ISG15 dysregulates endoplasmic reticulum-mitochondrial contacts and calcium homeostasis in Ataxia telangiectasia

Dysregulation of endoplasmic reticulum and mitochondrial (ER:Mit) contacts and mitochondrial calcium (mitCa²⁺) homeostasis are found in several neurodegenerative disorders, including Ataxia Telangiectasia (A-T). However, the cellular basis of these defects remains unclear. Previously, we demonstrated that the aberrantly elevated Interferon-Stimulated Gene 15 (ISG15) pathway inhibits protein polyubiquitylation, its dependent protein turnover, and mitophagy pathways in A-T. Literature indicates that silencing of mitochondrial ubiquitin ligase 1 (MUL1) stabilizes mitofusin2 (MFN2) and attenuates mitCa²⁺ uptake from ER to Mit (mitCa²⁺_influx) in primary neurons. We have replicated these findings in apparently healthy fibroblasts. We hypothesized that elevated ISG15 may inhibit ubiquitin-dependent MUL1-mediated degradation of MFN2 and dysregulate ER:Mit contacts and mitCa²⁺ homeostasis in A-T fibroblasts. Concurrently, MFN2 is stabilized in A-T, MUL1-silenced A-T, MUL1/ISG15-silenced A-T vs ISG15-silenced A-T fibroblasts. Moreover, the number of ER:Mit contacts is increased in A-T vs ISG15-silenced A-T fibroblasts. Notably, mitCa²⁺_efflux is significantly attenuated in A-T vs ISG15-silenced A-T fibroblasts in which mitCa²⁺_efflux is restored to levels comparable to those observed in normal fibroblasts. The mitCa²⁺_efflux remains attenuated in MUL1 and MUL1/ISG15-silenced A-T fibroblasts. We conclude that ISG15 impairs MUL1/MFN2-mediated regulation of ER:Mit contacts and attenuates mitCa²⁺_efflux, which may, in turn, cause Ca²⁺ overload-mediated mitochondrial damage in A-T. These findings suggest that ISG15 silencers may correct mitochondrial abnormalities and improve mitochondrial health in A-T patients and in those with other neurodegenerative disorders in which ISG15 is elevated, such as ALS.