The sequence of the mouse genome is a key informational tool for understanding the contents of the human genome and a key experimental tool for biomedical research. Here, we report the results of an international collaboration to produce a high-quality draft sequence of the mouse genome. We also present an initial comparative analysis of the mouse and human genomes, describing some of the insights that can be gleaned from the two sequences. We discuss topics including the analysis of the evolutionary forces shaping the size, structure and sequence of the genomes; the conservation of large-scale synteny across most of the genomes; the much lower extent of sequence orthology covering less than half of the genomes; the proportions of the genomes under selection; the number of protein-coding genes; the expansion of gene families related to reproduction and immunity; the evolution of proteins; and the identification of intraspecies polymorphism.

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Initial sequencing and comparative analysis of the mouse genome.

For a long time, lysosomes were considered merely to be cellular 'incinerators' involved in the degradation and recycling of cellular waste. However, now there is compelling evidence indicating that lysosomes have a much broader function and that they are involved in fundamental processes such as secretion, plasma membrane repair, signalling and energy metabolism. Furthermore, the essential role of lysosomes in autophagic pathways puts these organelles at the crossroads of several cellular processes, with significant implications for health and disease. The identification of a master regulator, transcription factor EB (TFEB), that regulates lysosomal biogenesis and autophagy has revealed how the lysosome adapts to environmental cues, such as starvation, and targeting TFEB may provide a novel therapeutic strategy for modulating lysosomal function in human disease.

Signals from the lysosome: a control centre for cellular clearance and energy metabolism.

The occurrence in nature of erythrocyte-agglutinating proteins has been known since the turn of the 19th century. By the 1960s it became apparent that such proteins also agglutinate other types of cells, and that many of them are sugar-specific. These cell-agglutinating and sugar-specific proteins have been named lectins. Although shown to occur widely in plants and to some extent also in invertebrates, very few lectins had been isolated until the early 1970s, and they had attracted little attention. This attitude changed with the demonstration that lectins are extremely useful tools for the investigation of carbohydrates on cell surfaces, in particular of the changes that the latter undergo in malignancy, as well as for the isolation and characterization of glycoproteins. In subsequent years numerous lectins have been isolated from plants as well as from microorganisms and animals, and during the past two decades the structures of hundreds of them have been established. Concurrently, it was shown that lectins function as recognition molecules in cell-molecule and cell-cell interactions in a variety of biological systems. Here we present a brief account of 100-plus years of lectin research and show how these proteins have become the focus of intense interest for biologists and in particular for the glycobiologists among them.

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History of lectins: from hemagglutinins to biological recognition molecules

The transient receptor potential (TRP) protein superfamily consists of a diverse group of cation channels that bear structural similarities to Drosophila TRP. TRP channels play important roles in nonexcitable cells; however, an emerging theme is that many TRP-related proteins are expressed predominantly in the nervous system and function in sensory physiology. The TRP superfamily is divided into seven subfamilies, the first of which is composed of the "classical" TRPs" (TRPC subfamily). Some TRPCs may be store-operated channels, whereas others appear to be activated by production of diacylglycerol or regulated through an exocytotic mechanism. Many members of a second subfamily (TRPV) function in sensory physiology and respond to heat, changes in osmolarity, odorants, and mechanical stimuli. Two members of the TRPM family function in sensory perception and three TRPM proteins are chanzymes, which contain C-terminal enzyme domains. The fourth and fifth subfamilies, TRPN and TRPA, include proteins with many ankyrin repeats. TRPN proteins function in mechanotransduction, whereas TRPA1 is activated by noxious cold and is also required for the auditory response. In addition to these five closely related TRP subfamilies, which comprise the Group 1 TRPs, members of the two Group 2 TRP subfamilies, TRPP and TRPML, are distantly related to the group 1 TRPs. Mutations in the founding members of these latter subfamilies are responsible for human diseases. Each of the TRP subfamilies are represented by members in worms and flies, providing the potential for using genetic approaches to characterize the normal functions and activation mechanisms of these channels.

The TRP Superfamily of Cation Channels

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Secretory Granule Exocytosis

Mammalian sperm acrosome: formation, contents, and function.

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Mammalian fertilization: a carbohydrate-mediated event.

The Biological and Functional Significance of the Sperm Acrosome and Acrosomal Enzymes in Mammalian Fertilization

In previous reports from this laboratory, we identified the presence of a novel alpha-D-mannosidase on the surface of rat, mouse, hamster, and human spermatozoa [J Cell Biol 1989; 109:1257-1267 and Biol Reprod 1990; 42:843-858]. Since it has been suggested that mannosyl residues on the egg zona pellucida may be important for sperm-egg binding, studies were undertaken to examine the potential role of the sperm alpha-D-mannosidase during fertilization. Incubation of mouse spermatozoa in the presence of increasing concentrations of the inhibitory sugars, alpha-methyl mannoside, alpha-methyl glucoside, D-mannose, or D-mannitol, resulted in a dose-dependent decrease in the number of spermatozoa bound per egg without a deleterious effect on sperm motility or on the sperm acrosome, and a dose-dependent inhibition of the sperm mannosidase activity. Galactose, however had no effect on sperm-egg binding or on sperm mannosidase activity. Two nucleotide sugars (UDP-GlcNAc and UDP-gal) were also tested and shown to reduce sperm-egg binding but with only a minimal effect on sperm mannosidase activity. In additional studies, spermatozoa incubated in the presence of a mannose-containing oligosaccharide exhibited a dramatic reduction in sperm-egg binding that correlated with a similar inhibition of sperm mannosidase activity. The oligosaccharide substrate did not affect sperm motility or the sperm acrosome. These studies suggest that the sperm alpha-D-mannosidase may play an important role during fertilization.

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Inhibition of the mouse sperm surface alpha-D-mannosidase inhibits sperm-egg binding in vitro.

During the course of a study of glycoprotein processing mannosidases in the rat epididymis, we have made an intriguing discovery regarding the presence of a novel alpha-D-mannosidase on the rat sperm plasma membranes. Unlike the sperm acrosomal "acid" mannosidase which has a pH optimum of 4.4, the newly discovered alpha-D-mannosidase has a pH optimum of 6.2, and 6.5 when assayed in sperm plasma membranes and intact spermatozoa, respectively. In addition, the two enzymes show different substrate specificity. The acrosomal alpha-D-mannosidase is active mainly towards synthetic substrate, p-nitrophenyl alpha-D-mannopyranoside, whereas the sperm plasma membrane alpha-D-mannosidase shows activity mainly towards mannose-containing oligosaccharides. Evidence is presented which suggest that the sperm plasma membrane alpha-D-mannosidase is different from several processing mannosidases previously characterized from the rat liver. The newly discovered alpha-D-mannosidase appears to be an intrinsic plasma membrane component, since washing of the purified membranes with buffered 0.4 M NaCl did not release the enzyme in soluble form. The enzyme requires nonionic detergent (Triton X-100) for complete solubilization. The enzyme is activated by Co2+ and Mn2+. However, Cu2+ and Zn2+ are potent inhibitors of the sperm plasma membrane alpha-D-mannosidase. At a concentration of 0.1 mM, these divalent cations caused nearly complete inactivation of the sperm enzyme. In addition methyl-alpha-D-mannoside, methyl-alpha-D-glucoside, mannose, 2-deoxy-D-glucose, and D-mannosamine are inhibitors of the sperm surface alpha-D-mannosidase. The physiological role of the newly discovered enzyme is not yet known. Several published reports in three species, including the rat, suggest that the sperm surface alpha-D-mannosidase may have a role in binding to mannose-containing saccharides presumably present on the zona pellucida.

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Daulat R.P. Tulsiani

Papers

Mammalian sperm acrosome: formation, contents, and function.

Mammalian fertilization: a carbohydrate-mediated event.

The Biological and Functional Significance of the Sperm Acrosome and Acrosomal Enzymes in Mammalian Fertilization

Inhibition of the mouse sperm surface alpha-D-mannosidase inhibits sperm-egg binding in vitro.

Novel alpha-D-mannosidase of rat sperm plasma membranes: characterization and potential role in sperm-egg interactions.