MRAP Protein Drives Thymus-to-Fat Conversion Behind Age-Related Immune Decline
Scientists identify MRAP as a key molecular driver of thymic fat replacement, linking thymosin-α1 and FoxO1 signaling to immune aging.
Summary
As we age, the thymus — the organ responsible for producing T cells — progressively fills with fat, crippling immune function. This study identifies MRAP (melanocortin-2 receptor accessory protein) as a critical driver of this transformation. Thymic mesenchymal stromal cells (tMSCs) were found to preferentially differentiate into fat cells rather than bone, unlike MSCs from other tissues. MRAP expression surges in tMSCs during adipogenesis, and knocking it out dramatically reduces fat cell formation. The thymic peptide thymosin-α1 was found to trigger MRAP expression via the FoxO1 signaling pathway. Single-cell RNA sequencing of human thymus tissue confirmed that tMSCs and adipocytes accumulate with age, pointing to MRAP as a promising target to slow thymic fat deposition and preserve immune function during aging.
Detailed Summary
Thymic involution — the progressive replacement of immune-active tissue with fat that accelerates after puberty and leaves the thymus largely adipose by age 50 — is one of the most consequential but least understood processes in immunological aging. This Nature Communications study provides a mechanistic explanation, centering on thymic mesenchymal stromal cells (tMSCs) and a protein called MRAP.
Researchers isolated tMSCs from mouse thymus and characterized them as a fibroblast-like population negative for epithelial and hematopoietic markers but positive for canonical MSC surface proteins (CD29, CD105, Sca-1). When placed in adipogenic or osteogenic culture conditions, tMSCs showed a striking asymmetry: robust lipid droplet formation and strong induction of PPARγ and CEBPα (adipogenic genes), but only minimal osteogenic response — a sharp contrast to dental pulp MSCs (dpMSCs), which differentiated readily into bone-forming cells but not fat cells.
Transcriptomic comparison (RNA-seq) between tMSCs and dpMSCs identified MRAP as the most differentially upregulated gene in tMSCs. Under adipogenic stimulation, MRAP expression increased ~500–600-fold in tMSCs versus dpMSCs, and ~40-fold over untreated tMSC controls. Silencing MRAP with siRNA dramatically reduced adipogenic gene expression (PPARγ, CEBPα, Fabp4) and lipid accumulation without affecting osteogenic markers. Mrap knockout mice confirmed reduced thymic adipogenesis in vivo, with older Mrap-deficient mice showing preserved thymocyte numbers.
The study then investigated what triggers MRAP. Thymosin-α1, a naturally occurring thymic peptide, was found to strongly upregulate MRAP expression in tMSCs through the FoxO1 transcription factor. ChIP assays confirmed direct FoxO1 binding to the MRAP promoter. Inhibiting FoxO1 blocked thymosin-α1-driven MRAP induction and subsequent adipogenesis, while FoxO1 overexpression enhanced it. Crucially, these findings translated to human tissue: human thymic MSCs exposed to thymosin-α1 also differentiated into adipocytes in a MRAP-dependent manner.
Single-cell RNA sequencing analysis of human thymic tissue across age groups revealed progressive accumulation of tMSC and adipocyte clusters in older individuals, with MRAP expression enriched in aging tMSC populations. Together, the data construct a coherent pathway: aging elevates thymosin-α1 → FoxO1 activation → MRAP upregulation → tMSC adipogenesis → thymic fat replacement. This positions MRAP as a druggable node to potentially slow or reverse thymic involution and help maintain T cell output in aging populations.
Key Findings
- tMSCs preferentially differentiate into adipocytes over osteoblasts, unlike dental pulp MSCs from the same animals.
- MRAP is the most upregulated gene in tMSCs versus dpMSCs and rises ~500-fold during adipogenic stimulation.
- MRAP knockdown or knockout markedly reduces thymic fat cell formation and preserves thymocyte numbers in older mice.
- Thymosin-α1 induces MRAP expression via direct FoxO1 binding to the MRAP promoter, driving adipogenesis.
- Single-cell RNA-seq of human thymus confirms age-related accumulation of tMSCs and adipocytes with high MRAP activity.
Methodology
Mouse tMSCs and human thymic MSCs were isolated, characterized by flow cytometry, and compared to dental pulp MSCs via RNA-seq and functional differentiation assays. MRAP function was tested by siRNA knockdown and Mrap knockout mice; FoxO1 involvement was confirmed by ChIP assay. Human aging data were derived from published single-cell RNA-seq datasets of thymic tissue across age groups.
Study Limitations
Mrap full-knockout mice could not be aged beyond 7 months, limiting long-term in vivo evidence. The study does not establish whether blocking MRAP improves measurable immune function in vivo. Causal directionality in human aging relies on observational single-cell data rather than interventional human studies.
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