Analysis of Kallikrein 6, Acetyl-α-Tubulin, and Aquaporin 1 and 2 Expression Patterns During Normal Human Nephrogenesis and in Congenital Anomalies of the Kidney and Urinary Tract (CAKUT)

<b>Background/Objectives</b>: The human kallikrein-related peptidase 6 (KLK6), a serine protease with trypsin-like properties, belongs to the 15-member kallikrein (<i>KLK</i>) gene family and is predominantly recognized for its role in oncogenesis, neurodegenerative disorders, and skin conditions. Aquaporins (AQPs) are integral membrane proteins that facilitate water transport across cell membranes. AQP1 is constitutively active in the kidneys and plays a crucial role in reabsorbing filtered water, while AQP2 is regulated by vasopressin and is essential for maintaining body fluid homeostasis. The primary objective of the present study is to investigate the spatio-temporal expression patterns of KLK6, AQP1, and AQP2 throughout normal human nephrogenesis and congenital kidney and urinary tract (CAKUT) abnormalities: duplex kidneys, horseshoe kidneys, and dysplastic kidneys. <b>Methods</b>: An immunofluorescence analysis of KLK6, AQP1, and AQP2 was performed on 37 paraffin-embedded fetal kidney samples. The area percentage of KLK6 in the kidney cortex was calculated in normal developing samples during developmental phases 2, 3, and 4 and compared with CAKUT samples. <b>Results</b>: KLK6 exhibits distinct spatiotemporal expression patterns during human kidney development, with consistent localization in proximal tubules. Its subcellular positioning shifts from the basolateral cytoplasm in early phases to the apical cytoplasm in later stages, which may be strategically positioned to act on its substrate in either the peritubular space or the tubular fluid. KLK6 expression followed a quadratic trajectory, peaking at Ph4. This marked increase in the final developmental phase aligns with its strong expression in mature kidneys, suggesting a potential role in proximal tubule differentiation and functional maturation through facilitating extracellular matrix remodeling and activating proteinase-activated receptors, modulating the signaling pathways that are essential for tubular development. In duplex kidneys, structural abnormalities such as ureteral obstruction and hydronephrosis may upregulate KLK6 as part of a reparative response, while its downregulation could impair epithelial remodeling and cytoskeletal integrity, exacerbating dysplastic phenotypes. <b>Conclusions</b>: These findings highlight the potential of KLK6 involvement in normal kidney development and the pathology of CAKUT.