SLK-mediated phosphorylation of paxillin is required for focal adhesion turnover and cell migration

Role of the Ste20-like kinase SLK in podocyte adhesion

SLK controls the cytoskeleton, cell adhesion, and migration. Podocyte-specific deletion of SLK in mice leads to podocyte injury as mice age and exacerbates injury in experimental focal segment glomerulosclerosis (FSGS; adriamycin nephrosis). We hypothesized that adhesion proteins may be substrates of SLK. In adriamycin nephrosis, podocyte ultrastructural injury was exaggerated by SLK deletion. Analysis of a protein kinase phosphorylation site dataset showed that podocyte adhesion proteins-paxillin, vinculin, and talin-1 may be potential SLK substrates. In cultured podocytes, deletion of SLK increased adhesion to collagen. Analysis of paxillin, vinculin, and talin-1 showed that SLK deletion reduced focal adhesion complexes (FACs) containing these proteins mainly in adriamycin-induced injury; there was no change in FAC turnover (focal adhesion kinase Y397 phosphorylation). In podocytes, paxillin S250 showed basal phosphorylation that was slightly enhanced by SLK; however, SLK did not phosphorylate talin-1. In adriamycin nephrosis, SLK deletion did not alter glomerular expression/localization of talin-1 and vinculin, but increased focal adhesion kinase phosphorylation modestly. Therefore, SLK decreases podocyte adhesion, but FAC proteins in podocytes are not major substrates of SLK in health and disease.

The Rac-GEF Tiam1 controls integrin-dependent neutrophil responses

Rac GTPases are required for neutrophil adhesion and migration, and for the neutrophil effector responses that kill pathogens. These Rac-dependent functions are impaired when neutrophils lack the activators of Rac, Rac-GEFs from the Prex, Vav, and Dock families. In this study, we demonstrate that Tiam1 is also expressed in neutrophils, governing focal complexes, actin cytoskeletal dynamics, polarisation, and migration, in a manner depending on the integrin ligand to which the cells adhere. Tiam1 is dispensable for the generation of reactive oxygen species but mediates degranulation and NETs release in adherent neutrophils, as well as the killing of bacteria. In vivo, Tiam1 is required for neutrophil recruitment during aseptic peritonitis and for the clearance of Streptococcus pneumoniae during pulmonary infection. However, Tiam1 functions differently to other Rac-GEFs. Instead of promoting neutrophil adhesion to ICAM1 and stimulating β2 integrin activity as could be expected, Tiam1 restricts these processes. In accordance with these paradoxical inhibitory roles, Tiam1 limits the fMLP-stimulated activation of Rac1 and Rac2 in adherent neutrophils, rather than activating Rac as expected. Tiam1 promotes the expression of several regulators of small GTPases and cytoskeletal dynamics, including αPix, Psd4, Rasa3, and Tiam2. It also controls the association of Rasa3, and potentially αPix, Git2, Psd4, and 14-3-3ζ/δ, with Rac. We propose these latter roles of Tiam1 underlie its effects on Rac and β2 integrin activity and on cell responses. Hence, Tiam1 is a novel regulator of Rac-dependent neutrophil responses that functions differently to other known neutrophil Rac-GEFs.

Revealing phosphorylation regulatory networks during embryogenesis of honey bee worker and drone (Apis mellifera)

Protein phosphorylation is known to regulate a comprehensive scenario of critical cellular processes. However, phosphorylation-mediated regulatory networks in honey bee embryogenesis are mainly unknown. We identified 6342 phosphosites from 2438 phosphoproteins and predicted 168 kinases in the honey bee embryo. Generally, the worker and drone develop similar phosphoproteome architectures and major phosphorylation events during embryogenesis. In 24 h embryos, protein kinases A play vital roles in regulating cell proliferation and blastoderm formation. At 48–72 h, kinase subfamily dual-specificity tyrosine-regulated kinase, cyclin-dependent kinase (CDK), and induced pathways related to protein synthesis and morphogenesis suggest the centrality to enhance the germ layer development, organogenesis, and dorsal closure. Notably, workers and drones formulated distinct phosphoproteome signatures. For 24 h embryos, the highly phosphorylated serine/threonine-protein kinase minibrain, microtubule-associated serine/threonine-protein kinase 2 (MAST2), and phosphorylation of mitogen-activated protein kinase 3 (MAPK3) at Thr564 in workers, are likely to regulate the late onset of cell proliferation; in contrast, drone embryos enhanced the expression of CDK12, MAPK3, and MAST2 to promote the massive synthesis of proteins and cytoskeleton. In 48 h, the induced serine/threonine-protein kinase and CDK12 in worker embryos signify their roles in the construction of embryonic tissues and organs; however, the highly activated kinases CDK1, raf homolog serine/threonine-protein kinase, and MAST2 in drone embryos may drive the large-scale establishment of tissues and organs. In 72 h, the activated pathways and kinases associated with cell growth and tissue differentiation in worker embryos may promote the configuration of rudimentary organs. However, kinases implicated in cytoskeleton organization in drone embryos may drive the blastokinesis and dorsal closure. Our hitherto most comprehensive phosphoproteome offers a valuable resource for signaling research on phosphorylation dynamics in honey bee embryos.

Interphase microtubule disassembly is a signaling cue that drives cell rounding at mitotic entry

At mitotic entry, reorganization of the actomyosin cortex prompts cells to round-up. Proteins of the ezrin, radixin, and moesin family (ERM) play essential roles in this process by linking actomyosin forces to the plasma membrane. Yet, the cell-cycle signal that activates ERMs at mitotic entry is unknown. By screening a compound library using newly developed biosensors, we discovered that drugs that disassemble microtubules promote ERM activation. We further demonstrated that disassembly of interphase microtubules at mitotic entry directs ERM activation and metaphase cell rounding through GEF-H1, a Rho-GEF inhibited by microtubule binding, RhoA, and its kinase effector SLK. We finally demonstrated that GEF-H1 and Ect2, another Rho-GEF previously identified to control actomyosin forces, act together to drive activation of ERMs and cell rounding in metaphase. In summary, we report microtubule disassembly as a cell-cycle signal that controls a signaling network ensuring that actomyosin forces are efficiently integrated at the plasma membrane to promote cell rounding at mitotic entry.

Ste20-like Kinase Is Critical for Inhibitory Synapse Maintenance and Its Deficiency Confers a Developmental Dendritopathy

The size and structure of the dendritic arbor play important roles in determining how synaptic inputs of neurons are converted to action potential output. The regulatory mechanisms governing the development of dendrites, however, are insufficiently understood. The evolutionary conserved Ste20/Hippo kinase pathway has been proposed to play an important role in regulating the formation and maintenance of dendritic architecture. A key element of this pathway, Ste20-like kinase (SLK), regulates cytoskeletal dynamics in non-neuronal cells and is strongly expressed throughout neuronal development. However, its function in neurons is unknown. We show that, during development of mouse cortical neurons, SLK has a surprisingly specific role for proper elaboration of higher, ≥ third-order dendrites both in male and in female mice. Moreover, we demonstrate that SLK is required to maintain excitation-inhibition balance. Specifically, SLK knockdown caused a selective loss of inhibitory synapses and functional inhibition after postnatal day 15, whereas excitatory neurotransmission was unaffected. Finally, we show that this mechanism may be relevant for human disease, as dysmorphic neurons within human cortical malformations revealed significant loss of SLK expression. Overall, the present data identify SLK as a key regulator of both dendritic complexity during development and inhibitory synapse maintenance.SIGNIFICANCE STATEMENT We show that dysmorphic neurons of human epileptogenic brain lesions have decreased levels of the Ste20-like kinase (SLK). Decreasing SLK expression in mouse neurons revealed that SLK has essential functions in forming the neuronal dendritic tree and in maintaining inhibitory connections with neighboring neurons.

AKT-mediated phosphorylation of Sox9 induces Sox10 transcription in a murine model of HER2-positive breast cancer

Background

Approximately 5–10% of HER2-positive breast cancers can be defined by low expression of the Ste20-like kinase, SLK, and high expression of SOX10. Our lab has observed that genetic deletion of SLK results in the induction of Sox10 and significantly accelerates tumor initiation in a HER2-induced mammary tumor model. However, the mechanism responsible for the induction of SOX10 gene expression in this context remains unknown.

Methods

Using tumor-derived cell lines from MMTV-Neu mice lacking SLK and biochemical approaches, we have characterized the signaling mechanisms and relevant DNA elements driving Sox10 expression.

Results

Biochemical and genetic analyses of the SOX10 regulatory region in SLK-deficient mammary tumor cells show that Sox10 expression is dependent on a novel −7kb enhancer that harbors three SoxE binding sites. ChIP analyses demonstrate that Sox9 is bound to those elements in vivo. Our data show that AKT can directly phosphorylate Sox9 in vitro at serine 181 and that AKT inhibition blocks Sox9 phosphorylation and Sox10 expression in SLK(-/-) tumor cells. AKT-mediated Sox9 phosphorylation increases its transcriptional activity on the Sox10 −7kb enhancer without altering its DNA-binding activity. Interestingly, analysis of murine and human mammary tumors reveals a direct correlation between the levels of active phospho-Sox9 S181 and Sox10 expression.

Conclusions

Our results have identified a novel Sox10 enhancer and validated Sox9 as a direct target for AKT. As Sox10 is a biomarker for triple-negative breast cancers (TNBC), these findings might have major implications in the targeting and treatment of those cancers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01435-6.

The Ste20-like kinase - a Jack of all trades?

Over the past 20 years, the Ste20-like kinase (SLK; also known as STK2) has emerged as a central regulator of cytoskeletal dynamics. Reorganization of the cytoskeleton is necessary for a plethora of biological processes including apoptosis, proliferation, migration, tissue repair and signaling. Several studies have also uncovered a role for SLK in disease progression and cancer. Here, we review the recent findings in the SLK field and summarize the various roles of SLK in different animal models and discuss the biochemical mechanisms regulating SLK activity. Together, these studies have revealed multiple roles for SLK in coupling cytoskeletal dynamics to cell growth, in muscle repair and in negative-feedback loops critical for cancer progression. Furthermore, the ability of SLK to regulate some systems appears to be kinase activity independent, suggesting that it may be an important scaffold for signal transduction pathways. These various findings reveal highly complex functions and regulation patterns of SLK in development and disease, making it a potential therapeutic target.

Research and Clinical Significance of the Differentially Expressed Genes TP63 and LMO4 in Human Immunodeficiency Virus-Related Penile Squamous Cell Carcinoma

To study the differential gene expression and clinical significance in human immunodeficiency virus-infected individuals (HIVIIs) with penile squamous cell carcinoma. At our hospital from 2019 to 2020, we selected six samples of HIV-related penile squamous cell carcinoma for the experimental group and six samples of non-HIV-related penile squamous cell carcinoma for the control group. Transcriptome sequencing of sample mRNAs was performed by high-throughput sequencing. Differential gene expression analysis, differential Gene Ontology (GO) enrichment analysis and differential Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were carried out, and the reads per kilobase per million reads (RPKM) value was used as a measure of gene expression. A total of 2418 differentially expressed genes were obtained, of which 663 were upregulated and 1755 were downregulated (absolute value of logFC >1 and p value <.05). On the basis of the significance of the GO enrichment analysis, we found that the tumor protein p63 (TP63) gene was significantly upregulated and that the LIM domain only 4 (LMO4) gene was significantly downregulated in the experimental group compared with the control group. KEGG pathway analysis of the differentially expressed genes revealed that DNA replication was the most significant pathway associated with the upregulated genes and cell adhesion molecule (CAM) metabolism was the most significant pathway associated with the downregulated genes. The gene expression profiles of HIV-related penile squamous cell carcinoma and non-HIV-related penile squamous cell carcinoma are significantly different and involve significant GO enrichment and KEGG metabolic pathways, and this is very meaningful for the study of non-AIDS-defining cancers (NADCs). Differential expression of genes may be an important target for the prevention of penile squamous cell carcinoma in HIVIIs.

Muscle-specific deletion of SLK/Stk2 enhances p38 activity and myogenesis in mdx mice

Duchenne's muscular dystrophy (DMD) is a severe muscle wasting disorder characterized by the loss of dystrophin expression, muscle necrosis, inflammation and fibrosis. Ongoing muscle regeneration is impaired by persistent cytokine stress, further decreasing muscle function. Patients with DMD rarely survive beyond their early 20s, with cardiac and respiratory dysfunction being the primary cause of death. Despite an increase in our understanding of disease progression as well as promising preclinical animal models for therapeutic intervention, treatment options for muscular dystrophy remain limited and novel therapeutic targets are required. Many reports suggest that the TGFβ signalling pathway is activated in dystrophic muscle and contributes to the pathology of DMD in part by impairing the differentiation of myoblasts into mature myofibers. Here, we show that in vitro knockdown of the Ste20-like kinase, SLK, can partially restore myoblast differentiation downstream of TGFβ in a Smad2/3 independent manner. In an mdx model, we demonstrate that SLK is expressed at high levels in regenerating myofibers. Muscle-specific deletion of SLK reduced leukocyte infiltration, increased myogenin and utrophin expression and enhanced differentiation. This was accompanied by resistance to eccentric contraction-induced injury in slow fiber type-enriched soleus muscles. Finally, we found that these effects were partially dependent on the upregulation of p38 signalling. Collectively, these results demonstrate that SLK downregulation can restore some aspects of disease progression in DMD.

Genetic ablation of SLK exacerbates glomerular injury in adriamycin nephrosis in mice

Ste20-like kinase SLK is critical for embryonic development and may play an important role in wound healing, muscle homeostasis, cell migration, and tumor growth. Mice with podocyte-specific deletion of SLK show albuminuria and damage to podocytes as they age. The present study addressed the role of SLK in glomerular injury. We induced adriamycin nephrosis in 3- to 4-mo-old control and podocyte SLK knockout (KO) mice. Compared with control, SLK deletion exacerbated albuminuria and loss of podocytes, synaptopodin, and podocalyxin. Glomeruli of adriamycin-treated SLK KO mice showed diffuse increases in the matrix and sclerosis as well as collapse of the actin cytoskeleton. SLK can phosphorylate ezrin. The complex of phospho-ezrin, Na⁺/H⁺ exchanger regulatory factor 2, and podocalyxin in the apical domain of the podocyte is a key determinant of normal podocyte architecture. Deletion of SLK reduced glomerular ezrin and ezrin phosphorylation in adriamycin nephrosis. Also, deletion of SLK reduced the colocalization of ezrin and podocalyxin in the glomerulus. Cultured glomerular epithelial cells with KO of SLK showed reduced ezrin phosphorylation and podocalyxin expression as well as reduced F-actin. Thus, SLK deletion leads to podocyte injury as mice age and exacerbates injury in adriamycin nephrosis. The mechanism may at least in part involve ezrin phosphorylation as well as disruption of the cytoskeleton and podocyte apical membrane structure.

Nanoporosity Stimulates Cell Spreading and Focal Adhesion Formation in Cells with Mutated Paxillin

We have evaluated the response to nanotopography of CHO-K1 cells that express wild-type paxillin or paxillin with mutations at serine 273 that inhibit phosphorylation. Cells were grown on nanoporous and polished titanium surfaces. With all cell types, immunofluorescence showed that adhesion and spreading were minimally affected on the treated surface and that the actin filaments were more abundant and well-aligned. Scanning electron microscopy revealed changes in cell shape and abundant filopodia with lateral nanoprotrusions in response to nanoporosity. Gene expression of proteins associated with cellular adhesion and protrusions was significantly increased on the nanoporous surface regardless of the cell type. In particular, α-actinin, Rac1, Cdc42, and ITGα1 were upregulated in S273 cells with alanine substitutions, whereas FAK, Pxn, and Src were downregulated, leading to improved focal adhesion formation. These findings suggest that the surface nanoporosity can "compensate for" the genetic mutations that affect the biomechanical relationship of cells to surfaces.

Centrosome-derived microtubule radial array, PCM-1 protein, and primary cilia formation

In animal cells, the centrosome nucleates and anchors microtubules (MT), forming their radial array. During interphase centrosome-derived MT, aster can either team up with other MT network or function in an autonomous manner. What is the function of the centrosome-derived MT aster? We suggested that it might play an important role in the formation of the primary cilium, the organelle obligatorily associated with the centrosome. PCM-1 (PeriCentriolar Matrix 1) protein, which participates in the organization of the primary cilium, is a part of pericentiolar satellites. They are transported to the centrosome along MTs by the motor protein dynein in a complex with its cofactor dynactin. Previously, we showed that SLK/LOSK phosphorylated the p150^Glued subunit of dynactin, thus promoting its centrosomal targeting followed by its participation in the retention of microtubules. Here, we found that under the repression of SLK/LOSK activity, the PCM-1 protein lost its pericentrosomal localization and was being dispersed throughout the cytoplasm. Despite that the alanine and glutamine mutants of p150^Glued had opposite effects on PCM-1 localization, they associated with PCM-1 to the same extent. The occurrence of primary cilia also significantly decreased when SLK/LOSK was repressed. These defects also correlated with a disturbance of the long-range transport in cells, whereas dynein-depending motility was intact. Treatment with the GSK-3β kinase inhibitor also resulted in the loss of the centrosome-derived MT aster, dispersion of PCM-1 over the cytoplasm, and reduction of primary cilia occurrence. Thus, kinases involved in the centrosome-derived MT aster regulation can indirectly control the formation of primary cilia in cells.

The ATP-dependent RNA helicase, DDX42 interacts with paxillin and regulates apoptosis and polarization of Ba/F3 cells

Paxillin is a focal adhesion adaptor protein, heavily phosphorylated at multiple tyrosine residues, as well as at serine 273 (S273), and is known to be critical for cytoskeleton rearrangement and cell migration. We previously found that paxillin plays a regulatory role in IL-3-dependent survival of Ba/F3 cells, a mouse pro-B cell line. In this study, by using overexpressed His6 tagged-paxillin as a bait, we found that DDX42, a DEAD-box RNA helicase, interacted with paxillin, inhibited apoptosis, and promoted polarization of Ba/F3 cells. His6 tagged-paxillin was stably overexpressed in Ba/F3 cells, pulled-down from cell lysates with Ni⁺-NTA beads, and analyzed by one-dimensional SDS-PAGE followed by LC–MS. We found that DDX42 co-precipitated with paxillin, as demonstrated by western blotting analysis of His6 tagged-paxillin precipitates with anti-DDX42 antibodies and His6 tagged-DDX42 precipitates with anti-paxillin antibodies. In addition, we observed a preferential interaction of DDX42 with the paxillin mutant, S273A, compared to the S273D mutant. Furthermore, DDX42 overexpression in Ba/F3 cells delayed the apoptosis induced by IL-3 deprivation and promoted restoration of the elongated shape in Ba/F3 cells induced by IL-3 re-supply after a 6 h-deprivation. These results suggested that DDX42 interacts with paxillin and participates in IL-3-dependent cell survival, as well as in the cytoskeletal rearrangements underlying polarization of Ba/F3 cells.

O-GlcNAcylation regulates integrin-mediated cell adhesion and migration via formation of focal adhesion complexes

O-GlcNAcylation is a post-translational modification of a protein serine or threonine residue catalyzed by O-GlcNAc transferase (OGT) in the nucleus and cytoplasm. O-GlcNAcylation plays important roles in the cellular signaling that affect the different biological functions of cells, depending upon cell type. However, whether or not O-GlcNAcylation regulates cell adhesion and migration remains unclear. Here, we used the doxycycline-inducible short hairpin RNA (shRNA) system to establish an OGT knockdown (KD) HeLa cell line and found that O-GlcNAcylation is a key regulator for cell adhesion, migration, and focal adhesion (FA) complex formation. The expression levels of OGT and O-GlcNAcylation were remarkably suppressed 24 h after induction of doxycycline. Knockdown of OGT significantly promoted cell adhesion, but it suppressed the cell migration on fibronectin. The immunostaining with paxillin, a marker for FA plaque, clearly showed that the number of FAs was increased in the KD cells compared with that in the control cells. The O-GlcNAcylation levels of paxillin, talin, and focal adhesion kinase were down-regulated in KD cells. Interestingly, the complex formation between integrin β1, focal adhesion kinase, paxillin, and talin was greatly increased in KD cells. Consistently, levels of active integrin β1 were significantly enhanced in KD cells, whereas they were decreased in cells overexpressing OGT. The data suggest a novel regulatory mechanism for O-GlcNAcylation during FA complex formation, which thereby affects integrin activation and integrin-mediated functions such as cell adhesion and migration.

Ste20-like kinase, SLK, a novel mediator of podocyte integrity

SLK is essential for embryonic development and may play a key role in wound healing, tumor growth, and metastasis. Expression and activation of SLK are increased in kidney development and during recovery from ischemic acute kidney injury. Overexpression of SLK in glomerular epithelial cells/podocytes in vivo induces injury and proteinuria. Conversely, reduced SLK expression leads to abnormalities in cell adhesion, spreading, and motility. Tight regulation of SLK expression thus may be critical for normal renal structure and function. We produced podocyte-specific SLK-knockout mice to address the functional role of SLK in podocytes. Mice with podocyte-specific deletion of SLK showed reduced glomerular SLK expression and activity compared with control. Podocyte-specific deletion of SLK resulted in albuminuria at 4-5 mo of age in male mice and 8-9 mo in female mice, which persisted for up to 13 mo. At 11-12 mo, knockout mice showed ultrastructural changes, including focal foot process effacement and microvillous transformation of podocyte plasma membranes. Mean foot process width was approximately twofold greater in knockout mice compared with control. Podocyte number was reduced by 35% in knockout mice compared with control, and expression of nephrin, synaptopodin, and podocalyxin was reduced in knockout mice by 20-30%. In summary, podocyte-specific deletion of SLK leads to albuminuria, loss of podocytes, and morphological evidence of podocyte injury. Thus, SLK is essential to the maintenance of podocyte integrity as mice age.

Transforming growth factor β-induced epithelial to mesenchymal transition requires the Ste20-like kinase SLK independently of its catalytic activity

Invasion can be stimulated in vitro using the soluble ligand transforming growth factor-β (TGFβ) to induce a process called epithelial-to-mesenchymal transition (EMT) characterized by cell-cell junction breakdown and an invasive phenotype. We have previously demonstrated a role for Ste20-like kinase SLK cell migration and invasion. Here we show that SLK depletion in NMuMG mammary epithelial cells significantly impairs their TGFβ-induced migration and invasion. Immunofluorescence studies show that a fraction of SLK localizes to E-cadherin-positive adherens junction and that SLK impairs the breakdown of cell-cell contacts. We find that SLK-depleted cultures express significantly lower levels of vimentin protein as well as Snai1 and E-cadherin mRNA levels following TGF-β treatment. Surprisingly, our data show that SLK depletion does not affect the activation and nuclear translocation of Smad3. Furthermore, we show that expression of a dominant negative kinase does not impair tight junction breakdown and rescues Snai1 mRNA expression levels. Together these data suggest that SLK plays a novel role in TGFβ-induced EMT, independent of Smads, in a kinase activity-independent manner.

Regulation of Ste20-like kinase, SLK, activity: Dimerization and activation segment phosphorylation

The Ste20-like kinase, SLK, has diverse cellular functions. SLK mediates organ development, cell cycle progression, cytoskeletal remodeling, cytokinesis, and cell survival. Expression and activity of SLK are enhanced in renal ischemia-reperfusion injury, and overexpression of SLK was shown to induce apoptosis in cultured glomerular epithelial cells (GECs) and renal tubular cells, as well as GEC/podocyte injury in vivo. The SLK protein consists of a N-terminal catalytic domain and an extensive C-terminal domain, which contains coiled-coils. The present study addresses the regulation of SLK activity. Controlled dimerization of the SLK catalytic domain enhanced autophosphorylation of SLK at T183 and S189, which are located in the activation segment. The full-length ectopically- and endogenously-expressed SLK was also autophosphorylated at T183 and S189. Using ezrin as a model SLK substrate (to address exogenous kinase activity), we demonstrate that dimerized SLK 1–373 or full-length SLK can effectively induce activation-specific phosphorylation of ezrin. Mutations in SLK, including T183A, S189A or T193A reduced T183 or S189 autophosphorylation, and showed a greater reduction in ezrin phosphorylation. Mutations in the coiled-coil region of full-length SLK that impair dimerization, in particular I848G, significantly reduced ezrin phosphorylation and tended to reduce autophosphorylation of SLK at T183. In experimental membranous nephropathy in rats, proteinuria and GEC/podocyte injury were associated with increased glomerular SLK activity and ezrin phosphorylation. In conclusion, dimerization via coiled-coils and phosphorylation of T183, S189 and T193 play key roles in the activation and signaling of SLK, and provide targets for novel therapeutic approaches.

Deletion of the Ste20-like kinase SLK in skeletal muscle results in a progressive myopathy and muscle weakness

Background

The Ste20-like kinase, SLK, plays an important role in cell proliferation and cytoskeletal remodeling. In fibroblasts, SLK has been shown to respond to FAK/Src signaling and regulate focal adhesion turnover through Paxillin phosphorylation. Full-length SLK has also been shown to be essential for embryonic development. In myoblasts, the overexpression of a dominant negative SLK is sufficient to block myoblast fusion.

Methods

In this study, we crossed the Myf5-Cre mouse model with our conditional SLK knockout model to delete SLK in skeletal muscle. A thorough analysis of skeletal muscle tissue was undertaken in order to identify defects in muscle development caused by the lack of SLK. Isometric force analysis was performed on adult knockout mice and compared to age-matched wild-type mice. Furthermore, cardiotoxin injections were performed followed by immunohistochemistry for myogenic markers to assess the efficiency muscle regeneration following SLK deletion.

Results

We show here that early deletion of SLK from the myogenic lineage does not markedly impair skeletal muscle development but delays the regenerative process. Interestingly, adult mice (~6 months) display an increase in the proportion of central nuclei and increased p38 activation. Furthermore, mice as young as 3 months old present with decreased force generation, suggesting that the loss of SLK impairs myofiber stability and function. Assessment of structural components revealed aberrant localization of focal adhesion proteins, such as FAK and paxillin. Our data show that the loss of SLK results in unstable myofibers resulting in a progressive myopathy. Additionally, the loss of SLK resulted in a delay in muscle regeneration following cardiotoxin injections.

Conclusions

Our results show that SLK is dispensable for muscle development and regeneration but is required for myofiber stability and optimal force generation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0119-1) contains supplementary material, which is available to authorized users.

Proteomic Analysis of the Peri-Infarct Area after Human Umbilical Cord Mesenchymal Stem Cell Transplantation in Experimental Stroke

Among various therapeutic approaches for stroke, treatment with human umbilical cord mesenchymal stem cells (hUC-MSCs) has acquired some promising results. However, the underlying mechanisms remain unclear. We analyzed the protein expression spectrum of the cortical peri-infarction region after ischemic stroke followed by treatment with hUC-MSCs, and found 16 proteins expressed differentially between groups treated with or without hUC-MSCs. These proteins were further determined by Gene Ontology term analysis and network with CD200-CD200R1, CCL21-CXCR3 and transcription factors. Three of them: Abca13, Grb2 and Ptgds were verified by qPCR and ELISA. We found the protein level of Abca13 and the mRNA level of Grb2 consistent with results from the proteomic analysis. Finally, the function of these proteins was described and the potential proteins that deserve to be further studied was also highlighted. Our data may provide possible underlying mechanisms for the treatment of stroke using hUC-MSCs.

Phosphotyrosine profiling of curcumin-induced signaling

BACKGROUND

Curcumin, derived from the rhizome Curcuma longa, is a natural anti-cancer agent and has been shown to inhibit proliferation and survival of tumor cells. Although the anti-cancer effects of curcumin are well established, detailed understanding of the signaling pathways altered by curcumin is still lacking. In this study, we carried out SILAC-based quantitative proteomic analysis of a HNSCC cell line (CAL 27) to investigate tyrosine signaling in response to curcumin.

RESULTS

Using high resolution Orbitrap Fusion Tribrid Fourier transform mass spectrometer, we identified 627 phosphotyrosine sites mapping to 359 proteins. We observed alterations in the level of phosphorylation of 304 sites corresponding to 197 proteins upon curcumin treatment. We report here for the first time, curcumin-induced alterations in the phosphorylation of several kinases including TNK2, FRK, AXL, MAPK12 and phosphatases such as PTPN6, PTPRK, and INPPL1 among others. Pathway analysis revealed that the proteins differentially phosphorylated in response to curcumin are known to be involved in focal adhesion kinase signaling and actin cytoskeleton reorganization.

CONCLUSIONS

The study indicates that curcumin may regulate cellular processes such as proliferation and migration through perturbation of the focal adhesion kinase pathway. This is the first quantitative phosphoproteomics-based study demonstrating the signaling events that are altered in response to curcumin. Considering the importance of curcumin as an anti-cancer agent, this study will significantly improve the current knowledge of curcumin-mediated signaling in cancer.

Glycogen synthase kinase 3β dictates podocyte motility and focal adhesion turnover by modulating paxillin activity: implications for the protective effect of low-dose lithium in podocytopathy

Aberrant focal adhesion turnover is centrally involved in podocyte actin cytoskeleton disorganization and foot process effacement. The structural and dynamic integrity of focal adhesions is orchestrated by multiple cell signaling molecules, including glycogen synthase kinase 3β (GSK3β), a multitasking kinase lately identified as a mediator of kidney injury. However, the role of GSK3β in podocytopathy remains obscure. In doxorubicin (Adriamycin)-injured podocytes, lithium, a GSK3β inhibitor and neuroprotective mood stabilizer, obliterated the accelerated focal adhesion turnover, rectified podocyte hypermotility, and restored actin cytoskeleton integrity. Mechanistically, lithium counteracted the doxorubicin-elicited GSK3β overactivity and the hyperphosphorylation and overactivation of paxillin, a focal adhesion-associated adaptor protein. Moreover, forced expression of a dominant negative kinase dead mutant of GSK3β highly mimicked, whereas ectopic expression of a constitutively active GSK3β mutant abolished, the effect of lithium in doxorubicin-injured podocytes, suggesting that the effect of lithium is mediated, at least in part, through inhibition of GSK3β. Furthermore, paxillin interacted with GSK3β and served as its substrate. In mice with doxorubicin nephropathy, a single low dose of lithium ameliorated proteinuria and glomerulosclerosis. Consistently, lithium therapy abrogated GSK3β overactivity, blunted paxillin hyperphosphorylation, and reinstated actin cytoskeleton integrity in glomeruli associated with an early attenuation of podocyte foot process effacement. Thus, GSK3β-modulated focal adhesion dynamics might serve as a novel therapeutic target for podocytopathy.

SLK/LOSK kinase regulates cell motility independently of microtubule organization and Golgi polarization

Cell motility is an essential complex process that requires actin and microtubule cytoskeleton reorganization and polarization. Such extensive rearrangement is closely related to cell polarization as a whole. The serine/threonine kinase SLK/LOSK is a potential regulator of cell motility, as it phosphorylates a series of cytoskeleton-bound proteins that collectively participate in the remodeling of migratory cell architecture. In this work, we report that SLK/LOSK is an indispensable regulator of cell locomotion that primarily acts through the small GTPase RhoA and the dynactin subunit p150(Glued). Both RhoA and dynactin affect cytoskeleton organization, polarization, and general cell locomotory activity to various extents. However, it seems that these events are independent of each other. Thus, SLK/LOSK kinase effectively functions as a switch that links all of the processes underlying cell motility to provide robust directional movement.

Regulation of Catalytic and Non-catalytic Functions of the Drosophila Ste20 Kinase Slik by Activation Segment Phosphorylation

Protein kinases carry out important functions in cells both by phosphorylating substrates and by means of regulated non-catalytic activities. Such non-catalytic functions have been ascribed to many kinases, including some members of the Ste20 family. The Drosophila Ste20 kinase Slik phosphorylates and activates Moesin in developing epithelial tissues to promote epithelial tissue integrity. It also functions non-catalytically to promote epithelial cell proliferation and tissue growth. We carried out a structure-function analysis to determine how these two distinct activities of Slik are controlled. We find that the conserved C-terminal coiled-coil domain of Slik, which is necessary and sufficient for apical localization of the kinase in epithelial cells, is not required for Moesin phosphorylation but is critical for the growth-promoting function of Slik. Slik is auto- and trans-phosphorylated in vivo. Phosphorylation of at least two of three conserved sites in the activation segment is required for both efficient catalytic activity and non-catalytic signaling. Slik function is thus dependent upon proper localization of the kinase via the C-terminal coiled-coil domain and activation via activation segment phosphorylation, which enhances both phosphorylation of substrates like Moesin and engagement of effectors of its non-catalytic growth-promoting activity.

Recruitment and activation of SLK at the leading edge of migrating cells requires Src family kinase activity and the LIM-only protein 4

The Ste20-like kinase SLK plays a pivotal role in cell migration and focal adhesion turnover and is regulated by the LIM domain-binding proteins Ldb1 and Ldb2. These adapter proteins have been demonstrated to interact with LMO4 in the organization of transcriptional complexes. Therefore, we have assessed the ability of LMO4 to also interact and regulate SLK activity. Our data show that LMO4 can directly bind to SLK and activate its kinase activity in vitro and in vivo. LMO4 can be co-precipitated with SLK following the induction of cell migration by scratch wounding and Cre-mediated deletion of LMO4 in conditional LMO4(fl/fl) fibroblasts inhibits cell migration and SLK activation. Deletion of LMO4 impairs Ldb1 and SLK recruitment to the leading edge of migrating cells. Supporting this, Src/Yes/Fyn-deficient cells (SYF) expressing very low levels of LMO4 do not recruit SLK to the leading edge. Re-expression of wildtype Myc-LMO4 in SYF cells, but not a mutant version, restores SLK localization and kinase activity. Overall, our data suggest that activation of SLK by haptotactic signals requires its recruitment to the leading edge by LMO4 in a Src-dependent manner. Furthermore, this establishes a novel cytosolic role for the transcriptional co-activator LMO4.

Essential role for the SLK protein kinase in embryogenesis and placental tissue development

BACKGROUND

Over the past decade, the Ste20-like kinase SLK, has been implicated in several signaling processes. SLK repression has been shown to impair cell cycle kinetics and inhibit FAK-mediated cell migration. Here, using a gene trapped allele, we have generated mice expressing a truncated form of the SLK kinase.

RESULTS

Our results show that an SLK-LacZ fusion protein is expressed in embryonic stem cells and in embryos throughout development. We find that the SLK-LacZ fusion protein is less efficient at phosphorylating substrates resulting in reduced cell proliferation within the embryos and angiogenic defects in the placentae of the homozygous mutant animals at embryonic day (E) 12.5. This results in marked developmental defects and apoptotic lesions in the embryos by E14.5.

CONCLUSIONS

Homozygotes expressing the SLK-LacZ fusion protein present with an embryonic lethal phenotype occurring between E12.5 and E14.5. Overall, we demonstrate a requirement for SLK kinase activity in the developing embryo and placenta.

SATB2 enhances migration and invasion in osteosarcoma by regulating genes involved in cytoskeletal organization

Osteosarcoma (OS) is the most common malignant bone tumor and the majority of recurrences are due to metastasis. However, the molecular mechanisms that regulate OS metastatic spread are largely unknown. In this study, we report that special AT-rich-binding protein 2 (SATB2) is highly expressed in OS cells and tumors. Short hairpin RNA-mediated knockdown of SATB2 (sh-SATB2) decreases migration and invasion of OS cells without affecting proliferation or viability. Microarray analysis identified genes that were differentially regulated by SATB2 including the actin-binding protein Epithelial Protein Lost In Neoplasm (EPLIN), which was upregulated in sh-SATB2 cells. Silencing EPLIN rescues the decreased invasion observed in sh-SATB2 cells. Pathway analyses of SATB2-regulated genes revealed enrichment of those involved in cytoskeleton dynamics, and increased stress fiber formation was detected in cells with SATB2 knockdown. Furthermore, sh-SATB2 cells exhibit increased RhoA, decreased Rac1 and increased phosphorylation of focal adhesion kinase (FAK) and paxillin. These findings identify SATB2 as a novel regulator of OS invasion, in part via effects on EPLIN and the cytoskeleton.

Decipher the dynamic coordination between enzymatic activity and structural modulation at focal adhesions in living cells

Focal adhesions (FAs) are dynamic subcellular structures crucial for cell adhesion, migration and differentiation. It remains an enigma how enzymatic activities in these local complexes regulate their structural remodeling in live cells. Utilizing biosensors based on fluorescence resonance energy transfer (FRET), we developed a correlative FRET imaging microscopy (CFIM) approach to quantitatively analyze the subcellular coordination between the enzymatic Src activation and the structural FA disassembly. CFIM reveals that the Src kinase activity only within the microdomain of lipid rafts at the plasma membrane is coupled with FA dynamics. FA disassembly at cell periphery was linearly dependent on this raft-localized Src activity, although cells displayed heterogeneous levels of response to stimulation. Within lipid rafts, the time delay between Src activation and FA disassembly was 1.2 min in cells seeded on low fibronectin concentration ([FN]) and 4.3 min in cells on high [FN]. CFIM further showed that the level of Src-FA coupling, as well as the time delay, was regulated by cell-matrix interactions, as a tight enzyme-structure coupling occurred in FA populations mediated by integrin αvβ₃, but not in those by integrin α₅β₁. Therefore, different FA subpopulations have distinctive regulation mechanisms between their local kinase activity and structural FA dynamics.

A combination of SILAC and nucleotide acyl phosphate labelling reveals unexpected targets of the Rsk inhibitor BI-D1870

Protein kinase inhibitors frequently have interesting effects that cannot be fully ascribed to the intended target kinase(s) but identifying additional targets that might explain the effects is not straightforward. By comparing two different inhibitors of the Rsk (p90 ribosomal S6 kinase) kinases, we found that the increasingly used compound BI-D1870 had biological effects in murine DCs (dendritic cells) that could not be solely ascribed to Rsk or other documented targets. We assessed the ability of BI-D1870 and a second Rsk inhibitor, BIX 02565 to protect enzyme active sites from reaction with biotinylated nucleotide acyl phosphates. Using SILAC (stable isotope labelling by amino acids in cell culture)-labelled DC lysates as a source of enzyme targets, we identify several kinases that interact with BI-D1870 but not with BIX 02565. We confirmed that these kinases, including Slk, Lok and Mst1, are inhibited by BI-D1870 but to a much lesser extent by BIX 02565 and that phosphorylation of some of their substrates is blocked by BI-D1870 in living cells. Our results suggest that the BI-D1870 inhibitor should be used with caution. The SILAC-based methodology we used should be useful for further comparative unbiased profiling of the target spectrum of kinase inhibitors with interesting biological effects under conditions that closely mimic those found in cells.

Paxillin suppresses the proliferation of HPS rat serum treated PASMCs by up-regulating the expression of cytoskeletal proteins

Hepatopulmonary syndrome (HPS) is a triad of advanced liver disease, intrapulmonary vasodilatation (IPVD), and arterial hypoxemia. The arterial hypoxemia induces pulmonary vascular remodelling (PVR). In recent studies, the role of the proliferation of pulmonary artery smooth muscle cells (PASMCs) in PVR associated with HPS has been established; the changes in cytoskeletal proteins play an essential role in the proliferation of PASMCs. Little is known about the relevance of cytoskeletal protein expression or the molecular mechanisms of PVR associated with HPS. In addition, it has been identified that paxillin could influence the cytoskeletal protein expression by some important signaling pathways in many diseases, including lung cancer and liver cancer. In this study, we found that HPS rat serum from a common bile duct ligation (CBDL) rat model decreased the expression of cytoskeletal proteins (α-actin, α-tubulin, and destrin) and enhanced the expression levels of paxillin mRNA and protein in PASMCs. After silencing paxillin with siRNA, we found that the down-regulation of cytoskeletal protein expression, induced by the HPS rat serum, was reversed. Additionally, we reported that HPS rat serum improved the proliferation of PASMCs and down-regulation of paxillin could significantly inhibit this variation. These findings suggest that the up-regulation of cytoskeletal protein expression, induced by the paxillin, may cause the dysregulation of PASMC proliferation as well as play a fundamental role in PVR associated with HPS. In conclusion, down-regulation of paxillin by siRNA results in the inhibition of the dysregulation of cytoskeletal proteins and proliferation of PASMCs, suggesting a potential therapeutic effect on PVR associated with HPS.

Ste20-like protein kinase SLK (LOSK) regulates microtubule organization by targeting dynactin to the centrosome

The protein kinase SLK (LOSK) phosphorylates the 1A isoform of the p150^Glued subunit of dynactin and targets it to the centrosome, where it maintains microtubule radial organization. In addition, dynactin phosphorylation is involved in Golgi reorientation in polarized cells.

The microtubule- and centrosome-associated Ste20-like kinase (SLK; long Ste20-like kinase [LOSK]) regulates cytoskeleton organization and cell polarization and spreading. Its inhibition causes microtubule disorganization and release of centrosomal dynactin. The major function of dynactin is minus end–directed transport along microtubules in a complex with dynein motor. In addition, dynactin is required for maintenance of the microtubule radial array in interphase cells, and depletion of its centrosomal pool entails microtubule disorganization. Here we demonstrate that SLK (LOSK) phosphorylates the p150^Glued subunit of dynactin and thus targets it to the centrosome, where it maintains microtubule radial organization. We show that phosphorylation is required only for centrosomal localization of p150^Glued and does not affect its microtubule-organizing properties: artificial targeting of nonphosphorylatable p150^Glued to the centrosome restores microtubule radial array in cells with inhibited SLK (LOSK). The phosphorylation site is located in a microtubule-binding region that is variable for two isoforms (1A and 1B) of p150^Glued expressed in cultured fibroblast-like cells (isoform 1B lacks 20 amino acids in the basic microtubule-binding domain). The fact that SLK (LOSK) phosphorylates only a minor isoform 1A of p150^Glued suggests that transport and microtubule-organizing functions of dynactin are distinctly divided between the two isoforms. We also show that dynactin phosphorylation is involved in Golgi reorientation in polarized cells.

Distinct roles for Ste20-like kinase SLK in muscle function and regeneration

BACKGROUND

Cell growth and terminal differentiation are controlled by complex signaling systems that regulate the tissue-specific expression of genes controlling cell fate and morphogenesis. We have previously reported that the Ste20-like kinase SLK is expressed in muscle tissue and is required for cell motility. However, the specific function of SLK in muscle tissue is still poorly understood.

METHODS

To gain further insights into the role of SLK in differentiated muscles, we expressed a kinase-inactive SLK from the human skeletal muscle actin promoter. Transgenic muscles were surveyed for potential defects. Standard histological procedures and cardiotoxin-induced regeneration assays we used to investigate the role of SLK in myogenesis and muscle repair.

RESULTS

High levels of kinase-inactive SLK in muscle tissue produced an overall decrease in SLK activity in muscle tissue, resulting in altered muscle organization, reduced litter sizes, and reduced breeding capacity. The transgenic mice did not show any differences in fiber-type distribution but displayed enhanced regeneration capacity in vivo and more robust differentiation in vitro.

CONCLUSIONS

Our results show that SLK activity is required for optimal muscle development in the embryo and muscle physiology in the adult. However, reduced kinase activity during muscle repair enhances regeneration and differentiation. Together, these results suggest complex and distinct roles for SLK in muscle development and function.