Crunching the numbers. Responsibly.
Alzheimer's disease (AD) is characterized by synaptic loss, which can result from dysfunctional microglial phagocytosis and complement activation. However, what signals drive aberrant microglia-mediated engulfment of synapses in AD is unclear. Here we report that secreted phosphoprotein 1 (SPP1/osteopontin) is upregulated predominantly by perivascular macrophages and, to a lesser extent, by perivascular fibroblasts. Perivascular SPP1 is required for microglia to engulf synapses and upregulate phagocytic markers including C1qa, Grn and Ctsb in presence of amyloid-β oligomers. Absence of Spp1 expression in AD mouse models results in prevention of synaptic loss. Furthermore, single-cell RNA sequencing and putative cell-cell interaction analyses reveal that perivascular SPP1 induces microglial phagocytic states in the hippocampus of a mouse model of AD. Altogether, we suggest a functional role for SPP1 in perivascular cells-to-microglia crosstalk, whereby SPP1 modulates microglia-mediated synaptic engulfment in mouse models of AD.
Strengths
STED super-resolution imaging, correlative light-electron microscopy, single-molecule FISH, single-cell RNA sequencing, genetic knockouts, and in vitro rescue all triangulate the same causal chain from perivascular SPP1 to microglial engulfment.
↳ Figs. 1-5 and Extended Data Figs. 1-6
The work localizes SPP1 predominantly to perivascular macrophages rather than microglia in the adult hippocampus, a reassignment supported by reporter mice, CLEM, and RiboTag data, reframing a known AD marker as an upstream functional signal.
↳ Results (SPP1 is expressed by PVMs and fibroblasts); Fig. 2
Crossing Spp1 knockout to the AppNL-F model prevents microglial phagocytic marker upregulation and rescues synapse loss despite amyloid challenge, and extracellular SPP1 restores engulfment in knockout microglia, supporting an extrinsic signaling mechanism.
↳ Fig. 4 and Extended Data Figs. 4-5
Limitations
Several pivotal readouts rest on low animal numbers, including n=2 for one Homer1 engulfment quantification and n=3 from a single independent experiment for the STED SPP1 measurement, which limits statistical confidence in individual findings.
↳ Fig. 1e,f; Fig. 3h; Fig. 5e
The NicheNet ligand-target predictions are based on sparse sorted-cell numbers (around 200 PVMs and 350 PVFs per genotype) and Spp1 was not even detected in the sorted PVMs; only select hits are orthogonally validated, so the network claims carry substantial uncertainty.
↳ Fig. 5; Methods (scRNA-seq library preparation)
Human evidence is limited to descriptive postmortem perivascular SPP1 localization with no functional or pharmacological validation, and no therapeutic intervention targeting SPP1 is tested, leaving a substantial gap between mouse genetics and clinical utility.
↳ Fig. 1j, Fig. 2i; Discussion
Evaluations converged on the view that this is a competently executed, multi-modal mechanistic study whose central claim is well-supported by orthogonal methods and a clean genetic loss-of-function rescue. Divergence centered on how heavily to weight the small per-experiment sample sizes in the imaging and engulfment readouts (for example n=2 in Fig. 3h and n=3 from one experiment in Fig. 1e,f) and the predictive-only nature of the NicheNet analysis, which is built on very few sorted perivascular cells. These located limitations held the Methodological Rigour score just below the upper band despite the genuine breadth of the methodology. On impact, the genetic demonstration is meaningful but the absence of any pharmacological test and the confinement to hippocampal CA1, together with descriptive-only human data, anchor the impact assessment in the preliminary range. The Discussion's generalization toward conserved, disease-general crosstalk modestly tempered the context assessment.
Strong4.0/5.0
The score reflects a genuine conceptual advance: identifying perivascular macrophages (not microglia) as the predominant brain source of SPP1 and showing that genetic Spp1 ablation prevents synaptic loss despite amyloid challenge. The advance is proportionate to the loss-of-function evidence and reframes SPP1 from a disease-associated marker to a functional upstream signal.
Absence of Spp1 expression in AD mouse models results in prevention of synaptic loss
The score reflects a strong multi-modal design (STED, CLEM, smFISH-IHC, scRNA-seq, genetic knockouts, in vitro rescue) with appropriate statistical handling, offset by small per-experiment sample sizes in several key readouts and a predictive-only computational component. The convergence of orthogonal methods partially mitigates the limited statistical power of individual experiments.
total of n = 3 mice examined over one independent experiment
The score reflects a logically ordered narrative from perivascular localization through genetic perturbation to network analysis, with disciplined qualifying language for predictive findings. The only friction is the four-genotype comparison scheme, which is inherent to the design rather than a writing failure.
we suggest a functional role for SPP1 in perivascular cells-to-microglia crosstalk
The score reflects substantive engagement with the literature on microglial phagocytosis, complement, SPP1 biology, and perivascular cell identity, with acknowledged limitations and future directions. It is tempered by Discussion passages that generalize toward conserved, disease-general crosstalk beyond the directly tested models.
SPP1 activation might represent a conserved molecular response to perturbed perivascular homeostasis
Watch-outs4 of 4 checks
The paper is internally consistent across abstract, figures, and results, with multiple orthogonal methods converging on the same conclusion and the scRNA-seq vs smFISH/RiboTag Spp1 detection discrepancy explicitly explained as an isolation artifact. The one noted item is that Discussion passages generalize from mouse-model findings toward broader AD-relevant and disease-general crosstalk beyond the directly tested evidence.
Ethics approvals are declared for animal work (UK Home Office/UCL, Jackson Laboratory IACUC, Weizmann Institute Animal Care Committee) and human postmortem tissue (NHS ethics committee, license 12,198; Queen Square Brain Bank with informed consent). These are declared-but-unverified at the editorial layer; no undisclosed COI was identified; data is shared only via linked source files with no named repository and no project-specific code, which is the noted item.
Flags: 2 declared / 5 total
All 85 cited references resolved to real publications via PubMed, Crossref, or OpenAlex.
No retraction notice found in Retraction Watch.
Sources: Retraction Watch ✓PubPeer (coming soon)
Medium3.3/5.0
The score reflects a well-documented problem (synaptic loss in AD) with a specific, secreted, potentially druggable target (SPP1) and an identifiable cell source (PVMs), with stakeholders visible but no named programs. The application pathway remains research-stage.
may offer opportunities to specifically manipulate and block microglia-mediated synaptic engulfment
The score reflects proof-of-concept established via genetic loss-of-function in mouse models, with human evidence limited to descriptive postmortem localization and no pharmacological intervention tested. The work characterizes a mechanism but does not yet provide intervention guidance.
we also found a striking presence of SPP1 immunoreactivity along the vasculature in the hippocampus of postmortem AD patient tissues
The score reflects support from two disease paradigms (AppNL-F genetic and acute oAB ICV) plus cross-species human localization, balanced against confinement to hippocampal CA1 SLM with documented region-specificity. Generalization across brain regions and disease settings remains uncertain.
The SPP1 upregulation was region-specific, that is, in hippocampus but not cerebellum
The score reflects a contribution that fills a specific literature gap and supplies concrete follow-up hypotheses (TGF-B, integrins, calreticulin) plus a shared reporter reagent, while primarily refining a pathway already under active investigation. The available Spp1TdT line facilitates future research.
available as B6J.Spp1tm1(tdTomato)Msasn/J (stock 33731) from The Jackson Laboratory
Nabu’s assessment, alongside the field’s view.
Evaluation Metadata
AI-powered, human-governed. Something look off? Contact us to request a review.