Early precision of radial patterning of the mouse cochlea is achieved by a linear BMP signaling gradient and is further refined by SOX2

Positional information encoded in signaling molecules is essential for early patterning in the prosensory domain of the developing cochlea. The cochlea contains an exquisite repeating pattern of sensory hair cells and supporting cells. This requires precision in the morphogen signals that set the initial radial compartment boundaries, but this has not been investigated. To measure gradient formation and morphogenetic precision in developing cochlea, we developed a quantitative image analysis procedure measuring SOX2 and pSMAD1/5/9 profiles in mouse embryos at embryonic day (E)12.5, E13.5, and E14.5. Intriguingly, we found that the pSMAD1/5/9 profile forms a linear gradient in the medial [~]75% of the PSD during E12.5 and E13.5. This is a surprising activity readout for a diffusive BMP4 ligand secreted from a tightly constrained lateral region1,2 since morphogens typically form exponential or power-law gradient shapes. This is meaningful for gradient interpretation because while linear profiles offer the theoretically highest information content and distributed precision for patterning, a linear morphogen gradient has not yet been observed. In addition to the information-optimized linear profile, we found that while pSMAD1/5/9 is stable during this timeframe, an accompanying gradient of SOX2 shifts dynamically. Third, we see through joint decoding maps of pSMAD1/5/9 and SOX2 that there is a high-fidelity mapping between signaling activity and position in the regions soon to become Kollikers organ and the organ of Corti, where radial patterns are more intricate than lateral regions. Mapping is ambiguous in the prosensory domain precursory to the outer sulcus, where cell fates are uniform. Altogether, this research provides new insights into the precision of early morphogenetic patterning cues in the radial cochlea prosensory domain. Summary ParagraphThe organ of Corti is the precisely patterned group of cells in the cochlea responsible for transforming sound energy into our perception of hearing. Morphogenetic signals encoding positional information are crucial for the early stages of patterning along the developing cochleas radial axis. SOX2 and pSMAD1/5/9 are transcription factors that together serve as an integrative readout of morphogen activity during E12.5 to E14.5 in the developing mouse cochlea. However, the role of spatiotemporal precision in these signals is unknown. Here we show that pSMAD1/5/9 forms a linear profile to establish a domain spanning reference frame of positional information and that SOX2 further refines precision. We found that the pSMAD1/5/9 signal retains its linear shape across at least 24 h of development while SOX2 dynamically shifts. The stable linear pSMAD1/5/9 profile provides a global reference point of radial positional information, while the SOX2 profile improves local precision with steep slopes. Furthermore, a linear profile from a diffusive ligand is unexpected, suggesting unidentified mechanisms of BMP regulation unique to this system. A version of the source-sink model for creating a linear morphogen profile modified from its original formulation3 is explored in this system, enabling a tight fit between the BMP model and pSMAD1/5/9 data. We expect the methods and results shown here to be a starting point for increased precision in cochlear morphogen activity measurements to enable further modeling and experimental inquiry. This combination of quantitative mechanistic explanation for how signals form, along with quantitative interpretations of their decoding properties, revealing why they form a certain way, together form a potent basis for biological discovery and may even be applied to the design of synthetic systems.