Specific developmental profiles of lysosomal and brush border enzymuria in the human

Targeted design of a recombinant tracer for SPECT renal imaging

Rationale: A robust radiopharmaceutical has high uptake in the target and low retention in non-target tissues. However, traditional tracers for renal imaging that chemically chelate ^99mTc are excreted through the renal route with transient resident time in the kidney. Following a rational design approach, we constructed a protein-based radiotracer, designated PBT-Fc, to sequentially bind tubular neonatal Fc-receptor and subsequently proximal tubular basement membrane for its targeted sequestration in kidney parenchyma. In this process, the tracer participates in physiologic glomerular filtration and tubular reabsorption while escaping lysosomal catabolism and urinary clearance. Methods: To specifically target renal receptors in navigating the urinary passage in the kidney, we produced a recombinant fusion protein with two separate functional parts: a polybasic PBT segment derived from human Vascular Endothelial Growth Factor and Fc segment of IgG1. The chimeric fusion of PBT-Fc was labeled with radionuclide ^99mTc and tested in rodent models of kidney diseases. Planar scintigraphy and single-photon emission computerized tomography (SPECT) were performed to evaluate renal-specificity of the tracer. Results: When injected in mouse and rat, following a brief 10 - 15 min dynamic redistribution phase in circulation, ~ 95% of the [^99mTc]-PBT-Fc signal was concentrated in the kidney and lasted for hours without urinary loss or surrounding tissue activities. Long-lasting tracer signals in the kidney cortex in conjunction with SPECT greatly augmented the image quality in detecting pathological lesions in a variety of disease models, including ischemic acute kidney injury, drug-induced renal toxicity, and chronic kidney disease from renin-angiotensin system (RAS) overactivation. Conclusion: Exclusive renal retention of the recombinant radiotracer greatly facilitated static-phase signal acquisition by SPECT and achieved submillimeter spatial resolution of kidney alternations in glomerular and tubular disease models.

Urinary Activities of Proximal Tubule Enzymes in Neonates Treated with Gentamicin

In order to determine the nephrotoxicity of gentamicin, an aminoglycoside antibiotic, activity of the enzymes dominantly localized in proximal tubules, i.e. alanine aminopeptidase (AAP), γ-glutamyl transferase (GGT) and N-acetylβ-D-glucosaminidase (NAG) was examined. Determinations were performed in 12-h urine samples of 30 neonates i.v. receiving gentamicin against Gram negative infections in daily doses of 5.0 mg/kg body mass for 10 consecutive days. The activities of the same enzymes were measured in 12-h urine samples of 30 examinées of the control group. The groups consisted of neonates of both sexes. The pretreatment period lasted for 5 days. On day 8 of gentamicin application, statistically significant differences in the activity of AAP and GGT expressed in U/mmol creatinine between the gentamicin-receiving and control group (p<0.01) were found. No significant differences in NAG activity of the gentamicin-treated group in comparison with the control were recorded during the 10-day gentamicin therapy. It can be concluded that 10-day treatment of neonates with usually prescribed gentamicin doses results in mild nephrotoxic changes close to the end of the therapy accompanied by increased activity of both urinary AAP and GGT, known as very sensitive indicators of nephrotoxicity. During the same treatment period no changes in NAG activity were observed, meaning that the antibiotic causes no severe injuries to proximal tubule cells at the level of cellular organelles.

Evaluation of renal glomerular and tubular functional and structural integrity in neonates

BACKGROUND

Renal cells are not fully differentiated at birth, representing a major risk in preterm infants. We evaluated glomerular and tubular functional integrity as well as structural integrity of renal tubules among healthy full-term and preterm infants as well as diseased preterm infants.

METHODS

A total of 50 newborns (10 healthy full-term, 10 healthy preterm, and 30 diseased preterm, at 38.9 +/- 1.10, 34.2 +/- 0.92, and 32 +/- 2.47 weeks gestational age, respectively) were included in the present study. Glomerular function was assessed by measuring urinary levels of both microalbumin and immunoglobulin G as well as serum creatinine levels, whereas the proximal tubular function was investigated by measuring the urinary levels of both alpha1-microglobulin and beta2-microglobulin as well as retinol-binding protein. Also, distal tubular reabsorption capacity was investigated by assessing fractional excretion of sodium. Moreover, the structural integrity of renal proximal tubules was studied by measuring the urinary activities of both the brush-border membrane enzyme leucine-aminopeptidase (LAP) and the lysosomal enzyme N-acetyl-beta-D-glucosaminidase. The preceding investigations were done on both the first and third days of life of all 50 newborns.

RESULTS

Glomerular and tubular function and structure was relatively impaired at birth among both healthy and diseased preterm as well as healthy full-term neonates and improved rapidly thereafter. The diseased preterm neonates showed worse renal function and structure with minimal improvement regardless of the underlying sickness.

CONCLUSION

Renal insufficiency and renal immaturity could be evaluated using enzymuria and low- and high-molecular-weight proteinuria as noninvasive methods.

Lysosomal enzymes in preterm infants with bronchopulmonary dysplasia: a potential diagnostic marker

Some lysosomal glycohydrolases (N-acetyl-beta-D-glucosaminidase and their major isoenzymes, beta-D-glucuronidase, alpha-D-galactosidase, beta-D-galactosidase and alpha-D-glucosidase) were investigated in the plasma of 36 preterm infants with respiratory distress, 11 of whom developed bronchopulmonary dysplasia (BPD), in order to evaluate the role of the lysosomal apparatus in the disease. Enzyme activity was assayed fluorimetrically; the major N-acetyl-beta-D-glucosaminidase (NAG) isoenzymes were separated using a routine chromatofocusing procedure; the diagnostic efficiency was evaluated by Bayes theorem. The mean levels of almost all glycohydrolases considered were significantly higher in BPD than in non-BPD infants. Among NAG major isoenzymes, an increase was found only in form A. No variation was evident in the plasma levels of glycohydrolases during dexamethasone therapy. Data from a retrospective analysis performed in all preterms considered, show that alpha-D-galactosidase and beta-D-galactosidase differentiate a posteriori BPD and non-BPD subjects. These enzymes, after a priori verification of their diagnostic potential in preterm infants at risk of BPD development, could acquire an important predictive value.

Urinary enzyme changes in newborns with unconjugated hyperbilirubinemia

Various changes in renal function caused by unconjugated hyperbilirubinemia in newborns have been suggested in previous reports. Disclosing an injury in renal tubulus epithelium is feasible by measurement of urinary enzymes. Thus, renal function tests and urinary enzymes in 25 terms newborns with unconjugated hyperbilirubinemia were evaluated before and after phototherapy. Ten healthy term newborns without hyperbilirubinemia formed the control group. Mean values of the variables obtained before and after phototherapy in the study group and in the controls were, respectively: urine osmolality (osm/kg H2O): 0.147 +/- 0.009, 0.174 +/- 0.011, and 0.153 +/- 0.018; endogenous creatinine clearance (mL/min per 1.73 m2): 45.7 +/- 2.15, 46.0 +/- 1.6 and 46.7 +/- 3.9; fractional excretion of sodium (%): 1.27 +/- 0.30, 0.79 +/- 0.19 and 1.24 +/- 0.07; tubular phosphorus reabsorption (%): 85.8 +/- 3.3, 87.8 +/- 2.8 and 86.6 +/- 1.7; urinary N-acetyl-beta-D glucosaminidase/creatinine (IU/mg): 0.617 +/- 0.226, 0.574 +/- 0.214 and 0.619 +/- 0.210; fractional excretion of alkaline phosphatase (%): 0.422 +/- 0.103, 1.001 +/- 0.374 and 0.596 +/- 0.201; fractional excretion of lactic dehydrogenase (LDH; %): 0.102 +/- 0.019, 0.121 +/- 0.023 and 0.119 +/- 0.041; fractional excretion of AST (%): 0.433 +/- 0.127, 0.530 +/- 0.113 and 0.502 +/- 0.074; fractional excretion of alanine aminotransferase (ALT; %) 0.856 +/- 0.413, 1.619 +/- 1.076 and 1.066 +/- 0.366. No significant difference was found between these values before and after phototherapy in the study group, or between the values before phototherapy in hyperbilirubinemic neonates and in the control group. In conclusion, unconjugated hyperbilirubinemia up to a serum level of 18.4 mg/dL in term neonates does not seem to result in injury of normal tubulus epithelium as shown by urinary enzyme levels.