Galectin-3 is associated with prostasomes in human semen

Abstract

Galectin-3 is a β-galactoside-binding protein involved in immunomodulation, cell interactions, cancer progression, and pathogenesis of infectious organisms. We report the identification and characterization of galectin-3 in human semen. In the male reproductive tract, the ~30 kDa galectin-3 protein was identified in testis, epididymis, vas deferens, prostate, seminal vesicle, and sperm protein extracts. In seminal plasma, galectin-3 was identified in the soluble fraction and in prostasomes, cholesterol-rich, membranous vesicles that are secreted by the prostate and incorporated into seminal plasma during ejaculation. Two-dimensional immunoblot analysis of purified prostasomes identified five galectin-3 isoelectric variants with a pI range of 7.0 to 9.2. Affinity purification and tandem mass spectrometry of β-galactoside-binding proteins from prostasomes confirmed the presence of galectin-3 in prostasomes and identified a truncated galectin-3 variant. The intact galectin-3 molecule contains a carbohydrate recognition domain and a non-lectin domain that interacts with protein and lipid moieties. The identification of a monovalent galectin-3 fragment with conserved carbohydrate-binding activity indicates the functional relevance of this truncation and suggests a regulatory mechanism for galectin-3 in prostasomes. Surface biotinylation studies suggested that galectin-3 and the truncated galectin-3 variant are localized to the prostasome surface. Prostasomes are proposed to function in immunosuppression and regulation of sperm function in the female reproductive tract, are implicated in facilitating sexually-transmitted infections, and are indicated in prostate cancer progression. Given the overlap in functional significance, the identification of galectin-3 in prostasomes lays the groundwork for future studies of prostasomes in reproduction, disease transmission, and cancer progression.

Prostasomes: extracellular vesicles from the prostate

“…CD46 is also known as a receptor for measles virus, and prostasomes have been shown to inhibit infectivity of this virus, probably by capture of the virus by CD46, as a kind of 'mock cell' (Kitamura et al 1995). Another immunomodulatory prostasomal surface protein is galectin-3 (Jones et al 2010), a b-galactoside-binding protein that plays a role in immunomodulation, pathogen-host interactions, cell-cell and cell-matrix interactions and cancer progression (Dumic et al 2006). Galectin-3 lacks a signal sequence for transport into the endoplasmic reticulum and is therefore secreted via a non-classical pathway.…”

The galectin lattice at a glance

“…Galectins are synthesized in the cytoplasm and interact with cell surface glycans following their secretion by a non-classical exocytic pathway (i.e. not via the ER/ Golgi secretory route), that is likely to be an exosome-mediated secretory route (Hughes, 1999;Jones et al, 2010). The β-galactoside epitopes are di-to tetra-saccharide sequences that are widely found in oligosaccharide modifications (N-and O-glycans) to glycoproteins and glycolipids of vertebrate tissues.…”

Identification of Distinct Populations of Prostasomes That Differentially Express Prostate Stem Cell Antigen, Annexin A1, and GLIPR2 in Humans1

“…Other tissues within the male genital tract, such as the vesicular glands, which produce the majority of seminal fluid, and the vasa deferentia, may also contribute membrane vesicles to the seminal plasma [3,4]. In this respect, while the term prostasomes is often used to refer to the collective array of membrane vesicles isolated from seminal plasma [5][6][7][8], it would be more appropriate to define prostasomes as those membrane vesicles that are actually produced by the prostatic epithelium. Prostasomes are ;0.1 lm vesicles that have been proposed to be formed within multivesicular bodies in prostate epithelial cells and released into the seminal fluid following fusion of these multivesicular bodies with the plasma membrane of the containing cell [1,9,10].…”

“…Prostasomes could also help to protect sperm cells from immune responses within the female reproductive tract by, for example, inhibiting the complement pathway [24,25], inhibiting lymphocyte proliferation [26], and inhibiting monocyte and neutrophil phagocytosis [27], possibly via contained immunomodulatory proteins such as galectin 3 [8] or CD48 [28]. Although it is possible that a single type of prostasome is able to function in a variety of seemingly unrelated processes, we consider it more likely that distinct populations of vesicles carry out different tasks.…”

“…Prostasomes were collected from the seminal fluid of individual donors(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) and immunoblotted for CD9 (A). The samples were probed on parallel blots for PSCA using mouse anti-human PSCA (B) or rabbit antihuman PSCA (C).…”