Developmental biology. Making one cell from two

Expression of meltrin-alpha mRNA is not restricted to fusagenic cells

Meltrin-alpha is a myoblast gene product reported to be required for cell fusion [Yagami-Hiromasa et al. (1995): Nature 377:652-656]. Because Northern blots revealed expression only in muscle and bone, the suggestion was made that meltrin-alpha is expressed exclusively by fusagenic cells in these tissues (myoblast and osteoclast). We studied expression of meltrin-alpha mRNA in a panel of tissues and cell lines using the polymerase chain reaction and found it widely expressed. Meltrin-alpha mRNA was readily detected in the osteoblast, the most abundant cell type in bone. In situ hybridization analysis on sections of neonatal mice revealed high levels of expression in the trabecular meshwork of long bones, the basal regions of the dermis and its underlying mesenchyme. We conclude that expression of meltrin-alpha mRNA is not restricted to fusagenic cells and that, in bone, the osteoblast is the major source.

Non-secretory multiple myeloma with multinucleated giant plasma cells

We report a case of non-secretory multiple myeloma with unusual cytological features. The plasma cells were multinucleated and contained up to forty nuclei. All nuclei had regular outlines without multilobulated and convoluted slopes. DNA content measurement demonstrated that all nuclei of uni- and multinucleated cells were diploid. All plasma cells contained cytoplasmic alpha chain but light chains and their corresponding transcripts were absent. There is no clear explanation concerning multinuclearity. In addition, hypotheses regarding non-secretion of immunoglobulin in non-secretory multiple myeloma and in other B-cell neoplasias are discussed.

The mammalian acrosome reaction: gateway to sperm fusion with the oocyte?

Mammalian sperm undergo discharge of a single, anterior secretory granule following their attachment to the zona pellucida surrounding the oocyte. This secretory discharge is known for historical reasons as the acrosome reaction. It fulfils a number of purposes and without it, sperm are unable to penetrate the zona pellucida and fuse with the oocyte. In this review, we focus on the role of the acrosome reaction in the development of fusion competence in sperm. Any naturally occurring membrane fusion has two major sequential steps: a docking or adhesion step, in which two membranes adhere, and a fusion step, in which their lipid bilayers are destabilized and merged and a cellular compartment is either created or destroyed. Recent evidence suggests that there is an important role for oocyte integrins and sperm-bound disintegrins in mammalian sperm/oocyte adhesion and fusion. The fusion mechanism employed by sperm remains poorly understood, however, and circumstantial evidence suggests it is more complex than the interaction between a single protein species and its target. Sperm/oocyte fusion is probably the most accessible eukaryotic model for intercellular fusion currently available, partly because it is temporally separated from gene expression. Elucidation of the mechanism of sperm/oocyte fusion may throw light on the mechanism of other intercellular fusions such as myoblast fusion, and the evolutionary relationship of intercellular membrane fusion to intracellular membrane fusion.

ADAMs and cell fusion

Members of the ADAM family (membrane proteins with a disintegrin and metalloprotease domain) have been implicated in several cell-interactive events, including cell-cell fusion. Recent evidence implicates three ADAMs, fertilin-alpha, fertilin-beta, and meltrin-alpha, in sperm-egg fusion and myoblast fusion. In light of the large number and wide tissue distribution of the ADAMs, they may also participate in other cell-cell fusion events. As ADAMs are also found in both vertebrates and invertebrates, some features of cell-cell fusion reactions may be conserved throughout the animal kingdom.