TNF reverse signalling in the developing peripheral nervous system

Tumour necrosis factor (TNF) is an extensively well characterised proinflammatory cytokine. It is expressed as a type two membrane glycoprotein that is active both as a membrane-integrated ligand and as a soluble ligand following proteolytic release of the ectodomain from the cell membrane. TNF signals via two receptors, TNFR1 and TNFR2. In the immune system, it has been shown that these receptors can function as ligands for membrane-integrated TNF and initiate TNF reverse signalling. I was a member of a team that discovered, characterised and evaluated the physiological significance of TNF reverse signalling in the nervous system. We showed that TNFR1 is expressed in tissues innervated by sympathetic neurons and that this initiates TNF reverse signalling in postnatal sympathetic axons, which in turn enhances their growth and branching locally. Using a tissue whole mount method to visualize sympathetic fibres, I found that the innervation of multiple tissues that receive their innervation exclusively or predominantly from the paravertebral sympathetic chain is defective both in mice lacking TNF and mice lacking TNFR1. Sympathetic fibres reach these tissues in these mice but fail to grow and branch extensively in these tissues. In contrast, tissues that receive their sympathetic innervation predominantly from prevertebral ganglia are either unaffected, in mice lacking TNF and TNFR1, or hyperinnervated. Using live calcium imaging, pharmacological blockers of calcium channels and shRNA gene knockdown, I obtained evidence that T-type calcium channels are required for the effects of TNF reverse signalling on axon growth. I also showed that TNF reverse signalling enhances the growth of sensory axons, during an earlier stage in development than sympathetic neurons. This work establishes that TNF reverse signalling is widely involved in regulating axon growth in the developing peripheral nervous system