Calcitriol increases frataxin levels and restores mitochondrial function in cell models of Friedreich Ataxia.

Elena Britti,A Sanz-Alcázar,Marta Llovera,Joaquim Ros,Jordi Tamarit,Stefka Mincheva-Tasheva,Fabien Delaspre,Marta Medina-Carbonero,Rosa Purroy

doi:10.1042/bcj20200331

Abstract

Friedreich ataxia (FA) is a neurodegenerative disease caused by the deficiency of frataxin, a mitochondrial protein. In primary cultures of dorsal root ganglia neurons, we showed that frataxin depletion resulted in decreased levels of the mitochondrial calcium exchanger NCLX, neurite degeneration and apoptotic cell death. Here, we describe that frataxin-deficient dorsal root ganglia neurons display low levels of ferredoxin 1 (FDX1), a mitochondrial Fe/S cluster-containing protein that interacts with frataxin and, interestingly, is essential for the synthesis of calcitriol, the active form of vitamin D. We provide data that calcitriol supplementation, used at nanomolar concentrations, is able to reverse the molecular and cellular markers altered in DRG neurons. Calcitriol is able to recover both FDX1 and NCLX levels and restores mitochondrial membrane potential indicating an overall mitochondrial function improvement. Accordingly, reduction in apoptotic markers and neurite degeneration was observed and, as a result, cell survival was also recovered. All these beneficial effects would be explained by the finding that calcitriol is able to increase the mature frataxin levels in both, frataxin-deficient DRG neurons and cardiomyocytes; remarkably, this increase also occurs in lymphoblastoid cell lines derived from FA patients. In conclusion, these results provide molecular bases to consider calcitriol for an easy and affordable therapeutic approach for FA patients.

Highlights

Friedreich ataxia (FA) is caused by decreased expression of the mitochondrial protein frataxin due to large expansions of GAA triplet repeats in the first intron of the gene
The clue that frataxin depletion could be connected to vitamin D synthesis came from data [49] revealing that, on a human cell model of granulosa cells, frataxin depletion resulted in a deficient steroid synthesis
The results shown here reveal that ferredoxin 1 (FDX1) levels are significantly below of those found in normal dorsal root ganglia (DRG) neurons or in Friedreich Ataxia (FA) lymphoblastoid cells (Figs, 1 and S1) supporting the rationale that calcitriol supplementation should be able to recover cell survival after frataxin depletion

Summary

Introduction

Friedreich ataxia (FA) is caused by decreased expression of the mitochondrial protein frataxin due to large expansions of GAA triplet repeats in the first intron of the gene. Using primary cultures of frataxindeficient DRG, we observed alterations of several parameters compatible with calcium mishandling such as a decrease in mitochondrial membrane potential (Ψm), increased fodrin cleavage by calpain and caspase (two calcium-activated proteases) and Bax induction. These events leading to apoptotic cell death, can be minimized either by supplementing cultures with BAPTA, a calcium chelator, or by TAT-BH4, the antiapoptotic domain of Bcl-xL fused to TAT peptide [9]. In endothelial cells submitted to oxidative stress/hypoxic conditions, calcitriol supplementation diminishes cellular damage by increasing the expression of SOD1 and VGEF [31] Connected to this finding, the positive effects of calcitriol in delaying

Methods

Results

Conclusion