Developmental and tissue-specific expression of alpha 1-microglobulin mRNA in the rat

A rat liver cDNA library was constructed in the lambda gt11 expression vector. Three clones expressing alpha 1-microglobulin, an immunosuppressive plasma protein, were detected by screening with rabbit antiserum against rat alpha 1-microglobulin. The alpha 1-microglobulin activity from one of the clones, 6b, was confirmed with monoclonal antibodies in a solid phase radioimmunoassay. The nucleotide sequence of the fragment (165 base pairs) was determined, and the translated amino acid sequence (55 amino acids) showed a 75% homology to human alpha 1-microglobulin (position 122-176). Southern blots of restriction endonuclease-digested rat DNA indicated two distinct genes with alpha 1-microglobulin homology when probed with radioactive cDNA fragment from clone 6b. Northern blots showed the presence of a single mRNA species in rat liver, and the level was low in 1-month-old animals, increased to reach a maximum during adulthood (3 months), and decreased with aging (12 months). The alpha 1-microglobulin concentration in rat serum showed the same age dependence between 1 and 12 months, with the highest values at 3 months. Embryonic development (8.5-day to 17.5-day) was studied using total fetal RNA, and expression of alpha 1-microglobulin mRNA was detected in low amounts only at day 15.5. alpha 1-Microglobulin mRNA levels, studied by an RNA dot blot assay, were high in liver and kidney, low in brain and testis, and none were found in hypothalamus and spleen cells.