Membrane-microdomain localization of amyloid β-precursor protein (APP) C-terminal fragments is regulated by phosphorylation of the cytoplasmic Thr668 residue

Amyloid β-precursor protein (APP) is primarily cleaved by α- or β-secretase to generate membrane-bound, C-terminal fragments (CTFs). In turn, CTFs are potentially subject to a second, intramembrane cleavage by γ-secretase, which is active in a lipid raft-like membrane microdomain. Mature APP (N- and O-glycosylated APP), the actual substrate of these secretases, is phosphorylated at the cytoplasmic residue Thr(668) and this phosphorylation changes the overall conformation of the cytoplasmic domain of APP. We found that phosphorylated and nonphosphorylated CTFs exist equally in mouse brain and are kinetically equivalent as substrates for γ-secretase, in vitro. However, in vivo, the level of the phosphorylated APP intracellular domain peptide (pAICD) generated by γ-cleavage of CTFs was very low when compared with the level of nonphosphorylated AICD (nAICD). Phosphorylated CTFs (pCTFs), rather than nonphosphorylated CTFs (nCTFs), were preferentially located outside of detergent-resistant, lipid raft-like membrane microdomains. The APP cytoplasmic domain peptide (APP(648-695)) with Thr(P)(668) did not associate with liposomes composed of membrane lipids from mouse brain to which the nonphosphorylated peptide preferentially bound. In addition, APP lacking the C-terminal 8 amino acids (APP-ΔC8), which are essential for membrane association, decreased Aβ generation in N2a cells. These observations suggest that the pCTFs and CTFΔC8 are relatively movable within the membrane, whereas the nCTFs are susceptible to being anchored into the membrane, an interaction made available as a consequence of not being phosphorylated. By this mechanism, nCTFs can be preferentially captured and cleaved by γ-secretase. Preservation of the phosphorylated state of APP-CTFs may be a potential treatment to lower the generation of Aβ in Alzheimer disease.

P2 receptors in renal pathophysiology

Our knowledge and understanding of the P2 receptor signalling system in the kidney have increased significantly in the last ten years. The broad range of physiological roles proposed for this receptor system and the variety of P2 receptor subtypes found in the kidney suggest that any disturbance of function may contribute to several pathological processes. So far, most reports of a possible pathophysiological role for this system in the kidney have focussed on polycystic kidney disease, where abnormal P2 receptor signalling might be involved in cyst expansion and disease progression, and on the P2X(7) receptor, a unique P2X subtype, which when activated enhances inflammatory cytokine release and production, and also cell death. Expression of this particular receptor is upregulated in some forms of chronic renal injury and inflammatory diseases. Further studies of adenosine triphosphate signalling and P2 receptor expression in renal disorders could provide us with novel insights into the role of these receptors in both normal and abnormal kidney function.

P2X7 deficiency attenuates renal injury in experimental glomerulonephritis

The P2X7 receptor is a ligand-gated cation channel that is normally expressed by a variety of immune cells, including macrophages and lymphocytes. Because it leads to membrane blebbing, release of IL-1beta, and cell death by apoptosis or necrosis, it is a potential therapeutic target for a variety of inflammatory diseases. Although the P2X7 receptor is usually not detectable in normal renal tissue, we previously reported increased expression of both mRNA and protein in mesangial cells and macrophages infiltrating the glomeruli in animal models of antibody-mediated glomerulonephritis. In this study, we used P2X7-knockout mice in the same experimental model of glomerulonephritis and found that P2X7 deficiency was significantly renoprotective compared with wild-type controls, evidenced by better renal function, a striking reduction in proteinuria, and decreased histologic glomerular injury. In addition, the selective P2X7 antagonist A-438079 prevented the development of antibody-mediated glomerulonephritis in rats. These results support a proinflammatory role for P2X7 in immune-mediated renal injury and suggest that the P2X7 receptor is a potential therapeutic target.

Lymphocytes from P2X7-deficient mice exhibit enhanced P2X7 responses

The purinergic receptor P2X(7) is expressed on immune cells, and its stimulation results in the release of IL-1beta from macrophages. Its absence, as evidenced from the analysis of two independent strains of P2X(7)-deficient mice, results in reduced susceptibility to inflammatory disease, and the molecule is an important, potential therapeutic target in autoimmunity. However, P2X(7) has also been detected in several neuronal cell types, although its function and even its presence in these cells are highly contested, with anti-P2X(7) antibodies staining brain tissue from both strains of P2X(7)(-/-) mice identically to wild-type mice. It has therefore been suggested that neurons express a distinct "P2X(7)-like" protein that has similar antibody recognition epitopes to P2X(7) and some properties of the genuine receptor. In this study, we show that whereas P2X(7) activity is absent from macrophages and dendritic cells in P2X(7)(-/-) animals, T cells from one gene-deficient strain unexpectedly exhibit higher levels of P2X(7) activity than that found in cells from control, unmanipulated C57BL/6 mice. A potential mechanism for this tissue-specific P2X(7) expression in P2X(7)(-/-) animals is discussed, as is the implication that the immune and indeed neuronal functions of P2X(7) may have been underestimated.

Sequential shrinkage and swelling underlie P2X7-stimulated lymphocyte phosphatidylserine exposure and death

Patterns of change in cell volume and plasma membrane phospholipid distribution during cell death are regarded as diagnostic means of distinguishing apoptosis from necrosis, the former being associated with cell shrinkage and early phosphatidylserine (PS) exposure, whereas necrosis is associated with cell swelling and consequent lysis. We demonstrate that cell volume regulation during lymphocyte death stimulated via the purinergic receptor P2X7 is distinct from both. Within seconds of stimulation, murine lymphocytes undergo rapid shrinkage concomitant with, but also required for, PS exposure. However, within 2 min shrinkage is reversed and swelling ensues ending in cell rupture. P2X7-induced shrinkage and PS translocation depend upon K+ efflux via KCa3.1, but use a pathway of Cl- efflux distinct from that previously implicated in apoptosis. Thus, P2X7 stimulation activates a novel pathway of cell death that does not conform to those conventionally associated with apoptosis and necrosis. The mixed apoptotic/necrotic phenotype of P2X7-stimulated cells is consistent with a potential role for this death pathway in lupus disease.

Regulatory T Cells Are Resistant to Apoptosis via TCR but Not P2X7

Regulatory T cells (Tregs) are relatively autoreactive yet, paradoxically, have been found to display normal sensitivity to thymic deletion. The relationship between self-avidity, apoptosis, and the selection of Tregs therefore remains unclear. We show that thymic Tregs develop efficiently, even at low self-avidity, and are moderately resistant to apoptosis in comparison to conventional thymocytes. Consistent with this, although conventional self-reactive T cell populations undergo chronic peripheral deletion, self-reactive Tregs are largely spared removal. Similarly, the distribution of Tregs among peripheral CD4(+) cells exhibits a linear inverse relationship with CD45RB expression, indicating relative apoptosis resistance of Tregs in chronic responses to environmental Ags. We also show that appropriate controls for CD45RB levels are important for comparisons of Treg and conventional T cell activity. When thus controlled, and contrary to previous reports, Tregs exhibit normal sensitivity to cell death through TCR-independent stimuli, such as the purinergic receptor, P2X(7). Finally, although absence of CD45 in gene-targeted mice results in profound T cell hyporesponsiveness, there is little or no effect on thymic Treg frequency. In summary, the data support a model in which signal strength plays little part in Treg lineage specification, though moderate resistance of self-reactive Tregs to apoptosis may result in progressive biasing of peripheral Treg TCRs toward autoreactivity in comparison to those of conventional T cells.

Phosphatidylserine exposure in B lymphocytes: a role for lipid packing

Plasma membrane lipids are usually distributed asymmetrically, with phosphatidylserine (PS) confined to the inner leaflet. PS exposure at the outer leaflet occurs early in apoptosis, but it is also constitutive on some nonapoptotic cell populations where it plays a role in cell signaling. How PS is transported (“flopped”) to the cell surface is unknown. Contrary to previous reports that normal murine B lymphocytes lack lipid asymmetry, we show that PS is normally restricted to the inner leaflet of these cells. PS exposure on normal B cells did, however, occur spontaneously ex vivo. Consistent with the hypothesis that loss of PS asymmetry is regulated by CD45, PS is constitutively exposed on viable, CD45-deficient B cells. We show that calcium-stimulated PS exposure in B cells is strain variable, ABCA1 independent, and both preceded by and dependent on a decrease in lipid packing. This decrease in lipid packing is concomitant with cell shrinkage and consequent membrane distortion, both of which are potently inhibited by blockers of volume-regulatory K+ and Cl- ion channels. Thus, changes in plasma membrane organization precede PS translocation. The data suggest a model in which PS redistribution may occur by a translocase-independent mechanism at energetically favorable sites of membrane perturbation where lipid packing is decreased.

Inhibitors of amyloid toxicity based on beta-sheet packing of Abeta40 and Abeta42

Amyloid fibrils associated with Alzheimer's disease and a wide range of other neurodegenerative diseases have a cross beta-sheet structure, where main chain hydrogen bonding occurs between beta-strands in the direction of the fibril axis. The surface of the beta-sheet has pronounced ridges and grooves when the individual beta-strands have a parallel orientation and the amino acids are in-register with one another. Here we show that in Abeta amyloid fibrils, Met35 packs against Gly33 in the C-terminus of Abeta40 and against Gly37 in the C-terminus of Abeta42. These packing interactions suggest that the protofilament subunits are displaced relative to one another in the Abeta40 and Abeta42 fibril structures. We take advantage of this corrugated structure to design a new class of inhibitors that prevent fibril formation by placing alternating glycine and aromatic residues on one face of a beta-strand. We show that peptide inhibitors based on a GxFxGxF framework disrupt sheet-to-sheet packing and inhibit the formation of mature Abeta fibrils as assayed by thioflavin T fluorescence, electron microscopy, and solid-state NMR spectroscopy. The alternating large and small amino acids in the GxFxGxF sequence are complementary to the corresponding amino acids in the IxGxMxG motif found in the C-terminal sequence of Abeta40 and Abeta42. Importantly, the designed peptide inhibitors significantly reduce the toxicity induced by Abeta42 on cultured rat cortical neurons.

Membrane phosphatidylserine distribution as a non-apoptotic signalling mechanism in lymphocytes

Phosphatidylserine (PS) exposure is normally associated with apoptosis and the removal of dying cells. We observed that PS is exposed constitutively at high levels on T lymphocytes that express low levels of the transmembrane tyrosine phosphatase CD45RB. CD45 was shown to be a negative regulator of PS translocation in response to various signals, including activation of the ATP receptor P2X(7). Changes in PS distribution were shown to modulate several membrane activities: Ca(2+) and Na(+) uptake through the P2X(7) cation channel itself; P2X(7)-stimulated shedding of the homing receptor CD62L; and reversal of activity of the multidrug transporter P-glycoprotein. The data identify a role for PS distribution changes in signal transduction, rapidly modulating the activities of several membrane proteins. This seems to be an all-or-none effect, coordinating the activity of most or all the molecules of a target protein in each cell. The data also suggest a new approach to circumventing multidrug resistance.

A novel missense mutation in ABCA1 results in altered protein trafficking and reduced phosphatidylserine translocation in a patient with Scott syndrome

Scott syndrome (SS) is a bleeding disorder characterized by a failure to expose phosphatidylserine (PS) to the outer leaflet of the platelet plasma membrane. Because the adenosine triphosphate (ATP)–binding cassette transporter A1 (ABCA1) is implicated in the exofacial translocation of PS, we assessed its role in the pathophysiology of a patient with SS. Substantially reduced levels of ABCA1 mRNA were found in the patient's leukocytes, compared with controls. The SS patient was heterozygous for a novel missense mutation c.6064G>A (ABCA1 R1925Q), absent from unaffected family members and controls. Both mutant and wild-type alleles were reduced in mRNA expression, and no causative mutation for this phenomenon was identified in the ABCA1 gene or its proximal promoter, suggesting a putative second mutation in a trans-acting regulatory gene may also be involved in the disorder in this patient. In vitro expression studies showed impaired trafficking of ABCA1 R1925Q to the plasma membrane. Overexpression of wild-type ABCA1 in SS lymphocytes complemented the Ca2+-dependent PS exposure at the cell surface. These data identify a mutation in ABCA1 that contributes to the defective PS translocation phenotype in our patient with SS.

Structural role of glycine in amyloid fibrils formed from transmembrane alpha-helices

Amyloid fibrils associated with diseases such as Alzheimer's are often derived from the transmembrane helices of membrane proteins. It is known that the fibrils have a cross-beta-sheet structure where main chain hydrogen bonding occurs between beta-strands in the direction of the fibril axis. However, the structural basis for how the membrane-spanning helix is converted into a beta-sheet or how protofibrils associate into fibrils is not known. Here, we use a model peptide corresponding to a portion of the single transmembrane helix of glycophorin A to investigate the structural role of glycine in amyloid-like fibrils formed from transmembrane helices. Glycophorin A contains a GxxxG motif that is found in many transmembrane sequences including that of the amyloid precursor protein and prion protein. We propose that glycine, which mediates helix interactions in membrane proteins, also provides key packing motifs when it occurs in beta-sheets. We show that glycines in the glycophorin A transmembrane helix promote extended beta-strand formation when the helix partitions into aqueous environments and stabilize the packing of beta-sheets in the formation of amyloid-like fibrils. We demonstrate that fibrillization can be disrupted with a new class of inhibitors that target the molecular grooves created by glycine.

The P2X7 receptor is a candidate product of murine and human lupus susceptibility loci: a hypothesis and comparison of murine allelic products

Systemic lupus erythematosus and its murine equivalent, modelled in the New Zealand Black and New Zealand White (NZB × NZW)F₁ hybrid strain, are polygenic inflammatory diseases, probably reflecting an autoimmune response to debris from cells undergoing programmed cell death. Several human and murine loci contributing to disease have been defined. The present study asks whether the proinflammatory purinergic receptor P2X₇, an initiator of a form of programmed cell death known as aponecrosis, is a candidate product of murine and human lupus susceptibility loci. One such locus in (NZB × NZW)F₁ mice is lbw3, which is situated at the distal end of NZW chromosome 5. We first assess whether NZB mice and NZW mice carry distinct alleles of the P2RX₇ gene as expressed by common laboratory strains, which differ in sensitivity to ATP stimulation. We then compare the responses of NZB lymphocytes, NZW lymphocytes and (NZB × NZW)F₁ lymphocytes to P2X₇ stimulation. NZB and NZW parental strains express the distinct P2X₇-L and P2X₇-P alleles of P2RX₇, respectively, while lymphocytes from these and (NZB × NZW)F₁ mice differ markedly in their responses to P2X₇ receptor stimulation. NZB mice and NZW mice express functionally distinct alleles of the proinflammatory receptor, P2X₇. We show that current mapping suggests that murine and human P2RX₇ receptor genes lie within lupus susceptibility loci lbw3 and SLEB4, and we argue that these encode a product with the functional characteristics consistent with a role in lupus. Furthermore, we argue that aponecrosis as induced by P2X₇ is a cell death mechanism with characteristics that potentially have particular relevance to disease pathogenesis.

Multidrug transporter activity in lymphocytes

Multidrug transporters play a dual role in haematopoietic cells, mediating the efflux of xenobiotics and regulating cell migration. For several reasons including the lack of specific antibodies, reports of multidrug transporter distribution on lymphocytes conflict. Murine B cells have been reported to completely lack transporter activity. Through analysis of parental and 'knockout' mice we show that, contrary to previous studies, murine B and T lymphocytes possess at least three active multidrug transporters and also a hitherto unrecognised drug-specific import activity. Surprisingly, the drug specificity of P-glycoprotein appears cell type dependent. The data indicate that a range of developmentally regulated, multidrug transporters can impose a barrier to treatment of immune disorders.

Major histocompatibility complex class I shedding and programmed cell death stimulated through the proinflammatory P2X7 receptor: a candidate susceptibility gene for NOD diabetes

It has been hypothesized that type 1 diabetes is initiated by neonatal physiological pancreatic beta-cell death, indicating that the early stages of this autoimmune response may reflect a dysregulated response to immune "danger" signals. One potential danger signal is ATP, high concentrations of which stimulate the purinergic receptor P2X7 on hematopoietic cells. We compared the sensitivity of lymphocytes from model type 1 diabetic (NOD) and control (C57BL/10) mice to activation of this pathway. Stimulation of the P2X7 receptor of NOD mice resulted in more pronounced shedding of the lymphocyte homing receptor CD62L and in increased programmed cell death. Levels of major histocompatibility complex class I molecules, which have previously been reported to be poorly expressed on NOD lymphocytes, were initially normal, but the molecules were shed preferentially from NOD cells after P2X7 receptor stimulation. Thus, although NOD lymphocytes have been considered resistant to programmed cell death, they are highly sensitive to that stimulated through the P2X7 receptor. Because NOD mice express a low activation threshold allele of the P2X7 receptor and the P2X7 gene maps to a locus associated with disease, P2X7 is a good candidate susceptibility gene for NOD diabetes.

Leukocyte ABCA1 gene expression is associated with fasting glucose concentration in normoglycemic men

Adenosine triphosphate (ATP)-binding cassette transporter A1 (ABCA1) mediates the efflux of cholesterol to apolipoprotein A1, a process necessary for high-density lipoprotein (HDL) formation and reverse cholesterol transport. In patients with Tangier disease, mutations in ABCA1 result in low circulating HDL-cholesterol and predisposition to coronary heart disease (CHD). ABCA1 gene expression is decreased in diabetic mice. In humans, glycated hemoglobin (HbA(1c)) predicted future CHD events, even within the normal range. We hypothesised that leukocyte ABCA1 gene expression would be inversely associated with indices of glycemia in normoglycemic men. Fasting blood samples were taken from 32 healthy, nonsmoking, normoglycemic men (age 23 to 46 years). ABCA1, peroxisome proliferator-activated receptor gamma (PPARgamma), and liver X receptor alpha (LXRalpha) gene expressions in circulating leukocytes were measured using TaqMan technology. Significant inverse associations between ABCA1 gene expression and both fasting glucose concentration (r = -0.49, P =.008) and age (r = -0.39, P =.043) were found. There was no association with HbA(1c) (r = -0.23, P =.238) or HDL-cholesterol concentration (r = 0.02, P =.904). In a multiple regression model, fasting glucose remained a significant independent predictor (P =.037), whereas age did not (P =.226). Mechanisms underlying the association were explored; there were no significant associations between fasting glucose concentration and leukocyte PPARgamma gene expression, or between fasting glucose concentration and leukocyte LXRalpha gene expression. This is the first demonstration of an association between ABCA1 gene expression and fasting glucose concentration in vivo.

Detection of Alzheimer's amyloid in transgenic mice using magnetic resonance microimaging

The presence of amyloid-beta (Abeta) plaques in the brain is a hallmark pathological feature of Alzheimer's disease (AD). Transgenic mice overexpressing mutant amyloid precursor protein (APP), or both mutant APP and presenilin-1 (APP/PS1), develop Abeta plaques similar to those in AD patients, and have been proposed as animal models in which to test experimental therapeutic approaches for the clearance of Abeta. However, at present there is no in vivo whole-brain imaging method to detect Abeta plaques in mice or men. A novel method is presented to detect Abeta plaques in the brains of transgenic mice by magnetic resonance microimaging (muMRI). This method uses Abeta1-40 peptide, known for its high binding affinity to Abeta, magnetically labeled with either gadolinium (Gd) or monocrystalline iron oxide nanoparticles (MION). Intraarterial injection of magnetically labeled Abeta1-40, with mannitol to transiently open the blood-brain barrier (BBB), enabled the detection of many Abeta plaques. Furthermore, the numerical density of Abeta plaques detected by muMRI and by immunohistochemistry showed excellent correlation. This approach provides an in vivo method to detect Abeta in AD transgenic mice, and suggests that diagnostic MRI methods to detect Abeta in AD patients may ultimately be feasible.

IKCa1 activity is required for cell shrinkage, phosphatidylserine translocation and death in T lymphocyte apoptosis

Apoptotic cell volume decrease (AVD) and exposure of phosphatidylserine (PtdSer) at the cell surface are early events in apoptosis. However, the ion channels responsible for AVD, and their relationship to PtdSer translocation and cell death are poorly understood. Real-time analysis of calcium-induced apoptosis in lymphocytes and thymocytes showed that AVD occurs rapidly, and precedes PtdSer translocation. Blockers of the K(+) channel IKCa1 completely inhibited AVD. Blockade of IKCa1, and hence AVD, also completely prevented PtdSer translocation and cell death. Thus, IKCa1-mediated AVD is the earliest-defined essential step in calcium-induced apoptosis, required for both PtdSer translocation and cell death.

Functional analysis of candidate ABC transporter proteins for sitosterol transport

Two ATP-binding cassette (ABC) proteins, ABCG5 and ABCG8, have recently been associated with the accumulation of dietary cholesterol in the sterol storage disease sitosterolemia. These two 'half-transporters' are assumed to dimerize to form the complete sitosterol transporter which reduces the absorption of sitosterol and related molecules in the intestine by pumping them back into the lumen. Although mutations altering ABCG5 and ABCG8 are found in affected patients, no functional demonstration of sitosterol transport has been achieved. In this study, we investigated whether other ABC transporters implicated in lipid movement and expressed in tissues with a role in sterol synthesis and absorption, might also be involved in sitosterol transport. Transport by the multidrug resistance P-glycoprotein (P-gp; Abcb1), the multidrug resistance-associated protein (Mrp1; Abcc1), the breast cancer resistance protein (Bcrp; Abcg2) and the bile salt export pump (Bsep; Abcb11) was assessed using several assays. Unexpectedly, none of the candidate proteins mediated significant sitosterol transport. This has implications for the pathology of sitosterolemia. In addition, the data suggest that otherwise broad-specific ABC transporters have acquired specificity to exclude sitosterol and related sterols like cholesterol presumably because the abundance of cholesterol in the membrane would interfere with their action; in consequence, specific transporters have evolved to handle these sterols.

Phosphorylation-dependent regulation of the interaction of amyloid precursor protein with Fe65 affects the production of beta-amyloid

Neuronal Fe65 is an adapter protein that interacts with the cytoplasmic domain of the beta-amyloid precursor protein (APP). Although the interaction has been reported to occur between the second phosphotyrosine interaction domain of Fe65 and the YENPTY motif in the cytoplasmic domain of APP, the regulatory mechanism and biological function of this interaction remain unknown. We report here that (i) a single amino acid mutation at the Thr-668 residue of APP695, located 14 amino acids toward the amino-terminal end from the (682)YENPTY(687) motif, reduced the interaction between members of the Fe65 family of proteins and APP, whereas interaction of APP with the phosphotyrosine interaction domain of other APP binders such as X11-like and mammalian disabled-1 was not influenced by this mutation; (ii) the phosphorylation of APP at Thr-668 diminished the interaction of APP with Fe65 by causing a conformational change in the cytoplasmic domain that contains the Fe65-binding motif, YENPTY; and (iii) the expression of Fe65 slightly suppressed maturation of APP and decreased production of beta-amyloid (Abeta). Mutation at Thr-668 of APP abolished the effect of Fe65 on APP maturation. This mutation blocked the Fe65-dependent suppression of Abeta production and resulted in the release of increased levels of Abeta in the presence of Fe65. We previously reported that during maturation of APP in neurons, the protein is specifically phosphorylated at Thr-668 and undergoes O-glycosylation. The present results suggest that the phosphorylation of O-glycosylated mature APP at Thr-668 causes a conformational change in its cytoplasmic domain that prevents binding of Fe65 in neurons and may lead to an alteration in the production of Abeta.

pH-dependent fibrillogenesis of a VkappaIII Bence Jones protein

Disorders of immunoglobulin (Ig) synthesis that occur in malignant plasma-cell proliferation may result in either granular (LCDD) or fibrillar (AL) tissue deposition of light-chain monoclonal components. The structural features that govern the transition from soluble polypeptides to either fibrillar or granular conformational states remain undefined. Among the many factors presumed to play a role in these transitions the net charge of the molecule has been associated with folding conformation changes. The majority of the proteins involved in AL amyloidosis show acidic isoelectric points (pI 3.8-5.2), whereas most L chains with basic pIs deposit in granular patterns. In our studies a 12 kD VkappaIII fragment was purified as the main component of the fibrils isolated from myocardium and adipose tissue of the pericardium obtained post-mortem from an individual with systemic AL amyloidosis. An apparently identical 12 kD VL fragment with the same N-terminal sequence constituted the BJ protein present in the urine. This urinary protein exhibited strikingly cathodic electrophoretic mobility on agarose gels and lacked retention by anionic exchange chromatography matrices, indicative of a highly basic pI (>10). When it was subjected to in vitro fibril-formation experiments, the BJ protein adopted a fibrillar conformation only at acidic pHs, remaining aggregated but not fibrillar at physiological pH. The data indicate that a specific tissue deposition pattern involves not only structural properties of the protein but rather more complex mechanisms in which acidic micro-environments may contribute to the stabilization of amyloidogenic conformations.

Dual Valpha T cells

The assumption that T cells can only express a single receptor for antigen has in recent years been shown to be incorrect. However, the finding that a substantial number of T cells express two distinct antigen receptors at the cell surface raises a number of questions. In particular, it has been suggested that cells expressing low levels of a self-reactive T cell receptor may escape self-tolerance mechanisms and in certain situations trigger the onset of autoimmune disease. Such a hypothesis in turn raises questions central to the understanding of the nature of T cell recognition and the process of thymocyte maturation.

Isoaspartyl post-translational modification triggers autoimmune responses to self-proteins

The normal functioning immune system is programmed to attack foreign pathogens and other foreign proteins while maintaining tolerance to self-proteins. The mechanisms by which tolerance is broken in the initiation of autoimmunity are not completely understood. In the present study, mice immunized with the murine cytochrome c peptide 90-104 showed no response by the B or T cell compartments. However, immunization with the isoaspartyl form of this peptide, where the linkage of Asp(93) to Leu(94) occurs through the beta-carboxyl group, resulted in strong B and T cell autoimmune responses. Antibodies elicited by immunization with the isoaspartyl form of self-peptide were cross-reactive in binding to both isoforms of cytochrome c peptide and to native cytochrome c self-protein. In a similar manner, immunization of mice with the isoaspartyl form of a peptide autoantigen of human systemic lupus erythematosus (SLE) resulted in strong B and T cell responses while mice maintained tolerance to the normal aspartyl form of self-antigen. Isoaspartyl linkages within proteins are enhanced in aging and stressed cells and arise under physiological conditions. These post-translationally modified peptides may serve as an early immunologic stimulus in autoimmune disease.

Chronic deletion, escape from deletion and activation of mouse mammary tumor virus superantigen-reactive T cells in C57BL/10 mice

Though C57BL/10 mice express the mouse mammary tumor virus superantigens (sag) encoded by Mtv-8 and Mtv-9, it has been thought that these sag do not bind to the MHC class II molecule H2-Ab and consequently do not affect the T cell repertoire. However, we show that cells bearing TCR Vbeta chains specific for Mtv-8 and -9 sag are chronically deleted in C57BL/10 mice. Thymocytes and peripheral T cells escaping deletion by Mtv sag display a small reduction in the level of cell surface CD4. T cells escaping thymic deletion respond variably to endogenous Mtv sag with some, but not all, reactive populations appearing overrepresented in the activated/memory subset. The data suggest that in normal mice fine modulation of coreceptor expression levels may be a common way by which thymocytes escape elimination, that systems utilizing potentially Mtv sag-reactive TCR on a C57BL background may be inappropriate for the measurement of the affinity of TCR/MHC/peptide interactions required in thymic selection, and that detection of the activity of human sag may be aided by analysis of CD4 levels and activation markers on T cells in conjunction with studies of the frequency of cells bearing specific TCRVbeta chains.

Selection of dual Valpha T cells

Incomplete allelic exclusion of TCRa gene rearrangement permits the generation of dual Valpha T cells, though the issues of their frequency and whether both alphabeta pairs participate in thymic selection have not been resolved. Both questions have been investigated using lymphocytes from mice hemizygous at the TCRa locus and consequently unable to express two rearranged TCRa genes, as background controls. The data presented show that both the frequency of dual Valpha T cells and the relative expression levels of co-expressed Valpha chains are variable and are determined by thymic selection. Possession of a Valpha chain which is inefficiently positively selected appears to increase the likelihood that a second Valpha chain will be co-expressed, whilst the relative cell surface levels of a given pair of Valpha chains differ between CD4 and CD8 subsets. Further, for some but not all Valpha pairs, dual Valpha T cells appear to express elevated levels of surface TCR. Finally, contrary to previous claims, dual Valpha T cells do not appear to be relatively frequent amongst immature thymocytes.

T cell repertoire formation displays characteristics of qualitative models of thymic selection

The use of T cell receptor elements varies between mouse strains, reflecting a balance between positive and negative selection. The presence of H-2E biases V alpha and V beta usage through major histocompatibility class II isotype preferences of V elements, and mammary tumor virus-dependent, negative selection. Quantitative models of thymic selection predict that negative selection equates to 'excess' positive selection, whereas qualitative models suggest that positive and negative selection are opposing forces. This report attempts to distinguish between the models by assessing whether, at the level of the T cell repertoire, positive and negative selection have quantitative or qualitative characteristics. The data show that the effect of bearing V alpha and V beta regions which are both preferentially (or negatively) selected in the presence of H-2E is additive or synergistic, whilst positive stimuli counteract negative ones. The data thus provide support for qualitative models of thymic selection.

Streptozotocin-induced diabetes in mice lacking alphabeta T cells

Multiple low-dose streptozotocin (MD-STZ) is widely used for the experimental induction of diabetes, but, as non-obese diabetic (NOD)-scid/scid mice have been found to display enhanced susceptibility to MD-STZ, whether or not the model is genuinely autoimmune and T cell-mediated has been unclear. Mice bearing a targeted mutation of the T cell receptor (TCR) alpha-chain were therefore used to assess whether TCR alphabeta+ cells are involved in the diabetogenic effects of MD-STZ injections. Young NOD mice lacking TCR alphabeta cells, when given five daily injections of 40 mg/kg STZ, developed diabetes at low frequency (2/12), despite the widespread destruction of pancreatic islet cells. By comparison, most normal control mice became hyperglycaemic (12/23). We conclude that whilst much of the tissue destruction observed in this model is due to the direct toxic effect of STZ, a significant amount is also due to the action of TCR alphabeta cells tipping the balance between tolerable and clinically damaging action on islet cells.

Mice lacking alpha beta + T cells are resistant to the induction of experimental autoimmune encephalomyelitis

T cells of the gamma delta subset have been found to localise to demyelinated lesions in both multiple sclerosis and experimental autoimmune encephalomyelitis (EAE) and have been implicated in the pathogenesis of both diseases. We have assessed mice carrying a targeted mutation of the T cell receptor-alpha locus which consequently lack T cell receptor (TCR) alpha beta cells but have an intact gamma delta(+)-T cell population for their susceptibility to EAE. No disease was found in any of the mutant mice, nor was any infiltration of the CNS detected. These data show that, at least in the absence of TCR-alpha beta cells. TCR-gamma delta cells are not able to elicit the pathology associated with EAE.

Non-obese diabetic mice hemizygous at the T cell receptor alpha locus are susceptible to diabetes and sialitis

To test the hypothesis that T cells carrying two T cell receptor (TCR) alpha chains play a role in autoimmunity, we backcrossed the non-obese diabetic (NOD) strain with one carrying a TCR alpha gene disrupted by homologous recombination. Mice carrying one copy of the disrupted gene are incapable of generating T cells carrying two cell surface TCR alpha chains. Our early results suggested that either dual TCR alpha T cells play a role in insulin-dependent diabetes mellitus (IDDM) induction in NOD mice or that a locus co-segregating with the disrupted TCR alpha locus protected mice from diabetes induction. From the analysis both of mice in which the region co-segregating with the disrupted TCR alpha locus is minimized and of the F1 offspring of NOD mice with the 129 strain (TCR alpha hemizygous mice), the apparent protective effect of the absence of dual TCR alpha T cells is lost; thus, such cells do not appear to play a critical role in autoimmune disease in NOD mice.

Dual T cell receptor alpha chain T cells in autoimmunity

Allelic exclusion at the T cell receptor alpha locus TCR-alpha is incomplete, as demonstrated by the presence of a number of T lymphocyte clones carrying two expressed alpha chain products. Such dual alpha chain T cells have been proposed to play a role in autoimmunity, for example, because of a second TCR-alpha beta pair having bypassed negative selection by virtue of low expression. We examined this hypothesis by generating mice of various autoimmunity-prone strains carrying a hemizygous targeted disruption of the TCR-alpha locus, therefore unable to produce dual alpha chain T cells. Normal mice have a low but significant proportion of T cells expressing two cell-surface TCR-alpha chains that could be enumerated by comparison to TCR-alpha hemizygotes, which have none. Susceptibility to various autoimmune diseases was analyzed in TCR-alpha hemizygotes that had been backcrossed to disease-prone strains for several generations. The incidence of experimental allergic encephalomyelitis and of lupus is not affected by the absence of dual TCR-alpha cells. In contrast, nonobese diabetic (NOD) TCR alpha hemizygotes are significantly protected from cyclophosphamide-accelerated insulitis and diabetes. Thus, dual alpha T cells may play an important role in some but not all autoimmune diseases. Furthermore, since protected and susceptible NOD mice both show strong spontaneous responses to glutamic acid decarboxylase, responses to this antigen, if necessary for diabetetogenesis, are not sufficient.

The T cell response of HLA-DR transgenic mice to human myelin basic protein and other antigens in the presence and absence of human CD4

Analysis of HLA class II transgenic mice has progressed in recent years from analysis of single chain HLA class II transgenes with expression of mixed mouse/human heterodimers to double transgenic mice expressing normal human heterodimers. Previous studies have used either HLA transgenic mice in which there is a species-matched interaction with CD4 or mice which lack this interaction. Since both systems are reported to generate HLA-restricted responses, the matter of the requirement for species-matched CD4 remains unclear. We have generated triple transgenic mice expressing three human transgenes, DRA, DRB, and CD4, and compared HLA-restricted responses to peptide between human-CD4+ (Hu-CD4+) and Hu-CD4- littermates. We saw no difference between Hu-CD4+ and Hu-CD4- groups, supporting the notion that for some responses at least the requirement for species-matched CD4 may not be absolute. Evidence for positive selection of mouse T cell receptors in HLA-DR transgenic mice came both from the acquisition of new, HLA-restricted responses to various peptides and from an increased frequency of T cells using the TCR V beta 4 gene segment. An important goal with respect to the analysis of function in HLA transgenic mice is the clarification of mechanisms which underpin the recognition of self-antigens in human autoimmune disease. As a first step towards 'humanized' disease models in HLA transgenic mice, we analyzed the responses of HLA-DR transgenic mice to the human MPB 139-154 peptide which has been implicated as an epitope recognized by T cells of multiple sclerosis patients. We obtained T cell responses to this epitope in transgenic mice but not in nontransgenic controls. This study suggests that HLA transgenic mice will be valuable in the analysis of HLA-restricted T cell epitopes implicated in human disease and possibly in the design of new disease models.

Random activation of a transgene under the control of a hybrid hCD2 locus control region/Ig enhancer regulatory element

Locus control regions such as those of human CD2 and beta-globin differ from classical enhancers in that, whereas the former confer high level, copy-dependent, position-independent expression to linked genes in transgenic mice, the latter do not, expression levels being dependent on the site of integration. We report that the position independence of the CD2 locus control region is modified by coupling it to the immunoglobulin heavy chain enhancer. Whilst in the majority of transgenic lines the Ig heavy chain enhancer has little or no effect on T cell expression of the hCD2 transgene, in others transgene expression is non-specifically extinguished in a proportion of lymphoid cells. The transgenic locus chromatin appears inaccessible to DNase I in these cells, which do not express the gene. Furthermore, mice homozygous for the hybrid hCD2-Ig heavy chain enhancer construct contain T cells with both an active and an inactive transgene. The 'decision' to express or repress the gene appears to be a random process which involves each chromosome separately, occurs at early stages in differentiation and is heritable by daughter cells. These data suggest the possibility that stochastic decisions might control a number of biological processes.

HLA-DR and H-2E transgenes differentially mediate TCR-specific positive selection

The use of HLA transgenic mice in models of immunity and disease assumes that human MHC molecules are able to contribute toward the positive selection of the mouse TCR repertoire. As an initial step towards analysis of this we have compared the relative ability of DR alpha/E beta or E alpha/E beta complexes to induce T cell receptor (TCR) positive selection in H-2Ea and HLA-DRA transgenic mice lacking endogenous E alpha. The results show that, like E alpha/E beta, the hybrid DR alpha/E beta complexes are capable of mediating positive selection of V beta 2+, V beta 6+, and V beta 10+ cells. However, differences were found between the effects of the two transgenes. Thus, while V beta 6+ cells were efficiently selected in both H-2Ea and DRA transgenic mice, positive selection of V beta 10+ cells was less apparent in the DRA transgenic mice. Variation between Ea and DRA transgenic mice is consistent with the notion that this process is dependent on differential binding of endogenous peptides to the E alpha/E beta and DR alpha/E beta complexes. Furthermore, contrary to expectations, in neither set of mice was positive selection limited solely to the CD4+ subset. Thus, examples were found in which V beta-specific positive selection was confined to either the CD4+ or CD8+ subsets, and others in which both subpopulations were concomitantly increased. In the case of V beta 2 positive selection, H-2Ea transgenic mice showed expansion of these cells in both the CD4+ and CD8+ subpopulations while in DRA transgenic mice this occurred predominantly in the CD8+ subpopulation.

Mouse mammary tumor virus-mediated T-cell receptor negative selection in HLA-DRA transgenic mice

Products of specific mouse Mtv genes expressed in association with mouse MHC class II products cause the deletion of T cells expressing particular TCR V beta gene segments. These endogenous deletion ligands have been termed superantigens due to their ability to negatively select entire T-cell families, as defined by V beta-chain usage. In most cases, deletion is preferentially effected through interaction of the Mtv ligand with H-2E products. Although human DR alpha shares only 75% identity with the E alpha chain of H-2E, it has previously been shown to substitute for the mouse homologue in its capacity to induce the deletion of V beta 11- and V beta 17a-bearing T cells. In the present study, we have undertaken a more comprehensive analysis of the interaction of mixed DR alpha/E beta pairs with various endogenous Mtv integrants in various mouse backgrounds, leading to negative selection of particular V beta families. We show in this paper that transgenic DR alpha/E beta can also efficiently interact with products of Mtv-7, causing deletion of both V beta 6+ and V beta 7+ cells. Deletion of V beta 11+ T cells in DRA transgenic mice carrying Mtv-8 and -9, however, was less efficient than in control H-2Ea transgenic mice. These data and those from other MHC transgenic mouse studies show that while the class II alpha chain can influence the interaction with superantigen, it is the identity of the beta chain that seems to be critical.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and use of an internal amino acid sequencing standard peptide

A sequenceable internal standard has been made that can be added to samples destined for automated amino acid sequencing and that provides a means for continuously monitoring instrument performance. The 41-residue standard peptide is composed of two unnatural amino acids, norleucine and succinyl-lysine, and has the structure [norleucine-(succinyl-lysine)4]8-norleucine. The phenylthiohydantoin derivatives of the latter two amino acids are well separated via reversed-phase HPLC from the phenylthiohydantoin derivatives of the 20 naturally occurring amino acids. In addition, the standard peptide can be readily synthesized and it has excellent solubility, stability, and sequencing characteristics. The placement of norleucine at every fifth residue in this peptide permits accurate repetitive yield and carryover calculations following runs that are as short as six cycles. Use of this internal standard permits the early detection of suboptimal instrument performance and is especially helpful in differentiating between two common reasons (blocked NH2-terminus versus an instrument problem) for the failure of an automated protein sequencer to provide an NH2-terminal sequence.

Anergy and suppression in B-cell responses

Two main ideas have been put forward to explain the unexpectedly low anti-hapten antibody titres which can result from pre-priming a mouse with carrier before hapten-carrier immunization. The first involves the interaction of a network of idiotype-specific suppressor T cells, the second instead arguing for the role of intrinsic B-cell anergy. This paper proposes that the data available can equally be interpreted as reflecting the suboptimal interaction between T and B cells at differing stages of maturity, provided that memory B cells can be divided into two subsets. Further, it is suggested that these considerations must be taken into account in the analysis of B-cell anergy in receptor transgenic mice.

The implications of the failure to generate autoantibody-producing hybridomas from rat erythrocyte-immunized mice

Injection of mice with rat erythrocytes (RRBC) has long been thought to provide an experimental model in which suppressor T cells (Ts) control autoimmunity. The basis of this is that whilst mice immunized with RRBC produce an antibody response, of which a proportion cross-reacts with autologous red cells, the RRBC-immunized recipients of RRBC-primed spleen cells make no, or little, autoantibody, and secondly because the transfer of this autoantibody-specific suppression can be abrogated by T-cell depletion of transferred spleen cells. Here an alternative explanation of these phenomena is described.

The influence of adjuvants on the generation of autoantibody and specific suppression in rat erythrocyte-immunized mice

A number of adjuvants were investigated for their ability to modulate either the autoimmune response induced in mice by immunization with rat erythrocytes (RRBC) or the ability of RRBC-primed spleen cells to suppress the induction of anti-red cell autoimmunity in recipient mice. The inability of the agents used to do so is discussed on the background of models used to explain the generation of suppression in this system.

The influence of I-E on the generation of autoantibody and specific suppression in rat erythrocyte-immunized mice

A proportion of the antibodies produced by mice in response to the injection of rat erythrocytes (RRBC) cross-react with autologous red cells. When spleen cells from mice so immunized are transferred to naive syngeneic recipients, the recipient mice produce high anti-RRBC antibody titres but little or no autoantibody. This phenomenon has been attributed to the action of suppressor T cells. To date, the only mouse strain which consistently fails to demonstrate specific suppression of the autoantibody response is the SJL, which lacks the I-E molecules suggested to be important in the generation of suppressor T cells. The results presented here show that some, but not all, I-E negative strains of mice are capable of exhibiting transferable suppression of RRBC-induced antibodies.

Shuffling of amino acid sequence: an important control in synthetic peptide studies of nucleic acid-binding domains. Binding properties of fragments of a conserved eukaryotic RNA binding motif

We have used synthetic peptides to study a conserved RNA binding motif in yeast poly(A)-binding protein. Two peptides, 45 and 44 amino acids in length, corresponding to amino and carboxyl halves of a 90-amino acid RNA-binding domain in the protein were synthesized. While the amino-terminal peptide had no significant affinity for nucleic acids, the carboxyl-terminal peptide-bound nucleic acids with similar characteristics to that for the entire 577 residue yeast poly(A)-binding protein. In 100 mM NaCl, the latter peptide retained over 50% of the intrinsic binding free energy of the protein, as well as, similar RNA versus DNA binding specificity. However, shuffling of the sequence of this 44 residue peptide had surprisingly little effect on its nucleic acid binding properties suggesting the overriding importance of amino acid composition as opposed to primary sequence. Deletion studies on the 44 residue peptide with the "correct" sequence succeeded in identifying amino acids important for conferring RNA specificity and for increasing our understanding of the molecular basis for nucleic acid binding by synthetic peptides. The shuffled peptide study, however, clearly indicates that considerable caution must be exercised before extrapolating results of structure/function studies on synthetic peptide analogues to the parent protein.

Synthetic peptide analogs of DARPP-32 (Mr 32,000 dopamine- and cAMP-regulated phosphoprotein), an inhibitor of protein phosphatase-1. Phosphorylation, dephosphorylation, and inhibitory activity

Synthetic peptides based on the threonine phosphorylation site and proposed inhibitory site of DARPP-32 (dopamine- and cAMP-regulated phosphoprotein, Mr = 32,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) were prepared and analyzed as substrates for cAMP-dependent protein kinase and protein phosphatases-1c, -2Ac (the catalytic subunits of protein phosphatase-1 and 2A, respectively) and -2B, and as inhibitors of protein phosphatase-1c. Studies of the kinetics of phosphorylation of the peptides by cAMP-dependent protein kinase indicated an important role in facilitating phosphorylation for the region COOH-terminal to the phosphorylatable threonyl residue. Studies of the dephosphorylation of the phosphopeptides demonstrated that they were effectively dephosphorylated by protein phosphatase-2A and -2B and poorly dephosphorylated by protein phosphatase-1. The active inhibitory region of phospho-DARPP-32 was analyzed by determining the effects of synthetic phosphopeptides on the activity of protein phosphatase-1c. Phospho-D32-(8-48) and phospho-D32-(8-38) inhibited protein phosphatase-1c with IC50 values of 2 x 10(-8) and 4 x 10(-8) M, respectively, compared with an IC50 of 8 x 10(-9) M for intact phospho-DARPP-32. Phospho-D32-(9-38) was equipotent with phospho-D32-(8-38); however, further NH2-terminal deletions resulted in marked reductions in IC50 values. An analog of an active DARPP-32 phosphopeptide containing a phosphoseryl residue in place of the phosphothreonyl residue also exhibited a much reduced IC50. These data identify the essential inhibitory region of phospho-DARPP-32 as residues 9-38, which contains the phosphorylation site (Thr34). This region exhibits extensive amino acid sequence identity with phosphatase inhibitor-1, a distinct inhibitor of protein phosphatase-1. Kinetic studies of the inhibition of protein phosphatase-1c by phospho-D32-(9-38), a potent inhibitor, as well as by phospho-D32-(10-38), a weak inhibitor, indicated a mixed competitive/noncompetitive mechanism of inhibition, as has been previously found for both intact phospho-DARPP-32 and intact phospho-inhibitor-1. These findings support the hypothesis that a 30-amino acid domain in the NH2-terminal region of phospho-DARPP-32 is sufficient for the inhibition of protein phosphatase-1.

Epitope-specific regulation and the literature

The antibody response to a hapten conjugated to a protein can, in a number of situations, be suppressed by prior immunization with the carrier protein alone. The study of this finding appears to have been handicapped by previous literature on the subject having been consistently misquoted. It is to be hoped that recognition of this will allow a better understanding of the limitations to such suppression, and hence the mechanism by which it may act.

p10 single-stranded nucleic acid binding protein from murine leukemia virus binds metal ions via the peptide sequence Cys26-X2-Cys29-X4-His34-X4-Cys39

The RNA binding protein of 56 residues encoded by the extreme 3' region of the gag gene of Rauscher murine leukemia virus (MuLV) has been chemically synthesized by a solid-phase synthesis approach. Since the peptide contains a Cys26-X2-Cys29-X4-His34-X2-Cys39 sequence that is shared by all retroviral gag polyproteins which has been proposed to be a metal binding region, it was of considerable interest to examine the metal binding properties of the complete p10 protein. As postulated, p10 binds the metal ions Cd(II), Co(II), and Zn(II). The Co(II) protein shows a set of d-d absorption bands typical of a tetrahedral Co(II) complex at 695 (epsilon = 565 M-1 cm-1), 642 (epsilon = 655 M-1 cm-1), and 615 nm (epsilon = 510 M-1 cm-1) and two intense bands at 349 (epsilon = 2460 M-1 cm-1) and 314 nm (epsilon = 4240 M-1 cm-1) typical of Co(II)----(-)S- charge transfer. The ultraviolet absorption spectrum also indicates Cd(II) binding by the appearance of a Cd(II)----(-)S- charge-transfer band at 255 nm. The 113Cd NMR spectrum of 113Cd(II)-p10 reveals one signal at delta = 648 ppm. This chemical shift correlates well with that predicted for ligation of 113Cd(II) to three -S- from the three Cys residues of p10. The chemical shift of 113Cd(II)-p10 changes by only 4 ppm upon binding of d(pA)6, indicating that the chelate complex is little changed by oligonucleotide binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of the p10 single-stranded nucleic acid binding protein from murine leukemia virus

The p10 murine leukemia virus (MuLV) protein is a basic single-stranded nucleic acid binding protein encoded by the extreme 3' region of the gag gene of MuLV type C. It contains the Cys-X2-Cys-X4-His-X4-Cys sequence shared by all retroviral gag polyproteins. A similar sequence is found in the gene 32 single-stranded DNA binding protein of bacteriophage T4 and is believed to be the zinc binding region of the protein. Solid phase synthesis of p10 was carried out based on the known primary structure of the native protein, with the exception that the acetamidomethyl (Acm) derivative of cysteine was incorporated at all three cysteine positions. The structure of the synthetic p10 was confirmed by direct amino-acid sequencing, as well as by amino acid analysis and FAB mass spectrometry of endoproteinase Lys-C peptides derived from p10. A Chou and Fasman analysis of the primary sequence predicts that p10 contains 9% beta strand and/or sheet and 36% alpha helix. Circular dichroism experiments carried out on the Acm derivatized peptide gave somewhat different results, in that they suggest that p10 contains approximately 70% random coil, less than 30% beta strand and/or sheet and less than 10% alpha helix. With a Ka of greater than 10(8) M-1 for single-stranded RNA, the synthetic peptide binds as tightly as the p10 protein does when isolated directly from infected HTG-2 cells. The Acm groups can be removed from the synthetic p10 peptide by the use of mercuric acetate, followed by treatment with dithiothreitol to sequester the mercuric ion.(ABSTRACT TRUNCATED AT 250 WORDS)