Elizabeth M. Bradshaw

Mechanisms linking CD33 variants to Alzheimer Disease (AD) are poorly defined. Here, we combine structural, cellular, and genetic analyses to delineate how the CD33 ^M splice isoform, upregulated in carriers of CD33 risk alleles, modulates microglial function. We show that CD33 ^M ectodomain dimerizes, enabling binding of large multi-sialylated molecules. We demonstrate that another AD risk protein - clusterin (CLU) ± Aβ oligomers (but not ApoE) binds with nanomolar avidity to CD33 ^M , but not CD33 ^m . We show that in human monocytes CD33 ^M :CLU binding induces CD33 ^M ITIM phosphorylation, recruits SHP-1, suppresses Aβ phagocytosis, and impairs clearance of amyloid plaques. We identify a soluble CD33 ^M ectodomain fragment (sCD33 ^M ) - absent from CD33 ^m -expressing cells - which could contribute to the role of CD33 ^M in AD. Genetic analyses confirm that CD33:CLU interaction modulates amyloid burden, cognition, and disease risk. These findings define a mechanistic CLU:Aβ:CD33 ^M axis, highlighting CD33 ^M dimerization and ligand-binding sites as potential therapeutic targets.

Meningeal immune cells monitor the central nervous system (CNS) and influence neuroinflammation in mice, but the human leptomeningeal immune landscape and the changes that occur in this immunological niche in neurodegeneration remain underexplored. Here we performed single-cell RNA and T cell receptor (TCR) sequencing of 99,625 high-quality immune cells from 57 leptomeninges and brain samples from donors with Alzheimer’s disease (AD), amyotrophic lateral sclerosis and Parkinson’s disease and found that although the leptomeninges are home to highly clonally expanded CD8 tissue-resident memory (T_RM) T cells, the maximal level of clonal expansion was decreased in AD in comparison to non-neurodegenerative controls. Intra-patient paired tissue analysis further revealed that brain and leptomeningeal TCR repertoires share significant similarities, but tissue-specific clones emerge in AD. Finally, in AD, the degree of CD8 T_RM clonal expansion was positively correlated with microglial TGFB2, suggesting that brain and leptomeningeal immune cells coordinate their activities in AD. In addition to identifying key inflammatory dynamics in the human degenerating CNS, this study establishes a foundational resource for future studies that could inform treatment for AD and other neuroinflammatory diseases.

While genetics implicate a central role for dysregulated innate immunity in Alzheimer’s disease (AD), the contributions of peripheral myeloid cells, such as monocytes, have been largely overlooked in favor of microglia. Here, we investigate whether AD-associated loci, specifically rs3865444 in the CD33 locus and rs1057233 in the SPI1 locus, converge on shared functional pathways in monocytes in the context of amyloid-beta peptide 1-42 (Aβ1-42) as an immune stimulus. To do so, we isolated monocytes from peripheral blood mononuclear cells (PBMCs) from healthy individuals and exposed them to aggregated Aβ1-42. In this study, we identify functional convergence of the CD33 and SPI1 AD risk variants in the context of aggregated Aβ, both resulting in reduced phagocytosis and loss of surface TREM2 expression, demonstrating an interaction between genetics and environment to reduce myeloid cell fitness. These findings highlight that peripheral monocytes, like brain-resident microglia, are genetically and functionally linked to AD risk, underscoring their importance as accessible immune cells that contribute to disease susceptibility and progression.

While genetics implicate a central role for dysregulated innate immunity in Alzheimer’s disease (AD), the contributions of peripheral myeloid cells, such as monocytes, have been largely overlooked in favor of microglia. Here, we investigate whether AD associated loci, specifically rs3865444 in the CD33 locus and rs1057233 in the SPI1 locus, converge on shared functional pathways in monocytes in the context of amyloid-beta peptide 1-42 (Aβ1-42) as an immune stimulus. To do so, we isolated monocytes from peripheral blood mononuclear cells (PBMCs) from healthy individuals and exposed them to aggregated Aβ1-42. In this study, we identify functional convergence of the CD33 and SPI1 AD risk variants in the context of aggregated Aβ, both resulting in reduced phagocytosis and loss of surface TREM2 expression, demonstrating an interaction between genetics and environment to reduce myeloid cell fitness. These findings highlight that peripheral monocytes, like brain-resident microglia, are genetically and functionally linked to AD risk, underscoring their importance as accessible immune cells that contribute to disease susceptibility and progression.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease marked by progressive motor neuron loss, with TDP-43 pathology present in over 90% of cases. While neuroinflammation is a recognized hallmark, the role of microglia in ALS pathogenesis remains incompletely understood. Here, we demonstrate that TDP-43 regulates microglial function via triglyceride metabolism. Using shRNA-mediated TARDBP knockdown in human monocyte-derived microglia-like cells (MDMi), we observed suppressed cholesterol biosynthesis, upregulation of fatty acid metabolism genes, lipid droplet accumulation, enhanced phagocytic activity, and increased IL-1β production. Inhibiting diacylglycerol acyltransferase (DGAT) enzymes reduced lipid droplet formation, phagocytosis, and IL-1β, directly linking the triglyceride pathway to microglial activation. Patient-derived MDMi from both sporadic and TARDBP -mutant ALS cases showed overlapping as well as distinct alterations, some of which were reversed by DGAT inhibition. Our findings identify dysregulated triglyceride metabolism as a novel pathway through which TDP-43 mediates microglial dysfunction, highlighting a potential therapeutic target for ALS. Graphical Highlights: TDP-43 nuclear depletion causes increased LD, driven by triglyceride accumulation. TDP-43 nuclear depletion causes increased phagocytosis and pro-inflammatory cytokine expression. Inhibiting triglyceride synthesis using DGAT inhibitors rescues LD and pro-inflammatory phenotype in TDP-43 depleted MDMi ALS patient-derived MDMi display increased LD and IL1B expression, rescued by DGAT inhibitors

The innate immune gene CD 33, encoding a myeloid inhibitory sialic acid-binding receptor, is associated with Alzheimer’s disease (AD) susceptibility. The AD-associated rs3865444 ^CC risk variant reduces splicing of the sialic acid-binding domain and increases expression of the full-length (sialic acid-binding) CD33 isoform seven-fold compared to the rs3865444 ^AA protective genotype. Here we identify CD45 as an immune cell-specific sialic acid-dependent cis CD33 binding partner, whose phosphatase activity is inhibited by CD33. Overexpression of CD33 or loss of CD45 contributes to impaired microglial clearance of amyloid beta and amyloid beta-induced loss of dendritic spines in microglial-neuronal co-cultures, aligning with a detrimental effect of CD33-mediated inhibition of CD45. CD33-CD45 interaction frequency was increased in monocytes from individuals with the rs3865444 ^CC risk variant compared to rs3865444 ^AA , as well as in AD compared to controls, independent of genotype. Furthermore, an interaction between CD33 and PTPRC (encoding CD45) gene expression in human brain tissue was associated with a pathological diagnosis of AD and global burden of AD pathology. Our findings thus establish a functional interaction between CD33 and CD45 relevant to AD susceptibility and systemic myeloid dysfunction in this disease.

The protein alpha-synuclein (αSyn) plays a pivotal role in the pathogenesis of synucleinopathies, including Parkinson’s disease and multiple system atrophy, with growing evidence indicating that lipid dyshomeostasis is a key phenotype in these neurodegenerative disorders. Previously, we identified that αSyn localizes, at least in part, to mitochondria-associated endoplasmic reticulum membranes (MAMs), which are transient functional domains containing proteins that regulate lipid metabolism, including the de novo synthesis of phosphatidylserine. In the present study, we analyzed the lipid composition of postmortem human samples, focusing on the substantia nigra pars compacta of Parkinson’s disease and controls, as well as three less affected brain regions of Parkinson’s donors. To further assess synucleinopathy-related lipidome alterations, similar analyses were performed on the striatum of multiple system atrophy cases. Our data reveal region- and disease-specific changes in the levels of lipid species. Specifically, our data revealed alterations in the levels of specific phosphatidylserine species in brain areas most affected in Parkinson’s disease. Some of these alterations, albeit to a lesser degree, are also observed in multiple system atrophy. Using induced pluripotent stem cell-derived neurons, we show that αSyn regulates phosphatidylserine metabolism at MAM domains, and that αSyn dosage parallels the perturbation in phosphatidylserine levels. These findings support the notion that αSyn pathophysiology is linked to the dysregulation of lipid homeostasis, which may contribute to the vulnerability of specific brain regions in synucleinopathy. These findings have significant therapeutic implications.

Large-scale genetic studies have identified numerous genetic risk factors that suggest a central role for innate immune cells in susceptibility to Alzheimer’s disease (AD). CD33, an immunomodulatory transmembrane sialic acid binding protein expressed on myeloid cells, was identified as one such genetic risk factor associated with Alzheimer’s disease. Several studies explored the molecular outcomes of genetic variation at the CD33 locus. It has been determined that the risk variant associated with AD increases the expression of the large isoform of CD33 (CD33M) in innate immune cells and alters its biological functions. CD33 is thought to signal via the interaction of its ITIM motif and the protein tyrosine phosphatase, SHP-1. Here, we utilize different molecular and computational approaches to investigate how AD-associated genetic variation in CD33 affects its interaction with SHP-1 in human microglia and microglia-like cells. Our findings demonstrate a genotype-dependent interaction between CD33 and SHP-1, which may functionally contribute to the AD risk associated with this CD33 variant. We also found that CD33-PTPN6 (SHP-1) gene–gene interactions impact AD-related traits, while CD33-PTPN11 (SHP-2) interactions do not.

Large-scale genetic studies have identified numerous genetic risk factors that suggest a central role for innate immune cells in susceptibility to Alzheimer’s disease (AD). CD33, an immunomodulatory transmembrane sialic-acid binding protein expressed on myeloid cells, was identified as one such genetic risk factor associated with Alzheimer’s disease. Several studies explored the molecular outcomes of genetic variation at the CD33 locus. It has been determined that the risk variant associated with AD increases the expression of the large isoform of CD33 (CD33M) in innate immune cells and alters its biological functions. CD33 is thought to signal via the interaction of its ITIM domain and the protein tyrosine phosphatase, SHP-1. Here, we utilize different molecular and computational approaches to investigate how AD-associated genetic variation in CD33 affects its interaction with SHP-1 in human microglia and microglia-like cells. Our findings demonstrate a genotype-dependent interaction between CD33 and SHP-1, which may functionally contribute to the AD risk associated with this CD33 variant. We also found that gene-gene interactions impact AD-related traits.