gms | German Medical Science

Organizing centres emit signalling molecules that specify different neuronal cell types at precise positions along the anterior-posterior and dorsal-ventral axes of the neural tube during development. The Otx2 gene is critical for the specification and regionalization of forebrain and midbrain regions. In transgenic mice, ectopically Otx2 expression in the anterior hindbrain using a knock-in strategy into the En1 locus results in a caudal shift of the midbrain–hindbrain organizer (MHO). This in turn, leads to an expansion of the midbrain dopaminergic (mid-DA) neuronal population. Complementary to this, the extension of the hindbrain serotonergic cell group is decreased. These changes are preserved in adulthood, and the additional, ectopic dopaminergic neurons project to the striatum, which is the right projection target area. The purpose of the study is to evaluate the functional properties of the ectopically expanded mid-DA neurons in vivo.

The ectopically expanded mid-DA neurons and the respective wild type area as well as ventral mesencephalon (VM) from Otx2 transgenic (Tg) and wild type (wt) E13 mouse embryos were explanted. Tissue was dissociated, and subsequently transplanted into the striatum of 6-hydroxydopamine unilaterally lesioned immunosuppressed rats (n=total of 56 rats). Lesion and transplantation effects were evaluated by apomorphine and amphetamine drug induced rotations, performed 6 weeks after lesion and 2 and 5 weeks after grafting.

In the apomorphine rotations, both VM groups showed a significant compensation (Tg 56%, wt 53%) 5 weeks after transplantation. Furthermore, we saw a strong tendency for compensation in the ectopically expanded mid-DA neurons, whereas no such an effect could be seen in their wt equivalent.

A complete compensation was already observed in the two weeks as well as in the five weeks amphetamine rotations, not only in the Tg and wt grafted VM groups, but also in the grafts derived from ectopically expanded mid-DA neurons. Their wt equivalent showed no compensation, similar to the sham transplanted controls.

In conclusion, our results revealed that the ectopically Otx2 expression in the anterior hindbrain resulting in caudally expanded mid-DA neurons demonstrate similar functional properties as their counterparts of the VM. Taken together, the 6-OHDA rat model for Parkinson’s disease is a valuable tool for in vivo investigations of the functional capacity of genetically modified DA progenitor cells.