Functions of Nogo-A in the Innervation and Mineralisation of Rodent Teeth

Abstract

Nogo-A is best described as an inhibitor of axon regeneration in the adult central nervous system (CNS). At the cell surface, Nogo-A signals through receptor complexes to trigger the disorganisation of the cytoskeleton and the downregulation of growth-related genes. Blocking Nogo-A treatments have since been developed into human clinical trials for CNS regeneration. Nogo-A is also involved in the development and physiology of the CNS. Furthermore, its expression and functions extend beyond neuronal tissues. Indeed, Nogo-A regulates diverse cellular processes such as cell migration, differentiation, and apoptosis of non-neural cells. Despite being a promising therapeutic target, the roles of Nogo-A in peripheral innervation and peripheral organs remain poorly investigated. In this context, teeth represent a powerful model to study organ development and innervation. Adult teeth are richly innervated by sensory neurons originating from the trigeminal ganglion (tgg), who respond to electrical, thermal, mechanical, and chemical stimulation. During development, neuroregulatory molecules precisely regulate the neuronal patterning of the dental tissues. Intriguingly, innervation of the dental pulp, the central soft tissue of the tooth, happens late during its morphogenesis compared to other tgg-innervated tissues. In addition, some of these neuroregulatory molecules are also directly involved in dental cell differentiation events, therefore coordinating neuronal patterning with tooth morphogenesis. Novel therapeutic strategies have been designed using the properties of these molecules for the regeneration of dental tissues. In this dissertation, we investigated the roles of Nogo-A in the tooth innervation and morphogenesis. We found that Nogo-A is expressed in trigeminal neurons and in dental tissues during development, and that this expression persists postnatally. Using a mouse genetic model, we observed that the deletion of Nogo-A (Nogo-A KO) leads to a significant reduction of tooth innervation, already from the early stages of tooth innervation. Acute blocking with Nogo-A antibodies affected the outgrowth of wild-type (WT) tgg in explant cultures and resulted in the premature innervation of co-cultured tooth germs. RNA sequencing analyses of WT and Nogo-A KO tgg pointed towards dysfunctions of synapses and interference with Neurotrophin signalling. Furthermore, roots of mature teeth of Nogo-A KO mice exhibited increased volume of dentin, the predominant hard tissue of the tooth. Likewise, blocking Nogo-A antibodies promoted mineralisation of primary cultures established from dental pulps and bone marrows, while treatment with an active Nogo-A protein fragment significantly reduced their mineralisation potential. Taken together, our results identify Nogo-A as a regulator of both tooth innervation and tooth mineralisation, providing a link between tooth morphogenesis and neuronal patterning.