Neurotrophins are a family of proteins that influence neurons through cell signaling. Specifically, neurotrophins bind to a cell’s receptors to “send a message” to the cells, and thereby cause a reaction or change from the cell. And neurotrophins bind to two classes of receptors: the p75, and the Trk families of Tyrosine kinase receptors.
p75 is a low-affinity neurotrophin receptor that all neurotrophins can bind to, and is also a member of the tumor necrosis super family. It seems to me that when the p75 receptor is activated, it facilitates both cellular apoptosis and survival depending on the cellular circumstance.
Trk receptors stands for “Tropomyosin-related kinase” receptors. Trk receptors when activated either directly or indirectly (through signaling cascade) regulate the synaptic strength and plasticity of neurons, affect neuronal survival & differentiation, and also affect the function of neurons.
The 3 main types of Trk receptors include TrkA, TrkB, and TrkC. Each of these receptors have different binding affinities to different types of neurotrophins, i.e. different Trk receptors interact with neurotrophins at different strengths. Like how well different keys fit into a lock. A good example of binding affinities are with caffeine & theobromine, where caffeine & theobromine both target the adenosine receptor. But because caffeine is a better fit than theobromine, it explains why a cup of coffee tends to wake you up better than a cup of hot chocolate.
The TrkA receptor has the highest affinity to binding to Nerve Growth Factor (NGF). The TrkB receptor has the highest affinity to binding to BDNF and NT-4. And the TrkC receptor has the highest affinity (and has little activation to other ligands or neurotrophins) to binding with NT-3
TrkA receptors mediates the effects of NGF. TrkB receptors binds and is activated by BDNF, NT-4, and NT-3. And TrkC binds and is activated only by NT-3.
- The p75 Neurotrophin Receptor Activates Akt (Protein Kinase B) through a Phosphatidylinositol 3-Kinase-dependent Pathway [J. Biol. Chem.]