UCLA

Mutant mice with a deletion of Reelin (Reln), or both lipoprotein receptors, or Disabled-1 (Dab1) exhibit neuronal positioning errors, heat hypersensitivity and mechanical insensitivity. Despite the extensive nociceptive abnormalities, anatomical alterations in the lumbar dorsal horn neurons that participate in Reelin signaling were not yet identified. This thesis will identify these Dab1 and Reelin dorsal horn neurons and characterize their positioning errors associated with the nociceptive abnormalities in Reelin-pathway mutants. In the first study, we found that 70% of Dab1-labeled laminae I-II neurons were glutamatergic as they co-expressed the transcription factor Lmx1b. Dab1-Lmx1b neurons were increased within the IB4 layer and reduced within the lateral reticulated area and lateral spinal nucleus (LSN) of Reln-/- versus Reln+/+ mice. Additionally, Dab1-Lmx1b neurons participated in nociceptive circuits as they expressed Fos following noxious thermal or mechanical stimulation. Importantly, we determined that mispositioned Dab1 neurons that co-expressed Neurokinin-1-receptors contribute specifically to the heat hypersensitivity of dab1-/- mice. The second study asked whether the loss of Reelin laminae I-II neurons contributes to the mechanical insensitivity of dab1-/- mice. We found that 83% of Reelin laminae I-II neurons co-expressed Lmx1b. Although Reelin-Lmx1b and Dab1-Lmx1b neurons were independent populations, together they comprised 37% of laminae I-II glutamatergic neurons. Reelin-Lmx1b neurons sustained similar positioning errors to the Dab1-Lmx1b neurons, that is, with more of these neurons within the IB4 layer and less in the lateral reticulated area and LSN of dab1-/- compared to dab1+/+ mice. The area of laminae I-IIouter was reduced in both Reln and dab1 mutants, but the IB4 layer did not differ between genotypes. We examined the migratory patterns of Reelin and Dab1 neurons and found that they arose from both the early-born (dI5) and late-born (dILB) Lmx1b-expressing populations during embryonic development. When Dab1 was absent, the migration of Reelin and Reelin-Lmx1b neurons were relatively normal whereas without Reelin, Dab1 and Dab1-Lmx1b neurons exhibited clear migratory errors. In combination, we determined that the neuroanatomical abnormalities in Reln-/- and dab1-/- dorsal horn are due to the disruption of the Reelin-Dab1-signaling pathway, and these positioning errors contribute to the nociceptive alterations displayed by the mutant mice.