Studies of Hereditary Neurological Disease: Disease Mechanisms

National Institute Of Neurological Disorders And Stroke

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Abstract

Recently our research has focused on three hereditary motor neuron diseases: spinal and bulbar muscular atrophy (SBMA) due to polyglutamine expansion in the androgen receptor (AR), autosomal recessive spinal muscular atrophy (SMA) due to deficiency of the protein SMN, and amyotrophic lateral sclerosis type 4 (ALS4) due to mutation in senataxin. Specific research accomplishments include the following: (1) ALS4 is a rare, early-onset, autosomal dominant form of ALS, characterized by slow disease progression and sparing of respiratory musculature. Dominant, gain-of-function mutations in the senataxin gene (SETX) cause ALS4, but the mechanistic basis for motor neuron toxicity is unknown. SETX is a RNA-binding protein with a highly conserved helicase domain, but does not possess a low-complexity domain, making it unique among ALS-linked disease proteins. We derived ALS4 mouse models by expressing two different senataxin gene mutations (R2136H and L389S) via transgenesis and knock-in gene targeting. Both approaches yielded SETX mutant mice that develop neuromuscular phenotypes and motor neuron degeneration. Neuropathological characterization of SETX mice revealed nuclear clearing of TDP-43, accompanied by TDP-43 cytosolic mislocalization, consistent with the hallmark pathology observed in human ALS patients. Postmortem material from ALS4 patients exhibited TDP-43 mislocalization in spinal cord motor neurons, and motor neurons from SETX ALS4 mice displayed enhanced stress granule formation. Immunostaining analysis for nucleocytoplasmic transport proteins Ran and RanGAP1 uncovered nuclear membrane abnormalities in the motor neurons of SETX ALS4 mice, and nuclear import was delayed in SETX ALS4 cortical neurons, indicative of impaired nucleocytoplasmic trafficking. SETX ALS4 mice thus recapitulated ALS disease phenotypes in association with TDP-43 mislocalization and provided insight into the basis for TDP-43 histopathology, linking SETX dysfunction to common pathways of ALS motor neuron degeneration. (2) SETX has a normal function in resolution of RNA/DNA hybrids, and an increase in this activity may contribute to the pathophysiology of ALS4. RNA/DNA hybrids form when RNA hybridizes with its template DNA generating a three-stranded structure known as the R-loop. Knowledge of how they form and resolve, as well as their functional roles, is limited. Here, by pull-down assays followed by mass spectrometry, we identified 803 proteins that bind to RNA/DNA hybrids. Because these proteins were identified using in vitro assays, we confirmed that they bind to R-loops in vivo. They include proteins that are involved in a variety of functions, including most steps of RNA processing. The proteins are enriched for K homology (KH) and helicase domains. Among them, more than 300 proteins preferred binding to hybrids than double-stranded DNA. These proteins serve as starting points for mechanistic studies to elucidate what RNA/DNA hybrids regulate and how they are regulated. (3) Accurate modeling of human neuronal cell biology has been a long-standing challenge. However, methods to differentiate human induced pluripotent stem cells (iPSCs) to neurons have recently provided experimentally tractable cell models. Numerous methods that use small molecules to direct iPSCs into neuronal lineages have arisen in recent years. Unfortunately, these methods entail numerous challenges, including poor efficiency, variable cell type heterogeneity, and lengthy, expensive differentiation procedures. We recently developed a new method to generate stable transgenic lines of human iPSCs with doxycycline-inducible transcription factors at safe-harbor loci. Using a simple two-step protocol, these lines can be inducibly differentiated into either cortical (i3 Neurons) or lower motor neurons (i3 LMN) in a rapid, efficient, and scalable manner (Wang et al., 2017). In this manuscript, we describe a set of protocols to assist investigators in the culture and genetic engineering of iPSC lines to enable transcription factor-mediated differentiation of iPSCs into i3 Neurons or i3 LMNs, and we present neuronal culture conditions for various experimental applications.