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

Abstract

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.

Keywords: cell polarity; intermediate filament network; neural stem cell; organelle distribution; signal transduction.

Fig. 1

Schematic illustration showing the localization of the seven kinds of IF networks in the cytoplasm of a polarized cell. Red lines indicate IF networks. An apex network (nw.) exists under the apical cell membrane. A peripheral network is distributed just beneath the basolateral cell membrane. A granule-associated network surrounds a mass of secretory granules. A Golgi-associated network surrounds the Golgi apparatus. A radial network (R) is located from the perinuclear region to the specific area of the cell membrane. A juxtanuclear network (J) surrounds the nucleus. An entire cytoplasmic network is distributed throughout the entire cytoplasm.

Fig. 2

Structural model of IF protein. The central α-helical rod domain is subdivided into the coil segments 1A, 1B, 2A and 2B by the short non-helical linker regions L₁, L₁₂ and L₂. The rod domain is flanked by the non-helical N-terminal head domain and the C-terminal tail domain.

Fig. 3

Immunostaining of vimentin in a chondrocyte of rabbit tracheal cartilage. Vimentin IFs are concentrated in a perinuclear region from which they appear to radiate to the cell periphery. Bar=5 µm.

Fig. 4

Immunostaining of keratin 5 and 18 in the absorptive cells of the rabbit duodenum. Bar=5 µm. A: Keratin 5 (K5) is localized as a thin layer (arrow) in apical areas of the cell. B: The localization of keratin 18 (K18) resembles that of keratin 5. C: Double immunofluorescence staining of keratin 5 (red) and keratin 18 (green) confirms tight co-localization of both keratin proteins as an apex network (arrow).

Fig. 5

Immunoelectron microscopic staining of keratin 18 (K18) in absorptive cells of the rabbit duodenum. Keratin 18-positive filaments (arrowheads) are localized just under the terminal web (asterisk) and anchored to a desmosome (arrow). Some of them are observed in the terminal web and in the upper cytoplasm of the terminal web. Bar=0.5 µm.

Fig. 6

Peripheral network and granule-associated network. A: Immunofluorescence staining of keratin 20 (K20) in a goblet cell of the rabbit duodenum. Keratin 20 (red) is distributed just beneath the basolateral cell membrane (arrows) and around a mass of mucigen granules (arrowheads). Bar=5 µm. B: Immunoelectron microscopical staining of keratin 20 in mucous cells of the rabbit duodenal gland. Keratin 20-containing filaments exist just under the cell membrane (arrows) and around the mucigen granules (arrowheads). Bar=2 µm.

Fig. 7

Golgi-associated network in absorptive cells of the rabbit duodenum. A: Double immunofluorescence staining of keratin 8 (K8: red) and 14 (K14: green). Both keratins are co-localized as specific ring structures (arrows) in the supranuclear region of the cell. Bar=5 µm. B: Immunoelectron microscopical staining of keratin 14. Keratin 14 is localized around the Golgi apparatus (arrows). Bar=0.5 µm.

Fig. 8

Schematic representation of changes in the composition of the Golgi-associated network during the migration of absorptive cells along the crypt-villus axis. The Golgi-associated network consists of keratin 8/14 filaments (K8/K14) alone in the immature absorptive cells at the upper crypt. This network is reinforced by the addition of actin filaments at the villus base and keratin 7/17 filaments (K7/K17) at the mid-villus to keratin 8/14 filaments following maturation of the Golgi apparatus.

Fig. 9

Radial network and juxtanuclear network. Bar=5 µm. A: Strong staining of keratin 20 (K20: red) is observed in a few particular columnar cells (E) scattered throughout the villous and cryptic epithelia of the rabbit duodenum. These keratin 20-positive columnar cells were confirmed to be enteroendocrine cells as shown in Figure 10A. Keratin 20 is localized around the nucleus and from the edge of the nucleus to the apical cell membrane. B: Vimentin (Vim)-positive columnar cells (M) are scattered throughout the villus epithelium of the rabbit small intestine. These cells were confirmed to be M cells by their ultrastructural and histochemical features [61]. In these cells, vimentin is localized around the nucleus and from the edge of the nucleus to the cell membrane surrounding intraepithelial lymphocytes (L). C: Double immunofluorescence staining of vimentin (Vim: red) and keratin 18 (K18: green) in the M cells of rabbit ileum. In the M cell, vimentin forms the radial and juxtanuclear networks, and keratin 18 form the apex and peripheral networks.

Fig. 10

Immunoelectron micrographs of radial and juxtanuclear networks. Bar=2 µm. A: A keratin 20 (K20)-positive columnar cell (E) was confirmed to be an enteroendocrine cell, because all keratin 20-positive columnar cells accumulate many secretory granules in their basal cytoplasm. In these cells, keratin 20-containing filaments (arrows) are detected in the perinuclear region and between this region and the apical cell membrane. B: In the M cell (M), vimentin (Vim: arrows) is localized around the nucleus and from the edge of the nucleus to the cell membrane, which touches intraepithelial lymphocytes (L).

Fig. 11

Schematic representation of the embryonic and postnatal neurogenesis in the developing and adult rabbit spinal ganglia. Ovoid cells, which originate from the vimentin-positive neural crest, differentiate into NF-positive pseudounipolar neurons, GFAP/vimentin-positive glial cells (satellite cells and Schwann cells), and keratin-positive polymorphic cells during prenatal life. The polymorphic cells, which express keratin 8, keratin 14, nestin, NF-L and GFAP, differentiate into pseudounipolar neurons and glial cells after birth. When the polymorphic cells differentiate into neurons, the immature neurons transiently express these five kinds of IF proteins as a Golgi-associated filament network (GAN).

Fig. 12

Double immunofluorescence staining of vimentin (Vim: red) and nestin (Nes: green) in the rudiments of the rabbit spinal ganglion at 15 days of gestation. Small cells (Sm) express vimentin alone and ovoid cells (Ov) express nestin alone, but spindle-shaped cells (Sp) co-express both proteins. Mesenchymal cells (M) also express vimentin. Bar=10 µm.

Fig. 13

Double immunofluorescence staining of two kinds of intermediate filament proteins in the adult rabbit spinal ganglia. Bar=10 µm. A: Keratin 8 (green) and keratin 14 (red) are detected throughout the entire cytoplasm of a few polymorphic cells (arrow). B: Keratin 8 (red) and nestin (green) are also detected throughout the entire cytoplasm of a few polymorphic cells (arrow).

Fig. 14

Schematic representation of changes in the composition of the entire cytoplasmic network during the migration of absorptive cells along the villus axis. This network consists of keratin 7/17 filaments (K7/K17) alone in the immature absorptive cells at the villus base. This network is reinforced by the addition of keratin 5/18 filaments (K5/K18) following maturation of the cell. Just before cell exfoliation, keratin 20 (K20) enters this network.

Fig. 15

Immunofluorescence staining of lamin A (red) in the rabbit spinal ganglion. Strong staining is observed at the nuclear rim. N, nucleus of the pseudounipolar neuron. S, nucleus of the satellite cell. Bar=5 µm.