Planarian dorsoventral Netrins control a muscle midline signaling center and regulate blastema formation

Abstract

Integration of positional information across body axes is likely critical for whole-body regeneration to define the territories of missing tissue in three dimensions with fidelity. The body-wall musculature in planarians expresses patterning factors regulating the anteroposterior, dorsoventral, and mediolateral axes, but how this information coordinates is not fully understood. Using analysis of scRNAseq data, we identify pitx and islet transcription factors as regulators of a dorsal muscle midline signaling center expressing a the BMP/Activin decoy receptor bambi-2. Dorsal midline cells are comprised of longitudinal muscle and are required for blastema formation in fragments lacking a pre-existing midline. In laterally amputated animals regenerating an entirely new midline, bambi-2+ cells are initially formed at the dorsal-most region of the excised tissue fragment, then dynamically spread and ultimately reset to restore bilateral symmetry, rather than forming at the initial geometric mediolateral midpoint. We further identify a system of dorsoventral Netrin and Netrin receptor signals expressed from body-wall muscle that control midline identity and blastema morphology. Ventral and laterally expressed netrins -1, -4, and -5 signal via dorsally-enriched netrin repulsion receptors unc-5C, unc-5E, and dcc-2, which together limit mediolateral spread of bambi-2+ dorsal midline muscle and enable muscle targeting across the midline. Our results suggest a model in which ventral determinants dictate mediolateral information important for blastema patterning.