A truncated Reelin protein is produced but not secreted in the 'Orleans' reeler mutation (Reln[rl-Orl]).
Reelin is the protein defective in reeler mutant mice [I. Bar, C. Lambert de Rouvroit, I. Royaux, D.B. Krizman, C. Dernoncourt, D. Ruelle, M.C. Beckers, A.M. Goffinet, A YAC contig containing the reeler locus with preliminary characterization of candidate gene fragments, Genomics 26 (1995) 543-549; G. D'Arcangelo, G.G. Miao, S.C. Chen, H.D. Soares, J.I. Morgan, T. Curran, A protein related to extracellular matrix proteins deleted in the mouse mutant reeler, Nature 374 (1995) 719-723; S. Hirotsune, T. Takahara, N. Sasaki, K. Hirose, A. Yoshiki, T. Ohashi, M. Kusakabe, Y. Murakami, M. Muramatsu, S. Watanabe, K. Nakao, M. Katsuki, Y. Hayashizaki, The reeler gene encodes a protein with an EGF-like motif expressed by pioneer neurons, Nature Genet. 10 (1995) 77-83]. In the Orleans allele of reeler (symbol: Reln[rl-Orl]), a 220 nucleotide deletion is present in the 3' region of the Reelin message, resulting in a frame shift with production of a predicted protein amputated from its C-terminal amino acids. In this study, we first show that the predicted truncated protein indeed exists in Orleans reeler mice, using several anti-Reelin antibodies. Three antibodies are directed against epitopes located in the N-terminal region of the protein, namely: monoclonal antibody CR-50 [M. Ogawa, T. Miyata, K. Nakajima, K. Yagyu, M. Seike, K. Ikenaka, H. Yamamoto, K. Mikoshiba, The reeler gene-associated antigen on Cajal-Retzius neurons is a crucial molecule for laminar organization of cortical neurons, Neuron 14 (1995) 899-912] (epitope region between Reelin residues 251-407), monoclonal antibody G10 (epitope located between amino acids 199 and 244) and the polyclonal antipeptide rp4 (positions 381-399). A fourth antibody, antipeptide rp5, reacts with the C-terminal (3443-3461) Reelin sequence. In normal embryos, all four antibodies stained cells in the marginal zone with features of Cajal-Retzius cells. While N-terminal specific antibodies detected Reelin immunoreactivity in mouse embryos homozygous for the reeler-Orleans mutation, no staining was obtained with the rp5 antibody, showing the presence of a truncated protein. Moreover, although Reelin could be detected at the surface of living Cajal-Retzius cells of normal mice, it was not revealed after vital staining of embryonic cortex from Orleans reeler mice. These results indicate that the C-terminal region of Reelin is essential for its secretion and suggest that the Orleans reeler phenotype is due to defective Reelin secretion rather than to secretion of an inactive protein.