Maintenance of neurotransmitter identity by Hox proteins through a homeostatic mechanism

Abstract

Hox transcription factors play fundamental roles during early patterning, but they are also expressed continuously, from embryonic stages through adulthood, in the nervous system. However, the functional significance of their sustained expression remains unclear. In C. elegans motor neurons (MNs), we find that LIN-39 (Scr/Dfd/Hox4-5) is continuously required during post-embryonic life to maintain neurotransmitter identity, a core element of neuronal function. LIN-39 acts directly to co-regulate genes that define cholinergic identity (e.g., unc-17/VAChT, cho-1/ChT). We further show that LIN-39, MAB-5 (Antp/Hox6-8) and the transcription factor UNC-3 (Collier/Ebf) operate in a positive feedforward loop to ensure continuous and robust expression of cholinergic identity genes. Finally, we identify a two-component design principle for homeostatic control of Hox gene expression in adult MNs: Hox transcriptional autoregulation is counterbalanced by negative UNC-3 feedback. These findings uncover a noncanonical role for Hox proteins during post-embryonic life, critically broadening their functional repertoire from early patterning to the control of neurotransmitter identity.

A molecular atlas of adult C. elegans motor neurons reveals ancient diversity delineated by conserved transcription factor codes

“…In C. elegans , Hox genes ceh-13 , lin-39 , mab-5 , and egl-5 are expressed both in developing and adult MNs ( Figure 2 ). 28 , 71 In mice, Hox genes are expressed in developing mouse MNs, 26 , 72 and we found that their expression persists into adulthood ( Figure 6C ). The sustained Hox expression, from embryo to adult, in specific MNs along the rostrocaudal axis points toward non-canonical roles in the adult nervous system.…”

“…In C. elegans , combinations of TFs have been described for several neuron types, 30 , 31 , 58 – 62 but studies of TF function in adult neurons remain rare. 63 In the case of nerve cord MNs, only a handful (e.g., unc-3/EBF , cfi-1/ARID , bnc-1/BNC , mab-9/TBX , lin-39/HOX ) 28 , 64 – 66 of the 234 TFs we found to be expressed in adult MNs are known to have bona fide roles on adult MN identity and function. This can be addressed by auxin-inducible TF degradation in adult stages to bypass developmental effects caused by constitutive TF alleles.…”

“…Recent work showed that lin-39 ( Scr/Dfd/Hox4–5 ) is required in the adult to maintain the neuro-transmitter identity of nerve cord MNs. 28 The continuous expression of ceh-13 , mab-5 , and egl-5 suggest that these Hox genes, such as lin-39 , may also play critical roles in maintenance of adult MN identity. Recent findings suggest parallel roles for Hox genes in other species.…”

“… 4 , 26 , 27 Emerging evidence in C. elegans suggests that HOX genes are expressed in adult MNs and required to maintain cholinergic MN identity. 28 , 29 However, a comprehensive description of TF families expressed in adult C. elegans MNs is currently needed. Such knowledge will lay the groundwork for future studies to dissect TF functional roles in adult MNs.…”

Cell context-dependent CFI-1/ARID3 functions control neuronal terminal differentiation

“…UNC-3 and the Hox proteins LIN-39 and MAB-5 are known terminal selectors in C. elegans nerve cord motor neurons. 45 , 46 , 58 , 59 As such, they are continuously required from embryonic to adult stages to activate expression of multiple terminal differentiation genes. 34 , 45 On the other hand, our constitutive (null alleles) and conditional (temporally controlled protein depletion) approaches also revealed a continuous requirement for CFI-1 in motor neurons ( Figure 4 ).…”