Abstract
Osteoclasts or their precursors interact with the glycoprotein-enriched matrix of bone during extravasation from the vasculature, and upon attachment prior to resorption. Reverse transcriptase-PCR studies showed that two new alternatively spliced forms of chicken galectin-3, termed Gal-3TM1 and Gal-3TR1, were enriched and preferentially expressed in highly purified chicken osteoclast-like cells. Gal-3TM1 and Gal-3TR1 mRNA were also detected in chicken intestinal tissue, but not in kidney, liver, or lung. Gal-3TM1 and Gal-3TR1 messages both contain an open reading frame encoding a predicted 70-amino acid TM1 sequence inserted between the N-terminal Gly/Pro repeat domain and the carbohydrate recognition domain (exons 3 and 4). Gal-3TR1 mRNA contains an additional 241-bp sequence, which encodes a truncated open reading frame between the 4th and 5th exons, and, whose translation is expected to terminate within the carbohydrate recognition domain encompassing exons 4, 5, and 6. Immunoblotting and affinity chromatography showed that purified osteoclast preparations and intestinal homogenates contained a 36-kDa lactose-binding galectin. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analyses on chymotryptic peptides from the 36-kDa lectin confirmed its identity as Gal-3TM1. The TM1 insert contains a single transmembrane-spanning region and a leucine zipper-like stalk domain that is predicted to position the intact carbohydrate recognition domain of Gal-3TM1 on the exterior surface of the plasma membrane. Immunofluorescent staining of chicken osteoclasts confirmed the expression of Gal-3TM1 at the plasma membrane. Gal-3TM1 is the first example of a galectin superfamily member capable of being expressed as a soluble protein and as a transmembrane protein.
Highlights
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AF479564 and AF479565
A variety of carbohydrate receptors are known to be expressed by monocyte/macrophages, including selectins [1], galectin-3 (Mac-2, IgEBP) [2], mannose receptor [3], and sialoadhesin [4]
These studies clearly demonstrate that the vitronectin receptor and its ligands play an important role in osteoclast attachment, several pieces of conflicting data are most explained by the existence of one or more additional cell adhesion receptors [7, 10, 11]
Summary
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AF479564 and AF479565. Carbohydrate receptors on osteoclasts and osteoclast precursors could play a functional role in mediating cell-matrix interactions. Galectin-3 exhibits diverse functions, e.g. in pre-RNA splicing [22], binding of circulating or extracellular matrix glycoproteins like IgE or Mac-2 binding protein on cell surfaces [23, 24], and as a regulator of apoptosis and cell growth [25]. Taken together, these data suggest a potential role for galectin-3 in osteoclast function. We report here characterization of two new alternatively spliced sequences of galectin-3 expressed exclusively by chicken osteoclasts, Gal-3TM1 (galectin-3 containing transmembranespanning region and leucine zipper motif) and Gal3-TR1 (galectin-3 containing transmembrane-spanning region, leucine zipper motif, and truncated carbohydrate recognition domain)
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