Figure 1. Neurotrophin signaling pathway
Neurotrophins are a family of proteins that play an essential role in the survival, development and function of neurons. They significantly regulate axonal and dendritic growth and guidance, synaptic structure and connections, neurotransmitter release, long-term potentiation (LTP) and synaptic plasticity. Neurotrophins prevent the associated neurons from initiating programmed cell death, which allows the neurons to survive. They also induce the differentiation of progenitor cells to form neurons. During the neurogenesis, neurotrophins are chemicals that help to stimulate and control this process. The alterations of neurotrophin levels show various effects on a wide series of phenomena, including myelination, regeneration, pain, aggression, depression and substance abuse. Nevertheless, neurotrophins are not necessary for development of neuronal circuits since they are not found in Drosophila and C.elegans. Neurotrophins are initially synthesized as precursors or proneurotrophins which are cleaved to produce the mature proteins. Pro-neurotrophins are cleaved intracellularly by FURIN, an endopeptidase with specificity for the consensus sequence Arg-X-Lys/Arg-Arg, or by pro-convertases at a highly conserved dibasic amino-acid cleavage site to release carboxy-terminal mature proteins. All mature proteins are about 12 kDa in size, forming the stable and non-covalent dimers.
There are four types of neurotrophins in mammals in total: nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). They are derived from common ancestral genes and share sequence and structural similarities. Nerve growth factor (NGF), the prototypical growth factor, is critical for the survival and maintenance of sympathetic and sensory neurons. After being released from the target cells, NGF binds and activates its receptor TrkA on the neuron. Brain-derived neurotrophic factor (BDNF) is a neurotrophic factor found originally in the brain, but actually it is also expressed in the central neural system, retina, motor neurons, kidneys and the prostate. BDNF plays a significant role in supporting the survival of existing neurons and it also contributes to the growth and differentiation of new neurons and synapses through axonal and dendritic sprouting. BDNF is also one of the most active substances that stimulate the generation of neurons, namely the neurogenesis. Neurotrophin-3 (NT-3), a neurotrophic factor in the NGF-family of neurotrophins, has the similar functions as BDNF’s and acts on certain neurons of the peripheral and central nervous system. NT-3 is unique among the neurotrophins since it is able to activate two of the receptor tyrosine kinase neurotrophin receptors (TrkC and TrkB). Neurotrophin-4 (NT-4) which is also known as NT4, NT5, NTF4, and NT-4/5 is a neurotrophic factor. NT-4 mainly binds TrkB tyrosine receptor kinase to initiate its signaling pathway.
The neurotrophins interact with two entirely distinct classes of receptors: p75 neurotrophin receptor (p75NTR) and tropomyosin receptor kinase (Trk). p75 neurotrophin receptor (p75NTR), the first receptor to be discovered, is a receptor which can bind each of the neurotrophins with a low affinity. p75NTR belongs to the tumor necrosis receptor superfamily. It has an extracellular domain with four cysteine-rich motifs, a single transmembrane domain and a cytoplasmic domain with a “death domain” which shares the structural similarity with those present in other members of the tumor necrosis receptor superfamily. Especially, it is the four cysteine-rich motifs in extracellular domain that associate with NGF dimers.
Figure 2. Neurotrophin receptors
The second class of neurotrophin receptors is the Trk receptor tyrosine kinase family which includes four members. Trk receptors are directly bound and dimerized by neurotrophins, which lead to the activation of the tyrosine kinases present in their cytoplasmic domains. Each neurotrophin has its specificity for particular receptor. NGF binds preferentially to TrkA; BDNF and NT-4 are specific for TrkB. Both of them are actually of low affinity. While NT-3 is able to activate all Trk receptors, it mainly binds to TrkC with higher affinity. The most important site where Trk receptors interact with neurotrophins has been localized to the most proximal immunoglobulin (Ig) domain of each receptor which regulates the strength and specificity of binding between neurotrophins and Trk receptors. In addition, the interaction between the neurotrophin and its receptor can be affected by receptor dimerization, structural modifications or association with the p75 receptor.
In addition to binding to neurotrophin, the p75 receptor can also act as a coreceptor for Trk receptors. The existence of p75 enhances the affinity of TrkA for NGF and its specificity for cognate neurotrophins, which leads to the higher ligand selectivity conferred on the Trk receptors by the p75 receptor.
Figure 3. The model of Trk/p75 coreceptor
Neurotrophin signaling pathway
Neurotrophin signaling cascade
Although Trk and p75 receptors can form co-receptors, they still show independent signaling properties and separate downstream signal transduction pathways which make great contributions to individual physiological responses. Trk receptors mediate neural cells differentiation and survival signaling by activating extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3K) and phospholipase Cγ (PLC-γ) pathways. The binding of neurotrophin as dimers to Trk receptor induces Trk receptor dimerization which results in trans-autophosphorylation and the activation of intracellular signaling cascades. The activated Trk receptors recruit and increase the phosphorylation of PLC-γ and Src homologous and collagen-like adaptor protein (Shc). The interaction between Trk receptor and Shc enhances the activities of phosphatidylinositol 3-kinase (PI3K) and protein kinase B (Akt). The phosphorylated Shc also activates Ras via Grb2 and SOS protein, which further promotes the extracellular signal-regulated kinase (ERK) pathway. Rap1 exerts its positive impacts on ERK pathway mediated by activated Ras from an endosomal location. Different from Trk’s adaptor proteins, p75 receptors activated by ligand binding predominantly increase Jun N-terminal kinase (JNK), NF-κB, ceramide and modulate RhoA activity. The cytoplasmic domain of p75 is critical to these interactions with adaptor proteins which also include neurotrophin-receptor interacting factor (NRIF), neurotrophin-associated cell death executor (NADE), neurotrophin-receptor-interacting MAGE homologue (NRAGE), Schwann cell 1 (SC1) and receptor-interacting protein 2 (RIP2).
Many key components of other pathways mediate neurotrophin signaling, such as ERK, Akt, PLC, PKC, Ras, JNK and NF-κB, which means neurotrophin signaling pathway regulates a series of downstream signaling pathways. After the binding of neurotrophin, activated Trk phosphorylates Shc to activate the ERK pathway through Ras, which in turn influences transcriptional events, such as the induction of the cyclic AMP-response element binding (CREB) transcription factor. This regulation is of great importance during the neural cells differentiation and survival. The small G protein Rap1 secreted from endosome also makes a contribution to this process. As for the signaling via p75 receptor, the downstream signaling JNK and NF-κB are also activated, via Rac1 and RIP2 respectively. The JNK activity is essential for NGF-dependent death and the NF-κB mediates a survival response which is similar to the behavior of other tumor necrosis factor receptors.
The activation of Trk receptors not only promotes other signaling pathways but is also related to ion channels controls. NGF can produce an approximately 30-fold increase in proton-evoked currents via TRPV1 when it interacts with TrkA. The recruitment of PLC-γ to TrkA is essential for the NGF-mediated potentiation of channel activity and the related events. With the association of BDNF, TrkB increases tyrosine phosphorylation of NMDA and voltage-gated potassium channels, which blocks postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated currents. This further decides activity-dependent changes of synaptic efficacy.
Figure 4. The interaction between ion channels with Trk receptors
The effects of neurotrophin signaling pathway depend on a series of factors, including the level of neurotrophin, the affinity of binding to its transmembrane receptors and the duration and intensity of downstream signaling cascades that are initiated after the receptor is activated. The factors accounting for the inhibition of neurotrophin signaling pathway are tightly regulated by a range of different mechanisms, including dephosphorylation and degradation.
For example, the protein tyrosine phosphatase SH2 domain-containing phosphatase -2 (SHP-2) has a transient association with the Trk receptors upon neurotrophin binding and then provides an important counterbalance to TrkB activation. Another related phosphatase named SHP-1 can dephosphorylate residues Y674 and Y675 in the TrkA tyrosine kinase activation loop, which significantly inhibits the activity of TrkA and then impedes the further signaling. The protein tyrosine phosphatase receptor zeta (PTPRz) acts the same as SHP-1. In addition to dephosphorylation, the degradation mediated by ubiquitin can also negatively regulate neurotrophin signaling pathway. Ubiquitin ligase TRAF6 degrades TrkA through poly-ubiquitination. The ubiquitin ligase Cbl has also been shown that it is key in the ligand-dependent ubiquitination of TrkA and subsequent targeting of the receptor to lysosomes for degradation and then inhibition of the signaling cascade.
Relationship with diseases
Neurotrophin signaling pathway involves in many neurodegenerative disorders, such as Alzheimer's disease, Huntington's disease and psychiatric disorders, including depression and substance abuse. The polymorphism in the pro-domain of BDNF caused by a single amino-acid change is highly associated with bipolar disorders, depression and schizophrenia. As a highly conserved protein, BDNF may cause severe disorders in health once mutations occur. Since neurotrophins also have essential influences on synaptic connection and plasticity, as well as neuro-transmission, some mis-regulations can cause damage or even irreversible damage to neural circuits.