Developmental stage-specific role of Frs adapters as mediators of FGF receptor signaling in the oligodendrocyte lineage cells.

FGF signaling is important for numerous cellular processes and produces diverse cellular responses. Our recent studies using mice conditionally lacking FGF-Receptor-1 (Fgfr1) or Fgfr2 during different stages of myelinogenesis revealed that Fgfr signaling is first required embryonically for the specification of oligodendrocyte progenitors (OPCs) and then later postnatally for the growth of the myelin sheath during active myelination but not for OPC proliferation, differentiation, or ensheathment of axons. What intracellular signal transduction pathways are recruited immediately downstream of Fgfrs and mediate these distinct developmentally regulated stage-specific responses remain unclear. The adapter protein Fibroblast-Growth-Factor-Receptor-Substrate-2 (Frs2) is considered a key immediate downstream target of Fgfrs. Therefore, here, we investigated the in vivo role of Frs adapters in the oligodendrocyte lineage cells, using a novel genetic approach where mice were engineered to disrupt binding of Frs2 to Fgfr1 or Fgfr2, thus specifically uncoupling Frs2 and Fgfr signaling. In addition, we used conditional mutants with complete ablation of Frs2 and Frs3. We found that Frs2 is required for specification of OPCs in the embryonic telencephalon downstream of Fgfr1. In contrast, Frs2 is largely dispensable for transducing Fgfr2-mediated signals for the growth of the myelin sheath during postnatal myelination, implying the potential involvement of other adapters downstream of Fgfr2 for this function. Together, our data demonstrate a developmental stage-specific function of Frs2 in the oligodendrocyte lineage cells. This contextual requirement of adapter proteins, downstream of Fgfrs, could partly explain the distinct responses elicited by the activation of Fgfrs during different stages of myelinogenesis.