Upregulation of the p75 low-affinity neurotrophin receptor by phagocytically active perivascular active cells in the rat neural lobe.

Expression of the p75 low-affinity neurotrophin receptor (p75NTR) was investigated immunocytochemically at the light and ultrastructural level during the axonal degeneration that follows partial denervation of the rat neural lobe (NL) and following systemic administration of lipopolysaccharide (LPS). A significant increase in the intensity and extent of p75NTR immunoreactivity in the NL of partially denervated animals compared with age-matched, sham-operated controls was observed at 5-10 days postdenervation, with immunoreactivity returning to control values by 35 days. Dual-label confocal comparison of p75NTR localization with that of the C3bi complement receptor, a microglial marker, and S100, an astrocyte-specific Ca2+-binding protein, revealed no colocalization. Immunoelectron-microscopic examination demonstrated that the p75NTR immunoreactivity is present in a subpopulation of cells located within the extensive perivascular space of the NL. No examples of p75NTR-immunoreactive pituicytes or endothelia were observed at the light or ultrastructural level. Dense p75NTR immunoreactivity was frequently observed surrounding endocytotic omega profiles of plasmalemma engulfing extracellular debris as well as lining vacuoles within the cytoplasm of perivascular cells. The association of p75NTR with phagocytosis was confirmed by confocal microscopy, showing the presence of p75NTR in all cells expressing the ED-1 antigen, which is restricted to the lysosomal membrane of phagocytes (Damoiseaux et al. 1994). Likewise, a marked increase in p75NTR and ED-1 immunoreactivity was observed in the NL following systemic administration of LPS. These results suggest a strong correlation between modulation of p75NTR immunoreactivity and conditions that induce high levels of phagocytic activity by perivascular cells in the NL of the rat. Implications for understanding the mechanisms by which phagocytes may support compensatory responses to neuronal injury are discussed.