Biochemical and functional characterization of a novel neuron-glia adhesion molecule that is involved in neuronal migration

Genotypic and phenotypic diversity in X-linked retinoschisis: Findings from a South Indian patient cohort

PURPOSE

Retinoschisis is a distinctive condition characterized by intraretinal layer clefts, primarily associated with X-linked recessive inheritance due to RS1 gene mutations. This study aims to uncover the RS1 mutation spectrum in a cohort of 22 X-linked retinoschisis cases from South India and emphasizes the genotypic and phenotypic associations within patients harboring only RS1 mutations.

METHODS

A total of 22 probands were suspected of having X-linked retinoschisis. All study subjects underwent ophthalmic investigations, including assessments of visual acuity, fundus examination, optical coherence tomography (OCT), and electroretinogram (ERG). RS1 gene screening was conducted using Sanger sequencing, and the pathogenicity of the variants was assessed through Sorting Intolerant from Tolerant (SIFT) and PolyPhen-2 in silico tools.

RESULTS

The study found that the probands had an average visual acuity of 0.79 ± 0.39 log of minimum angle of resolution (logMAR), ranging from 0.17 to 1.77. During fundus examination, the probands exhibited a characteristic spoke wheel-like pattern in the macular region. Furthermore, OCT analysis revealed distinct alterations in the inner retinal microstructure, and ERG results consistently showed a reduction in b-wave amplitude. Eventually, Sanger sequencing results showed hemizygous mutations in the RS1 gene in only 12 probands, including a novel missense mutation in the RS1 gene's signal sequence.

CONCLUSION

This study provides valuable insights into the spectrum of RS1 mutations in X-linked retinoschisis probands from South India. It reveals distinct genotypic-phenotypic associations and highlights the clinical manifestations associated with the disease pathogenesis.

Upregulation of alveolar fluid clearance is not sufficient for Na+,K+-ATPase β subunit-mediated gene therapy of LPS-induced acute lung injury in mice

Acute Lung Injury/Acute Respiratory Distress Syndrome (ALI/ARDS) is characterized by diffuse alveolar damage and significant edema accumulation, which is associated with impaired alveolar fluid clearance (AFC) and alveolar-capillary barrier disruption, leading to acute respiratory failure. Our previous data showed that electroporation-mediated gene delivery of the Na⁺, K⁺-ATPase β1 subunit not only increased AFC, but also restored alveolar barrier function through upregulation of tight junction proteins, leading to treatment of LPS-induced ALI in mice. More importantly, our recent publication showed that gene delivery of MRCKα, the downstream effector of β1 subunit-mediated signaling towards upregulation of adhesive junctions and epithelial and endothelial barrier integrity, also provided therapeutic potential for ARDS treatment in vivo but without necessarily accelerating AFC, indicating that for ARDS treatment, improving alveolar capillary barrier function may be of more benefit than improving fluid clearance. In the present study, we investigated the therapeutical potential of β2 and β3 subunits, the other two β isoforms of Na⁺, K⁺-ATPase, for LPS-induced ALI. We found that gene transfer of either the β1, β2, or β3 subunits significantly increased AFC compared to the basal level in naïve animals and each gave similar increased AFC to each other. However, unlike that of the β1 subunit, gene transfer of the β2 or β3 subunit into pre-injured animal lungs failed to show the beneficial effects of attenuated histological damage, neutrophil infiltration, overall lung edema, or increased lung permeability, indicating that β2 or β3 gene delivery could not treat LPS induced lung injury. Further, while β1 gene transfer increased levels of key tight junction proteins in the lungs of injured mice, that of either the β2 or β3 subunit had no effect on levels of tight junction proteins. Taken together, this strongly suggests that restoration of alveolar-capillary barrier function alone may be of equal or even more benefit than improving AFC for ALI/ARDS treatment.

Altered expression levels of miR-144-3p and ATP1B2 are associated with schizophrenia

Objectives: Schizophrenia is a devastating mental disease. Various microRNAs were proven to be associated with schizophrenia. Altered microRNA-144-3p (miR-144-3p) levels were found in various neurological and psychotic disorders. Beta2-subunit of Na(+)/K(+)-ATPase (ATP1B2) regulates neuronal migration and cell growth during brain development through the PI3K/Akt/mTOR pathway. The present study explored the associations of miR-144-3p and ATP1B2 with schizophrenia and their mutual interaction.Methods: A schizophrenic animal model employing repeated MK-801 administration was established and 293 T cells over-expressing miR-144-3p were constructed by lentivirus. The in vitro and in vivo levels of miR-144-3p, ATP1B2, and the PI3K/Akt/mTOR pathway were examined by qRT-PCR and Western Blots. The interaction between miR-144-3p and ATP1B2 was predicted and assessed by using bioinformatic methods and a luciferase reporter gene assay, respectively.Results: MiR-144-3p expression was elevated in the schizophrenic rat hippocampus. ATP1B2 was down-regulated in schizophrenic patients by analysing GEO datasets. Additionally, miR-144-3p can directly bind with ATP1B2. Furthermore, the ATP1B2 expression and PI3K/Akt/mTOR phosphorylation levels were down-regulated in the 293 T cells over-expressing miR-144-3p and schizophrenic rat hippocampus, which could be reversed by risperidone.Conclusions: This study revealed that up-regulated miR-144-3p might be associated with schizophrenia through down-regulating ATP1B2, implicating new targets of schizophrenia treatment.

The β2-Subunit (AMOG) of Human Na+, K+-ATPase Is a Homophilic Adhesion Molecule

The β₂ subunit of Na⁺, K⁺-ATPase was originally identified as the adhesion molecule on glia (AMOG) that mediates the adhesion of astrocytes to neurons in the central nervous system and that is implicated in the regulation of neurite outgrowth and neuronal migration. While β₁ isoform have been shown to trans-interact in a species-specific mode with the β₁ subunit on the epithelial neighboring cell, the β₂ subunit has been shown to act as a recognition molecule on the glia. Nevertheless, none of the works have identified the binding partner of β₂ or described its adhesion mechanism. Until now, the interactions pronounced for β₂/AMOG are heterophilic cis-interactions. In the present report we designed experiments that would clarify whether β₂ is a cell–cell homophilic adhesion molecule. For this purpose, we performed protein docking analysis, cell–cell aggregation, and protein–protein interaction assays. We observed that the glycosylated extracellular domain of β₂/AMOG can make an energetically stable trans-interacting dimer. We show that CHO (Chinese Hamster Ovary) fibroblasts transfected with the human β₂ subunit become more adhesive and make large aggregates. The treatment with Tunicamycin in vivo reduced cell aggregation, suggesting the participation of N-glycans in that process. Protein–protein interaction assay in vivo with MDCK (Madin-Darby canine kidney) or CHO cells expressing a recombinant β₂ subunit show that the β₂ subunits on the cell surface of the transfected cell lines interact with each other. Overall, our results suggest that the human β₂ subunit can form trans-dimers between neighboring cells when expressed in non-astrocytic cells, such as fibroblasts (CHO) and epithelial cells (MDCK).

Therapeutic targeting of membrane-associated proteins in central nervous system tumors

The activity of the most complex system, the central nervous system (CNS) is profoundly regulated by a huge number of membrane-associated proteins (MAP). A minor change stimulates immense chemical changes and the elicited response is organized by MAP, which acts as a receptor of that chemical or channel enabling the flow of ions. Slight changes in the activity or expression of these MAPs lead to severe consequences such as cognitive disorders, memory loss, or cancer. CNS tumors are heterogeneous in nature and hard-to-treat due to random mutations in MAPs; like as overexpression of EGFRvIII/TGFβR/VEGFR, change in adhesion molecules α5β3 integrin/SEMA3A, imbalance in ion channel proteins, etc. Extensive research is under process for developing new therapeutic approaches using these proteins such as targeted cytotoxic radiotherapy, drug-delivery, and prodrug activation, blocking of receptors like GluA1, developing viral vector against cell surface receptor. The combinatorial approach of these strategies along with the conventional one might be more potential. Henceforth, our review focuses on in-depth analysis regarding MAPs aiming for a better understanding for developing an efficient therapeutic approach for targeting CNS tumors.

Na/K-ATPase beta1-subunit associates with neuronal growth regulator 1 (NEGR1) to participate in intercellular interactions

Neuronal growth regulator 1 (NEGR1) is a GPI-anchored membrane protein that is involved in neural cell adhesion and communication. Multiple genome wide association studies have found that NEGR1 is a generic risk factor for multiple human diseases, including obesity, autism, and depression. Recently, we reported that Negr1^−/− mice showed a highly increased fat mass and affective behavior. In the present study, we identified Na/K-ATPase, beta1-subunit (ATP1B1) as an NEGR1 binding partner by yeast two-hybrid screening. NEGR1 and ATP1B1 were found to form a relatively stable complex in cells, at least partially co-localizing in membrane lipid rafts. We found that NEGR1 binds with ATP1B1 at its C-terminus, away from the binding site for the alpha subunit, and may contribute to intercellular interactions. Collectively, we report ATP1B1 as a novel NEGR1-interacting protein, which may help deciphering molecular networks underlying NEGR1-associated human diseases.

Spider venom components decrease glioblastoma cell migration and invasion through RhoA-ROCK and Na+/K+-ATPase β2: potential molecular entities to treat invasive brain cancer

BACKGROUND

Glioblastoma (GB) cells have the ability to migrate and infiltrate the normal parenchyma, leading to the formation of recurrent tumors often adjacent to the surgical extraction site. We recently showed that Phoneutria nigriventer spider venom (PnV) has anticancer effects mainly on the migration of human GB cell lines (NG97 and U-251). The present work aimed to investigate the effects of isolated components from the venom on migration, invasiveness, morphology and adhesion of GB cells, also evaluating RhoA-ROCK signaling and Na⁺/K⁺-ATPase β2 (AMOG) involvement.

METHODS

Human (NG97) GB cells were treated with twelve subfractions (SFs-obtained by HPLC from PnV). Migration and invasion were evaluated by scratch wound healing and transwell assays, respectively. Cell morphology and actin cytoskeleton were shown by GFAP and phalloidin labeling. The assay with fibronectin coated well plate was made to evaluate cell adhesion. Western blotting demonstrated ROCK and AMOG levels and a ROCK inhibitor was used to verify the involvement of this pathway. Values were analyzed by the GraphPad Prism software package and the level of significance was determinate using one-way analysis of variance (ANOVA) followed by Dunnett's multiple comparisons test.

RESULTS

Two (SF1 and SF11) of twelve SFs, decreased migration and invasion compared to untreated control cells. Both SFs also altered actin cytoskeleton, changed cell morphology and reduced adhesion. SF1 and SF11 increased ROCK expression and the inhibition of this protein abolished the effects of both subfractions on migration, morphology and adhesion (but not on invasion). SF11 also increased Na⁺/K⁺-ATPase β2.

CONCLUSION

All components of the venom were evaluated and two SFs were able to impair human glioblastoma cells. The RhoA effector, ROCK, was shown to be involved in the mechanisms of both PnV components. It is possible that AMOG mediates the effect of SF11 on the invasion. Further investigations to isolate and biochemically characterize the molecules are underway.

Glioma malignancy is linked to interdependent and inverse AMOG and L1 adhesion molecule expression

BACKGROUND

Gliomas account for the majority of primary human brain tumors and remain a challenging neoplasm for cure due to limited therapeutic options. Cell adhesion molecules play pivotal roles in the growth and progression of glial tumors. Roles of the adhesion molecules on glia (AMOG) and L1CAM (L1) in glioma cells have been shown to correlate with tumorigenesis: Increased expression of L1 and decreased expression of AMOG correlate with degree of malignancy.

METHODS

We evaluated the interdependence in expression of these molecules by investigating the role of AMOG in vitro via modulation of L1 expression and analyzing apoptosis and cell senescence of glioma cells.

RESULTS

Immunohistochemical staining of normal human cortical and glioma tissue microarrays demonstrated that AMOG expression was lower in human gliomas compared to normal tissue and is inversely correlated with the degree of malignancy. Moreover, reduction of AMOG expression in human glioblastoma cells elevated L1 expression, which is accompanied by decreased cell apoptosis as well as senescence.

CONCLUSION

AMOG and L1 interdependently regulate their expression levels not only in U-87 MG cells but also in U251 and SHG44 human glioma cell lines. The capacity of AMOG to reduce L1 expression suggests that methods for increasing AMOG expression may provide a therapeutic choice for the management of glial tumors with high expression of L1.

Targeting β2 subunit of Na+/K+-ATPase induces glioblastoma cell apoptosis through elevation of intracellular Ca2

Glioblastoma (GBM) is the most frequent brain cancer with poor prognosis and few therapies and urgently requires effective treatments. Na+/K+-ATPase is considered as a target for GBM therapy and development of anticancer drugs. Cardiac glycosides bind the Na+/K+-ATPase α subunit to inhibit enzymatic activity and are promising candidates for anticancer drug development including GBM. However, the comparatively higher doses required for effective anticancer actions cause severe cardiotoxicity. Selectively targeting the ATPase Na+/K+ transporting subunit beta 2 (ATP1B2) that is not expressed in the heart might avoid the cardiotoxicity. However, the effect of targeting ATP1B2 in GBM remains unknown. In this study, we found that high ATP1B2 expression is significantly associated with poor prognosis of patients with GBM. ATP1B2 silencing in GBM cells resulted in remarkably cell cycle arrest at the G2/M phase and apoptosis with concomitant increase in intracellular Ca2+ and activation of p38 kinase, similar to Na+/K+-ATPase inhibition by the classic cardiac glycoside digoxin. ATP1B2 is expressed higher in glioblastoma stem-like cells (GSCs) than in GBM cells and its downregulation induces apoptosis of GSCs. Furthermore, inducible ATP1B2 knockdown significantly inhibit tumor growth in vivo. Our data suggest ATP1B2 has potential as a therapeutic target for GBM.

A Functional Interaction Between Na,K-ATPase β2-Subunit/AMOG and NF2/Merlin Regulates Growth Factor Signaling in Cerebellar Granule Cells

The Na,K-ATPase, consisting of a catalytic α-subunit and a regulatory β-subunit, is a ubiquitously expressed ion pump that carries out the transport of Na⁺ and K⁺ across the plasma membranes of most animal cells. In addition to its pump function, Na,K-ATPase serves as a signaling scaffold and a cell adhesion molecule. Of the three β-subunit isoforms, β₁ is found in almost all tissues, while β₂ expression is mostly restricted to brain and muscle. In cerebellar granule cells, the β₂-subunit, also known as adhesion molecule on glia (AMOG), has been linked to neuron-astrocyte adhesion and granule cell migration, suggesting its role in cerebellar development. Nevertheless, little is known about molecular pathways that link the β₂-subunit to its cellular functions. Using cerebellar granule precursor cells, we found that the β₂-subunit, but not the β₁-subunit, negatively regulates the expression of a key activator of the Hippo/YAP signaling pathway, Merlin/neurofibromin-2 (NF2). The knockdown of the β₂-subunit resulted in increased Merlin/NF2 expression and affected downstream targets of Hippo signaling, i.e., increased YAP phosphorylation and decreased expression of N-Ras. Further, the β₂-subunit knockdown altered the kinetics of epidermal growth factor receptor (EGFR) signaling in a Merlin-dependent mode and impaired EGF-induced reorganization of the actin cytoskeleton. Therefore, our studies for the first time provide a functional link between the Na,K-ATPase β₂-subunit and Merlin/NF2 and suggest a role for the β₂-subunit in regulating cytoskeletal dynamics and Hippo/YAP signaling during neuronal differentiation.

Identification of the retinoschisin-binding site on the retinal Na/K-ATPase

X-linked juvenile retinoschisis (XLRS) is a hereditary retinal dystrophy, caused by mutations in the RS1 gene which encodes the secreted protein retinoschisin. In recent years, several molecules have been proposed to interact with retinoschisin, including the retinal Na/K-ATPase, L-voltage gated Ca²⁺ channels, and specific sugars. We recently showed that the retinal Na/K-ATPase consisting of subunits ATP1A3 and ATP1B2 is essential for anchoring retinoschisin to plasma membranes and identified the glycosylated ATP1B2 subunit as the direct interaction partner for retinoschisin. We now aimed to precisely map the retinoschisin binding domain(s) in ATP1B2. In general, retinoschisin binding was not affected after selective elimination of individual glycosylation sites via site-directed mutagenesis as well as after full enzymatic deglycosylation of ATP1B2. Applying the interface prediction tool PresCont, two putative protein-protein interaction patches (“patch I” and “patch II”) consisting each of four hydrophobic amino acid stretches on the ATP1B2 surface were identified. These were consecutively altered by site-directed mutagenesis. Functional assays with the ATP1B2 patch mutants identified patch II and, specifically, the associated amino acid at position 240 (harboring a threonine in ATP1B2) as crucial for retinoschisin binding to ATP1B2. These and previous results led us to suggest an induced-fit binding mechanism for the interaction between retinoschisin and the Na/K-ATPase, which is dependent on threonine 240 in ATP1B2 allowing the accommodation of hyperflexible retinoschisin spikes by the associated protein-protein interaction patch on ATP1B2.

Differential expression patterns of sodium potassium ATPase alpha and beta subunit isoforms in mouse brain during postnatal development

The sodium potassium ATPase (Na⁺/K⁺ ATPase) is essential for the maintenance of a low intracellular Na⁺ and a high intracellular K⁺ concentration. Loss of function of the Na⁺/K⁺ ATPase due to mutations in Na⁺/K⁺ ATPase genes, anoxic conditions, depletion of ATP or inhibition of the Na⁺/K⁺ ATPase function using cardiac glycosides such as digitalis, causes a depolarization of the resting membrane potential. While in non-excitable cells, the uptake of glucose and amino acids is decreased if the function of the Na⁺/K⁺ ATPase is compromised, in excitable cells the symptoms range from local hyper-excitability to inactivating depolarization. Although several studies have demonstrated the differential expression of the various Na⁺/K⁺ ATPase alpha and beta isoforms in the brain tissue of rodents, their expression profile during development has yet to be thoroughly investigated. An immunohistochemical analysis of postnatal day 19 mouse brain showed ubiquitous expression of Na⁺/K⁺ ATPase isoforms α1, β1 and β2 in both neurons and glial cells, whereas α2 was expressed mostly in glial cells and the α3 and β3 isoforms were expressed in neurons. Furthermore, we examined potential changes in the relative expression of the different Na⁺/K⁺ ATPase isoforms in different brain areas of postnatal day 6 and in adult 9 months old animals using immunoblot analysis. Our results show a significant up-regulation of the α1 isoform in cortex, hippocampus and cerebellum, whereas, the α2 isoform was significantly up-regulated in midbrain. The β3 isoform showed a significant up-regulation in all brain areas investigated. The up-regulation of the α3 isoform matched that of the β2 isoform which were both significantly up-regulated in cortex, hippocampus and midbrain, suggesting that the increased maturation of the neuronal network is accompanied by an increase in expression of α3/β2 complexes in these brain structures.

Cell adhesion and matricellular support by astrocytes of the tripartite synapse

Astrocytes contribute to the formation, function, and plasticity of synapses. Their processes enwrap the neuronal components of the tripartite synapse, and due to this close interaction they are perfectly positioned to modulate neuronal communication. The interaction between astrocytes and synapses is facilitated by cell adhesion molecules and matricellular proteins, which have been implicated in the formation and functioning of tripartite synapses. The importance of such neuron-astrocyte integration at the synapse is underscored by the emerging role of astrocyte dysfunction in synaptic pathologies such as autism and schizophrenia. Here we review astrocyte-expressed cell adhesion molecules and matricellular molecules that play a role in integration of neurons and astrocytes within the tripartite synapse.

A SINE Insertion in ATP1B2 in Belgian Shepherd Dogs Affected by Spongy Degeneration with Cerebellar Ataxia (SDCA2)

Spongy degeneration with cerebellar ataxia (SDCA) is a genetically heterogeneous neurodegenerative disorder with autosomal recessive inheritance in Malinois dogs, one of the four varieties of the Belgian Shepherd breed. Using a combined linkage and homozygosity mapping approach we identified an ∼10.6 Mb critical interval on chromosome 5 in a Malinois family with four puppies affected by cerebellar dysfunction. Visual inspection of the 10.6 Mb interval in whole-genome sequencing data from one affected puppy revealed a 227 bp SINE insertion into the ATP1B2 gene encoding the β₂ subunit of the Na⁺/K⁺-ATPase holoenzyme (ATP1B2:c.130_131insLT796559.1:g.50_276). The SINE insertion caused aberrant RNA splicing. Immunohistochemistry suggested a reduction of ATP1B2 protein expression in the central nervous system of affected puppies. Atp1b2 knockout mice had previously been reported to show clinical and neurohistopathological findings similar to the affected Malinois puppies. Therefore, we consider ATP1B2:c.130_131ins227 the most likely candidate causative variant for a second subtype of SDCA in Malinois dogs, which we propose to term spongy degeneration with cerebellar ataxia subtype 2 (SDCA2). Our study further elucidates the genetic and phenotypic complexity underlying cerebellar dysfunction in Malinois dogs and provides the basis for a genetic test to eradicate one specific neurodegenerative disease from the breeding population in Malinois and the other varieties of the Belgian Shepherd breed. ATP1B2 thus represents another candidate gene for human inherited cerebellar ataxias, and SDCA2-affected Malinois puppies may serve as a naturally occurring animal model for this disorder.

The Structure and Function of the Na,K-ATPase Isoforms in Health and Disease

The sodium and potassium gradients across the plasma membrane are used by animal cells for numerous processes, and the range of demands requires that the responsible ion pump, the Na,K-ATPase, can be fine-tuned to the different cellular needs. Therefore, several isoforms are expressed of each of the three subunits that make a Na,K-ATPase, the alpha, beta and FXYD subunits. This review summarizes the various roles and expression patterns of the Na,K-ATPase subunit isoforms and maps the sequence variations to compare the differences structurally. Mutations in the Na,K-ATPase genes encoding alpha subunit isoforms have severe physiological consequences, causing very distinct, often neurological diseases. The differences in the pathophysiological effects of mutations further underline how the kinetic parameters, regulation and proteomic interactions of the Na,K-ATPase isoforms are optimized for the individual cellular needs.

An immunoaffinity-based method for isolating ultrapure adult astrocytes based on ATP1B2 targeting by the ACSA-2 antibody

Astrocytes are a major cell type in the mammalian CNS. Astrocytes are now known to play a number of essential roles in processes including synapse formation and function, as well as blood-brain barrier formation and control of cerebral blood flow. However, our understanding of the molecular mechanisms underlying astrocyte development and function is still rudimentary. This lack of knowledge is at least partly due to the lack of tools currently available for astrocyte biology. ACSA-2 is a commercially available antibody originally developed for the isolation of astrocytes from young postnatal mouse brain, using magnetic cell-sorting methods, but its utility in isolating cells from adult tissue has not yet been published. Using a modified protocol, we now show that this tool can also be used to isolate ultrapure astrocytes from the adult brain. Furthermore, using a variety of techniques (including single-cell sequencing, overexpression and knockdown assays, immunoblotting, and immunohistochemistry), we identify the ACSA-2 epitope for the first time as ATP1B2 and characterize its distribution in the CNS. Finally, we show that ATP1B2 is stably expressed in multiple models of CNS injury and disease. Hence, we show that the ACSA-2 antibody possesses the potential to be an extremely valuable tool for astrocyte research, allowing the purification and characterization of astrocytes (potentially including injury and disease models) without the need for any specialized and expensive equipment. In fact, our results suggest that ACSA-2 should be a first-choice method for astrocyte isolation and characterization.

The X-linked juvenile retinoschisis protein retinoschisin is a novel regulator of mitogen-activated protein kinase signalling and apoptosis in the retina

X-linked juvenile retinoschisis (XLRS) is a hereditary retinal dystrophy in young males, caused by mutations in the RS1 gene. The function of the encoded protein, termed retinoschisin, and the molecular mechanisms underlying XLRS pathogenesis are still unresolved, although a direct interaction partner of the secreted retinoschisin, the retinal Na/K-ATPase, was recently identified. Earlier gene expression studies in retinoschisin-deficient (Rs1h^-/Y ) mice provided a first indication of pathological up-regulation of mitogen-activated protein (MAP) kinase signalling in disease pathogenesis. To further investigate the role for retinoschisin in MAP kinase regulation, we exposed Y-79 cells and murine Rs1h^-/Y retinae to recombinant retinoschisin and the XLRS-associated mutant RS1-C59S. Although normal retinoschisin stably bound to retinal cells, RS1-C59S exhibited a strongly reduced binding affinity. Simultaneously, exposure to normal retinoschisin significantly reduced phosphorylation of C-RAF and MAP kinases ERK1/2 in Y-79 cells and murine Rs1h^-/Y retinae. Expression of MAP kinase target genes C-FOS and EGR1 was also down-regulated in both model systems. Finally, retinoschisin treatment decreased pro-apoptotic BAX-2 transcript levels in Y-79 cells and Rs1h^-/Y retinae. Upon retinoschisin treatment, these cells showed increased resistance against apoptosis, reflected by decreased caspase-3 activity (in Y-79 cells) and increased photoreceptor survival (in Rs1h^-/Y retinal explants). RS1-C59S did not influence C-RAF or ERK1/2 activation, C-FOS or EGR1 expression, or apoptosis. Our data imply that retinoschisin is a novel regulator of MAP kinase signalling and exerts an anti-apoptotic effect on retinal cells. We therefore discuss that disturbances of MAP kinase signalling by retinoschisin deficiency could be an initial step in XLRS pathogenesis.

Selective Assembly of Na,K-ATPase α2β2 Heterodimers in the Heart: DISTINCT FUNCTIONAL PROPERTIES AND ISOFORM-SELECTIVE INHIBITORS

The Na,K-ATPase α₂ subunit plays a key role in cardiac muscle contraction by regulating intracellular Ca²⁺, whereas α₁ has a more conventional role of maintaining ion homeostasis. The β subunit differentially regulates maturation, trafficking, and activity of α-β heterodimers. It is not known whether the distinct role of α₂ in the heart is related to selective assembly with a particular one of the three β isoforms. We show here by immunofluorescence and co-immunoprecipitation that α₂ is preferentially expressed with β₂ in T-tubules of cardiac myocytes, forming α₂β₂ heterodimers. We have expressed human α₁β₁, α₂β₁, α₂β₂, and α₂β₃ in Pichia pastoris, purified the complexes, and compared their functional properties. α₂β₂ and α₂β₃ differ significantly from both α₂β₁ and α₁β₁ in having a higher K_0.5K⁺ and lower K_0.5Na⁺ for activating Na,K-ATPase. These features are the result of a large reduction in binding affinity for extracellular K⁺ and shift of the E₁P-E₂P conformational equilibrium toward E₁P. A screen of perhydro-1,4-oxazepine derivatives of digoxin identified several derivatives (e.g. cyclobutyl) with strongly increased selectivity for inhibition of α₂β₂ and α₂β₃ over α₁β₁ (range 22-33-fold). Molecular modeling suggests a possible basis for isoform selectivity. The preferential assembly, specific T-tubular localization, and low K⁺ affinity of α₂β₂ could allow an acute response to raised ambient K⁺ concentrations in physiological conditions and explain the importance of α₂β₂ for cardiac muscle contractility. The high sensitivity of α₂β₂ to digoxin derivatives explains beneficial effects of cardiac glycosides for treatment of heart failure and potential of α₂β₂-selective digoxin derivatives for reducing cardiotoxicity.

Na,K-ATPase Isozymes in Colorectal Cancer and Liver Metastases

The goal of this study was to define Na,K-ATPase α and β subunit isoform expression and isozyme composition in colorectal cancer cells and liver metastases. The α1, α3, and β1 isoforms were the most highly expressed in tumor cells and metastases; in the plasma membrane of non-neoplastic cells and mainly in a cytoplasmic location in tumor cells. α1β1 and α3β1 isozymes found in tumor and metastatic cells exhibit the highest and lowest Na⁺ affinity respectively and the highest K⁺ affinity. Mesenchymal cell isozymes possess an intermediate Na⁺ affinity and a low K⁺ affinity. In cancer, these ions are likely to favor optimal conditions for the function of nuclear enzymes involved in mitosis, especially a high intra-nuclear K⁺ concentration. A major and striking finding of this study was that in liver, metastasized CRC cells express the α3β1 isozyme. Thus, the α3β1 isozyme could potentially serve as a novel exploratory biomarker of CRC metastatic cells in liver.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Na⁺/K⁺-ATPase β2-subunit (AMOG) expression abrogates invasion of glioblastoma-derived brain tumor-initiating cells

BACKGROUND

Mechanisms of glioma invasion remain to be fully elucidated. Glioma cells within glioblastoma multiforme (GBM) range from well-differentiated tumor cells to less-differentiated brain tumor-initiating cells (BTICs). The β2-subunit of Na(+)/K(+)-ATPase, called the adhesion molecule on glia (AMOG), is highly expressed in normal glia but is thought to be universally downregulated in GBM. To test our hypothesis that expression of AMOG is heterogeneous in GBM and confers a less invasive phenotype, we compared it between BTICs and differentiated cells from patient-matched GBM and then tested GBM invasion in vitro after AMOG overexpression.

METHODS

Immunohistochemistry, immunoblotting, and real-time PCR were used to characterize AMOG protein and mRNA expression in tumor samples, BTICs, and differentiated cells. Matrigel invasion assay, scratch assay, and direct cell counting were used for testing in vitro invasion, migration, and proliferation, respectively.

RESULTS

While AMOG expression is heterogeneous in astrocytomas of grades II-IV, it is lost in most GBM. BTICs express higher levels of AMOG mRNA and protein compared with patient-matched differentiated tumor cells. Overexpression of AMOG decreased GBM cell and BTIC invasion without affecting migration or proliferation. Knockdown of AMOG expression in normal human astrocytes increased invasion.

CONCLUSIONS

AMOG expression inhibits GBM invasion. Its downregulation increases invasion in glial cells and may also represent an important step in BTIC differentiation. These data provide compelling evidence implicating the role of AMOG in glioma invasion and provide impetus for further investigation.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

Subunit isoform selectivity in assembly of Na,K-ATPase α-β heterodimers

To catalyze ion transport, the Na,K-ATPase must contain one α and one β subunit. When expressed by transfection in various expression systems, each of the four α subunit isoforms can assemble with each of the three β subunit isoforms and form an active enzyme, suggesting the absence of selective α-β isoform assembly. However, it is unknown whether in vivo conditions the α-β assembly is random or isoform-specific. The α(2)-β(2) complex was selectively immunoprecipitated by both anti-α(2) and anti-β(2) antibodies from extracts of mouse brain, which contains cells co-expressing multiple Na,K-ATPase isoforms. Neither α(1)-β(2) nor α(2)-β(1) complexes were detected in the immunoprecipitates. Furthermore, in MDCK cells co-expressing α(1), β(1), and β(2) isoforms, a greater fraction of the β(2) subunits was unassembled with α(1) as compared with that of the β(1) subunits, indicating preferential association of the α(1) isoform with the β(1) isoform. In addition, the α(1)-β(2) complex was less resistant to various detergents than the α(1)-β(1) complex isolated from MDCK cells or the α(2)-β(2) complex isolated from mouse brain. Therefore, the diversity of the α-β Na,K-ATPase heterodimers in vivo is determined not only by cell-specific co-expression of particular isoforms, but also by selective association of the α and β subunit isoforms.

X-linked juvenile retinoschisis: clinical diagnosis, genetic analysis, and molecular mechanisms

X-linked juvenile retinoschisis (XLRS, MIM 312700) is a common early onset macular degeneration in males characterized by mild to severe loss in visual acuity, splitting of retinal layers, and a reduction in the b-wave of the electroretinogram (ERG). The RS1 gene (MIM 300839) associated with the disease encodes retinoschisin, a 224 amino acid protein containing a discoidin domain as the major structural unit, an N-terminal cleavable signal sequence, and regions responsible for subunit oligomerization. Retinoschisin is secreted from retinal cells as a disulphide-linked homo-octameric complex which binds to the surface of photoreceptors and bipolar cells to help maintain the integrity of the retina. Over 190 disease-causing mutations in the RS1 gene are known with most mutations occurring as non-synonymous changes in the discoidin domain. Cell expression studies have shown that disease-associated missense mutations in the discoidin domain cause severe protein misfolding and retention in the endoplasmic reticulum, mutations in the signal sequence result in aberrant protein synthesis, and mutations in regions flanking the discoidin domain cause defective disulphide-linked subunit assembly, all of which produce a non-functional protein. Knockout mice deficient in retinoschisin have been generated and shown to display most of the characteristic features found in XLRS patients. Recombinant adeno-associated virus (rAAV) mediated delivery of the normal RS1 gene to the retina of young knockout mice result in long-term retinoschisin expression and rescue of retinal structure and function providing a 'proof of concept' that gene therapy may be an effective treatment for XLRS.

Na(+), K(+)-ATPase subunit composition in a human chondrocyte cell line; evidence for the presence of α1, α3, β1, β2 and β3 isoforms

Membrane transport systems participate in fundamental activities such as cell cycle control, proliferation, survival, volume regulation, pH maintenance and regulation of extracellular matrix synthesis. Multiple isoforms of Na(+), K(+)-ATPase are expressed in primary chondrocytes. Some of these isoforms have previously been reported to be expressed exclusively in electrically excitable cells (i.e., cardiomyocytes and neurons). Studying the distribution of Na(+), K(+)-ATPase isoforms in chondrocytes makes it possible to document the diversity of isozyme pairing and to clarify issues concerning Na(+), K(+)-ATPase isoform abundance and the physiological relevance of their expression. In this study, we investigated the expression of Na(+), K(+)-ATPase in a human chondrocyte cell line (C-20/A4) using a combination of immunological and biochemical techniques. A panel of well-characterized antibodies revealed abundant expression of the α1, β1 and β2 isoforms. Western blot analysis of plasma membranes confirmed the above findings. Na(+), K(+)-ATPase consists of multiple isozyme variants that endow chondrocytes with additional homeostatic control capabilities. In terms of Na(+), K(+)-ATPase expression, the C-20/A4 cell line is phenotypically similar to primary and in situ chondrocytes. However, unlike freshly isolated chondrocytes, C-20/A4 cells are an easily accessible and convenient in vitro model for the study of Na(+), K(+)-ATPase expression and regulation in chondrocytes.

The Na+-K+-ATPase as self-adhesion molecule and hormone receptor

Thanks to the homeostasis of the internal milieu, metazoan cells can enormously simplify their housekeeping efforts and engage instead in differentiation and multiple forms of organization (tissues, organs, systems) that enable them to produce an astonishing diversity of mammals. The stability of the internal milieu despite drastic variations of the external environment (air, fresh or seawater, gastrointestinal fluids, glomerular filtrate, bile) is due to transporting epithelia that can adjust their specific permeability to H(2)O, H(+), Na(+), K(+), Ca(2+), and Cl(-) over several orders of magnitude and exchange substances with the outer milieu with exquisite precision. This exchange is due to the polarized expression of membrane proteins, among them Na(+)-K(+)-ATPase, an oligomeric enzyme that uses chemical energy from ATP molecules to translocate ions across the plasma membrane of epithelial cells. Na(+)-K(+)-ATPase presents two types of asymmetries: the arrangement of its subunits, and its expression in one pole of the epithelial cell ("polarity"). In most epithelia, polarity consists of the expression of Na(+)-K(+)-ATPase towards the intercellular space and arises in part from the interaction of the extracellular segment of the β-subunit with another β-subunit present in a Na(+)-K(+)-ATPase molecule expressed by a neighboring cell. In addition to enabling the Na(+)-K(+)-ATPase to transport ions and water vectorially, this position exposes its receptors to ouabain and analogous cardiotonic steroids, which are present in the internal milieu because these were secreted by endocrine cells.

Assembly with the Na,K-ATPase alpha(1) subunit is required for export of beta(1) and beta(2) subunits from the endoplasmic reticulum

The level of the heterodimeric Na,K-ATPase is tightly controlled in epithelia to maintain appropriate transport function. The catalytic Na,K-ATPase alpha subunit is not able to exit the ER or catalyze ion transport unless assembled with the beta subunit. However, requirements for the ER exit of the Na,K-ATPase beta subunit that plays an additional, ion-transport-independent, role in intercellular adhesion are not clear. Exogenous beta(1) or beta(2) subunits expressed in renal MDCK cells replace endogenous beta(1) subunits in the alpha-beta complexes in the ER, resulting in a decrease in the amount of the alpha(1)-bound endogenous beta(1) subunits by 47-61% with no change in the amount of alpha(1) subunits. Disruption of the alpha(1)-beta association by mutations in defined alpha(1)-interacting regions of either beta(1) or beta(2) subunits results in the ER retention and rapid degradation of unassembled mutants. Hence, the ER quality control system allows export only of assembled alpha-beta complexes to the Golgi, thereby maintaining an equimolar ratio of alpha and beta subunits in the plasma membrane, whereas the number of alpha(1) subunits in the ER determines the amount of the alpha-beta complexes.

Spontaneous calcium spike activity in embryonic spinal neurons is regulated by developmental expression of the Na+, K+-ATPase beta3 subunit

Different types and patterns of spontaneous electrical activity drive many aspects of neuronal differentiation. Neurons in the developing Xenopus spinal cord exhibit calcium spikes, which regulate gene transcription and neurotransmitter specification. The ionic currents necessary for spike production have been described. However, the mechanisms that generate the onset of this activity and the basis of its regulation remain unclear. Although signaling molecules appear to act on plasma membrane receptors to trigger calcium spike activity, other mechanisms for spontaneous calcium spike regulation may exist as well. Here, we analyze the developmental expression of the Na(+), K(+)-ATPase beta3 subunit in Xenopus tropicalis embryos and show that its levels are downregulated at a time during embryonic development that coincides with the onset of prominent calcium spike activity in spinal neurons. Inhibition of an earlier increase in beta3 expression leads to more depolarized resting membrane potentials and results in later reduction of spike activity. This suppression of beta3 levels also reduces expression of the store-operated calcium channel subunit, Orai1. These findings suggest that the Na(+), K(+)-ATPase plays a role in initiating calcium spike activity and regulating calcium homeostasis.

Comparative epigenomics of human and mouse mammary tumors

Gene silencing by aberrant epigenetic chromatin alteration is a well-recognized event contributing to tumorigenesis. Although genetically engineered tumor-prone mouse models have proven a powerful tool in understanding many aspects of carcinogenesis, to date few studies have focused on epigenetic alterations in mouse tumors. To uncover epigenetically silenced tumor suppressor genes (TSGs) in mouse mammary tumor cells, we conducted initial genome-wide screening by combining the treatment of cultured cells with the DNA demethylating drug 5-aza-2'-deoxycytidine (5-azadC) and the histone deacetylase inhibitor trichostatin A (TSA) with expression microarray. By conducting this initial screen on EMT6 cells and applying protein function and genomic structure criteria to genes identified as upregulated in response to 5-azadC/TSA, we were able to identify two characterized breast cancer TSGs (Timp3 and Rprm) and four putative TSGs (Atp1B2, Dusp2, FoxJ1 and Smpd3) silenced in this line. By testing a panel of 10 mouse mammary tumor lines, we determined that each of these genes is commonly hypermethylated, albeit with varying frequency. Furthermore, by examining a panel of human breast tumor lines and primary tumors we observed that the human orthologs of ATP1B2, FOXJ1 and SMPD3 are aberrantly hypermethylated in the human disease whereas DUSP2 was not hypermethylated in primary breast tumors. Finally, we examined hypermethylation of several genes targeted for epigenetic silencing in human breast tumors in our panel of 10 mouse mammary tumor lines. We observed that the orthologs of Cdh1, RarB, Gstp1, RassF1 genes were hypermethylated, whereas neither Dapk1 nor Wif1 were aberrantly methylated in this panel of mouse tumor lines. From this study, we conclude that there is significant, but not absolute, overlap in the epigenome of human and mouse mammary tumors.

Cell adhesion molecules in the central nervous system

Cell-cell adhesion molecules play key roles at the intercellular junctions of a wide variety of cells, including interneuronal synapses and neuron-glia contacts. Functional studies suggest that adhesion molecules are implicated in many aspects of neural network formation, such as axon-guidance, synapse formation, regulation of synaptic structure and astrocyte-synapse contacts. Some basic cell biological aspects of the assembly of junctional complexes of neurons and glial cells resemble those of epithelial cells. However, the neuron specific junctional machineries are required to exert neuronal functions, such as synaptic transmission and plasticity. In this review, we describe the distribution and function of cell adhesion molecules at synapses and at contacts between synapses and astrocytes.

Prion protein regulates glutamate-dependent lactate transport of astrocytes

Prion-related protein (PrP) is a neural cell adhesion molecule involved in neurite outgrowth, neuronal survival, and synaptic function. In search of novel binding partners for PrP, we identified the alpha2/beta2-Na+/K+-ATPase and showed that this astroglial ATPase interacts directly with the immunoglobulin superfamily adhesion molecule basigin. In cultured astrocytes, PrP is involved in regulating lactate transport via the astroglial monocarboxylate transporter 1 (MCT1) and in conjunction with alpha2/beta2-ATPase and basigin. Lactate transport via MCT1 is glutamate dependent and regulated by glutamate receptor 2 (GluR2)-containing AMPA receptors with which PrP interacts. The functional interplay between PrP, GluR2, alpha2/beta2-ATPase, basigin, and MCT1 in regulating lactate transport of astrocytes may be functional in the metabolic cross talk between astrocytes and neurons, most likely under stress.

Retinoschisin (RS1), the Protein Encoded by the X-linked Retinoschisis Gene, Is Anchored to the Surface of Retinal Photoreceptor and Bipolar Cells through Its Interactions with a Na/K ATPase-SARM1 Complex

Retinoschisin or RS1 is a discoidin domain-containing protein encoded by the gene responsible for X-linked retinoschisis (XLRS), an early onset macular degeneration characterized by a splitting of the retina. Retinoschisin, expressed and secreted from photoreceptors and bipolar cells as a homo-octameric complex, associates with the surface of these cells where it serves to maintain the cellular organization of the retina and the photoreceptor-bipolar synaptic structure. To gain insight into the role of retinoschisin in retinal cell adhesion and the pathogenesis of XLRS, we have investigated membrane components in retinal extracts that interact with retinoschisin. Unlike the discoidin domain-containing blood coagulation proteins Factor V and Factor VIII, retinoschisin did not bind to phospholipids or retinal lipids reconstituted into unilamellar vesicles or immobilized on microtiter plates. Instead, co-immunoprecipitation studies together with mass spectrometric-based proteomics and Western blotting showed that retinoschisin is associated with a complex consisting of Na/K ATPase (alpha3, beta2 isoforms) and the sterile alpha and TIR motif-containing protein SARM1. Double labeling studies for immunofluorescence microscopy confirmed the co-localization of retinoschisin with Na/K ATPase and SARM1 in photoreceptors and bipolar cells of retina tissue. We conclude that retinoschisin binds to Na/K ATPase on photoreceptor and bipolar cells. This interaction may be part of a novel SARM1-mediated cell signaling pathway required for the maintenance of retinal cell organization and photoreceptor-bipolar synaptic structure.

Families of neural adhesion molecules

The neural cell adhesion molecules L1 and N-CAM share a common carbohydrate epitope that is recognized by the monoclonal antibodies L2 and HNK-1. The L2/HNK-1 epitope is also present on the myelin-associated glycoprotein (MAG) and secreted J1 glycoprotein, both of which have been identified as cell adhesion molecules. Each of the four adhesion molecules is differentially expressed during development on distinct cell types. Expression of the L2/HNK-1 epitope is regulated independently of the protein backbone, is phylogenetically conserved, and plays a role in cell-cell and, particularly, cell-substrate interactions. Another set of glycoproteins shares a common carbohydrate epitope designated L3. This epitope is present on the novel adhesion molecule on glia (AMOG), L1 and MAG, but not on J1 and N-CAM. As in the L2/HNK-1 family, the number of glycoproteins expressing this epitope is not yet known. It is therefore possible that heterogeneities in carbohydrate structures are associated with different sets of adhesion molecules and may have functional implications.

Identification of novel genes associated with astrocytoma progression using suppression subtractive hybridization and real-time reverse transcription-polymerase chain reaction

To identify novel genes involved in glioma progression we performed suppression subtractive hybridization combined with cDNA array analysis on 4 patients with primary low-grade gliomas of World Health Organization (WHO) grade II that recurred as secondary glioblastomas (WHO grade IV). Eight genes showing differential expression between primary and recurrent tumors in 3 of the 4 patients were selected for further analysis using real-time reverse transcription-PCR on a series of 10 pairs of primary low-grade and recurrent high-grade gliomas as well as 42 astrocytic gliomas of different WHO grades. These analyses revealed that 5 genes, i.e., AMOG (ATP1B2, 17p13.1), APOD (3q26.2-qter), DMXL1 (5q23.1) DRR1 (TU3A, 3p14.2) and PSD3 (KIAA09428/HCA67/EFA6R, 8p22), were expressed at significantly lower levels in secondary glioblastomas as compared to diffuse astrocytomas of WHO grade II. In addition, AMOG, DRR1 and PSD3 transcript levels were significantly lower in primary glioblastomas than in diffuse astrocytomas. Treatment of glioma cell lines with 5-aza-2'-deoxycytidine and trichostatin A resulted in increased expression of AMOG and APOD transcripts. Sequencing of sodium bisulfite-modified DNA demonstrated AMOG promoter hypermethylation in the glioma cell lines and 1 primary anaplastic astrocytoma with low AMOG expression. Taken together, we identified interesting novel candidate genes that likely contribute to glioma progression and provide first evidence for a role of epigenetic silencing of AMOG in malignant glioma cells.

Akt-dependent cell size regulation by the adhesion molecule on glia occurs independently of phosphatidylinositol 3-kinase and Rheb signaling

The role of cell adhesion molecules in mediating interactions with neighboring cells and the extracellular matrix has long been appreciated. More recently, these molecules have been shown to modulate intracellular signal transduction cascades critical for cell growth and proliferation. Expression of adhesion molecule on glia (AMOG) is downregulated in human and mouse gliomas, suggesting that AMOG may be important for growth regulation in the brain. In this report, we examined the role of AMOG expression on cell growth and intracellular signal transduction. We show that AMOG does not negatively regulate cell growth in vitro or in vivo. Instead, expression of AMOG in AMOG-deficient cells results in a dramatic increase in cell size associated with protein kinase B/Akt hyperactivation, which occurs independent of phosphatidylinositol 3-kinase activation. AMOG-mediated Akt phosphorylation specifically activates the mTOR/p70S6 kinase pathway previously implicated in cell size regulation, but it does not depend on tuberous sclerosis complex/Ras homolog enriched in brain (Rheb) signaling. These data support a novel role for a glial adhesion molecule in cell size regulation through selective activation of the Akt/mTOR/S6K signal transduction pathway.

Expression, regulation and function of Na,K-ATPase in the lens

Na,K-ATPase is responsible for maintaining the correct concentrations of sodium and potassium in lens cells. Na,K-ATPase activity is different in the two cell types that make up the lens, epithelial cells and fibers; specific activity in the epithelium is higher than in fibers. In some parts of the fiber mass Na,K-ATPase activity is barely detectable. There is a large body of evidence that suggests Na,K-ATPase-mediated ion transport by the epithelium contributes significantly to the regulation of ionic composition in the entire lens. In some species different Na,K-ATPase isoforms are present in epithelium and fibers but in general, fibers and epithelium express a similar amount of Na,K-ATPase protein. Turnover of Na,K-ATPase by protein synthesis may contribute to preservation of high Na,K-ATPase activity in the epithelium. In ageing lens fibers, oxidation, and glycation may decrease Na,K-ATPase activity. Na,K-ATPase activity in lens fibers and epithelium also may be subject to regulation as the result of protein tyrosine phosphorylation. Moreover, activation of G protein-coupled receptors by agonists such as endothelin-1 elicits changes of Na,K-ATPase activity. The asymmetrical distribution of Na,K-ATPase activity in the epithelium and fibers may contribute to ionic currents that flow in and around the lens. Studies on human cataract and experimental cataract in animals reveal changes of Na,K-ATPase activity but no clear pattern is evident. However, there is a convincing link between abnormal elevation of lens sodium and the opacification of the lens cortex that occurs in age-related human cataract.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Involvement of cell surface HSP90 in cell migration reveals a novel role in the developing nervous system

Heat shock protein HSP90 plays important roles in cellular regulation, primarily as a chaperone for a number of key intracellular proteins. We report here that the two HSP90 isoforms, alpha and beta, also localize on the surface of cells in the nervous system and are involved in their migration. A 94-kDa surface antigen, the 4C5 antigen, which was previously shown to be involved in migration processes during development of the nervous system, is shown to be identical to HSP90alpha using mass spectrometry analysis. This identity is further confirmed by immunoprecipitation experiments and by induction of 4C5 antigen expression in heat shock-treated embryonic rat brain cultures. Moreover, immunocytochemistry on live cerebellar rat cells reveals cell surface localization of both HSP90alpha and -beta. Cell migration from cerebellar and sciatic nerve explants is inhibited by anti-HSP90alpha and anti-HSP90beta antibodies, similarly to the inhibition observed with monoclonal antibody 4C5. Moreover, immunostaining with rhodamine-phalloidin of migrating Schwann cells cultured in the presence of antibodies against both alpha and beta isoforms of HSP90 reveals that HSP90 activity is associated with actin cytoskeletal organization, necessary for lamellipodia formation.

Axonal Regeneration of Fish Optic Nerve after Injury

Since Sperry's work in the 1950s, it has been known that the central nervous system (CNS) neurons of lower vertebrates such as fish and amphibians can regenerate after axotomy, whereas the CNS neurons of mammals become apoptotic after axotomy. The goldfish optic nerve (ON) is one of the most studied animal models of CNS regeneration. Morphological changes in the goldfish retina and tectum after ON transection were first researched in the 1970s-1980s. Many biochemical studies of neurite outgrowth-promoting substances were then carried out in the 1980s-1990s. Many factors have been reported to be active substances that show increased levels during fish ON regeneration, as shown by using various protein chemistry techniques. However, there are very few molecular cloning techniques for studying ON regeneration after injury. In this review article, we summarize the neurite outgrowth-promoting factors reported by other researchers and describe our strategies for searching for ON regenerating molecules using a differential hybridization technique in the goldfish visual system. The process of goldfish ON regeneration after injury is very long. It takes about half a year from the start of axonal regrowth to complete restoration of vision. The process has been classified into three stages: early, middle and late. We screened for genes with increased expression during regeneration using axotomized goldfish retinal and tectal cDNA libraries and obtained stage-specific cDNA clones that were upregulated in the retina and tectum. We further discuss functional roles of these molecules in the regeneration processes of goldfish ON.

Development of midline glial populations at the corticoseptal boundary

Three midline glial populations are found at the corticoseptal boundary: the glial wedge (GW), glia within the indusium griseum (IGG), and the midline zipper glia (MG). Two of these glial populations are involved in axonal guidance at the cortical midline, specifically development of the corpus callosum. Here we investigate the phenotypic and molecular characteristics of each population and determine whether they are generated at the same developmental stage. We find that the GW is derived from the radial glial scaffold of the cortex. GW cells initially have long radial processes that extend from the ventricular surface to the pial surface, but by E15 loose their pial attachment and extend only part of the way to the pial surface. Later in development the radial morphology of cells within the GW is replaced by multipolar astrocytes, providing supportive evidence that radial glia can transform into astrocytes. IGG and MG do not have a radial morphology and do not label with the radial glial markers, Nestin and RC2. We conclude that the GW and IGG have different morphological and molecular characteristics and are born at different stages of development. IGG and MG have many phenotypic and molecular characteristics in common, indicating that they may represent a common population of glia that becomes spatially distinct by the formation of the corpus callosum.

Characterization of nucleotide pyrophosphatase-5 as an oligomannosidic glycoprotein in rat brain

Membrane glycoproteins of neural cells play crucial roles in axon guidance, synaptogenesis, and neuronal transmission. We have here characterized membrane glycoproteins containing terminal alpha-mannose residues in rat brain membranes. Affinity purification using Galanthus nivalis agglutinin, that is highly specific for terminal alpha-mannose residues, revealed a 50-kDa protein as well as 80-kDa SHPS-1 and 45-kDa beta2 subunit of Na,K-ATPase in rat brain membranes. Combination of N-terminal peptide sequencing and mass spectrometry indicated that the 50-kDa protein was rat nucleotide pyrophosphatase-5 (NPP-5). In contrast to other NPPs, NPP-5 was a type-I transmembrane protein. Northern blot analysis showed that NPP-5 was highly expressed in brain, but also expressed in other peripheral tissues. However, we could not detect either the NPP activity or the lysophospholipase D activity in the immunoprecipitates with antibodies to NPP-5 from rat brain membranes. These data, therefore, suggest that NPP-5 is a neural oligomannosidic glycoprotein that may participate in neural cell communications.

AMOG/beta2 and glioma invasion: does loss of AMOG make tumour cells run amok?

The beta2 subunit of Na,K-ATPase, initially described as adhesion molecule on glia (AMOG), has been shown to mediate neurone-astrocyte adhesion as well as neural cell migration in vitro. We have investigated the expression of AMOG/beta2 in human gliomas and its effect on glioma cell adhesion and migration. Compared to normal astrocytes of human brain, AMOG/beta2 expression levels of neoplastic astrocytes were down-regulated in biopsy specimens and inversely related to the grade of malignancy. One rat and four human glioma cell lines showed complete loss of AMOG. To investigate the function of AMOG/beta2, its expression was re-established by transfecting an expression plasmid into AMOG/beta2-negative C6 rat glioma cells. In vitro assays revealed increased adhesion and decreased migration on matrigel of AMOG/beta2-positive cells as compared to their AMOG/beta2-negative counterparts. We conclude that increasing loss of AMOG/beta2 during malignant progression parallels and may underlie the extensive invasion pattern of malignant gliomas.

The immunoglobulin-superfamily molecule basigin is a binding protein for oligomannosidic carbohydrates: an anti-idiotypic approach

Recognition molecules that carry carbohydrate structures regulate cell interactions during development and play important roles in synaptic plasticity and regeneration in the adult. Glycans appear to be involved in these interactions. We have searched for binding proteins for oligomannosidic structures using the L3 antibody directed against high mannose-type glycans in an anti-idiotypic approach. A selected monoclonal anti-idiotype antibody was used for affinity chromatography and identified basigin as a binding protein from mouse brain detergent lysates. Basigin was found to bind to high mannose-carrying cell recognition molecules, such as myelin-associated glycoprotein, L1, the beta2-subunit of Na+/K+-ATPase and an oligomannosidic neoglycolipid. Furthermore, basigin was involved in outgrowth of astrocytic processes in vitro. A striking homology between the first immunoglobulin (Ig)-like domain of basigin and the fourth Ig-like domain of NCAM, previously shown to bind to oligomannosidic glycans, and the lectin domain of the mannose receptor confirms that basigin is an oligomannose binding lectin. To our knowledge this is the first report that anti-idiotypic antibodies can be used to identify binding partners for carbohydrates.

The Adhesion Molecule on Glia (AMOG) Is Widely Expressed by Astrocytes in Developing and Adult Mouse Brain

Adhesion molecule on glia (AMOG) is a 45 - 50 kD cell surface glycoprotein structurally similar to the Na, K-ATPase beta-subunit and associated with the catalytic subunit of this enzyme. Previous immunofluorescence results had suggested that AMOG is transiently expressed on Bergmann glia during mouse cerebellar development, and antibody-inhibition results have implicated it in the migration of granule neurons. We report that, while AMOG mRNA is detected in Bergmann glia during the migratory period, this astrocyte derivative continues to express AMOG mRNA at similar levels in adult mice suggesting a functional role for AMOG in adulthood. Evidence from RNA and protein blot analyses that AMOG is present before birth, increasing about ten fold in adult mouse brain and cerebellum is also provided. RNA blot analysis of astrocyte-enriched cell populations and in situ hybridization results show that astrocytes synthesize AMOG mRNA in all regions of the developing and adult brain. In the adult, AMOG mRNA is more abundant in grey than white matter and, among grey matter regions, highest in cerebellar cortex. These results indicate a relationship between density of neuronal elements and AMOG expression. It is further speculated that AMOG is part of a Na,K-ATPase complex expressed preferentially by astrocytes in mouse brain.

Mouse glioma gene expression profiling identifies novel human glioma-associated genes

Based on previous studies demonstrating increased RAS activity in human astrocytomas, we developed a transgenic mouse model (B8) that targets an activated RAS molecule to astrocytes. Within 3 to 4 months after birth, these mice develop high-grade astrocytomas that are histologically identical to human astrocytomas. To characterize genetic events associated with B8 mouse astrocytoma formation, we employed comparative gene expression profiling of wild-type cultured mouse astrocytes, non-neoplastic B8 astrocytes, B8 astrocytoma cultures, and two other astrocytoma cultures from independently derived RAS transgenic mouse lines. We identified several classes of gene expression changes, including those associated with the non-neoplastic state in the B8 transgenic mouse, those associated with astrocytoma formation, and those specifically associated with only one of the three independently derived transgenic mouse astrocytomas. Differential expression of several unique genes was confirmed at the protein level in both the RAS transgenic mouse astrocytomas and two human glioblastoma multiforme cell lines. Furthermore, reexpression of one of these downregulated astrocytoma-associated proteins, GAP43, resulted in C6 glioma cell growth suppression. The use of this transgenic mouse model to identify novel genetic changes that might underlie the pathogenesis of human high-grade astrocytomas provides a unique opportunity to discover future targets for brain tumor therapy.

Functional role of a glycolipid in directional movements of neurons

Migration of neurons from their site of origin to their final destination is a critical and universal step in the formation of the complex structure of the nervous system. The migratory process is thought to be governed in part by genetically and epigenetically defined sequences of signals which are interpreted by migrating cells. The molecular mechanisms that underlie neuronal migration have been the subject of intense investigation. As in other developmental processes, many molecules must participate in neuronal migration. Some molecules, such as cell adhesion molecules and motor proteins, may contribute to discrete steps in the migration act; others, like extracellular signaling molecules, may regulate the activation and/or termination of the migration program. In this article we review findings from our group that demonstrate the functional role(s) of a specific glycolipid in neuronal migration and neurite outgrowth in the developing and adult nervous system.