**6. OXPHOS therapies: The place for Ca2+ modulating drugs**

OXPHOS disorders are complex and heterogeneous group of multisystem diseases. The fact that they can result from mutations in hundreds of genes distributed across all of the chromosomes as well as the mtDNA, render the understanding of causative factors and the identification of common disease-related factors difficult. Accordingly effective therapeutic interventions are still not readily available. There are two main approaches to mitochondrial disease therapy: genetic and metabolic pharmacological (for recent review see (Roestenberg et al., 2011) and (Wallace et al., 2010)).

New approaches for genetic therapies for nDNA-encoded mitochondrial diseases as well as for mtDNA diseases are beginning to offer alternatives for individuals suffering from these devastating disorders. For mtDNA, these approaches include: (*a*) import of normal mtDNA polypeptides into the mitochondrion to complement the mtDNA defect, (*b*) reduction of the proportion of mutant mtDNAs (heteroplasmy shifting), and (*c*) direct medication of the mtDNA. Researchers are focusing also on the possible use of stem cell as a medication of OXPHOS disorders. However, these approaches are not as likely to relieve the devastating symptoms suffered by individuals with bioenergetic diseases.

The pharmacological approach includes the use of: (a) cofactors that increase the production of ATP (coQ, Idebenone, and succinate), (b) vitamins and metabolic supplements (thiamine, riboflavine, carnitine and L-arginine), (c) reactive oxygen species scavengers and mitochondrial antioxidants (CoQ/Idebenone, Vitamin E and Vitamin C), (d) modulators of PTP (cyclosporin A), and (e) regulators of mitochondrial biogenesis (bezafibrate and sirtuin analogs).

Current interventions based on metabolic correction include the use of mitochondrialtargeted drugs (compounds and peptides targeted to the mitochondrial matrix) such as mitoquinone "MitoQ", a derivative of coenzyme Q10, and SS-peptides, Szesto-Schiller peptides, a novel class of small cell permeable peptide antioxidants.

Another alternative to rescue mitochondrial bioenergetics defects is the use the mitochondrial Na+/Ca2+ exchanger inhibitor benzothiazepine CGP37157 (Cox & Matlib, 1993). CGP37157 normalized aberrant mitochondrial Ca2+ handling during hormone stimulation of cybrid cells carrying the tRNALys mutation associated with MERRF syndrom (Brini et al., 1999). Short-term pre-treatment with CGP37157 (1 μM, 2 min) fully normalized the amplitude of the hormone-induced mitochondrial Ca2+ signal in fibroblasts from patients with isolated complex I deficiency (Visch et al., 2004), without altering this

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**10. References** 

42).

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parameter in healthy fibroblasts. Similar result was obtained recently in a study including a large number of patient fibroblasts with complex I deficiency (Willems et al., 2009). Also the reduced maximal [ATP] in the mitochondrial matrix and cytosol were fully normalized by CGP37157 treatment. The effect of CGP37157 was independent of the presence of extracellular Ca2+, excluding a stimulatory effect on Ca2+ entry across the plasma membrane (Willems et al., 2009).

It is worth to mention that CGP37157 may also stimulate the IP3-induced release of Ca2+ from intracellular stores. In addition to these effects, CGP37157 was demonstrated to inhibit capacitative store refilling (Malli et al., 2005; Poburko et al., 2007). As far as its specificity is concerned, recent studies suggest that CGP37157 can also directly act on L-type Ca2+ channels (Thu le et al., 2006). Thus the use of this drug will hamper Ca2+-stimulated processes that depend on Ca2+ entry across the plasma membrane (Luciani et al., 2007).

All over, these findings suggest that the mitochondrial Na+/Ca2+ exchanger is a potential target for drugs aiming to restore or improve Ca2+-stimulated mitochondrial ATP synthesis in OXPHOS deficiencies and highlight the role of Ca2+ deregulation in the development of mitochondrial and cellular pathology in OXPHOS diseases.
