Chromosomal assignment and expression pattern of the murine Lasp-1 gene

The CXCR4-Dependent LASP1-Ago2 Interaction in Triple-Negative Breast Cancer

The CXCR4-LASP1 axis is an emerging target in the field of breast cancer metastasis. C-X-C chemokine receptor type 4 (CXCR4) mediates directed cell migration when activated by its cognate ligand CXCL12. LIM and SH3 Protein 1 (LASP1) is a critical node in the CXCR4 signaling pathway, as its deficiency blocks CXCR4-dependent Matrigel invasion. The mechanism by which LASP1 facilitates this invasive ability of tumor cells when CXCR4 is activated is unknown. Our previous proteomics work had revealed several components of the RNA interference (RNAi) machinery as being potential LASP1 interacting proteins. Here we report that argonaute 2 (Ago2), a protein with central involvement in RNAi, associates with LASP1 in triple-negative breast cancer (TNBC) cells. We demonstrate that LASP1 co-immunoprecipitates with Ago2 endogenously in a CXCL12-dependent manner, with further confirmation of this interaction by proximity ligation assay. Furthermore, this association is specific to CXCR4 as it can be abrogated by the CXCR4 antagonist, AMD3465. By GST-pulldown approach, we identify that LASP1 directly binds to Ago2 through its LIM and SH3 domains, and that this binding is dictated by the S146 and Y171 phosphorylation sites of LASP1. Additionally, the phosphorylation status of LASP1 affected tumor suppressor microRNA (miRNA) Let-7a-guided Ago2 activity. Levels of several endogenous targets of Let-7a were found to be altered including C-C chemokine receptor type 7 (CCR7), which is another critical chemokine receptor involved in metastasis to lymph nodes. Our results suggest a novel role for the LASP1-Ago2 module in shaping the RNAi landscape, functionally impacting the invasive ability of cancer cells.

New Frontiers for the Cytoskeletal Protein LASP1

In the recent two decades, LIM and SH3 protein 1 (LASP1) has been developed from a simple actin-binding structural protein to a tumor biomarker and subsequently to a complex, nuclear transcriptional regulator. Starting with a brief historical perspective, this review will mainly compare and contrast LASP1 and LASP2 from the angle of the newest data and importantly, examine their role in transcriptional regulation. We will summarize the current knowledge through pictorial models and tables including the roles of different microRNAs in the differential regulation of LASP1 levels and patient outcome rather than specify in detail all tumor entities. Finally, the novel functional roles of LASP1 in secretion of vesicles, expression of matrix metalloproteinases and transcriptional regulation as well as the activation of survival and proliferation pathways in different cancer types are described.

Role of microRNA-21 in uveal melanoma cell invasion and metastasis by regulating p53 and its downstream protein

AIM

To reveal the insight mechanism of liver metastasis in uveal melanoma, we investigated cell functions of microRNA-21 in three different uveal melanoma cell lines and analyze the relationship of target gene p53 and its downstream targets.

METHODS

Quantitative reverse transcription polymerase chain reaction (qRT-PCR) was used to detect microRNA-21 expression in normal uveal tissue and uveal melanoma cell lines. Lenti-virus expression system was used to construct OCM-1, MuM-2B and M619 cell line with stable overexpression and inhibition of microRNA-21. In vitro cell function tests such as cell proliferation, cell apoptosis, cell circle and abilities of migration and invasion were examined by MTT, BrdU assay, flow cytometry, transwell assay and Matrigel invasion assay respectively. The target gene was predicted by bioinformatics and confirmed by using a dual luciferase reporter assay. The expression of p53 and its suspected downstream targets LIM and SH3 protein 1 (LASP1) and glutathione S transferase pi (GST-Pi) were determined by qRT-PCR in mRNA level and Western blotting analysis in protein level. Finally, the effect of microRNA-21 in a xenograft tumor model was assessed in four-week-old BALB/c nude mice.

RESULTS

Compared to normal uveal melanoma, expressions of microRNA-21 were significantly higher in uveal melanoma cell lines. Overexpression of microRNA-21 promoted proliferation, migration, and invasion of OCM-1, M619 and MuM-2B cells, while inhibition of microRNA-21 reveal opposite effects. Wild type p53 was identified as a target gene of microRNA-21-3p, and proved by dual luciferase reporter assay. Up-regulated microRNA-21 inhibited the expression of wild type p53 gene, and the increased expression of LASP1 in mRNA level and protein level, while down-regulated microRNA-21 presented opposite way. However, GST-pi showed the potential pattern as expected, but relative mRNA level showed no statistically significant difference in OCM-1 cells. Furthermore, the mRNA expression of GST-pi was decreased in microRNA-21 overexpressing MuM-2B, and increased in M619 cells with inhibition of microRNA-21. In vivo, inhibition of microRNA-21 reduced tumor growth with statistically significant difference.

CONCLUSION

These findings provide novel insight into molecular etiology of microRNA-21 in uveal melanoma cell lines, and suggest that microRNA-21 might be a potential candidate for the diagnosis and prognostic factor of human uveal melanoma.

Roles of Nebulin Family Members in the Heart

The members of the nebulin protein family, including nebulin, nebulette, LASP-1, LASP-2, and N-RAP, contain various numbers of nebulin repeats and bind to actin, but are otherwise heterogeneous with regard to size, expression pattern, and function. This review focuses on the roles of nebulin family members in the heart. Nebulin is the largest member predominantly expressed in skeletal muscle, where it stretches along the thin filament. In heart, nebulin is detectable only at low levels and its absence has no apparent effects. Nebulette is similar in structure to the nebulin C-terminal Z-line region and specifically expressed in heart. Nebulette gene mutations have been identified in dilated cardiomyopathy patients and transgenic mice overexpressing nebulette mutants partially recapitulate the human pathology. In contrast, nebulette knockout mice show no functional phenotype, but exhibit Z-line widening. LASP-2 is an isoform of nebulette expressed in multiple tissues, including the heart. It is present in the Z-line and intercalated disc and able to bind and cross-link filamentous actin. LASP-1 is similar in structure to LASP-2, but expressed only in non-muscle tissue. N-RAP is present in myofibril precursors during myofibrillogenesis and thought to be involved in myofibril assembly, while it is localized at the intercalated disc in adult heart. Additional in vivo models are required to provide further insights into the functions of nebulin family members in the heart.

An update on the LIM and SH3 domain protein 1 (LASP1): a versatile structural, signaling, and biomarker protein

The gene encoding the LIM and SH3 domain protein (LASP1) was cloned two decades ago from a cDNA library of breast cancer metastases. As the first protein of a class comprising one N-terminal LIM and one C-terminal SH3 domain, LASP1 founded a new LIM-protein subfamily of the nebulin group. Since its discovery LASP1 proved to be an extremely versatile protein because of its exceptional structure allowing interaction with various binding partners, its ubiquitous expression in normal tissues, albeit with distinct expression patterns, and its ability to transmit signals from the cytoplasm into the nucleus. As a result, LASP1 plays key roles in cell structure, physiological processes, and cell signaling. Furthermore, LASP1 overexpression contributes to cancer aggressiveness hinting to a potential value of LASP1 as a cancer biomarker.

In this review we summarize published data on structure, regulation, function, and expression pattern of LASP1, with a focus on its role in human cancer and as a biomarker protein. In addition, we provide a comprehensive transcriptome analysis of published microarrays (n=2,780) that illustrates the expression profile of LASP1 in normal tissues and its overexpression in a broad range of human cancer entities.

LASP1, a Newly Identified Melanocytic Protein with a Possible Role in Melanin Release, but Not in Melanoma Progression

The LIM and SH3 protein 1 (LASP1) is a focal adhesion protein. Its expression is increased in many malignant tumors. However, little is known about the physiological role of the protein. In the present study, we investigated the expression and function of LASP1 in normal skin, melanocytic nevi and malignant melanoma. In normal skin, a distinct LASP1 expression is visible only in the basal epidermal layer while in nevi LASP1 protein is detected in all melanocytes. Melanoma exhibit no increase in LASP1 mRNA compared to normal skin. In melanocytes, the protein is bound to dynamin and mainly localized at late melanosomes along the edges and at the tips of the cell. Knockdown of LASP1 results in increased melanin concentration in the cells. Collectively, we identified LASP1 as a hitherto unknown protein in melanocytes and as novel partner of dynamin in the physiological process of membrane constriction and melanosome vesicle release.

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

Targeted disruption of the Lasp-1 gene is linked to increases in histamine-stimulated gastric HCl secretion

Lasp-1 (LIM and SH3 domain protein 1) is a multidomain actin-binding protein that is differentially expressed within epithelial tissues and brain. In the gastric mucosa, Lasp-1 is highly expressed in the HCl-secreting parietal cell, where it is prominently localized within the F-actin-rich subcellular regions. Histamine-induced elevation of parietal cell [cAMP]i increases Lasp-1 phosphorylation, which is correlated with activation of HCl secretion. To determine whether Lasp-1 is involved in the regulation of HCl secretion in vivo, we generated a murine model with a targeted disruption of the Lasp-1 gene. Lasp-1-null mice had slightly lower body weights but developed normally and had no overt phenotypic abnormalities. Basal HCl secretion was unaffected by loss of Lasp-1, but histamine stimulation induced a more robust acid secretory response in Lasp-1-null mice compared with wild-type littermates. A similar effect of histamine was observed in isolated gastric glands on the basis of measurements of accumulation of the weak base [14C]aminopyrine. In addition, inhibition of the acid secretory response to histamine by H2 receptor blockade with ranitidine proceeded more slowly in glands from Lasp-1-null mice. These findings support the conclusion that Lasp-1 is involved in the regulation of parietal HCl secretion. We speculate that cAMP-dependent phosphorylation of Lasp-1 alters interactions with F-actin and/or endocytic proteins that interact with Lasp-1, thereby regulating the trafficking/activation of the H+, K+-ATPase (proton pump).

The LIM and SH3 domain protein family: structural proteins or signal transducers or both?

LIM and SH3 Protein 1 (LASP-1) was initially identified from a cDNA library of metastatic axillary lymph nodes (MLN) more than a decade ago. It was found to be overexpressed in human breast and ovarian cancer and became the first member of a newly defined LIM-protein subfamily of the nebulin group characterized by the combined presence of LIM and SH3 domains. LASP2, a novel LASP1-related gene was first identified and characterized in silico. Subsequently it proved to be a splice variant of the Nebulin gene and therefore was also termed LIM/nebulette. LASP-1 and -2 are highly conserved in their LIM, nebulin-like and SH3 domains but differ significantly at their linker regions. Both proteins are ubiquitously expressed and involved in cytoskeletal architecture, especially in the organization of focal adhesions. Here we present the first systematic review to summarize all relevant data concerning their domain organization, expression profiles, regulating factors and function. We compile evidence that both, LASP-1 and LASP-2, are important during early embryo- and fetogenesis and are highly expressed in the central nervous system of the adult. However, only LASP-1 seems to participate significantly in neuronal differentiation and plays an important functional role in migration and proliferation of certain cancer cells while the role of LASP-2 is more structural. The increased expression of LASP-1 in breast tumours correlates with high rates of nodal-metastasis and refers to a possible relevance as a prognostic marker.

Abnormal cytoskeletal protein expression in cultured skin fibroblasts from type 1 diabetes mellitus patients with nephropathy: A proteomic approach

Diabetic nephropathy (DN) develops in about 40% of insulin-dependent type 1 diabetes mellitus (T1DM) patients, and is associated not only with diabetes duration and metabolic control, but also with a genetic predisposition. Constitutive alterations of cytoskeletal proteins may play a role in the development of DN. We investigated the expression of these proteins in cultured skin fibroblasts, obtained from long-term T1DM patients with and without DN but comparable metabolic control, and from matched healthy subjects, by means of 2-DE electrophoresis and MS-MALDI analyses. In T1DM with DN, compared to the other two groups, quantitative analyses revealed an altered expression of 17 spots (p<0.05-p<0.01), corresponding to 12 unique proteins. In T1DM with DN, beta-actin and three isoforms of tubulin beta-2 chain, tropomodulin-3, and LASP-1 were decreased, whereas two tubulin beta-4 chain isoforms, one alpha actinin-4 isoform, membrane-organizing extension spike protein (MOESIN), FLJ00279 (corresponding to a fragment of myosin heavy chain, non-muscle type A), vinculin, a tropomyosin isoform, and the macrophage capping protein were increased. A shift in caldesmon isoforms was also detected. These results demonstrate an association between DN and the constitutive expression of cytoskeleton proteins in cultured skin fibroblasts from T1DM with DN, which may retain pathophysiologycal implications.

A lasp family protein of Ciona intestinalis

Lasp-1 and lasp-2 are actin-binding proteins that contain a LIM domain, nebulin repeats, and an SH3 domain and they are significantly conserved in mammalian and avian. Lasp-1 is widely expressed in nonmuscle tissues and lasp-2 is specifically expressed in the brain. Genes encoding proteins homologous to lasp-1 and lasp-2 were deposited in the genome/cDNA database of invertebrates such as sea urchins, nematodes, and insects; however, function of their proteins have not been studied in detail. In this study, we analyzed the gene structure, actin-binding activity, and expression of the lasp protein of the ascidian Ciona intestinalis (Ci lasp). A single gene encoding lasp protein was found in the ascidian, and the amino acid sequences of Ci lasp and other invertebrate lasp proteins exhibited similarity to vertebrate lasp-1 and lasp-2 to the same extent. A part of the exon-intron boundaries was conserved between the vertebrate lasp-1, the vertebrate lasp-2 and the invertebrate lasp genes. Ci lasp exhibited actin-binding activity in a co-sedimentation assay. In situ hybridization revealed that the expression of Ci lasp mRNA was apparent in nervous system of early embryos and was detected in various tissues in young adults. This suggests that the functions of invertebrate lasp proteins might include the functions of vertebrate lasp-1 and lasp-2.

Linker region of nebulin family members plays an important role in targeting these molecules to cellular structures

The nebulin family of actin-binding proteins plays an essential role in cytoskeletal dynamics and actin filament stability. All of the family members are modular proteins with their key defining structural feature being the presence of the 35-residue nebulin modules. The family members now include nebulin, nebulette, N-RAP, LASP-1, and LIM-nebulette. Nebulin and nebulette are associated with the thin filament/Z-line junction of striated muscle. LASP-1 and LIM-nebulette are found within focal adhesions, and N-RAP is associated with muscle cellular junctions. Although much investigation has focused on the role of the interactions between nebulin modules and actin, each of these proteins contains other domains that are essential for their cellular targeting and functions. The serine-rich linker region of nebulette has previously been shown to serve just such a purpose by targeting the association of the nebulin modules to the cardiac Z-line in cultured cardiomyocytes. In this report, we analyze the targeting functions of the homologous regions of LASP-1 and LIM-nebulette in their incorporation into focal adhesions. We have found that the linker region of LASP-1 is indeed important for its cellular localization and that the shortened linker region of LIM-nebulette drives the association of nebulin modules to focal adhesions.

Chromosomal assignment of LASP1 and LASP2 genes and organization of the LASP2 gene in chicken

Lasp-1 and lasp-2 are actin-binding proteins that contain a LIM domain, two nebulin repeats and an SH3 domain with significant identity. We determined the chromosomal locations of the LASP1 and LASP2 genes in chicken by fluorescence in situ hybridization. The LASP1 gene was localized to a pair of microchromosomes and the LASP2 gene was localized to chromosome 2p3.1, indicating that the chromosomal locations of the LASP1 and LASP2 genes are highly conserved between chicken and human. The comparison of genomic and cDNA sequences of chicken lasp-2 and nebulette, a nebulin-related protein in muscle, suggested that both the corresponding mRNAs shared exons in the same manner as their human homologues. When compared with the domain structure of nebulette, another nebulin repeat was predicted for lasp-2, and all the nebulin repeats of lasp-2 were better conserved than those in nebulette. We also found the exon boundaries in nebulin repeats of lasp-2 were similar to those of other nebulin-related proteins.

Phosphorylation of mouse LASP-1 on threonine 156 by cAMP- and cGMP-dependent protein kinase

LIM and SH3 domain protein (LASP-1) is a specific focal adhesion protein involved in cell migration. Overlay studies demonstrate that LASP-1 directly binds to the proline-rich domains of zyxin, lipoma preferred partner (LPP), and vasodilator-stimulated phosphoprotein (VASP), with zyxin being the most prominent interacting partner. Despite the LIM/zinc-finger domain, hypothesized to be involved in homodimerization, LASP-1 exists as a monomer. In vitro phosphorylation of recombinant mouse LASP-1 by cAMP- and cGMP-dependent protein kinase (PKA and PKG, respectively) occurs at serine 61, serine 99, and threonine 156 whereas in intact cells mouse LASP-1 is phosphorylated only at threonine 156. This site is different from the known in vivo phosphorylation sites in human (serine 146) and rabbit (serine 99 and serine 146). Nevertheless, immunofluorescence of LASP-1 in human and mouse mesangial cells revealed no difference in subcellular distribution. Exposure of the cells to forskolin induced a translocation of both, human and mouse LASP-1, from the focal contacts to the cell interior without affecting F-actin structure. Immunoblotting of LASP-1 in various mouse and human tissues detected a similar prominent expression in non-muscle tissue. Altogether, our data suggest so far no functional differences between human and mouse LASP-1.

Zyxin interacts with the SH3 domains of the cytoskeletal proteins LIM-nebulette and Lasp-1

Zyxin is a versatile component of focal adhesions in eukaryotic cells. Here we describe a novel binding partner of zyxin, which we have named LIM-nebulette. LIM-nebulette is an alternative splice variant of the sarcomeric protein nebulette, which, in contrast to nebulette, is expressed in non-muscle cells. It displays a modular structure with an N-terminal LIM domain, three nebulin-like repeats, and a C-terminal SH3 domain and shows high similarity to another cytoskeletal protein, Lasp-1 (LIM and SH3 protein-1). Co-precipitation studies and results obtained with the two-hybrid system demonstrate that LIM-nebulette and Lasp-1 interact specifically with zyxin. Moreover, the SH3 domain from LIM-nebulette is both necessary and sufficient for zyxin binding. The SH3 domains from Lasp-1 and nebulin can also interact with zyxin, but the SH3 domains from more distantly related proteins such as vinexin and sorting nexin 9 do not. On the other hand, the binding site in zyxin is situated at the extreme N terminus as shown by site-directed mutagenesis. LIM-nebulette and Lasp-1 use the same linear binding motif. This motif shows some similarity to a class II binding site but does not contain the classical PXXP sequence. LIM-nebulette reveals a subcellular distribution at focal adhesions similar to Lasp-1. Thus, LIM-nebulette, Lasp-1, and zyxin may play an important role in the organization of focal adhesions.

A novel LIM and SH3 protein (lasp-2) highly expressing in chicken brain

From eluates of F-actin affinity chromatography of chicken brain, we identified a novel actin-binding protein (lasp-2) whose gene was predicted in silico. We cloned cDNA of chicken lasp-2 and analyzed its structure, expression, activity, and localization with lasp-1 (LIM and SH3 protein 1), a previously identified actin-binding protein closely related to lasp-2. Chicken lasp-2 showed high homology to mammalian putative lasp-2. Both chicken lasp-1 and chicken lasp-2 have N-terminal LIM domains, C-terminal SH3 domains, and internal nebulin repeats. However, lasp-2 is greatly different from lasp-1 in the sequence between the second nebulin repeat and a SH3 domain, and the region is conserved in chicken, mouse, and human. As expected from its structural similarity to lasp-1, lasp-2 possessed actin-binding activity and localized with actin filament in filopodia of neuroblastoma. In contrast to lasp-1, which is widely distributed in non-muscle tissues, lasp-2 was highly expressed in brain.

Bovine mammary gene expression profiling using a cDNA microarray enhanced for mammary-specific transcripts

A cDNA microarray resource enhanced for transcripts specific to the bovine mammary gland (BMAM) has been developed and used in pilot studies to examine gene expression profiles in the mammary gland. One goal driving development of this resource was to shed some light on the pathways and mechanisms specifically related to bovine mammary gland growth and development. To accomplish this, gene expression patterns from bovine adipose, liver, adrenal, lymph, spleen, thymus, gut, and developing mammary tissue were compared using the BMAM microarray. We have thus identified a putative set of 16 genes being preferentially expressed in developing mammary gland. Another of our long-term goals is to elucidate the genes and pathways associated with bovine lactation and involution and to use these as a model for human mammary gland development as it relates to human breast cancer risks. To begin this process, we conducted a pilot study, comparing gene expression profiles of lactating bovine mammary tissue against nonlactating tissue on the BMAM microarray. Our results have yielded many novel and interesting genes exhibiting differential expression in lactating mammary tissue, including oncogenes (VAV3, C-myc), mediators of apoptosis (Caspase 8), and cell cycle regulators (LASP1).

The human LASP1 gene is fused to MLL in an acute myeloid leukemia with t(11;17)(q23;q21)

The MLL gene at chromosome 11q23 is frequently rearranged in acute leukemia. Here we report the identification of a new MLL fusion partner in the case of an infant with AML-M4 and a t(11;17)(q23;q21) translocation. Fluorescence in situ hybridization (FISH) and RT-PCR analyses indicated a rearrangement of the MLL gene, but no fusion with previously identified MLL fusion partners at 17q, such as AF17 or MSF. Rapid amplification of cDNA ends (RACE) revealed an in-frame fusion of MLL to LASP1, a gene that is amplified and overexpressed in breast cancer. Retroviral transduction of myeloid progenitors demonstrated that MLL/LASP1 is the fourth known fusion of MLL with a cytoplasmic protein that has no in vitro transformation capability, thus establishing a potential subgroup among the MLL fusion proteins.

Proteomics analysis of the neurodegeneration in the brain of tau transgenic mice

Protein tau, a major microtubule-binding protein in the brain, comprises six isoforms generated through alternative mRNA splicing. A dysfunctional form of mutant and normal tau is associated or implicated in the pathogenesis of several neurodegenerative disorders. The neuropathological hallmark of these tau-opathies are intraneuronal depositions of fibrillary aggregates of which neurofibrillary tangles are most common. Several distinct transgene mouse models confirmed that tau protein can cause neurodegeneration directly. This study was aimed at identifying proteins that might play a role in the cellular disturbances caused by overexpression of the longest isoform of human tau in the brain of transgenic mice. We found 34 proteins which differed in integrated intensity by a factor of at least 1.5. These proteins could be sorted into several categories. Some of the phenotypic characteristics found in the htau transgenic mice could be related to proteins found in this study. Several proteins are linked to processes involving apoptosis and neuronal death and have been discussed in papers describing neurodegenerative disorders.

The LIM and SH3 domain-containing protein, lasp-1, may link the cAMP signaling pathway with dynamic membrane restructuring activities in ion transporting epithelia

Lasp-1 is a unique LIM and src homology 3 (SH3) domain-containing protein that was initially identified as a 40 kDa cAMP-dependent phosphoprotein in the HCl-secreting gastric parietal cell. Because cAMP is a potent stimulator of parietal cell acid secretion, we have hypothesized that changes in lasp-1 phosphorylation might be involved in the regulation of ion transport-related activities, perhaps by modulating interactions among cytoskeletal and/or vesicle-associated proteins. In this study, we demonstrate that the cAMP-dependent acid secretory agonist, histamine, induces a rapid, sustained rise in parietal cell lasp-1 phosphorylation and this increase in phosphorylation is closely correlated with the acid secretory response. In addition, elevation of intracellular cAMP concentrations appear to induce a partial redistribution of lasp-1 from the cell cortex, where it predominates along with the gamma-isoform of actin in unstimulated cells, to the beta-actin enriched, apically-directed intracellular canalicular region, which is the site of active proton transport in the parietal cell. Additional studies demonstrate that although lasp-1 mRNA and protein are expressed in a wide range of tissues, the expression is specific for certain actin-rich cell types present within these tissues. For example, gastric chief cells, which contain relatively little F-actin and secrete the enzyme, pepsinogen, by regulated exocytosis, do not appear to express lasp-1. Similarly, lasp-1 was not detected in pancreatic acinar cells, which secrete enzymes by similar mechanisms and also contain relatively low levels of F-actin. Lasp-1 also was not detectable in proximal tubules in the kidney, in gastrointestinal smooth muscle, heart or skeletal muscle. In contrast, expression was prominent in the cortical regions of ion-transporting duct cells in the pancreas and in the salivary parotid gland as well as in certain F-actin-rich cells in the distal tubule/collecting duct. Interestingly, moderate levels of expression were also detected in podocytes present in renal glomeruli and in vascular endothelium. In primary cultures of gastric fibroblasts, lasp-1 was present mainly within the tips of lamellipodia and at the leading edges of membrane ruffles. Taken together these results support the hypothesis that the lasp-1 plays an important role in the regulation of dynamic actin-based, cytoskeletal activities. Agonist-dependent changes in lasp-1 phosphorylation may also serve to regulate actin-associated ion transport activities, not only in the parietal cell but also in certain other F-actin-rich secretory epithelial cell types.

Def-2, -3, -6 and -8, novel mouse genes differentially expressed in the haemopoietic system

To identify developmentally regulated genes during myeloid differentiation, a self-inactivating retroviral gene-trap vector carrying a beta-galactosidase-neomycin (SA/lacZ/neo) fusion gene was constructed and used to infect myeloid progenitor cells (FDCP-Mix A4). G418-resistant and beta-galactosidase positive cell lines (gene-trap integration [GTI] clones) were established and induced to differentiate in vitro into either macrophages or granulocytes. Expression of the trapped loci was monitored at a single-cell level by analysing the mature cell types for beta-galactosidase activity. All 37 GTI clones tested showed down-regulation either during granulocyte or both granulocytic and macrophage differentiation. The endogenous coding regions fused to the SA/lacZ/neo reporter gene were isolated from eight clones. Molecular analysis revealed that half of them represented novel mouse genes (def-2, -3, -6 and -8) which we confirmed to be differentially expressed in primary haemopoietic tissues. Database searches revealed no significant similarities for def-2 (associated with haemopoietic progenitors) and def-8 (expressed most strongly in peripheral leucocytes). Def-6, which is down-regulated upon the differentiation into myeloid as well as erythroid lineages, was found to be closely related but not identical with the recently described B-cell-specific switch recombinase SWAP-70. Def-3, which is down-regulated upon differentiation into granulocytes but expressed in progenitor cells and macrophages, defines a novel family of RNA binding proteins.

Characterization of nebulette and nebulin and emerging concepts of their roles for vertebrate Z-discs

Nebulin is an 800 kDa large actin-binding protein specific to skeletal muscle and thought to act as a molecular template that regulates the length of thin filaments. Recently, a 100 kDa nebulin-like protein has been described in the avian cardiac muscle and referred to as nebulette. We have determined the full-length (8 kb) cDNA sequence of the human nebulette. Its open reading frame (3044 bp) encodes a 109 kDa protein that shares extensive similarity with the C-terminal region of human nebulin. The C-terminal regions of nebulin and nebulette are identical in domain organization and share a family of highly related C-terminal repeats, a serine-rich domain with potential phosphorylation sites, and an SH3 domain. Immunoelectron-microscopy suggests that the C-terminal 30 kDa of nebulin and nebulette filaments integrate into the Z-disc lattice, whereas their N termini appear to project into the I-band. Gene mapping studies assign the human nebulette gene to chromosome 10p12, whereas the nebulin gene has been previously assigned to 2q21. Evolutionary constraints appear to have maintained identical modular arrangements in these two independent genes. Comparison of nebulin and nebulette cDNAs demonstrates that a subgroup of repeats within the C-terminal regions is regulated tissue-specifically and stage-dependently during development of both molecules. This leads to a substantial diversity of nebulin and nebulette isoforms. Their further study is likely to provide insights into how they contribute to the molecular diversity of Z-discs from different muscle tissues and fiber types.