Incoherent Feed-Forward Regulatory Loops Control Segregation of C-Mechanoreceptors, Nociceptors, and Pruriceptors

Abstract

Mammalian skin is innervated by diverse, unmyelinated C fibers that are associated with senses of pain, itch, temperature, or touch. A key developmental question is how this neuronal cell diversity is generated during development. We reported previously that the runt domain transcription factor Runx1 is required to coordinate the development of these unmyelinated cutaneous sensory neurons, including VGLUT3ϩ low-threshold c-mechanoreceptors (CLTMs), MrgprD ϩ polymodal nociceptors, MrgprA3 ϩ pruriceptors, MrgprB4 ϩ c-mechanoreceptors, and others. However, how these Runx1-dependent cutaneous sensory neurons are further segregated is poorly illustrated. Here, we find that the Runx1-dependent transcription factor gene Zfp521 is expressed in, and required for establishing molecular features that define, VGLUT3 ϩ CLTMs. Furthermore, Runx1 and Zfp521 form a classic incoherent feedforward loop (I-FFL) in controlling molecular identities that normally belong to MrgprD ϩ neurons, with Runx1 and Zfp51 playing activator and repressor roles, respectively (in genetic terms). A knock-out of Zfp521 allows prospective VGLUT3 lineage neurons to acquire MrgprD ϩ neuron identities. Furthermore, Runx1 might form other I-FFLs to regulate the expression of MrgprA3 and MrgprB4, a mechanism preventing these genes from being expressed in Runx1-persistent VGLUT3 ϩ and MrgprD ϩ neurons. The evolvement of these I-FFLs provides an explanation for how modality-selective sensory subtypes are formed during development and may also have intriguing implications for sensory neuron evolution and sensory coding.

Roles of Runx Genes in Nervous System Development

“…Both Runx1 and Runx3 are expressed in developing DRG sensory neurons, mostly in nonoverlapping patterns. Mouse Runx1 is preferentially expressed in several nociceptive neurons, including pain-related nociceptors and thermoceptors (Levanon et al 2002;Chen et al 2006a;Kramer et al 2006;Marmigere et al 2006;Lou et al 2013Lou et al , 2015. In contrast, Runx3 expression marks for the most part proprioceptive neurons (Inoue et al 2002;Levanon et al 2002;Chen et al 2006b;Kramer et al 2006;Yoshikawa et al 2007;Inoue et al 2008;Lallemend and Ernfors, 2012).…”

“…+ cells become 'non-peptidergic' sensory neurons defined by specific molecular traits and innervation of skin epidermis and hair follicle targets (Chen et al 2006a;Kramer et al 2006;Marmigere et al 2006;Luo et al 2007;Yoshikawa et al 2007;Gascon et al 2010;Yang et al 2013;Lou et al 2015). Runx1 is essential for both the initial separation of TrkA + and TrkA − sensory neuron lineages and the subsequent generation of cutaneous sensory neuron diversity, a process mediated in part by the transient or persistent nature of Runx1 expression after the separation of TrkA and Runx1 expression (Lou et al 2015).…”

“…Runx1 is essential for both the initial separation of TrkA + and TrkA − sensory neuron lineages and the subsequent generation of cutaneous sensory neuron diversity, a process mediated in part by the transient or persistent nature of Runx1 expression after the separation of TrkA and Runx1 expression (Lou et al 2015). During non-peptidergic neuronal fate specification, Runx1 orchestrates transcriptional mechanisms that directly or indirectly regulate the expression of numerous genes defining the nonpeptidergic phenotype.…”

“…As mentioned, Runx1 also plays key roles in the further specification of several cutaneous sensory neuronal subtypes, including polymodal nociceptors, pruciceptors and other neurons associated with pain, as well as specific types of mechanoreceptors (Lou et al 2013(Lou et al , 2015Yang et al 2013). As an example, Runx1 regulates the development of specific unmyelinated lowthreshold mechanoreceptors that persistently express vesicular glutamate transporter 3 (VGLUT3) and mediate pleasant touch and/or pain (Lou et al 2013).…”

Noxious or Non-noxious Inputs to Oxytocin Neurons: Possible Roles in the Control of Behaviors

“…There appear to be, at least, two subtypes of C fibers with low-threshold mechanoreceptors that contribute to pleasant touch: first, neurons that express vesicular glutamate transporter, tyrosine hydroxylase [40], a chemokine-like protein TAFA4 and the Runx1-dependent transcription factor Zfp521 [42,43], and second, neurons that express the Masrelated G-protein-coupled receptor MrgprB4 [74].…”