Gain-of-function, focal segmental glomerulosclerosis Trpc6 mutation minimally affects susceptibility to renal injury in several mouse models

Abstract

Mutations in TRPC6 are a cause of autosomal dominant focal segmental glomerulosclerosis in humans. Many of these mutations are known to have a gain-of-function effect on the non-specific cation channel function of TRPC6. In vitro studies have suggested these mutations affect several signaling pathways, but in vivo studies have largely compared wild-type and Trpc6-deficient rodents. We developed mice carrying a gain-of-function Trpc6 mutation encoding an E896K amino acid change, corresponding to a known FSGS mutation in TRPC6. Homozygous mutant Trpc6 animals have no appreciable renal pathology, and do not develop albuminuria until very advanced age. The Trpc6E896K mutation does not impart susceptibility to PAN nephrosis. The animals show a slight delay in recovery from the albumin overload model. In response to chronic angiotensin II infusion, Trpc6E896K/E896K mice have slightly greater albuminuria initially compared to wild-type animals, an effect that is lost at later time points, and a statistically non-significant trend toward more glomerular injury. This phenotype is nearly opposite to that of Trpc6-deficient animals previously described. The Trpc6 mutation does not appreciably impact renal interstitial fibrosis in response to either angiotensin II infusion, or folate-induced kidney injury. TRPC6 protein and TRPC6-agonist induced calcium influx could not be detected in glomeruli. In sum, these findings suggest that a gain-of-function Trpc6 mutation confers only a mild susceptibility to glomerular injury in the mouse.

An inactivating human TRPC6 channel mutation without focal segmental glomerulosclerosis

“…Furthermore, transgenic overexpression of either wild-type or mutant TRPC6 (P111Q and E896K GOF) in podocytes causes only modest albuminuria and mild histological changes with incomplete penetrance in mice [ 48 ]. Human autosomal dominant FSGS-causing mutations in TRPC6 [ 49 ], ACTN4 [ 50 ], and INF2 [ 51 ] are unable to induce a renal phenotype when present in heterozygous mice. Although homozygous Actn4 or Inf 2 mutant mice show enhanced sensitivity to injury [ 50 , 52 ], this is not observed in homozygous mutant TRPC6 (E896K GOF) mice exhibiting no susceptibility to PAN nephrosis [ 49 ].…”

“…Human autosomal dominant FSGS-causing mutations in TRPC6 [ 49 ], ACTN4 [ 50 ], and INF2 [ 51 ] are unable to induce a renal phenotype when present in heterozygous mice. Although homozygous Actn4 or Inf 2 mutant mice show enhanced sensitivity to injury [ 50 , 52 ], this is not observed in homozygous mutant TRPC6 (E896K GOF) mice exhibiting no susceptibility to PAN nephrosis [ 49 ]. The reason for the relative lack of a renal phenotype in the TRPC6 knock-in/-out animals remains unclear.…”

The Effects of TRPC6 Knockout in Animal Models of Kidney Disease

“…Thus, TRPC6-M131T is the mouse homolog of human TRPC6-M132T, which was discovered in an individual with severe FSGS with a childhood onset and which produces a very large gain-of-function in heterologous expression systems [ 36 ]. The available studies at least suggest that gain-of-function TRPC6 mutations that produce later onset nephrosis in humans tend to produce less severe kidney disease in mice [ 45 , 46 , 47 ]. Familial nephrotic syndromes are quite rare, and only a small proportion of the familial forms are due to mutations in TRPC6 .…”

“…Finally, one needs to consider the issue of lifespan and how it affects the development of a kidney disease. A human with a TRPC6 mutation may experience renal failure in the third decade of life [ 35 ], which would be considered quite young relative to a normal human lifespan, and yet an adult mouse constitutively over-expressing the same TRPC6 variant may barely show any effects [ 46 , 47 ]. Perhaps this reflects the fact that laboratory mice only live for 24–30 months, or perhaps it reflects the fact that mice have a great deal of spare capacity in their kidneys.…”