Lysophosphatidylcholine mediates fast decline in kidney function in diabetic kidney disease.

Some patients with diabetic kidney disease (DKD) show a fast progression of kidney dysfunction and are known as a "fast decliner" (FD). Therefore, it is critical to understand pathomechanisms specific for fast decline. Here, we performed a comprehensive metabolomic analysis of patients with stage G3 DKD and identified increased urinary lysophosphatidylcholine (LPC) in fast decline. This was confirmed by quantification of urinary LPC using mass spectrometry and identified urinary LPC containing saturated fatty acids palmitic (16:0) and stearic (18:0) acids was increased in FDs. The upsurge in urinary LPC levels was correlated with a decline in estimated glomerular filtration rate after 2.5 years. To clarify a pathogenic role of LPC in FD, we studied an accelerated rat model of DKD and observed an increase in LPC (16:0) and (18:0) levels in the urine and kidney tubulointerstitium as the disease progressed. These findings suggested that local dysregulation of lipid metabolism resulted in excessive accumulation of this LPC species in the kidney. Our in vitro studies also confirmed LPC-mediated lipotoxicity in cultured proximal tubular cells. LPC induced accumulation of lipid droplets via activation of peroxisome proliferator-activated receptor-δ followed by upregulation of the lipid droplet membrane protein perilipin 2 and decreased autophagic flux, thereby inducing organelle stress and subsequent apoptosis. Thus, LPC (16:0) and (18:0) may mediate a fast progression of DKD and may serve as a target for novel therapeutic approaches.

Metal chaperone, NhpC, involved in the metallocenter biosynthesis of nitrile hydratase.

Pseudomonas chlororaphis B23 yields nitrile hydratase (NHase) used for the production of 5-cyanovaleramide at the industrial level. Although the nhpC gene (known as P47K) located just downstream of the NHase structural genes (nhpAB) has been important for efficient NHase expression, the key role of nhpC remains poorly studied. Here, we purified two NHases expressed in the presence and absence of nhpC, respectively, and characterized them. The purified NHase expressed with nhpC proved to be an iron-containing holo-NHase, while the purified one expressed without nhpC was identified as an apo-NHase, which was iron-deficient. These findings indicated that nhpC would play a crucial role in the post-translational incorporation of iron into the NHase active site as a metal chaperone. In the overall amino acid sequence of NhpC, only the N-terminus exhibited similarities to the CobW protein involved in cobalamin biosynthesis, the UreG and HypB proteins essential for the metallocenter biosynthesis of urease and hydrogenase, respectively. NhpC contains a P-loop motif known as a nucleotide-binding site, and Lys23 and Thr24 are conserved in the P-loop motif in NhpC. Expression analysis of NHase formed in the presence of each mutant NhpC (i.e., K23A and T24A) resulted in immunodetectable production of a mutant NhpC and remarkable expression of NHase lacking the enzyme activity. These findings suggested that an intact P-loop containing Lys23 and Thr24 would be essential for the NhpC function in vivo for the post-translational metallocenter assembly of NHase.

Positron emission tomography imaging of renal mitochondria is a powerful tool in the study of acute and progressive kidney disease models.

Mitochondrial dysfunction plays a critical role in the pathogenesis of kidney diseases via ATP depletion and reactive oxygen species overproduction. Nonetheless, few studies have reported the renal mitochondrial status clinical settings, partly due to a paucity of methodologies. Recently, a positron emission tomography probe, 18F-BCPP-BF, was developed to non-invasively visualize and quantitate the renal mitochondrial status in vivo. Here, 18F-BCPP-BF positron emission tomography was applied to three mechanistic kidney disease models in rats: kidney ischemia-reperfusion, 5/6 nephrectomy and anti-glomerular basement membrane glomerulonephritis. In rats with ischemia-reperfusion, a slight decrease in the kidney uptake of 18F-BCPP-BF was accompanied by morphological abnormality of the mitochondria in the proximal tubular cells after three hours of reperfusion, when the kidney function was slightly declined. In 5/6 nephrectomy and rats with anti-glomerular basement membrane glomerulonephritis, the kidney uptake of 18F-BCPP-BF cumulatively decreased with impairment of the kidney function, which was accompanied by a reduction of mitochondrial protein and a pathological tubulointerstitial exacerbation rather than glomerular injury. The 18F-BCPP-BF uptake in the injured kidney was suggested to represent the volume of healthy tubular epithelial cells with normally functioning mitochondria. Thus, this positron emission tomography probe can be a powerful tool for studying the pathophysiological meanings of the mitochondrial status in kidney disease.