TH expression and activity
Dopamine is the main catecholamine neurotransmitter in the brain (Marsden, 2006; Klein et al. , 2018). The main dopamine sources are two neighboring areas of the midbrain: the substantia nigra associated with the dorsal striatum (nigrostriatal dopaminergic pathway) and the ventral tegmental area (VTA) that sends dopaminergic afferents to the ventral striatum (mesolimbic pathway) and the prefrontal cortex (mesocortical pathway) (Gantz et al. , 2018). While mesolimbic and mesocortical dopamine is the key neurotransmitter of the reward system participating in goal-directed behavior and emotional reactions, nigrostriatal dopamine is mainly involved in regulation of locomotor function and operational learning (Reynolds & Flores, 2021). TH is a rate-limiting enzyme of dopamine biosynthesis which is specifically expressed in the cell bodies and processes of catecholamine-producing neurons (Klein et al. , 2018). Activity of TH is modulated by phosphorylation at a number of sites including Ser40 and Ser31. It was shown that Ser40 is the main activating site phosphorylation of which strongly potentiates TH activity, while phosphorylation at Ser31 activates TH in a lesser extent (Dunkley et al. , 2004).
Our data revealed that 3-day SI did not affect expression and phosphorylation of TH in the SNc. In the SNc of HU mice, the expression of TH phosphorylated at Ser40 was significantly reduced in comparison with both GH and SI mice. We also observed a decrease in TH phosphorylated at Ser31 and total TH in HU mice in comparison to SI. Altogether, these data indicated a decrease in the activity of dopamine biosynthesis in the SNc that was caused by HU conditions. Interestingly, changes in dopamine system in response to gravitational unloading and loss of support afferentation were observed not only in mammals (Popovaet al. , 2015, 2020; Tsybko et al. , 2015; Kulikova et al. , 2017), but also in nematode C. elegans , in which both real and simulated microgravity decreased dopamine metabolism, while adding of contact stimuli reversed these changes (Sudevan et al. , 2022).
Moreover, our data showed decreased TH expression in the dopaminergic terminals innervating the dorsal striatum in HU mice as compared with isolated animals. Similarly to SNc, these data indicated the negative effect of unloading. However, striatal expression of pTH(Ser40) was decreased both in HU and SI mice suggesting that this indicator of TH activity was negatively affected mainly by SI. Thus, in the dorsal striatum both HU and isolation contributed to impaired dopamine innervation.
In opposite, TH phosphorylation at Ser31 was increased in the dorsal striatum of HU mice. While phosphorylation of TH at Ser40 participates in the up-regulation of its catalytic activity (Dunkley et al. , 2004), the role of this post-translational modification is far from clarity. In vivo analysis of rats demonstrated that content of pTH(Ser31) in the terminals of dopaminergic neurons, particularly in the striatum, was larger than in SN, while there was no difference in the content of pTH(Ser40) between striatum and SN (Salvatore & Pruett, 2012). The authors supposed that pTH(Ser31) can play a role of the autonomous regulator of dopamine synthesis in the SN terminals (Salvatore & Pruett, 2012). Latest data revealed that pTH(Ser31) can be a transport form of TH which is loaded into vesicles and transferred from the cell body to the axon terminals (Jorge-Finnigan et al. , 2017). Moreover, in rodent model of Parkinson’s disease pTH(Ser31) was decreased in the striatum, but increased in SN, against the background of progressive TH loss (Salvatore, 2014). Thus, we supposed that increased TH phosphorylation at Ser31 in the striatum of HU mice could provide an adaptive mechanism aimed to compensate the impairment of dopamine synthesis in dopaminergic neurons of SNc.