Objective Sorting mechanisms that cause the amyloid precursor protein (APP) and the β-secretases and γ-secretases to colocalize in the same compartment play an important role in the regulation of Aβ production in Alzheimer’s disease (AD). We and others have reported that genetic variants in the Sortilin-related receptor (SORL1) increased the risk of AD, that SORL1 is involved in trafficking of APP, and that under expression of SORL1 leads to overproduction of Aβ. Here we explored the role of one of its homologs, the sortilin-related VPS10 domain containing receptor 1 (SORCS1), in AD. Methods We analyzed the genetic associations between AD and 16 SORCS1–single nucleotide polymorphisms (SNPs) in 6 independent data sets (2,809 cases and 3,482 controls). In addition, we compared SorCS1 expression levels of affected and unaffected brain regions in AD and control brains in microarray gene expression and real-time polymerase chain reaction (RT-PCR) sets, explored the effects of significant SORCS1-SNPs on SorCS1 brain expression levels, and explored the effect of suppression and overexpression of the common SorCS1 isoforms on APP processing and Aβ generation. Results Inherited variants in SORCS1 were associated with AD in all datasets (0.001 < p < 0.049). In addition, SorCS1 influenced APP processing. While overexpression of SorCS1 reduced γ-secretase activity and Aβ levels, the suppression of SorCS1 increased γ-secretase processing of APP and the levels of Aβ. Interpretations These data suggest that inherited or acquired changes in SORCS1 expression or function may play a role in the pathogenesis of AD.

SUMMARY Tuberous Sclerosis Complex (TSC) is a neurodevelopmental disease caused by TSC1 or TSC2 mutations and subsequent activation of the mTORC1 kinase. Upon mTORC1 activation, anabolic metabolism, which requires mitochondria, is induced, yet at the same time the principal pathway for mitochondrial turnover, autophagy, is compromised. How mTORC1 activation impacts mitochondrial turnover in neurons remains unknown. Here we demonstrate impaired mitochondrial homeostasis in neuronal in vitro and in vivo models of TSC. We find that Tsc1/2-deficient neurons accumulate mitochondria in cell bodies but are depleted of axonal mitochondria, including those supporting presynaptic sites. Axonal and global mitophagy of damaged mitochondria is impaired, suggesting that decreased turnover may act upstream of impaired mitochondrial metabolism. Importantly, blocking mTORC1 or inducing mTOR-independent autophagy restores mitochondrial homeostasis. Our study clarifies the complex relationship between the TSC-mTORC1 pathway, autophagy and mitophagy, and defines mitochondrial homeostasis as a therapeutic target for TSC and related diseases.

Genetic variants in the sortilin-related receptor (SORL1) and the sortilin-related vacuolar protein sorting 10 (VPS10) domain-containing receptor 1 (SORCS1) are associated with increased risk of Alzheimer's disease (AD), declining cognitive function and altered amyloid precursor protein (APP) processing. We explored whether other members of the (VPS10) domain-containing receptor protein family (the sortilin-related VPS10 domain-containing receptors 2 and 3 (SORCS2 and SORCS3) and sortilin (SORT1)) would have similar effects either independently or together. We conducted the analyses in a large Caucasian case control data set (n=11 840 cases, 10 931 controls) to determine the associations between single nucleotide polymorphisms (SNPs) in all the five homologous genes and AD risk. Evidence for interactions between SNPs in the five VPS10 domain receptor family genes was determined in epistatic statistical models. We also compared expression levels of SORCS2, SORCS3 and SORT1 in AD and control brains using microarray gene expression analyses and assessed the effects of these genes on γ-secretase processing of APP. Several SNPs in SORL1, SORCS1, SORCS2 and SORCS3 were associated with AD. In addition, four specific linkage disequilibrium blocks in SORCS1, SORCS2 and SORCS3 showed additive epistatic effects on the risk of AD (P⩽0.0006). SORCS3, but not SORCS2 or SORT1, showed reduced expression in AD compared with control brains, but knockdown of all the three genes using short hairpin RNAs in HEK293 cells caused a significant threefold increase in APP processing (from P<0.001 to P<0.05). These findings indicate that in addition to SORL1 and SORCS1, variants in other members of the VPS10 domain receptor family (that is, SORCS1, SORCS2, SORCS3) are associated with AD risk and alter APP processing. More importantly, the results indicate that variants within these genes have epistatic effects on AD risk.

In a host of neurodegenerative diseases Tau, a microtubuleassociated protein, aggregates into insoluble lesions within neurons. Previous studies have utilized cyanine dyes as Tau aggregation inhibitors in vitro. Herein we utilize cyanine dye 3,3-diethyl-9-methyl-thiacarbocyanine iodide (C11) to modulate Tau polymerization in two model systems, an organotypic slice culture model derived from Tau transgenic mice and a split green fluorescent protein complementation assay in Tau-expressing cells. In slice cultures, submicromolar concentrations (0.001 M) of C11 produced a significant reduction of aggregated Tau and a corresponding increase in unpolymerized Tau. In contrast, treatment with a 1 M dose promoted aggregation of Tau. These results were recapitulated in the complementation assay where administration of 1 M C11 produced a significant increase in polymerized Tau relative to control, whereas treatment of cells with 0.01 M C11 resulted in a marked reduction of aggregated Tau. In the organotypic slice cultures, modulation of Tau aggregation was independent of changes in phosphorylation at disease and microtubule binding relevant epitopes for both dosing regimes. Furthermore, treatment with 0.001 M C11 resulted in a decrease in both total filament mass and number. There was no evidence of apoptosis or loss of synaptic integrity at either dose, however, whereas submicromolar concentrations of C11 did not interfere with microtubule binding, higher doses resulted in a decrease in the levels of microtubule-bound Tau. Overall, a cyanine dye can dissociate aggregated Tau in an ex vivo model of tauopathy with little toxicity and exploration of the use of these type of dyes as therapeutic agents is warranted.

Saitohin is a gene unique to humans and their closest relatives, the function of which is not yet known. Saitohin contains a single polymorphism (Q7R), and its Q and R alleles belong to the H1 and H2 tau haplotype, respectively. The Saitohin Q allele confers susceptibility to several neurodegenerative diseases. To get a handle on Saitohin function, we used it as a bait in a yeast two-hybrid screen. By this assay and subsequent co-immunoprecipitation and glutathione S-transferase pull-down assays, we discovered and confirmed that Saitohin interacts with peroxiredoxin 6, a unique member of that family that is bifunctional and the levels of which increase in Pick disease. The strength of the interaction appeared to be allelespecific, giving the first distinction between the two forms of Saitohin. Saitohin (STH),2 an intronless gene encoding an open reading frame of 128 amino acids, is located in the intron between exons 9 and 10 of the human tau gene (1). It bears no obvious homology to any known protein or motif, and its expression pattern is very similar to that of tau in human tissues. The DNA sequences homologous to the human STH gene reveal an intact, highly conserved open reading frame in the primates most closely related to humans (chimpanzee, bonobo, and gorilla) but not in other primates or in rodents (2, 3). STH provides an evolutionary locus that separates humans and their closest relatives from other mammals.A single nucleotide polymorphism of human STH has been identified that changes glutamine residue 7 to arginine (Q7R (1)). This polymorphism is associated with the two tau gene haplotypes: the Q allele with H1, the R allele with H2 (2, 4). The Q allele is the most common haplotype in humans, but all nonhuman primates are homozygous for the R allele, which makes the Q allele a human-specific marker (2, 3). Hence, the R allele is the ancestral haplotype, whereas the Q allele evolved after the human lineages separated from the other primates.In addition to evolution studies, the R allele of STH was initially found to be associated with Alzheimer disease (1), although subsequent data showing association of the R allele with Alzheimer disease have been inconsistent (2, 4 -10). Recent work may have resolved the conflicting results by indicating that the R allele is associated with Alzheimer disease specifically resulting from the ApoE4 susceptibility factor (11). Furthermore, the Q allele has been shown to be over-represented in several neurodegenerative diseases: progressive supranuclear palsy (12, 13), frontotemporal dementia (4), and Parkinson disease (14). Taken together, these results suggest that identifying a function of STH could implicate several proteins and/or pathway(s) as potential contributors in these neurodegenerative diseases as well.To discover putative functions of STH, we used it as a bait in a yeast two-hybrid screening. We discovered that it interacts with a unique member of the peroxiredoxin family, peroxiredoxin 6 (Prdx6), which has both an antioxidant function and phospholipase act...

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Saitohin (STH) is a gene unique to humans and their closest relatives whose function is not yet known. STH contains a single polymorphism (Q7R); the Q allele is human-specific and confers susceptibility to several neurodegenerative diseases. In previous work, we discovered that STH interacts with Peroxiredoxin 6 (Prdx6), a unique member of that family which is bifunctional and whose levels increase in Pick’s disease. In this study, we report that STH also interacts with tau and the non-receptor tyrosine kinase c-Abl (Abl). Furthermore, Abl phosphorylates STH on its single tyrosine residue and STH increases tyrosine phosphorylation by Abl. The effect of Saitohin on Abl-mediated phosphorylation appears to be allele-specific, providing evidence for a new cellular function for STH.

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