Maintenance of neuronal identity in C. elegans and beyond: Lessons from transcription and chromatin factors

Abstract

Neurons are remarkably long-lived, non-dividing cells that must maintain their functional features (e.g., electrical properties, chemical signaling) for extended periods of time - decades in humans. How neurons accomplish this incredible feat is poorly understood. Here, we review recent advances, primarily in the nematode C. elegans, that have enhanced our understanding of the molecular mechanisms that enable post-mitotic neurons to maintain their functionality across different life stages. We begin with "terminal selectors" - transcription factors necessary for the establishment and maintenance of neuronal identity. We highlight new findings on five terminal selectors (CHE-1 [Glass], UNC-3 [Collier/Ebf1-4], LIN-39 [Scr/Dfd/Hox4-5], UNC-86 [Acj6/Brn3a-c], AST-1 [Etv1/ER81]) from different transcription factor families (ZNF, COE, HOX, POU, ETS). We compare the functions of these factors in specific neuron types of C. elegans with the actions of their orthologs in other invertebrate (D. melanogaster) and vertebrate (M. musculus) systems, highlighting remarkable functional conservation. Finally, we reflect on recent findings implicating chromatin-modifying proteins, such as histone methyltransferases and Polycomb proteins, in the control of neuronal terminal identity. Altogether, these new studies on transcription factors and chromatin modifiers not only shed light on the fundamental problem of neuronal identity maintenance, but also outline mechanistic principles of gene regulation that may operate in other long-lived, post-mitotic cell types.

Keywords: C. elegans; Chromatin-modifying proteins; D. melanogaster; Maintenance; Mus musculus; Neuronal identity; Terminal selectors; Transcription factors.

Figure 1.. Terminal selectors of neuronal identity.

Terminal selector transcription factors can operate alone, or in combinations, to establish and maintain neuronal terminal identity. They directly regulate expression of terminal identity genes and intermediary transcription factors through binding at the cis-regulatory regions of these genes. Continuous terminal selector expression, required for neuronal identity maintenance, is ensured through direct positive autoregulation.

Figure 2.. Terminal selector CHE-1 controls ASE identity.

A. Schematic of ASE chemosensory neurons in the C. elegans head. B. CHE-1 directly promotes terminal identity gene and intermediary transcription factor expression through binding at its cognate site in the cis-regulatory regions of these genes. Continuous CHE-1 expression is ensured through autoregulation. C. ASE subtype diversification via a bistable feedback loop. CHE-1 specifies both ASEL and ASER subtype-specific fates through activation of a downstream bistable feedback loop. Initiation of ASER fate by the transcription factor COG-1 (later maintained by FOZI-1) is antagonized by DIE-1 (via the miRNA lsy-6) in the ASEL.

Figure 3:. UNC-3 is a terminal selector of cholinergic MN identity.

A. Five (colored) cholinergic MN subtypes used for C. elegans locomotion. The 50 cells that comprise these subtypes intermingle along the ventral nerve cord. B. UNC-3/Ebf activates, via direct binding at its cognate site (COE motif) in cis-regulatory regions, a diverse suite of genes essential for cholinergic MN function. Schematic modified from C. UNC-3 broadly activates both shared and subtype-specific terminal identity genes for cholinergic MNs. Subtype diversity is established and maintained by various subtype-specific repressor proteins (R1, R2) that antagonize UNC-3’s ability to activate terminal identity genes.

Figure 4:. Hox factors collaborate with terminal selectors to control MN identity along the rostro-caudal axis in C. elegans and mice.

A. Four of the six C. elegans Hox genes are expressed continuously from development through adulthood in specific groups of MNs, depending on their positions in the ventral nerve cord. Hox factors thus intersect with UNC-3 in a region-specific manner. B. Distinct Hox genes and UNC-3 collaborate in a region-specific manner to establish and maintain the spatial identities of MNs along the rostro-caudal axis of the C. elegans nerve cord. C. Hox factors and UNC-3 collaborate to regulate terminal identity genes in a feed-forward-loop (FFL): both Hox and UNC-3 directly activate terminal identity genes; Hox directly activates UNC-3 expression; Hox expression is amplified by positive autoregulation and limited by negative feedback from UNC-3. This interaction may serve to buffer terminal identity gene expression against fluctuations in the levels of either activator. Panels A-C modified from D. Hoxc8 is required to maintain MN terminal identity in the mouse spinal cord. E. Hoxa5 is needed in the mouse brainstem to maintain neuronal terminal identity and function.