FN1 (Fibronectin 1) Notes - ISOFORMS Project

Key Isoform Biology

FN1 is a classic example of developmental/tissue-specific splicing with 17 named isoforms.

Core Alternative Splicing Events

The key splicing events involve:
1. EDA domain (Extra Domain A, also called EIIIA) - alternative exon
2. EDB domain (Extra Domain B, also called EIIIB) - alternative exon
3. IIICS region (Variable region, V region) - can be partially or completely included

Functional Isoform Groups

Isoform Type EDA EDB Expression Function
Plasma FN - - Hepatocytes, soluble Wound healing, blood clot
Cellular FN + +/- Fibroblasts, solid ECM assembly, embryonic development
Oncofetal FN + + Embryonic, tumors Angiogenesis, tumor progression

Critical Functional Differences

EDA-containing isoforms:
- Promote cell adhesion more strongly
- Essential for wound healing
- Re-expressed in cancer (oncofetal antigen)
- EDA-null mice have normal development but impaired wound healing

EDB-containing isoforms:
- Strong pro-angiogenic activity
- Expressed during embryonic development
- Re-expressed in tumor angiogenesis
- Target for cancer imaging/therapy (antibody L19)

EDA/EDB double-null mice:
- Cardiovascular defects
- Embryonic lethality or early postnatal death
- Demonstrates functional importance of these domains

UniProt Isoform Summary

UniProt lists 17 isoforms including:
- Isoform 7: Contains EDB domain
- Isoform 8: Lacks EDA domain
- Isoform 9: Lacks both EDA and EDB, uses V64 variant
- Isoform 16: Migration stimulation factor (MSF)

Expected Annotation Issues

  1. "Angiogenesis" - may be specific to EDB+ isoforms
  2. "Wound healing" - involves EDA+ isoforms
  3. "Embryonic development" - EDA+/EDB+ isoforms re-expressed
  4. "Cell adhesion" - all isoforms, but strength varies

Key References