Diabetes is the leading cause of end-stage renal disease, one of the nation’s most debilitating and expensive medical conditions. In 2016 Medicare spent $35 billion caring for more than 725,000 Americans whose kidneys had failed. Short of mandating universal diabetes treatment, regular exercise and low-calorie diets, little can be done to stem the rising tide of kidney failure — unless scientists can figure out why exactly the kidney’s filtration units, the glomeruli, stop working.