Distinct functions of SR proteins in alternative pre-mRNA splicing

Alternative splicing of precursor messenger RNAs (pre-mRNAs) is a common mechanism of regulating gene expression. SR proteins are a family of pre-mRNA splicing factors that are structurally related and evolutionarily conserved. Any member of the SR family can complement a splicing-deficient extract that lacks the entire family of SR proteins. Here it is demonstrated that particular SR proteins have distinct functions in alternative pre-mRNA splicing in vitro. In addition, SR proteins are differentially expressed in a variety of tissues. These results suggest a fundamental role for SR proteins in the regulation of alternative splicing.

Adhesion of a chicken myeloblast cell line to fibrinogen and vitronectin through a beta 1-class integrin

The adhesive interactions of circulating blood cells are tightly regulated, receptor-mediated events. To establish a model for studies on regulation of cell adhesion, we have examined the adhesive properties of the HD11 chick myeloblast cell line. Function-perturbing antibodies were used to show that integrins containing the beta 1 subunit mediate HD11 cell attachment to several distinct extracellular matrix proteins, specifically fibronectin, collagen, vitronectin, and fibrinogen. This is the first evidence that an integrin heterodimer in the beta 1 family functions as a receptor for fibrinogen. While the alpha v beta 1 heterodimer has been shown to function as a vitronectin receptor on some cells, this heterodimer could not be detected on HD11 cells. Instead, results suggest that the beta 1 subunit associates with different, unidentified alpha subunit(s) to form receptors for vitronectin and fibrinogen. Results using function-blocking antibodies also demonstrate that on these cells, additional receptors for vitronectin are formed by alpha v beta 3 and alpha v associated with an unidentified 100-kD beta subunit. The adhesive interactions of HD11 cells with these extracellular matrix ligands were shown to be regulated by lipopolysaccharide treatment, making the HD11 cell line attractive for studies of mechanisms regulating cell adhesion. In contrast to primary macrophage which rapidly exhibit enhanced adhesion to laminin and collagen upon activation, activated HD11 cells exhibited reduced adhesion to most extracellular matrix constituents.

Cell-surface regulation of beta 1-integrin activity on developing retinal neurons

Integrins are a family of alpha beta heterodimeric receptors that mediate cell-cell and cell-substratum interactions. Integrin binding to extracellular ligands regulates cell adhesion, shape, motility, intracellular signalling and gene expression. Mechanisms that regulate integrin function are, therefore, central to the participation of integrins in a diverse set of cellular events. Here we report the identification of TASC, a monoclonal antibody to a novel epitope on the integrin beta 1 subunit, which inhibits cell adhesion to vitronectin but promotes adhesion to laminin and collagen types I and IV. We show that developing retinal neurons that have lost responsiveness to laminin regain the ability to bind laminin in the presence of TASC. Thus, beta 1-class integrins are likely to occupy multiple affinity states that can be modulated at the cell surface.

Vitronectin and thrombospondin promote retinal neurite outgrowth: developmental regulation and role of integrins

Extracellular matrix (ECM) glycoproteins regulate neuronal development and axonal growth. In this paper, the ECM glycoprotein vitronectin was identified and localized in the embryonic chick neuroretina. To identify potentially important neurite outgrowth-promoting molecules, responses of embryonic chick retinal neurons to vitronectin and thrombospondin, another retinal ECM constituent, were examined. These neurons were shown to attach and extend neurites on either glycoprotein. Integrins containing the alpha v or beta 1 subunits mediate both responses to vitronectin and neurite outgrowth on thrombospondin. Attachment to thrombospondin was inhibited by heparin, suggesting that neurons also utilize a proteoglycan or sulfated glycolipid as a receptor for this glycoprotein. Thus, retinal neurons use specific receptors to interact with vitronectin and thrombospondin, two glycoproteins present in the embryonic neuroretina, suggesting roles for these ligands and their receptors in retinal development.

N-cadherin, NCAM, and integrins promote retinal neurite outgrowth on astrocytes in vitro

Retinal ganglion neurons extend axons that grow along astroglial cell surfaces in the developing optic pathway. To identify the molecules that may mediate axon extension in vivo, antibodies to neuronal cell surface proteins were tested for their effects on neurite outgrowth by embryonic chick retinal neurons cultured on astrocyte monolayers. Neurite outgrowth by retinal neurons from embryonic day 7 (E7) and E11 chick embryos depended on the function of a calcium-dependent cell adhesion molecule (N-cadherin) and beta 1-class integrin extracellular matrix receptors. The inhibitory effects of either antibody on process extension could not be accounted for by a reduction in the attachment of neurons to astrocytes. The role of a third cell adhesion molecule, NCAM, changed during development. Anti-NCAM had no detectable inhibitory effects on neurite outgrowth by E7 retinal neurons. In contrast, E11 retinal neurite outgrowth was strongly dependent on NCAM function. Thus, N-cadherin, integrins, and NCAM are likely to regulate axon extension in the optic pathway, and their relative importance varies with developmental age.

N-cadherin and integrins: two receptor systems that mediate neuronal process outgrowth on astrocyte surfaces

Receptor-mediated interactions between neurons and astroglia are likely to play a crucial role in the growth and guidance of CNS axons. Using antibodies to neuronal cell surface proteins, we identified two receptor systems mediating neurite outgrowth on cultured astrocytes. N-cadherin, a Ca2(+)-dependent cell adhesion molecule, functions prominently in the outgrowth of neurites on astrocytes by E8 and E14 chick ciliary ganglion (CG) neurons. beta 1-class integrin ECM receptor heterodimers function less prominently in E8 and not at all in E14 neurite outgrowth on astrocytes. The lack of effect of integrin beta 1 antibodies on E14 neurite outgrowth reflects an apparent loss of integrin function, as assayed by E14 neuronal attachment and process outgrowth on laminin. N-CAM appeared not to be required for neurite outgrowth by either E8 or E14 neurons. Since N-cadherin and integrin beta 1 antibodies together virtually eliminated E8 CG neurite outgrowth on cultured astrocytes, these two neuronal receptors are probably important in regulating axon growth on astroglia in vivo.

Embryonic neural retinal cell response to extracellular matrix proteins: developmental changes and effects of the cell substratum attachment antibody (CSAT)

Cell attachment and neurite outgrowth by embryonic neural retinal cells were measured in separate quantitative assays to define differences in substrate preference and to demonstrate developmentally regulated changes in cellular response to different extracellular matrix glycoproteins. Cells attached to laminin, fibronectin, and collagen IV in a concentration-dependent fashion, though fibronectin was less effective for attachment than the other two substrates. Neurite outgrowth was much more extensive on laminin than on fibronectin or collagen IV. These results suggest that different substrates have distinct effects on neuronal differentiation. Neural retinal cell attachment and neurite outgrowth were inhibited on all three substrates by two antibodies, cell substratum attachment antibody (CSAT) and JG22, which recognize a cell surface glycoprotein complex required for cell interactions with several extracellular matrix constituents. In addition, retinal cells grew neurites on substrates coated with the CSAT antibodies. These results suggest that cell surface molecules recognized by this antibody are directly involved in cell attachment and neurite extension. Neural retinal cells from embryos of different ages varied in their capacity to interact with extracellular matrix substrates. Cells of all ages, embryonic day 6 (E6) to E12, attached to collagen IV and CSAT antibody substrates. In contrast, cell attachment to laminin and fibronectin diminished with increasing embryonic age. Age-dependent differences were found in the profile of proteins precipitated by the CSAT antibody, raising the possibility that modifications of these proteins are responsible for the dramatic changes in substrate preference of retinal cells between E6 and E12.