Expression of rat insulin-like growth factor binding protein-6 in the brain, spinal cord, and sensory ganglia

Insulin-like growth factors (IGFs) are important trophic factors during development as well as in the adult or damaged nervous system. Their trophic actions are modulated by interactions with six distinct IGF binding proteins. The mRNA expression profiles of binding proteins 2, 4 and 5 in the normal developing and adult CNS are well characterized and are shown to have distinctive, non-overlapping distributions. The IGF binding protein-6 (BP6) is also expressed in the CNS, however, details regarding its mRNA expression distribution in the developing and adult nervous system is limited. BP6 has the unique property of preferentially binding the IGF-II ligand. Coupled with the fact that this ligand is the most abundantly expressed IGF in the adult CNS, this suggests that the IGF-II/BP6 complex has a unique role in modulating IGF-II function in the adult brain. In this report the anatomical distribution of BP6 messenger RNA in the developing and adult rat nervous system is presented. In the embryonic animal the CNS expression is tightly restricted to trigeminal ganglia and, relative to the rest of the embryo, this structure has the highest expression. The expression in the forebrain and cerebellum does not occur until after postnatal day 21 and then is primarily associated with GABAergic interneurons. The highest levels of expression in the adult animal are in the hindbrain, spinal cord, cranial ganglia, and dorsal root ganglia. These nuclei in the hindbrain and periphery that express BP6 are all associated with the coordination of sensorimotor function in the cerebellum, which indicates an important role for the BP6/IGF-II complex in the function and maintenance of these systems.