Intraneuronal signaling pathways of metallothionein

Metallothionein (MT) belongs to a family of metal-binding cysteine-rich proteins comprising several structurally related proteins implicated in tissue protection and regeneration after injuries and functioning as antiapoptotic antioxidants in neurological disorders. This has been demonstrated in animals receiving MT treatment and in mice with endogenous MT overexpression or null mutation during various experimental models of neuropathology, and also in patients with Alzheimer's disease and amyotrophic lateral sclerosis. Exogenously applied MT increases neurite outgrowth and neuronal survival in rat cerebellar, hippocampal, dopaminergic, and cortical neurons in vitro. In this study, the intraneuronal signaling involved in MT-mediated neuritogenesis was examined. The MT-induced neurite outgrowth in cultures of cerebellar granule neurons was dependent on activation of a heterotrimeric G-protein-coupled pathway but not on protein tyrosine kinases or on receptor tyrosine kinases. Activation of phospholipase C was necessary for MT-induced neurite outgrowth, and furthermore it was shown that inhibition of several intracellular protein kinases, such as protein kinase A, protein kinase C, phosphatidylinositol 3-kinase, Ca(2+)/calmodulin kinase-II, and mitogen-activated protein kinase kinase, abrogated the MT-mediated neuritogenic response. In addition, exogenously applied MT resulted in a decrease in phosphorylation of intraneuronal kinases implicated in proinflammatory reactions and apoptotic cell death, such as glycogen synthase-serine kinase 3alpha, Jun, and signal transducer and activator of transcription 3. This paper elucidates the intraneuronal molecular signaling involved in neuroprotective effects of MT.