Horizontal basal cells self-govern their neurogenic potential during injury-induced regeneration of the olfactory epithelium

TrajectoryGeometry suggests cell fate decisions can involve branches rather than bifurcations

Differentiation of multipotential progenitor cells is a key process in the development of any multi-cellular organism and often continues throughout its life. It is often assumed that a bi-potential progenitor develops along a (relatively) straight trajectory until it reaches a decision point where the trajectory bifurcates. At this point one of two directions is chosen, each direction representing the unfolding of a new transcriptional programme. However, we have lacked quantitative means for testing this model. Accordingly, we have developed the R package TrajectoryGeometry. Applying this to published data we find several examples where, rather than bifurcate, developmental pathways branch. That is, the bipotential progenitor develops along a relatively straight trajectory leading to one of its potential fates. A second relatively straight trajectory branches off from this towards the other potential fate. In this sense only cells that branch off to follow the second trajectory make a 'decision'. Our methods give precise descriptions of the genes and cellular pathways involved in these trajectories. We speculate that branching may be the more common behaviour and may have advantages from a control-theoretic viewpoint.

An in vitro model of acute horizontal basal cell activation reveals gene regulatory networks underlying the nascent activation phase

While horizontal basal cells (HBCs) make minor contributions to olfactory epithelium (OE) regeneration during homeostatic conditions, they possess a potent, latent capacity to activate and subsequently regenerate the OE following severe injury. Activation requires, and is mediated by, the downregulation of the transcription factor (TF) TP63. In this paper, we describe the cellular processes that drive the nascent stages of HBC activation. The compound phorbol 12-myristate 13-acetate (PMA) induces a rapid loss in TP63 protein and rapid enrichment of HOPX and the nuclear translocation of RELA, previously identified as components of HBC activation. Using bulk RNA sequencing (RNA-seq), we find that PMA-treated HBCs pass through various stages of activation identifiable by transcriptional regulatory signatures that mimic stages identified in vivo. These temporal stages are associated with varying degrees of engraftment and differentiation potential in transplantation assays. Together, these data show that our in vitro HBC activation system models physiologically relevant features of in vivo HBC activation and identifies new candidates for mechanistic testing.

Spatiotemporal dynamics exhibited by horizontal basal cells reveal a pro-neurogenic pathway during injury-induced olfactory epithelium regeneration

Horizontal basal cells (HBCs) mediate olfactory epithelium (OE) regeneration following severe tissue injury. The dynamism of the post-injury environment is well illustrated by in silico modeling of RNA sequencing data that demonstrate an evolving HBC transcriptome. Unfortunately, spatiotemporally dynamic processes occurring within HBCs in situ remain poorly understood. Here, we show that HBCs at 24 h post-OE injury spatially redistribute a constellation of proteins, which, in turn, helped to nominate Rac1 as a regulator of HBC differentiation during OE regeneration. Using our primary culture model to activate HBCs pharmacologically, we demonstrate that concurrent Rac1 inhibition attenuates HBC differentiation potential. This in vitro functional impairment manifested in vivo as decreased HBC differentiation into olfactory sensory neurons following HBC-specific Rac1 conditional knockout. Taken together, our data potentiate the design of hyposmia-alleviating therapies and highlight aspects of in situ HBC spatiotemporal dynamics that deserve further investigation.