The SLC36 transporter Pathetic is required for neural stem cell proliferation and for brain growth under nutrition restriction

Abstract: Background: Drosophila neuroblasts (NBs) are neural stem cells whose maintenance relies on Notch activity. NBs proliferate throughout larval stages to generate a large number of adult neurons. Their proliferation is protected under conditions of nutrition restriction but the mechanisms responsible are not fully understood. As amino acid transporters (Solute Carrier transporters, SLCs), such as SLC36, have important roles in coupling nutrition inputs to growth pathways, they may have a role in this process. For example, an SLC36 family transporter Pathetic (Path) that supports body size and neural dendrite growth in Drosophila, was identified as a putative Notch target in genome-wide studies. However, its role in sustaining stem cell proliferation and maintenance has not been investigated. This study aimed to investigate the function of Path in the larval NBs and to determine whether it is involved in protecting them from nutrient deprivation. Methods: The expression and regulation of Path in the Drosophila larval brain was analysed using a GFP knock-in allele and reporter genes containing putative Notch regulated enhancers. Path function in NB proliferation and overall brain growth was investigated under different nutrition conditions by depleting it from specific cell types in the CNS, using mitotic recombination to generate mutant clones or by directed RNA-interference. Results: Path is expressed in both NBs and glial cells in the Drosophila CNS. In NBs, path is directly targeted by Notch signalling via Su(H) binding at an intronic enhancer, PathNRE. This enhancer is responsive to Notch regulation both in cell lines and in vivo. Loss of path in neural stem cells delayed proliferation, consistent with it having a role in NB maintenance. Expression from pathNRE was compromised in conditions of amino acid deprivation although other Notch regulated enhancers are unaffected. However, NB-expressed Path was not required for brain sparing under amino acid deprivation. Instead, it appears that Path is important in glial cells to help protect brain growth under conditions of nutrient restriction. Conclusions: We identify a novel Notch target gene path that is required in NBs for neural stem cell proliferation, while in glia it protects brain growth under nutrition restriction