Using cDNA representational difference analysis to identify markers present in chondrocytes but not osteoblasts, Steck et al. (2001) isolated a cDNA encoding CRTAC1, which they called CEP68. The deduced 653-amino acid CRTAC1 protein has a signal peptide and a C-terminal EGF (131530)-like domain. SDS-PAGE analysis showed expression of a 70-kD protein, close to the predicted size of 68 kD. RT-PCR analysis confirmed the cultured chondrocyte-specific expression of CRTAC1 and also detected expression in bone and cartilage primary tissue. CRTAC1 expression was inducible in mesenchymal stem cells cultured in chondrogenic medium. Northern blot analysis revealed expression of a 3.0-kb transcript in cultured chondrocytes as well as in lung; no expression was detected in heart, placenta, liver, skeletal muscle, pancreas, and blood. A 2.4-kb CRTAC1 transcript was detected in brain. Steck et al. (2001) proposed that CRTAC1 may be an extracellular matrix protein expressed in bone, cartilage, and cultured chondrocytes.

By database analysis, Steck et al. (2007) identified 2 splice variants of CRTAC1 that used different 3-prime terminal exons. The variants, CRTAC1A and CRTAC1B, encode deduced 661- and 645-amino acid proteins, respectively. Both proteins contain 4 FG (phe-gly)-GAP (gly-ala-pro) repeat domains, an integrin (see 192968)-binding RGD (arg-gly-asp) motif, a UnvB/ASPIC domain, and an EGF-like calcium-binding domain. CRTAC1A has 5 putative O-glycosylation sites at its C terminus that are absent in CRTAC1A. RT-PCR with exon-specific primers detected expression of CRTAC1A in cartilage and lung and both CRTAC1A and CRTAC1B in brain. EST database analysis revealed CRTAC1B expression in brain, eye, lens, pineal gland, and lung. In situ hybridization and immunohistochemical analysis detected CRTAC1 in deep zone chondrocytes of human articular cartilage. CRTAC1B orthologs were present in vertebrates, but not invertebrates, and CRTAC1A appeared to be human specific.

Sato et al. (2011) cloned mouse Crtac1b, which they called Lotus. The deduced 646-amino acid protein contains 4 FG-GAP domains, followed by a UnvB/ASPIC domain, an EGF-type calcium-binding domain, and a C-terminal transmembrane domain. In situ hybridization revealed Lotus expression in several brain regions of embryonic day-15.5 mouse, including olfactory bulb, hippocampus, cerebral cortex, thalamus, and spinal cord. Immunohistochemical analysis of cultured olfactory bulb neurons revealed Lotus expression in growth cones and neurite shafts, where it colocalized with Ngr1 (RTN4R; 605566) and an Ngr1 ligand, Nogoa (604475).

Using Western blot analysis, Steck et al. (2007) found that CRTAC1A was secreted by cultured human chondrocytes. Mutation and in vitro biochemical analyses revealed that only secreted CRTAC1A was O-glycosylated and that the last putative O-glycosylation site, thr626, was used. Real-time PCR showed that BMP4 (112262) stimulation elevated CRTAC1 expression in human chondrocytes.

Specificity of neural connections is determined by repulsive and attractive axon guidance molecules and their receptors. Using chromophore-assisted light inactivation, Sato et al. (2011) identified Lotus as a membrane surface protein in developing mouse lateral olfactory tract axons. Screening a mouse olfactory bulb expression library revealed that Lotus interacted with Ngr1, but not with other Nogo receptor family members. Lotus did not localize to the lateral olfactory tract in the absence of Ngr1. Overexpression of Lotus inhibited binding between Ngr1 and Nogo66, the 66-amino acid extracellular domain of Nogoa.

By genomic sequence analysis, Steck et al. (2001) determined that the CRTAC1 gene contains 14 exons and spans at least 74 kb.

Steck et al. (2007) determined that the CRTAC1 gene spans over 164 kb and contains 16 exons, including the penultimate exon 15B and the 3-prime terminal exon 15A, which are used by the CRTAC1B and CRTAC1A transcripts, respectively.

By genomic sequence analysis, Steck et al. (2001) mapped the CRTAC1 gene to chromosome 10.

By genomic sequence analysis, Steck et al. (2007) mapped the CRTAC1 gene to chromosome 10q22. The 3-prime exon of the CRTAC1 gene overlaps the 3-prime exon of the GOLGA7B gene (614189), which is transcribed from the opposite strand.

Sato et al. (2011) found that olfactory bulb axons projected caudally in an abnormally defasciculated manner in brains of Lotus -/- mouse embryos. Defasciculation was not observed in Lotus -/- Ngr1 -/- embryos, suggesting that the defect was due to unopposed Ngr1 signaling. Nogo66 induced growth cone collapse in wildtype cultured neurons, but not in neurons cultured from Lotus -/- embryos. Sato et al. (2011) concluded that Lotus functions in axon bundling by antagonizing Nogo66/Ngr1-induced repulsive signaling.