Immunohistochemical localization of biliverdin reductase in rat brain: age related expression of protein and...

Biliverdin reductase regulates heme oxygenase activity by removing the inhibitory product of the oxygenase activity, biliverdin; and reducing it to bilirubin. The other products of the oxygenase are carbon monoxide and Fe. To date, biliverdin reductase remains unique among all enzymes described by using 2 different cofactors (NADPH and NADH) at different pH ranges. The present study reports on the developmentally regulated changes in the pattern of protein expression and the level of biliverdin reductase transcript in rat brain. Biliverdin reductase activity of the brain cytosol with both NADPH (pH 8.7) and NADH (pH 6.7) exhibited developmental changes with the activity increasing after birth, reaching an adult level by day 28 postpartum. When analyzed by Western blotting the immunoreactive protein detected increased as the animal matured (day 1 to 28 postparturition). Northern blot hybridization of RNA isolated from rat brain revealed the presence of approximately 1.5 kb biliverdin reductase transcript at all stages of development ranging from 1 day post partum to 20 months. The level of the transcript was developmentally regulated and a gradual increase ( approximately 4-fold) was observed from day 1 after birth to adulthood and was maintained in 20 month old animals. Cellular localization, using immunohistochemical technique, revealed age-related pattern of expression of the reductase in select regions such as the cortex, substantia nigra, hippocampus and in the cerebellum; the changes, however, did not follow the same pattern. To elaborate, in the cortex, the reductase expression increased when 7-day-old animals were compared with young adults (2 months old) and then declined in the 20-month-old animals. In the substantia nigra the level of reductase expression progressively declined with age when 7-day-old neonate, 2- and 20-month-old animals were compared. In the hippocampus, a distinct reductase-expressing cell population residing between CA1 and the dentate gyrus was observed in the 7-day-old animals; these cells were not detected in the adults (2 or 20 months old). In the cerebellum, the expression of the reductase reflected the developmental organization of this region. We postulate that age-dependent increase of the brain reductase at the transcript and protein levels in the course of maturation serves to control heme oxygenase activity which also displays a developmental pattern in the organ. As such, the reductase modulates generation of biologically active heme degradation products; bilirubin, carbon monoxide and Fe.