Mapping the microtubule binding regions of calponin

Mechanoregulation and function of calponin and transgelin

It is well known that chemical energy can be converted to mechanical force in biological systems by motor proteins such as myosin ATPase. It is also broadly observed that constant/static mechanical signals potently induce cellular responses. However, the mechanisms that cells sense and convert the mechanical force into biochemical signals are not well understood. Calponin and transgelin are a family of homologous proteins that participate in the regulation of actin-activated myosin motor activity. An isoform of calponin, calponin 2, has been shown to regulate cytoskeleton-based cell motility functions under mechanical signaling. The expression of the calponin 2 gene and the turnover of calponin 2 protein are both under mechanoregulation. The regulation and function of calponin 2 has physiological and pathological significance, as shown in platelet adhesion, inflammatory arthritis, arterial atherosclerosis, calcific aortic valve disease, post-surgical fibrotic peritoneal adhesion, chronic proteinuria, ovarian insufficiency, and tumor metastasis. The levels of calponin 2 vary in different cell types, reflecting adaptations to specific tissue environments and functional states. The present review focuses on the mechanoregulation of calponin and transgelin family proteins to explore how cells sense steady tension and convert the force signal to biochemical activities. Our objective is to present a current knowledge basis for further investigations to establish the function and mechanisms of calponin and transgelin in cellular mechanoregulation.

Evolution and function of calponin and transgelin

Calponin and transgelin (originally named SM22) are homologous cytoskeleton proteins that regulate actin-activated myosin motor functions in smooth muscle contraction and non-muscle cell motility during adhesion, migration, proliferation, phagocytosis, wound healing, and inflammatory responses. They are abundant cytoskeleton proteins present in multiple cell types whereas their physiological functions remain to be fully established. This focused review summarizes the evolution of genes encoding calponin and transgelin and their isoforms and discusses the structural similarity and divergence in vertebrate and invertebrate species in the context of functions in regulating cell motility. As the first literature review focusing on the evolution of the calponin-transgelin family of proteins in relevance to their structure-function relationship, the goal is to outline a foundation of current knowledge for continued investigations to understand the biological functions of calponin and transgelin in various cell types during physiological and pathological processes.

Two distinct regions of calponin share common binding sites on actin resulting in different modes of calponin-actin interaction

Calponins are a small family of proteins that alter the interaction between actin and myosin II and mediate signal transduction. These proteins bind F-actin in a complex manner that depends on a variety of parameters such as stoichiometry and ionic strength. Calponin binds G-actin and F-actin, bundling the latter primarily through two distinct and adjacent binding sites (ABS1 and ABS2). Calponin binds other proteins that bind F-actin and considerable disagreements exist as to how calponin is located on the filament, especially in the presence of other proteins. A study (Galkin, V.E., Orlova, A., Fattoum, A., Walsh, M.P. and Egelman, E.H. (2006) J. Mol. Biol. 359, 478-485.), using EM single-particle reconstruction has shown that there may be four modes of interaction, but how these occur is not yet known. We report that two distinct regions of calponin are capable of binding some of the same sites on actin (such as 18-28 and 360-372 in subdomain 1). This accounts for the finding that calponin binds the filament with different apparent geometries. We suggest that the four modes of filament binding account for differences in stoichiometry and that these, in turn, arise from differential binding of the two calponin regions to actin. It is likely that the modes of binding are reciprocally influenced by other actin-binding proteins since members of the alpha-actinin group also adopt different actin-binding positions and bind actin principally through a domain that is similar to calponin's ABS1.

Gene expression patterns in isolated keloid fibroblasts

Keloid scars after skin trauma are a significant clinical problem, especially in black populations, in which the incidence of keloids has been estimated at 4-16%. Keloids are abnormal dermal proliferative scars secondary to dysregulated wound healing. Despite several biochemical studies on the role of extracellular matrix proteins and growth factors during keloid formation, we still do not know what molecules and signals induce this change. Fibroblasts are thought to be the major inductive cell for keloid scar formation. The aim of this study was to identify gene expression patterns that characterize keloid fibroblasts; identifying such genetic disequilibrium may shed light on the molecular signaling events responsible for keloid formation. In this study, we performed gene expression analysis of fibroblasts isolated from keloid lesions from three individuals in comparison with the fibroblasts isolated from normal skin using the Affymetrix U133a chip (22,284 genes and expression sequence tags). We found through J5 test score expression analysis that among 22,284 genes, there were 43 genes that were overexpressed and five genes were underexpressed in keloid fibroblasts when compared with dermal fibroblasts from persons without keloids. The overexpression of three genes not previously reported as being up-regulated in keloids (annexin A2, Transgelin, and RPS18) was confirmed by real-time polymerase chain reaction. Certain overexpressed genes were similar to previous biochemical observations on the protein levels of these overexpressed genes during keloid formation. We also report for the first time that a few tumor-related genes are overexpressed in keloid fibroblasts.

Change in the shape and density of dendritic spines caused by overexpression of acidic calponin in cultured hippocampal neurons

Dendritic spines are morphing structures believed to provide a cellular substrate for synaptic plasticity. It has been suggested that the actin cytoskeleton is the target of molecular mechanisms regulating spine morphology. Here we hypothesized that acidic calponin, an actin-binding protein, is one of the key regulators of actin filaments during spine plasticity. Our data showed that the overexpression of acidic calponin-GFP (green fluorescent protein) in primary cultures of rat hippocampal neurons causes an elongation of spines and an increase of their density as compared with those of GFP-expressing neurons. These effects required the actin-binding domains of acidic calponin. The close apposition of the presynatic marker synaptophysin to these long spines and the presence of specific postsynaptic markers actin, PSD-95, NR1, and GluR1 suggested the existence of functional excitatory synaptic contacts. Indeed, electrophysiological data showed that the postsynaptic overexpression of acidic calponin enhanced the frequency of miniature excitatory postsynaptic currents as compared with that of GFP-expressing neurons, but did not affect their properties such as amplitude, rise time, and half width. Studies in heterologous cells revealed that acidic calponin reorganized the actin filaments and stabilized them. Taken together, these findings show that acidic calponin regulates dendritic spine morphology and density, likely via regulation of the actin cytoskeleton reorganization and dynamic. Furthermore, the acidic calponin-induced spines are able to establish functional glutamatergic synapses. Such data suggest that acidic calponin is a key factor in the regulation of spine plasticity and synaptic activity.

ROK-induced cross-link formation stiffens passive muscle: reversible strain-induced stress softening in rabbit detrusor

Passive mechanical properties of strips of rabbit detrusor smooth muscle were examined and found by cyclic loading in a calcium-free solution to display viscoelastic softening and strain-induced stress softening (strain softening). Strain softening, or the Mullins effect, is a loss of stiffness attributed to the breakage of cross-links, and appeared irreversible in detrusor even after the return of spontaneous rhythmic tone during 120 min of incubation in a calcium-containing solution. However, 3 min of KCl or carbachol (CCh)-induced contraction permitted rapid regeneration of the passive stiffness lost to strain softening, and 3 μM of the RhoA kinase (ROK) inhibitor Y-27632 prevented this regeneration. The degree of ROK-induced passive stiffness was inversely dependent on muscle length over a length range where peak CCh-induced force was length independent. Thus rabbit detrusor displayed variable passive stiffness both strain- and activation-history dependent. In conclusion, activation of ROK by KCl or CCh increased passive stiffness softened by muscle strain and thereby attributed to cross-links that remained stable during tissue incubation in a calcium-free solution. Degradation of this signaling system could potentially contribute to urinary incontinence.

PGF2alpha induced differential expression of genes involved in turnover of extracellular matrix in rat decidual cells

In the rat, the decidual tissue is an important component for maternal recognition of pregnancy. Decidualization can be induced by either the implantation of the blastocyst or by artificial stimuli. The process of decidua formation or decidualization, is characterized by growth and differentiation of endometrial stromal cells. Prostaglandin F2alpha (PGF2alpha) has been shown to be involved in inhibition of implantation, alteration of embryo development, induction of luteal regression, and the mediation of pregnancy loss induced by microorganism infections. In order to establish a direct role for PGF2alpha in decidual function, we have evaluated its effects on the expression of an extensive array of genes using primary decidual cell culture. Upon treatment with PGF2alpha sixty genes were significantly down-regulated whereas only six genes were up-regulated (from a total of 1176 genes studied). Interestingly, the majority of the genes inhibited by PGF2alpha are either directly or indirectly involved in the turnover of the extracellular matrix (ECM). Genes such as gelatinase A (MMP2), cathepsin L, tissue inhibitor metalloproteinases 2 (TIMP2) and 3 (TIMP3), plasminogen activator inhibitor1 (PAI1), tissue type plasminogen activator (tPA), urokinase plasminogen activator (tPA), endothelin 1, calponin, carboxypeptidase D and calponin acidic were down regulated. The opposite effect was observed for prostromelysin 53 kDa (proMMP3), plasma proteinase I alpha and alpha 1 antiproteinase, all of which were significantly up-regulated by PGF2alpha. The results strongly suggest that the abortificient role of elevated levels of PGF2alpha after implantation is due, in large part, to inhibition of genes involved in the normal turnover of the extracellular matrix necessary for decidual formation.

A plant-specific kinesin binds to actin microfilaments and interacts with cortical microtubules in cotton fibers

A novel kinesin, GhKCH1, has been identified from cotton (Gossypium hirsutum) fibers. GhKCH1 has a centrally located kinesin catalytic core, a signature neck peptide of minus end-directed kinesins, and a unique calponin homology (CH) domain at its N terminus. GhKCH1 and other CH domain-containing kinesins (KCHs) belong to a distinct branch of the minus end-directed kinesin subfamily. To date the KCH kinesins have been found only in higher plants. Because the CH domain is often found in actin-binding proteins, we proposed that GhKCH1 might play a role in mediating dynamic interaction between microtubules and actin microfilaments in cotton fibers. In an in vitro actin-binding assay, GhKCH1's N-terminal region including the CH domain interacted directly with actin microfilaments. In cotton fibers, GhKCH1 decorated cortical microtubules in a punctate manner. Occasionally GhKCH1 was found to be associated with transverse-cortical actin microfilaments, but never with axial actin cables in cotton fibers. Localization of GhKCH1 on cortical microtubules was independent of the integrity of actin microfilaments. Thus, GhKCH1 may play a role in organizing the actin network in coordination with the cortical microtubule array. These data also suggest that flowering plants may employ unique KCHs to coordinate actin microfilaments and microtubules during cell growth.

Tyrosine phosphorylation of calponins. Inhibition of the interaction with F-actin

The phosphorylation-dephosphorylation of serine and threonine residues of calponin is known to modulate in vitro its interaction with F-actin and is thought to regulate several biological processes in cells, involving either of the calponin isoforms. Here, we identify, for the first time, tyrosine-phosphorylated calponin h3 within COS 7 cells, before and after their transfection with the pSV vector containing cDNA encoding the cytoplasmic, Src-related, tyrosine kinase, Fyn. We then describe the specific tyrosine phosphorylation in vitro of calponin h1 and calponin h3 by this kinase. 32P-labeling of tyrosine residues was monitored by combined autoradiography, immunoblotting with a specific phosphotyrosine monoclonal antibody and dephosphorylation with the phosphotyrosine-specific protein phosphatase, YOP. PhosphorImager analyses showed the incorporation of maximally 1.4 and 2.0 mol of 32P per mol of calponin h3 and calponin h1, respectively. As a result, 75% and 68%, respectively, of binding to F-actin was lost by the phosphorylated calponins. Furthermore, F-actin, added at a two- or 10-fold molar excess, did not protect, but rather increased, the extent of 32P-labeling in both calponins. Structural analysis of the tryptic phosphopeptides from each 32P-labeled calponin revealed a single, major 32P-peptide in calponin h3, with Tyr261 as the phosphorylation site. Tyr261 was also phosphorylated in calponin h1, together with Tyr182. Collectively, the data point to the potential involvement, at least in living nonmuscle cells, of tyrosine protein kinases and the conserved Tyr261, located in the third repeat motif of the calponin molecule, in a new level of regulation of the actin-calponin interaction.