The p75 neurotrophin receptor: at the crossroad of neural repair and death.

The strong repair and pro-survival functions of neurotrophins at their primary receptors, TrkA, TrkB and TrkC, have made them attractive candidates for treatment of nervous system injury and disease. However, difficulties with the clinical implementation of neurotrophin therapies have prompted the search for treatments that are stable, easier to deliver and allow more precise regulation of neurotrophin actions. Recently, the p75 neurotrophin receptor (p75(NTR)) has emerged as a potential target for pharmacological control of neurotrophin activity, supported in part by studies demonstrating 1) regulation of neural plasticity in the mature nervous system, 2) promotion of adult neurogenesis and 3) increased expression in neurons, macrophages, microglia, astrocytes and/or Schwann cells in response to injury and neurodegenerative diseases. Although the receptor has no intrinsic catalytic activity it interacts with and modulates the function of TrkA, TrkB, and TrkC, as well as sortilin and the Nogo receptor. This provides substantial cellular and molecular diversity for regulation of neuron survival, neurogenesis, immune responses and processes that support neural function. Upregulation of the p75(NTR) under pathological conditions places the receptor in a key position to control numerous processes necessary for nervous system recovery. Support for this possibility has come from recent studies showing that small, non-peptide p75(NTR) ligands can selectively modify pro-survival and repair functions. While a great deal remains to be discovered about the wide ranging functions of the p75(NTR), studies summarized in this review highlight the immense potential for development of novel neuroprotective and neurorestorative therapies.

Introduction

Neurotrophins and their receptors have the potential to contribute enormously to the restoration of the central nervous system (CNS) and peripheral nervous system (PNS) following injury. Considerable effort has been expended to take therapeutic advantage of their strong survival and growth promoting properties but such efforts have faced significant hurdles due to technical difficulties associated with peptide delivery and the complexity of neurotrophin signaling. Efforts to circumvent these hurdles continue and include the use of better delivery strategies and the development of novel neurotrophin receptor ligands. Each member of the neurotrophin family, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) interacts with specific tropomyosin regulated kinase (Trk) receptors which regulate growth and survival via activation of PI3K-Akt, extracellular signal-regulated kinase (Erk) and phospholipase C-γ (PLCγ) pathways. In addition to the Trk receptors, both the mature neurotrophins and their pro-neurotrophin precursors can bind to the p75 neurotrophin receptor (p75NTR), a single membrane spanning protein in the tumor necrosis factor (TNF) receptor family. Unlike the Trk receptors which autophosphorylate after ligand engagement, the p75NTR does not have intrinsic catalytic activity. Instead, it partners with the three neurotrophin receptors, TrkA, TrkB, TrkC, as well as non-neurotrophin receptors, sortilin and Nogo. These various interactions, summarized in , provide the opportunity for the p75NTR to influence a wide range of cellular functions.

Figure 1

Potential signaling partners of the p75 neurotrophin receptor (p75NTR) following nervous system injury.

The p75NTR is a single membrane-spanning receptor in the tumor necrosis factor (TNF) death domain containing receptor family. Four cysteine-rich extracellular domains bind each of the four neurotrophins: NGF, BDNF, NT-3 or NT-4 with low affinity and their respective pro-forms with high affinity. The intracellular portion of the receptor does not contain a catalytic domain to autoactivate the receptor. Thus, the receptor functions largely via interactions with other effector proteins. The p75NTR can interact with at least three different receptor classes which mediate different outcomes: Trk receptors, sortilin and Nogo receptor. 1) The p75NTR-Trk receptor complex increases the affinity of the mature neurotrophin/Trk interaction and enhances pro-survival and growth signaling via PI3KAkt, ERK or PLCγ pathways. In addition, expression on neural progenitors supports proliferation and differentiation. 2) Interactions of p75NTR with sortilin allow high affinity binding of the pro-neurotrophins to the p75NTR/sortilin complex leading to apoptosis. However, the role of sortilin may be diverse as it also supports neuron survival via its prominent sorting functions which include anterograde trafficking of neurotrophin receptors. 3) Interactions of the p75NTR with the Nogo receptor and Lingo-1 play a role in the control of growth. Binding of myelin proteins to the complex activates RhoA by displacement of Rho-GDI and concurrently suppress Rac leading to collapse of growth cones, neurite retraction and decreases in spine density. These various interactions afford the p75NTR the capacity to play a pivotal role in the regulation of numerous processes that determine the fate and functions of the cell. The role of the p75NTR in response to injury is largely unknown and is a potential source of new strategies to facilitate recovery of the damaged nervous system. NGF: Nerve growth factor; BDNF: brain-derived neurotrophic factor; NT-3: neurotrophin-3; NT-4: neurotrophin-4; Trk: tropomyosin regulated kinase; ERK: extracellular signal-regulated kinase; PLCγ: phospholipase C-γ.

When complexed with Trk receptors, the p75NTR increases the affinity of the mature neurotrophins and supports pro-survival and pro-growth signaling (Hempstead et al., 1991). In contrast to the mature neurotrophins, pro-neurotrophins bind with high affinity to the p75NTR/sortilin complex, which often leads to the activation of apoptotic pathways and death (Friedman, 2000; Beattie et al., 2002; Nykjaer et al., 2004). In complex with Nogo receptor and Lingo-1, the p75NTR supports growth cone retraction in response to myelin-derived proteins (Wang et al., 2002; Wong et al., 2002; Mi et al., 2004). Each of these p75NTR mediated processes contributes in different ways to the development, maturation and maintenance of the nervous system (Kraemer et al., 2014).

During development, the p75NTR plays a well-defined role in synapse strengthening and neuron selection. With the notable exception of cholinergic cells of the basal forebrain, the p75NTR is highly down regulated as the nervous system matures. While the low level of expression suggested that the p75NTR may have limited function in the adult nervous system, several new studies have identified important functions of the p75NTR such as hippocampal synapse modification (Rosch et al., 2005; Woo et al., 2005; Zagrebelsky et al., 2005) and the regulation of neurogenesis (Bernabeu and Longo, 2010). In addition, many studies have now shown that the p75NTR can increase dramatically in response to injury or disease in both the PNS and CNS. These observations highlight the expanding role of the p75NTR in the regulation of nervous system function in both normal and pathological states.

As recently reviewed (Ibanez and Simi, 2012; Kraemer et al., 2014; Meeker and Williams, 2014), increased neuronal expression of the p75NTR has been documented in response to axotomy (Zhou et al., 1996; Johnson et al., 1999), neural damage, intraocular pressure (Wei et al., 2007), seizures (Roux et al., 1999; Ozbas-Gerceker et al., 2004) and ischemia (Kokaia et al., 1998). In addition to neurons, injury-induced increases in p75NTR expression may occur in Schwann cells (Taniuchi et al., 1986; Gschwendtner et al., 2003; Zhou and Li, 2007; Provenzano et al., 2008), astrocytes (Choi and Friedman, 2009), oligodendrocytes (Beattie et al., 2002) or microglia/macrophages (Dowling et al., 1999; Wong et al., 2010). Why does expression of the p75NTR increase in response to damage? This is still an emerging area of research but there are several notable possibilities. The p75NTR may 1) enhance survival of injured neurons by increasing the efficiency of Trk activation, 2) trigger apoptosis to eliminate damaged cells while minimizing inflammatory responses, 3) provide a supportive environment for appropriate re-growth and/or 4) control inflammation.

Survival versus Apoptosis: the Decision to Live or Die

The ability of the p75NTR to support either the pro-survival actions of the Trk receptors or the induction of apoptosis with sortilin, places it in a position to make major life and death decisions for the cell. Given this diversity, it is not too surprising that many studies have been published in support of either death or survival functions of the p75NTR following injury (Meeker and Williams, 2014).

In p75NTR deficient mice, less cell death is seen after axotomy (Sorensen et al., 2003), seizures (Troy et al., 2002) or ligation by proneurotrophins (Teng et al., 2005). Similar effects are seen in sortilin knockout mice suggesting that the p75NTR/sortilin interaction underlies the loss of the cells (Nykjaer and Willnow, 2012). An increased capacity for interactions of the p75NTR with sortilin following injury may recapitulate its functions in the developing nervous system where apoptosis plays an important role in eliminating neurons that make weak connections in a “survival of the fittest” strategy. After injury, a similar strategy may be essential for effective resolution of injury while minimizing inflammation.

Knockdown of the p75NTR also reduces damage in neurodegenerative disease models (Underwood and Coulson, 2008). In the superoxide dismutase 1 (SOD1) mutant mouse, an amyotrophic lateral sclerosis (ALS) model which develops severe neurodegeneration, the expression of p75NTR correlated with the extent of degeneration (Shepheard et al., 2014) and p75NTR knockdown delayed disease progression (Turner et al., 2003). In animal models of Alzheimer's disease (AD), the p75NTR contributes to amyloid β (Aβ)-induced neural damage (Yang et al., 2008). In humans with AD, increases in p75NTR expression relative to TrkA have been suggested to be responsible for the loss of cholinergic neurons (Counts et al., 2004; Costantini et al., 2006). Increases in proNGF in AD (Fahnestock et al., 2001) indicate that the neurotrophin environment is favorable for p75NTR/sortilin signaling and supports the theory that age-related neural damage is facilitated by a shift toward proNGF-mediated signaling. A similar shift to p75NTR expression in response to injury has been suggested to foster damage and loss of neurons.

While the above findings implicate p75NTR involvement in the loss of neural cells, many other studies support the idea that the p75NTR plays an essential role in protection and recovery of the nervous system (Meeker and Williams, 2014). For example, expression of the p75NTR in proliferating satellite glia in the injured dorsal root ganglion (DRG) correlates with the appearance of sympathetic sprouting which is attenuated in p75NTR null mice (Hannila and Kawaja, 2005). In studies of motor neuron injury, less motor neuron survival was seen after the introduction of grafted Schwann cells deficient in p75NTR (Tomita et al., 2007). BDNF, a positive modulator of myelination during peripheral nerve recovery, is dependent on the p75NTR and knockout of the p75NTR impairs re-myelination of injured sciatic nerve (Song et al., 2006). Chu et al. (2007) observed that the p75NTR was essential for functional recovery after spinal cord injury. In cultured hippocampal neurons subjected to excitotoxic concentrations of glutamate, the p75NTR was necessary to see protective NGF/TrkA signaling (Culmsee et al., 2002). Thus, in many studies, the p75NTR appears to play a pivotal role in support of neuron survival and regrowth.

The seemingly contradictory observations noted above reflect the complex and diverse roles of the p75NTR, where some conditions may support repair while other conditions facilitate neural damage and apoptosis. The pivotal role of the p75NTR in crucial decisions regarding cell fate and efforts to support nervous system recovery following injury provides opportunities to therapeutically influence the recovery process. However, the implementation of new therapies will require a better understanding of how opposing effects of the p75NTR are regulated.

Neural Plasticity and Re-growth

The regulation of proper growth and synaptic plasticity also depends on p75NTR functions. Depending on the context, the p75NTR may either support or restrict new growth. Restrictions to axonal growth in the damaged nervous system are thought to be due, in part, to the growth inhibiting effects of myelin proteins (Nogo, MAG, MOG) that bind to the p75NTR/Nogo receptor complex. Since excess activity of this pathway is thought to restrict neural recovery after injury, many therapeutic efforts to overcome these restrictions have focused on suppression of p75NTR/Nogo receptor activity (Fry et al., 2007). However, the p75NTR may also play a positive role in recovery by suppressing inappropriate growth and therefore restricting new growth and re-innervation to appropriate targets. Thus, efforts to prevent growth restrictions post-injury have to be balanced with appropriate guidance of axonal outgrowth and development of synaptic connections.

The role of the p75NTR in synapse modification is still emerging, and there is a need to better understand how the synapse is regulated by the p75NTR under normal conditions as well as in response to injury. In rat hippocampal slices, proBDNF enhanced long-term depression in a p75NTR-dependent fashion, indicating that the p75NTR plays a normal role in the regulation of synaptic plasticity (Woo et al., 2005) perhaps by modifying AMPA glutamate receptor subunit expression (Rosch et al., 2005) and/or regulation of spine density (Zagrebelsky et al., 2005). The impact of increases in p75NTR expression in response to injury on synaptic function remains to be determined and is fertile ground for studies of nervous system recovery.

The role of the p75NTR in neurogenesis of adult neural progenitors may also play a role in recovery of function following injury. Neurospheres generated from p75NTR-positive cells have been shown to be selectively neurogenic, possibly via a synergistic effect with BDNF or NGF stimulation (Young et al., 2007). Adult mice deficient in p75NTR show reduced proliferation/maturation of hippocampal dentate progenitors (Bernabeu and Longo, 2010). The functional implications of these observations are still being explored but are likely to impact the extensive efforts to develop neuroregenerative therapies designed to enhance production of new neurons from progenitors.

Immune Cell Activity and Clearance

Finally, a new and relatively unexplored area that may ultimately have a significant impact on nervous system recovery is revealed by the increased expression of the p75NTR on macrophages and microglia under different pathological conditions (Meeker and Williams 2014). Exploration of the functions of the p75NTR in macrophages and microglia has been limited but the expression of Trk receptors, sortilin and Nogo receptors indicate that the same signaling pathways are present that exist in neurons. Several studies suggest that neurotrophins may regulate monocyte/macrophage chemotaxis and retention at sites of injury (Fry et al., 2007; David et al., 2008; Yan et al., 2012) and promote recovery from spinal cord injury (Zhu et al., 2010). The p75NTR/Nogo receptor complex may play a role in resolution of inflammation by facilitating macrophage clearance. In Nogo receptor deficient mice with sciatic nerve crush injury, macrophages fail to leave the area following myelin regeneration (Fry et al., 2007). The repulsive interactions of myelin proteins with Nogo appear to be necessary for migration from the site of injury and ultimate resolution of the inflammation. Actions of neurotrophins may also participate in the functional differentiation of macrophages/microglia and this may provide new avenues for regulation of the wide range of supportive or deleterious actions of these cells in the injured nervous system.

The p75NTR as a Therapeutic Target

While the diverse and often opposing roles of the p75NTR pose significant challenges for attempts to develop therapies, recent studies have clearly demonstrated the feasibility of efforts to pharmacologically modify p75NTR function (Longo and Massa, 2013). Nanomolar concentrations of the experimental compound, LM11A-31, a non-peptide structural mimetic of loop1 of NGF, have been shown to have neurotropic properties and to offer potent neuroprotection from Aβ induced damage in mouse and culture models of AD (Yang et al., 2008; Nguyen et al., 2014), spinal cord injury (Tep et al., 2013), traumatic brain injury (Shi et al., 2013), virus-induced inflammation (Meeker et al., 2012) and chemotherapeutic toxicity (James et al., 2008) in the absence of pro-apoptotic effects. This therapeutic approach was based, in part, on the concept that small molecules might regulate neurotrophin receptor dimerization, activation and/or interactions with co-receptors in ways that do not necessarily recapitulate the full actions of the naturally occurring peptides. Because the binding may be unconventional, the compounds were identified through functional screens designed to reveal neurotrophic, pro-survival activity. Therapeutic efficacy has now been evaluated in a variety of in vitro and in vivo models. LM11A-31 inhibits neuritic dystrophy and impairments in long-term potentiation (LTP) induced in neural cultures treated with Aβ; a protective effect not recapitulated by NGF (Yang et al., 2008). In a mouse model of AD, 3 months of oral administration decreased CNS pathology and suppressed the development of behavioral deficits (Nguyen et al., 2014). In a model of spinal cord injury with robust upregulation of p75NTR expression, LM11A-31 promoted recovery in tests of motor function in the absence of effects on pain sensitivity (Tep et al., 2013). Protection from RhoA-dependent degeneration of DRG cultures exposed to chemotherapeutic agents (James et al., 2008) and the dendritic beading, pruning and calcium dysregulation associated with viral inflammation (Meeker et al., 2012) suggest that prevention of cytoskeletal damage may be a common effect that confers protection in a variety of pathological circumstances. The precise pharmacological mechanisms of protection are under investigation but may include facilitation of Akt signaling and inhibition of proNGF binding. The ability to selectively modify specific neurotrophin actions with small ligands is also seen with the experimental TrkA ligand, MT2, which has been shown to influence the TrkA tyrosine phosphorylation pattern, resulting in NGF-like pro-survival activity with significantly less effect on differentiation (Scarpi et al., 2012). Although these efforts are still in relatively early stages of development they provide strong proof of concept that small molecules targeted to specific domains of the p75NTR can exert fine control over neurotrophin receptor signaling. Thus, the p75NTR is a novel and versatile target for the development of neurotrophin based therapies.

Conclusion

As evidenced in the above studies, the p75NTR supports a wide range of activity throughout the CNS. The ability to interact with all Trk receptors, sortilin, and the NOGO receptor affords the p75NTR with the opportunity to control many different signaling pathways. Established roles for p75NTR in neurogenesis, regulation of sprouting, synaptogenesis and pruning of unwanted synapses puts it in the center of major efforts to promote recovery of the nervous system. The many functions of the p75NTR are still being defined but should yield major insights into the processes that regulate neuronal growth, survival and plasticity under both normal and pathological conditions. The challenge of future studies will be to determine the role of the various p75NTR-dependent pathways in response to injury as well as to determine the appropriate balance of activity that maximally supports recovery. The pivotal role of the p75NTR in so many processes necessary for nervous system recovery and the promising results from p75NTR-targeted interventions makes the receptor an attractive target for the development of novel neuroprotective and neurorestorative therapies.