Dysregulation of brain iron metabolism and iron accumulation is known to be associated with ageing and AD, although underlying mechanisms remain unclear. It is known that iron load and inflammation regulate the synthesis of hepcidin, the main iron regulatory protein.
In particular, the inflammation-modulating cytokine interleukin-6 (IL-6), also known to modulate brain-muscle crosstalk, is involved in the activation of hepcidin synthesis in the brain.
Although regular physical exercise is known to have a beneficial effect on total body iron metabolism and anti-inflammatory action, the role of regular exercise on iron homeostasis in the brain and in the context of AD remains unclear.
The levels of iron and iron-related proteins were analysed in the brain and skeletal muscle.
The researchers also investigated the potential involvement of iron in the crosstalk between the brain and periphery upon regular exercise.
The results suggest that IL-6 is a key modulator of hepcidin in exercise-induced brain iron modulation. These findings help to better understand why regular exercise is beneficial in AD, and may provide new insight for disease prevention or effective treatment approaches.
The study was conducted in the Neurobiology of Disease laboratory led by Associate Professor Katja Kanninen at the University of Eastern Finland.
Funding: The study was supported by the Academy of Finland, the Sigrid Juselius foundation, the Finnish Cultural Foundation, and the University of Eastern Finland.
Excess of iron in any tissue may induce oxidative stress and impair tissue function. In the skeletal muscle, oxidative stress not only causes muscle damage but also negatively impacts its endocrine function. The skeletal muscle is a source of myokines, which are cytokines produced and released by skeletal muscle capable of exerting protective effects on other tissues, including the neuronal tissue (Besse-Patin et al., 2014; Dai et al., 2018; Liu et al., 2018).
This is supported by the observation that regular exercise, which pronouncedly increases myokine biosynthesis, reduces the risk of various diseases, including Parkinson’s disease (PD) and Alzheimer’s disease (AD) (Chen et al., 2005; Santos-Lozano et al., 2016). Conversely, disruption of balance between muscle protein synthesis and degradation, resulting from a wide variety of conditions, including cancer, immobilization (or disuse), denervation, or iron overload, can lead to oxidative stress dependent skeletal muscle atrophy and impairment of myokine synthesis (Tisdale, 2004; Argiles et al., 2014).
Although muscle function is widely studied in terms of adaptive changes induced by exercise, or atrophy induced by some morbidities, much less is known about its possible illness-related role as an endocrine tissue, and about the interconnections between oxidative stress and myokine production. This topic will probably represent one of the hot new areas in the study of pathomechanisms of neurodegeneration and other diseases.
Iron overload is a known contributor to multiple degenerative diseases, including liver fibrosis, heart attack, and cancer (Stevens et al., 1994; Klipstein-Grobusch et al., 1999; Ong and Farooqui, 2005). Importantly, excess iron accumulation in the brain is linked to neurodegenerative disorders (Bartzokis et al., 1999, 2000). Some neurodegenerative diseases are associated with the failure of muscle function (Busse et al., 2008).
However, little is known about the link between iron accumulation in the muscle and neurodegeneration. Some interesting results come from studies on amyotrophic lateral sclerosis (ALS), a neurodegenerative disease characterized by a selective loss of motor neurons (Gajowiak et al., 2015).
These findings, as well as current knowledge about iron metabolism in the skeletal muscle and its possible influence on neurodegenerative diseases, will be discussed in the current review.
Dysregulation of Iron Metabolism and Skeletal Muscle Atrophy
Loss of muscle mass is caused by an imbalance between protein synthesis and muscle fiber degradation. Two main degradation pathways can be hyperactivated during muscle dystrophy: the ubiquitin-proteasome and autophagy-lysosome systems. These two pathways require ATP and are believed to serve separate functions. Proteasomes degrade myofibrillar and short-lived proteins (Solomon and Goldberg, 1996; Clarke et al., 2007; Fielitz et al., 2007; Cohen et al., 2009), whereas autophagy-lysosomes remove long-lived proteins and organelles (Levine and Kroemer, 2008; Mizushima et al., 2008).
The ubiquitin-proteasome system relies on a cascade of enzymatic reactions that culminate in the labeling of substrate proteins with ubiquitin chains, for degradation by the 26S proteasome. E3 ubiquitin ligases confer substrate specificity and play a crucial role in this system. Two E3 ubiquitin ligases are essential for the development of skeletal muscle atrophy: muscle atrophy F-box (MAFbx)/atrogin-1 and muscle RING finger-1 (MuRF1). They are responsible for the selection and ubiquitination of myofibrillar proteins for subsequent proteosomal degradation (Bodine et al., 2001; Gomes et al., 2001).
The autophagy-lysosomal pathway involves sequestration of substrates within vacuoles called autophagosomes. These vacuoles subsequently fuse with lysosomes, and the cargo is hydrolyzed by lysosomal hydrolases. This process is controlled by autophagy-specific gene products, including Beclin 1.
Crucial stages of the pathway rely on the transfer of small ubiquitin-like molecules (LC3 and others) from the conjugation system to the membranes, to allow their growth into double-membrane autophagosomes that engulf portions of the cytoplasm (Kabeya et al., 2000; Mizushima et al., 2004).
Interestingly, both pathways are upregulated during atrophy by Forkhead box (FOX) O3a, which regulates the transcription of genes coding for atrogin-1, MuRF1, LC3B, and its homolog Gabarap 1, as well as Beclin 1 (Sandri et al., 2004; Mammucari et al., 2007; Zhao et al., 2007). Transcriptional activity of FOXO3a is regulated by posttranslational modifications. Regulation by Akt has been most extensively investigated; the protein phosphorylates FOXO3a on Thr32 and Ser253, leading to its cytosolic retention by 14-3-3 (Brunet et al., 1999).
Consequently, factors that activate Akt, such as insulin or the growth factor/phosphatidylinositide 3-kinase (PI3K) pathway, cause FOXO3a inactivation and prevent the synthesis of proteins involved in muscle atrophy. By contrast, phosphorylation of Ser413/588 of FOXO3a by AMP-activated protein kinase, a protein that becomes activated during energy deficit (exercise, hypoxia, or nutritional stress), leads to its activation and, subsequently, the induction of protein degradation pathways (Sandri, 2010; Sanchez et al., 2012).
The relationship between muscle iron metabolism and muscle atrophy with age or disease is unclear; however, recent reports have shed some light on these processes. Ikeda et al. (2016) showed that iron administration results in a decrease of skeletal muscle mass in mouse. The molecular mechanism of this phenomenon involved the induction of oxidative stress and inhibition of the Akt-FOXO3a pathway, hence, upregulation of atrogin-1 and MuRF1. Silencing of FOXO3a expression in C2C12 myotube cells or application of ROS scavenger, TEMPOL, suppress iron-induced expression of atrogin-1 and MuRF1, and prevent cell atrophy (Ikeda et al., 2016). Furthermore, Huang et al. (2013) demonstrated that mouse fed a high-iron diet exhibits elevated AMP-activated protein kinase activity and impaired insulin signaling in the skeletal muscle and liver. These effects are abrogated by co-treatment with N-acetyl cysteine (Huang et al., 2013).
Another regulator of the degradation pathways is tumor necrosis factor receptor-associated factor (TRAF6). It induces the expression of muscle-specific E3 ubiquitin ligases and autophagy-related molecules in the skeletal muscle on denervation and in Lewis lung carcinoma tumor-bearing mouse (Paul et al., 2010). It is worth noting that iron might stimulate this signaling pathway which was shown in hepatic macrophages (Zhong et al., 2012). Thus, iron accumulation in the skeletal muscle may play an underlying role in skeletal muscle atrophy (Figure 1).
Iron Accumulation in the Skeletal Muscle
Under physiological conditions, most iron is stored in the liver, spleen, and bone marrow; however, a high amount of iron has also been detected in the skeletal muscle. It has been determined that the total amount of stored iron in the skeletal muscle is comparable with that in the liver in healthy individuals (Torrance et al., 1968). Furthermore, muscle storage of iron can increase, e.g., in individuals with iron overload.
Iron metabolism seems to be tightly controlled. It is not entirely clear why under some conditions iron accumulates in the skeletal muscle and/or other tissues as well. It has been demonstrated that diet rich in highly bioavailable forms of iron promotes high iron stores, whereas foods containing phytate and other natural iron chelators reduce these stores.
Conversely, under some pathological conditions, excessive iron accumulation is observed regardless of the diet. For example, in an animal model of ALS, the amount of iron and iron storage proteins, ferritin L and ferritin H, is elevated in the skeletal muscles and neurons (Jeong et al., 2009; Halon-Golabek et al., 2018).
Skeletal muscle iron accumulation has also been observed after immobilization (Kondo et al., 1992). Further, hepatic iron content significantly increases after 2 weeks of a high-fructose diet (Ackerman et al., 2005). These data clearly indicate that tissue iron accumulation is not always associated with the consumption of food with high-iron content (Tsuchiya et al., 2013) but, rather, with impaired tissue iron metabolism. The mechanism of iron transport into a cell is well understood; however, the changes in iron metabolism that are responsible for excess iron accumulation are not fully known.
Iron overload can negatively affect skeletal muscle function, as it can induce oxidative stress (Schafer et al., 1981). Intracellular ROS formation is strongly associated with the amount of free iron. Lowering the levels of catalytic free iron in a cell by using chelators always results in reduced ROS formation and changes the composition of free radical species. For example, formation of the hydroxyl radical is iron-dependent.
It is not clear why increased iron stores correlate with enhanced iron-dependent oxidative stress since iron, stored mostly in ferritin, does not stimulate ROS formation. Despite this, a positive correlation between oxidative DNA damage and body iron stores has been observed (Barollo et al., 2004; Sullivan, 2004). Iron may affect the clinical course of diseases associated with the pathological disorders of the muscle.
We have recently shown that in a transgenic rat bearing the G93A hmSOD1 gene (an animal model of familial ALS), iron levels in the muscle increased with the development of disease, and that was accompanied by increased oxidative stress (Halon et al., 2014; Halon-Golabek et al., 2018). Taken together, similarly to the brain, liver, and some other tissues, under certain conditions, the skeletal muscle may accumulate too much iron, which contributes to ROS formation.
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6436082/
Original Research:
“Regular Physical Exercise Modulates Iron Homeostasis in the 5xFAD Mouse Model of Alzheimer’s Disease” by Katja Kanninen et al. International Journal of Molecular Sciences
