Glomerular permeability: in vivo tracer studies with polyanionic and polycationic ferritins

Area of Science:

• Nephrology
• Molecular Biology
• Biophysics

Background:

• The glomerular filtration barrier selectively restricts molecule passage based on size and charge.
• Understanding the role of molecular charge in glomerular permeability is crucial for diagnosing kidney diseases.

Purpose of the Study:

• To investigate the influence of molecular charge on glomerular permeability using ferritin tracers in vivo.
• To elucidate the mechanisms of charge-dependent restriction in the glomerular filtration barrier.

Main Methods:

• Intravenous injection of native anionic ferritin and cationized ferritin derivatives into mice and rats.
• Kidney examination using electron microscopy to analyze ferritin molecule distribution.
• Assessment of ferritin tracer penetration across different layers of the glomerular basement membrane (GBM).

Main Results:

• Native anionic ferritin was largely excluded from the GBM, while cationized ferritin derivatives showed increased penetration correlated with their isoelectric points (pI).
• All filtered ferritin molecules were restricted at the filtration slits, appearing in podocyte phagosomes.
• Renal hemodynamics did not affect ferritin movement into the GBM, indicating a primary role for charge-based filtration.

Conclusions:

• Molecular charge is a critical determinant of glomerular permeability, with cationic molecules exhibiting greater filtration.
• The findings support a charge-dependent filtration mechanism in the mammalian glomerulus, similar to polyanionic gel systems.
• Glomerular restriction of plasma proteins involves charge-based exclusion, partly due to polyanionic glycoproteins in the GBM.