Tomoyuki Yamanaka

Mammalian Lgl Forms a Protein Complex with PAR-6 and aPKC Independently of PAR-3 to Regulate Epithelial Cell Polarity

BACKGROUND Epithelial cells have apicobasal polarity and an asymmetric junctional complex that provides the bases for development and tissue maintenance. In both vertebrates and invertebrates, the evolutionarily conserved protein complex, PAR-6/aPKC/PAR-3, localizes to the subapical region and plays critical roles in the establishment of a junctional complex and cell polarity. In Drosophila, another set of proteins called tumor suppressors, such as Lgl, which localize separately to the basolateral membrane domain but genetically interact with the subapical proteins, also contribute to the establishment of cell polarity. However, how physically separated proteins interact remains to be clarified. RESULTS We show that mammalian Lgl competes for PAR-3 in forming an independent complex with PAR-6/aPKC. During cell polarization, mLgl initially colocalizes with PAR-6/aPKC at the cell-cell contact region and is phosphorylated by aPKC, followed by segregation from apical PAR-6/aPKC to the basolateral membrane after cells are polarized. Overexpression studies establish that increased amounts of the mLgl/PAR-6/aPKC complex suppress the formation of epithelial junctions; this contrasts with the previous observation that the complex containing PAR-3 promotes it. CONCLUSIONS These results indicate that PAR-6/aPKC selectively interacts with either mLgl or PAR-3 under the control of aPKC activity to regulate epithelial cell polarity.

aPKC Acts Upstream of PAR-1b in Both the Establishment and Maintenance of Mammalian Epithelial Polarity

BACKGROUND aPKC and PAR-1 are required for cell polarity in various contexts. In mammalian epithelial cells, aPKC localizes at tight junctions (TJs) and plays an indispensable role in the development of asymmetric intercellular junctions essential for the establishment and maintenance of apicobasal polarity. On the other hand, one of the mammalian PAR-1 kinases, PAR-1b/EMK1/MARK2, localizes to the lateral membrane in a complimentary manner with aPKC, but little is known about its role in apicobasal polarity of epithelial cells as well as its functional relationship with aPKC. RESULTS We demonstrate that PAR-1b is essential for the asymmetric development of membrane domains of polarized MDCK cells. Nonetheless, it is not required for the junctional localization of aPKC nor the formation of TJs, suggesting that PAR-1b works downstream of aPKC during epithelial cell polarization. On the other hand, aPKC phosphorylates threonine 595 of PAR-1b and enhances its binding with 14-3-3/PAR-5. In polarized MDCK cells, T595 phosphorylation and 14-3-3 binding are observed only in the soluble form of PAR-1b, and okadaic acid treatment induces T595-dependent dissociation of PAR-1b from the lateral membrane. Furthermore, T595A mutation induces not only PAR-1b leakage into the apical membrane, but also abnormal development of membrane domains. These results suggest that in polarized epithelial cells, aPKC phosphorylates PAR-1b at TJs, and in cooperation with 14-3-3, promotes the dissociation of PAR-1b from the lateral membrane to regulate PAR-1b activity for the membrane domain development. CONCLUSIONS These results suggest that mammalian aPKC functions upstream of PAR-1b in both the establishment and maintenance of epithelial cell polarity.

PAR‐6 regulates aPKC activity in a novel way and mediates cell‐cell contact‐induced formation of the epithelial junctional complex

Background PAR‐6, aPKC and PAR‐3 are polarity proteins that co‐operate in the establishment of cell polarity in Caenorhabditis elegans and Drosophila embryos. We have recently shown that mammalian aPKC is required for the formation of the epithelia‐specific cell‐cell junctional structure. We have also revealed that a mammalian PAR‐6 forms a ternary complex with aPKC and ASIP/PAR‐3, and localizes at the most apical end of the junctional complex in epithelial cells.

Interaction between PAR-3 and the aPKC–PAR-6 complex is indispensable for apical domain development of epithelial cells

The evolutionarily conserved polarity proteins PAR-3, atypical protein kinase C (aPKC) and PAR-6 critically regulate the apical membrane development required for epithelial organ development. However, the molecular mechanisms underlying their roles remain to be clarified. We demonstrate that PAR-3 knockdown in MDCK cells retards apical protein delivery to the plasma membrane, and eventually leads to mislocalized apical domain formation at intercellular regions in both two-dimensional and three-dimensional culture systems. The defects in PAR-3 knockdown cells are efficiently rescued by wild-type PAR-3, but not by a point mutant (S827/829A) that lacks the ability to interact with aPKC, indicating that formation of the PAR-3–aPKC–PAR-6 complex is essential for apical membrane development. This is in sharp contrast with tight junction maturation, which does not necessarily depend on the aPKC–PAR-3 interaction, and indicates that the two fundamental processes essential for epithelial polarity are differentially regulated by these polarity proteins. Importantly, highly depolarized cells accumulate aPKC and PAR-6, but not PAR-3, on apical protein-containing vacuoles, which become targeted to PAR-3-positive primordial cell-cell contact sites during the initial stage of the repolarization process. Therefore, formation of the PAR-3–aPKC–PAR-6 complex might be required for targeting of not only the aPKC–PAR-6 complex but also of apical protein carrier vesicles to primordial junction structures.

Lgl mediates apical domain disassembly by suppressing the PAR-3-aPKC-PAR-6 complex to orient apical membrane polarity

The basolateral tumor suppressor protein Lgl is important for the regulation of epithelial cell polarity and tissue morphology. Recent studies have shown a physical and functional interaction of Lgl with another polarity-regulating protein machinery, the apical PAR-3-aPKC-PAR-6 complex, in epithelial cells. However, the mechanism of Lgl-mediated regulation of epithelial cell polarity remains obscure. By an siRNA method, we here show that endogenous Lgl is required for the disassembly of apical membrane domains in depolarizing MDCK cells induced by Ca2+ depletion. Importantly, this Lgl function is mediated by the suppression of the apical PAR-3-aPKC-PAR-6 complex activity. Analysis using 2D- or 3D-cultured cells in collagen gel suggests the importance of this suppressive regulation of Lgl on the collagen-mediated re-establishment of apical membrane domains and lumen formation. These results indicate that basolateral Lgl plays a crucial role in the disassembly of apical membrane domains to induce the orientation of apical membrane polarity, which is mediated by the suppression of apical PAR-3-aPKC-PAR-6 complex activity.

Mutant Huntingtin reduces HSP70 expression through the sequestration of NF-Y transcription factor

In Huntingtons disease (HD), mutant Huntingtin, which contains expanded polyglutamine stretches, forms nuclear aggregates in neurons. The interactions of several transcriptional factors with mutant Huntingtin, as well as altered expression of many genes in HD models, imply the involvement of transcriptional dysregulation in the HD pathological process. The precise mechanism remains obscure, however. Here, we show that mutant Huntingtin aggregates interact with the components of the NF‐Y transcriptional factor in vitro and in HD model mouse brain. An electrophoretic mobility shift assay using HD model mouse brain lysates showed reduction in NF‐Y binding to the promoter region of HSP70, one of the NF‐Y targets. RT–PCR analysis revealed reduced HSP70 expression in these brains. We further clarified the importance of NF‐Y for HSP70 transcription in cultured neurons. These data indicate that mutant Huntingtin sequesters NF‐Y, leading to the reduction of HSP70 gene expression in HD model mice brain. Because suppressive roles of HSP70 on the HD pathological process have been shown in several HD models, NF‐Y could be an important target of mutant Huntingtin.

Role of Lgl/Dlg/Scribble in the regulation of epithelial junction, polarity and growth.

Genetic studies in Drosophila have revealed that three tumor suppressors, Discs large (Dlg), Scribble (Scrib) and Lethal giant larvae (Lgl), which localize to the basolateral region of epithelial cells, cooperatively regulate cell polarity, junction formation and cell growth in epithelial cells. Subsequent studies in Drosophila, vertebrates and C. elegans have shown the evolutionary conservation of some of their functions in epithelial cells. Also, these studies revealed the importance of antagonistic interactions between these tumor suppressors and apical polarity regulators such as Crumbs and aPKC for the establishment of apical-basal polarity with organized cell-cell junctions and regulation of cell growth in epithelial cells.

Intranuclear Degradation of Polyglutamine Aggregates by the Ubiquitin-Proteasome System

Huntington disease and its related autosomal-dominant polyglutamine (pQ) neurodegenerative diseases are characterized by intraneuronal accumulation of protein aggregates. Studies on protein aggregates have revealed the importance of the ubiquitin-proteasome system as the front line of protein quality control (PQC) machinery against aberrant proteins. Recently, we have shown that the autophagy-lysosomal system is also involved in cytoplasmic aggregate degradation, but the nucleus lacked this activity. Consequently, the nucleus relies entirely on the ubiquitin-proteasome system for PQC. According to previous studies, nuclear aggregates possess a higher cellular toxicity than do their cytoplasmic counterparts, however degradation kinetics of nuclear aggregates have been poorly understood. Here we show that nuclear ubiquitin ligases San1p and UHRF-2 each enhance nuclear pQ aggregate degradation and rescued pQ-induced cytotoxicity in cultured cells and primary neurons. Moreover, UHRF-2 is associated with nuclear inclusion bodies in vitro and in vivo. Our data suggest that UHRF-2 is an essential molecule for nuclear pQ degradation as a component of nuclear PQC machinery in mammalian cells.

Direct binding of Lgl2 to LGN during mitosis and its requirement for normal cell division

The Drosophila tumor suppressor protein lethal (2) giant larvae (l(2)gl) is involved in asymmetric cell division during development and epithelial cell polarity through interaction with the aPKC·Par-6 complex. We showed here that Lgl2, a mammalian homolog of l(2)gl, directly bound to LGN, a mammalian homolog of Partner of inscuteable in HEK293 cells. The C-terminal tail of Lgl2 bound to LGN with a Kd value of about 56 nm. Endogenous Lgl2 formed a complex with aPKC, Par-6, and LGN. This complex formation was enhanced in metaphase of the synchronized cells by treatment with thymidine and nocodazole. Immunofluorescence staining of the complex was the strongest at the cell periphery of the metaphase cells. Overexpression of the C-terminal tail of Lgl2 induced mis-localization of the nuclear mitotic apparatus protein NuMA and disorganization of the mitotic spindle during mitosis, eventually causing formation of multiple micronuclei. Knockdown of endogenous Lgl (Lgl1 and Lgl2) also induced disorganization of the mitotic spindle, thereby causing formation of multiple micronuclei. The binding between Lgl2 and LGN played a role in the mitotic spindle organization through regulating formation of the LGN·NuMA complex. These results indicate that Lgl2 forms a Lgl2·Par-6·aPKC·LGN complex, which responds to mitotic signaling to establish normal cell division.

Intracellular polarity protein PAR‐1 regulates extracellular laminin assembly by regulating the dystroglycan complex

Cell polarity depends on extrinsic spatial cues and intrinsic polarity proteins including PAR‐aPKC proteins. In mammalian epithelial cells, cell–cell contacts provide spatial cues that activate the aPKC‐PAR‐3‐PAR‐6 complex to establish the landmark of the initial cellular asymmetry. PAR‐1, a downstream target of the aPKC‐PAR‐3‐PAR‐6 complex, mediates further development of the apical and basolateral membrane domains. However, the relationships between the PAR‐aPKC proteins and other extrinsic spatial cues provided by the extracellular matrix (ECM) remain unclear. Here, we show that PAR‐1 colocalizes with laminin receptors and is required for the assembly of extracellular laminin on the basal surface of epithelial cells. Furthermore, PAR‐1 regulates the basolateral localization of the dystroglycan (DG) complex, one of the laminin receptors essential for basement membrane formation. We also show that PAR‐1 interacts with the DG complex and is required for the formation of a functional DG complex. These results reveal the presence of a novel inside‐out pathway in which an intracellular polarity protein regulates the ECM organization required for epithelial cell polarity and tissue morphogenesis.