Investigation of Amphiregulin-mediated neuro-immune crosstalk.

The immune and the nervous system are closely connected with each other, the one influencing the functioning of the other. Nevertheless, the mechanisms by which these two systems communicate with each other are still poorly understood.

Neuregulins (NRGs) and neurotrophic factors such as nerve growth factor (NGF) represent some of the best-characterized mediators of neuro-immune crosstalk. These molecules are expressed by leukocytes and are involved in the maintenance, proliferation and survival of peripheral neurons as well as the outgrowth of neural dendritic spines at nerve terminals.

So far, mast cells have been characterized as the main source of these factors. Mast cells are strategically located at nerve endings; therefore they can easily mediate axonal guidance during inflammation. One of the best-established examples of such guidance is the NGF-mediated innervation of inflamed tissues, which leads to nerve fibre remodelling and enhanced pain sensation at inflamed tissues.

We have found that the EGF-like growth factor Amphiregulin can also be expressed under inflammatory situations by leukocytes such as mast cells or tissue residential macrophages, and is known to contribute to tissue repair. Amphiregulin was originally named Schwannoma-derived Growth Factor (SDGF) and it interacts with a receptor closely related to NRG receptors. Amphiregulin signals via the EGFR, NRGs signal via the EGFR-like receptors ErbB2, ErbB3 and ErbB4. Furthermore, and similar to NGF, Amphiregulin binds its receptor with extreme low affinity. This low affinity of interaction is associated with the outgrowth of neurites from cultured ganglia and the differentiation of the neuronal cell line PC-12, (Kimura & Schubert J Cell Bio,1992) which both of these factors induce. These data suggest that Amphiregulin could function as a neurotrophin.

Two recent publications support the view that Amphiregulin derived from cells of the immune system may play a key role in the nervous system. First, tissue residential macrophages are essential for the innervation of brown-adipose tissue (Wolf et al., Nature Immunology 2017), and secondly, tissue-residential regulatory T-cells promote myelin regeneration upon damage of the central nervous system (Dombrowski et al., Nature Neurosciences 2017). Both of these leukocyte populations are well-established sources of Amphiregulin. Importantly, mice with a macrophage-specific deletion of Amphiregulin (LysM-cre x Aregfl/fl mice) show a similar phenotype to that described in first publication. These data suggest that leukocyte-derived Amphiregulin may directly contribute to tissue innervation and neuronal regeneration upon wounding.

In this proposal we will test this hypothesis by characterizing tissue innervation in Amphiregulin-deficient mice under steady state and under inflammatory conditions, and their capacity to regenerate peripheral nerves after a crush injury..