Death of injured neurons caused by the precursor of nerve growth factor.

B ecause the adult mammalian brain has a very limited capacity to replace neurons lost after lesion (1), understanding the mechanisms regulating their survival or elimination is of special significance. A study in this issue of PNAS (2) reveals that cutting the axons of a neuronal population involved in movement control leads to a progressive and dramatic increase of pro-nerve growth factor (NGF) in brain fluids. Pro-NGF kills injured neurons by virtue of its high affinity binding to a receptor that is induced after axotomy, the neurotrophin receptor p75. Another recent study also indicates that neurons may not be the only targets of pro-NGF-mediated killing: after partial transaction of the spinal cord, oligodendrocytes may also be eliminated by the mechanism described by Harrington and colleagues (3). In mammals, the neurotrophin family consists of four genes that encode structurally related proteins that are proteolytically processed and secreted in the extracellular space (for review, see ref. 4). In the brain, the secretion of neurotrophins is regulated by the activity of the neurons that synthesize them, and after secretion, these proteins affect many important aspects of neuronal function, including synaptic transmission and excitability (for review, see ref. 5). As their names suggest, neurotrophins are best known for their ‘‘trophic’’ roles on neurons, typically including the promotion of nerve growth and the prevention of the death of embryonic neurons. These two properties were used to purify NGF and brain-derived neurotrophic factor (BDNF), respectively (for review, see ref. 4). Protein sequencing work revealed that the activity of these two neurotrophins is contained in what was shown by cDNA cloning to correspond to the C-terminal half of a larger precursor protein (Fig. 1). A first big surprise came when an intriguing report indicated that the neurotrophin receptor p75 causes the death of the cell line in which it was overexpressed (6). It was later found that the application of antibodies to NGF (including those used in the present study) or to the p75 receptor both blocked the elimination of cells in the developing chick retina (7). These results suggested that even in vivo, neurotrophins may not always prevent the death of neurons, but also precipitate it. More recently, another surprising report indicated that unprocessed, pro-NGF binds with high affinity to the p75 receptor but not to TrkA, the tyrosine kinase receptor mediating the trophic actions of NGF (8). The current report by Harrington and colleagues (2) is the first to demonstrate that pro-NGF can actually be detected in extracellular fluids, suggesting a significant biological role for this protein in the adult brain after lesion. Like many other secreted growth factors, neurotrophins are synthesized as prepro-proteins. Until recently, essentially all of the work on neurotrophins was performed with the processed proteins. It was generally assumed that the N-glycosylated precursors merely represent transient intermediates allowing proper folding and appropriate disulfide bridging of the mature proteins. In addition, unlike processed neurotrophins that are highly conserved between different species, the sequences of the neurotrophin precursors differ substantially. With regard to biosynthesis, these sequences seem to be interchangeable, at least when tested in heterologous systems. For example, substituting the prosequence of pro-NT3 by that of proBDNF (or vice versa) does not change the yield of mature, biologically active neurotrophin dimers (9). An additional reason why little attention has been paid to pro-neurotrophins in a physiological context is the presence of a typical cluster of basic amino acids immediately preceding the mature sequence, a potential target for several, widely distributed proteases including plasmin, furins, and matrix metalloproteinases (8). This cleavage site was thought to make it unlikely that pro-neurotrophins would be secreted from intact cells in significant amounts. However, experiments with hippocampal neurons infected with vaccinia virus encoding prepro-BDNF revealed that these neurons release substantial amounts pro-BDNF (10). In addition, a frequent polymorphism was recently detected in the pro-sequence of the human bdnf gene that is predictive of memory performance. This Val–Met substitution is thought to affect the intracellular trafficking of pro-BDNF, an