Hypothesis: Ordered Collagen Fragment Clearance via CD206‑Dependent Lysosomal Exocytosis Maintains Adipose Proteostasis

Core idea In expanding adipose tissue, a proteostatic program actively sequesters pericellular collagen I fragments through CD206‑mediated endocytosis in M2‑like macrophages, followed by lysosomal processing and exocytosis of harmless peptides. When this flux is overwhelmed—by hypoxia‑driven TGF‑β up‑regulation of collagen synthesis, AMPK suppression that diminishes lysosomal biogenesis, or impaired CD206 recycling—the system stalls. Accumulated bioactive fragments then engage integrin α2β1 on adipocytes, triggering NF‑κB‑driven inflammation and insulin resistance. Thus, fibrosis reflects a failure of an ordered clearance pathway, not a passive buildup of garbage.

Novel mechanistic twist We propose that collagen VI, beyond its structural role, acts as a molecular “landing pad” that presents collagen I fragments to CD206 on macrophage surfaces. Collagen VI’s triple‑helix domains bind specific gelatinase‑generated motifs, increasing the local concentration of substrates for CD206. Simultaneously, collagen VI‑derived cryptic peptides (generated by MMP‑9) act as autocrine agonists for the macrophage receptor CXCR4, promoting a survival‑biased M2 phenotype that sustains CD206 expression. In obesity, hypoxia‑induced ROS oxidizes critical methionine residues in collagen VI, reducing its affinity for both fragments and CXCR4 ligands, thereby uncoupling the scaffold from its signaling function.

Testable predictions

1. Rescue by collagen VI mimetics – Injecting a synthetic collagen VI peptide that preserves the MMP‑9 cleavage site will restore CD206‑mediated fragment uptake and reduce fibrosis in obese mice, even without altering TGF‑β levels.
2. ROS sensitivity – Macrophages expressing a methionine‑to‑leucine mutant collagen VI (oxidation‑resistant) will retain fragment‑binding capacity under hypoxic conditions and protect adipocytes from inflammation, whereas wild‑type collagen VI will not.
3. Lysosomal exocytosis dependence – Inhibiting TFEB nuclear translocation (with curcumin) or blocking SNAP‑23‑dependent exocytosis (with botulinum toxin light chain B) will abolish the protective effect of CD206 overexpression, leading to fragment accumulation despite normal endocytic uptake.
4. Integrin α2β1 blockade – Treating obese mice with a function‑blocking antibody against integrin α2β1 will attenuate NF‑κB activation and improve insulin tolerance, confirming that the pathogenic signal originates from uncleared fragments rather than total collagen load.

Falsifiability If any of the above interventions fail to alter fibrosis or metabolic readouts as predicted—for example, oxidation‑resistant collagen VI does not improve macrophage clearance, or lysosomal exocytosis inhibition does not worsen outcomes despite CD206 overexpression—then the hypothesis that an ordered, CD206‑collagen VI‑lysosomal axis protects adipose tissue would be refuted. Conversely, consistent support across these orthogonal assays would substantiate the view that fibrosis is a breakdown of a regulated clearance system, opening therapeutic avenues that boost fragment handling rather than merely suppress collagen synthesis.

References [1] https://www.pnas.org/doi/10.1073/pnas.2313185121 [2] https://pubmed.ncbi.nlm.nih.gov/27179976/ [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC9990697/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC2648231/