Macrophages facilitate peripheral nerve regeneration by organizing regeneration tracks through Plexin-B2.

The regeneration of peripheral nerves is guided by regeneration tracks formed through an interplay of many cell types, but the underlying signaling pathways remain unclear. Here, we demonstrate that macrophages are mobilized ahead of Schwann cells in the nerve bridge after transection injury to participate in building regeneration tracks. This requires the function of guidance receptor Plexin-B2, which is robustly up-regulated in infiltrating macrophages in injured nerves. Conditional deletion of Plexin-B2 in myeloid lineage resulted in not only macrophage misalignment but also matrix disarray and Schwann cell disorganization, leading to misguided axons and delayed functional recovery. Plexin-B2 is not required for macrophage recruitment or activation but enables macrophages to steer clear of colliding axons, in particular the growth cones at the tip of regenerating axons, leading to parallel alignment postcollision. Together, our studies unveil a novel reparative function of macrophages and the importance of Plexin-B2-mediated collision-dependent contact avoidance between macrophages and regenerating axons in forming regeneration tracks during peripheral nerve regeneration.

Role of forward and reverse signaling in Eph receptor and ephrin mediated cell segregation.

Eph receptor and ephrin signaling has a major role in segregating distinct cell populations to form sharp borders. Expression of interacting Ephs and ephrins typically occurs in complementary regions, such that polarised activation of both components occurs at the interface. Forward signaling through Eph receptors can drive cell segregation, but it is unclear whether reverse signaling through ephrins can also contribute. We have tested the role of reverse signaling, and of polarised versus non-polarised activation, in assays in which contact repulsion drives cell segregation and border sharpening. We find that polarised forward signaling drives stronger segregation than polarised reverse signaling. Nevertheless, reverse signaling contributes since bidirectional Eph and ephrin activation drives stronger segregation than unidirectional forward signaling alone. In contrast, non-polarised Eph activation drives little segregation. We propose that although polarised forward signaling is the principal driver of segregation, reverse signaling enables bidirectional repulsion which prevents mingling of each population into the other.

Patterning spinal nerves and vertebral bones.

A prominent anatomical feature of the peripheral nervous system is the segmentation of mixed (motor, sensory and autonomic) spinal nerves alongside the spinal cord. During early development their axon growth cones avoid the developing vertebral elements by traversing the anterior/cranial half of each somite-derived sclerotome, so ensuring the separation of spinal nerves from vertebral bones as axons extend towards their peripheral targets. A glycoprotein expressed on the surface of posterior half-sclerotome cells confines growth cones to the anterior half-sclerotomes by contact repulsion. A closely similar glycoprotein is expressed in avian and mammalian grey matter, where we hypothesize it may have evolved to regulate neural plasticity in birds and mammals.