Podocyte specific knockout of the natriuretic peptide clearance receptor is podocyte protective in focal segmental glomerulosclerosis

Abstract

Natriuretic peptides (NPs) bind to glomerular podocytes and attenuate glomerular injury. The beneficial effects of NPs are negatively regulated by the NP clearance receptor (NPRC), which is highly expressed in podocytes. To determine if inhibiting NPRC is podocyte protective, we examined the effects of deleting NPRC in both cultured podocytes and in vivo. We found that: 1.Both atrial NP and C-type NP inhibit podocyte apoptosis in cultured podocytes, but these podocyte protective effects are significantly attenuated in cells expressing NPRC, and 2. Atrial NP was significantly more effective than CNP at inhibiting the apoptotic response. Consistent with the protective actions of NPs, podocyte specific knockout of NPRC reduced albuminuria, glomerular sclerosis and tubulointerstitial inflammation in a mouse model of focal segmental glomerulosclerosis. These beneficial actions were associated with: 1. Decreased expression of the myofibroblast marker alpha-smooth muscle actin, 2. Reduced expression of the extracellular matrix proteins collagen 4-alpha-1 and fibronectin, and 3. Preserved expression of the podocyte proteins nephrin and podocin. Inhibiting NP clearance may be a useful therapeutic approach to treat glomerular diseases.

Fig 1. (1A) CNP is produced by podocytes and binds to both NPRB and NPRC on glomerular podocytes.

(1B) Knockdown (KD) of NPRC increases CNP levels by inhibiting clearance of CNP from the circulation. (1C) Four shRNA constructs (1, 2, 3 & 4) and a scrambled control were used to KD NPRC in cultured podocytes. Constructs 1 and 2 provided the most effective KD of NPRC. (1D & 1E) CNP expression in cell culture medium was significantly increased by KD of NPRC in the both the presence and absence of the neprilysin inhibitor LBQ657. (1F) Generation of cGMP by low dosages of CNP was enhanced by NPRC KD in cultured podocytes compared to controls. (1G) Serum deprivation significantly increased apoptosis in control podocytes and KD of NPRC inhibited the apoptotic response in the absence of exogenous NPs. In addition, treatment with the NPRB antagonist P19 inhibited the podocyte protective actions of NPRC KD, resulting in an increase in apoptosis. * P <  0.005 **P <  0.001 versus control cells.

Fig 2. (A) NPRC was highly expressed in WT podocytes compared to either NPRA or NPRB in the presence of absence of NPRC KD.

In addition, NPRC KD effectively decreased NPRC mRNA in cultured podocytes. (B) NPRB expression was downregulated in NPRC KD cells. (C) Both ANP and CNP increased cGMP generation in a stepwise fashion. The increase in cGMP generation was statistically significant for the KD cells at the 10 nM concentrations compared to both unstimulated KD cells and WT podocytes stimulated with either ANP or CNP. (D) Apoptosis induced by serum deprivation was inhibited by KD of NPRC. ANP treatment alone further inhibited podocyte apoptosis and combined treatment with ANP and NPRC KD reduced the apoptotic response toward baseline. In contrast, CNP alone had little effect on podocyte apoptosis, but NPRC KD reduced the apoptotic response in the presence or absence of 10 nM CNP. **P < 0.0001 versus other NPRs shown in Fig 2A.

Fig 3. Cre-mediated recombination in vivo.

(3A & 3B). These studies used a mouse expressing a reporter allele that contains a cell membrane-localized red fluorescence transgene (td-Tomato) with widespread expression in all tissues and cell types. Expression of Cre recombinase induces a cell membrane-localized green fluorescence protein (EGFP). To examine Cre-medicated recombination in vivo, “double” TG mice (NPHS2-rtTA, tetO-Cre) mice were crossed with the TG reporter animals to create triple TG mice (NPHS2-rtTA, tetO-Cre, td-Tomato). In these TG mice, treatment with doxycycline induces green fluorescence specifically in podocytes, with red fluorescence confined to other cell types (panel 3A). In contrast, little EGFP expression was observed in controls (panel 3B). (3C) Quantitative RT-PCR demonstrated a significant decrease in expression NPRC in enriched glomerular preparations from NPRC KO mice compared to controls. (3D & 3E). NPRC protein was significantly reduced in glomerular preparations form podocyte specific KO mice compared to WT mice. Expression of NPRC in kidney cortices was similar in WT and KO kidneys.

Fig 4. Effect of podocyte specific NPRC KO on systemic BP and albuminuria.

(4A) Systolic BP was similarly decreased in both groups of Gq mice compared to controls, but this difference was not statistically significant. (4B) Treatment with PAN induced heavy proteinuria at day 10 and day 14 in WT Gq mice. The increase in albuminuria was significantly decreased by NPRC-KO at both day 10 and day 14 following PAN injection.

Fig 5. Effect of podocyte specific NPRC KO on renal histopathology.

(5A, 5B) Representative pictures of GS in Gq mice and Gq-KO mice. (5C) NPRC-KO significant reduced GS in Gq-KO mice compared to Gq mice. (5D).NPRC-KO significantly reduced TI inflammation in Gq-KO mice compared to Gq mice. (5E) Tubular injury was similar in both groups of Gq mice. (NOTE: data in Figs 5C, 5D and 5E are presented as a percentage of mice with the indicated histologic abnormalities, but the statistical evaluation was performed using the number of mice with the specified histologic abnormality).

Fig 6. Effect of NPRC KO on fibrotic markers.

(6A) The myofibroblast marker alpha-SMA was significantly increased in glomerular preparations from WT Gq mice compared to controls. The increase in alpha-SMA was significantly reduced by podocyte specific KO of NPRC. (6B, 6C & 6D) The ECM proteins fibronectin and the alpha-1 chains of collagen 4 (CO4 alpha-1) and collagen 1 (Col1 alpha-1) were significantly increased in Gq mice compared to controls. Podocyte specific KO of NPRC decreased expression of both fibronectin and Col4 alpha-1, which was statistically significant for Col4 alpha-1. In contrast, NPRC KO had little effect on Col1 alpha-1 expression. (6E, 6F, 6G & 6H) Protein levels of the myofibroblast marker alpha-SMA and fibronectin were significantly increased in glomerular preparations from WT Gq mice compared to controls. Podocyte specific KO of NPRC significantly reduced expression of both alpha-SMA and the ECM protein fibronectin. NOTE: Quantitative RT-PCR data is presented in logarithmic format.

Fig 7. Effect of NPRC KO on podocyte proteins.

(7A & 7B) Expression of nephrin and podocin mRNAs were significantly decreased in glomerular preparations from Gq mice compared to controls and expression of both podocyte proteins were preserved by podocyte specific KO of NPRC. (7C & 7D) The pattern of nephrin protein expression was similar to the mRNA results, but the differences were not statistically significant.

Fig 8. Effect of NPRC KO on renal function.

Cystatin C levels were significantly increased in serum of Gq mice compared to controls. KO of NPRC decreased cystatin C levels in Gq mice but the difference was not statistically significant.