Loss of TDP-43 induces synaptic dysfunction that is rescued by UNC13A splice-switching ASOs.

TDP-43 loss of function induces multiple splicing changes, including a cryptic exon in the amyotrophic lateral sclerosis and fronto-temporal lobar degeneration risk gene UNC13A, leading to nonsense-mediated decay of UNC13A transcripts and loss of protein. UNC13A is an active zone protein with an integral role in coordinating pre-synaptic function. Here, we show TDP-43 depletion induces a severe reduction in synaptic transmission, leading to an asynchronous pattern of network activity. We demonstrate that these deficits are largely driven by a single cryptic exon in UNC13A. Antisense oligonucleotides targeting the UNC13A cryptic exon robustly rescue UNC13A protein levels and restore normal synaptic function, providing a potential new therapeutic approach for ALS and other TDP-43-related disorders.

TDP-43 loss induces extensive cryptic polyadenylation in ALS/FTD.

Nuclear depletion and cytoplasmic aggregation of the RNA-binding protein TDP-43 is the hallmark of ALS, occurring in over 97% of cases. A key consequence of TDP-43 nuclear loss is the de-repression of cryptic exons. Whilst TDP-43 regulated cryptic splicing is increasingly well catalogued, cryptic alternative polyadenylation (APA) events, which define the 3' end of last exons, have been largely overlooked, especially when not associated with novel upstream splice junctions. We developed a novel bioinformatic approach to reliably identify distinct APA event types: alternative last exons (ALE), 3'UTR extensions (3'Ext) and intronic polyadenylation (IPA) events. We identified novel neuronal cryptic APA sites induced by TDP-43 loss of function by systematically applying our pipeline to a compendium of publicly available and in house datasets. We find that TDP-43 binding sites and target motifs are enriched at these cryptic events and that TDP-43 can have both repressive and enhancing action on APA. Importantly, all categories of cryptic APA can also be identified in ALS and FTD post mortem brain regions with TDP-43 proteinopathy underlining their potential disease relevance. RNA-seq and Ribo-seq analyses indicate that distinct cryptic APA categories have different downstream effects on transcript and translation. Intriguingly, cryptic 3'Exts occur in multiple transcription factors, such as ELK1, SIX3, and TLX1, and lead to an increase in wild-type protein levels and function. Finally, we show that an increase in RNA stability leading to a higher cytoplasmic localisation underlies these observations. In summary, we demonstrate that TDP-43 nuclear depletion induces a novel category of cryptic RNA processing events and we expand the palette of TDP-43 loss consequences by showing this can also lead to an increase in normal protein translation.

Mis-spliced transcripts generate de novo proteins in TDP-43-related ALS/FTD.

Functional loss of TDP-43, an RNA binding protein genetically and pathologically linked to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), leads to the inclusion of cryptic exons in hundreds of transcripts during disease. Cryptic exons can promote the degradation of affected transcripts, deleteriously altering cellular function through loss-of-function mechanisms. Here, we show that mRNA transcripts harboring cryptic exons generated de novo proteins in TDP-43-depleted human iPSC-derived neurons in vitro, and de novo peptides were found in cerebrospinal fluid (CSF) samples from patients with ALS or FTD. Using coordinated transcriptomic and proteomic studies of TDP-43-depleted human iPSC-derived neurons, we identified 65 peptides that mapped to 12 cryptic exons. Cryptic exons identified in TDP-43-depleted human iPSC-derived neurons were predictive of cryptic exons expressed in postmortem brain tissue from patients with TDP-43 proteinopathy. These cryptic exons produced transcript variants that generated de novo proteins. We found that the inclusion of cryptic peptide sequences in proteins altered their interactions with other proteins, thereby likely altering their function. Last, we showed that 18 de novo peptides across 13 genes were present in CSF samples from patients with ALS/FTD spectrum disorders. The demonstration of cryptic exon translation suggests new mechanisms for ALS/FTD pathophysiology downstream of TDP-43 dysfunction and may provide a potential strategy to assay TDP-43 function in patient CSF.

Creation of de novo cryptic splicing for ALS/FTD precision medicine.

A system enabling the expression of therapeutic proteins specifically in diseased cells would be transformative, providing greatly increased safety and the possibility of pre-emptive treatment. Here we describe "TDP-REG", a precision medicine approach primarily for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), which exploits the cryptic splicing events that occur in cells with TDP-43 loss-of-function (TDP-LOF) in order to drive expression specifically in diseased cells. In addition to modifying existing cryptic exons for this purpose, we develop a deep-learning-powered algorithm for generating customisable cryptic splicing events, which can be embedded within virtually any coding sequence. By placing part of a coding sequence within a novel cryptic exon, we tightly couple protein expression to TDP-LOF. Protein expression is activated by TDP-LOF in vitro and in vivo, including TDP-LOF induced by cytoplasmic TDP-43 aggregation. In addition to generating a variety of fluorescent and luminescent reporters, we use this system to perform TDP-LOF-dependent genomic prime editing to ablate the UNC13A cryptic donor splice site. Furthermore, we design a panel of tightly gated, autoregulating vectors encoding a TDP-43/Raver1 fusion protein, which rescue key pathological cryptic splicing events. In summary, we combine deep-learning and rational design to create sophisticated splicing sensors, resulting in a platform that provides far safer therapeutics for neurodegeneration, potentially even enabling preemptive treatment of at-risk individuals.

Mis-spliced transcripts generate de novo proteins in TDP-43-related ALS/FTD.

Functional loss of TDP-43, an RNA-binding protein genetically and pathologically linked to ALS and FTD, leads to inclusion of cryptic exons in hundreds of transcripts during disease. Cryptic exons can promote degradation of affected transcripts, deleteriously altering cellular function through loss-of-function mechanisms. However, the possibility of de novo protein synthesis from cryptic exon transcripts has not been explored. Here, we show that mRNA transcripts harboring cryptic exons generate de novo proteins both in TDP-43 deficient cellular models and in disease. Using coordinated transcriptomic and proteomic studies of TDP-43 depleted iPSC-derived neurons, we identified numerous peptides that mapped to cryptic exons. Cryptic exons identified in iPSC models were highly predictive of cryptic exons expressed in brains of patients with TDP-43 proteinopathy, including cryptic transcripts that generated de novo proteins. We discovered that inclusion of cryptic peptide sequences in proteins altered their interactions with other proteins, thereby likely altering their function. Finally, we showed that these de novo peptides were present in CSF from patients with ALS. The demonstration of cryptic exon translation suggests new mechanisms for ALS pathophysiology downstream of TDP-43 dysfunction and may provide a strategy for novel biomarker development.

TDP-43 loss and ALS-risk SNPs drive mis-splicing and depletion of UNC13A.

Variants of UNC13A, a critical gene for synapse function, increase the risk of amyotrophic lateral sclerosis and frontotemporal dementia1-3, two related neurodegenerative diseases defined by mislocalization of the RNA-binding protein TDP-434,5. Here we show that TDP-43 depletion induces robust inclusion of a cryptic exon in UNC13A, resulting in nonsense-mediated decay and loss of UNC13A protein. Two common intronic UNC13A polymorphisms strongly associated with amyotrophic lateral sclerosis and frontotemporal dementia risk overlap with TDP-43 binding sites. These polymorphisms potentiate cryptic exon inclusion, both in cultured cells and in brains and spinal cords from patients with these conditions. Our findings, which demonstrate a genetic link between loss of nuclear TDP-43 function and disease, reveal the mechanism by which UNC13A variants exacerbate the effects of decreased TDP-43 function. They further provide a promising therapeutic target for TDP-43 proteinopathies.

Unanchored tri-NEDD8 inhibits PARP-1 to protect from oxidative stress-induced cell death.

NEDD8 is a ubiquitin-like protein that activates cullin-RING E3 ubiquitin ligases (CRLs). Here, we identify a novel role for NEDD8 in regulating the activity of poly(ADP-ribose) polymerase 1 (PARP-1) in response to oxidative stress. We show that treatment of cells with H2O2 results in the accumulation of NEDD8 chains, likely by directly inhibiting the deneddylase NEDP1. One chain type, an unanchored NEDD8 trimer, specifically bound to the second zinc finger domain of PARP-1 and attenuated its activation. In cells in which Nedp1 is deleted, large amounts of tri-NEDD8 constitutively form, resulting in inhibition of PARP-1 and protection from PARP-1-dependent cell death. Surprisingly, these NEDD8 trimers are additionally acetylated, as shown by mass spectrometry analysis, and their binding to PARP-1 is reduced by the overexpression of histone de-acetylases, which rescues PARP-1 activation. Our data suggest that trimeric, acetylated NEDD8 attenuates PARP-1 activation after oxidative stress, likely to delay the initiation of PARP-1-dependent cell death.

The AAA + ATPase Thorase is neuroprotective against ischemic injury.

Neuronal preconditioning in vitro or in vivo with a stressful but non-lethal stimulus leads to new protein expression that mediates a profound neuroprotection against glutamate excitotoxicity and experimental stroke. The proteins that mediate neuroprotection are relatively unknown and under discovery. Here we find that the expression of the AAA + ATPase Thorase is induced by preconditioning stimulation both in vitro and in vivo. Thorase provides neuroprotection in an ATP-dependent manner against oxygen-glucose deprivation (OGD) neurotoxicity or glutamate N-Methyl-D-aspartate (NMDA) receptor-mediated excitotoxicity in vitro. Knock-down of Thorase prevents the establishment of preconditioning induced neuroprotection against OGD or NMDA neurotoxicity. Transgenic overexpression of Thorase provides neuroprotection in vivo against middle cerebral artery occlusion (MCAO)-induced stroke in mice, while genetic deletion of Thorase results in increased injury in vivo following stroke. These results define Thorase as a neuroprotective protein and understanding Thorase signaling could offer a new therapeutic strategy for the treatment of neurologic disorders.

UBQLN2 Mediates Autophagy-Independent Protein Aggregate Clearance by the Proteasome.

Clearance of misfolded and aggregated proteins is central to cell survival. Here, we describe a new pathway for maintaining protein homeostasis mediated by the proteasome shuttle factor UBQLN2. The 26S proteasome degrades polyubiquitylated substrates by recognizing them through stoichiometrically bound ubiquitin receptors, but substrates are also delivered by reversibly bound shuttles. We aimed to determine why these parallel delivery mechanisms exist and found that UBQLN2 acts with the HSP70-HSP110 disaggregase machinery to clear protein aggregates via the 26S proteasome. UBQLN2 recognizes client-bound HSP70 and links it to the proteasome to allow for the degradation of aggregated and misfolded proteins. We further show that this process is active in the cell nucleus, where another system for aggregate clearance, autophagy, does not act. Finally, we found that mutations in UBQLN2, which lead to neurodegeneration in humans, are defective in chaperone binding, impair aggregate clearance, and cause cognitive deficits in mice.

Characterization of the mammalian family of DCN-type NEDD8 E3 ligases.

Cullin-RING ligases (CRL) are ubiquitin E3 enzymes that bind substrates through variable substrate receptor proteins and are activated by attachment of the ubiquitin-like protein NEDD8 to the cullin subunit. DCNs are NEDD8 E3 ligases that promote neddylation. Mammalian cells express five DCN-like (DCNL) proteins but little is known about their specific functions or interaction partners. We found that DCNLs form stable stoichiometric complexes with CAND1 and cullins that can only be neddylated in the presence of a substrate adaptor. These CAND-cullin-DCNL complexes might represent 'reserve' CRLs that can be rapidly activated when needed. We further found that all DCNLs interact with most cullin subtypes, but that they are probably responsible for the neddylation of different subpopulations of any given cullin. This is consistent with the fact that the subcellular localization of DCNLs in tissue culture cells differs and that they show unique tissue-specific expression patterns in mice. Thus, the specificity between DCNL-type NEDD8 E3 enzymes and their cullin substrates is only apparent in well-defined physiological contexts and related to their subcellular distribution and restricted expression.

Neuronal activity regulates hippocampal miRNA expression.

Neuronal activity regulates a broad range of processes in the hippocampus, including the precise regulation of translation. Disruptions in proper translational control in the nervous system are associated with a variety of disorders that fall in the autistic spectrum. MicroRNA (miRNA) represent a relatively recently discovered player in the regulation of translation in the nervous system. We have conducted an in depth analysis of how neuronal activity regulates miRNA expression in the hippocampus. Using deep sequencing we exhaustively identify all miRNAs, including 15 novel miRNAs, expressed in hippocampus of the adult mouse. We identified 119 miRNAs documented in miRBase but less than half of these miRNA were expressed at a level greater than 0.1% of total miRNA. Expression profiling following induction of neuronal activity by electroconvulsive shock demonstrates that most miRNA show a biphasic pattern of expression: rapid induction of specific mature miRNA expression followed by a decline in expression. These results have important implications into how miRNAs influence activity-dependent translational control.

The AAA+ ATPase Thorase regulates AMPA receptor-dependent synaptic plasticity and behavior.

The synaptic insertion or removal of AMPA receptors (AMPAR) plays critical roles in the regulation of synaptic activity reflected in the expression of long-term potentiation (LTP) and long-term depression (LTD). The cellular events underlying this important process in learning and memory are still being revealed. Here we describe and characterize the AAA+ ATPase Thorase, which regulates the expression of surface AMPAR. In an ATPase-dependent manner Thorase mediates the internalization of AMPAR by disassembling the AMPAR-GRIP1 complex. Following genetic deletion of Thorase, the internalization of AMPAR is substantially reduced, leading to increased amplitudes of miniature excitatory postsynaptic currents, enhancement of LTP, and elimination of LTD. These molecular events are expressed as deficits in learning and memory in Thorase null mice. This study identifies an AAA+ ATPase that plays a critical role in regulating the surface expression of AMPAR and thereby regulates synaptic plasticity and learning and memory.