Fragile X syndrome, the most common form of inherited intellectual disability, is caused most often by a lack of fragile X mental retardation protein (FMRP). However, the mechanism remains unclear and effective treatment is lacking. Here we show that a loss of FMRP leads to activation of adult neural stem cells (NSCs) and a subsequent reduction in neuronal production. We identified ubiquitin ligase MDM2 as a target of FMRP. FMRP regulates Mdm2 mRNA stability, and loss of FMRP results in elevated mRNA and MDM2 protein levels. We further found that increased MDM2 levels lead to reduced P53 in NSCs, which alters NSC proliferation and differentiation. Treatment with Nutlin-3, a small molecule undergoing clinical trials for cancer, specifically inhibits MDM2 and P53 interaction, and rescues the neurogenic and cognitive deficits in FMRP-deficient mice. Our data unveil a regulatory role for FMRP and a potential new treatment for fragile X syndrome.

Fragile X syndrome (FXS) is the most prevalent inherited intellectual disability, resulting from a loss of fragile X mental retardation protein (FMRP). Patients with FXS suffer lifelong cognitive disabilities, but the function of FMRP in the adult brain and the mechanism underlying age-related cognitive decline in FXS is not fully understood. Here, we report that a loss of FMRP results in increased protein synthesis of histone acetyltransferase EP300 and ubiquitination-mediated degradation of histone deacetylase HDAC1 in adult hippocampal neural stem cells (NSCs). Consequently, FMRP-deficient NSCs exhibit elevated histone acetylation and age-related NSC depletion, leading to cognitive impairment in mature adult mice. Reducing histone acetylation rescues both neurogenesis and cognitive deficits in mature adult FMRP-deficient mice. Our work reveals a role for FMRP and histone acetylation in cognition and presents a potential novel therapeutic strategy for treating adult FXS patients.

Fragile X syndrome results from a loss of the RNA-binding protein fragile X mental retardation protein (FMRP). How FMRP regulates neuronal development and function remains unclear. Here, we show that FMRP-deficient immature neurons exhibit impaired dendritic maturation, altered expression of mitochondrial genes, fragmented mitochondria, impaired mitochondrial function, and increased oxidative stress. Enhancing mitochondrial fusion partially rescued dendritic abnormalities in FMRP-deficient immature neurons. We show that FMRP deficiency leads to reduced Htt mRNA and protein levels and that HTT mediates FMRP regulation of mitochondrial fusion and dendritic maturation. Mice with hippocampal Htt knock-down and Fmr1 knockout mice showed similar behavioral deficits that could be rescued by treatment with a mitochondrial fusion compound. Our data unveil mitochondrial dysfunction as a contributor to the impaired dendritic maturation of FMRP-deficient neurons and suggest a role for interactions between FMRP and HTT in the pathogenesis of Fragile X syndrome.

CRISPR/Cas9 guided gene-editing is a potential therapeutic tool, however application to neurodegenerative disease models has been limited. Moreover, conventional mutation correction by gene-editing would only be relevant for the small fraction of neurodegenerative cases that are inherited. Here we introduce a CRISPR/Cas9-based strategy in cell and animal models to edit endogenous amyloid precursor protein (APP) at the extreme C-terminus and reciprocally manipulate the amyloid pathway, attenuating APP-β-cleavage and Aβ production, while up-regulating neuroprotective APP-α-cleavage. APP N-terminus and compensatory APP-homologues remain intact, with no apparent effects on neurophysiology in vitro. Robust APP-editing is seen in human iPSC-derived neurons and mouse brains with no detectable off-target effects. Our strategy likely works by limiting APP and BACE-1 approximation, and we also delineate mechanistic events that abrogates APP/BACE-1 convergence in this setting. Our work offers conceptual proof for a selective APP silencing strategy.

In the central nervous system, oligodendrocytes produce myelin sheaths that enwrap neuronal axons to provide trophic support and increase conduction velocity. New oligodendrocytes are produced throughout life through a process referred to as oligodendrogenesis. Oligodendrogenesis consists of three canonical stages: the oligodendrocyte precursor cell (OPC), the premyelinating oligodendrocyte (preOL), and the mature oligodendrocyte (OL). However, the generation of oligodendrocytes is inherently an inefficient process. Following precursor differentiation, a majority of premyelinating oligodendrocytes are lost, likely due to apoptosis. If premyelinating oligodendrocytes progress through this survival checkpoint, they generate new myelinating oligodendrocytes in a process we have termed integration. In this review, we will explore the intrinsic and extrinsic signaling pathways that influence preOL survival and integration by examining the intrinsic apoptotic pathways, metabolic demands, and the interactions between neurons, astrocytes, microglia, and premyelinating oligodendrocytes. Additionally, we will discuss similarities between the maturation of newly generated neurons and premyelinating oligodendrocytes. Finally, we will consider how increasing survival and integration of preOLs has the potential to increase remyelination in multiple sclerosis. Deepening our understanding of premyelinating oligodendrocyte biology may open the door for new treatments for demyelinating disease and will help paint a clearer picture of how new oligodendrocytes are produced throughout life to facilitate brain function.

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Parvalbumin interneurons (PVIs) are affected in many psychiatric disorders including schizophrenia (SCZ), however the mechanism remains unclear. FXR1 , a high confident risk gene for SCZ, is indispensable but its role in the brain is largely unknown. We show that deleting FXR1 from PVIs of medial prefrontal cortex (mPFC) leads to reduced PVI excitability, impaired mPFC gamma oscillation, and SCZ-like behaviors. PVI-specific translational profiling reveals that FXR1 regulates the expression of Cacna1h /Cav3.2 a T-type calcium channel implicated in autism and epilepsy. Inhibition of Cav3.2 in PVIs of mPFC phenocopies whereas elevation of Cav3.2 in PVIs of mPFC rescues behavioral deficits resulted from FXR1 deficiency. Stimulation of PVIs using a gamma oscillation-enhancing light flicker rescues behavioral abnormalities cause by FXR1 deficiency in PVIs. This work unveils the function of a newly identified SCZ risk gene in SCZ-relevant neurons and identifies a therapeutic target and a potential non-invasive treatment for psychiatric disorders.

The gradual accumulation of amyloid-b (Ab) is a neuropathologic hallmark of Alzheimer's disease (AD); playing a key role in disease progression. Ab is generated by the sequential cleavage of amyloid precursor protein (APP) by b-and g-secretases, with BACE-1 (b-site APP cleaving enzyme-1) cleavage as the rate limiting step [1][2][3] . CRISPR/Cas9 guided gene-editing is emerging as a promising tool to edit pathogenic mutations and hinder disease progression 4,5,6 . However, few studies have applied this technology to neurologic diseases 7-9 . Besides technical caveats such as low editing efficiency in brains and limited in vivo validation 7 , the canonical approach of 'mutation-correction' would only be applicable to the small fraction of neurodegenerative cases that are inherited (i.e. < 10% of AD, Parkinson's, ALS); with a new strategy needed for every gene. Moreover, feasibility of CRISPR/Cas9 as a therapeutic possibility in sporadic AD has not been explored. Here we introduce a strategy to edit endogenous APP at the extreme C-terminus and reciprocally manipulate the amyloid pathway -attenuating bcleavage and Ab, while up-regulating neuroprotective a-cleavage. APP N-terminus, as well as compensatory APP homologues remain intact, and key physiologic parameters remain unaffected. Robust APP-editing is seen in cell lines, cultured neurons, human embryonic stem cells/iPSC-neurons, and mouse brains. Our strategy works by limiting the physical association of APP and BACE-1, and we also delineate the mechanism that abrogates APP/BACE-1 interaction in this setting. Our work offers an innovative 'cut and silence' gene-editing strategy that could be a new therapeutic paradigm for AD.Our broad idea is to rationally edit small segments of wild-type (WT) proteins known to play key roles in the progression of sporadic disease, with the ultimate goal of attenuating their pathologic activity. As endogenous proteins expectedly play physiologic roles as well, it is also important to conserve the normal function of these molecules, as far as possible. Motivated by this idea, we designed sgRNAs targeting the extreme C-terminus of mouse APP, and one of these led to robust APP-editing, as shown in Fig. 1 (see protospacer adjacent motif -PAM -site and genomic target recognized by the sgRNA in Fig. 1a). APP-editing resulted in attenuated staining with an antibody (Y188) recognizing the extreme C-terminus of APP that is distal to the sgRNA-targeting site (neuroblastoma cells shown in Fig. 1b, see Y188 epitope in Fig. 1a). APP-sgRNA also attenuated the production of APP C-terminal fragments (CTFs; Fig. 1c, d -time-course of editing in Fig. 1e). However, an antibody recognizing the APP N-terminus (22C11) showed no differences between control and sgRNA-treated samples (Fig. 1c), suggesting that the editing only affected the short intracellular C-terminus.Genomic deep-sequencing confirmed efficient editing of mouse APP at the expected target (Fig. 1f).Though the abovementioned TGG PAM is conserved in both mouse and human APP, the upstre...