Proteomics analysis of the non-muscle myosin heavy chain IIa-enriched actin-myosin complex reveals multiple functions within the podocyte

BORG family proteins in physiology and human disease

The binder of rho GTPases (BORG)/Cdc42 effector proteins (Cdc42EP) family is composed of five Rho GTPase binding proteins whose functions and mechanism of actions are of emerging interest. Here, we review recent findings pertaining to the family as a whole and consider how these change our understanding of cellular organization. Recent studies have implicated BORGs in both fundamental physiology and in human diseases, mainly cancers. An emerging pattern suggests that BORG family members cancer-promoting properties are related to their ability to regulate the cytoskeleton, with many impacting the organization of acto-myosin stress fibers. This is consistent with the broader literature indicating that BORG family members are regulators of both the septin and actin cytoskeleton networks. The exact mechanism through which BORGs modify the cytoskeleton is not clear, but we consider here a few data-supported and speculative possibilities. Finally, we delve into how the Rho GTPase Cdc42 modifies BORG function in cells. This remains open-ended as Cdc42's effects on BORGs appear cell type- and cell state-dependent. Collectively, these data point to the importance of the BORG family and suggest broader themes in their function and regulation.

Urine-Derived Epithelial Cells as Models for Genetic Kidney Diseases

Epithelial cells exfoliated in human urine can include cells anywhere from the urinary tract and kidneys; however, podocytes and proximal tubular epithelial cells (PTECs) are by far the most relevant cell types for the study of genetic kidney diseases. When maintained in vitro, they have been proven extremely valuable for discovering disease mechanisms and for the development of new therapies. Furthermore, cultured patient cells can individually represent their human sources and their specific variants for personalized medicine studies, which are recently gaining much interest. In this review, we summarize the methodology for establishing human podocyte and PTEC cell lines from urine and highlight their importance as kidney disease cell models. We explore the well-established and recent techniques of cell isolation, quantification, immortalization and characterization, and we describe their current and future applications.

Modulation of proteomic and inflammatory signals by Bradykinin in podocytes

Podocyte damage is one of the hallmarks of diabetic nephropathy leading to proteinuria and kidney damage. The underlying mechanisms of podocyte injury are not well defined. Bradykinin (BK) was shown to contribute to diabetic kidney disease. Here, we evaluated the temporal changes in proteome profile and inflammatory signals of podocytes in response to BK (10⁻⁷M). Protein profile was evaluated by liquid chromatography mass Spectrometry (LC-MS/MS) analysis. Proteome profile analysis of podocytes treated with BK (10⁻⁷M) for 3 and 6 h, revealed 61 proteins that were differentially altered compared to unstimulated control podocytes. Pathway enrichment analysis suggested inhibition of cell death pathways, engagement of cytoskeletal elements and activation of inflammatory pathways. One of the inflammatory proteins that was identified to be induced by BK treatment is Prostaglandin (PG) H Synthase-2 (Cyclooxygenase-2, COX-2). In addition, BK significantly induced the production and release of PGE₂ and this effect was inhibited by both COX-2 and MEK Kinase inhibitors, demonstrating that the production of PGE₂ by BK is mediated via COX-2 and MAPK-dependent mechanisms. These findings provide a global understanding of the effector modulated proteome in response to BK and also reveal BK as an important modulator of inflammation and a potential player in podocyte injury.

The Extracellular Matrix Receptor Discoidin Domain Receptor 1 Regulates Collagen Transcription by Translocating to the Nucleus

BACKGROUND

The discoidin domain receptor 1 (DDR1) is activated by collagens, upregulated in injured and fibrotic kidneys, and contributes to fibrosis by regulating extracellular matrix production, but how DDR1 controls fibrosis is poorly understood. DDR1 is a receptor tyrosine kinase (RTK). RTKs can translocate to the nucleus via a nuclear localization sequence (NLS) present on the receptor itself or a ligand it is bound to. In the nucleus, RTKs regulate gene expression by binding chromatin directly or by interacting with transcription factors.

METHODS

To determine whether DDR1 translocates to the nucleus and whether this event is mediated by collagen-induced DDR1 activation, we generated renal cells expressing wild-type or mutant forms of DDR1 no longer able to bind collagen. Then, we determined the location of the DDR1 upon collagen stimulation. Using both biochemical assays and immunofluorescence, we analyzed the steps involved in DDR1 nuclear translocation.

RESULTS

We show that although DDR1 and its natural ligand, collagen, lack an NLS, DDR1 is present in the nucleus of injured human and mouse kidney proximal tubules. We show that DDR1 nuclear translocation requires collagen-mediated receptor activation and interaction of DDR1 with SEC61B, a component of the Sec61 translocon, and nonmuscle myosin IIA and β-actin. Once in the nucleus, DDR1 binds to chromatin to increase the transcription of collagen IV, a major collagen upregulated in fibrosis.

CONCLUSIONS

These findings reveal a novel mechanism whereby activated DDR1 translates to the nucleus to regulate synthesis of profibrotic molecules.

Non-muscle Myosin II: Role in Microbial Infection and Its Potential as a Therapeutic Target

Currently, the major measures of preventing and controlling microbial infection are vaccinations and drugs. However, the appearance of drug resistance microbial mounts is main obstacle in current anti-microbial therapy. One of the most ubiquitous actin-binding proteins, non-muscle myosin II (NM II) plays a crucial role in a wide range of cellular physiological activities in mammals, including cell adhesion, migration, and division. Nowadays, growing evidence indicates that aberrant expression or activity of NM II can be detected in many diseases caused by microbes, including viruses and bacteria. Furthermore, an important role for NM II in the infection of some microbes is verified. Importantly, modulating the expression of NM II with small hairpin RNA (shRNA) or the activity of it by inhibitors can affect microbial-triggered phenotypes. Therefore, NM II holds the promise to be a potential target for inhibiting the infection of microbes and even treating microbial-triggered discords. In spite of these, a comprehensive view on the functions of NM II in microbial infection and the regulators which have an impact on the roles of NM II in this context, is still lacking. In this review, we summarize our current knowledge on the roles of NM II in microbial-triggered discords and provide broad insights into its regulators. In addition, the existing challenge of investigating the multiple roles of NM II in microbial infection and developing NM II inhibitors for treating these microbial-triggered discords, are also discussed.

MYH9 E1841K Mutation Augments Proteinuria and Podocyte Injury and Migration

Intronic variants of the MYH9 gene that encodes the nonmuscle myosin heavy chain IIA are associated with diabetic nephropathy in European Americans and with sickle cell disease-associated nephropathy. However, the causal functional variants of MYH9 have remained elusive. Rare missense mutations in MYH9 cause macrothrombocytopenia and are occasionally associated with development of nephropathy. The E1841K mutation is among the common MYH9 missense mutations and has been associated with nephropathy in some carriers. To determine the contribution of the E1841K mutation in kidney disease, we studied the effects of the E1841K mutation in mice subjected to high salt or angiotensin II (Ang II) as models of hypertension and in mice subjected to renal mass reduction as a model of CKD. Despite similar levels of BP among wild-type (MYH9^+/+ ) mice and mice heterozygous (MYH9^+/E1841K ) and homozygous (MYH9^{E1841K/E1841K} ) for the mutation in each model, MYH9^{E1841K/E1841K} mice exhibited mildly increased albuminuria in response to high salt; severe albuminuria, nephrinuria, FSGS, and podocyte foot effacement in Ang II-induced hypertension; and early mortality in the renal mass reduction model. Treatment with candesartan during Ang II-induced hypertension attenuated kidney disease development in MYH9^{E1841K/E1841K} mice. In vitro, isolated primary podocytes from MYH9^{E1841K/E1841K} mice exhibited increased lamellipodia formation and reorganization of F-actin stress fibers. Wound healing assays revealed that MYH9^+/+ podocytes had the lowest migration rate, followed by MYH9^+/E1841K then MYH9^{E1841K/E1841K} podocytes. In conclusion, the MYH9 E1841K variant alters podocyte cytoskeletal structure and renders podocytes more susceptible to injury after a damaging stimulus.

Injury-induced actin cytoskeleton reorganization in podocytes revealed by super-resolution microscopy

The architectural integrity of tissues requires complex interactions, both between cells and between cells and the extracellular matrix. Fundamental to cell and tissue homeostasis are the specific mechanical forces conveyed by the actomyosin cytoskeleton. Here we used super-resolution imaging methods to visualize the actin cytoskeleton in the kidney glomerulus, an organized collection of capillaries that filters the blood to make the primary urine. Our analysis of both mouse and human glomeruli reveals a network of myosin IIA-containing contractile actin cables within podocyte cell bodies and major processes at the outer aspects of the glomerular tuft. These likely exert force on an underlying network of myosin IIA-negative, noncontractile actin fibers present within podocyte foot processes that function to both anchor the cells to the glomerular basement membrane and stabilize the slit diaphragm against the pressure of fluid flow. After injuries that disrupt the kidney filtration barrier and cause foot process effacement, the podocyte's contractile actomyosin network relocates to the basolateral surface of the cell, manifesting as sarcomere-like structures juxtaposed to the basement membrane. Our findings suggest a new model of the podocyte actin cytoskeleton in health and disease and suggest the existence of novel mechanisms that regulate podocyte architecture.

Septin 7 reduces nonmuscle myosin IIA activity in the SNAP23 complex and hinders GLUT4 storage vesicle docking and fusion

Glomerular epithelial cells, podocytes, are insulin responsive and can develop insulin resistance. Here, we demonstrate that the small GTPase septin 7 forms a complex with nonmuscle myosin heavy chain IIA (NMHC-IIA; encoded by MYH9), a component of the nonmuscle myosin IIA (NM-IIA) hexameric complex. We observed that knockdown of NMHC-IIA decreases insulin-stimulated glucose uptake into podocytes. Both septin 7 and NM-IIA associate with SNAP23, a SNARE protein involved in GLUT4 storage vesicle (GSV) docking and fusion with the plasma membrane. We observed that insulin decreases the level of septin 7 and increases the activity of NM-IIA in the SNAP23 complex, as visualized by increased phosphorylation of myosin regulatory light chain. Also knockdown of septin 7 increases the activity of NM-IIA in the complex. The activity of NM-IIA is increased in diabetic rat glomeruli and cultured human podocytes exposed to macroalbuminuric sera from patients with type 1 diabetes. Collectively, the data suggest that the activity of NM-IIA in the SNAP23 complex plays a key role in insulin-stimulated glucose uptake into podocytes. Furthermore, we observed that septin 7 reduces the activity of NM-IIA in the SNAP23 complex and thereby hinders GSV docking and fusion with the plasma membrane.

•

Septin 7, nonmuscle myosin heavy chain IIA (NMHC-IIA) and SNAP23 form a complex.

•

Knockdown of septin 7 increases NM-IIA activity in the SNAP23 complex.

•

Insulin decreases septin 7 level and increases NM-IIA activity in the SNAP23 complex.

•

Septin 7 hinders GSV docking/fusion by reducing NM-IIA activity in the SNAP23 complex.

Non-muscle myosin-IIA is critical for podocyte f-actin organization, contractility, and attenuation of cell motility

Several glomerular pathologies resulting from podocyte injury are linked to genetic variation involving the MYH9 gene, which encodes the heavy chain of non-muscle myosin-IIA (NM-IIA). However, the functional role of NM-IIA has not been studied extensively in podocytes. We hypothesized that NM-IIA is critical for maintenance of podocyte structure and mechanical function. To test this hypothesis, we studied murine podocytes in vitro subjected to blebbistatin inhibition of NM-II activity, or RNA interference-mediated, isoform-specific ablation of Myh9 gene and protein (NM-IIA) or its paralog Myh10 gene and protein (NM-IIB). Using quantitative immunofluorescence microscopy, traction force microscopy, and attachment and "wound healing" assays, we found that NM-IIA ablation altered podocyte actin cytoskeletal structure and focal adhesion distribution, decreased cell attachment and contractility, and increased cell motility. Blebbistatin treatment had similar effects. NM-IIB ablation produced cells that exhibited poor attachment, but cytoskeletal structural organization, contractility and motility were maintained. These findings indicate that NM-IIA is essential for maintenance of podocyte cytoskeletal structure and mechanical function in vitro, and NM-IIB does not replace it in this role when NM-IIA expression is altered. We conclude that critical podocyte functions may be affected by MYH9 mutations or disease-associated haplotypes. © 2016 Wiley Periodicals, Inc.

The contractome – a systems view of actomyosin contractility in non-muscle cells

Actomyosin contractility is a highly regulated process that affects many fundamental biological processes in each and every cell in our body. In this Cell Science at a Glance article and the accompanying poster, we mined the literature and databases to map the contractome of non-muscle cells. Actomyosin contractility is involved in at least 49 distinct cellular functions that range from providing cell architecture to signal transduction and nuclear activity. Containing over 100 scaffolding and regulatory proteins, the contractome forms a highly complex network with more than 230 direct interactions between its components, 86 of them involving phosphorylation. Mapping these interactions, we identify the key regulatory pathways involved in the assembly of actomyosin structures and in activating myosin to produce contractile forces within non-muscle cells at the exact time and place necessary for cellular function.

Afadin regulates RhoA/Rho-associated protein kinase signaling to control formation of actin stress fibers in kidney podocytes

The function of kidney podocytes is closely associated with actin cytoskeleton. Rho family small GTPase RhoA promotes stress fiber assembly through Rho-associated protein kinase (ROCK)-dependent myosin II phosphorylation and plays an important role in maintenance of actin stress fibers of podocytes. However, little is known how stress fiber assembly is regulated in podocytes. Here, we found that afadin, an actin filament-binding protein, is required for RhoA/ROCK-dependent formation of actin stress fibers in rat podocyte C7 cells. We show that depletion of afadin in C7 cells induced loss of actin stress fibers. Conversely, forced expression of afadin increased the formation of actin stress fibers. Depletion of afadin inactivated RhoA and reduced the phosphorylation of myosin II. Moreover, the DIL domain of afadin appears to be responsible for actin stress fiber formation. Thus, afadin mediates RhoA/ROCK signaling and contributes to the formation of actin stress fibers in podocyte cells.