ApoL1 Overexpression Drives Variant-Independent Cytotoxicity

J Am Soc Nephrol. 2018 Mar;29(3):869-879. doi: 10.1681/ASN.2016121322. Epub 2017 Nov 27.

Abstract

Coding variants in the APOL1 gene are associated with kidney diseases in African ancestral populations; yet, the underlying biologic mechanisms remain uncertain. Variant-dependent autophagic and cytotoxic cell death have been proposed as pathogenic pathways mediating kidney injury. To examine this possibility, we conditionally expressed APOL1-G0 (reference), -G1, and -G2 (variants) using a tetracycline-regulated system in HEK293 cells. Autophagy was monitored biochemically and cell death was measured using multiple assays. We measured intracellular Na+ and K+ content with atomic absorption spectroscopy and APOL1-dependent currents with whole-cell patch clamping. Neither reference nor variant APOL1s induced autophagy. At high expression levels, APOL1-G0, -G1, and -G2 inserted into the plasma membrane and formed pH-sensitive cation channels, causing collapse of cellular Na+ and K+ gradients, phosphorylation of p38 mitogen-activated protein kinase, and cell death, without variant-dependent differences. APOL1-G0 and -G2 exhibited similar channel properties in whole-cell patch clamp experiments. At low expression levels, neither reference nor variant APOL1s localized on the plasma membrane, Na+ and K+ gradients were maintained, and cells remained viable. Our results indicate that APOL1-mediated pore formation is critical for the trypanolytic activity of APOL1 and drives APOL1-mediated cytotoxicity in overexpression systems. The absence of cytotoxicity at physiologic expression levels suggests variant-dependent intracellular K+ loss and cytotoxicity does not drive kidney disease progression.

Keywords: autophagy; cell death; genetic renal disease; kidney.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Apolipoprotein L1 / genetics*
  • Apolipoprotein L1 / physiology
  • Autophagy / genetics*
  • Calcium / metabolism
  • Cell Membrane / physiology
  • Gene Expression / drug effects
  • Genetic Variation*
  • Genotype
  • HEK293 Cells
  • Humans
  • Ion Channels
  • Kidney Diseases / genetics*
  • Patch-Clamp Techniques
  • Phosphorylation
  • Potassium / metabolism*
  • Sodium / metabolism*
  • Tetracycline / pharmacology
  • p38 Mitogen-Activated Protein Kinases / metabolism

Substances

  • APOL1 protein, human
  • Apolipoprotein L1
  • Ion Channels
  • Sodium
  • p38 Mitogen-Activated Protein Kinases
  • Tetracycline
  • Potassium
  • Calcium