Immunohistochemical Localization of Napsin and Its Potential Role in Protein Catabolism in Renal Proximal Tubules.

Abstract

In a previous in situ hybridization study, we demonstrated the mRNA expression of napsin, an aspartic protease of the pepsin family, in the kidney, lung, and lymphoid organs of mice. However, findings on the cellular localization of napsin at the protein level are controversial, and no information on the subcellular localization is available. The present immunohistochemical study revealed the cellular and subcellular localization of napsin in mice and rats, and also analyzed the influences of chemical-induced proteinuria on the renal expression of this enzyme in rats. Immunohistochemistry using a polyclonal antibody against mouse napsin showed that napsin immunoreactivity was noticeable in lysosomes of renal proximal tubule cells and in lamellar bodies of pulmonary type II alveolar cells. In the lung, immunoreactivity was also found in lysosomes of alveolar macrophages and on the surface of type I alveolar cells; the immunoreactivities in these cells may be due to the uptake and adhesion of napsin secreted from type II alveolar cells, since they did not express napsin mRNA. Conversely, immunoreactivity for napsin was undetectable in B lymphocytes with intense mRNA expression. In puromycin- or doxorubicin-induced proteinuria, napsin mRNA expression was markedly elevated in renal proximal tubules, showing characteristic distribution patterns. Immunostaining of kidneys with proteinuria showed intense immunoreactivity for napsin in congested and enlarged lysosomes, called protein absorption droplets. These results indicate that napsin functions as a lysosomal protease and is involved in protein catabolism in renal proximal tubules.

Cathepsin H and napsin A are active in the alveoli and increased in alveolar proteinosis

“…Napsin A may be involved in many extracellular enzymatic processes. In line with the extracellular functions of napsin A is its secretion from the kidney into the urine of normal human subjects [16,17,34]. All this fits well into the more general paradigm that enzymatic activities, previously believed to be responsible for entirely intracellular processes, may also have specific functions after secretion or release into the extracellular environment.…”

Proteomic-Based Identification of Cleaved Urinary β2-microglobulin as a Potential Marker for Acute Tubular Injury in Renal Allografts

“…Recently, two members of the aspartic protease family have been detected in human urine (cathepsin D (18) and napsin A (19)). Both enzymes are primarily located in lysosomes in tubular epithelial cells and are involved in protein degradation (20)(21)(22). The degradation of intact urinary b2-m by cathepsin D has been demonstrated, however, only two of the reported cleavage sites found by N-terminal sequencing are consistent with the 26 found in this study (18).…”

Involvement of a Novel Q-SNARE, D12, in Quality Control of the Endomembrane System

“…We screened a total of 9000 clones of a murine hematopoietic stem cell cDNA library and observed several clones expressing the EGFP signal in the ER. These included SEP15 (a 15-kDa selenoprotein) (26) and KDAP (kidney-derived aspartic protease-like protein, Napsin) (27), both of which contain signal sequences. We also obtained clones encoding the cytoplasmic proteins Gng10 (guanine nucleotide binding protein ␥ 10), whose signal was mostly in the cytosol, and SAP18 (Sin3-associated polypeptide), whose signal was found in the ER (data not shown).…”