Chronophin mediates an ATP-sensing mechanism for cofilin dephosphorylation and neuronal cofilin-actin rod formation

Bioenergetic and excitotoxic determinants of cofilactin rod formation

Cofilactin rods (CARs), which are 1:1 aggregates of cofilin-1 and actin, lead to neurite loss in ischemic stroke and other disorders. The biochemical pathways driving CAR formation are well-established, but how these pathways are engaged under ischemic conditions is less clear. Brain ischemia produces both ATP depletion and glutamate excitotoxicity, both of which have been shown to drive CAR formation in other settings. Here, we show that CARs are formed in cultured neurons exposed to ischemia-like conditions: oxygen-glucose deprivation (OGD), glutamate, or oxidative stress. Of these conditions, only OGD produced significant ATP depletion, showing that ATP depletion is not required for CAR formation. Moreover, the OGD-induced CAR formation was blocked by the glutamate receptor antagonists MK-801 and kynurenic acid; the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors GSK2795039 and apocynin; as well as an ROS scavenger. The findings identify a biochemical pathway leading from OGD to CAR formation in which the glutamate release induced by energy failure leads to activation of neuronal glutamate receptors, which in turn activates NADPH oxidase to generate oxidative stress and CARs.

Resveratrol Preconditioning Protects Against Ischemia-Induced Synaptic Dysfunction and Cofilin Hyperactivation in the Mouse Hippocampal Slice

Perturbations in synaptic function are major determinants of several neurological diseases and have been associated with cognitive impairments after cerebral ischemia (CI). Although the mechanisms underlying CI-induced synaptic dysfunction have not been well defined, evidence suggests that early hyperactivation of the actin-binding protein, cofilin, plays a role. Given that synaptic impairments manifest shortly after CI, prophylactic strategies may offer a better approach to prevent/mitigate synaptic damage following an ischemic event. Our laboratory has previously demonstrated that resveratrol preconditioning (RPC) promotes cerebral ischemic tolerance, with many groups highlighting beneficial effects of resveratrol treatment on synaptic and cognitive function in other neurological conditions. Herein, we hypothesized that RPC would mitigate hippocampal synaptic dysfunction and pathological cofilin hyperactivation in an ex vivo model of ischemia. Various electrophysiological parameters and synaptic-related protein expression changes were measured under normal and ischemic conditions utilizing acute hippocampal slices derived from adult male mice treated with resveratrol (10 mg/kg) or vehicle 48 h prior. Remarkably, RPC significantly increased the latency to anoxic depolarization, decreased cytosolic calcium accumulation, prevented aberrant increases in synaptic transmission, and rescued deficits in long-term potentiation following ischemia. Additionally, RPC upregulated the expression of the activity-regulated cytoskeleton associated protein, Arc, which was partially required for RPC-mediated attenuation of cofilin hyperactivation. Taken together, these findings support a role for RPC in mitigating CI-induced excitotoxicity, synaptic dysfunction, and pathological over-activation of cofilin. Our study provides further insight into mechanisms underlying RPC-mediated neuroprotection against CI and implicates RPC as a promising strategy to preserve synaptic function after ischemia.

Cofilin Inhibitor Protects against Traumatic Brain Injury-Induced Oxidative Stress and Neuroinflammation

Microglial activation and failure of the antioxidant defense mechanisms are major hallmarks in different brain injuries, particularly traumatic brain injury (TBI). Cofilin is a cytoskeleton-associated protein involved in actin binding and severing. In our previous studies, we identified the putative role of cofilin in mediating microglial activation and apoptosis in ischemic and hemorrhagic conditions. Others have highlighted the involvement of cofilin in ROS production and the resultant neuronal death; however, more studies are needed to delineate the role of cofilin in oxidative stress conditions. The present study aims to investigate the cellular and molecular effects of cofilin in TBI using both in vitro and in vivo models as well as the first-in-class small-molecule cofilin inhibitor (CI). An in vitro H2O2-induced oxidative stress model was used in two different types of cells, human neuroblastoma (SH-SY5Y) and microglia (HMC3), along with an in vivo controlled cortical impact model of TBI. Our results show that treatment with H2O2 increases the expression of cofilin and slingshot-1 (SSH-1), an upstream regulator of cofilin, in microglial cells, which was significantly reduced in the CI-treated group. Cofilin inhibition significantly attenuated H2O2-induced microglial activation by reducing the release of proinflammatory mediators. Furthermore, we demonstrate that CI protects against H2O2-induced ROS accumulation and neuronal cytotoxicity, activates the AKT signaling pathway by increasing its phosphorylation, and modulates mitochondrial-related apoptogenic factors. The expression of NF-E2-related factor 2 (Nrf2) and its associated antioxidant enzymes were also increased in CI-treated SY-SY5Y. In the mice model of TBI, CI significantly activated the Nrf2 and reduced the expression of oxidative/nitrosative stress markers at the protein and gene levels. Together, our data suggest that cofilin inhibition provides a neuroprotective effect in in vitro and in vivo TBI mice models by inhibiting oxidative stress and inflammatory responses, the pivotal mechanisms involved in TBI-induced brain damage.

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.

Characterization of phalloidin-negative nuclear actin filaments in U2OS cells expressing cytoplasmic actin-EGFP

Actin is a major structural component of the cytoskeleton in eukaryotic cells including fungi, plants and animals, and exists not only in the cytoplasm as cytoskeleton but also in the nucleus. Recently, we developed a novel actin probe, β-actin-EGFP fusion protein, which exhibited similar monomeric to filamentous ratio as that of endogenous actin, in contrast to the widely used EGFP-β-actin fusion protein that over-assembles in cells. Unexpectedly, this novel probe visualized an interconnected meshwork of slightly curved beam-like bundles of actin filaments in the nucleus of U2OS cells. These structures were not labeled with rhodamine phalloidin, Lifeact-EGFP or anti-actin antibodies. In addition, immunofluorescence staining and expression of cofilin-EGFP revealed that this nuclear actin structures contained cofilin. We named these actin filaments as phalloidin negative intranuclear (PHANIN) actin filaments. Since PHANIN actin filaments could not be detected by general detection methods for actin filaments, we propose that PHANIN actin filaments are different from previously reported nuclear actin structures.

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.

Cofilin and Neurodegeneration: New Functions for an Old but Gold Protein

Cofilin is an actin-binding protein that plays a major role in the regulation of actin dynamics, an essential cellular process. This protein has emerged as a crucial molecule for functions of the nervous system including motility and guidance of the neuronal growth cone, dendritic spine organization, axonal branching, and synaptic signalling. Recently, other important functions in cell biology such as apoptosis or the control of mitochondrial function have been attributed to cofilin. Moreover, novel mechanisms of cofilin function regulation have also been described. The activity of cofilin is controlled by complex regulatory mechanisms, with phosphorylation being the most important, since the addition of a phosphate group to cofilin renders it inactive. Due to its participation in a wide variety of key processes in the cell, cofilin has been related to a great variety of pathologies, among which neurodegenerative diseases have attracted great interest. In this review, we summarized the functions of cofilin and its regulation, emphasizing how defects in these processes have been related to different neurodegenerative diseases.

Differential mechanisms of synaptic plasticity for susceptibility and resilience to chronic social defeat stress in male mice

Mood dysregulation refers to the inability of a person to control their negative emotions, and it is linked to various stressful experiences. Dysregulated neural synaptic plasticity and actin-filament dynamics are important regulators of stress response in animal models. However, until now, there is no evidence to differential the mechanisms of synaptic plasticity and actin-filament dynamics in stress susceptibility and stress-resistant. Here we found that depression-like behaviour was observed in the susceptible group following chronic social defeat stress (CSDS) exposure, but not in stress-resistant mice. High-frequency stimulation-induced long-term potentiation (LTP) was impaired in the CSDS-induced depression-susceptible group. Further, the levels of pro-brain derived neurotrophic factor (BDNF), mature BDNF, PSD-95, phosphorylated CaMKII, and phosphorylated Cofilin, an actin-filament dynamics regulator, were reduced in CSDS-induced depression-susceptible mice unlike in stress-resistant mice. These results demonstrate that synaptic plasticity-related molecules, such as BDNF and phosphorylated Cofilin, are important for maintaining synaptic functions and structure in mice that experience more stress.

Cofilin, a Master Node Regulating Cytoskeletal Pathogenesis in Alzheimer's Disease

 The defining pathological hallmarks of Alzheimer’s disease (AD) are proteinopathies marked by the amyloid-β (Aβ) peptide and hyperphosphorylated tau. In addition, Hirano bodies and cofilin-actin rods are extensively found in AD brains, both of which are associated with the actin cytoskeleton. The actin-binding protein cofilin known for its actin filament severing, depolymerizing, nucleating, and bundling activities has emerged as a significant player in AD pathogenesis. In this review, we discuss the regulation of cofilin by multiple signaling events impinging on LIM kinase-1 (LIMK1) and/or Slingshot homolog-1 (SSH1) downstream of Aβ. Such pathophysiological signaling pathways impact actin dynamics to regulate synaptic integrity, mitochondrial translocation of cofilin to promote neurotoxicity, and formation of cofilin-actin pathology. Other intracellular signaling proteins, such as β-arrestin, RanBP9, Chronophin, PLD1, and 14-3-3 also impinge on the regulation of cofilin downstream of Aβ. Finally, we discuss the role of activated cofilin as a bridge between actin and microtubule dynamics by displacing tau from microtubules, thereby destabilizing tau-induced microtubule assembly, missorting tau, and promoting tauopathy.

Role of Cofilin in Alzheimer's Disease

Alzheimer's disease (AD) is a degenerative neurological disease and has an inconspicuous onset and progressive development. Clinically, it is characterized by severe dementia manifestations, including memory impairment, aphasia, apraxia, loss of recognition, impairment of visual-spatial skills, executive dysfunction, and changes in personality and behavior. Its etiology is unknown to date. However, several cellular biological signatures of AD have been identified such as synaptic dysfunction, β-amyloid plaques, hyperphosphorylated tau, cofilin-actin rods, and Hirano bodies which are related to the actin cytoskeleton. Cofilin is one of the most affluent and common actin-binding proteins and plays a role in cell motility, migration, shape, and metabolism. They also play an important role in severing actin filament, nucleating, depolymerizing, and bundling activities. In this review, we summarize the structure of cofilins and their functional and regulating roles, focusing on the synaptic dysfunction, β-amyloid plaques, hyperphosphorylated tau, cofilin-actin rods, and Hirano bodies of AD.

PD-1 suppresses the maintenance of cell couples between cytotoxic T cells and target tumor cells within the tumor

The killing of tumor cells by CD8⁺ T cells is suppressed by the tumor microenvironment, and increased expression of inhibitory receptors, including programmed cell death protein-1 (PD-1), is associated with tumor-mediated suppression of T cells. To find cellular defects triggered by tumor exposure and associated PD-1 signaling, we established an ex vivo imaging approach to investigate the response of antigen-specific, activated effector CD8⁺ tumor-infiltrating lymphocytes (TILs) after interaction with target tumor cells. Although TIL-tumor cell couples readily formed, couple stability deteriorated within minutes. This was associated with impaired F-actin clearing from the center of the cellular interface, reduced Ca²⁺ signaling, increased TIL locomotion, and impaired tumor cell killing. The interaction of CD8⁺ T lymphocytes with tumor cell spheroids in vitro induced a similar phenotype, supporting a critical role of direct T cell-tumor cell contact. Diminished engagement of PD-1 within the tumor, but not acute ex vivo blockade, partially restored cell couple maintenance and killing. PD-1 thus contributes to the suppression of TIL function by inducing a state of impaired subcellular organization.

Cofilin-Mediated Actin Stress Response Is Maladaptive in Heat-Stressed Embryos

Environmental stress threatens the fidelity of embryonic morphogenesis. Heat, for example, is a teratogen. Yet how heat affects morphogenesis is poorly understood. Here, we identify a heat-inducible actin stress response (ASR) in Drosophila embryos that is mediated by the activation of the actin regulator Cofilin. Similar to ASR in adult mammalian cells, heat stress in fly embryos triggers the assembly of intra-nuclear actin rods. Rods measure up to a few microns in length, and their assembly depends on elevated free nuclear actin concentration and Cofilin. Outside the nucleus, heat stress causes Cofilin-dependent destabilization of filamentous actin (F-actin) in actomyosin networks required for morphogenesis. F-actin destabilization increases the chance of morphogenesis mistakes. Blocking the ASR by reducing Cofilin dosage improves the viability of heat-stressed embryos. However, improved viability correlates with restoring F-actin stability, not rescuing morphogenesis. Thus, ASR endangers embryos, perhaps by shifting actin from cytoplasmic filaments to an elevated nuclear pool.

Figard et al. show that heat stress induces an actin stress response (ASR) in early Drosophila embryos. This ASR is mediated by a heat-induced increase in Cofilin activity. Increased Cofilin activity destabilizes F-actin structures required for morphogenesis. In addition, the Cofilin-mediated ASR reduces embryo viability.

Ca2+ functions as a molecular switch that controls the mutually exclusive complex formation of pyridoxal phosphatase with CIB1 or calmodulin

Pyridoxal 5'-phosphate (PLP) is an essential cofactor for neurotransmitter metabolism. Pyridoxal phosphatase (PDXP) deficiency in mice increases PLP and γ-aminobutyric acid levels in the brain, yet how PDXP is regulated is unclear. Here, we identify the Ca2+ - and integrin-binding protein 1 (CIB1) as a PDXP interactor by yeast two-hybrid screening and find a calmodulin (CaM)-binding motif that overlaps with the PDXP-CIB1 interaction site. Pulldown and crosslinking assays with purified proteins demonstrate that PDXP directly binds to CIB1 or CaM. CIB1 or CaM does not alter PDXP phosphatase activity. However, elevated Ca2+ concentrations promote CaM binding and, thereby, diminish CIB1 binding to PDXP, as both interactors bind in a mutually exclusive way. Hence, the PDXP-CIB1 complex may functionally differ from the PDXP-Ca2+ -CaM complex.

Cofilin dysregulation alters actin turnover in frataxin-deficient neurons

Abnormalities in actin cytoskeleton have been linked to Friedreich's ataxia (FRDA), an inherited peripheral neuropathy characterised by an early loss of neurons in dorsal root ganglia (DRG) among other clinical symptoms. Despite all efforts to date, we still do not fully understand the molecular events that contribute to the lack of sensory neurons in FRDA. We studied the adult neuronal growth cone (GC) at the cellular and molecular level to decipher the connection between frataxin and actin cytoskeleton in DRG neurons of the well-characterised YG8R Friedreich's ataxia mouse model. Immunofluorescence studies in primary cultures of DRG from YG8R mice showed neurons with fewer and smaller GCs than controls, associated with an inhibition of neurite growth. In frataxin-deficient neurons, we also observed an increase in the filamentous (F)-actin/monomeric (G)-actin ratio (F/G-actin ratio) in axons and GCs linked to dysregulation of two crucial modulators of filamentous actin turnover, cofilin-1 and the actin-related protein (ARP) 2/3 complex. We show how the activation of cofilin is due to the increase in chronophin (CIN), a cofilin-activating phosphatase. Thus cofilin emerges, for the first time, as a link between frataxin deficiency and actin cytoskeleton alterations.

Actin(g) on mitochondria - a role for cofilin1 in neuronal cell death pathways

Actin dynamics, the coordinated assembly and disassembly of actin filaments (F-actin), are essential for fundamental cellular processes, including cell shaping and motility, cell division or organelle transport. Recent studies highlighted a novel role for actin dynamics in the regulation of mitochondrial morphology and function, for example, through mitochondrial recruitment of dynamin-related protein 1 (Drp1), a key factor in the mitochondrial fission machinery. Mitochondria are dynamic organelles, and permanent fission and fusion is essential to maintain their function in energy metabolism, calcium homeostasis and regulation of reactive oxygen species (ROS). Here, we summarize recent insights into the emerging role of cofilin1, a key regulator of actin dynamics, for mitochondrial shape and function under physiological conditions and during cellular stress, respectively. This is of peculiar importance in neurons, which are particularly prone to changes in actin regulation and mitochondrial integrity and function. In neurons, cofilin1 may contribute to degenerative processes through formation of cofilin-actin rods, and through enhanced mitochondrial fission, mitochondrial membrane permeabilization, and the release of cytochrome c. Overall, mitochondrial impairment induced by dysfunction of actin-regulating proteins such as cofilin1 emerge as important mechanisms of neuronal death with relevance to acute brain injury and neurodegenerative diseases, such as Parkinson's or Alzheimer's disease.

Much More Than a Scaffold: Cytoskeletal Proteins in Neurological Disorders

Recent observations related to the structure of the cytoskeleton in neurons and novel cytoskeletal abnormalities involved in the pathophysiology of some neurological diseases are changing our view on the function of the cytoskeletal proteins in the nervous system. These efforts allow a better understanding of the molecular mechanisms underlying neurological diseases and allow us to see beyond our current knowledge for the development of new treatments. The neuronal cytoskeleton can be described as an organelle formed by the three-dimensional lattice of the three main families of filaments: actin filaments, microtubules, and neurofilaments. This organelle organizes well-defined structures within neurons (cell bodies and axons), which allow their proper development and function through life. Here, we will provide an overview of both the basic and novel concepts related to those cytoskeletal proteins, which are emerging as potential targets in the study of the pathophysiological mechanisms underlying neurological disorders.

Brain ischemic insult induces cofilin rod formation leading to synaptic dysfunction in neurons

Ischemic stroke not only induces neuron death in the infarct area but also structural and functional damage of the surviving neurons in the surrounding peri-infarct area. In the present study, we first identified cofilin rod, a pathological rod-like aggregation, formed in neurons of in vivo ischemic stroke animal model and induced neuronal impairment. Cofilin rods formed only on the ipsilateral side of the middle cerebral artery occlusion and reperfusion (MCAO-R) rat brain and showed the highest density in peri-infarct area. Our real-time live cell imaging, immunostaining and patch clamp studies showed that cofilin rod formation in neurons led to dendritic mitochondrial transportation failure, as well as impairment of synaptic structure and functions. Overexpression of LIM kinase or activation of its upstream regulator Rho, suppressed ischemia-induced cofilin rod formation and showed protective effect on synaptic function and structure impairment in both cultured neurons and MCAO-R rat model. In summary, our results demonstrate a novel mechanism of ischemic stroke-induced neuron injury in peri-infarct area and provide a potential target for the protection of neuronal structure and function against brain ischemia insult.

Dual role of cofilin in APP trafficking and amyloid-β clearance

The accumulation of amyloid-β (Aβ) plays a pivotal early event in the pathogenesis of Alzheimer's disease (AD). In the brain, neurons produce Aβ by the proteolytic processing of amyloid precursor protein (APP) through the endocytic pathway, whereas microglia mediate Aβ clearance also via endocytic mechanisms. Previous studies have shown the critical importance of cofilin, a filamentous actin-severing protein, in actin dynamics and pathogen-triggered endocytic processes. Moreover, the binding of Aβ42 oligomers to β1-integrin triggers the cofilin activation, and in turn, cofilin promotes the internalization of surface β1-integrin. However, a role for cofilin in APP processing and Aβ metabolism has not been investigated. In this study, we found that knockdown of cofilin in Chinese hamster ovary 7WD10 cells and primary neurons significantly reduces Aβ production by increasing surface APP (sAPP) levels. Expression of active (S3A) but not inactive (S3E) cofilin reduces sAPP levels by enhancing APP endocytosis. Accordingly, Aβ deposition in APP and presenilin 1 (PS1) transgenic mice is significantly reduced by genetic reduction of cofilin (APP/PS1;cofilin^+/-). However, the reduction of Aβ load in APP/PS1;cofilin^+/- mice is paradoxically associated with significantly increased ionized calcium-binding adaptor molecule 1-positive microglial activation surrounding Aβ deposits. Primary microglia isolated from cofilin^+/- mice demonstrate significantly enhanced state of activation and greater ability to uptake and clear Aβ42, which is reversed with the active (S3A) but not inactive (S3E) form of cofilin. These results taken together indicate a significant role for cofilin in Aβ accumulation via dual and opposing endocytic mechanisms of promoting Aβ production in neurons and inhibiting Aβ clearance in microglia.-Liu, T., Woo, J.-A. A., Yan, Y., LePochat, P., Bukhari, M. Z., Kang, D. E. Dual role of cofilin in APP trafficking and amyloid-β clearance.

Nuclear Actin: From Discovery to Function

While actin was discovered in the nucleus over 50 years ago, research lagged for decades due to strong skepticism. The revitalization of research into nuclear actin occurred after it was found that cellular stresses induce the nuclear localization and alter the structure of actin. These studies provided the first hints that actin has a nuclear function. Subsequently, it was established that the nuclear import and export of actin is highly regulated. While the structures of nuclear actin remain unclear, it can function as monomers, polymers, and even rods. Furthermore, even within a given structure, distinct pools of nuclear actin that can be differentially labeled have been identified. Numerous mechanistic studies have uncovered an array of functions for nuclear actin. It regulates the activity of RNA polymerases, as well as specific transcription factors. Actin also modulates the activity of several chromatin remodeling complexes and histone deacetylases, to ultimately impinge on transcriptional programing and DNA damage repair. Further, nuclear actin mediates chromatin movement and organization. It has roles in meiosis and mitosis, and these functions may be functionally conserved from ancient bacterial actin homologs. The structure and integrity of the nuclear envelope and sub-nuclear compartments are also regulated by nuclear actin. Furthermore, nuclear actin contributes to human diseases like cancer, neurodegeneration, and myopathies. Here, we explore the early discovery of actin in the nucleus and discuss the forms and functions of nuclear actin in both normal and disease contexts. Anat Rec, 301:1999-2013, 2018. © 2018 Wiley Periodicals, Inc.

Cofilin-actin rod formation in neuronal processes after brain ischemia

Functional impairment after brain ischemia results in part from loss of neuronal spines and dendrites, independent of neuronal death. Cofilin-actin rods are covalently linked aggregates of cofilin-1 and actin that form in neuronal processes (neurites) under conditions of ATP depletion and oxidative stress, and which cause neurite degeneration if not disassembled. ATP depletion and oxidative stress occur with differing severity, duration, and time course in different ischemic conditions. Here we evaluated four mouse models of brain ischemia to define the conditions that drive formation of cofilin-actin rods. Three of the models provide early reperfusion: transient middle cerebral artery occlusion (MCAo), transient bilateral common carotid artery occlusion (CCAo), and cardiac arrest / cardiopulmonary resuscitation (CA/CPR). Early reperfusion restores ATP generating capacity, but also induces oxidative stress. The fourth model, photothrombotic cortical infarction, does not provide reperfusion. Cofilin-actin rods were formed in each of these models, but with differing patterns. Where acute reperfusion occurred, rod formation was maximal within 4 hours after reperfusion. Where infarction occurred, rods continued to form for at least 24 hours after ischemic onset, and extended into the adjacent non-ischemic tissue. Interventions that limit cofilin-actin rod formation may help to preserve integrity of neuronal processes in permanent ischemia.

Binucleate germ cells in Caenorhabditis elegans are removed by physiological apoptosis

Cell death plays a major role during C. elegans oogenesis, where over half of the oogenic germ cells die in a process termed physiological apoptosis. How germ cells are selected for physiological apoptosis, or instead become oocytes, is not understood. Most oocytes produce viable embryos when apoptosis is blocked, suggesting that physiological apoptosis does not function to cull defective germ cells. Instead, cells targeted for apoptosis may function as nurse cells; the germline is syncytial, and all germ cells appear to contribute cytoplasm to developing oocytes. C. elegans has been a leading model for the genetics and molecular biology of apoptosis and phagocytosis, but comparatively few studies have examined the cell biology of apoptotic cells. We used live imaging to identify and examine pre-apoptotic germ cells in the adult gonad. After initiating apoptosis, germ cells selectively export their mitochondria into the shared pool of syncytial cytoplasm; this transport appears to use the microtubule motor kinesin. The apoptotic cells then shrink as they expel most of their remaining cytoplasm, and close off from the syncytium. Shortly thereafter the apoptotic cells restructure their microtubule and actin cytoskeletons, possibly to maintain cell integrity; the microtubules form a novel, cortical array of stabilized microtubules, and actin and cofilin organize into giant cofilin-actin rods. We discovered that some apoptotic germ cells are binucleate; the binucleate germ cells can develop into binucleate oocytes in apoptosis-defective strains, and appear capable of producing triploid offspring. Our results suggest that the nuclear layer of the germline syncytium becomes folded during mitosis and growth, and that binucleate cells arise as the layer unfolds or everts; all of the binucleate cells are subsequently removed by apoptosis. These results show that physiological apoptosis targets at least two distinct populations of germ cells, and that the apoptosis machinery efficiently recognizes cells with two nuclei.

Many germ cells die by apoptosis during the development of animal oocytes, including more than half of all germ cells in the model system C. elegans. How individual germ cells are selected for apoptosis, or survival, is not known. Here we study the cell biology of apoptosis. The C. elegans gonad is a syncytium, with nearly 1000 germ “cells” connected to a shared, core cytoplasm. Once apoptosis is initiated, germ cells selectively transport their mitochondria into the gonad core, apparently using the microtubule motor protein kinesin. The apoptotic cells next constrict, expelling most of their remaining cytoplasm into the core, and close off from the gonad core. The microtubule and actin cytoskeletons are remodeled and stabilized, presumably to maintain the integrity of the dying cell. The apoptotic cells form giant cofilin-actin rods, similar to rods described in stressed cultured cells and in human myopathies and neuropathies such as Alzheimer’s and Huntington’s disease. We show that some germ cells are binucleate; these cells appear to form during germline morphogenesis, and are removed by apoptosis. These results demonstrate heterogeneity between oogenic germ cells, and show that the apoptosis machinery efficiently recognizes and removes cells with two nuclei.

Chronophin activation is necessary in Doxorubicin-induced actin cytoskeleton alteration

Although doxorubicin (Dox)-induced oxidative stress is known to be associated with cytotoxicity, the precise mechanism remains unclear. Genotoxic stress not only generates free radicals, but also affects actin cytoskeleton stability. We showed that Dox-induced RhoA signaling stimulated actin cytoskeleton alterations, resulting in central stress fiber disruption at early time points and cell periphery cortical actin formation at a later stage, in HeLa cells. Interestingly, activation of a cofilin phosphatase, chronophin (CIN), was initially evoked by Dox-induced RhoA signaling, resulting in a rapid phosphorylated cofilin turnover leading to actin cytoskeleton remodeling. In addition, a novel interaction between CIN and 14-3-3ζ was detected in the absence of Dox treatment. We demonstrated that CIN activity is quite contrary to 14-3-3ζ binding, and the interaction leads to enhanced phosphorylated cofilin levels. Therefore, initial CIN activation regulation could be critical in Dox-induced actin cytoskeleton remodeling through RhoA/cofilin signaling. [BMB Reports 2017; 50(6): 335-340].

Wild-type p53 enhances endothelial barrier function by mediating RAC1 signalling and RhoA inhibition

Inflammation is the major cause of endothelial barrier hyper‐permeability, associated with acute lung injury and acute respiratory distress syndrome. This study reports that p53 “orchestrates” the defence of vascular endothelium against LPS, by mediating the opposing actions of Rac1 and RhoA in pulmonary tissues. Human lung microvascular endothelial cells treated with HSP90 inhibitors activated both Rac1‐ and P21‐activated kinase, which is an essential element of vascular barrier function. 17AAG increased the phosphorylation of both LIMK and cofilin, in contrast to LPS which counteracted those effects. Mouse lung microvascular endothelial cells exposed to LPS exhibited decreased expression of phospho‐cofilin. 17AAG treatment resulted in reduced levels of active cofilin. Silencing of cofilin pyridoxal phosphate phosphatase (PDXP) blocked the LPS‐induced hyper‐permeability, and P53 inhibition reversed the 17AAG‐induced PDXP down‐regulation. P190RHOGAP suppression enhanced the LPS‐triggered barrier dysfunction in endothelial monolayers. 17AAG treatment resulted in P190RHOGAP induction and blocked the LPS‐induced pMLC2 up‐regulation in wild‐type mice. Pulmonary endothelial cells from “super p53” mice, which carry additional p53‐tg alleles, exhibited a lower response to LPS than the controls. Collectively, our findings help elucidate the mechanisms by which p53 operates to enhance barrier function.

Social isolation suppresses actin dynamics and synaptic plasticity through ADF/cofilin inactivation in the developing rat barrel cortex

Exposure to a stressful environment early in life can cause psychiatric disorders by disrupting circuit formation. Actin plays central roles in regulating neuronal structure and protein trafficking. We have recently reported that neonatal isolation inactivated ADF/cofilin, the actin depolymerizing factor, resulted in a reduced actin dynamics at spines and an attenuation of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor delivery in the juvenile rat medial prefrontal cortex (mPFC), leading to altered social behaviours. Here, we investigated the impact of neonatal social isolation in the developing rat barrel cortex. Similar to the mPFC study, we detected an increase in stable actin fraction in spines and this resulted in a decreased synaptic AMPA receptor delivery. Thus, we conclude that early life social isolation affects multiple cortical areas with common molecular changes.

Decelerated neurodegeneration after intravitreal injection of α-synuclein antibodies in a glaucoma animal model

Although elevated intraocular pressure (IOP) remains the major risk factor in glaucoma, neurodegenerative processes continue despite effective IOP lowering. Altered α-synuclein antibody (Abs) levels have been reported to play a crucial role. This study aimed at identifying whether α-synuclein Abs are capable to decelerate neuronal decay while providing insights into proteomic changes. Four groups of Sprague Dawley rats received episcleral vein occlusion: (1) CTRL, no intravitreal injection, n = 6, (2) CTRL IgG, intravitreal injection of unspecific IgG, n = 5, (3) Buffer, intravitreal injection of buffer, n = 6, (4), α-synuclein Ab, intravitreal injection of α-synuclein Ab, n = 5. IOP and retinal nerve fiber layer thickness (RNFLT) were monitored and immunohistochemistry, microarray and proteomic analysis were performed. RNFLT was reduced in CTRL, CTRL IgG and Buffer group (all p < 0.01) and α-synuclein Ab group (p = 0.17). Axon and RGC density showed an increased neurodegeneration in CTRL, CTRL IgG and Buffer group (all p < 0.01) and increased neuronal survival in α-synuclein Ab group (p = 0.38 and 0.06, respectively) compared with fellow eyes. Proteomic analysis revealed alterations of cofilin 1 and superoxide dismutase 1 expression. This data indicate that α-synuclein Ab might indirectly modulate the actin cytoskeleton organization and negatively regulate apoptotic processes via cofilin 1 and superoxide dismutase 1.

Peptide regulation of cofilin activity in the CNS: A novel therapeutic approach for treatment of multiple neurological disorders

Cofilin is a ubiquitous protein which cooperates with many other actin-binding proteins in regulating actin dynamics. Cofilin has essential functions in nervous system development including neuritogenesis, neurite elongation, growth cone pathfinding, dendritic spine formation, and the regulation of neurotransmission and spine function, components of synaptic plasticity essential for learning and memory. Cofilin's phosphoregulation is a downstream target of many transmembrane signaling processes, and its misregulation in neurons has been linked in rodent models to many different neurodegenerative and neurological disorders including Alzheimer disease (AD), aggression due to neonatal isolation, autism, manic/bipolar disorder, and sleep deprivation. Cognitive and behavioral deficits of these rodent models have been largely abrogated by modulation of cofilin activity using viral-mediated, genetic, and/or small molecule or peptide therapeutic approaches. Neuropathic pain in rats from sciatic nerve compression has also been reduced by modulating the cofilin pathway within neurons of the dorsal root ganglia. Neuroinflammation, which occurs following cerebral ischemia/reperfusion, but which also accompanies many other neurodegenerative syndromes, is markedly reduced by peptides targeting specific chemokine receptors, which also modulate cofilin activity. Thus, peptide therapeutics offer potential for cost-effective treatment of a wide variety of neurological disorders. Here we discuss some recent results from rodent models using therapeutic peptides with a surprising ability to cross the rodent blood brain barrier and alter cofilin activity in brain. We also offer suggestions as to how neuronal-specific cofilin regulation might be achieved.

Cytoskeleton-Associated Risk Modifiers Involved in Early and Rapid Progression of Sporadic Creutzfeldt-Jakob Disease

A high priority in the prion field is to identify pre-symptomatic events and associated profile of molecular changes. In this study, we demonstrate the pre-symptomatic dysregulation of cytoskeleton assembly and its associated cofilin-1 pathway in strain and brain region-specific manners in MM1 and VV2 subtype-specific Creutzfeldt-Jakob disease at clinical and pre-clinical stage. At physiological level, PrP^C interaction with cofilin-1 and phosphorylated form of cofilin (p-cofilin(Ser3)) was investigated in primary cultures of mouse cortex neurons (PCNs) of PrP^C wild-type and knockout mice (PrP^-/-). Short-interfering RNA downregulation of active form of cofilin-1 resulted in the redistribution/downregulation of PrP^C, increase of activated form of microglia, accumulation of dense form of F-actin, and upregulation of p-cofilin(Ser3). This upregulated p-cofilin(Ser3) showed redistribution of expression predominantly in the activated form of microglia in PCNs. At pathological level, cofilin-1 expression was significantly altered in cortex and cerebellum in both humans and mice at pre-clinical stage and at early symptomatic clinical stage of the disease. Further, to better understand the possible mechanism of dysregulation of cofilin-1, we also demonstrated alterations in upstream regulators; LIM kinase isoform 1 (LIMK1), slingshot phosphatase isoform 1 (SSH1), RhoA-associated kinase (Rock2), and amyloid precursor protein (APP) in sporadic Creutzfeldt-Jakob disease MM1 mice and in human MM1 and VV2 frontal cortex and cerebellum samples. In conclusion, our findings demonstrated for the first time a key pre-clinical response of cofilin-1 and the associated pathway in prion disease.

Cofilin Mediates LPS-Induced Microglial Cell Activation and Associated Neurotoxicity Through Activation of NF-κB and JAK-STAT Pathway

Microglial cells are activated in response to different types of injuries or stress in the CNS. Such activation is necessary to get rid of the injurious agents and restore tissue homeostasis. However, excessive activation of microglial cells is harmful and contributes to secondary injury. Pertinently, microglial cell activity was targeted in many preclinical and clinical studies but such strategy failed in clinical trials. The main reason behind the failed attempts is the complexity of the injury mechanisms which needs either a combination therapy or targeting a process that is involved in multiple pathways. Cofilin is a cytoskeleton-associated protein involved in actin dynamics. In our previous study, we demonstrated the role of cofilin in mediating neuronal apoptosis during OGD conditions. Previous studies on microglia have shown the involvement of cofilin in ROS formation and phagocytosis. However, additional studies are needed to delineate the role of cofilin in microglial cell activation. Therefore, in the current study, we investigated the role of cofilin in LPS-induced microglial cell activation using cofilin siRNA knockdown paradigms. The viability of differentiated PC12 cells was used as a measure of the neurotoxic potential of conditioned medium derived from cofilin siRNA-transfected and LPS-activated microglial cells. Cofilin knockdown significantly inhibited LPS-induced microglial cell activation through NF-κB and JAK-STAT pathways. The release of proinflammatory mediators (NO, TNF-α, iNOS, and COX2) as well as microglial proliferation and migration rates were significantly reduced by cofilin knockdown. Furthermore, differentiated PC12 cells were protected from the neurotoxicity induced by conditioned medium derived from cofilin-transfected and LPS-activated microglial cells. In conclusion, we demonstrated that cofilin is involved in the cascade of microglial cell activation and further validates our previous study on cofilin's role in mediating neuronal apoptosis. Together, our results suggest that cofilin could present a common target in neurons and microglial cells and might prove to be a promising therapy for different brain injury mechanisms including stroke.

Deficiency of Cks1 Leads to Learning and Long-Term Memory Defects and p27 Dependent Formation of Neuronal Cofilin Aggregates

In mitotic cells, the cyclin-dependent kinase (CDK) subunit protein CKS1 regulates S phase entry by mediating degradation of the CDK inhibitor p27. Although mature neurons lack mitotic CDKs, we found that CKS1 was actively expressed in post-mitotic neurons of the adult hippocampus. Interestingly, Cks1 knockout (Cks1-/-) mice exhibited poor long-term memory, and diminished maintenance of long-term potentiation in the hippocampal circuits. Furthermore, there was neuronal accumulation of cofilin-actin rods or cofilin aggregates, which are associated with defective dendritic spine maturation and synaptic loss. We further demonstrated that it was the increased p27 level that activated cofilin by suppressing the RhoA kinase-mediated inhibitory phosphorylation of cofilin, resulting in the formation of cofilin aggregates in the Cks1-/- neuronal cells. Consistent with reports that the peptidyl-prolyl-isomerase PIN1 competes with CKS1 for p27 binding, we found that inhibition of PIN1 diminished the formation of cofilin aggregates through decreasing p27 levels, thereby activating RhoA and increasing cofilin phosphorylation. Our results revealed that CKS1 is involved in normal glutamatergic synapse development and dendritic spine maturation in adult hippocampus through modulating p27 stability.

The Effects of Disease Models of Nuclear Actin Polymerization on the Nucleus

Actin plays a crucial role in regulating multiple processes within the nucleus, including transcription and chromatin organization. However, the polymerization state of nuclear actin remains controversial, and there is no evidence for persistent actin filaments in a normal interphase nucleus. Further, several disease pathologies are characterized by polymerization of nuclear actin into stable filaments or rods. These include filaments that stain with phalloidin, resulting from point mutations in skeletal α-actin, detected in the human skeletal disease intranuclear rod myopathy, and cofilin/actin rods that form in response to cellular stressors like heatshock. To further elucidate the effects of these pathological actin structures, we examined the nucleus in both cell culture models as well as isolated human tissues. We find these actin structures alter the distribution of both RNA polymerase II and chromatin. Our data suggest that nuclear actin filaments result in disruption of nuclear organization, which may contribute to the disease pathology.

The M. tuberculosis HAD phosphatase (Rv3042c) interacts with host proteins and is inhibited by Clofazimine

Mycobacterium tuberculosis codes for a HAD-phosphatase, Rv3042c (MtSerB2), that has earlier been characterized as a metabolic enzyme. Here we demonstrate that MtSerB2 is secreted into the cytosol of infected macrophages and is found in bronchoalveolar lavage samples of tuberculosis patients. MtSerB2 induces significant cytoskeleton rearrangements through cofilin activation and affects the expression of genes that regulate actin dynamics. It specifically interacts with HSP90, HSP70 and HSP27 that block apoptotic pathways and not with other HSPs. It actively dephosphorylates MAPK-p38 and NF-kappa B p65 that play crucial roles in inflammatory and immune responses. This in turn leads to down-regulation of Interleukin 8, a chemotactic and inflammatory cytokine. Finally, during evaluation of inhibitors against MtSerB2 we found that Clofazimine, a drug being evaluated for XDR and MDR tuberculosis, inhibits MtSerB2 phosphatase activity and reverses the above effects and interactions with host proteins. Overall, the study identifies that MtSerB2 has new functions that might help the pathogen to evade the host's immune response.

Actin dynamics and cofilin-actin rods in alzheimer disease

Cytoskeletal abnormalities and synaptic loss, typical of both familial and sporadic Alzheimer disease (AD), are induced by diverse stresses such as neuroinflammation, oxidative stress, and energetic stress, each of which may be initiated or enhanced by proinflammatory cytokines or amyloid-β (Aβ) peptides. Extracellular Aβ-containing plaques and intracellular phospho-tau-containing neurofibrillary tangles are postmortem pathologies required to confirm AD and have been the focus of most studies. However, AD brain, but not normal brain, also have increased levels of cytoplasmic rod-shaped bundles of filaments composed of ADF/cofilin-actin in a 1:1 complex (rods). Cofilin, the major ADF/cofilin isoform in mammalian neurons, severs actin filaments at low cofilin/actin ratios and stabilizes filaments at high cofilin/actin ratios. It binds cooperatively to ADP-actin subunits in F-actin. Cofilin is activated by dephosphorylation and may be oxidized in stressed neurons to form disulfide-linked dimers, required for bundling cofilin-actin filaments into stable rods. Rods form within neurites causing synaptic dysfunction by sequestering cofilin, disrupting normal actin dynamics, blocking transport, and exacerbating mitochondrial membrane potential loss. Aβ and proinflammatory cytokines induce rods through a cellular prion protein-dependent activation of NADPH oxidase and production of reactive oxygen species. Here we review recent advances in our understanding of cofilin biochemistry, rod formation, and the development of cognitive deficits. We will then discuss rod formation as a molecular pathway for synapse loss that may be common between all three prominent current AD hypotheses, thus making rods an attractive therapeutic target. © 2016 Wiley Periodicals, Inc.

Cofilin as a Promising Therapeutic Target for Ischemic and Hemorrhagic Stroke

Neurovascular unit (NVU) is considered as a conceptual framework for investigating the mechanisms as well as developing therapeutic targets for ischemic and hemorrhagic stroke. From a molecular perspective, oxidative stress, excitotoxicity, inflammation, and disruption of the blood brain barrier are broad pathophysiological frameworks on the basis on which potential therapeutic candidates for ischemic and hemorrhagic stroke could be discussed. Cofilin is a potent actin-binding protein that severs and depolymerizes actin filaments in order to generate the dynamics of the actin cytoskeleton. Although studies of the molecular mechanisms of cofilin-induced reorganization of the actin cytoskeleton have been ongoing for decades, the multicellular functions of cofilin and its regulation in different molecular pathways are expanding beyond its primary role in actin cytoskeleton. This review focuses on the role of cofilin in oxidative stress, excitotoxicity, inflammation, and disruption of the blood brain barrier in the context of NVU as well as how and why cofilin could be studied further as a potential target for ischemic and hemorrhagic stroke.

Cellular prion protein: A co-receptor mediating neuronal cofilin-actin rod formation induced by β-amyloid and proinflammatory cytokines

Increasing evidence suggests that proteins exhibiting "prion-like" behavior cause distinct neurodegenerative diseases, including inherited, sporadic and acquired types. The conversion of cellular prion protein (PrP(C)) to its infectious protease resistant counterpart (PrP(Res)) is the essential feature of prion diseases. However, PrP(C) also performs important functions in transmembrane signaling, especially in neurodegenerative processes. Beta-amyloid (Aβ) synaptotoxicity and cognitive dysfunction in mouse models of Alzheimer disease are mediated by a PrP(C)-dependent pathway. Here we review how this pathway converges with proinflammatory cytokine signaling to activate membrane NADPH oxidase (NOX) and generate reactive oxygen species (ROS) leading to dynamic remodeling of the actin cytoskeleton. The NOX signaling pathway may also be integrated with those of other transmembrane receptors clustered in PrP(C)-enriched membrane domains. Such a signal convergence along the PrP(C)-NOX axis could explain the relevance of PrP(C) in a broad spectrum of neurodegenerative disorders, including neuroinflammatory-mediated alterations in synaptic function following traumatic brain injury. PrP(C) overexpression alone activates NOX and generates a local increase in ROS that initiates cofilin activation and formation of cofilin-saturated actin bundles (rods). Rods sequester cofilin from synaptic regions where it is required for plasticity associated with learning and memory. Rods can also interrupt vesicular transport by occluding the neurite within which they form. Through either or both mechanisms, rods may directly mediate the synaptic dysfunction that accompanies various neurodegenerative disorders.

Chronophin coordinates cell leading edge dynamics by controlling active cofilin levels

Cofilin, a critical player of actin dynamics, is spatially and temporally regulated to control the direction and force of membrane extension required for cell locomotion. In carcinoma cells, although the signaling pathways regulating cofilin activity to control cell direction have been established, the molecular machinery required to generate the force of the protrusion remains unclear. We show that the cofilin phosphatase chronophin (CIN) spatiotemporally regulates cofilin activity at the cell edge to generate persistent membrane extension. We show that CIN translocates to the leading edge in a PI3-kinase-, Rac1-, and cofilin-dependent manner after EGF stimulation to activate cofilin, promotes actin free barbed end formation, accelerates actin turnover, and enhances membrane protrusion. In addition, we establish that CIN is crucial for the balance of protrusion/retraction events during cell migration. Thus, CIN coordinates the leading edge dynamics by controlling active cofilin levels to promote MTLn3 cell protrusion.

Synergistic action of dendritic mitochondria and creatine kinase maintains ATP homeostasis and actin dynamics in growing neuronal dendrites

The distribution of mitochondria within mature, differentiated neurons is clearly adapted to their regional physiological needs and can be perturbed under various pathological conditions, but the function of mitochondria in developing neurons has been less well studied. We have studied mitochondrial distribution within developing mouse cerebellar Purkinje cells and have found that active delivery of mitochondria into their dendrites is a prerequisite for proper dendritic outgrowth. Even when mitochondria in the Purkinje cell bodies are functioning normally, interrupting the transport of mitochondria into their dendrites severely disturbs dendritic growth. Additionally, we find that the growth of atrophic dendrites lacking mitochondria can be rescued by activating ATP-phosphocreatine exchange mediated by creatine kinase (CK). Conversely, inhibiting cytosolic CKs decreases dendritic ATP levels and also disrupts dendrite development. Mechanistically, this energy depletion appears to perturb normal actin dynamics and enhance the aggregation of cofilin within growing dendrites, reminiscent of what occurs in neurons overexpressing the dephosphorylated form of cofilin. These results suggest that local ATP synthesis by dendritic mitochondria and ATP-phosphocreatine exchange act synergistically to sustain the cytoskeletal dynamics necessary for dendritic development.

Pathogenic microbes manipulate cofilin activity to subvert actin cytoskeleton

Actin-depolymerizing factor (ADF)/cofilin proteins are key players in controlling the temporal and spatial extent of actin dynamics, which is crucial for mediating host-pathogen interactions. Pathogenic microbes have evolved molecular mechanisms to manipulate cofilin activity to subvert the actin cytoskeletal system in host cells, promoting their internalization into the target cells, modifying the replication niche and facilitating their intracellular and intercellular dissemination. The study of how these pathogens exploit cofilin pathways is crucial for understanding infectious disease and providing potential targets for drug therapies.

Cofilin Inhibition Restores Neuronal Cell Death in Oxygen-Glucose Deprivation Model of Ischemia

Ischemia is a condition associated with decreased blood supply to the brain, eventually leading to death of neurons. It is associated with a diverse cascade of responses involving both degenerative and regenerative mechanisms. At the cellular level, the changes are initiated prominently in the neuronal cytoskeleton. Cofilin, a cytoskeletal actin severing protein, is known to be involved in the early stages of apoptotic cell death. Evidence supports its intervention in the progression of disease states like Alzheimer's and ischemic kidney disease. In the present study, we have hypothesized the possible involvement of cofilin in ischemia. Using PC12 cells and mouse primary cultures of cortical neurons, we investigated the potential role of cofilin in ischemia in two different in vitro ischemic models: chemical induced oxidative stress and oxygen-glucose deprivation/reperfusion (OGD/R). The expression profile studies demonstrated a decrease in phosphocofilin levels in all models of ischemia, implying stress-induced cofilin activation. Furthermore, calcineurin and slingshot 1L (SSH) phosphatases were found to be the signaling mediators of the cofilin activation. In primary cultures of cortical neurons, cofilin was found to be significantly activated after 1 h of OGD. To delineate the role of activated cofilin in ischemia, we knocked down cofilin by small interfering RNA (siRNA) technique and tested the impact of cofilin silencing on neuronal viability. Cofilin siRNA-treated neurons showed a significant reduction of cofilin levels in all treatment groups (control, OGD, and OGD/R). Additionally, cofilin siRNA-reduced cofilin mitochondrial translocation and caspase 3 cleavage, with a concomitant increase in neuronal viability. These results strongly support the active role of cofilin in ischemia-induced neuronal degeneration and apoptosis. We believe that targeting this protein mediator has a potential for therapeutic intervention in ischemic brain injury and stroke.

NAMPT-mediated salvage synthesis of NAD+ controls morphofunctional changes of macrophages

Functional morphodynamic behavior of differentiated macrophages is strongly controlled by actin cytoskeleton rearrangements, a process in which also metabolic cofactors ATP and NAD(H) (i.e. NAD+ and NADH) and NADP(H) (i.e. NADP+ and NADPH) play an essential role. Whereas the link to intracellular ATP availability has been studied extensively, much less is known about the relationship between actin cytoskeleton dynamics and intracellular redox state and NAD+-supply. Here, we focus on the role of nicotinamide phosphoribosyltransferase (NAMPT), found in extracellular form as a cytokine and growth factor, and in intracellular form as one of the key enzymes for the production of NAD+ in macrophages. Inhibition of NAD+ salvage synthesis by the NAMPT-specific drug FK866 caused a decrease in cytosolic NAD+ levels in RAW 264.7 and Maf-DKO macrophages and led to significant downregulation of the glycolytic flux without directly affecting cell viability, proliferation, ATP production capacity or mitochondrial respiratory activity. Concomitant with these differential metabolic changes, the capacity for phagocytic ingestion of particles and also substrate adhesion of macrophages were altered. Depletion of cytoplasmic NAD+ induced cell-morphological changes and impaired early adhesion in phagocytosis of zymosan particles as well as spreading performance. Restoration of NAD+ levels by NAD+, NMN, or NADP+ supplementation reversed the inhibitory effects of FK866. We conclude that direct coupling to local, actin-based, cytoskeletal dynamics is an important aspect of NAD+'s cytosolic role in the regulation of morphofunctional characteristics of macrophages.

Amyloid-β and proinflammatory cytokines utilize a prion protein-dependent pathway to activate NADPH oxidase and induce cofilin-actin rods in hippocampal neurons

Neurites of neurons under acute or chronic stress form bundles of filaments (rods) containing 1∶1 cofilin∶actin, which impair transport and synaptic function. Rods contain disulfide cross-linked cofilin and are induced by treatments resulting in oxidative stress. Rods form rapidly (5-30 min) in >80% of cultured hippocampal or cortical neurons treated with excitotoxic levels of glutamate or energy depleted (hypoxia/ischemia or mitochondrial inhibitors). In contrast, slow rod formation (50% of maximum response in ∼6 h) occurs in a subpopulation (∼20%) of hippocampal neurons upon exposure to soluble human amyloid-β dimer/trimer (Aβd/t) at subnanomolar concentrations. Here we show that proinflammatory cytokines (TNFα, IL-1β, IL-6) also induce rods at the same rate and within the same neuronal population as Aβd/t. Neurons from prion (PrP(C))-null mice form rods in response to glutamate or antimycin A, but not in response to proinflammatory cytokines or Aβd/t. Two pathways inducing rod formation were confirmed by demonstrating that NADPH-oxidase (NOX) activity is required for prion-dependent rod formation, but not for rods induced by glutamate or energy depletion. Surprisingly, overexpression of PrP(C) is by itself sufficient to induce rods in over 40% of hippocampal neurons through the NOX-dependent pathway. Persistence of PrP(C)-dependent rods requires the continuous activity of NOX. Removing inducers or inhibiting NOX activity in cells containing PrP(C)-dependent rods causes rod disappearance with a half-life of about 36 min. Cofilin-actin rods provide a mechanism for synapse loss bridging the amyloid and cytokine hypotheses for Alzheimer disease, and may explain how functionally diverse Aβ-binding membrane proteins induce synaptic dysfunction.

Rare Variants and Transcriptomics in Alzheimer disease

Alzheimer disease (AD) is the most common dementia in the elderly, still without effective treatment. Early-onset AD (EOAD) is caused by mutations in the genes APP, PSEN1 and PSEN2. Genome-wide association studies have identified >20 late-onset AD (LOAD) susceptibility genes with common variants of small risk, with the exception of APOE. We review rare susceptibility variants in LOAD with larger effects that have been recently identified in the EOAD gene APP and the newly discovered AD genes TREM2 and PLD3. Human genetic studies now consistently support the amyloid hypothesis of AD for both EOAD and LOAD. Moreover, they identified biological processes that overlap with human transcriptomics studies in AD across different tissues, such as inflammation, cytoskeletal organization, synaptic functions, etc. Transcriptomic profiles of pre-symptomatic AD-associated variant carriers already reflect specific molecular mechanisms reminiscent to those of AD patients. This might provide an avenue for personalized medicine.

Chronophin dimerization is required for proper positioning of its substrate specificity loop

Mammalian phosphatases of the haloacid dehalogenase (HAD) superfamily have emerged as important regulators of physiology and disease. Many of these enzymes are stable homodimers; however, the role of their dimerization is largely unknown. Here, we explore the function of the obligatory homodimerization of chronophin, a mammalian HAD phosphatase known to dephosphorylate pyridoxal 5'-phosphate (PLP) and serine/threonine-phosphorylated proteins. The exchange of two residues in the murine chronophin homodimerization interface (chronophin(A194K,A195K)) yields a constitutive monomer both in vitro and in cells. The catalytic activity of monomeric chronophin toward PLP is strongly impaired. X-ray crystallographic studies of chronophin(A194K,A195K) revealed that dimer formation is essential for an intermolecular arginine-arginine-tryptophan stacking interaction that positions a critical histidine residue in the substrate specificity loop of chronophin for PLP coordination. Analysis of all available crystal structures of HAD hydrolases that are grouped together with chronophin in the C2a-type structural subfamily uncovered a highly conserved mode of dimerization that results in intermolecular contacts involving the substrate specificity loop. Our results explain how the dimerization of HAD hydrolases contributes to their catalytic efficiency and substrate specificity.

Evolutionary and structural analyses of mammalian haloacid dehalogenase-type phosphatases AUM and chronophin provide insight into the basis of their different substrate specificities

Mammalian haloacid dehalogenase (HAD)-type phosphatases are an emerging family of phosphatases with important functions in physiology and disease, yet little is known about the basis of their substrate specificity. Here, we characterize a previously unexplored HAD family member (gene annotation, phosphoglycolate phosphatase), which we termed AUM, for aspartate-based, ubiquitous, Mg(2+)-dependent phosphatase. AUM is a tyrosine-specific paralog of the serine/threonine-specific protein and pyridoxal 5'-phosphate-directed HAD phosphatase chronophin. Comparative evolutionary and biochemical analyses reveal that a single, differently conserved residue in the cap domain of either AUM or chronophin is crucial for phosphatase specificity. We have solved the x-ray crystal structure of the AUM cap fused to the catalytic core of chronophin to 2.65 Å resolution and present a detailed view of the catalytic clefts of AUM and chronophin that explains their substrate preferences. Our findings identify a small number of cap domain residues that encode the different substrate specificities of AUM and chronophin.