Neurodegenerative diseases are characterized by the progressive degeneration of nerve cells. Some neurodegenerative diseases such as Alzheimer’s and Parkinson’s are caused by disorders in the mitochondria, which are organelles present in the eukaryotic cells of animals, plants and fungi, and their function is to produce energy.
1. Introduction
Luft et al. published in 1962 the first case of mitochondrial dysfunction in a 35-year-old woman affected by myopathy
[1]. Since then, there has been a continuous advancement of research that is leading to the understanding, from a medical perspective, of the role of mitochondria in health, diseases and aging. Many seemingly unrelated pathologies, such as neuropathies, Alzheimer’s, Parkinson’s, myopathies, ataxia, cancer and others, share common underlying pathophysiological procedures. These mechanisms include the production of reactive oxygen species (ROS) and the damage they cause to the mitochondrial genome, leading to mitochondrial dysfunction
[2,3,4][2][3][4]. As mitochondrial formation arises from the contribution of two distinct genomes, namely the nuclear DNA (nDNA) and the mitochondrial DNA (mtDNA)
[5,6[5][6][7],
7], genetic mutations in either the nDNA or the mtDNA, as well as genetic deletions in the mtDNA, can give rise to mitochondrial disease. These genetic mutations specifically result in defects in the mitochondrial oxidative phosphorylation system (OXPHOS)
[8], which are the underlying cause of mitochondrial diseases
[9,10,11][9][10][11]. Extensive research has been conducted on mutations occurring in nuclear genes associated with mitochondrial disorders, specifically focusing on their connection to diseases associated with premature aging
[12,13][12][13] and POLG-related disorders that exhibit neurological symptoms
[3,14,15][3][14][15]. Mitochondrial diseases represent the most common group of inherited metabolic dysfunction and are among the most prevalent types of inherited neurological disorders
[16]. Mitochondrial disorders resulting from recurrent mutations in mtDNA manifest as shared syndromes across unrelated families and populations
[2,17][2][17]. These disorders exhibit a wide range of clinical variations and can emerge at any age
[16]. Given that mitochondria exist in all cells of the body except for red blood cells, the resulting clinical symptoms may manifest in specific organs independently but frequently involve multiple systems in organs that have high energy requirements, such as the brain, skeletal system, muscles and heart
[16]. With reference to mutations in mtDNA or nDNA, from the point of view of clinical diagnosis, primary mitochondrial disease (PMD) and secondary mitochondrial dysfunction (SMD) are distinguished
[18,19][18][19]. The primary differentiation between PMD and SMD lies in the fact that PMD genes either directly encode OXPHOS proteins or influence OXPHOS function by affecting the production of the complex machinery required for the OXPHOS process. On the other hand, SMD can be caused by genes that do not encode OXPHOS proteins or affect their production. SMD is often associated with various hereditary nonmitochondrial diseases
[11,18,19][11][18][19]. There are several factors that determine the onset of mitochondrial dysfunction such as age, heredity factors, an unhealthy lifestyle or stress, environmental toxicity and more
[7,16,20,21,22,23,24][7][16][20][21][22][23][24]. Among the mitochondrial diseases, the most widely known are neurodegenerative diseases, which are mainly caused by genetic factors and by environmental factors
[7,24,25][7][24][25]. Neurodegenerative diseases are stress-inducing brain disorders characterized by behavioral, motor and cognitive impairments. Numerous studies conducted over the years have provided evidence that mitochondrial dysfunction serves as the underlying cause in the development of neurodegenerative diseases, which include Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), Amyotrophic Lateral Sclerosis (ALS) and Dementia
[26,27,28,29][26][27][28][29]. These diseases arise from deficiencies in mitochondrial function, which is primarily regulated by over 1000 proteins encoded by both the mitochondrial and nuclear genomes
[30]. Studies have shown that the majority, around 90%, of mitochondrial proteins are encoded by genes found in the cell nucleus. These proteins are synthesized by ribosomes in the cytosol and subsequently transported into mitochondria through a complex import machinery composed of multiple components
[5,6,31,32,33,34][5][6][31][32][33][34]. These proteins have various functions attributed to them, including respiration, metabolite transport, protein translocation, redox homeostasis and other processes that are interconnected within complex and dynamic networks. The malfunctioning of these systems could lead to the development of diseases
[35]. Despite mitochondria having their own genome, many nuclear-encoded mitochondrial ribosomal proteins (MRPs) are essential for proper organelle function. Mutations in Mrp genes, as indispensable components of the mitochondrial translation machinery, are detrimental to the OXPHOS system and associated with several neurodegenerative diseases in humans
[36,37,38,39][36][37][38][39].
2. Mitochondrial Neurodegenerative Diseases
The human mitochondrial genome consists of a circular molecular structure comprising approximately 16,569 pairs of nucleotides (
Figure 21). It contains a total of 24 mtDNA genes, including the small ribosomal RNA (12S rRNA) and large ribosomal RNA (16S rRNA) genes, as well as 22 transfer RNA (tRNA) genes necessary for translating the 13 respiratory chain proteins
[19,41,42][19][40][41].
Studies conducted over the past forty years have highlighted the role of mitochondria in both normal brain function and the pathogenesis of diseases originating from them (
Table 1)
[43][42]. Mitochondrial diseases result from hereditary or spontaneous mutations in mtDNA or nDNA, leading to abnormalities in the functions of proteins or RNA molecules typically found within mitochondria
[41,44,45][40][43][44]. Generally, the term “mitochondrial disease” describes a heterogeneous set of conditions caused by genetic defects in the assembly and/or function of OXPHOS proteins
[45][44]. In fact, mitochondria play a fundamental role in energy production and are essential in numerous cellular processes
[35,46][35][45]. Consequently, mitochondrial dysfunction often propagates and/or underlies many pathological states, including neurodegenerative diseases
[47,48][46][47] and the aging process
[49,50][48][49]. More recently, extensive research efforts have helped define some of the pathological mechanisms underlying neurodegenerative processes, such as the two most common neurodegenerative disorders: AD and PD.
Table 1.
Mitochondrial diseases
a
.