Mapping Projectome Heterogeneity of Subiculum Neuron Cell Types.

The subiculum (SUB) is the main output structure of the hippocampus, influencing diverse behaviors through its widespread cortical and subcortical connections. Our previous work creating the mouse Hippocampus Gene Expression Atlas (HGEA) identified four genetically distinct cellular layers across five columnar domains in the SUB, with gene expression boundaries corresponding to distinct connectivity patterns and brain-wide networks involved in spatial navigation, social behavior, and neuroendocrine regulation. Using the Digital Brain Mouse Projectome Atlas (MPA) tool, we conducted 'virtual tract-tracing' to assess whether connectivity patterns of single-neuron 3D reconstructions aligned with HGEA-defined SUB cell types. We classified 689 SUB projection neurons into 12 HGEA cell-type groups based on their laminar and columnar distributions, whose spatial organization recapitulated HGEA-defined 3D boundaries. Using this population sample, we performed a SUB cell-type census, characterized neuronal heterogeneity and projection prevalence, identified common and rare connectivity motifs and axonal collateralization patterns, and defined distinct projection themes for each SUB cell type. Together, this analysis integrates single-neuron and population-level data to advance understanding of SUB cell type organization and its contributions to brain-wide networks regulating diverse behaviors.