Calcium signalling in mitochondria
Our interests are understanding calcium regulation of mitochondrial function by way of the calcium-dependent mitochondrial carriers of aspartate-glutamate/AGCs, components of the malate aspartate shuttle (MAS), or ATP.Mg2+-Pi/SCaMCs. These carriers have Ca2+-binding motifs facing the intermembrane space and are not activated by matrix calcium. We also aim at learning the role of these carriers in health and disease.
In mammals, there are two isoforms of the aspartate-glutamate carrier: Aralar/AGC1 and citrin/AGC2. Aralar is the predominant AGC in the brain and skeletal muscle whereas citrin is the major AGC in liver hepatocites. Thus, although their function is similar, mutations in either isoform lead to very different pathologies.
Deficiency in Aralar/AGC1 is a rare disease with impaired neurodevelopment, epilepsy and brain hypomyelination. We have explored treatments for Aralar deficiency and found that β-hydroxybutyrate (βOHB), the main metabolic product of ketogenic diets, is able to overcome the defect in basal and workload-stimulated respiration in Aralar-deficient neurons and partially revert their failure to produce aspartate and NAA. In vivo administration of βOHB to Aralar-KO mice increases myelin synthesis and dopaminergic markers in these mice, suggesting βOHB administration as a potential treatment in Aralar deficiency. However, whether the defect in myelination of the Aralar-KO mouse is due to the lack of aralar in neurons or oligodendrocytes is unclear. To dissect the role of brain Aralar in the disease mechanism of AGC1 deficiency, we have ablated the expression of Aralar in specific brain cell types: mature neurons and oligodendrocyte progenitors and its lineage by using the CaMKIIα-driven and the NG2-driven Cre recombinase expression, respectively, in Aralarlox/lox mice.
Citrin deficiency is a urea cycle disorder with different manifestations. Citrin/AGC2 is mainly expressed in liver. In the frame of the Citrin Foundation, we are exploring the exogenous expression of Aralar, which has low expression in normal liver, as possible therapy for Citrin deficiency. We have generated Citrin-KO mice carrying liver-specific Aralar transgene and are studying the effect of the transgene in recovering liver MAS activity and other traits of Citrin deficiency reproduced in Citrin-KO mice. We also plan to determine the role of Citrin in the calcium regulation of metabolic pathways such as gluconeogenesis and ureagenesis, using primary hepatocyte cultures and hepatic cell lines.