Kohzo Nakayama

Neuronal nitric oxide synthase expression in neuronal cell differentiation

Expression of neuronal nitric oxide synthase (n-NOS) was investigated during neuronal cell differentiation. Trace amounts of n-NOS mRNA were detected in the primary culture of neural precursor cell (NPC) at embryonic day 10, and a high level of n-NOS mRNA was observed after a further 7 days cultivation of NPC. Both n-NOS activity and its mRNA level were also increased in the human neuroblastoma cell line, TGW, following trans-retinoic acid (RA)-induced neuronal differentiation. These findings indicate that there is positive regulation of n-NOS mRNA during neuronal cell differentiation. However TGW cell differentiation with trans-RA could not be reversed by treatment with specific inhibitors of NOS. Thus, the signal transduction mechanism which is involved in the induction of neuronal cell differentiation by trans-RA appears to be distinct from the NO-mediated pathway.

Fourth component of Xenopus laevis complement: cDNA cloning and linkage analysis of the frog MHC

ComplementC4 shows extensive structural and functional similarity to complementC3, hence these components are believed to have originated by gene duplication from a common ancestor. Although to dateC3 cDNA clones have been isolated from all major classes of extant vertebrates includingXenopus, C4 cDNA clones have been isolated from mammalian species only. We describe here the molecular cloning and structural analysis ofXenopus C4 cDNA. The cDNA sequence encoding the thioester region ofXenopus C4 was amplified by reverse transcriptase-polymerase chain reaction usingXenopus liver mRNA as a template, and then used to screen a liver cDNA library. The amino acid sequence ofXenopus C4 deduced from a clone containing the entire protein-coding sequence showed 39%, 30%, 25%, and 20% overall identity with those of human C4, C3, C5, and α2-macroglobulin, respectively. The predicted amino acid sequence consisted of a 22-residue putative signal peptide, a 634-residue β chain, a 732-residue α chain, and a 287-residue γ chain. Of 30 cysteine residues, 27 were found in exactly the same positions as in humanC4. Genomic Southern blotting analysis indicated thatC4 is a single copy gene inXenopus and is part of the frog MHC cluster. These results clearly demonstrate thatC3/C4 gene duplication and linkage between theC4 gene and the major histocompatibility complex predate mammalian/amphibian divergence.

Three extra copies of a C4-related gene in H-2w7 mice are C4/Slp hybrid genes generated by multiple recombinational events

Mice bearing the H-2w7 haplotype have five C4-related genes and constitutively express the Slp antigen. To understand the structure and evolution of the five C4-related genes of the C3H.W7 mouse, we have determined nucleotide sequences of the 5′ end region of these genes. A C4/Slp hybrid nature was confirmed for three of five C4-related genes as predicted previously by restriction enzyme analysis. The nucleotide sequences of the 5′ flanking regions of these three hybrid genes showed close similarity to that of the C4 gene, while the 3′ side of the ninth exon of the three hybrid genes showed close similarity to that of the Slp gene. In contrast, the regions between the first exon and the middle of the ninth exon of the three hybrid genes showed a mosaic structure of C4-like and Slp-like sequences. Moreover, the boundaries of the C4-like and Slp-like sequences were quite different among the three hybrid genes. The pattern of nucleotide sequence diversity in this region among the five C4-related sequences could be mainly explained not by point mutations but by gene conversions or unequal crossovers. These results suggest that multiple genetic recombinational events between two homologous sequences played an important role in the generation and diversification of the extra copies of the C4/Slp gene in the H-2w7 mouse.

Post-transcriptional regulation of complement C4 in low C4-producing strain of mouse

The expression of the fourth component of complement (C4) of the mouse can differ 20-fold and is determined byC4-high (C4h) orC4-low (C4l) alleles. To investigate the molecular mechanisms underlying the differences in C4 expression, we compared the transcriptional activity of theC4 genes between high and low C4-producer strains of mice (B10 and FM vs B10.BR) using nuclear transcriptional and chloramphenicol acetyltransferase (CAT) assays. We also compared the level of C4-specific RNA in total and nuclear RNA of the liver. The results revealed no significant difference in transcriptional activity betweenC4h andC4l genes. However, the steady-state levels of C4 mRNA are ten times lower inC4l strains than inC4h strains, suggesting that the major regulation of C4 plasma levels occurs at the post-transcriptional level.