Stable expression of α1-antitrypsin Portland in MDA-MB-231 cells increased MT1-MMP and MMP-9 levels, but reduced tumour progression

The membrane bound matrix metalloproteinase MT1-MMP plays roles in modulating cell movement, independent of its abilities to remodel the extracellular matrix. Unlike many MMPs, MT1-MMP is activated in the Golgi prior to secretion by a pro-protein convertase, primarily furin. Regulation of the activation of pro-MT1-MMP has been methodically investigated, as altering the level of the active protein has broad implications in both activating other pro-MMPs, including pro-MMP-2, and many subsequent remodelling events. Our previous work in MCF-7 cells has demonstrated that modest, and not extremely high, levels of active MT1-MMP manifests into altered cell morphology and movement. At this low but optimal amount of MT1-MMP protein, changes to MT1-MMP levels are always mirrored by MMP-9 and pERK levels, and always opposite to MMP-2 levels. In this study, stable expression of the furin inhibitor α1-antitrypsin Portland (α1-PDX) in MDA-MB-231 cells increased overall MT1-MMP levels, but cells maintained a 21% proportion of pro-MT1-MMP. The increase in MT1-MMP was mirrored by increases in MMP-9 and pERK, but a decrease in MMP-2. These changes were associated with increased NF-κB transcription. In vitro analysis showed that α1-PDX decreased cell protrusions and migration, and this manifested as decreased tumourigenesis when examined in vivo using a chick CAM assay.

Domain specific overexpression of TIMP-2 and TIMP-3 reveals MMP-independent functions of TIMPs during Xenopus laevis development

Extracellular matrix remodelling mediates many processes including cell migration and differentiation and is regulated through the enzymatic action of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). TIMPs are secreted proteins, consisting of structurally and functionally distinct N- and C-terminal domains. TIMP N-terminal domains inhibit MMP activity, whereas their C-terminal domains may have cell signalling activity. The in vivo role of TIMP N- and C-terminal domains in regulating developmental events has not previously been demonstrated. Here we investigated the roles of TIMP-2 and TIMP-3 N- and C-terminal domains in Xenopus laevis embryos. We show that overexpression of TIMP-2 N- and C-terminal domains results in severe developmental defects and death, as well as unique changes in MMP-2 and -9 expression, indicating that the individual domains may regulate MMPs through distinct mechanisms. In contrast, we show that only the N-terminal, but not the C-terminal domain of TIMP-3, results in developmental defects.

Thyroid hormone-induced expression of sonic hedgehog correlates with adult epithelial development during remodeling of the Xenopus stomach and intestine

Sonic hedgehog (Shh) was isolated from the Xenopus laevis intestine as an early thyroid hormone (TH) response gene. To investigate possible roles of TH-upregulated expression of Shh during metamorphosis, we raised a polyclonal antibody against Xenopus Shh and immunohistochemically examined the relationship between Shh expression and the larval-to-adult intestinal remodeling at the cellular level. Our results indicate that the epithelial-specific expression of Shh in the intestine spatiotemporally correlates well with active proliferation and/or initial differentiation of the secondary (adult) epithelial primordia that originate from stem cells, but not with apoptosis of the primary (larval) epithelium. Given the similar transformations of the stomach during metamorphosis, we also analyzed Shh expression in this organ and found similar correlations in the stomach, although the position of the adult epithelial primordia and their final differentiation in the stomach are different from those in the intestine. Furthermore, we show here that Shh expression is organ-autonomously induced by TH and its correlation with the adult epithelial development is reproduced in vitro in both the intestine and the stomach. More importantly, addition of recombinant Shh protein to the culture medium results in developmental anomalies of both organs. However, differentiation of the adult epithelium is more severely inhibited by exogenous Shh in the intestine than in the stomach. These results suggest that TH-upregulated expression of Shh plays important roles in the postembryonic gastrointestinal remodeling, but its roles are at least partially different between the intestine and the stomach.

Role of ECM remodeling in thyroid hormone-dependent apoptosis during anuran metamorphosis

Programmed cell death or apoptosis is an important aspect in organogenesis and tissue remodeling. It is precisely controlled both temporally and spatially during development. Amphibian metamorphosis is an excellent model to study developmental control of apoptosis in vertebrates. This process involves the transformation of essentially every organ/tissue as tadpoles change to frogs, yet is controlled by a single hormone, thyroid hormone (TH). Although different organs and tissues undergo vastly different developmental changes, including de novo development and total resorption, most require apoptotic elimination of at least some cell types. Such properties and the dependence on TH make frog metamorphosis a unique model to isolate and functionally characterize genes participating in the regulation of tissue specific cell death during organ development in vertebrates. Indeed, molecular studies of the TH-dependent gene regulation cascade have led to the discovery of a group of genes encoding matrix metalloproteinases (MMPs) participating in metamorphosis. In vivo and in vitro studies have provided strong evidence to support a role of MMP-mediated remodeling of the extracellular matrix in regulating apoptotic tissue remodeling during metamorphosis.

Overexpression of matrix metalloproteinases leads to lethality in transgenicXenopus laevis: Implications for tissue-dependent functions of matrix metalloproteinases during late embryonic development

The extracellular matrix (ECM) functions as the structural support of cells and as a medium for cell-cell interactions. It is understood to play critical roles in development. ECM remodeling is mediated largely through the action of matrix metalloproteinases (MMPs), a family of Zn2+-dependent proteases capable of degrading various proteinaceous components of the ECM. MMPs are expressed in many developmental and pathologic processes. However, few studies have been carried out to investigate the function of MMPs during embryogenesis and postembryonic organogenesis. By using Xenopus development as a model system, we have previously shown that several MMP genes are expressed from neurulation to the completion of embryogenesis in distinct tissues/organs, suggesting that ECM remodeling during mid- to late embryogenesis occurs in an organ-specific manner. By using the recently developed transgenic technology for Xenopus laevis, we overexpressed Xenopus MMPs stromelysin-3 (ST3) and collagenase-4 (Col4) under the control of a ubiquitous promoter and observed that embryos with overexpressed ST3 or Col4, but not the control green fluorescent protein (GFP), died in a dose-dependent manner during late embryogenesis. The specificity of this embryonic lethal phenotype was confirmed by the failure of a catalytically inactive mutant of ST3 to affect development. Finally, overexpression of a mammalian membrane type-MMP also led to late embryonic lethality in Xenopus embryos, suggesting that membrane type-MMPs have functions in vivo for ECM remodeling, in addition to being activators of other pro-MMPs. These data together with the developmental expression of several MMPs during Xenopus development, suggest that MMPs play important roles during mid- to late embryogenesis and that proper regulation of MMP genes is critical for tissue morphogenesis and organogenesis.

Multiple stage-dependent roles for histone deacetylases during amphibian embryogenesis: implications for the involvement of extracellular matrix remodeling

Histone acetylation has long been implicated in the regulation of gene expression. Recently, a number of histone acetyltransferase and histone deacetylase genes have been identified and cloned. Molecular studies have shown that these enzymes influence transcriptional regulation as components of cofactor complexesthat interact with diversetranscription factors. However, relatively little is known about their function during development. Here, we make use of the ability to manipulate Xenopus laevis embryos in vitro to study the role of histone deacetylases in development. We first demonstrate that the histone deacetylase Rpd3 and its associated co-repressor Sin3A are coordinately expressed during embryogenesis. Rpd3 and Sin3A are known to be part of at least one large corepressor complex, which is involved in transcriptional regulation by many transcription factors, suggesting that deacetylase activity is important for embryogenesis through transcriptional regulation. Indeed, treating developing embryos with a specific histone deacetylase inhibitor, trichostatin A (TSA), leads to embryonic lethality with severe defects in the head and tail regions. Furthermore, the effects of TSA are stage-dependent with the severity of the defects decreasing when treatment is initiated at later stages. On the other hand, a sharp bend (kink) develops in the tail even when TSA treatment begins at tadpole hatching. We provide evidence that this tail defect may be in part due to the TSA-dependent inhibition of the expression of the matrix metalloproteinase gene stromelysin-3, which has been implicated in tail development through extracellular matrix remodeling.

Dual functions of thyroid hormone receptors during Xenopus development

Thyroid hormone (TH) plays a causative role in anuran metamorphosis. This effect is presumed to be manifested through the regulation of gene expression by TH receptors (TRs). TRs can act as both activators and repressors of a TH-inducible gene depending upon the presence and absence of TH, respectively. We have been investigating the roles of TRs during Xenopus laevis development, including premetamorphic and metamorphosing stages. In this review, we summarize some of the studies on the TRs by others and us. These studies reveal that TRs have dual functions in frog development as reflected in the following two aspects. First, TRs function initially as repressors of TH-inducible genes in premetamorphic tadpoles to prevent precocious metamorphosis, thus ensuring a proper period of tadpole growth, and later as activators of these genes to activate the metamorphic process. Second, TRs can promote both cell proliferation and apoptosis during metamorphosis, depending upon the cell type in which they are expressed.

Differential regulation of three thyroid hormone-responsive matrix metalloproteinase genes implicates distinct functions during frog embryogenesis

Matrix metalloproteinases (MMPs) are a family of Zn(2+)-dependent extracellular proteases capable of degrading various proteinaceous components of the extracellular matrix (ECM). They are expressed in developmental and pathological processes such as postlactation mammary gland involution and tumor metastasis. Relatively few studies have been carried out to investigate the function of MMPs during embryogenesis and postembryonic organ development. Using Xenopus development as a model system, we and others have previously isolated three MMP genes as thyroid hormone response genes. They have distinct temporal and organ-specific regulations during thyroid hormone-dependent metamorphosis. We demonstrate here that three MMPs-stromelysin-3 (ST3), collagenases-3 (Col3), and collagenases-4 (Col4)-also have distinct spatial and temporal expression profiles during embryogenesis. Consistent with earlier suggestions that ST3 is a direct thyroid hormone response gene whereas Col3 and Col4 are not, we show that precocious overexpression of thyroid hormone receptors in the presence of thyroid hormone lead to increased expression of ST3, but not Col3. Furthermore, our whole-mount in situ hybridizations reveal a tight but distinct association of individual MMPs with tissue remodeling in different regions of the animal during embryogenesis. These results suggest that ST3 is likely to play a role in ECM remodeling that facilitate apoptotic tissue remodeling or resorption, whereas Col3 and Col4 appear to participate in connective tissue degradation during development.

Transcriptional repression by XPc1, a new Polycomb homolog in Xenopus laevis embryos, is independent of histone deacetylase

The Polycomb group (Pc-G) genes encode proteins that assemble into complexes implicated in the epigenetic maintenance of heritable patterns of expression of developmental genes, a function largely conserved from Drosophila to mammals and plants. The Pc-G is thought to act at the chromatin level to silence expression of target genes; however, little is known about the molecular basis of this repression. In keeping with the evidence that Pc-G homologs in higher vertebrates exist in related pairs, we report here the isolation of XPc1, a second Polycomb homolog in Xenopus laevis. We show that XPc1 message is maternally deposited in a translationally masked form in Xenopus oocytes, with XPc1 protein first appearing in embryonic nuclei shortly after the blastula stage. XPc1 acts as a transcriptional repressor in vivo when tethered to a promoter in Xenopus embryos. We find that XPc1-mediated repression can be only partially alleviated by an increase in transcription factor dosage and that inhibition of deacetylase activity by trichostatin A treatment has no effect on XPc1 repression, suggesting that histone deacetylation does not form the basis for Pc-G-mediated repression in our assay.

Spatial and temporal regulation of collagenases-3, -4, and stromelysin -3 implicates distinct functions in apoptosis and tissue remodeling during frog metamorphosis

Matrix metalloproteinases (MMPs) are a family of extracellular proteases capable of degrading various proteinaceous components of the extracellular matrix (ECM). They have been implicated to play important roles in a number of developmental and pathological processes, such as tumor metastasis and inflammation. Relatively few studies have been carried out to investigate the function of MMPs during postembryonic organ-development. Using Xenopus laevis development as a model system, we demonstrate here that three MMPs, stromelysin-3 (ST3), collagenases-3 (Col3), and Col4, have distinct spatial and temporal expression profiles during metamorphosis as the tadpole transforms into a frog. In situ hybridizations reveal a tight, but distinct, association of individual MMPs with tissue remodeling in the tail and intestine during metamorphosis. In particular, ST3 expression is strongly correlated with apoptosis in both organs as demonstrated by analyses of serial sections with in situ hybridization for ST3 mRNA and TUNEL (terminal deoxyribonucleotidyl transferase-mediated dUTP-biotin nick end labeling) for apoptosis, respectively. On the other hand, Col3 and Col4 are present in regions where extensive connective tissue remodeling take place. These results indicate that ST3 is likely to play a role in ECM-remodeling that facilitate apoptotic tissue remodeling or resorption while Col3 and Col4 appear to participate in connective tissue degradation during development.

Molecular and cellular basis of tissue remodeling during amphibian metamorphosis

Amphibian metamorphosis involves systematic transformations of various tadpole organs/tissues. Three major types of changes take place during this process. These are remodeling, resorption, and de novo development, all of which appear to involve both cell proliferation and apoptosis (programmed cell death). All metamorphic changes are controlled by thyroid hormone (T3) and are organ-autonomous. Recent studies using primary cell cultures and a stably transformed cell line from tadpole tissues have implicated that T3 induces apoptosis cell-autonomously. This T3-induced, metamorphosis-associated apoptosis is similar to cell death in other animal species and involves similar cell death executioners. Both the activation of these executioners and the pathways leading to cell proliferation and differentiation are believed to be through transcriptional regulation by T3 receptors (TRs). TRs can activate or repress target gene transcription depending upon the presence or absence of T3, respectively. Many direct T3-response genes have been isolated and found to encode a variety of proteins that can affect both intra- and extra-cellular events. The determinations of the identities of these response genes through sequence analyses and studies on their expression profiles during development have provided strong clues toward their roles in metamorphosis. However, future studies using organ and cell culture systems and/or transient or stable transgenic technologies are required to understand how these genes transduce the T3 signal to activate the downstream cell death and proliferation/differentiation pathways.

Regulation of apoptosis during development: input from the extracellular matrix (review)

Programmed cell death or apoptosis is an important aspect in organogenesis and tissue remodeling during development. Extensive investigations have led to the identification of many genes that participate in the regulation of cell death execution. These include the caspases and nucleases, which are involved in the degradation of cellular proteins and nuclear DNA to initiate the irreversible death process. In addition, several families of proteins like Bcl-2 superfamily can either prevent or promote cell death. The function of these proteins are getting to be understood. On the other hand, how these proteins are regulated remains to be investigated. This is in part due to the presence of diverse upstream signals that can influence cell fate. One such signal is the remodeling of the extracellular matrix (ECM), which is largely due to the action of matrix metalloproteinases (MMPs). The regulation of MMPs and ECM remodeling has been shown to affect apoptosis in different systems, including the apoptotic remodeling of the intestine during Xenopus laevis metamorphosis and post-lactation involution of the mouse mammary gland. Current evidence suggests that ECM regulates cell fate at least in part through its membrane receptors, the integrins, which in turn send the signal through yet poorly understood pathways to the nucleus to regulate gene expression.

Auto-regulation of thyroid hormone receptor genes during metamorphosis: roles in apoptosis and cell proliferation

Amphibian metamorphosis is an excellent model system for studying postembryonic development in vertebrates. It involves specific degeneration of larval cells through programmed cell death with apoptotic morphology and selective proliferation and differentiation of adult cell types. Thyroid hormone (T3) plays a causative role in this process and the effects of T3 is presumed to be mediated by T3 receptors (TRs). Studies in other systems have suggested that TRs function as heterodimers formed with RXRs (9-cis retinoic acid receptors) and require the presence of various cofactor in transcriptional activation and repression in the presence and absence of T3, respectively. The T3-induced transcriptional activation leads to chromatin remodeling which may involve some of the cofactors. Recent investigation on receptor expression has implicated a role of TRs in T3-induced apoptosis in larval tissues and proliferation of adult cell types. Functional studies in tadpoles and developing embryos have provide strong support for such a role and further demonstrate the importance of RXR in mediating the effect of T3.

Regulation of SPARC expression during early Xenopus development: evolutionary divergence and conservation of DNA regulatory elements between amphibians and mammals

SPARC (Secreted Protein, Acidic, Rich in Cysteine/osteonectin/BM-40) is a highly conserved metal-binding extracellular matrix (ECM) glycoprotein which is first expressed by Xenopus embryos during late gastrulation/early neurulation (stage 12/13), by presumptive notochord and somitic cells. When animal cap explants of stage 9 embryos were cultured in vitro, SPARC expression was not detected until sibling embryos reached late neurula stage (stage 19). Addition of activin, a potent dorsal mesoderm inducer, to animal caps resulted in SPARC being expressed by the time sibling embryos reached stage 16. While basic fibroblast growth factor (bFGF), a ventral mesoderm inducer, had modest effects on SPARC mRNA expression, the combination of both activin and bFGF was synergistic. The appearance, however, of SPARC transcripts 11 h after the addition of activin and bFGF, indicates that unknown intermediates were likely to be involved in activating SPARC expression. In order to identify the potential intermediate regulatory factors which may activate and control SPARC expression, we examined the genomic organization of the 5' end of the Xenopus SPARC gene. No significant homology to the equivalent region that is highly conserved in the mouse, bovine and human SPARC genes was observed. Thus, while mammalian SPARC promoters lack TATA or CAAT boxes, the Xenopus gene contains a consensus TATA box. Moreover, promoter-proximal GGA-box repeats necessary for high level expression of mammalian SPARC are absent in Xenopus. When reporter constructs containing the 5' flanking region of the Xenopus gene were microinjected into two-cell embryos, 868 bp of 5' flanking DNA was sufficient to mimic the temporal and tissue-specific pattern of SPARC expression observed in whole embryos. While a bovine SPARC promoter reporter construct containing 740 bp of the 5' flanking DNA was expressed at a significant level in Xenopus embryos, significant differences in the cell-type expression of the reporter genes were obtained between the bovine and Xenopus constructs. The data indicate that zygotic activation of SPARC mRNA is mediated by regulatory factors acting downstream of major mesoderm induction events. The high DNA sequence conservation at the 5' end of mammalian SPARC genes is not conserved in Xenopus. These differences led to differences in their ability to direct tissue-specific gene expression in early Xenopus embryos.

Anteroposterior gradient of epithelial transformation during amphibian intestinal remodeling: immunohistochemical detection of intestinal fatty acid-binding protein

To determine whether the remodeling of the well-organized intestinal epithelium during amphibian metamorphosis is regionally regulated along the anteroposterior axis of the intestine, we raised a polyclonal antibody against the Xenopus laevis intestinal fatty acid-binding protein (IFABP), which is known to be specifically expressed in intestinal absorptive cells, and examined immunohistochemically the differentiation, proliferation, and apoptosis of the epithelial cells throughout X. laevis small intestine. During pre- and prometamorphosis, IFABP-immunoreactive (ir) epithelial cells were localized only in the anterior half of the larval intestine. At the beginning of metamorphic climax, apoptotic cells detected by nick end-labeling (TUNEL) suddenly increased in number in the entire larval epithelium, concurrently with the appearance of adult epithelial primordia. Subsequently, the adult primordia in the anterior part of the intestine developed more rapidly by active cell proliferation than those in the posterior part, and replaced the larval epithelial cells earlier than those in the posterior part. IFABP-ir cells in the adult epithelium were first detectable at the tips of newly formed folds in the proximal part of the intestine. Thereafter, IFABP expression gradually progressed both in the anteroposterior direction and in the crest-trough direction of the folds. These results suggest that developmental processes of the adult epithelium in the X. laevis intestine are regionally regulated along the anteroposterior axis of the intestine, which is maintained throughout metamorphosis, and along the trough-crest axis of the epithelial folds, which is newly established during metamorphosis. Furthermore, the regional differences in IFABP expression along the anteroposterior axis of the intestine were reproduced in organ cultures in vitro. In addition, IFABP expression was first down-regulated and then reactivated in vitro when the anterior part, but not the posterior part, of the larval intestine was treated with thyroid hormone (TH) for extended periods. Therefore, it seems that, in addition to TH, an endogenous factor(s) localized in the intestine itself with an anteroposterior gradient participates in the development of the adult epithelium during amphibian metamorphosis.

Both thyroid hormone and 9-cis retinoic acid receptors are required to efficiently mediate the effects of thyroid hormone on embryonic development and specific gene regulation in Xenopus laevis

Tissue culture transfection and in vitro biochemical studies have suggested that heterodimers of thyroid hormone receptors (TRs) and 9-cis retinoic acid receptors (RXRs) are the likely in vivo complexes that mediate the biological effects of thyroid hormone, 3,5,3'-triiodothyronine (T3). However, direct in vivo evidence for such a hypothesis has been lacking. We have previously reported a close correlation between the coordinated expression of TR and RXR genes and tissue-dependent temporal regulation of organ transformations during Xenopus laevis metamorphosis. By introducing TRs and RXRs either individually or together into developing Xenopus embryos, we demonstrate here that RXRs are critical for the developmental function of TRs. Precocious expression of TRs and RXRs together but not individually leads to drastic, distinct embryonic abnormalities, depending upon the presence or absence of T3, and these developmental effects require the same receptor domains as those required for transcriptional regulation by TR-RXR heterodimers. More importantly, the overexpressed TR-RXR heterodimers faithfully regulate endogenous T3 response genes that are normally regulated by T3 only during metamorphosis. That is, they repress the genes in the absence of T3 and activate them in the presence of the hormone. On the other hand, the receptors have no effect on a retinoic acid (RA) response gene. Thus, RA- and T3 receptor-mediated teratogenic effects in Xenopus embryos occur through distinct molecular pathways, even though the resulting phenotypes have similarities.

Ectopic expression of SPARC in Xenopus embryos interferes with tissue morphogenesis: identification of a bioactive sequence in the C-terminal EF hand

SPARC is a matricellular Ca(2+)-binding glycoprotein that exhibits both counteradhesive and antiproliferative effects on cultured cells. It is secreted by cells of various tissues as a consequence of morphogenesis, response to injury, and cyclic renewal and/or repair. In an earlier study with Xenopus embryos we had shown a highly specific and regulated pattern of SPARC expression. We now show that ectopic expression of SPARC before its normal embryonic activation produces severe anomalies, some of which are consistent with the functions of SPARC proposed from studies in vitro. Microinjection of SPARC RNA, protein, and peptides into Xenopus embryos before endogenous embryonic expression generated different but overlapping phenotypes. (a) Injection of SPARC RNA into one cell of a two-cell embryo resulted in a range of unilateral defects. (b) Precocious exposure of embryos to SPARC by microinjection of protein into the blastocoel cavity was associated with certain axial defects comparable to those obtained with SPARC RNA. (c) SPARC peptides containing follistatin-like and copper-binding sequences were without obvious effect, whereas SPARC peptide 4.2, corresponding to a disulfide-bonded, Ca(2+)-binding domain, was associated with a reduction in axial structures that led eventually to complete ventralization of the embryos. Histological analysis of ventralized embryos indicated that the morphogenetic events associated with gastrulation might have been inhibited. Microinjection of other Ca(2+)-binding glycoproteins, such as osteopontin and bone sialoprotein, resulted in phenotypes that were unique. We probed further the structural correlates of this region of SPARC in the context of tissue development. Co-injection of peptide 4.2 with Ca2+ or EGTA, and injection of peptide 4.2K (containing a mutated consensus Ca(2+)-binding sequence), demonstrated that the developmental defects associated with peptide 4.2 were independent of Ca2+. However, the disulfide bridge in this region of SPARC was found to be critical, as injection of peptide 4.2AA, a mutant lacking the cystine, generated no axial defects. We have therefore shown for the first time in vivo that the temporally inappropriate presence of SPARC is associated with perturbations in tissue morphogenesis. Moreover, we have identified at least one bioactive region of SPARC as the C-terminal disulfide-bonded, Ca(2+)-binding loop that was previously shown to be both counteradhesive and growth-inhibitory.

Transient expression of SPARC in the dorsal axis of early Xenopus embryos: correlation with calcium-dependent adhesion and electrical coupling

Our comprehension of the molecular mechanisms underlying embryogenesis has been greatly enhanced by the identification and characterization of associated extracellular matrix macromolecules. Using Xenopus laevis as a model, we investigated the expression and distribution of SPARC (Secreted Protein, Acidic, Rich in Cysteine; also called osteonectin and BM-40) during early embryonic development. SPARC has been found to be enriched in tissues undergoing rapid morphological development, differentiation, and remodeling. In Xenopus, SPARC transcripts are first expressed by primordial cells which give rise to the first embryonic tissues, the notochord and somites. SPARC RNA levels remained high throughout the rapid morphological development and differentiation phase of these tissues, and then rapidly decreased. Of particular interest, SPARC protein began to accumulate within the intersomitic clefts at the onset of trunk myotome contraction. The intersomitic enrichment of SPARC remained high as long as the myotomes remained electrically coupled, principally by gap junctions. As myotomes became innervated, SPARC expression decreased dramatically within the somites. SPARC was also found to be enriched within other tissues, such as the neural tube and epidermis. In addition, the selective spatial-temporal enrichment of SPARC suggests it makes important calcium-dependent contributions to early morphological development.

Molecular analysis of Xenopus laevis SPARC (Secreted Protein, Acidic, Rich in Cysteine). A highly conserved acidic calcium-binding extracellular-matrix protein

SPARC (Secreted Protein, Acidic, Rich in Cysteine) is expressed as a 1.6 kb mRNA in Xenopus laevis. On the basis of cDNA sequence analysis, Xenopus SPARC has a core Mr of 32643, with one potential N-glycosylation site. Western analysis of SPARC isolated from Xenopus long bone indicates that the mature protein has an Mr of 43,000. At the amino acid level, Xenopus SPARC has 78-79% sequence similarity to mouse, bovine and human SPARC. The least-conserved region is found within the N-terminal glutamic acid-rich domain, with the C-terminal Ca(2+)-binding domain being the most conserved. Adult Xenopus tissues show the same pattern of tissue-specific distribution of SPARC mRNAs as adult mouse.

Expression of SPARC/osteonectin in tissues of bony and cartilaginous vertebrates

To explore the biological functions of SPARC (secreted protein, acidic, rich in cysteine), a Ca(2+)-binding extracellular glycoprotein, we have examined its expression in an evolutionary diverse group of organisms. Similar patterns of SPARC mRNA expression were observed in adult mouse and rat tissues. SPARC transcripts represented 0.0002-0.0025% of the total RNA found in calvarium, lung, brain, and heart, whereas relatively low levels of SPARC RNA were detected in liver and kidney. Within nonmuscular tissues, a statistically significant correlation was observed between the tissue distribution of SPARC and cytoskeletal actin transcripts. Southern blot analysis revealed SPARC as a low or single-copy gene in an evolutionary diverse group of vertebrates. No hybridization signal was observed with the invertebrates examined. The tissue distribution of SPARC transcripts in the vertebrates examined was similar, except for sea lamprey and sea skate, two vertebrates that do not form mineralized bone. These data suggest that SPARC has multiple functions in mineralized and nonmineralized tissues of vertebrates.