Renal Tubular Transport and Catabolism of Proteins and Peptides

Kidney International, Vol. 16 (1979), pp. 271 -2 78 Renal tubular transport and catabolism of proteins and peptides FRANK A. CARONE, DARRYL R. PETERSON, SUZANNE OPARIL, and THEODORE N.PULLMAN Departments of Pathology, Physiology and Medicine, Northwestern University Medical School and Veterans Administration Lakeside Hospital, Chicago, Illinois, and Department of Medicine, University of Alabama, Birmingham, Alabama It is established that the kidney plays an impor- are not transported into tubular cells across the tant role in the metabolism of a number of protein, peritubular cell membrane, presumably due to the polypeptide, and small peptide molecules, including absence of an endocytic mechanism on this side of plasma proteins, growth hormone, L-chains of im- the tubular epithelium. munoglobulins, /32-microglobulin, lysozyme, in- Luminal uptake. Many morphologic, micro- sulin, proinsulin, parathyroid hormone, glucagon, puncture, microinfusion, and microperfusion stud- and small vasoactive peptides. Absorption, trans- ies have demonstrated that a wide variety of pro- port, and/or degradation of proteins or peptides are teins are absorbed across the luminal aspect of functions of the proximal tubule; there is little evi- proximal tubular cells [1, 2]. We quantified the up- dence that other segments of the nephron have the take of '251-labeled rabbit serum albumin micro- mechanisms for uptake or transport of these sub- perfused into isolated segments of the rabbit neph- stances. Indirect and direct studies indicate that a ron [3]. Proximal convoluted and proximal straight variety of proteins and polypeptides filtered at the segments accumulated 1251-albumin nearly linearly glomerulus are absorbed by the proximal tubule by as a function of time (Fig. 1), whereas cortical col- luminal endocytosis and hydrolyzed by lysosomal lecting segments did not accumulate measurable enzymes. Our recent studies suggest that small lin- amounts of protein. The rate of accumulation of al- ear peptides, consisting of eight to ten amino acids, bumin in the proximal convoluted tubule was 3.2 x are handled by the proximal tubule by a different 10-2 ng/mm/min, which was 2.6 times as great as mechanism. We have demonstrated that small lin- that in the proximal straight tubule. Assuming an ear peptides microinfused into proximal tubules are albumin concentration of 0.3 mg/dl in normal gb- hydrolyzed at the luminal surface of the brush bor- merular filtrate [4], we can calculate that the reab- der, which is rich in a variety of enzymes, by the sorptive capacity of the entire proximal tubule for process of membrane or contact digestion with albumin exceeds the amount of albumin filtered by reabsorption of most of the breakdown products. the normal glomerulus. The ultrastructural basis for reabsorption of la- Proximal tubular handling of proteins and large peptides beled albumin by proximal tubules was investigated Qualitative morphologic and indirect functional autoradiographically by Maunsbach with in vivo studies have demonstrated that proximal tubular microinfusion methods in the rat kidney [5] and in cells absorb protein from the luminal fluid and have our laboratory in isolated rabbit tubules micro- suggested a pathway by which protein undergoes perfused with '251-albumin [3]. Sequential studies intracellular digestion. These processes have been revealed that silver grains were initially located quantified directly. Although some findings suggest that certain proteins are transported intact across proximal epithelial cells, other investigations do not Received for publication March 13,1979 support this conclusion. Direct studies on isolated 0085-2538/7910016-0271$01.60 tubular segments indicate that albumin and insulin ©1979 by the International Society ofNephrology 271 272 Caroneet al PCT I E C, (0 PST C E1. -C CCL 40 80 120 160 200 Durationof perfusion, mlvi Fig.1.Accumulationof iodinated albumin as a function of time in proximal convoluted (PCT) proximal straight (PST) and cortical collecting (CCT) segments of the rabbit nephron. All tubules were perfused with a 21-mg/dl solution of iodinated rabbit albumin at 18 to 20 nI/mm at 370 C. (Reprinted with permission of J Cell Biol [3]) - -'ç •A Fig. 3. Electron microscope radioautograph with radioactive la- bel in dense bodies (arrows) in cells of the proximal tubule after 85 mm's perfusion with "51-albumin. (X15,200) (Reprintedwith S permission off Cell Biol [3]) over tubular invaginations at the base of the brush border and over small apical vesicles and later in larger membrane-bound apical vacuoles (Fig. 2). Fi- nally, grains becoming concentrated in cytoplasmic dense bodies (Fig. 3) were associated with acid phosphatase positive bodies, indicative of lysosomes. Similar ultrastructural findings were ob- served with labeled insulin in our laboratory [6] and with a number of other proteins in several experi- mental animals [1, 2]. There is evidence that the first step in endocytosis involves binding of protein to the luminal plasma membrane [2]. There is wide variation in the affinity of different proteins for the plasma membrane, which may be related to the number and chemical structure of membrane binding sites and '4- to the net charge on the protein molecules. Small amounts of certain proteins bind largely to the plasma membrane, whereas large amounts in tubu- Fig. 2. Electron microscopic radioautograph of a proximal tu- lar fluid appear mainly in endocytic vesicles un- bule perfused 10 mm with "51-albumin. Grains are located at the base of the brush border (small arrow), in small apical vesicles bound to the plasma membrane [2]. Thus, endo- (medium arrow), and in apical vacuoles (large arrow). (X26,510) cytosis may be largely specific when small quan- (Reprinted with permission of f Cell Biol [3]) tities of protein are reabsorbed due to membrane Tubular handling of proteins and pept ides 273 binding and less specific when large amounts of pro- 4.0 tein are reabsorbed. Several studies suggest that the membrane of apical invaginations and vesicles in 3.0 the endocytic process is replaced by a de novo syn- oxC c::i thesis of plasma membrane [7, 8] and not by down- contraIumnaI ward flow of brush border membrane [9]. Most evi- 2.0 dence favors the conclusion that protein is trans- ferred to lysosomes by fusion of endocytic vacuoles and preexisting lysosomes. 111.0 Lysosomes contain many hydrolytic enzymes which have been shown to digest a wide variety of 0 proteins. Lysosomal extracts isolated from renal PCT PST TAL CCT cortical homogenates have been used to quantify Fig.4. Comparison of luminal and contraluminal accumulation hydrolysis of albumin and other proteins. Hydroly- rates of albumin -in isolated renal tubules. Luminal uptake data is taken from Fig. I above. (Reprinted with permission of Am J sis of '251-albumin is maximal at a low pH, and the Physiol [16]) major labeled product of digestion is monoiodotryo- sine [10]. Other in vitro studies have demonstrated hydrolysis of absorbed protein within isolated intact 700 lysosomes [11] or within lysosomes of intact cells in kidney slice preparations [12]. Small-molecular- 600 weight metabolites of proteins diffuse out of lysosomes into the cell cytoplasm and interstitial fluid. 500 There is no evidence for nonendocytic reabsorption aC of protein on the luminal side of proximal tubular 400 cells or for release of intact reabsorbed protein from a lysosomes on the contraluminal side of the cells. .0300 Contraluminal uptake of protein. Although the C uptake of proteins from the luminal aspect of the 200 proximal tubules is well established, uptake from the contraluminal aspect is uncertain. Studies in intact kidneys, however, provide evidence for con- 100 traluminal uptake of certain proteins such as f32-mi- 0— croglobulin [13] and insulin [14]. Because the base- Luminal Contraluminal ment membranes of isolated perfused tubules are Fig.5. Luminal and contraluminal 1251-insulin accumulation by moderately permeable to albumin, as demonstrated isolated perfused proximal convoluted tubules. (Reprinted with by Welling and Grantham [15], it is possible that in- permission of Am J Physiol [6]) terstitial protein is in contact with the basilar membranes of tubular cells. We studied the con- due to the absence of a prominant endocytic mecha- traluminal uptake of albumin directly in isolated nism or the lack of specific receptor sites on the bas- perfused segments of proximal thick ascending limb ilar side of tubular cells. and cortical collecting tubules of the rabbit in- Intercellular and transcellular transport of intact cubated in '251-albumin [16]. After 2 to 90 mi con- proteins. In normal mature kidneys, intercellular traluminal uptake of albumin was negligible in all transport of proteins has not been demonstrated. In segments compared to luminal uptake in the proxi- the immature kidney [17] and in certain abnormal mal tubules (Fig. 4). In a companion study [6], we states, such as elevated tubular hydrostatic pres- also found that contraluminal uptake of '251-insulin sure [18], proteins may pass between cells through was negligible compared to luminal uptake (Fig. 5). cellular junctional complexes. On the other hand, It is known that albumin and insulin accumulate transcellular transport of intact protein by renal tu- within proximal tubular cells from the luminal side bules remains a controversial topic. A number of where endocytosis is prominent, but not in cells of studies provide evidence both for and against a collecting tubules, where endocytosis is not pro- mechanism for the transcellular transport of intact nounced. Absence of significant contraluminal up- proteins [2]. This problem may be resolved by di- take of albumin, or insulin, or both is presumably rect studies that quantify and characterize protein 274 Carone et a! and protein catabolites absorbed and released by peptide was microinfused into proximal nephrons tubular cells. with excess unlabeled L-lle, urinary recovery of 14C greatly exceeded that seen with 14C-AII alone and Renal handling of small peptides increased directly with distance of the infusion site The kidney has been shown to degrade circulat- from the glomerulus (Fig.

Load more