Expression of the EGF receptor family members ErbB2, ErbB3, and ErbB4 in germinal zones of the developing brain and in neurosphere cultures containing CNS stem cells.

The epidermal growth factor receptor family consists of four related tyrosine kinases: the epidermal growth factor receptor (EGF-R or ErbB), ErbB2, ErbB3, and ErbB4. These receptors are capable of extensive cross-activation upon the binding of their ligands - the EGF family of peptides for EGF-R and the neuregulins for ErbB3 and ErbB4. Since EGF-R is expressed by proliferating cells in the central nervous system (CNS), including multipotent CNS stem cells, we examined the expression of ErbB2, ErbB3 and ErbB4 in the germinal epithelia of the developing rat brain using in situ hybridization. ErbB2 and ErbB4 mRNAs were widely distributed within the germinal zones as early as E12. However, as development proceeded, ErbB2 mRNA was mainly present within the layers of cells immediately adjacent to the ventricular surface - the ventricular zone, while ErbB4 mRNA was predominantly expressed by subventricular zone cells, in the regions where these specialized germinal epithelia were present. ErbB3 mRNA distribution within germinal epithelia was more restricted, primarily confined to the diencephalon and rostral midbrain. Cultured neurospheres, which contain CNS stem cells, expressed ErbB2, ErbB4 and, to a lesser extent, ErbB3 protein as demonstrated by Western blot analysis. This expression declined during following differentiation. Heregulin-beta1, a neuregulin, had no effect on the proliferative capacity of neurospheres. Overall, our results indicate that ErbB2, ErbB3 and ErbB4 may play important and distinct roles in the genesis of the CNS. However, our in vitro data do not support a role for neuregulins in proliferation, per se, of CNS stem cells.