Cestode transmission patterns

The paradox of high prevalence but low probability of having an egg develop to an adult has been resolved by the evolution of 3 major and basic strategies involving transmission: evolution of life cycles interpolated into host biology; presentation of infective stages that increase probability of contact between host and parasite; and increase in reproductive potential. The rarity of direct cycles confirms that cycles in themselves, with at least 2 hosts, are a key element of cestode success because they provide a vehicle for dispersal and transmission of infective stages. Transmission is primarily by passive stages that become incorporated through intermediate hosts or accidentally in the food chain. High host specificity results from efficient transmission pathways but may represent a fragile system for the evolution of the species. Probability of transmission is increased through diversity of intermediate hosts, making eggs more susceptible to ingestion and by behavioral manipulation of hosts by parasite stages. Spatial and temporal aspects of transmission may be increased through paratenesis. Asexual proliferation of immature stages is uncommon and is favored where there is selective predation; such proliferation may be part of a transmission strategy of colonial cestodes that require high infrapopulations in order to survive. Hyperapolysis may be part of a transmission strategy used by the Tetraphyllidea, Trypanorhyncha, and Lecanicephalidea to increase proglottid production. The dynamics of transmission for cestodes of humans and domestic animals require a different perspective than those of wild hosts. All strategies are reviewed within the framework of certain cestode morphological and ecological constraints. A total of 11 figures and 48 references complements the text.     

Cestode systematics and phylogeny move forward

This paper represents a meeting report for the Fifth International Workshop on Cestode Systematics and Phylogeny held at the Institute of Parasitology, Academy of Sciences of the Czech Republic, České Budějovice, 18–22 July 2005. The major topics discussed included (i) the progress in cestode systematics during 2002–2005, (ii) the use of the life-cycle data in phylogenetic studies, (iii) the utilisation of new morphological and molecular characters in cestode systematics and phylogeny, and (iv) the ongoing work on the completion of the Global Cestode Database.     

Secretory lysosomes

Regulated secretion of stored secretory products is important in many cell types. In contrast to professional secretory cells, which store their secretory products in specialized secretory granules, some secretory cells store their secretory proteins in a dual-function organelle, called a secretory lysosome. Functionally, secretory lysosomes are unusual in that they serve both as a degradative and as a secretory compartment. Recent work shows that cells with secretory lysosomes use new sorting and secretory pathways. The importance of these organelles is highlighted by several genetic diseases, in which immune function and pigmentation--two processes that normally involve secretory lysosomes--are impaired.     

Cestode regulation of inflammation and inflammatory diseases

Helminth parasites are masters of immune regulation; a likely prerequisite for long-term survival by circumventing their hosts’ attempt to eradicate them. From a translational perspective, knowledge of immune events as a response to infection with a helminth parasite could be used to reduce the intensity of unwanted inflammatory reactions. Substantial data have accumulated showing that inflammatory reactions that promote a variety of auto-inflammatory diseases are dampened as a consequence of infection with helminth parasites, via either the mobilization of an anti-worm spectrum of immune events or by the direct effect of secretory/excretory bioactive immunomodulatory molecules released from the parasite. However, many issues are outstanding in the definition of the mechanism(s) by which infection with helminth parasites can affect the outcome, positively or negatively, of concomitant disease. We focus on a subgroup of this complex group of metazoan parasites, the cestodes, summarizing studies from rodent models that illustrate if, and by what mechanisms, infection with tapeworms ameliorate or exaggerate disease in their host. The ability of infection with cestodes, or other classes of helminth, to worsen a disease course or confer susceptibility to intracellular pathogens should be carefully considered in the context of ‘helminth therapy’. In addition, poorly characterised cestode extracts can regulate murine and human immunocyte function, yet the impact of these in the context of autoimmune or allergic diseases is poorly understood. Thus, studies with cestodes, as representative helminths, have helped cement the concept that infection with parasitic helminths can inhibit concomitant disease; however, issues relating to long-term effects, potential side-effects, mixed pathogen infections and purification of immunomodulatory molecules from the parasite remain as challenges that need to be addressed in order to achieve the use of helminths as anti-inflammatory agents for human diseases.     

Conversion of dense lysosomes into light lysosomes during hepatocytic autophagy

Incubation of isolated rat hepatocytes under conditions which support maximal autophagy (amino acid-free medium) caused a marked alteration in the density distribution of lysosomes in continuous metrizamide gradients (mean peak density reduced from 1.14 to 1.09 g/ml). The autophagic sequestration inhibitor 3-methyladenine (3MA) partially prevented the density shift, presumably by stopping the formation of light autophagosomes which otherwise fuse with dense lysosomes and thereby alter the lysosomal density.     

Regulating secretory lysosomes

Secretory lysosomes are lysosomes which are capable of undergoing regulated secretion in response to external stimuli. Many cells of the immune system use secretory lysosomes to release proteins involved in their specialised effector mechanisms. Precisely how lysosomal secretion is regulated in each of these cell types is now the study of much research as these mechanisms control the ability of each of these cells to function. Studies on a number of human genetic diseases have identified some key proteins in controlling secretory lysosome release, and now many interacting partners have been identified. The different regulatory components seem to vary from one cell type to another, providing a multitude of ways for fine tuning the release of secretory lysosomes.     

The proteome of lysosomes

Lysosomes are organelles of eukaryotic cells that are critically involved in the degradation of macromolecules mainly delivered by endocytosis and autophagocytosis. Degradation is achieved by more than 60 hydrolases sequestered by a single phospholipid bilayer. The lysosomal membrane facilitates interaction and fusion with other compartments and harbours transport proteins catalysing the export of catabolites, thereby allowing their recycling. Lysosomal proteins have been addressed in various proteomic studies that are compared in this review regarding the source of material, the organelle/protein purification scheme, the proteomic methodology applied and the proteins identified. Distinguishing true constituents of an organelle from co-purifying contaminants is a central issue in subcellular proteomics, with additional implications for lysosomes as being the site of degradation of many cellular and extracellular proteins. Although many of the lysosomal hydrolases were identified by classical biochemical approaches, the knowledge about the protein composition of the lysosomal membrane has remained fragmentary for a long time. Using proteomics many novel lysosomal candidate proteins have been discovered and it can be expected that their functional characterisation will help to understand functions of lysosomes at a molecular level that have been characterised only phenomenologically so far and to generally deepen our understanding of this indispensable organelle.     

The Nervous System of Early Larval Stages of the Cestode Diphyllobothrium dendriticum

The nervous system (NS) of the coracidium and procercoid larval stages of the pseudophyllidean cestode Diphyllobothrium dendriticum was examined by whole mount immunocytochemistry and electron microscopy. Antisera against the bioamine serotonin and the molluscan cardioexcitatory peptide FMRF-amide gave a positive immunoreaction in the NS of the procercoid. Ultrastructurally, three types of nervous elements were discerned. Ultrastructural evidence of both aminergic and peptidergic neurons in the coracidium is provided. The NS of these early larval stages is simply organized. The serotonergic part of the NS shows a beginning centralization in the procercoid, while the peptidergic neurons are more peripherally distributed.