JUN and arthritic joint disease: Upon binding to trkA, NGF induces its auto-phosphorylation and subsequently the activation of both phospholipase PLCγ and protein kinase C, which in turn activates the mitogen-activated protein kinase pathway involving the c-jun N-terminal, the p38, and the extracellular-regulated protein kinases (ERK1/2) all of which have been identified in arthritis as well.