Huntingtin Is Required for Epithelial Polarity through RAB11A-Mediated Apical Trafficking of PAR3-aPKC

Decoding the role of microtubules: a trafficking road for vesicle

Background: In eukaryotic cells, intracellular and extracellular vesicle transport systems are ubiquitous and tightly linked. This process involves well-defined initiation and termination points, as well as mechanisms for vesicle recycling. During transport, cytoskeletal components serve as "roads" to prevent disordered vesicular movement and to ensure efficient transport, particularly through microtubules. Microtubules primarily facilitate the long-distance transport of vesicles. The dynamic nature of microtubule structure makes its stability sensitive to proteins, drugs, and post-translational modifications such as acetylation, which in turn regulate microtubule-dependent vesicular transport. Furthermore, motor proteins interact with microtubules and bind to cargoes via their tail domains, driving vesicle transport along microtubules and determining the directionality of movement. Aim of review: To elucidate the detailed processes and mechanisms of microtubules-regulated long-distance vesicle transport, providing a comprehensive overview of current research in this area. Key scientific concepts of review: This review provides an in-depth analysis of microtubule-mediated vesicle transport, emphasizing the molecular mechanisms involved. It examines vesicle transport between organelles, the impact of microtubule characteristics on this process, and the role of motor proteins in vesicle dynamics. Additionally, it summarizes diseases associated with abnormal microtubule-mediated vesicle transport, aiming to offer insights for the treatment of related conditions.

The dynamics of tubulogenesis in development and disease

Tubes are crucial for the function of many organs in animals given their fundamental roles in transporting and exchanging substances to maintain homeostasis within an organism. Therefore, the development and maintenance of these tube-like structures within organs is a vital process. Tubes can form in diverse ways, and advances in our understanding of the molecular and cellular mechanisms underpinning these different modes of tubulogenesis have significant impacts in many biological contexts, including development and disease. This Review discusses recent progress in understanding developmental mechanisms underlying tube formation.

Pleiotropic effects of mutant huntingtin on retinopathy in two mouse models of Huntington's disease

Huntington's disease (HD) is caused by the expansion of a CAG repeat, encoding a string of glutamines (polyQ) in the first exon of the huntingtin gene (HTTex1). This mutant huntingtin protein (mHTT) with extended polyQ forms aggregates in cortical and striatal neurons, causing cell damage and death. The retina is part of the central nervous system (CNS), and visual deficits and structural abnormalities in the retina of HD patients have been observed. Defects in retinal structure and function are also present in the R6/2 and R6/1 HD transgenic mouse models that contain a gene fragment to express mHTTex1. We investigated whether these defects extend to the zQ175KI mouse model which is thought to be more representative of the human condition because it was engineered to contain the extended CAG repeat within the endogenous HTT locus. We found qualitatively similar phenotypes between R6/1 and zQ175KI retinae that include the presence of mHTT aggregates in retinal neurons, cone loss, downregulation of rod signaling proteins and abnormally elongated photoreceptor connecting cilia. In addition, we present novel findings that mHTT disrupts cell polarity in the photoreceptor cell layer and the retinal pigment epithelium (RPE). Furthermore, we show that the RPE cells from R6/1 mice contain mHTT nuclear inclusions, adding to the list of non-neuronal cells with mHTT aggregates and pathology. Thus, the eye may serve as a useful system to track disease progression and to test therapeutic intervention strategies for HD.

CeRNA Network Reveals the Circular RNA Characterization in Goat Ear Fibroblasts Reprogramming into Mammary Epithelial Cells

Circular RNAs (circRNAs) are a type of non-coding RNA that play a crucial role in the development and lactation of mammary glands in mammals. A total of 107 differentially expressed circRNAs (DE circRNAs) were found, of which 52 were up-regulated and 55 were down-regulated. We also found that DE circRNA host genes were mainly involved in GO terms related to the development process of mammary epithelial cells and KEGG pathways were mostly related to mammary epithelial cells, lactation, and gland development. Protein network analysis found that DE circRNAs can competitively bind to miRNAs as key circRNAs by constructing a circRNA-miRNA-mRNA network. CircRNAs competitively bind to miRNAs (miR-10b-3p, miR-671-5p, chi-miR-200c, chi-miR-378-3p, and chi-miR-30e-5p) involved in goat mammary gland development, mammary epithelial cells, and lactation, affecting the expression of core genes (CDH2, MAPK1, ITGB1, CAMSAP2, and MAPKAPK5). Here, we generated CiMECs and systematically explored the differences in the transcription profile for the first time using whole-transcriptome sequencing. We also analyzed the interaction among mRNA, miRNA, and cirRNA and predicted that circRNA plays an important role in the maintenance of mammary epithelial cells.

Huntington's disease and brain development

Huntington's disease is a rare inherited neurological disorder that generally manifests in mild-adulthood. The disease is characterized by the dysfunction and the degeneration of specific brain structures leading progressively to psychiatric, cognitive and motor disorders. The disease is caused by a mutation in the gene coding for huntingtin and, although it appears in adulthood, embryos carry the mutated gene from their development in utero. Studies based on mouse models and human stem cells have reported altered developmental mechanisms in disease conditions. However, does the mutation affect development in humans? Focusing on the early stages of brain development in human fetuses carrying the HD mutation, we have identified abnormalities in the development of the neocortex, the structure that ensure higher cerebral functions. Altogether, these studies suggests that developmental defects could contribute to the onset symptoms in adults, changing the perspective on disease and thus the health care of patients.

Rab11 and Its Role in Neurodegenerative Diseases

Vesicles mediate the trafficking of membranes/proteins in the endocytic and secretory pathways. These pathways are regulated by small GTPases of the Rab family. Rab proteins belong to the Ras superfamily of GTPases, which are significantly involved in various intracellular trafficking and signaling processes in the nervous system. Rab11 is known to play a key role especially in recycling many proteins, including receptors important for signal transduction and preservation of functional activities of nerve cells. Rab11 activity is controlled by GEFs (guanine exchange factors) and GAPs (GTPase activating proteins), which regulate its function through modulating GTP/GDP exchange and the intrinsic GTPase activity, respectively. Rab11 is involved in the transport of several growth factor molecules important for the development and repair of neurons. Overexpression of Rab11 has been shown to significantly enhance vesicle trafficking. On the other hand, a reduced expression of Rab11 was observed in several neurodegenerative diseases. Current evidence appears to support the notion that Rab11 and its cognate proteins may be potential targets for therapeutic intervention. In this review, we briefly discuss the function of Rab11 and its related interaction partners in intracellular pathways that may be involved in neurodegenerative processes.

Huntingtin coordinates dendritic spine morphology and function through cofilin-mediated control of the actin cytoskeleton

Compelling evidence indicates that in Huntington's disease (HD), mutation of huntingtin (HTT) alters several aspects of early brain development such as synaptogenesis. It is not clear to what extent the partial loss of wild-type HTT function contributes to these abnormalities. Here we investigate the function of HTT in the formation of spines. Although larger spines normally correlate with more synaptic activity, cell-autonomous depletion of HTT leads to enlarged spines but reduced excitatory synaptic function. We find that HTT is required for the proper turnover of endogenous actin and to recruit AMPA receptors at active synapses; loss of HTT leads to LIM kinase (LIMK) hyperactivation, which maintains cofilin in its inactive state. HTT therefore influences actin dynamics through the LIMK-cofilin pathway. Loss of HTT uncouples spine structure from synaptic function, which may contribute to the ultimate development of HD symptoms.

Therapeutic Targeting of Rab GTPases: Relevance for Alzheimer's Disease

Rab GTPases (Rabs) are small proteins that play crucial roles in vesicle transport and membrane trafficking. Owing to their widespread functions in several steps of vesicle trafficking, Rabs have been implicated in the pathogenesis of several disorders, including cancer, diabetes, and multiple neurodegenerative diseases. As treatments for neurodegenerative conditions are currently rather limited, the identification and validation of novel therapeutic targets, such as Rabs, is of great importance. This review summarises proof-of-concept studies, demonstrating that modulation of Rab GTPases in the context of Alzheimer's disease (AD) can ameliorate disease-related phenotypes, and provides an overview of the current state of the art for the pharmacological targeting of Rabs. Finally, we also discuss the barriers and challenges of therapeutically targeting these small proteins in humans, especially in the context of AD.

Huntingtin CAG expansion impairs germ layer patterning in synthetic human 2D gastruloids through polarity defects

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an expansion of the CAG repeats in the huntingtin gene (HTT). Although HD has been shown to have a developmental component, how early during human embryogenesis the HTT-CAG expansion can cause embryonic defects remains unknown. Here, we demonstrate a specific and highly reproducible CAG length-dependent phenotypic signature in a synthetic model for human gastrulation derived from human embryonic stem cells (hESCs). Specifically, we observed a reduction in the extension of the ectodermal compartment that is associated with enhanced activin signaling. Surprisingly, rather than a cell-autonomous effect, tracking the dynamics of TGFβ signaling demonstrated that HTT-CAG expansion perturbs the spatial restriction of activin response. This is due to defects in the apicobasal polarization in the context of the polarized epithelium of the 2D gastruloid, leading to ectopic subcellular localization of TGFβ receptors. This work refines the earliest developmental window for the prodromal phase of HD to the first 2 weeks of human development, as modeled by our 2D gastruloids.

Summary: 2D gastruloids of isogenic human embryonic stem cells modeling Huntington's Disease reveal that huntingtin CAG expansion perturbs the spatial restriction of the activin response in the context of the polarized epithelium.

The molecular basis of mammary gland development and epithelial differentiation

Our understanding of the molecular events underpinning the development of mammalian organ systems has been increasing rapidly in recent years. With the advent of new and improved next-generation sequencing methods, we are now able to dig deeper than ever before into the genomic and epigenomic events that play critical roles in determining the fates of stem and progenitor cells during the development of an embryo into an adult. In this review, we detail and discuss the genes and pathways that are involved in mammary gland development, from embryogenesis, through maturation into an adult gland, to the role of pregnancy signals in directing the terminal maturation of the mammary gland into a milk producing organ that can nurture the offspring. We also provide an overview of the latest research in the single-cell genomics of mammary gland development, which may help us to understand the lineage commitment of mammary stem cells (MaSCs) into luminal or basal epithelial cells that constitute the mammary gland. Finally, we summarize the use of 3D organoid cultures as a model system to study the molecular events during mammary gland development. Our increased investigation of the molecular requirements for normal mammary gland development will advance the discovery of targets to predict breast cancer risk and the development of new breast cancer therapies.

Striatal Projection Neurons Require Huntingtin for Synaptic Connectivity and Survival

Huntington's disease (HD) is caused by an autosomal dominant polyglutamine expansion mutation of Huntingtin (HTT). HD patients suffer from progressive motor, cognitive, and psychiatric impairments, along with significant degeneration of the striatal projection neurons (SPNs) of the striatum. HD is widely accepted to be caused by a toxic gain-of-function of mutant HTT. However, whether loss of HTT function, because of dominant-negative effects of the mutant protein, plays a role in HD and whether HTT is required for SPN health and function are not known. Here, we delete Htt from specific subpopulations of SPNs using the Cre-Lox system and find that SPNs require HTT for motor regulation, synaptic development, cell health, and survival during aging. Our results suggest that loss of HTT function in SPNs could play a critical role in HD pathogenesis.

Huntington's disease alters human neurodevelopment

Although Huntington's disease is a late-manifesting neurodegenerative disorder, both mouse studies and neuroimaging studies of presymptomatic mutation carriers suggest that Huntington's disease might affect neurodevelopment. To determine whether this is actually the case, we examined tissue from human fetuses (13 weeks gestation) that carried the Huntington's disease mutation. These tissues showed clear abnormalities in the developing cortex, including mislocalization of mutant huntingtin and junctional complex proteins, defects in neuroprogenitor cell polarity and differentiation, abnormal ciliogenesis, and changes in mitosis and cell cycle progression. We observed the same phenomena in Huntington's disease mouse embryos, where we linked these abnormalities to defects in interkinetic nuclear migration of progenitor cells. Huntington's disease thus has a neurodevelopmental component and is not solely a degenerative disease.

Rab11-mediated recycling endosome role in nervous system development and neurodegenerative diseases

STUDY

Membrane trafficking process is significant for the complex and precise regulatory of the nervous system. Rab11, as a small GTPase of the Rab superfamily, controls endocytic vesicular trafficking to the cell surface after sorting in recycling endosome (RE), coordinating with its receptors to maintain neurological function.

MATERIALS AND METHODS

This article reviewed the literature of Rab11 in nervous system.

RESULTS

Rab11-positive vesicles targeted transport growth-related molecules, such as integrins, protrudin, tropomyosin receptor kinase (Trk) A/B receptor and AMPA receptor (AMPAR) to membrane surface to promote the regeneration capacity of axon and dendrites and maintain synaptic plasticity. In addition, many studies have shown that the expression of Rab11 is decreased in multiple neurodegenerative diseases, which is highly correlated with the process of production, transport and clearance of disease-related pathological proteins. And overexpression or increased activity of Rab11 can greatly improve the defect of membrane trafficking and slow down the disease process.

CONCLUSION

With increasing research efforts on Rab11-dependent membrane trafficking mechanisms, a potential target for nerve regeneration and neurodegenerative diseases will be provided.

Analysis of the Circadian Regulation of Cancer Hallmarks by a Cross-Platform Study of Colorectal Cancer Time-Series Data Reveals an Association with Genes Involved in Huntington's Disease

Accumulating evidence points to a link between circadian clock dysfunction and the molecular events that drive tumorigenesis. Here, we investigated the connection between the circadian clock and the hallmarks of cancer in an in vitro model of colorectal cancer (CRC). We used a cross-platform data normalization method to concatenate and compare available microarray and RNA-sequencing time series data of CRC cell lines derived from the same patient at different disease stages. Our data analysis suggests differential regulation of molecular pathways between the CRC cells and identifies several of the circadian and likely clock-controlled genes (CCGs) as cancer hallmarks and circadian drug targets. Notably, we found links of the CCGs to Huntington’s disease (HD) in the metastasis-derived cells. We then investigated the impact of perturbations of our candidate genes in a cohort of 439 patients with colon adenocarcinoma retrieved from the Cancer Genome Atlas (TCGA). The analysis revealed a correlation of the differential expression levels of the candidate genes with the survival of patients. Thus, our study provides a bioinformatics workflow that allows for a comprehensive analysis of circadian properties at different stages of colorectal cancer, and identifies a new association between cancer and HD.

Cancer: From Wild-Type to Mutant Huntingtin

Huntingtin (HTT) is a scaffold protein mostly known because it gives rise to the severe and incurable inherited neurological disorder Huntington’s disease (HD) when mutated. The Huntingtin gene (HTT) carries a polymorphic trinucleotide expansion of CAGs in exon 1 that ranges from 9 to 35 in the non-HD affected population. However, if it exceeds 35 CAG repeats, the altered protein is referred to as mutant HTT and leads to the development of HD. Given the wide spectrum of severe symptoms developed by HD individuals, wild-type and mutant HTT have been mostly studied in the context of this disorder. However, HTT expression is ubiquitous and several peripheral symptoms in HD have been described, suggesting that HTT is of importance, not only in the central nervous system (CNS), but also in peripheral organs. Accordingly, HTT and mutant HTT may interfere with non-brain-related diseases. Correlative studies have highlighted a decreased cancer incidence in the HD population and both wild-type and mutant HTT have been implicated in tumor progression. In this review, we describe the current evidence linking wild-type and mutant HTT to cancer and discuss how CAG polymorphism, HTT function, and partners may influence carcinogenesis and metastatic progression.

Paxillin-dependent regulation of apical-basal polarity in mammary gland morphogenesis

Establishing apical-basal epithelial cell polarity is fundamental for mammary gland duct morphogenesis during mammalian development. While the focal adhesion adapter protein paxillin is a well-characterized regulator of mesenchymal cell adhesion signaling, F-actin cytoskeleton remodeling and single cell migration, its role in epithelial tissue organization and mammary gland morphogenesis in vivo has not been investigated. Here, using a newly developed paxillin conditional knockout mouse model with targeted ablation in the mammary epithelium, in combination with ex vivo three-dimensional organoid and acini cultures, we identify new roles for paxillin in the establishment of apical-basal epithelial cell polarity and lumen formation, as well as mammary gland duct diameter and branching. Paxillin is shown to be required for the integrity and apical positioning of the Golgi network, Par complex and the Rab11/MyoVb trafficking machinery. Paxillin depletion also resulted in reduced levels of apical acetylated microtubules, and rescue experiments with the HDAC6 inhibitor tubacin highlight the central role for paxillin-dependent regulation of HDAC6 activity and associated microtubule acetylation in controlling epithelial cell apical-basal polarity and tissue branching morphogenesis.

Morphology, Migration, and Transcriptome Analysis of Schwann Cell Culture on Butterfly Wings with Different Surface Architectures

It has been shown that material surface topography greatly affects cell attachment, growth, proliferation, and differentiation. However, the underlying molecular mechanisms for cell-material interactions are still not understood well. Here, two kinds of butterfly wings with different surface architectures were employed for addressing such an issue. Papilio ulysses telegonus (P.u.t.) butterfly wing surface is composed of micro/nanoconcaves, whereas Morpho menelaus (M.m.) butterfly wings are decorated with grooves. RSC96 cells grown on M.m. wings showed a regular sorting pattern along with the grooves. On the contrary, the cells seeded on P.u.t. wings exhibited random arrangement. Transcriptome sequencing and bioinformatics analysis revealed that huntingtin (Htt)-regulated lysosome activity is a potential key factor for determining cell growth behavior on M.m. butterfly wings. Gene silence further confirmed this notion. In vivo experiments showed that the silicone tubes fabricated with M.m. wings markedly facilitate rat sciatic nerve regeneration after injury. Lysosome activity and Htt expression were greatly increased in the M.m. wing-fabricated graft-bridged nerves. Collectively, our data provide a theoretical basis for employing butterfly wings to construct biomimetic nerve grafts and establish Htt lysosome as a crucial regulator for cell-material interactions.

Sorting nexin-21 is a scaffold for the endosomal recruitment of huntingtin

The endo-lysosomal network serves an essential role in determining the fate of endocytosed transmembrane proteins and their associated proteins and lipids. Sorting nexins (SNXs) play a central role in the functional organisation of this network. Comprising over 30 proteins in humans, SNXs are classified into sub-groups based on the presence of additional functional domains. Sorting nexin-20 (SNX20) and sorting nexin-21 (SNX21) comprise the SNX-PXB proteins. The presence of a predicted protein-protein interaction domain, termed the PX-associated B (PXB) domain, has led to the proposal that they function as endosome-associated scaffolds. Here, we used unbiased quantitative proteomics to define the SNX21 interactome. We reveal that the N-terminal extension of SNX21 interacts with huntingtin (Htt) whereas the PXB domain appears to associate with septins, a family of cytoskeletal- and membrane-associated proteins. In establishing that these interactions are sufficient for SNX21 to recruit Htt and septins on to an endosomal population, we reveal a scaffolding function for this sorting nexin. Our work paves the way for a more-detailed mechanistic analysis of the role(s) of the SNX-PXB proteins in endosomal biology.

Summary: A potential scaffolding function for SNX21 paves the way for a more-detailed mechanistic analysis of the role(s) of this protein in endosomal biology.

PAR3-PAR6-atypical PKC polarity complex proteins in neuronal polarization

Polarity is a fundamental feature of cells. Protein complexes, including the PAR3-PAR6-aPKC complex, have conserved roles in establishing polarity across a number of eukaryotic cell types. In neurons, polarity is evident as distinct axonal versus dendritic domains. The PAR3, PAR6, and aPKC proteins also play important roles in neuronal polarization. During this process, either aPKC kinase activity, the assembly of the PAR3-PAR6-aPKC complex or the localization of these proteins is regulated downstream of a number of signaling pathways. In turn, the PAR3, PAR6, and aPKC proteins control various effector molecules to establish neuronal polarity. Herein, we discuss the many signaling mechanisms and effector functions that have been linked to PAR3, PAR6, and aPKC during the establishment of neuronal polarity.

Principles and approaches for reproducible scoring of tissue stains in research

Evaluation of tissues is a common and important aspect of translational research studies. Labeling techniques such as immunohistochemistry can stain cells/tissues to enhance identification of specific cell types, cellular activation states, and protein expression. While qualitative evaluation of labeled tissues has merit, use of semiquantitative and quantitative scoring approaches can greatly enhance the rigor of the tissue data. Adhering to key principles for reproducible scoring can enhance the quality and reproducibility of the tissue data so as to maximize its biological relevance and scientific impact.

Insane in the apical membrane: Trafficking events mediating apicobasal epithelial polarity during tube morphogenesis

The creation of cellular tubes is one of the most vital developmental processes, resulting in the formation of most organ types. Cells have co-opted a number of different mechanisms for tube morphogenesis that vary among tissues and organisms; however, generation and maintenance of cell polarity is fundamental for successful lumenogenesis. Polarized membrane transport has emerged as a key driver not only for establishing individual epithelial cell polarity, but also for coordination of epithelial polarization during apical lumen formation and tissue morphogenesis. In recent years, much work has been dedicated to identifying membrane trafficking regulators required for lumenogenesis. In this review we will summarize the findings from the past couple of decades in defining the molecular machinery governing lumenogenesis both in 3D tissue culture models and during organ development in vivo.

Rab11a promotes proliferation and invasion through regulation of YAP in non-small cell lung cancer

Rab11a, an evolutionarily conserved Rab GTPases, plays important roles in intracellular transport and has been implicated in cancer progression. However, its role in human non-small cell lung cancer (NSCLC) has not been explored yet. In this study, we discovered that Rab11a protein was upregulated in 57/122 NSCLC tissues. Rab11a overexpression associated with advanced TNM stage, positive nodal status and poor patient prognosis. Rab11a overexpression promoted proliferation, colony formation, invasion and migration with upregulation of cyclin D1, cyclin E, and downregulation of p27 in NSCLC cell lines. Nude mice xenograft demonstrated that Rab11a promoted in vivo cancer growth. Importantly, we found that Rab11a induced YAP protein and inhibited Hippo signaling. Depletion of YAP abolished the effects of Rab11a on cell cycle proteins and cell proliferation. Furthermore, immunoprecipitation showed that Rab11a interacted with YAP in lung cancer cells. In conclusion, the present study suggestes that Rab11a serves as an important oncoprotein and a regulator of YAP in NSCLC.

Is Huntingtin Dispensable in the Adult Brain?

Huntingtin (HTT) is an essential protein during early embryogenesis and the development of the central nervous system (CNS). Conditional knock-out of mouse Huntingtin (Htt) expression in the CNS beginning during neural development, as well as reducing Htt expression only during embryonic and early postnatal stages, results in neurodegeneration in the adult brain. These findings suggest that HTT is important for the development and/or maintenance of the CNS, but they do not address the question of whether HTT is required specifically in the adult CNS for its normal functions and/or homeostasis. Recently, it was reported that although removing Htt expression in young adult mice causes lethality due to acute pancreatitis, loss of Htt expression in the adult brain is well tolerated and does not result in either motor deficits or neurodegeneration for up to 7 months after Htt inactivation. However, recent studies have also demonstrated that HTT participates in several cellular functions that are important for neuronal homeostasis and survival including sensing reactive oxygen species (ROS), DNA damage repair, and stress responses, in addition to its role in selective macroautophagy. In this review, HTT's functions in development and in the adult CNS will be discussed in the context of these recent discoveries, together with a discussion of their potential impact on the design of therapeutic strategies for Huntington's disease (HD) aimed at lowering total HTT expression.

A comprehensive in silico analysis of huntingtin and its interactome

A polyglutamine expansion of the N-terminal region of huntingtin (Htt) causes Huntington's disease, a severe neurodegenerative disorder. Htt huge multidomain structure, the presence of disordered regions, and the lack of sequence homologs of known structure, so far prevented structural studies of Htt, making the study of its structure-function relationships very difficult. In this work, the presence and location of five Htt ordered domains (named from Hunt1 to Hunt5) has been detected and the structure of these domains has been predicted for the first time using a combined threading/ab initio modeling approach. This work has led to the identification of a previously undetected HEAT repeats region in the Hunt3 domain. Furthermore, a putative function has been assigned to four out of the five domains. Hunt1 and Hunt5, displaying structural similarity with the regulatory subunit A of protein phosphatase 2A, are predicted to play a role in regulating the phosphorylation status of cellular proteins. Hunt2 and Hunt3 are predicted to be homologs of two yeast importins and to mediate vescicles transport and protein trafficking. Finally, a comprehensive analysis of the Htt interactome has been carried out and is discussed to provide a global picture of the Htt's structure-function relationships.

The transcriptional repressor Blimp1 is expressed in rare luminal progenitors and is essential for mammary gland development

Mammary gland morphogenesis depends on a tight balance between cell proliferation, differentiation and apoptosis, to create a defined functional hierarchy within the epithelia. The limited availability of stem cell/progenitor markers has made it challenging to decipher lineage relationships. Here, we identify a rare subset of luminal progenitors that express the zinc finger transcriptional repressor Blimp1, and demonstrate that this subset of highly clonogenic luminal progenitors is required for mammary gland development. Conditional inactivation experiments using K14-Cre and WAPi-Cre deleter strains revealed essential functions at multiple developmental stages. Thus, Blimp1 regulates proliferation, apoptosis and alveolar cell maturation during puberty and pregnancy. Loss of Blimp1 disrupts epithelial architecture and lumen formation both in vivo and in three-dimensional (3D) primary cell cultures. Collectively, these results demonstrate that Blimp1 is required to maintain a highly proliferative luminal subset necessary for mammary gland development and homeostasis.

Highlighted article: In the mouse mammary gland, Blimp1 marks a rare progenitor population, and is required for cell proliferation and polarity as well as efficient milk production.

Preclinical Evaluation of a Lentiviral Vector for Huntingtin Silencing

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder resulting from a polyglutamine expansion in the huntingtin (HTT) protein. There is currently no cure for this disease, but recent studies suggest that RNAi to downregulate the expression of both normal and mutant HTT is a promising therapeutic approach. We previously developed a small hairpin RNA (shRNA), vectorized in an HIV-1-derived lentiviral vector (LV), that reduced pathology in an HD rodent model. Here, we modified this vector for preclinical development by using a tat-independent third-generation LV (pCCL) backbone and removing the original reporter genes. We demonstrate that this novel vector efficiently downregulated HTT expression in vitro in striatal neurons derived from induced pluripotent stem cells (iPSCs) of HD patients. It reduced two major pathological HD hallmarks while triggering a minimal inflammatory response, up to 6 weeks after injection, when administered by stereotaxic surgery in the striatum of an in vivo rodent HD model. Further assessment of this shRNA vector in vitro showed proper processing by the endogenous silencing machinery, and we analyzed gene expression changes to identify potential off-targets. These preclinical data suggest that this new shRNA vector fulfills primary biosafety and efficiency requirements for further development in the clinic as a cure for HD.

Molecular mechanisms of asymmetric divisions in mammary stem cells

Stem cells have the remarkable ability to undergo proliferative symmetric divisions and self-renewing asymmetric divisions. Balancing of the two modes of division sustains tissue morphogenesis and homeostasis. Asymmetric divisions of Drosophila neuroblasts (NBs) and sensory organ precursor (SOP) cells served as prototypes to learn what we consider now principles of asymmetric mitoses. They also provide initial evidence supporting the notion that aberrant symmetric divisions of stem cells could correlate with malignancy. However, transferring the molecular knowledge of circuits underlying asymmetry from flies to mammals has proven more challenging than expected. Several experimental approaches have been used to define asymmetry in mammalian systems, based on daughter cell fate, unequal partitioning of determinants and niche contacts, or proliferative potential. In this review, we aim to provide a critical evaluation of the assays used to establish the stem cell mode of division, with a particular focus on the mammary gland system. In this context, we will discuss the genetic alterations that impinge on the modality of stem cell division and their role in breast cancer development.

VE-cadherin interacts with cell polarity protein Pals1 to regulate vascular lumen formation

Blood vessel tubulogenesis requires the establishment of apicobasal polarity of endothelial cells. A novel interaction is described of the cell adhesion molecule VE-cadherin with the cell polarity protein Pals1. The activity of VE-cadherin in regulation of endothelial lumen formation depends on its interaction with both Pals1 and Par3.

Blood vessel tubulogenesis requires the formation of stable cell-to-cell contacts and the establishment of apicobasal polarity of vascular endothelial cells. Cell polarity is regulated by highly conserved cell polarity protein complexes such as the Par3-aPKC-Par6 complex and the CRB3-Pals1-PATJ complex, which are expressed by many different cell types and regulate various aspects of cell polarity. Here we describe a functional interaction of VE-cadherin with the cell polarity protein Pals1. Pals1 directly interacts with VE-cadherin through a membrane-proximal motif in the cytoplasmic domain of VE-cadherin. VE-cadherin clusters Pals1 at cell–cell junctions. Mutating the Pals1-binding motif in VE-cadherin abrogates the ability of VE-cadherin to regulate apicobasal polarity and vascular lumen formation. In a similar way, deletion of the Par3-binding motif at the C-terminus of VE-cadherin impairs apicobasal polarity and vascular lumen formation. Our findings indicate that the biological activity of VE-cadherin in regulating endothelial polarity and vascular lumen formation is mediated through its interaction with the two cell polarity proteins Pals1 and Par3.

Complex Polarity: Building Multicellular Tissues Through Apical Membrane Traffic

The formation of distinct subdomains of the cell surface is crucial for multicellular organism development. The most striking example of this is apical-basal polarization. What is much less appreciated is that underpinning an asymmetric cell surface is an equally dramatic intracellular endosome rearrangement. Here, we review the interplay between classical cell polarity proteins and membrane trafficking pathways, and discuss how this marriage gives rise to cell polarization. We focus on those mechanisms that regulate apical polarization, as this is providing a number of insights into how membrane traffic and polarity are regulated at the tissue level.

Nuclear fallout provides a new link between aPKC and polarized cell trafficking

Background

Cell polarity, essential for cell physiology and tissue coherence, emerges as a consequence of asymmetric localization of protein complexes and directional trafficking of cellular components. Although molecules required in both processes are well known their relationship is still poorly understood.

Results

Here we show a molecular link between Nuclear Fallout (Nuf), an adaptor of Rab11-GTPase to the microtubule motor proteins during Recycling Endosome (RE) trafficking, and aPKC, a pivotal kinase in the regulation of cell polarity. We demonstrate that aPKC phosphorylates Nuf modifying its subcellular distribution. Accordingly, in aPKC mutants Nuf and Rab11 accumulate apically indicating altered RE delivery. We show that aPKC localization in the apico-lateral cortex is dynamic. When we block exocytosis, by means of exocyst-sec mutants, aPKC accumulates inside the cells. Moreover, apical aPKC concentration is reduced in nuf mutants, suggesting aPKC levels are maintained by recycling.

Conclusions

We demonstrate that active aPKC interacts with Nuf, phosphorylating it and, as a result, modifying its subcellular distribution. We propose a regulatory loop by which Nuf promotes aPKC apical recycling until sufficient levels of active aPKC are reached. Thus, we provide a novel link between cell polarity regulation and traffic control in epithelia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0253-6) contains supplementary material, which is available to authorized users.

Rab35 GTPase couples cell division with initiation of epithelial apico-basal polarity and lumen opening

Establishment and maintenance of apico-basal polarity in epithelial organs must be tightly coupled with cell division, but the underlying molecular mechanisms are largely unknown. Using 3D cultures of renal MDCK cells (cysts), we found that the Rab35 GTPase plays a crucial role in polarity initiation and apical lumen positioning during the first cell division of cyst development. At the molecular level, Rab35 physically couples cytokinesis with the initiation of apico-basal polarity by tethering intracellular vesicles containing key apical determinants at the cleavage site. These vesicles transport aPKC, Cdc42, Crumbs3 and the lumen-promoting factor Podocalyxin, and are tethered through a direct interaction between Rab35 and the cytoplasmic tail of Podocalyxin. Consequently, Rab35 inactivation leads to complete inversion of apico-basal polarity in 3D cysts. This novel and unconventional mode of Rab-dependent vesicle targeting provides a simple mechanism for triggering both initiation of apico-basal polarity and lumen opening at the centre of cysts.

Establishment and maintenance of apico-basal polarity in epithelial organs needs to be tightly coupled with cell division. Here the authors show that the Rab35 GTPase tethers intracellular vesicles containing key apical determinants at the cleavage site, connecting cytokinesis to apico-basal polarity.

Dominant-Negative Effects of Adult-Onset Huntingtin Mutations Alter the Division of Human Embryonic Stem Cells-Derived Neural Cells

Mutations of the huntingtin protein (HTT) gene underlie both adult-onset and juvenile forms of Huntington’s disease (HD). HTT modulates mitotic spindle orientation and cell fate in mouse cortical progenitors from the ventricular zone. Using human embryonic stem cells (hESC) characterized as carrying mutations associated with adult-onset disease during pre-implantation genetic diagnosis, we investigated the influence of human HTT and of an adult-onset HD mutation on mitotic spindle orientation in human neural stem cells (NSCs) derived from hESCs. The RNAi-mediated silencing of both HTT alleles in neural stem cells derived from hESCs disrupted spindle orientation and led to the mislocalization of dynein, the p150^Glued subunit of dynactin and the large nuclear mitotic apparatus (NuMA) protein. We also investigated the effect of the adult-onset HD mutation on the role of HTT during spindle orientation in NSCs derived from HD-hESCs. By combining SNP-targeting allele-specific silencing and gain-of-function approaches, we showed that a 46-glutamine expansion in human HTT was sufficient for a dominant-negative effect on spindle orientation and changes in the distribution within the spindle pole and the cell cortex of dynein, p150^Glued and NuMA in neural cells. Thus, neural derivatives of disease-specific human pluripotent stem cells constitute a relevant biological resource for exploring the impact of adult-onset HD mutations of the HTT gene on the division of neural progenitors, with potential applications in HD drug discovery targeting HTT-dynein-p150^Glued complex interactions.