The cell recognition molecule, cognin, mediates choline acetyltransferase activity in embryonic chick retina

Cloning of chicken choline acetyltransferase and its expression in early embryonic retina

The enzyme choline acetyltransferase [EC 2.3.1.6] (ChAT) synthesizes the neurotransmitter acetylcholine that plays a key morphogenic role in vertebrate retina development. As the embryonic avian retina is particularly useful for morphogenetic studies, we cloned the complete coding region of chicken ChAT cDNA. At the deduced amino acid level, chicken ChAT is approximately 76% identical to mammalian ChAT proteins. We also report here the cloning of the complete 5' end of the complex cholinergic locus. This locus contains both the ChAT gene and the nested intronless gene for the vesicular acetylcholine transporter (VAChT). The genomic organization of the 5' end of the chicken cholinergic locus is similar to that reported in other vertebrate species. A 5.7 kb mRNA corresponding to the ChAT message was detected in both embryonic retina and post-hatch brain. An analysis of the ChAT mRNA in embryonic chick retina shows that the message can be detected by E6 and its level increased during early retinal development. Vertebrate ChAT mRNAs can contain one or more of three non-coding exons, M, N or R and by RT-PCR we demonstrate, at least, a chicken ChAT mRNA containing exon M.

The cell adhesion molecule retina cognin is a cell surface protein disulfide isomerase that uses disulfide exchange activity to modulate cell adhesion

The retina cell adhesion molecule, R-cognin, shares cDNA sequence with protein disulfide isomerase (PDI) but has a different molecular size and subcellular location. We asked whether R-cognin originated from a unique PDI gene transcript or was a product of posttranscriptional processing. The 3'-terminal partial cDNA clone for R-cognin was extended by both 5' RACE and by PCR from sequence near the 5' end of the PDI-translated region. The cDNA sequence was compared to those of chicken, bovine, and human PDI. The R-cognin cDNA sequence was identical to that of chicken PDI and differed by less than 10% from mammalian PDI proteins. The role of the disulfide exchange activity characteristic of both proteins was studied by assessing the cell-aggregation-enhancing ability and tissue specificity of R-cognin and recombinant human PDI and its derivatives. Chicken and normal human PDI proteins showed tissue- and developmental-specific enhancement of cell aggregation identical to R-cognin, and this activity was blocked by inactivation of the -WCGHC- motifs which function in disulfide exchange. Dependence of retina cell aggregation on disulfide exchange activity was shown by blocking that activity with the inhibitor, DTNB, or with a recombinant human PDI with the -WCGHC- motif cysteines mutated. The results suggest that one -WCGHC- motif in R-cognin is sufficient and that the more C-terminal motif is most active. We conclude that R-cognin is a tissue-specific protein product of the standard PDI chicken gene. The -WCGHC- motif in mature R-cognin is necessary, but not sufficient, for cell adhesion.

Thioreductase activity of retina cognin and its role in cell adhesion

Retina cognin (R-cognin) is a 50-kDa protein on the surface of embryonic chick retina cells that mediates cell-cell recognition and neuronal differentiation. It is developmental stage- and tissue-specific in its expression. The partial cDNA clone for R-cognin is nearly identical to that of chicken protein disulfide isomerase (chicken PDI) and enzyme with thioreductase activity. The R-cognin clone extends from beyond the 3' polyadenylation site up to the boundary between PDI exons 1 and 2, with the putative R-cognin equivalent of PDI exon 1 remaining uncloned. The question posed here was whether the sequence-specific properties of PDI were significant in the action of R-cognin. We show that R-cognin, like PDI, has thioreductase activity as revealed by RNase renaturation enzymatic assays. We then asked if this thioreductase activity was involved in the mediation of cell adhesion and recognition in developing chick retina. We show, through cell aggregation assays, that both R-cognin and chicken PDI enhance chick retina cell aggregation but not that of cells from other CNS tissues. We also show that treating R-cognin and chicken PDI with the thioreductase inhibitor 5,5'-dithio-bis (2-nitrobenzoic acid), which covalently binds to the functional cysteines of the thioreductase active sites, reduces the enhancement of cell aggregation. Thus R-cognin acts, in part, by catalyzing a covalent protein-protein linkage at the cell surface.

Co-localization of the insulin receptor, jun protein and choline acetyltransferase in embryonic chick retina

Previous work demonstrated that the availability of insulin to the embryonic chick retina at a critical developmental stage stimulated the activity of the acetylcholine synthetic enzyme, choline acetyltransferase (ChAT) and that this increase required the AP-1 transcription factor, c-jun. Here it is shown that immediately following a 2-5 min exposure to insulin there is, in the amacrine and ganglion cells of the chick embryo retina, a transient increase in the level of jun protein followed by a long-lasting increase in ChAT. These and previous results show that insulin receptor activation is necessary for the characteristic retina developmental increase in ChAT protein and that this increase is preceded by a transient increase in the synthesis of the transcription factor c-jun in the same retina cells. The data demonstrate an intracellular signal transduction pathway from the developmentally-activated insulin receptor through c-jun to ChAT and cholinergic differentiation.

Developmental localization of retina cognin synthesis by in situ hybridization

Retina cognin (R-cognin) is a 50 kDa protein involved in cell recognition and neuronal differentiation during development of the embryonic chick retina. Initial characterization of a partial cDNA encoding R-cognin revealed a striking similarity to the cDNA encoding protein disulfide isomerase (PDI), a 57 kDa multifunctional protein. The exact nature of the relationship between R-cognin and PDI is not known; however, both proteins appear to be encoded by the same gene. In the present study, we developed cRNA probes to examine the expression of R-cognin and PDI transcripts in embryonic chick retina and liver. In the retina, the amount of transcript decreased with embryonic age, in parallel to a similar decrease in R-cognin protein. In the liver, where PDI is prominently expressed, the amount of transcript was not developmentally regulated. The spatial and temporal pattern of expression of the R-cognin-encoding retinal transcript was examined by in situ hybridization. R-cognin mRNA was expressed in cells across the retina early in retinogenesis, but became restricted to the cells of the inner retina later in development. This pattern of expression was the same as the developmental pattern of R-cognin protein [Dobi et al., Invest. Ophthalmol. Vis. Sci. 27, (1986) p. 323-329], thus, demonstrating that this secreted protein functions at the surface of the cells where it is transcribed.

Causal role for jun protein in the stimulation of choline acetyltransferase by insulin in embryonic chick retina

Previous work showed that the availability of insulin to the embryonic chick retina at a critical developmental stage stimulated the activity of the acetylcholine synthetic enzyme, choline acetyltransferase (ChAT) (R. E. Hausman et al., 1991, Dev. Brain Res. 59, 31-37). Here we show that a 2- to 5-min exposure to insulin results in a greater than 24 hr elevation in ChAT protein. Immediately following exposure to insulin there is a transient increase in the level of jun protein followed by an increase in ChAT. The stimulation of ChAT protein is not the result of an overall stimulation of protein synthesis as other proteins are not affected. Exposure of the cells to antisense oligonucleotide to jun, but not to sense oligonucleotide, reduces the increase in both jun and ChAT. These and previous results suggest that insulin is necessary for the characteristic increase in ChAT protein during retina development and that this increase requires the transient synthesis of jun.

Effects of cell signaling on the development of GABA receptors in chick retina neurons

R-cognin, a cell recognition molecule, and insulin are known to play significant roles in GABAergic differentiation in the developing chick retina. In the present study, the effects of insulin and R-cognin on post-synaptic (GABAceptive) differentiation were investigated. In ovo binding of [3H]GABA and [3H]flunitrazepam ([3H]Flu) to the GABA and benzodiazepine (BZD) receptors, respectively, remained at low levels during early embryogenesis but increased sharply from mid-embryogenesis through hatching, increases which also occur in cultured neurons from early-embryonic (E7) and mid-embryonic (E11) chick retina. E7 neurons respond to insulin treatment (100 ng/ml) with increased [3H]Flu binding but no change in [3H]GABA binding. Cognin antibody (10 micrograms/ml) treatment of E7 neurons caused no significant inhibition of the developmental increases in binding of either radioligand. Insulin in E11 cultures led to greater developmental increases in binding sites for both radioligands, but exposure to cognin antibody was without significant effect. These data, along with previous studies, indicate that GABAergic differentiation in developing chick retina is regulated, in part, by insulin and cognin-mediated cell signaling. Insulin also regulates post-synaptic (GABAceptive) differentiation whereas cognin-mediated interactions are relatively insignificant.

cDNA for R-cognin: homology with a multifunctional protein

Retina cognin (R-cognin) is a developmentally regulated 50-kDa protein that was isolated from chicken embryo retina cell membranes. It mediates the adhesion and reaggregation in vitro of retina cells from chicken and mouse embryos, but not of cells from other tissues, and may be involved in neuronal differentiation. We report here the cloning of a cDNA for R-cognin. A chicken embryo retina cDNA library was constructed in lambda gt11 vector and was screened with polyclonal R-cognin antiserum, yielding several immunoreactive clones. Antiserum prepared to the R-cognin-beta-galactosidase fusion protein produced by one recombinant lysogen recognized the 50-kDa R-cognin protein derived from retina cell membranes. This antiserum inhibited the reaggregation of dissociated retina cells and immunostained chicken embryo retina tissue in a pattern similar to that obtained with R-cognin antiserum. In vitro translation of RNA from a cDNA subclone yielded a 50-kDa protein that was recognized by R-cognin antiserum on a Western blot. By these criteria we identify the cDNA clone as representative of the gene encoding R-cognin. This cDNA is nearly identical to a major portion of the cDNA for the multifunctional protein that is the beta subunit of prolyl 4-hydroxylase and has both protein disulfide isomerase activity and thyroid hormone-binding activity. These findings demonstrate that R-cognin differs from other cell adhesion molecules and suggest possible mechanisms for its action in cell adhesion and neuronal differentiation.