Peripheral nerve injury can result in loss of cutaneous sensory function, with associated defects in protective
reflexes and tactile function, as well as neuropathic pain. While regeneration from injured nerve is the best-
studied mechanism of reinnervation following injury, it is inherently slow even with surgical augmentation.
Collateral sprouting from neighboring uninjured neurons represents an additional mechanism for recovery.
Although axonal regeneration has been studied extensively, much less is known about the mechanistic basis or
functional consequences of collateral sprouting. After peripheral nerve injury, macrophages are recruited into or
proliferate within the injured nerve stump and areas undergoing Wallerian degeneration, where they play pivotal
roles in phagocytic removal of neuronal and myelin debris and facilitate the regeneration of injured axons.
However, changes and potential functions of macrophages in denervated skin after nerve injury remain less
studied. In this project, we will explore the hypothesis that macrophages promote skin reinnervation by collateral
sprouting of uninjured nociceptors after peripheral nerve injury. In Aim 1, we will characterize changes in the
abundance, localization, morphology, and phenotype of macrophages in denervated skin over time and define
the anatomical relationship of these cells to sprouting nociceptive fibers. In Aim 2, we will test the hypothesis that
macrophages are essential for normal nociceptor sprouting and functional sensory recovery by quantifying
sprouting-mediated skin reinnervation and behavioral sensory recovery after SNI surgery in mice in which
macrophages have been globally or locally depleted. In Aim 3, to define the mechanisms by which macrophages
promote collateral sprouting after nerve injury, we will assess whether macrophage depletion alters peripheral
nerve injury-induced upregulation of sprouting-associated genes (SAGs) in spared nociceptive neurons. We also
will use in vitro DRG neuron culture to examine the impact of prior in vivo injury, with or without macrophage
depletion, on neurite outgrowth and branching. Finally, we will ask whether in vitro exposure of DRG neurons to
either paw skin or macrophages isolated after nerve injury alters axon outgrowth or branching. Success in this
proposal will allow us to elucidate the importance of cutaneous macrophages in anatomical and functional
sensory recovery by collateral sprouting. It will also help to identify candidate transcriptional and cell biological
changes driving these effects and establish a system with which to subsequently identify the responsible
secreted macrophages effectors. Based on our findings, targeting of the recruitment, differentiation, survival, or
effector function of macrophages might be explored as therapeutic strategies to promote the progression of
sensory recovery and/or limit unwanted events such as chronic pain after peripheral nerve injury.
