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.

Sox10-Deficient Drug-Resistant Melanoma Cells Are Refractory to Oncolytic RNA Viruses

Targeted therapy resistance frequently develops in melanoma due to intratumor heterogeneity and epigenetic reprogramming. This also typically induces cross-resistance to immunotherapies. Whether this includes additional modes of therapy has not been fully assessed. We show that co-treatments of MAPKi with VSV-based oncolytics do not function in a synergistic fashion; rather, the MAPKis block infection. Melanoma resistance to vemurafenib further perturbs the cells' ability to be infected by oncolytic viruses. Resistance to vemurafenib can be induced by the loss of SOX10, a common proliferative marker in melanoma. The loss of SOX10 promotes a cross-resistant state by further inhibiting viral infection and replication. Analysis of RNA-seq datasets revealed an upregulation of interferon-stimulated genes (ISGs) in SOX10 knockout populations and targeted therapy-resistant cells. Interestingly, the induction of ISGs appears to be independent of type I IFN production. Overall, our data suggest that the pathway mediating oncolytic resistance is due to the loss of SOX10 during acquired drug resistance in melanoma.

Periostin gene expression in neu-positive breast cancer cells is regulated by a FGFR signaling cross talk with TGFβ/PI3K/AKT pathways

Background

Breast cancer is a highly heterogeneous disease with multiple drivers and complex regulatory networks. Periostin (Postn) is a matricellular protein involved in a plethora of cancer types and other diseases. Postn has been shown to be involved in various processes of tumor development, such as angiogenesis, invasion, cell survival and metastasis. The expression of Postn in breast cancer cells has been correlated with a more aggressive phenotype. Despite extensive research, it remains unclear how epithelial cancer cells regulate Postn expression.

Methods

Using murine tumor models and human TMAs, we have assessed the proportion of tumor samples that have acquired Postn expression in tumor cells. Using biochemical approaches and tumor cell lines derived from Neu+ murine primary tumors, we have identified major regulators of Postn gene expression in breast cancer cell lines.

Results

Here, we show that, while the stromal compartment typically always expresses Postn, about 50% of breast tumors acquire Postn expression in the epithelial tumor cells. Furthermore, using an in vitro model, we show a cross-regulation between FGFR, TGFβ and PI3K/AKT pathways to regulate Postn expression. In HER2-positive murine breast cancer cells, we found that basic FGF can repress Postn expression through a PKC-dependent pathway, while TGFβ can induce Postn expression in a SMAD-independent manner. Postn induction following the removal of the FGF-suppressive signal is dependent on PI3K/AKT signaling.

Conclusion

Overall, these results reveal a novel regulatory mechanism and shed light on how breast tumor cells acquire Postn expression. This complex regulation is likely to be cell type and cancer specific as well as have important therapeutic implications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01487-8.

Antiviral Potential of the Antimicrobial Drug Atovaquone against SARS-CoV-2 and Emerging Variants of Concern

The antimicrobial medication malarone (atovaquone/proguanil) is used as a fixed-dose combination for treating children and adults with uncomplicated malaria or as chemoprophylaxis for preventing malaria in travelers. It is an inexpensive, efficacious, and safe drug frequently prescribed around the world. Following anecdotal evidence from 17 patients in the provinces of Quebec and Ontario, Canada, suggesting that malarone/atovaquone may present some benefits in protecting against COVID-19, we sought to examine its antiviral potential in limiting the replication of SARS-CoV-2 in cellular models of infection. In VeroE6 expressing human TMPRSS2 and human lung Calu-3 epithelial cells, we show that the active compound atovaquone at micromolar concentrations potently inhibits the replication of SARS-CoV-2 and other variants of concern including the alpha, beta, and delta variants. Importantly, atovaquone retained its full antiviral activity in a primary human airway epithelium cell culture model. Mechanistically, we demonstrate that the atovaquone antiviral activity against SARS-CoV-2 is partially dependent on the expression of TMPRSS2 and that the drug can disrupt the interaction of the spike protein with the viral receptor, ACE2. Additionally, spike-mediated membrane fusion was also reduced in the presence of atovaquone. In the United States, two clinical trials of atovaquone administered alone or in combination with azithromycin were initiated in 2020. While we await the results of these trials, our findings in cellular infection models demonstrate that atovaquone is a potent antiviral FDA-approved drug against SARS-CoV-2 and other variants of concern in vitro.

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.

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.

Assessing the efficacy of androgen receptor and Sox10 as independent markers of the triple-negative breast cancer subtype by transcriptome profiling

The Androgen Receptor (AR) has recently garnered a lot of attention as a potential biomarker and therapeutic target in hormone-dependent cancers, including breast cancer. However, several inconsistencies exist within the literature as to which subtypes of breast cancer express AR or whether it can be used to define its own unique subtype. Here, we analyze 1246 invasive breast cancer samples from the Cancer Genome Atlas and show that human breast cancers that have been subtyped based on their HER2, ESR1, or PGR expression contain four clusters of genes that are differentially expressed across all subtypes. We demonstrate that Sox10 is highly expressed in approximately one-third of all HER2/ESR1/PGR-low tumors and is a candidate biomarker of the triple-negative subtype. Although AR expression is acquired in many breast cancer cases, its expression could not define a unique subtype. Despite several reports stating that AR expression is acquired in HER2/ESR1/PGR triple-negative cancers, here we show that a low percentage of these cancers express AR (~20%). In contrast, AR is highly expressed in HER2-positive or ESR1/PGR-positive cancers (> 95%). Although AR expression cannot be used as an independent subtype biomarker, our analysis shows that routine evaluation of AR expression in tumors which express HER2, ESR1 and/or PGR may identify a unique subset of tumors which would benefit from anti-androgen based therapies.

Focal Adhesion Kinase Inhibitors in Combination with Erlotinib Demonstrate Enhanced Anti-Tumor Activity in Non-Small Cell Lung Cancer

Blockade of epidermal growth factor receptor (EGFR) activity has been a primary therapeutic target for non-small cell lung cancers (NSCLC). As patients with wild-type EGFR have demonstrated only modest benefit from EGFR tyrosine kinase inhibitors (TKIs), there is a need for additional therapeutic approaches in patients with wild-type EGFR. As a key component of downstream integrin signalling and known receptor cross-talk with EGFR, we hypothesized that targeting focal adhesion kinase (FAK) activity, which has also been shown to correlate with aggressive stage in NSCLC, would lead to enhanced activity of EGFR TKIs. As such, EGFR TKI-resistant NSCLC cells (A549, H1299, H1975) were treated with the EGFR TKI erlotinib and FAK inhibitors (PF-573,228 or PF-562,271) both as single agents and in combination. We determined cell viability, apoptosis and 3-dimensional growth in vitro and assessed tumor growth in vivo. Treatment of EGFR TKI-resistant NSCLC cells with FAK inhibitor alone effectively inhibited cell viability in all cell lines tested; however, its use in combination with the EGFR TKI erlotinib was more effective at reducing cell viability than either treatment alone when tested in both 2- and 3-dimensional assays in vitro, with enhanced benefit seen in A549 cells. This increased efficacy may be due in part to the observed inhibition of Akt phosphorylation when the drugs were used in combination, where again A549 cells demonstrated the most inhibition following treatment with the drug combination. Combining erlotinib with FAK inhibitor was also potent in vivo as evidenced by reduced tumor growth in the A549 mouse xenograft model. We further ascertained that the enhanced sensitivity was irrespective of the LKB1 mutational status. In summary, we demonstrate the effectiveness of combining erlotinib and FAK inhibitors for use in known EGFR wild-type, EGFR TKI resistant cells, with the potential that a subset of cell types, which includes A549, could be particularly sensitive to this combination treatment. As such, further evaluation of this combination therapy is warranted and could prove to be an effective therapeutic approach for patients with inherent EGFR TKI-resistant NSCLC.

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.

Loss of periostin/OSF-2 in ErbB2/Neu-driven tumors results in androgen receptor-positive molecular apocrine-like tumors with reduced Notch1 activity

INTRODUCTION

Periostin (Postn) is a secreted cell adhesion protein that activates signaling pathways to promote cancer cell survival, angiogenesis, invasion, and metastasis. Interestingly, Postn is frequently overexpressed in numerous human cancers, including breast, lung, colon, pancreatic, and ovarian cancer.

METHODS

Using transgenic mice expressing the Neu oncogene in the mammary epithelium crossed into Postn-deficient animals, we have assessed the effect of Postn gene deletion on Neu-driven mammary tumorigenesis.

RESULTS

Although Postn is exclusively expressed in the stromal fibroblasts of the mammary gland, Postn deletion does not affect mammary gland outgrowth during development or pregnancy. Furthermore, we find that loss of Postn in the mammary epithelium does not alter breast tumor initiation or growth in mouse mammary tumor virus (MMTV)-Neu expressing mice but results in an apocrine-like tumor phenotype. Surprisingly, we find that tumors derived from Postn-null animals express low levels of Notch protein and Hey1 mRNA but increased expression of androgen receptor (AR) and AR target genes. We show that tumor cells derived from wild-type animals do not proliferate when transplanted in a Postn-null environment but that this growth defect is rescued by the overexpression of active Notch or the AR target gene prolactin-induced protein (PIP/GCDFP-15).

CONCLUSIONS

Together our data suggest that loss of Postn in an ErbB2/Neu/HER2 overexpression model results in apocrine-like tumors that activate an AR-dependent pathway. This may have important implications for the treatment of breast cancers involving the therapeutic targeting of periostin or Notch signaling.

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.

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.

Ste20-like kinase SLK, at the crossroads: a matter of life and death

Reorganization of the cytoskeleton is necessary for apoptosis, proliferation, migration, development and tissue repair. However, it is well established that mutations or overexpression of key regulators contribute to the phenotype and progression of several pathologies such as cancer. For instance, c-src mutations and the overexpression of FAK have been implicated in the invasive and metastatic process, suggesting that components of the motility system may represent a new class of therapeutic targets. Over the last several years, we and others have established distinct roles for the Ste20-like kinase SLK, encompassing apoptosis, growth, motility and development. Here, we review the SLK field from its initial cloning to the most recent findings from our laboratory. We summarize the various roles of SLK and the biochemical mechanisms that regulate its activity. These various findings reveal very complex functions and pattern of regulation for SLK in development and cancer, making it a potential therapeutic target.

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

Focal adhesion turnover is a complex process required for cell migration. We have previously shown that the Ste20-like kinase (SLK) is required for cell migration and efficient focal adhesion (FA) turnover in a FA kinase (FAK)-dependent manner. However, the role of SLK in this process remains unclear. Using a candidate substrate approach, we show that SLK phosphorylates the adhesion adapter protein paxillin on serine 250. Serine 250 phosphorylation is required for paxillin redistribution and cell motility. Mutation of paxillin serine 250 prevents its phosphorylation by SLK in vitro and results in impaired migration in vivo as evidenced by an accumulation of phospho-FAK-Tyr397 and altered FA turnover rates. Together, our data suggest that SLK phosphorylation of paxillin on serine 250 is required for FAK-dependent FA dynamics.

Focal adhesion kinase inhibitors are potent anti-angiogenic agents

Focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase and scaffold protein localized to focal adhesions, is uniquely positioned at the convergence point of integrin and receptor tyrosine kinase signal transduction pathways. FAK is overexpressed in many tumor cells, hence various inhibitors targeting its activity have been tested for anti-tumor activity. However, the direct effects of these pharmacologic agents on the endothelial cells of the vasculature have not been examined. Using primary human umbilical vein endothelial cells (HUVEC), we characterized the effects of two FAK inhibitors, PF-573,228 and FAK Inhibitor 14 on essential processes for angiogenesis, such as migration, proliferation, viability and endothelial cell tube formation. We observed that treatment with either FAK Inhibitor 14 or PF-573,228 resulted in reduced HUVEC viability, migration and tube formation in response to vascular endothelial growth factor (VEGF). Furthermore, we found that PF-573,228 had the added ability to induce apoptosis of endothelial cells within 36 h post-drug administration even in the continued presence of VEGF stimulation. FAK inhibitors also resulted in modification of the actin cytoskeleton within HUVEC, with observed increased stress fiber formation in the presence of drug. Given that endothelial cells were sensitive to FAK inhibitors at concentrations well below those reported to inhibit tumor cell migration, we confirmed their ability to inhibit endothelial-derived FAK autophosphorylation and FAK-mediated phosphorylation of recombinant paxillin at these doses. Taken together, our data indicate that small molecule inhibitors of FAK are potent anti-angiogenic agents and suggest their utility in combinatorial therapeutic approaches targeting tumor angiogenesis.

The Ldb1 and Ldb2 transcriptional cofactors interact with the Ste20-like kinase SLK and regulate cell migration

Cell migration involves a multitude of signals that converge on cytoskeletal reorganization, essential for development, immune responses, and tissue repair. Here, we show that the microtubule-associated Ste20 kinase SLK, required for cell migration, interacts with the LIM domain binding transcriptional cofactor proteins Ldb1/CLIM2 and Ldb2/CLIM1/NLI. We demonstrate that Ldb1 and 2 bind directly to the SLK carboxy-terminal AT1-46 homology domain in vitro and in vivo. We find that Ldb1 and -2 colocalize with SLK in migrating cells and that both knockdown and overexpression of either factor results in increased motility. Supporting this, knockdown of Ldb1 increases focal adhesion turnover and enhances migration in fibroblasts. We propose that Ldb1/2 function to maintain SLK in an inactive state before its activation. These findings highlight a novel function for Ldb1 and -2 and expand their role to include the control of cell migration.

The Ste20-like kinase SLK is required for ErbB2-driven breast cancer cell motility

The Ste20-like kinase, SLK, is involved in the control of cell motility through its effects on actin reorganization and focal adhesion turnover. Here we investigated the role of SLK in chemotaxis downstream of the tyrosine kinase receptor, HER2/ErbB2/Neu, which is frequently overexpressed in human breast cancers. Our results show that SLK is required for the efficient cell migration of human and mouse mammary epithelial cell lines in the presence of the Neu activator, heregulin, as a chemoattractant. SLK activity is stimulated by heregulin treatment or by overexpression of activated Neu. Phosphorylation of tyrosine 1201 or tyrosines 1226/7 on Neu is a key event for SLK activation and cell migration, and cancer cell invasion mediated by these tyrosines is inhibited by kinase-inactive SLK. Signaling pathway inhibitors show that Neu-mediated SLK activation is dependent on MEK, PI3K, PLCgamma and Shc signaling. Furthermore, heregulin-stimulated SLK activity requires signals from the focal adhesion proteins, FAK and src. Finally, phospho-FAK analysis shows that SLK is required for Neu-dependent focal adhesion turnover. Together, these studies define an interaction between Neu and SLK signaling in the regulation of cancer cell motility.

A novel role for the Ste20 kinase SLK in adhesion signaling and cell migration

With over 60 members, the Sterile 20 family of kinases has been implicated in numerous biological processes, including growth, survival, apoptosis and cell migration. Recently, we have shown that, in addition to cell death, the Ste20-like kinase SLK is required for efficient cell migration in fibroblasts. We have observed that SLK is involved in cell motility through its effect on actin reorganization and microtubule-induced focal adhesion turnover. Scratch wounding of confluent monolayers results in SLK activation. The induction of SLK kinase activity requires the scaffold FAK and a MAPK-dependent pathway. However, its recruitment to the leading edge of migrating fibroblasts requires the activity of the Src family kinases. Since SLK is microtubule-associated, it may represent one of the signals delivered to focal contacts that induces adhesions turnover. A speculative model is proposed to illustrate the mechanism of SLK activation and recruitment at the leading edge of migrating cells.

FAK/src-family dependent activation of the Ste20-like kinase SLK is required for microtubule-dependent focal adhesion turnover and cell migration

Cell migration involves a multitude of signals that converge on cytoskeletal reorganization, essential for development, immune responses and tissue repair. Using knockdown and dominant negative approaches, we show that the microtubule-associated Ste20-like kinase SLK is required for focal adhesion turnover and cell migration downstream of the FAK/c-src complex. Our results show that SLK co-localizes with paxillin, Rac1 and the microtubules at the leading edge of migrating cells and is activated by scratch wounding. SLK activation is dependent on FAK/c-src/MAPK signaling, whereas SLK recruitment to the leading edge is src-dependent but FAK independent. Our results show that SLK represents a novel focal adhesion disassembly signal.

Cortical spreading depression releases ATP into the extracellular space and purinergic receptor activation contributes to the induction of ischemic tolerance

Cortical Spreading Depression (CSD) is a well-studied model of preconditioning that provides a high degree of tolerance to a subsequent ischemic event in the brain. The present study was undertaken in order to determine whether the release of ATP during CSD could contribute to the induction of ischemic tolerance. Direct measurement of ATP levels during CSD indicates that with each CSD wave ATP is released into the extracellular space at levels exceeding 100 microM. Cultures of rat primary cortical neurons exposed to low levels of extracellular ATP developed tolerance to subsequent oxygen-glucose deprivation (OGD) or metabolic hypoxia. The preconditioning effect requires new protein synthesis and develops with time, suggesting that a complex genomic response is required for the induction of tolerance. Multiple purinergic receptors are involved in mediating tolerance, with P2Y receptor activation having the greatest effect. Although extracellular adenosine or glutamate may make a small contribution, most of the tolerance was found to be induced independently of adenosine or glutamate receptor activation. Multiple signal transduction pathways mediate the response to extracellular ATP with the protein kinase A pathway and activation of phospholipase C contributing the most. The results are consistent with the proposal that CSD releases ATP into the extracellular space and the subsequent activation of P2Y receptors makes a major contribution to the induction of ischemic tolerance in the brain.

Transient expression of Nxf, a bHLH-PAS transactivator induced by neuronal preconditioning, confers neuroprotection in cultured cells

Cortical spreading depression (CSD) induces waves of neuronal depolarization that confer neuroprotection to subsequent ischemic events in the rat brain. To gain insights into the molecular mechanisms elicited by CSD, we used representational difference analysis (RDA) to identify mRNAs induced by potassium depolarization in vivo. Using this approach, we have isolated a cDNA encoding the SIM2-related bHLH-PAS protein Nxf. Our results confirm that Nxf mRNA and protein are rapidly and transiently expressed in cortical neurons following CSD. Reporter assays show that Nxf is a transcriptional activator that associates with the bHLH-PAS sub-class co-factor ARNT2. Adenovirus-mediated expression of epitope-tagged Nxf results in cell death and the direct activation of the Bax gene in cultured cells. However, RNA interference studies show that endogenous Nxf is required for optimal neuroprotection by preconditioning in cultured F-11 cells. Together, our data indicate that Nxf is a novel bHLH-PAS transactivator transiently induced by preconditioning and that its sustained expression is detrimental. The identification of Nxf may represent an important step in our understanding of the molecular mechanisms of brain preconditioning and injury.

v-Src-dependent Down-regulation of the Ste20-like Kinase SLK by Casein Kinase II

We have previously shown that the Ste20-like kinase SLK is a microtubule-associated protein inducing actin stress fiber disassembly. Here, we show that v-Src expression can down-regulate SLK activity. This down-regulation is independent of focal adhesion kinase but requires v-Src kinase activity and membrane translocation. SLK down-regulation by v-Src is indirect and is accompanied by SLK hyperphosphorylation on serine residues. Deletion analysis revealed that casein kinase II (CK2) sites at position 347/348 are critical for v-Src-dependent modulation of SLK activity. Further studies show that CK2 can directly phosphorylate SLK at these positions and that inhibition of CK2 in v-Src-transformed cells results in normal kinase activity. Finally, CK2 and SLK can be co-localized in fibroblasts spreading on fibronectin-coated substrates, suggesting a mechanism whereby SLK may be regulated at sites of actin remodeling, such as membrane lamellipodia and ruffles, through CK2.

Regulation of expression of early growth response transcription factors in rat primary cortical neurons by extracellular ATP

The zinc finger transcription factor early growth response-1 (Egr-1, NGFI-A, zif268, Krox 24, TIS8, ZENK) is upregulated immediately in the brain by cortical spreading depression (CSD) and other preconditioning stimuli and thus might participate in regulation of the overall genomic response to preconditioning. In the present study, the induction of expression of Egr-1 and other early growth response family members was characterized in rat primary cortical neuronal cultures. In neuronal cultures in vitro, depolarization or exposure to extracellular glutamate caused a 4-fold increase in egr-1 mRNA while exposure to extracellular ATP caused a 10-fold increase. The presence of mRNA encoding for multiple types of purinergic receptors was confirmed by RT-PCR. A number of nucleotide agonists proved effective in eliciting an increase in egr-1 mRNA. Over a limited range of concentration, the most effective agonists were ATP > ADP > alpha, beta-methylene ATP > UTP > cAMP > UDP > AMP > adenosine. Pertussis toxin, suramin, reactive blue 2, PPADS, DPCPX and inhibitors of Protein Kinase C, Protein Kinase A and PI3 kinase significantly reduced the upregulation of egr-1 by exposure to extracellular ATP. These findings suggest that neuronal metabotropic purinergic receptor activation contributes to the induction of early growth response transcription factors and may provide a target that can be manipulated to increase ischemic tolerance of the brain in vivo.

The Ste20-like kinase SLK is required for cell cycle progression through G2

We have previously shown that the Ste20-like kinase SLK is a microtubule-associated protein that can regulate actin reorganization during cell adhesion and spreading (Wagner, S., Flood, T. A., O'Reilly, P., Hume, K., and Sabourin, L. A. (2002) J. Biol. Chem. 277, 37685-37692). Because of its association with the microtubule network, we investigated whether SLK plays a role in cell cycle progression, a process that requires microtubule dynamics during mitosis. Consistent with microtubule association in exponentially growing cells, our results showed that SLK co-localizes with the mitotic spindle in cells undergoing mitosis. Expression of a kinase-inactive mutant or SLK small interfering RNAs inhibited cell proliferation and resulted in an accumulation of quiescent cells stimulated to re-enter the cell cycle in the G2 phase. Cultures expressing the mutant SLK displayed a normal pattern of cyclin D, E, and B expression but failed to down-regulate cyclin A levels, suggesting that they cannot proceed through M phase. In addition, these cultures displayed low levels of both phospho-H3 and active p34/cdc2 kinase. Overexpression of active SLK resulted in ectopic spindle assembly and the induction of cell cycle re-entry of Xenopus oocytes, suggesting that SLK is required for progression through G2 upstream of H1 kinase activation.

Ste20-like kinase SLK displays myofiber type specificity and is involved in C2C12 myoblast differentiation

Cell growth and terminal differentiation are controlled by complex signaling cascades that regulate the expression of specific subsets of genes implicated in cell fate and morphogenic processes. We have recently cloned and characterized a novel Ste20-like kinase termed SLK that is associated with adhesion structures during cell adhesion and spreading. However, the specific function of SLK is poorly understood. To gain further insight into the role of SLK, we have characterized its activity, expression, and distribution in skeletal muscle and during the in vitro differentiation of C2C12 myoblasts. Although SLK is expressed ubiquitously in adult tissues, our results show that it is predominantly expressed in muscle masses during development. Furthermore, SLK activity is upregulated during the differentiation of C2C12 myoblasts. In addition, we have found that SLK localizes presynaptically at neuromuscular junctions and that it is preferentially expressed in types I and IIA myofibers at major myofibrillar striations. Supporting a role in myoblast function and differentiation, SLK expression is induced in Myf5- and Pax7-positive activated satellite cells during regeneration and expression of dominant negative SLK in C2C12 cultures impairs myoblast fusion, suggesting a role for SLK in muscle cell differentiation.

Inhibition of myogenesis in transgenic mice expressing the human DMPK 3'-UTR

Myotonic dystrophy (DM1) is a multisystemic disorder caused by a CTG repeat expansion within the 3'-UTR of the DMPK gene. DM1 is characterized by delayed muscle development, muscle weakness and wasting, cardiac conduction abnormalities, cognitive defects and cataracts. Recent studies have demonstrated that the disease mechanism involves a dominant gain-of-function conferred upon mutant transcripts by expanded repeats. However, further attempts to model aspects of DM muscle pathology in cultured myoblasts suggest that 3'-UTR sequences flanking the CTG repeat tract are also required for full expression of the disease phenotype. Here, we report that overexpression of the DMPK 3'-UTR including either wild-type (11) or expanded (91) CTG repeats results in aberrant and delayed muscle development in fetal transgenic mice. In addition, transgenic animals with both expanded and wild-type CTG repeats display muscle atrophy at 3 months of age. Primary myoblast cultures from both 11 and 91 repeat mice display reduced fusion potential, but a greater reduction is observed in the 91 repeat cultures. Taken together, these data indicate that overexpression of the DMPK 3'-UTR interferes with normal muscle development in mice and that this is exacerbated by inclusion of a mutant repeat. This suggests that the delayed muscle development in DM1 involves an interplay between the expanded CTG repeat and adjacent 3'-UTR sequences.

PDZK1: I. A major scaffolder in brush borders of proximal tubular cells11See Editorial by Moe, p. 1916

BACKGROUND

In proximal tubular cells, PDZK1 (NaPi-Cap1) has been implicated in apical expression of the Na+-dependent phosphate cotransporter (NaPi-IIa) via interaction with its C-terminus. PDZK1 represents a multidomain protein consisting of four PDZ domains and thus is believed to have a broader specificity besides NaPi-IIa.

METHODS

We subjected single PDZ domains derived from PDZK1 either to yeast two-hybrid screens or yeast trap assays. Different pull-down assays and blot overlays were applied to corroborate the PDZK1-mediated interactions in vitro. Co-localization of interacting proteins with PDZK1 in proximal tubular cells was assessed by immunohistochemistry.

RESULTS

In the yeast screens, the most abundant candidate protein to interact with PDZK1 was the membrane-associated protein of 17 kD (MAP17). Besides MAP17, C-terminal parts of following transporters were also identified: NaPi-IIa, solute carrier SLC17A1 (NaPi-I), Na+/H+ exchanger (NHE-3), organic cation transporter (OCTN1), chloride-formate exchanger (CFEX), and urate-anion exchanger (URAT1). In addition, other regulatory factors were found among the clones, such as a protein kinase A (PKA)-anchoring protein (D-AKAP2) and N+/H+ exchanger regulator factor (NHERF-1). All interactions of itemized proteins with PDZK1 were affirmed by in vitro techniques. Apart from PDZK1, strong in vitro interactions of NHERF-1 were also observed with the solute transporters (excluding MAP17) and D-AKAP2. All identified proteins were immunolocalized in proximal tubular cells, wherein all membrane proteins co-localized with PDZK1 in brush borders.

CONCLUSION

We hypothesize that PDZK1 and NHERF-1 establish an extended network beneath the apical membrane to which membrane proteins and regulatory components are anchored.

Expression of the Ste20-like kinase SLK during embryonic development and in the murine adult central nervous system

Cell growth and terminal differentiation are controlled by complex signaling cascades that regulate the expression of specific subsets of genes controlling cell fate and morphogenic processes. We have recently cloned and characterized a novel Ste20-like kinase termed SLK (Sabourin et al., Mol. Cel. Biol. 20 (2000) 684). However, the specific function of SLK is poorly understood. To gain further insights into the role of SLK we have characterized its activity, expression and distribution in the CNS during embryonic development and in the adult brain. Although SLK is expressed ubiquitously in adult tissues, our results show that it is expressed preferentially in neuronal lineages during development. We find that SLK is preferentially expressed in the neurons and neuroepithelium of the developing embryo and can be detected at 10.5 and 12.5 days post-coitum (dpc) in the forebrain, midbrain and hindbrain of the developing CNS. At later stages (14.5 dpc), SLK is expressed in the hypothalamus region, all layers of the neural tube, dorsal root ganglion and in the proliferating ependymal layers. Surprisingly, following middle cerebral artery occlusion, SLK expressing neuronal cells are lost and SLK is localized to phagocytic macrophages/microglia. These results suggest a functional role for SLK in early neuronal development as well as in the adult CNS.

Association of the Ste20-like kinase (SLK) with the microtubule. Role in Rac1-mediated regulation of actin dynamics during cell adhesion and spreading

Cytoskeletal remodeling events are tightly regulated by signal transduction systems that impinge on adhesion components and modulators of cellular architecture. We have previously shown that the Ste20-like kinase (SLK) can induce apoptosis through the induction of actin disassembly and cellular retraction (Sabourin, L. A., Tamai, K., Seale, P., Wagner, J., and Rudnicki, M. A. (2000) Mol. Cell. Biol. 20, 684-696). Using immunofluorescence studies, we report that SLK is redistributed with adhesion components at large podosome-like adhesion sites in fibronectin-stimulated fibroblasts. However, in vitro kinase assays demonstrate that its activity is not modulated following fibronectin stimulation. Double immunofluorescence studies in exponentially growing or spreading fibroblasts show that SLK is associated with the microtubule network and can be coprecipitated with alpha-tubulin. Furthermore, the stimulation of adhesion site formation by microtubule-disrupting agents induces the relocalization of SLK with unpolymerized alpha-tubulin to large vinculin-containing adhesion complexes. Using microinjection studies, we show that ectopic expression of activated SLK induces the disassembly of actin stress fibers, a process that can be inhibited by dominant negative Rac1. Significantly, endogenous SLK can be colocalized with Rac1 in spreading cells on FN. Finally, the overexpression of SLK by adenoviral infection inhibits cell spreading on fibronectin. These results suggest that SLK is part of a microtubule-associated complex that is targeted to adhesion sites and implicated in the regulation of cytoskeletal dynamics.

Cortical spreading depression transiently activates MAP kinases

Cortical spreading depression (CSD) has been shown to have neuroprotective effects when administered in advance of cerebral ischemia. The mechanism by which CSD induces its neuroprotective effect however remains to be elucidated. Since MAP kinases have been shown to impart neuroprotection in ischemic preconditioning paradigms, we attempted to determine the role CSD may have in the activation of MAPK. We show that CSD is capable of increasing the phosphorylation of ERK in a MEK-dependent manner. This phosphorylation is, however, transient, as phosphorylated ERK levels return to control levels 45 min after 2 h of CSD elicitation. Immunohistochemical analysis reveals that the phosphorylated form of ERK is located ubiquitously in cells of the CSD-treated cortex while CSD-elicited MEK phosphorylation resides solely in the nuclei. These data suggest that CSD may act via the MAP kinase pathways to mediate preconditioning.

Pax7 is required for the specification of myogenic satellite cells

The paired box transcription factor Pax7 was isolated by representational difference analysis as a gene specifically expressed in cultured satellite cell-derived myoblasts. In situ hybridization revealed that Pax7 was also expressed in satellite cells residing in adult muscle. Cell culture and electron microscopic analysis revealed a complete absence of satellite cells in Pax7(-/-) skeletal muscle. Surprisingly, fluorescence-activated cell sorting analysis indicated that the proportion of muscle-derived stem cells was unaffected. Importantly, stem cells from Pax7(-/-) muscle displayed almost a 10-fold increase in their ability to form hematopoietic colonies. These results demonstrate that satellite cells and muscle-derived stem cells represent distinct cell populations. Together these studies suggest that induction of Pax7 in muscle-derived stem cells induces satellite cell specification by restricting alternate developmental programs.

Myotonic dystrophy (DM) protein kinase levels in congenital and adult DM patients

Myotonic dystrophy is caused by a (CTG)n trinucleotide repeat expansion located in the 3' untranslated region of the myotonic dystrophy protein kinase gene (DMPK). To date, the disease mechanism has proven elusive. The mutation would not be expected to affect kinase function and yet the disease is inherited in a dominant fashion. Mutant DMPK transcripts have been demonstrated to be retained in affected cell nuclei which could reduce DMPK protein levels and cause disease by haploinsufficiency. An alternate hypothesis is that the expansion confers a toxic gain of function on the transcript. In previous studies, various 52-55 kDa proteins have been detected using antisera targeted against DMPK and a decline of two of these candidates in disease tissues was reported. Current information now suggests that these proteins are not products of the myotonic dystrophy gene. We have characterised an antiserum which has been confirmed to recognise authentic 71 and 80 kDa isoforms of DMPK. Determination of the kinase levels in disease tissues with controls for patient age and tissue integrity demonstrates a modest overexpression in adult patients. In tissues from severely affected congenital patients only a slight decline is seen. This data argues against DMPK haploinsufficiency as a disease mechanism.

Caspase 3 cleavage of the Ste20-related kinase SLK releases and activates an apoptosis-inducing kinase domain and an actin-disassembling region

We have demonstrated that a novel Ste20-related kinase, designated SLK, mediates apoptosis and actin stress fiber dissolution through distinct domains generated by caspase 3 cleavage. Overexpression of SLK in C2C12 myoblasts stimulated the disassembly of actin stress fibers and focal adhesions and induced apoptosis, as determined by annexin V binding and terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling analysis. SLK was cleaved by caspase 3 in vitro and in vivo during c-Myc-, tumor necrosis factor alpha, and UV-induced apoptosis. Furthermore, cleavage of SLK released two domains with distinct activities: an activated N-terminal kinase domain that promoted apoptosis and cytoskeletal rearrangements and a C-terminus domain that disassembled actin stress fibers. Moreover, our analysis has identified a novel conserved region (termed the AT1-46 homology domain) that efficiently promotes stress fiber disassembly. Finally, transient transfection of SLK also activated the c-Jun N-terminal kinase signaling pathway. Our results suggest that caspase-activated SLK represents a novel effector of cytoskeletal remodeling and apoptosis.

The molecular regulation of myogenesis

Over the past years, several studies have unraveled important mechanisms by which the four myogenic regulatory factors (MRFs: MyoD, Myf-5, myogenin, and MRF4) control the specification and the differentiation of the muscle lineage. Early experiments led to the hypothesis that these factors were redundant and could functionally replace one another. However, recent experiments using in vivo and in vitro models have demonstrated that in fact different aspects of the myogenic program are controlled by different factors in vivo, suggesting that these factors play distinct roles during myogenesis. The activity of the MRFs during proliferation and differentiation of muscle precursor cells has clearly been demonstrated to be dependent on specific cell-cycle control mechanisms as well as distinct interactions with other regulatory molecules, such as the ubiquitously expressed E proteins and several other transcription factors. Furthermore, the observation that the MRFs can recruit chromatin remodeling proteins has shed some light on the mechanisms by which the MRFs activate gene expression. Recently, a functional role for MyoD during satellite cell activation and muscle repair has been identified in vivo, which cannot be substituted for by the other MRFs. This has put forward the hypothesis that these factors also play specific biological roles following muscle injury and repair.

Induction of apoptosis by SLK, a Ste20-related kinase

We have cloned and characterized a novel murine Ste20-related kinase designated SLK. SLK displays high homology to the Ste20-related kinase LOK, and is more distantly related to MST1 and 2, both Ste20-like kinases. In addition, SLK displays high homology to microtubule and nuclear associated protein (M-NAP) and AT1-46, both of unknown function. SLK is ubiquitously expressed as multiple mRNAs in tissues and cell lines and is downregulated by mitogen depletion in differentiating myoblasts. Biochemical characterization showed that SLK overexpression activates c-Jun amino-terminal kinase 1 (JNK1). However, in vitro kinase assays indicated that SLK was not activated in response to various growth factors or stress-inducing agents. Immunofluorescence studies revealed that SLK colocalized to distinct cytosolic domains, preferentially at the periphery of the cells. In addition, prolonged overexpression of SLK in cultured fibroblasts resulted in apoptosis as demonstrated by annexin-V and TUNEL staining. Our results suggest that SLK belongs to a new family of protein kinases, mediating activation of the stress response pathway through a novel signaling cascade.

Reduced differentiation potential of primary MyoD-/- myogenic cells derived from adult skeletal muscle

To gain insight into the regeneration deficit of MyoD-/- muscle, we investigated the growth and differentiation of cultured MyoD-/- myogenic cells. Primary MyoD-/- myogenic cells exhibited a stellate morphology distinct from the compact morphology of wild-type myoblasts, and expressed c-met, a receptor tyrosine kinase expressed in satellite cells. However, MyoD-/- myogenic cells did not express desmin, an intermediate filament protein typically expressed in cultured myoblasts in vitro and myogenic precursor cells in vivo. Northern analysis indicated that proliferating MyoD-/- myogenic cells expressed fourfold higher levels of Myf-5 and sixfold higher levels of PEA3, an ETS-domain transcription factor expressed in newly activated satellite cells. Under conditions that normally induce differentiation, MyoD-/- cells continued to proliferate and with delayed kinetics yielded reduced numbers of predominantly mononuclear myocytes. Northern analysis revealed delayed induction of myogenin, MRF4, and other differentiation-specific markers although p21 was upregulated normally. Expression of M-cadherin mRNA was severely decreased whereas expression of IGF-1 was markedly increased in MyoD-/- myogenic cells. Mixing of lacZ-labeled MyoD-/- cells and wild-type myoblasts revealed a strict autonomy in differentiation potential. Transfection of a MyoD-expression cassette restored cytomorphology and rescued the differentiation deficit. We interpret these data to suggest that MyoD-/- myogenic cells represent an intermediate stage between a quiescent satellite cell and a myogenic precursor cell.

Definition of regulatory sequence elements in the promoter region and the first intron of the myotonic dystrophy protein kinase gene

Myotonic dystrophy is the most common inherited adult neuromuscular disorder with a global frequency of 1/8000. The genetic defect is an expanding CTG trinucleotide repeat in the 3'-untranslated region of the myotonic dystrophy protein kinase gene. We present the in vitro characterization of cis regulatory elements controlling transcription of the myotonic dystrophy protein kinase gene in myoblasts and fibroblasts. The region 5' to the initiating ATG contains no consensus TATA or CCAAT box. We have mapped two transcriptional start sites by primer extension. Deletion constructs from this region fused to the bacterial chloramphenicol acetyltransferase reporter gene revealed only subtle muscle specific cis elements. The strongest promoter activity mapped to a 189-base pair fragment. This sequence contains a conserved GC box to which the transcription factor Sp1 binds. Reporter gene constructs containing a 2-kilobase pair first intron fragment of the myotonic dystrophy protein kinase gene enhances reporter activity up to 6-fold in the human rhabdomyosarcoma myoblast cell line TE32 but not in NIH 3T3 fibroblasts. Co-transfection of a MyoD expression vector with reporter constructs containing the first intron into 10 T1/2 fibroblasts resulted in a 10-20-fold enhancement of expression. Deletion analysis of four E-box elements within the first intron reveal that these elements contribute to enhancer activity similarly in TE32 myoblasts and 10 T1/2 fibroblasts. These data suggest that E-boxes within the myotonic dystrophy protein kinase first intron mediate interactions with upstream promoter elements to up-regulate transcription of this gene in myoblasts.

Overexpression of 3'-untranslated region of the myotonic dystrophy kinase cDNA inhibits myoblast differentiation in vitro

The genetic defect underlying myotonic dystrophy (DM) has been identified as an unstable CTG trinucleotide repeat amplification in the 3'-untranslated region (3'-UTR) of the DM kinase gene (DMK). Individuals with the most severe congenital form display a marked delay in muscle terminal differentiation. To gain insight into the role of DMK during myogenesis, we have examined the effect of DMK overexpression on the terminal differentiation of the murine myoblast cell line C2C12. We demonstrate that a 4-10-fold constitutive overexpression of DMK mRNA in myoblasts caused a marked inhibition of terminal differentiation. Surprisingly, this activity was mapped to a 239-nucleotide region of the 3'-UTR of the DMK transcript. When the DMK 3'-UTR was placed downstream of a reporter gene, the same inhibition of myogenesis was observed. Following the induction of differentiation of myoblast clones overexpressing the DMK 3'-UTR, the levels of myogenin mRNA were reduced by approximately 4-fold, whereas the steady state levels of mef-2c transcripts were not affected. These data suggest that overexpression of the DMK 3'-UTR may interfere with the expression of musclespecific mRNAs leading to a delay in terminal differentiation.

Investigation of myotonic dystrophy kinase isoform translocation and membrane association

Myotonic dystrophy is caused by the expansion of a CTG repeat found in the 3'-untranslated region of the myotonic dystrophy kinase. The mechanism of disease and the role of the kinase are currently obscure. Here we begin the investigation of domain structure/function correlations to aid in determining its normal function. Expressed full-length protein and protein truncated before a C-terminal hydrophobic domain were compared. In vitro, signal peptide function and protection of kinase by microsomal membranes were absent; thus, it is not translocated, as previously proposed. However, full-length kinase expressed in insect cells was found in fractions enriched for membranes and decorated mitochondria. The truncated form was found primarily in the cytosol. The kinase was present as two self-associated, disulfide-linked complexes. The majority of full-length kinase was found in the larger of the two complexes, while almost all of the truncated form was found in the smaller. Thus, the C-terminal region confers a higher order of self-association. Furthermore, full-length kinase expressed in COS-1 cells was present as high molecular weight complex, while the truncated form was present as monomer species. These experiments indicate that the myotonic dystrophy kinase is not membrane-integrated, but that it may have a molecular organization which favors peripheral association with membranes.

Isolation of a novel G protein-coupled receptor (GPR4) localized to chromosome 19q13.3

We present the cloning and sequencing of the human gene for a novel G-protein coupled receptor (GPR4), from the critical myotonic dystrophy (DM) region on chromosome 19q13.3. The homologous porcine gene was isolated and sequenced as well. The genes of both species are intronless and contain an open reading frame encoding a protein of 362 amino acids. In human, two isoforms of GPR4 are expressed, differing in their 3' untranslated region due to the use of alternate polyadenylation signals and measuring approximately 2.8 and 1.8 kb, respectively. Northern blot analysis showed that GPR4 is widely expressed, with higher levels in kidney, heart, and especially lung, where it is at least fivefold greater than in other tissues. Sequence analysis suggests that GPR4 is a peptide receptor and shares strongest homologies with purinergic receptors and receptors for angiotensin II, platelet activating factor, thrombin, and bradykinin.

Suppression of programmed death and G1 arrest in B-cell hybridomas by interleukin-6 is not accompanied by altered expression of immediate early response genes

The murine B-cell hybridoma B9 requires interleukin-6 (IL-6) for its survival and proliferation in vitro. We show here that withdrawal of IL-6 from B9 cultures results in programmed death, concomitant with arrest of the cells in the G1 phase of the cell cycle. Unlike several other systems that undergo programmed cell death, no induction of transcripts corresponding to the testosterone-repressed message-2 or transglutaminase genes is observed during this process. Upon readdition of IL-6 to G1-arrested B9 cells, viability is maintained and entry into S phase occurs after a lag period of 10 to 12 hr. Northern blot analysis showed that the immediate-early mRNAs normally induced shortly after growth factor stimulation in quiescent fibroblasts (c-fos, c-jun, Egr-1, c-myc, JE, and KC), and other growth-related genes (2F1, c-Ha-ras, and p53), are either not induced or remain unchanged during G1 to S phase progression. A correlation was found, however, between the temporal pattern of expression of several G1/S phase genes (dihydrofolate reductase, thymidine kinase, transferrin receptor, and histone H3) and DNA synthesis. These results demonstrate that IL-6-induced viability and growth of hybridoma (and, presumably, plasmacytoma) cells is mediated via novel signal transduction pathways.

Comparative analysis of retroviral vector expression in mouse embryonal carcinoma cells

A series of replication-defective retroviral vectors were assessed for their ability to efficiently transfer functional genes into undifferentiated cells. In these vectors (designated handicapped because of a deletion of enhancer and promoter sequences in the viral long terminal repeat) transcription of inserted genes is under the control of internal promoters. Although a composite promoter composed of a mutant polyoma virus enhancer (PyF441) coupled to the herpes simplex virus thymidine kinase promoter was anticipated to function efficiently, it was found to be significantly inferior to the mouse X-chromosome phosphoglycerate kinase (pgk-1) promoter, in its ability to express the selectable neomycin phosphotransferase gene in mouse embryonal carcinoma cells. The pHMB vector, which contains the pgk-1 promoter, was shown to confer the drug-resistant phenotype at high frequencies to F9 and P19 cells. This vector might prove to be of general utility for efficient gene expression in other developmental contexts.