Isolation and characterization of different C-terminal fragments of dystrophin expressed in Escherichia coli

Polymorphisms in Heat Shock Proteins A1B and A1L (HOM) as Risk Factors for Oesophageal Carcinoma in Northeast India

BACKGROUND

To investigate polymorphisms in heat shock proteins A1B and A1L (HOM) and associated risk of oesophageal carcinoma in Northeast India.

MATERIALS AND METHODS

The study includes oesophageal cancer (ECA) patients attending general outpatient department (OPD) and endoscopic unit of Gauhati Medical College. Patients were diagnosed based on endoscopic and histopathological findings. Genomic DNA was typed for HSPA1B1267 and HSPA1L2437 SNPs using the polymerase chain reaction with restriction fragment length polymorphisms.

RESULTS

A total of 78 cases and 100 age-sex matched healthy controls were included in the study with a male: female ratio of 5:3 and a mean age of 61.4±8.5 years. Clinico-pathological evaluation showed 84% had squamous cell carcinoma and 16% were adenocarcinoma. Dysphagia grades 4 (43.5%) and 5 (37.1%) were observed by endoscopic and hispathological evaluation. The frequency of genomic variation of A1B from wild type A/A to heterozygous A/G and mutant G/G showed a positive association [chi sq=19.9, p= <0.05] and the allelic frequency also showed a significant correlation [chi sq=10.3, with cases vs. controls, OR=0.32, p≤0.05]. The genomic variation of A1L from wild T/T to heterozygous T/C and mutant C/C were found positively associated [chi sq= 7.02, p<0.05] with development of ECA. While analyzing the allelic frequency, there was no significant association [chi sq= 3.19, OR=0.49, p=0.07]. Among all the risk factors, betel quid [OR =9.79, Chi square= 35.0, p<0.05], tobacco [OR = 2.95, chi square=10.6, p<0.05], smoking [OR=3.23, chi square=10.1, p<0.05] demonstrated significant differences between consumers vs. non consumers regarding EC development. Alcohol did not show any significant association [OR= 1.34, chi square=0.69, p=0.4] independently.

CONCLUSIONS

It can be concluded that the present study provides marked evidence that polymorphisms of HSP70 A1B and HSP70 A1L genes are associated with the development of ECA in a population in Northeast India, A1B having a stronger influence. Betel quid consumption was found to be a highly significant risk factor, followed by smoking and tobacco chewing. Although alcohol was not a potent risk factor independently, alcohol consumption along with tobacco, smoking and betel nut was found to contribute to development of ECA.

Biochemical and histochemical analysis of 71 kDa dystrophin isoform (Dp71f) in rat brain

Dp71 is a member of the dystrophin family and the most abundant dmd gene product in the brain. In the present study, we focused on a short dystrophin transcript named Dp71f, which is alternatively spliced when exon 78 is absent The topographic localization of this protein in the encephalon has not been properly described yet, nor its cellular or subcellular localization, and even less its functions. Dp71f was found to be a cytoplasmic 70 kDa protein and localized in all encephalon regions studied. Double labeling using specific markers for various cell types confirmed Dp71f distribution in the cytoplasm of all cell types studied. Labeling was more conspicuous near the nucleus and diminished towards the periphery of cells. In some cases, we observed cells that were positive for actin and Dp71f in regions corresponding to lamellipodia-like structures. Dp71f and Dp71d isoforms were differently distributed. Our study is the first specific and unambiguous description of the topography and cellular localization patterns of Dp71f in brain, suggesting that Dp71f is a ubiquitous protein.

Contribution of the different modules in the utrophin carboxy-terminal region to the formation and regulation of the DAP complex

The carboxy-terminal region of utrophin, like the homologous proteins dystrophin, Drp2 and dystrobrevins, contains structural domains frequently involved in protein-protein interaction. These domains (WW, EF hands, ZZ and H1-H2) mediate recognition and binding to a multicomponent complex of proteins, also known as dystrophin-associated proteins (DAPs) for their association with dystrophin, the product of the gene, mutated in Duchenne muscular dystrophy. We have exploited phage display and in vitro binding assays to study the recognition specificity of the different domains of the utrophin carboxy-terminus. We found that none of the carboxy-terminal domains of utrophin, when isolated from its structural context, selects specific ligand peptides from a phage-displayed peptide library. By contrast, panning with an extended region containing the WW, EF hands, and ZZ domain defines the consensus binding motif, PPxY which is also found in beta-dystroglycan, a component of the DAP complex that interacts with utrophin in several tissues. WW-mediated binding to PPxY peptides and to beta-dystroglycan requires the presence of the EF hands and ZZ domain. When the ZZ domain is either deleted or engaged in binding to calmodulin, the utrophin beta-dystroglycan complex cannot be formed. These findings suggest a potential regulatory mechanism by means of which the attachment of utrophin to the DAP complex can be modulated by the Ca(2+)-dependent binding of calmodulin. The remaining two motifs found in the carboxy-terminus (H1-H2) mediate the formation of utrophin-dystrobrevin hybrids but do not select ligands in a repertoire of random nonapeptides.

Fluorescence tracing of intracellular proteins

Developments in fluorescence microscopy and the availability of fluorescently labeled antibodies and probes for localization of molecules and organelles have made the microscope an indispensable tool with which one can map specific molecules to subcellular loci allowing deep insight into cell and organelle biology. Furthermore, confocal microscopy permits analysis of the three dimensional architecture of cells that could not be accomplished by conventional light microscopy. The goal of fluorescence protein tracing by microscopy is to visualize cellular constituents and general cytoarchitecture as close to native organization as possible. To achieve this, and to preserve cellular structure in the best possible manner, the specimen is usually fixed chemically. Here I review several standard fixation, permeabilization and labeling schemes followed by examples of several standard imaging techniques.

WW and EF hand domains of dystrophin-family proteins mediate dystroglycan binding

The membrane-spanning dystrophin glycoprotein complex mediates an indirect linkage between the actin-based cytoskeleton and the extracellular matrix. Although expressed by diverse cell types, genetic lesions of members of this complex result in muscular dystrophy phenotypes emphasizing the importance of these interactions in muscle cells. We have characterized interactions between dystrophin family members and dystroglycan: cytoskeletal and transmembrane components of the complex, respectively. Our results demonstrate that both the WW and EF hand domains of dystrophin and utrophin, an autosomal homologue of dystrophin, directly bind the cytoplasmic domain of dystroglycan. Furthermore, alpha-dystrobrevin, a more distantly related dystrophin family member which lacks a WW domain but contains the EF hand domain, binds dystroglycan. This is the first demonstration of a direct interaction between a dystrobrevin or utrophin and dystroglycan, and has implications for the organization of the dystrophin glycoprotein complex and the use of dystrophin homologues in muscular dystrophy therapy.

alpha-actinin-2 is a new component of the dystrophin-glycoprotein complex

The human skeletal muscle yeast two-hybrid cDNA library was screened with the carboxyl-terminal region (the last 200 amino acids) of dystrophin. Two interacting clones were identified corresponding to alpha-actinin-2 and actin. Interactions between alpha-actinin, actin, and dystrophin were confirmed by the ligand-blotting technique, by colocalization of dystrophin and alpha-actinin-2 to the isolated skeletal muscle sarcolemmal vesicles and to the plasma membranes isolated from C2C12 myoblasts, and by indirect immunolocalization of dystrophin and alpha-actinin-2 in skeletal muscle cells. This is the first identification of a direct interaction between alpha-actinin, actin, and the carboxyl-terminal region of dystrophin. We propose that dystrophin forms lateral, multicontact association with actin and that binding of alpha-actinin-2 to the carboxyl-terminus of dystrophin is the communication link between the integrins and the dystrophin/dystrophin-glycoprotein complex.

The WW domain of dystrophin requires EF-hands region to interact with beta-dystroglycan

Skeletal muscle dystrophin is a 427 kDa protein thought to act as a link between the actin cytoskeleton and the extracellular matrix. Perturbations of the dystrophin-associated complex, for example, between dystrophin and the transmembrane glycoprotein beta-dystroglycan, may lead to muscular dystrophy. Previously, the cysteine-rich region and first half of the carboxy-terminal domain of dystrophin were shown to interact with beta-dystroglycan through a stretch of fifteen amino acids at the carboxy-terminus of beta-dystroglycan. This region of dystrophin implicated in binding beta-dystroglycan contains four modular protein domains: a WW domain, two putative Ca2+-binding EF-hand motifs, and a putative zinc finger ZZ domain. The WW domain is a globular domain of 38-40 amino acids with two highly conserved tryptophan residues spaced 20-22 amino acids apart. A subset of WW domains was shown to bind ligands that contain a Pro-Pro-x-Tyr core motif (where x is any amino acid). Here we elucidate the role of the WW domain of dystrophin and surrounding sequence in binding beta-dystroglycan. We show that the WW domain of dystrophin along with the EF-hand motifs binds to the carboxy-terminus of beta-dystroglycan. Through site-specific mutagenesis and in vitro binding assays, we demonstrate that binding of dystrophin to the carboxy-terminus of beta-dystroglycan occurs via a beta-dystroglycan Pro-Pro-x-Tyr core motif. Targeted mutagenesis of conserved WW domain residues reveals that the dystrophin/beta-dystroglycan interaction occurs primarily through the WW domain of dystrophin. Precise mapping of this interaction could aid in therapeutic design.

Effect of adopting a new histological grading system of acute rejection after heart transplantation

BACKGROUND

Treatment policy of acute rejection after heart transplantation has been changed after adopting the ISHLT endomyocardial biopsy grading system in 1991.

OBJECTIVE

To determine the effect of this policy change on clinical outcome after transplantation.

METHODS

The outcome of 147 patients who had a transplant before (early group, median follow up 96 months) and 114 patients who had a transplant after (late group, median follow up 41 months) the introduction of the ISHLT biopsy grading system was studied retrospectively. Initially "moderate rejection" according to Billingham's conventional criteria was treated. From January 1991 grade 3A and higher was considered to require intensification of immunosuppression.

RESULTS

There were some differences between the two groups: recipients (50 v 44 years) as well as donors (28 v 24 years) were older in the "late group" and more patients of this group received early anti-T cell prophylaxis (92% v 56%). Despite more extensive use of early prophylaxis more rejection episodes were diagnosed (2.4 v 1.4) and considerably more courses of rejection treatment were instituted in the late compared with the early group (3.2 v 1.5). There were no deaths because of rejection in the late group, however, more infections occurred within the first year (mean 1.8 v 1.4) and more non-skin malignancies within the first 41 months were diagnosed (8 of 57 v 6 of 147, 95% CIs of difference includes 0). The incidence of graft vascular disease in the late group has been comparable to the early group until now.

CONCLUSION

The interpretation of the ISHLT grading system resulted in lowering of the threshold for the diagnosis of rejection thereby increasing the number of rejections and subsequently the immunosuppressive load and its complications.

The membrane-cytoskeleton interface: the role of dystrophin and utrophin

Recent studies with transgenic animals have considerably advanced our knowledge of the roles of dystrophin and utrophin in both muscle and non-muscle tissues. Rigorous analyses of the roles of the various mdx mutations in mice, as well as the use of artificial transgenes in an mdx background, are beginning to define the functional importance of various regions of the dystrophin protein in normal muscle. Furthermore, recent biochemical analyses have revealed new insights into the role and organization of dystrophin at the membrane-cytoskeleton interface. Transgenic approaches have also revealed surprising and encouraging results with respect to utrophin. Against expectations, the long-awaited utrophin knockout mice have a remarkably mild phenotype with only subtle changes in neuromuscular junction architecture. On the other hand, mdx mice transgenic for a mini-utrophin construct showed rescue of the muscular dystrophy phenotype, clearly an encouraging finding with obvious therapeutic possibilities. These and other recent findings are discussed in the context of the structure and function of dystrophin and utrophin at the membrane-cytoskeleton interface.

Calreticulin inhibits glucocorticoid- but not cAMP-sensitive expression of tyrosine aminotransferase gene in cultured McA-RH7777 hepatocytes

Calreticulin is a ubiquitously expressed Ca2+ binding protein of the endoplasmic reticulum which inhibits DNA binding and transcriptional activation by steroid hormone receptors. In this study the effects of calreticulin on tyrosine aminotransferase (TAT) gene expression in cultured McA-RH7777 hepatocytes was investigated. McA-RH7777 cells were stably transfected with calreticulin expression vector to generate cells overexpressing the protein. The transcriptional activity of the TAT gene, which is glucocorticoid-sensitive and cAMP-dependent, was investigated in the mock transfected McA-RH7777 and in cells overexpressing calreticulin (designated McA-11 and McA-17). In the presence of dexamethasone or the cAMP analog (CTP-cAMP) expression of the TAT gene was induced in mock transfected McA-RH7777 cells by approximately 4.5 and 5 fold, respectively. In McA-11 and McA-17 cells, overexpressing calreticulin, glucocorticoid-sensitive expression of the TAT gene was significantly inhibited, however, the CTP-cAMP-dependent expression of the TAT gene was not affected. The ability of calreticulin to inhibit glucocorticoid-sensitive TAT gene expression but not the cAMP-dependent expression of the gene suggests that the protein affects specifically the action of transcription pathways involving steroid receptors or transcription factors containing KxFF(K/R)R-like motifs. Calreticulin may play an important role in the regulation of glucocorticoid-sensitive pathway of expression of the hepatocytes specific genes during development.

Endoplasmic reticulum form of calreticulin modulates glucocorticoid-sensitive gene expression

Calreticulin is a ubiquitously expressed Ca2+-binding protein of the endoplasmic reticulum (ER), which inhibits DNA binding in vitro and transcriptional activation in vivo by steroid hormone receptors. Transient transfection assays were carried out to investigate the effects of different intracellular targeting of calreticulin on transactivation mediated by glucocorticoid receptor. BSC40 cells were transfected with either calreticulin expression vector (ER form of calreticulin) or calreticulin expression vector encoding calreticulin minus leader peptide, resulting in cytoplasmic localization of the recombinant protein. Transfection of BSC40 cells with calreticulin expression vector encoding the ER form of the protein led to 40-50% inhibition of the dexamethasone-sensitive stimulation of luciferase expression. However, in a similar experiment, but using the calreticulin expression vector encoding cytoplasmic calreticulin, dexamethasone-stimulated activation of the luciferase reporter gene was inhibited by only 10%. We conclude that the ER, but not cytosolic, form of calreticulin is responsible for inhibition of glucocorticoid receptor-mediated gene expression. These effects are specific to calreticulin, since overexpression of the ER lumenal proteins (BiP, ERp72, or calsequestrin) has no effect on glucocorticoid-sensitive gene expression. The N domain of calreticulin binds to the DNA binding domain of the glucocorticoid receptor in vitro; however, we show that the N+P domain of calreticulin, when synthesized without the ER signal sequence, does not inhibit glucocorticoid receptor function in vivo. Furthermore, expression of the N domain of calreticulin and the DNA binding domain of glucocorticoid receptor as fusion proteins with GAL4 in the yeast two-hybrid system revealed that calreticulin does not interact with glucocorticoid receptor under these conditions. We conclude that calreticulin and glucocorticoid receptor may not interact in vivo and that the calreticulin-dependent modulation of the glucocorticoid receptor function may therefore be due to a calreticulin-dependent signaling from the ER.

Phosphorylation of the carboxyl-terminal region of dystrophin

Dystrophin is a protein product of the gene responsible for Duchenne and Becker muscular dystrophy. The protein is localized to the inner surface of sarcolemma and is associated with a group of membrane (glyco)proteins. Dystrophin links cytoskeletal actins via the dystrophin-associated protein complex to extracellular matrix protein, laminin. This structural organization implicates the role of dystrophin in stabilizing the sarcolemma of muscle fibers. Precisely how dystrophin functions is far from clear. The presence of an array of isoforms of the C-terminal region of dystrophin suggests that dystrophin may have functions other than structural. In agreement, many potential phosphorylation sites are found in the C-terminal region of dystrophin, and the C-terminal region of dystrophin is phosphorylated both in vitro and in vivo by many protein kinases, including MAP kinase, p34cdc2 kinase, CaM kinase, and casein kinase, and is dephosphorylated by calcineurin. The C-terminal domain of dystrophin is also a substrate for hierarchical phosphorylation by casein kinase-2 and GSK-3. These observations, in accordance with the finding that the cysteine-rich region binds to Ca2+, Zn2+, and calmodulin, suggest an active involvement of dystrophin in transducing signals across muscle sarcolemma. Phosphorylation-dephosphorylation of the C-terminal region of dystrophin may play a role in regulating dystrophin-protein interactions and (or) transducing signal from the extracellular matrix via the dystrophin molecule to the cytoskeleton.

Phosphorylation of the carboxyl terminal region of dystrophin by mitogen-activated protein (MAP) kinase

Dystrophin is the 427-kDa protein product of the Duchenne muscular dystrophy gene (DMD). The function of this protein remains to be elucidated. We have recently reported that dystrophin is phosphorylated, in vivo, in rat skeletal muscle primary cell culture (RE Milner, JL Busaan, CFB Holmes, JH Wang, M Michalak (1993) J Biol Chem 268:21901-21905). This observation suggests that protein phosphorylation may have some role in modulating the function of dystrophin or its interaction with membrane associate dystroglycan. We report here that the carboxyl-terminal of dystrophin is phosphorylated by the MAP kinase p44mpk (mitogen-activated protein kinase), from the sea star oocytes and by soluble extracts of rabbit skeletal muscle. Importantly we showed that native dystrophin in isolated sarcolemmal vesicles is phosphorylated by sea star p44mpk Partial purification and immunological analysis show that a mammalian kinase related to p44mpk is present in the skeletal muscle extracts and that it contributes to phosphorylation of the carboxyl-terminal of dystrophin. This kinase phosphorylates dystrophin on a threonine residue(s). We conclude that phosphorylation of dystrophin may play an important role in the function of this cytoskeletal protein.

Dystrophin and utrophin: the missing links!

There is considerable sequence homology between dystrophin and utrophin, both at the protein and DNA level, and consequently it was assumed that their domain structures and functions would be similar. As more of the detailed biochemical and cell biological properties of these two proteins become known, so it becomes clear that there are subtle if not significant differences between them. We review recent findings and present new hypotheses into the structural and functional properties of the actin-binding domain, central coiled-coil region and regulatory/membrane protein-binding regions of dystrophin and utrophin.

Cloning, expression and characterization of two putative calcium-binding sites in human non-erythroid alpha-spectrin

The C-terminus of alpha-spectrins contains two putative calcium-binding sites or EF-hands. To characterize the binding, we have isolated clones from a human fetal liver cDNA library and expressed several fragments comprising either one or both of these sites. When the isolated clones were sequenced, we found that three consecutive nucleotides differed compared to the published sequence. The discrepancy affected two codons in the first of the two putative calcium sites. These codons translated into glutamate and phenylalanine, which are identical to the residues present at the same position in other alpha-spectrins. In the presence of magnesium, only recombinant peptides comprising the second putative site bound calcium as determined by a calcium overlay assay. Although the first putative EF-hand appeared to bind some calcium in the absence of magnesium, no binding could be detected under stringent conditions. Therefore, it is likely that the second EF-hand constitutes the only functional calcium-binding site in the C-terminus of human non-erythroid alpha-spectrin. Since peptides comprising the second EF-hand bound calcium nearly as well as intact spectrin, it is also apparent that the second EF-hand constitutes the major binding site for calcium in spectrin. The relative change in negative ellipticity, induced by the binding of calcium, indicates a dissociation constant of approximately 120 microM.

Calcium/calmodulin-dependent regulation of the NH2-terminal F-actin binding domain of utrophin

The cytoskeletal proteins utrophin, dystrophin and alpha-actinin are predicted to form antiparallel dimers thus potentially bringing their NH2-terminal F-actin binding domains in close proximity to their EF-hand containing COOH-terminal domains. This arrangement would allow for calcium-dependent regulation of F-actin binding. We tested this hypothesis by determining the effect of the ubiquitous calcium binding protein calmodulin on their F-actin binding capabilities. Binding of the NH2-terminal F-actin binding domain of utrophin to F-actin was inhibited by increasing concentrations of calmodulin in a calcium-dependent manner. The homologous F-actin binding domains from dystrophin and alpha-actinin were not regulated by calmodulin in the presence or absence of calcium. These findings have implications for the structural organisation of utrophin dimers and also for the replacement of dystrophin by over-expression of utrophin in dystrophic muscle.

Characterization and localization of a 77 kDa protein related to the dystrophin gene family

The Duchenne muscular dystrophy gene gives rise to transcripts of several lengths. These mRNAs differ in their coding content and tissue distribution. The 14 kb mRNA encodes dystrophin, a 427 kDa protein found in muscle and brain, and the short transcripts described encode DP71, a 77 kDa protein found in various organs. These short transcripts have many features common to the deduced primary structure of dystrophin, especially in the cysteine-rich specific C-terminal domains. The dystrophin C-terminal domain could be involved in membrane anchorage via the glycoprotein complex, but such a functional role for these short transcript products has yet to be demonstrated. Here we report the first isolation of a short transcript product from saponin-solubilized cardiac muscle membranes using alkaline buffer and affinity chromatography procedures. This molecule was found to be glycosylated and could be easily dissociated from cardiac muscle and other non-muscle tissues such as brain and liver. DP71-specific monoclonal antibody helped to identify this molecule as being related to the dystrophin gene family. Immunofluorescence analysis of bovine or chicken cardiac muscle showed a periodic distribution of DP71 in transverse T tubules and this protein was co-localized with the dystrophin glycoprotein complex in the Z-disk area.

Molecular organization at the glycoprotein-complex-binding site of dystrophin. Three dystrophin-associated proteins bind directly to the carboxy-terminal portion of dystrophin

Direct interaction between the C-terminal portion of dystrophin and dystrophin-associated proteins was investigated. The binding of dystrophin to each protein was reconstituted by overlaying bacterially expressed dystrophin fusion proteins onto the blot membranes to which dystrophin-associated proteins were transferred after separation by SDS/PAGE with the following results. (a) Among the components of the glycoprotein complex which links dystrophin to the sarcolemma, a 43-kDa dystrophin-associated glycoprotein binds directly to dystrophin. Although at least one of the binding sites of this protein resides within the cysteine-rich domain of dystrophin, a contribution of additional amino acid residues within the first half of the C-terminal domain was also suggested for more secure binding. (b) Two other proteins also directly bind to dystrophin. Their binding sites are suggested to reside within the last half of the C-terminal domain which is alternatively spliced depending on the tissue type. Previously, based on the enzyme digestion experiments, we showed that the binding site for the glycoprotein complex on dystrophin is present within the cysteine-rich domain and the first half of the C-terminal domain [Suzuki, A., Yoshida, M., Yamamoto, H. & Ozawa, E. (1992) FEBS Lett. 308, 154-160]. Here, we have extended this work and found that the region which is involved in interaction with the complex is widely extended to the entire length of this part of the molecule. On the basis of the present results, we propose a model of molecular architecture at the binding site for the complex on dystrophin.

The structural and functional diversity of dystrophin

Duchenne and Becker muscular dystrophies are caused by defects of the dystrophin gene. Expression of this large X-linked gene is under elaborate transcriptional and splicing control. At least five independent promoters specify the transcription of their respective alternative first exons in a cell-specific and developmentally controlled manner. Three promoters express full-length dystrophin, while two promoters near the C terminus express the last domains in a mutually exclusive manner. Six exons of the C terminus are alternatively spliced, giving rise to several alternative forms. Genetic, biochemical and anatomical studies of dystrophin suggest that a number of distinct functions are subserved by its great structural diversity. Extensive studies of dystrophin may lead to an understanding of the cause and perhaps a rational treatment for muscular dystrophy.