People of recent sub-Saharan African ancestry develop kidney failure much more frequently than other groups. A large fraction of this disparity is due to two coding sequence variants in the APOL1 gene. Inheriting two copies of these APOL1 risk variants, known as G1 and G2, causes high rates of focal segmental glomerulosclerosis (FSGS), HIV-associated nephropathy and hypertension-associated end-stage kidney disease. Disease risk follows a recessive mode of inheritance, which is puzzling given the considerable data that G1 and G2 are toxic gain-of-function variants. We developed coisogenic bacterial artificial chromosome (BAC) transgenic mice harboring either the wild-type (G0), G1 or G2 forms of human APOL1. Expression of interferon gamma (IFN-γ) via plasmid tail vein injection results in upregulation of APOL1 protein levels together with robust induction of heavy proteinuria and glomerulosclerosis in G1/G1 and G2/G2 but not G0/G0 mice. The disease phenotype was greater in G2/G2 mice. Neither heterozygous (G1/G0 or G2/G0) risk variant mice nor hemizygous (G1/−, G2/−) mice had significant kidney injury in response to IFN-γ, although the heterozygous mice had a greater proteinuric response than the hemizygous mice, suggesting that the lack of significant disease in humans heterozygous for G1 or G2 is not due to G0 rescue of G1 or G2 toxicity. Studies using additional mice (multicopy G2 and a non-isogenic G0 mouse) supported the notion that disease is largely a function of the level of risk variant APOL1 expression. Together, these findings shed light on the recessive nature of APOL1-nephropathy and present an important model for future studies.

UNC51-like kinase-1 (ULK1) is the catalytic component of the autophagy pre-initiation complex that stimulates autophagy via phosphorylation of ATG14, BECLN1 and other autophagy proteins. ULK1 has also been shown to specifically promote mitophagy but the mechanistic basis of how has remained unclear. Here we show that ULK1 phosphorylates the BNIP3 mitochondrial cargo receptor on a critical serine residue (S17) adjacent to its amino terminal LIR motif. ULK1 similarly phosphorylates BNIP3L on S35. Phosphorylation of BNIP3 on S17 by ULK1 promotes interaction with LC3 and mitophagy. ULK1 interaction also promotes BNIP3 protein stability by limiting its turnover at the proteasome. The ability of ULK1 to regulate BNIP3 protein stability depends on an intact “BH3” domain and deletion of its “BH3” domain reduces BNIP3 turnover and increases BNIP3 protein levels independent of ULK1. In summary ULK1 promotes mitophagy by both stabilization of BNIP3 protein and via phosphorylation of S17 to stimulate interaction with LC3.

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Approximately 20% of patients with myeloproliferative neoplasms (MPNs) harbor mutations in the gene calreticulin (CALR), with 80% of those mutations classified as either type 1 or type 2. While type 2 CALR mutant proteins retain many of the Ca2+ binding sites present in the wild type protein, type 1 CALR mutant proteins lose these residues. The functional consequences of this differential loss of Ca2+ binding sites remain yet unexplored. Here, we show that the loss of Ca2+ binding residues in the type 1 mutant CALR protein directly impairs its Ca2+ binding ability, which in turn leads to depleted endoplasmic reticulum (ER) Ca2+ and subsequent activation of the IRE1a/XBP1 pathway of the unfolded protein response. Genetic or pharmacological inhibition of IRE1a/XBP1 signaling induces cell death only in type 1 mutant but not type 2 mutant or wild type CALR-expressing cells, and abrogates type 1 mutant CALR-driven MPN disease progression in vivo.