NF1 and SPRED1/2 cooperate through RAS-MAPK-independent functions

Neurofibromin, the protein product of the neurofibromatosis type 1 (NF1) gene, requires the direct binding interaction with SPRED to negatively regulate the RAS-MAPK pathway. Although the region of neurofibromin that stimulates the intrinsic GTPase activity of RAS represents only a small percentage of the entire protein, a large degree of the NF1 structural domains and their correlating mechanistic functions remain elusive. Here, we demonstrate RAS-independent biochemical and signaling functions regulated by the coordinate control of NF1 and SPRED1/2. Utilizing CRISPR-Cas9 methods to ablate NF1 or SPRED1/2 in isogenic "RASless" mouse embryonic fibroblast (MEF) cell lines expressing either the KRAS4b wild-type variant or an oncogenic KRAS-mutation, we show loss of SPRED1/2 phenocopies NF1 loss and their cooperation is required to modulate MAPK-AKT signaling. Moreover, NF1 or SPRED1/2 loss also resulted in a potent suppression of the RAS family GTPases, RRAS and RRAS2, occurring independently of RAS or AKT pathway activation. A transcriptome microarray analysis of the NF1 or SPRED1/2 knockout MEF cells revealed a specific subset of RAS-independent, NF1-SPRED1/2-dependent gene signatures, in which these same genes were also directly regulated by the RAS-GTPase function of neurofibromin. The modulation of these NF1-SPRED1/2-dependent downstream signaling effectors were further corroborated in Schwann cell models derived from Neurofibromatosis type I patients that consisted of either plexiform neurofibroma cells or unaffected nerve cells abrogated of NF1 or neurofibromin RAS-GAP activity. Taken together, this study provides RAS-independent functions that are dependent on the cooperation of NF1 and SPRED1/2 in a manner that is uncoupled from canonical MAPK signaling.