Downregulation of Hsp90 and the antimicrobial peptide Mtk suppresses poly(GR)-induced neurotoxicity in C9ORF72-ALS/FTD.

GGGGCC repeat expansion in the C9ORF72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Repeat RNAs can be translated into dipeptide repeat proteins, including poly(GR), whose mechanisms of action remain largely unknown. In an RNA-seq analysis of poly(GR) toxicity in Drosophila, we found that several antimicrobial peptide genes, such as metchnikowin (Mtk), and heat shock protein (Hsp) genes are activated. Mtk knockdown in the fly eye or in all neurons suppresses poly(GR) neurotoxicity. These findings suggest a cell-autonomous role of Mtk in neurodegeneration. Hsp90 knockdown partially rescues both poly(GR) toxicity in flies and neurodegeneration in C9ORF72 motor neurons derived from induced pluripotent stem cells (iPSCs). Topoisomerase II (TopoII) regulates poly(GR)-induced upregulation of Hsp90 and Mtk. TopoII knockdown also suppresses poly(GR) toxicity in Drosophila and improves survival of C9ORF72 iPSC-derived motor neurons. These results suggest potential novel therapeutic targets for C9ORF72-ALS/FTD.

Antagonistic feedback loops involving Rau and Sprouty in the Drosophila eye control neuronal and glial differentiation.

During development, differentiation is often initiated by the activation of different receptor tyrosine kinases (RTKs), which results in the tightly regulated activation of cytoplasmic signaling cascades. In the differentiation of neurons and glia in the developing Drosophila eye, we found that the proper intensity of RTK signaling downstream of fibroblast growth factor receptor (FGFR) or epidermal growth factor receptor required two mutually antagonistic feedback loops. We identified a positive feedback loop mediated by the Ras association (RA) domain-containing protein Rau that sustained Ras activity and counteracted the negative feedback loop mediated by Sprouty. Rau has two RA domains that together showed a binding preference for GTP (guanosine 5'-triphosphate)-loaded (active) Ras. Rau homodimerized and was found in large-molecular weight complexes. Deletion of rau in flies decreased the differentiation of retinal wrapping glia and induced a rough eye phenotype, similar to that seen in alterations of Ras signaling. Further, the expression of sprouty was repressed and that of rau was increased by the COUP transcription factor Seven-up in the presence of weak, but not constitutive, activation of FGFR. Together, our findings reveal another regulatory mechanism that controls the intensity of RTK signaling in the developing neural network in the Drosophila eye.