A new view of macula densa cell protein synthesis.

Macula densa (MD) cells, a chief sensory cell type in the nephron, are endowed with unique microanatomical features including a high density of protein synthetic organelles and secretory vesicles in basal cell processes ("maculapodia") that suggest a so far unknown high rate of MD protein synthesis. The present study aimed to explore the rate and regulation of MD protein synthesis and their effects on glomerular function using novel transgenic mouse models, newly established fluorescence cell biology techniques and intravital microscopy. Sox2-tdTomato kidney tissue sections and the O-propargyl puromycin (OPP)-incorporation based fluorescence imaging assay showed that MD cells have the highest level of protein synthesis within the kidney cortex followed by intercalated cells and podocytes. Genetic gain-of-function (gof) of mTOR signaling specifically in MD cells (in MD-mTORgof mice) or their physiological activation by low salt diet resulted in further significant increases in the synthesis of MD proteins. Specifically, these included both classic and recently identified MD-specific proteins such as cyclooxygenase 2 (COX2), microsomal prostaglandin E2 synthase 1 (mPGES1) and Pappalysin 2 (Pappa2), respectively. Intravital imaging of the kidney using multiphoton microscopy showed significant increases in afferent and efferent arteriole and glomerular capillary diameters and blood flow in MD-mTORgof mice coupled with elevated glomerular filtration rate. The presently identified high rate of MD protein synthesis that is regulated by mTOR signaling is a novel component of the physiological activation and glomerular hemodynamic regulatory functions of MD cells that remains to be fully characterized.