Metabolic and behavioral effects of neurofibromin result from differential recruitment of MAPK and mTOR signaling

TL;DR

This study shows that different symptoms of neurofibromatosis type 1 are caused by distinct signaling pathways, suggesting potential for targeted treatments.

Contribution

The study identifies distinct signaling pathways responsible for behavioral and metabolic effects in neurofibromatosis type 1 using a Drosophila model.

Findings

Behavioral effects of neurofibromin are mediated by MEK signaling without requiring Akt.

Metabolic effects require coordinated MEK/ERK and Akt/mTOR/S6K/4E-BP signaling.

Loss of neurofibromin causes metabolic dysregulation in interneurons and muscle, with altered muscle mitochondria.

Abstract

Neurofibromatosis type 1 results from mutations in the NF1 gene and its encoded neurofibromin protein. This condition produces multiple symptoms, including tumors, behavioral alterations, and metabolic changes. Molecularly, neurofibromin mutations affect Ras activity, influencing multiple downstream signaling pathways, including MAPK (Raf/MEK/ERK) and PI3K/Akt/mTOR signaling. This pleiotropy raises the question of which pathways could be targeted to treat the disease symptoms, and whether different phenotypes driven by neurofibromin mutations exhibit similar or diverging dependence on the signaling pathways downstream of Ras. To test this, we examined metabolic and behavioral alterations in the genetically tractable Drosophila neurofibromatosis type 1 model. In vivo genetic analysis revealed that behavioral effects of neurofibromin were mediated by MEK signaling, with no necessity for Akt. In contrast, metabolic effects of neurofibromin were mediated by coordinated actions MEK/ERK and Akt/mTOR/S6K/4E-BP signaling. At the systemic level, loss of neurofibromin dysregulated metabolism via molecular effects in interneurons and muscle. These changes were accompanied by altered muscle mitochondria morphology, with no concomitant changes in neuronal ultrastructure or neuronal mitochondria. Overall, this suggests that neurofibromin mutations affect multiple signaling cascades downstream of Ras, which differentially affect metabolic and behavioral neurofibromatosis type 1 phenotypes.

The genetic disorder neurofibromatosis type 1 drives multiple symptoms in humans, including increasing risk for behavioral disorders and altering metabolism. The genetic mutations that underlie the disorder change cellular signaling pathways in multiple ways. In this study, we asked whether the multiple symptoms in the disorder could be caused by the different changes in cellular signaling pathways by testing them one at a time in a powerful genetic model, the fruit fly, Drosophila melanogaster. Mimicking the disorder by reducing levels of the neurofibromin protein increases the activity of the Ras signaling pathway and activates several downstream signaling pathways, including MEK/ERK and Akt/mTOR. Flies with reduced Nf1 exhibit several characteristics that are reminiscent of the human symptoms, including behavioral changes and metabolic alterations. With a series of genetic experiments, we examined how the two cellular signaling pathways drive behavioral changes and metabolic alterations. The results showed that multiple signaling molecules were required for Nf1-dependent effects on metabolism, while the behavioral effects were more selectively modulated by one of the two pathways (MEK/ERK). These results suggest that different symptoms in neurofibromatosis type 1 may result from different cellular signaling mechanisms. If so, they may be independently targetable with different interventions. The study sets the stage to examine other signaling pathways and to model other symptoms in the disease in animal models.

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