in motoneuron loss in SMA. In this study, we characterized actin rods formed in a SMA cell culture model analyzing their composition by LC–MS-based proteomics. Besides actin and cofilin, we identified proteins involved in processes such as ubiquitination, translation or protein folding to be bound to actin rods. This suggests their sequestration to actin rods, thus impairing important cellular functions. Moreover, we showed the involvement of the cytoskeletal protein profilin2 and its upstream effectors RhoA/ROCK in actin rod assembly in SMA. These findings implicate that the formation of actin rods exerts detrimental effects on motoneuron homeostasis by affecting actin dynamics and disturbing essential cellular pathways.

New Insights into Cellular Functions of Nuclear Actin

Simple Summary

It is well known that actin forms a cytoplasmic network of microfilaments, the part of the cytoskeleton, in the cytoplasm of eukaryotic cells. The presence of nuclear actin was elusive for a very long time. Now, there is a very strong evidence that actin plays many important roles in the nucleus. Here, we discuss the recently discovered functions of the nuclear actin pool. Actin does not have nuclear localization signal (NLS), so its import to the nucleus is facilitated by the NLS-containing proteins. Nuclear actin plays a role in the maintenance of the nuclear structure and the nuclear envelope breakdown. It is also involved in chromatin remodeling, and chromatin and nucleosome movement necessary for DNA recombination, repair, and the initiation of transcription. It also binds RNA polymerases, promoting transcription. Because of the multifaceted role of nuclear actin, the future challenge will be to further define its functions in various cellular processes and diseases.

Abstract

Actin is one of the most abundant proteins in eukaryotic cells. There are different pools of nuclear actin often undetectable by conventional staining and commercial antibodies used to identify cytoplasmic actin. With the development of more sophisticated imaging and analytical techniques, it became clear that nuclear actin plays a crucial role in shaping the chromatin, genomic, and epigenetic landscape, transcriptional regulation, and DNA repair. This multifaceted role of nuclear actin is not only important for the function of the individual cell but also for the establishment of cell fate, and tissue and organ differentiation during development. Moreover, the changes in the nuclear, chromatin, and genomic architecture are preamble to various diseases. Here, we discuss some of the newly described functions of nuclear actin.

Cofilin: A Promising Protein Implicated in Cancer Metastasis and Apoptosis

Cofilin is an actin-binding protein that regulates filament dynamics and depolymerization. The over-expression of cofilin is observed in various cancers, cofilin promotes cancer metastasis by regulating cytoskeletal reorganization, lamellipodium formation and epithelial-to-mesenchymal transition. Clinical treatment of cancer regarding cofilin has been explored in aspects of tumor cells apoptosis and cofilin related miRNAs. This review addresses the structure and phosphorylation of cofilin and describes recent findings regarding the function of cofilin in regulating cancer metastasis and apoptosis in tumor cells.

The actin cytoskeleton orchestra in Entamoeba histolytica

Years of evolution have kept actin conserved throughout various clades of life. It is an essential protein starring in many cellular processes. In a primitive eukaryote named Entamoeba histolytica, actin directs the process of phagocytosis. A finely tuned coordination between various actin-binding proteins (ABPs) choreographs this process and forms one of the virulence factors for this protist pathogen. The ever-expanding world of ABPs always has space to accommodate new and varied types of proteins to the earlier existing repertoire. In this article, we report the identification of 390 ABPs from Entamoeba histolytica. These proteins are part of diverse families that have been known to regulate actin dynamics. Most of the proteins are primarily uncharacterized in this organism; however, this study aims to annotate the ABPs based on their domain arrangements. A unique characteristic about some of the ABPs found is the combination of domains present in them unlike any other reported till date. Calponin domain-containing proteins formed the largest group among all types with 38 proteins, followed by 29 proteins with the infamous BAR domain in them, and 23 proteins belonging to actin-related proteins. The other protein families had a lesser number of members. Presence of exclusive domain arrangements in these proteins could guide us to yet unknown actin regulatory mechanisms prevalent in nature. This article is the first step to unraveling them.

Metalloprotease-mediated cleavage of PlexinD1 and its sequestration to actin rods in the motoneuron disease spinal muscular atrophy (SMA)

Cytoskeletal rearrangement during axon growth is mediated by guidance receptors and their ligands which act either as repellent, attractant or both. Regulation of the actin cytoskeleton is disturbed in Spinal Muscular Atrophy (SMA), a devastating neurodegenerative disease affecting mainly motoneurons, but receptor-ligand interactions leading to the dysregulation causing SMA are poorly understood. In this study, we analysed the role of the guidance receptor PlexinD1 in SMA pathogenesis. We showed that PlexinD1 is cleaved by metalloproteases in SMA and that this cleavage switches its function from an attractant to repellent. Moreover, we found that the PlexinD1 cleavage product binds to actin rods, pathological aggregate-like structures which had so far been described for age-related neurodegenerative diseases. Our data suggest a novel disease mechanism for SMA involving formation of actin rods as a molecular sink for a cleaved PlexinD1 fragment leading to dysregulation of receptor signaling.

Functions of actin-interacting protein 1 (AIP1)/WD repeat protein 1 (WDR1) in actin filament dynamics and cytoskeletal regulation

Actin-depolymerizing factor (ADF)/cofilin and actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), are conserved among eukaryotes and play critical roles in dynamic reorganization of the actin cytoskeleton. AIP1 preferentially promotes disassembly of ADF/cofilin-decorated actin filaments but exhibits minimal effects on bare actin filaments. Therefore, AIP1 has been often considered to be an ancillary co-factor of ADF/cofilin that merely boosts ADF/cofilin activity level. However, genetic and cell biological studies show that AIP1 deficiency often causes lethality or severe abnormalities in multiple tissues and organs including muscle, epithelia, and blood, suggesting that AIP1 is a major regulator of many biological processes that depend on actin dynamics. This review summarizes recent progress in studies on the biochemical mechanism of actin filament severing by AIP1 and in vivo functions of AIP1 in model organisms and human diseases.