Reelin dorsal horn neurons co‐express Lmx1b and are mispositioned in disabled‐1 mutant mice
Mice missing either Reelin or Disabled‐1 (Dab1) exhibit dorsal horn neuronal positioning errors and display heat hypersensitivity and mechanical insensitivity. Reelin binds its receptors, Apolipoprotein E receptor 2 and Very‐low‐density lipoprotein receptor, leading to the recruitment and phosphorylation of Dab1 and activation of downstream pathways that regulate neuronal migration. Previously, we reported that 70% of Dab1 laminae I‐II neurons co‐expressed LIM homeobox transcription factor 1 beta (Lmx1b). Here we asked if Reelin‐expressing dorsal horn neurons co‐express Lmx1b, are mispositioned in dab1 mutants, and contribute to nociceptive abnormalities. About 90% of Reelin‐labeled neurons are Lmx1b‐positive in laminae I‐II, confirming that most Reelin and Dab1 neurons are glutamatergic. We determined that Reelin‐Lmx1b and Dab1‐Lmx1b dorsal horn neurons are separate populations, and together, comprise 37% of Lmx1b‐positive cells within and above the Isolectin B4 (IB4) layer in wild‐type mice. Compared to wild‐type mice, dab1 mutants have a reduced area of laminae I‐II outer (above the IB4 layer), more Reelin‐Lmx1b neurons within the IB4 layer, and fewer Reelin‐Lmx1b neurons within the lateral reticulated area of lamina V and lateral spinal nucleus. Interestingly, both Reelin‐ and Dab1‐labeled dorsal horn neurons sustain similar positioning errors in mutant mice. After noxious thermal and mechanical stimulation, Reelin, Lmx1b, and Reelin‐Lmx1b neurons expressed Fos in laminae I‐II and the lateral reticulated area in wild‐type mice, and therefore participate in nociceptive circuits. Together, our data suggest that disruption of the Reelin‐signaling pathway results in neuroanatomical abnormalities that contribute to the nociceptive changes that characterize these mutant mice.