Brain region-specific susceptibility of Lewy body pathology in synucleinopathies is governed by α-synuclein conformations.

The protein α-synuclein, a key player in Parkinson's disease (PD) and other synucleinopathies, exists in different physiological conformations: cytosolic unfolded aggregation-prone monomers and helical aggregation-resistant multimers. It has been shown that familial PD-associated missense mutations within the α-synuclein gene destabilize the conformer equilibrium of physiologic α-synuclein in favor of unfolded monomers. Here, we characterized the relative levels of unfolded and helical forms of cytosolic α-synuclein in post-mortem human brain tissue and showed that the equilibrium of α-synuclein conformations is destabilized in sporadic PD and DLB patients. This disturbed equilibrium is decreased in a brain region-specific manner in patient samples pointing toward a possible "prion-like" propagation of the underlying pathology and forms distinct disease-specific patterns in the two different synucleinopathies. We are also able to show that a destabilization of multimers mechanistically leads to increased levels of insoluble, pathological α-synuclein, while pharmacological stabilization of multimers leads to a "prion-like" aggregation resistance. Together, our findings suggest that these disease-specific patterns of α-synuclein multimer destabilization in sporadic PD and DLB are caused by both regional neuronal vulnerability and "prion-like" aggregation transmission enabled by the destabilization of local endogenous α-synuclein protein.

MicroRNA-132 provides neuroprotection for tauopathies via multiple signaling pathways.

MicroRNAs (miRNA) regulate fundamental biological processes, including neuronal plasticity, stress response, and survival. Here, we describe a neuroprotective function of miR-132, the miRNA most significantly downregulated in neurons in Alzheimer's disease. We demonstrate that miR-132 protects primary mouse and human wild-type neurons and more vulnerable Tau-mutant neurons against amyloid ß-peptide (Aß) and glutamate excitotoxicity. It lowers the levels of total, phosphorylated, acetylated, and cleaved forms of Tau implicated in tauopathies, promotes neurite elongation and branching, and reduces neuronal death. Similarly, miR-132 attenuates PHF-Tau pathology and neurodegeneration, and enhances long-term potentiation in the P301S Tau transgenic mice. The neuroprotective effects are mediated by direct regulation of the Tau modifiers acetyltransferase EP300, kinase GSK3ß, RNA-binding protein Rbfox1, and proteases Calpain 2 and Caspases 3/7. These data suggest miR-132 as a master regulator of neuronal health and indicate that miR-132 supplementation could be of therapeutic benefit for the treatment of Tau-associated neurodegenerative disorders.

Developmental Regulation of Mitochondrial Apoptosis by c-Myc Governs Age- and Tissue-Specific Sensitivity to Cancer Therapeutics.

It is not understood why healthy tissues can exhibit varying levels of sensitivity to the same toxic stimuli. Using BH3 profiling, we find that mitochondria of many adult somatic tissues, including brain, heart, and kidneys, are profoundly refractory to pro-apoptotic signaling, leading to cellular resistance to cytotoxic chemotherapies and ionizing radiation. In contrast, mitochondria from these tissues in young mice and humans are primed for apoptosis, predisposing them to undergo cell death in response to genotoxic damage. While expression of the apoptotic protein machinery is nearly absent by adulthood, in young tissues its expression is driven by c-Myc, linking developmental growth to cell death. These differences may explain why pediatric cancer patients have a higher risk of developing treatment-associated toxicities.

Lessons from a Rare Familial Dementia: Amyloid and Beyond.

Here we review the similarities between a rare inherited disorder, familial British dementia (FBD), and the most common of all late-life neurological conditions, Alzheimer's diseases (AD). We describe the symptoms, pathology and genetics of FBD, the biology of the BRI2 protein and mouse models of FBD and familial Danish dementia. In particular, we focus on the evolving recognition of the importance of protein oligomers and aberrant processing of the amyloid ß-protein precursor (APP) - themes that are common to both FBD and AD. The initial discovery that FBD is phenotypically similar to AD, but associated with the deposition of an amyloid peptide (ABri) distinct from the amyloid ß-protein (Aß) led many to assume that amyloid production alone is sufficient to initiate disease and that ABri is the molecular equivalent of Aß. Parallel with work on Aß, studies of ABri producing animal models and in vitro ABri toxicity experiments caused a revision of the amyloid hypothesis and a focus on soluble oligomers of Aß and ABri. Contemporaneous other studies suggested that loss of the ABri precursor protein (BRI2) may underlie the cognitive deficits in FBD. In this regard it is important to note that BRI2 has been shown to interact with and regulate the processing of APP, and that mutant BRI2 leads to altered cleavage of APP. A synthesis of these results suggests that a "two-hit mechanism" better explains FBD than earlier toxic gain of function and toxic loss of function models. The lessons learned from the study of FBD imply that the molecular pathology of AD is also likely to involve both aberrant aggregation (in AD, Aß) and altered APP processing. With regard to FBD, we propose that the C-terminal 11 amino acid of FBD-BRI2 interfere with both the normal function of BRI2 and promotes the production of cystine cross-linked toxic ABri oligomers. In this scenario, loss of BRI2 function leads to altered APP processing in as yet underappreciated ways. Given the similarities between FBD and AD it seems likely that study of the structure of ABri oligomers and FBD-induced changes in APP metabolites will further our understanding of AD.

Real-Time Monitoring of Alzheimer's-Related Amyloid Aggregation via Probe Enhancement-Fluorescence Correlation Spectroscopy.

This work describes the use of fluorescence correlation spectroscopy (FCS) and a novel amyloid-binding fluorescent probe, ARCAM 1, to monitor the aggregation of the Alzheimer's disease-associated amyloid ß-peptide (Aß). ARCAM 1 exhibits a large increase in fluorescence emission upon binding to Aß assemblies, making it an excellent candidate for probe enhancement FCS (PE-FCS). ARCAM 1 binding does not change Aß aggregation kinetics. It also exhibits greater dynamic range as a probe in reporting aggregate size by FCS in Aß, when compared to thioflavin T (ThT) or an Aß peptide modified with a fluorophore. Using fluorescent burst analysis (via PE-FCS) to follow aggregation of Aß, we detected soluble aggregates at significantly earlier time points compared to typical bulk fluorescence measurements. Autocorrelation analysis revealed the size of these early Aß assemblies. These results indicate that PE-FCS/ARCAM 1 based assays can detect and provide size characterization of small Aß aggregation intermediates during the assembly process, which could enable monitoring and study of such aggregates that transiently accumulate in biofluids of patients with Alzheimer's and other neurodegenerative diseases.

C-Terminally Truncated Forms of Tau, But Not Full-Length Tau or Its C-Terminal Fragments, Are Released from Neurons Independently of Cell Death.

Recent evidence suggests that tau aggregation may spread via extracellular release and subsequent uptake by synaptically connected neurons, but little is known about the processes by which tau is released or the molecular forms of extracellular tau. To gain insight into the nature of extracellular tau, we used highly sensitive ELISAs, which, when used in tandem, are capable of differentiating between full-length (FL) tau, mid-region-bearing fragments, and C-terminal (CT) fragments. We applied these assays to the systematic study of the conditioned media of N2a cells, induced pluripotent stem cell-derived human cortical neurons, and primary rat cortical neurons, each of which was carefully assessed for viability. In all three neuronal models, the bulk of extracellular tau was free-floating and unaggregated and <0.2% was encapsulated in exosomes. Although most intracellular tau was FL, the majority of extracellular tau was CT truncated and appeared to be released both actively by living neurons and passively by dead cells. In contrast, only a small amount of extracellular tau was aggregation-competent tau (i.e., contained the microtubule-binding regions) and this material appears to be released solely due to a low level of cell death that occurs in all cell culture systems. Importantly, amyloid ß-protein (Aß)-induced neuronal compromise significantly increased the quantity of all forms of extracellular tau, but the presence of Aß before detectable cell compromise did not increase extracellular tau. Collectively, these results suggest that factors that induce neuronal death are likely to be necessary to initiate the extracellular spread of tau aggregation. SIGNIFICANCE STATEMENT: Recent studies suggest that the transfer of tau between neurons underlies the characteristic spatiotemporal progression of neurofibrillary pathology. We searched for tau in the conditioned medium of N2a cells, induced pluripotent stem cell-derived human cortical neurons, and primary rat cortical neurons and analyzed the material present using four different tau ELISAs. We demonstrate that the majority of tau released from healthy neurons is C-terminally truncated and lacks the microtubule-binding region (MTBR) thought necessary for self-aggregation. A small amount of MTBR-containing tau is present outside of cells, but this appears to be solely due to cell death. Therefore, if propagation of tau aggregation is mediated by extracellular tau, our findings suggest that neuronal compromise is required to facilitate this process.

The Familial British Dementia Mutation Promotes Formation of Neurotoxic Cystine Cross-linked Amyloid Bri (ABri) Oligomers.

Familial British dementia (FBD) is an inherited neurodegenerative disease believed to result from a mutation in the BRI2 gene. Post-translational processing of wild type BRI2 and FBD-BRI2 result in the production of a 23-residue long Bri peptide and a 34-amino acid long ABri peptide, respectively, and ABri is found deposited in the brains of individuals with FBD. Similarities in the neuropathology and clinical presentation shared by FBD and Alzheimer disease (AD) have led some to suggest that ABri and the AD-associated amyloid ß-protein (Aß) are molecular equivalents that trigger analogous pathogenic cascades. But the sequences and innate properties of ABri and Aß are quite different, notably ABri contains two cysteine residues that can form disulfide bonds. Thus we sought to determine whether ABri was neurotoxic and if this activity was regulated by oxidation and/or aggregation. Crucially, the type of oxidative cross-linking dramatically influenced both ABri aggregation and toxicity. Cyclization of Bri and ABri resulted in production of biologically inert monomers that showed no propensity to assemble, whereas reduced ABri and reduced Bri aggregated forming thioflavin T-positive amyloid fibrils that lacked significant toxic activity. ABri was more prone to form inter-molecular disulfide bonds than Bri and the formation of covalently stabilized ABri oligomers was associated with toxicity. These results suggest that extension of the C-terminal of Bri causes a shift in the type of disulfide bonds formed and that structures built from covalently cross-linked oligomers can interact with neurons and compromise their function and viability.