Regulation of interkinetic nuclear migration by cell cycle-coupled active and passive mechanisms in the developing brain.

A hallmark of neurogenesis in the vertebrate brain is the apical-basal nuclear oscillation in polarized neural progenitor cells. Known as interkinetic nuclear migration (INM), these movements are synchronized with the cell cycle such that nuclei move basally during G1-phase and apically during G2-phase. However, it is unknown how the direction of movement and the cell cycle are tightly coupled. Here, we show that INM proceeds through the cell cycle-dependent linkage of cell-autonomous and non-autonomous mechanisms. During S to G2 progression, the microtubule-associated protein Tpx2 redistributes from the nucleus to the apical process, and promotes nuclear migration during G2-phase by altering microtubule organization. Thus, Tpx2 links cell-cycle progression and autonomous apical nuclear migration. In contrast, in vivo observations of implanted microbeads, acute S-phase arrest of surrounding cells and computational modelling suggest that the basal migration of G1-phase nuclei depends on a displacement effect by G2-phase nuclei migrating apically. Our model for INM explains how the dynamics of neural progenitors harmonize their extensive proliferation with the epithelial architecture in the developing brain.

Seven kinds of intermediate filament networks in the cytoplasm of polarized cells: structure and function.

Intermediate filaments (IFs) are involved in many important physiological functions, such as the distribution of organelles, signal transduction, cell polarity and gene regulation. However, little information exists on the structure of the IF networks performing these functions. We have clarified the existence of seven kinds of IF networks in the cytoplasm of diverse polarized cells: an apex network just under the terminal web, a peripheral network lying just beneath the cell membrane, a granule-associated network surrounding a mass of secretory granules, a Golgi-associated network surrounding the Golgi apparatus, a radial network locating from the perinuclear region to the specific area of the cell membrane, a juxtanuclear network surrounding the nucleus, and an entire cytoplasmic network. In this review, we describe these seven kinds of IF networks and discuss their biological roles.

Transient expression of keratin during neuronal development in the adult rabbit spinal ganglion.

A few neurons of the adult rabbit spinal ganglion express keratin. To examine the characters of these keratin-positive neurons, six kinds of intermediate filament proteins, namely keratin 8, keratin 14, nestin, vimentin, neurofilament 68 (NF-L) and glial fibrillary acidic protein (GFAP), were investigated immunohistochemically in developing and adult rabbit spinal ganglia. At 15 days of gestation, the spinal ganglion increased rapidly in volume and mainly consisted of three kinds of cells: small cells expressing vimentin, spindle-shaped cells co-expressing vimentin and nestin, and ovoid cells with an eccentric nucleus expressing nestin. Since some ovoid cells co-expressed nestin with either NF-L or GFAP, the ovoid cell may be considered to be an embryonic neural stem cell of the ganglion. In addition, a few keratin-positive polymorphic cells could be observed among these three kinds of cells. These polymorphic cells expressed five kinds of intermediate filament proteins, namely keratin 8, keratin 14, nestin, NF-L and GFAP. These cells were also detected in newborn and adult ganglia. A few neurons in the adult ganglion also expressed these five kinds of proteins as a Golgi-associated network. However, neurons expressing these proteins could not be detected in embryonic and newborn ganglia. Therefore, it may be considered that the keratin-positive polymorphic cell is a postnatal neural stem cell of the ganglion and that neurons transiently express keratin when polymorphic cells differentiate into neurons.

Keratin 20 expressed in the endocrine and exocrine cells of the rabbit duodenum.

The expression of intermediate filaments is sensitively reflected in cell function. To examine the involvement of keratin in a secretory function, 15 kinds of keratin (keratin-2, 3, 4, 5, 6, 7, 8, 10, 13, 14, 16, 17, 18, 19, 20) were detected immunohistochemically and immunoelectron microscopically in the rabbit duodenum. Four types of secretory cells existed in the rabbit duodenum: enteroendocrine cells and goblet cells in the epithelium and mucous and serous cells in the duodenal glands. Among the 15 kinds of keratin, keratin 20 was selectively expressed in all these secretory cells. However, localization of keratin 20 in the endocrine cells differed from that in three types of exocrine cells. In the enteroendocrine cells, keratin 20-containing filaments formed a juxtanuclear network from which they extended to the apical cell membrane. These filaments may play a role in intracellular signal transduction, since the apical cell membrane contains some receptors for binding a specific extracellular signal. In the exocrine cells, on the other hand, keratin 20-containing filaments existed just beneath the cell membrane. These filaments may play some role in maintaining cell shape, which is remarkably changed during the secretory cycle.

Golgi-associated filament networks in duct epithelial cells of rabbit submandibular glands: immunohistochemical light and electron microscopic studies.

The duct epithelial cells of rabbit submandibular glands expressed keratin 8, keratin 14, and actin in the supranuclear region, and these cytoskeletal proteins formed ring structures, approximately 1-2.5 microm in diameter. Ultrastructurally, these ring structures were observed as a 'Golgi-associated filament network' surrounding Golgi apparatuses. Double immunofluorescent studies showed that keratin 8 and keratin 14 formed keratin 8/14 filaments, and that these filaments and actin filaments colocalized as components of the Golgi-associated filament network. Our studies suggested that the Golgi-associated filament network maintains the complex structure and location of the Golgi apparatus of the duct epithelium of rabbit submandibular glands. Tubulin was distributed diffusely throughout the cytoplasm of columnar cells, but no special relationship was found between tubulin and the Golgi apparatus.

Vimentin-positive cells in the villus epithelium of the rabbit small intestine.

In rabbit intestinal epithelium, vimentin intermediate filaments are selectively expressed in the M cells of follicle-associated epithelium (FAE). To find intestinal epithelial cells belonging to the M cell lineage, vimentin was detected immunohistochemically in the rabbit small and large intestines. Vimentin-positive columnar cells were scattered throughout the villus epithelium of the small intestine. In their cytoplasm, vimentin was located from the perinuclear region to the cell membrane touching intraepithelial lymphocytes. These cells had microvilli shorter than those of absorptive cells, and the alkaline phosphatase activity of the microvilli was markedly weaker than that of absorptive cell microvilli. Glycoconjugates on the surface of the microvilli were alcian blue positive and periodic acid-Schiff negative. The morphological and histochemical features of these vimentin-positive villus epithelial cells differed from those of adjacent absorptive cells and closely resembled those of the M cells in FAE covering Peyer's patches and solitary lymphatic nodules. These results suggest that the vimentin-positive cells in the villus epithelium belong to the M cell lineage.