COL4A1 and diabetic kidney disease: Glomerular FGF1 expression (Figure 7A) was positively associated with the levels of GFR, while the levels of glomerular NRP1 (Figure 7B), tubular COL4A1 (Figure 7C), and tubular NRP1 (Figure 7D) were negatively related to the levels of GFR, suggesting that cell cross-talk-related mechanisms contribute to the development of DKD.