Basic fibroblast growth factor and fibroblast growth factor receptor-1 in the human hippocampal formation

Basic fibroblast growth factor (bFGF) is an important mitogen and neurotrophic factor that binds and signals through the high-affinity receptor, fibroblast growth factor receptor 1 (FGFR1). However, only a limited amount of information is available concerning the molecular forms and anatomical distribution of fibroblast growth factors (FGFs) in the normal human brain. We found multiple bFGF and FGFR1 mRNA transcripts which vary in expression pattern across human brain regions. Using in situ hybridization and immunohistochemistry, we localized bFGF and FGFR1 mRNA and protein to cells in the normal adult human hippocampus and caudal entorhinal cortex (ERC). The majority of pyramidal neurons contained FGFR1 mRNA and protein in the mesial temporal lobe, with neurons in the CA2/CA3 region demonstrating the highest levels of FGFR1 mRNA. In contrast to FGFR1, bFGF mRNA expression was detected at very low levels in a small fraction of the neurons in the human hippocampus and caudal ERC. While bFGF mRNA may be expressed at low levels in neurons, bFGF-immunopositive cells with astrocytic features were detected throughout the mesial temporal lobe in rats, monkeys and humans. bFGF immunoreactive processes are found traversing the dentate gyrus, and bFGF immunoreactive cells are found in the neurogenic subgranular zone in all three mammalian species studied. The anatomical distribution of these two FGF family members suggests that bFGF is endogenously positioned to be involved in ongoing neurogenesis in the adult hippocampus, and that FGF trophic signaling to differentiated neurons could involve the release of astrocytic bFGF acting on neuronal FGFR1 in the normal adult human hippocampus.