Transgenic Fv-4 mice resistant to Friend virus.

Fv-4 is a mouse gene that confers resistance to infection with ecotropic retroviruses. A candidate Fv-4 gene was cloned previously and found to resemble the 3' half of a murine leukemia virus (MuLV). To study the effect of this gene in vivo, we generated two transgenic mouse strains carrying the Fv-4 env gene under control of its presumed natural promoter, a cellular sequence unrelated to retroviruses. Transgenic progeny expressed a 3-kb Fv-4 env RNA in all of the organs and tissues examined, as well as an Fv-4 envelope antigen on the surface of thymocytes and spleen cells, similar to mice carrying the natural Fv-4 gene. One of the two transgenic strains (designated Fv4-2) expressed three to nine times as much transgene RNA and protein as the other strain (Fv4-11). When challenged with a Friend virus complex containing up to 10(4) XC PFU of Friend MuLV, Fv4-2 mice were completely resistant to development of splenomegaly and had no detectable ecotropic virus in the spleen or blood, confirming that the cloned Fv-4 gene is responsible for resistance to ecotropic MuLV in vivo. In contrast, Fv4-11 mice were only partially resistant, developing viremia and splenomegaly at the highest inoculum dose but recovering from viremia several weeks after inoculation with 10-fold less virus. The phenotype of recovery from viremia in Fv4-11 mice was unexpected and suggests that low levels of expression of the Fv-4 gene enhance the effectiveness of the immune response.

Lens protein gene expression: alpha-crystallins and MIP.

The crystallin genes encode the major soluble proteins of the lens. Some of the crystallin genes are expressed exclusively in the lens while others are also expressed in different tissues. The two alpha-crystallin genes, alpha A and alpha B, differ in their tissue specificity. Transcription of the alpha A-crystallin gene occurs only in the lens, while the alpha B-crystallin gene is also expressed in other tissues, including heart, skeletal muscle, kidney, lung and brain. MIP (also called MP26), the major intrinsic protein of the lens fiber membranes, is also expressed exclusively in the lens. Correct expression of both alpha-crystallin and MIP are required for normal lens function. Here we review our studies on the molecular basis of expression of the alpha-crystallin and MIP genes in the lens. The 5' flanking sequences containing the initiation site of transcription of the alpha A-crystallin, alpha B-crystallin and MIP genes were fused to the bacterial chloramphenicol acetyltransferase (CAT) gene, and the expression of this reporter gene was studied in transient assays and transgenic mice. DNA sequences flanking the 5' end of the alpha A-crystallin gene contain regulatory elements responsible for the lens-specific expression and developmental regulation of the CAT gene in transgenic mice. Interestingly, although some of the murine alpha A-crystallin regulatory sequences are conserved in the human and chicken genes, different functional regulatory elements appear to control the expression of the murine and chicken alpha A-crystallin genes. The 5' flanking sequence of the alpha B-crystallin gene preferentially directs expression of the CAT gene to the lens and to skeletal muscle. Different regulatory elements of the alpha B-crystallin gene appear to be responsible for its transcription in various tissues. The 5' flanking sequence of the MIP gene also contains regulatory elements that direct expression of the CAT gene to lens cells; these sequences are not functional in transfected non-lens cells and are different from the cis regulatory elements controlling alpha-crystallin gene expression. The multiplicity of cis-regulatory elements controlling the transcription of these three genes indicates the complexity of the mechanisms that regulate gene expression in the lens.