MLP-deficient mice exhibit a disruption of cardiac cytoarchitectural organization, dilated cardiomyopathy, and heart failure

The PDZ domain of the LIM protein enigma binds to beta-tropomyosin

PDZ and LIM domains are modular protein interaction motifs present in proteins with diverse functions. Enigma is representative of a family of proteins composed of a series of conserved PDZ and LIM domains. The LIM domains of Enigma and its most related family member, Enigma homology protein, bind to protein kinases, whereas the PDZ domains of Enigma and family member actin-associated LIM protein bind to actin filaments. Enigma localizes to actin filaments in fibroblasts via its PDZ domain, and actin-associated LIM protein binds to and colocalizes with the actin-binding protein alpha-actinin-2 at Z lines in skeletal muscle. We show that Enigma is present at the Z line in skeletal muscle and that the PDZ domain of Enigma binds to a skeletal muscle target, the actin-binding protein tropomyosin (skeletal beta-TM). The interaction between Enigma and skeletal beta-TM was specific for the PDZ domain of Enigma, was abolished by mutations in the PDZ domain, and required the PDZ-binding consensus sequence (Thr-Ser-Leu) at the extreme carboxyl terminus of skeletal beta-TM. Enigma interacted with isoforms of tropomyosin expressed in C2C12 myotubes and formed an immunoprecipitable complex with skeletal beta-TM in transfected cells. The association of Enigma with skeletal beta-TM suggests a role for Enigma as an adapter protein that directs LIM-binding proteins to actin filaments of muscle cells.

Cardiac myosin heavy chains lacking the light chain binding domain cause hypertrophic cardiomyopathy in mice

Myosin is a chemomechanical motor that converts chemical energy into the mechanical work of muscle contraction. More than 40 missense mutations in the cardiac myosin heavy chain (MHC) gene and several mutations in the two myosin light chains cause a dominantly inherited heart disease called familial hypertrophic cardiomyopathy. Very little is known about the biochemical defects in these alleles and how the mutations lead to disease. Because removal of the light chain binding domain in the lever arm of MHC should alter myosin's force transmission but not its catalytic function, we tested the hypothesis that such a mutant MHC would act as a dominant mutation in cardiac muscle. Hearts from transgenic mice expressing this mutant myosin are asymmetrically hypertrophied, with increases in mass primarily restricted to the cardiac anterior wall. Histological examination demonstrates marked cellular hypertrophy, myocyte disorganization, small vessel coronary disease, and severe valvular pathology that included thickening and plaque formation. Skinned myocytes and multicellular preparations from transgenic hearts exhibited decreased Ca2+ sensitivity of tension and decreased relaxation rates after flash photolysis of diazo 2. These experiments demonstrate that alterations in myosin force transmission are sufficient to trigger the development of hypertrophic cardiomyopathy.

Dilated cardiomyopathy of Doberman pinschers: retrospective histomorphologic evaluation of heart from 32 cases

Dilated cardiomyopathy of Doberman Pinschers (DCDP) is a progressive disease often presenting with a history of episodic weakness and syncope, or with clinical signs of predominantly left-sided congestive heart failure. A systematic dissection and histomorphologic evaluation of the heart from 32 Doberman Pinschers with a clinical diagnosis of dilated cardiomyopathy revealed a highly specific location for the characteristic myocardial lesions. The lesions of DCDP were found only in the left ventricular free wall, and in 30 cases, the lesions were characterized by myofiber degeneration and atrophy, and replacement of myocardium by dense bundles of collagen and clusters of adipocytes. In the two remaining hearts, myofiber atrophy and degeneration were accompanied by collagen deposition, but not adipocytes. In stained longitudinal (base to apex) tissue sections of the left ventricle, the lesions of DCDP were usually apparent to the unaided eye; appearing as a central linear pale zone, aligned in the long axis of the ventricular free wall. The lesions did not contain inflammatory cell infiltrates and often involved > 50% of ventricular wall.

Structural features of LIM kinase that control effects on the actin cytoskeleton

LIM kinase phosphorylates and inactivates the actin binding/depolymerizing factor cofilin and induces actin cytoskeletal changes. Several unique structural features within LIM kinase were investigated for their roles in regulation of LIM kinase activity. Disruption of the second LIM domain or the PDZ domain or deletion of the entire amino terminus increased activity in vivo measured as increasing aggregation of the actin cytoskeleton. A kinase-deleted alternate splice product was identified and characterized. This alternate splice product and a kinase inactive mutant inhibited LIM kinase in vivo, indicating that the amino terminus suppresses activity of the kinase domain. Mutation of threonine 508 in the activation loop to valine abolished activity whereas replacement with 2 glutamic acid residues resulted in a fully active enzyme. Dephosphorylation of LIM kinase inhibited cofilin phosphorylation. Mutation of the basic insert in the activation loop inhibited activity in vivo, but not in vitro. These results indicate phosphorylation is an essential regulatory feature of LIM kinase and indicate that threonine 508 and the adjacent basic insert sequences of the activation loop are required for this process. A combination of structural features are thus involved in receiving upstream signals that regulate LIM kinase-induced actin cytoskeletal reorganization.

Loss of a gp130 cardiac muscle cell survival pathway is a critical event in the onset of heart failure during biomechanical stress

Biomechanical stress is a major stimulus for cardiac hypertrophy and the transition to heart failure. By generating mice that harbor a ventricular restricted knockout of the gp130 cytokine receptor via Cre-IoxP-mediated recombination, we demonstrate a critical role for a gp130-dependent myocyte survival pathway in the transition to heart failure. Such conditional mutant mice have normal cardiac structure and function, but during aortic pressure overload, these mice display rapid onset of dilated cardiomyopathy and massive induction of myocyte apoptosis versus the control mice that exhibit compensatory hypertrophy. Thus, cardiac myocyte apoptosis is a critical point in the transition between compensatory cardiac hypertrophy and heart failure. gp130-dependent cytokines may represent a novel therapeutic strategy for preventing in vivo heart failure.

Viral myocarditis: identification of five differentially expressed genes in coxsackievirus B3-infected mouse heart

Differences in host susceptibility to viral myocarditis caused by a given strain of coxsackievirus B3 (CVB3) are known to be largely related to host genetic factors. Little is known, however, about the key genes that encode determinants (mediators) of myocarditis development or the nature of injury. To identify these genes and further understand the molecular mechanisms of the disease process, we have used a murine model and the differential display technique to fingerprint mRNAs from CVB3-infected mouse hearts. Total RNA was extracted from hearts of 4- and 10-week-old A/J(H-2(a)) mice at day 4 after CVB3 infection, and mRNAs were detected by reverse transcriptase-polymerase chain reaction and subsequently analyzed on polyacrylamide DNA sequencing gels. The differentially displayed bands were confirmed by Northern hybridization using the bands as cDNA probes. Twenty-eight upregulated or downregulated bands were selected from the sequencing gels; among these, 2 upregulated and 3 downregulated cDNA fragments were confirmed by Northern hybridization. DNA sequence analysis and GenBank searching have determined that 4 of the 5 candidate genes are homologous to genes encoding Mus musculus inducible GTPase, mouse mitochondrial hydrophobic peptide (a subunit of NADH dehydrogenase), mouse beta-globin, and Homo sapiens cAMP-regulated response element binding protein (CREB) binding protein (CBP), respectively. The remaining candidate gene matches an unpublished cDNA clone, M musculus Nip21 mRNA (GenBank accession number, AF035207), which is homologous to human Nip2, a Bcl-2 binding protein. Our data suggest preliminarily that both structural and nonstructural genes are involved in myocarditis development. For the structural gene, beta-globin, we further confirmed its downregulation at the protein level by measuring the mean cell volume of red blood cells and found it was marginally reduced in the CVB3-infected group (P<0.06), with no change in hemoglobin concentration. Cardiac myoglobin concentration was also measured and found to be decreased (P<0.005), with a parallel decrease in total soluble protein in the CVB3-infected mouse myocardium (P<0.01). We also noted that the ratio of myoglobin to total protein was not significantly changed; this may be due to the downregulation of additional genes in the host heart, a number being observed on the differential display gels. The significant downregulation of beta-globin major gene expression in the heart may be relevant to impaired cardiac function in both the early and late postinfection period. The other identified nonstructural genes are known to be involved in regulation of gene expression, signal transduction pathways, and apoptotic cell death. The altered expression of structural and nonstructural genes may play important roles in the mediation of myocarditis development and perhaps other pathological processes in the heart.

Muscle LIM protein: expressed in slow muscle and induced in fast muscle by enhanced contractile activity

To identify early changes in gene expression during the fast-to-slow transition induced by chronic low-frequency stimulation, total RNA was extracted from 12-h-stimulated tibialis anterior (TA) muscles of rats and amplified by differential display RT-PCR. Among the signals of differentially expressed mRNAs, a cDNA approximately 300 bp in length, which was almost undetectable in control TA muscles but prominent in stimulated TA and normal soleus muscles, was identified. This cDNA was cloned and identified as corresponding to the mRNA of the muscle LIM protein (MLP). Its differential expression in control, stimulated TA, and soleus muscles was verified by Northern blotting. Antibodies against MLP were used to identify by immunoblot analysis a protein of 22 kDa, the predicted molecular mass of MLP. Immunohistochemistry revealed strong reactivity for MLP in all fibers of normal soleus muscle and faint staining of some type IIA and type I fibers in control TA muscle. These fibers increased in number and staining intensity in 4-day-stimulated TA muscle. MLP thus seems to play an essential role during the rearrangement of cytoskeletal and/or myofibrillar structures in transforming adult muscle fibers.

Familial dilated cardiomyopathy locus maps to chromosome 2q31

BACKGROUND

Inherited gene defects are an important cause of dilated cardiomyopathy. Although the chromosome locations of some defects and 1 disease gene (actin) have been identified, the genetic etiologies of most cases of familial dilated cardiomyopathy remain unknown.

METHODS AND RESULTS

We clinically evaluated 3 generations of a kindred with autosomal dominant transmission of dilated cardiomyopathy. Nine surviving and affected individuals had early-onset disease (ventricular chamber dilation during the teenage years and congestive heart failure during the third decade of life). The disease was nonpenetrant in 2 obligate carriers. To identify the causal gene defect, linkage studies were performed. A new dilated cardiomyopathy locus was identified on chromosome 2 between loci GCG and D2S72 (maximum logarithm of odds [LOD] score=4.86 at theta=0). Because the massive gene encoding titin, a cytoskeletal muscle protein, resides in this disease interval, sequences encoding 900 amino acid residues of the cardiac-specific (N2-B) domain were analyzed. Five sequence variants were identified, but none segregated with disease in this family.

CONCLUSIONS

A dilated cardiomyopathy locus (designated CMD1G) is located on chromosome 2q31 and causes early-onset congestive heart failure. Although titin remains an intriguing candidate gene for this disorder, a disease-causing mutation is not present in its cardiac-specific N2-B domain.

The LIM protein, CRP1, is a smooth muscle marker

LIM domains are double zinc-finger motifs found in many proteins that play central roles in cell differentiation. Members of the cysteine-rich protein (CRP) family display two LIM domains and are implicated in muscle development. Here we describe the characterization of one member of this family, CRP1, in the mouse. We have isolated and sequenced murine cDNAs that encode CRP1. We have determined by Northern analysis and in situ hybridization that CRP1 expression is developmentally regulated in the embryonic mouse and displays organ specific regulation in adults. The gene encoding CRP1 is expressed in the smooth muscle cells (SMCs) of the dorsal aorta at E9.5, thus illustrating that CRP1 is an early marker for SMC differentiation at that site. As development proceeds, CRP1 transcripts are observed throughout the SMC lineage, with minimal, transient expression detected in skeletal and cardiac muscle. Interestingly, although several markers of mature smooth muscle are already expressed, CRP1 expression in the bladder is not upregulated until the onset of bladder expansion at embryonic day 16.5, at which time its expression becomes very prominent. CRP1 expression persists into adulthood with prominent expression observed in both vascular and visceral smooth muscle. The results reported here define CRP1 as a general marker of smooth muscle lineages.

Enteroviral protease 2A cleaves dystrophin: evidence of cytoskeletal disruption in an acquired cardiomyopathy

Enteroviruses such as Coxsackievirus B3 can cause dilated cardiomyopathy, but the mechanism of this pathology is unknown. Mutations in cytoskeletal proteins such as dystrophin cause hereditary dilated cardiomyopathy, but it is unclear if similar mechanisms underlie acquired forms of heart failure. We demonstrate here that purified Coxsackievirus protease 2A cleaves dystrophin in vitro as predicted by computer analysis. Dystrophin is also cleaved during Coxsackievirus infection of cultured myocytes and in infected mouse hearts, leading to impaired dystrophin function. In vivo, dystrophin and the dystrophin-associated glycoproteins alpha-sarcoglycan and beta-dystroglycan are morphologically disrupted in infected myocytes. We suggest a molecular mechanism through which enteroviral infection contributes to the pathogenesis of acquired forms of dilated cardiomyopathy.

The fourth member of the FHL family of LIM proteins is expressed exclusively in the testis

We have determined the sequence of a novel LIM protein, termed FHL4. It was found to contain the defining secondary structural arrangement of the FHL (formerly SLIM) family of LIM proteins. This sequence was assembled using EST sequences deposited on the dbEST database and from sequencing of PCR fragments, all of which were derived from murine testis cDNA. Northern analysis of a wide range of murine tissues demonstrated that the expression of FHL4 mRNA was restricted to the testis. Using in situ hybridisation, FHL4 mRNA was found to be associated with the seminiferous epithelium and not with the interstitial tissue surrounding the seminiferous tubules. It is suggested that mFHL4 is associated with the process of spermatogenesis.

The LIM proteins FHL1 and FHL3 are expressed differently in skeletal muscle

We have determined the complete mRNA sequence of FHL3 (formerly SLIM2). We have confirmed that it is a member of the family of LIM proteins that share a similar secondary protein structure, renamed as Four-and-a-Half-LIM domain (or FHL) proteins in accordance with this structure. The "half-LIM" domain is a single zinc finger domain that may represent a subfamily of LIM domains and defines this particular family of LIM proteins. The distribution of FHL mRNA expression within a variety of murine tissues is complex. Both FHL1 and FHL3 were expressed in a number of skeletal muscles while FHL2 was expressed at high levels in cardiac muscle. Localisation of FHL3 to human chromosome 1 placed this gene in the proximity of, but not overlapping with, alleles associated with muscle diseases. FHL1 and FHL3 mRNAs were reciprocally expressed in the murine C2C12 skeletal muscle cell line and this suggested that the pattern of expression was linked to key events in myogenesis.

Mitochondrial disease in superoxide dismutase 2 mutant mice

Oxidative stress has been implicated in many diseases. The chief source of reactive oxygen species within the cell is the mitochondrion. We have characterized a variety of the biochemical and metabolic effects of inactivation of the mouse gene for the mitochondrial superoxide dismutase (CD1-Sod2(tm1Cje)). The Sod2 mutant mice exhibit a tissue-specific inhibition of the respiratory chain enzymes NADH-dehydrogenase (complex I) and succinate dehydrogenase (complex II), inactivation of the tricarboxylic acid cycle enzyme aconitase, development of a urine organic aciduria in conjunction with a partial defect in 3-hydroxy-3-methylglutaryl-CoA lyase, and accumulation of oxidative DNA damage. These results indicate that the increase in mitochondrial reactive oxygen species can result in biochemical aberrations with features reminiscent of mitochondrial myopathy, Friedreich ataxia, and 3-hydroxy-3-methylglutaryl-CoA lyase deficiency.

Characterization of the porcine Lhx3/LIM-3/P-Lim LIM homeodomain transcription factor

Lhx3/LIM-3/P-Lim is a LIM homeodomain transcription factor which is essential in mice for the development of anterior and intermediate lobes of the pituitary gland. We report the cloning and characterization of porcine Lhx3. The porcine Lhx3 protein exhibits strong similarity to murine Lhx3 within the amino terminal LIM domains and the homeodomain, however, it is diverged in regions outside these motifs. Expression vectors for porcine Lhx3 activated murine and porcine alpha-glycoprotein reporter genes in transfection assays, and recombinant porcine Lhx3 protein specifically bound to a target site within the porcine alpha-glycoprotein gene upstream sequence. In addition, porcine Lhx3 synergistically induced transcription from prolactin enhancer/promoter reporter genes in cooperation with the Pit-1 pituitary transcription factor. Porcine Lhx3 protein interacted with Pit-1 protein in solution and also with the LIM domain-binding protein NLI/Lbd1/CLIM. Together, these data indicate that many aspects of Lhx3 function in the mammalian pituitary are conserved and that Lhx3 may be involved in the activation of trophic hormone genes during early and late stages of pituitary organogenesis. Divergence in the Lhx3 amino acid sequence between mammalian species may suggest distinct activities for this protein in some species and may help identify important functional domains of this key developmental transcription factor.

A conserved LIM protein that affects muscular adherens junction integrity and mechanosensory function in Caenorhabditis elegans

We describe here the molecular and functional characterization of the Caenorhabditis elegans unc-97 gene, whose gene product constitutes a novel component of muscular adherens junctions. UNC-97 and homologues from several other species define the PINCH family, a family of LIM proteins whose modular composition of five LIM domains implicates them as potential adapter molecules. unc-97 expression is restricted to tissue types that attach to the hypodermis, specifically body wall muscles, vulval muscles, and mechanosensory neurons. In body wall muscles, the UNC-97 protein colocalizes with the beta-integrin PAT-3 to the focal adhesion-like attachment sites of muscles. Partial and complete loss-of-function studies demonstrate that UNC-97 affects the structural integrity of the integrin containing muscle adherens junctions and contributes to the mechanosensory functions of touch neurons. The expression of a Drosophila homologue of unc-97 in two integrin containing cell types, muscles, and muscle-attached epidermal cells, suggests that unc-97 function in adherens junction assembly and stability has been conserved across phylogeny. In addition to its localization to adherens junctions UNC-97 can also be detected in the nucleus, suggesting multiple functions for this LIM domain protein.

Exclusion of muscle specific actinin-associated LIM protein (ALP) gene from 4q35 facioscapulohumeral muscular dystrophy (FSHD) candidate genes

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant disorder for which no candidate gene has yet been identified. The gene corresponding to one of the novel human cDNAs that we cloned on the basis of a muscle restricted expression pattern [Piétu G, Alibert O, Guichard B, et al. Genome Res 1996;6:492-503] was mapped in the region of the FSHD1A genetic locus, i.e. one of the loci involved in this muscular dystrophy. The corresponding encoded protein contains a PDZ and a LIM domain, two protein-protein interaction domains, and was very recently shown to bind alpha-actinin-2 and was named ALP (actinin-associated LIM protein) [Xia H, Winokur S, Kuo W, Altherr M, Bredt D. J Cell Biol 1997;139:507-515]. We raised a specific polyclonal anti-ALP serum against an ALP recombinant polypeptide to evaluate the size, level of expression and subcellular localization of ALP in three patients, clearly diagnosed with FSHD disease. Quantitative or qualitative alterations of ALP expression have not been detected in any of them, thus prompting us to exclude ALP as a FSHD gene candidate.

Embryonic expression suggests an important role for CRP2/SmLIM in the developing cardiovascular system

Proteins of the LIM family are critical regulators of development and differentiation in various cell types. We have described the cloning of cysteine-rich protein 2/smooth muscle LIM protein (CRP2/SmLIM), a LIM-only protein expressed in differentiated vascular smooth muscle cells. As a first step toward understanding the potential functions of CRP2/SmLIM, we analyzed its expression after gastrulation in developing mice and compared the expression of CRP2/SmLIM with that of the other 2 members of the CRP subclass, CRP1 and CRP3/MLP. In situ hybridization in whole-mount and sectioned embryos showed that CRP2/SmLIM was expressed in the sinus venosus and the 2 cardiac chambers at embryonic day 9. Vascular expression of CRP2/SmLIM was first seen at embryonic day 10. At subsequent time points, CRP2/SmLIM expression decreased in the heart but remained high in the vasculature. CRP1 was expressed both in vascular and nonvascular tissues containing smooth muscle cells, whereas CRP3/MLP was expressed only in tissues containing striated muscle. These patterns of expression were maintained in the adult animal and suggest an important role for this gene family in the development of smooth and striated muscle.

Disruption of the p70(s6k)/p85(s6k) gene reveals a small mouse phenotype and a new functional S6 kinase

Recent studies have shown that the p70(s6k)/p85(s6k) signaling pathway plays a critical role in cell growth by modulating the translation of a family of mRNAs termed 5'TOPs, which encode components of the protein synthetic apparatus. Here we demonstrate that homozygous disruption of the p70(s6k)/p85(s6k) gene does not affect viability or fertility of mice, but that it has a significant effect on animal growth, especially during embryogenesis. Surprisingly, S6 phosphorylation in liver or in fibroblasts from p70(s6k)/p85(s6k)-deficient mice proceeds normally in response to mitogen stimulation. Furthermore, serum-induced S6 phosphorylation and translational up-regulation of 5'TOP mRNAs were equally sensitive to the inhibitory effects of rapamycin in mouse embryo fibroblasts derived from p70(s6k)/p85(s6k)-deficient and wild-type mice. A search of public databases identified a novel p70(s6k)/p85(s6k) homolog which contains the same regulatory motifs and phosphorylation sites known to control kinase activity. This newly identified gene product, termed S6K2, is ubiquitously expressed and displays both mitogen-dependent and rapamycin-sensitive S6 kinase activity. More striking, in p70(s6k)/p85(s6k)-deficient mice, the S6K2 gene is up-regulated in all tissues examined, especially in thymus, a main target of rapamycin action. The finding of a new S6 kinase gene, which can partly compensate for p70(s6k)/p85(s6k) function, underscores the importance of S6K function in cell growth.

Advances in molecular genetics of dilated cardiomyopathy. The Heart Muscle Disease Study Group

In clinical surveys, familial dilated cardiomyopathy (FDC) has been demonstrated in 20% to 30% of patients. In these patients, the cause of the disease lies at the DNA level. Molecular genetic studies represent the tools for the understanding of the etiology of FDC and are currently producing relevant advances: 6 different loci have been mapped so far. The only known disease gene is the dystrophin gene causing X-linked dilated cardiomyopathy, but other cytoskeletal proteins, such as adhalin, could be involved. In familial right ventricular cardiomyopathy (or arrhythmogenic right ventricular dysplasia) characterized by isolated or prevalent right ventricular involvement, three further disease loci have been identified.

Transgenic expression of replication-restricted enteroviral genomes in heart muscle induces defective excitation-contraction coupling and dilated cardiomyopathy

Numerous studies have implicated Coxsackievirus in acute and chronic heart failure. Although enteroviral nucleic acids have been detected in selected patients with dilated cardiomyopathy, the significance of such persistent nucleic acids is unknown. To investigate the mechanisms by which restricted viral replication with low level expression of Coxsackieviral proteins may be able to induce cardiomyopathy, we generated transgenic mice which express a replication-restricted full-length Coxsackievirus B3 (CVB3) cDNA mutant (CVB3DeltaVP0) in the heart driven by the cardiac myocyte-specific myosin light chain-2v (MLC-2v) promoter. CVB3DeltaVP0 was generated by mutating infectious CVB3 cDNA at the VP4/VP2 autocatalytic cleavage site from Asn-Ser to Lys-Ala. Cardiac-specific expression of this cDNA leads to synthesis of positive- and negative-strand viral RNA in the heart without formation of infectious viral progeny. Histopathologic analysis of transgenic hearts revealed typical morphologic features of myocardial interstitial fibrosis and in some cases degeneration of myocytes, thus resembling dilated cardiomyopathy in humans. There was also an increase in ventricular atrial natriuretic factor mRNA levels, demonstrating activation of the embryonic program of gene expression typical of ventricular hypertrophy and failure. Echocardiographic analysis demonstrated the presence of left ventricular dilation and decreased systolic function in the transgenic mice compared with wild-type littermates, evidenced by increased ventricular end-diastolic and end-systolic dimensions and decreased fractional shortening. Analysis of isolated myocytes from transgenic mice demonstrate that there is defective excitation-contraction coupling and a decrease in the magnitude of isolated cell shortening. These data demonstrate that restricted replication of enteroviral genomes in the heart can induce dilated cardiomyopathy with excitation-contraction coupling abnormalities similar to pressure overload models of dilated cardiomyopathy.

Structure of cysteine- and glycine-rich protein CRP2. Backbone dynamics reveal motional freedom and independent spatial orientation of the lim domains

Members of the cysteine- and glycine-rich protein family (CRP1, CRP2, and CRP3) contain two zinc-binding LIM domains, LIM1 (amino-terminal) and LIM2 (carboxyl-terminal), and are implicated in diverse cellular processes linked to differentiation, growth control, and pathogenesis. Here we report the solution structure of full-length recombinant quail CRP2 as determined by multi-dimensional triple-resonance NMR spectroscopy. The structural analysis revealed that the global fold of the two LIM domains in the context of the full-length protein is identical to the recently determined solution structures of the isolated individual LIM domains of quail CRP2. There is no preference in relative spatial orientation of the two domains. This supports the view that the two LIM domains are independent structural and presumably functional modules of CRP proteins. This is also reflected by the dynamic properties of CRP2 probed by 15N relaxation values (T1, T2, and nuclear Overhauser effect). A model-free analysis revealed local variations in mobility along the backbone of the two LIM domains in the native protein, similar to those observed for the isolated domains. Interestingly, fast and slow motions observed in the 58-amino acid linker region between the two LIM domains endow extensive motional freedom to CRP2. The dynamic analysis indicates independent backbone mobility of the two LIM domains and rules out correlated LIM domain motion in full-length CRP2. The finding that the LIM domains in a protein encompassing multiple LIM motifs are structurally and dynamically independent from each other supports the notion that these proteins may function as adaptor molecules arranging two or more protein constituents into a macromolecular complex.

Expression of a cysteine-rich protein (CRP) encoding gene during early development of the trout

Members of the cysteine-rich protein (CRP) define a subclass of LIM-only proteins implicated mainly in muscle differentiation. Until now, very little is known concerning the expression of CRP encoding genes during vertebrate development. We describe here the isolation of a trout (Oncorhynchus mykiss) gene encoding a cysteine-rich protein (TCRP) and the pattern of its mRNA accumulation during embryogenesis, focusing on somitogenesis. TCRP encodes a putative protein with two LIM domains linked to a short glycine-rich region that displays 86%, 76%, 67% identity with chicken CRP2, CRP1 and MLP/CRP3 proteins, respectively. Whole-mount in situ hybridisation showed that TCRP transcript is first detected just before somitogenesis in the paraxial mesoderm, while it is absent in the axial structures. During somitogenesis, the expression of TCRP was observed caudally in the elongating presomitic mesoderm and in the last formed somites. The labelling for TCRP was found to fade as the somites mature. At the end of the somitogenesis, TCRP transcripts accumulation was restricted to pronephros and bronchial arches.

Regulation of actin dynamics through phosphorylation of cofilin by LIM-kinase

Cell division, cell motility and the formation and maintenance of specialized structures in differentiated cells depend directly on the regulated dynamics of the actin cytoskeleton. To understand the mechanisms of these basic cellular processes, the signalling pathways that link external signals to the regulation of the actin cytoskeleton need to be characterized. Here we identify a pathway for the regulation of cofilin, a ubiquitous actin-binding protein that is essential for effective depolymerization of actin filaments. LIM-kinase 1, also known as KIZ, is a protein kinase with two amino-terminal LIM motifs that induces stabilization of F-actin structures in transfected cells. Dominant-negative LIM-kinasel inhibits the accumulation of the F-actin. Phosphorylation experiments in vivo and in vitro provide evidence that cofilin is a physiological substrate of LIM-kinase 1. Phosphorylation by LIM-kinase 1 inactivates cofilin, leading to accumulation of actin filaments. Constitutively active Rac augmented cofilin phosphorylation and LIM-kinase 1 autophosphorylation whereas phorbol ester inhibited these processes. Our results define a mechanism for the regulation of cofilin and hence of actin dynamics in vivo. By modulating the stability of actin cytoskeletal structures, this pathway should play a central role in regulating cell motility and morphogenesis.

Expression of a beta-adrenergic receptor kinase 1 inhibitor prevents the development of myocardial failure in gene-targeted mice

Heart failure is accompanied by severely impaired beta-adrenergic receptor (betaAR) function, which includes loss of betaAR density and functional uncoupling of remaining receptors. An important mechanism for the rapid desensitization of betaAR function is agonist-stimulated receptor phosphorylation by the betaAR kinase (betaARK1), an enzyme known to be elevated in failing human heart tissue. To investigate whether alterations in betaAR function contribute to the development of myocardial failure, transgenic mice with cardiac-restricted overexpression of either a peptide inhibitor of betaARK1 or the beta2AR were mated into a genetic model of murine heart failure (MLP-/-). In vivo cardiac function was assessed by echocardiography and cardiac catheterization. Both MLP-/- and MLP-/-/beta2AR mice had enlarged left ventricular (LV) chambers with significantly reduced fractional shortening and mean velocity of circumferential fiber shortening. In contrast, MLP-/-/betaARKct mice had normal LV chamber size and function. Basal LV contractility in the MLP-/-/betaARKct mice, as measured by LV dP/dtmax, was increased significantly compared with the MLP-/- mice but less than controls. Importantly, heightened betaAR desensitization in the MLP-/- mice, measured in vivo (responsiveness to isoproterenol) and in vitro (isoproterenol-stimulated membrane adenylyl cyclase activity), was completely reversed with overexpression of the betaARK1 inhibitor. We report here the striking finding that overexpression of this inhibitor prevents the development of cardiomyopathy in this murine model of heart failure. These findings implicate abnormal betaAR-G protein coupling in the pathogenesis of the failing heart and point the way toward development of agents to inhibit betaARK1 as a novel mode of therapy.

Dilated cardiomyopathy in transgenic mice expressing a dominant-negative CREB transcription factor in the heart

Idiopathic-dilated cardiomyopathy (IDC) is a common primary myocardial disease of unknown etiology characterized by progressive biventricular failure, cardiac dilatation, and premature mortality. Here we show that transgenic mice expressing a dominant-negative form of the CREB transcription factor (CREBA133) under the control of the cardiac myocyte-specific alpha-MHC promoter develop dilated cardiomyopathy that closely resembles many of the anatomical, physiological, and clinical features of human IDC. Between 2 and 20 wk of age, these mice develop four chamber cardiac dilatation, decreased systolic and diastolic left ventricular function, and attenuated contractile responses to the beta-adrenergic agonist, isoproterenol. Histologically, the CREBA133 hearts demonstrated both atrophic and hypertrophied fibers as well as significant interstitial fibrosis. These anatomical and hemodynamic changes were associated with hepatic congestion and peripheral edema, intracardiac thrombi, and premature mortality. Taken together, these results implicate CREB as an important regulator of cardiac myocyte function and provide a genetic model of dilated cardiomyopathy which should facilitate studies of both the pathogenesis and therapy of this clinically important disorder.

LIM domains of cysteine-rich protein 1 (CRP1) are essential for its zyxin-binding function

Previous studies have demonstrated that the adhesion-plaque protein, zyxin, interacts specifically with a 23 kDa protein, called the cysteine-rich protein 1 (CRP1), which has been implicated in myogenesis. Primary sequence analyses have revealed that both zyxin and CRP1 exhibit multiple copies of a structural motif called the LIM domain. LIM domains, which are defined by the consensus CX2CX16-23HX2CX2CX2CX16-23CX2-3(C,H,D), are found in a variety of proteins that are involved in cell growth and differentiation. Recent studies have established that LIM domains are zinc-binding structures that mediate specific protein-protein interactions. For example, in the case of the zyxin-CRP1 interaction, one of zyxin's three LIM domains is necessary and sufficient for binding to CRP1. Because the CRP1 molecule is comprised primarily of two LIM domains, we were interested in the possibility that the binding site for zyxin on CRP1 might also be contained within a single LIM domain. Consistent with the hypothesis that the LIM domains of CRP1 are critical for the protein's zyxin-binding function, zinc-depleted CRP1 displays a reduced zyxin-binding activity. However, domain mapping analyses have revealed that neither of the two individual LIM domains of CRP1 can support a wild-type interaction with zyxin. Collectively, our results suggest that the binding site for zyxin on CRP1 is not contained within a single contiguous sequence of amino acids. Instead, the interaction appears to rely on the co-ordinate action of a number of residues that are displayed in both of CRP1's LIM domains.

Molecular cloning, characterization, and promoter analysis of the mouse Crp2/SmLim gene. Preferential expression of its promoter in the vascular smooth muscle cells of transgenic mice

Several members of the LIM protein family have important roles in development and differentiation. We recently isolated a rat cDNA encoding a new member of this family, CRP2/SmLIM, that contains two LIM domains and is expressed preferentially in vascular smooth muscle cells (VSMC). To study the molecular mechanisms that regulate VSMC-specific transcription of the Crp2/SmLim gene, we cloned the cDNA and gene of mouse Crp2/SmLim. Mouse Crp2/SmLim is a single copy gene of six exons and five introns spanning approximately 20 kilobases of genomic DNA. By 5'-rapid amplification of cDNA ends and S1 nuclease protection assay, we determined that the transcription start site is an A residue 80 base pairs 5' of the translation initiation codon. A TATA-like sequence is located 27 base pairs 5' of the transcription start site, and there are potential cis-acting elements (GATA, Sp1, AP-2, E box, CCAC box, and GArC motif) in the 5'-flanking sequence. In transient transfection assays in rat aortic smooth muscle cells in primary culture, 5 kilobases of the Crp2/SmLim 5'-flanking sequence generated a high level of luciferase reporter gene activity. By deletion analysis and gel mobility shift assay, we found that the region between bases -74 and -39 of this 5 kilobase DNA fragment binds Sp1 and confers basal promoter activity in the Crp2/SmLim gene. In vitro, the 5-kilobase fragment was active in multiple cell types. In vivo, however, the 5-kilobase fragment directed high level expression of the lacZ reporter gene preferentially in the VSMC of transgenic mice, indicating the presence of VSMC-specific element(s) in this fragment.

Molecular cloning and characterization of FHL2, a novel LIM domain protein preferentially expressed in human heart

A full-length cDNA clone encoding a novel LIM-only protein was isolated and sequenced from a human fetal heart cDNA library. This full-length clone consists of 1416 base pairs and has a predicted open reading frame (ORF) encoding 279 amino acids. The ORF of this polypeptide codes for the human heart-specific four and a half LIM-only protein 2 (FHL2). It possesses an extra zinc finger that is a half LIM domain and four repeats of LIM domain. When the human FHL2 cDNA probe was used to hybridize with poly-A RNA of various human tissues, a very strong signal could be seen in heart tissues, and only moderately low signals could be detected in placenta, skeletal muscle and ovary. Virtually no signal could be detected in brain, lung, liver, kidney, pancreas, spleen, thymus, prostate, testis, small intestine, colon or peripheral blood leukocyte. FHL2 was mapped to chromosome 2q12-q13 by fluorescent in-situ hybridization (FISH).

Effects of growth hormone and insulin-like growth factor I in experimental heart failure

These studies suggest that IGF-I and GH have generally favourable effects on the failing heart. They further demonstrate the ability of the severely depressed and failing heart to respond to the trophic and inotropic effects of GH. There is, however, a need for a better understanding of the mechanism of the contractility effect, the character of the hypertrophy observed (whether it is a more favourable type than that secondary to mechanical overload) and the vascular actions, both trophic and vasodilatory. In addition, the degree to which high-dose ACE inhibition or angiotensin II receptor blockade may inhibit some of these effects requires further study. Finally, it is clear that additional experimental studies and clinical trials are needed to investigate the long-term effects of GH on morbidity and mortality in heart failure, as well as the possible side-effects and other actions, such as the potential of GH to enhance skeletal muscle size and strength.

Molecular structure of the sarcomeric Z-disk: two types of titin interactions lead to an asymmetrical sorting of alpha-actinin

The sarcomeric Z-disk, the anchoring plane of thin (actin) filaments, links titin (also called connectin) and actin filaments from opposing sarcomere halves in a lattice connected by alpha-actinin. We demonstrate by protein interaction analysis that two types of titin interactions are involved in the assembly of alpha-actinin into the Z-disk. Titin interacts via a single binding site with the two central spectrin-like repeats of the outermost pair of alpha-actinin molecules. In the central Z-disk, titin can interact with multiple alpha-actinin molecules via their C-terminal domains. These interactions allow the assembly of a ternary complex of titin, actin and alpha-actinin in vitro, and are expected to constrain the path of titin in the Z-disk. In thick skeletal muscle Z-disks, titin filaments cross over the Z-disk centre by approximately 30 nm, suggesting that their alpha-actinin-binding sites overlap in an antiparallel fashion. The combination of our biochemical and ultrastructural data now allows a molecular model of the sarcomeric Z-disk, where overlapping titin filaments and their interactions with the alpha-actinin rod and C-terminal domain can account for the essential ultrastructural features.

Isolation of differentially expressed genes upon immunoglobulin class switching by a subtractive hybridization method using uracil DNA glycosylase

Immunoglobulin class switch recombination enables B lymphocytes to sequentially express antibodies that have identical specificities but that differ in class and effector function. Although several cis elements required for class switch recombination have been identified, few trans -acting factors which are directly related to class switching have been found. Previously we have developed an efficient in vitro class switching system using a cell line, CH12F3-2. To clarify the molecular mechanism of class switching, we intended to isolate genes induced with interleukin (IL)-4, transforming growth factor (TGF)-beta and CD40L using the in vitro class switching system. For that purpose, an improved method for making subtracted cDNA libraries, using uracil DNA glycosylase, has been developed. This method can overcome a general problem of subtraction, that rare cDNAs are easily lost. This new subtraction method was applied to the CH12F3-2 switching system to isolate genes induced by stimulations with IL-4, TGF-beta and CD40L, and cDNAs encoding deiodinase 1 and SS32, an alternatively spliced form of the muscle LIM protein, were obtained. Their expression patterns in response to various combinations of stimuli and the time courses of the induction supported the usefulness of this method.

The role of cytoskeletal proteins in cardiomyopathies

Cardiomyopathies are serious heart muscle disorders in children and adults, which result in morbidity and premature death. These disorders include hypertrophic cardiomyopathy, dilated cardiomyopathy and restrictive cardiomyopathy. Recently, mutations in seven genes, all encoding sarcomeric proteins, have been identified as causes of familial hypertrophic cardiomyopathy. The genes include those encoding the beta-myosin heavy chain, alpha-tropomyosin, cardiac troponin T, myosin binding protein-C, myosin essential light chain, myosin regulatory light chain, and troponin I. Advances in the understanding of dilated cardiomyopathy have been made recently as well and it appears as if cytoskeletal proteins play a central role. Dystrophin has been identified as the gene responsible for X-linked dilated cardiomyopathy and this protein, which is also responsible for Duchenne and Becker muscular dystrophy, plays an important role in myocyte and cardiomyocyte function. Mutations in other cytoskeletal proteins such as metavinculin, alpha-dystroglycan, alpha- and gamma-sarcoglycan, and muscle LIM protein have also been found to result in dilated cardiomyopathy, suggesting that cytoskeletal proteins play a central role in cardiac function.

Energy deprivation and a deficiency in downstream metabolic target genes during the onset of embryonic heart failure in RXRalpha−/− embryos

RXRalpha null mutant mice display ocular and cardiac malformations, liver developmental delay, and die from cardiac failure around embryonic day (E) 14.5 pc. To dissect the molecular basis of the RXRalpha-associated cardiomyopathy, we performed subtractive hybridization and systematically characterized putative downstream target genes that were selectively lacking in the mutant embryos, both at early (E10.5) and late (E13.5) stages of mouse embryonic development. Approximately 50% of the subtracted clones (61/115) encoded proteins involved in intermediary metabolism and electron transport, suggesting an energy deficiency in the RXRalpha−/− embryos. In particular, clone G1, which encodes subunit 14.5b of the NADH-ubiquinone dehydrogenase complex, displayed a dose-dependent expression in the wild-type, heterozygous and RXRalpha mutant mice. This gene was also downregulated in a retinoid-deficient rat embryo model. ATP content and medium Acyl-CoA dehydrogenase mRNA were lower in RXRalpha mutant hearts compared to wild-type mice. Ultrastructural studies showed that the density of mitochondria per myocyte was higher in the RXRalpha mutant compared to wild-type littermates. We propose a model whereby defects in intermediary metabolism may be a causative factor of the RXRalpha−/− phenotype and resembles an embryonic form of dilated cardiomyopathy.

Cardiac defects and altered ryanodine receptor function in mice lacking FKBP12

FKBP12, a cis-trans prolyl isomerase that binds the immunosuppressants FK506 and rapamycin, is ubiquitously expressed and interacts with proteins in several intracellular signal transduction systems. Although FKBP12 interacts with the cytoplasmic domains of type I receptors of the transforming growth factor-beta (TGF-beta) superfamily in vitro, the function of FKBP12 in TGF-beta superfamily signalling is controversial. FKBP12 also physically interacts stoichiometrically with multiple intracellular calcium release channels including the tetrameric skeletal muscle ryanodine receptor (RyR1). In contrast, the cardiac ryanodine receptor, RyR2, appears to bind selectively the FKBP12 homologue, FKBP12.6. To define the functions of FKBP12 in vivo, we generated mutant mice deficient in FKBP12 using embryonic stem (ES) cell technology. FKBP12-deficient mice have normal skeletal muscle but have severe dilated cardiomyopathy and ventricular septal defects that mimic a human congenital heart disorder, noncompaction of left ventricular myocardium. About 9% of the mutants exhibit exencephaly secondary to a defect in neural tube closure. Physiological studies demonstrate that FKBP12 is dispensable for TGF-beta-mediated signalling, but modulates the calcium release activity of both skeletal and cardiac ryanodine receptors.

Cardiac muscle cell hypertrophy and apoptosis induced by distinct members of the p38 mitogen-activated protein kinase family

p38 mitogen-activated protein (MAP) kinase activities were significantly increased in mouse hearts after chronic transverse aortic constriction, coincident with the onset of ventricular hypertrophy. Infection of cardiomyocytes with adenoviral vectors expressing upstream activators for the p38 kinases, activated mutants of MAP kinase kinase 3b(E) (MKK3bE) and MAP kinase kinase 6b(E) (MKK6bE), elicited characteristic hypertrophic responses, including an increase in cell size, enhanced sarcomeric organization, and elevated atrial natriuretic factor expression. Overexpression of the activated MKK3bE in cardiomyocytes also led to an increase in apoptosis. The hypertrophic response was enhanced by co-infection of an adenoviral vector expressing wild type p38 beta, and was suppressed by the p38 beta dominant negative mutant. In contrast, the MKK3bE-induced cell death was increased by co-infection of an adenovirus expressing wild type p38 alpha, and was suppressed by the dominant negative p38 alpha mutant. This provides the first evidence in any cell system for divergent physiological functions for different members of the p38 MAP kinase family. The direct involvement of p38 pathways in cardiac hypertrophy and apoptosis suggests a significant role for p38 signaling in the pathophysiology of heart failure.

Myofibril degeneration caused by tropomodulin overexpression leads to dilated cardiomyopathy in juvenile mice

Loss of myofibril organization is a common feature of chronic dilated and progressive cardiomyopathy. To study how the heart compensates for myofibril degeneration, transgenic mice were created that undergo progressive loss of myofibrils after birth. Myofibril degeneration was induced by overexpression of tropomodulin, a component of the thin filament complex which determines and maintains sarcomeric actin filament length. The tropomodulin cDNA was placed under control of the alpha-myosin heavy chain gene promoter to overexpress tropomodulin specifically in the myocardium. Offspring with the most severe phenotype showed cardiomyopathic changes between 2 and 4 wk after birth. Hearts from these mice present characteristics consistent with dilated cardiomyopathy and a failed hypertrophic response. Histological analysis showed widespread loss of myofibril organization. Confocal microscopy of isolated cardiomyocytes revealed intense tropomodulin immunoreactivity in transgenic mice together with abnormal coincidence of tropomodulin and alpha-actinin reactivity at Z discs. Contractile function was compromised severely as determined by echocardiographic analyses and isolated Langendorff heart preparations. This novel experimentally induced cardiomyopathy will be useful for understanding dilated cardiomyopathy and the effect of thin filament-based myofibril degeneration upon cardiac structure and function.

Accumulation of muscle ankyrin repeat protein transcript reveals local activation of primary myotube endcompartments during muscle morphogenesis

The characteristic shapes and positions of each individual body muscle are established during the process of muscle morphogenesis in response to patterning information from the surrounding mesenchyme. Throughout muscle morphogenesis, primary myotubes are arranged in small parallel bundles, each myotube spanning the forming muscles from end to end. This unique arrangement potentially assigns a crucial role to primary myotube end regions for muscle morphogenesis. We have cloned muscle ankyrin repeat protein (MARP) as a gene induced in adult rat skeletal muscle by denervation. MARP is the rodent homologue of human C-193 (Chu, W., D.K. Burns, R.A. Swerick, and D.H. Presky. 1995. J. Biol. Chem. 270:10236-10245) and is identical to rat cardiac ankyrin repeat protein. (Zou, Y., S. Evans, J. Chen, H.-C. Kuo, R.P. Harvey, and K.R. Chien. 1997. Development. 124:793-804). In denervated muscle fibers, MARP transcript accumulated in a unique perisynaptic pattern. MARP was also expressed in large blood vessels and in cardiac muscle, where it was further induced by cardiac hypertrophy. During embryonic development, MARP was expressed in forming skeletal muscle. In situ hybridization analysis in mouse embryos revealed that MARP transcript exclusively accumulates at the end regions of primary myotubes during muscle morphogenesis. This closely coincided with the expression of thrombospondin-4 in adjacent prospective tendon mesenchyme, suggesting that these two compartments may constitute a functional unit involved in muscle morphogenesis. Transfection experiments established that MARP protein accumulates in the nucleus and that the levels of both MARP mRNA and protein are controlled by rapid degradation mechanisms characteristic of regulatory early response genes. The results establish the existence of novel regulatory muscle fiber subcompartments associated with muscle morphogenesis and denervation and suggest that MARP may be a crucial nuclear cofactor in local signaling pathways from prospective tendon mesenchyme to forming muscle and from activated muscle interstitial cells to denervated muscle fibers.

Molecular remodeling of cardiac contractile function

A number of techniques are now available that allow the contractile apparatus of the heart to be altered in a defined manner. This review focuses on those approaches that result in germ-line transmission of the remodeling event(s). Thus the desired modifications can be propagated stably throughout multiple generations and result in the creation of stable, new animal models. Necessarily, such stable changes need to be performed at the level of the genome, and two distinct but complementary approaches have been developed: transgenesis and gene targeting. Each results in the stable modification of the mammalian genome. Via gene targeting or gene ablation of sequences encoding various components of the sarcomere, the contractile apparatus of the heart can be altered dramatically. Ablating a gene may lead to a loss in function, which can help establish a function of the candidate sequence. Gene targeting can also be used to effect changes in the sequences encoding a functional domain of the contractile protein or at a single-amino acid residue, resulting in the establishment of precise structure-function relationships. With the use of transgenesis, the contractile apparatus of the heart can also be significantly remodeled. These approaches are rapidly creating a group of animals in which altered contractile protein complements will lead to a fundamental understanding of the structure-function relationships that underlie the function of the heart at the molecular, biochemical, whole organ, and whole animal levels.

Comparison of three members of the cysteine-rich protein family reveals functional conservation and divergent patterns of gene expression

Members of the cysteine-rich protein (CRP) family are evolutionarily conserved proteins that have been implicated in the processes of cell proliferation and differentiation. In particular, one CRP family member has been shown to be an essential regulator of cardiac and skeletal muscle development. Each of the three vertebrate CRP isoforms characterized to date is composed of two copies of the zinc-binding LIM domain with associated glycine-rich repeats. In this study, we have addressed the biological significance of the CRP multigene family by comparing the subcellular distributions, biochemical properties, and expression patterns of CRP1, CRP2, and CRP3/MLP. Our data reveal that all three CRP family members, when expressed in adherent fibroblasts, associate specifically with the actin cytoskeleton. Moreover, all three CRP isoforms are capable of interacting with the cytoskeletal proteins alpha-actinin and zyxin. Together, these observations suggest that CRP family members may exhibit overlapping cellular functions. Differences between the three CRPs are evident in their protein expression patterns in chick embryos. CRP1 expression is detected in a variety of organs enriched in smooth muscle. CRP2 is restricted to arteries and fibroblasts. CRP3/MLP is dominant in organs enriched in striated muscle. CRP isoform expression is also developmentally regulated in the chick. Our findings suggest that the three CRP family members perform similar functions in different muscle derivatives. The demonstration that all members of the CRP family are associated with cytoskeletal components that have been implicated in the assembly and organization of filamentous actin suggests that CRPs contribute to muscle cell differentiation via effects on cytoarchitecture.

Actinin-associated LIM protein: identification of a domain interaction between PDZ and spectrin-like repeat motifs

PDZ motifs are protein-protein interaction domains that often bind to COOH-terminal peptide sequences. The two PDZ proteins characterized in skeletal muscle, syntrophin and neuronal nitric oxide synthase, occur in the dystrophin complex, suggesting a role for PDZ proteins in muscular dystrophy. Here, we identify actinin-associated LIM protein (ALP), a novel protein in skeletal muscle that contains an NH2-terminal PDZ domain and a COOH-terminal LIM motif. ALP is expressed at high levels only in differentiated skeletal muscle, while an alternatively spliced form occurs at low levels in the heart. ALP is not a component of the dystrophin complex, but occurs in association with alpha-actinin-2 at the Z lines of myofibers. Biochemical and yeast two-hybrid analyses demonstrate that the PDZ domain of ALP binds to the spectrin-like motifs of alpha-actinin-2, defining a new mode for PDZ domain interactions. Fine genetic mapping studies demonstrate that ALP occurs on chromosome 4q35, near the heterochromatic locus that is mutated in fascioscapulohumeral muscular dystrophy.

Cardiac compartment-specific overexpression of a modified retinoic acid receptor produces dilated cardiomyopathy and congestive heart failure in transgenic mice

Retinoids play a critical role in cardiac morphogenesis. To examine the effects of excessive retinoid signaling on myocardial development, transgenic mice that overexpress a constitutively active retinoic acid receptor (RAR) controlled by either the alpha- or beta-myosin heavy chain (MyHC) promoter were generated. Animals carrying the alpha-MyHC-RAR transgene expressed RARs in embryonic atria and in adult atria and ventricles, but developed no signs of either malformations or disease. In contrast, beta-MyHC-RAR animals, where expression was activated in fetal ventricles, developed a dilated cardiomyopathy that varied in severity with transgene copy number. Characteristic postmortem lesions included biventricular chamber dilation and left atrial thrombosis; the incidence and severity of these lesions increased with increasing copy number. Transcript analyses showed that molecular markers of hypertrophy, alpha-skeletal actin, atrial natriuretic factor and beta-MyHC, were upregulated. Cardiac performance of transgenic hearts was evaluated using the isolated perfused working heart model as well as in vivo, by transthoracic M-mode echocardiography. Both analyses showed moderate to severe impairment of left ventricular function and reduced cardiac contractility. Thus, expression of a constitutively active RAR in developing atria and/ or in postnatal ventricles is relatively benign, while ventricular expression during gestation can lead to significant cardiac dysfunction.

CRP1, a LIM domain protein implicated in muscle differentiation, interacts with alpha-actinin

Members of the cysteine-rich protein (CRP) family are LIM domain proteins that have been implicated in muscle differentiation. One strategy for defining the mechanism by which CRPs potentiate myogenesis is to characterize the repertoire of CRP binding partners. In order to identify proteins that interact with CRP1, a prominent protein in fibroblasts and smooth muscle cells, we subjected an avian smooth muscle extract to affinity chromatography on a CRP1 column. A 100-kD protein bound to the CRP1 column and could be eluted with a high salt buffer; Western immunoblot analysis confirmed that the 100-kD protein is alpha-actinin. We have shown that the CRP1-alpha-actinin interaction is direct, specific, and saturable in both solution and solid-phase binding assays. The Kd for the CRP1-alpha-actinin interaction is 1.8 +/- 0.3 microM. The results of the in vitro protein binding studies are supported by double-label indirect immunofluorescence experiments that demonstrate a colocalization of CRP1 and alpha-actinin along the actin stress fibers of CEF and smooth muscle cells. Moreover, we have shown that alpha-actinin coimmunoprecipitates with CRP1 from a detergent extract of smooth muscle cells. By in vitro domain mapping studies, we have determined that CRP1 associates with the 27-kD actin-binding domain of alpha-actinin. In reciprocal mapping studies, we showed that alpha-actinin interacts with CRP1-LIM1, a deletion fragment that contains the NH2-terminal 107 amino acids (aa) of CRP1. To determine whether the alpha-actinin binding domain of CRP1 would localize to the actin cytoskeleton in living cells, expression constructs encoding epitope-tagged full-length CRP1, CRP1-LIM1(aa 1-107), or CRP1-LIM2 (aa 108-192) were microinjected into cells. By indirect immunofluorescence, we have determined that full-length CRP1 and CRP1-LIM1 localize along the actin stress fibers whereas CRP1-LIM2 fails to associate with the cytoskeleton. Collectively these data demonstrate that the NH2-terminal part of CRP1 that contains the alpha-actinin-binding site is sufficient to localize CRP1 to the actin cytoskeleton. The association of CRP1 with alpha-actinin may be critical for its role in muscle differentiation.