Inverted Formin 2 Regulates Actin Dynamics by Antagonizing Rho/Diaphanous-related Formin Signaling
*Division of Nephrology, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MassachusettsJournal of the American Society of Nephrology (Impact Factor: 9.34). 04/2013; 24(6). DOI: 10.1681/ASN.2012080834
Mutations in inverted formin 2 INF2 are a common cause of familial FSGS. INF2 interacts with diaphanous-related formins (mDia) and antagonizes mDia-mediated actin polymerization in response to active Rho signaling, suggesting that dysregulation of these pathways may mediate the development of INF2-related FSGS. However, the precise mechanisms by which INF2 regulates actin-dependent podocyte behavior remain largely unknown. Here, we investigated the possible role of INF2 in both lamellipodia-associated actin dynamics and actin-dependent slit diaphragm (SD) protein trafficking by manipulating the expression of INF2 and the activity of Rho/mDia signaling in cultured podocytes. Activation of mDia in the absence of INF2 led to defective formation of lamellipodia and abnormal SD trafficking. Effects of mutations disrupting the INF2-mDia interaction suggested the specificity of the mDia-antagonizing effect of INF2 in maintaining the lamellipodium. Furthermore, we found that SD trafficking requires INF2 interaction with lipid raft components. In summary, INF2 regulates lamellipodial actin dynamics and the trafficking of slit diaphragm proteins by opposing Rho/mDia-mediated actin polymerization. Thus, in podocytes, INF2 appears to be an important modulator of actin-dependent behaviors that are under the control of Rho/mDia signaling.
- "La maggior parte delle mutazioni sono raggruppate negli esoni che codificano per il dominio diafano inibitorio (DID) della molecola, e quelle delle forme sindromiche sono per lo più situate tra due DID-binding pockets, e producono alterazioni funzionali più gravi della proteina rispetto alle mutazioni delle forme non-sindromiche. Il DID media l'autoinibizione di INF2 attraverso la sua interazione con il dominio diafano C-terminale e consente a INF2 di accelerare la polimerizzazione/depolimerizzazione di actina e di regolare il targeting alla membrana cellulare mediante la formazione di complessi con Rho, Cdc42, MAL (myelin and lymphocyte protein), e MAL2 sia nei podociti che nelle cellule di Schwann  (full text)  (full text). "
Article: [Podocytes: genetics and biology][Show abstract] [Hide abstract]
ABSTRACT: Progresses in podocyte biology have been strictly connected with genetic advances; the identification of genes mutated in familial and sporadic forms of nephrotic syndrome has been followed by functional studies of the encoded proteins, revealing numerous properties of the cell. The molecules uncovered so far belong to three main categories: a) proteins located at the slit diaphragm, the intercellular junction which laterally connects podocyte processes and is responsible for selectivity of the glomerular filter, b) molecules involved in regulation of actin dynamics, which are essential for the maintenance of podocyte structure and function, and c) molecules belonging to intracellular organelles, such as mitochondria and lysosomes, which are central players in podocyte metabolism. Considering the key role of the podocyte in health and disease of the glomerular filter, better knowledge of this cell is a pre-requisite for developing targeted therapies of glomerular diseases.
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- "In cultured cells, we have found that this prevents the dampening effect of INF2 on Rho/Dia signaling, leading to imbalanced actin dynamics, perturbation of actin polymerization, and disruption of actin based cellular remodeling processes (e.g. lamellipodia formation and trafficking to the membrane of cultured podocytes (Sun et al., 2013)). "
ABSTRACT: Mutations in Inverted Formin 2 (INF2), a diaphanous formin family protein that regulates actin cytoskeleton dynamics, cause focal segmental glomerulosclerosis (FSGS) and Charcot-Marie-Tooth Disease (CMT) in humans. In addition to directly remodeling actin filaments in vitro, we have shown that INF2 regulates intracellular actin dynamics and actin dependent cellular behavior by opposing Rhoa/Dia signaling. As a step towards a better understanding of the human kidney disease, we wanted to explore the relevance of these findings to the in vivo situation. We used dose dependent knockdown of INF2 to first define an in vivo model and establish an overt glomerular phenotype in zebrafish. This simple assay was validated by rescue with wild type INF2 confirming the specificity of the findings. The edema, podocyte dysfunction, and an altered glomerular filtration barrier observed in the zebrafish pronephros correlate with mistrafficking of glomerular slit diaphragm proteins, defective slit-diaphragm signaling, and disinhibited diaphanous formin (mDia) activity. In contrast to wild-type human INF2, INF2 mutants associated with kidney disease fail to rescue the zINF2 morphant phenotype. Of particular interest, this INF2 knockdown phenotype is also rescued by loss of either RhoA or Dia2. This simple assay allows the demonstration that INF2 functions, at least in part, to modulate Dia-mediated Rho signaling, and that disease causing mutations specifically impair this regulatory function. These data support a model in which disease-associated diaphanous inhibitory domain (DID) mutants in INF2 interfere with its binding to and inhibition of Dia, leading to uncontrolled Rho/Dia signaling and perturbed actin dynamics. Methods to fine tune Rho signaling in the glomerulus may lead to new approaches to therapy in humans.
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ABSTRACT: Nephrotic syndrome, characterized by massive proteinuria, hypoalbuminemia and edema, is one of the most common kidney diseases in children. Although glucocorticoids (GCs), the mainstay of therapy for over 50 years, are effective in most children, more than 20% develop GC resistant nephrotic syndrome (SRNS), among whom focal segmental glomerular sclerosis (FSGS) is a frequent pathological outcome and the cause of endstage renal disease with a prevalence of 4% in the USA. Despite its clinical importance, the molecular basis of SRNS is unknown. In recent years, researchers have not only gained a new understanding of the roles of structural and functional abnormalities in GC receptors (GRs) in GC resistance, but have also gradually discovered close relationships between GC resistance in idiopathic nephrotic syndrome and podocyte-related molecules, like slit diaphragm (SD) molecules and so on. Here we mainly discussed these molecules and their physiological as well as pathological effects, including nephrin, podocin, CD2-associated protein (CD2AP), a-actinin-4, transient receptor potential cation channel 6 (TRPC6), phospholipase C epsilon-1 (PLCe1), Wilms' tumor suppressor gene 1 (WT1), Lmx1b, LAMB2, myosin 1e (MOY1E) and inverted formin 2 (IFN2). Mitochondrial cytopathies are also involved in GC resistance and well-reviewed [1, 2], which will not be discussed in detail in this review. To those SRNS without any genetic defects, immunological disturbances are always involved and should be stressed. In this article, recent progress in research on the mechanisms of GC resistance in idiopathic nephrotic syndrome is reviewed.
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