BGP-15 prevents the death of neurons in a mouse model of familial dysautonomia

Abstract

Hereditary sensory and autonomic neuropathy type III, or familial dysautonomia [FD; Online Mendelian Inheritance in Man (OMIM) 223900], affects the development and long-term viability of neurons in the peripheral nervous system (PNS) and retina. FD is caused by a point mutation in the gene IKBKAP/ELP1 that results in a tissue-specific reduction of the IKAP/ELP1 protein, a subunit of the Elongator complex. Hallmarks of the disease include vasomotor and cardiovascular instability and diminished pain and temperature sensation caused by reductions in sensory and autonomic neurons. It has been suggested but not demonstrated that mitochondrial function may be abnormal in FD. We previously generated an Ikbkap/Elp1 conditional-knockout mouse model that recapitulates the selective death of sensory (dorsal root ganglia) and autonomic neurons observed in FD. We now show that in these mice neuronal mitochondria have abnormal membrane potentials, produce elevated levels of reactive oxygen species, are fragmented, and do not aggregate normally at axonal branch points. The small hydroxylamine compound BGP-15 improved mitochondrial function, protecting neurons from dying in vitro and in vivo, and promoted cardiac innervation in vivo. Given that impairment of mitochondrial function is a common pathological component of neurodegenerative diseases such as amyotrophic lateral sclerosis and Alzheimer's, Parkinson's, and Huntington's diseases, our findings identify a therapeutic approach that may have efficacy in multiple degenerative conditions.T he autonomic nervous system is essential for homeostasis, and its disruption in familial dysautonomia (FD) can have fatal consequences resulting from cardiovascular instability, respiratory dysfunction, and/or sudden death during sleep (1-3). In addition to developmental decreases in the number of sensory and autonomic neurons, FD patients undergo a progressive loss of peripheral neurons and retinal ganglion cells. The latter loss may ultimately lead to blindness (4, 5). More than 98% of FD cases result from a single base substitution (IVS20+6T > C) in the IKBKAP/ELP1 gene (3, 6). This mutation is carried by 1 in 27 to 1 in 32 Ashkenazi Jews (3, 6). The protein encoded by the IKBKAP/ELP1 gene, IKAP/ELP1, is a scaffolding protein for the six-subunit Elongator complex (ELP1-ELP6), which modifies tRNAs during translation (7). Why reduction in the IKAP/ ELP1 protein results in neuronal death is unknown, and we have sought to elucidate the cellular and molecular mechanisms that cause progressive neurodegeneration in FD to help identify treatments that improve neuronal function and viability.Accumulating evidence indicates that cells from FD patients and from mouse models with deletions in the Elongator subunits Ikbkap/Elp1 or Elp3 experience intracellular stress (4,5,(8)(9)(10) resulting from the direct and/or indirect consequences of impaired translation (7, 11). The C terminus of IKAP/ELP1 has been shown to bind c-Jun N-terminal kinase (JNK) and to regulate JNK cytosolic stress signaling (12)....

Retina-specific loss of Ikbkap/Elp1 causes mitochondrial dysfunction that leads to selective retinal ganglion cell degeneration in a mouse model of familial dysautonomia

“…Our recent study revealed that mitochondria of embryonic Ikbkap CKO dorsal root ganglia neurons are depolarized, produce elevated levels of reactive oxygen species (ROS), are fragmented, and do not aggregate normally at axonal branch points ( Ohlen et al, 2017 ). In support of a mitochondrial deficit in FD, FD patients have a temporal optic nerve degeneration that is reminiscent of LHON and DOA, both of which are caused by mutations that affect mitochondrial function ( Carelli et al, 2009 ; Chevrollier et al, 2008 ; Kirches, 2011 ; Newman and Biousse, 2004 ; Yu-Wai-Man et al, 2011 ; Zanna et al, 2008 ).…”

“…Recently, we demonstrated that mitochondria of Ikbkap CKO embryonic PNS neurons were fragmented, had disrupted membrane potential and had increased ROS ( Ohlen et al, 2017 ). In this new study, we show that mitochondria of Ikbkap -deficient retinal neurons are also morphologically and functionally impaired ( Figs 4 - 6 ), demonstrating that mitochondrial dysfunction occurs in both developing and adult neurons in the PNS and CNS in the absence of IKAP.…”

Animal and cellular models of familial dysautonomia

“…FD patients also suffer frequently from rhabdomyolysis, which can result from mitochondrial-induced myopathy [59]. These findings provoked a detailed investigation of mitochondrial function in DRG neurons from Wnt1-cre;Ikbkap LoxP/LoxP mice [60], which revealed that mitochondria in Ikbkap depleted DRG neurons were depolarized, fragmented and released elevated levels of reactive oxygen species (ROS) compared to their control littermate DRG neurons. Furthermore, this study also showed that the small hydroxylamine, BGP-15 which has been shown in numerous studies to be cytoprotective by exerting anti-oxidative damage effects, could not only significantly improve mitochondrial function in the CKO neurons, but could also prevent their death both in vitro and in vivo [60].…”

“…These findings provoked a detailed investigation of mitochondrial function in DRG neurons from Wnt1-cre;Ikbkap LoxP/LoxP mice [60], which revealed that mitochondria in Ikbkap depleted DRG neurons were depolarized, fragmented and released elevated levels of reactive oxygen species (ROS) compared to their control littermate DRG neurons. Furthermore, this study also showed that the small hydroxylamine, BGP-15 which has been shown in numerous studies to be cytoprotective by exerting anti-oxidative damage effects, could not only significantly improve mitochondrial function in the CKO neurons, but could also prevent their death both in vitro and in vivo [60]. These findings and those from several other of the studies described here demonstrate not only the power of mouse models to provide the necessary cell types for elucidation of the pathophysiological mechanisms that cause the FD phenotype, but also their effectiveness as a readily accessible and defined system for testing potential therapeutics.…”

Development of the Autonomic Nervous System: Clinical Implications

“…[156][157][158] Furthermore, by studying the cell biology of developing neurons in FD mouse models, it has become clear that in the absence of Elp1, neurons undergo intracellular stress, marked by elevated p53, increased pJNK, depolarized mitochondria, increased levels of reactive oxygen species, and impaired mitochondrial function. 152,159 Rett Syndrome…”