Angiotensin -converting enzyme 2 (ACE2) is a regulator of the renin angiotensin system involved in acute lung failure, cardiovascular functions and severe acute respiratory syndrome (SARS) infections in mammals1,2,3. A gene encoding a homologue to ACE2, termed collectrin (Tmem27), has been identified in immediate proximity to the ace2 locus4. The in vivo function of collectrin was unclear. Here we report that targeted disruption of collectrin in mice results in a severe defect in renal amino acid uptake owing to downregulation of apical amino acid transporters in the kidney. Collectrin associates with multiple apical transporters and defines a novel group of renal amino acid transporters. Expression of collectrin in Xenopus oocytes and Madin–Darby canine kidney (MDCK) cells enhances amino acid transport by the transporter B0AT1. These data identify collectrin as a key regulator of renal amino acid uptake.
ACE2 is a chimaeric protein that has emerged from the duplication of two genes: homology with ACE at the catalytic domain and homology with collectrin in the membrane proximal domain (Fig. 1a). In the mouse, collectrin is expressed in the kidneys4 and pancreas5,6 and to a lesser extent in the intestine (duodenum, jejunum and ileum), liver, heart and stomach (Supplementary Fig. 1). To assess the in vivo function of collectrin, we generated collectrin knockout mice (Fig. 1b). Mice were crossed with the transgenic Cre deleter strain to ubiquitously delete collectrin7 (Supplementary Fig. 2a). The null mutation of collectrin was verified by the absence of collectrin messenger RNA transcripts and protein (Supplementary Fig. 2b, c). ACE2 expression in the kidneys was not altered in collectrin mutant mice (Supplementary Fig. 2c). Collectrin null mice were born at the expected mendelian frequency, indistinguishable from their heterozygous and wild-type littermates. Collectrin mutant males (collectrin-/y) and females (collectrin-/-) were fertile and we failed to observe any overt morphological alterations in all organs analysed, including kidney, for up to six months of age (Fig. 1c).
Collectrin was originally reported to be expressed in collecting ducts4 and β-islet cells of the pancreas5,6. We therefore assessed whether collectrin is involved in renal salt and/or glucose balance. Na+ excretion in urine was comparable in collectrin+/y and collectrin-/y mice. Loss of collectrin also did not affect the levels of calcium, potassium, phosphate, chloride, urea, uric acid or creatinine in urine and blood (Supplementary Table 1). collectrin mutant mice had the same glucose concentration as their wild-type littermates in serum and urine. Surprisingly, storage of the urine samples collected from mutant mice at 4 °C resulted in the formation of large white precipitates (Fig. 1d). Morphological analysis revealed large needle-like crystals in the urine of the collectrin null mice whereas such crystals were never observed in wild-type mice (Fig. 1e). Biochemically, these crystals were not composed of common urinary crystal components such as ammonium urate, sodium urate, calcium oxalate dihydrate or calcium phosphate. High performance liquid chromatography (HPLC) analyses showed that these crystals contained ∼10% phenylalanine (Phe) and 90% tyrosine (Tyr) (see Fig. 1f). When animals were fed a tyrosine-free and phenylalanine-free diet or when the animals were fasting, no crystals were formed in the urine of collectrin null mice (Fig. 1d), indicating that the observed changes are not caused by tyrosinaemia8,9. Thus, loss of collectrin expression results in the formation of tyrosine/phenylalanine crystals in the urine.
To determine whether collectrin deficiency caused a more general defect in amino acid handling in the kidneys, we assessed urinary amino acid contents. Whereas in wild-type mice almost all amino acids are reabsorbed from the urine, deletion of collectrin resulted in severe leakage of all amino acids (Fig. 1g). Concentrations of the amino acids were also lower in the serum of mutant mice, although amino acids were still present (Supplementary Fig. 3). Urinary output was higher in collectrin-/y mice (3.31 ± 0.25 ml urine per 24 h) as compared to collectrin+/y littermates (1.65 ± 0.28 ml urine per 24 h) which may be owing to the higher concentration of osmotically active solutes in urine from collectrin-deficient mice. Water restriction for 24 h demonstrated that collectrin-deficient mice were not able to concentrate urine to the same degree, confirming that the higher urinary output is due to a primarily renal defect (Supplementary Fig. 4). Thus, collectrin is critical for normal amino acid reabsorption in the kidney.
Amino acid reabsorption occurs in the proximal tubules of the kidney10. Immunohistochemical staining showed that collectrin is indeed expressed at the luminal side of brush border membranes in proximal tubules (Fig. 2a), which is a specialized microenvironment involved in amino acid reabsorption11. Biochemically, we confirmed specific expression of collectrin in brush border membrane vesicles (BBMV) isolated from the kidney of collectrin+/y, but not collectrin-/y mice (Fig. 2b). BBMV isolates were free from contamination (assessed by the absence of the collecting duct marker aquaporin 2; Fig. 2b). In addition, collectrin mRNA was found to be highly expressed in proximal tubules (Fig. 2c). Thus, collectrin is localized in brush border membranes of proximal tubules.
Because collectrin is predicted to be a type I transmembrane protein having only one transmembrane helix4, it was unlikely that collectrin itself forms an amino acid transport pore. Because collectrin expression parallels that of the B0 cluster of the solute carrier SLC6 proteins B0AT1, XT3s1/SIT1, XT2 and XT3, a recently identified subfamily of Na+-dependent transporters of neutral amino acids12,13,14,15,16,17, we assessed whether loss of collectrin may affect these amino acid transporters. Immunoblots of BBMVs revealed that the neutral amino acid transporter B0AT1 was markedly downregulated in collectrin-/y mice (Fig. 2d and Supplementary Fig. 5a, b). Protein expression of the three other members of the SLC6 family XT3s1/SIT1, XT2 and XT3, and expression of EAAC1, the apical transporter responsible for glutamate/aspartate reabsorption18, was also markedly downregulated (Fig. 2e). By contrast, expression of b0,+AT (also known as slc7a9) (Fig. 2e), the exchanger mediating apical uptake of cystine, arginine, lysine and ornithine19, and expression of the basolateral amino acid transporter subunits 4F2hc and LAT2 (ref. 10) remained unchanged (Supplementary Fig. 5b). Expression of the sodium/phosphate co-transporter Na+/Pi IIa was also not affected in collectrin null mice (Fig. 2d). mRNA expression of all these transporters was comparable in collectrin+/y and collectrin-/y mice (Supplementary Fig. 6). Immunoblotting of total kidney membranes demonstrated that the abundance of these amino acid transporters was also decreased to a similar extent as observed in BBMVs (Supplementary Fig. 5b).
To test whether reduced expression resulted in decreased amino acid transport activity, we isolated BBMVs and measured Na+-dependent amino acid uptake rates. As expected from the urinary loss of B0AT1 substrates, the uptake rate of glutamine and phenylalanine in BBMV from kidneys of collectrin null mice was decreased compared to control littermates, whereas uptake of phosphate was not affected (Fig. 2f). These data show that loss of collectrin is associated with reduced expression and impaired activity of apical, Na+-dependent amino acid transporters in the kidney.
To identify the molecular pathway by which collectrin regulates amino acid transporters, we first examined whether collectrin can associate with B0AT1 and/or other transporters. Immunoprecipitations from BBMV lysates showed direct binding between collectrin and B0AT1, XT2 and XT3 in collectrin+/y, but not collectrin-/y mice (Fig. 3a). Moreover, collectrin co-localized with B0AT1 in the early proximal S1 tubule (Fig. 3b). As a control, collectrin did not associate with b0,+AT (Fig. 3a). The specificity of the collectrin band was confirmed by pre-incubation with the antigenic peptide (Supplementary Fig. 7a, b). Renal amino acid transporters such as LAT2 or y+LAT1 form functional heterodimers with a second subunit, 4F2hc (ref. 10). In all known instances, this heterodimerization is stabilized by disulphide bonds20. Under reducing conditions (+DTT), collectrin appears as a single band with gel mobility of ∼44 kDa, whereas B0AT1 can be detected as a single band that corresponds to a monomeric form of B0AT1 (Fig. 3c). Unlike 4F2hc and its catalytic subunits, B0AT1 and collectrin migrate under non-reducing (no dithiothreitol, -DTT) conditions with the same mobility as under reducing conditions (Fig. 3c), indicating that the association between collectrin and B0AT1 as well as other transporters occurs through non-covalent interactions. Thus, collectrin co-localizes and specifically associates with a subgroup of renal amino acid transporters.
To define the molecular mechanism of collectrin-regulated amino acid transport, we examined the effect of collectrin on B0AT1 function using Xenopus laevis oocytes and polarized MDCK cells. Two days of expression of B0AT1 alone in Xenopus oocytes conferred only a low transport activity for the B0AT1 substrate l-isoleucine16 (Fig. 4a). Collectrin alone did not exhibit any transport activity. However, co-expression of B0AT1 with collectrin resulted in a striking increase in l-isoleucine uptake (Fig. 4a). As a control, co-expression of collectrin with the Na+/phosphate IIa co-transporter in oocytes did not affect the rates and kinetics of phosphate uptake (Fig. 4b). To assess whether collectrin changes binding of l-isoleucine to its transporter B0AT1, we measured the apparent affinity (Km) for l-isoleucine uptake (Fig. 4c). Co-transport of l-isoleucine with Na+ generated a saturable, reversible inward current with an apparent Km for l-isoleucine of 0.99 ± 0.17 mM when B0AT1 was expressed alone, and of 0.78 ± 0.10 mM when B0AT1 was co-expressed with collectrin. Thus, the collectrin-induced increase in l-isoleucine transport is not due to a change in apparent affinity, but due to an increase in the maximal transport rate of B0AT1, probably as a result of increased surface expression. Indeed, expression of collectrin allowed functional surface expression of B0AT1 and apical uptake of l-isoleucine in polarized kidney MDCK cells (Fig. 4d). Thus, collectrin enhances functional surface expression of the SLC6 family transporter B0AT1.
We conclude that genetic inactivation of collectrin in mice results in a major defect in renal amino acid reabsorption. Collectrin controls protein expression and function of apical amino acid transporters in the brush border membranes of proximal tubules. Mechanistically, collectrin associates with several amino acid transporters, enhances their surface expression, and thereby increases amino acid transport function. These data identify the ACE2 homologue collectrin as a regulator of renal amino acid uptake and may provide a molecular explanation for aminoaciduria in Hartnup disease, Fanconi syndrome, or diabetes.
collectrin knockout mice
A gene-targeting vector containing three loxP sites flanking exon 2 and a Neo selection cassette was constructed. Chimaeric progeny were crossed to the C57BL/6J Cre deleter line7 to generate complete null animals.
mRNA and protein analyses
Real-time polymerase chain reaction (PCR) was performed as described12. For protein analyses, total cell lysates and BBMVs of adult mouse kidney were prepared21 and probed with antibodies to mouse b0,+AT, NaPi IIa, aquaporin 2, collectrin, XT2, XT3, SIT1, B0AT1, ACE2, LAT2 and 4F2. For immunoprecipitations, brush border membrane proteins were probed with antibodies to B0AT1, XT2, XT3 and b0,+AT.
Urine and plasma analyses
Urine was collected over 24 h in a metabolic cage. Urinary crystals were observed under a magnification of ×1,000. For HPLC analysis19, crystals were dissolved in 0.1 M HCl. Na+, K+ and Cl- were assayed by indirect potentiometry. Phosphate was determined using the phosphomolybdate method. Calcium levels were obtained by cresolphthalein. Glucose, urea and uric acid were measured using Reflovet (Roche).
Histology and immunohistochemistry
Paraffin sections were stained with haematoxylin and eosin (H&E). Immunohistochemistry was performed with an affinity purified anti-collectrin antibody. B0AT1 was detected using a rabbit anti-mouse B0AT1 antibody.
Amino acid transporter activities
Uptake of 3H-Phe, 14C-Gln and 32PO4 was determined in BBMVs at 60 s. The final amino acid concentration was 1 mM. Measurements were done in parallel in the presence of either Na+ or K+. Expression studies and uptake measurements in Xenopus laevis oocytes and kidney MDCK cells were performed as described22,23.
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We thank M. J. Crackower, A. Leibbrandt, Y. Imai, K. Kuba and many others for their contributions. Supported by grants from The National Bank of Austria, The Austrian Ministry of Science and Education, IMBA, an EU Marie Curie Excellence grant, and EUGeneHeart to J.M.P., from the Swiss National Science Foundation to F.V. and EUGINDAT to F.V. and C.A.W.
Author Contributions U.D, R.S., C.R., C.B., G.S., A.R., S.A., J.A.P., D.S., S.M.R.C., V.M., T.R. and C.A.W. performed experiments. U.D., F.V., C.A.W. and J.M.P. planned experiments and wrote the manuscript.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Danilczyk, U., Sarao, R., Remy, C. et al. Essential role for collectrin in renal amino acid transport. Nature 444, 1088–1091 (2006). https://doi.org/10.1038/nature05475
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