UC San Diego

Entorhinal cortex (EC) cooperates with the hippocampal formation (HPF) in episodic memory encoding and spatial navigation, and it is one of the first human brain regions to degenerate in Alzheimer’s disease (AD). EC layer II (L2) contains unique projection neurons, including Reln+ stellate neurons (SNs) and clusters of Calb1+ pyramidal neurons (PNs), that are selectively vulnerable to AD and remain poorly understood. Therefore, we aim to better characterize these neuron types with a focus on neurogenic trajectories and gene expression patterns.Chapter 1 (Introduction) provides key background on the following: cortex organization; EC structure, function, and neuron types; classic “inside out” neocortical development versus current understanding of EC development; EC in disease; and transcriptional profiling of EC cells. Chapter 2 is an original study into developmental timelines and trajectories of EC principal neurons, in which we address the hypothesis that some EC neurons undergo specialized migration and differentiation. Through thymidine analog birthdating, we demonstrate that murine EC diverges from classic “inside out” neocortical neurogenesis with a population of early-born L2 SNs (but not PNs) that develops concurrently with deeper layers. We also dismiss the possibility precocious SNs may derive directly from early, potentially multipotent neuroepithelial cells (NECs) or radial glial cells (RGCs) by lineage tracing both PNs and SNs from neuron-fated intermediate progenitor cells (IPCs) despite their differing birthdates. Chapter 3 details preliminary work in establishing a comparative atlas of EC neurons with the aim of better understanding (1) intrinsic properties that may influence differential disease susceptibility across neuron types and (2) functional significance of an early-born SN population. We perform single nucleus RNA sequencing (snRNAseq) on neuronally-enriched nuclei from murine and human EC, annotating clusters based on marker gene expression and laminar positioning. In a new collaboration with the Scheuermann Lab at J. Craig Venter Institute, we determine minimal marker genes that define each cell cluster, elucidating several new PN and SN markers. We also begin to probe differentially-enriched biological pathways between neuron types. Finally, Chapter 4 provides a Summary and Future Work, including proposed experiments for understanding early SN development and plans for ongoing snRNAseq analysis.