Assembly and molecular order of two-dimensional peptoid nanosheets through the oil–water interface

Peptoid nanosheets are a recently discovered class of 2D nanomaterial that form from the self-assembly of a sequence-specific peptoid polymer at an air–water interface. Nanosheet formation occurs first through the assembly of a peptoid monolayer and subsequent compression into a bilayer structure. These bilayer materials span hundreds of micrometers in lateral dimensions and have the potential to be used in a variety of applications, such as in molecular sensors, artificial membranes, and as catalysts. This paper reports that the oil–water interface provides another opportunity for growth of these unique and highly ordered peptoid sheets. The monolayers formed at this interface are found through surface spectroscopic measurements to be highly ordered and electrostatic interactions between the charged moieties, namely carboxylate and ammonium residues, of the peptoid are essential in the ability of these peptoids to form ordered nanosheets at the oil–water interface. Expanding the mechanism of peptoid nanosheet formation to the oil–water interface and understanding the crucial role of electrostatic interactions between peptoid residues in nanosheet formation is essential for increasing the complexity and functionality of these nanomaterials.Recent advances in the design, synthesis, and characterization of 2D nanomaterials with atomic precision have given rise to ultrathin materials with unprecedented functionality (1). Specifically, organic 2D nanomaterials hold promise as biocompatible materials that can be chemically tailored and built from the bottom up, through the self-assembly of small molecule, protein, or polymer building blocks (2–5). Polymer-based 2D nanomaterials, in particular, hold promise as templates for bottom-up assembly of circuits, semiconductors, and organic–inorganic composite materials (6–8) as well as high surface area membranes for filtration, catalysis, and sensing (9, 10). Material properties, such as precisely defined porosity, metal or protein recognition sites, and other reactive groups (5, 11, 12), can be patterned directly into the polymer sequence. Peptoids are a highly designable class of polymer, recently designed to assemble into 2D peptoid nanosheets (13–15). Synthesized from a chemically diverse set of cheap, commercially available building blocks (16), peptoids have an exact monomer sequence that can direct chain folding into higher order nanostructures. Peptoid nanosheets span hundreds of micrometers in lateral dimensions, are only 3 nm thick, and are freely floating in water. The outer surfaces of the nanosheet can be readily functionalized with a controlled spatial density of binding sites (5), and the nanosheets are stable across a wide range of temperatures and pH values (14), making peptoid nanosheets a rugged, highly designable platform for bottom-up assembly of 2D nanomaterials and composite materials.Peptoid nanosheets have previously been made through a unique assembly mechanism involving the organization of molecular units at the air–water (air–H₂O) interface (15, 17). Here we report that peptoid nanosheets can also form at an oil–water interface. A key intermediate in the nanosheet assembly pathway is the formation of a peptoid monolayer. Adsorption at a 2D fluid interface preorganizes the linear peptoid chains into the orientation needed for bilayer formation, with aromatic groups preferentially displayed on the nonpolar side of the monolayer and polar groups on the aqueous side (Fig. 1). Upon compression, the monolayer buckles into a bilayer, where the nonpolar side of the monolayer becomes the interior, structural core of the nanosheet. Aromatic–aromatic interactions on the nonpolar side of the monolayer likely contribute to the nanosheet’s ordered atomic structure and stability. Importantly, the spacing between peptoid chains in the monolayer mirrors the spacing observed in the resultant nanosheet. Thus, the lateral assembly of the chains in the peptoid monolayer is crucial for dictating the structure and stability of the resultant peptoid nanosheets. Substituting oil in place of air, as the nonpolar phase, opens up opportunities to engineer the nanosheet structure and production method. For example, the oil phase could contain chemical reagents, serve to minimize evaporation of the aqueous phase, or enable microfluidic production of nanosheets.Fig. 1.Schematic illustration of the assembly pathway for peptoid nanosheet formation at the oil–water interface.Engineering peptoid nanosheet structure requires precise control of the monolayer intermediate. Thus, a molecular level understanding of the monolayer is required. For example, the chemical cross-linking of neighboring chains, as a means of improving nanosheet stability, requires knowledge of the chain conformation. Ultimately, a true understanding of the relationship between monomer sequence and nanosheet structure requires atomic-level structural data. To date, most of the structural data on nanosheets are from scattering and microscopy measurements (13–15), which provide excellent information on the nanoscale ordering, but little on chain conformation. The study of chain conformation and supramolecular interaction between neighboring chains in the monolayer intermediate requires surface-sensitive spectroscopic techniques. To this end, we use total internal reflection (TIR) vibrational sum frequency (VSF) spectroscopy (18) and interfacial tension measurements to study peptoid assembly at the carbon tetrachloride–aqueous (CCl₄–H₂O) interface. VSF spectroscopy is a well-established surface-selective technique that produces vibrational spectra of interfacial molecules (19). As with traditional vibrational spectroscopic techniques, peak center frequencies and widths of VSF spectra are sensitive to the chemical environments of the functional groups, and can thus provide details about specific intermolecular forces at play during the assembly of a monolayer. VSF spectroscopy additionally has the capability of probing molecular orientation; signal is only generated when molecules adsorb to the interface with their functional groups aligned in a net orientation relative to the plane of the interface. Based on these merits, VSF spectroscopy is an optimal technique to study the molecular-level structural and conformational details of the key peptoid monolayer intermediate.To elucidate the role of polymer side-chains in the supramolecular assembly, we take advantage of the precision sequence control of peptoid polymers. Exact peptoid sequences can be designed and synthesized using the submonomer solid-phase method (20). The ability to synthesize an exact polymer sequence enables atomic-level control over molecular design parameters, such as polymer chain length, sequence patterning, and side-chain chemistry. Thus, peptoid polymers serve as convenient building blocks for the systematic study of structure–property relationships between molecular design and nanosheet material properties. In this work, we compare the interfacial adsorption and assembly of a previously reported (5, 14) peptoid 1 (Fig. 2A) with its carboxyl analog, peptoid 2 (Fig. 2B), wherein the amine residues of 1 have been replaced with carboxyl groups. Peptoid 1 was found to be more stable than an alternating charge structure (14). In this way, we isolate the contribution of the charged functional groups from the aromatic groups, and show that the charged functional groups play an important role in the sequence design and 2D assembly of 1.Fig. 2.Chemical structures of (A) peptoid 1 and (B) peptoid 2 investigated in this work. Peptoid 1 formed nanosheets whereas 2 did not.     

Fe-vacancy order and superconductivity in tetragonal β-Fe1-xSe

Several superconducting transition temperatures in the range of 30–46 K were reported in the recently discovered intercalated FeSe system (A_1-xFe_2-ySe₂, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal β-Fe_1-xSe). Three types of iron-vacancy order were found through analytical electron microscopy, and one was identified to be nonsuperconducting and magnetic at low temperature. This discovery suggests that the rich-phases found in A_1-xFe_2-ySe₂ are not exclusive in Fe-Se and related superconductors. In addition, the magnetic β-Fe_1-xSe phases with particular iron-vacancy orders are more likely to be the parent phase of the FeSe superconducting system instead of the previously assigned β-Fe_1+δTe.The iron pnictide superconductors have opened the door to a new way to obtain superconductivity at very high temperatures. β-Fe_1+δSe is remarkable among those superconductors in that it contains the essential electronic and structural constituents required for superconductivity without the conceptual complexity seen in other systems (1). Previous studies showed that the superconducting property of β-Fe_1+δSe made under high-temperature thermodynamic conditions is very sensitive to stoichiometry (1, 2). In the Fe-Se binary phase diagram (2–4), the PbO-type tetragonal structure (the β phase) only stabilized at the Fe-rich side (δ = 0.01–0.04), whereas bulk superconductivity was observed in samples with δ close to 0.01 (5). McQueen et al. showed no superconductivity for samples with δ = 0.03 (5). On the other hand, the fact that only one superconducting phase has been reported in FeSe, unlike the other Fe-As–based superconductors that exhibit clear doping dependence of superconductivity and the absence of superconductivity in FeTe, led to the suggestion that FeTe is the nonsuperconducting parent compound of FeSe (6). Thus, the phase diagram derived from this picture shows very different features compared with other Fe-As–based superconductors (6, 7). In this work, we use low-temperature synthesis methods to prepare β-Fe_1-xSe for a wide range of compositions, which allows for the determination for the composition-dependent electronic behavior for this important superconducting system.The recent discovered alkali/alkaline-intercalated iron selenide (A_1-xFe_2-ySe₂) superconductors with rich superconducting phases, where A = K, Rb, Cs, Tl, attracted great attention not only due to its high superconducting transition temperature (T_c, up to 46 K) (8), but also because of their dissimilar characteristics compared with other iron-based superconductors, especially its seemingly intrinsic multiphase nature and the presence of iron vacancies and orders in the nonsuperconducting regime (9–13). The most frequently observed Fe-vacancy order in A_1-xFe_2-ySe₂ is the × × 1 superstructure, which yields a phase of A_0.8Fe_1.6Se₂ (or A₂Fe₄Se₅). Scanning tunneling microscopy (STM) (11, 14, 15) and transport studies (12, 13, 16, 17) showed that A₂Fe₄Se₅ is an antiferromagnetic (AFM) insulator. Neutron scattering measurements (9) revealed a blocked checkerboard AFM with magnetic moments along the c axis for A₂Fe₄Se₅, ordered at a temperature as high as >500 K, with an unexpected large ordered magnetic moment of ∼3.3 μ_B/Fe at 10 K. Experiments have further shown that the type of vacancy and magnetic orders is highly sensitive to the stoichiometry (x and y) of A_1-xFe_2-ySe₂. Reports have shown the existence of other Fe-vacancy order with the forms × × 1 (10), × 2 × 1 (13, 18), and × × 1 (19). However, the magnetic properties such as the type and transition temperature of the magnetic order are far less studied compared with that of the K₂Fe₄Se₅ phase. In addition, there were also results showing in K_1-xFe_2-ySe₂ samples with a typical T_c = 31 K and additional superconducting phase with T_c = 44 K (20), whereas no clear identification of the new phases was available.The complexity of phases and phase separation during crystal preparation in A_1-xFe_2-ySe₂ make it difficult to conclusively verify the phase-property relationship, even for the superconducting phases. β-Fe_1+δSe, on the other hand, has the simplest structure among all iron-based superconductor families. Several surprising results related to the Fe-Se system appeared in the literature during the last few years, including the enhancement of T_c to about 40 K under high pressure (21–23) and the intriguing extremely high T_c (with a superconducting energy gap of ∼20 meV) in molecular beam epitaxy (MBE)-grown single-layer FeSe (24–26). We also demonstrated the presence of a superconducting-like feature with T_c close to 40 K in samples of nano-dimensional form (27). Therefore, it is quite natural to ask whether the presence of the complex phases observed in A_1-xFe_2-ySe₂ compounds and Fe-vacancy order exist in samples without alkaline metals. Here we present the first discovery of iron vacancies and three types of vacancy orders in tetragonal β-Fe_1-xSe, characterized by analytical transmission electron microscopy (TEM). Our observations imply that an unprecedented phase diagram should be considered in the Fe-Se superconductors.     

Genomes of Strongylocentrotus franciscanus and Lytechinus variegatus: are there any genomic explanations for the two order of magnitude difference in the lifespan of sea urchins?

Sea urchins are marine invertebrates of extreme diversity of life span. Red sea urchin S. franciscanus is among the longest living creatures of the Ocean. Its lifetime is estimated to exceed a century, while the green sea urchin L. variegatus hardly survives more than four years. We sequenced and compared the genomes of these animals aiming at determination of the genetic basis of their longevity difference. List of genes related to the longevity of other animal species was created and used for homology search among the genomic data obtained in this study. Aminoacid sequences of longevity related proteins of S. franciscanus and L. variegatus as well as from a set of model species, were aligned and grouped on the basis of the species lifespan. Aminoacid residues specific for a longevity group were identified. Proteins containing aminoacids whose identity correlated with the lifespan were clustered on the basis of their function.     

Microscopic identification of the order parameter governing liquid–liquid transition in a molecular liquid

A liquid–liquid transition (LLT) in a single-component substance is an unconventional phase transition from one liquid to another. LLT has recently attracted considerable attention because of its fundamental importance in our understanding of the liquid state. To access the order parameter governing LLT from a microscopic viewpoint, here we follow the structural evolution during the LLT of an organic molecular liquid, triphenyl phosphite (TPP), by time-resolved small- and wide-angle X-ray scattering measurements. We find that locally favored clusters, whose characteristic size is a few nanometers, are spontaneously formed and their number density monotonically increases during LLT. This strongly suggests that the order parameter of LLT is the number density of locally favored structures and of nonconserved nature. We also show that the locally favored structures are distinct from the crystal structure and these two types of orderings compete with each other. Thus, our study not only experimentally identifies the structural order parameter governing LLT, but also may settle a long-standing debate on the nature of the transition in TPP, i.e., whether the transition is LLT or merely microcrystal formation.Liquid-liquid transition (LLT) is an intriguing phenomenon in which a liquid transforms into another one via a first-order transition. This means that there can be more than two liquid states for a single-component substance. Despite its counterintuitive nature, there have recently been many pieces of experimental and numerical evidence for the existence of LLT, for various liquids such as water (1–5), aqueous solutions (6–8), triphenyl phosphite (9–12), l-butanol (13), phosphorus (14), silicon (15, 16), germanium (17), and Y₂O₃–Al₂O₃ (18, 19). This suggests that the LLT may be rather universally observed for various types of liquids. However, none of the LLTs reported so far is free from criticisms (20, 21), mainly because these LLTs take place under experimentally difficult conditions [e.g., at high temperature and pressure (14, 15, 17–19)] or in a supercooled state below the melting point (1–3, 5–7, 9, 10), where the transition is inevitably contaminated by microcrystal formation. The latter is not limited to experiments but arises in numerical simulations, often causing many controversies [LLT (22–25) vs. crystallization (26–28)]. For ST2 water, however, this issue has recently been settled by an extensive simulation study by Palmer et al. (4).One of the hottest and long-standing debates is on the nature of the transition found in a molecular liquid, triphenyl phosphite (TPP), by Kivelson and his coworkers (29). The transition is very easy to access experimentally, because it takes place at ambient pressure and at a temperature range between 230 and 210 K and the transformation speed is slow enough to follow the kinetics. Since the finding of this transition (29, 30), many researchers thus have been interested in this intriguing phenomenon and there have been hot discussions on the nature of the transition (20, 21). Some people interpreted this as a liquid-associated phenomenon (9, 10, 31, 32), but others interpret it differently. All of the controversies come from the fact that this transition accompanies microcrystal formation and thus the final state, which is called “glacial phase,” often contains microcrystallites. This led many researchers to explain the transition by non-LLT scenarios, which include a defect-ordered phase scenario predicted by a frustration limited domain theory (29, 30, 33, 34), a microcrystallization scenario (35–38), and a liquid-crystal or plastic-crystal phase scenario (39). Each scenario captures a certain feature of the glacial phase, but fails in explaining all of the experimental results in a consistent manner. Similar situations are often seen in other candidates of LLTs, such as l-butanol [LLT (13) vs. microcrystallization (40–43)], confined water [LLT (5) vs. other phenomena (44–46)], and aqueous solutions [LLT (6, 7) vs. microcrystallization (8, 28, 47, 48)]. For TPP, however, some pieces of experimental evidence supportive of the LLT scenario rather than the microcrystallization scenario have recently been reported (11, 12).We propose a two-order-parameter (TOP) model of a liquid to explain LLT (20, 49). The main point of this model is that it is necessary to consider the spatiotemporal hierarchical nature of a liquid to understand LLT. More specifically, we argue that in addition to density order parameter ρ describing a gas–liquid transition, we need an additional scalar order parameter S, which is the number density of locally favored structures (LFS). In this model, LLT is a consequence of the cooperative ordering of the scalar nonconserved order parameter S, i.e., the cooperative formation of LFS. In other words, LLT is regarded as a gas–liquid-like transition of LFS: one liquid is a gas state of LFS (low-S state), and the other is its liquid state (high-S state). Recently, it was proposed by Anisimov and coworkers (50, 51) that the thermodynamic ordering field conjugate to the order parameter is the conversion equilibrium constant, which further characterizes the nature of LLT. We explained our experimental observation of LLT in TPP in terms of this model (9, 10). We also studied the phase transition dynamics and the physical and chemical properties of the second liquid state (liquid II), which were also explained by the model (20, 21).However, we have not had any direct experimental evidence for the formation of such LFS up to now; thus, an open question is, what is the relevant order parameter governing LLT, although the link of the order parameter to the enthalpy (9, 10), the refractive index (or, density) (9, 10, 29, 30), and the polarity associated with local molecular ordering (12) has been suggested for LLT in TPP. There have been structural studies on LLT by X-ray and neutron scattering measurements, focusing on local liquid structures at an inter- and intramolecular scale (36, 38, 52–54) and mesoscopic structures (34, 55). However, there has been no experimental evidence for the presence of locally favored structures, which characterize the liquid state uniquely, or the order parameter has still not been identified from a microscopic viewpoint.Here we study the structural change of TPP during LLT by time-resolved small- and wide-angle X-ray scattering measurements, which cover a length scale from a single molecule size ( ～  1 nm) to more than tens of nanometers. We show, to our knowledge, the first direct evidence for the presence of LFS and the temporal increase upon the liquid I-to-liquid II transformation. Furthermore, we also find an indication of the formation of microcrystallites during LLT. However, we reveal that LFS and microcrystallites have different sizes and growth kinetics, indicating that although they sometimes appear simultaneously during the process of LLT, LLT itself is driven by the formation of LFS and not by that of microcrystallites. We also discover that LFS are destroyed upon crystallization, clearly indicating not only that these two types of orderings are competing with each other but also that LFS is a structure unique to the liquid state. Our findings provide a comprehensive view on the long-standing controversy on the origin of the glacial phase, which was discovered by Kivelson and his coworkers (29, 30), and show that the fraction of LFS may be the relevant order parameter of LLT. This suggests that a liquid can have a spatiotemporal hierarchical structure at a low temperature, contrary to the common picture of a high-temperature liquid where the structure is random and homogeneous beyond the molecular size.     

Gerontology across the professions and the Atlantic: Students’ reflective views and voices

ABSTRACT

Health professions students need to have increasing exposure to interprofessional and international experience in developing the knowledge and skills needed to work with older adults. As students, the authors explore in this article the significant elements of our learning that took place in a blended Gerontology Across the Professions and the Atlantic course for participants from the United States, Canada, and Norway. These factors focus on the following aspects of this course: (1) weekly online topic discussions and learning experiences, (2) group case studies and presentations, (3) international perspectives, (4) interprofessional perspectives, and (5) the final course seminar in Bergen. The authors end their discussion by sharing sidebar stories of their experiences in this course that brought together the powerful, transformative elements of interprofessional and international insights into the challenges of geriatric care in the future.     

Functional requirements of AID’s higher order structures and their interaction with RNA-binding proteins

Activation-induced cytidine deaminase (AID) is essential for the somatic hypermutation (SHM) and class-switch recombination (CSR) of Ig genes. Although both the N and C termini of AID have unique functions in DNA cleavage and recombination, respectively, during SHM and CSR, their molecular mechanisms are poorly understood. Using a bimolecular fluorescence complementation (BiFC) assay combined with glycerol gradient fractionation, we revealed that the AID C terminus is required for a stable dimer formation. Furthermore, AID monomers and dimers form complexes with distinct heterogeneous nuclear ribonucleoproteins (hnRNPs). AID monomers associate with DNA cleavage cofactor hnRNP K whereas AID dimers associate with recombination cofactors hnRNP L, hnRNP U, and Serpine mRNA-binding protein 1. All of these AID/ribonucleoprotein associations are RNA-dependent. We propose that AID’s structure-specific cofactor complex formations differentially contribute to its DNA-cleavage and recombination functions.Activation-induced cytidine deaminase (AID), which is expressed in antigen-stimulated mature B cells, is essential for Ig somatic hypermutation (SHM) and class-switch recombination (CSR) (1, 2). AID induces DNA breaks at the variable (V) and switch (S) regions during SHM and CSR, respectively (3, 4). Although both processes are initiated by AID-induced DNA cleavage, point mutations at the V region are executed mostly by error-prone DNA repair whereas CSR is accomplished by recombination of cleaved ends at donor and acceptor S regions (5, 6). However, the detailed mechanisms by which AID carries out the two mechanistically distinct functions for SHM and CSR have yet to be uncovered (7). Studies on AID mutants revealed that AID’s N- and C-terminal domains are distinctly required for its DNA-cleavage and recombination functions, respectively (8–10). Mutations at the N terminus of AID impair SHM as well as CSR whereas those at the C terminus abrogate CSR only and show increased SHM activity. Recent studies demonstrated that the CSR process after DNA cleavage, including the synapsis formation between cleaved ends, is impaired with the C-terminally defective AID, indicating that AID’s C terminus confers a CSR-specific recombination function, independent of AID’s DNA cleavage function, by an unknown mechanism (11, 12).AID belongs to the APOBEC (apolipoprotein B mRNA-editing enzyme catalytic polypeptide) family of cytidine deaminases (CDDs) and shows high sequence homology with APOBEC1 (A1) (1, 13, 14), which edits apolipoprotein B (APOB) mRNA. The APOB mRNA editing ability of A1 is highly dependent on its cofactors, A1CF/ACF (15, 16) and RBM47 (17), both of which belong to the heterogeneous nuclear ribonucleoprotein (hnRNP) family. Recently, two A1CF-like hnRNPs, hnRNP K and hnRNP L, were identified as the cofactors of AID and found to be involved in the cleavage and recombination of DNA, respectively (18). Because the N and C termini of AID differentially regulate two functions of AID—cleavage and recombination, respectively—we speculated that the AID termini would be critical for function-coupled cofactor association. For instance, the N or C terminus of AID may function as a molecular switch that induces an AID–AID interaction, enabling AID to exert distinct physiological functions through its association with cofactors. Regrettably, however, there is little structural information available that can explain any of AID’s regulatory modes of action, including its cofactor association mechanisms, in the context of its physiological functions.Although a significant amount of structural information is available for a number of APOBEC family members, the 3D structures of A1 and AID are yet to be resolved (19, 20). The CDD family of enzymes exists in nature in a variety of structural forms, including monomeric, dimeric, and tetrameric forms, and comparative structural modeling using the yeast CDD structure predicts a dimeric structure for both A1 and AID (21, 22). On the other hand, homology modeling with the APOBEC2 (A2) crystal structure, which seems to be a tetramer composed of two head-to-head interacting dimers, predicts that AID forms a tetramer (23). Notably, A2 was later reported to exist as a monomer in solution (24). Similarly, an atomic force microscopic (AFM) study found that AID exists in the cell predominantly as a monomer associated with a single-strand DNA substrate (25). However, the same AFM dataset was interpreted differently by another group of investigators, who concluded that AID probably forms an A2-like tetramer in solution (26). The modeling of AID’s catalytic pocket in reference to eight APOBEC family members suggested that most of the AID–DNA complex remains in an inactive state due to occlusion by the substrate DNA, which may explain its weak catalytic activity for cleaving DNA in vitro (27).One of the limitations of the computational modeling of AID’s structure is that AID’s N-and C-terminal sequences are substantially different from those of other APOBEC members and thus reside outside the modeling template. Although the structural outcome of a protein can differ by a variety of reasons, including the methods applied (28), none of the AID studies mentioned above explain why the C-terminal deletion of AID leads to the loss of CSR function only. Therefore, model-based computational simulation may not explain the physiological structure–function relationship of AID in B cells.Here, we explored AID’s structure–function relationship using a bimolecular fluorescence complementation (BiFC) assay, which detects homo- or heteromeric protein–protein interactions in live cells (29, 30). For the homomeric interaction assay, the target protein is fused to two nonfluorescent halves of a green or red fluorescent protein. An interaction between two of the target proteins brings the two nonfluorescent halves of the fluorescent protein into close proximity, reconstituting the fluorescence. The BiFC assay thus allows a rapid analysis of the dimerization of a protein of interest in live cells.By combining this assay with other biochemical approaches, such as coimmunoprecipitation (co-IP) and glycerol gradient sedimentation, we revealed the presence of both monomeric and dimeric forms of AID in analyzed cells. Intriguingly, C-terminal AID mutants that lost CSR function showed a severe dimerization defect, suggesting that AID’s C terminus is required to stabilize the dimeric structure that is required for CSR. We also showed that the AID monomer and dimer associate with different RNA-binding proteins (RBPs) to form ribonucleoprotein (RNP) complexes. Based on these findings, we propose that the monomeric AID–RNP complex includes hnRNP K (18) and contributes to the DNA cleavage function of AID whereas the dimeric AID–RNP complexes include hnRNP L (18), hnRNP U (31), or Serpine mRNA-binding protein 1 (SERBP1) (32) and contribute to the recombination step of CSR.     

What are the risks and the benefits of current and emerging weight-loss medications?

Obesity is epidemic; new medications and therapeutic options are urgently needed to reduce the associated health care burden. The initial clinical strategy for weight loss is lifestyle modification involving a combination of diet, exercise, and behavior change. However, it is difficult for many to achieve and maintain weight loss solely through this approach. Only two drugs, orlistat and sibutramine, have been approved by the US Food and Drug Administration (FDA) to treat obesity long term, and both medications have undesirable side effects, leaving an enormous unmet need for efficacious and safe therapy for obesity. Other medications with weight-loss effects have been approved by the FDA for short-term treatment of obesity or for disorders other than obesity, but these also have potential adverse effects. This article discusses the perceived benefits and risks of these approved medications along with emerging drugs that have shown weight-loss effects.     

Inflammatory bowel disease and the genes for the natural resistance-associated macrophage protein-1 and the interferon-Á receptor 1

The genes for the interferon-γ receptor1 and the natural resistance-associated macrophage protein1 (NRAMP1) control the immune response to intracellular microbial pathogens. Such pathogens, in particular Mycobacterium paratuberculosis, have been implicated in the pathogenesis of Crohn's disease. We studied markers in the genes for NRAMP1 and two mutations in the interferon-γ receptor in relation to inflammatory bowel disease (IBD) in the following groups: 270 healthy individuals, 74 patients with Crohn's disease, 72 patients with ulcerative colitis, and 40 patients with primary sclerosing cholangitis. We studied the allele frequencies of two restriction fragment length polymorphisms in the gene for NRAMP1 and the prevalence of two mutations in the interferon-γ receptor1 gene. The markers in the NRAMP1 gene were not associated with inflammatory bowel disease. Also, the mutations in the interferon-γ receptor1 were not found in the 186 IBD patients. Genetic markers in NRAMP1 are thus not associated with IBD. Therefore this gene is not likely to play a role in the pathogenesis of IBD. The mutation in the interferon-γ receptor was not found in our IBD patients group. Accepted: 13 October 1998