CD300 antigen like family member G: A novel Ig receptor like protein exclusively expressed on capillary endothelium

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Abstract

We report the characteristics of CD300LG, a member of the CD300 antigen like family. Its genomic structure is similar in both mouse and human, and at least four isoforms exist in both species. The amino acid sequence of the immunoglobulin (Ig) V like domain of CD300LG showed approximately 35% identity to those of the polymeric Ig receptor (pIgR) and Fcα/μR. Interestingly, mouse CD300LG proteins were uniquely expressed on capillary endothelium. Immunoelectron microscopy revealed that mouse CD300LG is localized on both apical and basolateral plasma membranes, as well as on intracellular vesicular structures, in the capillary endothelium. Transcytosis assays using polarized MDCK epithelial cells showed that CD300LG could be transcytosed bidirectionally. Furthermore, CD300LG exogenously expressed on HeLa cells could take up IgA2 and IgM, but not IgG. These results suggest that CD300LG might play an important role in molecular traffic across the capillary endothelium.

Section snippets

Materials and methods

Animals. C57BL/6J mice between 7 and 10 weeks old were obtained from CLEA Japan. All animal experiments were performed in accordance with the Guidelines for the Care and Use of Laboratory Animals in RIKEN.

cDNA clones and plasmid construction. An IMAGE consortium cDNA clone (clone ID No. 4913418) covering the coding sequence of mouse CD300LG-α was obtained from Invitrogen Corp., Carlsbad, CA. To obtain other isoforms of mouse CD300L, oligo(dT) primed cDNAs of the mouse heart were amplified by

Cloning and molecular characterization of CD300LG

To identify the novel Fc receptors for IgA, we made use of large databases of ESTs with Ig domains to identify those which showed homology with a region of the Ig domain conserved significantly between pIgR and Fcα/μR. As a result, we found a mouse Ig receptor like gene that encodes a putative type-I membrane protein containing one Ig domain having approximately 35% identity to those of pIgR and Fcα/μR, respectively. The gene was named CD300 antigen like family member G (CD300LG) or CMRF35

Discussion

In the present study, we demonstrated that CD300LG shows restricted expression on capillary endothelium in mouse, in contrast to the fact that many genes of this family are expressed on a variety of hematopoietic cell types [9], [10], [11], [25], [26], [27], [28], [29]. And human CD300LG transcripts were expressed strongly in the human heart and placenta where capillaries are highly developed. In mouse and human, the CD300L family is located on chromosomes 11 and 17, respectively. Whereas

Acknowledgments

We thank Drs. T. Murakami, A. Yoshino, and K. Kawano for critical reading of the manuscript. This study was supported in part by Grants-in-Aid for Young Scientists (B) (H.T.), Scientific Research (H.O.), Scientific Research in Priority Areas (H.O.), and Protein 3000 Project (H.O.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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