Molecular Mechanisms of Wound Healing
Introduction
Beyond the cellular phases of wound healing lie intricate molecular networks that orchestrate tissue repair. Growth factors, cytokines, chemokines, and matrix components communicate through complex signaling cascades to guide cells from injury to healed tissue.
This chapter explores the key molecular players in wound healing and the mechanisms underlying impaired healing in clinical conditions.
Growth Factors in Wound Healing
Major Growth Factor Families
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Growth Factor Functions
| Factor | Primary Source | Primary Target | Main Functions |
|---|
| PDGF | Platelets, macrophages | Fibroblasts, SMCs | Chemotaxis, proliferation |
| EGF | Platelets, macrophages | Keratinocytes | Re-epithelialization |
| KGF (FGF-7) | Fibroblasts | Keratinocytes | Epithelial proliferation |
| FGF-2 | Macrophages, endothelium | Endothelial cells, fibroblasts | Angiogenesis |
| VEGF | Keratinocytes, macrophages | Endothelial cells | Angiogenesis |
| TGF-β1 | Platelets, macrophages | Fibroblasts | Matrix synthesis, scarring |
| TGF-β3 | Keratinocytes | Fibroblasts | Reduced scarring |
| IGF-1 | Fibroblasts, macrophages | Multiple | Proliferation, metabolism |
| HGF | Mesenchymal cells | Keratinocytes | Migration, proliferation |
TGF-β: Master Regulator of Healing and Scarring
TGF-β Signaling Pathway
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TGF-β Isoforms and Scarring
| Isoform | Effect on Scarring | Evidence |
|---|
| TGF-β1 | Pro-fibrotic | Keloid overexpression |
| TGF-β2 | Pro-fibrotic | Similar to TGF-β1 |
| TGF-β3 | Anti-fibrotic | Fetal wound healing (scarless) |
Angiogenesis: VEGF and Beyond
VEGF-Driven Angiogenesis
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Additional Angiogenic Factors
| Factor | Role | Source |
|---|
| FGF-2 (bFGF) | Potent angiogenic | Macrophages |
| Angiopoietin-1 | Vessel maturation | Pericytes |
| Angiopoietin-2 | Vessel destabilization | Endothelium |
| PDGF-BB | Pericyte recruitment | Platelets |
| HGF | Angiogenic, morphogenic | Mesenchyme |
Matrix Metalloproteinases (MMPs)
MMP Classes and Substrates
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MMP/TIMP Balance
| Situation | MMP Activity | TIMP Activity | Outcome |
|---|
| Normal healing | Balanced | Balanced | Controlled remodeling |
| Chronic wound | ↑↑↑ | ↓ | Excessive degradation |
| Keloid/fibrosis | ↓ | ↑↑ | Excessive matrix accumulation |
Myofibroblast: Key Effector Cell
Myofibroblast Differentiation
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Myofibroblast Features
| Feature | Description |
|---|
| Marker | α-Smooth muscle actin (α-SMA) |
| Contractile | Stress fibers, focal adhesions |
| Synthetic | High collagen I/III production |
| Origin | Local fibroblasts, BMDCs, EMT, EndoMT |
| Fate | Apoptosis (normal) or persistence (fibrosis) |
Impaired Wound Healing
Diabetic Wound Pathophysiology
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Chronic Wound Environment
| Feature | Acute Wound | Chronic Wound |
|---|
| Growth factors | Abundant, active | Low, sequestered, degraded |
| MMPs | Regulated | ↑↑↑ Elevated |
| TIMPs | Balanced | ↓ Reduced |
| Proteases | Controlled | ↑↑ Excessive |
| Senescent cells | Few | Many |
| Bacteria | Controlled | Biofilm formation |
| Matrix | Progressive deposition | Degraded |
| Macrophages | M1→M2 transition | Stuck in M1 |
Abnormal Scarring
Keloids vs. Hypertrophic Scars
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Molecular Differences
| Feature | Hypertrophic Scar | Keloid |
|---|
| TGF-β | ↑ | ↑↑↑ |
| MMP-1 | ↓ | ↓↓ |
| TIMP-1 | ↑ | ↑↑ |
| Collagen | Type III predominant | Type I and III |
| Myofibroblasts | Present, transient | Persistent |
| p53 | Normal | Abnormal regulation |
Keloid Genetic Predisposition
Genome-wide association studies have identified susceptibility loci for keloid formation:
| Chromosomal Locus | Population | Inheritance Pattern |
|---|
| Chromosome 1 | Japanese, Chinese | Autosomal dominant |
| Chromosome 2 | Japanese | Autosomal dominant |
| Chromosome 3 | Japanese | Autosomal dominant |
| Chromosome 7 | Multiple | Case-control identified |
| Chromosome 15 | Japanese | Case-control identified |
Keloid keratinocytes exhibit elevated expression of genes involved in wound healing, cell motility, and angiogenesis (including VEGF). Keloid tissue contains increased M2 macrophages, mast cells, and "keloid-associated lymphoid tissue" (KALT), suggesting chronic inflammation perpetuates keloid growth.
CTGF and Wnt/β-catenin in Fibrosis
Connective tissue growth factor (CTGF/CCN2) is a downstream mediator of TGF-β signaling and a key driver of keloid pathogenesis:
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Fetal Wound Healing: Scarless Paradigm
Characteristics of Fetal Wounds
Early gestational wounds (first and second trimester) heal without scarring—a phenomenon that has driven research into anti-fibrotic therapies.
| Feature | Adult Wound | Fetal Wound |
|---|
| Fibrin clot | Present | Absent |
| Platelet degranulation | Robust | Minimal |
| Inflammation | Intense | Sparse, poorly differentiated |
| Mast cells | Normal numbers | Reduced |
| Tenascin C | Normal | ↑ Elevated |
| Hyaluronic acid | Normal levels | ↑ Prolonged elevation |
| TGF-β1/β2 | High | Low |
| TGF-β3 | Low | ↑ Elevated |
TGF-β Isoform Balance
The ratio of TGF-β isoforms is critical for scarless healing:
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Lin28 and Metabolic Reprogramming
The highly conserved RNA-binding protein Lin28 (expressed during embryogenesis) promotes tissue repair in adult tissue by reprogramming cellular metabolism. Lin28 enhances glycolysis and anabolic pathways, shifting cells toward a regenerative phenotype reminiscent of fetal tissues.
Therapeutic Growth Factors
Clinical Applications
| Agent | Target | Application | Status |
|---|
| Becaplermin (PDGF-BB) | PDGF receptor | Diabetic foot ulcers | FDA approved |
| EGF (topical) | EGF receptor | Diabetic ulcers | Available in some countries |
| VEGF gene therapy | VEGFR | Ischemic wounds | Investigational |
| TGF-β3 (avotermin) | TGF-β receptors | Scar reduction | Discontinued (Phase III failure) |
| Platelet-rich plasma | Multiple | Various wounds | Variable evidence |
Summary: Molecular Orchestration
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Key Clinical Pearls
| Topic | Pearl |
|---|
| PDGF | Only FDA-approved growth factor (becaplermin) for diabetic ulcers |
| TGF-β1 | Pro-fibrotic; excess = keloids |
| VEGF | Hypoxia-induced; essential for granulation tissue |
| MMP excess | Hallmark of chronic wounds; target for therapy |
| Myofibroblast | α-SMA+ cells drive contraction and matrix; must apoptose |
| Diabetic wounds | Stuck in inflammatory phase; M1→M2 failure |