Reversible Axonal Dystrophy by Calcium Modulation in Frataxin-Deficient Sensory Neurons of YG8R Mice

Abstract

Friedreich’s ataxia (FRDA) is a peripheral neuropathy involving a loss of proprioceptive sensory neurons. Studies of biopsies from patients suggest that axonal dysfunction precedes the death of proprioceptive neurons in a dying-back process. We observed that the deficiency of frataxin in sensory neurons of dorsal root ganglia (DRG) of the YG8R mouse model causes the formation of axonal spheroids which retain dysfunctional mitochondria, shows alterations in the cytoskeleton and it produces impairment of axonal transport and autophagic flux. The homogenous distribution of axonal spheroids along the neurites supports the existence of continues focal damages. This lead us to propose for FRDA a model of distal axonopathy based on axonal focal damages. In addition, we observed the involvement of oxidative stress and dyshomeostasis of calcium in axonal spheroid formation generating axonal injury as a primary cause of pathophysiology. Axonal spheroids may be a consequence of calcium imbalance, thus we propose the quenching or removal extracellular Ca2+ to prevent spheroids formation. In our neuronal model, treatments with BAPTA and o-phenanthroline reverted the axonal dystrophy and the mitochondrial dysmorphic parameters. These results support the hypothesis that axonal pathology is reversible in FRDA by pharmacological manipulation of intracellular Ca2+ with Ca2+ chelators or metalloprotease inhibitors, preventing Ca2+-mediated axonal injury. Thus, the modulation of Ca2+ levels may be a relevant therapeutic target to develop early axonal protection and prevent dying-back neurodegeneration.

Transcriptional profiling of isogenic Friedreich ataxia induced pluripotent stem cell-derived neurons

“…FRDA neurons show a decrease in mitochondrial protein transcript levels, such as components of the ATP synthase complex and of complex I (black module), all of which have been described in FRDA (23,27,(49)(50)(51). We observe consistent changes in ECM organization, focal adhesion and related signaling (brown and magenta modules and SNs), possibly linked to cytoskeletal abnormalities that have been reported in FRDA cells (52)(53)(54). We also identify changes in chemical synapsis transmission (turquoise module, Fig.…”

Disarrangement of Endoplasmic reticulum-mitochondria communication impairs Ca2+ homeostasis in FRDA

“…In the last years we described both in neuroblastoma cells (Bolinches-Amorós et al, 2014) as well as in frataxin deficient sensory neurons of the YG8R mouse model (Mollá et al, 2017) a clear loss of buffering Ca 2+ capacity in mitochondria leading to increased levels of Ca 2+ in the cytosol. In agreement, Ca 2+ chelators were able to improve FRDA conditions (Mollá et al, 2017).…”

“…In the last years we described both in neuroblastoma cells (Bolinches-Amorós et al, 2014) as well as in frataxin deficient sensory neurons of the YG8R mouse model (Mollá et al, 2017) a clear loss of buffering Ca 2+ capacity in mitochondria leading to increased levels of Ca 2+ in the cytosol. In agreement, Ca 2+ chelators were able to improve FRDA conditions (Mollá et al, 2017). More recently, Abeti and co-workers also observed that primary cultures of cerebellar granule neurons and cardiomyocytes from the YG8R mouse model presented a reduced mitochondrial Ca 2+ uptake that was concomitantly accompanied by lower Ca 2+ amount in the mitochondria and in the ER (Abeti et al, 2018a).…”

“…Other processes like mitochondrial energy conversion (Ristow et al, 2000) and oxidative phosphorylation (González-Cabo et al, 2005) have been described regarding frataxin, as well as its participation in oxidative stress regulation by reducing the production of reactive oxygen species (ROS) (Napoli et al, 2006;Santos et al, 2010;Tamarit et al, 2016). Moreover, frataxin has an important role in proper calcium (Ca 2+ ) handling (Bolinches-Amorós et al, 2014;Mollá et al, 2017), whose effect in neurons is the formation of multiple axonal spheroids mainly caused by Ca 2+ imbalance.…”

Cofilin dysregulation alters actin turnover in frataxin-deficient neurons

“…The state of phosphorylation of cofilin-1 (inactivation) is regulated by three mechanisms consisting on the regulation of the physiological levels of ATP 49 , the oxidative stress [49][50][51][52] , and the cytoplasmic Ca 2+ levels (reviewed in 53 ). These three processes are affected in the frataxin-deficient DRG neurons from YG8R mice 25 and are known to induce the activation of phosphatases chronophin (CIN) that dephosphorylates cofilin-1. It is well described that the hyperactivation of cofilin-1 has other unhealthy consequences, such as the formation of pathogenic inclusions in axons known as actin rods [49][50][51] .…”