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.

Polarity and differential inheritance--universal attributes of life?

When and why did cell polarization arise? Recent work in bacteria and yeast suggests that polarization may have evolved to restrict senescence to one daughter during division by enabling the differential segregation of damaged material. In more complex organisms, polarity functions have diversified to permit the differential inheritance of centrosomes, RNAs, proteins, and membranes, which is essential for the generation of diverse cell types from stem cells and for morphogenesis.

Sustained activation of the HER1-ERK1/2-RSK signaling pathway controls myoepithelial cell fate in human mammary tissue.

Human mammary glands arise from multipotent progenitor cells, which likely respond both to cell-autonomous and to extrinsic cues. However, the identity of these cues and how they might act remain unclear. We analyzed HER1 ligand effects on mammary morphogenesis using a three-dimensional organoid model generated from human breast tissue that recapitulates both qualitatively and quantitatively the normal ductal network in situ. Strikingly, different HER1 ligands generate distinct patterns of cell fate. Epidermal growth factor (EGF) causes a massive expansion of the myoepithelial lineage. Amphiregulin, in contrast, enables normal ductal development. These differences cannot be ascribed to preferential apoptosis or proliferation of differentiated cell populations, but are dependent on HER1 signal intensity. Inhibition of the extracellular signal-regulated kinase 1/2 (ERK1/2) effector RSK prevents the EGF-induced myoepithelial expansion. Notably, mouse mammary organoids are much less responsive to HER1 ligands. Little is known about the myoepithelial lineage or about growth factor effects on mammary progenitor differentiation, and our studies provide an important window into human mammary development that reveals unexpected differences from the mouse model.

The Par3/aPKC interaction is essential for end bud remodeling and progenitor differentiation during mammary gland morphogenesis.

Mammalian polarity proteins have been studied predominantly in cell culture systems, and little is known about their functions in vivo. To address this issue, we used a shRNA lentiviral system to manipulate gene expression in mouse mammary stem/progenitor cells. Transplantation of Par3-depleted stem/progenitor cells into the mammary fat pad severely disrupted mammary development, and glands were characterized by ductal hyperplasia, luminal filling, and highly disorganized end bud structures that were unable to remodel into normal ductal structures. Unexpectedly, Par3-depleted mammary glands also had an expanded progenitor population. We identified a novel function for the atypical protein kinase C (aPKC)-binding domain of Par3 in restricting Par3 and aPKC to the apical region in mammary epithelia in vivo, and found that mammary morphogenesis is dependent on the ability of Par3 to directly bind aPKC. These results reveal a new function for Par3 in the regulation of progenitor differentiation and epithelial morphogenesis in vivo and demonstrate for the first time an essential requirement for the Par3-aPKC interaction.

Loss of the polarity protein PAR3 activates STAT3 signaling via an atypical protein kinase C (aPKC)/NF-κB/interleukin-6 (IL-6) axis in mouse mammary cells.

PAR3 suppresses tumor growth and metastasis in vivo and cell invasion through matrix in vitro. We propose that PAR3 organizes and limits multiple signaling pathways and that inappropriate activation of these pathways occurs without PAR3. Silencing Pard3 in conjunction with oncogenic activation promotes invasion and metastasis via constitutive STAT3 activity in mouse models, but the mechanism for this is unknown. We now show that loss of PAR3 triggers increased production of interleukin-6, which induces STAT3 signaling in an autocrine manner. Activation of atypical protein kinase C ι/λ (aPKCι/λ) mediates this effect by stimulating NF-κB signaling and IL-6 expression. Our results suggest that PAR3 restrains aPKCι/λ activity and thus prevents aPKCι/λ from activating an oncogenic signaling network.

NRMT is an alpha-N-methyltransferase that methylates RCC1 and retinoblastoma protein.

The post-translational methylation of alpha-amino groups was first discovered over 30 years ago on the bacterial ribosomal proteins L16 and L33 (refs 1, 2), but almost nothing is known about the function or enzymology of this modification. Several other bacterial and eukaryotic proteins have since been shown to be alpha-N-methylated. However, the Ran guanine nucleotide-exchange factor, RCC1, is the only protein for which any biological function of alpha-N-methylation has been identified. Methylation-defective mutants of RCC1 have reduced affinity for DNA and cause mitotic defects, but further characterization of this modification has been hindered by ignorance of the responsible methyltransferase. All fungal and animal N-terminally methylated proteins contain a unique N-terminal motif, Met-(Ala/Pro/Ser)-Pro-Lys, indicating that they may be targets of the same, unknown enzyme. The initiating Met is cleaved, and the exposed alpha-amino group is mono-, di- or trimethylated. Here we report the discovery of the first alpha-N-methyltransferase, which we named N-terminal RCC1 methyltransferase (NRMT). Substrate docking and mutational analysis of RCC1 defined the NRMT recognition sequence and enabled the identification of numerous new methylation targets, including SET (also known as TAF-I or PHAPII) and the retinoblastoma protein, RB. Knockdown of NRMT recapitulates the multi-spindle phenotype seen with methylation-defective RCC1 mutants, demonstrating the importance of alpha-N-methylation for normal bipolar spindle formation and chromosome segregation.

Codependent functions of RSK2 and the apoptosis-promoting factor TIA-1 in stress granule assembly and cell survival.

Stress granules aid cell survival in response to environmental stressors by acting as sites of translational repression. We report an unanticipated link between stress granules and the serine/threonine kinase RSK2. In stressed breast cells, endogenous RSK2 colocalizes in granules with TIA-1 and poly(A)-binding protein 1, and the sequestration of RSK2 and TIA-1 exhibits codependency. The RSK2 N-terminal kinase domain controls the direct interaction with the prion-related domain of TIA-1. Silencing RSK2 decreases cell survival in response to stress. Mitogen releases RSK2 from the stress granules and permits its nuclear import via a nucleocytoplasmic shuttling sequence in the C-terminal domain. Nuclear accumulation is dependent on TIA-1. Surprisingly, nuclear localization of RSK2 is sufficient to enhance proliferation through induction of cyclin D1, in the absence of other active signaling pathways. Hence, RSK2 is a pivotal factor linking the stress response to survival and proliferation.

Posttranslational modification of CENP-A influences the conformation of centromeric chromatin.

Centromeres are chromosomal loci required for accurate segregation of sister chromatids during mitosis. The location of the centromere on the chromosome is not dependent on DNA sequence, but rather it is epigenetically specified by the histone H3 variant centromere protein A (CENP-A). The N-terminal tail of CENP-A is highly divergent from other H3 variants. Canonical histone N termini are hotspots of conserved posttranslational modification; however, no broadly conserved modifications of the vertebrate CENP-A tail have been previously observed. Here, we report three posttranslational modifications on human CENP-A N termini using high-resolution MS: trimethylation of Gly1 and phosphorylation of Ser16 and Ser18. Our results demonstrate that CENP-A is subjected to constitutive initiating methionine removal, similar to other H3 variants. The nascent N-terminal residue Gly1 becomes trimethylated on the α-amino group. We demonstrate that the N-terminal RCC1 methyltransferase is capable of modifying the CENP-A N terminus. Methylation occurs in the prenucleosomal form and marks the majority of CENP-A nucleosomes. Serine 16 and 18 become phosphorylated in prenucleosomal CENP-A and are phosphorylated on asynchronous and mitotic nucleosomal CENP-A and are important for chromosome segregation during mitosis. The double phosphorylation motif forms a salt-bridged secondary structure and causes CENP-A N-terminal tails to form intramolecular associations. Analytical ultracentrifugation of phospho-mimetic CENP-A nucleosome arrays demonstrates that phosphorylation results in greater intranucleosome associations and counteracts the hyperoligomerized state exhibited by unmodified CENP-A nucleosome arrays. Our studies have revealed that the major modifications on the N-terminal tail of CENP-A alter the physical properties of the chromatin fiber at the centromere.

Does FACS perturb gene expression?

Fluorescence activated cell sorting is the technique most commonly used to separate primary mammary epithelial sub-populations. Many studies incorporate this technique before analyzing gene expression within specific cellular lineages. However, to our knowledge, no one has examined the effects of fluorescence activated cell sorting (FACS) separation on short-term transcriptional profiles. In this study, we isolated a heterogeneous mixture of cells from the mouse mammary gland. To determine the effects of the isolation and separation process on gene expression, we harvested RNA from the cells before enzymatic digestion, following enzymatic digestion, and following a mock FACS sort where the entire cohort of cells was retained. A strict protocol was followed to minimize disruption to the cells, and to ensure that no subpopulations were enriched or lost. Microarray analysis demonstrated that FACS causes minimal disruptions to gene expression patterns, but prior steps in the mammary cell isolation process are followed by upregulation of 18 miRNA's and rapid decreases in their predicted target transcripts. © 2015 International Society for Advancement of Cytometry.

N-terminal alpha-methylation of RCC1 is necessary for stable chromatin association and normal mitosis.

Regulator of chromatin condensation 1 (RCC1) is the only known guanine nucleotide-exchange factor for the Ran GTPase and has pivotal roles in nucleo-cytoplasmic transport, mitosis, and nuclear-envelope assembly. RCC1 associates dynamically with chromatin through binding to histones H2A and/or H2B in a Ran-regulated manner. Here, we report that, unexpectedly, the amino-terminal serine or proline residue of RCC1 is uniquely methylated on its alpha-amino group. Methylation requires removal of the initiating methionine, and the presence of proline and lysine at positions 3 and 4, respectively. Methylation-defective mutants of RCC1 bind less effectively than wild-type protein to chromatin during mitosis, which causes spindle-pole defects. We propose a bimodal attachment mechanism for RCC1 in which the tail promotes stable RCC1 association with chromatin through DNA binding in an alpha-N-methylation-dependent manner. These data provide the first known function for N-terminal protein methylation.

Genome-wide screen reveals APC-associated RNAs enriched in cell protrusions.

RNA localization is important for the establishment and maintenance of polarity in multiple cell types. Localized RNAs are usually transported along microtubules or actin filaments and become anchored at their destination to some underlying subcellular structure. Retention commonly involves actin or actin-associated proteins, although cytokeratin filaments and dynein anchor certain RNAs. RNA localization is important for diverse processes ranging from cell fate determination to synaptic plasticity; however, so far there have been few comprehensive studies of localized RNAs in mammalian cells. Here we have addressed this issue, focusing on migrating fibroblasts that polarize to form a leading edge and a tail in a process that involves asymmetric distribution of RNAs. We used a fractionation scheme combined with microarrays to identify, on a genome-wide scale, RNAs that localize in protruding pseudopodia of mouse fibroblasts in response to migratory stimuli. We find that a diverse group of RNAs accumulates in such pseudopodial protrusions. Through their 3' untranslated regions these transcripts are anchored in granules concentrated at the plus ends of detyrosinated microtubules. RNAs in the granules associate with the adenomatous polyposis coli (APC) tumour suppressor and the fragile X mental retardation protein (FMRP). APC is required for the accumulation of transcripts in protrusions. Our results suggest a new type of RNA anchoring mechanism as well as a new, unanticipated function for APC in localizing RNAs.

NRMT2 is an N-terminal monomethylase that primes for its homologue NRMT1.

NRMT (N-terminal regulator of chromatin condensation 1 methyltransferase) was the first eukaryotic methyltransferase identified to specifically methylate the free α-amino group of proteins. Since the discovery of this N-terminal methyltransferase, many new substrates have been identified and the modification itself has been shown to regulate DNA-protein interactions. Sequence analysis predicts one close human homologue of NRMT, METTL11B (methyltransferase-like protein 11B, now renamed NRMT2). We show in the present paper for the first time that NRMT2 also has N-terminal methylation activity and recognizes the same N-terminal consensus sequences as NRMT (now NRMT1). Both enzymes have similar tissue expression and cellular localization patterns. However, enzyme assays and MS experiments indicate that they differ in their specific catalytic functions. Although NRMT1 is a distributive methyltransferase that can mono-, di- and tri-methylate its substrates, NRMT2 is primarily a monomethylase. Concurrent expression of NRMT1 and NRMT2 accelerates the production of trimethylation, and we propose that NRMT2 activates NRMT1 by priming its substrates for trimethylation.

A size filter at the Golgi regulates apical membrane protein sorting.

Despite decades of research, apical sorting of epithelial membrane proteins remains incompletely understood. We noted that apical cytoplasmic domains are smaller than those of basolateral proteins; however, the reason for this discrepancy is unknown. Here we used a synthetic biology approach to investigate whether a size barrier at the Golgi apparatus might hinder apical sorting of proteins with large cytoplasmic tails. We focused on Crb3, Ace2 and Muc1 as representative apical proteins with short cytoplasmic tails. By incorporating a streptavidin-binding peptide, these proteins can be trapped in the endoplasmic reticulum until addition of biotin, which triggers synchronous release to the Golgi and subsequent transport to the apical cortex. Strikingly, increasing the size of their cytoplasmic domains caused partial mislocalization to the basolateral cortex and significantly delayed Golgi departure. Moreover, N-glycosylation of 'large' Crb3 was delayed, and 'small' Crb3 segregated into spatially distinct Golgi regions. Biologically, Crb3 forms a complex through its cytoplasmic tail with the Pals1 protein, which could also delay departure, but although associated at the endoplasmic reticulum and Golgi, Pals1 disassociated before Crb3 departure. Notably, a non-dissociable mutant Pals1 hampered the exit of Crb3. We conclude that, unexpectedly, a size filter at the Golgi facilitates apical sorting of proteins with small cytoplasmic domains and that timely release of Pals1, to reduce cytoplasmic domain size, is essential for normal Crb3 sorting.

The PAR-6 polarity protein regulates dendritic spine morphogenesis through p190 RhoGAP and the Rho GTPase.

The majority of excitatory synaptic transmission in the brain occurs at dendritic spines, which are actin-rich protrusions on the dendrites. The asymmetric nature of these structures suggests that proteins regulating cell polarity might be involved in their formation. Indeed, the polarity protein PAR-3 is required for normal spine morphogenesis. However, this function is independent of association with atypical protein kinase C (aPKC) and PAR-6. Here we show that PAR-6 together with aPKC plays a distinct but essential role in spine morphogenesis. Knockdown of PAR-6 inhibits spine morphogenesis, whereas overexpression of PAR-6 increases spine density, and these effects are mediated by aPKC. Using a FRET biosensor, we further show that p190 RhoGAP and RhoA act downstream of the PAR-6/aPKC complex. These results define a role for PAR-6 and aPKC in dendritic spine biogenesis and maintenance, and reveal an unexpected link between the PAR-6/aPKC complex and RhoA activity.

The polarity protein PAR-3 and TIAM1 cooperate in dendritic spine morphogenesis.

PAR-3 (partitioning-defective gene 3) is essential for cell polarization in many contexts, including axon specification. However, polarity proteins have not been implicated in later steps of neuronal differentiation, such as dendritic spine morphogenesis. Here, we show that PAR-3 is necessary for normal spine development in primary hippocampal neurons. Depletion of PAR-3 causes the formation of multiple filopodia- and lamellipodia-like dendritic protrusions - a phenotype similar to neurons expressing activated Rac. PAR-3 regulates spine formation by binding the Rac guanine nucleotide-exchange factor (GEF) TIAM1, and spatially restricting it to dendritic spines. Thus, a balance of PAR-3 and TIAM1 is essential to modulate Rac-GTP levels and to allow spine morphogenesis.

Structural insight into filament formation by mammalian septins.

Septins are GTP-binding proteins that assemble into homo- and hetero-oligomers and filaments. Although they have key roles in various cellular processes, little is known concerning the structure of septin subunits or the organization and polarity of septin complexes. Here we present the structures of the human SEPT2 G domain and the heterotrimeric human SEPT2-SEPT6-SEPT7 complex. The structures reveal a universal bipolar polymer building block, composed of an extended G domain, which forms oligomers and filaments by conserved interactions between adjacent nucleotide-binding sites and/or the amino- and carboxy-terminal extensions. Unexpectedly, X-ray crystallography and electron microscopy showed that the predicted coiled coils are not involved in or required for complex and/or filament formation. The asymmetrical heterotrimers associate head-to-head to form a hexameric unit that is nonpolarized along the filament axis but is rotationally asymmetrical. The architecture of septin filaments differs fundamentally from that of other cytoskeletal structures.

Efficient detection of RNA-protein interactions using tethered RNAs.

The diverse localization of transcripts in cells suggests that there are many specific RNA-protein interactions that have yet to be identified. Progress has been limited, however, by the lack of a robust method to detect and isolate the RNA-binding proteins. Here we describe the use of an RNA aptamer, scaffolded to a tRNA, to create an affinity matrix that efficiently pulls down transcript-specific RNA-binding proteins from cell lysates. The addition of the tRNA scaffold to a Streptavidin aptamer (tRSA) increased binding efficiency by ∼ 10-fold. The tRSA system with an attached G-quartet sequence also could efficiently and specifically capture endogenous Fragile X Mental Retardation Protein (FMRP), which recognizes this RNA sequence. An alternative method, using biotinylated RNA, captured FMRP less efficiently than did our tRSA method. Finally we demonstrate the identification of novel RNA-binding proteins that interact with intron2 or 3'-UTR of the polarity protein Crumbs3 transcript. Proteins captured by these RNA sequences attached to the tRNA scaffold were identified by mass spectrometry. GFP-tagged versions of these proteins also showed specific interaction with either the Crb3 intron2 or 3'-UTR. Our tRSA technique should find wide application in mapping the RNA-protein interactome.

A junction-dependent mechanism drives murine mammary cell intercalation for ductal elongation.

The luminal epithelium of the mammary gland is organized into monolayers; however, it originates from multilayered terminal end buds (TEBs) during development. Although apoptosis provides a plausible mechanism for cavitation of the ductal lumen, it doesn't account for ductal elongation behind TEBs. Spatial calculations in mice suggest that most TEB cells integrate into the outermost luminal layer to generate elongation. We developed a quantitative cell culture assay that models intercalation into epithelial monolayers. We found that tight junction proteins play a key role in this process. ZO-1 puncta form at the new cellular interface and resolve into a new boundary as intercalation proceeds. Deleting ZO-1 suppresses intercalation both in culture and in cells transplanted into mammary glands via intraductal injection. Cytoskeletal rearrangements at the interface are critical for intercalation. These data identify luminal cell rearrangements necessary for mammary development and suggest a mechanism for integration of cells into an existing monolayer.

A Size Filter Regulates Apical Protein Sorting.

Despite decades of research, apical sorting of epithelial membrane proteins remains incompletely understood. We noted that apical cytoplasmic domains are smaller than those of basolateral proteins; however, the reason for this discrepancy is unknown. We investigated whether a size barrier at the trans-Golgi network (TGN) might hinder apical sorting of proteins with large cytoplasmic tails. We focused on Crb3 and Ace2 as representative apical proteins with short cytoplasmic tails. By incorporating a streptavidin-binding peptide, these proteins can be trapped in the endoplasmic reticulum (ER) until addition of biotin, which triggers synchronous release to the Golgi and subsequent transport to the apical cortex. Strikingly, departure from the Golgi could be significantly delayed simply by increasing cytoplasmic bulk. Moreover, large and small Crb3 segregated into spatially distinct Golgi regions as detected by super resolution imaging. Biologically, Crb3 forms a complex through its cytoplasmic tail with the Pals1 protein, which could also delay departure, but although associated at the ER and Golgi, we found that Pals1 disassociates prior to Crb3 departure. Notably, a non-dissociable mutant Pals1 hampers the exit of Crb3. We conclude that an unexpected mechanism involving a size filter at the TGN facilitates apical sorting of proteins with small cytoplasmic domains and that timely release of Pals1, to reduce cytoplasmic domain size, is essential for the normal kinetics of Crb3 sorting.