Techniques for the visualisation of cytoskeletal components in Dictyostelium discoideum

Dictyostelium discoideum: a model system for ultrastructural analyses of cell motility and development

Dictyostelium occupies an interesting niche in the grand scheme of model organisms. On the one hand, it is a compact, highly motile single cell that presents numerous opportunities to investigate the fundamental mechanisms of signal transduction, cell movement, and pathogen infection. However, upon starvation, individual cells enter a developmental pathway that involves cell aggregation, cell-cell adhesion, pattern formation, and differentiation. Thus, Dictyostelium is also well known as a basic model for studying developmental processes. Electron microscopy (EM) has played a large role in both the unicellular and the multicellular life stages, for example, providing image detail for structure/function relationships of cytoskeletal proteins, the deposition of cellulose fibrils in maturing spores, and the identification of intercellular junctional complexes. Powerful combinations of robust molecular genetic tools, high-resolution light microscopy, and EM methods make this organism an attractive model for imaging dynamic cell processes. This chapter serves to highlight the past and current EM approaches that have advanced our understanding of how cells and proteins function.

Freeze-substitution: origins and applications

Freeze-substitution is a physicochemical process in which biological specimens are immobilized and stabilized for microscopy. Water frozen within cells is replaced by organic solvents at subzero temperatures. Freeze-substitution is widely used for ultrastructural and immunocytochemical analyses of cells by transmission and scanning electron microscopy. Less well recognized is its superiority over conventional chemical fixation in preserving labile and rare tissue antigens for immunocytochemistry by light microscopy. In the postgenome era, the focus of molecular genetics will shift from analyzing DNA sequence structure to elucidating the function of gene networks, the intercellular effects of polygenetic diseases, and the conformational rearrangements of proteins in situ. Novel strategies will be needed to integrate knowledge of chemical structures of normal and abnormal macromolecules with the physiology and developmental biology of cells and tissues from whole organisms. This review summarizes the progress and future prospects of freeze-substitution for such explorations.

Intracellular growth of Legionella pneumophila in Dictyostelium discoideum, a system for genetic analysis of host-pathogen interactions

Conditions were established in which Legionella pneumophila, an intracellular bacterial pathogen, could replicate within the unicellular organism Dictyostelium discoideum. By several criteria, L. pneumophila grew by the same mechanism within D. discoideum as it does in amoebae and macrophages. Bacteria grew within membrane-bound vesicles associated with rough endoplasmic reticulum, and L. pneumophila dot/icm mutants, blocked for growth in macrophages and amoebae, also did not grow in D. discoideum. Internalized L. pneumophila avoided degradation by D. discoideum and showed evidence of reduced fusion with endocytic compartments. The ability of L. pneumophila to grow within D. discoideum depended on the growth state of the cells. D. discoideum grown as adherent monolayers was susceptible to L. pneumophila infection and to contact-dependent cytotoxicity during high-multiplicity infections, whereas D. discoideum grown in suspension was relatively resistant to cytotoxicity and did not support intracellular growth. Some known D. discoideum mutants were examined for their effect on growth of L. pneumophila. The coronin mutant and the myoA/B double myosin I mutant were more permissive than wild-type strains for intracellular growth. Growth of L. pneumophila in a G(beta) mutant was slightly reduced compared to the parent strain. This work demonstrates the usefulness of the L. pneumophila-D. discoideum system for genetic analysis of host-pathogen interactions.

Preparation of basal cell membranes for scanning probe microscopy

Scanning probe microscopy has the potential for investigating membranes in a physiological environment. We prepared with a lysis-squirting protocol basal cell membranes, that are suitable for scanning probe microscopy. Investigations using atomic force microscopy under liquid revealed cellular filaments which correlated perfectly with fluorescently stained actin filaments. Globular structures with a diameter as little as 10 nm could be resolved by stripping cytoplasmic components from the membranes. Therefore, cytoplasmic sides of supported basal cell membranes prove useful to gain high resolution with scanning probe microscopy in studies of plasma membrane associated structures and processes under buffer solution.

Visualization of the subsarcolemmal cytoskeleton network of mouse skeletal muscle cells by en face views and application to immunoelectron localization of dystrophin

The ultrastructural organization of the highly interconnected filamentous network underneath the sarcolemma as well as the role played by the muscle protein dystrophin within this cytoskeleton remain yet unclear. More accurate information has been obtained by using a method which provides three-dimensional en face views of large membrane areas applied to mouse cultured myotubes and isolated adult skeletal muscle fibres. Two levels have been distinguished in the cytoskeleton underlying the sarcolemma: the submembranous level, partly integrated into the membrane, and the cortical level, invading the proximal cytoplasmic space. Few differences have been found between the membrane cytoskeletons of myotubes issued from 14-day-old cultures and those of adult fibres. The comparison was done with cells where dystrophin is missing (mdx mouse muscle): surprisingly, the lack of dystrophin does not induce obvious or dramatic ultrastructural disorganization, either in the cortical cytoskeletal network or in the submembranous one. Immunogold labelling of either the central-rod or the C-terminal domain of dystrophin is not located among the cortical network. This study provides additional data on the spatial ordering of subsarcolemmal cytoskeletal elements: dystrophin does not appear as a filamentous structure entirely located among subsarcolemmal cytoskeleton but seems partly embedded in membranous material.