The power of genetic screening: identifying genes that alter nervous system shape in Drosophila

There is no simple way to make a brain, even in a creature as small as a fruit fly. As an embryonic fly develops into adulthood, its central nervous system (CNS) expands almost 100-fold in mass. Neuronal, glial, immune, and vascular cells—in both the CNS and the peripheral nervous system (PNS)—must work in harmony to build the structures responsible for controlling movement and behavior. Since structure dictates function, the size and shape of the CNS must be tightly regulated, but the genes and pathways involved in the process have yet to be fully described.

In a recent study published in the September issue of G3: Genes|Genomes|Genetics, Lacin et al. use the power of forward genetics in Drosophila larvae to identify genes controlling nervous system shape. Using the robust genetic manipulation toolkit available in Drosophila, they further identify a glial subtype-specific molecular profile that functionally subdivides glia along the peripheral-central axis.

Their screen used the classic mutagenesis agent ethyl methanesulfonate (EMS) to randomly introduce mutations, generating more than 12,000 mutant lines that carried mutations specifically on the second chromosome. The authors screened for larval mutants with dramatically altered CNS shapes, sorting them into three categories: widened, elongated, or misshapen. Through a combination of genetic mapping, complementation analysis, and whole genome sequencing, they identified 50 mutant alleles across 17 genes that encode transcription factors, enzymes, signaling receptors, tumor suppressors, and basement membrane proteins.

Four of the mutant alleles were found in the senseless-2 (sens-2) gene, which encodes a zinc-finger domain transcription factor; these alleles caused massive elongation of the ventral nerve cord (the Drosophila equivalent to the spinal cord) that manifested very early in the first-instar larvae (see Figure 1). To understand the cellular basis for the mutant sens-2 CNS elongation phenotype, the authors generated an antibody against the Sens-2 protein and found it localized to most glia on peripheral nerves—but not in any CNS glial cells.

To determine whether sens-2’s role in determining ventral nerve cord length was specific to its presence in peripheral glia, the authors selectively knocked down its expression in those cells using the Gal4-UAS system. They found that sens-2 expression in peripheral glia is necessary to control CNS structure, and loss in those cells accounted for the observed elongation phenotype. Restoration of sens-2 expression rescued the elongation phenotype.

Lacin et al. were able to establish sens-2 as a marker distinguishing specific glial subtypes along the CNS-PNS axis with a profound impact on gross nervous system structure. In the future, the authors aim to investigate transcriptional targets of sens-2, which could help illuminate mechanisms governing glial development and differentiation in the PNS.

In recent years, the use of expensive -omics technologies to discover cellular heterogeneity at scale has become quite popular in neuroscience research, and the genes identified in these studies need validation and characterization. Here, Lacin et al. present a powerful demonstration that classical genetic studies in invertebrate model systems are still effective at powering neurogenetics and cellular heterogeneity research—at a fraction of the cost.