Genome-wide analysis of the bHLH gene family in planarians identifies factors required for adult neurogenesis and neuronal regeneration

Abstract

In contrast to most well-studied model organisms, planarians have a remarkable ability to completely regenerate a functional nervous system from a pluripotent stem cell population. Thus, planarians provide a powerful model to identify genes required for adult neurogenesis in vivo. We analyzed the basic helix-loop-helix (bHLH) family of transcription factors, many of which are crucial for nervous system development and have been implicated in human diseases. However, their potential roles in adult neurogenesis or central nervous system (CNS) function are not well understood. We identified 44 planarian bHLH homologs, determined their patterns of expression in the animal and assessed their functions using RNAi. We found nine bHLHs expressed in stem cells and neurons that are required for CNS regeneration. Our analyses revealed that homologs of coe, hes (hesl-3) and sim label progenitors in intact planarians, and following amputation we observed an enrichment of coe + and sim + progenitors near the wound site. RNAi knockdown of coe, hesl-3 or sim led to defects in CNS regeneration, including failure of the cephalic ganglia to properly pattern and a loss of expression of distinct neuronal subtype markers. Together, these data indicate that coe, hesl-3 and sim label neural progenitor cells, which serve to generate new neurons in uninjured or regenerating animals. Our study demonstrates that this model will be useful to investigate how stem cells interpret and respond to genetic and environmental cues in the CNS and to examine the role of bHLH transcription factors in adult tissue regeneration.

Wnt/Notum spatial feedback inhibition controls neoblast differentiation to regulate reversible growth of the planarian brain

“…Neoblasts are a heterogeneous population that includes both pluripotent cells and distinct subpopulations specified for production of multiple differentiated tissues (Adler et al, 2014;Cowles et al, 2013;Currie and Pearson, 2013;Forsthoefel et al, 2012;Lapan and Reddien, 2012;Marz et al, 2013;Scimone et al, 2011Scimone et al, , 2014avan Wolfswinkel et al, 2014;Vásquez-Doorman and Petersen, 2014;Vogg et al, 2014;Wenemoser et al, 2012). We hypothesized that notum and wnt11-6 might regulate brain cell production by controlling numbers of brain cell progenitors marked by expression of lineage-specific transcription factors.…”

“…We examined two additional neoblast subpopulations that contribute to the brain, lhx1/5-1 Pearson, 2013) (Fig. S10C,D) and coe + smedwi-1 + cells that form a variety of neuronal subtypes [including cpp-1 + , chat + , gad + , tph + , th + and tbh + neurons (Cowles et al, 2013)]. In all cases, notum (RNAi) animals had fewer brain progenitors and wnt11-6(RNAi) animals had more brain progenitors (Fig.…”

“…Growth from feeding or epimorphic regeneration depends on parenchymal cells termed neoblasts that are the only proliferating cells in adult planarians (Baguñà, 1976;Newmark and Sánchez Alvarado, 2000;Reddien et al, 2005b). Neoblasts are a heterogeneous population containing both adult pluripotent stem cells (Wagner et al, 2011) and more lineage-restricted dividing cells (Adler et al, 2014;Cowles et al, 2013;Currie and Pearson, 2013;Forsthoefel et al, 2012;Lapan and Reddien, 2012;Marz et al, 2013;Scimone et al, 2014aScimone et al, ,b, 2011van Wolfswinkel et al, 2014;Vásquez-Doorman and Petersen, 2014;Vogg et al, 2014;Wenemoser et al, 2012). Tissue removal results in proliferative activation of neoblasts thought to be necessary for production of missing tissue types within the regeneration blastema.…”

The nervous system of Xenacoelomorpha: a genomic perspective

“…bHLHs are regulators of nervous system development; they control aspects such as neural fate commitment, cellular subtype specification, migratory behaviour and axonal guidance (Bertrand et al, 2002;Guillemot, 2007). Moreover, recent studies have detected bHLHs expressed in stem cells and neuronal precursors required for CNS regeneration (Cowles et al, 2013). These neuronal precursor proteins possess a highly conserved bHLH DNA-binding domain which, as the name suggests, is composed of a basic region for DNA binding and two α-helices, interrupted by a variable loop region.…”

teashirt is required for head-versus-tail regeneration polarity in planarians

“…Planarian regeneration requires a population of proliferative cells called neoblasts, which includes pluripotent stem cells (Wagner et al, 2011). Neoblasts can specialize to produce lineage-specific progenitors for the regeneration of missing target tissues (Lapan and Reddien, 2011;Scimone et al, 2011Scimone et al, , 2014aWenemoser et al, 2012;Cowles et al, 2013;Currie and Pearson, 2013;Reddien, 2013;van Wolfswinkel et al, 2014), and must receive appropriate instructions during regeneration in order to replace missing tissues correctly. The choice of whether to regenerate a head or a tail at transverse amputation planes, sometimes called regeneration polarity, is a classic problem in regeneration and a paradigm for molecular dissection of the positional information that guides regeneration (Morgan, 1898;Reddien, 2011).…”