Familial focal segmental glomerulosclerosis (FSGS)-linked α-actinin 4 (ACTN4) protein mutants lose ability to activate transcription by nuclear hormone receptors

Bioinformatics Analysis Reveals a Shared Pathway for Common Forms of Adult Nephrotic Syndrome

Key Points

Dysregulation of the focal adhesion pathway is present in the three most common forms of glomerular disease, that is, Focal segmental glomerulosclerosis, membranous nephropathy, and minimal change disease. Zyxin is seen to be upregulated in the glomerular compartment of patients with the three most common forms of glomerular disease.

Background

Focal segmental glomerulosclerosis, membranous nephropathy, and minimal change disease are common causes of nephrotic syndrome. Although triggers for these diseases differ, disease progression may share common molecular mechanisms. The aim of this study was to investigate the presence of molecular pathways that are dysregulated across these glomerular diseases.

Methods

The gene expression dataset GSE200828 from the Nephrotic Syndrome Study Network study was obtained from the Gene Expression Omnibus database. R and Python packages, Cytoscape software, and online tools (DAVID and STRING) were used to identify core genes and topologically relevant nodes and molecular pathways. Single-cell RNA sequencing analysis was applied to identify the expression patterns of core genes across kidney cell types in glomerular compartments.

Results

A total of 1087 differentially expressed genes were identified, including 691 upregulated genes and 396 downregulated genes, which are common in all three forms of nephrotic syndrome compared with kidney donor controls (FDR P <0.01). A multiapproach bioinformatics analysis narrowed down to 28 similarly dysregulated genes across the three proteinuric glomerulopathies. The most topologically relevant nodes belonged to the adherens junction, focal adhesion, and cytoskeleton pathways, where zyxin covers all of those gene ontology terms.

Conclusions

We report that dysregulation of cell adhesion complexes was present in the three most common forms of glomerular disease. Zyxin could be a biomarker in all three common forms of nephrotic syndrome. If further functional studies confirm its role in their development, zyxin could be a potential therapeutic target.

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

The Potential Role of Exosomal Proteins in Prostate Cancer

Prostate cancer is the most prevalent malignant tumor in men across developed countries. Traditional diagnostic and therapeutic methods for this tumor have become increasingly difficult to adapt to today’s medical philosophy, thus compromising early detection, diagnosis, and treatment. Prospecting for new diagnostic markers and therapeutic targets has become a hot topic in today’s research. Notably, exosomes, small vesicles characterized by a phospholipid bilayer structure released by cells that is capable of delivering different types of cargo that target specific cells to regulate biological properties, have been extensively studied. Exosomes composition, coupled with their interactions with cells make them multifaceted regulators in cancer development. Numerous studies have described the role of prostate cancer-derived exosomal proteins in diagnosis and treatment of prostate cancer. However, so far, there is no relevant literature to systematically summarize its role in tumors, which brings obstacles to the later research of related proteins. In this review, we summarize exosomal proteins derived from prostate cancer from different sources and summarize their roles in tumor development and drug resistance.

Case report and literature review: A de novo pathogenic missense variant in ACTN4 gene caused rapid progression to end-stage renal disease

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) is a histopathological diagnosis of the sclerosis of glomeruli and the damage to renal podocytes. FSGS affects the filtration function of the kidneys and results in nephrotic syndrome (NS) in children and adults. FSGS is a clinically and genetically heterogeneous disorder. FSGS-1 [OMIM #603278] is one of the progressive hereditary renal diseases. It is caused by heterozygous variants of the actinin alpha 4 (ACTN4) [OMIM*604638] gene on chromosome 19q13.2 in a dominant inheritance (AD) manner. With the recent development of whole-exome sequencing (WES), 22 (including our case) pathogenic or likely pathogenic variants have been identified in ACTN4 gene.

CASE PRESENTATION

We reported a 17-year-old Chinese girl who was hospitalized with foamy urine, nausea and vomiting. Laboratory tests revealed increased levels of serum creatinine and urea nitrogen. Ultrasonography demonstrated bilaterally reduced size of kidneys. The primary diagnoses were NS and chronic kidney disease stage 5 (CKD5). The hemodialysis was initiated in 48 h after admission. After 4 months of treatment, the patient received an allogeneic kidney transplantation from her father. A novel heterozygous missense variant c.494C > T (p.A165V) in the ACTN4 gene was found by WES in the patient. This variant was confirmed by Sanger sequencing. The computational simulation of the stability of mutant protein (p.A165V) was decreased. Interatomic interactions of the p.A165V site were increased, and it might be associated with the increased ubiquitylation in the vicinity of the mutant site.

CONCLUSION

As per the guidelines of the American College of Medical Genetics and Genomics for interpreting sequence variants, the novel heterozygous missense variant was pathogenic (PS2 + PM1 + PM2 + PP3 + PP4). It should be noted that the early onset of severe proteinuria with a poor prognosis is an important and universal symptom for most genetic FSGS. If necessary, genetic screening is recommended.

α-Actinin 4 Links Vasopressin Short-Term and Long-Term Regulation of Aquaporin-2 in Kidney Collecting Duct Cells

Water permeability of the kidney collecting ducts is regulated by the peptide hormone vasopressin. Between minutes and hours (short-term), vasopressin induces trafficking of the water channel protein aquaporin-2 to the apical plasma membrane of the collecting duct principal cells to increase water permeability. Between hours and days (long-term), vasopressin induces aquaporin-2 gene expression. Here, we investigated the mechanisms that bridge the short-term and long-term vasopressin-mediated aquaporin-2 regulation by α-actinin 4, an F-actin crosslinking protein and a transcription co-activator of the glucocorticoid receptor. Vasopressin induced F-actin depolymerization and α-actinin 4 nuclear translocation in the mpkCCD collecting duct cell model. Co-immunoprecipitation followed by immunoblotting showed increased interaction between α-actinin 4 and glucocorticoid receptor in response to vasopressin. ChIP-PCR showed results consistent with α-actinin 4 and glucocorticoid receptor binding to the aquaporin-2 promoter. α-actinin 4 knockdown reduced vasopressin-induced increases in aquaporin-2 mRNA and protein expression. α-actinin 4 knockdown did not affect vasopressin-induced glucocorticoid receptor nuclear translocation, suggesting independent mechanisms of vasopressin-induced nuclear translocation of α-actinin 4 and glucocorticoid receptor. Glucocorticoid receptor knockdown profoundly reduced vasopressin-induced increases in aquaporin-2 mRNA and protein expression. In the absence of glucocorticoid analog dexamethasone, vasopressin-induced increases in glucocorticoid receptor nuclear translocation and aquaporin-2 mRNA were greatly reduced. α-actinin 4 knockdown further reduced vasopressin-induced increase in aquaporin-2 mRNA in the absence of dexamethasone. We conclude that glucocorticoid receptor plays a major role in vasopressin-induced aquaporin-2 gene expression that can be enhanced by α-actinin 4. In the absence of vasopressin, α-actinin 4 crosslinks F-actin underneath the apical plasma membrane, impeding aquaporin-2 membrane insertion. Vasopressin-induced F-actin depolymerization in one hand facilitates aquaporin-2 apical membrane insertion and in the other hand frees α-actinin 4 to enter the nucleus where it binds glucocorticoid receptor to enhance aquaporin-2 gene expression.

Alpha-actinin-4 is essential for maintaining normal trophoblast proliferation and differentiation during early pregnancy

BACKGROUND

Proper differentiation of trophoblasts in the human placenta is essential for a successful pregnancy, whereas abnormal regulation of this process may lead to adverse pregnancy outcomes, especially preeclampsia (PE). However, the underlying mechanism of trophoblast differentiation remains unclear. Previous studies have reported the involvement of alpha-actinin-4 (ACTN4) in the actin cytoskeleton dynamics and motility. Hence, we hypothesized that ACTN4 may act as an important regulator in the normal proliferation and differentiation of trophoblasts during early pregnancy.

METHOD

To test this hypothesis, we collected villous tissues from women undergoing a legal pregnancy termination during 6-10 weeks of gestation and explanted them for cell culture and siRNA transfection. We also obtained placental tissues from PE patients and healthy pregnant women and isolated the primary cytotrophoblast (CTB) cells. The expression of ACTN4 in the CTBs of placental villi and during the differentiation of CTBs into STBs was detected by immunofluorescence, immunohistochemistry (IHC), and EdU proliferation assays. Besides, villous explant, Matrigel invasion, transwell migration assay, and Wound-healing assay were performed to identify the possible role of ACTN4 in the outgrowth of explants and the invasion, migration, and proliferation of cell column trophoblasts (CCTs). Western blot analysis was carried out to compare the protein expression level of AKT, Snail activities, and epithelial-to-mesenchymal transition (EMT) in the villi or HTR8/SVneo cells with ACTN4 knockdown.

RESULTS

ACTN4 was highly expressed in CTB cells and interstitial extravillous trophoblast (iEVT) cells but not found in the syncytiotrophoblast (STB) cells in the first trimester villi. Downregulation of ACTN4 led to reduced trophoblast proliferation and explant outgrowth ex vivo, as well as iEVT invasion and migration in vitro due to disrupt of actin filaments organization. Such ACTN4 inhibition also decreased AKT and Snail activities and further impeded the EMT process. In addition, ACTN4 expression was found to be downregulated in the iEVTs from preeclamptic placentas.

CONCLUSIONS

Our findings suggest that ACTN4 may act as an important regulator of trophoblast proliferation and differentiation during early pregnancy, and dysregulation of this protein may contribute to preeclampsia pathogenesis.

The Effect of Interleukin-4 and Dexamethasone on RNA-Seq-Based Transcriptomic Profiling of Human Podocytes: A Potential Role in Minimal Change Nephrotic Syndrome

Interleukin-4 (IL-4) expression is implicated in the pathogenesis of nephrotic syndrome (NS). This study aimed to investigate the changes in the transcriptomes of human podocytes induced by IL-4 treatment and to analyze whether these changes could be affected by simultaneous steroid treatment. Three groups of human podocytes were treated with control, IL-4, and IL-4 plus dexamethasone (DEX), respectively. We performed whole-transcriptome sequencing to identify differentially expressed genes (DEGs) between the groups. We investigated relevant biological pathways using Gene Ontology (GO) enrichment analyses. We also attempted to compare and validate the DEGs with the genes listed in PodNet, a literature-based database on mouse podocyte genes. A total of 176 genes were differentially expressed among the three groups. GO analyses showed that pathways related to cytoskeleton organization and cell signaling were significantly enriched. Among them, 24 genes were listed in PodNet, and 12 of them were previously reported to be associated with IL-4-induced changes in human podocytes. Of the 12 genes, the expression levels of BMP4, RARB, and PLCE1 were reversed when podocytes were simultaneously treated with DEX. In conclusion, this study explored changes in the transcriptome profiles of human podocytes treated with IL-4. Few genes were reported in previous studies and were previously validated in experiments with human podocytes. We speculate that IL-4 may exert pathogenic effects on the transcriptome of human podocytes, and a few genes may be involved in the pathogenesis.

Changes in the levels of α-actinin-4 in differentiating human myeloid leukemia cells induced by retinoic acid

Retinoic acid (RA) induces granulocytic differentiation and inhibits the growth of human promyelocytic leukemia HL60 cells. α-Actinin-4 is a member of the α-actinin family, which exhibits unique mechanosensory regulation. Herein, we elucidated the effects of RA on α-actinin-4 expression during cell differentiation. RA increased the levels of α-actinin-4 protein significantly, while mRNA expression remained unchanged. In addition, RA treatment altered the intracellular localization of α-actinin-4 from the nucleus to the cytoplasm. Cells pretreated with RA, maintained α-actinin-4 protein levels after cycloheximide treatment as compared with control cells. The amount of ubiquitylated α-actinin-4 protein in RA-treated cells was less than in control cells. These results indicate that RA may inhibit nuclei transport and proteasomal degradation of α-actinin-4 protein. α-Actinin-4 may play a significant role in RA-induced differentiation, including the promotion of cytomorphology changes.

The Expressions and Mechanisms of Sarcomeric Proteins in Cancers

The sarcomeric proteins control the movement of cells in diverse species, whereas the deregulation can induce tumours in model organisms and occurs in human carcinomas. Sarcomeric proteins are recognized as oncogene and related to tumor cell metastasis. Recent insights into their expressions and functions have led to new cancer therapeutic opportunities. In this review, we appraise the evidence for the sarcomeric proteins as cancer genes and discuss cancer-relevant biological functions, potential mechanisms by which sarcomeric proteins activity is altered in cancer.

Analysis of the uterine lumen in fertility-classified heifers: II. Proteins and metabolites†

Survival and growth of the bovine conceptus is dependent on endometrial secretions or histotroph. Previously, serial blastocyst transfer was used to classify heifers as high fertile (HF), subfertile (SF), or infertile (IF). Here, we investigated specific histotroph components (proteins and metabolites) in the uterine lumen of day 17 fertility-classified heifers. Interferon tau (IFNT) was more abundant in uterine lumenal fluid (ULF) of pregnant HF than SF animals as the conceptus was longer in HF heifers. However, no differences in endometrial expression of selected classical and nonclassical interferon-stimulated genes (ISGs) were observed, suggesting that IFNT signaling in the endometrium of pregnant HF and SF heifers was similar. Pregnancy significantly increased the abundance of several proteins in ULF. Based on functional annotation, the abundance of a number of proteins involved in energy metabolism, oxidative stress, amino acid metabolism, and cell proliferation and differentiation were greater in the ULF of pregnant HF than SF heifers. Metabolomics analysis found that pregnancy only changed the metabolome composition of ULF from HF heifers. The majority of the metabolites that increased in the ULF of pregnant HF as compared to SF heifers were associated with energy and amino acid metabolism. The observed differences in ULF proteome and metabolome are hypothesized to influence uterine receptivity with consequences on conceptus development and survival in fertility-classified heifers.

α-Actinin-4 regulates cancer stem cell properties and chemoresistance in cervical cancer

Cancer stem cells (CSCs) initiate tumors and possess the properties of self-renewal and differentiation. Since they are responsible for chemoresistance, CSCs are known to be a key factor in cancer recurrence. α-Actinin-4 (ACTN4) is an actin-binding protein that is involved in muscle differentiation and cancer metastasis. It promotes epithelial to mesenchymal transition and cell cycle progression via β-catenin stabilization in cervical cancer. In the present study, we investigated the role of ACTN4 in regulating cancer cell stemness and chemoresistance in cervical cancer. Results from the gene expression database analysis showed that ACTN4 mRNA expression was elevated in cancerous cervices when compared with normal cervices. Furthermore, ACTN4 knockdown suppressed sphere formation and CSC proliferation. It also decreased CSC size and CD44high/CD24low cell population. ACTN4-knockdown CSCs were sensitive to anticancer drugs, which was observed by down-regulation of the ATP-binding cassette family G2 involved in drug resistance. Finally, ACTN4-knockdown CSCs formed reduced tumors in vivo when compared with control CSCs. Overall, these findings suggest that ACTN4 regulates CSC properties and contributes to chemoresistance in cervical cancer.

Mechanisms of Scarring in Focal Segmental Glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) presents with scar in parts of some glomeruli and often progresses to global and diffuse glomerulosclerosis. Podocyte injury is the initial target in primary FSGS, induced by a circulating factor. Several gene variants, for example, APOL1, are associated with increased susceptibility to FSGS. Primary FSGS may be due to genetic mutation in key podocyte genes. Increased work stress after loss of nephrons, epigenetic mechanisms, and various profibrotic pathways can contribute to progressive sclerosis, regardless of the initial injury. The progression of FSGS lesions also involves crosstalk between podocytes and other kidney cells, such as parietal epithelial cells, glomerular endothelial cells, and even tubular epithelial cells. New insights related to these mechanisms could potentially lead to new therapeutic strategies to prevent progression of FSGS.

Proteostasis as a therapeutic target in glomerular injury associated with mutant α-actinin-4

Mutations in α-actinin-4 (actinin-4) result in hereditary focal segmental glomerulosclerosis (FSGS) in humans. Actinin-4 mutants induce podocyte injury because of dysregulation of the cytoskeleton and proteotoxicity. Injury may be associated with endoplasmic reticulum (ER) stress and polyubiquitination of proteins. We assessed if the chemical chaperone 4-phenylbutyrate (4-PBA) can ameliorate the proteotoxicity of an actinin-4 mutant. Actinin-4 K255E, which causes FSGS in humans (K256E in the mouse), showed enhanced ubiquitination, accelerated degradation, aggregate formation, and enhanced association with filamentous (F)-actin in glomerular epithelial cells (GECs). The mutant disrupted ER function and stimulated autophagy. 4-PBA reduced actinin-4 K256E aggregation and its tight association with F-actin. Transgenic mice that express actinin-4 K256E in podocytes develop podocyte injury, proteinuria, and FSGS in association with glomerular ER stress. Treatment of these mice with 4-PBA in the drinking water over a 10-wk period significantly reduced albuminuria and ER stress. Another drug, celastrol, which enhanced expression of ER and cytosolic chaperones in GECs, tended to reduce actinin-4 aggregation but did not decrease the tight association of actinin-4 K256E with F-actin and did not reduce albuminuria in actinin-4 K256E transgenic mice. Thus, chemical chaperones, such as 4-PBA, may represent a novel therapeutic approach to certain hereditary glomerular diseases.

Investigational drugs in development for focal segmental glomerulosclerosis

INTRODUCTION

Focal segmental glomerulosclerosis is an important cause of end stage kidney disease and is a paradigm for the study of glomerular scarring. There are no FDA approved treatments for this condition. Current therapies, assessed based on reduction in proteinuria, are generally effective in a subset of patients which suggests that FSGS is a heterogeneous group of glomerular disorders or podocytopathies that converge on a common histopathological phenotype. Areas covered: We searched for investigational drugs agents that target different pathophysiological pathways using the key words 'FSGS' and 'podocyte' in American and European clinical trial registers (clinicaltrials.gov; clinicaltrialsregister.eu). Published articles were searched in PubMed, Medline, the Web of Science and the Cochrane Central Register of Controlled Trials Library. Expert opinion: Progress is being made in defining the mechanism of action of subtypes of FSGS. Current and investigational therapies for FSGS target these different pathways of injury. It is anticipated that advances in systems biology will further refine the classification of FSGS by subdividing the disease based on the primary mechanism of glomerular injury, identify biomarkers to discriminate between different subtypes, and enable appropriate selection of appropriate therapy for each individual in accordance with the goals of precision medicine.

[Research advances in the protective effect of all-trans retinoic acid against podocyte injury]

All-trans retinoic acid (ATRA) is a vitamin A derivative and plays an important role in the regulation of cell aggregation, differentiation, apoptosis, proliferation, and inflammatory response. In recent years, some progress has been made in the role of ATRA in renal diseases, especially its protective effect on podocytes. This article reviews the research advances in podocyte injury, characteristics of ATRA, podocyte differentiation and regeneration induced by ATRA, and the protective effect of ATRA against proliferation, deposition of fibers, and apoptosis.

Endocytic Trafficking at the Mature Podocyte Slit Diaphragm

Endocytic trafficking couples cell signaling with the cytoskeletal dynamics by organizing a crosstalk between protein networks in different subcellular compartments. Proteins residing in the plasma membrane are internalized and transported as cargo in endocytic vesicles (i.e., endocytosis). Subsequently, cargo proteins can be delivered to lysosomes for degradation or recycled back to the plasma membrane. The slit diaphragm is a modified tight junction connecting foot processes of the glomerular epithelial cells, podocytes. Signaling at the slit diaphragm plays a critical role in the kidney while its dysfunction leads to glomerular protein loss (proteinuria), manifesting as nephrotic syndrome, a rare condition with an estimated incidence of 2-4 new cases per 100,000 each year. Relatively little is known about the role of endocytic trafficking in podocyte signaling and maintenance of the slit diaphragm integrity. This review will focus on the role of endocytic trafficking at the mature podocyte slit diaphragm.

α Actinin 4 (ACTN4) Regulates Glucocorticoid Receptor-mediated Transactivation and Transrepression in Podocytes

Glucocorticoids are a general class of steroids that possess renoprotective activity in glomeruli through their interaction with the glucocorticoid receptor. However, the mechanisms by which glucocorticoids ameliorate proteinuria and glomerular disease are not well understood. In this study, we demonstrated that α actinin 4 (ACTN4), an actin-cross-linking protein known to coordinate cytoskeletal organization, interacts with the glucocorticoid receptor (GR) in the nucleus of human podocytes (HPCs), a key cell type in the glomerulus critical for kidney filtration function. The GR-ACTN4 complex enhances glucocorticoid response element (GRE)-driven reporter activity. Stable knockdown of ACTN4 by shRNA in HPCs significantly reduces dexamethasone-mediated induction of GR target genes and GRE-driven reporter activity without disrupting dexamethasone-induced nuclear translocation of GR. Synonymous mutations or protein expression losses in ACTN4 are associated with kidney diseases, including focal segmental glomerulosclerosis, characterized by proteinuria and podocyte injury. We found that focal segmental glomerulosclerosis-linked ACTN4 mutants lose their ability to bind liganded GR and support GRE-mediated transcriptional activity. Mechanistically, GR and ACTN4 interact in the nucleus of HPCs. Furthermore, disruption of the LXXLL nuclear receptor-interacting motif present in ACTN4 results in reduced GR interaction and dexamethasone-mediated transactivation of a GRE reporter while still maintaining its actin-binding activity. In contrast, an ACTN4 isoform, ACTN4 (Iso), that loses its actin-binding domain is still capable of potentiating a GRE reporter. Dexamethasone induces the recruitment of ACTN4 and GR to putative GREs in dexamethasone-transactivated promoters, SERPINE1, ANGPLT4, CCL20, and SAA1 as well as the NF-κB (p65) binding sites on GR-transrepressed promoters such as IL-1β, IL-6, and IL-8 Taken together, our data establish ACTN4 as a transcriptional co-regulator that modulates both dexamethasone-transactivated and -transrepressed genes in podocytes.

Podocyte-actin dynamics in health and disease

Genetic studies of hereditary forms of nephrotic syndrome have identified several proteins that are involved in regulating the permselective properties of the glomerular filtration system. Further extensive research has elucidated the complex molecular basis of the glomerular filtration barrier and clearly established the pivotal role of podocytes in the pathophysiology of glomerular diseases. Podocyte architecture is centred on focal adhesions and slit diaphragms - multiprotein signalling hubs that regulate cell morphology and function. A highly interconnected actin cytoskeleton enables podocytes to adapt in order to accommodate environmental changes and maintain an intact glomerular filtration barrier. Actin-based endocytosis has now emerged as a regulator of podocyte integrity, providing an impetus for understanding the precise mechanisms that underlie the steady-state control of focal adhesion and slit diaphragm components. This Review outlines the role of actin dynamics and endocytosis in podocyte biology, and discusses how molecular heterogeneity in glomerular disorders could be exploited to deliver more rational therapeutic interventions, paving the way for targeted medicine in nephrology.

Introducing STRaNDs: shuttling transcriptional regulators that are non-DNA binding

Many proteins originally identified as cytoplasmic - including many associated with the cytoskeleton or cell junctions - are increasingly being found in the nucleus, where they have specific functions. Here, we focus on proteins that translocate from the cytoplasm to the nucleus in response to external signals and regulate transcription without binding to DNA directly (for example, through interaction with transcription factors). We propose that proteins with such characteristics are classified as a distinct group of extracellular signalling effectors, and we suggest the term STRaND (shuttling transcriptional regulators and non-DNA binding) to refer to this group. Crucial roles of STRaNDs include linking cell morphology and adhesion with changes in transcriptional programmes in response to signals such as mechanical stresses.

Variability of Gene Expression Identifies Transcriptional Regulators of Early Human Embryonic Development

An analysis of gene expression variability can provide an insightful window into how regulatory control is distributed across the transcriptome. In a single cell analysis, the inter-cellular variability of gene expression measures the consistency of transcript copy numbers observed between cells in the same population. Application of these ideas to the study of early human embryonic development may reveal important insights into the transcriptional programs controlling this process, based on which components are most tightly regulated. Using a published single cell RNA-seq data set of human embryos collected at four-cell, eight-cell, morula and blastocyst stages, we identified genes with the most stable, invariant expression across all four developmental stages. Stably-expressed genes were found to be enriched for those sharing indispensable features, including essentiality, haploinsufficiency, and ubiquitous expression. The stable genes were less likely to be associated with loss-of-function variant genes or human recessive disease genes affected by a DNA copy number variant deletion, suggesting that stable genes have a functional impact on the regulation of some of the basic cellular processes. Genes with low expression variability at early stages of development are involved in regulation of DNA methylation, responses to hypoxia and telomerase activity, whereas by the blastocyst stage, low-variability genes are enriched for metabolic processes as well as telomerase signaling. Based on changes in expression variability, we identified a putative set of gene expression markers of morulae and blastocyst stages. Experimental validation of a blastocyst-expressed variability marker demonstrated that HDDC2 plays a role in the maintenance of pluripotency in human ES and iPS cells. Collectively our analyses identified new regulators involved in human embryonic development that would have otherwise been missed using methods that focus on assessment of the average expression levels; in doing so, we highlight the value of studying expression variability for single cell RNA-seq data.

The biological role of actinin-4 (ACTN4) in malignant phenotypes of cancer

Invasion and metastasis are malignant phenotypes in cancer that lead to patient death. Cell motility is involved in these processes. In 1998, we identified overexpression of the actin-bundling protein actinin-4 in several types of cancer. Protein expression of actinin-4 is closely associated with the invasive phenotypes of cancers. Actinin-4 is predominantly expressed in the cellular protrusions that stimulate the invasive phenotype in cancer cells and is essential for formation of cellular protrusions such as filopodia and lamellipodia. ACTN4 (gene name encoding actinin-4 protein) is located on human chromosome 19q. ACTN4 amplification is frequently observed in patients with carcinomas of the pancreas, ovary, lung, and salivary gland, and patients with ACTN4 amplifications have worse outcomes than patients without amplification. In addition, nuclear distribution of actinin-4 is frequently observed in small cell lung, breast, and ovarian cancer. Actinin-4, when expressed in cancer cell nuclei, functions as a transcriptional co-activator. In this review, we summarize recent developments regarding the biological roles of actinin-4 in cancer invasion.

The role of alpha-actinin-4 in human kidney disease

Mutations in the Alpha-actinin-4 gene (ACTN4) cause a rare form of familial focal segmental glomerulosclerosis in humans. Individuals with kidney disease-associated ACTN4 mutations tend to have mild to moderate proteinuria, with many developing decreased kidney function progressing to end stage kidney disease. All of the disease-causing ACTN4 mutations identified to date are located within the actin-binding domain of the encoded protein, increasing its binding affinity to F-actin and leading to abnormal actin rich cellular aggregates. The identification of ACTN4 mutations as a cause of human kidney disease demonstrates a key cellular pathway by which alterations in cytoskeletal behavior can mediate kidney disease. Here we review the studies relevant to ACTN4 and its role in mediating kidney disease.

The podocyte cytoskeleton in health and in disease

The podocyte is a key cell in the selective filtering action of the glomerular capillary wall. Podocyte injury is of pathogenetic and prognostic significance in human glomerular disease; podocyte repair and regeneration are important therapeutic targets. In particular, podocyte function is dependent on the cells' actin cytoskeleton: this maintains their complex structure. Alterations in the actin cytoskeleton arise from a variety of genetic and acquired causes. Therapeutic agents that are beneficial in proteinuric disease may act at least partly by restoring the cell shape via effects on the actin cytoskeleton. Recent studies of podocytes in vivo and in vitro are described, highlighting clinically relevant observations and those that help us understand the ways in which we may harness nature's own mechanisms to repair and/or renew these specialized glomerular cells, with a particular focus on their actin cytoskeleton. Drugs that have beneficial effects on podocytes can improve our ability to treat important renal diseases including diabetic nephropathy. Currently available agents can be applied in this way and the rapid progress in the study of podocytes is highlighting new therapeutic targets that can bring even more specificity.

α-Actinin 4 potentiates nuclear factor κ-light-chain-enhancer of activated B-cell (NF-κB) activity in podocytes independent of its cytoplasmic actin binding function

Glomerular podocytes are highly specialized terminally differentiated cells that act as a filtration barrier in the kidney. Mutations in the actin-binding protein, α-actinin 4 (ACTN4), are linked to focal segmental glomerulosclerosis (FSGS), a chronic kidney disease characterized by proteinuria. Aberrant activation of NF-κB pathway in podocytes is implicated in glomerular diseases including proteinuria. We demonstrate here that stable knockdown of ACTN4 in podocytes significantly reduces TNFα-mediated induction of NF-κB target genes, including IL-1β and NPHS1, and activation of an NF-κB-driven reporter without interfering with p65 nuclear translocation. Overexpression of ACTN4 and an actin binding-defective variant increases the reporter activity. In contrast, an FSGS-linked ACTN4 mutant, K255E, which has increased actin binding activity and is predominantly cytoplasmic, fails to potentiate NF-κB activity. Mechanistically, IκBα blocks the association of ACTN4 and p65 in the cytosol. In response to TNFα, both NF-κB subunits p65 and p50 translocate to the nucleus, where they bind and recruit ACTN4 to their targeted promoters, IL-1β and IL-8. Taken together, our data identify ACTN4 as a novel coactivator for NF-κB transcription factors in podocytes. Importantly, this nuclear function of ACTN4 is independent of its actin binding activity in the cytoplasm.

The non-muscle functions of actinins: an update

α-Actinins are a major class of actin filament cross-linking proteins expressed in virtually all cells. In muscle, actinins cross-link thin filaments from adjacent sarcomeres. In non-muscle cells, different actinin isoforms play analogous roles in cross-linking actin filaments and anchoring them to structures such as cell-cell and cell-matrix junctions. Although actinins have long been known to play roles in cytokinesis, cell adhesion and cell migration, recent studies have provided further mechanistic insights into these functions. Roles for actinins in synaptic plasticity and membrane trafficking events have emerged more recently, as has a 'non-canonical' function for actinins in transcriptional regulation in the nucleus. In the present paper we review recent advances in our understanding of these diverse cell biological functions of actinins in non-muscle cells, as well as their roles in cancer and in genetic disorders affecting platelet and kidney physiology. We also make two proposals with regard to the actinin nomenclature. First, we argue that naming actinin isoforms according to their expression patterns is problematic and we suggest a more precise nomenclature system. Secondly, we suggest that the α in α-actinin is superfluous and can be omitted.

The p.R229Q variant of the NPHS2 (podocin) gene in focal segmental glomerulosclerosis and steroid-resistant nephrotic syndrome: a meta-analysis

While many previous studies have reported an association between the p.R229Q variant of the NPHS2 gene and focal segmental glomerulosclerosis (FSGS) or steroid-resistant nephrotic syndrome (SRNS), a conclusive relationship has not been defined. In this study, we performed a meta-analysis of the published data to investigate the impact of the p.R229Q polymorphism on FSGS and SRNS patients. Despite significant heterogeneity within some of the comparisons, the results revealed significantly higher risks of SRNS in individuals homozygous for the variant allele (OR 7.411, 95% confidence interval 1.876-29.436, p = 0.004) compared to homozygous non-variant individuals. However, the carrier rate of the p.R229Q variant was not significantly different between SRNS patients and steroid-sensitive nephrotic syndrome patients. No statistically significant differences in the p.R229Q carrier rate were observed between FSGS patients and controls or FSGS patients and patients with different pathology classifications. No notable differences in the p.R229Q carrier rate were found between patients and controls in any group with early-onset disease (onset age < 18). In conclusion, our meta-analysis suggests that for adult-onset disease (onset age > 18), the homozygous variant could be a potential predictor of hereditary nephrotic syndrome and that the p.R229Q allele cannot currently be considered a risk factor for predicting FSGS.

Peroxisome proliferator-activated receptor agonists in a battle against the aging kidney

As aging is a complex phenomenon characterized by intraindividual and interindividual diversities in the maintenance of the homeostatic condition of cells and tissues, changes in renal function are not uniform and depend on associated diseases and environmental factors. Multiple studies have investigated the possible underlying mechanisms of age-related decline in kidney function. Evolutionary, molecular, cellular and systemic theories have been postulated to explain the primary disease independent age-related changes and adaptive responses. As peroxisome proliferator-activated receptors (PPARs) are involved in a broad spectrum of biological processes, PPAR activation might have an effect on the prevention of cell senescence. In this review, we will focus on the experimental and clinical evidence of PPAR agonists in a battle against the aging kidney.

Actin-binding protein alpha-actinin 4 (ACTN4) is a transcriptional co-activator of RelA/p65 sub-unit of NF-kB

ACTN4 is an actin-binding protein that participates in cytoskeleton organisation. It resides both in the cytoplasm and nucleus and physically associates with various transcription factors. Here, we describe an effect of ACTN4 expression on transcriptional activity of the RelA/p65 subunit of NF-kB. We demonstrate that ACTN4 enhances RelA/p65-dependant expression of c-fos, MMP-3 and MMP-1 genes, but it does not affect TNC, ICAM1 and FN1 expression. Importantly, actin-binding domains of ACTN4 are not critical for the nuclear translocation and co-activation of RelA/p65-dependent transcription. Collectively, our data suggest that in the nucleus, ACTN4 functions as a selective transcriptional co-activator of RelA/p65.

Microarray analyses of glucocorticoid and vitamin D3 target genes in differentiating cultured human podocytes

Glomerular podocytes are highly differentiated epithelial cells that are key components of the kidney filtration units. Podocyte damage or loss is the hallmark of nephritic diseases characterized by severe proteinuria. Recent studies implicate that hormones including glucocorticoids (ligand for glucocorticoid receptor) and vitamin D3 (ligand for vitamin D receptor) protect or promote repair of podocytes from injury. In order to elucidate the mechanisms underlying hormone-mediated podocyte-protecting activity from injury, we carried out microarray gene expression studies to identify the target genes and corresponding pathways in response to these hormones during podocyte differentiation. We used immortalized human cultured podocytes (HPCs) as a model system and carried out in vitro differentiation assays followed by dexamethasone (Dex) or vitamin D3 (VD3) treatment. Upon the induction of differentiation, multiple functional categories including cell cycle, organelle dynamics, mitochondrion, apoptosis and cytoskeleton organization were among the most significantly affected. Interestingly, while Dex and VD3 are capable of protecting podocytes from injury, they only share limited target genes and affected pathways. Compared to VD3 treatment, Dex had a broader and greater impact on gene expression profiles. In-depth analyses of Dex altered genes indicate that Dex crosstalks with a broad spectrum of signaling pathways, of which inflammatory responses, cell migration, angiogenesis, NF-κB and TGFβ pathways are predominantly altered. Together, our study provides new information and identifies several new avenues for future investigation of hormone signaling in podocytes.

Membrane trafficking in podocyte health and disease

Podocytes are highly specialized epithelial cells localized in the kidney glomerulus. The distinct cell signaling events and unique cytoskeletal architecture tailor podocytes to withstand changes in hydrostatic pressure during glomerular filtration. Alteration of glomerular filtration leads to kidney disease and frequently manifests with proteinuria. It has been increasingly recognized that cell signaling and cytoskeletal dynamics are coupled more tightly to membrane trafficking than previously thought. Membrane trafficking coordinates the cross-talk between protein networks and signaling cascades in a spatially and temporally organized fashion and may be viewed as a communication highway between the cell exterior and interior. Membrane trafficking involves transport of cargo from the plasma membrane to the cell interior (i.e., endocytosis) followed by cargo trafficking to lysosomes for degradation or to the plasma membrane for recycling. Yet, recent studies indicate that the conventional classification does not fully reflect the complex and versatile nature of membrane trafficking. While the increasing complexity of elaborate protein scaffolds and signaling cascades is being recognized in podocytes, the role of membrane trafficking is less well understood. This review will focus on the role of membrane trafficking in podocyte health and disease.

Alpha-actinin 4 and tumorigenesis of breast cancer

Alpha-actinins (ACTNs) were originally identified as cytoskeletal proteins which cross-link filamentous actin to establish cytoskeletal architect that protects cells from mechanical stress and controls cell movement. Notably, unlike other ACTNs, alpha-actinin 4 (ACTN4) displays unique characteristics in signaling transduction, nuclear translocation, and gene expression regulation. Initial reports indicated that ACTN4 is part of the breast cancer cell motile apparatus and is highly expressed in the nucleus. These results imply that ACTN4 plays a role in breast cancer tumorigenesis. While several observations in breast cancer and other cancers support this hypothesis, little direct evidence links the tumorigenic phenotype with ACTN4-mediated pathological mechanisms. Recently, several studies have demonstrated that in addition to its role in coordinating cytoskeleton, ACTN4 interacts with signaling mediators, chromatin remodeling factors, and transcription factors including nuclear receptors. Thus, ACTN4 functions as a versatile promoter for breast cancer tumorigenesis and appears to be an ideal drug target for future therapeutic development.