Kinetic Measurements of Di- and Tripeptide and Peptidomimetic Drug Transport in Different Kidney Regions Using the Fluorescent Membrane Potential-Sensitive Dye, DiS-C3-(3)

Knock-out of dipeptidase CN2 in human proximal tubular cells disrupts dipeptide and amino acid homeostasis and para- and transcellular solute transport

AIM

Although of potential biomedical relevance, dipeptide metabolism has hardly been studied. We found the dipeptidase carnosinase-2 (CN2) to be abundant in human proximal tubules, which regulate water and solute homeostasis. We therefore hypothesized, that CN2 has a key metabolic role, impacting proximal tubular transport function.

METHODS

A knockout of the CN2 gene (CNDP2-KO) was generated in human proximal tubule cells and characterized by metabolomics, RNA-seq analysis, paracellular permeability analysis and ion transport.

RESULTS

CNDP2-KO in human proximal tubule cells resulted in the accumulation of cellular dipeptides, reduction of amino acids and imbalance of related metabolic pathways, and of energy supply. RNA-seq analyses indicated altered protein metabolism and ion transport. Detailed functional studies demonstrated lower CNDP2-KO cell viability and proliferation, and altered ion and macromolecule transport via trans- and paracellular pathways. Regulatory and transport protein abundance was disturbed, either as a consequence of the metabolic imbalance or the resulting functional disequilibrium.

CONCLUSION

CN2 function has a major impact on intracellular amino acid and dipeptide metabolism and is essential for key metabolic and regulatory functions of proximal tubular cells. These findings deserve in vivo analysis of the relevance of CN2 for nephron function and regulation of body homeostasis.

Hypertension alters the function and expression profile of the peptide cotransporters PEPT1 and PEPT2 in the rodent renal proximal tubule

Hypertension is a major risk factor for kidney and cardiovascular disease. The treatment of hypertensive individuals by selected ACE inhibitors and certain di-and tripeptides halts the progression of renal deterioration and extends life-span. Renal reabsorption of these low molecular weight substrates are mediated by the PEPT1 and PEPT2 cotransporters. This study aims to investigate whether hypertension and ageing affects renal PEPT cotransporters at gene, protein expression and distribution as well as function in the superficial cortex and the outer medulla of the kidney. Membrane vesicles from the brush border (BBMV) and outer medulla (OMMV) were isolated from the kidneys of young Wistar Kyoto (Y-WKY), young spontaneously hypertensive (Y-SHR), and middle aged SHR (M-SHR) rats. Transport activity was measured using the substrate, β-Ala-Lys (AMCA). Gene expression levels of PEPT genes were assessed with qRT-PCR while renal localisation of PEPT cotransporters was examined by immunohistochemistry with Western Blot validation. The K_m and V_max of renal PEPT1 were decreased significantly in SHR compared to WKY BBMV, whilst the Vmax of PEPT2 showed differences between SHR and WKY. By contrast to the reported cortical distribution of PEPT1, PEPT1-staining was detected in the outer medulla, whilst PEPT2 was expressed primarily in the cortex of all SHR; PEPT1 was significantly upregulated in the cortex of Y-SHR. These outcomes are indicative of a redistribution of PEPT1 and PEPT2 in the kidney proximal tubule under hypertensive conditions that has potential repercussions for nutrient handling and the therapeutic use of ACE inhibitors in hypertensive individuals.

An assessment of the transport mechanism and intraneuronal stability of L-carnosine

L-Carnosine (β-alanyl-L-histidine) is a well-known antioxidant and neuroprotector in various models on animals and cell cultures. However, while there is a plethora of data demonstrating its efficiency as a neuroprotector, there is a distinct lack of data regarding the mechanism of its take up by neurons. According to literature, cultures of rat astrocytes, SKPT cells and rat choroid plexus epithelial cells take up carnosine via the H⁺-coupled PEPT2 membrane transporter. We've assessed the effectiveness and mechanism of carnosine transport, and its stability in primary rat cortical culture neurons. We demonstrated that neurons take up carnosine via active transport with Km = 119 μM and a maximum velocity of 0.289 nmol/mg (prot)/min. Passive transport speed constituted 0.21∙10^-4 nmol/mg (prot)/min (with 119 μM concentration in the medium)-significantly less than active transport speed. However, carnosine concentrations over 12.5 mM led to passive transport speed becoming greater than active transport speed. Using PEPT2 inhibitor zofenopril, we demonstrated that PEPT2-dependent transport is one of the main modes of carnosine take up by neurons. Our experiments demonstrated that incubation with carnosine does not affect PEPT2 amount present in culture. At the same time, after removing carnosine from the medium, its elimination speed by culture cells reached 0.035 nmol/mg (prot)/min, which led to a decrease in carnosine quantity to control levels in culture within 1 h. Thus, carnosine is taken up by neurons with an effectiveness comparable to that of other PEPT2 substrates, but its elimination rate suggests that for effective use as a neuroprotector it's necessary to either maintain a high concentration in brain tissue, or increase the effectiveness of glial cell synthesis of endogenous carnosine and its shuttling into neurons, or use more stable chemical modifications of carnosine.