Miwako Nishizawa

Keratin attenuates tumor necrosis factor-induced cytotoxicity through association with TRADD

Keratin 8 and 18 (K8/18) are the major components of intermediate filament (IF) proteins of simple or single-layered epithelia. Recent data show that normal and malignant epithelial cells deficient in K8/18 are nearly 100 times more sensitive to tumor necrosis factor (TNF)–induced cell death. We have now identified human TNF receptor type 1 (TNFR1)–associated death domain protein (TRADD) to be the K18-interacting protein. Among IF proteins tested in two-hybrid systems, TRADD specifically bound K18 and K14, type I (acidic) keratins. The COOH-terminal region of TRADD interacted with the coil Ia of the rod domain of K18. Endogenous TRADD coimmunoprecipitated with K18, and colocalized with K8/18 filaments in human mammary epithelial cells. Overexpression of the NH2 terminus (amino acids 1–270) of K18 containing the TRADD-binding domain as well as overexpression of K8/18 in SW13 cells, which are devoid of keratins, rendered the cells more resistant to killing by TNF. We also showed that overexpressed NH2 termini of K18 and K8/18 were associated with endogenous TRADD in SW13 cells, resulting in the inhibition of caspase-8 activation. These results indicate that K18 may sequester TRADD to attenuate interactions between TRADD and activated TNFR1 and moderate TNF-induced apoptosis in simple epithelial cells.

Visualization of mitotic radial glial lineage cells in the developing rat brain by Cdc2 kinase-phosphorylated vimentin

Although accumulating data reveal patterns of proliferation, migration, and differentiation of neuronal lineage cells in the developing brain, gliogenesis in the brain has not been well elucidated. In the rat brain, vimentin is selectively expressed in radial glia and in their progeny, not in oligodendrocytes or neurons from embryonic day 15 (E15) until postnatal day 15 (P15). Here we examined mitotic radial glial lineage cells in the rat brain E17–P7, using the monoclonal antibody 4A4, which recognizes vimentin phosphorylated by a mitosis‐specific kinase, cdc2 kinase. In the neocortex, mainly radial glia in the ventricular zone, but not their progeny, underwent cell division. In contrast, not only radial glia but also various types of radial glial progeny including Bergmann glia continued to proliferate in the cerebellum. Radial glia in the neocortex divided horizontally, obliquely, and vertically against the ventricular surface. The percentage of the vertical division increased with progress in the stage of development, concurrently with the decrease of the population of horizontal divisions. Thus, the monoclonal antibody 4A4 provides an useful tool to label mitotic glia in the developing brain and revealed different patterns of gliogenesis in the neocortex and cerebellum. A possibility is discussed that the dynamics of mitotic orientation observed here may be related to the change of the pattern of gliogenesis during development. GLIA 23:191–199, 1998.

Densin-180 Interacts with δ-Catenin/Neural Plakophilin-related Armadillo Repeat Protein at Synapses

Densin-180, a protein purified from the postsynaptic density fraction of the rat forebrain, is the founding member of a newly described family of proteins termed the LAP (leucine-rich repeats and PSD-95/Dlg-A/ZO-1 (PDZ) domains) family that plays essential roles in establishment of cell polarity. To identify Densin-180-binding proteins, we screened a yeast two-hybrid library using the carboxyl-terminal fragment of Densin-180 containing PDZ domain as bait, and we isolated δ-catenin/neural plakophilin-related armadillo repeat protein (NPRAP) as a Densin-180-interacting protein. δ-catenin/NPRAP, a member of the armadillo repeat family, is a nervous system-specific adherens junction protein originally discovered as an interactor with presenilin-1, a protein involved in Alzheimers disease. Densin-180 PDZ domain binds the COOH terminus of δ-catenin/NPRAP containing the PDZ domain-binding sequence. Endogenous Densin-180 was co-immunoprecipitated with δ-catenin/NPRAP and N-cadherin. Although Densin-180 was reported to be a transmembrane protein, Densin-180 was not accessible to surface biotinylation in dissociated hippocampal neurons; hence Densin-180 may be a cytosolic protein. Densin-180 co-localized with δ-catenin/NPRAP at synapses in dissociated hippocampal neurons. We propose that Densin-180 is associated in vivo with δ-catenin/NPRAP and may be involved in organization of the synaptic cell-cell junction through interaction with the δ-catenin/NPRAP-N-cadherin complex.

Identification of Mrj, a DnaJ/Hsp40 Family Protein, as a Keratin 8/18 Filament Regulatory Protein

To elucidate the function of keratins 8 and 18 (K8/18), major components of the intermediate filaments of simple epithelia, we searched for K8/18-binding proteins by screening a yeast two-hybrid library. We report here that human Mrj, a DnaJ/Hsp40 family protein, directly binds to K18. Among the interactions between DnaJ/Hsp40 family proteins and various intermediate filament proteins that we tested using two-hybrid methods, Mrj specifically interacted with K18. Immunostaining with anti-Mrj antibody showed that Mrj colocalized with K8/18 filaments in HeLa cells. Mrj was immunoprecipitated not only with K18, but also with the stress-induced and constitutively expressed heat shock protein Hsp/c70. Mrj bound to K18 through its C terminus and interacted with Hsp/c70 via its N terminus, which contains the J domain. Microinjection of anti-Mrj antibody resulted in the disorganization of K8/18 filaments, without effects on the organization of actin filaments and microtubules. Taken together, these results suggest that Mrj may play an important role in the regulation of K8/18 filament organization as a K18-specific co-chaperone working together with Hsp/c70.

ERBIN associates with p0071, an armadillo protein, at cell‐cell junctions of epithelial cells

Working model for the ERBIN–p0071 interaction at cell‐cell adhesions. ERBIN is associated with p0071 at adherens junctions and desmosomes. p0071 is thought to interact with classic cadherins or desmosomal cadherins ( Hatzfeld 1999 ). The subcellular localization of ERBIN may be regulated by the Rho family and other signals controlling cell polarity.

Densin‐180, a synaptic protein, links to PSD‐95 through its direct interaction with MAGUIN‐1

Background: Densin‐180, a brain‐specific protein highly concentrated at the postsynaptic density (PSD), belongs to the LAP [leucine‐rich repeats and PSD‐95/Dlg‐A/ZO‐1 (PDZ) domains] family of proteins, some of which play fundamental roles in the establishment of cell polarity.

Identification of trichoplein, a novel keratin filament-binding protein.

Keratins 8 and 18 (K8/18) are major components of the intermediate filaments (IFs) of simple epithelia. We report here the identification of a novel protein termed trichoplein. This protein shows a low degree of sequence similarity to trichohyalin, plectin and myosin heavy chain, and is a K8/18-binding protein. Among interactions between trichoplein and various IF proteins that we tested using two-hybrid methods, trichoplein interacted significantly with K16 and K18, and to some extent with K5, K6a, K8 and K14. In in vitro co-sedimentation assays, trichoplein directly binds to K8/18, but not with vimentin, desmin, actin filaments or microtubules. An antibody raised against trichoplein specifically recognized a polypeptide with a relative molecular mass of 61 kDa in cell lysates. Trichoplein was immunoprecipitated using this antibody in a complex with K8/18 and immunostaining revealed that trichoplein colocalized with K8/18 filaments in HeLa cells. In polarized Caco-2 cells, trichoplein colocalized not only with K8/18 filaments in the apical region but also with desmoplakin, a constituent of desmosomes. In the absorptive cells of the small intestine, trichoplein colocalized with K8/18 filaments at the apical cortical region, and was also concentrated at desmosomes. Taken together, these results suggest that trichoplein is a keratin-binding protein that may be involved in the organization of the apical network of keratin filaments and desmosomes in simple epithelial cells.

Palmitoylation of ERBIN is required for its plasma membrane localization

LAP (leucine‐rich repeats (LRR) and PSD‐95/Dlg/ZO‐1 (PDZ)) family proteins, including Scribble, LET‐413, ERBIN, Densin‐180 and Lano, are involved in the regulation of cell polarity. The LRR domains of LAP proteins were reported to mediate their basolateral membrane localization and to be essential for their function. To further dissect the mechanism of the plasma membrane localization of ERBIN, we introduced various mutants of ERBIN into cultured cells and observed the intracellular localization. When an LRR domain mutant lacking amino acid residues 1–32 at the amino (N) terminal region was over‐expressed in cells, the mutant did not localize at the plasma membrane, but localized in the cytoplasm. We found that cysteines 14 and 16 at the N‐terminal region of ERBIN are in vivo palmitoylated. Over‐expressed mutants in which cysteine 14 and/or cysteine 16 were changed to serines did not localize at the plasma membrane, indicating that the palmitoylation of ERBIN is necessary for its plasma membrane localization. The over‐expressed 1–196 amino acids fragment of ERBIN, which lacked the latter half of LRR, was palmitoylated but did not localize at the plasma membrane. These results suggest that both palmitoylation and LRR are required for the plasma membrane localization of ERBIN.

DNA polymerases and DNA topoisomerases solubilized from nuclear matrices of regenerating rat livers

DNA topoisomerase activity together with the activities of DNA polymerase were detected in a form tightly associated with rat liver nuclear matrices. DNA polymerase activities were solubilized from the nuclear matrices of regenerating rat livers by sonic treatment followed by extraction of these activities with detergent and salt. The predominant activity was mainly alpha-polymerase as judged from the size determined by sucrose density gradient centrifugation. However, only beta-polymerase activity was detected in the matrix of normal rat livers. DNA topoisomerase activity, detected in both regenerating and normal liver nuclear matrices, showed a molecular size of about 4 S in sucrose gradient, and was active in the presence of EDTA. These results suggest that this enzyme belongs to type I topoisomerase.

Antitumor activity of 2-amino-4,4 alpha-dihydro-4 alpha, 7-dimethyl-3H-phenoxazine-3-one, a novel phenoxazine derivative produced by the reaction of 2-amino-5-methylphenol with bovine hemolysate.

2-Amino-4,4 alpha-dihydro-4 alpha,7-dimethyl-3H-phenoxazine-3-one (Phx) was synthesized by the reaction of 2-amino-5-methyl-phenol with bovine hemolysates. Since Phx is a phenoxazine derivative like actinomycin D, which exerts a strong anti-tumor effect by intercalating DNA, we examined the effects of Phx on cell proliferation and cell cycle progression in human epidermoid carcinoma cells (KB cells). Phx inhibited the proliferation of Kb cells in a dose-dependent manner. When KB cells were incubated for 9 h with medium containing 50 microM Phx, a transient accumulation of cells in S and G2/M phase was observed and at 24 h many of cells had lower DNA content. Although Phx had antitumor activity, the drug did not intercalate DNA, showing a different mode of action from actinomycin D.