Intravital imaging of podocyte calcium in glomerular injury and disease

Intracellular calcium ([Ca²⁺]i) signaling mediates physiological and pathological processes in multiple organs, including the renal podocyte; however, in vivo podocyte [Ca²⁺]i dynamics are not fully understood. Here we developed an imaging approach that uses multiphoton microscopy (MPM) to directly visualize podocyte [Ca²⁺]i dynamics within the intact kidneys of live mice expressing a fluorescent calcium indicator only in these cells. [Ca²⁺]i was at a low steady-state level in control podocytes, while Ang II infusion caused a minor elevation. Experimental focal podocyte injury triggered a robust and sustained elevation of podocyte [Ca²⁺]i around the injury site and promoted cell-to-cell propagating podocyte [Ca²⁺]i waves along capillary loops. [Ca²⁺]i wave propagation was ameliorated by inhibitors of purinergic [Ca²⁺]i signaling as well as in animals lacking the P2Y2 purinergic receptor. Increased podocyte [Ca²⁺]i resulted in contraction of the glomerular tuft and increased capillary albumin permeability. In preclinical models of renal fibrosis and glomerulosclerosis, high podocyte [Ca²⁺]i correlated with increased cell motility. Our findings provide a visual demonstration of the in vivo importance of podocyte [Ca²⁺]i in glomerular pathology and suggest that purinergic [Ca²⁺]i signaling is a robust and key pathogenic mechanism in podocyte injury. This in vivo imaging approach will allow future detailed investigation of the molecular and cellular mechanisms of glomerular disease in the intact living kidney.