A role for axon-glial interactions and Netrin-G1 signaling in the formation of low-threshold mechanoreceptor end organs

Abstract

Low-threshold mechanoreceptors (LTMRs) and their cutaneous end organs convert light mechanical forces acting on the skin into electrical signals that propagate to the central nervous system. In mouse hairy skin, hair follicle-associated longitudinal lanceolate complexes, which are end organs comprising of LTMR axonal endings that intimately associate with terminal Schwann cell (TSC) processes, mediate LTMR responses to hair deflection and skin indentation. Here, we characterized developmental steps leading to the formation of Aβ RA-LTMR and Aδ-LTMR lanceolate complexes. During early postnatal development, Aβ RA-LTMRs and Aδ-LTMRs extend and prune cutaneous axonal branches in close association with nascent TSC processes. Netrin-G1 is expressed in these developing Aβ RA-LTMR and Aδ-LTMR lanceolate endings, and Ntng1 ablation experiments indicate that Netrin-G1 functions in sensory neurons to promote lanceolate ending elaboration around hair follicles. The Netrin-G ligand (NGL-1), encoded by Lrrc4c, is expressed in TSCs, and ablation of Lrrc4c phenocopies the lanceolate complex deficits observed in Ntng1 mutants. Moreover, NGL-1–Netrin-G1 signaling is a general mediator of LTMR end organ formation across diverse tissue types demonstrated by the fact that Aβ RA-LTMR endings associated with Meissner corpuscles and Pacinian corpuscles are also compromised in the Ntng1 and Lrrc4c mutant mice. Thus, axon-glia interactions mediated by NGL-1–Netrin-G1 signaling promote LTMR end organ formation.Significance statementOur sense of touch is essential for fundamental tasks ranging from object recognition to social exchange. Yet, touch remains one of the least understood senses at the developmental level. Here, we investigate the formation of lanceolate complexes, which are mechanosensory end organs associated with hair follicles. The axons of touch sensory neurons innervating hairy skin extend into the skin at late embryonic and neonatal times, prune excessive branches during early postnatal development, and closely associate with non-myelinating glial cells, called terminal Schwann cells (TSCs) during formation of mature endings around hair follicles. Moreover, NGL-1 and its receptor Netrin-G1 mediate a molecular dialogue between nascent TSCs and sensory neuron axonal endings to promote mechanosensory end organ formation in both hairy and non-hairy (glabrous) skin.

Follicle-innervating Aδ-low threshold mechanoreceptors organize through a population-dependent mechanism

“…2D-F, 2G, Supplementary Table 1). These findings are largely in agreement with other studies and suggest that murine hair follicle innervation occurs during the early postnatal window, and that by the end of the first postnatal week hair follicle innervation has reached a grossly mature state (Meltzer et al, 2022b preprint; Peters et al, 2002).…”

“…4). These findings agree with recent work describing interactions between LTMR axons and terminal Schwann cells at the hair follicle during this time period (Meltzer et al, 2022b preprint).…”

“…3). Recent comparison of the development of Aβ RA-LTMRs and Aδ-LTMRs supports this conclusion, showing Aβ RA-LTMRs maturing at P3 and Aδ-LTMRs maturing after P6 (Meltzer et al, 2022b preprint). This temporal hierarchy parallels the maturation of hair follicle subtypes.…”

“…While BDNF derived from hair follicle-associated epithelial cells is important for the polarized innervation pattern and LLE maturation of TrkB + Aδ-LTMR axonal endings, it is dispensable for the initial attraction of Aδ-LTMR axons to hair follicles, and it is unknown whether it regulates receptive field size (Rutlin et al, 2014). Similarly, recent work has shown that the GPI-linked receptor Netrin-G1 is required in LTMRs for the proper maturation of terminal endings around hair follicles, but not for terminal axon branching or receptive field size (Meltzer et al, 2022b preprint). In contrast, ɣ-Protocadherins are important for the peripheral branching and innervation of non-guard hair follicles by Aβ-LTMRs, but not for the maturation of LLEs (Meltzer et al, 2022a preprint).…”

“…Together, these data suggest that the initial innervation of hair follicles and the morphological maturation of their terminal endings are distinct events regulated by different molecular pathways. Interestingly, terminal Schwann cells are the source for ligands/binding partners for both Netrin-G1 and ɣ-Protocadherins in LTMRs, demonstrating that they play a critical role in regulating hair follicle innervation (Meltzer et al, 2022a preprint; Meltzer et al, 2022b preprint).…”