Lipopolysaccharide structure modulates cationic biocide susceptibility and crystalline biofilm formation in Proteus mirabilis

Chlorhexidine (CHD) is a cationic biocide used ubiquitously in healthcare settings. Proteus mirabilis, an important pathogen of the catheterized urinary tract, and isolates of this species are often described as “resistant” to CHD-containing products used for catheter infection control. To identify the mechanisms underlying reduced CHD susceptibility in P. mirabilis, we subjected the CHD tolerant clinical isolate RS47 to random transposon mutagenesis and screened for mutants with reduced CHD minimum inhibitory concentrations (MICs). One mutant recovered from these screens (designated RS47-2) exhibited ~ 8-fold reduction in CHD MIC. Complete genome sequencing of RS47-2 showed a single mini-Tn5 insert in the waaC gene involved in lipopolysaccharide (LPS) inner core biosynthesis. Phenotypic screening of RS47-2 revealed a significant increase in cell surface hydrophobicity and serum susceptibility compared to the wildtype, and confirmed defects in LPS production congruent with waaC inactivation. Disruption of waaC was also associated with increased susceptibility to a range of other cationic biocides but did not affect susceptibility to antibiotics tested. Complementation studies showed that repression of smvA efflux activity in RS47-2 further increased susceptibility to CHD and other cationic biocides, reducing CHD MICs to values comparable with the most CHD susceptible isolates characterized. The formation of crystalline biofilms and blockage of urethral catheters was also significantly attenuated in RS47-2. Taken together, these data show that aspects of LPS structure and upregulation of the smvA efflux system function in synergy to modulate susceptibility to CHD and other cationic biocides, and that LPS structure is also an important factor in P. mirabilis crystalline biofilm formation.

The SWIS trial: Protocol of a pragmatic cluster randomised controlled trial of school based social work

BACKGROUND

Child and family social workers in the UK work closely with other agencies including schools and the police, and typically they are based in local authority offices. This study will evaluate the effectiveness of placing social workers in schools (SWIS) on the need for social care interventions. SWIS was piloted in three local authorities in 2018-2020, and findings from a feasibility study of the pilots suggests SWIS may operate through three key pathways: (1) by enhancing schools' response to safeguarding issues, (2) through increased collaboration between social workers, school staff, and parents, and (3) by improving relationships between social workers and young people.

METHODS

The study is a two-arm pragmatic cluster randomised controlled trial building on three feasibility studies which found SWIS to be promising. Social workers will work within secondary schools across local authorities in England. 280 mainstream secondary schools will be randomly allocated with a 1:1 ratio to SWIS or a comparison arm, which will be schools that continue as normal, without a social worker. The primary outcome will be the rate of Child Protection (Section 47) enquiries. Secondary outcomes will comprise rate of referrals to children's social care, rate of Child in Need (Section 17) assessments, days spent in care, and educational attendance and attainment. The study also includes an economic evaluation, and an implementation and process evaluation. Social care outcomes will be measured in July 2022, and educational outcomes will be measured in July 2023. Days in care will be measured at both time points.

DISCUSSION

Findings will explore the effectiveness and cost-effectiveness of SWIS on the need for social care interventions. A final report will be published in January 2024.

TRIAL REGISTRATION

The study was registered retrospectively with the International Standard Randomised Controlled Trial Number registry on 13.11.2020 (ISRCTN90922032).

Evaluation of the efficacy and impact of a clinical prediction tool to identify maltreatment associated with children's burns

BACKGROUND

An estimated 10%-24% of children attending emergency departments with a burn are maltreated.

OBJECTIVE

To test whether a clinical prediction tool (Burns Risk assessment for Neglect or abuse Tool; BuRN-Tool) improved the recognition of maltreatment and increased the referral of high-risk children to safeguarding services for assessment.

METHODS

A prospective study of children presenting with burns to four UK hospitals (2015-2018), each centre providing a minimum of 200 cases before and after the introduction of the BuRN-Tool. The proportions of children referred to safeguarding services were compared preintervention and postintervention, and the relationship between referral and the recommended cut-off for concern (BuRN-Tool score (BT-score) ≥3) was explored.

RESULTS

The sample was 2443 children (median age 2 years). Nurses and junior doctors mainly completed the BuRN-Tool, and a BT-score was available for 90.8% of cases. After intervention, 28.4% (334/1174) had a BT-score ≥3 and were nearly five times more likely to be discussed with a senior clinician than those with a BT-score <3 (65.3% vs 13.4%, p<0.001). There was no overall difference in the proportion of safeguarding referrals preintervention and postintervention. After intervention, the proportion of referrals for safeguarding concerns was greater when the BT-score was ≥3 (p=0.05) but not for scores <3 (p=0.60). A BT-score of 3 as a cut-off for referral had a sensitivity of 72.1, a specificity of 82.7 and a positive likelihood ratio of 4.2.

CONCLUSIONS

A BT-score ≥3 encouraged discussion of cases of concern with senior colleagues and increased the referral of <5 year-olds with safeguarding concerns to children's social care.

Contact burns: the influence of agents and mechanisms of injury on anatomical burn locations in children <5 years old and associations with child protection referrals

OBJECTIVE

To identify how causative agents and mechanisms of injury influence the location of an accidental contact burn in children and whether these factors differ in cases referred for child protection (CP) assessment.

DESIGN

Prospective multicentre cross-sectional study.

SETTING

20 hospital sites across England and Wales, including: emergency departments, minor injury units and regional burn units.

PATIENTS

Children less than 5 years old who attended hospital for a contact burn (August 2015 to September 2018).

MAIN OUTCOME MEASURES

Location of burns with respect to agent and mechanism for accidental contact burns. Secondary outcome: mechanism, agent and location of burns referred for CP assessment.

RESULTS

816 accidental burns and 92 referrals for CP assessment. The most common for accidental burns: mechanism was reaching while stationary (68%, 553/816), agent was oven (24.5%, 200/816) and site was the hand (69.2%, 565/816). Burns to head and trunk were rare at 3.7% (30/816). The data enabled a tabulation of the locations of burns as predicted by agent and mechanism of injury. The location of the burn was most strongly influenced by mechanism.Burns from irons (p<0.01), caused by mechanisms independent of the child (p=0.01), unwitnessed burns (p<0.001) and burns to the head and trunk (p<0.001) were significantly more common among the children referred for CP assessment.

CONCLUSIONS

By overlaying agent, mechanism and site it was possible to tabulate and quantify simple narratives of accidental contact burns in population of young children. These findings have the potential to aid clinicians in recognising accidental contact burns.

Burns and Scalds Assessment Template: standardising clinical assessment of childhood burns in the emergency department

Objectives

The Burns and Scalds Assessment Template (BaSAT) is an evidence-based proforma coproduced by researchers and ED staff with the aim of (1) standardising the assessment of children attending ED with a burn, (2) improving documentation and (3) screening for child maltreatment. This study aimed to test whether the BaSAT improved documentation of clinical, contributory and causal factors of children’s burns.

Methods

A retrospective before-and-after study compared the extent to which information was recorded for 37 data fields after the BaSAT was introduced in one paediatric ED. Pre-BaSAT, a convenience sample of 50 patient records of children who had a burn was obtained from the hospital electronic database of 2007. The post-BaSAT sample included 50 randomly selected case notes from 2016/2017 that were part of another research project. Fisher's exact test and Mann-Whitney U tests were conducted to test for statistical significance.

Results

Pre-BaSAT, documentation of key data fields was poor. Post-BaSAT, this varied less between patients, and median completeness significantly (p<0.001) increased from 44% (IQR 4%–94%) to 96% (IQR 94%–100%). Information on ‘screening for maltreatment, referrals to social care and outcome’ was poorly recorded pre-BaSAT (median of 4% completed fields) and showed the greatest overall improvement (to 95%, p<0.001). Documentation of domestic violence at home and child’s ethnicity improved significantly (p<0.001) post-BaSAT; however, these were still not recorded in 36% and 56% of cases, respectively.

Conclusion

Introduction of the BaSAT significantly improved and standardised the key clinical data routinely recorded for children attending ED with a burn.

Educational outcomes of children in Wales with cerebral palsy

Background with rationaleIt is recognized that children with disability have special educational needs (SEN). They are likely to have poor school attendance and do not achieve well academically. Many children with a cerebral palsy (CP) have SEN but little is known about their educational provision or outcomes. Main AimTo investigate the educational experience of children in Wales with CP and describe the type of SEN and SEN provision; school attendance; achievement—teacher assessments at the end of the Foundation Phase and Key Stages 2 and 3 of the National Curriculum (NC)—and in General Certificate of Secondary Education (GCSE) examinations. Methods/ApproachData from the Pupil Level Annual School Census (PLASC), NC and GCSE results from 2009 to 2016 were linked with routine e-health records of primary and secondary health care data held in SAIL. The health care records were used to identify pupils who, potentially, had a cerebral palsy. ResultsThe linked data set included around 360,000 pupils per school census of whom 1200–1400 per school census were identified as having a CP, representing a crude prevalence of 0.4%. Adjusted for age, year and sex, there was no significant variation in prevalence by area deprivation. Around 60% of children with a CP have a statement of SEN; over a quarter of CP children are educated in special schools; CP children in mainstream (primary, middle and secondary) schools tended to miss more school sessions (~50% more) than other children and lower percentages achieved the expected levels at Key Stages 2 and 3 and the Level 2 GCSE threshold. Conclusion/Implications This work demonstrates the utility of record-linkage between health and education data to map, monitor and provide information to parents, carers and policymakers about education outcomes for this group of children to inform planning and service provision.

Review of preventative behavioural interventions for dermal and respiratory hazards

BACKGROUND

No previous systematic review of the evidence base has been undertaken to help occupational health professionals understand how to reliably lower the instance of occupational ill-health through reducing risk-taking behaviour.

AIMS

To evaluate the effectiveness and processes of occupational-based behavioural interventions for workers exposed to dermal and respiratory hazards.

METHODS

A systematic review was conducted. Sixteen electronic databases were searched using key words. Bibliography, health and safety websites and hand searches of key journals were also undertaken. Articles were included if they evaluated an intervention targeting workers' behavioural compliance, addressed dermal or respiratory hazards, used before and after measures with a control group comparison and used behaviour-related exposure indicators such as airborne exposure, health effects, behaviour observations and self-reported work practices. Data were extracted according to potential sources of bias, impact and behavioural change processes used.

RESULTS

Ten of 550 articles identified as potentially relevant were included. A predominance of small effect sizes, particularly for larger samples, demonstrated limited but positive impact upon exposure. Studies contained too much heterogeneity for reliable meta-analysis. None of the studies covered the full range of behaviour change components necessary for reducing exposure risk.

CONCLUSIONS

We conclude that future interventions could enhance their effectiveness through improving design quality, reporting and basing their content upon evidence-based behavioural change approaches. Using a comprehensive range of evidence-informed behaviour change ingredients should improve occupational health professional's ability to reliably reduce occupational ill-health where exposure cannot totally be designed out of the workplace.

Antifreeze proteins in Alaskan insects and spiders

Prior to this study, antifreeze proteins (AFPs) had not been identified in terrestrial arthropods from the Arctic or anywhere in Alaska. The hemolymph of 75 species of insects and six spiders from interior and arctic Alaska were screened for thermal hysteresis (a difference between the freezing and melting points), characteristic of the presence of AFPs. Eighteen species of insects and three spiders were shown to have AFPs. Ten of the insects with AFPs were beetles including the first species from the families Chrysomelidae, Pythidae, Silphidae and Carabidae. In addition, the first Neuropteran to have AFPs was identified, the lacewing Hemerobius simulans together with the second and third Diptera (the first Tipulids) and the second and third Hemiptera, the stinkbug Elasmostethus interstinctus (the first Pentatomid), and the water strider Limnoporus dissortis (the first Gerrid). Prior to this study, 33 species of insects and three spiders had been reported to have AFPs. Most AFP-producing terrestrial arthropods are freeze avoiding, and the AFPs function to prevent freezing. However, some of the AFP- producing insects identified in this study are known to be freeze tolerant (able to survive freezing) to very low temperatures (-40 to -70 degrees C).

The ANK repeat: a ubiquitous motif involved in macromolecular recognition

Many proteins rely on stable, noncovalent interactions with other macromolecules to perform their function. The identification of a repeated sequence motif, the ANK repeat, in diverse proteins whose common function involves binding to other proteins indicates one way nature may achieve a wide range of protein-protein interactions. In this article, we describe evidence that these ANK repeats are involved in the specific recognition of proteins and possibly DNA, and present a model for the folding of the motif.

Ankyrin-G coordinates assembly of the spectrin-based membrane skeleton, voltage-gated sodium channels, and L1 CAMs at Purkinje neuron initial segments

The axon initial segment is an excitable membrane highly enriched in voltage-gated sodium channels that integrates neuronal inputs and initiates action potentials. This study identifies Nav1.6 as the voltage-gated sodium channel isoform at mature Purkinje neuron initial segments and reports an essential role for ankyrin-G in coordinating the physiological assembly of Nav1.6, betaIV spectrin, and the L1 cell adhesion molecules (L1 CAMs) neurofascin and NrCAM at initial segments of cerebellar Purkinje neurons. Ankyrin-G and betaIV spectrin appear at axon initial segments by postnatal day 2, whereas L1 CAMs and Nav1.6 are not fully assembled at continuous high density along axon initial segments until postnatal day 9. L1 CAMs and Nav1.6 therefore do not initiate protein assembly at initial segments. betaIV spectrin, Nav1.6, and L1 CAMs are not clustered in adult Purkinje neuron initial segments of mice lacking cerebellar ankyrin-G. These results support the conclusion that ankyrin-G coordinates the physiological assembly of a protein complex containing transmembrane adhesion molecules, voltage-gated sodium channels, and the spectrin membrane skeleton at axon initial segments.

FIGQY phosphorylation defines discrete populations of L1 cell adhesion molecules at sites of cell-cell contact and in migrating neurons

Phosphorylation of neurofascin, a member of the L1 family of cell adhesion molecules (L1 CAMs), at the conserved FIGQY-tyrosine abolishes the ankyrin-neurofascin interaction. This study provides the first evidence, in Drosophila melanogaster and vertebrates, for the physiological occurrence of FIGQY phosphorylation in L1 family members. FIGQY tyrosine phosphorylation is localized at specialized cell junctions, including paranodes of sciatic nerve, neuromuscular junctions of adult rats and Drosophila embryos, epidermal muscle attachment sites of Drosophila, and adherens junctions of developing epithelial cells of rat and Drosophila. In addition, FIGQY-phosphorylated L1 CAMs are abundantly expressed in regions of neuronal migration and axon extension, including the embryonic cortex, the neonatal cerebellum and the rostral migratory stream, a region of continued neurogenesis and migration throughout adulthood in the rat. Based on our results, physiological FIGQY-tyrosine phosphorylation of the L1 family likely regulates adhesion molecule-ankyrin interactions establishing ankyrin-free and ankyrin-containing microdomains and participates in an ankyrin-independent intracellular signaling pathway at specialized sites of intercellular contact in epithelial and nervous tissue.

LAD-1, the Caenorhabditis elegans L1CAM homologue, participates in embryonic and gonadal morphogenesis and is a substrate for fibroblast growth factor receptor pathway-dependent phosphotyrosine-based signaling

This study shows that L1-like adhesion (LAD-1), the sole Caenorhabditis elegans homologue of the L1 family of neuronal adhesion molecules, is required for proper development of the germline and the early embryo and embryonic and gonadal morphogenesis. In addition, the ubiquitously expressed LAD-1, which binds to ankyrin-G, colocalizes with the C. elegans ankyrin, UNC-44, in multiple tissues at sites of cell-cell contact. Finally, we show that LAD-1 is phosphorylated in a fibroblast growth factor receptor (FGFR) pathway-dependent manner on a tyrosine residue in the highly conserved ankyrin-binding motif, FIGQY, which was shown previously to abolish the L1 family of cell adhesion molecule (L1CAM) binding to ankyrin in cultured cells. Immunofluorescence studies revealed that FIGQY-tyrosine-phosphorylated LAD-1 does not colocalize with nonphosphorylated LAD-1 or UNC-44 ankyrin but instead is localized to sites that undergo mechanical stress in polarized epithelia and axon-body wall muscle junctions. These findings suggest a novel ankyrin-independent role for LAD-1 related to FGFR signaling. Taken together, these results indicate that L1CAMs constitute a family of ubiquitous adhesion molecules, which participate in tissue morphogenesis and maintaining tissue integrity in metazoans.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

Association of protein kinase C(lambda) with adducin in 3T3-L1 adipocytes

There is evidence that the atypical protein kinases C (PKC(lambda), PKC(zeta)) participate in signaling from the insulin receptor to cause the translocation of glucose transporters from an intracellular location to the plasma membrane in adipocytes. In order to search for downstream effectors of these PKCs, we identified the proteins that were immunoprecipitated by an antibody against PKC(lambda/zeta) from lysates of 3T3-L1 adipocytes through peptide sequencing by mass spectrometry. The data show that PKC(lambda) is the major atypical PKC in these cells. Moreover, an oligomeric complex consisting of alpha- and gamma-adducin, which are cytoskeletal proteins, coimmunoprecipitated with PKC(lambda). Association of the adducins with PKC(lambda) was further indicated by the finding that the adducins coimmunoprecipitated proportionally with PKC(lambda) in repeated rounds of immunoprecipitation. Such an association is consistent with literature reports that the adducins contain a single major site for PKC phosphorylation in their carboxy termini. Using antibody against the phospho form of this site for immunoblotting, we found that insulin caused little or no increase in the phosphorylation of this site on the adducins in a whole cell lysate or on the small portion of the adducins that coimmunoprecipitated with PKC(lambda). PKC(lambda) and the adducins were located in both the cytosol and subcellular membranous fractions. The binding of PKC(lambda) to adducin may function to localize PKC(lambda) in 3T3-L1 adipocytes.

Fibronectin isoforms in megakaryocytes

Our studies have shown that megakaryocytes (MK) can synthesize fibronectin (FN) and alternatively spliced fibronectin, FN EIIIB. FN EIIIB is primarily present in embryonic, proliferating and migrating cells, and thought to be important for cell maturation. MK, but not nonmegakaryocytic bone marrow cells, contain FN EIIIB and thus, MK and platelets are among a small number of adult cells and tissues that synthesize and contain FN EIIIB. Thrombin can induce the secretion of general FN, but does not cause the secretion of FN EIIIB into the medium. Analysis of immunostained cells by confocal microscopy revealed that both general FN and FN EIIIB accumulated on the MK surface following thrombin treatment. Thus, FN EIIIB can be released only to be bound to the MK surface. The expression of FN EIIIB on the MK surface may have a unique role in MK migration and maturation.

Ankyrins and cellular targeting of diverse membrane proteins to physiological sites

Ankyrins are spectrin-binding proteins that associate via ANK repeats with a variety of ion channels/pumps, calcium release channels and cell adhesion molecules. Recent studies in mice indicate that ankyrins have a physiological role in restricting voltage-gated sodium channels and members of the L1 CAM family of cell adhesion molecules to excitable membranes in the central nervous system and in targeting calcium-release channels to the calcium homeostasis compartment of striated muscle.

Adducin: structure, function and regulation

Adducin is a ubiquitously expressed membrane-skeletal protein localized at spectrin-actin junctions that binds calmodulin and is an in vivo substrate for protein kinase C (PKC) and Rho-associated kinase. Adducin is a tetramer comprised of either alpha/beta or alpha/gamma heterodimers. Adducin subunits are related in sequence and all contain an N-terminal globular head domain, a neck domain and a C-terminal protease-sensitive tail domain. The tail domains of all adducin subunits end with a highly conserved 22-residue myristoylated alanine-rich C kinase substrate (MARCKS)-related domain that has homology to MARCKS protein. Adducin caps the fast-growing ends of actin filaments and also preferentially recruits spectrin to the ends of filaments. Both the neck and the MARCKS-related domains are required for these activities. The neck domain self-associates to form oligomers. The MARCKS-related domain binds calmodulin and contains the major phosphorylation site for PKC. Calmodulin, gelsolin and phosphorylation by the kinase inhibit in vitro activities of adducin involving actin and spectrin. Recent observations suggest a role for adducin in cell motility, and as a target for regulation by Rho-dependent and Ca2+-dependent pathways. Prominent physiological sites of regulation of adducin include dendritic spines of hippocampal neurons, platelets and growth cones of axons.

alpha-Actinin is a potent regulator of G protein-coupled receptor kinase activity and substrate specificity in vitro

G protein-coupled receptor kinases (GRKs) phosphorylate G protein-coupled receptors, thereby terminating receptor signaling. Herein we report that alpha-actinin potently inhibits all GRK family members. In addition, calcium-bound calmodulin and phosphatidylinositol 4,5-bisphosphate (PIP2), two regulators of GRK activity, coordinate with alpha-actinin to modulate substrate specificity of the GRKs. In the presence of calmodulin and alpha-actinin, GRK5 phosphorylates soluble, but not membrane-incorporated substrates. In contrast, in the presence of PIP2 and alpha-actinin, GRK5 phosphorylates membrane-incorporated, but not soluble substrates. Thus, modulation of alpha-actinin-mediated inhibition of GRKs by PIP2 and calmodulin has profound effects on both GRK activity and substrate specificity.

Abnormal cardiac Na(+) channel properties and QT heart rate adaptation in neonatal ankyrin(B) knockout mice

The cytoskeleton of the cardiomyocyte has been shown to modulate ion channel function. Cytoskeletal disruption in vitro alters Na(+) channel kinetics, producing a late Na(+) current that can prolong repolarization. This study describes the properties of the cardiac Na(+) channel and cardiac repolarization in neonatal mice lacking ankyrin(B), a cytoskeletal "adaptor" protein. Using whole-cell voltage clamp techniques, I(Na) density was lower in ankyrin(B)(-/-) ventricular myocytes than in wild-type (WT) myocytes (-307+/-26 versus -444+/-39 pA/pF, P<0.01). Ankyrin(B)(-/-) myocytes exhibited a hyperpolarizing shift in activation and inactivation kinetics compared with WT. Slower recovery from inactivation contributed to the negative shift in steady-state inactivation in ankyrin(B)(-/-). Single Na(+) channel mean open time was longer in ankyrin(B)(-/-) versus WT at test potentials (V(t)) of -40 mV (1.0+/-0.1 versus 0. 61+/-0.04 ms, P<0.05) and -50 mV (0.8+/-0.1 versus 0.39+/-0.05 ms, P<0.05). Ankyrin(B)(-/-) exhibited late single-channel openings at V(t) -40 and -50 mV, which were not seen in WT. Late I(Na) contributed to longer action potential durations measured at 90% repolarization (APD(90)) at 1 Hz stimulation in ankyrin(B)(-/-) compared with WT (354+/-26 versus 274+/-22 ms, P<0.05). From ECG recordings of neonatal mice, heart rates were slower in ankyrin(B)(-/-) than in WT (380+/-14 versus 434+/-13 bpm, P<0.01). Although the QT interval was similar in ankyrin(B)(-/-) and WT at physiological heart rates, QT-interval prolongation in response to heart rate deceleration was greater in ankyrin(B)(-/-). In conclusion, Na(+) channels in ankyrin(B)(-/-) display reduced I(Na) density and abnormal kinetics at the whole-cell and single-channel level that contribute to prolonged APD(90) and abnormal QT-rate adaptation.

A requirement for ankyrin binding to clathrin during coated pit budding

Recent studies suggest that the mobility of clathrin-coated pits at the cell surface are restricted by an actin cytoskeleton and that there is an obligate reduction in the amount of spectrin on membranes during coated pit budding. The spectrin-actin cytoskeleton associates with membranes primarily through ankyrins, which interact with the cytoplasmic region of numerous integral membrane proteins. We now report that the fourth repeat domain (D4) of ankyrin(R) binds to the N-terminal domain of clathrin heavy chain with high affinity. Addition of peptides containing the D4 region inhibited clathrin-coated pit budding in vitro. In addition, microinjection of D4 containing peptides blocked the endocytosis of fluorescent low density lipoprotein (LDL). Ankyrin(R) peptides that contained repeat domains other than D4 had no effect on either in vitro budding or internalization of LDL. Finally, immunofluorescence shows that ankyrin is uniformly associated with endosomes that contain fluorescent LDL. These results suggest that ankyrin plays a role in the budding of clathrin-coated pits during endocytosis.

Phosphorylation of adducin by Rho-kinase plays a crucial role in cell motility

Adducin is a membrane skeletal protein that binds to actin filaments (F-actin) and thereby promotes the association of spectrin with F-actin to form a spectrin-actin meshwork beneath plasma membranes such as ruffling membranes. Rho-associated kinase (Rho- kinase), which is activated by the small guanosine triphosphatase Rho, phosphorylates alpha-adducin and thereby enhances the F-actin-binding activity of alpha-adducin in vitro. Here we identified the sites of phosphorylation of alpha-adducin by Rho-kinase as Thr445 and Thr480. We prepared antibody that specifically recognized alpha-adducin phosphorylated at Thr445, and found by use of this antibody that Rho-kinase phosphorylated alpha-adducin at Thr445 in COS7 cells in a Rho-dependent manner. Phosphorylated alpha-adducin accumulated in the membrane ruffling area of Madin-Darby canine kidney (MDCK) epithelial cells and the leading edge of scattering cells during the action of tetradecanoylphorbol-13-acetate (TPA) or hepatocyte growth factor (HGF). The microinjection of Botulinum C3 ADP-ribosyl-transferase, dominant negative Rho-kinase, or alpha-adducinT445A,T480A (substitution of Thr445 and Thr480 by Ala) inhibited the TPA-induced membrane ruffling in MDCK cells and wound-induced migration in NRK49F cells. alpha-AdducinT445D,T480D (substitution of Thr445 and Thr480 by Asp), but not alpha-adducinT445A,T480A, counteracted the inhibitory effect of the dominant negative Rho-kinase on the TPA-induced membrane ruffling in MDCK cells. Taken together, these results indicate that Rho-kinase phosphorylates alpha-adducin downstream of Rho in vivo, and that the phosphorylation of adducin by Rho-kinase plays a crucial role in the regulation of membrane ruffling and cell motility.

Physiological roles of axonal ankyrins in survival of premyelinated axons and localization of voltage-gated sodium channels

440 kD ankyrin-B and 480/270 kD ankyrin-G are membrane skeletal proteins with closely related biochemical properties yet distinctive physiological roles in axons. These proteins associate with spectrin-actin networks and also bind to integral membrane proteins including the L1 CAM family of cell adhesion molecules and voltage-gated sodium channels. 440 kD ankyrin-B is expressed with L1 in premyelinated axon tracts, and is essential for survival of these axons, at least in the case of the optic nerve. 440 ankyrin-B may collaborate with L1 in transcellular structures that mediate axon fasciculation and mechanically stabilize axon bundles, although these proteins may also be involved in axon pathfinding. Ankyrin-B (-/-) mice exhibit loss of L1 from premyelinated axon tracts and a similar, although much more severe, phenotype to L1 (-/-) mice and humans with L1 mutations. Ankyrin-B and L1 thus are candidates to collaborate in the same structural pathway and defects in this pathway can lead to nervous system malformations and mental retardation. 480/270 kD ankyrin-G are highly concentrated along with the L1CAM family members neurofascin and NrCAM at nodes of Ranvier and axon initial segments. Voltage-gated sodium channels bind directly to ankyrins, and are likely to associate in a ternary complex containing neurofascin/NrCAM, and ankyrin-G. Mice with ankyrin-G expression abolished in the cerebellum exhibit loss of ability of Purkinje neurons to fire action potentials, as well as loss of restriction of neurofascin/NrCAM to axon initial segments. Ankyrin-G thus is a key component in assembly of functional components of the axon initial segment and possibly the node of Ranvier.

AnkyrinG is required for clustering of voltage-gated Na channels at axon initial segments and for normal action potential firing

Voltage-gated sodium channels (NaCh) are colocalized with isoforms of the membrane-skeletal protein ankyrinG at axon initial segments, nodes of Ranvier, and postsynaptic folds of the mammalian neuromuscular junction. The role of ankyrinG in directing NaCh localization to axon initial segments was evaluated by region-specific knockout of ankyrinG in the mouse cerebellum. Mutant mice exhibited a progressive ataxia beginning around postnatal day P16 and subsequent loss of Purkinje neurons. In mutant mouse cerebella, NaCh were absent from axon initial segments of granule cell neurons, and Purkinje cells showed deficiencies in their ability to initiate action potentials and support rapid, repetitive firing. Neurofascin, a member of the L1CAM family of ankyrin-binding cell adhesion molecules, also exhibited impaired localization to initial segments of Purkinje cell neurons. These results demonstrate that ankyrinG is essential for clustering NaCh and neurofascin at axon initial segments and is required for physiological levels of sodium channel activity.

Nervous system defects of AnkyrinB (-/-) mice suggest functional overlap between the cell adhesion molecule L1 and 440-kD AnkyrinB in premyelinated axons

The L1 CAM family of cell adhesion molecules and the ankyrin family of spectrin-binding proteins are candidates to collaborate in transcellular complexes used in diverse contexts in nervous systems of vertebrates and invertebrates. This report presents evidence for functional coupling between L1 and 440-kD ankyrinB in premyelinated axons in the mouse nervous system. L1 and 440-kD ankyrinB are colocalized in premyelinated axon tracts in the developing nervous system and are both down-regulated after myelination. AnkyrinB (-/-) mice exhibit a phenotype similar to, but more severe, than L1 (-/-) mice and share features of human patients with L1 mutations. AnkyrinB (-/-) mice exhibit hypoplasia of the corpus callosum and pyramidal tracts, dilated ventricles, and extensive degeneration of the optic nerve, and they die by postnatal day 21. AnkyrinB (-/-) mice have reduced L1 in premyelinated axons of long fiber tracts, including the corpus callosum, fimbria, and internal capsule in the brain, and pyramidal tracts and lateral columns of the spinal cord. L1 was evident in the optic nerve at postnatal day 1 but disappeared by postnatal day 7 in mutant mice while NCAM was unchanged. Optic nerve axons of ankyrinB (-/-) mice become dilated with diameters up to eightfold greater than normal, and they degenerated by day 20. These findings provide the first evidence for a role of ankyrinB in the nervous system and support an interaction between 440-kD ankyrinB and L1 that is essential for maintenance of premyelinated axons in vivo.

Structural requirements for association of neurofascin with ankyrin

This paper presents the first structural analysis of the cytoplasmic domain of neurofascin, which is highly conserved among the L1CAM family of cell adhesion molecules, and describes sequence requirements for neurofascin-ankyrin interactions in living cells. The cytoplasmic domain of neurofascin dimerizes in solution, has an asymmetric shape, and exhibits a reversible temperature-dependent beta-structure. Residues Ser56-Tyr81 are necessary for ankyrin binding but do not contribute to either dimerization or formation of structure. Transfected neurofascin recruits GFP-tagged 270-kDa ankyrinG to the plasma membrane of human embryo kidney 293 cells. Deletion mutants demonstrate that the sequence Ser56-Tyr81 contains the major ankyrin-recruiting activity of neurofascin. Mutations of the FIGQY tyrosine (Y81H/A/E) greatly impair neurofascin-ankyrin interactions. Mutation of human L1 at the equivalent tyrosine (Y1229H) is responsible for certain cases of mental retardation (Van Camp, G., Fransen, E., Vits, L., Raes, G., and Willems, P. J. (1996) Hum. Mutat. 8, 391). Mutations F77A and E73Q greatly impair ankyrin binding activity, whereas mutation D74N and a triple mutation of D57N/D58N/D62N result in less loss of ankyrin binding activity. These results provide evidence for a highly specific interaction between ankyrin and neurofascin and suggest that ankyrin association with L1 is required for L1 function in humans.

Restriction of 480/270-kD ankyrin G to axon proximal segments requires multiple ankyrin G-specific domains

AnkyrinG (-/-) neurons fail to concentrate voltage-sensitive sodium channels and neurofascin at their axon proximal segments, suggesting that ankyrinG is a key component of a structural pathway involved in assembly of specialized membrane domains at axon proximal segments and possibly nodes of Ranvier (Zhou, D., S. Lambert, D.L. Malen, S. Carpenter, L. Boland, and V. Bennett, manuscript submitted for publication). This paper addresses the mechanism for restriction of 270-kD ankyrinG to axon proximal segments by evaluation of localization of GFP-tagged ankyrinG constructs transfected into cultured dorsal root ganglion neurons, as well as measurements of fluorescence recovery after photobleaching of neurofascin- GFP-tagged ankyrinG complexes in nonneuronal cells. A conclusion is that multiple ankyrinG-specific domains, in addition to the conserved membrane-binding domain, contribute to restriction of ankyrinG to the axonal plasma membrane in dorsal root ganglion neurons. The ankyrinG-specific spectrin-binding and tail domains are capable of binding directly to sites on the plasma membrane of neuronal cell bodies and axon proximal segments, and presumably have yet to be identified docking sites. The serine-rich domain, which is present only in 480- and 270-kD ankyrinG polypeptides, contributes to restriction of ankyrinG to axon proximal segments as well as limiting lateral diffusion of ankyrinG-neurofascin complexes. The membrane-binding, spectrin-binding, and tail domains of ankyrinG also contribute to limiting the lateral mobility of ankyrinG-neurofascin complexes. AnkyrinG thus functions as an integrated mechanism involving cooperation among multiple domains heretofore regarded as modular units. This complex behavior explains ability of ankyrinB and ankyrinG to sort to distinct sites in neurons and the fact that these ankyrins do not compensate for each other in ankyrin gene knockouts in mice.

Adducin is an in vivo substrate for protein kinase C: phosphorylation in the MARCKS-related domain inhibits activity in promoting spectrin-actin complexes and occurs in many cells, including dendritic spines of neurons

Adducin is a heteromeric protein with subunits containing a COOH-terminal myristoylated alanine-rich C kinase substrate (MARCKS)-related domain that caps and preferentially recruits spectrin to the fast-growing ends of actin filaments. The basic MARCKS-related domain, present in alpha, beta, and gamma adducin subunits, binds calmodulin and contains the major phosphorylation site for protein kinase C (PKC). This report presents the first evidence that phosphorylation of the MARCKS-related domain modifies in vitro and in vivo activities of adducin involving actin and spectrin, and we demonstrate that adducin is a prominent in vivo substrate for PKC or other phorbol 12-myristate 13-acetate (PMA)-activated kinases in multiple cell types, including neurons. PKC phosphorylation of native and recombinant adducin inhibited actin capping measured using pyrene-actin polymerization and abolished activity of adducin in recruiting spectrin to ends and sides of actin filaments. A polyclonal antibody specific to the phosphorylated state of the RTPS-serine, which is the major PKC phosphorylation site in the MARCKS-related domain, was used to evaluate phosphorylation of adducin in cells. Reactivity with phosphoadducin antibody in immunoblots increased twofold in rat hippocampal slices, eight- to ninefold in human embryonal kidney (HEK 293) cells, threefold in MDCK cells, and greater than 10-fold in human erythrocytes after treatments with PMA, but not with forskolin. Thus, the RTPS-serine of adducin is an in vivo phosphorylation site for PKC or other PMA-activated kinases but not for cAMP-dependent protein kinase in a variety of cell types. Physiological consequences of the two PKC phosphorylation sites in the MARCKS-related domain were investigated by stably transfecting MDCK cells with either wild-type or PKC-unphosphorylatable S716A/S726A mutant alpha adducin. The mutant alpha adducin was no longer concentrated at the cell membrane at sites of cell-cell contact, and instead it was distributed as a cytoplasmic punctate pattern. Moreover, the cells expressing the mutant alpha adducin exhibited increased levels of cytoplasmic spectrin, which was colocalized with the mutant alpha adducin in a punctate pattern. Immunofluorescence with the phosphoadducin-specific antibody revealed the RTPS-serine phosphorylation of adducin in postsynaptic areas in the developing rat hippocampus. High levels of the phosphoadducin were detected in the dendritic spines of cultured hippocampal neurons. Spectrin also was a component of dendritic spines, although at distinct sites from the ones containing phosphoadducin. These data demonstrate that adducin is a significant in vivo substrate for PKC or other PMA-activated kinases in a variety of cells, and that phosphorylation of adducin occurs in dendritic spines that are believed to respond to external signals by changes in morphology and reorganization of cytoskeletal structures.

Adducin preferentially recruits spectrin to the fast growing ends of actin filaments in a complex requiring the MARCKS-related domain and a newly defined oligomerization domain

Adducin is a protein associated with spectrin and actin in membrane skeletons of erythrocytes and possibly other cells. Adducin has activities in in vitro assays of association with the sides of actin filaments, capping the fast growing ends of actin filaments, and recruiting spectrin to actin filaments. This study presents evidence that adducin exhibits a preference for the fast growing ends of actin filaments for recruiting spectrin to actin and for direct association with actin. beta-Adducin-(335-726) promoted recruitment of spectrin to gelsolin-sensitive sites at fast growing ends of actin filaments with half-maximal activity at 15 nM and to gelsolin-insensitive sites with half-maximal activity at 75 nM. beta-Adducin-(335-726) also exhibited a preference for actin filament ends in direct binding assays; the half-maximal concentration for binding of adducin to gelsolin-sensitive sites at filament ends was 60 nM, and the Kd for binding to lateral sites was 1.5 microM. The concentration of beta-adducin-(335-726) of 60 nM required for half-maximal binding to filament ends is in the same range as the concentration of 150 nM required for half-maximal actin capping activity. All interactions of adducin with actin require the myristoylated alanine-rich protein kinase C substrate-related domain as well as a newly defined oligomerization site localized in the neck domain of adducin. Surprisingly, the head domain of adducin is not required for spectrin-actin interactions, although it could play a role in forming tetramers. The relative activities of adducin imply that an important role of adducin in cells is to form a complex with the fast growing ends of actin filaments that recruits spectrin and prevents addition or loss of actin subunits.

Palmitoylation of neurofascin at a site in the membrane-spanning domain highly conserved among the L1 family of cell adhesion molecules

This report presents the first evidence that a member of the L1 family of nervous system cell-adhesion molecules is covalently modified by thioesterification with palmitate, and identifies a highly conserved cysteine in the predicted membrane-spanning domain as the site of modification. Neurofascin is constitutively palmitoylated at cysteine-1213 at close to a 1:1 molar stoichiometry. Kinetics of palmitate incorporation into neurofascin expressed in resting neuroblastoma cells indicate that the palmitate modification has the same turnover rate as the polypeptide chain and does not affect the protein stability of neurofascin. Palmitoylation of neurofascin expressed in dorsal root ganglion neurons is not required for delivery of neurofascin to the plasma membrane or targeting to axons. Palmitoylation also has no effect on ankyrin-binding activity of neurofascin, on the oligomeric state of neurofascin in solution, or on cell-adhesion activity of neurofascin expressed in neuroblastoma cells. A significant difference between native and C1213L neurofascin is that these proteins were localized in distinct fractions within a low-density membrane population enriched in signaling molecules. These results indicate a palmitate-dependent targeting of neurofascin to a specialized membrane microdomain.

Regulation of the association of adducin with actin filaments by Rho-associated kinase (Rho-kinase) and myosin phosphatase

The small GTPase Rho is believed to regulate the actin cytoskeleton and cell adhesion through its specific targets. We previously identified the Rho targets: protein kinase N, Rho-associated kinase (Rho-kinase), and the myosin-binding subunit (MBS) of myosin phosphatase. Here we purified MBS-interacting proteins, identified them as adducin, and found that MBS specifically interacted with adducin in vitro and in vivo. Adducin is a membrane-skeletal protein that promotes the binding of spectrin to actin filaments and is concentrated at the cell-cell contact sites in epithelial cells. We also found that Rho-kinase phosphorylated alpha-adducin in vitro and in vivo and that the phosphorylation of alpha-adducin by Rho-kinase enhanced the interaction of alpha-adducin with actin filaments in vitro. Myosin phosphatase composed of the catalytic subunit and MBS showed phosphatase activity toward alpha-adducin, which was phosphorylated by Rho-kinase. This phosphatase activity was inhibited by the phosphorylation of MBS by Rho-kinase. These results suggest that Rho-kinase and myosin phosphatase regulate the phosphorylation state of adducin downstream of Rho and that the increased phosphorylation of adducin by Rho-kinase causes the interaction of adducin with actin filaments.

The phosphorylation state of the FIGQY tyrosine of neurofascin determines ankyrin-binding activity and patterns of cell segregation

Cell-cell recognition and patterning of cell contacts have a critical role in mediating reversible assembly of a variety of transcellular complexes in the nervous system. This study provides evidence for regulation of cell interactions through modulation of ankyrin binding to neurofascin, a member of the L1CAM family of nervous system cell adhesion molecules. The phosphorylation state of the conserved FIGQY tyrosine in the cytoplasmic domain of neurofascin regulates ankyrin binding and governs neurofascin-dependent cell aggregation as well as cell sorting when neurofascin is expressed in neuroblastoma cells. These findings suggest a general mechanism for the patterning of cell contact based on external signals that regulate tyrosine phosphorylation of L1CAM members and modulate their binding to ankyrin.

Morphogenesis of the node of Ranvier: co-clusters of ankyrin and ankyrin-binding integral proteins define early developmental intermediates

AnkyrinG 480/270 kDa and three ankyrin-binding integral membrane proteins (neurofascin, NrCAM, and the voltage-dependent sodium channel) colocalize within a specialized domain of the spectrin-actin network found at axonal segments of nodes of Ranvier in myelinated axons. Before myelination in embryonic nerves, ankyrinG 480/270 kDa and the related ankyrin isoform ankyrinB 440 kDa are co-expressed along with NrCAM in an abundant, continuous distribution along the length of axons. This study has resolved intermediate stages in the developmental transition from a continuous distribution of ankyrinG 480/270 kDa in all axons to a highly polarized localization at the node of Ranvier in the developing rat sciatic nerve. The first detected event is formation of clusters containing the cell adhesion molecules neurofascin and NrCAM at sites independent of myelin-associated glycoprotein (MAG)-staining Schwann cell processes. Subsequent steps involve recruitment of ankyrinG 480/270 kDa and the voltage-dependent sodium channel to cluster sites containing cell adhesion molecules, and elaboration of MAG-staining Schwann cell processes adjacent to these cluster sites. Formation of the mature node of Ranvier results from the fusion of asynchronously formed pairs of clusters associated with MAG-positive Schwann cells flanking the site of presumed node formation. Studies with the hypomyelinating mutant mouse trembler demonstrate that the elaboration of compact myelin is not required for the formation of these clustered nodal intermediates. Clustering of neurofascin and NrCAM precedes redistribution of ankyrinG 480/270 kDa and the voltage-dependent sodium channel, suggesting that the adhesion molecules define the initial site for subsequent assembly of ankyrin and the voltage-dependent sodium channel.

Tyrosine phosphorylation at a site highly conserved in the L1 family of cell adhesion molecules abolishes ankyrin binding and increases lateral mobility of neurofascin

This paper presents evidence that a member of the L1 family of ankyrin-binding cell adhesion molecules is a substrate for protein tyrosine kinase(s) and phosphatase(s), identifies the highly conserved FIGQY tyrosine in the cytoplasmic domain as the principal site of phosphorylation, and demonstrates that phosphorylation of the FIGQY tyrosine abolishes ankyrin-binding activity. Neurofascin expressed in neuroblastoma cells is subject to tyrosine phosphorylation after activation of tyrosine kinases by NGF or bFGF or inactivation of tyrosine phosphatases with vanadate or dephostatin. Furthermore, both neurofascin and the related molecule Nr-CAM are tyrosine phosphorylated in a developmentally regulated pattern in rat brain. The FIGQY sequence is present in the cytoplasmic domains of all members of the L1 family of neural cell adhesion molecules. Phosphorylation of the FIGQY tyrosine abolishes ankyrin binding, as determined by coimmunoprecipitation of endogenous ankyrin and in vitro ankyrin-binding assays. Measurements of fluorescence recovery after photobleaching demonstrate that phosphorylation of the FIGQY tyrosine also increases lateral mobility of neurofascin expressed in neuroblastoma cells to the same extent as removal of the cytoplasmic domain. Ankyrin binding, therefore, appears to regulate the dynamic behavior of neurofascin and is the target for regulation by tyrosine phosphorylation in response to external signals. These findings suggest that tyrosine phosphorylation at the FIGQY site represents a highly conserved mechanism, used by the entire class of L1-related cell adhesion molecules, for regulation of ankyrin-dependent connections to the spectrin skeleton.

Induction of sodium channel clustering by oligodendrocytes

As oligodendrocytes wrap axons of the central nervous system (CNS) with insulating myelin sheaths, sodium channels that are initially continuously distributed along axons become segregated into regularly spaced gaps in the myelin called nodes of Ranvier. It is not known whether the regular spacing of nodes results from regularly spaced glial contacts or is instead intrinsically specified by the axonal cytoskeleton. Contact with Schwann cells induces clustering of sodium channels along the axons of peripheral neurons in vitro and in vivo. Similarly, it has been suggested that astrocyte contact induces clustering of sodium channels along CNS axons. Here we show that oligodendrocytes are necessary for clustering of sodium channels in vitro and in vivo. The induction, but not the maintenance, of sodium-channel clustering along the axons of highly purified rat retinal ganglion cells in culture depends on a protein secreted by oligodendrocytes. Surprisingly, the oligodendrocyte-induced clusters are regularly spaced at the predicted interval in the absence of glial-axonal contact. Mutant rats that are deficient in oligodendrocytes develop few axonal sodium channel clusters in vivo. These results demonstrate a crucial role for oligodendrocytes in inducing clustering of sodium channels.

Synthesis and secretion of von Willebrand factor and fibronectin in megakaryocytes at different phases of maturation

Our goals have been to define the biochemical characteristics of megakaryocytes during maturation that are critical for platelet assembly and release into the circulation and to introduce biochemical markers for megakaryocytes. To achieve these goals, we have studied fibronectin (FN) and von Willebrand factor (vWF), which are large adhesive proteins that are synthesized by megakaryocytes, stored in alpha granules, and thought to have a fundamental role in hemostasis. The study demonstrated that vWF is primarily synthesized in mature megakaryocytes, which synthesized 7.5 times more vWF than immature megakaryocytes. Brefeldin A, which blocks the exit of proteins from the rough endoplasmic reticulum (RER), inhibited the formation of vWF multimers but did not affect the synthesis of monomers and dimers in mature megakaryocytes. These data are consistent with the formation of vWF dimers in the RER and the assembly of vWF multimers in the trans- and post-golgi. The synthesis of both the 260-kD and 275-kD pro-vWF was detected. However, the synthesis of 275-kD pro-vWF and 220-kD mature vWF was only evident after 2 hours, suggesting that the transit time of nascent vWF through the RER is about 2 hours. Constitutive secretion of vWF was demonstrated in megakaryocytes. About 14.5% and 4.6% of synthesized vWF was secreted by mature and immature megakaryocytes, respectively. In contrast, the synthesis of FN monomers and dimers was established in immature megakaryocytes, and their synthesis in mature megakaryocytes was very similar. Constitutive secretion of FN was not seen in megakaryocytes. Brefeldin A did not inhibit the synthesis of FN dimers; thus, formation of FN dimers occurs in the RER. The demonstration that vWF and FN are synthesized at different phases of megakaryocyte maturation and that only vWF is constitutively secreted by megakaryocytes provides new information relevant to alpha granule formation and possibly bone marrow matrix assembly.

Modeling seasonal changes in intracellular freeze-tolerance of fat body cells of the gall fly Eurosta solidaginis (Diptera, Tephritidae)

Although seasonal changes in the freeze-tolerance of third-instar larvae of Eurosta solidaginis have been well documented for the whole organism, the nature of this cold-hardiness at the cellular level has not been examined. Seasonal changes in the survival of fat body cells from E. solidaginis larvae were assessed using fluorescent vital dyes after freezing at -10, -25 or -80 &deg;C for 24 h both in vivo and in vitro. Cells frozen in vitro were frozen with glycerol, with sorbitol (both of which enhanced cell survival) or without cryoprotectants. Both cellular and organismal survival were low in August when larvae were not freeze-tolerant, then increased dramatically during September and October before leveling off from November to January. This observation for cells frozen without cryoprotectants indicates that the cells themselves have adapted. The single most important factor influencing cell survival, as determined by logistic regression modeling, was the time of larval collection, which reflects the level of cold-hardiness achieved by field acclimation. Cells frozen in vivo exhibited greater survival than did those frozen in vitro, even with the addition of cryoprotectants. Since no differences were observed between cells frozen with glycerol or sorbitol, the role of the multi-component cryoprotectant system present in E. solidaginis should be investigated.

Identification of O-linked N-acetylglucosamine modification of ankyrinG isoforms targeted to nodes of Ranvier

AnkyrinGs of 270 and 480 kDa are localized at nodes of Ranvier and are candidates to couple the voltage-dependent sodium channel and neurofascin to the spectrin/actin network. This study presents evidence that these ankyrins contain O-linked GlcNAc residues and identifies as the site of glycosylation a serine-rich domain that distinguishes them from other ankyrin isoforms. The 480-kDa ankyrinG, extracted from brain membranes associated with wheat germ agglutinin-affinity columns, was [3H]galactose-labeled with UDP-[3H] galactose and galactosyltransferase, and cross-reacted with an antibody against O-GlcNAc monosaccharides. AnkyrinG-associated sugars are O-linked monosaccharides based on resistance to peptide-N-glycosidase F and analysis of saccharides released by beta-elimination. The serine-rich domain is the site of glycosylation based on wheat germ agglutinin binding activity of polypeptides produced by in vitro translation in reticulocyte lysates. Immunofluorescence revealed co-localization of ankyrinG and O-GlcNAc immunoreactivity at nodes of Ranvier. These observations suggest that ankyrin at the node of Ranvier is O-GlcNAc-glycosylated and are the first demonstration of a post-translational modification that is concentrated at the node of Ranvier and not in adjacent areas of myelinated axons.

Molecular composition of the node of Ranvier: identification of ankyrin-binding cell adhesion molecules neurofascin (mucin+/third FNIII domain-) and NrCAM at nodal axon segments

Neurofascin, NrCAM, L1, and NgCAM are a family of Ig/FNIII cell adhesion molecules that share ankyrin-binding activity in their cytoplasmic domains, and are candidates to form membrane-spanning complexes with members of the ankyrin family of spectrin-binding proteins in a variety of cellular contexts in the nervous system. Specialized forms of ankyrin, 270 kD and/or 480 kD ankyrinG are components of the membrane undercoat of axons at the node of Ranvier. This paper focuses on definition of the isoforms of ankyrin-binding cell adhesion molecules localized with ankyrinG at the nodal axon segment. The exon usage of two major forms of neurofascin was determined by isolation of full-length cDNAs and used to prepare isoform-specific antibodies. An isoform of neurofascin containing a mucin-like domain and lacking the third FNIII domain was concentrated at axon initial segments and colocalized at nodes of Ranvier with ankyrinG and the voltage-dependent sodium channel. An alternative form of neurofascin lacking the mucin-like domain and containing the third FNIII domain was present in unmyelinated axons. The antibody initially raised against neurofascin was used to screen a rat brain cDNA expression library. In addition to neurofascin, this screen yielded a clone with 80% sequence identity to NrCAM from chicken. The sequences of two full-length cDNAs are presented. NrCAM is most closely related to neurofascin among the other members of the L1/neurofascin/NgCAM family, with over 70% identity between cytoplasmic domains. NrCAM, visualized with antibodies specific for the ecto-domain, also was found to be coexpressed with neurofascin at nodes of Ranvier and at axon initial segments. This is the first characterization of defined neuronal cell adhesion molecules localized to axonal membranes at the node of Ranvier of myelinated axons.

Adducin regulation. Definition of the calmodulin-binding domain and sites of phosphorylation by protein kinases A and C

Adducin promotes association of spectrin with actin and caps the fast growing end of actin filaments. Adducin contains N-terminal core, neck, and C-terminal tail domains, is a substrate for protein kinases A (PKA) and C (PKC), and binds to Ca2+/calmodulin. Ser-726 and Ser-713 in the C-terminal MARCKS-related domains of alpha- and beta-adducin, respectively, were identified as the major phosphorylation sites common for PKA and PKC. PKA, in addition, phosphorylated alpha-adducin at Ser-408, -436, and -481 in the neck domain. Phosphorylation by PKA, but not PKC, reduced the affinity of adducin for spectrin-F-actin complexes as well as the activity of adducin in promoting binding of spectrin to F-actin. The myristoylated alanine-rich protein kinase C substrate-related domain of beta-adducin was identified as the dominant Ca2+-dependent calmodulin-binding site. Calmodulin-binding was inhibited by phosphorylation of beta-adducin and of a MARCKS-related domain peptide by PKA and PKC. Calmodulin in turn inhibited the rate, but not the extent, of phosphorylation of beta-adducin, but not alpha-adducin, by PKA and that of each subunit by PKC. These findings suggest a complex reciprocal relationship between regulation of adducin function by calmodulin binding and phosphorylation by PKA and PKC.

Identification of the spectrin subunit and domains required for formation of spectrin/adducin/actin complexes

Adducin is an actin-binding protein that has been proposed to function as a regulated assembly factor for the spectrin/actin network. This study has addressed the question of the subunit and domains of spectrin required for formation of spectrin/adducin/actin complexes in in vitro assays. Quantitative evidence is presented that the beta-spectrin N-terminal domain plus the first two alpha-helical domains are required for optimal participation of spectrin in spectrin/adducin/actin complexes. The alpha subunit exhibited no detectable activity either alone or following association with beta-spectrin. The critical domains of beta-spectrin involved in complex formation were determined using recombinant proteins expressed in bacteria. The N-terminal domain (residues 1-313) of beta-spectrin associated with F-actin with a Kd of 26 microM, and promoted adducin binding to F-actin with half-maximal activation at 110 nM. Addition of the first alpha-helical domain (residues 1-422) lowered the Kdfor F-actin by 4-fold to 6 microM, but also reduced the capacity by 3-fold and had no effect on interaction with adducin. Further addition of the second alpha-helical domain (residues 1-528) did not alter binding to F-actin but resulted in a 2-fold increased activity in promoting adducin binding with half-maximal activation at 50 nM. Addition of up to eight additional alpha-helical domains (residues 1-1388) resulted in no further change in F-actin binding or association with adducin. These results demonstrate an unanticipated role of the first repeat of beta-spectrin in actin binding activity and of the second repeat in association with adducin/actin, and imply the possibility of an extended contact between adducin, spectrin, and actin involving several actin subunits.

A new function for adducin. Calcium/calmodulin-regulated capping of the barbed ends of actin filaments

Adducin is a membrane skeleton protein originally described in human erythrocytes that promotes the binding of spectrin to actin and also binds directly to actin and bundles actin filaments. Adducin is associated with regions of cell-cell contact in nonerythroid cells, where it is believed to play a role in regulating the assembly of the spectrin-actin membrane skeleton. In this study we demonstrate a novel function for adducin; it completely blocks elongation and depolymerization at the barbed (fast growing) ends of actin filaments, thus functioning as a barbed end capping protein (Kcap approximately 100 nM). This barbed end capping activity requires the intact adducin molecule and is not provided by the NH2-terminal globular head domains alone nor by the COOH-terminal extended tail domains, which were previously shown to contain the spectrin-actin binding, calmodulin binding, and phosphorylation sites. A novel difference between adducin and other previously described capping proteins is that it is down-regulated by calmodulin in the presence of calcium. The association of stoichiometric amounts of adducin with the short erythrocyte actin filaments in the membrane skeleton indicates that adducin could be the functional barbed end capper in erythrocytes and play a role in restricting actin filament length. Our experiments also suggest novel possibilities for calcium regulation of actin filament assembly by adducin in erythrocytes and at cell-cell contact sites in nonerythroid cells.

Mechanism for binding site diversity on ankyrin. Comparison of binding sites on ankyrin for neurofascin and the Cl-/HCO3- anion exchanger

Ankyrins are a family of spectrin-binding proteins that associate with at least seven distinct membrane proteins, including ion transporters and cell adhesion molecules. The membrane-binding domain of ankyrin is comprised of a tandem array of 24 ANK repeats organized into four 6-repeat folding domains. Tandem arrays of ANK repeats have been proposed to mediate protein interactions in a variety of proteins including factors involved in the regulation of transcription and the cell cycle. This report provides several new insights into the versatility of ANK repeats of ankyrin in protein recognition, using neurofascin and the Cl-/HCO3- anion exchanger as model ligands and ankyrinR as the prototypic ankyrin. Different combinations of ANK repeat domains from this ankyrin form two distinct, high affinity binding sites for neurofascin. One site requires both repeat domains 3 and 4. The other site involves both repeat domains 2 and 3, although domain 2 has significant activity alone. The sites appear to be independent with Kd values of 3 and 14 nM, respectively. Both the Cl-/HCO3- anion exchanger and neurofascin can interact simultaneously with repeat domains 3 and 4, because neurofascin is unable to displace binding of the anion exchanger cytoplasmic domain to domains 3 and 4, despite having a 3-5-fold higher affinity. These results demonstrate two levels of diversity in the binding sites on ankyrin: one resulting from different combinations of ANK repeat domains and another from different determinants within the same combination of repeat domains. One consequence of this diversity is that ankyrin can accommodate two neurofascin molecules as well as the anion exchanger through interactions mediated by ANK repeats. The ability of ankyrin to simultaneously associate with multiple types of membrane proteins is an unanticipated finding with implications for the assembly of integral membrane proteins into specialized regions of the plasma membrane.