Excess Synaptojanin 1 Contributes to Place Cell Dysfunction and Memory Deficits in the Aging Hippocampus in Three Types of Alzheimer's Disease.

The phosphoinositide phosphatase synaptojanin 1 (SYNJ1) is a key regulator of synaptic function. We first tested whether SYNJ1 contributes to phenotypic variations in familial Alzheimer's disease (FAD) and show that SYNJ1 polymorphisms are associated with age of onset in both early- and late-onset human FAD cohorts. We then interrogated whether SYNJ1 levels could directly affect memory. We show that increased SYNJ1 levels in autopsy brains from adults with Down syndrome (DS/AD) are inversely correlated with synaptophysin levels, a direct readout of synaptic integrity. We further report age-dependent cognitive decline in a mouse model overexpressing murine Synj1 to the levels observed in human sporadic AD, triggered through hippocampal hyperexcitability and defects in the spatial reproducibility of place fields. Taken together, our findings suggest that SYNJ1 contributes to memory deficits in the aging hippocampus in all forms of AD.

Neuronal lysosomal dysfunction releases exosomes harboring APP C-terminal fragments and unique lipid signatures.

Defects in endolysosomal and autophagic functions are increasingly viewed as key pathological features of neurodegenerative disorders. A master regulator of these functions is phosphatidylinositol-3-phosphate (PI3P), a phospholipid synthesized primarily by class III PI 3-kinase Vps34. Here we report that disruption of neuronal Vps34 function in vitro and in vivo impairs autophagy, lysosomal degradation as well as lipid metabolism, causing endolysosomal membrane damage. PI3P deficiency also promotes secretion of unique exosomes enriched for undigested lysosomal substrates, including amyloid precursor protein C-terminal fragments (APP-CTFs), specific sphingolipids, and the phospholipid bis(monoacylglycero)phosphate (BMP), which normally resides in the internal vesicles of endolysosomes. Secretion of these exosomes requires neutral sphingomyelinase 2 and sphingolipid synthesis. Our results reveal a homeostatic response counteracting lysosomal dysfunction via secretion of atypical exosomes eliminating lysosomal waste and define exosomal APP-CTFs and BMP as candidate biomarkers for endolysosomal dysfunction associated with neurodegenerative disorders.

Endolysosomal dysfunction and exosome secretion: implications for neurodegenerative disorders.

Growing evidence suggests that endolysosomal and autophagic defects are key pathogenic processes in various neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). The causal relationship between these defects and neurodegeneration is supported by human genetic studies identifying disease mutations in genes controlling endolysosomal function and autophagy. The canonical view is that defects in these processes lead to impaired lysosomal clearance of proteins prone to form toxic oligomeric assemblies and/or aggregates, ultimately resulting in cellular pathologies that define these disorders. Because lysosomes mediate the clearance of a large number of lipids, lipid storage is frequently associated with compromised endolysosomal and autophagic function. However, an emerging notion, supported by our recent study on class III phosphatidylinositol 3-kinase (PI3K) Vps34, is that neuronal endolysosomal and autophagic dysfunction can manifest itself with the occurrence of physically damaged endomembranes and with the release of exosomes enriched for Amyloid Precursor Protein COOH-terminal fragments (APP-CTFs) as well as atypical phospholipid bis(monoacylglycero)phosphate (BMP). Here, we summarize our recent findings and their potential implications in the context of lysosomal biology, lipid signaling and neurodegenerative diseases.

Disruption of amyloid precursor protein ubiquitination selectively increases amyloid ß (Aß) 40 levels via presenilin 2-mediated cleavage.

Amyloid plaques, a neuropathological hallmark of Alzheimer's disease, are largely composed of amyloid ß (Aß) peptide, derived from cleavage of amyloid precursor protein (APP) by ß- and γ-secretases. The endosome is increasingly recognized as an important crossroad for APP and these secretases, with major implications for APP processing and amyloidogenesis. Among various post-translational modifications affecting APP accumulation, ubiquitination of cytodomain lysines may represent a key signal controlling APP endosomal sorting. Here, we show that substitution of APP C-terminal lysines with arginine disrupts APP ubiquitination and that an increase in the number of substituted lysines tends to increase APP metabolism. An APP mutant lacking all C-terminal lysines underwent the most pronounced increase in processing, leading to accumulation of both secreted and intracellular Aß40. Artificial APP ubiquitination with rapalog-mediated proximity inducers reduced Aß40 generation. A lack of APP C-terminal lysines caused APP redistribution from endosomal intraluminal vesicles (ILVs) to the endosomal limiting membrane, with a subsequent decrease in APP C-terminal fragment (CTF) content in secreted exosomes, but had minimal effects on APP lysosomal degradation. Both the increases in secreted and intracellular Aß40 were abolished by depletion of presenilin 2 (PSEN2), recently shown to be enriched on the endosomal limiting membrane compared with PSEN1. Our findings demonstrate that ubiquitin can act as a signal at five cytodomain-located lysines for endosomal sorting of APP. They further suggest that disruption of APP endosomal sorting reduces its sequestration in ILVs and results in PSEN2-mediated processing of a larger pool of APP-CTF on the endosomal membrane.