Extra-hepatic expression of serum albumin mRNA in mouse retina

Purpose. In some mammals, serum albumin protein exists in the interphotoreceptor space (IPS), the space between photoreceptor cells and the retinal pigment epithelium. Serum albumin is synthesized largely in the liver, though low levels of extra-hepatic expression have been documented in several other tissues, including fetal rat kidney, pancreas, lung, and heart. The purpose of this study was to investigate whether serum albumin protein and mRNA are found in mouse retina. Methods. Using albumin rabbit antibodies and HRP goat anti-(rabbit IgG), we performed immunoassays on mouse IFS washes to detect the presence of serum albumin protein. Protein extracts from IFS washes were subjected to Affigel Blue chromatography. This resin has an affinity for serum albumin. Reverse transcription-polymerase chain reaction (RT-PCR) of retina total RNA was performed to search for albumin mRNA. Also, real-time reverse transcription polymerase chain reaction (RT-RT-PCR) was employed to look at the levels of expression in different age groups. Results. A constituent of the IFS washes specifically bound and eluted from Affigel Blue column, suggesting that the washes contained serum albumin. SDS PAGE revealed that the size of the constituent was 67 kDa, the size of serum albumin. This 67 kDa band reacted with mouse serum antibody. An RT-PCR amplified fragment of serum albumin mRNA from retina displayed the expected size. The sequence of this fragment is identical to authentic serum albumin cDNA sequence. RPE and choroid were negative for serum albumin mRNA. However, rd1(-)/rd1(-) retina was positive, suggesting that at least some serum albumin is synthesized in the inner layers of the retina. RT-RT-PCR showed that serum albumin mRNA levels in whole retina reached a maximum at about postnatal day 15 and gradually decreased to about one-sixth of maximum at 12 months age. Conclusions. Serum albumin protein and mRNA were found in mouse IFS and retina, suggesting that the protein is synthesized in the retina. The previously demonstrated ability of serum albumin to bind fatty acids and retinoids and its presence in the mouse IFS suggest a role for serum albumin in transporting retinoids in the retina or IFS, especially at young ages when concentrations appear greatest.