Abnormal laminar position and dendrite development of interneurons in the reeler forebrain.

The majority of cortical and hippocampal interneurons originate in the subcortical telencephalon and migrate tangentially into pallial regions before settling in various cortical layers. The molecular cues that regulate final positioning of specific interneurons in cortical structures have not yet been identified. The positioning of radially migrating principal neurons of the cortex and hippocampus depends upon Reelin, an extracellular protein expressed near the pial surface during embryonic development that is absent in reeler mutant mice. To determine whether the layer specification of interneurons, like that of principal neurons, requires Reelin, we crossed reeler with transgenic mice that contain Green Fluorescent Protein (GFP)-expressing Inhibitory Neurons (GINs). These neurons express basal forebrain markers Dlx1/2 in normal and reeler mice. In normal mice, GINs express Reelin and are localized to specific layers of the cortex and hippocampus. In reeler mutant mice, we show that GINs migrate normally into the pallium, but fail to acquire proper layer position. Double labeling experiments indicate that the neurochemical profile of these interneurons is not generally altered in reeler mice. However, the extension of their cellular processes is abnormal. Quantitative analysis of GINs in the cortex revealed that they are hypertrophic, bearing longer neuritic branches than normal. Thus, the lack of Reelin signaling results in abnormal positioning and altered morphology of forebrain interneurons.

Reelin promotes hippocampal dendrite development through the VLDLR/ApoER2-Dab1 pathway.

Reelin is a secreted glycoprotein that regulates neuronal positioning in cortical brain structures through the VLDLR and ApoER2 receptors and the adaptor protein Dab1. In addition to cellular disorganization, dendrite abnormalities are present in the brain of reeler mice lacking Reelin. It is unclear whether these defects are due primarily to cellular ectopia or the absence of Reelin. Here we examined dendrite development in the hippocampus of normal and mutant mice and in dissociated cultures. We found that dendrite complexity is severely reduced in homozygous mice deficient in Reelin signaling both in vivo and in vitro, and it is also reduced in heterozygous mice in the absence of cellular ectopia. Addition of Reelin interfering antibodies, receptor antagonists, and Dab1 phosphorylation inhibitors prevented dendrite outgrowth from normal neurons, whereas addition of recombinant Reelin rescued the deficit in reeler cultures. Thus, the same signaling pathway controls both neuronal migration and dendrite maturation.

Interaction of reelin signaling and Lis1 in brain development.

Loss-of-function mutations in RELN (encoding reelin) or PAFAH1B1 (encoding LIS1) cause lissencephaly, a human neuronal migration disorder. In the mouse, homozygous mutations in Reln result in the reeler phenotype, characterized by ataxia and disrupted cortical layers. Pafah1b1(+/-) mice have hippocampal layering defects, whereas homozygous mutants are embryonic lethal. Reln encodes an extracellular protein that regulates layer formation by interacting with VLDLR and ApoER2 (Lrp8) receptors, thereby phosphorylating the Dab1 signaling molecule. Lis1 associates with microtubules and modulates neuronal migration. We investigated interactions between the reelin signaling pathway and Lis1 in brain development. Compound mutant mice with disruptions in the Reln pathway and heterozygous Pafah1b1 mutations had a higher incidence of hydrocephalus and enhanced cortical and hippocampal layering defects. Dab1 and Lis1 bound in a reelin-induced phosphorylation-dependent manner. These data indicate genetic and biochemical interaction between the reelin signaling pathway and Lis1.