Reelin Promotes Neuronal Orientation and Dendritogenesis during Preplate Splitting

Abstract

The secreted ligand Reelin is thought to regulate the translocation and positioning of prospective layer 6 (L6) neurons into the preplate, a plexus of neurons overlying the ventricular zone. We examined wild type and Reelin-deficient cortices and found that L6 neurons were equivalently positioned beneath the pia during the period of preplate splitting and initial cortical plate (CP) formation. The absence of detectable L6 ectopia in "reeler" cortices at this developmental time point indicates that Reelin-signaling might not regulate L6 neuron migration or gross positioning during preplate splitting. To explore the acute response of L6 neurons to Reelin, subpial injections of Reelin were made into Reelin-deficient explants. Reelin injection caused L6 neurons to orient their nuclei and polarize their Golgi toward the pia while initiating exuberant dendritic (MAP2+) outgrowth within 4 h. This rapid Reelin-dependent neuronal orientation and alignment created CP-like histology without any significant change in the mean position of the population of L6 neurons. Conversely, subplate cells and chondroitin sulfate proteoglycan immunoreactivity were found at significantly deeper positions from the pial surface after injection, suggesting that Reelin partially rescues preplate splitting within 4 h. Thus, Reelin has a direct role in promoting rapid morphological differentation and orientation of L6 neurons during preplate splitting.

Ectopic Reelin Induces Neuronal Aggregation with a Normal Birthdate-Dependent “Inside-Out” Alignment in the Developing Neocortex

“…The important finding in the transgenic mouse study was that, when the transgene was bred into reeler mice, the initial steps in the cortical development, including preplate splitting, were rescued (Magdaleno et al, 2002). Other groups also reported finding that ectopic sources of Reelin were capable of rescuing at least the early stages of cortical development Yoshida et al, 2006;Nichols and Olson, 2010). These results could be predicted based on the our findings in the present study, because the ascending processes of future layer VI neurons could be affected by the ectopic Reelin, resulting in their translocation through the subplate to split the preplate (Cooper, 2008).…”

<em>Ex utero</em> Electroporation and Whole Hemisphere Explants: A Simple Experimental Method for Studies of Early Cortical Development

“…This mouse strain contains a transgene that reports immature neurons of the excitatory cortical lineage 7,29,30 . Whole hemisphere cortical explants were prepared at E13, at a time point prior to preplate splitting in the dorsal neocortex (Figure 2A).…”

“…Prospective layer 6 cortical neurons (i.e. the first cortical neurons born in the VZ) then orient their somata in a stereotypical pattern and coalesce into a distinct layer within the PP 7 . These events split the preplate into a superficial marginal (future cortical layer 1) and a deep zone, the latter composed of subplate cells (transient cortical layer 7).…”

“…the first cortical neurons born in the VZ) then orient their somata in a stereotypical pattern and coalesce into a distinct layer within the PP 7 . These events split the preplate into a superficial marginal (future cortical layer 1) and a deep zone, the latter composed of subplate cells (transient cortical layer 7). This process, termed preplate splitting, is a foundational event in the future growth of the cerebral cortex 8 .…”

“…Throughout this period the developing neuron is surrounded by the appropriate ECM and cell types that would confront the corresponding cell in vivo. This system has already proved valuable in deciphering the cellular events that underlie ethanol toxicity 26 , layer 6 formation and preplate splitting 7,25 . …”