Characterization of a new human isoform of the enigma homolog family specifically expressed in skeletal muscle

Expression of PDLIM5 Spliceosomes and Regulatory Functions on Myogenesis in Pigs

Meat yield, determined by muscle growth and development, is an important economic trait for the swine industry and a focus of research in animal genetics and breeding. PDZ and LIM domain 5 (PDLIM5) are cytoskeleton-related proteins that play key roles in various tissues and cells. These proteins have multiple isoforms, primarily categorized as short (PDLIM5-short) and long (PDLIM5-long) types, distinguished by the absence and presence of an LIM domain, respectively. However, the expression patterns of swine PDLIM5 isoforms and their regulation during porcine skeletal muscle development remain largely unexplored. We observed that PDLIM5-long was expressed at very low levels in pig muscles and that PDLIM5-short and total PDLIM5 were highly expressed in the muscles of slow-growing pigs, suggesting that PDLIM5-short, the dominant transcript in pigs, is associated with a slow rate of muscle growth. PDLIM5-short suppressed myoblast proliferation and myogenic differentiation in vitro. We also identified two single nucleotide polymorphisms (-258 A > T and -191 T > G) in the 5' flanking region of PDLIM5, which influenced the activity of the promoter and were associated with muscle growth rate in pigs. In summary, we demonstrated that PDLIM5-short negatively regulates myoblast proliferation and differentiation, providing a theoretical basis for improving pig breeding programs.

Myofibrillar myopathy hallmarks associated with ZAK deficiency

The ZAK gene encodes two functionally distinct kinases, ZAKα and ZAKβ. Homozygous loss of function mutations affecting both isoforms causes a congenital muscle disease. ZAKβ is the only isoform expressed in skeletal muscle and is activated by muscle contraction and cellular compression. The ZAKβ substrates in skeletal muscle or the mechanism whereby ZAKβ senses mechanical stress remains to be determined. To gain insights into the pathogenic mechanism, we exploited ZAK-deficient cell lines, zebrafish, mice and a human biopsy. ZAK-deficient mice and zebrafish show a mild phenotype. In mice, comparative histopathology data from regeneration, overloading, ageing and sex conditions indicate that while age and activity are drivers of the pathology, ZAKβ appears to have a marginal role in myoblast fusion in vitro or muscle regeneration in vivo. The presence of SYNPO2, BAG3 and Filamin C (FLNC) in a phosphoproteomics assay and extended analyses suggested a role for ZAKβ in the turnover of FLNC. Immunofluorescence analysis of muscle sections from mice and a human biopsy showed evidence of FLNC and BAG3 accumulations as well as other myofibrillar myopathy markers. Moreover, endogenous overloading of skeletal muscle exacerbated the presence of fibres with FLNC accumulations in mice, indicating that ZAKβ signalling is necessary for an adaptive turnover of FLNC that allows for the normal physiological response to sustained mechanical stress. We suggest that accumulation of mislocalized FLNC and BAG3 in highly immunoreactive fibres contributes to the pathogenic mechanism of ZAK deficiency.

The unexpected versatility of ALP/Enigma family proteins

One of the most intriguing features of multicellular animals is their ability to move. On a cellular level, this is accomplished by the rearrangement and reorganization of the cytoskeleton, a dynamic network of filamentous proteins which provides stability and structure in a stationary context, but also facilitates directed movement by contracting. The ALP/Enigma family proteins are a diverse group of docking proteins found in numerous cellular milieus and facilitate these processes among others. In vertebrates, they are characterized by having a PDZ domain in combination with one or three LIM domains. The family is comprised of CLP-36 (PDLIM1), Mystique (PDLIM2), ALP (PDLIM3), RIL (PDLIM4), ENH (PDLIM5), ZASP (PDLIM6), and Enigma (PDLIM7). In this review, we will outline the evolution and function of their protein domains which confers their versatility. Additionally, we highlight their role in different cellular environments, focusing specifically on recent advances in muscle research using Drosophila as a model organism. Finally, we show the relevance of this protein family to human myopathies and the development of muscle-related diseases.

An Overview of the Cytoskeleton-Associated Role of PDLIM5

Regenerative medicine represented by stem cell technology has become one of the pillar medical technologies for human disease treatment. Cytoskeleton plays important roles in maintaining cell morphology, bearing external forces, and maintaining the effectiveness of cell internal structure, among which cytoskeleton related proteins are involved in and play an indispensable role in the changes of cytoskeleton. PDLIM5 is a cytoskeleton-related protein that, like other cytoskeletal proteins, acts as a binding protein. PDZ and LIM domain 5 (PDLIM5), also known as ENH (Enigma homolog), is a cytoplasmic protein with a molecular mass of about 63 KDa that consists of a PDZ domain at the N-terminus and three LIM domains at the C-terminus. PDLIM5 binds to the cytoskeleton and membrane proteins through its PDZ domain and interacts with various signaling molecules, including protein kinases and transcription factors, through its LIM domain. As a cytoskeleton-related protein, PDLIM5 plays an important role in regulating cell proliferation, differentiation and cell fate decision in multiple tissues and cell types. In this review, we briefly summarize the state of knowledge on the PDLIM5 gene, structural properties, and molecular functional mechanisms of the PDLIM5 protein, and its role in cells, tissues, and organ systems, and describe the possible underlying molecular signaling pathways. In the last part of this review, we will focus on discussing the limitations of existing research and the future prospects of PDLIM5 research in turn.

Mutations in PDLIM5 are rare in dilated cardiomyopathy but are emerging as potential disease modifiers

BACKGROUND

A causal genetic mutation is found in 40% of families with dilated cardiomyopathy (DCM), leaving a large percentage of families genetically unsolved. This prevents adequate counseling and clear recommendations in these families. We aim to identify novel genes or modifiers associated with DCM.

METHODS

We performed computational ranking of human genes based on coexpression with a predefined set of genes known to be associated with DCM, which allowed us to prioritize gene candidates for their likelihood of being involved in DCM. Top candidates will be checked for variants in the available whole-exome sequencing data of 142 DCM patients. RNA was isolated from cardiac biopsies to investigate gene expression.

RESULTS

PDLIM5 was classified as the top candidate. An interesting heterozygous variant (189_190delinsGG) was found in a DCM patient with a known pathogenic truncating TTN-variant. The PDLIM5 loss-of-function (LoF) variant affected all cardiac-specific isoforms of PDLIM5 and no LoF variants were detected in the same region in a control cohort of 26,000 individuals. RNA expression of PDLIM5 and its direct interactors (MYOT, LDB3, and MYOZ2) was increased in cardiac tissue of this patient, indicating a possible compensatory mechanism. The PDLIM5 variant cosegregated with the TTN-variant and the phenotype, leading to a high disease penetrance in this family. A second patient was an infant with a homozygous 10 kb-deletion of exon 2 in PDLIM5 resulting in early-onset cardiac disease, showing the importance of PDLIM5 in cardiac function.

CONCLUSIONS

Heterozygous PDLIM5 variants are rare and therefore will not have a major contribution in DCM. Although they likely play a role in disease development as this gene plays a major role in contracting cardiomyocytes and homozygous variants lead to early-onset cardiac disease. Other environmental and/or genetic factors are probably necessary to unveil the cardiac phenotype in PDLIM5 mutation carriers.

Deletion of Enigma Homologue from the Z-disc slows tension development kinetics in mouse myocardium

Enigma Homologue (ENH) is a component of the Z-disc, a structure that anchors actin filaments in the contractile unit of muscle, the sarcomere. Cardiac-specific ablation of ENH protein expression causes contractile dysfunction that ultimately culminates in dilated cardiomyopathy. However, whether ENH is involved in the regulation of myocardial contractility is unknown. To determine if ENH is required for the mechanical activity of cardiac muscle, we analyze muscle mechanics of isolated trabeculae from the hearts of ENH ^+/+ and ENH ^-/- mice. We detected no differences in steady-state mechanical properties but show that when muscle fibers are allowed to relax and then are restretched, the rate at which tension redevelops is depressed in ENH ^-/- mouse myocardium relative to that in ENH ^+/+ myocardium. SDS-PAGE analysis demonstrated that the expression of β-myosin heavy chain is increased in ENH ^-/- mouse myocardium, which could partially, but not completely, account for the depression in tension redevelopment kinetics. Using top-down proteomics analysis, we found that the expression of other thin/thick filament regulatory proteins is unaltered, although the phosphorylation of a cardiac troponin T isoform, cardiac troponin I, and myosin regulatory light chain is decreased in ENH ^-/- mouse myocardium. Nevertheless, these alterations are very small and thus insufficient to explain slowed tension redevelopment kinetics in ENH ^-/- mouse myocardium. These data suggest that the ENH protein influences tension redevelopment kinetics in mouse myocardium, possibly by affecting cross-bridge cycling kinetics. Previous studies also indicate that ablation of specific Z-disc proteins in myocardium slows contraction kinetics, which could also be a contributing factor in this study.

RBM20 and RBM24 cooperatively promote the expression of short enh splice variants

PDZ-LIM protein ENH1 is a scaffold protein for protein kinases and transcriptional regulators. While ENH1 promotes the hypertrophic growth of cardiomyocytes, its short splice variant (ENH3) prevents the hypertrophic growth. The mechanism underlying the alternative splicing of enh mRNA between ENH short and long isoforms has remained unknown. Here, we found that two splicing factors, RNA-binding motif 20 (RBM20) and RNA-binding motif 24 (RBM24) together promoted the expression of short enh splice variants and bound the 5' intronic region of exon 11 containing an in-phase stop codon. In addition, expression of both RBMs is repressed by hypertrophic stimulations. Collectively, our results suggest that, in healthy conditions, RBM20 and RBM24 cooperate to promote the expression of short ENH isoforms.

MicroRNA-17-92 regulates myoblast proliferation and differentiation by targeting the ENH1/Id1 signaling axis

Myogenesis is an important biological process that occurs during both skeletal muscle regeneration and postnatal growth. Growing evidence points to the critical role of microRNAs (miRNAs) in myogenesis. Our analysis of miRNA expression patterns reveal that miRNAs of miR-17-92 cluster are dramatically downregulated in C2C12 cells after myogenesis stimulation, are strongly induced in mouse skeletal muscle after injury and decrease steadily thereafter and are downregulated with age in skeletal muscle during mouse and porcine postnatal growth. However, their roles in muscle developmental processes remain elusive. We show that the miR-17-92 cluster promotes mouse myoblast proliferation but inhibits myotube formation. miR-17, -20a and -92a target the actin-associated protein enigma homolog 1 (ENH1). The silencing of ENH1 increased the nuclear accumulation of the inhibitor of differentiation 1 (Id1) and represses myogenic differentiation. Furthermore, the injection of adenovirus expressing miR-20a into the tibialia anterior muscle downregulates ENH1 and delays regeneration. In addition, the downregulation of miR-17-92 during myogenesis is transcriptionally regulated by E2F1. Overall, our results reveal a E2F1/miR-17-92/ENH1/Id1 regulatory axis during myogenesis.

Scaffold protein enigma homolog activates CREB whereas a short splice variant prevents CREB activation in cardiomyocytes

Enigma Homolog (ENH1 or Pdlim5) is a scaffold protein composed of an N-terminal PDZ domain and three LIM domains at the C-terminal end. The enh gene encodes for several splice variants with opposing functions. ENH1 promotes cardiomyocytes hypertrophy whereas ENH splice variants lacking LIM domains prevent it. ENH1 interacts with various Protein Kinase C (PKC) isozymes and Protein Kinase D1 (PKD1). In addition, the binding of ENH1's LIM domains to PKC is sufficient to activate the kinase without stimulation. The downstream events of the ENH1-PKC/PKD1 complex remain unknown. PKC and PKD1 are known to phosphorylate the transcription factor cAMP-response element binding protein (CREB). We tested whether ENH1 could play a role in the activation of CREB. We found that, in neonatal rat ventricular cardiomyocytes, ENH1 interacts with CREB, is necessary for the phosphorylation of CREB at ser133, and the activation of CREB-dependent transcription. On the contrary, the overexpression of ENH3, a LIM-less splice variant, inhibited the phosphorylation of CREB. ENH3 overexpression or shRNA knockdown of ENH1 prevented the CREB-dependent transcription. Our results thus suggest that ENH1 plays an essential role in CREB's activation and dependent transcription in cardiomyocytes. At the opposite, ENH3 prevents the CREB transcriptional activity. In conclusion, these results provide a first molecular explanation to the opposing functions of ENH splice variants.

PDLIM5 mediates PKCε translocation in PMA-induced growth cone collapse

Growth cone collapse is a critical repulsive response to various guidance cues for axon guidance. Protein kinase C epsilon (PKCε) plays important regulation roles in such responses. Translocation of PKCε from cytoplasm to membrane is crucial to archive its regulatory roles in this process. We previously reported that PDLIM5 could selectively recruit PKCε to its specific substrate in neurons. However, the molecular mechanism of PKCε translocation in the neuronal growth cone collapse remains elusive. Here, we demonstrated that PDLIM5 and PKCε co-existed in the nerve growth cones. By interacting with α-actinin, but not β-actin or β-tubulin, PDLIM5 might contribute to regulation of remodeling of the microfilaments in neurons. Meanwhile, PDLIM5 could also bind to PKCε to form PDLIM5-PKCε complexes in growth cones. In the primary cultured neurons, activation of PKCε by PMA resulted in translocation of both PKCε and PDLIM5 from cytoplasm to the membrane. Knockdown of either PDLIM5 or PKCε rescued the neuron from PMA-induced growth cone collapse. Furthermore, in neurons, application of PDLIM5 shRNA or over-expression of PDLIM5 LIM1-3 mutants reduced the amount of PKCε in the membrane. Together, these results suggest that PDLIM5 acts as a scaffold protein by mediated PKCε translocated to the membrane in PMA-induced growth cone collapse.

The physiological role of cardiac cytoskeleton and its alterations in heart failure

Cardiac muscle cells are equipped with specialized biochemical machineries for the rapid generation of force and movement central to the work generated by the heart. During each heart beat cardiac muscle cells perceive and experience changes in length and load, which reflect one of the fundamental principles of physiology known as the Frank-Starling law of the heart. Cardiac muscle cells are unique mechanical stretch sensors that allow the heart to increase cardiac output, and adjust it to new physiological and pathological situations. In the present review we discuss the mechano-sensory role of the cytoskeletal proteins with respect to their tight interaction with the sarcolemma and extracellular matrix. The role of contractile thick and thin filament proteins, the elastic protein titin, and their anchorage at the Z-disc and M-band, with associated proteins are reviewed in physiologic and pathologic conditions leading to heart failure. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé

Enigma homolog 1 promotes myogenic gene expression and differentiation of C2C12 cells

The Enigma homolog (ENH) gene generates several splicing variants. The initially identified ENH1 possesses one PDZ and three LIM domains, whereas ENH2~4 lack the latter domains. The splicing switch from ENH1 to LIM-less ENHs occurs during development/maturation of skeletal and heart muscles. We examined for the roles of ENH splicing variants in muscle differentiation using C2C12 cells. Cells stably expressing ENH1 exhibited significantly higher MyoD and myogenin mRNA levels before differentiation and after 5 days in low serum-differentiating medium than mock-transfected cells. ENH1-stable transformants also retained the ability to exhibit elongated morphology with well-extended actin fibers following differentiation. In contrast, cells stably expressing ENH3 or ENH4 did not show myotube-like morphology or reorganization of actin fibers following culture in the differentiating medium. Transient overexpression of ENH1 using adenovirus supported the increased expression of muscle marker mRNAs and the formation of well-organized stress fibers, whereas ENH4 overexpression prevented these morphological changes. Furthermore, specific suppression of ENH1 expression by RNAi caused a significant reduction in MyoD mRNA level and blocked the morphological changes. These results suggest that ENH1 with multiple protein-protein interaction modules is essential for differentiation of striated muscles, whereas ectopic expression of LIM-less ENH disrupts normal muscle differentiation.

Differential proteomic analysis of abnormal intramyoplasmic aggregates in desminopathy

Desminopathy is a subtype of myofibrillar myopathy caused by desmin mutations and characterized by protein aggregates accumulating in muscle fibers. The aim of this study was to assess the protein composition of these aggregates. Aggregates and intact myofiber sections were obtained from skeletal muscle biopsies of five desminopathy patients by laser microdissection and analyzed by a label-free spectral count-based proteomic approach. We identified 397 proteins with 22 showing significantly higher spectral indices in aggregates (ratio >1.8, p<0.05). Fifteen of these proteins not previously reported as specific aggregate components provide new insights regarding pathomechanisms of desminopathy. Results of proteomic analysis were supported by immunolocalization studies and parallel reaction monitoring. Three mutant desmin variants were detected directly on the protein level as components of the aggregates, suggesting their direct involvement in aggregate-formation and demonstrating for the first time that proteomic analysis can be used for direct identification of a disease-causing mutation in myofibrillar myopathy. Comparison of the proteomic results in desminopathy with our previous analysis of aggregate composition in filaminopathy, another myofibrillar myopathy subtype, allows to determine subtype-specific proteomic profile that facilitates identification of the specific disorder.

BIOLOGICAL SIGNIFICANCE

Our proteomic analysis provides essential new insights in the composition of pathological protein aggregates in skeletal muscle fibers of desminopathy patients. The results contribute to a better understanding of pathomechanisms in myofibrillar myopathies and provide the basis for hypothesis-driven studies. The detection of specific proteomic profiles in different myofibrillar myopathy subtypes indicates that proteomic analysis may become a useful tool in differential diagnosis of protein aggregate myopathies.

Experimental evidence for the involvement of PDLIM5 in mood disorders in hetero knockout mice

Background

Reports indicate that PDLIM5 is involved in mood disorders. The PDLIM5 (PDZ and LIM domain 5) gene has been genetically associated with mood disorders; it’s expression is upregulated in the postmortem brains of patients with bipolar disorder and downregulated in the peripheral lymphocytes of patients with major depression. Acute and chronic methamphetamine (METH) administration may model mania and the evolution of mania into psychotic mania or schizophrenia-like behavioral changes, respectively.

Methods

To address whether the downregulation of PDLIM5 protects against manic symptoms and cause susceptibility to depressive symptoms, we evaluated the effects of reduced Pdlim5 levels on acute and chronic METH-induced locomotor hyperactivity, prepulse inhibition, and forced swimming by using Pdlim5 hetero knockout (KO) mice.

Results

The homozygous KO of Pdlim5 is embryonic lethal. The effects of METH administration on locomotor hyperactivity and the impairment of prepulse inhibition were lower in Pdlim5 hetero KO mice than in wild-type mice. The transient inhibition of PDLIM5 (achieved by blocking the translocation of protein kinase C epsilon before the METH challenge) had a similar effect on behavior. Pdlim5 hetero KO mice showed increased immobility time in the forced swimming test, which was diminished after the chronic administration of imipramine. Chronic METH treatment increased, whereas chronic haloperidol treatment decreased, Pdlim5 mRNA levels in the prefrontal cortex. Imipramine increased Pdlim5 mRNA levels in the hippocampus.

Conclusion

These findings are partially compatible with reported observations in humans, indicating that PDLIM5 is involved in psychiatric disorders, including mood disorders.

Splicing transitions of the anchoring protein ENH during striated muscle development

The ENH (PDLIM5) protein acts as a scaffold to tether various functional proteins at subcellular sites via PDZ and three LIM domains. Splicing of the ENH primary transcript generates various products with different repertories of protein interaction modules. Three LIM-containing ENH predominates in neonatal cardiac tissue, whereas LIM-less ENHs are abundant in adult hearts, as well as skeletal muscles. Here we examine the timing of splicing transitions of ENH gene products during postnatal heart development and C2C12 myoblast differentiation. Real-time PCR analysis shows that LIM-containing ENH1 mRNA is gradually decreased during postnatal heart development, whereas transcripts with the short exon 5 appear in the late postnatal period and continues to increase until at least one month after birth. The splicing transition from LIM-containing ENH1 to LIM-less ENHs is also observed during the early period of C2C12 differentiation. This transition correlates with the emergence of ENH transcripts with the short exon 5, as well as the expression of myogenin mRNA. In contrast, the shift from the short exon 5 to the exon 7 occurs in the late differentiation period. The timing of this late event corresponds to the appearance of mRNA for the skeletal myosin heavy chain MYH4. Thus, coordinated and stepwise splicing transitions result in the production of specific ENH transcripts in mature striated muscles.

The costamere bridges sarcomeres to the sarcolemma in striated muscle

Costameres are sub-membranous, Z-line associated structures found in striated muscle. They have been shown to have important roles in transmission of force from the sarcomere to the sarcolemma and extracellular matrix, maintaining mechanical integrity of the sarcolemma, and orchestrating mechanically related signaling. The costamere is akin to the more well-known focal adhesion complex present in most cells. The Z-line is a critical structural anchor for the sarcomere, but it is also a hot-spot for muscle cell signaling. Therefore functionally, the costamere represents a two-way signaling highway tethered between the Z-line and the extracellular matrix, relaying mechanical stress signals from outside the cell to intracellular signaling networks. In this role it can modulate myofibril growth and contraction. The major force generated by sarcomeres is transduced in the lateral direction from the sarcomere to the extracellular matrix through the costamere. Two major protein complexes have been described at the costamere: the dystrophin-glycoprotein complex and the integrin-vinculin-talin complex. The importance of these two protein complexes in striated muscle function has between demonstrated both in human disease and mouse models. Members of the dystrophin glycoprotein complex and integrins have both been reported to interact directly with filamin-C, thus linking costameric complexes with those present at the Z-line. Moreover, studies from our labs and others have shown that the Z-line proteins belonging to the PDZ-LIM domain protein family, enigma homolog (ENH) and cypher, may directly or indirectly be involved in this linkage. The following review will focus on the protein components of this linkage, their function in force transmission, and how the dysfunction or loss of proteins within these complexes contributes to muscular disease.

Loss of enigma homolog protein results in dilated cardiomyopathy

RATIONALE

The Z-line, alternatively termed the Z-band or Z-disc, is a highly ordered structure at the border between 2 sarcomeres. Enigma subfamily proteins (Enigma, Enigma homolog protein, and Cypher) of the PDZ-LIM domain protein family are Z-line proteins. Among the Enigma subfamily, Cypher has been demonstrated to play a pivotal role in the structure and function of striated muscle, whereas the role of Enigma homolog protein (ENH) in muscle remains largely unknown.

OBJECTIVE

We studied the role of Enigma homolog protein in the heart using global and cardiac-specific ENH knockout mouse models.

METHODS AND RESULTS

We identified new exons and splice isoforms for ENH in the mouse heart. Impaired cardiac contraction and dilated cardiomyopathy were observed in ENH null mice. Mice with cardiac specific ENH deletion developed a similar dilated cardiomyopathy. Like Cypher, ENH interacted with Calsarcin-1, another Z-line protein. Moreover, biochemical studies showed that ENH, Cypher short isoform and Calsarcin-1 are within the same protein complex at the Z-line. Cypher short isoform and Calsarcin-1 proteins are specifically downregulated in ENH null hearts.

CONCLUSIONS

We have identified an ENH-CypherS-Calsarcin protein complex at the Z-line. Ablation of ENH leads to destabilization of this protein complex and dilated cardiomyopathy.

Splice variants of enigma homolog, differentially expressed during heart development, promote or prevent hypertrophy

AIMS

Proteins with a PDZ (for PSD-95, DLG, ZO-1) and one to three LIM (for Lin11, Isl-1, Mec-3) domains are scaffolding sarcomeric and cytoskeletal elements that form structured muscle fibres and provide for the link to intracellular signalling by selectively associating protein kinases, ion channels, and transcription factors with the mechanical stress-strain sensors. Enigma homolog (ENH) is a PDZ-LIM protein with four splice variants: ENH1 with an N-terminal PDZ domain and three C-terminal LIM domains and ENH2, ENH3, and ENH4 without LIM domains. We addressed the functional role of ENH alternative splicing.

METHODS AND RESULTS

We studied the expression of the four ENH isoforms in the heart during development and in a mouse model of heart hypertrophy. All four isoforms are expressed in the heart but the pattern of expression is clearly different between embryonic, neonatal, and adult stages. ENH1 appears as the embryonic isoform, whereas ENH2, ENH3, and ENH4 are predominant in adult heart. Moreover, alternative splicing of ENH was changed following induction of heart hypertrophy, producing an ENH isoform pattern similar to that of neonatal heart. Next, we tested a possible causal role of ENH1 and ENH4 in the development of cardiac hypertrophy. When overexpressed in rat neonatal cardiomyocytes, ENH1 promoted the expression of hypertrophy markers and increased cell volume, whereas, on the contrary, ENH4 overexpression prevented these changes.

CONCLUSION

Antagonistic splice variants of ENH may play a central role in the adaptive changes of the link between mechanical stress-sensing and signalling occurring during embryonic development and/or heart hypertrophy.

ALP/Enigma PDZ-LIM domain proteins in the heart

Actinin-associated LIM protein (ALP) and Enigma are two subfamilies of Postsynaptic density 95, discs large and zonula occludens-1 (PDZ)-Lin-11, Isl1 and Mec-3 (LIM) domain containing proteins. ALP family members have one PDZ and one LIM domain, whereas Enigma proteins contain one PDZ and three LIM domains. Four ALP and three Enigma proteins have been identified in mammals, each having multiple splice variants and unique expression patterns. Functionally, these proteins bind through their PDZ domains to alpha-actinin and bind through their LIM domains or other internal protein interaction domains to other proteins, including signaling molecules. ALP and Enigma proteins have been implicated in cardiac and skeletal muscle structure, function and disease, neuronal function, bipolar disorder, tumor growth, platelet and epithelial cell motility and bone formation. This review will focus on recent advances in the biological roles of ALP/Enigma PDZ-LIM domain proteins in cardiac muscle and provide insights into mechanisms by which mutations in these proteins are related to human cardiac disease.

Characterization of the interaction between Actinin-Associated LIM Protein (ALP) and the rod domain of alpha-actinin

Background

The PDZ-LIM proteins are a family of signalling adaptors that interact with the actin cross-linking protein, α-actinin, via their PDZ domains or via internal regions between the PDZ and LIM domains. Three of the PDZ-LIM proteins have a conserved 26-residue ZM motif in the internal region, but the structure of the internal region is unknown.

Results

In this study, using circular dichroism and nuclear magnetic resonance (NMR), we showed that the ALP internal region (residues 107–273) was largely unfolded in solution, but was able to interact with the α-actinin rod domain in vitro, and to co-localize with α-actinin on stress fibres in vivo. NMR analysis revealed that the titration of ALP with the α-actinin rod domain induces stabilization of ALP. A synthetic peptide (residues 175–196) that contained the N-terminal half of the ZM motif was found to interact directly with the α-actinin rod domain in surface plasmon resonance (SPR) measurements. Short deletions at or before the ZM motif abrogated the localization of ALP to actin stress fibres.

Conclusion

The internal region of ALP appeared to be largely unstructured but functional. The ZM motif defined part of the interaction surface between ALP and the α-actinin rod domain.

The ALP-Enigma protein ALP-1 functions in actin filament organization to promote muscle structural integrity in Caenorhabditis elegans

Mutations that affect the Z-disk-associated ALP-Enigma proteins have been linked to human muscular and cardiac diseases. Despite their clear physiological significance for human health, the mechanism of action of ALP-Enigma proteins is largely unknown. In Caenorhabditis elegans, the ALP-Enigma protein family is encoded by a single gene, alp-1; thus C. elegans provides an excellent model to study ALP-Enigma function. Here we present a molecular and genetic analysis of ALP-Enigma function in C. elegans. We show that ALP-1 and alpha-actinin colocalize at dense bodies where actin filaments are anchored and that the proper localization of ALP-1 at dense bodies is dependent on alpha-actinin. Our analysis of alp-1 mutants demonstrates that ALP-1 functions to maintain actin filament organization and participates in muscle stabilization during contraction. Reducing alpha-actinin activity enhances the actin filament phenotype of the alp-1 mutants, suggesting that ALP-1 and alpha-actinin function in the same cellular process. Like alpha-actinin, alp-1 also interacts genetically with a connectin/titin family member, ketn-1, to provide mechanical stability for supporting body wall muscle contraction. Taken together, our data demonstrate that ALP-1 and alpha-actinin function together to stabilize actin filaments and promote muscle structural integrity.

Characterization of the Enigma family in zebrafish

The three Enigma subfamily proteins, Enigma, Enigma homologue, and Cypher/ZASP belong to the PDZ and LIM encoding protein family, which is characterized by the presence of a PDZ- and one or more LIM domains. PDZ/LIM proteins play important biological roles, and all members have been shown to associate with the actin cytoskeleton. We describe here the splice form specific expression patterns for the three Enigma subfamily members during zebrafish embryogenesis. Whole-mount in situ hybridization revealed common and distinct expression patterns for the different PDZ or LIM domain encoding splice variants. We further studied the role of enigma in zebrafish development. Enigma knockdown appeared to be embryonic lethal shortly after the end of gastrulation and in few surviving embryos led to elongation defects and disorganized somites. In summary, we show here the temporal and spatial expression patterns of the three Enigma family members and their PDZ and LIM domain encoding splice forms during zebrafish embryogenesis. Our results suggest that enigma is important for the formation and organization of somites and might play an important role for actin cytoskeleton organization during development.

The protein ENH is a cytoplasmic sequestration factor for Id2 in normal and tumor cells from the nervous system

Id2 is a natural inhibitor of the basic helix-loop-helix transcription factors and the retinoblastoma tumor suppressor protein. Active Id2 prevents differentiation and promotes cell-cycle progression and tumorigenesis in the nervous system. A key event that regulates Id2 activity during differentiation is translocation from the nucleus to the cytoplasm. Here we show that the actin-associated protein enigma homolog (ENH) is a cytoplasmic retention factor for Id2. ENH contains three LIM domains, which bind to the helix-loop-helix domain of Id proteins in vitro and in vivo. ENH is up-regulated during neural differentiation, and its ectopic expression in neuroblastoma cells leads to translocation of Id2 from the nucleus to the cytoplasm, with consequent inactivation of transcriptional and cell-cycle-promoting functions of Id2. Conversely, silencing of ENH by RNA interference prevents cytoplasmic relocation of Id2 in neuroblastoma cells differentiated with retinoic acid. Finally, the differentiated neural crest-derived tumor ganglioneuroblastoma coexpresses Id2 and ENH in the cytoplasm of ganglionic cells. These data indicate that ENH contributes to differentiation of the nervous system through cytoplasmic sequestration of Id2. They also suggest that ENH is a restraining factor of the oncogenic activity of Id proteins in neural tumors.