Cdc42 controls progenitor cell differentiation and beta-catenin turnover in skin

CDC42 inhibition suppresses progression of incipient intestinal tumors

Mutations in the APC or β-catenin genes are well-established initiators of colorectal cancer, yet modifiers that facilitate the survival and progression of nascent tumor cells are not well defined. Using genetic and pharmacologic approaches in mouse colorectal cancer and human colorectal cancer xenograft models, we show that incipient intestinal tumor cells activate CDC42, an APC-interacting small GTPase, as a crucial step in malignant progression. In the mouse, Cdc42 ablation attenuated the tumorigenicity of mutant intestinal cells carrying single APC or β-catenin mutations. Similarly, human colorectal cancer with relatively higher levels of CDC42 activity was particularly sensitive to CDC42 blockade. Mechanistic studies suggested that Cdc42 may be activated at different levels, including at the level of transcriptional activation of the stem cell-enriched Rho family exchange factor Arhgef4. Our results indicate that early-stage mutant intestinal epithelial cells must recruit the pleiotropic functions of Cdc42 for malignant progression, suggesting its relevance as a biomarker and therapeutic target for selective colorectal cancer intervention.

Organization and execution of the epithelial polarity programme

Epithelial cells require apical-basal plasma membrane polarity to carry out crucial vectorial transport functions and cytoplasmic polarity to generate different cell progenies for tissue morphogenesis. The establishment and maintenance of a polarized epithelial cell with apical, basolateral and ciliary surface domains is guided by an epithelial polarity programme (EPP) that is controlled by a network of protein and lipid regulators. The EPP is organized in response to extracellular cues and is executed through the establishment of an apical-basal axis, intercellular junctions, epithelial-specific cytoskeletal rearrangements and a polarized trafficking machinery. Recent studies have provided insight into the interactions of the EPP with the polarized trafficking machinery and how these regulate epithelial polarization and depolarization.

Hair curvature: a natural dialectic and review

Although hair forms (straight, curly, wavy, etc.) are present in apparently infinite variations, each fibre can be reduced to a finite sequence of tandem segments of just three types: straight, bent/curly, or twisted. Hair forms can thus be regarded as resulting from genetic pathways that induce, reverse or modulate these basic curvature modes. However, physical interconversions between twists and curls demonstrate that strict one-to-one correspondences between them and their genetic causes do not exist. Current hair-curvature theories do not distinguish between bending and twisting mechanisms. We here introduce a multiple papillary centres (MPC) model which is particularly suitable to explain twisting. The model combines previously known features of hair cross-sectional morphology with partially/completely separated dermal papillae within single follicles, and requires such papillae to induce differential growth rates of hair cortical material in their immediate neighbourhoods. The MPC model can further help to explain other, poorly understood, aspects of hair growth and morphology. Separate bending and twisting mechanisms would be preferentially affected at the major or minor ellipsoidal sides of fibres, respectively, and together they exhaust the possibilities for influencing hair-form phenotypes. As such they suggest dialectic for hair-curvature development. We define a natural-dialectic (ND) which could take advantage of speculative aspects of dialectic, but would verify its input data and results by experimental methods. We use this as a top-down approach to first define routes by which hair bending or twisting may be brought about and then review evidence in support of such routes. In particular we consider the wingless (Wnt) and mammalian target of rapamycin (mTOR) pathways as paradigm pathways for molecular hair bending and twisting mechanisms, respectively. In addition to the Wnt canonical pathway, the Wnt/Ca(2+) and planar cell polarity (PCP) pathways, and others, can explain many alternatives and specific variations of hair bending phenotypes. Mechanisms for hair papilla budding or its division by bisection or fission can explain MPC formation. Epithelial-to-mesenchymal (EMT) and mesenchymal-to-epithelial (MET) transitions, acting in collaboration with epithelial-mesenchymal communications are also considered as mechanisms affecting hair growth and its bending and twisting. These may be treated as sub-mechanisms of an overall development from neural-crest stem cell (NCSC) lineages to differentiated hair follicle (HF) cell types, thus providing a unified framework for hair growth and development.

Rho GTPases and cancer

Rho GTPases are a family of small GTPases, which play an important role in the regulation of the actin cytoskeleton. Not surprisingly, Rho GTPases are crucial for cell migration and therefore highly important for cancer cell invasion and the formation of metastases. In addition, Rho GTPases are involved in growth and survival of tumor cells, in the interaction of tumor cells with their environment, and they are vital for the cancer supporting functions of the tumor stroma. Recent research has significantly improved our understanding of the regulation of Rho GTPase activity, the specificity of Rho GTPases, and their function in tumor stem cells and tumor stroma. This review summarizes these novel findings and tries to define challenging questions for future research.

The CRB1 and adherens junction complex proteins in retinal development and maintenance

The early developing retinal neuroepithelium is composed of multipotent retinal progenitor cells that differentiate in a time specific manner, giving rise to six major types of neuronal and one type of glial cells. These cells migrate and organize in three distinct nuclear layers divided by two plexiform layers. Apical and adherens junction complexes have a crucial role in this process by the establishment of polarity and adhesion. Changes in these complexes disturb the spatiotemporal aspects of retinogenesis, leading to retinal degeneration resulting in mild or severe impairment of retinal function and vision. In this review, we summarize the mouse models for the different members of the apical and adherens junction protein complexes and describe the main features of their retinal phenotypes. The knowledge acquired from the different mutant animals for these proteins corroborate their importance in retina development and maintenance of normal retinal structure and function. More recently, several studies have tried to unravel the connection between the apical proteins, important cellular signaling pathways and their relation in retina development. Still, the mechanisms by which these proteins function remain largely unknown. Here, we hypothesize how the mammalian apical CRB1 complex might control retinogenesis and prevents onset of Leber congenital amaurosis or retinitis pigmentosa.

MiR-126 promotes coxsackievirus replication by mediating cross-talk of ERK1/2 and Wnt/β-catenin signal pathways

Coxsackievirus B3 (CVB3) is one of the most prevalent causes of viral myocarditis and is associated with many other pathological conditions. CVB3 replication relies on host cellular machineries and causes direct damage to host cells. MicroRNAs have been found to regulate viral infections but their roles in CVB3 infection are still poorly understood. Here we describe a novel mechanism by which miR-126 regulates two signal pathways essential for CVB3 replication. We found that CVB3-induced ERK1/2 activation triggered the phosphorylation of ETS-1 and ETS-2 transcription factors, which induced miR-126 upregulation. By using both microRNA mimics and inhibitors, we proved that the upregulated miR-126 suppressed sprouty-related, EVH1 domain containing 1 (SPRED1) and in turn enhanced ERK1/2 activation. This positive feedback loop of ERK1/2-miR-126-ERK1/2 promoted CVB3 replication. Meanwhile, miR-126 expression stimulated GSK-3β activity and induced degradation of β-catenin through suppressing LRP6 and WRCH1, two newly identified targets in the Wnt/β-catenin pathway, which sensitized the cells to virus-induced cell death and increased viral progeny release to initiate new infections. Our results demonstrate that upregulated miR-126 upon CVB3 infection targets SPRED1, LRP6, and WRCH1 genes, mediating cross-talk between ERK1/2 and Wnt/β-catenin pathways, and thus promoting viral replication and contributes to the viral cytopathogenicity.

Cdc42 inhibits ERK-mediated collagenase-1 (MMP-1) expression in collagen-activated human keratinocytes

Following injury, keratinocytes switch gene expression programs from the one that promotes differentiation to the one that supports migration. A common feature of human wounds and ulcerations of any form is the expression of matrix metalloproteinase 1 (MMP-1; collagenase-1) by leading-edge basal keratinocytes migrating across the dermal or provisional matrix. Induction of MMP-1 occurs by signaling from the α2β1 integrin in contact with dermal fibrillar type I collagen, and the activity of MMP-1 is required for human keratinocytes to migrate on collagen. Thus, MMP-1 serves a critical role in the repair of damaged human skin. Here, we evaluated the mechanisms controlling MMP-1 expression in primary human keratinocytes from neonatal foreskin and adult female skin. Our results demonstrate that shortly following contact with type I collagen extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase were markedly activated, whereas c-Jun N-terminal kinase (JNK) phosphorylation remained at basal levels. ERK inhibition markedly blocked collagen-stimulated MMP-1 expression in keratinocytes. In contrast, inhibiting p38 or JNK pathways had no effect on MMP-1 production. Moreover, investigating the role of Rho GTPases revealed that Cdc42 attenuates MMP-1 expression by suppressing ERK activity. Thus, our data indicate that injured keratinocytes induce MMP-1 expression through ERK activation, and this process is negatively regulated by Cdc42 activity.

Defective tubulin organization and proplatelet formation in murine megakaryocytes lacking Rac1 and Cdc42

Rac1 and Cdc42 have redundant functions in platelet biogenesis. Deficiency of Rac1 and Cdc42 results in highly abnormal megakaryocyte morphology associated with severely defective tubulin organization.

Specific deletion of Cdc42 does not affect meiotic spindle organization/migration and homologous chromosome segregation but disrupts polarity establishment and cytokinesis in mouse oocytes

Oocyte-specific deletion of Cdc42 has little effect on meiotic spindle organization and migration to the cortex but inhibits polar body emission, although homologous chromosome segregation occurs. The failure of cytokinesis is due to loss of polarized Arp2/3 accumulation and actin cap formation, and thus the defective contract ring.

Mammalian oocyte maturation is distinguished by highly asymmetric meiotic divisions during which a haploid female gamete is produced and almost all the cytoplasm is maintained in the egg for embryo development. Actin-dependent meiosis I spindle positioning to the cortex induces the formation of a polarized actin cap and oocyte polarity, and it determines asymmetric divisions resulting in two polar bodies. Here we investigate the functions of Cdc42 in oocyte meiotic maturation by oocyte-specific deletion of Cdc42 through Cre-loxP conditional knockout technology. We find that Cdc42 deletion causes female infertility in mice. Cdc42 deletion has little effect on meiotic spindle organization and migration to the cortex but inhibits polar body emission, although homologous chromosome segregation occurs. The failure of cytokinesis is due to the loss of polarized Arp2/3 accumulation and actin cap formation; thus the defective contract ring. In addition, we correlate active Cdc42 dynamics with its function during polar body emission and find a relationship between Cdc42 and polarity, as well as polar body emission, in mouse oocytes.

Cdc42 coordinates proliferation, polarity, migration, and differentiation of small intestinal epithelial cells in mice

BACKGROUND & AIMS

Cdc42 is a Rho GTPase that regulates diverse cellular functions, including proliferation, differentiation, migration, and polarity. In the intestinal epithelium, a balance among these events maintains homeostasis. We used genetic techniques to investigate the role of Cdc42 in intestinal homeostasis and its mechanisms.

METHODS

We disrupted Cdc42 specifically in intestinal epithelial cells by creating Cdc42flox/flox-villin-Cre+ and Cdc42flox/flox-Rosa26-CreER+ mice. We collected intestinal and other tissues, and analyzed their cellular, molecular, morphologic, and physiologic features, compared with the respective heterozygous mice.

RESULTS

In all mutant mice studied, the intestinal epithelium had gross hyperplasia, crypt enlargement, microvilli inclusion, and abnormal epithelial permeability. Cdc42 deficiency resulted in defective Paneth cell differentiation and localization without affecting the differentiation of other cell lineages. In mutant intestinal crypts, proliferating stem and progenitor cells increased, compared with control mice, resulting in increased crypt depth. Cdc42 deficiency increased migration of stem and progenitor cells along the villi, caused a mild defect in the apical junction orientation, and impaired intestinal epithelium polarity, which can contribute to the observed defective intestinal permeability. The intestinal epithelium of the Cdc42flox/flox-villin-Cre+ and Cdc42flox/flox-Rosa26-CreER+ mice appeared similar to that of patients with microvillus inclusion disease. In the digestive track, loss of Cdc42 also resulted in crypt hyperplasia in the colon, but not the stomach.

CONCLUSIONS

Cdc42 regulates proliferation, polarity, migration, and differentiation of intestinal epithelial cells in mice and maintains intestine epithelial barrier and homeostasis. Defects in Cdc42 signaling could be associated with microvillus inclusion disease.

Cdc42 regulates Schwann cell radial sorting and myelin sheath folding through NF2/merlin-dependent and independent signaling

The Rho family GTPase Cdc42 has been implicated in developmental Schwann cell (SC) proliferation, providing sufficient SCs for radial sorting of axons preceding SC differentiation in the peripheral nervous system. We generated Cdc42 conditional knockout (Cdc42-CKO) mice and confirmed aberrant axon sorting in Cdc42-CKO nerves. In adult Cdc42-CKO nerves, blood vessels were enlarged, and mature Remak bundles containing small axons were absent. Abnormal infoldings and outfoldings of myelin sheaths developed in Cdc42-CKO nerves, mimicking pathological features of Charcot-Marie-Tooth (CMT) disease. The NF2/merlin tumor suppressor has been implicated up- and down-stream of Cdc42. In Cdc42-CKO;NF2-del double mutant mice, radial sorting defects seen in Cdc42-CKO nerves were rescued, while changes in myelin sheaths in Cdc42-CKO nerves were not. Phosphorylation of Focal adhesion kinase (FAK) and P-GSK3β, as well as expression of β-catenin were decreased in Cdc42-CKO nerves, and these changes were rescued by NF2/merlin mutation in Cdc42-CKO;NF2-del double mutant mice. Thus, Cdc42 regulates SC radial sorting in vivo through NF2/merlin dependent signaling pathways, while Cdc42 modulation of myelin sheath folding is NF2/merlin independent.

Cdc42 deficiency causes ciliary abnormalities and cystic kidneys

Ciliogenesis and cystogenesis require the exocyst, a conserved eight-protein trafficking complex that traffics ciliary proteins. In culture, the small GTPase Cdc42 co-localizes with the exocyst at primary cilia and interacts with the exocyst component Sec10. The role of Cdc42 in vivo, however, is not well understood. Here, knockdown of cdc42 in zebrafish produced a phenotype similar to sec10 knockdown, including tail curvature, glomerular expansion, and mitogen-activated protein kinase (MAPK) activation, suggesting that cdc42 and sec10 cooperate in ciliogenesis. In addition, cdc42 knockdown led to hydrocephalus and loss of photoreceptor cilia. Furthermore, there was a synergistic genetic interaction between zebrafish cdc42 and sec10, suggesting that cdc42 and sec10 function in the same pathway. Mice lacking Cdc42 specifically in kidney tubular epithelial cells died of renal failure within weeks of birth. Histology revealed cystogenesis in distal tubules and collecting ducts, decreased ciliogenesis in cyst cells, increased tubular cell proliferation, increased apoptosis, increased fibrosis, and led to MAPK activation, all of which are features of polycystic kidney disease, especially nephronophthisis. Taken together, these results suggest that Cdc42 localizes the exocyst to primary cilia, whereupon the exocyst targets and docks vesicles carrying ciliary proteins. Abnormalities in this pathway result in deranged ciliogenesis and polycystic kidney disease.

Divergent functions of the Rho GTPases Rac1 and Cdc42 in podocyte injury

Podocytes are highly specialized epithelial cells with complex actin cytoskeletal architecture crucial for maintenance of the glomerular filtration barrier. The mammalian Rho GTPases Rac1 and Cdc42 are molecular switches that control many cellular processes, but are best known for their roles in the regulation of actin cytoskeleton dynamics. Here we employed podocyte-specific Cre-lox technology and found that mice with deletion of Rac1 display normal podocyte morphology without glomerular dysfunction well into adulthood. Using the protamine sulfate model of acute podocyte injury, podocyte-specific deletion of Rac1 prevented foot process effacement. In a long-term model of chronic hypertensive glomerular damage, however, loss of Rac1 led to an exacerbation of albuminuria and glomerulosclerosis. In contrast, mice with podocyte-specific deletion of Cdc42 had severe proteinuria, podocyte foot process effacement, and glomerulosclerosis beginning as early as 10 days of age. In addition, slit diaphragm proteins nephrin and podocin were redistributed and cofilin was de-phosphorylated. Cdc42 is necessary for the maintenance of podocyte structure and function, but Rac1 is entirely dispensable in physiologic steady state. However, Rac1 has either beneficial or deleterious effects depending on the context of podocyte impairment. Thus, our study highlights the divergent roles of Rac1 and Cdc42 function in podocyte maintenance and injury.

Stage-specific functions of the small Rho GTPases Cdc42 and Rac1 for adult hippocampal neurogenesis

The molecular mechanisms underlying the generation, maturation, and integration of new granule cells generated throughout life in the mammalian hippocampus remain poorly understood. Small Rho GTPases, such as Cdc42 and Rac1, have been implicated previously in neural stem/progenitor cell (NSPC) proliferation and neuronal maturation during embryonic development. Here we used conditional genetic deletion and virus-based loss-of-function approaches to identify temporally distinct functions for Cdc42 and Rac1 in adult hippocampal neurogenesis. We found that Cdc42 is involved in mouse NSPC proliferation, initial dendritic development, and dendritic spine maturation. In contrast, Rac1 is dispensable for early steps of neuronal development but is important for late steps of dendritic growth and spine maturation. These results establish cell-autonomous and stage-specific functions for the small Rho GTPases Cdc42 and Rac1 in the course of adult hippocampal neurogenesis.

Live imaging of astrocyte responses to acute injury reveals selective juxtavascular proliferation

Astrocytes are thought to have important roles after brain injury, but their behavior has largely been inferred from postmortem analysis. To examine the mechanisms that recruit astrocytes to sites of injury, we used in vivo two-photon laser-scanning microscopy to follow the response of GFP-labeled astrocytes in the adult mouse cerebral cortex over several weeks after acute injury. Live imaging revealed a marked heterogeneity in the reaction of individual astrocytes, with one subset retaining their initial morphology, another directing their processes toward the lesion, and a distinct subset located at juxtavascular sites proliferating. Although no astrocytes actively migrated toward the injury site, selective proliferation of juxtavascular astrocytes was observed after the introduction of a lesion and was still the case, even though the extent was reduced, after astrocyte-specific deletion of the RhoGTPase Cdc42. Thus, astrocyte recruitment after injury relies solely on proliferation in a specific niche.

Yap- and Cdc42-dependent nephrogenesis and morphogenesis during mouse kidney development

Yap is a transcriptional co-activator that regulates cell proliferation and apoptosis downstream of the Hippo kinase pathway. We investigated Yap function during mouse kidney development using a conditional knockout strategy that specifically inactivated Yap within the nephrogenic lineage. We found that Yap is essential for nephron induction and morphogenesis, surprisingly, in a manner independent of regulation of cell proliferation and apoptosis. We used microarray analysis to identify a suite of novel Yap-dependent genes that function during nephron formation and have been implicated in morphogenesis. Previous in vitro studies have indicated that Yap can respond to mechanical stresses in cultured cells downstream of the small GTPases RhoA. We find that tissue-specific inactivation of the Rho GTPase Cdc42 causes a severe defect in nephrogenesis that strikingly phenocopies loss of Yap. Ablation of Cdc42 decreases nuclear localization of Yap, leading to a reduction of Yap-dependent gene expression. We propose that Yap responds to Cdc42-dependent signals in nephron progenitor cells to activate a genetic program required to shape the functioning nephron.

The mammalian kidney undergoes reiterative and stereotypical morphogenetic changes to create the elaborately convoluted adult nephron, the functional filtration unit of the kidney. How these sequential morphological events are controlled remains poorly understood. Here we show that the transcriptional activator Yap is essential in the developing murine kidney. Yap mutants have reduced nephrogenesis and defective morphogenesis. Yap function in nephrogenesis is independent of its previously described role in regulation of cell proliferation and apoptosis. Instead, Yap activity is needed for proper expression of a suite of genes that control cell signaling and cell structure. Remarkably, we find that ablation of Cdc42 phenocopies loss of Yap. We show that Cdc42 is essential for nuclear access of Yap, both in vivo and in tissue culture studies. Taken together, our work shows that Yap and Cdc42 are essential for the cell fate and morphogenesis decisions necessary to shape functioning nephrons, and suggests that Yap functions downstream of Cdc42 during kidney development.

CDC42 is required for tissue lamination and cell survival in the mouse retina

The small GTPase CDC42 has pleiotropic functions during development and in the adult. These functions include intra- as well as intercellular tasks such as organization of the cytoskeleton and, at least in epithelial cells, formation of adherens junctions. To investigate CDC42 in the neuronal retina, we generated retina-specific Cdc42-knockdown mice (Cdc42-KD) and analyzed the ensuing consequences for the developing and postnatal retina. Lack of CDC42 affected organization of the developing retina as early as E17.5, prevented correct tissue lamination, and resulted in progressive retinal degeneration and severely reduced retinal function of the postnatal retina. Despite the disorganization of the retina, formation of the primary vascular plexus was not strongly affected. However, both deeper vascular plexi developed abnormally with no clear layering of the vessels. Retinas of Cdc42-KD mice showed increased expression of pro-survival, but also of pro-apoptotic and pro-inflammatory genes and exhibited prolonged Müller glia hypertrophy. Thus, functional CDC42 is important for correct tissue organization already during retinal development. Its absence leads to severe destabilization of the postnatal retina with strong degeneration and loss of retinal function.

Cdc42-dependent structural development of auditory supporting cells is required for wound healing at adulthood

Cdc42 regulates the initial establishment of cytoskeletal and junctional structures, but only little is known about its role at later stages of cellular differentiation. We studied Cdc42′s role in vivo in auditory supporting cells, epithelial cells with high structural complexity. Cdc42 inactivation was induced early postnatally using the Cdc42^loxP/loxP;Fgfr3-iCre-ER^T2 mice. Cdc42 depletion impaired elongation of adherens junctions and F-actin belts, leading to constriction of the sensory epithelial surface. Fragmented F-actin belts, junctions containing ectopic lumens and misexpression of a basolateral membrane protein in the apical domain were observed. These defects and changes in aPKCλ/ι expression suggested that apical polarization is impaired. Following a lesion at adulthood, supporting cells with Cdc42 loss-induced maturational defects collapsed and failed to remodel F-actin belts, a process that is critical to scar formation. Thus, Cdc42 is required for structural differentiation of auditory supporting cells and this proper maturation is necessary for wound healing in adults.

CDC42 is required for structural patterning of the lung during development

The formation of highly branched epithelial structures is critical for the development of many essential organs, including lung, liver, pancreas, kidney and mammary glands. Elongation and branching of these structures require precise control of complex morphogenetic processes that are dependent upon coordinate regulation of cell shape, apical-basal polarity, proliferation, migration, and interactions among multiple cell types. Herein, we demonstrate that temporal-spatial regulation of epithelial cell polarity by the small GTPase, CDC42, is essential for branching morphogenesis of the developing lung. Epithelial cell-specific deletion of CDC42 in fetal mice disrupted epithelial cell polarity, the actin cytoskeleton, intercellular contacts, directional trafficking of proteins, proliferation and mitotic spindle orientation, impairing the organization and patterning of the developing respiratory epithelium and adjacent mesenchyme. Transition from a pseudostratified to a simple columnar epithelium was impaired, consistent with coordinate dysregulation of epithelial cell polarity, mitotic spindle orientation, and repositioning of mitotic cells within the epithelium during cell cycle progression. Expression of sonic hedgehog and its receptor, patched-1, was decreased, while fibroblast growth factor 10 expression in the mesenchyme was expanded, resulting in disruption of branching morphogenesis and bronchiolar smooth muscle formation in this model. CDC42 is required for spatial positioning of proliferating epithelial cells, as well as signaling interactions between the epithelium and mesenchyme and is, therefore, essential for formation and maintenance of the respiratory tract during morphogenesis of the fetal lung.

Rho-family GTPase Cdc42 controls migration of Langerhans cells in vivo

Epidermal Langerhans cells (LCs) of the skin represent the prototype migratory dendritic cell (DC) subtype. In the skin, they take up Ag, migrate to the draining lymph nodes, and contribute to Ag transport and immunity. Different depletion models for LCs have revealed contrasting roles and contributions of this cell type. To target the migratory properties of DCs, we generated mice lacking the Rho-family GTPase Cdc42 specifically in DCs. In these animals, the initial seeding of the epidermis with LCs is functional, resulting in slightly reduced Langerhans cell numbers. However, Cdc42-deficient LCs fail to leave the skin in steady state as well as upon stimulation, as they do not enter the skin-draining afferent lymph vessels. Similarly, also other Cdc42-deficient migratory DC subsets fail to home properly to the corresponding draining lymph nodes. We used this novel mouse model, in which LCs are locked out, to demonstrate that these cells contribute substantially to priming of Ag-specific CD4 and CD8 T cell responses upon epicutaneous immunization, but could not detect a role in the induction of contact hypersensitivity to various doses of hapten.

Myelin is dependent on the Charcot-Marie-Tooth Type 4H disease culprit protein FRABIN/FGD4 in Schwann cells

Studying the function and malfunction of genes and proteins associated with inherited forms of peripheral neuropathies has provided multiple clues to our understanding of myelinated nerves in health and disease. Here, we have generated a mouse model for the peripheral neuropathy Charcot-Marie-Tooth disease type 4H by constitutively disrupting the mouse orthologue of the suspected culprit gene FGD4 that encodes the small RhoGTPase Cdc42-guanine nucleotide exchange factor Frabin. Lack of Frabin/Fgd4 causes dysmyelination in mice in early peripheral nerve development, followed by profound myelin abnormalities and demyelination at later stages. At the age of 60 weeks, this was accompanied by electrophysiological deficits. By crossing mice carrying alleles of Frabin/Fgd4 flanked by loxP sequences with animals expressing Cre recombinase in a cell type-specific manner, we show that Schwann cell-autonomous Frabin/Fgd4 function is essential for proper myelination without detectable primary contributions from neurons. Deletion of Frabin/Fgd4 in Schwann cells of fully myelinated nerve fibres revealed that this protein is not only required for correct nerve development but also for accurate myelin maintenance. Moreover, we established that correct activation of Cdc42 is dependent on Frabin/Fgd4 function in healthy peripheral nerves. Genetic disruption of Cdc42 in Schwann cells of adult myelinated nerves resulted in myelin alterations similar to those observed in Frabin/Fgd4-deficient mice, indicating that Cdc42 and the Frabin/Fgd4-Cdc42 axis are critical for myelin homeostasis. In line with known regulatory roles of Cdc42, we found that Frabin/Fgd4 regulates Schwann cell endocytosis, a process that is increasingly recognized as a relevant mechanism in peripheral nerve pathophysiology. Taken together, our results indicate that regulation of Cdc42 by Frabin/Fgd4 in Schwann cells is critical for the structure and function of the peripheral nervous system. In particular, this regulatory link is continuously required in adult fully myelinated nerve fibres. Thus, mechanisms regulated by Frabin/Fgd4-Cdc42 are promising targets that can help to identify additional regulators of myelin development and homeostasis, which may crucially contribute also to malfunctions in different types of peripheral neuropathies.

Planar cell polarity effector gene Intu regulates cell fate-specific differentiation of keratinocytes through the primary cilia

Genes involved in the planar cell polarity (PCP) signaling pathway are essential for a number of developmental processes in mammals, such as convergent extension and ciliogenesis. Tissue-specific PCP effector genes of the PCP signaling pathway are believed to mediate PCP signals in a tissue- and cell type-specific manner. However, how PCP signaling controls the morphogenesis of mammalian tissues remains unclear. In this study, we investigated the role of inturned (Intu), a tissue-specific PCP effector gene, during hair follicle formation in mice. Tissue-specific disruption of Intu in embryonic epidermis resulted in hair follicle morphogenesis arrest because of the failure of follicular keratinocyte to differentiate. Targeting Intu in the epidermis resulted in almost complete loss of primary cilia in epidermal and follicular keratinocytes, and a suppressed hedgehog signaling pathway. Surprisingly, the epidermal stratification and differentiation programs and barrier function were not affected. These results demonstrate that tissue-specific PCP effector genes of the PCP signaling pathway control the differentiation of keratinocytes through the primary cilia in a cell fate- and context-dependent manner, which may be critical in orchestrating the propagation and interpretation of polarity signals established by the core PCP components.

Polarity and stratification of the epidermis

Polarity is a fundamental property of epithelial cells. In this review, we discuss our current knowledge of the polarity of a stratified epithelium, the epidermis, focusing on similarities and differences with simple epithelial models. We highlight how the differences in tissue architecture and physiology result in alterations in some aspects of cell polarity. In addition, we discuss one of the most prominent uses for cell polarity in the epidermis-orienting the mitotic spindle to drive the stratification and differentiation of this tissue during development.

Podocyte-specific loss of Cdc42 leads to congenital nephropathy

Rho family GTPases are molecular switches best known for their pivotal role in dynamic regulation of the actin cytoskeleton. The prototypic members of this family are Cdc42, Rac1, and RhoA; these GTPases contribute to the breakdown of glomerular filtration and the resultant proteinuria, but their functions in normal podocyte physiology remain poorly understood. Here, mice lacking Cdc42 in podocytes developed congenital nephropathy and died as a result of renal failure within 2 weeks after birth. In contrast, mice lacking Rac1 or RhoA in podocytes were overtly normal and lived to adulthood. Kidneys from Cdc42-mutant mice exhibited protein-filled microcysts with hallmarks of collapsing glomerulopathy, as well as extensive effacement of podocyte foot processes with abnormal junctional complexes. Furthermore, we observed aberrant expression of several podocyte markers and cell polarity proteins in the absence of Cdc42, indicating a disruption of the slit diaphragm. Kidneys from Rac1- and RhoA-mutant mice, however, had normal glomerular morphology and intact foot processes. A nephrin clustering assay suggested that Cdc42 deficiency, but not Rac1 or RhoA deficiency, impairs the polymerization of actin at sites of nephrin aggregates. Taken together, these data highlight the physiological importance of Cdc42, but not Rac1 or RhoA, in establishing podocyte architecture and glomerular function.

Cdc42 and Rab8a are critical for intestinal stem cell division, survival, and differentiation in mice

The constant self renewal and differentiation of adult intestinal stem cells maintains a functional intestinal mucosa for a lifetime. However, the molecular mechanisms that regulate intestinal stem cell division and epithelial homeostasis are largely undefined. We report here that the small GTPases Cdc42 and Rab8a are critical regulators of these processes in mice. Conditional ablation of Cdc42 in the mouse intestinal epithelium resulted in the formation of large intracellular vacuolar structures containing microvilli (microvillus inclusion bodies) in epithelial enterocytes, a phenotype reminiscent of human microvillus inclusion disease (MVID), a devastating congenital intestinal disorder that results in severe nutrient deprivation. Further analysis revealed that Cdc42-deficient stem cells had cell division defects, reduced capacity for clonal expansion and differentiation into Paneth cells, and increased apoptosis. Cdc42 deficiency impaired Rab8a activation and its association with multiple effectors, and prevented trafficking of Rab8a vesicles to the midbody. This impeded cytokinesis, triggering crypt apoptosis and disrupting epithelial morphogenesis. Rab8a was also required for Cdc42-GTP activity in the intestinal epithelium, where continued cell division takes place. Furthermore, mice haploinsufficient for both Cdc42 and Rab8a in the intestine demonstrated abnormal crypt morphogenesis and epithelial transporter physiology, further supporting their functional interaction. These data suggest that defects of the stem cell niche can cause MVID. This hypothesis represents a conceptual departure from the conventional view of this disease, which has focused on the affected enterocytes, and suggests stem cell-based approaches could be beneficial to infants with this often lethal condition.

Patterning skin by planar cell polarity: the multi-talented hair designer

In mammals, the skin can form complex global and local patterns to meet diverse functional requirements in different parts of the body. To date, the fundamental principles that underlie skin patterning remain poorly understood because of the involvement of multiple interacting processes. Genes involved in the planar cell polarity (PCP) signalling pathway, which is capable of polarizing cells within the planar plane of an epithelium, can control the orientation and differentiation of hair follicles, underlining their involvement in skin pattern formation. Here, we summarize recent progress that has been made to understand the PCP signalling pathway and its function in mammalian skin, including its role in hair follicle morphogenesis, ciliogenesis and wound healing. We argue that dissecting PCP signalling in the context of hair follicle formation might reveal many as-yet-undiscovered functions for PCP in the development, homeostasis and regeneration of skin.

Rho GTPase knockout induction in primary keratinocytes from adult mice

Primary keratinocytes are an important tool to investigate the molecular mechanism underlying the skin phenotype of mice with null mutations in Rho GTPase genes. If the RhoA gene deletion is conditional, the knockout can be induced in vitro by transfection with cre-IRES-GFP and sorting for GFP positive cells by flow cytometry. Such in vitro knockout will allow determining the cell autonomous functions of the Rho GTPase, independent of any in vivo interactions. Using the same method, also other expression vectors or knockdown constructs can be introduced into primary mouse keratinocytes.

Mechanisms of cardiogenesis in cardiovascular progenitor cells

Self-renewing cells of the vertebrate heart have become a major subject of interest in the past decade. However, many researchers had a hard time to argue against the orthodox textbook view that defines the heart as a postmitotic organ. Once the scientific community agreed on the existence of self-renewing cells in the vertebrate heart, their origin was again put on trial when transdifferentiation, dedifferentiation, and reprogramming could no longer be excluded as potential sources of self-renewal in the adult organ. Additionally, the presence of self-renewing pluripotent cells in the peripheral blood challenges the concept of tissue-specific stem and progenitor cells. Leaving these unsolved problems aside, it seems very desirable to learn about the basic biology of this unique cell type. Thus, we shall here paint a picture of cardiovascular progenitor cells including the current knowledge about their origin, basic nature, and the molecular mechanisms guiding proliferation and differentiation into somatic cells of the heart.

Notch signaling and the developing skin epidermis

The innermost (basal) layer of the skin epidermis consists of proliferative progenitors which give rise to multiple differentiating layers providing a barrier that keeps the inside of the body moist and protects the body from outside assaults by physical, environmental and biological factors. The epidermis is maintained throughout life through the proliferation of stem cells and differentiation of their progeny. Notch signaling pathway is a highly conserved molecular network that plays an essential role in cell fate determination during embryogenesis and also in postnatal life. Data from ongoing studies indicate that Notch signaling orchestrates the process of epidermal differentiation and proliferation through the sequential activity of different Notch ligands, receptors and downstream pathways.

The apical polarity determinant Crumbs 2 is a novel regulator of ESC-derived neural progenitors

ESCs undergoing neural differentiation in vitro display an intrinsic heterogeneity with a large subset of the cells forming polarized neural rosettes that maintain the neural progenitor microenvironment. This heterogeneity is not only necessary for normal development but also causes substantial technical challenges for practical applications. Here, we report a novel regulator of early neural progenitors, the apical polarity protein Crb2 (Crumbs homologue 2). Employing monolayer differentiation of mouse ESCs to model neurogenesis in vitro, we find that Crb2 is upregulated with Sox1 and Musashi at the onset of neuroepithelial specification and localizes to the apical side of neural rosettes. Stable Crb2-knockdown (KD) lines die at the onset of neural specification and fail to stabilize several apical polarity proteins. However, these cells are able to proliferate under self-renewing conditions and can be differentiated into mesodermal and endodermal lineages. Conversely, Crb2 overexpression during neural differentiation results in elevated levels of other apical polarity proteins and increases proliferation. Additionally, sustained overexpression of Crb2 reduces terminal differentiation into TuJ1-positive neurons. Furthermore, we demonstrate that Crb2 overexpression under self-renewing conditions increases glycogen synthase kinase (GSK)-3β inhibition, correlating with an increase in clonogenicity. To confirm the importance of GSK-3β inhibition downstream of Crb2, we show that Crb2-KD cells can be forced into neural lineages by blocking GSK-3β function and supplementing Epidermal Growth Factor (EGF) and basic Fibroblast Growth Factor (bFGF). Thus, this is the first demonstration that a member of the Crumbs family is essential for survival and differentiation of ESC-derived neural progenitors.

Retinal degeneration modulates intracellular localization of CDC42 in photoreceptors

PURPOSE

Rho GTPases such as RAS-related C3 botulinum substrate 1 (RAC1) and cell division cycle 42 homolog (S. cerevisiae; CDC42) have been linked to cellular processes including movement, development, and apoptosis. Recently, RAC1 has been shown to be a pro-apoptotic factor in the retina during light-induced photoreceptor degeneration. Here, we analyzed the role of CDC42 in the degenerating retina.

METHODS

Photoreceptor degeneration was studied in a mouse model for autosomal dominant retinitis pigmentosa (VPP) with or without a rod-specific knockdown of Cdc42, as well as in wild-type and Cdc42 knockdown mice after light exposure. Gene and protein expression were analyzed by real-time PCR, western blotting, and immunofluorescence. Retinal morphology and function were assessed by light microscopy and electroretinography, respectively.

RESULTS

CDC42 accumulated in the perinuclear region of terminal deoxynucleotidyl transferase dUTP nick end labeling-negative photoreceptors during retinal degeneration induced by excessive light exposure and in the rd1, rd10, and VPP mouse models of retinitis pigmentosa. The knockdown of Cdc42 did not affect retinal morphology or function in the adult mice and did not influence photoreceptor apoptosis or molecular signaling during induced and inherited retinal degeneration.

CONCLUSIONS

Retinal degeneration induces the accumulation of CDC42 in the perinuclear region of photoreceptors. In contrast to RAC1, however, lack of CDC42 does not affect the progression of degeneration. CDC42 is also dispensable for normal morphology and function of adult rod photoreceptor cells. RECEIVED: May 25, 2011 ACCEPTED: November 10, 2011.

Genetic deletion of cdc42 reveals a crucial role for astrocyte recruitment to the injury site in vitro and in vivo

It is generally suggested that astrocytes play important restorative functions after brain injury, yet little is known regarding their recruitment to sites of injury, despite numerous in vitro experiments investigating astrocyte polarity. Here, we genetically manipulated one of the proposed key signals, the small RhoGTPase Cdc42, selectively in mouse astrocytes in vitro and in vivo. We used an in vitro scratch assay as a minimal wounding model and found that astrocytes lacking Cdc42 (Cdc42Δ) were still able to form protrusions, although in a nonoriented way. Consequently, they failed to migrate in a directed manner toward the scratch. When animals were injured in vivo through a stab wound, Cdc42Δ astrocytes developed protrusions properly oriented toward the lesion, but the number of astrocytes recruited to the lesion site was significantly reduced. Surprisingly, however, lesions in Cdc42Δ animals, harboring fewer astrocytes contained significantly higher numbers of microglial cells than controls. These data suggest that impaired recruitment of astrocytes to sites of injury has a profound and unexpected effect on microglia recruitment.

The Rho GTPase Cdc42 is required for primary mammary epithelial cell morphogenesis in vitro

The Rho GTPase Cdc42 is overexpressed and hyperactivated in breast cancer, and several studies have described mechanisms by which it may promote tumor formation and progression. However, little is known about the role of Cdc42 during normal mammary epithelial cell (MEC) morphogenesis. Here we aimed to define the precise role for Cdc42 during primary mammary acinus formation in vitro. For these studies, MECs were isolated from Cdc42fl/fl conditional knockout mice, transduced with Adeno-cre-GFP virus to delete Cdc42 or Adeno-GFP control virus, and effects on morphogenesis were investigated using a three-dimensional (3D) culture assay. Interestingly, markedly fewer mammary acini developed in Cdc42 deficient cultures, and the acini that formed were significantly smaller and disorganized. Cellular proliferation and survival were reduced in the Cdc42 deficient acini. However, control and knockout MECs cultured as monolayers displayed similar cell cycle profiles, suggesting that Cdc42 is important for MEC proliferation in the context of 3D polarity. Overexpression of cyclin D1, which promotes cell cycle progression downstream of Cdc42, failed to rescue the defect in acinus size. Furthermore, lumen formation and apical-basal polarity were disrupted, and mitotic spindle orientation and Cdc42/aPKC polarity complex defects likely contributed to these phenotypes. Studies using dominant negative Cdc42 and siRNa to knockdown Cdc42 in MDcK and Caco-2 cell lines undergoing cystogenesis in 3D cultures revealed critical roles for Cdc42 in spindle orientation, polarity and lumen formation. Our studies, using complete knockout in primary epithelial cells, demonstrate that Cdc42 is not only an important regulator of polarity and lumen formation; it is also essential for proliferation and survival, which are key cellular processes that drive MEC morphogenesis in vitro and in vivo.

The small GTPase Cdc42 interacts with Niemann-Pick C1-like 1 (NPC1L1) and controls its movement from endocytic recycling compartment to plasma membrane in a cholesterol-dependent manner

Niemann-Pick C1-like 1 (NPC1L1) is a multi-transmembrane protein that mediates the absorption of dietary and biliary cholesterol through vesicular endocytosis. The subcellular localization of NPC1L1 is regulated by cholesterol. Cholesterol depletion induces the transport of NPC1L1 to plasma membrane (PM) from endocytic recycling compartment that requires MyoVb·Rab11a·Rab11-FIP2 triple complex, and cholesterol-replenishment renders the internalization of NPC1L1 together with cholesterol. Here, we find that GTP-bound Cdc42 interacts with NPC1L1. Cholesterol depletion regulates the activation of Cdc42 and enhances NPC1L1-Cdc42 interaction. Overexpression of constitutive GTP-bound Cdc42 mutant form or knockdown of Cdc42 inhibits the transport of NPC1L1 to the PM and disturbs the cholesterol-regulated binding of NPC1L1 to Rab11a, MyoVb, and actin. Knockdown of Cdc42 downstream effectors N-WASP or Arp3 also leads to the similar results. In liver-specific Cdc42 knock-out (Cdc42 LKO) mice, NPC1L1 fails to localize to bile canaliculi, and the biliary cholesterol cannot be efficiently reabsorbed. These results indicate that Cdc42 controls the cholesterol-regulated transport and localization of NPC1L1, and plays a role in cholesterol absorption.

Cdc42 controls vascular network assembly through protein kinase Cι during embryonic vasculogenesis

OBJECTIVE

The goal of this study was to determine the role of Cdc42 in embryonic vasculogenesis and the underlying mechanisms.

METHODS AND RESULTS

By using genetically modified mouse embryonic stem (ES) cells, we demonstrate that ablation of the Rho GTPase Cdc42 blocks vascular network assembly during embryoid body (EB) vasculogenesis without affecting endothelial lineage differentiation. Reexpression of Cdc42 in mutant EBs rescues the mutant phenotype, establishing an essential role for Cdc42 in vasculogenesis. Chimeric analysis revealed that the vascular phenotype is caused by inactivation of Cdc42 in endothelial cells rather than surrounding cells. Endothelial cells isolated from Cdc42-null EBs are defective in directional migration and network assembly. In addition, activation of atypical protein kinase Cι (PKCι) is abolished in Cdc42-null endothelial cells, and PKCι ablation phenocopies the vascular abnormalities of the Cdc42-null EBs. Moreover, the inhibitory phosphorylation of glycogen synthase kinase-3β (GSK-3β) at Ser9 depends on Cdc42 and PKCι, and expression of kinase-dead GSK-3β in Cdc42-null EBs promotes the formation of linear endothelial segments without branches. These results suggest that PKCι and GSK-3β are downstream effectors of Cdc42 during vascular morphogenesis.

CONCLUSIONS

Cdc42 controls vascular network assembly but not endothelial lineage differentiation by activating PKCι during embryonic vasculogenesis.

Gain-of-function mutations of ARHGAP31, a Cdc42/Rac1 GTPase regulator, cause syndromic cutis aplasia and limb anomalies

Regulation of cell proliferation and motility is essential for normal development. The Rho family of GTPases plays a critical role in the control of cell polarity and migration by effecting the cytoskeleton, membrane trafficking, and cell adhesion. We investigated a recognized developmental disorder, Adams-Oliver syndrome (AOS), characterized by the combination of aplasia cutis congenita (ACC) and terminal transverse limb defects (TTLD). Through a genome-wide linkage analysis, we detected a locus for autosomal-dominant ACC-TTLD on 3q generating a maximum LOD score of 4.93 at marker rs1464311. Candidate-gene- and exome-based sequencing led to the identification of independent premature truncating mutations in the terminal exon of the Rho GTPase-activating protein 31 gene, ARHGAP31, which encodes a Cdc42/Rac1 regulatory protein. Mutant transcripts are stable and increase ARHGAP31 activity in vitro through a gain-of-function mechanism. Constitutively active ARHGAP31 mutations result in a loss of available active Cdc42 and consequently disrupt actin cytoskeletal structures. Arhgap31 expression in the mouse is substantially restricted to the terminal limb buds and craniofacial processes during early development; these locations closely mirror the sites of impaired organogenesis that characterize this syndrome. These data identify the requirement for regulated Cdc42 and/or Rac1 signaling processes during early human development.

Cdc42 in oncogenic transformation, invasion, and tumorigenesis

The Rho family of GTPases represents a class of Ras-related signaling molecules often deregulated in cancer. Rho GTPases switch from a GDP-bound, inactive state to a GTP-bound, active state in response to extracellular stimuli such as mitogens and extracellular matrix. In addition, Rho GTPase signaling can be altered in response to cell intrinsic factors such as changes in oncogenic or tumor suppressor signaling. In their active form, these proteins bind to a number of effector molecules, activating signaling cascades which regulate a variety of cellular processes including cytoskeletal reorganization, cell cycle progression, cell polarity and transcription. Here, we focus on one Rho family member, Cdc42, which is overexpressed in a number of human cancers. Consistent with a role in the promotion of tumorigenesis, activating mutations in Cdc42 and guanine nucleotide exchange factors are transforming, while inhibition of Cdc42 activity can impinge on cellular transformation following the activation of oncoproteins or loss of tumor suppressor function. Furthermore, Cdc42 activity has been implicated in the invasive phenotype which characterizes tumor metastasis, further suggesting that Cdc42 may be a useful target for therapeutic intervention. However, several recent studies in mice have unveiled a putative tumor suppressor function of Cdc42 in several tissue types which may involve cell polarity maintenance, suggesting that the role of Cdc42 in cancer development is complex and may be cell type specific.

RhoA is dispensable for skin development, but crucial for contraction and directed migration of keratinocytes

RhoA is a small GTPase shown to be crucial for cytokinesis, stress fiber formation, and epithelial cell–cell contacts. Analyzing mice with a keratinocyte-restricted deletion of the RhoA gene, we find that RhoA is not required for skin development and maintenance but has specific functions in vitro.

RhoA is a small guanosine-5’-triphosphatase (GTPase) suggested to be essential for cytokinesis, stress fiber formation, and epithelial cell–cell contacts. In skin, loss of RhoA was suggested to underlie pemphigus skin blistering. To analyze RhoA function in vivo, we generated mice with a keratinocyte-restricted deletion of the RhoA gene. Despite a severe reduction of cofilin and myosin light chain (MLC) phosphorylation, these mice showed normal skin development. Primary RhoA-null keratinocytes, however, displayed an increased percentage of multinucleated cells, defective maturation of cell–cell contacts. Furthermore we observed increased cell spreading due to impaired RhoA-ROCK (Rho-associated protein kinase)-MLC phosphatase-MLC–mediated cell contraction, independent of Rac1. Rho-inhibiting toxins further increased multinucleation of RhoA-null cells but had no significant effect on spreading, suggesting that RhoB and RhoC have partially overlapping functions with RhoA. Loss of RhoA decreased directed cell migration in vitro caused by reduced migration speed and directional persistence. These defects were not related to the decreased cell contraction and were independent of ROCK, as ROCK inhibition by Y27632 increased directed migration of both control and RhoA-null keratinocytes. Our data indicate a crucial role for RhoA and contraction in regulating cell spreading and a contraction-independent function of RhoA in keratinocyte migration. In addition, our data show that RhoA is dispensable for skin development.

Cdc42 and Gsk3 modulate the dynamics of radial glial growth, inter-radial glial interactions and polarity in the developing cerebral cortex

Polarized radial glia are crucial to the formation of the cerebral cortex. They serve as neural progenitors and as guides for neuronal placement in the developing cerebral cortex. The maintenance of polarized morphology is essential for radial glial functions, but the extent to which the polarized radial glial scaffold is static or dynamic during corticogenesis remains an open question. The developmental dynamics of radial glial morphology, inter-radial glial interactions during corticogenesis, and the role of the cell polarity complexes in these activities remain undefined. Here, using real-time imaging of cohorts of mouse radial glia cells, we show that the radial glial scaffold, upon which the cortex is constructed, is highly dynamic. Radial glial cells within the scaffold constantly interact with one another. These interactions are mediated by growth cone-like endfeet and filopodia-like protrusions. Polarized expression of the cell polarity regulator Cdc42 in radial glia regulates glial endfeet activities and inter-radial glial interactions. Furthermore, appropriate regulation of Gsk3 activity is required to maintain the overall polarity of the radial glia scaffold. These findings reveal dynamism and interactions among radial glia that appear to be crucial contributors to the formation of the cerebral cortex. Related cell polarity determinants (Cdc42, Gsk3) differentially influence radial glial activities within the evolving radial glia scaffold to coordinate the formation of cerebral cortex.

Signaling role of Cdc42 in regulating mammalian physiology

Cdc42 is a member of the Rho GTPase family of intracellular molecular switches regulating multiple signaling pathways involved in actomyosin organization and cell proliferation. Knowledge of its signaling function in mammalian cells came mostly from studies using the dominant-negative or constitutively active mutant overexpression approach in the past 2 decades. Such an approach imposes a number of experimental limitations related to specificity, dosage, and/or clonal variability. Recent studies by conditional gene targeting of cdc42 in mice have revealed its tissue- and cell type-specific role and provide definitive information of the physiological signaling functions of Cdc42 in vivo.

Does GSK-3 provide a shortcut for PI3K activation of Wnt signalling?

Glycogen synthase kinase-3 (GSK-3) is a well-established downstream component of the phosphatidylinositol 3-kinase (PI3K) signalling pathway but is also a key enzyme in negatively regulating the canonical Wnt/β-catenin signalling pathway. Several recent studies argue that PKB (protein kinase B)-mediated inhibition of GSK-3 leads to β-catenin accumulation, but whether cross-talk actually exists between these two pathways is controversial. To elucidate the mechanisms of shared signalling components, further studies taking into account different components of the PI3K signalling pathway and different pools of GSK-3 or β-catenin are required.

Concise review: polarity in stem cells, disease, and aging

Adult somatic stem cells are central to homeostasis in tissues that present with a high cellular turnover like the skin, intestine, and the hematopoietic system. It is thought that polarity is particularly important with respect to fate decisions on stem cell division (symmetric or asymmetric) as well as for the maintenance of stem cell adhesion and quiescence (interaction with the niche). Consequently the failure to establish or regulate stem cell polarity might result in disease or tissue attrition. Members of the family of small RhoGTPases are known to exert an important role in regulating cell polarity. We summarize and discuss here recent views on the role of cell polarity in somatic stem cell function, aging, and disease, concluding that targeting cell polarity might be a novel approach to ameliorate or even revert aberrant somatic stem cell function. Stem Cells 2010; 28:1623–1629.

Migration of epithelial cells on laminins: RhoA antagonizes directionally persistent migration

Spatial and temporal expression of laminin isoforms is assumed to provide specific local information to neighboring cells. Here, we report the remarkably selective presence of LM-111 at the very tip of hair follicles where LM-332 is absent, suggesting that epithelial cells lining the dermal-epidermal junction at this location may receive different signals from the two laminins. This hypothesis was tested in vitro by characterizing with functional and molecular assays the comportment of keratinocytes exposed to LM-111 and LM-332. The two laminins induced morphologically distinct focal adhesions, and LM-332, but not LM-111, elicited persistent migration of keratinocytes. The different impact on cellular behavior was associated with distinct activation patterns of Rho GTPases and other signaling intermediates. In particular, while LM-111 triggered a robust activation of Cdc42, LM-332 provoked a strong and sustained activation of FAK. Interestingly, activation of Rac1 was necessary but not sufficient to promote migration because there was no directed migration on LM-111 despite Rac1 activation. In contrast, RhoA antagonized directional migration, since silencing of RhoA by RNA interference boosted unidirectional migration on LM-332. Molecular analysis of the role of RhoA strongly suggested that the mechanisms involve disassembly of cell-cell contacts, loss of the cortical actin network, mobilization of α6β4 integrin out of stable adhesions, and displacement of the integrin from its association with the insoluble pool of intermediate filaments.

Pleiotropic functions of Rho GTPase signaling: a Trojan horse or Achilles' heel for breast cancer treatment?

Rho GTPase signaling is altered in human breast tumors, and elevated expression and activation of Rho GTPases correlate with tumor progression, metastasis, and poor prognosis. Here we review the evidence that Rho signaling functions as a key regulator of cell cycle, mitosis, apoptosis, and invasion during breast cancer growth and progression and discuss whether these pleiotropic actions enhance or limit the targetability of this network. We propose that depending on the stage and subtype of breast cancer, targeting Rho signaling may have chemopreventative, anti-tumor, and anti-metastatic efficacy. An understanding of how Rho signaling is perturbed in specific stages and subtypes of breast cancer and how it functions in the context of the complex in vivo environment during the stochastic process of tumor formation and progression are necessary in order to effectively target this signaling network for breast cancer treatment.

Aberrant regulation of planar cell polarity in polycystic kidney disease

Mutations in PKD1, which encodes polycystin-1 (PC1), contribute to >85% of cases of autosomal dominant polycystic kidney disease (ADPKD). The planar cell polarity (PCP) pathway is necessary for the oriented cell division and convergent extension that establishes and maintains the structure of kidney tubules, but the role of this pathway in the pathophysiology of ADPKD is incompletely understood. Here, we show that inactivation of Pkd1 in postnatal developing mouse kidneys leads to a defect in oriented cell division in precystic kidney tubules. We also observed this defect in precystic Pkd1-inactivated mature kidneys subjected to ischemia-reperfusion injury as a "third hit." Cystic kidneys exhibited striking upregulation and activation of Frizzled 3 (Fz3), a regulator of PCP, and its downstream effector, CDC42. Precystic kidneys demonstrated upregulation of CDC42, but the localization of the polarity proteins Par3 and Par6 was similar to control. Fz3 was expressed on the cilia of cystic kidneys but barely detected on the cilia of normal kidneys. In vitro, PC1 and Fz3 antagonized each other to control CDC42 expression and the rate of cell migration in HEK293T cells. Taken together, our data suggest that PC1 controls oriented cell division and that aberrant PCP signaling contributes to cystogenesis.

GSK3 signalling in neural development

Recent evidence suggests that glycogen synthase kinase 3 (GSK3) proteins and their upstream and downstream regulators have key roles in many fundamental processes during neurodevelopment. Disruption of GSK3 signalling adversely affects brain development and is associated with several neurodevelopmental disorders. Here, we discuss the mechanisms by which GSK3 activity is regulated in the nervous system and provide an overview of the recent advances in the understanding of how GSK3 signalling controls neurogenesis, neuronal polarization and axon growth during brain development. These recent advances suggest that GSK3 is a crucial node that mediates various cellular processes that are controlled by multiple signalling molecules--for example, disrupted in schizophrenia 1 (DISC1), partitioning defective homologue 3 (PAR3), PAR6 and Wnt proteins--that regulate neurodevelopment.