Coronavirus Disease 2019 Cluster Originating in a Primary School Teachers' Room in Japan.

BACKGROUND: School closures are a subject of debate during the present coronavirus disease 2019 (COVID-19) pandemic. Because children are not the main driver of COVID-19 transmission in the community, school education must be prioritized in conjunction with appropriate infection prevention and control measures, as determined by local COVID-19 incidence. METHODS: We investigated the causes and transmission routes of a primary school cluster of COVID-19 that occurred during November and December 2020 in Niigata, Japan. RESULTS: In the cluster, the virus spread among teachers, then from teachers to students, and then to their family members. This primary school cluster comprised 26 infected patients and included teachers (13/33, 39%), students (9/211, 4%), and family members (4/65, 6%). The secondary attack rate from the 3 index teachers to the remaining 30 teachers was 33%; however, the rate to students was only 4%. Factors contributing to cluster formation include the fact that 2 of the index teachers continued working while symptomatic and that the environment and infection prevention measures in the teachers' room were inadequate. CONCLUSIONS: To open schools safely and without interruption, adequate measures to prevent COVID-19 infection in schools should be emphasized not only for children but also for teachers and their environment.

Par6B and atypical PKC regulate mitotic spindle orientation during epithelial morphogenesis.

Cdc42 plays an evolutionarily conserved role in promoting cell polarity and is indispensable during epithelial morphogenesis. To further investigate the role of Cdc42, we have used a three-dimensional matrigel model, in which single Caco-2 cells develop to form polarized cysts. Using this system, we previously reported that Cdc42 controls mitotic spindle orientation during cell division to correctly position the apical surface in a growing epithelial structure. In the present study, we have investigated the specific downstream effectors through which Cdc42 controls this process. Here, we report that Par6B and its binding partner, atypical protein kinase C (aPKC), are required to regulate Caco-2 morphogenesis. Depletion or inhibition of Par6B or aPKC phenocopies the loss of Cdc42, inducing misorientation of the mitotic spindle, mispositioning of the nascent apical surface, and ultimately, the formation of aberrant cysts with multiple lumens. Mechanistically, Par6B and aPKC function interdependently in this context. Par6B localizes to the apical surface of Caco-2 cysts and is required to recruit aPKC to this compartment. Conversely, aPKC protects Par6B from proteasomal degradation, in a kinase-independent manner. In addition, we report that depletion or inhibition of aPKC induces robust apoptotic cell death in Caco-2 cells, significantly reducing both cyst size and number. Cell survival and apical positioning depend upon different thresholds of aPKC expression, suggesting that they are controlled by distinct downstream pathways. We conclude that Par6B and aPKC control mitotic spindle orientation in polarized epithelia and, furthermore, that aPKC coordinately regulates multiple processes to promote morphogenesis.

Cdc42 controls spindle orientation to position the apical surface during epithelial morphogenesis.

The establishment of apical-basal polarity within a single cell and throughout a growing tissue is a key feature of epithelial morphogenesis. To examine the underlying mechanisms, the human intestinal epithelial cell line Caco-2 was grown in a three-dimensional matrix to generate a cystlike structure, where the apical surface of each epithelial cell faces a fluid-filled central lumen. A discrete apical domain is established as early as the first cell division and between the two daughter cells. During subsequent cell divisions, the apical domain of each daughter cell is maintained at the center of the growing structure through a combination of mitotic spindle orientation and asymmetric abscission. Depletion of Cdc42 does not prevent the establishment of apical-basal polarity in individual cells but rather disrupts spindle orientation, leading to inappropriate positioning of apical surfaces within the cyst. We conclude that Cdc42 regulates epithelial tissue morphogenesis by controlling spindle orientation during cell division.

Critical roles of actin-interacting protein 1 in cytokinesis and chemotactic migration of mammalian cells.

Cofilin regulates actin filament dynamics by stimulating actin filament disassembly and plays a critical role in cytokinesis and chemotactic migration. Aip1 (actin-interacting protein 1), also called WDR1 (WD-repeat protein 1), is a highly conserved WD-repeat protein in eukaryotes and promotes cofilin-mediated actin filament disassembly in vitro; however, little is known about the mechanisms by which Aip1 functions in cytokinesis and cell migration in mammalian cells. In the present study, we investigated the roles of Aip1 in cytokinesis and chemotactic migration of human cells by silencing the expression of Aip1 using siRNA (small interfering RNA). Knockdown of Aip1 in HeLa cells increased the percentage of multinucleate cells; this effect was reversed by expression of an active form of cofilin. In Aip1-knockdown cells, the cleavage furrow ingressed normally from anaphase to early telophase; however, an excessive accumulation of actin filaments was observed on the contractile ring in late telophase. These results suggest that Aip1 plays a crucial role in the completion of cytokinesis by promoting cofilin-mediated actin filament disassembly in telophase. We have also shown that Aip1 knockdown significantly suppressed chemokine-induced chemotactic migration of Jurkat T-lymphoma cells, and this was blocked by expression of an active form of cofilin. Whereas control cells mostly formed a single lamellipodium in response to chemokine stimulation, Aip1 knockdown cells abnormally exhibited multiple protrusions around the cells before and after cell stimulation. This indicates that Aip1 plays an important role in directional cell migration by restricting the stimulus-induced membrane protrusion to one direction via promoting cofilin activity.

LIM kinase-mediated cofilin phosphorylation during mitosis is required for precise spindle positioning.

The interaction of astral microtubules with cortical actin networks is essential for the correct orientation of the mitotic spindle; however, little is known about how the cortical actin organization is regulated during mitosis. LIM kinase-1 (LIMK1) regulates actin dynamics by phosphorylating and inactivating cofilin, an actin-depolymerizing protein. LIMK1 activity increases during mitosis. Here we show that mitotic LIMK1 activation is critical for accurate spindle orientation in mammalian cells. Knockdown of LIMK1 suppressed a mitosis-specific increase in cofilin phosphorylation and caused unusual cofilin localization in the cell cortex in metaphase, instability of cortical actin organization and astral microtubules, irregular rotation and misorientation of the spindle, and a delay in anaphase onset. Similar results were obtained by treating the cells with a LIMK1 in hibitor peptide or latrunculin A or by overexpressing a non-phosphorylatable cofilin(S3A) mutant. Furthermore, localization of LGN (a protein containing the repetitive Leu-Gly-Asn tripeptide motifs), an important regulator of spindle orientation, in the crescent-shaped cortical regions was perturbed in LIMK1 knockdown cells. Our results suggest that LIMK1-mediated cofilin phosphorylation is required for accurate spindle orientation by stabilizing cortical actin networks during mitosis.

Cell cycle-associated changes in Slingshot phosphatase activity and roles in cytokinesis in animal cells.

During cytokinesis the actomyosin-based contractile ring is formed at the equator, constricted, and then disassembled prior to cell abscission. Cofilin stimulates actin filament disassembly and is implicated in the regulation of contractile ring dynamics. However, little is known about the mechanism controlling cofilin activity during cytokinesis. Cofilin is inactivated by phosphorylation on Ser-3 by LIM-kinase-1 (LIMK1) and is reactivated by a protein phosphatase Slingshot-1 (SSH1). Here we show that the phosphatase activity of SSH1 decreases in the early stages of mitosis and is elevated in telophase and cytokinesis in HeLa cells, a time course correlating with that of cofilin dephosphorylation. SSH1 co-localizes with F-actin and accumulates onto the cleavage furrow and the midbody. Expression of a phosphatase-inactive SSH1 induces aberrant accumulation of F-actin and phospho-cofilin near the midbody in the final stage of cytokinesis and frequently leads to the regression of the cleavage furrow and the formation of multinucleate cells. Co-expression of cofilin rescued the inhibitory effect of phosphatase-inactive SSH1 on cytokinesis. These results suggest that SSH1 plays a critical role in cytokinesis by dephosphorylating and reactivating cofilin in later stages of mitosis.

Mitosis-specific activation of LIM motif-containing protein kinase and roles of cofilin phosphorylation and dephosphorylation in mitosis.

Actin filament dynamics play a critical role in mitosis and cytokinesis. LIM motif-containing protein kinase 1 (LIMK1) regulates actin reorganization by phosphorylating and inactivating cofilin, an actin-depolymerizing and -severing protein. To examine the role of LIMK1 and cofilin during the cell cycle, we measured cell cycle-associated changes in the kinase activity of LIMK1 and in the level of cofilin phosphorylation. Using synchronized HeLa cells, we found that LIMK1 became hyperphosphorylated and activated in prometaphase and metaphase, then gradually returned to the basal level as cells entered into telophase and cytokinesis. Although Rho-associated kinase and p21-activated protein kinase phosphorylate and activate LIMK1, they are not likely to be involved in mitosis-specific activation and phosphorylation of LIMK1. Immunoblot and immunofluorescence analyses using an anti-phosphocofilin-specific antibody revealed that the level of cofilin phosphorylation, similar to levels of LIMK1 activity, increased during prometaphase and metaphase then gradually declined in telophase and cytokinesis. Ectopic expression of LIMK1 increased the level of cofilin phosphorylation throughout the cell cycle and induced the formation of multinucleate cells. These results suggest that LIMK1 is involved principally in control of mitosis-specific cofilin phosphorylation and that dephosphorylation and reactivation of cofilin at later stages of mitosis play a critical role in cytokinesis of mammalian cells.