In vivo secretion and association of clusterin (SGP‐2) in luminal fluid with spermatozoa in the rat testis and epididymis

Abstract

Clusterin (sulfated glycoprotein-2) is a heterodimeric glycoprotein synthesized and secreted by rat Sertoli cells. An antigenically similar form is synthesized and secreted by the epididymis. The goal of this study was to define the epididymal regions in which clusterin is present and the regions in which clusterin is secreted and interacts with developing spermatozoa. Seminiferous tubule (STF), caput, corpus, and cauda fluids were collected by micropuncture and/or microperfusion and two-dimensional Western blot analysis was performed with a polyclonal antibody directed against Sertoli cell clusterin. Clusterin was found in both STF and epididymal fluid. STF contained predominantly the clusterin heavy chain (45 kd); however, a 70 Kd heterodimer was present under nonreducing conditions. Two subunits of clusterin with lower molecular weights (41 kd, heavy chain; 32 kd, light chain) and higher isoelectric points were present in the luminal fluid of all epididymal regions. The intraluminal levels of the heavy and light chains decreased from caput to cauda. Analysis by two-dimensional gel electrophoresis of proteins secreted directly into the epididymal luminal fluid revealed that clusterin was secreted by caput epithelium and not by the corpus and cauda epithelium. Western blots of membrane extracts from testicular, caput, and cauda spermatozoa revealed that testicular clusterin was associated with testicular sperm and epididymal clusterin with predominantly caput sperm. Our findings suggest that clusterin is secreted into the caput epididymal lumen, where it binds to sperm and then dissociates from sperm to be endocytosed by cells of the distal epididymal epithelium.

Proteomic Profiling of Regionalized Proteins in Rat Epididymis Indicates Consistency between Specialized Distribution and Protein Functions

“…Of these 28 proteins, 8 have been reported to have regionalized behavior within the epididymis, such as Gln synthetase 20 and clusterin. 21,22 Our results are highly accordant with the reports describing those proteins previously known to be regionalized (Table 1). In addition, our results are the first to reveal the regional distribution of four other previously identified epididymal proteins (Table 1).…”

Clusterin Overexpression is Responsible for the Anti-apoptosis Effect in a Mouse Neuroblastoma Cell Line, B103

“…In neuronal cells, early studies have shown that clusterin is accumulated in the apoptotic dying neurons in CA1 hippocampal pyramidal cells, and clusterin gene expression is strongly correlated with apoptosis (Rozovsky et al, 1994). Other studies have shown that an exogenously infected SGP-2 gene, does not induce apoptosis and that ApoJ expression is strongly associated with neuronal apoptosis in the olfactory mucosa (D'Mello et al, 1993;Michel et al, 1997). On the other hand, in the several photoreceptor neurons, clusterin is not causally involved in apoptosis, but may have cytoprotective functions (Jomary et al, 1999).…”

“…In neuronal cells, early studies have shown that clusterin is accumulated in apoptotic dying neurons in the CA1 hippocampal pyramidal cells, and clusterin gene expression is strongly related to apoptosis (Rozovsky et al, 1994). On the other hand, other studies have reported that an exogenously infected SGP-2 gene does not induce apoptosis (D'Mello et al, 1993). The biological function of clusterin associated with apoptosis in neurocytes is unclear.…”

“…It is encoded by a single gene and the translated product is internally cleaved to produce its α and subunit prior to its secretion from the cell (Wilson and Easterbrook-Smith, 2000). Early studies on the function of clusterin focused mainly on the reproductive tracts, the rete testis and the cauda epididymal fluids (Mattmueller and Hinton, 1991). Recent studies have revealed many functions of clusterin such as controlling the cell-cell interactions, regulating apoptosis, lipid transport, as a regulating complement and as a molecular chaperone (Wilson and Easterbrook-Smith, 2000;Humphreys et al, 1999).…”

Expression Pattern of Sulfated Glycoprotein-2 (SGP-2) mRNA in Rat Testes Exposed to Endocrine Disruptors

“…Sertoli cells and epididymal epithelial principal cells have been shown to synthesize and secrete a number of glycoproteins that have been suggested to play important roles in the regulation of spermatogenesis and sperm maturation [3]. One of the major proteins secreted by both Sertoli cells and epididymal epithelial cells is sulfated glycoprotein-2 (SGP-2) [4]. SGP-2 is a heavily glycosylated protein composed of two monomers that arise from a proteolytically-cleaved, single chain polypeptide precursor [5].…”