SUMMARY Metabolites in the kynurenine pathway of tryptophan degradation are thought to play an important role in neurodegenerative disorders such as Alzheimer’s disease and Huntington’s disease. Metabolites that cause glutamate receptor-mediated excitotoxicity and free radical formation are elevated in the blood and vulnerable brain regions in these diseases, while levels of the neuroprotective metabolite kynurenic acid are often decreased. Here we describe the synthesis and characterization of JM6, a novel small-molecule pro-drug inhibitor of kynurenine 3-monooxygenase (KMO). JM6 raises kynurenic acid and reduces extracellular glutamate in the brain after chronic oral administration by inhibiting KMO in blood. In a transgenic mouse model of Alzheimer’s disease, JM6 prevented spatial memory deficits, anxiety-related behavior, and synaptic loss. JM6 also extended life span, prevented synaptic loss, and decreased microglial activation in a mouse model of Huntington’s disease. These findings support a critical link between blood cells and neurodegeneration that is mediated by KMO and the kynurenine pathway.

Apolipoprotein E4 (apoE4) is the major genetic risk factor for Alzheimer's disease. However, the underlying mechanisms are unclear. We found that female apoE4 knock-in (KI) mice had an age-dependent decrease in hilar GABAergic interneurons that correlated with the extent of learning and memory deficits, as determined in the Morris water maze, in aged mice. Treating apoE4-KI mice with daily peritoneal injections of the GABA A receptor potentiator pentobarbital at 20 mg/kg for 4 weeks rescued the learning and memory deficits. In neurotoxic apoE4 fragment transgenic mice, hilar GABAergic interneuron loss was even more pronounced and also correlated with the extent of learning and memory deficits. Neurodegeneration and tauopathy occurred earliest in hilar interneurons in apoE4 fragment transgenic mice; eliminating endogenous Tau prevented hilar GABAergic interneuron loss and the learning and memory deficits. The GABA A receptor antagonist picrotoxin abolished this rescue, while pentobarbital rescued learning deficits in the presence of endogenous Tau. Thus, apoE4 causes age-and Tau-dependent impairment of hilar GABAergic interneurons, leading to learning and memory deficits in mice. Consequently, reducing Tau and enhancing GABA signaling are potential strategies to treat or prevent apoE4-related Alzheimer's disease.

Membrane fusion in the secretory pathway is mediated by soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins. Different fusion steps are thought to be effected by independent sets of SNAREs, but it is unclear whether specificity is determined by an intrinsic specificity of SNARE pairing or by upstream factors. Using a newly developed microscopy-based assay, we have investigated the SNARE specificity of homotypic early endosomal fusion. We show that early endosomes contain multiple sets of SNAREs, including, in addition to the putative early endosomal SNAREs, those involved in exocytosis and in fusion of late endosomes. We demonstrate that fusion is largely mediated by a complex formed by syntaxin 13, syntaxin 6, vti1a, and VAMP4, whereas the exocytic and late endosomal SNAREs play little or no role in the reaction. In contrast, proteoliposomes reconstituted with early endosomal SNAREs promiscuously fuse with liposomes containing exocytotic or late endosomal SNAREs. We conclude that the specificity of SNARE pairing does not suffice to determine the specificity of organelle fusion.endocytosis ͉ exocytosis ͉ PC12 cells S oluble N-ethylmaleimide (NEM)-sensitive factor attachment receptor (SNARE) proteins are represented by a superfamily of small membrane-associated proteins that mediate membrane fusion in the secretory pathway. All SNAREs possess a SNARE motif that is usually flanked by a C-terminal transmembrane domain (for recent reviews, see refs.

SNARE proteins mediate membrane fusion in eukaryotic cells. They contain conserved SNARE motifs that are usually located adjacent to a C-terminal transmembrane domain. SNARE motifs spontaneously assemble into four helix bundles, with each helix belonging to a different subfamily. Liposomes containing SNAREs spontaneously fuse with each other, but it is debated how the SNAREs are distributed between the membranes. Here, we report that the SNAREs mediating homotypic fusion of early endosomes fuse liposomes in five out of seven possible combinations, in contrast to previously studied SNAREs involved in heterotypic fusion events. The crystal structure of the early endosomal SNARE complex resembles that of the neuronal and late endosomal complexes, but differs in surface side-chain interactions. We conclude that homotypic fusion reactions may proceed with multiple SNARE topologies, suggesting that the conserved SNARE structure allows for flexibility in the initial interactions needed for fusion.