The neurotrophic signaling pathway describes the intracellular cascades activated by neurotrophin receptors to promote neuronal survival, differentiation, and synaptic plasticity. This pathway is fundamental to nervous system development and maintenance, and its dysregulation is implicated in multiple neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis[1].
This page describes the molecular mechanisms of neurotrophin receptor signaling. For information about specific neurotrophic factors, see Neurotrophic Factors in Neurodegeneration.
The Trk (tropomyosin receptor kinase) family includes TrkA, TrkB, and TrkC, each serving as high-affinity receptors for specific neurotrophins:
Ligand binding induces receptor dimerization and autophosphorylation at specific tyrosine residues, creating docking sites for downstream signaling adaptors[2].
The PI3K/AKT pathway is the primary mediator of neurotrophin-induced neuronal survival. Activated Trk receptors recruit PI3K to the membrane, where it phosphorylates PIP2 to generate PIP3. AKT (protein kinase B) is then recruited to the membrane and activated by PDK1-dependent phosphorylation. Active AKT phosphorylates multiple targets that promote cell survival, including BAD, caspase-9, and GSK-3β[3].
Key downstream effects:
The PI3K/AKT pathway is critically impaired in Alzheimer's disease, contributing to tau hyperphosphorylation and amyloid-β toxicity[4].
The RAS/ERK (extracellular signal-regulated kinase) pathway mediates the differentiation and plasticity effects of neurotrophins. Following Trk activation, RAS is recruited to the membrane and activated through a Grb2/SOS-dependent mechanism. Activated RAS initiates a kinase cascade involving Raf, MEK, and ERK[2:1].
Key downstream effects:
In neurodegeneration, ERK signaling has complex roles—it can be protective in some contexts but may contribute to pathological processes when chronically activated.
The PLC-γ pathway is activated by direct binding of phospholipase C-γ (PLC-γ) to phosphorylated Trk receptors. PLC-γ hydrolyzes PIP2 to generate IP3 and DAG, leading to calcium release from intracellular stores and activation of protein kinase C (PKC)[2:2].
Key downstream effects:
The p75 neurotrophin receptor (p75^NTR) can bind all neurotrophins with similar affinity and modulates the signaling outcomes of Trk receptors. When expressed alone, p75^NTR typically promotes apoptosis through activation of the NF-κB pathway and JNK kinase cascades[1:1].
The apoptotic signaling cascade initiated by p75^NTR involves:
In neurodegenerative diseases, p75^NTR expression is often upregulated in vulnerable neuronal populations, and this upregulation is associated with increased apoptosis.
p75^NTR can promote either survival or death depending on:
Pro-neurotrophins binding to p75^NTR/sortilin complexes preferentially trigger apoptosis, while mature neurotrophins promoting Trk-mediated survival.
In Alzheimer's disease, multiple components of neurotrophin signaling are disrupted:
In Parkinson's disease, neurotrophin signaling is compromised:
In ALS, neurotrophin signaling fails through:
The development of small molecule Trk agonists represents a promising approach to enhance neurotrophin signaling. Unlike native neurotrophins, these small molecules can cross the blood-brain barrier and are suitable for chronic oral administration.
AAV-mediated delivery of neurotrophin genes offers potential for sustained expression in target brain regions. Challenges include achieving appropriate spatial and temporal expression without causing side effects.
Chao, M.V., et al. Neurotrophin signaling in the nervous system. Cold Spring Harbor Perspectives in Biology. 2014. ↩︎ ↩︎
Patapoutian, A., Reichardt, L.F. Trk receptors: mediators of neurotrophin actions. Progress in Brain Research. 2001. ↩︎ ↩︎ ↩︎
Soehnlein, O., et al. Signal transduction pathways in neurotrophin receptor signaling. Cellular and Molecular Life Sciences. 2017. ↩︎
Huang, Y., et al. Dysregulation of neurotrophin signaling in neurodegenerative diseases. Nature Reviews Neurology. 2022. ↩︎ ↩︎