2009 Group 4 Project

From CellBiology

'"Tropomyosin-receptor-Kinase (Trk)"'


The tropomyosin-receptor kinase (trk) belongs to the family of tyrosine-protein kinase receptors (TK), which was discovered as a proto-oncogene that consisted of seven exons of ‘non-muscle tropomyosin’ gene fused to the cytoplasmic domains of a novel ‘tyrosine-kinase’ gene. Trks are commonly associated with cell survival, proliferation, neurite growth including axons & dendrites as well as regulatory function of ion channels and neurotransmitter receptors in immature neural cells. In addition, binding of ligands to trks can affect growth, plasticity and synaptic strength of neuronal junctions in the adult nervous system (ref), although it is notable that Ligand binding does not itself illicit an action potential event. Three trk genes have been identified in mammals: trkA (proto-oncogene), which serves as an NGF receptor, trkB and trkC, that each was subsequently identified due to their high homology to trkA(ref).

--Hamid/Arash Araghi 08:39, 14 May 2009 (EST)


  • Trk receptors are made of an extracellular segment that binds polypeptide ligands, transmembrane helix, and cytoplasmic segment where the tyrosine kinase catalytic activity takes place.
  • The extracellular segment of the Trk receptors has various range of distinct globular domains. They include immunoglobulin-like domains, fibronectin type III-like domains, cysteine rich domains, and epidermal growth factor-like domains.

Seven subfamilies of receptor tyrosine kinases

Each of the seven subfamilies are distinct in their own ways. Please see discussion. Include the above under this heading or make a new heading for it.

--Serkan Erkan 17:12, 2 May 2009 (EST)

  • TrK receptors act as dimeric transmembrane proteins. Distally, three cysteine-rich motiffs and two Leucine rich regions form a conserved NGF binding region which is common to all TrK's near the amino terminus(in addition, TrkA is possesed of an additional NGF binding region, see TrK A).
  • Additionally, the common binding region is flanked by a pair of extracellular immunoglobulin-like domains located proximally to the cell membrane. The juxtamembrane complex contains a variable amino acid sequence suggested to determine ligand bindng affinity and specificity and may bind directly to some ligands; The localization of these peptides appears to be non-specific in tertiary NGF-TrKA complex structures however their conformation within this complex appears to indicate direct participation in complex binding.
  • Further NGF binding site affinity is generated through collaboration with the p75 protein/neurotrophin receptor (p75NTR). Furthermore, p75NTR regulates NT-3-TrkA and NT-4/5-TrkB mediated receptor activation by blocking Neurotropin ligation by circumnavigating the prequisite of these neurotropins for receptor activation.


***Tyrosine receptor kinases*** play an important role in number of fundamental cellular processes such as cell migration, cell cycle, cell metabolism and survival as well as cell proliferation and differentiation. The activation of the tyrosine receptor kinases requires two processes: enhancement of intrinsic catalytic activity and creation of binding sites to recruit downstream signaling proteins. They are both achieved by the autophosphorylation of the tyrosine residues, a consequence of ligand-mediated oligomerisation. Upon their activation, tyrosine receptor kinases transduce extracellular signals to cytoplasm by autophosphorylation of the tyrosine residues on the receptors themselves and downstream signaling proteins within the cells. --Serkan Erkan 16:45, 2 May 2009 (EST)

Maintenance and Proliferation of undifferentiated Cell Lines

Dynamic Process

NGF binding to ligation region facilitates dimerisation of Trk Receptor monomers into the signal tranductive unit- activating the kinase unit. Phosphorylation of the autoregulatory loop tyrosines of the cytoplasmic domain further activates the kinase. Phosphorylation of an additional seven tyrosines within this region promotes signalling by providing docking sides for adaptor proteins which regulate signalling-cascade couples.

*Incomplete* Trk dynamics

Mechanism of Trk Activation via multiple Signalling pathways

Tropomyosine receptor kinases (Trk) enhance neuronal cell growth and differentiation of neurites via the activation of various signalling pathways. PKC, PLCy1, Ras/MAPK and PI3 are the four well known pathways, which exhibit this activity. The latter process involves the activation of transcription factors that will bind onto a specific target gene, thus trigerring neuronal cell division and/ differentiation via production of specific proteins that inhibit apoptosis.

PKC Pathway

To be completed by Dan

PLC-y1 Pathway

To be completed by Dan

Ras-MAPK (mitogen-activated protein kinase) Pathway

Activation of Ras-MAPK signalling pathway is predominantly essential for promoting neuronal cell growth and differentiation. Several pathways derive from Trk receptors for activation of Ras molecules, which begin by formation of ligand-bound mitogen (BDNF or NGF) at an extracellular domain of the trk, causing receptor dimerization. This subsequently will lead to phosphorylation of the intracellular parts of the receptors, which activates Guanine Exchange factors (GEFs), such as sos, Grb2 and Shc. GEFs trigger the exchange of GDP bound to inactive Ras to GTP, resulting Ras to activate. The presence of activated Ras molecules, stimulate signaling via a major downstream pathway known as MAP kinase (mitogen-activated protein kinase)-although activated Ras, would be also capable of triggering several other downstream pathways such as PLC-y1 and PI3-kinases. (Figure..awaiting for copyright approval..)

Mitogen-activated protein kinase (MAPK) pathway involves a series of signalling cascade. Raf is the first component of MAPK, activated by Ras-GTP on the plasma membrane. Raf then phosphorylates MEK1/2,which in turn activates the ERK1/2 by phosphorylation. ERK1/2 kinase would further phosphorylate a variety of downstream targets. These pathways may include phosphorylation of MAP 2-kinase by MAP 3-kinase on its serine and threonine amino-acid residues. Further phosphorylation of MAP- kinase by MAP 2-kinase on its serine and tyrosine amino-acid residues subsequently yields activated MAP-kinase. This results in activation of transcription factors that will lead to changes in gene expression. (Click here for well illustration of MAP-kinase pathway on You-tube) Nonetheless, aberrant Ras-MAPK signaling has been identified to be a possible factor responsible for the uncontrolled proliferation and malignancy (See “Cancer Biotherapy”, current research).

--Hamid Araghi 11:01, 20 May 2009 (EST)

PI3 (Phosphatidylinositol 3-kinase) Pathway

To be completed by Serkan

Current Research

Cancer biotherapy via targeting ‘Ras / Ras-MAP-kinase’ signalling pathway

Recent studies have hypothesized that receptors responsible for Ras activation would be effective target for treating human cancers (ref). Based on this hypothesis, since 1995, therapies utilizing the inhibition of Ras activation have aroused a major interest. Farnesylation is a process by which small G-proteins like Ras are activated and adhered to the cell membrane. In a recent paper (Sebti & Hamilton 2000), farnesyl transferase inhibitors were used in order to block the farnesylation of Ras proteins. The results revealed promising hopes in animal models but unfortunately have not been effective in human cancer treatment. The major downstream target of Ras-MAP-Kinase (MAPK) has also been taken as a subject for drug design. Sebolt and his colleagues showed that MAPK-inhibitors are effective in cancer therapy using mouse models (ref). It has been also suggested that Inhibition of the Ras-MAPK signaling pathway can serve as a potential therapy for NF1 in humans (ref).

Trk receptors can behave as signalling endosomes

In a recent publication, a hypothesized model was explained in which active Trk receptors are internalised into signaling endosomes during signalling transduction process (ref). The experiment that led to this understanding involved local application of NGF to axons and soma (cell bodies) of neuronal cells. It was shown that both ERK1/2 and ERK5 signaling cascades were activated. Surprisingly, application of NGF to neuronal cells at distance resulted only in activation of ERK5 pathway and NOT ERK1/2. It was then understood that activation of ERK5 in the cells somas required receptor internalisation due to inhibition by thermo-sensitive dynamin. Further investigation was conducted by local application of specific Trk-inhibitor (K252a) at either the distal axon or cell body, which blockaded the activation of ERK5 in the cell soma in the presence of NGF. Therefore, the hypothesized model in which active Trk receptors are internalised into signalling endosomes is inevitable. Nonetheless, the mechanism through which distance affects the specificity of signalling would be the future topic of investigation.

--Hamid Araghi 11:01, 20 May 2009 (EST)

The interplay between miRNA and Trks (tropomyosine-receptor-kinases)

Under construction


explain TrK Nomenclature



  • Autophosphorylation is the term used to define the phosphorylation of a kinase protein catalysed by its own enzymatic activity.
  • GTPases are a large family of enzymes that can bind and hydrolyse guanosine triphosphate(GTP).
  • NGF-Nerve growth Factor.
  • Exons-Expressed regions of eukaryotic genes.
  • Ras- small G-proteins that belong to the superfamily of monomeric GTPases. They are involved in receptor-mediated signal transduction pathways.
  • BDNF- Brain-derived neurotrophic growth factor is a protein that promotes activation of tropomyosine receptor kinases (trk).
  • NGF- Nerve growth factor is a protein that promotes activation of tropomyosine receptor kinases (trk).
  • Phosphorylation- A process by which a phosphate group is added to a protein that is required for its cellular activation.
  • ERK- Extracellular regulated kinase.
  • Apoptosis- Programmed cell death.
  • Farnesyl transferase inhibitors- a class of experimental cancer drug blocks farnesylation of Ras proteins.
  • Neurofibromatosis (NF)- an inherited disorder that causes production of benign tumours on nerve tissues. There are two types of NF; NF1 and NF2. NF1 is the most common disease amongst infants, also known as Recklinghausen disease.
  • Dynamin- GTPase enzyme responsible for endocytosis in eukaryotic cells.

2009 Group Projects

--Mark Hill 14:02, 19 March 2009 (EST) Please leave these links to all group projects at the bottom of your project page.

Group 1 Meiosis | Group 2 Cell Death - Apoptosis | Group 3 Cell Division | Group 4 Trk Receptors | Group 5 The Cell Cycle | Group 6 Golgi Apparatus | Group 7 Mitochondria | Group 8 Cell Death - Necrosis | Group 9 Nucleus | Group 10 Cell Shape