Direct visualization of Smad1/5/8-mediated transcriptional activity identifies podocytes and collecting ducts as major targets of BMP signalling in healthy and diseased kidneys

Bone morphogenetic protein 7 (BMP7) is a key determinant of renal response to injury, exhibiting strong protective as well as regenerative potential in a variety of experimental models. In vitro, beneficial effects of stimulation with BMP7 and other BMPs have been observed in many renal cell types. Still, it remains poorly understood which cells in the native kidney actually respond to BMPs in health and disease. Here, we report the use of BRE:gfp mice expressing green fluorescent protein (GFP) under the control of a pSmad1/5/8-specific BMP-responsive element (BRE) to directly visualize the spatiotemporal distribution of transcriptional activity downstream of canonical BMP signalling in healthy kidneys and in two distinct models of kidney disease. BRE-GFP signal coincided with expression of endogenous BMP target genes but, surprisingly, it was much more restricted than expected from the widespread distribution of pSmad1/5/8, a classical component of canonical BMP signal tranduction. BRE-GFP was mainly present in podocytes and collecting duct cells, and both glomerular and medullary BRE-GFP decreased following ischaemia-reperfusion injury as well as following unilateral ureteric obstruction, together with decreased BMP7, pSmad1/5/8 and BMP target gene expression. Remarkably, however, BRE-GFP was increased in injured proximal tubules in association with up-regulation of BMP receptors ALK2 and ALK3. Thus, native BMP transcriptional activity is much more restricted than previously suggested based on pSmad1/5/8 detection alone, and its response to injury varies according to cell type and nephron segment.