Cardiac sympathetic innervation is an unrecognized disease target in autosomal dominant optic atrophy

Abstract

The Optic Atrophy-1 protein (Opa1) controls mitochondrial (mt) fusion and cristae morphology, ATP production, maintenance of mtDNA, and apoptosis. Mutations in Opa1 gene cause Autosomal Dominant Optic Atrophy (ADOA), a neurodegenerative disease characterized by “retinal ganglion cell” death and visual loss. Recently, ADOA has been described as a “multi-systemic disease” also featuring myopathy, peripheral neuropathy, Parkinsonism and cardiac rhythm alterations. Sympathetic neurons (SNs) are the main extrinsic regulators of heart activity in basal and stress conditions, and their homeostasis strikingly depend on the correct mitochondrial dynamics. We here determined the effect of ADOA genetics on cardiac sympathetic innervation. The majority of Opa1 gene mutations result in truncated forms of the protein, leading to reduction in its cellular content. To study the impact of Opa1 haploinsufficiency on cardiac innervation, we generated transgenic mice with SN-specific Opa1 deletion (TH-Opa1+/− mice), which, in heterozygosity, causes 50% reduction of protein expression. Heart function of TH-Opa1+/− mice was assessed by ECHO and ECG telemetry. Cardiac innervation was evaluated by confocal IF, and whole-mount IF in tissue clarified heart blocks. In vitro assays achieved insights on the effects of Opa1 haploinsufficiency on SNs. SNs were also evaluated in skin biopsies from ADOA patients, and age- and sex-matched controls. We here found that SN density is significantly reduced in the skin of ADOA patients, indicating thatSNs are additional, previously neglected, targets of ADOA. To isolate the effects of Opa1 haploinsufficiency in SNs, we analyzed hearts of adult and aged TH-Opa1+/− mice, which showed intact morphology and contractile function. Looking at cardiac innervation, confocal IF and 3-D reconstruction of the SN network revealed a significant reduction in innervation density, which appeared evident already in adult mice, and worsened during ageing. Consistently, TH-Opa1+/− mice showed decreased Heart Rate Variability and increased incidence of arrhythmic beats. In vitro assays demonstrated the causal role of Opa1 haploinsufficiency in affecting SNs, by compromising TrkA/NGF signaling, which is key to sustain neuronal viability. Our data demonstrates that Opa1 haploinsufficiency also affects peripheral autonomic innervation, thus explaining extra-retinal symptoms experienced by ADOA patients.