Cortical neurons gradually attain a post-mitotic state

Once generated, neurons are thought to permanently exit the cell cycle and become irreversibly differentiated. However, neither the precise point at which this post-mitotic state is attained nor the extent of its irreversibility is clearly defined. 

Here we report that newly born neurons from the upper layers of the mouse cortex, despite initiating axon and dendrite elongation, continue to drive gene expression from the neural progenitor tubulin α1 promoter (Tα1p). These observations suggest an ambiguous post-mitotic neuronal state. 

Whole transcriptome analysis of sorted upper cortical neurons further revealed that neurons continue to express genes related to cell cycle progression long after mitotic exit until at least post-natal day 3 (P3). These genes are however down-regulated thereafter, associated with a concomitant up-regulation of tumor suppressors at P5. 

Interestingly, newly born neurons located in the cortical plate (CP) at embryonic day 18-19 (E18-E19) and P3 challenged with calcium influx are found in S/G2/M phases of the cell cycle, and still able to undergo division at E18-E19 but not at P3. 

At P5 however, calcium influx becomes neurotoxic and leads instead to neuronal loss. Our data delineate an unexpected flexibility of cell cycle control in early born neurons, and describe how neurons transit to a post-mitotic state.

Introduction

A longstanding orthodoxy in neuro-biology is that once formed, neurons never divide. Indeed, neurons are the quintessential 'post-mitotic' cell. Attempts to induce neurons to proliferate by either expressing oncogenes or by inactivating tumor suppressors are well documented in the literature, but have generally resulted in neuronal death instead. 

In addition, aberrant cell cycle re-entry precedes neuronal loss under various neuro-toxic conditions, and activation of cell cycle genes by neurons is elevated in certain neuro-degenerative disorders such as Alzheimer's disease. Together, these observations reinforce the notion that cell division is essentially incompatible with the survival of mature neurons. 

However, precisely when and how during its genesis a neuron attains this irreversible post-mitotic state is still poorly understood. The cell cycle machinery has a role during neuronal development. For instance, genes that regulate phases of the cell cycle also modulate neuronal migration, axon formation, and dendrite growth and branching. However, comparatively little is known about the timing that defines the terminally differentiated state in neurons, or in other words, when a neuron truly becomes a post-mitotic cell. 

It has been proposed that during the acquisition of morphological features that define a neuron (i.e. axon and dendrites formation), proteins associated with cell cycle regulation participate in these morphological changes as part of the developmental machinery distinct from the cell cycle control. 

Cell cycle exit and terminal differentiation generally are coordinated, although in some context they are separable, suggesting that cell cycle exit and differentiation could be different events. For instance, in Drosophila it was shown that neurons from retina could continue cycling and undergo mitosis while maintaining characteristics of terminal differentiation, such as axon formation. It is also possible, however, that newly born neurons are in an intermediate state between progenitors and neurons and that their post-mitotic nature is still not fully defined.

During neurogenesis in the cerebral cortex, neural progenitors divide to produce neurons in the ventricular zone (VZ) and sub-ventricular zone, which undergo further differentiation and migrate to their final destinations in the cortical plate in a systematic fashion. 

A tight coordination between these events is mediated by calcium signaling, which modulates various aspects of neurogenesis, including neural induction, migration, formation of neural circuits and neurotransmission. Importantly, calcium oscillations also affect the G1/S transition of the cell cycle and thereby regulate progenitor proliferation and differentiation. 

Based on these findings and with the aim of further investigating the functions of calcium signaling in early-born neurons, we used in situ live imaging, and directly characterized the effects of perturbing calcium dynamics in the developing neocortex. Here, we show unexpected cell cycle control flexibility in early born migrating neurons in the CP, which might depend on calcium buffering. These results suggest that neurons transit to a post-mitotic stage gradually.

https://www.nature.com/articles/cr201676