Activation of Autophagy Rescues Synaptic and Cognitive Deficits in Fragile X Mice
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Activation of autophagy rescues synaptic and cognitive deficits in fragile X mice Jingqi Yana,1, Morgan W. Porcha,1, Brenda Court-Vazqueza, Michael V. L. Bennetta,2, and R. Suzanne Zukina,2 aDominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, New York, NY 10461 Contributed by Michael V. L. Bennett, August 17, 2018 (sent for review May 29, 2018; reviewed by Claudia Bagni and Leonard K. Kaczmarek) Fragile X syndrome (FXS) is the most frequent form of heritable (17–19). Upon activation, mTOR phosphorylates Unc-51–like intellectual disability and autism. Fragile X (Fmr1-KO) mice exhibit autophagy-activating kinase 1 (ULK-1) at S757, a target of aberrant dendritic spine structure, synaptic plasticity, and cogni- mTORC1 and a well-established antiautophagy site (Fig. 1) tion. Autophagy is a catabolic process of programmed degradation (20). This, in turn, sequesters ULK-1 away from AMP kinase and recycling of proteins and cellular components via the lyso- (AMPK). AMPK phosphorylates and activates ULK-1 at S317. somal pathway. However, a role for autophagy in the pathophys- Upon activation, ULK-1 promotes phosphorylation and activa- iology of FXS is, as yet, unclear. Here we show that autophagic tion of Beclin-1 at S14, a critical step in the nucleation phase of flux, a functional readout of autophagy, and biochemical markers autophagy (21). Beclin-1 promotes lipidation of LC3-I to gen- of autophagy are down-regulated in hippocampal neurons of frag- erate its lipidated form LC3-II, enabling elongation of the lim- ile X mice. We further show that enhanced activity of mammalian iting membrane and the formation of autophagosomes (22). target of rapamycin complex 1 (mTORC1) and translocation of Autophagosomes act via protein adaptors such as p62 to deliver Raptor, a defining component of mTORC1, to the lysosome are proteins and organelles to lysosomes for degradation. causally related to reduced autophagy. Activation of autophagy Whereas it is well established that protein translation is ele- by delivery of shRNA to Raptor directly into the CA1 of living mice vated in neurons of fragile X mice (7, 10, 13–15, 23, 24), a role via the lentivirus expression system largely corrects aberrant for suppressed autophagy and protein degradation is, as yet, spine structure, synaptic plasticity, and cognition in fragile X mice. unclear. The present study was undertaken to examine the pos- Postsynaptic density protein (PSD-95) and activity-regulated sibility that autophagy, which is negatively regulated by phos- NEUROSCIENCE cytoskeletal-associated protein (Arc/Arg3.1), proteins implicated phorylation of mTOR on S2448 [(25), but see ref. 26], is reduced in spine structure and synaptic plasticity, respectively, are elevated in hippocampal neurons from fragile X mice. We focused on the in neurons lacking fragile X mental retardation protein. Activation hippocampus, which is central to cognition (27, 28). Using a of autophagy corrects PSD-95 and Arc abundance, identifying a combination of molecular, electrophysiological, and behavioral potential mechanism by which impaired autophagy is causally re- approaches, we found that autophagy is reduced in hippocampal lated to the fragile X phenotype and revealing a previously un- neurons of fragile X mice and that activation of autophagy appreciated role for autophagy in the synaptic and cognitive rescues, at least in part, aberrant dendritic spine morphology, deficits associated with fragile X syndrome. metabotropic glutamate receptor-dependent long-term depres- sion (mGluR-LTD), and cognition in fragile X mice. Moreover, fragile X syndrome | autophagy | cognition | autism | mTOR activation of autophagy largely corrects the abundance of the ragile X syndrome (FXS) is the most common heritable form Significance Fof intellectual disability and is a leading genetic cause of autism (1–4). Patients with FXS exhibit a complex and de- bilitating neurological phenotype including impaired cognition Fragile X syndrome (FXS) is the most common form of inherited and social interactions, delayed speech, hyperactivity, emotional intellectual disability and is a leading genetic cause of autism. lability, attentional deficits, resistance to change, seizures, sleep The mammalian target of rapamycin (mTOR) complex 1 cascade disorders, gross motor delays, hypersensitivity to sensory stimuli, is a central regulator of protein translation, cell growth, pro- liferation, survival, and autophagy. Findings in the present autistic behaviors, and autonomic dysfunction (1–4). In FXS, study demonstrate that autophagy and protein degradation expansion of a poly-CGG trinucleotide repeat located in the 5′ via the autophagy/lysosomal pathway are reduced in hippo- UTR of the fragile X mental retardation 1 (Fmr1) gene from campal neurons of Fmr1-KO mice, a model of human FXS. We ∼50 to >200 repeats results in hypermethylation and epigenetic show that excess mTOR activity is causally related to decreased silencing of the Fmr1 gene, causing FXS (5–7). Fragile X mental autophagy, which induces spine defects, exaggerated synaptic retardation protein (FMRP), the gene product of Fmr1,isan plasticity, and impaired cognition in Fmr1-KO mice. These RNA-binding protein implicated in cognitive dysfunction and findings increase our understanding of the etiology of FXS and autism that tightly regulates the trafficking, localization, and suggest components of the autophagy pathway as promising translation of a vast number of neuronal mRNAs critical to targets for amelioration of FXS in humans. neural development, synaptic plasticity, and dendritic spine ar- – chitecture (1, 8 10). An effective treatment for fragile X in hu- Author contributions: J.Y., M.W.P., M.V.L.B., and R.S.Z. designed research; J.Y., M.W.P., mans is still lacking. and B.C.-V. performed research; J.Y., M.W.P., M.V.L.B., and R.S.Z. analyzed data; and J.Y., Mammalian target of rapamycin (mTOR) is a serine/threonine M.W.P., M.V.L.B., and R.S.Z. wrote the paper. kinase that integrates external cues and is a central regulator of Reviewers: C.B., University of Lausanne; and L.K.K., Yale University School of Medicine. protein synthesis, cell growth, proliferation, survival, and auto- The authors declare no conflict of interest. phagy (11–14). Overactivation of mTOR complex 1 (mTORC1) Published under the PNAS license. at hippocampal synapses is a phenotypic feature of fragile X 1J.Y. and M.W.P. contributed equally to this work. humans, mice, and flies (7, 13, 15, 16). In neurons, mTORC1 is 2To whom correspondence may be addressed. Email: [email protected] or strategically positioned at pre- and postsynaptic sites where it [email protected]. promotes local protein translation (6–9) and serves as a brake This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. on autophagy (11–14). Autophagy is a catabolic process that 1073/pnas.1808247115/-/DCSupplemental. plays a pivotal role in synaptic remodeling and memory formation www.pnas.org/cgi/doi/10.1073/pnas.1808247115 PNAS Latest Articles | 1of10 Downloaded by guest on September 24, 2021 P S2448 mTOR Raptor deficit. Overactivation of mTORC1 in hippocampal neurons is a phenotypic feature common to FXS and other autism-like dis- Inhibitory signal Fmr1 S317 P S757 S14 orders (15, 23). Hippocampal neurons from -KO mice ex- P P AMPK ULK-1 Beclin LC3 -II p62 Autophagy hibit elevated phosphorylation of mTOR at S2448, indicating overactivation of mTOR (Fig. 3A). mTOR-dependent phos- phorylation of ULK-1 at S757, an antiautophagy site, prevents Atg7 assembly of ULK-1 with scaffolding protein Atg13 and adhesion Fig. 1. Schematic of mTOR and autophagy signaling. Upon activation, protein FIP200 (20) and halts the initiation of autophagy. mTOR phosphorylates ULK-1 at S757, a target of mTORC1 and a well- Phosphorylation of ULK-1 at S757 was markedly increased in established antiautophagy site. This, in turn, sequesters ULK-1 away from Fmr1-KO vs. WT neurons, consistent with down-regulated AMPK, which phosphorylates and activates ULK-1 at S317. Phosphorylation autophagy, with little or no change in ULK-1 abundance (Fig. of ULK-1 at S317 promotes phosphorylation and activation of Beclin-1 at 3B). In response to cellular stress, ULK-1 is phosphorylated at S14, a critical step in the nucleation phase of autophagy. Beclin- 1 promotes lipidation of LC3-I to generate its lipidated form LC3-II, en- S317 by AMPK to induce the initiation of autophagy (31). abling elongation of the limiting membrane and formation of autophago- Consistent with this, phosphorylation of ULK-1 at S317 was somes. Lipidation of LC3-I is an ubiquitin-like reaction that requires Atg7. decreased in neurons from KO vs. WT mice (Fig. 3C). Moreover, Autophagosomes act via protein adaptors such as p62 to deliver proteins phosphorylation and activation of Beclin-1 at S14, a downstream and organelles to lysosomes for degradation. postsynaptic density protein PSD-95 and the activity-regulated ACB 1.5 1.5 D 1.5 cytoskeletal-associated synaptic proteins Arc/Arg3.1 implicated LC3 in hippocampal neurons ** ** WT Fmr1 KO 1.0 1.0 1.0 β-actin in spine structure and synaptic plasticity. In summary, these LC3-I 19 kD I/ LC3-II 17 kD 0.5 0.5 0.5 LC3-II/LC3-1 findings document a causal relation between decreased auto- LC3- β-actin 42 kD LC3-II/β-actin 0 0 0 phagy and the synaptic and cognitive deficits in FXS and identify WT Fmr1 KO WT Fmr1 KO WT Fmr1 KO components of the autophagy pathway as potential therapeutic E Dendrites of hippocampal neurons targets for amelioration of the symptoms of FXS. LC3 MAP-2 Merge F 1.5 WT ** Results 1.0 Autophagy Is Down-Regulated in Hippocampal Neurons of Fragile X 0.5 Mice. Initiation of macroautophagy starts with autophagosome KO 0 Fmr1 LC3 puncta numberLC3 puncta WT KO formation, which depends on the lipidation of LC3-I to generate Fmr1 its lipidated counterpart LC3-II.