The acrosomal membrane of boar sperm: a Golgi-derived membrane poor in glycoconjugates

Insights into VPS13 properties and function reveal a new mechanism of eukaryotic lipid transport

The occurrence of protein mediated lipid transfer between intracellular membranes has been known since the late 1960's. Since these early discoveries, numerous proteins responsible for such transport, which often act at membrane contact sites, have been identified. Typically, they comprise a lipid harboring module thought to shuttle back and forth between the two adjacent bilayers. Recently, however, studies of the chorein domain protein family, which includes VPS13 and ATG2, has led to the identification of a novel mechanism of lipid transport between organelles in eukaryotic cells mediated by a rod-like protein bridge with a hydrophobic groove through which lipids can slide. This mechanism is ideally suited for bulk transport of bilayer lipids to promote membrane growth. Here we describe how studies of VPS13 led to the discovery of this new mechanism, summarize properties and known roles of VPS13 proteins, and discuss how their dysfunction may lead to disease.

Mechanism of Acrosome Biogenesis in Mammals

During sexual reproduction, two haploid gametes fuse to form the zygote, and the acrosome is essential to this fusion process (fertilization) in animals. The acrosome is a special kind of organelle with a cap-like structure that covers the anterior portion of the head of the spermatozoon. The acrosome is derived from the Golgi apparatus and contains digestive enzymes. With the progress of our understanding of acrosome biogenesis, a number of models have been proposed to address the origin of the acrosome. The acrosome has been regarded as a lysosome-related organelle, and it has been proposed to have originated from the lysosome or the autolysosome. Our review will provide a brief historical overview and highlight recent findings on acrosome biogenesis in mammals.

Protein transport and organization of the developing mammalian sperm acrosome

Experiments indicate that the mammalian acrosome develops as a result of a time-dependent sequence of events which involves protein incorporation into distinct regions or acrosomal domains. These domains can be characterized by electron microscopy and their isolation and partial purification are being accomplished. Recent success in isolating and characterizing major proteins that compromise the Golgi apparatus should accelerate knowledge of the interaction of the Golgi with the developing acrosome. Progress in this area is reviewed with the view that understanding the events involved in the transport of proteins from the Golgi apparatus to the acrosome and the mechanisms involved in positioning and modifying these proteins during spermiogenesis should provide a clearer understanding of how the acrosome develops in preparation for its role in fertilization.

Mycobacteria and human autoimmune disease: direct evidence of cross-reactivity between human lactoferrin and the 65-kilodalton protein of tubercle and leprosy bacilli

We document here by Western immunoblotting and immunogold ultracytochemistry that polyclonal antibodies against human lactoferrin (Lf) bind to tubercle and leprosy bacilli. In situ immunogold labeling of Mycobacterium leprae (present in armadillo liver and in human skin) and of Mycobacterium tuberculosis indicated that receptors for anti-Lf antibodies were present both on the cytoplasm and on the envelope of the bacilli. We found by immunoblotting that the 65-kDa heat shock protein is the major component of M. leprae and M. tuberculosis that is responsible for the binding of the anti-Lf probe. Furthermore, we show that anti-Lf immunoglobulin G eluted from the nitrocellulose-transferred mycobacterial 65-kDa protein band did bind back to Lf. Ultracytochemistry of biopsy samples of human lepromas showed that dead or severely damaged M. leprae was strongly marked by the anti-Lf antibodies; a similar pattern of immunogold marking was observed on M. leprae when antibodies against the 65-kDa mycobacterial protein were used. Our results offer direct evidence that the 65-kDa protein of leprosy and tubercle bacilli is recognized with specificity by antibodies against the human protein Lf. The Lf-65-kDa protein antigenic cross-reactivity may contribute to the formation of autoantibodies and immune complexes as well as to other autoimmune events that are frequent in tuberculosis and leprosy. Our immunocytochemical data also suggest that the cross-reactivity may persist for some time after the death of mycobacteria in infected hosts.

Freeze-fracture cytochemistry: a simplified guide and update on developments

A wide variety of methods by which cytochemistry and freeze-fracture can be successfully combined have recently become available. All these techniques are designed to provide information on the chemical nature of structural components revealed by freeze-fracture, but differ in how this is achieved, in precisely what type of information is obtained, and in which types of specimen can be studied. Colloidal gold labelling is the most widely used cytochemical technique in freeze-fracture cytochemistry, and for many of the methods it is indispensable. In principle, there are four points in which the cytochemical labelling step may be integrated into the standard freeze-fracture procedure: (i) before the specimen has been frozen, (ii) after it has been fractured and thawed, (iii) after platinum shadowing or (iv) after completion of the full replication sequence. Retention of the gold label so that it can be viewed with replicas can be achieved by depositing platinum and/or carbon upon the labelled surface, thereby partially entrapping the marker particles within the replica, or by retaining, attached to the replica, fragments of fractured membrane (or other cellular components) that would normally have been lost during the replica cleaning step. Another approach to visualizing the label is to use sections, either with portions of a replica included face-on, or for examining the fracture path through the sample (without replica). Recent developments have centered on the use of replicas to stabilize half-membrane leaflets; not only may these and associated attached components be retained for labelling just before mounting, but they provide a means for manipulating the specimen--specifically, turning it over during processing--so that additional structural information can be obtained. This article aims to explain how modern freeze-fracture cytochemistry works, and how the various techniques differ in what they can tell us about membranes and other cellular structures. With the effectiveness of many of the techniques now demonstrated, freeze-fracture cytochemistry is firmly established, alongside a range of related labelling techniques, for increasing application in cell and membrane biology in the 1990s.

Freeze-fracture cytochemistry: review of methods

"Freeze-fracture cytochemistry" encompasses a diversity of recently developed techniques in which freeze-fracture and cytochemistry are combined. Cytochemical labeling may, in principle, be integrated into one of three basic points in the standard freeze-fracture procedure; 1) before the specimen is frozen, 2) after it has been fractured, or 3) after it has been platinum shadowed and/or carbon coated. Visualization of the labeled cellular structures can be achieved by a variety of different methodologies. For example, the markers (usually colloidal gold particles) may be viewed embedded within a replica, or attached to it via fragments of membrane (or other cellular components). Sectioning is a central strategy in a number of techniques, either in combination with or in place of replication. The different combinations of methods that have been devised are not, for the most part, alternative ways of arriving at the same result; each provides quite distinct information about specific classes of membrane component or other structure in the cell. The purpose of this review is to present, within a single article, a systematic survey of the full range of techniques currently available in freeze-fracture cytochemistry. Emphasis is placed on explaining the principles underlying the methods and on illustrating their applications. With the success recently achieved, freeze-fracture cytochemistry has moved from the phase of experimental development to a position in which it may be expected increasingly to make significant contributions across a wide spectrum of problems in cell and membrane biology.

Widespread occurrence of calicin, a basic cytoskeletal protein of sperm cells, in diverse mammalian species

A novel cytoskeletal element consisting of dense webs of thin (3-14 nm) filaments surrounding the nucleus of the sperm head has recently been isolated and shown to be associated with certain major basic proteins. Using antibodies specific for calicin, a prominent Mr-60,000 cytoskeletal protein of the posterior calyx of bull sperm heads detected in immunoblotting on gel electrophoretically separated polypeptides as well as in immunofluorescence and immunoelectron microscopy, we show that the same--or an immunologically related--polypeptide occurs in sperm heads of other species with greatly different morphology, including human, boar, guinea pig, hamster, rat and mouse. The calicin localization in the various species is described and discussed in relation to the specific sperm morphology.

Backscattered electron imaging of lectin binding sites in tissues following freeze-fracture cytochemistry

Recently, cytochemical techniques have been applied for localizing membrane components; however, transmission electron microscopy only provides two-dimensional information about their distribution. Scanning electron microscopy, on the other hand, offers the possibility of examining the three-dimensional architecture of biological samples. The fracture-label cytochemical technique was combined with the backscattered electron imaging (BEI) mode of the scanning electron microscope to visualize the in vivo distribution of lectin binding sites on freeze-fractured biological membranes in tissues and cells. Pancreatic and testicular tissues, fixed with glutaraldehyde, were freeze-fractured and labeled with Helix pomatia lectin-gold or Ricinus communis I-gold complexes. The labeled specimens were then critical-point dried and replicated with platinum-carbon for routine transmission electron microscopy or with carbon alone for BEI. Lectin-gold labeling of fractured plasma and intracellular membranes observed with BEI showed a labeling pattern similar to that seen by the replica method. However, BEI-fracture-label provided additional information about the distribution of the labeling with respect to three-dimensional organization of tissues and cells. Large sample areas could be examined, making this technique particularly useful as a survey method for specimens that are either differentially labeled or composed of heterogenous cell populations.

Postreplication labeling of E-leaflet molecules: membrane immunoglobulins localized in sectioned, labeled replicas examined by TEM and HVEM

Conventional freeze-fracture techniques were combined with immunogold labeling and with plastic embedding and sectioning to analyze the distribution of membrane immunoglobulins (mIgs) and their associated intramembrane particles (IMPs) in E-face replicas of murine B-lymphocyte plasma membranes. Immunogold labels were applied to cells after the process of freeze-fracture and replication. Conventional stereoscopic transmission electron microscopic examination of sectioned, labeled replicas (SLRs) revealed that the gold-labeled mIgs were bound to and localized on the outer leaflets of split and replicated membranes. The gold labels were attached to the external determinants of the mIg molecules, which were retained beneath and contiguous with the replicated E-faces. The mIgs were also localized on the external surface of unreplicated microvilli. In addition, thick sections examined by high-voltage transmission electron microscopy (HVEM) revealed large expanses of replica with well-resolved IMPs. mIgs colocalized with small-diameter (less than 60 A) IMPs in E-face replicas of B-lymphocytes whose mIgs were patched by anti-immunoglobulin. Thus, postreplication E-surface labeling of split and replicated membranes is a high-resolution technique that is suitable for the study of membrane protein distribution in E-face replicas and contiguous nonreplicated tissue.

Preferential association of glycoproteins to the euchromatin regions of cross-fractured nuclei is revealed by fracture-label

We used fracture-label to establish ultrastructural localization of glycoproteins in cross-fractured nuclei of duodenal columnar and exocrine pancreatic cells. Mannose residues were detected in cell nuclei by labeling freeze-fractured tissues with concanavalin A-horseradish peroxidase X colloidal gold (Con A-HRP X CG) or direct concanavalin A X colloidal gold (Con A X CG); fucose residues were detected with Ulex Europaeus I X colloidal gold (UEA I X CG) markers. Areas of the three main intranuclear compartments (euchromatin, heterochromatin, and nucleolus) exposed by freeze-fracture were determined by automated image analysis. Colloidal gold particles bound to each nuclear subcompartment were counted and the results expressed in number of colloidal gold particles per square micrometer +/- SEM. Duodenal and pancreatic tissues fractured and labeled with Con A-HRP X CG complex or direct Con A X CG conjugates showed that the vast majority of Con A binding sites was confined to euchromatin regions with only sparse labeling of the heterochromatin and nucleolus. UEA I labeling of duodenal columnar cells showed that colloidal gold particles were almost exclusively confined to cross-fractured areas where euchromatin is exposed. Trypsinization of the fractured tissues before labeling with Con A and UEA I abolished 95-100% of the original label. Our results show that, within the nucleoplasm, mannose and fucose are residues of glycoproteins preferentially located within the regions of euchromatin.