SUMMARY Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are age-related neurodegenerative disorders with shared genetic etiologies and overlapping clinical and pathological features. Here we studied a novel ALS/FTD family and identified the P362L mutation in the low complexity domain (LCD) of T-cell-restricted intracellular antigen-1 (TIA1). Subsequent genetic association analyses showed an increased burden of TIA1 LCD mutations in ALS patients compared to controls (P = 8.7×10−6). Postmortem neuropathology of five TIA1 mutations carriers showed a consistent pathological signature with numerous round, hyaline, TAR DNA-binding protein 43 (TDP-43)-positive inclusions. TIA1 mutations significantly increased the propensity of TIA1 protein to undergo phase transition. In live cells, TIA1 mutations delayed stress granule (SG) disassembly and promoted the accumulation of non-dynamic SGs that harbored TDP-43. Moreover, TDP-43 in SGs became less mobile and insoluble. The identification of TIA1 mutations in ALS/FTD reinforces the importance of RNA metabolism and SG dynamics in ALS/FTD pathogenesis.

Background: TREM2 is associated with several neurodegenerative diseases. Results: ApoE bound to TREM2 and increased phagocytosis of apoptotic neurons by microglia. Alzheimer disease (AD) risk-associated TREM2-R47H mutant had a reduced binding to apoE. Conclusion: ApoE is a novel ligand for TREM2. Interaction between apoE and TREM2 likely regulates phagocytosis of apoEbound apoptotic neurons. Significance: Interaction between two AD risk-associated proteins modulates microglial function.

ABCA1 is an ATP-binding cassette protein that transports cellular cholesterol and phospholipids onto high density lipoproteins (HDL) in plasma. Lack of ABCA1 in humans and mice causes abnormal lipidation and increased catabolism of HDL, resulting in very low plasma apoA-I, apoA-II, and HDL. Herein, we have used Abca1 ؊/؊ mice to ask whether ABCA1 is involved in lipidation of HDL in the central nervous system (CNS). ApoE is the most abundant CNS apolipoprotein and is present in HDL-like lipoproteins in CSF. We found that Abca1 ؊/؊ mice have greatly decreased apoE levels in both the cortex (80% reduction) and the CSF (98% reduction). CSF from Abca1 ؊/؊ mice had significantly reduced cholesterol as well as small apoE-containing lipoproteins, suggesting abnormal lipidation of apoE. Astrocytes, the primary producer of CNS apoE, were cultured from Abca1 ؉/؉ , ؉/؊ , and ؊/؊ mice, and nascent lipoprotein particles were collected. Abca1 ؊/؊ astrocytes secreted lipoprotein particles that had markedly decreased cholesterol and apoE and had smaller apoE-containing particles than particles from Abca1 ؉/؉ astrocytes. These findings demonstrate that ABCA1 plays a critical role in CNS apoE metabolism. Since apoE isoforms and levels strongly influence Alzheimer's disease pathology and risk, these data suggest that ABCA1 may be a novel therapeutic target.