Cytoskeletal dynamics in animal morphogenesis

The regulation of proteins associated with the cytoskeleton by hepatitis B virus X protein during hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is a major malignant disease worldwide, and chronic hepatitis B virus (HBV) infection is one of the primary causes for this type of cancer. Hepatitis B virus X protein (HBx) is a non-structural protein encoded by the viral genome that has significant effects on the pathogenesis of HCC. With the development of high-throughput assays and technologies, the abnormal HBx-induced expression of certain cellular proteins with assorted biological functions has been investigated. These target proteins identified by various methods include specific proteins associated with the cellular cytoskeleton, which contribute to HBx-induced hepatocarcinogenesis. In addition, the cytoskeletal proteins deregulated by HBx are involved in cell morphogenesis, adhesion, migration and proliferation. This review aims to summarize the current understanding of the expression profiles of HBx-associated cytoskeletal proteins, as well as their complex functions and underlying mechanisms in hepatocarcinogenesis. Considering that the potential therapeutics for various types of tumors may function through the stabilization of cytoskeletal proteins in order to restrict cellular movement and limit intracellular processes, clarifying the mechanisms underlying protein-associated cytoskeleton dysregulation by HBx may provide novel possibilities and potent therapeutic targets for HBV-associated HCC.

Structure and function of the pore canals of the sea urchin madreporite

The madreporite is one of the most enigmatic organs of the echinoderms. It connects the internal cavity of the water-vascular system to the external seawater through its many pores which are lined with ciliated epithelium. Its physiological function, even the nature of transport, if any, through its pores has been controversial since the 19th century. We report here that the pores of the echinoid madreporite are capable of changing their diameter in response to stimulation and that their cilia support bidirectional transport, drawing water into the water-vascular system while expelling larger particles. A reversible constriction of the pore was induced by acetylcholine (ACh). At > 10 -7 M, ACh reduced the pore diameter to about 70% in 2 mins and to 60% in 6 mins. Atropine (10 -4 M), but not d -tubocurarine (10 -4 M), blocked the response to ACh (10 -7 M). Adrenaline (10 -5 M) had no effect on the pore diameter. These results suggest that the size of the pore is under cholinergic control. Observations of isolated pore-canal tissue indicated that the changes in pore size are not accompanied by changes in volume of the cells surrounding the pores. Electron microscopy showed no muscle cells in or near the pore-canal tissue. In the apical region of the ciliated columnar epithelial cell lining the pore canal, there are fine filaments attached to the adherens junctions. This region of the cell was positively stained with rhodamine-phalloidin, suggesting that the filaments consist of F-actin. It is possible that the filaments are involved in the pore closure response. We studied the water flow through isolated single pore canals, using Indian ink, and found that the water flows inwards while particles larger than ca. 1 μM move in the opposite direction. When the pores became narrower by the action of ACh, complete blocking of the water flow was observed. It is possible that the madreporite controls the volume or the pressure of the fluid in the water-vascular system by both the ciliary-driven water flow and the pore closure response.

Cytoskeletal features of the syncytial epidermis of the tapeworm Hymenolepis diminuta

A hallmark feature of parasitic platyhelminths is a cytoarchitecturally unusual syncytial epidermis composed of a peripheral layer of continuous cytoplasm (the ectocytoplasm) connected to underlying nucleated cell bodies by small cytoplasmic bridges. The helminth epidermis, or tegument, plays important roles in protection and nutrient acquisition; cestodes, in fact, completely lack a gastrointestinal tract and absorb all nutritive material through the tegument. Perhaps not surprisingly, the cestode tegument bears certain resemblances to the mucosal epithelium of the vertebrate small intestine, including the possession of a microvillous brush border upon the surface of the ectocytoplasm. In contrast to the intestinal epithelial cell, however, very little is known concerning the nature and organization of the cytoskeleton within the helminth epidermis. Therefore, a number of different microscopical preparative techniques were used to examine the tegument of the tapeworm Hymenolepis diminuta for the presence and distribution of microfilaments, intermediate filaments, and microtubules. It was found that both actin-containing microfilaments and intermediate-sized filaments are present but are restricted to specific locations along the plasmalemmae of the ectocytoplasm. In contrast, microtubules are found throughout the tegument, and are concentrated in the supranuclear regions of the perikarya and in the cytoplasmic bridges interconnecting the perikarya and ectocytoplasm. Unlike brush borders of most other epithelia, the cestode epidermal brush border lacks a filamentous terminal web and is instead associated with microtubules. A network of fine filaments, 5-8 nm in diameter but distinct from actin-containing microfilaments, runs throughout the ectocytoplasm and appears to interlink tegumental vesicles. These fine filaments may represent the primary "skeletal" system responsible for maintaining the structure of the tegumental cytoplasm.