Nerve growth factor signaling of p75 induces differentiation and ceramide-mediated apoptosis in Schwann cells cultured from degenerating nerves

Novel technique for peripheral nerve reconstruction in the absence of an artificial conduit

The purpose of this study is to promote nerve regeneration across a peripheral nerve gap, using a biologic, tissue-engineered nerve (TEN), containing a high density of viable Schwann cells (SCs) in the absence of supportive foreign materials and a tubular system. Isolated SCs from adult rat sciatic nerve were seeded onto biodegradable constructs and implanted into the backs of nude mice to create TENs. Six weeks later, the constructs were harvested, implanted into surgically created sciatic nerve gaps in rats without supportive artificial conduits and compared with both an autograft group and a silicone conduit group using SCs. Two months later, functional assessment was evaluated by walking track analysis and the implanted lesions were imaged by transmission electron microscopy. The axonal number and sciatic function index of the TEN were significantly higher than those of the silicone group and achieved a comparable level to the autograft group. The results indicate that the large number of SCs within their own extracellular matrix appeared sufficient to enable neuronal growth across a nerve gap in the absence of an artificial conduit and that these circumstances may have a positive effect on the supplement of growth factors from the surrounding tissues of implanted TEN.

Nerve growth factor and wound healing

The wound healing process following tissue injury consists of a highly regulated sequence of events. Besides many biological activities on both neuronal and nonneuronal cells, nerve growth factor (NGF) has been proposed as an important component of wound healing and tissue repair process in vivo and in vitro. For example, NGF accelerates the rate of wound healing both in normal mice and healing-impaired diabetic mice, and has a potent pharmacological effect in the treatment for ulcer of the skin and cornea in humans. This review summarizes the evidence for the role of NGF in wound healing and tissue repair, and introduces its clinical utility as a therapeutic agent for various diseases.

Tumour necrosis factor-alpha- vs. growth factor deprivation-promoted cell death: distinct converging pathways

Perturbations of neuronal physiological homeostasis are likely to underscore neuronal demise/impairments that are reportedly associated with aging of the central nervous system and age-related neurodegenerative diseases such as Alzheimer's disease (AD). A number of age- and/or disease-associated neurotoxic events has been described. These include abnormally modified proteins such as beta amyloid and hyper-phosphorylated Tau, cytokines such as tumour necrosis factor-alpha (TNFalpha), high levels of free radicals conducive to oxidative stress, and impaired/decreased neuronal trophic support by neurotrophic factors. Overall, it could be argued that toxic events in the aged brain are either active, such as those due to a direct action of cytokines, or passive, such as those due to lack of growth factor support. It is therefore conceivable that cellular responses to such diverse toxic stimuli are different, suggesting that interventions should be targeted accordingly. In order to begin answering this question, we determined in PC12 cells the time course of activity, in response to TNFalpha (active) or growth factor withdrawal (passive), of protein kinase c-zeta (PKCzeta), nuclear factor kappa B (NFkappaB), caspases 3 and 8, and poly (ADP-ribose) polymerase (PARP), key signal transduction elements associated with modulation of cell death/survival in PC12 cells. We found that the overall activity of PKCzeta, NFkappaB and caspase 8 was significantly different depending on the apoptotic initiator. The pattern of caspase 3 and PARP activity, however, was not statistically different between serum-free- and TNFalpha-induced cell death conditions. This suggests that two distinct cell responses are elicited that converge at caspase 3, which then induces downstream events involved in the execution of a common apoptotic programme. These results contribute to the aim of differentially targeting neuronal death in the aged brain (characterized by neurotrophic factor impairments) or in the diseased brain (e.g. AD, characterized by elevated levels of pro-inflammatory cytokines).

Activation of the mitogen-activated protein kinase pathway through p75NTR: a common mechanism for the neurotrophin family

Neurotrophins interact with two distinct classes of cell-surface receptors, the Trk receptor tyrosine kinase family and the common neurotrophin receptor p75(NTR). For many years, the biological role of p75(NTR) remained obscure, being relegated to modulating Trk binding of neurotrophins. Recently, the importance of p75(NTR) as a signaling receptor in itself has become increasingly clear. The signals initiated by p75(NTR) are likely to be as complex as those for the Trk family and probably depend on the cell system in which such signaling is being studied. In this study, all members of the neurotrophin family were demonstrated to be capable of stimulating p75(NTR)-mediated activation of the mitogen-activated protein kinase (MAPK) family (ERK1,2). This activation is rapid and transient, peaking at 5-15 min, depending on the cell system. The classical MAPK cascade consists of the reaction series Ras-Raf-MEK-MAPK. The p75(NTR)-induced MAPK activation is MEK dependent but Raf independent. This result implies that neurotrophin activation of p75(NTR) results in some cascade (as yet unknown) that bypasses Raf and converges on MEK to result in activation of MAPK. This activated MAPK is then able to translocate to the nucleus. The effect of this MAPK activation on cell survival is dependent on cell type. These results support the concept that signaling from the p75(NTR) receptor is more diverse and extensive than previously believed.

Differential cross-regulation of TrkA and TrkC tyrosine kinase receptors with p75

The neurotrophins neurotrophin-3 (NT-3), brain-derived growth factor (BDNF) and nerve growth factor (NGF) bind to the p75 receptor, but each neurotrophin also binds a more selective Trk receptor (e.g. TrkA-NGF and TrkC-NT-3). The biochemical signals following engagement of either Trk or p75 with ligands are well understood, but long-term biological outcomes (trophic, proapoptotic or differentiative) remain unclear because they are cell/tissue specific. For example, Trk receptors are usually trophic but when overexpressed they can be proapoptotic in neuroblastomas and medulloblastomas. We hypothesized that coexpression of Trk and p75 receptors may lead to cross-regulation of signals and different biological outcomes; and used receptor-selective ligands to study cross-regulation by these receptors. We show that in the absence of Trk activation, expression of TrkC is permissive of p75 trophic and differentiation signals induced by p75 ligands, whereas expression of TrkA abolishes trophic and differentiation signals induced by p75 ligands. In contrast, in the presence of Trk activation, p75 ligands can regulate TrkA-mediated survival and TrkC-mediated differentiation. Therefore, a complex homeostasis of p75-selective and Trk-selective signals may determine the fate of cells expressing both receptors.

Functional inhibition of the p75 receptor using a small interfering RNA

The neurotrophin receptor p75(NTR) mediates a wide variety of biological effects. Consistent with the function in controlling the survival and neurite formation, p75(NTR) is expressed during the developmental stages of the nervous system. Importantly, p75(NTR) is re-expressed in various pathological conditions and is suggested to contribute to the inhibition of neuronal regeneration and the death of the neurons. Here we develop a tool to knock down the expression of p75(NTR) by employing a small interfering RNA (siRNA). The siRNA for p75(NTR) effectively reduces the expression of endogenous p75(NTR) both in Schwann cells and dorsal root ganglion neurons in vitro. NGF-induced cell death in Schwann cells and the neurite retraction in DRG neurons induced by myelin-associated glycoprotein are attenuated by the siRNA. Inhibition of p75(NTR) in specific pathological conditions by the siRNA may provide a potential therapeutic agent.

Signals that determine Schwann cell identity

While the signals that direct neural crest cells to choose the glial lineage and generate Schwann cell precursors are still obscure, studies both in vivo and in vitro indicate that the survival and differentiation of these cells to form Schwann cells is regulated by at least two signals, neuregulin-1 and endothelin. We know little about the signals that cause some immature Schwann cells to choose myelin differentiation, while other cells form non-myelinating cells. Three transcription factors, Sox-10, Oct-6 and Krox-20, have been shown to play key roles in the Schwann cell lineage. The transcription factor Krox-20 has been identified as a major target of the signals that induce myelin differentiation. Gene transfer experiments in vitro show that this protein has a remarkable ability to promote a large number of phenotypic changes in immature Schwann cells that characterize the transition of these cells to myelinating cells. Furthermore, Krox-20 shows important functional interactions with neuregulin and transforming growth factor beta (TGFbeta), two factors that have been implicated in the regulation of myelination in postnatal nerves. Another signal of importance in developing peripheral nerves, Desert Hedgehog, secreted by Schwann cells directs formation of the peripheral nerve connective tissue sheaths. Ongoing gene screening experiments are likely to reveal new genes of interest in this system.