Group V Secretory Phospholipase A2 Is Involved in Tubular Integrity and Sodium Handling in the Kidney

Abstract

Group V (GV) phospholipase A2 (PLA2) is a member of the family of secreted PLA2 (sPLA2) enzymes. This enzyme has been identified in several organs, including the kidney. However, the physiologic role of GV sPLA2 in the maintenance of renal function remains unclear. We used mice lacking the gene encoding GV sPLA2 (Pla2g5-/-) and wild-type breeding pairs in the experiments. Mice were individually housed in metabolic cages and 48-h urine was collected for biochemical assays. Kidney samples were evaluated for glomerular morphology, renal fibrosis, and expression/activity of the (Na+ + K+)-ATPase α1 subunit. We observed that plasma creatinine levels were increased in Pla2g5-/- mice following by a decrease in creatinine clearance. The levels of urinary protein were higher in Pla2g5-/- mice than in the control group. Markers of tubular integrity and function such as γ-glutamyl transpeptidase, lactate dehydrogenase, and sodium excretion fraction (FENa+) were also increased in Pla2g5-/- mice. The increased FENa+ observed in Pla2g5-/- mice was correlated to alterations in cortical (Na+ + K+) ATPase activity/ expression. In addition, the kidney from Pla2g5-/- mice showed accumulation of matrix in corticomedullary glomeruli and tubulointerstitial fibrosis. These data suggest GV sPLA2 is involved in the maintenance of tubular cell function and integrity, promoting sodium retention through increased cortical (Na+ + K+)-ATPase expression and activity.

Fig 1. Mild glomerular morphologic changes are observed in Pla2g5^−/− mice.

PAS reagent was used for analysis of the mesangial surface of corticomedullary (A, B) and subcapsular glomeruli (C, D), as described in the Materials and Methods. Representative photomicrographs (magnification 40×) of (A) the corticomedullary glomerulus and (C) the subcapsular glomerulus. (B) Quantitative analysis of the corticomedullary and (D) subcapsular glomeruli (n = 6 per group). The results are expressed as means ± SE. *Statistically significant in relation to WT mice (P < 0.05).

Fig 2. Urinary tubular enzymes and collagen deposition, markers of tubular injury, are increased in Pla2g-5^−/− mice.

(A) LDH (B) and γGT activities were measured in urine samples as markers of tubular injury (n = 8 per group). Collagen deposition in the renal cortex was visualized by Picrosirius Red staining. (C) Representative photomicrographs (magnification 40×) of collagen deposition in the renal cortex of WT and Pla2g-5^−/− mice. (D) Quantitative analysis of the collagen deposition (n = 6 per group). The results are expressed as means ± SE. *Statistically significant in relation to WT mice (P < 0.05).

Fig 3. GV sPLA₂ promotes sodium retention.

(A) Urinary sodium excretion (U_Na⁺V), (B) clearance of sodium (C_Na⁺), (C) osmolar clearance, and (D) FE_Na⁺ in WT and Pla2g-5^−/− mice. The number of mice analyzed is given in Table 1. The results are expressed as means ± SE. *Statistically significant in relation to WT mice (P < 0.05).

Fig 4. GV sPLA2 upregulates activity and expression of cortical (Na+ + K+)-ATPase.

Expression and activity of (Na⁺ + K⁺)-ATPase in WT and Pla2g-5^−/− mice. ATPase activity from the renal cortex (A) and medulla homogenate (C) was determinate by the colorimetric method. Immunoblotting was performed for the (Na⁺ + K⁺)-ATPase α1 subunit in (B) the renal cortex and (D) the medullar preparation of both WT and Pla2g5^−/− mice, as described in the Materials and Methods (n = 8 per group). The results are expressed as means ± SE. *Statistically significant in relation to WT mice (P < 0.05).