Integrin receptors for extracellular matrix are critical for cell motility, but the signals that determine when to stop are not known. Analysis of distal tip cell (DTC) migration during gonadogenesis in Caenorhabditis elegans has revealed the importance of transcription factor vab-3/Pax6 in regulating the ␣ integrin genes, ina-1 and pat-2. Utilizing vab-3 mutants, we show that the downregulation of ina-1 is necessary for DTC migration cessation and the up-regulation of pat-2 is required for directionality. These results demonstrate concomitant, but distinct roles in migration for each integrin. Notably, transcriptional control of migration termination provides a new mechanism for regulation of morphogenesis and organ size.Supplemental material is available at http://www.genesdev.org.Received January 24, 2007; revised version accepted May 21, 2007. Integrins are heterodimeric transmembrane receptors consisting of ␣ and ␤ subunits that link the extracellular matrix (ECM) to the actin cytoskeleton via a wide array of intracellular proteins (Hynes 2002). Through these interactions, integrins regulate cell migration, division, differentiation, and adhesion. Coordinated regulation of integrin adhesive strength over space and time determines the speed and directionality of cell migration in vitro (Palecek et al. 1997;Ridley et al. 2003), and abnormal cell migration has been linked to certain developmental defects caused by loss of integrins in vivo (Bouvard et al. 2001). However, detailed in vivo analyses of integrins in migration are complicated by embryonic lethality of integrin-null mutants and by pleiotropic, redundant, or overlapping functions among the 18 ␣ and eight ␤ chains in mammals. Some of the experimental constraints inherent in studies on mice can be bypassed by using the nematode Caenorhabditis elegans, which has two conserved integrin receptors composed of an ina-1/␣ or pat-2/␣ subunit associated with the pat-3/␤ integrin subunit (Cox et al. 2004).Post-embryonic gonadogenesis in C. elegans provides an excellent model of cell migration during development. The shape of the hermaphrodite gonad is dictated by migration of two leader cells called the distal tip cells (DTCs) (Hubbard and Greenstein 2000). The DTCs begin to migrate away from the gonad primordium on the ventral ECM of the nematode in larval stage L2 (Fig. 1A). During the third larval stage, they turn and migrate to the dorsal side, followed by a second turn and migration toward the mid-body of the nematode throughout the L4 stage. Migration ends on the dorsal surface approximately opposite the vulva, resulting in two U-shaped gonad arms coincident with the onset of adulthood (Fig. 1A,B). Executing the DTC migratory program requires the coordinated action of matrix metalloproteases, the netrin signaling system, plus integrins and other signaling molecules that regulate the cytoskeleton (Hubbard and Greenstein 2000;Cram et al. 2006). Molecular regulation of gonad size and the signals that stop DTC migration are not known.The integrin heterod...

SummaryIntegrin receptors for extracellular matrix (ECM) are critical determinants of biological processes. Regulation of integrin expression is one way for cells to respond to changes in the ECM, to integrate intracellular signals, and to obtain appropriate adhesion for cell motility, proliferation, and differentiation. Transcriptional and post-translational mechanisms for changing the integrin repertoire at the cell surface have recently been described. These mechanisms work through transcriptional regulation that alters the proportions of one integrin relative to another, referred to as integrin switching, or through localized regulation of integrin-ECM interactions, thus providing exquisite control over cell rearrangements during tissue morphogenesis and remodeling. These integrin regulatory pathways may also be important targets in such emerging fields as tissue engineering and regenerative medicine.

In mammals, secretion of GDNF by the metanephrogenic mesenchyme is essential for branching morphogenesis of the ureteric bud and, thus, metanephric development. However, the expression pattern of GDNF and its receptor complex-the GPI-linked ligand-binding protein, GFRalpha-1, and the Ret tyrosine kinase signaling protein-indicates that it could operate at early steps in kidney development as well. Furthermore, the developing nephric systems of fish and amphibian embryos express components of the GDNF signaling system even though they do not make a metanephros. We provide evidence that GDNF signaling through GFRalpha-1 is sufficient to direct pathfinding of migrating pronephric duct cells in axolotl embryos by: (1) demonstrating that application of soluble GFRalpha-1 to an embryo lacking all GPI-linked proteins rescues PND migration in a dose-dependent fashion, (2) showing that application of excess soluble GFRalpha-1 to a normal embryo inhibits migration and that inhibition is dependent upon GDNF-binding activity, and (3) showing that the PND will migrate toward a GDNF-soaked bead in vivo, but will fail to migrate when GDNF is applied uniformly to the flank. These data suggest that PND pathfinding is accomplished by migration up a gradient of GDNF.

Initiation of excretory system development in all vertebrates requires (1) delamination of the pronephric and pronephric duct rudiments from intermediate mesoderm at the ventral border of anterior somites, and (2) extension of the pronephric duct to the cloaca. Pronephric duct extension is the central event in nephric system development; the pronephric duct differentiates into the tubule that carries nephric filtrate out of the body and induces terminal differentiation of adult kidneys. Early studies concluded that pronephric ducts formed by means of in situ segregation of pronephric duct tissue from lateral mesoderm ventral to the forming somites; more recent studies highlight caudal migration of the pronephric duct as the major morphogenetic mechanism. The purpose of this review is to provide the historical background on studies of the mechanisms of amphibian pronephric duct extension, to review evidence showing that different amphibians perform pronephric duct morphogenesis in different ways, and to suggest future studies that may help illuminate the molecular basis of the mechanisms that have evolved in amphibians to extend the pronephric duct to the cloaca. Developmental Dynamics 226:1-11, 2003.

Integrin signaling relies on multiple, distinct pathways to impact a diverse set of cell behaviors. The Rho family of GTPases are well-established downstream signaling partners of integrins that regulate cell shape, polarity, and migration. The nematode C. elegans provides a simple in vivo system for studying both integrins and the Rho family. Our previous work showed that the C. elegans α integrin cytoplasmic tails have tissue-specific functions during development. Here, we use chimeric α integrins to show that the cytoplasmic tails can rescue the loss of the Rho family of GTPases in three cell types in the somatic gonad. Knockdown of rho-1 by RNAi causes defects in sheath cell actin organization, ovulation, and vulva morphology. Chimeric α integrin ina-1 with the pat-2 cytoplasmic tail can rescue both actin organization and ovulation after rho-1 RNAi, yet cannot restore vulva morphology. Knockdown of cdc-42 by RNAi causes defects in sheath cell actin organization, ovulation, vulva morphology, and distal tip cell migration. Chimeric α integrin pat-2 with the ina-1 cytoplasmic tail can rescue vulva morphology defects and distal tip cell migration after cdc-42 RNAi, yet cannot restore sheath cell actin organization or ovulation. Disruption of Rac yields the same phenotype in distal tip cells regardless of α integrin cytoplasmic tail composition. Taken together, the cytoplasmic tails of α integrins can bypass signaling from members of the Rho family of GTPases during development.