The role of MAB-5, a Hox transcription factor, in establishing and maintaining neuronal identity and connectivity

/Abstract A nervous system requires a wide diversity of cell types for an organism to properly function. This diversity must be both established during development and maintained over the life of an animal. Neuronal cell type identity is characterized by distinct patterns of gene expression, morphology, connectivity and function. The neurons derived from the two Q neuroblasts, QL and QR, in the model organism C. elegans provide a genetically and experimentally accessible model for elucidating mechanisms of generating and maintaining neuronal cell fate and connectivity. The Q neuroblasts ultimately generate three pairs of neurons through identical differentiation patterns, with QL producing three neurons on the left side of the animal (named PQR, PVM and SDQL), and QR producing three neurons on the right (named AQR, AVM, and SDQR). From single- cell RNA sequencing experiments, we have demonstrated that the left and right neurons in each of the three pairs are transcriptionally distinct, despite their similar lineages. Electron microscopy reconstructions show that although SDQL and SDQR contact similar sets of cells, they synapse onto distinct postsynaptic partners. This proposal is aimed at understanding the molecular mechanisms underlying the differences between the left and right members of each pair of Q-derived neurons (AQR/PQR, AVM/PVM and SDQR/SDQL). Previous studies and our preliminary data have shown that the Hox transcription factor MAB-5 (ANTP in Drosophila/Hox6-8 in vertebrates) is expressed in QL descendent but not QR descendent neurons. Using endogenous reporters of neuropeptide expression, we have validated transcriptional differences between SDQL and SDQR. We have shown that mab-5, the gene encoding MAB-5, is required for the both the initial differential expression of the neuropeptide nlp-64 between SDQR and SDQL and for maintaining this differential expression after development. We hypothesize that MAB-5 is the major transcription factor driving transcriptional differences between the left and right members of all three Q-derived neuron pairs. We also hypothesize that MAB-5 regulates the differences in the postsynaptic partners between SDQR and SDQL. In Aim 1, we will examine the effect of mab-5 deletion and ectopic expression in QR-derived neurons on downstream gene expression, using both endogenous reporters throughout development and single-cell RNA sequencing to assess transcriptome wide differences in mab-5 loss of function mutants. In Aim 2, we will use the auxin-inducible degron system to degrade MAB-5 after initial development and use single-cell RNA sequencing to detect transcriptome wide changes. This will test our hypothesis that MAB-5 is required continuously to maintain cell identity. In Aim 3, we will label SDQ synapses and test the hypothesis that loss of MAB-5 will cause SDQL to acquire SDQR-like connectivity and ectopic expression of MAB-5 will cause SDQR to acquire SDQL-like connectivity. Our proposed work will broaden our understanding of the generation and maintenance of cell diversity and synaptic specificity. Project Number: 1R15NS145107-01A1 | Fiscal Year: 2026 | NIH Institute/Center: National Institute of Neurological Disorders and Stroke (NINDS) | Principal Investigator: SETH TAYLOR | Institution: BRIGHAM YOUNG UNIVERSITY, PROVO, UT | Award Amount: $550,159 | Activity Code: R15 | Study Section: Neurogenesis and Cell Fate Study Section[NCF] View on NIH RePORTER: https://reporter.nih.gov/project-details/11360342