The cGAS-STING-autophagy pathway: Novel perspectives in neurotoxicity induced by manganese exposure

Chronic high-level heavy metal exposure increases the risk of developing different neurodegenerative diseases. Chronic excessive manganese (Mn) exposure is known to lead to neurodegenerative diseases. In addition, some evidence suggests that autophagy dysfunction plays an important role in the pathogenesis of various neurodegenerative diseases. Over the past decade, the DNA-sensing receptor cyclic GMP-AMP synthase (cGAS) and its downstream signal-efficient interferon gene stimulator (STING), as well as the molecular composition and regulatory mechanisms of this pathway have been well understood. The cGAS-STING pathway has emerged as a crucial mechanism to induce effective innate immune responses by inducing type I interferons in mammalian cells. Moreover, recent studies have found that Mn²⁺ is the second activator of the cGAS-STING pathway besides dsDNA, and inducing autophagy is a primitive function for the activation of the cGAS-STING pathway. However, overactivation of the immune response can lead to tissue damage. This review discusses the mechanism of neurotoxicity induced by Mn exposure from the cGAS-STING-autophagy pathway. Future work exploiting the cGAS-STING-autophagy pathway may provide a novel perspective for manganese neurotoxicity.

Manganese methionine hydroxy analog chelated affects growth performance, trace element deposition and expression of related transporters of broilers

The present study aimed to evaluate the effects of manganese methionine hydroxyl analog chelated (Mn-MHAC) as a manganese (Mn) source on growth performance and trace element deposition in broilers. A total of 432 Arbor Acres commercial female broilers were fed a basal corn-soybean diet containing Mn at 25.64 mg/kg diet for 10 d. They were then randomly assigned to 6 groups, including a control group (the basal diet), a Mn sulfate group (the basal diet supplemented with Mn at 100 mg/kg diet), and 4 Mn-MHAC groups (the basal diet supplemented with Mn-MHAC at 25, 50, 75 and 100 mg Mn/kg diet, respectively). The results showed that compared with the control group, groups supplemented with Mn-MHAC had a positive effect on BW (quadratic, P = 0.017) and ADG (quadratic, P = 0.017). Moreover, the Mn-MHAC (50 mg Mn/kg diet) group had significantly greater BW and ADG (P < 0.05) compared with the other Mn-MHAC groups. Trace element deposition results also showed that tibial Mn increased (linear or quadratic, P = 0.002 and 0.009, respectively) in groups fed diets with increased levels of Mn-MHAC. In contrast, Fe deposition decreased both in the heart (linear, P = 0.020) and tibia (P < 0.05). In addition, the Mn-MHAC supplement noticeably lowered serum Mn-SOD activity (linear or quadratic, P = 0.048 and 0.019, respectively). The relative mRNA levels of divalent metal transporter 1 (DMT1) (P = 0.024), ferroportin 1 (FPN1) (P = 0.049), and Cu transporter-1(CTR1) (P < 0.001) in the duodenum, as well as CTR1 in the jejunum and ileum (P = 0.040 and 0.011, respectively) were higher in the Mn-supplemented group than in the control group. Furthermore, the relative mRNA level of DMT1 in the jejunum and ileum of broilers in the Mn-MHAC group (50 mg Mn/kg diet) did not differ from those in the control group, but was lower than those in the Mn sulfate group (P < 0.05). In conclusion, Mn-MHAC dietary supplementation improved the growth performance and trace element deposition in broilers. From this study, we recommend the optimum Mn-MHAC level to meet the Mn requirement of broilers is 50 to 75 mg Mn/kg diet.

Sudden sensorineural hearing loss and polymorphisms in iron homeostasis genes: new insights from a case-control study

Background. Even if various pathophysiological events have been proposed as explanations, the putative cause of sudden hearing loss remains unclear. Objectives. To investigate and to reveal associations (if any) between the main iron-related gene variants and idiopathic sudden sensorineural hearing loss. Study Design. Case-control study. Materials and Methods. A total of 200 sudden sensorineural hearing loss patients (median age 63.65 years; range 10-92) were compared with 400 healthy control subjects. The following genetic variants were investigated: the polymorphism c.-8CG in the promoter of the ferroportin gene (FPN1; SLC40A1), the two isoforms C1 and C2 (p.P570S) of the transferrin protein (TF), the amino acidic substitutions p.H63D and p.C282Y in the hereditary hemochromatosis protein (HFE), and the polymorphism c.-582AG in the promoter of the HEPC gene, which encodes the protein hepcidin (HAMP). Results. The homozygous genotype c.-8GG of the SLC40A1 gene revealed an OR for ISSNHL risk of 4.27 (CI 95%, 2.65-6.89; P = 0.001), being overrepresented among cases. Conclusions. Our study indicates that the homozygous genotype FPN1 -8GG was significantly associated with increased risk of developing sudden hearing loss. These findings suggest new research should be conducted in the field of iron homeostasis in the inner ear.

Dietary manganese supplementation influences the expression of transporters involved in iron metabolism in chickens

To investigate the effects of dietary manganese (Mn) supplementation on iron (Fe) metabolism, a total of 480 50-week-old hens were fed the basal diet (control, 24.35 mg Mn/kg) without Mn supplementation for 6 weeks to reduce Mn storage in the body. Hens were then randomly assigned to one of three treatments, which included the control and control added with 60 or 300 mg Mn/kg diet (M-Mn or H-Mn). Duodenum, heart, liver, and tibia were collected in hens after 12-week feeding period. No significant differences were observed in egg production, feed/egg ratio, shell breaking strength, and shell thickness among different treatments. Compared with control or M-Mn, H-Mn decreased (P < 0.05) serum Fe concentration, while increased (P < 0.05) total Fe-binding capacity (TIBC). The Fe concentration decreased (P < 0.05) in duodenum, and tended to reduce (P < 0.10) in liver from control to M-Mn and to H-Mn; whereas, dietary Mn supplementation did not influence (P > 0.10) Fe concentration in the heart and tibia. In conjunction with reduced Fe retention, DMT1 mRNA expression decreased (P < 0.05) with dietary Mn concentration increasing in the duodenum and liver. Duodenal FPN1 mRNA level was higher (P < 0.05) in H-Mn group than that in control or M-Mn group, while hepatic FPN1 mRNA expression was lower (P < 0.05) in M-Mn or H-Mn group when compared with control. The results demonstrated that dietary Mn supplementation decreased Fe concentration in duodenum and liver of hens, which may be related to the alteration of DMT1 and FPN1 expression in these tissues.

Manganese source affects manganese transport and gene expression of divalent metal transporter 1 in the small intestine of broilers

In the present study, two experiments were conducted to investigate the effect of Mn source on Mn transport and the expression of a Mn transporter, divalent metal transporter 1 (DMT1), in the small intestine of broilers. In Expt 1, in situ ligated duodenal loops from Mn-deficient chicks (29-d-old) were perfused with solutions containing 0-8.74 mmol Mn/l from either MnSO4, or one of two organic chelates of Mn and amino acids with moderate (OM) or strong (OS) chelation strength (Q(f)) up to 30 min. In Expt 2, Mn-deficient intact broilers (14-d-old) were fed a control diet (12.45 mg Mn/kg) or the control diet supplemented with 100 mg Mn/kg as one of all Mn sources for 14 d. The uptake kinetics of Mn from different Mn sources in the ligated duodenal loops followed a saturable process as determined by regression analysis of concentration-dependent uptake rates. The maximum transport rate (Jmax) and K(m) values, and DMT1 mRNA levels in the ligated duodenal loops were higher (P < 0.01) for OM and OS than for MnSO4. DMT1 mRNA levels were much higher (P < 0.01) in the duodenum than in the jejunum and ileum. Both DMT1 mRNA levels in the duodenum and plasma Mn contents from the hepatic portal vein of intact chicks on day 14 post-feeding increased (P < 0.05) in the following order: control < MnSO4 < OM < OS. These results indicated that organic Mn sources with stronger Q(f) showed higher Mn transport and absorption, and DMT1 might be involved in the regulation of organic Mn transport in the proximal small intestine of broilers.

Kinetics of manganese absorption in ligated small intestinal segments of broilers

Two experiments were conducted with 28-day-old male commercial broilers to study mechanisms of Mn absorption and the effect of Mn treatment on divalent metal transporter 1 (DMT1) mRNA levels in ligated segments from different intestinal regions of broilers. The results from experiment 1 showed that the amount of Mn absorption was asymptotic with respect to time within 40 min after perfusion of the duodenal, jejunal, and ileal segments of broilers with 2.18 mmol/L of Mn as MnSO(4). In experiment 2, a kinetic study of Mn absorption was performed with duodenal, jejunal, and ileal loops perfused with solutions containing 0, 0.13, 0.27, 0.54, 1.09, 2.18, 4.37, or 8.74 mmol/L of Mn as MnSO(4). Manganese concentrations in perfusates were determined at 5 min after perfusion. In the control group and in the group treated with 2.18 mmol/L Mn as MnSO(4), DMT1 mRNA levels of ligated intestinal regions at 30 min after perfusion were analyzed by real-time reverse transcription PCR. The kinetic curves of Mn absorption showed that Mn absorption was a carrier-mediated process in the duodenum and jejunum. The maximum absorption rate (J(max)) in duodenal segments was greater (P < 0.05) than that in the jejunum (94.08 vs. 81.17 nmol/cm per min). There was no significant difference (P = 0.85) in the Michaelis-Menten constant (K(m)) values between the duodenum and jejunum (3.41 vs. 3.60 mmol/L). In the ileum of Mn-deficient broilers, the most probable mechanism of Mn absorption was a nonsaturable diffusion process, and the diffusive constant (P; means +/- SE) was 2.42 x 10(-2) +/- 5.22 x 10(-4) cm(2)/min. The DMT1 mRNA levels in the duodenum and jejunum of broilers were greater (P < 0.001) than the level in the ileum. The DMT1 mRNA level in the small intestine of broilers in the Mn treatment group decreased significantly (P < 0.001) compared with that of the control. The different mechanisms of Mn absorption found in different intestinal segments suggest that the ileum is the main site of Mn absorption in the small intestine of broilers.