The enemy from within: Miss-localization of a compromised receptor as a mechanism for TrkAIII oncogenic activity

There is growing evidence that the mislocalisation of receptor tyrosine kinase oncogenes underpins downstream oncogenic signalling. Here, we highlight our recent work characterising the mechanism that underpins mislocalisation and subsequent oncogenic activity of the alternative TrkAIII splice variant of the tropomyosin related kinase A (TrkA) receptor, in human neuroblastoma cells. In primary neuroblastomas, expression of fully spliced TrkA associates with low-stage disease and better prognosis, whereas TrkAIII expression associates with advanced-stage disease and worse prognosis. In neuroblastoma models TrkA and TrkAIII exhibit opposite tumour suppressing and oncogenic activity, respectively. In an attempt to further understand the basis of this diametrically opposite behaviour, intracellular trafficking and activation TrkA and TrkAIII receptors was compared in SH-SY5Y neuroblastoma cells. We found that TrkAIII oncogenic activity originates from miss-localisation and spontaneous activation within the alternative membrane substrate context of the endoplasmic reticulum-Golgi intermediate (ERGIC)-COP-I vesicle compartment. This results from altered trafficking caused by interphase restricted spontaneous receptor activation, which impedes TrkAIII transport from the ERGIC to the Golgi network, associated with retrograde transport of activated TrkAIII from the ERGIC back to the endoplasmic reticulum (ER). Therefore, spontaneous TrkAIII activation within ERGIC/COPI membranes, facilitated by omission of the extracellular D4 spontaneous activation-prevention domain, sets-up self-perpetuating TrkAIII recycling between the ER and ERGIC. This mechanism ensures continuous accumulation of this compromised receptors above the spontaneous activation threshold of the ERGIC/COPI compartment, resulting in oncogenic signalling through IP3K from this altered substrate context. Furthermore, chronic ER stress caused by TrkAIII recycling back to the ER induces a protective ER-stress response, and also the recruitment of active TrkAIII to the centrosome, altering centrosome behaviour. These different tumour-promoting mechanisms all result from mislocalization and spontaneous activation of TrkAIII within the alternative substrate context of the ERGIC/COPI compartment and can be prevented by TrkA tyrosine kinase inhibitors.