Several lines of evidence suggest that polymerization of the amyloid β‐peptide (Aβ) into amyloid plaques is a pathogenic event in Alzheimer’s disease (AD). Aβ is produced from the amyloid precursor protein as the result of sequential proteolytic cleavages by β‐secretase and γ‐secretase, and it has been suggested that these enzymes could be targets for treatment of AD. γ‐Secretase is an aspartyl protease complex, containing at least four transmembrane proteins. Studies in cell lines have shown that γ‐secretase is partially localized to lipid rafts, which are detergent‐resistant membrane microdomains enriched in cholesterol and sphingolipids. Here, we studied γ‐secretase in detergent‐resistant membranes (DRMs) prepared from human brain. DRMs prepared in the mild detergent CHAPSO and isolated by sucrose gradient centrifugation were enriched in γ‐secretase components and activity. The DRM fraction was subjected to size‐exclusion chromatography in CHAPSO, and all of the γ‐secretase components and a lipid raft marker were found in the void volume (> 2000 kDa). Co‐immunoprecipitation studies further supported the notion that the γ‐secretase components are associated even at high concentrations of CHAPSO. Preparations from rat brain gave similar results and showed a postmortem time‐dependent decline in γ‐secretase activity, suggesting that DRMs from fresh rat brain may be useful for γ‐secretase activity studies. Finally, confocal microscopy showed co‐localization of γ‐secretase components and a lipid raft marker in thin sections of human brain. We conclude that the active γ‐secretase complex is localized to lipid rafts in human brain.

The amyloid-cascade hypothesis posits that the role of amyloid β-peptide (Aβ) in Alzheimer disease (AD) involves polymerization into structures that eventually are deposited as amyloid plaques. During this process, neurotoxic oligomers are formed that induce synaptic loss and neuronal death. Several different isoforms of Aβ are produced, of which the 40 and 42 residue variants (Aβ40 and Aβ42) are the most common. Aβ42 has a strong tendency to form neurotoxic aggregates and is involved in AD pathogenesis. Longer Aβ isoforms, like the less studied Aβ43, are gaining attention for their higher propensity to aggregate into neurotoxic oligomers. To further investigate Aβ43 in AD, we conducted a quantitative study on Aβ43 levels in human brain. We homogenized human brain tissue and prepared fractions of various solubility; tris buffered saline (TBS), sodium dodecyl sulfate (SDS) and formic acid (FA). Levels of Aβ43, as well as Aβ40 and Aβ42, were quantified using ELISA. We compared quantitative data showing Aβ levels in occipital and frontal cortex from sporadic (SAD) and familial (FAD) AD cases, as well as non-demented (ND) controls. Results showed Aβ43 present in each fraction from the SAD and FAD cases, while its level was lower than the detection limit in the majority of the ND-cases. Aβ42 and Aβ43 were enriched in the less soluble fractions (SDS and FA) of SAD and FAD cases in both occipital and frontal cortex. Thus, although the total levels of Aβ43 in human brain are low compared to Aβ40 and Aβ42, we suggest that Aβ43 could initiate the formation of oligomers and amyloid plaques and thereby be crucial to AD pathogenesis.

Protein glycosylation is crucial for the central nervous system and brain functions, including processes that are defective in Alzheimer disease (AD) such as neurogenesis, synaptic function, and memory formation. Still, the roles of glycans in the development of AD are relatively unexplored. Glycomics studies of cerebrospinal fluid (CSF) have previously shown altered glycosylation pattern in patients with different stages of cognitive impairment, including AD, compared to healthy controls. As a consequence, we hypothesized that the glycan profile is altered in the brain of patients with AD and analyzed the asparagine‐linked (N‐linked) glycan profile in hippocampus and cortex in AD and control brain. Glycans were enzymatically liberated from brain glycoproteins and analyzed by liquid chromatography‐tandem mass spectrometry (LC‐MS/MS). Eleven glycans showed significantly different levels in hippocampus compared to cortex in both control and AD brain. Two glycans in cortex and four in hippocampus showed different levels in AD compared to control brain. All glycans that differed between controls and AD brain had similar structures with one sialic acid, at least one fucose and a confirmed or potential bisecting N‐acetylglucosamine (GlcNAc). The glycans that were altered in AD brain differed from those that were altered in AD CSF. One glycan found to be present in significantly lower levels in both hippocampus and cortex in AD compared to control contained a structurally and functionally interesting epitope that we assign as a terminal galactose decorated with fucose and sialic acid. Altogether, these studies suggest that protein glycosylation is an important component in the development of AD and warrants further studies. image

In Alzheimer's disease (AD), tau undergoes abnormal post‐translational modifications and aggregations. Impaired intracellular degradation pathways further exacerbate the accumulation of pathological tau. A new strategy – targeted protein degradation – recently emerged as a modality in drug discovery where bifunctional molecules bring the target protein close to the degradation machinery to promote clearance. Since 2016, this strategy has been applied to tau pathologies and attracted broad interest in academia and the pharmaceutical industry. However, a systematic review of recent studies on tau degradation mechanisms is lacking. Here we review tau degradation mechanisms (the ubiquitin–proteasome system and the autophagy–lysosome pathway), their dysfunction in AD, and tau‐targeted degraders, such as proteolysis‐targeting chimeras and autophagy‐targeting chimeras. We emphasize the need for a continuous exploration of tau degradation mechanisms and provide a future perspective for developing tau‐targeted degraders, encouraging researchers to work on new treatment options for AD patients.Highlights: Post‐translational modifications, aggregation, and mutations affect tau degradation. A vicious circle exists between impaired degradation pathways and tau pathologies. Ubiquitin plays an important role in complex degradation pathways. Tau‐targeted degraders provide promising strategies for novel AD treatment.

The transmembrane protein assembly γ-secretase is a key protease in regulated intramembrane processing (RIP) of around 100 type-1 transmembrane proteins. Importantly, it has a pathological role in Alzheimer disease (AD) as it generates the neurotoxic amyloid β-peptide from the amyloid precursor protein (APP). Studies on γ-secretase location are therefore crucial both from a biological and a therapeutic perspective. Despite several years of efforts in many laboratories, it is not clear where in the neuron γ-secretase exerts it’s activities. Technical challenges include the fact that the active enzyme contains four protein components and that most subcellular compartments cannot be spatially resolved by traditional light microscopy. Here, we have used a powerful combination of the two nanoscopy techniques STORM and STED microscopy to visualize the location of γ-secretase in neurons using an active-site specific probe, with a focus on the synapse. We show that γ-secretase is present in both the pre-and postsynaptic compartments. We further show that the enzyme is enriched very close to the synaptic cleft in the postsynaptic membrane, as well as to NMDA receptors, demonstrating that γ-secretase is present in the postsynaptic plasma membrane. Importantly, the expression of γ-secretase increased in the pre- and postsynaptic compartments with the size of the synapse, suggesting a correlation between γ-secretase activity and synapse maturation. Thus, our data shows the synaptic location with high precision in three dimensions and settles the long-lasting debate on the synaptic location of γ-secretase.