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Acrosome
Acrosome GS Kopf, University of Pennsylvania School of Medicine, Philadelphia, PA, USA
© 2001 Elsevier Inc. All rights reserved.
This article is reproduced from the previous edition, volume 1, pp 3–4, © 2001, Elsevier Inc.
The acrosome is a vesicle overlying the nucleus of both inverte brate and vertebrate sperm composed of nonenzymatic and enzymatic proteins generally arranged as a matrix; these proteins have been demonstrated in some cases to play specific roles in the fertilization process. The contents of the acrosome are released prior to sperm–egg fusion in a regulated secretory event called the acrosome reaction. The morphology of the acrosome varies between species and the mechanics of the acrosome reaction vary widely between invertebrates and verte brates. This article will focus specifically on the acrosome of mammalian sperm. The acrosome is a product of the Golgi complex and is synthesized and assembled during spermiogenesis. The con tents of the acrosome include structural and nonstructural, nonenzymatic and enzymatic components, and this secretory vesicle is delimited by both inner and outer acrosomal mem branes. These components appear to play important roles in the establishment and maintenance of the acrosomal matrix, in the dispersion of the acrosomal matrix, in the penetration of the egg’s zona pellucida, and possibly in the interaction between the sperm and egg plasma membranes. This vesicle is finally confined within the plasma membrane overlying the entire sperm surface. There remain several questions pertaining to the formation and maturation of this organelle. For example, although prominent biogenesis of the acrosome occurs during the Golgi and cap phases of spermiogenesis, it is not clear whether it is during this developmental process that this organelle actually starts to develop. Furthermore, the acrosome is composed of multiple component proteins, but little is known regarding whether the synthesis of all of these components occurs at the same time or whether the synthesis is ordered and coordinates. Experimental evidence to date suggests the latter mechanism. The mechanisms by which these acrosomal components are targeted to this organelle during biogenesis are also not known. Although spermatogenic cells possess functional mannose-6 phosphate/insulin-like growth factor II receptors, it is not clear whether these receptors play a role in the transport of glyco proteins to the acrosome or whether targeting occurs primarily through the ‘default’ pathway seen in the transport of proteins in other secretory systems. Finally, once these components are packaged into the acrosome, the functional significance of additional processing of these components (i.e., posttransla tional modifications, movement within the organelle) during sperm residence in the testis and/or during residence in the extratesticular male reproductive organs (i.e., epididymis, vas deferens) is not clear. In some species (e.g., guinea pig and mouse), the formation of specific protein domains within the acrosome has been clearly demonstrated, but the mechanism by which this
Brenner’s Encyclopedia of Genetics, 2nd edition, Volume 1
compartmentalization is established is poorly understood and an understanding of the biological role of this compart mentalization is only starting to be realized. Answers to all of these questions will no doubt become apparent when a systematic evaluation of the proteins comprising the acrosome is undertaken with respect to transcription, translation, and posttranslational modifications. An understanding of these processes may greatly further our knowledge of the role of the acrosome in fertilization since it is becoming apparent that this secretory vesicle may have multiple functions (see below). It should also be noted that individuals whose sperm have poorly formed acrosomes or lack acrosomes altogether display inferti lity; this speaks to the importance of this organelle in the normal fertilization process. In any event, studies focused on the synthesis and processing of acrosomal components should be considered in the context of the acrosome functioning as a secretory granule and not a modified lysosome, as has been historically suggested. Although the fusion of the plasma membrane overlying the acrosome and the outer acrosomal membrane constitutes the acrosome reaction, it must be emphasized that this process is very complex and likely involves many of the steps constituting regulated exocytotic processes in other cell types. Such steps might include membrane priming, docking, and fusion. Therefore, this process can also be referred to as acrosomal exocytosis. Recent data support the idea that sperm capacita tion, an extratesticular maturational process that normally occurs in the female reproductive tract and confers fertilization competence to the sperm, may comprise signal transduction events that ready the plasma and outer acrosomal membranes for subsequent fusion during the process of acrosomal exocy tosis. Acrosomal exocytosis is regulated by ligand-induced signal transduction events in which the physiologically relevant ligand is the zona pellucida, an oocyte-specific extracellular matrix. Specific components of the zona pellucida are respon sible for species-specific binding of the sperm and subsequent acrosomal exocytosis. These events are likely mediated by sperm membrane-associated zona pellucida binding proteins and/or receptors; the identity and mode of action of such proteins is still quite controversial. Resultant exocytosis involves the point fusion and vesiculation of the plasma mem brane overlying the acrosome with the outer acrosomal membrane, thus creating hybrid membrane vesicles. The mole cular mechanisms involved in this fusion and vesiculation process are not known. The resultant fusion of these mem branes leads to the subsequent exposure of the acrosomal contents to the extracellular environment. Both the exposed soluble and insoluble components of the acrosome may play important roles in the binding of the acrosome-reacted sperm to the zona pellucida, as well as the subsequent penetration of
doi:10.1016/B978-0-12-374984-0.00008-5
9
10
Acrosome
the acrosome-reacted sperm through the zona pellucida. Although this exocytotic event can be induced by both physiological stimuli and pharmacological agents, the molecu lar mechanisms by which these different stimuli and agents function to induce exocytosis may be dramatically different.
See also: Fertilization.
© 2001 Elsevier Inc. All rights reserved.
This article is reproduced from the previous edition, volume 1, pp 3–4, © 2001, Elsevier Inc.
The acrosome is a vesicle overlying the nucleus of both inverte brate and vertebrate sperm composed of nonenzymatic and enzymatic proteins generally arranged as a matrix; these proteins have been demonstrated in some cases to play specific roles in the fertilization process. The contents of the acrosome are released prior to sperm–egg fusion in a regulated secretory event called the acrosome reaction. The morphology of the acrosome varies between species and the mechanics of the acrosome reaction vary widely between invertebrates and verte brates. This article will focus specifically on the acrosome of mammalian sperm. The acrosome is a product of the Golgi complex and is synthesized and assembled during spermiogenesis. The con tents of the acrosome include structural and nonstructural, nonenzymatic and enzymatic components, and this secretory vesicle is delimited by both inner and outer acrosomal mem branes. These components appear to play important roles in the establishment and maintenance of the acrosomal matrix, in the dispersion of the acrosomal matrix, in the penetration of the egg’s zona pellucida, and possibly in the interaction between the sperm and egg plasma membranes. This vesicle is finally confined within the plasma membrane overlying the entire sperm surface. There remain several questions pertaining to the formation and maturation of this organelle. For example, although prominent biogenesis of the acrosome occurs during the Golgi and cap phases of spermiogenesis, it is not clear whether it is during this developmental process that this organelle actually starts to develop. Furthermore, the acrosome is composed of multiple component proteins, but little is known regarding whether the synthesis of all of these components occurs at the same time or whether the synthesis is ordered and coordinates. Experimental evidence to date suggests the latter mechanism. The mechanisms by which these acrosomal components are targeted to this organelle during biogenesis are also not known. Although spermatogenic cells possess functional mannose-6 phosphate/insulin-like growth factor II receptors, it is not clear whether these receptors play a role in the transport of glyco proteins to the acrosome or whether targeting occurs primarily through the ‘default’ pathway seen in the transport of proteins in other secretory systems. Finally, once these components are packaged into the acrosome, the functional significance of additional processing of these components (i.e., posttransla tional modifications, movement within the organelle) during sperm residence in the testis and/or during residence in the extratesticular male reproductive organs (i.e., epididymis, vas deferens) is not clear. In some species (e.g., guinea pig and mouse), the formation of specific protein domains within the acrosome has been clearly demonstrated, but the mechanism by which this
Brenner’s Encyclopedia of Genetics, 2nd edition, Volume 1
compartmentalization is established is poorly understood and an understanding of the biological role of this compart mentalization is only starting to be realized. Answers to all of these questions will no doubt become apparent when a systematic evaluation of the proteins comprising the acrosome is undertaken with respect to transcription, translation, and posttranslational modifications. An understanding of these processes may greatly further our knowledge of the role of the acrosome in fertilization since it is becoming apparent that this secretory vesicle may have multiple functions (see below). It should also be noted that individuals whose sperm have poorly formed acrosomes or lack acrosomes altogether display inferti lity; this speaks to the importance of this organelle in the normal fertilization process. In any event, studies focused on the synthesis and processing of acrosomal components should be considered in the context of the acrosome functioning as a secretory granule and not a modified lysosome, as has been historically suggested. Although the fusion of the plasma membrane overlying the acrosome and the outer acrosomal membrane constitutes the acrosome reaction, it must be emphasized that this process is very complex and likely involves many of the steps constituting regulated exocytotic processes in other cell types. Such steps might include membrane priming, docking, and fusion. Therefore, this process can also be referred to as acrosomal exocytosis. Recent data support the idea that sperm capacita tion, an extratesticular maturational process that normally occurs in the female reproductive tract and confers fertilization competence to the sperm, may comprise signal transduction events that ready the plasma and outer acrosomal membranes for subsequent fusion during the process of acrosomal exocy tosis. Acrosomal exocytosis is regulated by ligand-induced signal transduction events in which the physiologically relevant ligand is the zona pellucida, an oocyte-specific extracellular matrix. Specific components of the zona pellucida are respon sible for species-specific binding of the sperm and subsequent acrosomal exocytosis. These events are likely mediated by sperm membrane-associated zona pellucida binding proteins and/or receptors; the identity and mode of action of such proteins is still quite controversial. Resultant exocytosis involves the point fusion and vesiculation of the plasma mem brane overlying the acrosome with the outer acrosomal membrane, thus creating hybrid membrane vesicles. The mole cular mechanisms involved in this fusion and vesiculation process are not known. The resultant fusion of these mem branes leads to the subsequent exposure of the acrosomal contents to the extracellular environment. Both the exposed soluble and insoluble components of the acrosome may play important roles in the binding of the acrosome-reacted sperm to the zona pellucida, as well as the subsequent penetration of
doi:10.1016/B978-0-12-374984-0.00008-5
9
10
Acrosome
the acrosome-reacted sperm through the zona pellucida. Although this exocytotic event can be induced by both physiological stimuli and pharmacological agents, the molecu lar mechanisms by which these different stimuli and agents function to induce exocytosis may be dramatically different.
See also: Fertilization.