**1. Introduction**

28 Bioenergetics

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Mitochondria are double membrane-bound organelles that not only constitute the "cellular power plants" but also are crucially involved in cell survival, apoptosis, redox control, Ca2+ homeostasis and many metabolic and biosynthetic pathways.

The mitochondria generate energy by oxidizing hydrogen derived from dietary carbohydrate (TCA: tricarboxylic acid cycle) and lipids (beta-oxidation) with oxygen to generate heat and energy in the form of ATP (Adenosine triphosphate). Energy generation in mitochondria occurs primarily through oxidative phosphorylation (OXPHOS), a process in which electrons are passed along a series of carrier molecules called the electron transport chain (ETC). This chain is composed of four multisubunit assemblies that are embedded in the mitochondrial inner membrane: complex I (NADH:ubiquinone oxidoreductase; EC 1.6.5.3), complex II (succinate:ubiquinone oxidoreductase; EC 1.3.5.1), complex III (ubiquinol:cytochrome-*c* oxidoreductase; EC 1.10.2.2) and complex IV (cytochrome-*c* oxidase; EC1.9.3.1). Complexes I, III and IV actively translocate protons from the matrix into the intermembrane space using energy extracted from electrons passing through the chain. These electrons are liberated from NADH and FADH2, at complexes I and II, respectively, where they are donated to the lipophilic electron carrier coenzyme Q for further transport to complex III. From there, electrons are shuttled to complex IV by cytochrome-*c*. At this complex, electrons are finally used for the reduction of oxygen to water (Hatefi, 1985; Saraste, 1999) (Figure 1 A).

The energy released by the flow of electrons through the ETC and the flux of protons out of the mitochondrial inner membrane creates a capacitance across the mitochondrial inner membrane, the electrochemical gradient (∆P) composed of an electrical potential (Δψ) and a concentration ratio (ΔpH). The potential energy stored in ∆P is coupled to ATP synthesis by complex V (F0/F1-ATP-synthase; EC 3.6.1.34). As protons flow back into mitochondrial

<sup>\*</sup> Corresponding Author

Mitochondrial Calcium Signalling: Role in Oxidative Phosphorylation Diseases 31

sperm and mature oocytes, in which mtDNA copy numbers are ≈ 102 and ≈ 105, respectively. In general, there are believed to be two to ten copies of DNA per mitochondrion. The sequences of mtDNAs from unrelated individuals in human populations typically differ by about 0.3 %. Most individuals, however, have a single mtDNA sequence variant in all their cells (homoplasmy). mtDNA transmission occurred exclusively through the maternal lineage. Almost all of the nDNA-encoded OXPHOS subunits have been characterized at the cDNA level and several at the genomic level in humans. In general, the chromosomal distribution of the genes seems to be random, and expression of most gene products is ubiquitous but

Richard Scarpulla and co-workers have provided important insight into the regulatory mechanisms that are involved in the transcriptional control of OXPHOS genes (Gugneja et al., 1996; Huo & Scarpulla, 1999; Wu et al., 1999). They identified the nuclear respiratory factors NRF1 and NRF2, which act on overlapping subsets of nuclear genes that are involved in the biogenesis of the respiratory chain. Recent mammalian studies have identified PGC1 as a crucial regulator of cardiac mitochondrial number and function in response to energy demand (Lehman et al., 2000). Analysis of the expression pattern of OXPHOS genes revealed that their regulation might also be exerted post-transcriptionally

Among the inborn errors of metabolism, mitochondrial disorders are the most frequent with an estimated incidence of at least 1 in 10,000 births (reviewed in (Smeitink et al., 2001)). Although the term mitochondrial disorder is very broad, it usually refers to diseases that are caused by disturbances in the OXPHOS system. After the first description, ≈ 40 years ago, of a patient with "loose coupling" — a defect in the coupling between mitochondrial respiration and phosphorylation — by Luft and collaborators (Luft et al., 1962), thousands of patients have been diagnosed by measurement of OXPHOS-system enzyme activities. The great complexity of the OXPHOS system, which consists of proteins, some encoded by the mitochondrial genome and others by the nuclear genome, may explain the wide variety of clinical phenotypes that are associated with genetic defects in oxidative phosphorylation. Disease-causing defects can occur in a single OXPHOS complex (isolated deficiency) or multiple complexes at the same time (combined deficiency). OXPHOS diseases give rise to a variety of clinical manifestations, particularly in organs and tissues with high-energy demand such as brain (encephalopathies), heart (cardiomyopathies), skeletal muscle (myopathies) and

liver (hepatopathies) (reviewed in (Finsterer, 2006a, 2006b; Schaefer et al., 2004)).

OXPHOS system to malfunction (Wallace, 2005).

We have also to consider the presence of fundamental differences between mitochondrial genetics and Mendelian genetics when studying human OXPHOS diseases. These differences are linked to maternal inheritance of mtDNA, polyplasmy, heteroplasmy and the threshold effect, whereby a critical number of mutated mtDNAs must be present for the

One of the frequent OXPHOS disorders is Leigh Syndrome (OMIM 256000), an early-onset progressive neurodegenerative disorder, leading to death mostly within a few years after the onset of the symptoms. This disorder is characterized by lesions of necrosis and capillary proliferation in variable regions of the central nervous system. Clinical signs and symptoms comprise muscular hypotonia, developmental delay, abnormal eye movements, seizures, respiratory irregularities and failure to thrive. Other mitochondrial disorders caused by

predominates in tissues or organs with a high energy demand.

(Di Liegro et al., 2000).

**2.2 OXPHOS diseases** 

matrix through complex V, ADP and Pi are bound, condensed and released as ATP. With Complex V, the ETC complexes constitute the OXPHOS system. The OXPHOS system generates the vast majority of cellular ATP during oxidative metabolism. Some of the ATP is used for the mitochondrion's own needs, but most of it is transported outside the organelle by the adenine nucleotide translocator (ANT) and used for diverse cell functions (Hatefi, 1985; Saraste, 1999) (Figure 1 A).

Fig. 1. Schematic view of of mitochondrial OXPHOS system (A), and. mitochondrial Ca2+ influx and efflux mechanisms (B). Large size arrows indicate the interplay between mitochondrial Ca2+ signaling and OXPHOS.
