MEK/ERK Signaling Regulates Reconstitution of the Dopaminergic Nerve Circuit in the Planarian Dugesia japonica.

Planarian Dugesia japonica is a flatworm that can autonomously regenerate its own body after an artificial amputation. A recent report showed the role of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK/ERK) pathway in the head morphogenesis during the planarian regeneration process after amputation; however, neuron-specific regeneration mechanisms have not yet been reported. Here, whether MEK/ERK pathway was involved in the dopaminergic neuronal regeneration in planarians was investigated. Planarians regenerated their body within 14 days after amputation; however, the head region morphogenesis was inhibited by MEK inhibitor U0126 (3 or 10 μM). Furthermore, the number of planarian tyrosine hydroxylase (DjTH)-positive dopaminergic neurons in the regenerated head region was also decreased by U0126. The 6-hydroxydopamine (6-OHDA), a dopaminergic neurotoxin, can decrease the number of dopaminergic neurons; however, planarians can regenerate dopaminergic neurons after injecting 6-OHDA into the intestinal tract. MEK inhibitor PD98059 (30 μM) or U0126 (10 μM) significantly decreased dopaminergic neurons 5 days after the 6-OHDA injection. During the regeneration process of dopaminergic neurons, phosphorylated histone H3 (H3P)-positive stem cells known as "neoblasts" were increased in the head region; however, MEK inhibitors significantly decreased the number of H3P-positive neoblasts. These results suggested that dopaminergic neuronal regeneration in planarian was regulated by the MEK/ERK pathway.