A role for SSeCKS, a major protein kinase C substrate with tumour suppressor activity, in cytoskeletal architecture, formation of migratory processes, and cell migration during embryogenesis

Role of A-Kinase Anchoring Protein 12 in the Central Nervous System

A-kinase anchoring protein (AKAP) 12 is a scaffolding protein that anchors various signaling proteins to the plasma membrane. These signaling proteins include protein kinase A, protein kinase C, protein phosphatase 2B, Src-family kinases, cyclins, and calmodulin, which regulate their respective signaling pathways. AKAP12 expression is observed in the neurons, astrocytes, endothelial cells, pericytes, and oligodendrocytes of the central nervous system (CNS). Its physiological roles include promoting the development of the blood-brain barrier, maintaining white-matter homeostasis, and even regulating complex cognitive functions such as long-term memory formation. Under pathological conditions, dysregulation of AKAP12 expression levels may be involved in the pathology of neurological diseases such as ischemic brain injury and Alzheimer's disease. This minireview aimed to summarize the current literature on the role of AKAP12 in the CNS.

Targeted disruption of PKC from AKAP signaling complexes

Protein Kinase C (PKC) is a member of the AGC subfamily of kinases and regulates a wide array of signaling pathways and physiological processes. Protein-protein interactions involving PKC and its scaffolding partners dictate the spatiotemporal dynamics of PKC activity, including its access to activating second messenger molecules and potential substrates. While the A Kinase Anchoring Protein (AKAP) family of scaffold proteins universally bind PKA, several were also found to scaffold PKC, thereby serving to tune its catalytic output. Targeting these scaffolding interactions can further shed light on the effect of subcellular compartmentalization on PKC signaling. Here we report the development of two hydrocarbon stapled peptides, CSTAD5 and CSTAD6, that are cell permeable and bind PKC to disrupt PKC-gravin complex formation in cells. Both constrained peptides downregulate PMA-induced cytoskeletal remodeling that is mediated by the PKC-gravin complex as measured by cell rounding. Further, these peptides downregulate PKC substrate phosphorylation and cell motility. To the best of our knowledge, no PKC-selective AKAP disruptors have previously been reported and thus CSTAD5 and CSTAD6 are novel disruptors of PKC scaffolding by AKAPs and may serve as powerful tools for dissecting AKAP-localized PKC signaling.

Exploration of Genetic Variants within the Goat A-Kinase Anchoring Protein 12 (AKAP12) Gene and Their Effects on Growth Traits

Simple Summary

AKAP12, the family of A-kinase anchoring proteins (AKAPs), plays an important role in the regulation of growth and development. There have been no corresponding studies of the effect of the AKAP12 gene on growth traits in goats. In our previous study, 7 bp (intron 3) and 13 bp (3′UTR) indels within the AKAP12 gene significantly influenced AKAP12 gene expression. This study expected to identify the association between these two genetic variations and growth-related traits in 1405 Shaanbei white cashmere (SBWC) goats. The P1–7 bp indel locus was significantly correlated with height at hip cross (HHC; p < 0.05) and the P2–13 bp indel locus was associated with body weight, body length, chest depth, chest width, hip width, chest circumference and cannon (bone) circumference in SBWC goats (p < 0.05). These results prove that the AKAP12 gene plays an important role in the growth and development of goats.

Abstract

The A-kinase anchoring protein 12 gene (AKAP12) is a scaffold protein, which can target multiple signal transduction effectors, can promote mitosis and cytokinesis and plays an important role in the regulation of growth and development. In our previous study, P1–7 bp (intron 3) and P2–13 bp (3′UTR) indels within the AKAP12 gene significantly influenced AKAP12 gene expression. Therefore, this study aimed to identify the association between these two genetic variations and growth-related traits in Shaanbei white cashmere goats (SBWC) (n = 1405). Herein, we identified two non-linkage insertions/deletions (indels). Notably, we found that the P1–7 bp indel mutation was related to the height at hip cross (HHC; p < 0.05) and the P2–13 bp indel was associated with body weight, body length, chest depth, chest width, hip width, chest circumference and cannon (bone) circumference in SBWC goats (p < 0.05). Overall, the two indels’ mutations of AKAP12 affected growth traits in goats. Compared to the P1–7 bp indel, the P2–13 bp indel is more suitable for the breeding of goat growth traits.

Everything you ever wanted to know about PKA regulation and its involvement in mammalian sperm capacitation

The 3', 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) is a tetrameric holoenzyme comprising a set of two regulatory subunits (PKA-R) and two catalytic (PKA-C) subunits. The PKA-R subunits act as sensors of cAMP and allow PKA-C activity. One of the first signaling events observed during mammalian sperm capacitation is PKA activation. Thus, understanding how PKA activity is restricted in space and time is crucial to decipher the critical steps of sperm capacitation. It is widely accepted that PKA specificity depends on several levels of regulation. Anchoring proteins play a pivotal role in achieving proper localization signaling, subcellular targeting and cAMP microdomains. These multi-factorial regulation steps are necessary for a precise spatio-temporal activation of PKA. Here we discuss recent understanding of regulatory mechanisms of PKA in mammalian sperm, such as post-translational modifications, in the context of its role as the master orchestrator of molecular events conducive to capacitation.

Akap12beta supports asymmetric heart development via modulating the Kupffer’s vesicle formation in zebrafish

The vertebrate body plan is accomplished by left-right asymmetric organ development and the heart is a representative asymmetric internal organ which jogs to the left-side. Kupffer’s vesicle (KV) is a spherical left-right organizer during zebrafish embryogenesis and is derived from a cluster of dorsal forerunner cells (DFCs). Cadherin1 is required for collective migration of a DFC cluster and failure of DFC collective migration by Cadherin1 decrement causes KV malformation which results in defective heart laterality. Recently, loss of function mutation of A-kinase anchoring protein 12 (AKAP12) is reported as a high-risk gene in congenital heart disease patients. In this study, we demonstrated the role of akap12β in asymmetric heart development. The akap12β, one of the akap12 isoforms, was expressed in DFCs which give rise to KV and akap12β-deficient zebrafish embryos showed defective heart laterality due to the fragmentation of DFC clusters which resulted in KV malformation. DFC-specific loss of akap12β also led to defective heart laterality as a consequence of the failure of collective migration by cadherin1 reduction. Exogenous akap12β mRNA not only restored the defective heart laterality but also increased cadherin1 expression in akap12β morphant zebrafish embryos. Taken together, these findings provide the first experimental evidence that akap12β regulates heart laterality via cadherin1.

A-Kinase Anchor Protein 12 Is Required for Oligodendrocyte Differentiation in Adult White Matter

Oligodendrocyte precursor cells (OPCs) give rise to oligodendrocytes in cerebral white matter. However, the underlying mechanisms that regulate this process remain to be fully defined, especially in adult brains. Recently, it has been suggested that signaling via A-kinase anchor protein 12 (AKAP12), a scaffolding protein that associates with intracellular molecules such as protein kinase A, may be involved in Schwann cell homeostasis and peripheral myelination. Here, we asked whether AKAP12 also regulates the mechanisms of myelination in the CNS. AKAP12 knockout mice were compared against wild-type (WT) mice in a series of neurochemical and behavioral assays. Compared with WTs, 2-months old AKAP12 knockout mice exhibited loss of myelin in white matter of the corpus callosum, along with perturbations in working memory as measured by a standard Y-maze test. Unexpectedly, very few OPCs expressed AKAP12 in the corpus callosum region. Instead, pericytes appeared to be one of the major AKAP12-expressing cells. In a cell culture model system, conditioned culture media from normal pericytes promoted in-vitro OPC maturation. However, conditioned media from AKAP12-deficient pericytes did not support the OPC function. These findings suggest that AKAP12 signaling in pericytes may be required for OPC-to-oligodendrocyte renewal to maintain the white matter homeostasis in adult brain. Stem Cells 2018;36:751-760.

SSeCKS/AKAP12 scaffolding functions suppress B16F10-induced peritoneal metastasis by attenuating CXCL9/10 secretion by resident fibroblasts

SSeCKS/Gravin/AKAP12 (SSeCKS) is a kinase scaffolding protein known to suppress metastasis by attenuating tumor-intrinsic PKC- and Src-mediated signaling pathways [1]. In addition to downregulation in metastatic cells, in silico analyses identified SSeCKS downregulation in prostate or breast cancer-derived stroma, suggesting a microenvironmental cell role in controlling malignancy. Although orthotopic B16F10 and SM1WT1[Braf^V600E] mouse melanoma tumors grew similarly in syngeneic WT or SSeCKS-null (KO) mice, KO hosts exhibited 5- to 10-fold higher levels of peritoneal metastasis, and this enhancement could be adoptively transferred by pre-injecting naïve WT mice with peritoneal fluid (PF), but not non-adherent peritoneal cells (PC), from naïve KO mice. B16F10 and SM1WT1 cells showed increased chemotaxis to KO-PF compared to WT-PF, corresponding to increased PF levels of multiple inflammatory mediators, including the Cxcr3 ligands, Cxcl9 and 10. Cxcr3 knockdown abrogated enhanced chemotaxis to KO-PF and peritoneal metastasis in KO hosts. Conditioned media from KO peritoneal membrane fibroblasts (PMF), but not from KO-PC, induced increased B16F10 chemotaxis over controls, which could be blocked with Cxcl10 neutralizing antibody. KO-PMF exhibited increased levels of the senescence markers, SA-β-galactosidase, p21^waf1 and p16^ink4a, and enhanced Cxcl10 secretion induced by inflammatory mediators, lipopolysaccharide, TNFα, IFNα and IFNγ. SSeCKS scaffolding-site mutants and small molecule kinase inhibitors were used to show that the loss of SSeCKS-regulated PKC, PKA and PI3K/Akt pathways are responsible for the enhanced Cxcl10 secretion. These data mark the first description of a role for stromal SSeCKS/AKAP12 in suppressing metastasis, specifically by attenuating signaling pathways that promote secretion of tumor chemoattractants in the peritoneum.

Structural environment built by AKAP12+ colon mesenchymal cells drives M2 macrophages during inflammation recovery

Macrophages exhibit phenotypic plasticity, as they have the ability to switch their functional phenotypes during inflammation and recovery. Simultaneously, the mechanical environment actively changes. However, how these dynamic alterations affect the macrophage phenotype is unknown. Here, we observed that the extracellular matrix (ECM) constructed by AKAP12+ colon mesenchymal cells (CMCs) generated M2 macrophages by regulating their shape during recovery. Notably, rounded macrophages were present in the linear and loose ECM of inflamed colons and polarized to the M1 phenotype. In contrast, ramified macrophages emerged in the contracted ECM of recovering colons and mainly expressed M2 macrophage markers. These contracted structures were not observed in the inflamed colons of AKAP12 knockout (KO) mice. Consequently, the proportion of M2 macrophages in inflamed colons was lower in AKAP12 KO mice than in WT mice. In addition, clinical symptoms and histological damage were more severe in AKAP12 KO mice than in WT mice. In experimentally remodeled collagen gels, WT CMCs drove the formation of a more compacted structure than AKAP12 KO CMCs, which promoted the polarization of macrophages toward an M2 phenotype. These results demonstrated that tissue contraction during recovery provides macrophages with the physical cues that drive M2 polarization.

AKAP-mediated feedback control of cAMP gradients in developing hippocampal neurons

Cyclic AMP (cAMP) and protein kinase A (PKA), classical examples of spatially compartmentalized signaling molecules, are critical axon determinants that regulate neuronal polarity and axon formation, yet little is known about micro-compartmentalization of cAMP and PKA signaling and its role in developing neurons. Here, we revealed that cAMP forms a gradient in developing hippocampal neurons, with higher cAMP levels in more distal regions of the axon compared to other regions of the cell. Interestingly, this cAMP gradient changed according to the developmental stage and depended on proper anchoring of PKA by A-kinase anchoring proteins (AKAPs). Disrupting PKA anchoring to AKAPs increased the cAMP gradient in early-stage neurons and led to enhanced axon elongation. Our results provide new evidence for a local negative feedback loop, assembled by AKAPs, for the precise control of a growth-stage-dependent cAMP gradient to ensure proper axon growth.

Protein kinase C and chemical-induced abnormal palate development

The protein kinase C (PKC) family of proteins mediates the action of growth factors and other ligands by activating a network of transcription factors that bind to TRE sequences in the promoters of many genes that regulate cell proliferation, differentiation, extracellular matrix synthesis, apoptosis and others in a cell type-, isozymeand context-specific manner. The critical role of PKC in embryonic development is indicated by early death of embryos in which one or more of these isozymes are inactivated. Our studies together with others show that palatal PKC signalling is functional and may be essential for normal palate development. Although single gene knockouts have failed to exhibit the cleft palate (CP) phenotype, owing to compensation by other kinases, many chemicals including the mycotoxin, secalonic acid D, disrupt palatal PKC signalling leading to altered palatal mesenchymal gene expression. The potential relevance of such effects to chemical-induced CP is discussed.

FRET biosensors reveal AKAP-mediated shaping of subcellular PKA activity and a novel mode of Ca(2+)/PKA crosstalk

Scaffold proteins play a critical role in cellular homeostasis by anchoring signaling enzymes in close proximity to downstream effectors. In addition to anchoring static enzyme complexes, some scaffold proteins also form dynamic signalosomes that can traffic to different subcellular compartments upon stimulation. Gravin (AKAP12), a multivalent scaffold, anchors PKA and other enzymes to the plasma membrane under basal conditions, but upon [Ca(2+)]i elevation, is rapidly redistributed to the cytosol. Because gravin redistribution also impacts PKA localization, we postulate that gravin acts as a calcium "switch" that modulates PKA-substrate interactions at the plasma membrane, thus facilitating a novel crosstalk mechanism between Ca(2+) and PKA-dependent pathways. To assess this, we measured the impact of gravin-V5/His expression on compartmentalized PKA activity using the FRET biosensor AKAR3 in cultured cells. Upon treatment with forskolin or isoproterenol, cells expressing gravin-V5/His showed elevated levels of plasma membrane PKA activity, but cytosolic PKA activity levels were reduced compared with control cells lacking gravin. This effect required both gravin interaction with PKA and localization at the plasma membrane. Pretreatment with calcium-elevating agents thapsigargin or ATP caused gravin redistribution away from the plasma membrane and prevented gravin from elevating PKA activity levels at the membrane. Importantly, this mode of Ca(2+)/PKA crosstalk was not observed in cells expressing a gravin mutant that resisted calcium-mediated redistribution from the cell periphery. These results reveal that gravin impacts subcellular PKA activity levels through the spatial targeting of PKA, and that calcium elevation modulates downstream β-adrenergic/PKA signaling through gravin redistribution, thus supporting the hypothesis that gravin mediates crosstalk between Ca(2+) and PKA-dependent signaling pathways. Based on these results, AKAP localization dynamics may represent an important paradigm for the regulation of cellular signaling networks.

AKAP6 inhibition impairs myoblast differentiation and muscle regeneration: Positive loop between AKAP6 and myogenin

Skeletal muscle regeneration occurs continuously to repair muscle damage incurred during normal activity and in chronic disease or injury. Herein, we report that A-kinase anchoring protein 6 (AKAP6) is important for skeletal myoblast differentiation and muscle regeneration. Compared with unstimulated skeletal myoblasts that underwent proliferation, differentiated cells show significant stimulation of AKAP6 expression. AKAP6 knockdown with siRNA effectively halts the formation of myotubes and decreases the expression of the differentiation markers myogenin and myosin heavy chain. When shAKAP6-lentivirus is delivered to mice with cardiotoxin (CTX)-induced muscle injury, muscle regeneration is impaired compared with that of mice injected with control shMock-lentivirus. The motor functions of mice infected with shAKAP6-lentivirus (CTX+shAK6) are significantly worse than those of mice infected with shMock-lentivirus (CTX+shMock). Mechanistic analysis showed that AKAP6 promotes myogenin expression through myocyte enhancer factor 2A (MEF2A). Notably, myogenin increases AKAP6 expression as well. The results of chromatin immunoprecipitation and luciferase assays showed that myogenin binds to an E-box site on the AKAP6 promoter. Taken together, our findings demonstrate a novel interplay between AKAP6 and myogenin, and we suggest that AKAP6 is an important regulator of myoblast differentiation, myotube formation, and muscle regeneration.

Akap12 is essential for the morphogenesis of muscles involved in zebrafish locomotion

Swimming behavior in fish is driven by coordinated contractions of muscle fibers. In zebrafish, slow muscle cell migration is crucial for the formation of the muscle network; slow myoblasts, which arise from medial adaxial cells, migrate radially to the lateral surface of the trunk and tail during embryogenesis. This study found that the zebrafish A-kinase anchoring protein (akap)12 isoforms akap12α and akap12β are required for muscle morphogenesis and locomotor activity. Embryos deficient in akap12 exhibited reduced spontaneous coiling, touch response, and free swimming. Akap12-depleted slow but not fast muscle cells were misaligned, suggesting that the behavioral abnormalities resulted from specific defects in slow muscle patterning; indeed, slow muscle cells and muscle pioneers in these embryos showed abnormal migration in a cell-autonomous manner. Taken together, these results suggest that akap12 plays a critical role in the development of zebrafish locomotion by regulating the normal morphogenesis of muscles.

Differential requirement for Src family tyrosine kinases in the initiation, progression, and metastasis of prostate cancer

Prostate cancer (CaP) recurrence after androgen ablation therapy remains a significant cause of mortality in aging men. Malignant progression and metastasis are typically driven by genetic and epigenetic changes controlled by the androgen receptor (AR). However, evidence suggests that activated nonreceptor tyrosine kinases, including those of the Src family kinases (SFK), directly phosphorylate AR, thereby activating its transcriptional activity in the absence of serum androgen levels. To ascertain whether CaP progression and metastasis require SFK members, an autochthonous transgenic adenocarcinoma (AD) of the mouse prostate (TRAMP) model was crossed into Src-, Lyn- or Fyn-null backgrounds. Primary-site CaP formation was dependent on Src, to a lesser extent, Lyn, but not Fyn. Only Src(-) (/) (-);TRAMP prostate tumors were marked by reactive stroma. SFK deficiency did not affect progression to neuroendocrine (NE) disease, although there were fewer new cancer cases initiating after 34 weeks in the SFK(-/-);TRAMP mice compared with TRAMP controls. Of note, 15% to 21% of older (>33 weeks) Lyn- or Fyn-null TRAMP mice lacking primary-site tumors suffered from aggressive metastatic AD growths, compared with 3% of TRAMP mice. Taken with the data that TRAMP mice lacking Src or Lyn exhibited fewer macroscopic metastases compared with Fyn(-) (/) (-);TRAMP and TRAMP controls, this suggests that SFK can either promote or suppress specific parameters of metastatic growth, possibly depending on cross-talk with primary tumors. These data identify critical, yet potentially opposing roles played by various SFKs in the initiation and metastatic potential of CaP using the TRAMP model.

IMPLICATIONS

Genetically defined mouse models indicate a critical role for Src tyrosine kinase in CaP initiation and metastatic progression.

Expression of SRC suppressed C kinase substrate in rat neural tissues during inflammation

Src-suppressed C kinase substrate (SSeCKS), an in vivo and in vitro protein kinase C substrate, is a major lipopolysaccharide (LPS) response protein which markedly upregulated in several organs, including brain, lung, heart, kidney etc., indicating a possible role of SSeCKS in inflammatory process. However, the expression and biological function of SSeCKS during neuronal inflammation remains to be elucidated, so we established an inflammatory model injected with LPS to investigate the gene expression patterns of SSeCKS in neural tissues by using TaqMan quantitative real-time PCR and immunohistochemistry in rat. Real-time PCR showed that LPS stimulated the expression of SSeCKS mRNA in a dose- and time-dependent manner in sciatic nerves, spinal cords and dorsal root ganglions. Immunohistochemistry showed that SSeCKS colocalized with nerve fibers in sciatic nerve after LPS administration, but there was no colocalization between SSeCKS and Schwann cells. In addition, SSeCKS colocalized with neurons which existed in dorsal root ganglions and spinal cords. These findings indicated that SSeCKS might play some important roles in sciatic nerve fibers and neurons in spinal cords and dorsal root ganglions after LPS injection.

Receptor-mediated Ca2+ and PKC signaling triggers the loss of cortical PKA compartmentalization through the redistribution of gravin

A-Kinase Anchoring Proteins (AKAPs) direct the flow of cellular information by positioning multiprotein signaling complexes into proximity with effector proteins. However, certain AKAPs are not stationary but can undergo spatiotemporal redistribution in response to stimuli. Gravin, a 300kD AKAP that intersects with a diverse signaling array, is localized to the plasma membrane but has been shown to translocate to the cytosol following the elevation of intracellular calcium ([Ca(2+)]i). Despite the potential for gravin redistribution to impact multiple signaling pathways, the dynamics of this event remain poorly understood. In this study, quantitative microscopy of cells expressing gravin-EGFP revealed that Ca(2+) elevation caused the complete translocation of gravin from the cell cortex to the cytosol in as little as 60s of treatment with ionomycin or thapsigargin. In addition, receptor mediated signaling was also shown to cause gravin redistribution following ATP treatment, and this event required both [Ca(2+)]i elevation and PKC activation. To understand the mechanism for Ca(2+) mediated gravin dynamics, deletion of calmodulin-binding domains revealed that a fourth putative calmodulin binding domain called CB4 (a.a. 670-694) is critical for targeting gravin to the cell cortex despite its location downstream of gravin's membrane-targeting domains, which include an N-terminal myristoylation site and three polybasic domains. Finally, confocal microscopy of cells co-transfected with gravin-EYFP and PKA RII-ECFP revealed that gravin redistribution mediated by ionomycin, thapsigargin, and ATP each triggered the gravin-dependent loss of PKA localized at the cell cortex. Our results support the hypothesis that gravin redistribution regulates cross-talk between PKA-dependent signaling and receptor-mediated events involving Ca(2+) and PKC.

AKAP13 Rho-GEF and PKD-binding domain deficient mice develop normally but have an abnormal response to β-adrenergic-induced cardiac hypertrophy

BACKGROUND

A-kinase anchoring proteins (AKAPs) are scaffolding molecules that coordinate and integrate G-protein signaling events to regulate development, physiology, and disease. One family member, AKAP13, encodes for multiple protein isoforms that contain binding sites for protein kinase A (PKA) and D (PKD) and an active Rho-guanine nucleotide exchange factor (Rho-GEF) domain. In mice, AKAP13 is required for development as null embryos die by embryonic day 10.5 with cardiovascular phenotypes. Additionally, the AKAP13 Rho-GEF and PKD-binding domains mediate cardiomyocyte hypertrophy in cell culture. However, the requirements for the Rho-GEF and PKD-binding domains during development and cardiac hypertrophy are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

To determine if these AKAP13 protein domains are required for development, we used gene-trap events to create mutant mice that lacked the Rho-GEF and/or the protein kinase D-binding domains. Surprisingly, heterozygous matings produced mutant mice at Mendelian ratios that had normal viability and fertility. The adult mutant mice also had normal cardiac structure and electrocardiograms. To determine the role of these domains during β-adrenergic-induced cardiac hypertrophy, we stressed the mice with isoproterenol. We found that heart size was increased similarly in mice lacking the Rho-GEF and PKD-binding domains and wild-type controls. However, the mutant hearts had abnormal cardiac contractility as measured by fractional shortening and ejection fraction.

CONCLUSIONS

These results indicate that the Rho-GEF and PKD-binding domains of AKAP13 are not required for mouse development, normal cardiac architecture, or β-adrenergic-induced cardiac hypertrophic remodeling. However, these domains regulate aspects of β-adrenergic-induced cardiac hypertrophy.

Pivotal Role of AKAP12 in the Regulation of Cellular Adhesion Dynamics: Control of Cytoskeletal Architecture, Cell Migration, and Mitogenic Signaling

Cellular dynamics are controlled by key signaling molecules such as cAMP-dependent protein kinase (PKA) and protein kinase C (PKC). AKAP12/SSeCKS/Gravin (AKAP12) is a scaffold protein for PKA and PKC which controls actin-cytoskeleton reorganization in a spatiotemporal manner. AKAP12 also acts as a tumor suppressor which regulates cell-cycle progression and inhibits Src-mediated oncogenic signaling and cytoskeletal pathways. Reexpression of AKAP12 causes cell flattening, reorganization of the actin cytoskeleton, and the production of normalized focal adhesion structures. Downregulation of AKAP12 induces the formation of thickened, longitudinal stress fibers and the proliferation of adhesion complexes. AKAP12-null mouse embryonic fibroblasts exhibit hyperactivation of PKC, premature cellular senescence, and defects in cytokinesis, relating to the loss of PKC scaffolding activity by AKAP12. AKAP12-null mice exhibit increased cell senescence and increased susceptibility to carcinogen-induced oncogenesis. The paper describes the regulatory and scaffolding functions of AKAP12 and how it regulates cell adhesion, signaling, and oncogenic suppression.

Retinoid-induced expression and activity of an immediate early tumor suppressor gene in vascular smooth muscle cells

Retinoids are used clinically to treat a number of hyper-proliferative disorders and have been shown in experimental animals to attenuate vascular occlusive diseases, presumably through nuclear receptors bound to retinoic acid response elements (RARE) located in target genes. Here, we show that natural or synthetic retinoids rapidly induce mRNA and protein expression of a specific isoform of A-Kinase Anchoring Protein 12 (AKAP12β) in cultured smooth muscle cells (SMC) as well as the intact vessel wall. Expression kinetics and actinomycin D studies indicate Akap12β is a retinoid-induced, immediate-early gene. Akap12β promoter analyses reveal a conserved RARE mildly induced with atRA in a region that exhibits hyper-acetylation. Immunofluorescence microscopy and protein kinase A (PKA) regulatory subunit overlay assays in SMC suggest a physical association between AKAP12β and PKA following retinoid treatment. Consistent with its designation as a tumor suppressor, inducible expression of AKAP12β attenuates SMC growth in vitro. Further, immunohistochemistry studies establish marked decreases in AKAP12 expression in experimentally-injured vessels of mice as well as atheromatous lesions in humans. Collectively, these results demonstrate a novel role for retinoids in the induction of an AKAP tumor suppressor that blocks vascular SMC growth thus providing new molecular insight into how retiniods may exert their anti-proliferative effects in the injured vessel wall.

Carcinogen-induced squamous papillomas and oncogenic progression in the absence of the SSeCKS/AKAP12 metastasis suppressor correlate with FAK upregulation

The ability of SSeCKS/Gravin/AKAP12 (SSeCKS) to negatively regulate cell cycle progression is thought to relate to its spatiotemporal scaffolding activity for key signaling molecules such as protein kinase A and C, calmodulin and cyclins. SSeCKS is downregulated upon progression to malignancy in many cancer types, including melanoma and nonmelanoma skin cancer. The forced re-expression of SSeCKS is especially potent in suppressing metastasis through the inhibition of VEGF-mediated neovascularization. We have previously shown that SSeCKS-null (KO) mice exhibit hyperplasia and focal dysplasia in the prostate marked by activated Akt. To address whether KO mice exhibit increased skin carcinogenesis, WT and KO C57BL/6 mice were treated topically with 12-O-tetradecanoylphorbol-13-acetate and 7,12-dimethylbenzanthracene. Compared to WT mice, KO mice developed squamous papillomas more rapidly and in greater numbers and also exhibited significantly increased progression to squamous cell carcinoma. Untreated KO epidermal layers were thicker than those in age-matched WT mice and exhibited significantly increased levels of FAK and phospho-ERK1/2, known mediators of carcinogen-induced squamous papilloma progression to carcinoma. Compared to protein levels in WT mouse embryo fibroblasts (MEF), SSeCKS levels were increased in FAK-null cells, whereas FAK levels were increased in SSeCKS-null cells. RNAi studies in WT MEF cells suggest that SSeCKS and FAK attenuate each other's expression. Our study implicates a role for SSeCKS in preventing of skin cancer progression possibly through negatively regulating FAK expression.

Temporal profile of Src, SSeCKS, and angiogenic factors after focal cerebral ischemia: correlations with angiogenesis and cerebral edema

A better understanding of the underlying mechanisms of angiogenesis and vascular permeability is necessary for the development of therapeutic strategies for ischemic injury. The purpose of this study was to examine the spatial and temporal expression of Src and Src-suppressed C kinase substrate (SSeCKS) in brain after middle cerebral artery occlusion (MCAO) and elucidate the relationships among Src, SSeCKS, and the key angiogenic factors present after stroke. Rats were subjected to either MCAO or sham operation. Reverse transcriptase-polymerase chain reaction and Western blotting results revealed that Src gradually increased starting as early as 2 h after MCAO and remained high for 1 day. In contrast, SSeCKS decreased after MCAO. Src expression correlated positively with that of vascular endothelial growth factor and angiopoietin-2, and negatively with that of SSeCKS, angiopoietin-1, and zonula occludens-1. However, SSeCKS had the reverse correlations. Changes in the expression of these factors correlated with the progress of angiogenesis and cerebral edema. Dynamic temporal changes in Src and SSeCKS expression may modulate angiogenesis and cerebral edema formation after focal cerebral ischemia.

Rb-dependent cellular senescence, multinucleation and susceptibility to oncogenic transformation through PKC scaffolding by SSeCKS/AKAP12

A subset of AKAPs (A Kinase Anchoring Proteins) regulate signaling and cytoskeletal pathways through the spaciotemporal scaffolding of multiple protein kinases (PK) such as PKC and PKA, and associations with the plasma membrane and the actin-based cytoskeleton. SSeCKS/Gravin/Akap12 expression is severely downregulated in many advanced cancers and exhibits tumor- and metastasis-suppressing activity. akap12-null (KO) mice develop prostatic hyperplasia with focal dysplasia, but the precise mechanism how Akap12 prevents oncogenic progression remains unclear. Here, we show that KO mouse embryonic fibroblasts (MEF) exhibit premature senescence marked by polyploidy and multinucleation, and by increased susceptibility to oncogenic transformation. Although p53 and Rb pathways are activated in the absence of Akap12, senescence is dependent on Rb. Senescence is driven by the activation of PKCα, which induces p16(Ink4a)/Rb through a MEK-dependent downregulation of Id1, and PKCδ, which downregulates Lats1/Warts, a mitotic exit network kinase required for cytokinesis. Our data strongly suggest that Akap12 controls Rb-mediated cell aging and oncogenic progression by directly scaffolding and attenuating PKCα/δ.

SSeCKS promote beta-amyloid-induced PC12 cells neurotoxicity by up-regulating tau phosphorylation in Alzheimer's disease

In Alzheimer's disease, beta-amyloid peptide (Abeta) could induce tau hyperphosphorylation which is the major cause of neuron apoptosis. However, the underlying mechanisms in the process remain unclear. In this study, Abeta-induced apoptosis and tau phosphorylation were investigated in differentiated PC12 cells. This Abeta-induced tau phosphorylation paralleled with the increase of expression and phosphorylation of Src-suppressed protein kinase C substrate (SSeCKS). By knocking down the expression of SSeCKS, Abeta-induced apoptosis and tau hyperphosphorylation in PC12 cells were partially rescued, and were increased further due to the overexpression of SSeCKS in PC12 cells. Also, the cell apoptosis and tau hyperphosphorylation were strongly decreased when the cells were pretreated with the protein kinase C inhibitor, Gö6983. In addition, Abeta-induced tau phosphorylation was also partially decreased due to the overexpression of SSeCKS in PC12cells. In summary, our data indicate that SSeCKS may play a critical role in Abeta-induced PC12 cells apoptosis through its phosphorylation.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.

Interleukin-17F-induced pulmonary microvascular endothelial monolayer hyperpermeability via the protein kinase C pathway

BACKGROUND

Interleukin (IL)-17F is involved in lung inflammation, but the effect of IL-17F on endothelial permeability and its signaling pathway remain ill-defined. The current study sought to investigate the effect of IL-17F on endothelium and assess the role of protein kinase C (PKC) and src-suppressed C kinase substrate (SSeCKS) in this process.

METHODS

Rat pulmonary microvascular endothelial monolayers were constructed to determine changes of permeability as measured by means of FITC-dextran and Hank's solution flux across monolayers and transendothelial electrical resistance with or without IL-17F and PKC inhibitors. Additional monolayers were stained using FITC-phalloidin for filamentous actin (F-actin). The gene expression of SSeCKS was analyzed by the reverse transcription-polymerase chains. Alterations of SSeCKS protein were investigated by immunoblotting and immunoprecipitation.

RESULTS

IL-17F increased endothelial monolayer permeability in a dose- and time-dependent manner. F-actin staining revealed that permeability changes were accompanied by reorganization of cytoskeleton. In the presence of PKC inhibitors, the IL-17F-induced hyperpermeability and reorganization of F-actin were attenuated. The gene and protein expression of SSeCKS were conspicuously elevated after IL-17F challenge. The process of SSeCKS phosphorylation followed a time course that mirrored the time course of hyperpermeability induced by IL-17F. IL-17F-induced SSeCKS phosphorylation was abrogated after PKC inhibitors pretreatment. The translocation of SSeCKS from the cytosol to the membrane and a significant increase in the SSeCKS association with the cytoskeleton were found after IL-17F treatment.

CONCLUSIONS

IL-17F is an important mediator of increased endothelial permeability. PKC and SSeCKS are integral signaling components essential for IL-17F-induced hyperpermeability.

Gravin dynamics regulates the subcellular distribution of PKA

Gravin, a multivalent A-kinase anchoring protein (AKAP), localizes to the cell periphery in several cell types and is postulated to target PKA and other binding partners to the plasma membrane. An N-terminal myristoylation sequence and three regions rich in basic amino acids are proposed to mediate this localization. Reports indicating that phorbol ester affects the distribution of SSeCKS, the rat orthologue of gravin, further suggest that PKC may also regulate the subcellular distribution of gravin, which in turn may affect PKA distribution. In this study, quantitative confocal microscopy of cells expressing full-length and mutant gravin-EGFP constructs lacking the proposed targeting domains revealed that either the N-myristoylation site or the polybasic regions were sufficient to target gravin to the cell periphery. Moreover, phorbol ester treatment induced redistribution of gravin-EGFP from the cell periphery to a juxtanuclear vesicular compartment, but this required the presence of the N-myristoylation site. Confocal microscopy further revealed that not only did gravin-EGFP target a PKA RII-ECFP construct to the cell periphery, but PKC activation resulted in redistribution of the gravin and PKA constructs to the same subcellular site. It is postulated that this dynamic response by gravin to PKC activity may mediate PKC dependent control of PKA activity.

Loss of the SSeCKS/Gravin/AKAP12 gene results in prostatic hyperplasia

SSeCKS/Gravin/AKAP12 (SSeCKS) is a kinase scaffolding protein that encodes metastasis-suppressor activity through the suppression of Src-mediated oncogenic signaling and vascular endothelial growth factor expression. SSeCKS expression is down-regulated in Src- and Ras-transformed fibroblasts, in human cancer cell lines and in several types of human cancer, including prostate. Normal human and mouse prostates express abundant SSeCKS in secretory epithelial cells and, to a lesser extent, in the surrounding mesenchyme. Here, we show that the loss of SSeCKS results in prostatic hyperplasia in the anterior and ventral lobes as well as increased levels of apoptosis throughout the prostate. Dysplastic foci were observed less frequently but were associated with the loss of E-cadherin staining and the loss of high molecular weight cytokeratin-positive basal epithelial cells. SSeCKS-null prostate tissues expressed significantly higher relative levels of AKT(poS473) compared with wild-type controls, suggesting that SSeCKS attenuates phosphatidylinositol-3-OH kinase signaling. The data suggest that SSeCKS-null mice have increased susceptibility for oncogenic transformation in the prostate.

Spatiotemporal patterns of SSeCKS expression after rat spinal cord injury

Src suppressed C kinase substrate (SSeCKS) was identified as a PKC substrate/PKC-binding protein, which plays a role in mitogenic regulatory activity and has a function in the control of cell signaling and cytoskeletal arrangement. However its distribution and function in the central nervous system (CNS) lesion remain unclear. In this study, we mainly investigated the mRNA and protein expression and cellular localization of SSeCKS during spinal cord injury (SCI). Real-time PCR and Western blot analysis revealed that SSeCKS was present in normal whole spinal cord. It gradually increased, reached a peak at 3 days for its mRNA level and 5 days for its protein level after SCI, and then declined during the following days. In ventral horn, the expression of SSeCKS underwent a temporal pattern that was similar with the whole spinal cord in both mRNA and protein level. However, in dorsal horn, the mRNA and protein for SSeCKS expression were significantly increased at 1 day for its mRNA level and 3 days for its protein level, and then gradually declined to the baseline level, ultimately up-regulated again from 7 to 14 days. The protein expression of SSeCKS was further analysed by immunohistochemistry. The positively stained areas for SSeCKS changed with the similar pattern to that of protein expression detected by immunoblotting analysis. Double immunofluorescence staining showed that SSeCKS immunoreactivity (IR) was found in neurons, astrocytes, oligodendrocytes of spinal cord tissues within 5 mm from the lesion site. Importantly, injury-induced expression of SSeCKS was co-labeled by active caspase-3 (apoptotic marker), Tau-1 (the marker for pathological oligodendrocyte) and beta-1,4-galactosyltransferase 1 (GalT). All the results suggested that SSeCKS might play important roles in spinal cord pathophysiology and further research is needed to have a good understanding of its function and mechanism.

Developmental regulation of SSeCKS expression in rat brain

SSeCKS (src suppressed C kinase substrate) was identified as a PKC substrate/PKC-binding protein, which plays a role in mitogenic regulatory activity and has a function in the control of cell signaling and cytoskeletal arrangement. Previous studies showed that expression of SSeCKS mRNA and protein levels were developmentally regulated in rat testis and the molecular might have some effects on the process of spermiogenesis. Here we carried out experiments to investigate the expression of SSeCKS in rat brain. Western blot analysis indicated that SSeCKS could be detected in the whole brain of developing rat embryos and reached its peak at 1 week after birth, while during mature period, its level was decreasing. Regional-distribution analysis showed that the expression pattern of SSeCKS in telencephalon, hippocampus and diencephalons was in accordance with the result from whole brain both in mRNA and protein level. However, in cerebellum, SSeCKS was almost in the same level, and in brainstem, the expression level was higher in 4-week-old rat brain than in 1-week-old one. Immunohistochemistry results showed SSeCKS was in diffused and granule-like distribution. Double immunofluorescence staining showed that it was expressed by some GFAP positive cells. All the results suggested that SSeCKS might affect brain development and further research is needed to have a good understanding of its function and mechanism.

Gravin regulates mesodermal cell behavior changes required for axis elongation during zebrafish gastrulation

Convergent extension of the mesoderm is the major driving force of vertebrate gastrulation. During this process, mesodermal cells move toward the future dorsal side of the embryo, then radically change behavior as they initiate extension of the body axis. How cells make this transition in behavior is unknown. We have identified the scaffolding protein and tumor suppressor Gravin as a key regulator of this process in zebrafish embryos. We show that Gravin is required for the conversion of mesodermal cells from a highly migratory behavior to the medio-laterally intercalative behavior required for body axis extension. In the absence of Gravin, paraxial mesodermal cells fail to shut down the protrusive activity mediated by the Rho/ROCK/Myosin II pathway, resulting in embryos with severe extension defects. We propose that Gravin functions as an essential scaffold for regulatory proteins that suppress the migratory behavior of the mesoderm during gastrulation, and suggest that this function also explains how Gravin inhibits invasive behaviors in metastatic cells.

Lipopolysaccharide induces expression of SSeCKS in rat lung microvascular endothelial cell

Src-suppressed C kinase substrate (SSeCKS) plays a role in membrane-cytoskeletal remodeling to regulate mitogenesis, cell differentiation, and motility. Previous study showed that lipopolysaccharide (LPS) induced a selective and strong expression of SSeCKS in the vascular endothelial cells of lung. Here we show that LPS stimulation elevated expression of SSeCKS mRNA and protein in Rat pulmonary microvascular endothelial cell (RPMVEC). LPS potentiated SSeCKS phosphorylation in a time- and dose-dependent manner, and partly induced translocation of SSeCKS from the cytosol to the membrane after LPS challenge. The PKC inhibitor, Calphostin C, significantly decreased LPS-induced phosphorylation of SSeCKS, inhibited SSeCKS translocation and actin cytoskeleton reorganization after LPS challenge, suggesting that PKC may play a role in LPS-induced SSeCKS translocation and actin rearrangement. We conclude that SSeCKS is located downstream of PKC and that SSeCKS and PKC are both necessary for LPS-induced stress fiber formation.

AKAP12 regulates human blood-retinal barrier formation by downregulation of hypoxia-inducible factor-1alpha

Many diseases of the eye such as retinoblastoma, diabetic retinopathy, and retinopathy of prematurity are associated with blood-retinal barrier (BRB) dysfunction. Identifying the factors that contribute to BRB formation during human eye development and maintenance could provide insights into such diseases. Here we show that A-kinase anchor protein 12 (AKAP12) induces BRB formation by increasing angiopoietin-1 and decreasing vascular endothelial growth factor (VEGF) levels in astrocytes. We reveal that AKAP12 downregulates the level of hypoxia-inducible factor-1alpha (HIF-1alpha) protein by enhancing the interaction of HIF-1alpha with pVHL (von Hippel-Lindau tumor suppressor protein) and PHD2 (prolyl hydroxylase 2). Conditioned media from AKAP12-overexpressing astrocytes induced barriergenesis by upregulating the expression of tight junction proteins in human retina microvascular endothelial cells (HRMECs). Compared with the retina during BRB maturation, AKAP12 expression in retinoblastoma patient tissue was markedly reduced whereas that of VEGF was increased. These findings suggest that AKAP12 may induce BRB formation through antiangiogenesis and barriergenesis in the developing human eye and that defects in this mechanism can lead to a loss of tight junction proteins and contribute to the development of retinal pathologies such as retinoblastoma.

AKAP12 induces apoptotic cell death in human fibrosarcoma cells by regulating CDKI-cyclin D1 and caspase-3 activity

AKAP12 (A-Kinase anchoring protein 12) is a protein kinase C substrate and a potential tumor suppressor. AKAP12 is down-regulated by several oncogenes and strongly suppressed in various cancers including prostate, ovarian and breast cancers. AKAP12 acts as a regulator of mitogenesis by anchoring key signal proteins such as PKA, PKC, and cyclins. In this study, AKAP12 was found to suppress tumor cell viability by inducing apoptosis via caspase-3 in HT1080 cells. This AKAP12-induced apoptosis was associated with a decreased expression of Bcl-2 and increased expression of Bax. Moreover, AKAP12-transfectant strongly induced the expression of Cip1/p21 and Kip1/p27, but resulted in a decrease in cyclin D1 involved in G(1) progression. Accordingly, these results suggest that AKAP12 may play an important role in tumor growth suppression by inducing apoptosis with the regulation of multiple molecules in the cell cycle progression.

Microarray analysis of the fetal hippocampus in the Emx2 mutant

Deficiency in the transcription factor Emx2 causes a specific alteration of hippocampal development, which has been well analyzed morphologically. We are currently using microarrays and in situ hybridization to characterize gene expression in the Emx2 mutant hippocampus. In this report on our preliminary results for the fetal stage, we identify a group of genes for most of which a putative relation to Emx2 pathways has not been previously recognized. Some candidates are development genes or are involved in functional maturation, and show expression in the hippocampal plate and/or developing dentate gyrus. A second class of candidates label neuronal, glial or vascular structures in the outer marginal zone, and likely represent markers for cell populations specifically absent in the mutant. Our results point at pathways and processes altered in the mutant, particularly the Notch and chemokine pathways, the processes of cell migration, axonal guidance and angiogenesis, and the relation of pia and Cajal-Retzius cells with hippocampal morphogenesis.

Neuronal acetylcholine receptor alpha9-subunit: a possible central nervous system autoantigen

Nicotinic acetylcholine receptor (AChR) is a membrane glycoprotein composed of five subunits. Muscle AChR is consist of two alpha1 and one each beta, delta, and epsilon subunits, whereas the neuronal AChR molecules are made up of various combinations of alpha (alpha2-alpha10) and beta (beta1-beta4) subunits. Myasthenia gravis (MG) develops as a result of an autoimmune attack against muscular AChR. While the prevailing symptom is muscle weakness, very rarely MG patients may develop additional central nervous system (CNS) symptoms. The majority of the anti-AChR antibodies responsible from disease induction is directed against alpha1 subunit of AChR. There is considerable identity between muscular alpha1 and neuronal alpha9 subunits. Preliminary studies showed antibodies reactive with the CNS antigens in the serum samples of mice with experimental autoimmune myasthenia gravis (EAMG). Also, alpha9 was present in the CNS in widespread locations and the binding pattern of anti-alpha9 antibody was reminiscent of that of serum samples of some of the mice with EAMG. Serum anti-AChR antibodies of myasthenic patients might be cross-reacting with CNS AChR subunits and thus inducing CNS symptoms. Neuronal AChR alpha9-subunit might be a major target antigen in this process.

SSeCKS/Gravin/AKAP12 attenuates expression of proliferative and angiogenic genes during suppression of v-Src-induced oncogenesis

Background

SSeCKS is a major protein kinase C substrate with kinase scaffolding and metastasis-suppressor activity whose expression is severely downregulated in Src- and Ras-transformed fibroblast and epithelial cells and in human prostate, breast, and gastric cancers. We previously used NIH3T3 cells with tetracycline-regulated SSeCKS expression plus a temperature-sensitive v-Src allele to show that SSeCKS re-expression inhibited parameters of v-Src-induced oncogenic growth without attenuating in vivo Src kinase activity.

Methods

We use cDNA microarrays and semi-quantitative RT-PCR analysis to identify changes in gene expression correlating with i) SSeCKS expression in the absence of v-Src activity, ii) activation of v-Src activity alone, and iii) SSeCKS re-expression in the presence of active v-Src.

Results

SSeCKS re-expression resulted in the attenuation of critical Src-induced proliferative and pro-angiogenic gene expression including Afp, Hif-1α, Cdc20a and Pdgfr-β, and conversely, SSeCKS induced several cell cycle regulatory genes such as Ptpn11, Gadd45a, Ptplad1, Cdkn2d (p19), and Rbbp7.

Conclusion

Our data provide further evidence that SSeCKS can suppress Src-induced oncogenesis by modulating gene expression downstream of Src kinase activity.

The expression of src-suppressed C kinase substrate (SSeCKS) and uptake of exogenous particles in endothelial and reticular cells

Src-suppressed C kinase substrate (SSeCKS), a potent tumor suppressor, plays a role in membrane-cytoskeletal remodeling to regulate mitogenesis, cell differentiation, and motility. Our previous study showed that lipopolysaccharide (LPS) induced a selective and strong expression of SSeCKS in the vascular endothelial cells of several organs, such as hepatic sinusoids, and in the reticular cells of lymphoid organs. In the present immunocyto-chemical study, we determined the detailed cellular and subcellular localization of SSeCKS in mouse tissues after LPS administration, and examined the involvement of SSeCKS in the uptake of exogenous particles. SSeCKS immunoreactivity in the liver and lymph nodes was below the detectable level under normal conditions. After LPS stimulation, an intense immunoreactivity for SSeCKS became noticeable in sinusoidal endothelial cells of the liver and medullary reticular cells of the lymph node. Electron-microscopically, the immunoreactivity was localized predominantly along the cytoplasmic membrane of both cell types. These cells in normal mice incorporated a small amount of injected particles (carbon particles and latex beads), while after LPS stimulation, the uptake of particles increased in terms of the amount and extent of the uptaking sites. Endothelial cells and reticular cells without SSeCKS expression could not incorporate any particles even after LPS stimulation. The subcellular localization of SSeCKS in endothelial cells correlated with some pinocytic pits and phago-lysosomes, although a diffuse distribution of SSeCKS in the cytoplasm was also visible. Taken together, these findings indicate that SSeCKS expression in endothelial cells and reticular cells is a functional index of the reticulo-endothelial system and is involved in the uptake of particles from blood and lymph circulation.

SSeCKS regulates angiogenesis and tight junction formation in blood-brain barrier

The blood-brain barrier (BBB) is essential for maintaining brain homeostasis and low permeability. BBB maintenance is important in the central nervous system (CNS) because disruption of the BBB may contribute to many brain disorders, including Alzheimer disease and ischemic stroke. The molecular mechanisms of BBB development remain ill-defined, however. Here we report that src-suppressed C-kinase substrate (SSeCKS) decreases the expression of vascular endothelial growth factor (VEGF) through AP-1 reduction and stimulates expression of angiopoietin-1 (Ang-1), an antipermeability factor in astrocytes. Conditioned media from SSeCKS-overexpressing astrocytes (SSeCKS-CM) blocked angiogenesis in vivo and in vitro. Moreover, SSeCKS-CM increased tight junction proteins in endothelial cells, consequently decreasing [3H]sucrose permeability. Furthermore, immunoreactivity to SSeCKS gradually increased during the BBB maturation period, and SSeCKS-expressing astrocytes closely interacted with zonula occludens (ZO)-1-expressing blood vessels in vivo. Collectively, our results suggest that SSeCKS regulates BBB differentiation by modulating both brain angiogenesis and tight junction formation.

Ligand-specific control of src-suppressed C kinase substrate gene expression

The src-suppressed C-kinase substrate, SSeCKS, is now recognized as a key regulator of cell signaling and cytoskeletal dynamics. However, few ligands that control SSeCKS expression have been identified. We report that platelet-derived growth factor-BB (PDGF-BB), lysophosphatidic acid (LPA), and eicosapentaenoic acid (EPA) potently modulate SSeCKS gene expression in cultured smooth muscle (RASM) cells relative to other bioactive ligands tested. In addition, EPA-dependent regulation of SSeCKS expression correlates with distinct changes in cell morphology and adhesion in RASM cells. Independent evidence that ligand-specific control of SSeCKS expression links to the regulation of cell adhesion and morphology was obtained using ras-transformed fibroblasts, KNRK. Sodium butyrate (NaB) upregulates SSeCKS mRNA and protein expression corresponding to increased cell-spreading and adhesion. In addition, ectopic expression of recombinant SSeCKS recapitulates attributes of NaB-induced morphogenesis in KNRK cells. The data provide novel evidence that SSeCKS functions in PDGF-BB-, LPA-, EPA-, and NaB-mediated cell signaling.

Mitogen-induced, FAK-dependent tyrosine phosphorylation of the SSeCKS scaffolding protein

The ability of mitogens to rapidly induce tyrosine phosphorylation of cellular proteins has been taken as evidence of participation in subsequent signaling pathways. SSeCKS, a major protein kinase C (PKC) substrate with protein scaffolding and tumor suppressive properties, becomes tyrosine phosphorylated in NIH3T3 and rodent embryo fibroblasts after short-term treatment with epidermal growth factor (EGF), platelet-derived growth factor (PDGF), or fetal calf serum in the presence of pervanadate, but not by treatment with insulin or insulin-like growth factor-1. The relative phosphotyrosine level on SSeCKS was higher in actively dividing cells than in confluent cultures. Tyrosine phosphorylation of SSeCKS was apparent in cells deficient in Src, Fyn, Yes, or Abl tyrosine kinases or in NIH3T3 cells expressing a temperature-sensitive v-Src allele, but not in FAK-deficient embryo fibroblasts. Purified FAK or Src enzyme failed to directly phosphorylate SSeCKS in vitro. EGF failed to induce SSeCKS tyrosine phosphorylation in FAK-/- fibroblasts, indicating that the EGF receptor is probably not the direct kinase of SSeCKS. Phosphorylation under these conditions was rescued by the transient reexpression of wt-FAK but not FAK mutated at Y397, a major autophosphorylation and SH2-based docking site. Adhesion of FAK+/+ cells to fibronectin failed to significantly induce SSeCKS tyrosine phosphorylation although FAK was activated, suggesting that SSeCKS phosphorylation is mediated through a growth factor receptor-FAK rather than an integrin-FAK pathway. Moreover, PDGF could induce SSeCKS tyrosine phosphorylation in the absence of FAK activation, suggesting a role for FAK SH2-based docking rather than kinase activity. Immunofluorescence analysis showed that in FAK-/- cells, SSeCKS costains along F-actin stress fibers, in contrast to FAK+/+ cells, where most SSeCKS stains at the cell edge and along a cortical cytoskeletal matrix. This correlated with increased coprecipitation of SSeCKS with biotin-phalloidin-bound F-actin from FAK-/- compared to FAK+/+ cell lysates. Similarly, bacterially expressed, unphosphorylated SSeCKS cosedimented with F-actin in ultracentrifugation assays. These data suggest that mitogen-induced, FAK-dependent tyrosine phosphorylation of SSeCKS modulates its binding to the actin-based cytoskeleton, suggesting a role for SSeCKS in mitogen-induced cytoskeletal reorganization.

SSeCKS immunolabeling in rat primary sensory neurons

SSeCKS (src suppressed C kinase substrate) is a protein kinase C substrate that may play a role in tumor suppression. Recently described in fibroblasts, testes and mesangial cells, SSeCKS may have a function in the control of cell signaling and cytoskeletal arrangement. To investigate the distribution of SSeCKS throughout the nervous system, representative sections of brain, spinal cord and dorsal root ganglia were processed using immunofluorescence. Labeling of central axonal collaterals of primary sensory neurons was observed in the dorsal horn at all spinal levels. SSeCKS-immunoreactivity was also observed in the cerebellum, medulla and sensory ganglia (including trigeminal ganglia). The pattern and distribution of anti-SSeCKS labeling in dorsal root ganglia and the dorsal horn of the spinal cord was similar to that observed for other markers of small primary sensory neurons. Therefore, the coexistence of SSeCKS with substance P, CGRP and acid phosphatase was examined in sections of sensory ganglia, spinal cord and medulla using double immunofluorescent labeling for SSeCKS and substance P/CGRP or sequential SSeCKS immunofluorescence and acid phosphatase/fluoride-resistant acid phosphatase enzyme histochemistry. A small portion of the SSeCKS-labeled cell bodies appeared to represent a subpopulation of substance P (4.8%) and CGRP (4.7%) containing neurons, while 45.0% contained fluoride-resistant acid phosphatase reactivity. These results indicate that SSeCKS has a restricted distribution within the nervous system and that expression of this protein may reflect the specific signaling requirements of a distinct population of nociceptive sensory neurons.

Induction of Src-suppressed C kinase substrate (SSeCKS) in vascular endothelial cells by bacterial lipopolysaccharide

We isolated cDNA of the mouse homologue of the src-suppressed C kinase substrate (SSeCKS) and analyzed the effects of lipopolysaccharide (LPS) injection on the tissue expression pattern of this protein. Northern blotting analysis showed that SSeCKS mRNA was expressed abundantly in the testis but at undetectable levels in other tissues of untreated control mice. Intraperitoneal administration of LPS strongly induced SSeCKS mRNA expression in the lung, heart, liver, spleen, kidney, lymph node, adrenal gland, and pituitary gland, as well as in the brain. In lung and spleen, the SSeCKS mRNA levels increased almost 10-fold at 1 hr after LPS injection and persisted at high levels until 4 hr. Both in situ hybridization and immunohistochemical studies revealed that LPS administration conspicuously elevated expression of SSeCKS mRNA and protein in vascular endothelial cells of several organs. Ectopic expression of SSeCKS caused loss of cytoplasmic F-actin fibers in the mouse endothelial cell line LEII. These results indicate that SSeCKS is one of the major LPS-responsive proteins and may participate in alteration of cytoskeletal architecture in endothelial cells during inflammation.

Emerging targets: molecular mechanisms of cell contact-mediated growth control

Contact inhibition of cell proliferation evokes a unique cellular program of growth arrest compared with stress, age, or other physical constraints. The last decade of research on genes activated by cell-cell contact has uncovered features of transmembrane signaling, cytoskeletal reorganization, and transcriptional control that initiate and maintain a quiescent phenotype. This review will focus on mechanisms controlling contact inhibition of cell proliferation, highlighting specific gene expression responses that are activated by cell-cell contact. Although a temporal framework for imposition of these mechanisms has not yet been well described, contact inhibition of cell proliferation clearly requires their coordinated function. Novel targets for intervention in proliferative disorders are emerging from these studies.

Functional interaction between the SSeCKS scaffolding protein and the cytoplasmic domain of β1,4-galactosyltransferase

The β1,4-galactosyltransferase family contains at least seven unique gene products, of which β1,4-galactosyltransferase I (GalT) is the most exhaustively studied. GalT exists in the Golgi complex, similar to many other glycosyltransferases, as well as on the cell surface, where it functions as a signaling receptor for extracellular glycoside ligands. When expressed on the surface, GalT associates with the cytoskeleton and, upon ligand-induced aggregation, induces cell-type specific intracellular signal cascades. In an effort to define the mechanisms by which surface GalT exerts these intracellular effects, we used the yeast two-hybrid system to identify proteins that specifically interact with the GalT cytoplasmic domain.

The yeast two-hybrid screen identified two distinct clones (1.12 and 2.52) that showed identity to portions of SSeCKS (Src Suppressed CKinase Substrate). SSeCKS is a previously defined kinase and cytoskeleton scaffolding protein whose subcellular distribution and functions are remarkably similar to those attributed to GalT. Both SSeCKS and GalT have been localized to the perinuclear/Golgi region as well as to filopodia/lamellipodia. SSeCKS and GalT have been implicated in regulating cell growth, actin filament dynamics, and cell spreading. Interestingly, 1.12 and 2.52-GFP constructs were localized to subcellular domains that correlated with the two purported subcellular distributions for GalT; 2.52 being confined to the Golgi, whereas 1.12 localized primarily to filopodia. Coimmunoprecipitation assays demonstrate stable binding between the GalT cytoplasmic domain and the 1.12 and 2.52 domains of SSeCKS in appropriately transfected cells. Similar assays demonstrate binding between the endogenous GalT and SSeCKS proteins also. Coimmunoprecipitation assays were performed in both directions and produced similar results (i.e. using either anti-GalT domain or anti-SSeCKS domain antibodies as the precipitating reagent). A functional interaction between the GalT cytoplasmic domain and SSeCKS was illustrated by the ability of either the 1.12 or 2.52 SSeCKS domain to restore a normal adhesive phenotype in cells overexpressing the TL-GFP dominant negative construct. TL-GFP is composed of the GalT cytoplasmic and transmembrane domains fused to GFP, and leads to a loss of cell adhesion on laminin by displacing the endogenous GalT from its cytoskeleton binding sites. This is the first reported interaction between a glycosyltransferase and a scaffolding protein, and suggests that SSeCKS serve to integrate the various functions ascribed to the GalT cytoplasmic domain.