*3.3. The c Subunit of ATP Synthase and Neurodevelopmental Disorders*

Fragile X-related disorders are due to a dynamic mutation of the CGG repeat in the *FMR1* gene on chromosome X, encoding for the RNA-binding protein FMRP. In patients these disorders are associated to mental retardation and neurocognitive deficits. In primary human-derived fibroblasts, mitochondrial morphology is altered and displays "donutshaped" organelles [148]. The mouse model of fragile X syndrome, the knockout of *FMR1*, resembled the human phenotype. Mouse brain mitochondria displayed a decreased ATP synthesis, while showing higher activities of the isolated respiratory chain complexes than in controls, suggesting a possible defect at the level of ATP synthase [149]. More recently, Elizabeth Jonas and coworkers showed that fragile X-affected neurons from mouse synthesized lower levels of cellular ATP [150]. These authors observed for the first time an increased level of the ATP synthase β subunit, an accumulation of the c subunit in insoluble aggregates in brain mitochondria and the presence of a proton leak in the F<sup>o</sup> sector of the enzyme [150]. This finding can explain the decreased ATP synthesis in spite of a fully active respiratory chain shown in *FMR1-* knockout mouse [149]. The presence of c subunit aggregates was also revealed in the group of neuronal ceroid-lipofuscinoses diseases that are linked by common clinical and pathological features falling under the description of Batten disease. Although the ceroid-lipofuscinoses present pathologically as lysosomal storage diseases, there is severe but selective neurodegeneration that leads to the clinical signs of dementia, blindness, seizures, and premature death. It was the accumulation of subunit c of mitochondrial ATP synthase in lysosomes of ovine tissues that are models of ceroidlipofuscinosis that first drew attention to the possible association of mitochondrial dysfunction with the pathogenesis of these diseases. It was reasoned that accumulation of storage bodies containing the c subunit within lysosomes was the consequence of a defect in its catabolic pathway [151]. This hypothesis was extended to include its initial disassembly from the "F<sup>o</sup> complex domain" of ATP synthase in the inner mitochondrial membrane [152]. This would be the initial step in the catabolic pathway and could depend on enzymes such as phospholipases rather than a protease [152]. Ca2+ causes a decrease of ATPase activity in isolated liver mitochondria from the ovine model of the disease, in comparison to controls displaying higher ATPase activity in the presence of Ca2+ [152], a fact that might be explained by proton leak but awaits further studies to be clarified.

#### **4. Conclusions**

The mitochondrial ATP synthase is a multi-subunit complex fundamental for the mitochondrial function and ATP synthesis under physiological conditions. In this review, we gave an update on the involvement of this mitochondrial enzyme in human diseases, such as encephalo- and cardiomyopathies of mitochondrial or nuclear origin, cardiovascular, neurodegenerative diseases or neurocognitive disorders, ranging from those that are caused by specific ATP synthase gene mutations to those that are instead initiated by other factors but are promoted by dysfunctions in the enzyme assembly and catalytic activity.

We here analyzed altered expressions of ATP synthase genes, enzyme subunit composition, post-translational modifications and interactions as causes of altered ATP synthase complex assembly and activity in human diseases, leading to mitochondrial morphology alterations and cell death.

**Author Contributions:** Conceptualization, V.G., A.B. and C.G.; writing—original draft preparation, V.G. and A.B.; writing—review and editing, V.G., A.B., C.G. and S.C. and funding acquisition, V.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research in the authors' laboarory received funding from AIRC under the MFAG 2017-ID. 20316 and PRIN 2017–ID. 201789LFKB projects, P.I. Giorgio Valentina.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data or in the writing of the manuscript.
