Transcription factor hlh-2/E/Daughterless drives expression of α integrin ina-1 during DTC migration in C. elegans.

Integrins are involved in a vast number of cell behaviors due to their roles in adhesion and signaling. The regulation of integrin expression is of particular interest as a mechanism to drive developmental events and for the role of altered integrin expression profiles in cancer. Dynamic regulation of the expression of integrin receptors is required for the migration of the distal tip cell (DTC) during gonadogenesis in Caenorhabditis elegans. α integrin ina-1 is required for DTC motility, yet is up-regulated by an unknown mechanism. Analysis of the promoter for α integrin ina-1 identified two E-box sequences that are required for ina-1 expression in the DTC. Knockdown of transcription factor hlh-2, an established E-box binding partner and ortholog of E/Daughterless, prevented expression of a transcriptional fusion of the ina-1 promoter to RFP and blocked DTC migration. Similarly, knockdown of hlh-2 also prevented expression of a translational fusion of the genomic ina-1 gene to GFP while blocking DTC migration. Knockdown of HLH-2 binding partner MIG-24 also reduced ina-1 expression and DTC migration. Overall, these results show that the transcription factor hlh-2 is required for up-regulation of ina-1 at the onset of DTC migration.

α integrin cytoplasmic tails can rescue the loss of Rho-family GTPase signaling in the C. elegans somatic gonad.

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.

α Integrin cytoplasmic tails have tissue-specific roles during C. elegans development.

Integrin signaling impacts many developmental processes. The complexity of these signals increases when multiple, unique integrin heterodimers are expressed during a single developmental event. Since integrin heterodimers have different signaling capabilities, the signals originating at each integrin type must be separated in the cell. C. elegans have two integrin heterodimers, α INA-1/ß PAT-3 and α PAT-2/ß PAT-3, which are expressed individually or simultaneously, based on tissue type. We used chimeric α integrins to assess the role of α integrin cytoplasmic tails during development. Chimeric integrin ina-1 with the pat-2 cytoplasmic tail rescued lethality and maintained neuron fasciculation in an ina-1 mutant. Interestingly, the pat-2 tail was unable to completely restore distal tip cell migration and vulva morphogenesis. Chimeric integrin pat-2 with the ina-1 cytoplasmic tail had a limited ability to rescue a lethal mutation in pat-2, with survivors showing aberrant muscle organization, yet normal distal tip cell migration. In a wild type background, α integrin pat-2 with the ina-1 cytoplasmic tail had a dominant negative effect which induced muscle disorganization, cell migration defects and lethality. These results show the α integrin cytoplasmic tails impact unique cellular behaviors that vary by tissue type during development.

Temporal and spatial regulation of integrins during development.

Integrin 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.

Control of C. elegans hermaphrodite gonad size and shape by vab-3/Pax6-mediated regulation of integrin receptors.

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 alpha integrin genes, ina-1 and pat-2. Utilizing vab-3 mutants, we show that the down-regulation 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.

Organogenesis: cutting to the chase.

Gonad morphogenesis in Caenorhabditis elegans requires two secreted proteases. Recent studies show that alterations of the extracellular matrix component fibulin-1 rescue gonadogenesis in the absence of these proteases. This finding is a critical step toward understanding the role of extracellular matrix in organogenesis.

Pronephric duct extension in amphibian embryos: migration and other mechanisms.

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.