Monocyte proinflammatory phenotypic control by ephrin type A receptor 4 mediates neural tissue damage

Abstract

Circulating monocytes have emerged as key regulators of the neuroinflammatory milieu in a number of neuropathological disorders. Ephrin type-A receptor 4 (Epha4) receptor tyrosine kinase, a prominent axon guidance molecule, has recently been implicated in the regulation of neuroinflammation. Using a mouse model of brain injury and GFP bone marrow (BM) chimeric approach, we find neuroprotection and lack of significant motor deficits that is marked by reduced monocyte/macrophage cortical infiltration, and increased number of arginase-1-postivity in the absence of BM-derived Epha4. This was accompanied by a shift in monocyte gene profile from pro-to anti-inflammatory that includes increased Tek (Tie2 receptor) expression. Inhibition of Tie2 attenuated enhanced expression of M2-like genes in cultured Epha4-null monocyte/macrophages. In Epha4 BMdeficient mice, cortical-isolated GFP+ monocyte/macrophages displayed a phenotypic shift from classical to an intermediate subtype, which displayed reduced Ly6c hi concomitant with increased Ly6c lo -and Tie2-expressing populations. Furthermore, clodronate liposome-mediated monocyte depletion mimicked these effects in wild-type but resulted in attenuation of phenotype in Epha4 BM-deficient mice suggesting control over monocyte polarization not recruitment dictates tissue damage. Thus, coordination of monocyte pro-inflammatory polarization by Epha4 is a key regulatory step mediating neural tissue damage.

Atypical Neurogenesis, Astrogliosis, and Excessive Hilar Interneuron Loss Are Associated With the Development of Post-Traumatic Epilepsy

“…Controlled Cortical Impact (CCI). Mice were injured as previously described [39][40][41][42][43]. Briefly, ketamine 100mg/kg, xylazine 10mg/kg, and Buprenorphine SR (0.5 mg/kg) were administered subcutaneously for anesthetic and analgesia before surgery.…”

“…Images were acquired using a Nikon ECLIPSE Ti2 Inverted confocal microscope with a motorized stage and a Nikon C2 laser system. Five coronal sections were analyzed by a blinded investigator using the Optical Fractionator probe from MBF StereoInvestigator software (MicroBrightField, Williston, VT, USA) and an upright Olympus BX51TRF motorized microscope (Olympus America, Center Valley, PA, USA) as previously described [39,40,42,43]. Contours for the DG and hilus were created, and the optical fractionator's grid size was set at 150x150mm with a 75x75mm counting frame.…”

Cross-Talk and Subset Control of Microglia and Associated Myeloid Cells in Neurological Disorders

“…It is well established that these monocyte subsets infiltrate the brain following traumatic injury in mice and that the CCR2-CCL2 axis is essential for the recruitment of the Ly6C hi subset [ 51 ]. Classical monocytes are involved in phagocytosis and pro-inflammatory responses; intermediate cells exclusively express CCR5 and mediate antigen presentation, while non-classical cells are involved in complement and Fc gamma-mediated phagocytosis and adhesion [ 52 , 53 , 54 ]. A subset of non-classical monocytes express CD9 and SLAN, which suggests increased efferocytosis and migration functions compared to the SLAN-negative population [ 53 , 55 ].…”

“…Monocyte depletion studies demonstrated that peripheral-derived monocytes contribute to the pathophysiology of several neurological disorders, including but not limited to Alzheimer’s disease (AD), intracerebral hemorrhage, traumatic brain injury, and multiple sclerosis [ 51 , 52 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 ]. Mice receiving intravenous injections of clodronate had reduced numbers of CCR2+Ly6c hi monocytes infiltrating the brain following injury, suggesting subset control modulates tissue damage [ 52 ]. Importantly, the use of CCR2-/- mice revealed that altering monocyte influx directly affects the production of type1 interferon genes in resident microglial subsets [ 47 ].…”

“…It should be noted that the source of irradiation, gamma vs. X-ray, and shielding from irradiation differs across studies. Microglia replacement has not been demonstrated using X-ray irradiation [ 7 , 52 , 118 ] but shows extensive repopulation in gamma-irradiated conditions [ 101 , 115 ]. Due to the recent discovery of a skull bone marrow and myeloid cell reservoir, it is plausible that engraftment may also occur as a result of this population [ 11 , 119 , 120 , 121 ].…”