Quantitative immunoelectron microscopy of type VI collagen in glomeruli in type I diabetic patients

Background: The nature of the extracellular matrix (ECM) accumulating in the glomerular basement membrane (GBM) and mesangium (Mes) in diabetes is unknown. Type IV collagen (Col IV) as estimated by quantitative immunoelectron microscopy was reduced in type I diabetic patients (D) with rapid ("fast-track") compared with slow ("slow-track") development of diabetic nephropathy (DN) lesions and controls (C). Col VI is another ECM component suggested to account for Mes matrix (MM) expansion in DN. Methods: Col VI ECM density was evaluated in eight "slow-track" (Mes fractional volume [Vv(Mes/glom)] <0.32, duration >20 years) and seven "fast-track" patients [Vv(Mes/glom) >0.37, duration <20 years diabetes] and in eight C. Quantitative immunoelectron microscopy was performed using polyclonal antibodies to Col VI. Gold particle density (PD_G) in MM and the inner layer (IL) of the GBM was measured using stereologic methods. Results: GBM IL PD_G was decreased in both fast-track (1.7 ± 1.6/μm², mean ± SD, P < 0.002) and slow-track (3.9 ± 2.4, P < 0.02) D versus C (10.8 ± 7.9). GBM IL PD_G was also lower in the fast-track versus slow-track D (P < 0.04). Mes matrix PD_G/μm² was decreased in fast-track D (3.2 ± 3.6) versus C (14.1 ± 14.6, P < 0.02); a similar trend was seen in slow-track D (5.7 ± 5.6, P < 0.1). There was no significant difference in MM PD_G between the slow-track and fast-track D. Conclusion: Col VI density in MM and GBM is decreased in diabetic patients with slowly and rapidly developing renal lesions. This leaves the nature of ECM accumulation in DN unexplained. At least in part, glomerular ECM compositional change is related to diabetes per se and may be independent of the severity of lesions.