Inversion of layer-specific cadherin expression profiles and maintenance of cytoarchitectonic areas in the allocortex of the reeler mutant mouse
Cadherins are calcium-depending cell adhesion proteins that play critical roles in brain morphogenesis and wiring. They provide an adhesive code for the development of cortical layers, due to their homophilic interactions and their restricted spatiotemporal expression patterns. In the adult organism, cadherins are involved in the maintenance and plasticity of neuronal circuits that play a role in learning. A well-known model for studying corticogenesis is the reeler mouse model. Numerous investigations of neocortical development suggest that, in the reeler mutant mouse, the lack of the protein Reelin results in cell-type and region-dependent changes of the neocortical layers. To investigate in detail how layer formation and regionalization is perturbed in the phylogenetically older archicortex of the adult reeler mutant mouse, we studied the expression of 11 different cadherins (Cdh4, Cdh7, Cdh8, Cdh11, Pcdh1, Pcdh7, Pcdh8, Pcdh9, Pcdh10, Pcdh17, and Pcdh19) and of the transcription factors ER81 and Cux2 by in situ hybridization in the (peri-)archicortex. All cadherins studied show a layer-specific expression in the (peri-)archicortex of the wildtype brain. In the archicortex of the reeler mutant, the cadherin-expressing cell layers are dispersed in the radial dimension, whereas in the periarchicortex the superficial and deep layers are inverted, both in the adult and during development. Possibly, this inversion relates to the histoarchitectural division of the reeler entorhinal cortex into an external and an internal zone. The regionalized, gradient-like expression of the cadherins is preserved in the reeler mutant mouse.