Podocytes are lost from glomeruli before completing apoptosis

Area of Science:

• Nephrology
• Cell Biology
• Immunology

Background:

• Podocyte apoptosis is well-documented in vitro but poorly characterized in vivo.
• Discrepancies in observing podocyte apoptosis in vitro versus in vivo require investigation.
• Human CD25 (hCD25)-directed immunotoxin LMB2 is used to study podocyte cell death.

Purpose of the Study:

• To investigate the discrepancy in podocyte apoptosis observation between in vitro and in vivo settings.
• To elucidate the mechanism of podocyte cell death and clearance in vivo.
• To determine the role of glomerular filtration in podocyte detachment and apoptosis.

Main Methods:

• Cultured podocytes expressing hCD25 were treated with LMB2.
• NEP25 mice (expressing hCD25) were treated with LMB2 to study in vivo podocyte apoptosis.
• Unilateral ureteral obstruction and kidney organoid models were used to assess the effect of glomerular filtration.

Main Results:

• LMB2 induced caspase-dependent apoptosis in cultured podocytes with typical apoptotic features.
• In vivo, LMB2 eliminated hCD25-expressing podocytes, but TUNEL staining was absent, with only occasional cleaved caspase-3/lamin A positivity.
• TUNEL-positive podocytes were found in urine, and obstructed kidneys showed increased cleaved lamin A-positive podocytes, suggesting filtration-facilitated detachment.

Conclusions:

• Activated podocytes detach and are lost in urine before nuclear fragmentation, explaining the lack of definitive apoptosis markers in vivo.
• Glomerular filtration actively facilitates podocyte detachment from the glomerulus.
• This filtration-dependent detachment mechanism clarifies the infrequent observation of classical podocyte apoptosis in vivo.