Figure 15-3. Prosurvival and pro-death signaling by neurotrophins. Domain structure of proneurotrophins and their enzymatically processed mature forms.

G was added in vitro. In vitro, these effects have been shown using cultured neurons or oligodendrocytes. In vivo, infusion of the pro-domain or mature domainspecific antisera to mice subjected to CNS injury can block the binding of endogenous, secreted proNGF to p75-containing receptor complexes and can rescue corticospinal neuron death. These results, together with published studies from a host of groups using oligodendroglia, suggest that proNGF is a naturally occurring, pathophysiologic ligand that can initiate apoptosis of neurons and glia in response to injury. Furthermore, agents that impair binding of proNGF to its receptors can result in enhanced survival in vivo.

Where is proNGF synthesized in the context of spinal cord injury? Current evidence suggests that proNGF is released from astrocytes or microglia that are “activated” via the intracellular production of peroxynitrite (Hempstead, 2006; Yune et al., 2007; Domeniconi et al., 2007). Activation of astrocytes may result from cytokine or growth factor stimulation (Figure 15-2). Scavengers of peroxynitrite or mitochondrially targeted antioxidants can prevent the release of pro-death factors such as proneurotrophins. Indeed, recent studies suggest that the neuroprotective antibiotic minocycline can inhibit the production of proNGF in microglia to protect oligodendrocytes after spinal cord injury. Of note, minocycline is currently in clinical trials for SCI, but the preclinical results have not been uniformly reproduced between laboratories (Pinzon et al., 2008; Stirling et al., 2004). The results raise the interesting possibility

that the inflammasome in neurons, possibly acting via caspase-1, caspase-11, and other proteins, acts to enhance release of cytokines such as interleukin (IL)- 1β and IL-18. These cytokines can then feedback on microglia or astrocytes to stimulate release of extrinsic apoptogenic factors such as proNGF, Fas, and tumor necrosis factor alpha (TNF-α).

How does binding of proNGF, Fas, or TNF-α to oligodendrocytes induce death? proNGF, Fas, and TNF-α all activate the JNK pathway as a downstream event after binding to their cognate receptors (Figure 15-2). JNK proteins can mediate apoptotic events via two arms. The first arm is a transcriptional one involving the phosphorylation of a number of transcription factors, including possibly c-jun. It appears that more than one JNK isoform (there are three) must be inhibited to prevent death, and as expected, each of the JNKs may have multiple transcription targets. Alternatively, JNKs can regulate the mitochondrial apoptotic pathway directly by facilitating cytochrome c release in culture. In oligodendrocytes after SCI, the primary JNK isoform that is activated is JNK3. JNK3 phosphorylates and thereby destabilizes myeloid cell leukemia sequence-1 (Mcl-1) (Li et al., 2007) (Figure 15-2). JNK3 facilitates degradation of Mcl-1 by disrupting its interaction with Pin-1 (phosphorylation specific propyl isomerase never in a mitosis gene a [NIMA] 1). Pin-1 regulates the stability of a number of important mitogen-activated protein kinase substrates, including p53, β-catenin, and the nuclear factor kappa β subunit, p65. In the nervous system, Pin-1 functions primarily as an anti-death protein, as Pin-1– deficient mice develop a progressive neuropathy and tau hyperphosphorylation. Consistent with these findings, Pin-1–deficient oligodendrocytes are more sensitive to apoptosis after spinal cord injury. Mcl-1 represses apoptosis by inhibiting bax function at the mitochondria, downstream of its activation and translocation to the organelle (Germain et al., 2008). Indeed, bax-deficient mice demonstrate durable, enhanced oligodendrocyte survival after spinal cord injury (Dong et al., 2003). Together these studies suggest that extrinsic factors have an important role in triggering delayed oligodendrocyte death.

7.1. Activation of p21 waf1/cip1: Targeting extrinsic and intrinsic pathways to death

Other studies have provided novel insight into the gene expression changes associated with neuronal and oligodendrocyte death in the subacute phase after injury. Twenty-four hours after injury, increased message and protein levels of a cassette of genes that are