Over the last 10 years, our knowledge ofextracellular proteolysis has progressed dramatically. Different enzymatic cascades cooperate to achieve extracellular matrix (ECM)I degradation, and a number of participant proteins have been characterized and cloned. Physiological inhibitors have been identified for most of these enzymes. Also, the concept of focused proteolysis, through binding of enzymes and inhibitors to specific regions ofthe extracellular milieu, has received broad experimental support. Finally, the biosynthesis of many of the relevant proteases and inhibitors has been shown to be under the control of hormones and growth factors. Plasminogen activators (PAs) and their inhibitors (PAIs) are thought to be key participants in the balance ofproteolytic and antiproteolytic activities that regulates matrix turnover. This article summarizes the evidence that supports this contention, discusses the role of PAspecific cell surface binding sites, and also draws attention to a number of instances in which the presence of PAs cannot be reconciled with an exclusive function in ECM degradation.The plasminogen activator/plasmin system ENZYMES PAs are serine proteases of tryptic specificity. Two enzymes, differing mostly in the domain organization and function of their noncatalytic regions, have been identified in mammals: urokinase-type PA (uPA) and tissue-type PA (tPA) (1). They are the products of distinct genes, and are secreted as singlechain (sc) proteins; whereas sc tPA is active, sc uPA is essentially inactive (pro-uPA) (2). Cleavage of pro-uPA by plasmin, kallikrein, Factor XIIa or cathepsin B (3) yields the disulfidelinked two-chain active enzyme. The two PAs have distinct targeting determinants in their noncatalytic regions: the "growth factor domain" of uPA directs the binding of the enzyme (and that ofpro-uPA) to a plasma membrane receptor (4,5), whereas other structural domains in tPA (the "finger" region and the "kringles") allow its binding to fibrin and other It is impossible to select a small set of references that would provide appropriate coverage of the entire field discussed. We have thus included references to reviews and to recent papers that can be used to trace back earlier work. We apologize to all our colleagues whose contributions are not adequately referenced, as well as to our readers.Receivedfor components of the ECM (6). These and perhaps additional interactions, for instance with heparinlike molecules, could have a dramatic effect on the focusing of PA-controlled proteolysis (7). The different extracellular addressing ofthe two PAs suggests that they play different biological roles.Plasminogen is the prefered substrate for PAs, but other molecules may also be cleaved by one or the other PA; for instance, avian uPA exerts a plasmin-independent effect on the morphology of chick fibroblasts (8). Plasminogen is present in plasma and extracellular fluids at a 1-2 AM concentration, in the range of the Km of the activation reaction. It can associate with fibrin and other proteins via lysi...

The primary event in mammalian sexual development is the differentiation of the bipotential gonads into either testes or ovaries. Our understanding of the molecular pathways specifying gonadal differentiation is still incomplete. To identify the initial molecular changes accompanying gonadal differentiation in mice, we have performed a large-scale transcriptional analysis of XX and XY Sf1-positive gonadal cells during sex determination. In both male and female genital ridges, a robust genetic program is initiated pre-dating the first morphological changes of the differentiating gonads. Between E10.5 and E13.5, 2306 genes were expressed in a sex-specific manner in the somatic compartment of the gonads; 1223 were overexpressed in XX embryos and 1083 in XY embryos. Although sexually dimorphic genes were scattered throughout the mouse genome, we identified chromosomal regions hosting clusters of genes displaying similar expression profiles. The cyclin-dependent kinase inhibitors Cdkn1a and Cdkn1c are overexpressed in XX gonads at E11.5 and E12.5, suggesting that the increased proliferation of XY gonads relative to XX gonads may result from the overexpression of cell cycle inhibitors in the developing ovaries. These studies define the major characteristics of testicular and ovarian transcriptional programs and reveal the richness of signaling processes in differentiation of the bipotential gonads into testes and ovaries.

In mice, gonads are formed shortly before embryonic day 10.5 by the thickening of the mesonephros and consist of somatic cells and migratory primordial germ cells. The male sex-determining process is set in motion by the sex-determining region of the Y chromosome (Sry), which triggers differentiation of the Sertoli cell lineage. In turn, Sertoli cells function as organizing centres and direct differentiation of the testis. In the absence of Sry expression, neither XX nor XY gonads develop testes, and alterations in Sry expression are often associated with abnormal sexual differentiation. The molecular signalling mechanisms by which Sry specifies the male pathway and models the undifferentiated gonad are unknown. Here we show that the insulin receptor tyrosine kinase family, comprising Ir, Igf1r and Irr, is required for the appearance of male gonads and thus for male sexual differentiation. XY mice that are mutant for all three receptors develop ovaries and show a completely female phenotype. Reduced expression of both Sry and the early testis-specific marker Sox9 indicates that the insulin signalling pathway is required for male sex determination.

Abstract. To assess in vivo the postulated participation of urokinase-type (u-PA) and tissue-type (t-PA) plasminogen activators in processes involving tissue remodeling and cell migration, we have studied the cellular distribution of u-PA and t-PA mRNAs during mouse oogenesis and embryo implantation. By in situ hybridizations, we detected t-PA mRNA in oocytes and u-PA mRNA in granulosa and thecal cells from preovulatory follicles. These findings are compatible with a role for plasminogen activators in oogenesis and follicular disruption. We demonstrated the presence of u-PA mRNA in the invasive and migrating trophoblast cells of 5.5-and 6.5-d-old embryos. At 7.5 days, u-PA mRNA was predominantly localized to trophoblast cells that had reached the deep layers of the uterine wall, while the peripheral trophoblast cells surrounding the presomite stage embryo were devoid of specific signal. In 8.5-d-old embryos abundant u-PA mRNA expression resumed transiently in the giant trophoblast cells at the periphery of the embryo and in the trophoblast cells of the ectoplacental cone, to become undetectable in 10.5-d-old embryos. These observations establish the in vivo expression of the u-PA gene by invading and migrating trophoblast cells in a biphasic time pattern; they are in agreement with the proposed involvement of the enzyme in the extracellular proteolysis accompanying embryo implantation.

Ontogenic relationships between the different types of endocrine cells in the islets of Langerhans were explored by generating transgenic mouse embryos in which cells transcribing the glucagon, insulin, or pancreatic polypeptide genes were destroyed through the promoter-targeted expression of the diphtheria toxin A chain. Embryos lacking glucagon-or insulin-containing cells did not exhibit alterations in the development of the nontargeted islet cell types, whereas embryos lacking pancreatic polypeptide gene-expressing cells also lacked pancreatic insulin-and somatostatin-containing cells. These results show that neither glucagon nor insulin gene-expressing cells are essential for the differentiation of the other islet endocrine-cell types. These results also suggest that pancreatic polypeptide gene-expressing cells are indispensable for the differentiation of islet .8 and 6 cells because the former produce a necessary paracrine or endocrine factor and/or operate through a cell-lineage relationship.