Actin filaments responsible for the location of the nucleus in the lentil statocyte are sensitive to gravity

The location of the nucleus in statocytes of lentil roots grown: I), at 1 g on the ground, 2), on a 1 g centrifuge in space, 3), in simulated microgravity on a slowly rotating clinostat (0.9 rmp) 4), in microgravity in space was investigated and statistically evaluated. In cells differentiated at 1 g on the ground, the nuclear membrane was almost in contact with the plasmalemma lining the proximal cell wall, whereas in statocytes of roots grown on the clinostat there was a distance of 0.47 μm horizontal clinorotation) and of 0.76 μm vertical clinorotation) between these membranes. However, in microgravity the nucleus was the most displaced, 0.87 μm from the proximal cell wall. Centrifugation of vertically grown roots in the root-tip direction showed that the threshold of centrifugal force to detach all nuclei from the proximal cell wall was about 40 g. In statocytes developed in the presence of cytochalasin B at 1 g the nuclei were sedimented on the amyloplasts at the distal cell pole, demonstrating that the location of the nucleus depends on actin filaments. The results obtained are in agreement with the hypothesis that gravity causes a tension of actin filaments and that this part of the cytoskeleton undergoes a relaxation in microgravity.     

Actin filaments responsible for the location of the nucleus in the lentil statocyte are sensitive to gravity

The location of the nucleus in statocytes or lentil roots grown: 1), at 1 g on the ground, 2), on a 1 g centrifuge in space, 3), in simulated microgravity on a slowly rotating clinostat (0.9 rmp) 4), in microgravity in space was investigated and statistically evaluated. In cells differentiated at 1 g on the ground, the nuclear membrane was almost in contact with the plasmalemma lining the proximal cell wall, whereas in statocytes of roots crown on the clinostat there was a distance of 0.47 micrometers (horizontal clinorotation) and or 0.76 micrometers (vertical clinorotation) between these membranes. However, in microgravity the nucleus was the most displaced, 0.87 micrometers from the proximal cell wall. Centrifugation of vertically grown roots in the root-tip direction showed that the threshold of centrifugal force to detach all nuclei from the proximal cell wall was about 40 g. In statocytes developed in the presence of cytochalasin B at 1 g the nuclei were sedimented on the amyloplasts at the distal cell pole, demonstrating that the location of the nucleus depends on actin filaments. The results obtained are in agreement with the hypothesis that gravity causes a tension of actin filaments and that this part of the cytoskeleton undergoes a relaxation in microgravity.     

Actin filaments line up acrossTradescantia epidermal cells,anticipating wound-induced divison planes

Summary This paper describes the role of actin filaments in setting up the phragmosome — the transvacuolar device that anticipates the division plane — and in forming a supracellular system that seems to override cell boundaries. Tradescantia leaf epidermal cells were induced to divide by wounding the leaf. New division planes formed parallel to slits, and encircled puncture wounds — the new division planes lining up across cells, instead of the joints being off-set as in normal, unwounded tissue. Within 30 min after wounding, rhodamine phalloidin staining showed that a belt of fine, cortical actin filaments formed parallel to the wound. In the next stage, migration of nuclei to a wall adjacent to the wound, involved pronounced association of actin filaments with the nucleus. Migration could be inhibited with cytochalasin D, confirming the role of actin in traumatotaxis. Later still, actin strands were seen to line up from cell to cell, parallel to the wound, anticipating the future division plane. Next, actin filaments accumulated in this anticlinal plane, throughout the depth of the cell, thereby contributing to the formation of the phragmosome. The phragmosome has been shown in previous work (Flanders et al. 1990) to contain microtubules that bridge nucleus to cortex, and is now found to contain actin filaments. Actin filaments are therefore involved in the key stages of nuclear migration and division plane alignment. The supracellular basis of actin alignment is discussed.Dedicated to the memory of Professor Oswald Kiermayer     

Actin up: regulation of podocyte structure and function by components of the actin cytoskeleton

Podocytes of the renal glomerulus are unique cells with a complex cellular organization consisting of a cell body, major processes and foot processes. Podocyte foot processes form a characteristic interdigitating pattern with foot processes of neighboring podocytes, leaving in between the filtration slits that are bridged by the glomerular slit diaphragm. The highly dynamic foot processes contain an actin-based contractile apparatus comparable to that of smooth muscle cells or pericytes. Mutations affecting several podocyte proteins lead to rearrangement of the actin cytoskeleton, disruption of the filtration barrier and subsequent renal disease. The fact that the dynamic regulation of the podocyte cytoskeleton is vital to kidney function has led to podocytes emerging as an excellent model system for studying actin cytoskeleton dynamics in a physiological context.     

Actin in oogenesis

Summary

The laser-scanning confocal microscope employed in conjunction with various specific agents and antibodies conjugated to fluorescent dyes reveals details of the actin scaffolding of developing oocytes and the nuclei of attendant cells. The employment of DNase I followed by anti-DNase I antibody has been particularly useful in revealing otherwise cryptic actin-containing structures. The cortical cytoskeleton of developing moth eggs was found to bind both poly (A)+RNA and RNA Pol II. Exposure to cytochalasin D disrupted the actin of the cortex, and at the same time caused redistribution of the proteins and RNA associated with the cytoskeleton. Cytochalasin also had dramatic effects on the structure of nuclei of nurse and follicle cells. Taken in context of the actin network in nuclei uncovered by DNase-anti-DNase treatment, these results suggest that actin plays a major structural and perhaps functional role in insect nuclei.     

Actin regulation in the malaria parasite

Many intracellular pathogens hijack host cell actin or its regulators for cell-to-cell spreading. In marked contrast, apicomplexan parasites, obligate intracellular, single cell eukaryotes that are phylogenetically older than the last common ancestor of animals and plants, employ their own actin cytoskeleton for active motility through tissues and invasion of host cells. A hallmark of actin-based motility of the malaria parasite is a minimal set of proteins that potentially regulate microfilament dynamics. An intriguing feature of the Plasmodium motor machinery is the virtual absence of elongated filamentous actin in vivo. Despite this unusual actin regulation sporozoites, the transmission stages that are injected into the mammalian host by Anopheles mosquitoes, display fast (1-3 μm/s) extracellular motility. Experimental genetics and analysis of recombinant proteins have recently contributed to clarify some of the cellular roles of apicomplexan actin monomer- and filament-binding proteins in parasite life cycle progression. These studies established that the malaria parasite employs multiple proteins that bind actin to form pools of readily polymerizable monomers, a prerequisite for fast formation of actin polymers. The motile extracellular stages of Plasmodium parasites are an excellent in vivo model system for functional characterization of actin regulation in single cell eukaryotes.     

Actin in Xenopus Development: Indirect Immunofluorescence Study of Actin Localization

Actin was studied in Xenopus unfertilized eggs and early developmental stages. Immunochemical proof is given of structural differences between Xenopus laevis muscle actin and nonmuscle cell actin. Actin localization and changes of actin aggregation during Xenopus development were observed using indirect immunofluorescence. We have also tried to explain the presence of an actin shell around the yolk platelets that appeared in our experiments.