The integrin subunit is necessary for gastrulation in sea urchin embryos

Abstract

Integrins are a family of cell adhesion molecules reported to mediate cellular interactions essential for normal embryonic morphogenesis. Here we describe a β integrin subunit that is expressed during early embryogenesis in the sea urchin embryo and appears to be necessary for normal development. The deduced amino acid sequence of βL is similar to vertebrate β integrin subunits, but is most closely related to the sea urchin βG subunit. Northern blots show that βL is expressed at all stages with maximum expression beginning during gastrulation. Immunolocalization and in situ RNA hybridization show that in blastulae βL is expressed in the blastoderm and by the ring of bottle cells in the vegetal plate during the initial phase of gastrulation. Presumptive secondary mesenchyme cells express high levels of βL throughout elongation of the archenteron and in the pluteus βL is expressed by blastocoelar cells, skeletal mesenchyme, and pigment cells. Antibodies and Fab fragments against βL block spreading of dissociated embryonic cells on RGD (arginine-glycine-aspartate)containing substrates. Treating embryos with anti-βL antibodies blocks the initial phase of gastrulation and interferes with the organization of actin filaments. Prior to gastrulation, the antibodies cause thickening of the blastoderm and later in development defects in skeletal patterning result. Probing for antibody in treated embryos indicates that it penetrates the ectoderm to cells within the blastocoel and is actively endocytosed. We propose that βL forms receptors that bind to RGD-containing ligands and anchors actin filaments. These receptors appear to be essential in several aspects of morphogenesis.