Conserved role for Ataxin‐2 in mediating endoplasmic reticulum dynamics

Abstract

Ataxin-2, a conserved RNA-binding protein, is implicated in the late-onset neurodegenerative disease Spinocerebellar ataxia type-2 (SCA2). SCA2 is characterized by shrunken dendritic arbors and torpedo-like axons within the Purkinje neurons of the cerebellum. Torpedo-like axons have been described to contain displaced endoplasmic reticulum (ER) in the periphery of the cell; however, the role of Ataxin-2 in mediating ER function in SCA2 is unclear. We utilized the Caenorhabditis elegans and Drosophila homologs of Ataxin-2 (ATX-2 and DAtx2, respectively) to determine the role of Ataxin-2 in ER function and dynamics in embryos and neurons. Loss of ATX-2 and DAtx2 resulted in collapse of the ER in dividing embryonic cells and germline, and ultrastructure analysis revealed unique spherical stacks of ER in mature oocytes and fragmented and truncated ER tubules in the embryo. ATX-2 and DAtx2 reside in puncta adjacent to the ER in both C. elegans and Drosophila embryos. Lastly, depletion of DAtx2 in cultured Drosophila neurons recapitulated the shrunken dendritic arbor phenotype of SCA2. ER morphology and dynamics were severely disrupted in these neurons. Taken together, we provide evidence that Ataxin-2 plays an evolutionary conserved role in ER dynamics and morphology in C. elegans and Drosophila embryos during development and in fly neurons, suggesting a possible SCA2 disease mechanism.

Atxn2-CAG100-KnockIn mouse spinal cord shows progressive TDP43 pathology associated with cholesterol biosynthesis suppression

“…Figure S7). As common denominator, all these pathways reflect progressive expression adjustments at the diverse sites of physiological ATXN2 localization in young unstressed tissue: ATXN2 was previously shown to interact with the EGF / insulin receptor and downstream AKT/MAPK signaling [41, 95, 133], to inhibit the mTORC1 growth complex in three species [10, 37, 96], to reside at the endoplasmic reticulum (ER) [36, 208] where cholesterol biosynthesis and calcium storage occur [62, 65, 107, 209], to modulate ribosomal mRNA translation [46, 138, 175], to interact with actin/actinin [105, 174] and to modulate adipogenesis [95, 185]. Thus, both transcriptome profiles are compatible with the concept that expanded ATXN2 irritates its various subcellular interactomes and triggers many gradual expression adaptations that may compensate most, but not all dysfunctions.…”

“…Various molecular pathways are irritated by the sequestration of ATXN2 interactor molecules into aggregates, principally the role of stress granules in the defense against toxic RNAs. Several other altered pathways such as ribosomal translation, calcium homeostasis and cholesterol biogenesis occur at the endoplasmic reticulum, where ATXN2 was shown to play a crucial role for structure and dynamics according to studies in C. elegans and D. melanogaster [36]. SCA2 pathogenesis has considerable overlap with the mechanisms documented in other ataxias, dystonias, and with ALS-associated features such as RNA toxicity and endoplasmic reticulum dysfunction [200].…”

Axonal Endoplasmic Reticulum Dynamics and Its Roles in Neurodegeneration

“…Tubular ER Morphogenesis ARL6IP1 ER tubulation (Yamamoto et al, 2014;Dong et al, 2018) Yes (Dong et al, 2018) Ataxin-2 Tubular ER network organization (del Castillo et al, 2019) Yes (Ralser et al, 2005) ATLs Homotypic membrane fusion between ER tubules (Hu et al, 2009;Orso et al, 2009) Yes (Zhu et al, 2003;Lee et al, 2009;Park et al, 2010;Behrendt et al, 2019;Krols et al, 2019) (Rocha et al, 2009) Lipid transfer protein Osbp associates with ER-endosomes MCSs (Dong et al, 2016) Lipid transfer protein ORP5 mediates ER-endosome tethering by interacting with NPC1 (Du et al, 2011) (Johnson et al, 2018) or Osbp-related lipid transfer proteins (Stefan et al, 2011;Petkovic et al, 2014;Chung et al, 2015) ER and Calcium Dynamics…”

“…For example, the small GTPases Rab10 and Rab18 regulate tubular ER morphology: depletion of Rab10 produces expansion of cisternal ER and fewer ER tubules (English and Voeltz, 2013), and loss of Rab18 from ER tubules, by depletion of the Rab3GAP complex (which is also a Rab18 GEF), causes fragmentation of the ER tubular network and spread of ER sheets (Gerondopoulos et al, 2014). Another example is the RNA-binding protein Ataxin-2, also associated (Fowler and O'Sullivan, 2016) HSP (Novarino et al, 2014;Wakil et al, 2019) Ataxin-2 Tubular ER morphogenesis KD: short and bulged neurites (del Castillo et al, 2019) ALS (Elden et al, 2010) SCA (Pulst et al, 1996) ATL1 Tubular ER morphogenesis LDs growth regulation KD: Reduced axon growth (Zhu et al, 2006) *KD: Decreased spontaneous release; impaired anterograde transport (De Gregorio et al, 2017); impaired regeneration (Levitan et al, 2016;Rao et al, 2016) *LOF: Fragmented presynaptic ER; Decreased evoked transmitter release (Summerville et al, 2016); increased number of presynaptic terminals (Lee et al, 2009) *OE: Decreased spontaneous release; reduced anterograde transport (De Gregorio et al, 2017); Decreased evoked transmitter release (Summerville et al, 2016) HSN (Guelly et al, 2011) HSP (Zhao et al, 2001) ATL3 Tubular ER morphogenesis Autophagy-mediated tubular ER turnover LOF: Decreased mitochondrial number (Krols et al, 2019) LOF: Impaired neurite outgrowth (Behrendt et al, 2019) *KD: Decreased spontaneous release; reduced anterograde transport (De Gregorio et al, 2017); impaired regeneration (Levitan et al, 2016;Rao et al, 2016) *LOF: Fragmented presynaptic ER; Decreased evoked transmitter release (Summerville et al, 2016); increased number of presynaptic terminals (Lee et al, 2009) *OE: Decreased spontaneous release; reduced anterograde transport (De Gregorio et al, 2017); Decreased evoked transmitter release (Summerville et al, 2016) HSN (Fischer et al, 2014;<...>…”

In Spino-Cerebellar Tissue, Ataxin-2 Expansion Affects Ceramide-Sphingomyelin Metabolism

“…Which of these molecular dysregulations might represent a primary event under direct influence of ATXN2 polyQ expansion, which other dysregulations might constitute secondary consequences? All ELOVL isoforms act in the endoplasmic reticulum, where most of the ATXN2 protein has its physiological localization and plays an important role for ER dynamics [43,150]. Similarly, CERS1 and CERS2 have an ER/Golgi distribution, as well as the protein encoded by Sptlc2.…”