**9. Conclusions**

It is unclear why dopaminergic neurons preferentially degenerate in PD and DRD. Many factors may contribute to this, including mitochondrial dysfunction [165,166], oxidative stress, decreased glutathione content [167], increased iron levels [168], and production of oxygen radicals through the combination of dopamine and tetrahydrobiopterin [169,170]. Here, we present clues to understanding this selective degeneration of dopamine-containing neurons, which are sensitive to dopamine/biopterin deficiency and α-synuclein invasion. The consequent elevation of TH phosphorylation is followed by the degradation of pSer40-TH by the ubiquitin-proteasome system. Interestingly, proteasomal inhibition results in TH aggregation, whereas choline acetyltransferase does not show such aggregations [70]. Owing to such characteristics of TH protein to aggregate and be degraded easily, especially pSer40-TH, handling phosphorylated TH is somehow tricky for dopaminergic neurons. The formation of insoluble inclusions of pSer40-TH further reduces cytoplasmic operable TH. We sugges<sup>t</sup> that the negative spiral mechanism of TH phosphorylation-induced degradation is involved in the loss of nigrostriatal TH protein in PD and DRD (Figure 5).

**Figure 5.** Schematic model for the pathways of degradation and accumulation of tyrosine hydroxylase (TH) protein. The ubiquitin-proteasome system degrades phosphorylated TH at serine 40 (pSer40-TH). Otherwise, pSer40-TH is accumulated in the proteasomal deficient state and forms insoluble aggregates.

In the present article, we have reviewed the molecular mechanism of the loss of mesencephalic TH protein in PD and DRD. We conclude that the ubiquitin-proteasome system participates in the degradation of phosphorylated TH. The mechanism of ubiquitin-proteasome-linked dopaminergic pathogenesis might help explain the dopaminergic neuron-selective loss of TH protein in PD and DRD. These insights may lead to more focused e fforts to develop therapeutics and strategies to prevent the onset of neurodegeneration in PD and DRD.

**Author Contributions:** Conceptualization, I.K. and K.F.; writing—original draft preparation, I.K.; writing—review and editing, I.K. and K.F.; funding acquisition, K.F. and I.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded in part by the Strategic Research Program for Brain Sciences of the Japan Agency for Medical Research and Development, AMED (JP18dm0107071 and JP19dm0107071) to K.F., as well as by the Japan Society for the Promotion of Science, KAKENHI (19K07097), Kobayashi Foundation, and Intelligent Cosmos Academic Foundation to I.K.

**Acknowledgments:** We appreciate Hiroshi Ichinose (Tokyo Institute of Technology) for helpful discussion on the proteasomal degradation of tyrosine hydroxylase protein. We also acknowledge Emeritus Toshiharu Nagatsu at Fujita Health University for valuable discussion on the characteristics of tyrosine hydroxylase.

**Conflicts of Interest:** The authors declare no conflicts of interest.
