Urinary protein composition is jointly determined by plasma filtration, tubular reabsorption, active secretion by the nephron, and protein release from injured epithelial cells. Different proteins correspond to distinct nephron segments and pathological processes and can therefore be used to identify abnormalities in glomerular filtration, impaired proximal tubular reabsorption, reduced thick ascending limb function, and tubular epithelial cell injury.

Keywords: urinary proteins; renal tubular injury; UMOD; KIM-1; NGAL; cystatin C; acute kidney injury; chronic kidney disease

1 Basic Concepts of Renal Tubule-Associated Urinary Proteins

1.1 Major Sources of Urinary Proteins

Urinary proteins arise mainly through four pathways: plasma proteins enter the primary urine through glomerular filtration; filtered low-molecular-weight proteins escape complete reabsorption by the proximal tubule; nephron epithelial cells actively synthesize and secrete proteins into the tubular lumen; and injured cells increase urinary protein levels through membrane ectodomain shedding, extracellular vesicle release, or plasma membrane rupture. Proteins originating from urinary tract epithelial cells, immune cells, and reproductive system secretions may also affect the protein composition of final urine.

1.2 Glomerular Filtration, Tubular Reabsorption, and Active Secretion

The glomerular filtration barrier consists of fenestrated endothelial cells, the glomerular basement membrane, and the podocyte slit diaphragm. It restricts the entry of plasma proteins into the primary urine according to molecular size, charge, and structure. Low-molecular-weight proteins such as cystatin C and β2-microglobulin can be filtered and are subsequently reabsorbed and degraded in lysosomes of proximal tubular cells, primarily through the megalin–cubilin receptor system. In contrast, proteins such as UMOD and EGF are synthesized by specific tubular epithelial cells, and their urinary levels mainly depend on the number, differentiation state, and secretory capacity of these cells.

1.3 Basic Patterns of Urinary Protein Changes

An increase in urinary protein does not necessarily indicate a single type of injury. Increased filtration barrier permeability, failure of proximal tubular reabsorption, injury-induced protein expression, and plasma membrane rupture can all increase urinary protein levels. Conversely, loss of functional nephrons, depletion of secretory cells, or impaired protein transport may reduce urinary protein levels. Before interpreting a test result, the source of the protein and the route by which it enters the urine should first be established.

Table 1. Major mechanisms underlying changes in urinary proteins

2 Major Renal Tubule-Associated Urinary Proteins

2.1 UMOD: A Secreted Protein Derived from the Thick Ascending Limb

Uromodulin (UMOD), also known as Tamm–Horsfall protein, is produced primarily by epithelial cells in the thick ascending limb of the loop of Henle and the early distal convoluted tubule. UMOD undergoes folding and glycosylation in the endoplasmic reticulum and Golgi apparatus. It is subsequently localized to the apical plasma membrane through a glycosylphosphatidylinositol anchor, cleaved by hepsin, released into the urine, and assembled into filamentous polymers.UMOD participates in NKCC2- and ROMK-associated ion transport, urine concentration, entrapment of urinary pathogens, and regulation of crystal aggregation. Its urinary level may also reflect the number of functional epithelial cells in the thick ascending limb.

2.2 KIM-1: An Injury-Induced Proximal Tubular Protein

Kidney injury molecule-1 (KIM-1), encoded by the HAVCR1 gene, is a type I transmembrane glycoprotein. KIM-1 expression is low in normal kidney tissue but is markedly upregulated in proximal tubular epithelial cells following ischemic injury, toxic exposure, or inflammatory damage.Its extracellular domain can be cleaved by metalloproteases and released into the urine, making urinary KIM-1 a widely used marker of proximal tubular injury. Cell-surface KIM-1 can also participate in the recognition of apoptotic cells and cellular debris, giving it characteristics of both an injury biomarker and a regulator of tissue repair.

2.3 NGAL: A Protein Associated with Acute Injury and Inflammation

Neutrophil gelatinase-associated lipocalin (NGAL), also known as LCN2, can be rapidly upregulated following tubular stress. Urinary NGAL may increase before changes in serum creatinine become detectable, making it suitable for monitoring the early stages of acute injury.NGAL can also be produced by neutrophils, the liver, the lungs, and other tissues. Infection and systemic inflammation may therefore introduce interference from extrarenal sources. NGAL results should be interpreted together with urinary KIM-1, renal function, and inflammatory status.

2.4 Cystatin C and β2-Microglobulin: Indicators of Proximal Tubular Reabsorption

Cystatin C and β2-microglobulin are both low-molecular-weight proteins that can be filtered by the glomerulus. After entering the primary urine, they are normally reabsorbed and degraded by proximal tubular cells, resulting in low concentrations in normal urine.When proximal tubular uptake or lysosomal processing is impaired, urinary cystatin C and β2-microglobulin increase. Serum cystatin C is mainly used to evaluate glomerular filtration, whereas urinary cystatin C more specifically reflects defective proximal tubular reabsorption. The interpretation therefore differs according to the sample type.

2.5 Clusterin, Osteopontin, and EGF: Proteins Associated with Injury and Repair

Clusterin is a stress-responsive, chaperone-like glycoprotein that may increase during proteostasis disruption, oxidative stress, and tissue injury. Osteopontin participates in immune-cell recruitment, crystal adhesion, and extracellular matrix remodeling. Urinary EGF is mainly derived from distal nephron epithelial cells, and its level is associated with the number of functional tubules and their repair capacity. Chronic nephron loss and interstitial fibrosis are frequently accompanied by reduced urinary EGF.These three proteins respectively reflect stress protection, inflammatory remodeling, and tubular functional reserve and should not be treated as equivalent injury indicators.

Table 2. Comparison of major renal tubule-associated urinary proteins

3 Molecular Mechanisms of Urinary Protein Changes in Kidney Injury

3.1 Increased Glomerular Filtration Load

Damage to the glomerular filtration barrier may allow large amounts of albumin and other plasma proteins to enter the primary urine. Filtered proteins that exceed the reabsorptive capacity of the proximal tubule not only appear in the urine but may also activate inflammatory signaling, oxidative stress, and lysosomal stress responses in proximal tubular cells.Under these conditions, urinary protein changes reflect both primary glomerular abnormalities and secondary tubular injury. The site of injury therefore cannot be determined solely from total urinary protein levels.

3.2 Impaired Proximal Tubular Reabsorption

The proximal tubule recovers low-molecular-weight proteins from the primary urine through megalin, cubilin, and associated endocytic systems. Ischemia, mitochondrial dysfunction, and drug toxicity may reduce ATP availability and suppress receptor-mediated endocytosis, vesicular transport, and lysosomal degradation.Increased urinary cystatin C and β2-microglobulin may therefore result from reduced reabsorptive capacity and do not require increased expression of the corresponding proteins in kidney tissue.

3.3 Injury-Induced Expression and Protein Release

Following exposure to ischemia, oxidative stress, or toxic compounds, tubular cells can initiate injury-response programs and upregulate KIM-1, NGAL, and clusterin. The KIM-1 ectodomain can be released through proteolytic cleavage, whereas NGAL and clusterin can enter the tubular lumen through secretory pathways. Increases in these proteins indicate an active cellular stress response and do not necessarily mean that the cells have already died.

3.4 Reduced Tubular Secretory Function

UMOD and EGF are actively produced by specific nephron segments. Chronic tubular atrophy, cellular dedifferentiation, or loss of functional nephrons can reduce their urinary levels.

Pathogenic UMOD variants may also cause protein misfolding and retention in the endoplasmic reticulum, leading to intracellular UMOD accumulation despite reduced urinary UMOD. Reduced secretory proteins and increased injury-induced proteins may therefore occur simultaneously.

3.5 Cell Death and Disruption of the Epithelial Barrier

Severe injury may induce apoptosis, necrosis, or other forms of lytic cell death, releasing intracellular proteins and enzymes into the tubular lumen. Epithelial cell detachment and disruption of intercellular junctions also increase abnormal permeability between the tubular lumen and interstitium.LDH release, PI uptake, and ATP depletion can be used to evaluate cellular integrity. However, these measurements primarily reflect the severity of cellular injury and cannot replace nephron segment-specific indicators such as KIM-1 and UMOD.

4 Urinary Protein Changes in Different Kidney Diseases

4.1 Acute Kidney Injury

Urinary KIM-1 and NGAL may increase during the early stages of acute kidney injury, whereas serum creatinine generally changes only after a substantial reduction in glomerular filtration rate. UMOD and EGF may decrease because of suppressed tubular function or cellular injury.Different proteins exhibit distinct temporal patterns. Results obtained during early injury, peak injury, and the repair phase should therefore not be interpreted according to the same criteria.

4.2 Chronic Kidney Disease

Chronic kidney disease is accompanied by nephron loss, tubular atrophy, interstitial inflammation, and fibrosis. Reduced urinary UMOD and EGF may reflect diminished numbers of functional nephrons and reduced secretory capacity, whereas persistent increases in KIM-1, clusterin, and osteopontin may indicate active injury and tissue remodeling.A single marker cannot simultaneously represent injury activity and residual renal function. A combination of urinary proteins derived from different sources is more suitable for constructing a functionally complementary biomarker panel.

4.3 Drug-Induced Kidney Injury

Cisplatin can accumulate in proximal tubular cells and cause mitochondrial injury, oxidative stress, and cell death. Gentamicin can enter proximal tubular cells through receptor-mediated endocytosis and induce lysosomal and mitochondrial damage. Aristolochic acid A can form DNA adducts and cause persistent tubulointerstitial injury.These models commonly exhibit increased KIM-1, NGAL, and low-molecular-weight urinary proteins, together with changes in cell viability, ATP levels, and membrane integrity.

4.4 Hereditary Tubulointerstitial Kidney Disease

Pathogenic UMOD variants can cause autosomal dominant tubulointerstitial kidney disease. Misfolded mutant proteins are retained in the endoplasmic reticulum, progressively inducing stress in thick ascending limb cells, tubular atrophy, and interstitial fibrosis.Patients frequently exhibit reduced urinary UMOD, progressive loss of kidney function, hyperuricemia, and early-onset gout. The key pathogenic event is not UMOD deficiency itself but the intracellular toxicity caused by the misfolded protein.

4.5 Urinary Tract Infection and Crystalline Kidney Injury

High-mannose structures on UMOD can bind the FimH adhesin of uropathogenic Escherichia coli, forming bacterial aggregates that can be excreted in the urine. UMOD and osteopontin can also influence the nucleation, aggregation, and epithelial adhesion of calcium oxalate and other crystals.Protein concentration, glycosylation, polymerization state, urinary pH, and ionic strength can all alter these effects. In kidney stone research, protective or procrystallization effects therefore cannot be inferred from protein abundance alone.

Table 3. Biomarker combinations in different disease contexts

5 Products for Renal Tubule-Associated Urinary Protein Research

5.1 Renal Tubule-Associated Proteins and Genetic Tools

5.2 Antibodies and Urinary Protein Detection Reagents

5.3 Products for Renal Tubular Injury Models

5.4 Products for Assessing Cellular Injury and Function

Renal tubule-associated urinary proteins originate from distinct processes, including filtration, reabsorption, active secretion, and injury-associated release. UMOD and EGF mainly reflect the secretory function of specific nephron segments, KIM-1 and NGAL indicate active injury, and cystatin C and β2-microglobulin reflect impaired proximal tubular reabsorption. The renal biological significance of urinary proteins can be interpreted accurately only by considering their sources, directions of change, and underlying disease context.

For more related articles, please see below:

[1] Urinary Protein Biomarker Analysis in Kidney Injury: Detection Significance of Albumin, Transferrin, IgG, and α2-Macroglobulin

[2] Renal Function Biomarker Research: Integrated Evaluation of Filtration Function, Tubular Injury, and Urinary Indicators

[3] Oxalate Metabolism Research Methods: Analysis of Oxalate Production, Degradation, Transport, and Kidney Stone Mechanisms

[4] Catalytic Mechanisms of Creatinine Metabolism-Related Enzymes and Their Applications in Renal Function Testing