Expression of SPAM1 (PH-20) in the Murine Kidney Is Not Accompanied by Hyaluronidase Activity: Evidence for Potential Roles in Fluid and Water Reabsorption

Abstract

Background:A role for <X_Underline>S</X_Underline>perm <X_Underline>A</X_Underline>dhesion <X_Underline>M</X_Underline>olecule <X_Underline>1</X_Underline> (SPAM1) hyaluronidase in murine kidney, where Spam1 transcript levels have been reported to be higher in males, has not been clarified. Methods:Spam1 RNA and protein were studied using RT-PCR, in situhybridization, Western blotting, immunohistochemistry and hyaluronic acid substrate gel electrophoresis. Urine volume and osmolality were studied in wild-type and Spam1 null mice. Results: While RT-PCR supported a tendency of higher RNA expression in males, no sex difference for the protein was detectable in the cortex, medulla, and urine. Transcripts were predominantly localized in the proximal tubules and glomeruli, with lower levels in the medulla. Similarly, Western blotting and immunohistochemistry revealed that SPAM1 is more abundant in the cortex. Hyaluronidase activity was absent at neutral and acidic pH: suggesting non-enzymatic role(s) for SPAM1. Wild-type and Spam1 null mice given free access to water showed significantly reduced urine volumes (p < 0.01; n = 12) in the latter.Baseline urine osmolality was similar in both, leading to a significantly (p < 0.05) lower osmolar output in the nulls. After water deprivation (24 h), a significant (p < 0.01) increase in urine osmolality was seen only for wild-type mice. Conclusion: SPAM1 is implicated in fluid reabsorption and urine concentration.