E-cadherin polarity is determined by a multifunction motif mediating lateral membrane retention through ankyrin-G and apical-lateral transcytosis through clathrin

Live imaging of excitable axonal microdomains in ankyrin-G-GFP mice

The axon initial segment (AIS) constitutes not only the site of action potential initiation, but also a hub for activity-dependent modulation of output generation. Recent studies shedding light on AIS function used predominantly post-hoc approaches since no robust murine in vivo live reporters exist. Here, we introduce a reporter line in which the AIS is intrinsically labeled by an ankyrin-G-GFP fusion protein activated by Cre recombinase, tagging the native Ank3 gene. Using confocal, superresolution, and two-photon microscopy as well as whole-cell patch-clamp recordings in vitro, ex vivo, and in vivo, we confirm that the subcellular scaffold of the AIS and electrophysiological parameters of labeled cells remain unchanged. We further uncover rapid AIS remodeling following increased network activity in this model system, as well as highly reproducible in vivo labeling of AIS over weeks. This novel reporter line allows longitudinal studies of AIS modulation and plasticity in vivo in real-time and thus provides a unique approach to study subcellular plasticity in a broad range of applications.

TRIM46 is not required for axon specification or axon initial segment formation in vivo

Vertebrate nervous systems use the axon initial segment (AIS) to initiate action potentials and maintain neuronal polarity. The microtubule-associated protein tripartite motif containing 46 (TRIM46) was reported to regulate axon specification, AIS assembly, and neuronal polarity through the bundling of microtubules in the proximal axon. However, these claims are based on TRIM46 knockdown in cultured neurons. To investigate TRIM46 function in vivo , we examined TRIM46 knockout mice. Contrary to previous reports, we find that TRIM46 is dispensable for AIS formation and maintenance, and axon specification. TRIM46 knockout mice are viable, have normal behavior, and have normal brain structure. Thus, TRIM46 is not required for AIS formation, axon specification, or nervous system function. We also show TRIM46 enrichment in the first ∼100 μm of axon occurs independently of ankyrinG (AnkG), although AnkG is required to restrict TRIM46 only to the AIS. Our results suggest an unidentified protein may compensate for loss of TRIM46 in vivo and highlight the need for further investigation of the mechanisms by which the AIS and microtubules interact to shape neuronal structure and function.

SIGNIFICANCE STATEMENT

A healthy nervous system requires the polarization of neurons into structurally and functionally distinct compartments, which depends on both the axon initial segment (AIS) and the microtubule cytoskeleton. In contrast to previous reports, we show that the microtubule-associated protein TRIM46 is not required for axon specification or AIS formation in mice. Our results emphasize the need for further investigation of the mechanisms by which the AIS and microtubules interact to shape neuronal structure and function.

Inhibitory synaptic transmission is impaired in the Kölliker-Fuse of male, but not female, Rett syndrome mice

Rett syndrome (RTT) is a severe neurodevelopmental disorder that mainly affects females due to silencing mutations in the X-linked MECP2 gene. One of the most troubling symptoms of RTT is breathing irregularity, including apneas, breath-holds, and hyperventilation. Mice with silencing mutations in Mecp2 exhibit breathing abnormalities similar to human patients and serve as useful models for studying mechanisms underlying breathing problems in RTT. Previous work implicated the pontine, respiratory-controlling Kölliker-Fuse (KF) in the breathing problems in RTT. The goal of this study was to test the hypothesis that inhibitory synaptic transmission is deficient in KF neurons from symptomatic male and female RTT mice. We performed whole cell voltage-clamp recordings from KF neurons in acute brain slices to examine spontaneous and electrically evoked inhibitory post-synaptic currents (IPSCs) in RTT mice and age- and sex-matched wild-type mice. The frequency of spontaneous IPSCs was reduced in KF neurons from male RTT mice but surprisingly not in female RTT mice. In addition, electrically evoked IPSCs were less reliable in KF neurons from male, but not female, RTT mice, which was positively correlated with paired-pulse facilitation, indicating decreased probability of release. KF neurons from male RTT mice were also more excitable and exhibited shorter-duration action potentials. Increased excitability of KF neurons from male mice was not explained by changes in axon initial segment length. These findings indicate impaired inhibitory neurotransmission and increased excitability of KF neurons in male but not female RTT mice and suggest that sex-dependent mechanisms contribute to breathing problems in RTT.NEW & NOTEWORTHY Kölliker-Fuse (KF) neurons in acute brain slices from male Rett syndrome (RTT) mice receive reduced inhibitory synaptic inputs compared with wild-type littermates. In female RTT mice, inhibitory transmission was not different in KF neurons compared with controls. The results from this study show that sex-specific alterations in synaptic transmission occur in the KF of RTT mice.

The Prognosis of Cancer Depends on the Interplay of Autophagy, Apoptosis, and Anoikis within the Tumor Microenvironment

Within the tumor microenvironment, the fight between the immune system and cancer influences tumor transformation. Metastasis formation is an important stage in the progression of cancer. This process is aided by cellular detachment and resistance to anoikis, which are achieved by altering intercellular signaling. Autophagy, specifically pro-survival autophagy, aids cancer cells in developing treatment resistance. Numerous studies have shown that autophagy promotes tumor growth and resistance to anoikis. To regulate protective autophagy, cancer-related genes phosphorylate both pro- and anti-apoptotic proteins. Apoptosis, a type of controlled cell death, eliminates damaged or unwanted cells. Anoikis is a type of programmed cell death in which cells lose contact with the extracellular matrix. The dysregulation of these cellular pathways promotes tumor growth and spread. Apoptosis, anoikis, and autophagy interact meticulously and differently depending on the cellular circumstances. For instance, autophagy can protect cancer cells from apoptosis by removing cellular components that are damaged and might otherwise trigger apoptotic pathways. Similarly, anoikis dysregulation can trigger autophagy by causing cellular harm and metabolic stress. In order to prevent or treat metastatic disease, specifically, targeting these cellular mechanisms may present a promising prospect for cancer therapy. This review discourses the state of our understanding of the molecular and cellular mechanisms underlying tumor transformation and the establishment of metastatic tumors. To enhance the prognosis for cancer, we highlight and discuss potential therapeutic approaches that target these processes and genes involved in them.

Dynamic interactions between E-cadherin and Ankyrin-G mediate epithelial cell polarity maintenance

E-cadherin is an essential cell‒cell adhesion protein that mediates canonical cadherin-catenin complex formation in epithelial lateral membranes. Ankyrin-G (AnkG), a scaffold protein linking membrane proteins to the spectrin-based cytoskeleton, coordinates with E-cadherin to maintain epithelial cell polarity. However, the molecular mechanisms governing this complex formation and its relationships with the cadherin-catenin complex remain elusive. Here, we report that AnkG employs a promiscuous manner to encapsulate three discrete sites of E-cadherin by the same region, a dynamic mechanism that is distinct from the canonical 1:1 molar ratio previously described for other AnkG or E-cadherin-mediated complexes. Moreover, we demonstrate that AnkG-binding-deficient E-cadherin exhibited defective accumulation at the lateral membranes and show that disruption of interactions resulted in cell polarity malfunction. Finally, we demonstrate that E-cadherin is capable of simultaneously anchoring to AnkG and β-catenin, providing mechanistic insights into the functional orchestration of the ankyrin-spectrin complex with the cadherin-catenin complex. Collectively, our results show that complex formation between E-cadherin and AnkG is dynamic, which enables the maintenance of epithelial cell polarity by ensuring faithful targeting of the adhesion molecule-scaffold protein complex, thus providing molecular mechanisms for essential E-cadherin-mediated complex assembly at cell‒cell junctions.

Extracellular Tau Oligomers Damage the Axon Initial Segment

BACKGROUND

In Alzheimer's disease (AD) brain, neuronal polarity and synaptic connectivity are compromised. A key structure for regulating polarity and functions of neurons is the axon initial segment (AIS), which segregates somatodendritic from axonal proteins and initiates action potentials. Toxic tau species, including extracellular oligomers (xcTauOs), spread tau pathology from neuron to neuron by a prion-like process, but few other cell biological effects of xcTauOs have been described.

OBJECTIVE

Test the hypothesis that AIS structure is sensitive to xcTauOs.

METHODS

Cultured wild type (WT) and tau knockout (KO) mouse cortical neurons were exposed to xcTauOs, and quantitative western blotting and immunofluorescence microscopy with anti-TRIM46 monitored effects on the AIS. The same methods were used to compare TRIM46 and two other resident AIS proteins in human hippocampal tissue obtained from AD and age-matched non-AD donors.

RESULTS

Without affecting total TRIM46 levels, xcTauOs reduce the concentration of TRIM46 within the AIS and cause AIS shortening in cultured WT, but not TKO neurons. Lentiviral-driven tau expression in tau KO neurons rescues AIS length sensitivity to xcTauOs. In human AD hippocampus, the overall protein levels of multiple resident AIS proteins are unchanged compared to non-AD brain, but TRIM46 concentration within the AIS and AIS length are reduced in neurons containing neurofibrillary tangles.

CONCLUSION

xcTauOs cause partial AIS damage in cultured neurons by a mechanism dependent on intracellular tau, thereby raising the possibility that the observed AIS reduction in AD neurons in vivo is caused by xcTauOs working in concert with endogenous neuronal tau.

Palmitoylation controls the stability of 190 kDa ankyrin-G in dendritic spines and is regulated by ZDHHC8 and lithium

Introduction

AnkG, encoded by the ANK3 gene, is a multifunctional scaffold protein with complex isoform expression: the 480 and 270 kDa isoforms have roles at the axon initial segment and node of Ranvier, whereas the 190 kDa isoform (AnkG-190) has an emerging role in the dendritic shaft and spine heads. All isoforms of AnkG undergo palmitoylation, a post-translational modification regulating protein attachment to lipid membranes. However, palmitoylation of AnkG-190 has not been investigated in dendritic spines. The ANK3 gene and altered expression of AnkG proteins are associated with a variety of neuropsychiatric and neurodevelopmental disorders including bipolar disorder and are implicated in the lithium response, a commonly used mood stabilizer for bipolar disorder patients, although the precise mechanisms involved are unknown.

Result

Here, we showed that Cys70 palmitoylation stabilizes the localization of AnkG-190 in spine heads and at dendritic plasma membrane nanodomains. Mutation of Cys70 impairs AnkG-190 function in dendritic spines and alters PSD-95 scaffolding. Interestingly, we find that lithium reduces AnkG-190 palmitoylation thereby increasing its mobility in dendritic spines. Finally, we demonstrate that the palmitoyl acyl transferase ZDHHC8, but not ZDHHC5, increases AnkG-190 stability in spine heads and is inhibited by lithium.

Discussion

Together, our data reveal that palmitoylation is critical for AnkG-190 localization and function and a potential ZDHHC8/AnkG-190 mechanism linking AnkG-190 mobility to the neuronal effects of lithium.

Polarized transport of membrane and secreted proteins during lumen morphogenesis

A ubiquitous feature of animal development is the formation of fluid-filled cavities or lumina, which transport gases and fluids across tissues and organs. Among different species, lumina vary drastically in size, scale, and complexity. However, all lumen formation processes share key morphogenetic principles that underly their development. Fundamentally, a lumen simply consists of epithelial cells that encapsulate a continuous internal space, and a common way of building a lumen is via opening and enlarging by filling it with fluid and/or macromolecules. Here, we discuss how polarized targeting of membrane and secreted proteins regulates lumen formation, mainly focusing on ion transporters in vertebrate model systems. We also discuss mechanistic differences observed among invertebrates and vertebrates and describe how the unique properties of the Na+/K+-ATPase and junctional proteins can promote polarization of immature epithelia to build lumina de novo in developing organs.

Mechanisms of formation and functions of the early embryonic cavities

As the early mouse embryo develops, fundamental steps include the sequential formation of the first lumens in the murine conceptus. The first cavity established in the pre-implantation embryo is the blastocoel, followed by the emergence of the proamniotic cavity during the peri-implantation stages. The mouse embryo is a dynamic system which switches its modes of lumenogenesis before and after implantation. The blastocoel emerges in between the basolateral membranes, whereas the proamniotic cavity is formed on the apical interface. Defects in the sculpting of these luminal spaces are associated with developmental abnormalities and embryonic lethality. Here, we review the mechanisms by which these early embryonic cavities are formed and discuss the cavities in terms of their common and stage-specific principles of lumenogenesis and their functions.

Biliverdin reductase bridges focal adhesion kinase to Src to modulate synaptic signaling

Synapses connect discrete neurons into vast networks that send, receive, and encode diverse forms of information. Synaptic function and plasticity, the neuronal process of adapting to diverse and variable inputs, depend on the dynamic nature of synaptic molecular components, which is mediated in part by cell adhesion signaling pathways. Here, we found that the enzyme biliverdin reductase (BVR) physically links together key focal adhesion signaling molecules at the synapse. BVR-null (BVR^-/-) mice exhibited substantial deficits in learning and memory on neurocognitive tests, and hippocampal slices in which BVR was postsynaptically depleted showed deficits in electrophysiological responses to stimuli. RNA sequencing, biochemistry, and pathway analyses suggested that these deficits were mediated through the loss of focal adhesion signaling at both the transcriptional and biochemical level in the hippocampus. Independently of its catalytic function, BVR acted as a bridge between the primary focal adhesion signaling kinases FAK and Pyk2 and the effector kinase Src. Without BVR, FAK and Pyk2 did not bind to and stimulate Src, which then did not phosphorylate the N-methyl-d-aspartate (NMDA) receptor, a critical posttranslational modification for synaptic plasticity. Src itself is a molecular hub on which many signaling pathways converge to stimulate NMDAR-mediated neurotransmission, thus positioning BVR at a prominent intersection of synaptic signaling.

The largest isoform of Ankyrin-G is required for lattice structure of the axon initial segment

Alzheimer's disease (AD) is the most frequent neurodegenerative disease and a common dementia in elderly individuals. Previous studies found a strong correlation between axon initial segment (AIS) defects and AD, but it remains unclear whether AD itself changes the arrangement of AIS components, and the mechanisms by which adaptor proteins and ion channels in the AIS are disturbed in AD are not well understood. With super-resolution structured illumination microscopy (SIM) revealing axonal structures, here we imaged the lattice structure of completely assembled AIS in APP/PS1 neurons. By analyzing the images with Gaussian fitting and 1D mean autocorrelation, we found dual spacings (∼200 nm and ∼370 nm) of Ankyrin-G (AnkG), Na_v1.2 and βIV-spectrin in AD model APP/PS1 mice due to the low-expressed 480-kDa AnkG. To identify the roles of each AnkG isoform, two isoforms were separately expressed in neurons from AnkG conditional knockout mice. Mice rescued with 270-kDa AnkG displayed dual spacings of AnkG components in cultured neurons and impaired in spatial memory, while transgenic mice expressing 480-kDa AnkG showed a normal molecular distribution in the AIS and normal cognitive performance. Our findings provide new insight into the mechanisms underlying impaired cognition associated with neurodegenerative diseases such as AD.

Mechanisms and consequences of casein kinase II and ankyrin-3 regulation of the epithelial Na+ channel

Activity of the Epithelial Na+ Channel (ENaC) in the distal nephron fine-tunes renal sodium excretion. Appropriate sodium excretion is a key factor in the regulation of blood pressure. Consequently, abnormalities in ENaC function can cause hypertension. Casein Kinase II (CKII) phosphorylates ENaC. The CKII phosphorylation site in ENaC resides within a canonical "anchor" ankyrin binding motif. CKII-dependent phosphorylation of ENaC is necessary and sufficient to increase channel activity and is thought to influence channel trafficking in a manner that increases activity. We test here the hypothesis that phosphorylation of ENaC by CKII within an anchor motif is necessary for ankyrin-3 (Ank-3) regulation of the channel, which is required for normal channel locale and function, and the proper regulation of renal sodium excretion. This was addressed using a fluorescence imaging strategy combining total internal reflection fluorescence (TIRF) microscopy with fluorescence recovery after photobleaching (FRAP) to quantify ENaC expression in the plasma membrane in living cells; and electrophysiology to quantify ENaC activity in split-open collecting ducts from principal cell-specific Ank-3 knockout mice. Sodium excretion studies also were performed in parallel in this knockout mouse. In addition, we substituted a key serine residue in the consensus CKII site in β-ENaC with alanine to abrogate phosphorylation and disrupt the anchor motif. Findings show that disrupting CKII signaling decreases ENaC activity by decreasing expression in the plasma membrane. In the principal cell-specific Ank-3 KO mouse, ENaC activity and sodium excretion were significantly decreased and increased, respectively. These results are consistent with CKII phosphorylation of ENaC functioning as a "switch" that favors Ank-3 binding to increase channel activity.

A Rare Variant of ANK3 Is Associated With Intracranial Aneurysm

Intracranial aneurysm (IA) is a cerebrovascular disorder in which abnormal dilation of a blood vessel results from weakening of the blood vessel wall. The aneurysm may rupture, leading to subarachnoid hemorrhage with severe outcomes. This study was conducted to identify the genetic factors involved in the etiology of IA. Whole-exome sequencing was performed in three IA-aggregate families to identify candidate variants. Further association studies of candidate variants were performed among sporadic cases and controls. Bioinformatic analysis was used to predict the functions of candidate genes and variants. Twenty variants were identified after whole-exome sequencing, among which eight were selected for replicative association studies. ANK3 c.4403G>A (p.R1468H) was significantly associated with IA (odds ratio 4.77; 95% confidence interval 1.94–11.67; p-value = 0.00019). Amino acid R1468 in ANK3 was predicted to be located in the spectrin-binding domain of ankyrin-G and may regulate the migration of vascular endothelial cells and affect cell–cell junctions. Therefore, the variation p.R1468H may cause weakening of the artery walls, thereby accelerating the formation of IA. Thus, ANK3 is a candidate gene highly related to IA.

Ankyrin G organizes membrane components to promote coupling of cell mechanics and glucose uptake

The response of cells to forces is critical for their function and occurs via rearrangement of the actin cytoskeleton¹. Cytoskeletal remodelling is energetically costly^2,3, yet how cells signal for nutrient uptake remains undefined. Here we present evidence that force transmission increases glucose uptake by stimulating glucose transporter 1 (GLUT1). GLUT1 recruitment to and retention at sites of force transmission requires non-muscle myosin IIA-mediated contractility and ankyrin G. Ankyrin G forms a bridge between the force-transducing receptors and GLUT1. This bridge is critical for enabling cells under tension to tune glucose uptake to support remodelling of the actin cytoskeleton and formation of an epithelial barrier. Collectively, these data reveal an unexpected mechanism for how cells under tension take up nutrients and provide insight into how defects in glucose transport and mechanics might be linked.

Deciphering epiblast lumenogenesis reveals proamniotic cavity control of embryo growth and patterning

During the peri-implantation stages, the mouse embryo radically changes its appearance, transforming from a hollow-shaped blastocyst to an egg cylinder. At the same time, the epiblast gets reorganized from a simple ball of cells to a cup-shaped epithelial monolayer enclosing the proamniotic cavity. However, the cavity's function and mechanism of formation have so far been obscure. Through investigating the cavity formation, we found that in the epiblast, the process of lumenogenesis is driven by reorganization of intercellular adhesion, vectoral fluid transport, and mitotic paracellular water influx from the blastocoel into the emerging proamniotic cavity. By experimentally blocking lumenogenesis, we found that the proamniotic cavity functions as a hub for communication between the early lineages, enabling proper growth and patterning of the postimplantation embryo.

Ankyrin-G regulates forebrain connectivity and network synchronization via interaction with GABARAP

GABAergic circuits are critical for the synchronization and higher order function of brain networks. Defects in this circuitry are linked to neuropsychiatric diseases, including bipolar disorder, schizophrenia, and autism. Work in cultured neurons has shown that ankyrin-G plays a key role in the regulation of GABAergic synapses on the axon initial segment and somatodendritic domain of pyramidal neurons, where it interacts directly with the GABAA receptor-associated protein (GABARAP) to stabilize cell surface GABAA receptors. Here, we generated a knock-in mouse model expressing a mutation that abolishes the ankyrin-G/GABARAP interaction (Ank3 W1989R) to understand how ankyrin-G and GABARAP regulate GABAergic circuitry in vivo. We found that Ank3 W1989R mice exhibit a striking reduction in forebrain GABAergic synapses resulting in pyramidal cell hyperexcitability and disruptions in network synchronization. In addition, we identified changes in pyramidal cell dendritic spines and axon initial segments consistent with compensation for hyperexcitability. Finally, we identified the ANK3 W1989R variant in a family with bipolar disorder, suggesting a potential role of this variant in disease. Our results highlight the importance of ankyrin-G in regulating forebrain circuitry and provide novel insights into how ANK3 loss-of-function variants may contribute to human disease.

Feedback-Driven Assembly of the Axon Initial Segment

The axon initial segment (AIS) is a unique neuronal compartment that plays a crucial role in the generation of action potential and neuronal polarity. The assembly of the AIS requires membrane, scaffolding, and cytoskeletal proteins, including Ankyrin-G and TRIM46. How these components cooperate in AIS formation is currently poorly understood. Here, we show that Ankyrin-G acts as a scaffold interacting with End-Binding (EB) proteins and membrane proteins such as Neurofascin-186 to recruit TRIM46-positive microtubules to the plasma membrane. Using in vitro reconstitution and cellular assays, we demonstrate that TRIM46 forms parallel microtubule bundles and stabilizes them by acting as a rescue factor. TRIM46-labeled microtubules drive retrograde transport of Neurofascin-186 to the proximal axon, where Ankyrin-G prevents its endocytosis, resulting in stable accumulation of Neurofascin-186 at the AIS. Neurofascin-186 enrichment in turn reinforces membrane anchoring of Ankyrin-G and subsequent recruitment of TRIM46-decorated microtubules. Our study reveals feedback-based mechanisms driving AIS assembly.

Bilirubin Links Heme Metabolism to Neuroprotection by Scavenging Superoxide

Bilirubin is one of the most frequently measured metabolites in medicine, yet its physiologic roles remain unclear. Bilirubin can act as an antioxidant in vitro, but whether its redox activity is physiologically relevant is unclear because many other antioxidants are far more abundant in vivo. Here, we report that depleting endogenous bilirubin renders mice hypersensitive to oxidative stress. We find that mice lacking bilirubin are particularly vulnerable to superoxide (O₂^⋅-) over other tested reactive oxidants and electrophiles. Whereas major antioxidants such as glutathione and cysteine exhibit little to no reactivity toward O₂^⋅-, bilirubin readily scavenges O₂^⋅-. We find that bilirubin's redox activity is particularly important in the brain, where it prevents excitotoxicity and neuronal death by scavenging O₂^⋅- during NMDA neurotransmission. Bilirubin's unique redox activity toward O₂^⋅- may underlie a prominent physiologic role despite being significantly less abundant than other endogenous and exogenous antioxidants.

Ankyrin-G regulated epithelial phenotype is required for mouse lens morphogenesis and growth

Epithelial cell polarity, adhesion, proliferation, differentiation and survival are essential for morphogenesis of various organs and tissues including the ocular lens. The molecular mechanisms regulating the lens epithelial phenotype however, are not well understood. Here we investigated the role of scaffolding protein ankyrin-G (AnkG) in mouse lens development by conditional suppression of AnkG expression using the Cre-LoxP recombination approach. AnkG, which serves to link integral membrane proteins to the spectrin/actin cytoskeleton, was found to distribute predominantly to the lateral membranes of lens epithelium with several isoforms of the protein being detected in the mouse lens. Conditional deficiency of AnkG impaired mouse lens morphogenesis starting from embryonic stage E15.5, with neonatal (P1) AnkG cKO lenses exhibiting overt abnormalities in shape, size, epithelial cell height, sheet length and lateral membrane assembly together with defective fiber cell orientation relative to lenses from littermate AnkG floxed or Cre expressing mice. Severe disruptions in E-cadherin/β-catenin-based adherens junctions, and the membrane organization of spectrin-actin cytoskeleton, ZO-1, connexin-50 and Na⁺-K⁺-ATPase were noted in AnkG deficient lenses, along with detection in lens epithelium of α-smooth muscle actin, a marker of epithelial to mesenchymal transition. Moreover, lens epithelial cell proliferation and survival were severely compromised while differentiation appears to be normal in AnkG deficient mouse lenses. Collectively, these results indicate that AnkG regulates establishment of the epithelial phenotype via lateral membrane assembly, stabilization of E-cadherin-based cell-cell junctions, polarity and membrane organization of transport and adhesion proteins and the spectrin-actin skeleton, and provide evidence for an obligatory role for AnkG in lens morphogenesis and growth.

βIV Spectrinopathies Cause Profound Intellectual Disability, Congenital Hypotonia, and Motor Axonal Neuropathy

βIV spectrin links ankyrinG (AnkG) and clustered ion channels at axon initial segments (AISs) and nodes of Ranvier to the axonal cytoskeleton. Here, we report bi-allelic pathogenic SPTBN4 variants (three homozygous and two compound heterozygous) that cause a severe neurological syndrome that includes congenital hypotonia, intellectual disability, and motor axonal and auditory neuropathy. We introduced these variants into βIV spectrin, expressed these in neurons, and found that 5/7 were loss-of-function variants disrupting AIS localization or abolishing phosphoinositide binding. Nerve biopsies from an individual with a loss-of-function variant had reduced nodal Na+ channels and no nodal KCNQ2 K+ channels. Modeling the disease in mice revealed that although ankyrinR (AnkR) and βI spectrin can cluster Na+ channels and partially compensate for the loss of AnkG and βIV spectrin at nodes of Ranvier, AnkR and βI spectrin cannot cluster KCNQ2- and KCNQ3-subunit-containing K+ channels. Our findings define a class of spectrinopathies and reveal the molecular pathologies causing nervous-system dysfunction.

Generation of a biotinylatable Sox2 mouse model to identify Sox2 complexes in vivo

Sox2 is a Sry-box containing family member of related transcription factors sharing homology in their DNA binding domain. Sox2 is important during different stages of development, and previously we showed that Sox2 plays an important role in branching morphogenesis and epithelial cell differentiation in lung development. The transcriptional activity of Sox2 depends on its interaction with other proteins, leading to ‘complex-specific’ DNA binding and transcriptional regulation. In this study, we generated a mouse model containing a biotinylatable-tag targeted at the translational start site of the endogenous Sox2 gene (bioSox2). This tag was biotinylated by the bacterial birA protein and the resulting bioSox2 protein was used to identify associating partners of Sox2 at different phases of lung development in vivo (the Sox2 interactome). Homozygous bioSox2 mice are viable and fertile irrespective of the biotinylation of the bio tag, indicating that the bioSox2 gene is normally expressed and the protein is functional in all tissues. This suggests that partners of Sox2 are most likely able to associate with the bioSox2 protein. BioSox2 complexes were isolated with high affinity using streptavidin beads and analysed by MALDI-ToF mass spectrometry analysis. Several of the identified binding partners are already shown to have a respiratory phenotype. Two of these partners, Wdr5 and Tcf3, were validated to confirm their association in Sox2 complexes. This bioSox2 mouse model will be a valuable tool for isolating in vivo Sox2 complexes from different tissues.

The role of βII spectrin in cardiac health and disease

Spectrins are large, flexible proteins comprised of α-β dimers that are connected head-to-head to form the canonical heterotetrameric spectrin structure. Spectrins were initially believed to be exclusively found in human erythrocytic membrane and are highly conserved among different species. βII spectrin, the most common isoform of non-erythrocytic spectrin, is found in all nucleated cells and forms larger macromolecular complexes with ankyrins and actins. Not only is βII spectrin a central cytoskeletal scaffolding protein involved in preserving cell structure but it has also emerged as a critical protein required for distinct physiologic functions such as posttranslational localization of crucial membrane proteins and signal transduction. In the heart, βII spectrin plays a vital role in maintaining normal cardiac membrane excitability and proper cardiac development during embryogenesis. Mutations in βII spectrin genes have been strongly linked with the development of serious cardiac disorders such as congenital arrhythmias, heart failure, and possibly sudden cardiac death. This review focuses on our current knowledge of the role βII spectrin plays in the cardiovascular system in health and disease and the potential future clinical implications.

Ankyrin-G isoform imbalance and interneuronopathy link epilepsy and bipolar disorder

ANK3, encoding the adaptor protein Ankyrin-G (AnkG), has been implicated in bipolar disorder by genome-wide association studies. ANK3 has multiple alternative first exons, and a bipolar disorder-associated ANK3 variant has been shown to reduce the expression of exon 1b. Here we identify mechanisms through which reduced ANK3 exon 1b isoform expression disrupts neuronal excitation-inhibition balance. We find that parvalbumin (PV) interneurons and principal cells differentially express ANK3 first exon subtypes. PV interneurons express only isoforms containing exon 1b, whereas excitatory principal cells express exon 1e alone or both 1e and 1b. In transgenic mice deficient for exon 1b, PV interneurons lack voltage-gated sodium channels at their axonal initial segments and have increased firing thresholds and diminished action potential dynamic range. These mice exhibit an Ank3 gene dosage-dependent phenotype including behavior changes modeling bipolar disorder, epilepsy and sudden death. Thus ANK3's important association with human bipolar susceptibility may arise from imbalance between AnkG function in interneurons and principal cells and resultant excessive circuit sensitivity and output. AnkG isoform imbalance is a novel molecular endophenotype and potential therapeutic target.

Genetic disruption of ankyrin-G in adult mouse forebrain causes cortical synapse alteration and behavior reminiscent of bipolar disorder

Genome-wide association studies have implicated the ANK3 locus in bipolar disorder, a major human psychotic illness. ANK3 encodes ankyrin-G, which organizes the neuronal axon initial segment (AIS). We generated a mouse model with conditional disruption of ANK3 in pyramidal neurons of the adult forebrain (Ank-G cKO). This resulted in the expected loss of pyramidal neuron AIS voltage-gated sodium and potassium channels. There was also dramatic loss of markers of afferent GABAergic cartridge synapses, resembling the cortical microcircuitry changes in brains from psychotic patients, and suggesting disinhibition. Expression of c-fos was increased in cortical pyramidal neurons, consistent with increased neuronal activity due to disinhibition. The mice showed robust behavioral phenotypes reminiscent of aspects of human mania, ameliorated by antimania drugs lithium and valproate. Repeated social defeat stress resulted in repeated episodes of dramatic behavioral changes from hyperactivity to "depression-like" behavior, suggestive of some aspects of human bipolar disorder. Overall, we suggest that this Ank-G cKO mouse model recapitulates some of the core features of human bipolar disorder and indicates that cortical microcircuitry alterations during adulthood may be involved in pathogenesis. The model may be useful for studying disease pathophysiology and for developing experimental therapeutics.

First de novo ANK3 nonsense mutation in a boy with intellectual disability, speech impairment and autistic features

Ankyrin-G, encoded by ANK3, plays an important role in neurodevelopment and neuronal function. There are multiple isoforms of Ankyrin-G resulting in differential tissue expression and function. Heterozygous missense mutations in ANK3 have been associated with autism spectrum disorder. Further, in three siblings a homozygous frameshift mutation affecting only the longest isoform and a patient with a balanced translocation disrupting all isoforms were documented. The latter four patients were affected by a variable degree of intellectual disability, attention deficit hyperactivity disorder and autism. Here, we report on a boy with speech impairment, intellectual disability, autistic features, macrocephaly, macrosomia, chronic hunger and an altered sleeping pattern. By trio-whole-exome sequencing, we identified the first de novo nonsense mutation affecting all ANK3 transcripts. Thus, our data expand the phenotype of ANK3-associated diseases and suggest an isoform-based, phenotypic continuum between dominant and recessive ANK3-associated pathologies.

Behavioural characterization of AnkyrinG deficient mice, a model for ANK3 related disorders

ANK3 encodes AnkyrinG (AnkG), a member of the Ankyrin family that is expressed in several different isoforms in many tissues. A unique serine-rich domain and tail domain in the two largest isoforms of AnkG (270 and 480kDa), restrict AnkG to the axon initial segment and nodes of Ranvier of myelinated neurons. At these sites, AnkG is a master regulator, coordinating the strict clustering of components necessary for proper action potential initiation and propagation along the axon. These components include voltage-gated sodium channels, potassium channels and members of the L1 cell adhesion molecule family. Genetic variation in the ANK3 gene has been linked to a range of neuropsychiatric and neurodevelopmental disorders in human, including schizophrenia, bipolar disorder, intellectual disability and autism spectrum disorders. Here, we study the effect of reduced expression of the large isoforms of Ank3 on cognition and behaviour using a heterozygous knockout mouse model. In three independent behavioural tests, being the open field test, elevated plus maze and social interaction test, we found evidence for increased anxiety in our Ank3 mouse model. Besides, we observed specific neuroanatomical defects in heterozygous knockout mice, including a smaller cingulate cortex, granular retrosplenial cortex, primary motor cortex and fimbria of the hippocampus.

Ankyrin G Expression Regulates Apical Delivery of the Epithelial Sodium Channel (ENaC)

The epithelial sodium channel (ENaC) is the limiting entry point for Na⁺ reabsorption in the distal kidney nephron and is regulated by numerous hormones, including the mineralocorticoid hormone aldosterone. Previously we identified ankyrin G (AnkG), a cytoskeletal protein involved in vesicular transport, as a novel aldosterone-induced protein that can alter Na⁺ transport in mouse cortical collecting duct cells. However, the mechanisms underlying AnkG regulation of Na⁺ transport were unknown. Here we report that AnkG expression directly regulates Na⁺ transport by altering ENaC activity in the apical membrane. Increasing AnkG expression increased ENaC activity while depleting AnkG reduced ENaC-mediated Na⁺ transport. These changes were due to a change in ENaC directly rather than through alterations to the Na⁺ driving force created by Na⁺/K⁺-ATPase. Using a constitutively open mutant of ENaC, we demonstrate that the augmentation of Na⁺ transport is caused predominantly by increasing the number of ENaCs at the surface. To determine the mechanism of AnkG action on ENaC surface number, changes in rates of internalization, recycling, and membrane delivery were investigated. AnkG did not alter ENaC delivery to the membrane from biosynthetic pathways or removal by endocytosis. However, AnkG did alter ENaC insertion from constitutive recycling pathways. These findings provide a mechanism to account for the role of AnkG in the regulation of Na⁺ transport in the distal kidney nephron.

Cadherin tales: Regulation of cadherin function by endocytic membrane trafficking

Cadherins are the primary adhesion molecules in adherens junctions and desmosomes and play essential roles in embryonic development. Although significant progress has been made in understanding cadherin structure and function, we lack a clear vision of how cells confer plasticity upon adhesive junctions to allow for cellular rearrangements during development, wound healing and metastasis. Endocytic membrane trafficking has emerged as a fundamental mechanism by which cells confer a dynamic state to adhesive junctions. Recent studies indicate that the juxtamembrane domain of classical cadherins contains multiple endocytic motifs, or "switches," that can be used by cellular membrane trafficking machinery to regulate adhesion. The cadherin-binding protein p120-catenin (p120) appears to be the master regulator of access to these switches, thereby controlling cadherin endocytosis and turnover. This review focuses on p120 and other cadherin-binding proteins, ubiquitin ligases, and growth factors as key modulators of cadherin membrane trafficking.

Isoforms of Spectrin and Ankyrin Reflect the Functional Topography of the Mouse Kidney

The kidney displays specialized regions devoted to filtration, selective reabsorption, and electrolyte and metabolite trafficking. The polarized membrane pumps, channels, and transporters responsible for these functions have been exhaustively studied. Less examined are the contributions of spectrin and its adapter ankyrin to this exquisite functional topography, despite their established contributions in other tissues to cellular organization. We have examined in the rodent kidney the expression and distribution of all spectrins and ankyrins by qPCR, Western blotting, immunofluorescent and immuno electron microscopy. Four of the seven spectrins (αΙΙ, βΙ, βΙΙ, and βΙΙΙ) are expressed in the kidney, as are two of the three ankyrins (G and B). The levels and distribution of these proteins vary widely over the nephron. αΙΙ/βΙΙ is the most abundant spectrin, found in glomerular endothelial cells; on the basolateral membrane and cytoplasmic vesicles in proximal tubule cells and in the thick ascending loop of Henle; and less so in the distal nephron. βΙΙΙ spectrin largely replaces βΙΙ spectrin in podocytes, Bowman’s capsule, and throughout the distal tubule and collecting ducts. βΙ spectrin is only marginally expressed; its low abundance hinders a reliable determination of its distribution. Ankyrin G is the most abundant ankyrin, found in capillary endothelial cells and all tubular segments. Ankyrin B populates Bowman’s capsule, podocytes, the ascending thick loop of Henle, and the distal convoluted tubule. Comparison to the distribution of renal protein 4.1 isoforms and various membrane proteins indicates a complex relationship between the spectrin scaffold, its adapters, and various membrane proteins. While some proteins (e.g. ankyrin B, βΙΙΙ spectrin, and aquaporin 2) tend to share a similar distribution, there is no simple mapping of different spectrins or ankyrins to most membrane proteins. The implications of this data are discussed.

An Adaptable Spectrin/Ankyrin-Based Mechanism for Long-Range Organization of Plasma Membranes in Vertebrate Tissues

Ankyrins are membrane-associated proteins that together with their spectrin partners are responsible for micron-scale organization of vertebrate plasma membranes, including those of erythrocytes, excitable membranes of neurons and heart, lateral membrane domains of columnar epithelial cells, and striated muscle. Ankyrins coordinate functionally related membrane transporters and cell adhesion proteins (15 protein families identified so far) within plasma membrane compartments through independently evolved interactions of intrinsically disordered sequences with a highly conserved peptide-binding groove formed by the ANK repeat solenoid. Ankyrins are coupled to spectrins, which are elongated organelle-sized proteins that form mechanically resilient arrays through cross-linking by specialized actin filaments. In addition to protein interactions, cellular targeting and assembly of spectrin/ankyrin domains also critically depend on palmitoylation of ankyrin-G by aspartate-histidine-histidine-cysteine 5/8 palmitoyltransferases, as well as interaction of beta-2 spectrin with phosphoinositide lipids. These lipid-dependent spectrin/ankyrin domains are not static but are locally dynamic and determine membrane identity through opposing endocytosis of bulk lipids as well as specific proteins. A partnership between spectrin, ankyrin, and cell adhesion molecules first emerged in bilaterians over 500 million years ago. Ankyrin and spectrin may have been recruited to plasma membranes from more ancient roles in organelle transport. The basic bilaterian spectrin-ankyrin toolkit markedly expanded in vertebrates through gene duplications combined with variation in unstructured intramolecular regulatory sequences as well as independent evolution of ankyrin-binding activity by ion transporters involved in action potentials and calcium homeostasis. In addition, giant vertebrate ankyrins with specialized roles in axons acquired new coding sequences by exon shuffling. We speculate that early axon initial segments and epithelial lateral membranes initially were based on spectrin-ankyrin-cell adhesion molecule assemblies and subsequently served as "incubators," where ion transporters independently acquired ankyrin-binding activity through positive selection.

Ankyrin-G Inhibits Endocytosis of Cadherin Dimers

Dynamic regulation of endothelial cell adhesion is central to vascular development and maintenance. Furthermore, altered endothelial adhesion is implicated in numerous diseases. Therefore, normal vascular patterning and maintenance require tight regulation of endothelial cell adhesion dynamics. However, the mechanisms that control junctional plasticity are not fully understood. Vascular endothelial cadherin (VE-cadherin) is an adhesive protein found in adherens junctions of endothelial cells. VE-cadherin mediates adhesion through trans interactions formed by its extracellular domain. Trans binding is followed by cis interactions that laterally cluster the cadherin in junctions. VE-cadherin is linked to the actin cytoskeleton through cytoplasmic interactions with β- and α-catenin, which serve to increase adhesive strength. Furthermore, p120-catenin binds to the cytoplasmic tail of cadherin and stabilizes it at the plasma membrane. Here we report that induced cis dimerization of VE-cadherin inhibits endocytosis independent of both p120 binding and trans interactions. However, we find that ankyrin-G, a protein that links membrane proteins to the spectrin-actin cytoskeleton, associates with VE-cadherin and inhibits its endocytosis. Ankyrin-G inhibits VE-cadherin endocytosis independent of p120 binding. We propose a model in which ankyrin-G associates with and inhibits the endocytosis of VE-cadherin cis dimers. Our findings support a novel mechanism for regulation of VE-cadherin endocytosis through ankyrin association with cadherin engaged in lateral interactions.

Cryopreservation and recovery of human endometrial epithelial cells with high viability, purity, and functional fidelity

OBJECTIVE

To develop a protocol for cryopreservation and recovery of human endometrial epithelial cells (eECs) retaining molecular and functional characteristics of endometrial epithelium in vivo.

DESIGN

In vitro study using human endometrial cells.

SETTING

University research laboratory.

PATIENT(S)

Endometrial biopsies were obtained from premenopausal women undergoing benign gynecologic procedures.

INTERVENTION(S)

Primary eECs were cryopreserved in 1% fetal bovine serum/10% dimethylsulfoxide in Defined Keratinocyte Serum-Free Medium (KSFM). Recovered cells were observed for endometrial stromal fibroblast (eSF) contamination and subsequently evaluated for morphology, gene expression, and functional characteristics of freshly cultured eECs and in vivo endometrial epithelium.

MAIN OUTCOME MEASURE(S)

Analysis of eEC morphology and the absence of eSF contamination; evaluation of epithelial-specific gene and protein expression; assessment of epithelial polarity.

RESULT(S)

Endometrial epithelial cells recovered after cryopreservation (n = 5) displayed epithelial morphology and expressed E-cadherin (CDH1), occludin (OCLN), claudin1 (CLDN1), and keratin18 (KRT18). Compared with eSF, recovered eECs displayed increased (P<.05) expression of epithelial-specific genes AREG, CDH1, DEFB4A, MMP7, and WNT7A, while exhibiting low-to-undetectable (P<.05) stromal-specific genes COL6A3, HOXA11, MMP2, PDGFRB, and WNT5A. Recovered eECs secreted levels of cytokines and growth factors similarly to freshly cultured eECs. Recovered eECs could form a polarized monolayer with high transepithelial electrical resistance (TER) and impermeability to small molecules, and expressed apical/basolateral localization of CDH1 and apical localization of OCLN.

CONCLUSION(S)

We have developed a protocol for cryopreservation of eECs in which recovered cells after thawing demonstrate morphologic, transcriptomic, and functional characteristics of human endometrial epithelium in vivo.

Dual pulse-chase microscopy reveals early divergence in the biosynthetic trafficking of the Na,K-ATPase and E-cadherin

The trafficking of newly synthesized Na,K-ATPase and E-cadherin is observed in polarized epithelial cells. E-cadherin’s exit from the Golgi complex is not susceptible to 19°C temperature block. Furthermore, these proteins exit the Golgi and are delivered to the basolateral cell surface in separate vascular carriers.

Recent evidence indicates that newly synthesized membrane proteins that share the same distributions in the plasma membranes of polarized epithelial cells can pursue a variety of distinct trafficking routes as they travel from the Golgi complex to their common destination at the cell surface. In most polarized epithelial cells, both the Na,K-ATPase and E-cadherin are localized to the basolateral domains of the plasma membrane. To examine the itineraries pursued by newly synthesized Na,K-ATPase and E-cadherin in polarized MDCK epithelial cells, we used the SNAP and CLIP labeling systems to fluorescently tag temporally defined cohorts of these proteins and observe their behaviors simultaneously as they traverse the secretory pathway. These experiments reveal that E-cadherin is delivered to the cell surface substantially faster than is the Na,K-ATPase. Furthermore, the surface delivery of newly synthesized E-cadherin to the plasma membrane was not prevented by the 19°C temperature block that inhibits the trafficking of most proteins, including the Na,K-ATPase, out of the trans-Golgi network. Consistent with these distinct behaviors, populations of newly synthesized E-cadherin and Na,K-ATPase become separated from one another within the trans-Golgi network, suggesting that they are sorted into different carrier vesicles that mediate their post-Golgi trafficking.

Dynamic spectrin/ankyrin-G microdomains promote lateral membrane assembly by opposing endocytosis

Current physical models for plasma membranes emphasize dynamic 10- to 300-nm compartments at thermodynamic equilibrium but subject to thermal fluctuations. However, epithelial lateral membranes contain micrometer-sized domains defined by an underlying membrane skeleton composed of spectrin and its partner ankyrin-G. We demonstrate that these spectrin/ankyrin-G domains exhibit local microtubule-dependent movement on a time scale of minutes and encounter most of the lateral membranes within an hour. Spectrin/ankyrin-G domains exclude clathrin and clathrin-dependent cargo, and inhibit both receptor-mediated and bulk endocytosis. Moreover, inhibition of endocytosis fully restores lateral membrane height in spectrin- or ankyrin-G-depleted cells. These findings support a non-equilibrium cellular-scale model for epithelial lateral membranes, where spectrin/ankyrin-G domains actively patrol the plasma membrane, analogous to "window washers," and promote columnar morphology by blocking membrane uptake.

Exon organization and novel alternative splicing of Ank3 in mouse heart

Ankyrin-G is an adaptor protein that links membrane proteins to the underlying cytoskeletal network. Alternative splicing of the Ank3 gene gives rise to multiple ankyrin-G isoforms in numerous tissues. To date, only one ankyrin-G isoform has been characterized in heart and transcriptional regulation of the Ank3 gene is completely unknown. In this study, we describe the first comprehensive analysis of Ank3 expression in heart. Using a PCR-based screen of cardiac mRNA transcripts, we identify two new exons and 28 alternative splice variants of the Ank3 gene. We measure the relative expression of each splice variant using quantitative real-time PCR and exon-exon boundary spanning primers that specifically amplify individual Ank3 variants. Six variants are rarely expressed (<1%), while the remaining variants display similar expression patterns in three hearts. Of the five first exons in the Ank3 gene, exon 1d is only expressed in heart and skeletal muscle as it was not detected in brain, kidney, cerebellum, and lung. Immunoblot analysis reveals multiple ankyrin-G isoforms in heart, and two ankyrin-G subpopulations are detected in adult cardiomyocytes by immunofluorescence. One population co-localizes with the voltage-gated sodium channel Na_V1.5 at the intercalated disc, while the other population expresses at the Z-line. Two of the rare splice variants excise a portion of the ZU5 motif, which encodes the minimal spectrin-binding domain, and these variants lack β-spectrin binding. Together, these data demonstrate that Ank3 is subject to complex splicing regulation resulting in a diverse population of ankyrin-G isoforms in heart.

Discs large 5, an Essential Gene in Drosophila, Regulates Egg Chamber Organization

Discs large 5 (Dlg5) is a member of the MAGUK family of proteins that typically serve as molecular scaffolds and mediate signaling complex formation and localization. In vertebrates, Dlg5 has been shown to be responsible for polarization of neural progenitors and to associate with Rab11-positive vesicles in epithelial cells. In Drosophila, however, the function of Dlg5 is not well-documented. We have identified dlg5 as an essential gene that shows embryonic lethality. dlg5 embryos display partial loss of primordial germ cells (PGCs) during gonad coalescence between stages 12 and 15 of embryogenesis. Loss of Dlg5 in germline and somatic stem cells in the ovary results in the depletion of both cell lineages. Reduced expression of Dlg5 in the follicle cells of the ovary leads to a number of distinct phenotypes, including defects in egg chamber budding, stalk cell overgrowth, and ectopic polar cell induction. Interestingly, loss of Dlg5 in follicle cells results in abnormal distribution of a critical component of cell adhesion, E-cadherin, shown to be essential for proper organization of egg chambers.

Giant ankyrin-G: a critical innovation in vertebrate evolution of fast and integrated neuronal signaling

Axon initial segments (AISs) and nodes of Ranvier are sites of clustering of voltage-gated sodium channels (VGSCs) in nervous systems of jawed vertebrates that facilitate fast long-distance electrical signaling. We demonstrate that proximal axonal polarity as well as assembly of the AIS and normal morphogenesis of nodes of Ranvier all require a heretofore uncharacterized alternatively spliced giant exon of ankyrin-G (AnkG). This exon has sequence similarity to I-connectin/Titin and was acquired after the first round of whole-genome duplication by the ancestral ANK2/ANK3 gene in early vertebrates before development of myelin. The giant exon resulted in a new nervous system-specific 480-kDa polypeptide combining previously known features of ANK repeats and β-spectrin-binding activity with a fibrous domain nearly 150 nm in length. We elucidate previously undescribed functions for giant AnkG, including recruitment of β4 spectrin to the AIS that likely is regulated by phosphorylation, and demonstrate that 480-kDa AnkG is a major component of the AIS membrane "undercoat' imaged by platinum replica electron microscopy. Surprisingly, giant AnkG-knockout neurons completely lacking known AIS components still retain distal axonal polarity and generate action potentials (APs), although with abnormal frequency. Giant AnkG-deficient mice live to weaning and provide a rationale for survival of humans with severe cognitive dysfunction bearing a truncating mutation in the giant exon. The giant exon of AnkG is required for assembly of the AIS and nodes of Ranvier and was a transformative innovation in evolution of the vertebrate nervous system that now is a potential target in neurodevelopmental disorders.

Giant ankyrin-G stabilizes somatodendritic GABAergic synapses through opposing endocytosis of GABAA receptors

GABAA-receptor-based interneuron circuitry is essential for higher order function of the human nervous system and is implicated in schizophrenia, depression, anxiety disorders, and autism. Here we demonstrate that giant ankyrin-G (480-kDa ankyrin-G) promotes stability of somatodendritic GABAergic synapses in vitro and in vivo. Moreover, giant ankyrin-G forms developmentally regulated and cell-type-specific micron-scale domains within extrasynaptic somatodendritic plasma membranes of pyramidal neurons. We further find that giant ankyrin-G promotes GABAergic synapse stability through opposing endocytosis of GABAA receptors, and requires a newly described interaction with GABARAP, a GABAA receptor-associated protein. We thus present a new mechanism for stabilization of GABAergic interneuron synapses and micron-scale organization of extrasynaptic membrane that provides a rationale for studies linking ankyrin-G genetic variation with psychiatric disease and abnormal neurodevelopment.

Glial ankyrins facilitate paranodal axoglial junction assembly

Neuron-glia interactions establish functional membrane domains along myelinated axons. These include nodes of Ranvier, paranodal axoglial junctions and juxtaparanodes. Paranodal junctions are the largest vertebrate junctional adhesion complex, and they are essential for rapid saltatory conduction and contribute to assembly and maintenance of nodes. However, the molecular mechanisms underlying paranodal junction assembly are poorly understood. Ankyrins are cytoskeletal scaffolds traditionally associated with Na(+) channel clustering in neurons and are important for membrane domain establishment and maintenance in many cell types. Here we show that ankyrin-B, expressed by Schwann cells, and ankyrin-G, expressed by oligodendrocytes, are highly enriched at the glial side of paranodal junctions where they interact with the essential glial junctional component neurofascin 155. Conditional knockout of ankyrins in oligodendrocytes disrupts paranodal junction assembly and delays nerve conduction during early development in mice. Thus, glial ankyrins function as major scaffolds that facilitate early and efficient paranodal junction assembly in the developing CNS.

A hierarchy of ankyrin-spectrin complexes clusters sodium channels at nodes of Ranvier

The scaffolding protein ankyrin-G is required for Na(+) channel clustering at axon initial segments. It is also considered essential for Na(+) channel clustering at nodes of Ranvier to facilitate fast and efficient action potential propagation. However, notwithstanding these widely accepted roles, we show here that ankyrin-G is dispensable for nodal Na(+) channel clustering in vivo. Unexpectedly, in the absence of ankyrin-G, erythrocyte ankyrin (ankyrin-R) and its binding partner βI spectrin substitute for and rescue nodal Na(+) channel clustering. In addition, channel clustering is also rescued after loss of nodal βIV spectrin by βI spectrin and ankyrin-R. In mice lacking both ankyrin-G and ankyrin-R, Na(+) channels fail to cluster at nodes. Thus, ankyrin R-βI spectrin protein complexes function as secondary reserve Na(+) channel clustering machinery, and two independent ankyrin-spectrin protein complexes exist in myelinated axons to cluster Na(+) channels at nodes of Ranvier.

Loss of PiT-1 results in abnormal endocytosis in the yolk sac visceral endoderm

PiT-1 protein is a transmembrane sodium-dependent phosphate (Pi) transporter. PiT-1 knock out (KO) embryos die from largely unknown causes by embryonic day (E) 12.5. We tested the hypothesis that PiT-1 is required for endocytosis in the embryonic yolk sac (YS) visceral endoderm (VE). Here we present data supporting that PiT-1 KO results in a YS remodeling defect and decreased endocytosis in the YS VE. The remodeling defect is not due to an upstream cardiomyocyte requirement for PiT-1, as SM22αCre-specific KO of PiT-1 in the developing heart and the YS mesodermal layer (ME) does not recapitulate the PiT-1 global KO phenotype. Furthermore, we find that high levels of PiT-1 protein localize to the YS VE apical membrane. Together these data support that PiT-1 is likely required in YS VE. During normal development maternal immunoglobulin (IgG) is endocytosed into YS VE and accumulates in the apical side of the VE in a specialized lysosome termed the apical vacuole (AV). We have identified a reduction in PiT-1 KO VE cell height and a striking loss of IgG accumulation in the PiT-1 KO VE. The endocytosis genes Tfeb, Lamtor2 and Snx2 are increased at the RNA level. Lysotracker Red staining reveals a loss of distinct AVs, and yolk sacs incubated ex vivo with phRODO Green Dextran for Endocytosis demonstrate a functional loss of endocytosis. As yolk sac endocytosis is controlled in part by microautophagy, but expression of LC3 had not been examined, we investigated LC3 expression during yolk sac development and found stage-specific LC3 RNA expression that is predominantly from the YS VE layer at E9.5. Normalized LC3-II protein levels are decreased in the PiT-1 KO YS, supporting a requirement for PiT-1 in autophagy in the YS. Therefore, we propose the novel idea that PiT-1 is central to the regulation of endocytosis and autophagy in the YS VE.

Ankyrin-G palmitoylation and βII-spectrin binding to phosphoinositide lipids drive lateral membrane assembly

Ankyrin-G and βII-spectrin colocalize at sites of cell-cell contact in columnar epithelial cells and promote lateral membrane assembly. This study identifies two critical inputs from lipids that together provide a rationale for how ankyrin-G and βII-spectrin selectively localize to Madin-Darby canine kidney (MDCK) cell lateral membranes. We identify aspartate-histidine-histidine-cysteine 5/8 (DHHC5/8) as ankyrin-G palmitoyltransferases required for ankyrin-G lateral membrane localization and for assembly of lateral membranes. We also find that βII-spectrin functions as a coincidence detector that requires recognition of both ankyrin-G and phosphoinositide lipids for its lateral membrane localization. DHHC5/8 and βII-spectrin colocalize with ankyrin-G in micrometer-scale subdomains within the lateral membrane that are likely sites for palmitoylation of ankyrin-G. Loss of either DHHC5/8 or ankyrin-G-βII-spectrin interaction or βII-spectrin-phosphoinositide recognition through its pleckstrin homology domain all result in failure to build the lateral membrane. In summary, we identify a functional network connecting palmitoyltransferases DHHC5/8 with ankyrin-G, ankyrin-G with βII-spectrin, and βII-spectrin with phosphoinositides that is required for the columnar morphology of MDCK epithelial cells.

Identification of distinct ChAT⁺ neurons and activity-dependent control of postnatal SVZ neurogenesis

Postnatal/adult SVZ neurogenesis is believed to be primarily controlled by neural stem cell (NSC)-intrinsic mechanisms, interacting with extracellular/niche-driven cues. Although behavioral paradigms and disease states have suggested possibilities for higher-level inputs, it is currently unknown if neural activity patterns from discrete circuits can directly regulate SVZ neurogenesis. We have identified a previously undescribed population of ChAT⁺ neurons residing within the rodent SVZ neurogenic niche. These neurons showed morphological and functional differences from neighboring striatal counterparts, and released acetylcholine locally in activity-dependent fashion. Optogenetic inhibition and stimulation of subependymal ChAT⁺ neurons in vivo showed that they are necessary and sufficient to control neurogenic proliferation. Furthermore, whole-cell recordings and biochemical experiments revealed direct SVZ NSC responses to local acetylcholine release, synergizing with FGF receptor activation to increase neuroblast production. These results uncovered an unknown gateway connecting SVZ neurogenesis to neuronal activity-dependent control, and possibilities for modulating neuroregenerative capacities in health and disease.

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.