Gradient of Wnt signaling facilitates Mef2 heterogeneity and limits commitment of the developmental muscle progenitor pool

Abstract

During skeletal muscle development, the timing and extent of lineage commitment towards differentiation must be coordinated to ensure proper tissue formation while preserving undifferentiated progenitors for adult stem cell function. This balance requires spatial and temporal regulation of cell fate and transcriptional regulators to act as key mediators of lineage progression. The transcription factor Mef2 is a key activator of myogenic differentiation across vertebrates and invertebrates. However, the mechanisms that spatially restrict Mef2 expression within the developing niche to limit muscle progenitor (MP) commitment towards differentiation remains less well characterized. Using the developmental flight muscle progenitor niche in Drosophila, a system that parallels vertebrate myogenesis, we investigated the transcriptional and spatial regulation of MP fate at a developmental timepoint when Mef2 expression begins to rise but precedes overt differentiation. We identified a spatially distinct subpopulation of MPs with low Mef2 expression and elevated Wnt/{beta}-catenin signaling. Within the broader MP pool, graded Wnt activity emerged as a key source of Mef2 heterogeneity: high Wnt activity led to strong repression of Mef2 via Armadillo/{beta}-catenin and TCF-dependent regulation. Moderate Wnt activity, meanwhile, not only repressed Mef2 expression but also sustained expression of zfh1, a conserved transcription factor linked to MP maintenance and adult muscle stem cell identity. Contrary to its well-established role in promoting myogenesis, Wnt/{beta}-catenin signaling in this early spatial context instead promotes a less committed state, leading to a subset of MPs with particular low Mef2 level. These findings highlight greater spatial complexity within the developing muscle progenitor niche than previously recognized, with potential implications for conserved strategies of muscle stem cell regulation across species.