New tree shrew Parkinson's model: a cost-effective alternative to monkey models

The surge in demand for experimental monkeys has led to a rapid increase in their associated costs. Consequently, there is a growing need for the development of a cost-effective model for Parkinsons disease (PD) that exhibits all core clinical and pathological phenotypes of PD. Evolutionarily, tree shrews (Tupaia belangeri) are much closer to primates in comparison to rodents and share more similar PD-related brain structures and movement ability with monkeys. As such, tree shrews represent an ideal small animal species for modeling PD. To develop a tree shrew PD model, we used the 1-Methyl-4-phenylpyridinium (MPP+) metabolite, derived from the well-established PD modeling drug 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), to induce lesions in the dopaminergic neurons of the unilateral substantia nigra. After determining the optimal modeling dosage, the tree shrews consistently exhibited and maintained all classic clinical manifestations of PD for a 5-month period. The symptoms closely resembled the ones observed in PD monkeys and included bradykinesia, rest tremor, postural instability, and apomorphine-induced rotations, a classic phenotype of unilateral PD models. Immunostaining showed a significant loss of dopaminergic neurons (approximately 95%) in the substantia nigra on the lesioned side of the brain, a crucial pathological marker of PD. Further cytomorphological analysis revealed that the size of nigral dopaminergic neurons in tree shrews exceeded that of rodents and more closely approximated that of macaques. Based on the principle that structure determines function, the morphological similarity between tree shrews and monkeys may be an important structural basis for the manifestation of the highly similar phenotypes between monkey and tree shrew PD models. Collectively, this study successfully developed a PD model in a small animal species that faithfully recapitulated the classical clinical symptoms and key pathological indicators of PD monkeys. In addition to the well-recognized monkey models, the tree shrew model provides a novel avenue for the evaluation of PD treatments and underlying mechanisms.