Kidney Albumin handling and reabsorption

• Albumin filters through the glomerulus, resulting in roughly 3.3 g filtered daily in human kidneys with a sieving coefficient of 0.00062.
• The proximal convoluted tubule reabsorbs 71% of filtered albumin, the loop of Henle and distal tubule 23%, and the collecting duct 3%.
• Albumin reabsorption dysfunction in proximal tubules, because of reduced megalin expression, can cause microalbuminuria in early-stage diabetes.
• Disorders of the glomerular filtration barrier, like podocyte detachment, glomerular basement membrane rupture, and slit diaphragm dysfunction in certain conditions cause massive nonselective proteinura.
• Selective albuminuria, coupled with foot process effacement tightly altered slits, occurs in people with minimal-change nephrotic syndrome.
• Albumin absorption increases in the podocyte cell body, possibly through albumin receptors in the low-dose puromycin model.
• The role of podocyte albumin transport must be investigated to understand the mechanism of selective albuminuria in minimal-change disease.

Introduction

• Kidneys maintain body fluid homeostasis via regulation of water, electrolyte, and acid-base balance, excreting uremic toxins, and producing hormones like vitamin D3, erythropoietin, and renin.
• Protein metabolism by the kidney has gone largely unnoticed.
• Renal proximal tubules actively reabsorb proteins smaller than albumin from the primitive urine filtered by the glomerulus; the proteins are broken down into amino acids in lysosomes before being returned to the circulation.
• The study focuses on glomerular albumin filtration, amount of tubular reabsorption, and relevant mechanisms and pathways in normal and diseases.
• Glomerular albumin filtration is thought to be restricted by the size and charge barriers of the glomerular basement membrane and the fine pores of the slit diaphragm.
• Glomerular albumin filtration may occur via diffusion back and forth across the GBM
• It is not known how albumin travels across podocyte foot processes covering the basement membrane in minimal-change nephrotic syndrome.

Albumin Filtration Under Normal Conditions

• Albumin comprises three domains, possessing a molecular weight of 69 kDa and a charge of -15.
• It has an ellipsoid shape, measuring 3.8 nm in diameter and 15 nm long.
• The glomerular slit-pores are 35 Å in diameter, with some variation in size.
• The Einstein radius of albumin is 35 Å, although some albumin molecules pass through the slit pores because of flexibility and ellipsoid shape
• Early micropuncture studies found concentrations of albumin from 3 to 728 µg/mL in primitive urine in Bowman's capsule in normal rats.
• This variation was a result of serum albumin in the peritubular capillaries during collection of tubular fluid.
• Renal tubules are initially punctured with an outer pipette, then four fractions of tubular fluid are collected using an inner pipette to avoid serum contamination.
• The value of albumin in Bowman's capsule fluid is 22.9 µg/mL, and the albumin-sieving coefficient is 0.00062 using this method.
• Isotope-labeled albumin clearance studies, which measured urinary excretion and tubular uptake, divided by the plasma isotope level, showed consistent values with fractional micropuncture data. – Albumin clearance tests following proximal tubular reabsorption blockade with L-lysine, low temperatures, or investigation of tubular reabsorption congenital abnormalities displayed values less than the micropuncture data.
• The excretion of albumin in Fanconi syndrome is 0.00008, roughly equal to the glomerular-sieving coefficient in the normal kidney.
• Nephron segments downstream from the proximal convoluted tubules may reabsorb 26% of filtered albumin, even with impaired proximal tubular albumin reabsorption in Fanconi syndrome.
• The glomerular albumin-sieving coefficient may be greater than 0.00011 in humans.
• A tritium-labeled albumin study found a sieving coefficient of 0.074, while a confocal microscopy study using Alexa-labeled albumin yielded a sieving coefficient of 0.0341.
• Out-of-focus fluorescence, sensitivity of the measurement methods, and removal of unbound labeling molecules are key aspects of limitation to larger estimates

Role of the Kidney in Protein Metabolism

• In rat nephrons, renal tubules absorb about 3 g of albumin daily in humans.
• Human kidneys reabsorb 1.9g of albumin a day, with a proximal tubule length of 6.5 mm and albumin reabsorption capacity of 99.9 × 10-3 ng/min/mm in the isolated rabbit proximal tubule [11].
• Nephron segments downstream from the proximal convoluted tubules reabsorb around 26% of filtered albumin, therefore total albumin reabsorption is at 2.6 g a day.
• Albumin molecules are taken up into lysosomes in the proximal tubule within 6–15 minutes, then degraded to amino acids after 30–120 minutes.
• Low molecular weight proteins are commonly filtered at the glomerulus with a sieving coefficient of 0.987, and 9.6 g reabsorbed daily.
• Hemodialysis does not compensate for protein metabolism tubular dysfunction in chronic renal failure, deposits of low molecular weight protein such as, B2-microglobulin, occur in organs, causing amyloidosis.

Microalbuminuria in Diabetic Nephropathy.

• Microalbuminuria, an early diabetes nephropathy marker, is believed to occur due to increased glomerular permeability and hyperfiltration.
• Proximal tubular albumin reabsorption was shown to decrease in the early stages of streptozotocin-induced diabetic nephropathy.
• Megalin, the receptor for albumin endocytosis in the proximal tubules, declines in diabetes, supporting proximal tubular disfunction
• Measuring albumin clearance after the lysine blockade of proximal reabsorption, utilizing isotope labeled-bovine serum albumin, also supports proximal tubule disfunction
• Albumin degrades to fragmented albumin by brush border enzymes in the proximal tubules, therefore the real value of albuminuria is possibly greater than what is detected in the urine
• RAS inhibitors can restore megalin expression, improve tubular albumin reabsorption function, and reduce albuminuria.
• Effective albumin reabsorption requires ligand-receptor dissociation and vesicular acidification via H+-ATPase, CLC-5, and NHE-3
• Renal tissue angiotensin II levels are elevated in diabetes.
• The endosomes acidification may decline from H+-ATPase inhibition lowering albumin reabsorption from renal angiotensin II.
• RAS inhibitors restore albumin metabolism in the proximal tubules as well as prevent intraglomerular hypertension disrupting glomerular permselectivity

Glomerular Albumin Filtration through Slit Pores in the Nephrotic Syndrome

• The filtration barrier consists of of fenestrated endothelium, glomerular basement membrane and slit diaphragm
• Fenestrated endothelium is covered by a negatively charged glycocalyx
• Glomerular size barrier is made up of laminin and type IV collagen; heparan sulfate charge barrier function as coarse barriers
• The slit diaphragm between foot processes; acts as a fine filter.
• The GBM or the slit diaphragm have been a key element in filtration barrier restriction controversy

Various Mechanisms of Proteinuria in Glomerular Diseases

• Microscopic images help to identify specific lesions that cause proteinuria in glomerular diseases.
• Glomerulonephritis cause GBM damage via inflammatory cells, resulting in nonselective proteinuria and hematuria.
• Podocyte detachment and apoptosis are seen in glomerular glomerulosclerosis and nephropathy with proteins leaking which leads to sclerosis.
• Podocyte detachment or apoptosis occurs by hemodynamic mechanisms and oxidative stress
• The mathematical model of permselectivity includes ultra structural GBM morphology as a assumed shunt pathway

Albuminuria in Minimal-Change Nephrotic Syndrome

• Reduced nephrin expression is normally the reason for protein leaks in the body
• Podocyte detachment is rare
• Podocyte slit pore density is at 80% decreased with 50% half of the sites having tight junctions, the slit diaphragm
• These occurrences may be affected by proteinuria and nephrin deficiencies which can lead to expanded slit pores.
• Albumins may be transferred by using endocytosis with exocytosis or an EB labeled protein that can produce greater glomerular filtration because of the glomerular system receptor known as FcRn, this filtration is a receptor mediated transcytosis that is key to the question of why "Clogging" does not occur.

Conclusion

• This paper determined the value of kidney function as 0.00062 as well as the fact that the protein deficiencies via megalin in patients with diabetic nephropathy
• The nephrin is key to discovering pore like functions in glomerular filtration. In selective nephrotic syndrome, proteinura may result from receptor-mediated mechanisms in glomerular filtration and mechanisms for treating nephrotic syndrome