The role of the p75 neurotrophin receptor (p75ntr, NGFR1/TNFRSF16) in injury and repair is complex; p75ntr is associated with both cell survival and cell death, depending upon its interactions with multiple co-receptors 1. P75ntr is important for neural development, but is only sparsely expressed in the adult brain. But after CNS injury, p75ntr is re-expressed in both neurons and glia 1. We showed years ago that the pro-form of NGF (pro- NGF), which binds to p75ntr, can induce oligodendrocyte (OL) apopotosis, and that p75ntr deletion protects OLs after spinal cord injury (SCI) 2. There are now many studies demonstrating that deletion or antagonism of p75ntr after CNS trauma can improve outcomes after SCI and traumatic brain injury (TBI)3,4, although protection is not always found 5. We recently found that a novel p75ntr antagonist provided highly significant protection from neuronal and OL cell death, and improved neurological outcomes when given for 7 days after TBI in both rats 6 and mice 7. Treatment reduced the number of pro-inflammatory monocytes in the blood, and reduced the invasion of CCR2+ monocytes into the lesion site after cortical contusion injury (CCI)-TBI in mice 7. P75ntr is expressed in many peripheral tissues including immune cells 8-11. We found that the p75ntr antagonist reduced the pro-inflammatory effects of LPS on mouse leukocytes in vitro 7. In addition, we found that the p75ntr antagonist inhibited LPS-induced production of inflammatory monocytes, suggesting that p75ntr is involved in myeloid cell differentiation and peripheral inflammation after TBI. We want to test that novel hypothesis. There are two aims:
AIM 1. We will assess the role of p75ntr in peripheral inflammatory responses by stimulating monocytes from wild-type (wt) vs p75ntr null mice with LPS in vitro. We will test the effects of the TLR4 agonist lipopolysaccharide (LPS) on myeloid cell differentiation in vitro in wild type (wt) C57Bl/6 myeloid cells vs myeloid cells from littermates with p75ntr deletion. We predict that KO of p75ntr will reduce NFkB signaling. As we are testing the in vitro effects of p75ntr deletion in the p75ntr KOs, we will generate mice with selective conditional knock-down of p75ntr in peripheral myeloid cells for the in vivo studies proposed for aim 2.
AIM 2. We will cross available p75ntr floxed, and myeloid cell-specific CRE/ERT mice to yield a new mouse line for testing the role of peripheral immune cell p75ntr in TBI. We will use a conditional genetic approach to examine the role of p75ntr in myeloid cells by crossing the p75ntr floxed mouse12 with available transgenics with cre expression in myeloid cells to produce myeloid cell-specific deletion of p75ntr (Csfr1cre/ERT aka CD115creER). These mice will be given tamoxifen or vehicle prior to CCI TBIs to reduce p75ntr expression in peripheral myeloid cells, and we will compare the effects on the differentiation of mono/MAC lineage cells in the circulation and their trafficking to the injured brain. Together, these studies focus on peripheral effects of p75ntr signaling after TBI & test the novel hypothesis that p75ntr signaling in peripheral myeloid cells is an important part of the post-TBI injury/repair cascade.
We have identified a p75 neurotrophin receptor (p75ntr) antagonist that protects the brain and reduces both CNS and systemic inflammation after traumatic brain injury in rats and mice. This suggests that the p75ntr, which is involved in brain development, repair and injury, is also important in systemic inflammation after TBI. We will use genetic deletions of p75ntr in mice to test this novel hypothesis.
