Senescent Cell-Derived SASP Gradients Reprogram CXCL12-Mediated Niche Signaling in Aged Bone Marrow

Hypothesis

In aged bone marrow, senescent stromal cells establish a spatially restricted senescence‑associated secretory phenotype (SASP) gradient that remodels CXCL12 retention and signaling, thereby displacing Cxcl12+Lepr+ skeletal stem/progenitor cells (SSPCs) from their perivascular niche and impairing hematopoietic support.

Mechanistic Rationale

• Senescent cells secrete IL‑6, TGF‑β, and MMPs that can cleave heparan sulfate proteoglycans (HSPGs) anchoring CXCL12 to the extracellular matrix, reducing its local concentration (1).
• Loss of matrix‑bound CXCL12 flattens the chemokine gradient that normally retains Pdgfra+Sca1+ SSPCs at the periosteum and Cxcl12+Lepr+ SSPCs in the marrow (2).
• Displaced SSPCs experience altered Notch and Wnt signaling, shifting them toward a fibro‑adipogenic phenotype, which further reinforces a pro‑fibrotic, immunosuppressive microenvironment.

Experimental Design

1. Sample preparation – Harvest femur/tibia from young (3 mo) and aged (24 mo) mice; include a cohort treated with a senolytic (dasatinib + quercetin) for 2 weeks.
2. Spatial transcriptomics – Apply Seq‑Scope‑X hydrogel expansion to achieve subcellular resolution; capture nuclear/cytoplasmic mRNA and generate gene‑expression matrices at ~0.5 µm voxel size.
3. Targeted markers – Quantify transcripts for senescence (Cdkn2a/p16^Ink4a, Il6, Tgfb1, Mmp9), CXCL12, Cxcr4, SSPC signatures (Pdgfra, Lepr, Sca1), and fibro‑adipogenic genes (Col1a1, Pparg).
4. Gradient analysis – Compute radial CXCL12 intensity profiles from sinusoidal/endothelial surfaces outward; compare slope, amplitude, and spatial variance across conditions.
5. Validation – Perform multiplex RNA‑ISH on adjacent sections for p16^Ink4a and CXCL12 to corroborate spatial patterns; assess SSPC localization via immunofluorescence for PDGFRα and LEPR.
6. Functional readout – Conduct CFU‑F assays and hematopoietic stem cell (HSC) competitive transplantation to link niche alterations to functional output.

Expected Outcomes & Falsifiability

• If hypothesis is correct: Aged marrow will show a shallow CXCL12 gradient (reduced slope) concomitant with elevated p16^Ink4a and SASP signals near the endosteal surface; SSPC transcripts will be markedly depleted from periosteal zones and enriched in ectopic fibro‑adipogenic clusters. Senolytic treatment should restore a steep CXCL12 gradient, re‑localize SSPCs to periosteal/perivascular niches, and rescue HSC engraftment.
• If hypothesis is false: CXCL12 gradients will be indistinguishable between young and aged marrow despite senescent marker accumulation, or SSPC localization will remain unchanged, indicating that SASP does not significantly modulate chemokine retention in this context.

Potential Confounds & Controls

• Account for age‑related changes in vascular density by staining for CD31 and normalizing CXCL12 signals to endothelial length.
• Include p16‑3MR reporter mice to visually confirm senescent cell clearance post‑senolytic.
• Use MMP inhibitor-treated controls to distinguish SASP protease effects from cytokine‑mediated signaling.

By directly linking subcellular‑resolution spatial transcriptomics of senescence effectors to niche‑level chemokine architecture, this proposal offers a clear, falsifiable mechanism by which aged stromal remodeling drives functional decline in the bone marrow microenvironment.