APOM Deficiency Affects Kidney Function in an Experimental Mouse Model of Alport Syndrome

Background: We have previously reported that glomerular APOM deficiency correlates with eGFR in patients with glomerular diseases enrolled in the Nephrotic Syndrome Study Network (NEPTUNE). However, little is known about the significance of APOM deficiency in kidney disease development and progression. With this study, we investigate if APOM deficiency occurs in Alport Syndrome (AS) and how ApoM deficiency may contribute to disease progression. We first analyzed glomerular APOM expression in a mouse model of progressive renal disease associated with AS (Col4a3 KO mice) and in podocytes isolated from Col4a3 KO mice. We found reduced mRNA and protein expression of APOM in glomeruli and podocytes isolated from Col4a3 KO mice. Methods: To investigate if APOM replenishment would improve renal function in this mouse model, we injected Col4a3 KO mice with human recombinant APOM (rhAPOM). Three groups of mice were studied, n=6 mice per group: (1) Col4a3 +/+ (WT) + vehicle, (2) Col4a3 -/- (KO) + vehicle, and (3) Col4a3 -/- (KO) + rhAPOM. rhAPOM was administered by weekly ip injection starting at 4 w of age and until sacrifice at 8 w of age. APOM protein levels in podocytes were measured by Western blot analysis. Serum and urine samples were used to determine blood urea nitrogen (BUN), creatinine, and urinary albumin/creatinine ratios (ACR). Triglyceride content (TG) was assayed using a colorimetric kit and lipid droplets (LD) content was using the OPERA screening system. Histological analysis was performed by Periodic Acid Schiff (PAS), renal fibrosis was evaluated by Picro Sirius Red (PSR), renal lipid content was determined by Oil Red O (ORO) and WT1 to detect podocytes number. Results: Glomerular APOM mRNA and protein expression were markedly reduced in Col4a3 KO mice when compared to WT mice. Treatment of Col4a3 KO podocytes with rhAPOM reduce TG levels and LD content and treatment of Col4a3 KO mice with rhAPOM normalized renal APOM levels and resulted in a significant reduction of serum BUN and creatinine levels, a reduced ACR. Conclusion: rhAPOM treatment protected from the development of glomerulosclerosis and kidney fibrosis.