Reducing p75 nerve growth factor receptor levels using antisense oligonucleotides prevents the loss of axotomized sensory neurons in the dorsal root ganglia of newborn rats

In vivo functions of p75NTR: challenges and opportunities for an emerging therapeutic target

The p75 neurotrophin receptor (p75^NTR) functions at the molecular nexus of cell death, survival, and differentiation. In addition to its contribution to neurodegenerative diseases and nervous system injuries, recent studies have revealed unanticipated roles of p75^NTR in liver repair, fibrinolysis, lung fibrosis, muscle regeneration, and metabolism. Linking these various p75^NTR functions more precisely to specific mechanisms marks p75^NTR as an emerging candidate for therapeutic intervention in a wide range of disorders. Indeed, small molecule inhibitors of p75^NTR binding to neurotrophins have shown efficacy in models of Alzheimer's disease (AD) and neurodegeneration. Here, we outline recent advances in understanding p75^NTR pleiotropic functions in vivo, and propose an integrated view of p75^NTR and its challenges and opportunities as a pharmacological target.

Implication of the neurotrophin receptor p75NTR in vascular diseases: beyond the eye

INTRODUCTION

The p75 neurotrophin receptor (p75^NTR) is a member of TNF-α receptor superfamily that bind all neurotrophins, mainly regulating their pro-apoptotic actions. Ischemia is a common pathology in different cardiovascular diseases affecting multiple organs, however the contribution of p75^NTR remains not fully addressed. The aim of this work is to review the current evidence through published literature studying the impact of p75^NTR receptor in ischemic vascular diseases.

AREAS COVERED

In the eye, several ischemic ocular diseases are associated with enhanced p75^NTR expression. Ischemic retinopathy including diabetic retinopathy, retinopathy of prematurity and retinal vein occlusion are characterized initially by ischemia followed by excessive neovascularization. Beyond the eye, cerebral ischemia, myocardial infarction and critical limb ischemia are ischemic cardiovascular diseases that are characterized by altered expression of neurotrophins and p75^NTR expression. We surveyed both clinical and experimental studies that examined the impact of p75^NTR receptor in ischemic diseases of eye, heart, brain and peripheral limbs.

EXPERT COMMENTARY

p75^NTR receptor is a major player in multiple ischemic vascular diseases affecting the eye, brain, heart and peripheral limbs with significant increases in its expression accompanying neuro-vascular injury. This has been addressed in the current review along with the beneficial vascular outcomes of p75^NTR inhibition.

BDNF isoforms: a round trip ticket between neurogenesis and serotonin?

The brain-derived neurotrophic factor, BDNF, was discovered more than 30 years ago and, like other members of the neurotrophin family, this neuropeptide is synthetized as a proneurotrophin, the pro-BDNF, which is further cleaved to yield mature BDNF. The myriad of actions of these two BDNF isoforms in the central nervous system is constantly increasing and requires the development of sophisticated tools and animal models to refine our understanding. This review is focused on BDNF isoforms, their participation in the process of neurogenesis taking place in the hippocampus of adult mammals, and the modulation of their expression by serotonergic agents. Interestingly, around this triumvirate of BDNF, serotonin, and neurogenesis, a series of recent research has emerged with apparently counterintuitive results. This calls for an exhaustive analysis of the data published so far and encourages thorough work in the quest for new hypotheses in the field. BDNF is synthetized as a pre-proneurotrophin. After removal of the pre-region, proBDNF can be cleaved by intracellular or extracellular proteases. Mature BDNF can bind TrkB receptors, promoting their homodimerization and intracellular phosphorylation. Phosphorylated-TrkB can activate three different signaling pathways. Whereas G-protein-coupled receptors can transactivate TrkB receptors, truncated forms can inhibit mBDNF signaling. Pro-BDNF binds p75(NTR) by its mature domain, whereas the pro-region binds co-receptors.

Peripheral nerve injury modulates neurotrophin signaling in the peripheral and central nervous system

Peripheral nerve injury disrupts the normal functions of sensory and motor neurons by damaging the integrity of axons and Schwann cells. In contrast to the central nervous system, the peripheral nervous system possesses a considerable capacity for regrowth, but regeneration is far from complete and functional recovery rarely returns to pre-injury levels. During development, the peripheral nervous system strongly depends upon trophic stimulation for neuronal differentiation, growth and maturation. The perhaps most important group of trophic substances in this context is the neurotrophins (NGF, BDNF, NT-3 and NT-4/5), which signal in a complex spatial and timely manner via the two structurally unrelated p75(NTR) and tropomyosin receptor kinase (TrkA, Trk-B and Trk-C) receptors. Damage to the adult peripheral nerves induces cellular mechanisms resembling those active during development, resulting in a rapid and robust increase in the synthesis of neurotrophins in neurons and Schwann cells, guiding and supporting regeneration. Furthermore, the injury induces neurotrophin-mediated changes in the dorsal root ganglia and in the spinal cord, which affect the modulation of afferent sensory signaling and eventually may contribute to the development of neuropathic pain. The focus of this review is on the expression patterns of neurotrophins and their receptors in neurons and glial cells of the peripheral nervous system and the spinal cord. Furthermore, injury-induced changes of expression patterns and the functional consequences in relation to axonal growth and remyelination as well as to neuropathic pain development will be reviewed.

The p75 neurotrophin receptor has nonapoptotic antineurotrophic actions in the basal forebrain

Because of controversy about the role of the p75 neurotrophin receptor (p75(NTR) ) in the cholinergic basal forebrain (CBF), we investigated this region in p75(NTR) third exon knockout mice that were congenic with 129/Sv controls. They express a shortened intracellular form of p75(NTR) , permitting detection of p75(NTR) -expressing cells. We performed separate counts of choline acetyltransferase (ChAT)-expressing and p75(NTR) -expressing neurons. In agreement with past reports, the number of ChAT-immunoreactive neurons in knockout mice was greater than in wild-type mice, and this was evident in each of the main anatomical divisions of the CBF. In contrast, the number of p75(NTR) -immunoreactive neurons did not differ between genotypes. The biggest increase in ChAT neurons (27%) was in the horizontal limb of the diagonal band of Broca (HDB), in which region the number of p75(NTR) -positive neurons was unchanged. Double staining revealed that some neurons in wild-type mice expressed p75(NTR) but not ChAT. In the knockout mice, all p75(NTR) -expressing neurons expressed ChAT. The increase in cholinergic neurons, therefore, was at least partially attributable to a higher proportion of ChAT immunoreactivity within the population of p75(NTR) -expressing neurons. Cholinergic neurons were also larger in knockout mice than in controls. In the hippocampal CA1 region, knockout mice had a greater number of cholinergic fibers. There was a 77% increase in hippocampal ChAT activity in knockout mice and a 38% increase in heterozygotes. The data do not support an apoptotic role but indicate a broad antineurotrophic role of p75(NTR) in the cholinergic basal forebrain.

Proteolytic processing of the p75 neurotrophin receptor: A prerequisite for signalling?: Neuronal life, growth and death signalling are crucially regulated by intra-membrane proteolysis and trafficking of p75(NTR)

The common neurotrophin receptor (p75(NTR) ) regulates various functions in the developing and adult nervous system. Cell survival, cell death, axonal and growth cone retraction, and regulation of the cell cycle can be regulated by p75(NTR) -mediated signals following activation by either mature or pro-neurotrophins and in combination with various co-receptors, including Trk receptors and sortilin. Here, we review the known functions of p75(NTR) by cell type, receptor-ligand combination, and whether regulated intra-membrane proteolysis of p75(NTR) is required for signalling. We highlight that the generation of the intracellular domain fragment of p75(NTR) is associated with many of the receptor functions, regardless of its ligand and co-receptor interactions.

Effects of hydrophobic amino acids and antibodies to nerve growth factor receptors on the development of splenic tissue culture from young and old rats

The effects of hydrophobic L-amino acids alone and in the presence of monoclonal antibodies to nerve growth factor receptors NGFRp75 (apoptosis inductors) were studied on organotypic culture of splenic lymphoid tissue from young (3 months) and old (24 months) rats. Nine amino acids inhibited cell proliferation in splenic explants from young rats. This was paralleled by hyperexpression of p53 proapoptotic protein. Only two amino acids stimulated apoptosis in explants from old rats. The inhibitory effects on the development of splenic explants from young and old rats were abolished in the presence of antibodies to NGFRp75. Hence, the group of hydrophobic amino acids mediates the proapoptotic effect in the lymphoid tissue of old and young rats through nerve growth factor low affinity receptors.

The physiology of neural injury and regeneration: The role of neurotrophic factors

Injured nerves regenerate slowly and often over long distances. Prolonged periods for regenerating nerves to make functional connections with denervated targets prolong the period of isolation of the neurons from the target (chronic axotomy) and of the denervation of Schwann cells in the distal nerve pathways (chronic denervation). In an animal model, we demonstrated that prolonged axotomy and chronic denervation severely reduce the regenerative capacity of neurons to less to 10%. Concurrent reduction in neurotrophic factors, including brain- and glial-derived neurotrophic factors (BDNF and GDNF) in axotomized neurons and denervated Schwann cells, suggest that these factors are required to sustain nerve regeneration. Findings that exogenous BDNF and GDNF did not increase numbers of neurons that regenerate their axons in freshly cut and repaired rat nerves, but did increase the numbers significantly after chronic axotomy, are consistent with the view that there is sufficient endogenous neurotrophic factor supply in axotomized motoneurons and denervated Schwann cells to support nerve regeneration but that the reduced supply must be supplemented when target reinnervation is delayed. In addition, findings that BDNF is essential for the effectiveness of brief low frequency electrical stimulation in promoting nerve growth, provides further support for a central role of BNDF in motor nerve regeneration.

LEARNING OUTCOMES

Readers of this article will gain an understanding of the basis for poor functional outcomes of peripheral nerve injuries, even when surgical repair is possible.

Deletion of p75NTR impairs regeneration of peripheral nerves in mice

AIMS

After peripheral nerve injury, p75NTR was upregulated in Schwann cells of the Wallerian degenerative nerves and in motor neurons but down-regulated in the injured sensory neurons. As p75NTR in neurons mediates signals of both neurotrophins and inhibitory factors, it is regarded as a therapeutic target for the treatment of neurodegeneration. However, its physiological function in the nerve regeneration is not fully understood. In the present study, we aimed to examine the role of p75NTR in the regeneration of peripheral nerves.

MAIN METHODS

In p75NTR knockout mice (exon III deletion), the sciatic nerves and facial nerves on one side were crushed and regenerating neurons in the facial nuclei and in the dorsal root ganglia were labelled by Fast Blue. The regenerating fibres in the sciatic nerve were also labelled by an anterograde tracer and by immunohistochemistry.

KEY FINDINGS

The results showed that the axonal growth of injured axons in the sciatic nerve of p75NTR mutant mice was significantly retarded. The number of regenerated neurons in the dorsal root ganglia and in the facial nuclei in p75NTR mutant mice was significantly reduced. Immunohistochemical staining of regenerating axons also showed the reduction in nerve regeneration in p75NTR mutant mice.

SIGNIFICANCE

Our data suggest that p75NTR plays an important role in the regeneration of injured peripheral nerves.

Protective effect of tripeptide in the presence of cyclophosphamide on the growth of cultured lymphoid tissue from rats of different age

We studied the effect of tripeptide T-38 (Lys-Glu-Asp) in the presence of cyclophosphamide on cell proliferation and apoptosis in explants of splenic lymphoid tissue from young and old rats. Peptide T-38 in a concentration of 0.05 ng/ml produced a stimulatory effect on the growth zone of the explants. Addition of 1 mg/ml cyclophosphamide to the culture medium suppressed cell proliferation, which was associated with enhanced expression of proapoptotic p53 protein. Under conditions of combined treatment with cyclophosphamide and T-38 no inhibiting effect of the cytostatics was observed. Thus, tripeptide T-38 in the presence of cytostatics produces a protective effect on cell proliferation in lymphoid tissue explants.

Effect of amino acids and antibodies against nerve growth factor receptors on the development of organotypic culture of lymphoid tissue

We studied the effects of 20 L-amino acids on organotypic culture of splenic lymphoid tissue from 3-month-old rats were studied in the presence of apoptosis-inducing monoclonal antibodies against low-affinity receptors for nerve growth factor NGFRp75. The influence of amino acids stimulating cell proliferation in explants (lysine, asparagine, and glutamic acid) did not depend on NGFRp75. Hydrophobic amino acids inhibiting the growth zone in isolated application and abolished the inhibition of explant development in the presence of antibodies against NGFRp75. These amino acids can mediate the proapoptotic effect on lymphoid tissue via low-affinity receptors for nerve growth factor.

Expression of nerve growth factor receptors is correlated with progression and prognosis of human pancreatic cancer

BACKGROUND AND AIM

The aim of the present study was to investigate the prognostic value of the two types of nerve growth factor receptors (NGFR), namely high-affinity receptor TrkA and low-affinity receptor p75NGFR, in pancreatic cancer.

METHODS

The mRNA expression of NGFR for TrkA and p75NGFR was examined in 56 human primary pancreatic cancers using real-time quantitative reverse transcription-polymerase chain reaction.

RESULTS

Nerve growth factor (NGF) receptors were found in all tumor specimens. It appears that the growth of pancreatic cancer cells stimulated by NGF depended on the expression levels and the ratio of TrkA to p75NGFR. TrkA and p75NGFR were negatively correlated and both were associated with abdominal or back pain and perineural invasion. Regarding this, patients with high TrkA expression levels exhibited more frequent perineural invasion and a higher degree of pain, whereas the results of p75NGFR were opposite. For Cox univariate analyses in the overall survival study, high expression of p75NGFR was associated with longer overall survival, but TrkA exhibited opposite effects and included an effect on perineural invasion and pain. Histoprognostic grading, tumor size and node involvement were not prognostic factors. In Cox multivariate analyses, TrkA and p75NGFR were both prognostic parameters.

CONCLUSIONS

The present study found that the expression of TrkA in pancreatic cancer is a marker of tumor aggressiveness. Conversely, we also found that elevated p75NGFR expression is associated with a favorable prognosis. We demonstrated that NGF exerts both stimulatory and inhibitory effects on pancreatic cancers, with the overall effect determined by the expression levels and the ratio of TrkA to p75NGFR.

Interactions between beta-neuregulin and neurotrophins in motor neuron apoptosis

Neuregulins play a major role in the formation and stabilization of neuromuscular junctions, and are produced by both motor neurons and muscle. Although the effects and mechanism of neuregulins on skeletal muscle (e.g. regulation of acetylcholine receptor expression) have been studied extensively, the effects of neuregulins on motor neurons remain unknown. We report that neuregulin-1beta (NRGbeta1) inhibited apoptosis of rat motor neurons for up to 7 days in culture by a phosphatidylinositol 3 kinase-dependent pathway and synergistically enhanced motor neuron survival promoted by glial-derived neurotrophic factor (GDNF). However, binding of neurotrophins, including brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), to the p75 neurotrophin receptor (p75NTR) abolished the neuregulin anti-apoptotic effect on motor neurons. Inhibitors of the c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase prevented motor neuron death caused by co-incubation of NRGbeta1 and BDNF or NGF, as well as by trophic factor deprivation. Motor neuron apoptosis resulting from both trophic factor deprivation and exposure to NRGbeta1 plus neurotrophins required the induction of neuronal nitric oxide synthase and peroxynitrite formation. Because motor neurons express both p75NTR and neuregulin erbB receptors during the period of embryonic programmed cell death, motor neuron survival may be the result of complex interactions between trophic and death factors, which may be the same molecules acting in different combinations.

Differential effects of endogenous brain-derived neurotrophic factor on the survival of axotomized sensory neurons in dorsal root ganglia: a possible role for the p75 neurotrophin receptor

After peripheral nerve injury, axotomized sensory neurons in dorsal root ganglia (DRG) undergo apoptosis and up-regulate brain-derived neurotrophic factor (BDNF). We tested whether endogenous BDNF plays any role in the survival of axotomized sensory neurons using in vitro and in vivo models. In the in vitro model, treatment with BDNF antibody significantly reduced apoptosis of sensory neurons in DRG explants from both adult and neonate rats and adult mice cultured for 48 h. Consistently, exogenous BDNF increased the percentage of apoptotic neurons in the DRGs from mice. The effects of the BDNF antibody and BDNF were not seen in DRGs from p75NTR(-/-) mice. In the in vivo model, sciatic nerve transection in neonatal rats decreased the total number of neurons in the injured DRG and treatment with antiserum to BDNF significantly exaggerated the loss of DRG neurons. Numbers of sensory neurons expressing BDNF and p75NTR in cultured DRGs increased but that expressing TrkB decreased. In contrast, sciatic nerve transection in vivo reduced the numbers of neurons expressing both p75NTR and TrkB but increased the numbers of cells expressing BDNF, 1 and 7 days after the surgery. These results suggest that BDNF may have differential effects on the survival of sensory neurons depending on the expression of p75NTR. While endogenous BDNF induced apoptosis of axotomized sensory neurons through p75NTR in vitro where more neurons expressed p75NTR, it prevented apoptosis in vivo where fewer neurons expressed p75NTR after sciatic nerve transection.

Differential effect of p75 neurotrophin receptor on expression of pro-apoptotic proteins c-jun, p38 and caspase-3 in dorsal root ganglion cells after axotomy in experimental diabetes

We have hypothesized that p75 neurotrophin receptor (p75(NTR))-mediated activation of the pro-apoptotic proteins c-jun, p38 and caspase-3 underlies the neuronal cell loss in dorsal root ganglia (DRG) neurons after axotomy in normal mice, and that this activation is exaggerated in experimental diabetes. To test this hypothesized relationship, we compared the expression of pro-apoptotic proteins in fifth lumbar DRG (L5DRG) neurons of wildtype Balb/c (p75+/+) mice and p75(NTR) knockout (p75-/-) mice, assigned to either non-diabetic control groups or to diabetic (1 month) groups, all with a unilateral sciatic nerve crush produced 10 days before tissue preparation. The absolute number of L5DRG neurons expressing immunoreactivities (IR) for phosphorylated c-jun (P-c-jun-IR), phosphorylated p-38 (P-p38-IR) and cleaved caspase-3 (caspase-3-IR) were estimated in semi-thick sections using the optical fractionator. Nerve crush increased the numbers of P-c-jun-IR and caspase-3-IR neurons in all four groups. On the crush side, diabetes did not exaggerate the increase of P-c-jun-IR or caspase-3-IR neurons in p75+/+ mice, whereas in p75-/- mice diabetes reduced the increase of P-c-jun-IR neurons. Also, in p75-/- mice there was fewer caspase-3-IR cells on the intact and crushed side in comparison with p75+/+ mice independent of the presence of diabetes. This study demonstrates that (1) diabetes of 1 month's duration does not potentiate the expression of three pro-apoptotic markers p38, caspase-3 and P-c-jun neither in intact neurons nor after nerve crush, and that (2) p75(NTR) is required for activation of the pro-apoptosis signal caspase-3 after nerve crush in both diabetic and non-diabetic mice.

Diverse functions of the p75 neurotrophin receptor

The pan-neurotrophin receptor p75NTR belongs to a large family of receptors, which includes tumor necrosis factor receptors, Fas and approximately 25 other members. The p75NTR is the first receptor to be cloned molecularly. Recent years have seen the emergence of a consensus regarding the signaling pathways activated by p75NTR and its potential biological function, although receptor characterization had not been targeted for some years. We now know that p75NTR has surprisingly diverse effects, ranging from cell death to regulation of axon elongation. This diversity can be explained by the complex formation of p75NTR with other receptors and multiple signaling molecules that interact with the intracellular domain of p75NTR.

The role of the p75 neurotrophin receptor in the morphology of dorsal root ganglion cells in streptozotocin diabetic mice: effects of sciatic nerve crush

AIMS/HYPOTHESIS

Neuronal dorsal root ganglion (DRG) cells seem to be vulnerable in diabetes. The aim of this study was to determine whether the p75 neurotrophin receptor stimulates perikaryal shrinkage and neuronal death, and further accelerates neuronal DRG cell loss after axotomy in a mouse model of diabetes.

METHODS

Nine non-diabetic BALB/c p75(+/+) mice, seven diabetic BALB/c p75(+/+) mice, nine non-diabetic p75(-/-) mice and nine diabetic p75(-/-) mice received a unilateral sciatic nerve crush 1 to 2 days after streptozotocin treatment. Tissues were fixed 28 days later by vascular perfusion, and the volume and number of the fifth lumbar DRG neurons were obtained using assumption-free stereological techniques.

RESULTS

In diabetic p75(+/+) mice there was a 9% reduction in the perikaryal volume of the DRG A cells ( p<0.05) and a 10% reduction in the perikaryal volume of the DRG B cells ( p<0.05) on the non-crushed side compared with in non-diabetic p75(+/+) mice. However, neuronal cell number was not reduced. Conversely, no perikaryal shrinkage of A cells or B cells occurred on the non-crushed side in diabetic p75(-/-) mice, and no neuronal cell loss was observed. Following nerve crush, there was a loss of B cells in non-diabetic p75(+/+) mice (37+/-6%) and in diabetic p75(+/+) mice (36+/-4%). In non-diabetic p75(-/-) mice, no neuronal cell loss occurred after crush, whereas in diabetic p75(-/-) mice the loss of B cells (14+/-4%) was small but significant ( p<0.02).

CONCLUSIONS/INTERPRETATION

In experimental diabetes the p75 neurotrophin receptor is involved in neuronal DRG cell body shrinkage without loss of neuronal DRG cells. Following sciatic nerve crush, DRG cell loss is not accelerated in diabetic p75(+/+) mice.

The p75 neurotrophin receptor can induce autophagy and death of cerebellar Purkinje neurons

The cellular mechanisms underlying Purkinje neuron death in various neurodegenerative disorders of the cerebellum are poorly understood. Here we investigate an in vitro model of cerebellar neuronal death. We report that cerebellar Purkinje neurons, deprived of trophic factors, die by a form of programmed cell death distinct from the apoptotic death of neighboring granule neurons. Purkinje neuron death was characterized by excessive autophagic-lysosomal vacuolation. Autophagy and death of Purkinje neurons were inhibited by nerve growth factor (NGF) and were activated by NGF-neutralizing antibodies. Although treatment with antisense oligonucleotides to the p75 neurotrophin receptor (p75ntr) decreased basal survival of cultured cerebellar neurons, p75ntr-antisense decreased autophagy and completely inhibited death of Purkinje neurons induced by trophic factor withdrawal. Moreover, adenoviral expression of a p75ntr mutant lacking the ligand-binding domain induced vacuolation and death of Purkinje neurons. These results suggest that p75ntr is required for Purkinje neuron survival in the presence of trophic support; however, during trophic factor withdrawal, p75ntr contributes to Purkinje neuron autophagy and death. The autophagic morphology resembles that found in neurodegenerative disorders, suggesting a potential role for this pathway in neurological disease.

The p75 neurotrophin receptor enhances TrkA signalling by binding to Shc and augmenting its phosphorylation

Nerve growth factor (NGF) is an important neuronal survival factor, especially during development. Optimal sensitivity of the survival response to NGF requires the presence of TrkA and the p75 neurotrophin receptor, p75(NTR). Signalling pathways used by TrkA are well established, but the mechanisms by which p75(NTR) enhances NGF signalling remain far from clear. A prevalent view is that p75(NTR) and TrkA combine to form a high-affinity receptor, but definitive evidence for this is still lacking. We therefore investigated the possibility that p75(NTR) and TrkA interact via their signal transduction pathways. Using antisense techniques to down-regulate p75(NTR) and TrkA, we found that p75(NTR) specifically enhanced phosphorylation of the 46- and 52-kDa isoforms of Shc during nerve growth factor-induced TrkA activation. p75(NTR) did not enhance tyrosine phosphorylation of other TrkA substrates. Serine phosphorylation of Akt, downstream of Shc activation, was also p75(NTR)-dependent. We consistently detected co-immunoprecipitation of p75(NTR) and Shc. These data indicate that p75(NTR) interacts with Shc physically, via a binding interaction, and functionally, by assisting its phosphorylation. Whilst providing evidence that p75(NTR) augments TrkA signal transduction, these results do not preclude the presence of a p75(NTR)-TrkA high-affinity NGF receptor.

The role of neurotransmission and the Chopper domain in p75 neurotrophin receptor death signaling

The role of p75 neurotrophin receptor (p75NTR) in mediating cell death is now well characterized, however, it is only recently that details of the death signaling pathway have become clearer. This review focuses on the importance of the juxtamembrane Chopper domain region of p75NTR in this process. Evidence supporting the involvement of K+ efflux, the apoptosome (caspase-9, apoptosis activating factor-1, APAF-1, and Bcl-xL), caspase-3, c-jun kinase, and p53 in the p75NTR cell death pathway is discussed and regulatory roles for the p75NTR ectodomain and death domain are proposed. The role of synaptic activity is also discussed, in particular the importance of neutrotransmitter-activated K+ channels acting as the gatekeepers of cell survival decisions during development and in neurodegenerative conditions.

Prevention of apoptotic but not necrotic cell death following neuronal injury by neurotrophins signaling through the tyrosine kinase receptor

Object. Neurotrophins prevent the death of neurons during embryonal development and have potential as therapeutic agents. During development, neuronal death occurs only by apoptosis and not by necrosis. Following injury, however, neurons can die by both processes. Data from prior studies have not clearly indicated whether neurotrophins can decrease apoptosis compared with necrosis. The goal of this study was to determine the effect of neurotrophin treatment on each of these processes following injury and to characterize the receptor(s) required.

Methods. The authors used an in vitro model of injury with the aid of primary cortical neurons obtained from rat embryos. After 9 days in culture and the elimination of glia, homogeneous and mature neurons were available for experimentation. Noxious stimuli were applied, including radiation, hypoxia, and ischemia. Subsequent cell death by apoptosis or necrosis was noted based on morphological and enzymatic assessments (such as lactate dehydrogenase [LDH] release) and assays for DNA fragmentation. The effect of treatment with nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 was determined. Finally, Western blot analyses were performed to note the neurotrophin receptor status in the neurons (tyrosine kinase receptors [Trks] and p75).

The authors studied different stimuli-induced cell death by using different processes. With the application of radiation, cells died primarily by apoptosis, as evidenced by cell shrinkage, the presence of apoptotic bodies, and specific DNA fragmentation. This was a delayed process (> 6 hours) that could be reduced by gene transcription or protein synthesis inhibitors. With ischemia, cells died immediately by necrosis, showing cell enlargement and rupture. Ischemic cell death was not affected by the inhibition of macromolecular synthesis. Hypoxia produced a mixture of the two cell death processes.

Both BDNF and neurotrophin-3 demonstrated protection against apoptotic cell death only. Statistically significant decreases of both LDH release and apoptosis-specific DNA fragmentation were noted following radiation and hypoxia, but not for ischemia. Nerve growth factor, unlike the other neurotrophins, did not affect apoptosis because a functional receptor, Trk A, was not expressed by the cortical neurons. There was expression of both Trk B and Trk C, which bind BDNF and neurotrophin-3.

Conclusions. These findings have significant clinical implications. Neurotrophins may only be effective in disorders in which apoptosis, and not necrosis, is the major process. Furthermore, the Trk signaling cascade must be activated for this response to occur. Because the expression of these receptors diminishes in adulthood, neurotrophin application may be most appropriate in the pediatric population.

p75 neurotrophin receptor signaling in the nervous system

The neurotrophin receptor p75(NTR) has long been known as a receptor for neurotrophins that promote survival and differentiation. Consistent with the role of neurotrophins, p75(NTR) is expressed during the developmental stages of the nervous system. However, p75(NTR) is re-expressed in various pathological conditions in the adult. We now know that p75(NTR) has the ability to elicit bi-directional signals, that result in the inhibition as well as the promotion of the neurite outgrowth. p75(NTR) is a key receptor for myelin-derived inhibitory cues that contribute to the lack of regeneration of the central nervous system.

Opposing effects of low and high-dose clozapine on survival of transgenic amyotrophic lateral sclerosis mice

Clozapine is a potent atypical neuroleptic or antipsychotic agent used to relieve symptoms of early-diagnosed schizophrenia. Aside from well-described dopamine and serotonin receptor blockade effects, clozapine may also be neuroprotective through its modulation of the p75 neurotrophin receptor (p75(NTR)) and superoxide dismutase 1 (SOD1) expression. The death-signalling activities of both p75(NTR) and mutant SOD1 are implicated in motor neuron degeneration in humans and transgenic mice with amyotrophic lateral sclerosis (ALS). We therefore investigated the effects of clozapine in cell culture and mouse models of ALS. Clozapine dose-dependently inhibited full-length and cleaved p75(NTR) but not SOD1 protein expression in the motor neuron-like (NSC-34) cell line. Furthermore, low concentrations of clozapine protected NSC-34 cells from paraquat-mediated superoxide toxicity, nerve growth factor (NGF)-induced death signalling, and serum deprivation, whereas high concentrations potentiated death. Systemic thrice-weekly administration of low and high-dose clozapine to mutant superoxide dismutase 1 (SOD1(G93A)) mice produced differential effects on disease onset and survival. Low-dose treatment was associated with delayed locomotor impairment and death, compared to high-dose clozapine, which accelerated paralysis and mortality (P < 0.05). Increased death was not attributable to toxicity, as clozapine-induced agranulocytosis was not detected from blood analysis. High-dose clozapine, however, produced extrapyramidal symptoms in mice manifest by hindlimb rigidity, despite reducing spinal cord p75(NTR) levels overall. These results suggest that although clozapine may exert p75(NTR)-mediated neuroprotective activity in vitro, its profound antagonistic effects on dopaminergic and serotonergic systems in vivo at high doses may exacerbate the phenotype of transgenic ALS mice.

Rho activation patterns after spinal cord injury and the role of activated Rho in apoptosis in the central nervous system

Growth inhibitory proteins in the central nervous system (CNS) block axon growth and regeneration by signaling to Rho, an intracellular GTPase. It is not known how CNS trauma affects the expression and activation of RhoA. Here we detect GTP-bound RhoA in spinal cord homogenates and report that spinal cord injury (SCI) in both rats and mice activates RhoA over 10-fold in the absence of changes in RhoA expression. In situ Rho-GTP detection revealed that both neurons and glial cells showed Rho activation at SCI lesion sites. Application of a Rho antagonist (C3-05) reversed Rho activation and reduced the number of TUNEL-labeled cells by approximately 50% in both injured mouse and rat, showing a role for activated Rho in cell death after CNS injury. Next, we examined the role of the p75 neurotrophin receptor (p75NTR) in Rho signaling. After SCI, an up-regulation of p75NTR was detected by Western blot and observed in both neurons and glia. Treatment with C3-05 blocked the increase in p75NTR expression. Experiments with p75NTR-null mutant mice showed that immediate Rho activation after SCI is p75NTR dependent. Our results indicate that blocking overactivation of Rho after SCI protects cells from p75NTR-dependent apoptosis.

Design and application of a peptide nucleic acid sequence targeting the p75 neurotrophin receptor

Novel antisense peptide nucleic acid (PNA) constructs targeting p75NTR as a potential therapeutic strategy for amyotrophic lateral sclerosis (ALS) were designed, synthesised and evaluated against phosphorothioate oligonucleotide sequences (PS-ODN). An 11-mer antisense PNA directed at the initiation codon dose-dependently inhibited p75NTR expression and death signalling by nerve growth factor in Schwann cell cultures. Inhibition of p75NTR production was not detected in cultures treated with the nonsense PNA or antisense PNA directed at the 3'-terminus sequence. The 19-mer PS-ODN sequences also failed to confer any activity against p75NTR but, unlike the PNA sequences, were toxic in vitro at comparable doses.

No further loss of dorsal root ganglion cells after axotomy in p75 neurotrophin receptor knockout mice

The role of the p75 neurotrophin receptor for neuronal survival after nerve crush was studied in L5 dorsal root ganglia (DRG) of knockout mice and controls with assumption-free stereological methods. Numbers of neuronal A- and B-cells were obtained using the optical fractionator and optical disector techniques. At birth, the total number of DRG neurons was 10,000 +/- 2,600 in control mice compared with 5,100 +/- 1,300 in p75 knockout mice. During postnatal development, 1,400 neuronal B-cell bodies were lost in p75 knockouts (2P < 0.05) and 1,100 in controls (NS), whereas the A-cell population remained stable. After a sciatic nerve crush, the total neuron loss in controls was 15.4% +/- 3.5% (2P < 0.05) and 22.7% +/- 5.1% (2P < 0.05) at days 14 and 42, respectively. In contrast, there was no loss in total number of neurons after crush in p75 knockout mice. Neuronal A-cell number was unchanged after the crush in p75 knockouts as well as in controls at both times. At 14 days, the population of B-cells was reduced by 24.8% +/- 3.6% in controls and by 6.1% +/- 3.5% in p75 knockouts, this difference being significant (2P < 0.001). At 42 days, the B-cell loss was 29.6% +/- 5.5% in controls and 4.2% +/- 6.4% in p75 knockouts (2P < 0.001). In conclusion, the lack of the p75 receptor results in neuronal DRG cells that are resistant to nerve injury, pointing to a role for the receptor in apoptosis.

In vivo cellular and molecular mechanisms of neuronal apoptosis in the mammalian CNS

Apoptosis has been recognized to be an essential process during neural development. It is generally assumed that about half of the neurons produced during neurogenesis die before completion of the central nervous system (CNS) maturation, and this process affects nearly all classes of neurons. In this review, we discuss the experimental data in vivo on naturally occurring neuronal death in normal, transgenic and mutant animals, with special attention to the cerebellum as a study model. The emerging picture is that of a dual wave of apoptotic cell death affecting central neurons at different stages of their life. The first wave consists of an early neuronal death of proliferating precursors and young postmitotic neuroblasts, and appears to be closely linked to cell cycle regulation. The second wave affects postmitotic neurons at later stages, and is much better understood in functional terms, mainly on the basis of the neurotrophic concept in its broader definition. The molecular machinery of late apoptotic death of postmitotic neurons more commonly follows the mitochondrial pathway of intracellular signal transduction, but the death receptor pathway may also be involved.Undoubtedly, analysis of naturally occurring neuronal death (NOND) in vivo will offer a basis for parallel and future studies aiming to elucidate the mechanisms of pathologic neuronal loss occurring as the result of conditions such as neurodegenerative disorders, trauma or ischemia.

Low-affinity neurotrophin receptor with targeted mutation of exon 3 is capable of mediating the death of axotomized neurons

1. In vivo studies have shown that the low-affinity 75 kDa neurotrophin receptor (p75NTR) is involved in axotomy-induced cell death of sensory and motor neurons. To further examine the importance of p75NTR in mediating neuronal death in vivo, we examined the effect of axotomy in the p75NTR-knockout mouse, which has a disrupted ligand-binding domain. 2. The extent of sensory and motor neuron loss in the p75NTR-knockout mouse following axotomy was not significantly different to that in wild-type mice. This suggests that disruption of the ligand-binding domain is insufficient to block the cell death process in axotomized neurons. 3. Immunohistochemical studies showed that axotomized neurons continue to express this mutant receptor with its intracellular death-signalling moiety intact. 4. Treatment with antisense oligonucleotides targeted against p75NTR resulted in significant reduction in the loss of axotomized neurons in the knockout mouse. 5. These data suggest that the intracellular domain of p75NTR is essential for death-signalling and that p75NTR can signal apoptosis, despite a disrupted ligand-binding domain.

Constitutive expression of the low-affinity neurotrophin receptor and changes during axotomy-induced death of sensory neurones in the neonatal rat dorsal root ganglion

Sensory neurones in the dorsal root ganglion (DRG) of the neonatal rat express the 75-kDa low-affinity neurotrophin receptor (p75NTR) and these neurones degenerate rapidly after axotomy. p75NTR belongs to the tumour necrosis factor superfamily, several members of which have a role in cell death and it is constitutively expressed within a subpopulation of DRG neurones. p75NTR has been implicated in mediating the degeneration of these neurones after axotomy. In this study, we characterize the expression of p75NTR in sensory neurones of the newborn rat DRG using immunohistochemistry. Furthermore, we investigate the change in constitutive expression pattern of p75NTR in these neurones following axotomy. In the C7 and C8 DRG of the newborn rat, p75NTR is expressed in approximately 70% of DRG neurones. Those expressing p75NTR can be classified into subpopulations with moderate or intense p75NTR expression, each present in approximately equal proportions. Whilst p75NTR expression is observed in neurones throughout the entire neuronal diameter range, a correlation exists between neuronal diameter and p75NTR expression intensity. We also found that the most vulnerable population following axotomy were those sensory neurones which constitutively express the highest levels of p75NTR, i.e. the large-diameter neurones.

Novel functional interactions between Trk kinase and p75 neurotrophin receptor in neuroblastoma cells

To understand the functional interactions between the TrkA and p75 nerve growth factor (NGF) receptors, we stably transfected LAN5 neuroblastoma cells with an expression vector for ET-R, a chimeric receptor with the extracellular domain of the epidermal growth factor receptor (EGFR), and the TrkA transmembrane and intracellular domains. EGF activated the ET-R kinase and induced partial differentiation. NGF, which can bind to endogenous p75, did not induce differentiation but enhanced the EGF-induced response, leading to differentiation of almost all cells. A mutated NGF, 3T-NGF, that binds to TrkA but not to p75 did not synergize with EGF. Enhancement of EGF-induced differentiation required at least nanomolar concentrations of NGF, consistent with the low-affinity p75 binding site. EGF may induce a limited number of neuronal cells because it also enhanced apoptosis. Both NGF and a caspase inhibitor reduced apoptosis and, thereby, enhanced differentiation. NGF seems to enhance survival through the phosphatidylinositol-3 kinase (PI3K) pathway. Consistent with this hypothesis, Akt, a downstream effector of the PI3K pathway, was hyperphosphorylated in the presence of EGF+NGF. These results demonstrate that TrkA kinase initiates differentiation, and p75 enhances differentiation by rescuing differentiating cells from apoptosis via the PI3K pathway. Even though both EGF and NGF are required for differentiation of LAN5/ET-R cells, only NGF is required for survival of the differentiated cells. In the absence of NGF, the cells die by an apoptotic mechanism, involving caspase-3. An anti-p75 antibody blocked the survival effect of NGF. Brain-derived neurotrophic factor also enhanced cell survival, indicating that in differentiated cells, NGF acts through the p75 receptor to prevent apoptosis.

Response of GAP-43 and p75 in human neuromas over time after traumatic injury

OBJECTIVE

GAP-43 and p75 are proteins that promote growth cone and neurite formation, elongation, and arborization in regenerating nerve axons. The objectives of this study were to determine whether GAP-43 and the low-affinity nerve growth factor receptor p75 are elevated in traumatic neuromas and whether there is a correlation between the relative amount of GAP-43 or p75 and demographic characteristics such as time elapsed between injury and repair.

METHODS

Traumatic neuromas from 21 randomly selected patients were studied, and the charts were reviewed. Specimens were collected at the time of nerve resection and grafting. Immunohistochemical analysis was performed on each sample and normal human nerve with antibodies to GAP-43 and p75. Western blot and computerized gel analyses were performed.

RESULTS

All neuroma specimens harvested within 13 months of injury exhibited markedly elevated GAP-43 levels compared with normal nerve. Specimens harvested at 14 months or more after injury showed precipitously lower GAP-43 levels, similar to or less than those of normal nerve. The correlation between the amount of intra-axonal GAP-43 and postinjury time interval was statistically significant, P = 0.0038. High GAP-43 levels were also correlated with transection injury, high postoperative sensory grade, and pain. p75 levels were elevated, without consistent variation in our population.

CONCLUSION

These preliminary data suggest that the expression of intra-axonal GAP-43 may vary over time after injury, remaining elevated for approximately the first year, then decreasing abruptly to normal or subnormal levels. These results correlate with clinical experience, indicating that peripheral nerves should be repaired relatively early if repair is indicated.

Modulation of nerve growth factor in peripheral organs by estrogen and progesterone

Nerve growth factor (NGF) synthesized in peripheral organs plays a critical role in the development and maintenance of the nervous system and also participates in processing nociceptive stimuli. Previous studies suggest that reproductive hormones may regulate the expression of NGF. Ovariectomies were performed on female mice, and mice were killed 24 h after hormone replacement to evaluate the effects of estrogen and progesterone on NGF in peripheral organs, specifically the uterus, bladder, heart, and salivary gland. Sham-operated intact mice and untreated ovariectomized mice served as controls. Immunohistochemistry demonstrated the presence of NGF, estrogen receptor-alpha, estrogen receptor-beta, and progesterone receptors in these organs. Ovariectomy caused a significant decrease in NGF protein content in the uterus, and short term treatment of ovariectomized mice with estrogen and/or progesterone increased uterine NGF mRNA and restored NGF protein to concentrations similar to intact control mice. Ovariectomy did not affect NGF protein concentrations in the salivary gland, but treatment of ovariectomized mice with estrogen alone or in conjunction with progesterone stimulated concentrations of NGF protein that exceeded those observed in intact control or ovariectomized, untreated mice. NGF mRNA was increased in salivary glands from ovariectomized mice treated with progesterone alone or in combination with estrogen relative to other groups. NGF protein content of the hearts of ovariectomized mice treated with estrogen alone or in conjunction with progesterone was increased relative to intact controls and ovariectomized, untreated mice, but neither ovariectomy or hormone replacement affected NGF mRNA content in the heart. NGF protein content of the bladder was unaffected by ovariectomy or hormone treatment, and bladder NGF mRNA was unaffected by ovariectomy or hormone treatment. Collectively, these results indicate that reproductive hormones have the capacity to regulate NGF message and protein in a manner that varies among organs. Fluctuations in the expression of NGF, in conjunction with other factors, may help to explain gender differences in pain sensation and inflammatory response.

Characterization of a p75(NTR) apoptotic signaling pathway using a novel cellular model

The p75 neurotrophin receptor (p75(NTR)) belongs to the tumor necrosis factor receptor/nerve growth factor receptor superfamily. In some cells derived from neuronal tissues it causes cell death through a poorly characterized pathway. We developed a neuronal system using conditionally immortalized striatal neurons, in which the expression of p75(NTR) is inducibly controlled by the ecdysone receptor. In these cells p75(NTR) induces apoptosis through its death domain in a nerve growth factor-independent manner. Caspases 9, 6, and 3 are activated by receptor expression indicating the activation of the common effector pathway of apoptosis. Cell death is blocked by a dominant negative form of caspase 9 and Bcl-X(L) consistent with a pathway that involves mitochondria. Significantly, the viral flice inhibitory protein E8 protects from p75(NTR)-induced cell death indicating that death effector domains are involved. A p75(NTR) construct with a deleted death domain dominantly interferes with p75(NTR) signaling, implying that receptor multimerization is required. However, in contrast to the other receptors of the family, p75(NTR)-mediated apoptosis does not involve the adaptor proteins Fas-associated death domain protein or tumor necrosis factor-associated death domain protein, and the apical caspase 8 is not activated. We conclude that p75(NTR) signals apoptosis by similar mechanisms as other death receptors but uses different adaptors and apical caspases.

Systemic administration of antisense p75(NTR) oligodeoxynucleotides rescues axotomised spinal motor neurons

The 75 kD low-affinity neurotrophin receptor (p75(NTR)) is expressed in developing and axotomised spinal motor neurons. There is now convincing evidence that p75(NTR) can, under some circumstances, become cytotoxic and promote neuronal cell death. We report here that a single application of antisense p75(NTR) oligodeoxynucleotides to the proximal nerve stumps of neonatal rats significantly reduces the loss of axotomised motor neurons compared to controls treated with nonsense oligodeoxynucleotides or phosphate-buffered saline. Our investigations also show that daily systemic intraperitoneal injections of antisense p75(NTR) oligodeoxynucleotides for 14 days significantly reduce the loss of axotomised motor neurons compared to controls. Furthermore, we found that systemic delivery over a similar period continues to be effective following axotomy when intraperitoneal injections were 1) administered after a delay of 24 hr, 2) limited to the first 7 days, or 3) administered every third day. In addition, p75(NTR) protein levels were reduced in spinal motor neurons following treatment with antisense p75(NTR) oligodeoxynucleotides. There were also no obvious side effects associated with antisense p75(NTR) oligodeoxynucleotide treatments as determined by behavioural observations and postnatal weight gain. Our findings indicate that antisense-based strategies could be a novel approach for the prevention of motor neuron degeneration associated with injuries or disease.

Localization of low affinity nerve growth factor receptor in the rat inferior olivary complex during development and plasticity of climbing fibres

The rat olivocerebellar pathway has a precise topography from an inferior olive (IOC) to Purkinje cells in the contralateral hemicerebellum. While its development and plasticity have been documented, the molecular mechanisms underlying these events are not fully elucidated. Neurotrophins are a family of growth factors with diverse roles in development and neuronal plasticity, acting through a two-receptor system, including a low affinity receptor (LNGFR) which binds all neurotrophins with similar affinity. Since neurotrophins are present in the cerebellum during early postnatal development when LNGFR is synthesized in the IOC, they may act as target-derived trophic agents for climbing fibres during development and plasticity. To assess this, standard immunohistochemistry was used to document the distribution of LNGFR in the rat IOC during climbing fibre development and until cerebellar development was complete at postnatal day 28 (P28). LNGFR immunoreactivity (LNGFR-IR) was detected in the IOC from P0 until P15, however after P7 it diminished in intensity and distribution, a change which indicates a relationship between cerebellar neurotrophins and climbing fibre development. After denervation of the left hemicerebellum, there was an apparent increase in inferior olivary LNGFR-IR that was concurrent with climbing fibre re-innervation. Thus the results of this study support the hypothesis that neurotrophins are involved in climbing fibre development and suggest a possible contribution to the plasticity of the olivocerebellar pathway.

Downregulation of the p75 neurotrophin receptor in tissue culture and in vivo, using beta-cyclodextrin-adamantane-oligonucleotide conjugates

Formation of complexes with beta-cyclodextrin derivatives via adamantyl groups was found to enhance the uptake and antisense efficacy of phosphorothioate oligos targeted to the p75 neurotrophin receptor in neuronally differentiated PC12 cells. After a 2-week course of systemic administration to mice (by intraperitoneal injection), there was evidence of a pronounced uptake of these oligos by the dorsal root ganglia (DRG), as well as by liver and kidney. There was no uptake by the brain. Consistent with uptake of antisense oligos by the DRG, systemic administration resulted in marked and consistent downregulation of p75 in DRG neurons. These results indicate that cyclodextrin-adamantane-oligo conjugates have great potential as agents to downregulate target genes in neurons, particularly in vivo in the peripheral nervous system.

Chopper, a new death domain of the p75 neurotrophin receptor that mediates rapid neuronal cell death

The cytoplasmic juxtamembrane region of the p75 neurotrophin receptor (p75(NTR)) has been found to be necessary and sufficient to initiate neural cell death. The region was named "Chopper" to distinguish it from CD95-like death domains. A 29-amino acid peptide corresponding to the Chopper region induced caspase- and calpain-mediated death in a variety of neural and non-neural cell types and was not inhibited by signaling through Trk (unlike killing by full-length p75(NTR)). Chopper triggered cell death only when bound to the plasma membrane by a lipid anchor, whereas non-anchored Chopper acted in a dominant-negative manner, blocking p75(NTR)-mediated death both in vitro and in vivo. Removal of the ectodomain of p75(NTR) increased the potency of Chopper activity, suggesting that it regulates the association of Chopper with downstream signaling proteins.

A comparison between antisense p75NTR oligonucleotides and neurotrophic factors in promoting the survival of postnatal sensory neurons in vitro

The 75-kDa low-affinity neurotrophin receptor (p75NTR) has been shown in previous reports to mediate neuronal cell death in vitro and in vivo under certain circumstances. Antisense oligonucleotides directed against p75NTR promote the survival of nerve growth factor-deprived dorsal root ganglia sensory neurons in vitro (Barrett, G.; Bartlett, P., Proc. Natl. Acad. Sci. USA 91:6501-6505; 1994) and axotomized dorsal root ganglia sensory neurons in vivo (Cheema, S. S.; Barrett, G. L.; Bartlett, P. F., J. Neurosci. Res. 46:239-245; 1996). In this study we compared the neuroprotective effects of antisense p75NRT oligonucleotides with two neurotrophic factors, namely nerve growth factor (NGF) and leukemia inhibitory factor, on cultured sensory neurons derived from postnatal day 7 and 14 rat dorsal root ganglia. After 3 d in culture, treatment with the neurotrophic factors had significant survival effects on sensory neuron cultures compared to treatment with basal medium (control). However, after 6 and 9 d in culture these rescue effects were not apparent. In contrast, antisense p75NTR oligonucleotides rescued significantly higher numbers of dorsal root ganglia sensory neurons after 6 and 9 d in culture than treatment with neurotrophic factors, sense oligonucleotides, and basal medium. Furthermore, antisense p75NTR oligonucleotides rescued trkA-, B-, and C-expressing neurons, while NGF and leukemia inhibitory factor targeted primarily the trkA-positive neurons. These findings suggest that antisense-based strategies that inhibit gene expression of cytotoxic molecules are more efficient at preventing postnatal sensory neuronal death in vitro than treatment with individual neurotrophic factors.

Role of neurotrophin receptor p75NTR in mediating neuronal cell death following injury

1. The neurotrophin receptor p75NTR has been shown to mediate neuronal cell death after nerve injury. 2. Down-regulation of p75NTR by antisense oligonucleotides is able to inhibit both sensory and motor neuron death and this treatment is more effective than treatment with growth factors. 3. p75NTR induces cell death by a unique death signalling pathway involving transcription factors (nuclear factor kappa B and c-jun), Bcl-2 family members and caspases.

Treatment of experimental autoimmune encephalomyelitis with antisense oligonucleotides against the low affinity neurotrophin receptor

Upregulated expression of the low-affinity neurotrophin receptor (p75) in the central nervous system (CNS) during experimental autoimmune encephalomyelitis (EAE) has recently been demonstrated. To investigate whether p75 plays a role in disease pathogenesis, we adopted a gene therapy approach, utilizing antisense oligonucleotides to downregulate p75 expression during EAE. Phosphorothioate antisense oligonucleotides (AS), nonsense oligonucleotides (NS) or phosphate buffered saline (PBS) were injected daily for 18 days after immunization of SJL/J (H-2s)-mice with myelin proteolipid protein (PLP) peptide 139-151. In the AS group, there was a statistically significant reduction in both the mean maximal disease score (1.85 in the AS, 2.94 in the NS and 2.75 in the PBS-groups, respectively, P < 0.025) and in the cumulative disease incidence ( approximately 60% in the AS group and approximately 90% in the control groups). Histological and immunohistochemical analysis showed reduced inflammation and demyelination, as well as reduced p75 expression at the blood-brain barrier (BBB) in the AS-treated mice in comparison with both control groups. There was no difference, however, in p75 expression on neural cells within the CNS between the three groups of mice. We conclude that p75 could play a proactive role in the pathogenesis of EAE and may exert its effect at the level of the BBB.

Enlarged cholinergic forebrain neurons and improved spatial learning in p75 knockout mice

The p75 low affinity neurotrophin receptor (p75) can induce apoptosis in various neuronal and glial cell types. Because p75 is expressed in the cholinergic neurons of the basal forebrain, p75 knockout mice may be expected to show an increased number of neurons in this region. Previous studies, however, have produced conflicting results, suggesting that genetic background and choice of control mice are critical. To try to clarify the conflicting results from previous reports, we undertook a further study of the basal forebrain in p75 knockout mice, paying particular attention to the use of genetically valid controls. The genetic backgrounds of p75 knockout and control mice used in this study were identical at 95% of loci. There was a small decrease in the number of cholinergic basal forebrain neurons in p75 knockout mice at four months of age compared with controls. This difference was no longer apparent at 15 months due to a reduction in numbers in control mice between the ages of 4 and 15 months. Cholinergic cell size in the basal forebrain was markedly increased in p75 knockout mice compared with controls. Spatial learning performance was consistently better in p75 knockout mice than in controls, and did not show any deterioration with age. The results indicate that p75 exerts a negative influence on the size of cholinergic forebrain neurons, but little effect on neuronal numbers. The markedly better spatial learning suggests that the function, as well as the size, of cholinergic neurons is negatively modulated by p75.

Differential actions of neurotrophins on apoptosis mediated by the low affinity neurotrophin receptor p75NTR in immortalised neuronal cell lines

The low affinity neurotrophin receptor (p75NTR) mediates apoptosis of a number of neuronal and non-neuronal cells but the signals leading to the apoptosis remain obscure. To reveal the mechanism of p75NTR-mediated apoptosis, a neural cell line expressing human p75NTR was established. The human cDNA fragment encoding for p75NTR was PCR-amplified, cloned into the retrovirus expression vector pXT-1 and transfected into the rat cerebellum cell line R2. The expression of p75NTR in the R2 cell line was demonstrated by both Northern blotting analysis and immunocytochemistry. Serum withdrawal induced dramatic apoptosis in p75NTR-expressing R2 cells (R2L1) but not in pXT-1 transfected control R2 cells (R2P). Reverse transcription polymerase chain reaction (RT-PCR) revealed that these cell lines express trkA and trkB but not trkC. The apoptosis of R2L1 cells triggered by the serum deprivation for 48 h was completely prevented by neurotrophin-3 and the antibody to p75NTR but only partially prevented by the nerve growth factor and brain derived neurotrophic factor. We conclude that the p75NTR mediates apoptosis of R2L1 cells by its intrinsic receptor effects requiring an unbound status of this receptor and that the apoptosis is prevented by neurotrophins or the antibody to p75NTR through distinct mechanisms.

Building blocks of pain: the regulation of key molecules in spinal sensory neurones during development and following peripheral axotomy

The pathways, synapses and molecules involved in pain processing in the newborn are not only required to trigger repair and recuperation but are also involved in the process of forming a mature nervous system. Sensory neurons in the dorsal root ganglion and dorsal horn express a phenomenal array of molecules which contribute to their structural and functional characteristics and many of these are developmentally regulated both pre- and postnatally. In order to understand nociceptive signalling and pain in the neonate we need a clear picture of that regulation. This review concentrates on the changing expression of selected key molecules, receptors and channels in the embryo, neonate and adult, which both characterise the sensory neuron and contribute to its response to painful stimuli in normal and pathological conditions.

Signaling of neuronal cell death by the p75NTR neurotrophin receptor

The neurotrophin receptor (p75NTR) is best known for mediating tropic support by participating in the formation of high-affinity nerve growth factor (NGF) receptor complexes with trkA, however, p75NTR more recently has been shown to act as a bona fide death-signaling receptor, which can signal independently of trkA. This article discusses the evidence for an active role of p75NTR in neuronal cell death and the mechanisms controlling this process, including roles for Bcl-2 family members, the c-jun stress kinase JNK, the transcription factor nuclear factor kappa B (NFkappaB), and caspases.

p75 neurotrophin receptor-mediated neuronal death is promoted by Bcl-2 and prevented by Bcl-xL

The p75 neurotrophin receptor (p75NTR) has been shown to mediate neuronal death through an unknown pathway. We microinjected p75NTR expression plasmids into sensory neurons in the presence of growth factors and assessed the effect of the expressed proteins on cell survival. We show that, unlike other members of the TNFR family, p75NTR signals death through a unique caspase-dependent death pathway that does not involve the "death domain" and is differentially regulated by Bcl-2 family members: the anti-apoptotic molecule Bcl-2 both promoted, and was required for, p75NTR killing, whereas killing was inhibited by its homologue Bcl-xL. These results demonstrate that Bcl-2, through distinct molecular mechanisms, either promotes or inhibits neuronal death depending on the nature of the death stimulus.