Microvillus

Abstract: Following homozygosity mapping in a single kindred, we identified nonsense and missense mutations in MYO5B, encoding type Vb myosin motor protein, in individuals with microvillus inclusion disease (MVID). MVID is characterized by lack of microvilli on the surface of enterocytes and occurrence of intracellular vacuolar structures containing microvilli. In addition, mislocalization of transferrin receptor in MVID enterocytes suggests that MYO5B deficiency causes defective trafficking of apical and basolateral proteins in MVID.

Abstract: Taste buds contain a variety of morphological and histochemical types of elongate cells. Serotonin, neuron-specific enolase (NSE), ubiquitin carboxyl terminal hydrolase (PGP 9.5), and neural cell adhesion molecule (N-CAM) all have been described as being present in the morphologically defined Type III taste cells in rats. In order to determine whether these substances coexist in a single cell, we undertook immunohistochemical and ultrastructural analysis of taste buds in rats. Double-label studies show that PGP 9.5 and NSE always colocalize. In contrast, PGP 9.5 and serotonin seldom colocalize. Further, whereas the serotonin-immunoreactive cells are always slender and elongate, the PGP 9.5/NSE population comprise two morphological types--one slender, the other broader and pyriform. Although gustducin-immunoreactive taste cells appear similar in overall shape to the pyriform PGP 9.5/NSE population, gustducin never colocalizes with PGP 9.5 or NSE. The serotonin-immunoreactive taste cells have an invaginated nucleus, synaptic contacts with nerve fibers, and taper apically to a single, large microvillus. These are all characteristics of Type III taste cells described previously in rabbits (Murray [1973] Ultrastructure of Sensory Organs I. Amsterdam: North Holland. p 1-81). PGP 9.5-immunoreactive taste cells exhibit two morphological varieties. One type is similar to the serotonin-immunoreactive population, containing an invaginated nucleus, synapses with nerve fibers, and a single large microvillus. The other type of PGP 9.5-immunoreactive taste cell has a large round nucleus and the apical end of the cell tapers to a tuft of short microvilli, which are characteristics of Type II taste cells. Thus, in rats, some Type III cells accumulate serotonin but do not express PGP 9.5, whereas others express PGP 9.5 but do not accumulate amines. Similarly, Type II taste cells come in at least two varieties: those immunoreactive for gustducin and those immunoreactive for PGP 9.5.

Abstract: Ezrin is a component of the microvillus cytoskeleton of a variety of polarized epithelial cells and is believed to function as a membrane-cytoskeletal linker. In this study, we isolated microvilli from human placental syncytiotrophoblast as a model system for biochemical analysis of ezrin function. In contrast to intestinal microvilli, ezrin is a major protein component of placental microvilli, comprising approximately 5% of the total protein mass and present at about one quarter of the molar abundance of actin. Gel filtration and chemical cross-linking studies demonstrated that ezrin exists mainly in the form of noncovalent dimers and higher order oligomers in extracts of placental microvilli. A novel form of ezrin, apparently representing covalently cross-linked adducts, was present as a relatively minor constituent of placental microvilli. Both oligomers and adducts remained associated with the detergent-insoluble cytoskeleton, indicating a tight interaction with actin filaments. Moreover, stimulation of human A431 carcinoma cells with EGF induces the rapid formation of ezrin oligomers in vivo, thus identifying a signal transduction pathway involving ezrin oligomerization coincident with microvillus assembly. In addition to time course studies, experiments with tyrosine kinase and tyrosine phosphatase inhibitors revealed a correlation between the phosphorylation of ezrin on tyrosine and the onset of oligomer formation, consistent with the possibility that phosphorylation of ezrin might be required for the generation of stable oligomers. Based on these observations, a model for the assembly of cell surface structures is proposed.

Abstract: We report on the internal ultrastructure of long, finger-like microvilli which cover the surface of the fertilized sea urchin egg. Eggs were attached to polylysine-coated surfaces; their upper portions were sheared away with a stream of buffer which left behind only their plasma membranes and adjacent cytoplasmic structures. Scanning electron microscopy (EM) of such fragments revealed intact thin protoplasmic projections radiating away from the body of the cortex. By transmission EM of cortices similarly prepared on grids, small bundles of microfilaments appear as cores within the thin cytoplasmic projections. These microfilaments are shown to be composed of actin by their ability to interact with muscle heavy meromyosin (HMM). HMM-decorated microfilaments possess repeating arrowheads which uniformly point toward the cell interior. Actin bundles in the microvilli of sea urchin eggs may mediate microvillus support and elongation.