Data for: Early radial positional information in the cochlea is optimized by a precise linear BMP gradient and enhanced by SOX2

Positional information encoded in signaling molecules is essential for
early patterning in the prosensory domain of the developing cochlea. The
sensory epithelium, the organ of Corti, contains an exquisite repeating
pattern of 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 up to the medial ~75%
of the PSD from the pSMAD1/5/9 peak in the lateral edge during E12.5 and
E13.5. This is a surprising activity readout for a diffusive BMP4 ligand
secreted from a tightly constrained lateral region 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.
Furthermore, this is unique to the cochlear epithelium as the pSMAD1/5/9
gradient is exponential in the surrounding mesenchyme. 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. Last, through joint decoding maps of pSMAD1/5/9 and SOX2, we
see that there is a high-fidelity mapping between signaling activity and
position in the regions that will become Kölliker's organ and the
organ of Corti. Mapping is ambiguous in the prosensory domain precursory
to the outer sulcus. Altogether, this research provides new insights into
the precision of early morphogenetic patterning cues in the radial cochlea
prosensory domain.