Over- or underfill: not all nephrotic states are created equal

Area of Science:

• Nephrology
• Pathophysiology
• Molecular Medicine

Background:

• The traditional "underfill" hypothesis for nephrotic syndrome edema posits urinary protein loss reduces oncotic pressure, causing fluid extravasation.
• Clinical observations challenge the "underfill" model, leading to the development of the "overfill" hypothesis, which suggests primary sodium retention.

Discussion:

• Proteinuria in nephrotic syndrome may contain proteinases like plasmin, activating the epithelial sodium channel (ENaC) in the collecting duct.
• This activation is proposed as a mechanism for primary sodium retention and subsequent edema.
• The study presents evidence of increased urinary plasmin and ENaC activation in children with nephrotic syndrome.

Key Insights:

• Urinary plasmin activation of ENaC offers a novel explanation for edema in nephrotic syndrome.
• This mechanism suggests potential therapeutic strategies targeting ENaC blockade, such as with amiloride.
• The persistence of the overfill versus underfill debate highlights the complexity of nephrotic syndrome pathophysiology.

Outlook:

• Further research is needed to fully elucidate the role of plasmin-ENaC signaling in nephrotic syndrome.
• Investigating why ENaC blockers are not empirically established treatments and understanding persistent underfilling signs is crucial.
• Resolving the overfill/underfill controversy will advance our understanding and treatment of nephrotic syndrome.