International Journal of Molecular Sciences Review Molecular Mechanisms Underlying the Beneficial Effects of Exercise on Brain Function and Neurological Disorders Kévin Nay 1,2, William J. Smiles 1 , Jacqueline Kaiser 1, Luke M. McAloon 1,2, Kim Loh 1,3, Sandra Galic 1, Jonathan S. Oakhill 1,2, Andrew L. Gundlach 3,4 and John W. Scott 1,2,4,* 1 St Vincent’s Institute of Medical Research, Fitzroy, Victoria 3065, Australia; [email protected] (K.N.); [email protected] (W.J.S.); [email protected] (J.K.); [email protected] (L.M.M.); [email protected] (K.L.); [email protected] (S.G.); [email protected] (J.S.O.) 2 Exercise and Nutrition Research Program, Mary MacKillop Institute for Health Research, Australian Catholic University, Melbourne, Victoria 3000, Australia 3 Department of Medicine, University of Melbourne, Parkville, Victoria 3010, Australia; andrew.gundlach@florey.edu.au 4 The Florey Institute of Neuroscience and Mental Health, Parkville, Victoria 3052, Australia * Correspondence: [email protected]; Tel.: +61-3-9288-3632 Abstract: As life expectancy has increased, particularly in developed countries, due to medical advances and increased prosperity, age-related neurological diseases and mental health disorders have become more prevalent health issues, reducing the well-being and quality of life of sufferers and their families. In recent decades, due to reduced work-related levels of physical activity, and key research insights, prescribing adequate exercise has become an innovative strategy to prevent or delay the onset of these pathologies and has been demonstrated to have therapeutic benefits when used Citation: Nay, K.; Smiles, W.J.; as a sole or combination treatment. Recent evidence suggests that the beneficial effects of exercise Kaiser, J.; McAloon, L.M.; Loh, K.; on the brain are related to several underlying mechanisms related to muscle–brain, liver–brain and Galic, S.; Oakhill, J.S.; Gundlach, A.L.; gut–brain crosstalk. Therefore, this review aims to summarize the most relevant current knowledge Scott, J.W. Molecular Mechanisms of the impact of exercise on mood disorders and neurodegenerative diseases, and to highlight the Underlying the Beneficial Effects of established and potential underlying mechanisms involved in exercise–brain communication and Exercise on Brain Function and their benefits for physiology and brain function. Neurological Disorders. Int. J. Mol. Sci. 2021, 22, 4052. https://doi.org/ Keywords: exercise; brain; BDNF; iron; microbiota; Alzheimer’s disease; Parkinson’s disease 10.3390/ijms22084052 Academic Editor: Metin Akay Received: 12 March 2021 1. Introduction Accepted: 12 April 2021 Both neurological and mental health disorders remain a significant medical challenge Published: 14 April 2021 worldwide. It has been estimated that roughly 30% of the world’s population have ex- perienced a mental disorder at some time in their life [1], while neurological disorders Publisher’s Note: MDPI stays neutral accounted for more than 9 million deaths in 2016 alone [2]. In 2018, a Lancet Commis- with regard to jurisdictional claims in sion report on mental health indicated that mental health disorders were on the rise in published maps and institutional affil- every country in the world and were predicted to cost the global economy $16 trillion by iations. 2030 [3]. Moreover, the global cost associated with dementia alone was estimated to reach $948 billion during 2016 [4]. Therefore, the need for effective strategies to prevent, treat and improve neurological and mental health disorders is now critical to reduce this massive, global economic burden. Copyright: © 2021 by the authors. In this regard, physical activity (exercise) has become a progressively common and Licensee MDPI, Basel, Switzerland. economically viable therapeutic strategy with positive effects on both neurological and This article is an open access article mental health [5–8]. Exercise can generally be classified as one of two different types, distributed under the terms and endurance and resistance. Endurance exercise is classically performed against a relatively conditions of the Creative Commons low load over a long duration, whereas resistance exercise is performed against a relatively Attribution (CC BY) license (https:// high load for a short duration [9]. In general, individuals who train with endurance creativecommons.org/licenses/by/ exercises develop better oxygen delivery and consumption within the skeletal muscle, 4.0/). Int. J. Mol. Sci. 2021, 22, 4052. https://doi.org/10.3390/ijms22084052 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 4052 2 of 18 whereas those who base their training on resistance exercises get bigger and stronger muscles [9]. However, most activities combine both endurance and resistance [9]. In this review, we outline the current knowledge of the impact of exercise on mood disorders and neurodegenerative disease, and highlight the potential underlying mechanisms involved in the organs-to-brain signalling axis in response to exercise. 2. Protective Role of Exercise in Preventing Mood Disorders and Neurodegenerative Diseases Over three decades ago, Farmer et al. [10] reported that sedentary behaviour may be an independent risk factor contributing to the depressive symptoms experienced in a large cohort of adults. Indeed, exercise has been consistently described as an effective treatment for depression [11]. Several randomized controlled trials demonstrated that physical activ- ity alone, or performed in combination with the administration of antidepressants, had a moderate-to-large effect on reducing depressive symptoms compared to physically inactive controls [12]. Interestingly, sensitivity to anxiety has also been inversely correlated with physical activity levels in healthy young adults [13]. In this regard, the positive impact of exercise on ameliorating symptoms associated with anxiety disorders (including general anxiety disorder, panic disorder, posttraumatic stress disorder, obsessive compulsive disor- der, social anxiety disorder and specific phobias) has been well documented [14]. Aside from mood disturbances, exercise can also improve the symptomatology in individuals diagnosed with schizophrenia [15,16], autism [17,18] and bipolar disorder [19]. Based on these data, the current consensus is to advocate for increasing habitual levels of physical activity to improve mood state and mental health [20]. Exercise therapy is also known to have a direct, positive impact on neurodegenerative diseases and acquired brain injuries. Parkinson’s disease (PD) is a progressive nervous system disorder related to cell damage in the brain that affects motor control and cognitive function. Indeed, the main pathological characteristics of PD are cell death in the basal ganglia (affecting up to 70% of the dopaminergic neurons in the area of the brain known as the substantia nigra) and the presence of Lewy bodies (accumulations of the protein α-synuclein) in many of the remaining neurons [21]. The most obvious early symptoms are tremor, rigidity, slowness of movement, and difficulty walking [22]. Clinical studies have demonstrated that physical exercise improves symptoms in PD patients [23,24]. In animal models of PD, exercise improved motor function through preservation of nigrostriatal dopaminergic neurons, protection of mitochondria and suppression of the nigrostriatal formation of Lewy bodies [25–27]. Alzheimer’s disease (AD) is another irreversible, progressive neurodegenerative disease characterized by the degeneration and the destruction of neurons and synapses in the cerebral cortex and certain subcortical regions, predominantly causing memory loss, anxiety and confusion [28]. It is known that a sedentary lifestyle might lead to an earlier AD onset. Recent studies have observed that exercise interventions were capable of attenuating the symptoms of neurodegeneration experienced by AD patients, with comparable observations in rodent models of AD [29–33]. In addition, Liang et al. [34] have shown that physical activity reduces the levels of AD-associated biomarkers (e.g., Pittsburgh compound-B, tau, phosphorylated tau) in the cerebrospinal fluid of cognitively normal older adults Huntington’s disease is an inherited disease that causes the progressive degeneration of nerve cells in the brain. Currently, there is no cure for this disease which has a broad impact on an individual’s functional ability that usually results in movement, cognitive and psychiatric disorders [35]. Here again, exercise training seems to be a safe and feasible treatment approach in patients with a possible positive effect on cognition and motor function in patients suffering from Huntington’s disease [36]. Multiple Sclerosis is a disease in which the immune system damages the protective covering of nerves, inducing nerve demyelination and neuronal dysfunction. The symp- toms of this disease include vision loss, pain on eye, fatigue and impaired coordination [37]. Within the last decade, exercise has been described as a potential treatment for patients with Int. J. Mol. Sci. 2021, 22, 4052 3 of 18 Multiple Sclerosis. Resistance and endurance exercise and aquatic therapy were shown to have positive impacts across a variety of specific symptoms of multiple sclerosis. This leads to a perceived increase in quality of life without any negative effects [38]. Some more recent studies have also shown encouraging results regarding the impact of exercise in patients with acquired traumatic brain injuries [39–41] and in analogous