Expression of nerve growth factor (NGF) isoforms in the rat uterus during pregnancy: Accumulation of precursor proNGF

The mechanisms that promote the transient degenerative changes in the uterus innervation during pregnancy remain incompletely understood. Signaling by the nerve growth factor ( NGF)- beta is important for maintaining the density of peripheral sympathetic innervation. Here, we analyzed the spatial and temporal expression of NGF isoforms in the rat uterus using RT- PCR, immunoblot analysis, and immunohistochemistry during pregnancy ( d 7, 14, and 21), and postpartum ( d 1, 8, and 22). Western blot analysis using antibodies to mature NGF-beta and to proNGF domain demonstrated a significant decrease in mature NGF- beta at gestational d 14 and 21 ( term pregnancy) and 1 d postpartum, which paralleled a remarkable accumulation of the 26 - 28-, 32-, and 60- kDa proNGF forms. There were diminished ratios of mature NGF- beta to proNGF independent of uterus growth on the same gestational days. Immunohistochemistry revealed a progressive NGF- beta decline throughout pregnancy in the myometrium and a near absence at term pregnancy, which contrasted with increased NGF immunostaining in the intermyometrial connective tissue layers. More importantly, proNGF- specific antibodies identified the increased NGF immunoreactivity in the intermyometrial layers at term pregnancy as proNGF and not mature NGF- beta. Alterations in the processing of NGF and accumulation of proNGF in the intermyometrial layers, where axonal degeneration occurs, may contribute significantly to the pregnancy-related uterine denervation and to the control of myometrial activity.