Regeneration of dopaminergic neurons after 6-hydroxydopamine-induced lesion in planarian brain

Planarians have robust regenerative ability dependent on X-ray-sensitive pluripotent stem cells, called neoblasts. Here, we report that planarians can regenerate dopaminergic neurons after selective degeneration of these neurons caused by treatment with a dopaminergic neurotoxin (6-hydroxydopamine; 6-OHDA). This suggests that planarians have a system to sense the degeneration of dopaminergic neurons and to recruit stem cells to produce dopaminergic neurons to recover brain morphology and function. We confirmed that X-ray-irradiated planarians do not regenerate brain dopaminergic neurons after 6-OHDA-induced lesioning, suggesting that newly generated dopaminergic neurons are indeed derived from pluripotent stem cells. However, we found that the majority of regenerated dopaminergic neurons were 5-bromo-2'-deoxyuridine-negative cells. Therefore, we carefully analyzed when proliferating stem cells became committed to become dopaminergic neurons during regeneration by a combination of 5-bromo-2'-deoxyuridine pulse-chase experiments, immunostaining/in situ hybridization, and 5-fluorouracil treatment. The results strongly suggested that G(2) -phase stem cells become committed to dopaminergic neurons in the mesenchymal space around the brain, after migration from the trunk region following S-phase. These new findings obtained from planarian regeneration provide hints about how to conduct cell-transplantation therapy for future regenerative medicine.     

Pleiotrophin receptor RPTP-zeta/beta expression is up-regulated by L-DOPA in striatal medium spiny neurons of parkinsonian rats

L-DOPA is still the drug of choice to treat Parkinson's disease although adverse side effects appear after several years of treatment. These are thought to be the consequence of plastic re-arrangements of the nigrostriatal connections, such as sprouting of the dopaminergic terminals or post-synaptic changes. Pleiotrophin, a trophic factor that we have shown to be up-regulated in the striatum of parkinsonian rats after long-term L-DOPA treatment may play a role in these plastic changes. To determine whether one of the three known pleiotrophin receptors [N-syndecan, receptor protein tyrosine phosphatase type zeta beta (RPTP-zeta/beta) and anaplastic lymphoma kinase] might be implicated in these putative plastic effects, we quantified their expression levels by real-time RT-PCR in the striatum and mesencephalon of rats with partial lesions of the nigrostriatal pathway undergoing L-DOPA treatment. Both pleiotrophin and RPTP-zeta/beta expression was up-regulated in the striatum but not in the mesencephalon of lesioned rats and RPTP-zeta/beta expression was even further increased by L-DOPA. The levels of the RPTP-zeta/beta protein were also increased in the striatum of L-DOPA-treated lesioned rats. Immunofluorescence labeling showed the protein to be constitutively expressed in striatal medium spiny neurons, which are innervated by both the corticostriatal glutamatergic and nigrostriatal dopaminergic systems. RPTP-zeta/beta might therefore be implicated in the plastic changes triggered by L-DOPA treatment and might merit further study as a potential candidate for Parkinon's disease therapy.