Reports suggest that two members of the novel immune-associated nucleotide (Ian) GTPase family, Ian1 and Ian5, play roles in T cell development. We performed real-time PCR analysis of the expression of Ian genes of the rat during T cell maturation, in macrophages and in cell lines. We found that all of the genes were expressed at relatively low levels at the early double-negative thymocyte stage but were expressed more strongly at later cell stages. Our study also revealed the fact that the previously reported Ian9, Ian10 and Ian11 genes are, instead, parts of a single gene for which we retain the name Ian9, potentially encoding a GTPase with a highly unusual triplicated structure. Antisera were developed against both Ian1 and Ian9. We established that Ian9 is produced as an approximately 75-kDa protein in both T cells and thymocytes. We observed that levels of both Ian1 and Ian9 proteins are profoundly reduced in T cells from lymphopenic rats as compared with wild-type rats. It was demonstrated that thymocytes and B cells from lymphopenic rats (Ian5 null) did not show enhanced sensitivity to gamma-irradiation-induced apoptosis.

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The Gimap/IAN family of GTPases has been implicated in the regulation of cell survival, particularly in lymphomyeloid cells. Prosurvival and prodeath properties have been described for different family members. We generated novel serological reagents to study the expression in rats of the prodeath family member Gimap4 (IAN1), which is sharply up-regulated at or soon after the stage of T cell-positive selection in the thymus. During these investigations we were surprised to discover a severe deficiency of Gimap4 expression in the inbred Brown Norway (BN) rat. Genetic analysis linked this trait to the Gimap gene cluster on rat chromosome 4, the probable cause being an AT dinucleotide insertion in the BN Gimap4 allele (AT(+)). This allele encodes a truncated form of Gimap4 that is missing 21 carboxyl-terminal residues relative to wild type. The low protein expression associated with this allele appears to have a posttranscriptional cause, because mRNA expression was apparently normal. Spontaneous and induced apoptosis of BN and wild-type T cells was analyzed in vitro and compared with the recently described mouse Gimap4 knockout. This revealed a “delayed” apoptosis phenotype similar to but less marked than that of the knockout. The Gimap4 AT(+) allele found in BN was shown to be rare in inbred rat strains. Nevertheless, when wild rat DNA samples were studied the AT(+) allele was found at a high overall frequency (∼30%). This suggests an adaptive significance for this hypomorphic allele.