TRP is a vital amino acid, and KP results in approximately 95% breakdown of this amino acid. Neuroactive metabolites of TRP degradation from the KP are concerned with the pathogenesis of a variety of neurodegenerative disorders such as AD, PD, HD, ALS, as well as autoimmune disorders such as MS [23]. Nowadays, due to the presence of numerous enzymes in the KP pathway, it has become an attractive target for the development of new therapies in neurological disorders. KP enzymes are important because they have located on the branch points of the KP and control neurotoxic and neuroprotective metabolites production. At the onset of neurodegenerative disorders, plasma and cerebrospinal fluid (CSF) levels of TRP are significantly reduced in patients, indicating KP activation in these diseases, followed by a disturbance in the balance of TRP metabolites [7]. There are many studies in the past decades to clarify the role of KP metabolites and enzymes in the pathogenesis of neurodegenerative and autoimmune disorders [6, 24]. Accordingly, Heilman et al. (2020) showed that a high QUIN / PIC ratio was strongly associated with an increase in symptoms in PD patients. They also believe that the QUIN / PIC ratio is regulated by the Acmsd enzyme and that reducing Acmsd activity can increase symptoms [25]. In 2013, Wu et al. indicated that the Acmsd enzyme leads to PIC production instead of QUIN in the KP. They showed that increased levels of Acmsd expression increased PIC expression in the cerebellum and hippocampus of triple-transgenic mice with Alzheimer's disease. However, they found that these PIC levels were insufficient to prevent QUIN toxicity. [26]. Moreover, in 2012 Schwarcz et al. propounded that neuroprotective metabolites of KP, such as PIC and KYNA can neutralize QUIN effects and therefore have neuroprotective properties [27].
In the present study, we investigated ACMSD as the major enzyme in the metabolic regulation of the KP pathway. Activation of the ACMSD enzyme leads to PIC conversion. PIC is an endogenous neuroprotective metabolite and an NMDA receptor antagonist that prevents QUIN-induced neurotoxicity [26, 28]. Based on literature reviews, the anti-inflammatory functions of PIC and KYNA have proven the regulatory role of KP metabolites in inflammation and immune responses. KP metabolites can moderate T cell functions by affecting through pro-inflammatory and anti-inflammatory cytokines. These proceedings together modulate the immune responses in diverse inflammatory disorders, especially neurodegenerative diseases [29]. Accordingly, ACMSD can be an attractive treatment option for treating or improving neurodegenerative disorders by increasing neurotransmitters, particularly QUIN, and decreasing the level of PIC as a neuroprotective metabolite [30]. This enzyme is the exclusively recognized enzyme capable of turning ACMS to PIC production and prevent the accumulation of QUIN from ACMS, so the ACMSD enzyme may contribute a major part to preventing the progression of diseases in which the KP is active and worsens the disease. Therefore, more experiments are required to verify the function of the ACMSD enzyme in ameliorating these disorders [20, 31]. To date, less importance has been given to KP and its enzymatic activity than to the role and importance of ACMSD. Therefore, by measuring Ido1 inflammatory gene and Ifn-γ gene as its regulator, we investigated the effect of Acmsd gene expression on inflammatory conditions and KP cycle. In inflammatory conditions, increased expression of IFN-γ, followed by increased expression of IDO1, increases the KYN/TRP ratio, which indicates TRP metabolism. Thus, in this conditions, the expression levels of IDO1 are significantly increased, which activates the KP and disrupts the balance of neurotoxic and neuroprotective metabolites [23, 32] In this regard, we examined the expression of Ifn-γ, Ido1 and Acmsd genes in RAW 264.7 unstimulated and LPS-stimulated mouse cells. According to the results of this study, the expression of Ifn-γ and Ido1 genes in LPS-stimulated cells was significantly increased compared to unstimulated cells, which indicates inflammation in these cells. Accordingly, increasing the expression level of Ido1 leads to the degradation of TRP and activation of the KP pathway. While Acmsd expression levels were significantly reduced in LPS-stimulated cells compared to non-stimulated cells. Decreased ACMSD expression level due to ACMS substrate accumulation leads to decreased PIC production and QUIN accumulation and finally indicates that in inflammatory conditions the severity of the disease may increase with KP activation. A decrease in ACMSD activity may therefore be neurotoxic [7, 33, 34]. As mentioned before, with the activation of the KP, the balance of neurotoxic and neuroprotective metabolites is disturbed, and the disease progresses and gets worse. However, if the balance of metabolites can be restored to normal with the enzymatic manipulations along the pathway, or by affecting the pathway-starting enzymes, the over-activation of the KP and the breakdown of TRP may be prevented. In these conditions, a reduction in disease progression may be observed. Currently, one of the broadest therapeutic goals is to focus on KP pathway enzymes. Most of the enzymes studied in previous studies included KMOs and KATs, and the ACMSD enzyme has received less attention [19, 21]. ACMSD may have promising and important effects in two ways in the course of diseases that are promoted by the KP. Firstly, by converting the ACMS to PIC it prevents the overproduction of QUIN. Secondly, according to our results, increased Acmsd gene expression led to decreased Ido1 gene expression and its regulator, Ifn-γ, which means that by decreasing the Ido1 expression, less TRP is broken down. As a result, it may modulate the intense activity of the KP and imbalance of its metabolites. Therefore, in this current research, when we attempted to clarify the role of Acmsd in inflammatory conditions, we found out overexpression of the Acmsd gene led to a down regulation in Ifn-γ, and Ido1 expression levels, an important enzyme that causes KP activation and TRP degradation in RAW 264.7 macrophage cells.
In conclusion, the findings of the present study showed that increasing the expression of Acmsd gene in inflammatory conditions in macrophage cells leads to a decrease in the expression level of Ido1 and Ifn-γ genes. The importance of these findings is important because, firstly, the decrease in Ifn-γ expression indicates a decrease in inflammation in cells, and secondly, because Ifn-γ regulates Ido1 expression, therefore the expression level of the Ido1 gene also decreases ifn- γ is reduced. Decreased Ido1 expression also reduces TRP degradation and less activation of the KP pathway, which is important in neurological diseases.