Reelin differentially shapes dendrite morphology of medial entorhinal cortical ocean and island cells

The function of medial entorhinal cortex layer II (MECII) excitatory neurons has been recently explored. MECII dysfunction underlies deficits in spatial navigation and working memory. MECII neurons comprise two major excitatory neuronal populations, pyramidal island and stellate ocean cells, in addition to the inhibitory interneurons. Ocean cells express reelin and surround clusters of island cells that lack reelin expression. The influence of reelin expression by ocean cells and interneurons on their own morphological differentiation and that of MECII island cells has remained unknown. To address this, we used a conditional reelin knockout (Reln^cKO) mouse to induce reelin deficiency postnatally in vitro and in vivo. Reelin deficiency caused dendritic hypertrophy of ocean cells, interneurons and only proximal dendritic compartments of island cells. Ca²⁺ recording showed that both cell types exhibited an elevation of calcium frequencies in Reln^cKO, indicating that the hypertrophic effect is related to excessive Ca²⁺ signalling. Moreover, pharmacological receptor blockade in Reln^cKO mouse revealed malfunctioning of GABA_B, NMDA and AMPA receptors. Collectively, this study emphasizes the significance of reelin in neuronal growth, and its absence results in dendrite hypertrophy of MECII neurons.