Tropomyosin receptor kinase A
Tropomyosin receptor kinase A (TrkA), also known as high affinity nerve growth factor receptor, neurotrophic tyrosine kinase receptor type 1, or TRK1-transforming tyrosine kinase protein is a protein that in humans is encoded by the NTRK1 gene.
This gene encodes a member of the neurotrophic tyrosine kinase receptor (NTKR) family. This kinase is a membrane-bound receptor that, upon neurotrophin binding, phosphorylates itself (autophosphorylation) and members of the MAPK pathway. The presence of this kinase leads to cell differentiation and may play a role in specifying sensory neuron subtypes. Mutations in this gene have been associated with congenital insensitivity to pain with anhidrosis, self-mutilating behaviors, intellectual disability and/or cognitive impairment and certain cancers. Alternate transcriptional splice variants of this gene have been found, but only three have been characterized to date.

# Function
TrkA is the high affinity catalytic receptor for the neurotrophin, Nerve Growth Factor, or "NGF".  As such, it mediates the multiple effects of NGF, which include neuronal differentiation and avoidance of programmed cell death.

# Family members
TrkA is part of a sub-family of protein kinases which includes TrkB and TrkC.  Also, there are other neurotrophic factors structurally related to NGF: BDNF (for Brain-Derived Neurotrophic Factor), NT-3 (for Neurotrophin-3) and NT-4 (for Neurotrophin-4). While TrkA mediates the effects of NGF, TrkB is bound and activated by BDNF, NT-4, and NT-3.  Further, TrkC binds and is activated by NT-3.
There is one other NGF receptor besides TrkA, called the "LNGFR" (for "Low Affinity Nerve Growth Factor Receptor"). As opposed to TrkA, the LNGFR plays a somewhat less clear role in NGF biology. Some researchers have shown the LNGFR binds and serves as a "sink" for neurotrophins. Cells which express both the LNGFR and the Trk receptors might therefore have a greater activity  since they have a higher "microconcentration" of the neurotrophin. It has also been shown, however, that in the absence of a co-expressed TrkA, the LNGFR may signal a cell to die via apoptosis  so therefore cells expressing the LNGFR in the absence of Trk receptors may die rather than live in the presence of a neurotrophin.

# Role in disease
TrkA was originally cloned from a colon tumor; the cancer occurred via a translocation, which resulted in the activation of the TrkA kinase domain.  However, TrkA itself does not appear to be an oncogene.
In one study, a total absence of TrkA receptor was found in keratoconus-affected corneas, along with an increased level of repressor isoform of Sp3 transcription factor.
Gene fusions involving NTRK1 have been shown to be oncogenic, leading to the constitutive TrkA activation.  In a research study by Vaishnavi A. et al., NTRK1 fusions are estimated to occur in 3.3% of lung cancer as assessed through next generation sequencing or fluoresence in situ hybridization.

# Regulation
The levels of distinct proteins can be regulated by the "ubiquitin/proteasome" system.  In this system, a small (78 kd)protein called "ubiquitin" is affixed to a target protein, and is thereby targeted for destruction by a structure called the "proteasome".  TrkA is targeted for proteasome-mediated destruction by an "E3 ubiquitin ligase" called NEDD-4. This mechanism may be a distinct way to control the survival of a neuron. The extent and maybe type of TrkA ubiquitiniation can be regulated by the other, unrelated receptor for NGF, p75NTR.

# Interactions
TrkA has been shown to interact with:
- Abl gene,
- FRS2,
- Grb2,
- MATK,
- NGFB,
- PLCG1,
- RICS,
- SQSTM1,
- SH2B1,
- SH2B2, and
- SHC1.

# Ligands
Small molecules such as amitriptyline and gambogic acid derivatives have been claimed to activate TrkA. Amitriptyline activates TrkA and facilitate the heterodimerisation of TrkA and TrkB in the absence of NGF. Binding of amitriptyline to TrkA occurs to the Leucine Rich Region (LRR) of the extracellular domain of the receptor, which is distinct from the NGF binding site. Amitryptiline possesses neurotrophic activity both in-vitro and in-vivo (mouse model). Gambogic amide, a derivative of gambogic acid, selectively activates TrkA (but not TrkB and TrkC) both in-vitro and in-vivo by interacting with the cytoplasmic juxtamembrane domain of TrkA.

# Role in cancer
Although originally identified as an oncogenic fusion in 1982, only recently has there been a renewed interest in the Trk family as it relates to its role in human cancers because of the identification of NTRK1 (TrkA), NTRK2 (TrkB) and NTRK3 (TrkC) gene fusions and other oncogenic alterations in a number of tumor types. A number of Trk inhibitors are (in 2015) in clinical trials and have shown early promise in shrinking human tumors.

## Inhibitors in development
Entrectinib (formerly RXDX-101) is an investigational drug developed by Ignyta, Inc., which has potential antitumor activity. It is a selective pan-trk receptor tyrosine kinase inhibitor (TKI) targeting gene fusions in trkA, trkB, and trkC (coded by NTRK1, NTRK2, and NTRK3 genes) that is currently in phase 2 clinical testing.