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.