Cell shape regulation by Gravin requires N-terminal membrane effector domains

Increased SSeCKS expression in rat hepatic stellate cells upon activation in vitro and in vivo

Recent reports suggest that src suppressed c kinase substrates (SSeCKS) are early inflammatory response protein. However, there is only scarce knowledge on the functional role of SSeCKS in liver under conditions of acute inflammation. In the present study, we investigated SSeCKS expression in liver after administration of carbon tetrachloride (CCl4) in rats and in isolated primary hepatic stellate cells (HSCs) upon activation on a plastic dish. We found that SSeCKS mRNA was hardly detectable in healthy liver tissue and further increased in carbon tetrachloride-mediated acute liver failure. SSeCKS protein expression was mainly found in hepatic stellate cells. In vitro, SSeCKS expression in activated rat HSCs was dramatically increased. The upregulation of SSeCKS protein expression in rat HSCs during activation in vitro and in vivo suggested the possibility of SSeCKS, an important part of function of the activated HSCs, perhaps through modulation of liver regeneration or formation of liver fibrosis after various injuries.

Protein phosphatase 1 β paralogs encode the zebrafish myosin phosphatase catalytic subunit

Background

The myosin phosphatase is a highly conserved regulator of actomyosin contractility. Zebrafish has emerged as an ideal model system to study the invivo role of myosin phosphatase in controlling cell contractility, cell movement and epithelial biology. Most work in zebrafish has focused on the regulatory subunit of the myosin phosphatase called Mypt1. In this work, we examined the critical role of Protein Phosphatase 1, PP1, the catalytic subunit of the myosin phosphatase.

Methodology/Principal Findings

We observed that in zebrafish two paralogous genes encoding PP1β, called ppp1cba and ppp1cbb, are both broadly expressed during early development. Furthermore, we found that both gene products interact with Mypt1 and assemble an active myosin phosphatase complex. In addition, expression of this complex results in dephosphorylation of the myosin regulatory light chain and large scale rearrangements of the actin cytoskeleton. Morpholino knock-down of ppp1cba and ppp1cbb results in severe defects in morphogenetic cell movements during gastrulation through loss of myosin phosphatase function.

Conclusions/Significance

Our work demonstrates that zebrafish have two genes encoding PP1β, both of which can interact with Mypt1 and assemble an active myosin phosphatase. In addition, both genes are required for convergence and extension during gastrulation and correct dosage of the protein products is required.

Emerging Roles for SSeCKS/Gravin/AKAP12 in the Control of Cell Proliferation, Cancer Malignancy, and Barriergenesis

Emerging data suggest that SSeCKS/Gravin/AKAP12 ("AKAP12"), originally identified as an autoantigen in cases of myasthenia gravis, controls multiple biological processes through its ability to scaffold key signaling proteins such as protein kinase (PK) C and A, calmodulin, cyclins, phosphoinositides, "long" β-1,4 galactosyltransferase (GalTase) isoform, Src, as well as the actin cytoskeleton in a spatiotemporal manner. Specialized functions attributed to AKAP12 include the suppression of cancer malignancy, especially aspects of metastatic progression, regulation of blood-brain and blood-retina barrier formation, and resensitization of β2-adrenergic pain receptors. Recent data identify a direct role for AKAP12 in cytokinesis completion, further suggesting a function as a negative regulator of cell senescence. The current review will discuss the emerging knowledge base of AKAP12-related biological roles and how the factors that affect AKAP12 expression or that interact with AKAP12 at the protein level control cancer progression and blood-tissue barrier formation.

Rho-regulated myosin phosphatase establishes the level of protrusive activity required for cell movements during zebrafish gastrulation

Rho-dependent amoeboid cell movement is a crucial mechanism in both tumor cell invasion and morphogenetic cell movements during fish gastrulation. Amoeboid movement is characterized by relatively non-polarized cells displaying a high level of bleb-like protrusions. During gastrulation, zebrafish mesodermal cells undergo a series of conversions from amoeboid cell behaviors to more mesenchymal and finally highly polarized and intercalative cell behaviors. We demonstrate that Myosin phosphatase, a complex of Protein phosphatase 1 and the scaffolding protein Mypt1, functions to maintain the precise balance between amoeboid and mesenchymal cell behaviors required for cells to undergo convergence and extension. Importantly, Mypt1 has different cell-autonomous and non-cell-autonomous roles. Loss of Mypt1 throughout the embryo causes severe convergence defects, demonstrating that Mypt1 is required for the cell-cell interactions involved in dorsal convergence. By contrast, mesodermal Mypt1 morphant cells transplanted into wild-type hosts undergo dorsally directed cell migration, but they fail to shut down their protrusive behavior and undergo the normal intercalation required for extension. We further show that Mypt1 activity is regulated in embryos by Rho-mediated inhibitory phosphorylation, which is promoted by non-canonical Wnt signaling. We propose that Myosin phosphatase is a crucial and tightly controlled regulator of cell behaviors during gastrulation and that understanding its role in early development also provides insight into the mechanism of cancer cell invasion.

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.