The molecular and biochemical basis of Duchenne muscular dystrophy

Duchenne and Becker muscular dystrophy (DMD, BMD) have both been clinically recognized for over 100 years, yet throughout much of that time nothing beyond clinical evaluation and supportive care during the disease course was available to patients. The identification of the molecular basis of DMD/BMD in 1986 paved the way for extensive progress toward the understanding, diagnosis and treatment of this disease.

Invariant chain can function as a chaperone protein for class II major histocompatibility complex molecules

During biosynthesis, class II major histocompatibility complex molecules are intimately associated with invariant chain (Ii). The Ii-class II association has been shown to block peptide-class II binding and to affect the ultimate conformation of class II expressed on the cell surface. To assess the biochemical basis for the effects of Ii on class II, we have analyzed the biosynthesis of class II in EL4 cells transfected with I-Ad with and without Ii. In these studies, we found that Ii had a profound effect on the biosynthesis of I-Ad. In the absence of Ii, class II could form dimers efficiently, but these dimers appeared to be misfolded and this altered conformation resulted in the loss of some monoclonal antibody epitopes and inefficient transport from the endoplasmic reticulum to the Golgi. In addition, class II that was transported through the Golgi accumulated an abnormally increased molecular mass associated with N-linked glycosylation. Subsequent transfection of Ii into these cells resulted in recovery of normal class II conformation, causing a restoration of monoclonal antibody epitopes, efficient intracellular transport, and normal glycosylation. Together, these data indicate that Ii can have a profound effect on the folding, transport, and modification of class II molecules and suggest that one function of Ii may be to act as a class II-specific chaperone.

Possible influences on the expression of X chromosome-linked dystrophin abnormalities by heterozygosity for autosomal recessive Fukuyama congenital muscular dystrophy

Abnormalities of dystrophin, a cytoskeletal protein of muscle and nerve, are generally considered specific for Duchenne and Becker muscular dystrophy. However, several patients have recently been identified with dystrophin deficiency who, before dystrophin testing, were considered to have Fukuyama congenital muscular dystrophy (FCMD) on the basis of clinical findings. Epidemiologic data suggest that only 1/3500 males with autosomal recessive FCMD should have abnormal dystrophin. To explain the observation of 3/23 FCMD males with abnormal dystrophin, we propose that dystrophin and the FCMD gene product interact and that the earlier onset and greater severity of these patients' phenotype (relative to Duchenne muscular dystrophy) are due to their being heterozygous for the FCMD mutation in addition to being hemizygous for Duchenne muscular dystrophy, a genotype that is predicted to occur in 1/175,000 Japanese males. This model may help explain the genetic basis for some of the clinical and pathological variability seen among patients with FCMD, and it has potential implications for understanding the inheritance of other autosomal recessive disorders in general. For example, sex ratios for rare autosomal recessive disorders caused by mutations in proteins that interact with X chromosome-linked gene products may display predictable deviation from 1:1.

Plasma membrane properties regulating the sensitivity of leukemia, lymphoma, and solid tumor cells to merocyanine 540-sensitized photoirradiation

Merocyanine 540 (MC 540) is a photosensitizing dye that has been used in a phase I clinical trial for the purging of leukemia and lymphoma cells from autologous bone marrow grafts. In this paper we examine the role of plasma membrane negative charge, plasma membrane fluidity, and plasma membrane hydrophobicity in the regulation of a cell's susceptibility to MC 540-sensitized photoirradiation. Among solid tumor cells, we found an inverse correlation between surface electronegativity, affinity for dye molecules, and susceptibility to MC 540-sensitized photoinactivation. That is, the least electronegative cells bound the highest amount of dye and were the most susceptible to dye-sensitized photoirradiation. By contrast, no such correlations were found among leukemia/lymphoma cells. This suggested that dye binding and susceptibility to MC 540-mediated photodynamic damages are regulated differently in hematopoietic/lymphopoietic and solid tumor cells.

Atomic Force Microscope Studies of Fullerene Films: Highly Stable C 60 fcc (311) Free Surfaces

Atomic force microscopy and x-ray diffractometry were used to study 1500 A-thick films of pure C(60) grown by sublimation in ultrahigh vacuum onto a CaF(2) (111) substrate. Topographs of the films did not reveal the expected close-packed structures, but they showed instead large regions that correspond to a face-centered cubic (311) surface and distortions of this surface. The open (311) structure may have a relatively low free energy because the low packing density contributes to a high entropy of the exposed surface.

Acyl-acyl carrier protein specificity of UDP-GlcNAc acyltransferases from gram-negative bacteria: relationship to lipid A structure

Lipid A, the component of lipopolysaccharide that provides the membrane anchor of the core and O-antigen sugars, is known to contain characteristic R-3-hydroxy fatty acids bound to the 2,2' (N-linked) and 3,3' (O-linked) positions of the glucosamine disaccharide in different gram-negative bacteria. The studies reported here show that it is the acyl-acyl carrier protein specificities of the enzymes UDP-GlcNAc-O-acyltransferase and UDP-3-O-[(R)-3-hydroxyacyl]-GlcN-N-acyltransferase that determine the nature of these fatty acids.

Farnesyl diphosphate synthetase. Molecular cloning, sequence, and expression of an essential gene from Saccharomyces cerevisiae

Farnesyl diphosphate (FPP) synthetase is a key enzyme in isoprenoid biosynthesis which supplies C15 precursors for several classes of essential metabolites including sterols, dolichols, and ubiquinones. The structural gene for FPP synthetase was isolated on a 4.5-kilobase EcoRI genomic restriction fragment from the yeast Saccharomyces cerevisiae. The clone encodes a 40,483-dalton polypeptide of 342 amino acids with a high degree of similarity to the protein encoded by a putative rat liver clone of FPP synthetase (Clarke, C. F., Tanaka, R. D., Svenson, K., Wamsley, M., Fogelman, A. M., and Edwards, P. A. (1987) Mol. Cell Biol. 7, 3138-3146) and to an active site protein fragment from avian liver FPP synthetase (Brems, D. N., Bruenger, E., and Rilling, H. C. (1981) Biochemistry 20, 3711-3718). When cloned into the yeast shuttle vector YRp17, the 4.5-kilobase EcoRI fragment directed a 2-3-fold over-expression of FPP synthetase activity in transformed yeast cells. The levels of expression were independent of culture growth phase and orientation of the insert, indicative of a functional promoter in the clone. Disruption of the FPP synthetase gene from a diploid yeast strain, followed by dissection and analysis of tetrads, demonstrates that the gene is an essential, single copy number gene in yeast. The gene for FPP synthetase resides on chromosome XI as judged from Southern blots of separated yeast chromosomes.

Isopentenyl diphosphate:dimethylallyl diphosphate isomerase. An improved purification of the enzyme and isolation of the gene from Saccharomyces cerevisiae

Isopentenyl diphosphate:dimethylallyl diphosphate isomerase (IPP isomerase) is an enzyme in the isoprenoid biosynthetic pathway which catalyzes the interconversion of the primary five-carbon homoallylic and allylic diphosphate building blocks. We report a substantially improved procedure for purification of this enzyme from Saccharomyces cerevisiae. An amino-terminal sequence (35 amino acids) was obtained from a highly purified preparation of IPP isomerase. Oligonucleotide probes based on the protein sequence were used to isolate the structural gene encoding IPP isomerase from a yeast lambda library. The cloned gene encodes a 33,350-dalton polypeptide of 288 amino acids. A 3.5-kilobase EcoRI fragment containing the gene was subcloned into the yeast shuttle vector YRp17. Upon transformation with plasmids containing the insert, a 5-6-fold increase in IPP isomerase activity was seen in transformed cells relative to YRp17 controls, confirming the identity of the cloned gene. This is the first reported isolation of the gene for IPP isomerase.

Priorities for nutrition content in a medical school curriculum: a national consensus of medical educators

The ASCN Committee on Medical/Dental School and Residency Nutrition Education conducted a series of activities to establish guidelines for nutrition core content in a medical school curriculum. These activities included mail surveys of medical-nutrition educators and a representative group of medical school curriculum administrators and a national consensus workshop of nutrition educators. Results indicated close agreement between the nutrition educators and curriculum administrators (r = 0.89, p less than 0.0001) on the importance ratings of 41 nutrition topics and on the number of hours of nutrition course work that medical schools should provide (44 vs 37 h, respectively, p = 0.14). There was consensus among the nutrition educators that 26 topics should be given priority ratings as essential for inclusion in medical course work. Further prioritization of these topics resulted in a listing of core content topics and subtopics to serve as a guide to administrators and educators for planning nutrition course work in a medical school curriculum.

Biosynthesis of lipid A in Escherichia coli: identification of UDP-3-O-[(R)-3-hydroxymyristoyl]-alpha-D-glucosamine as a precursor of UDP-N2,O3-bis[(R)-3-hydroxymyristoyl]-alpha-D-glucosamine

The lipid A disaccharide of the Escherichia coli envelope is synthesized from the two fatty acylated glucosamine derivatives UDP-N2,O3-bis[(R)-3-hydroxytetradecanoyl]-alpha-D- glucosamine (UDP-2,3-diacyl-GlcN) and N2,O3-bis[(R)-3-hydroxytetradecanoyl]-alpha-D-glucosamine 1-phosphate (2,3-diacyl-GlcN-1-P) [Ray, B. L., Painter, G., & Raetz, C. R. H. (1984) J. Biol. Chem. 259, 4852-4859]. We have previously shown that UDP-2,3-diacyl-GlcN is generated in extracts of E. coli by fatty acylation of UDP-GlcNAc, giving UDP-3-O-[(R)-3-hydroxymyristoyl]-GlcNAc as the first intermediate, which is rapidly converted to UDP-2,3-diacyl-GlcN [Anderson, M. S., Bulawa, C. E., & Raetz, C. R. H. (1985) J. Biol. Chem. 260, 15536-15541; Anderson, M. S., & Raetz, C. R. H. (1987) J. Biol. Chem. 262, 5159-5169]. We now demonstrate a novel enzyme in the cytoplasmic fraction of E. coli, capable of deacetylating UDP-3-O-[(R)-3-hydroxymyristoyl]-GlcNAc to form UDP-3-O-[(R)-3-hydroxymyristoyl]glucosamine. The covalent structure of the previously undescribed UDP-3-O-[(R)-3-hydroxymyristoyl] glucosamine intermediate was established by 1H NMR spectroscopy and fast atom bombardment mass spectrometry. This material can be made to accumulate in E. coli extracts upon incubation of UDP-3-O-[(R)-3- hydroxymyristoyl]-GlcNAc in the absence of the fatty acyl donor [(R)-3-hydroxymyristoyl]-acyl carrier protein. However, addition of the isolated deacetylation product [UDP-3-O-[(R)-3-hydroxymyristoyl] glucosamine] back to membrane-free extracts of E. coli in the presence of [(R)-3-hydroxymyristoyl]-acyl carrier protein results in rapid conversion of this compound into the more hydrophobic products UDP-2,3-diacyl-GlcN, 2,3-diacyl-GlcN-1-P, and O-[2-amino-2-deoxy-N2,O3- bis[(R)-3-hydroxytetradecanoyl]-beta-D-glucopyranosyl]-(1----6)-2-amino- 2-deoxy-N2,O3-bis[(R)-3-hydroxytetradecanoyl]-alpha-D- glucopyranose 1-phosphate (tetra-acyldisaccharide-1-P), demonstrating its competency as a precursor. In vitro incubations using [acetyl-3H]UDP-3-O-[(R)-3-hydroxymyristoyl]-GlcNAc confirmed release of the acetyl moiety in this system as acetate, not as some other acetyl derivative. The deacetylation reaction was inhibited by 1 mM N-ethylmaleimide, while the subsequent N-acylation reaction was not. Our observations provide strong evidence that UDP-3-O-[(R)-3-hydroxymyristoyl]glucosamine is a true intermediate in the biosynthesis of UDP-2,3-diacyl-GlcN and lipid A.

Identification of the neuropeptide transmitter proctolin in Drosophila larvae: characterization of muscle fiber-specific neuromuscular endings

The cellular localization of the peptide neurotransmitter proctolin was determined for larvae of the fruitfly Drosophila melanogaster. Proctolin was recovered from the CNS, hindgut, and segmental bodywall using reverse-phase HPLC, and characterized by bioassay, immunoassay, and enzymatic analysis. A small, stereotyped population of proctolin-immunoreactive neurons was found in the larval CNS. Several of the identified neurons may be efferents. In the periphery, proctolin-immunoreactive neuromuscular endings were identified on both visceral and skeletal muscle fibers. On the hindgut, the neuropeptide is associated with endings on intrinsic circular muscle fibers. We propose that the hindgut muscle fibers are innervated by central neurons homologous to previously described proctolinergic efferents of grasshoppers. The segmental bodywall innervation consists of a pattern of segment-specific junctions on several singly identifiable muscle fibers. While it is generally accepted that Drosophila muscle fibers are innervated by glutamatergic motoneurons, our data indicate that a specialized subset of muscle fibers are also innervated by peptidergic efferents.

Accumulation of lysophosphatidylinositol in RAW 264.7 macrophage tumor cells stimulated by lipid A precursors

N2,O3-Diacylglucosamine 1-phosphate (lipid X), a monosaccharide precursor of Escherichia coli lipid A, was used to stimulate RAW 264.7 macrophage tumor cells, and the effects on macrophage phospholipid metabolism were examined. The addition of E. coli lipid X to the medium of cells that had been uniformly labeled with 32Pi resulted in a 4-8-fold increase in the level of lysophosphatidylinositol. This effect was maximal at 5 microM lipid X. Lysophosphatidylinositol levels reached a maximum 45 min after stimulation, followed by a gradual decline to near normal levels within 2 h. The formation of lysophosphatidylinositol was dependent upon extracellular calcium and was almost completely inhibited when cycloheximide was added at the time of stimulation. The addition of the disaccharide lipid A precursor IVA, commercial lipopolysaccharide (1 microgram/ml), phorbol 12-myristate 13-acetate (10(-7) M), or calcium ionophore A23187 (10(-6) M) to these cells resulted in a similar increase in lysophosphatidylinositol levels, but phosphatidic acid was inactive. The stimulation by IVA and phorbol myristate acetate was blocked by cycloheximide, but the stimulation by lipopolysaccharide was only partially blocked. The stimulation by A23187 was unaffected by cycloheximide. The increase in lysophosphatidylinositol levels might be related to the stimulation of arachidonate release and prostaglandin synthesis that is also observed in cells treated with lipid A precursors. The disaccharide precursor, IVA, was at least 100 times more effective than lipid X at stimulating lysophosphatidylinositol formation and prostaglandin release. The relative ability of lipid X and IVA to stimulate these cells correlated well with their effects on other lipopolysaccharide-responsive systems. Macrophage tumor cells also had the ability to inactivate lipid X by dephosphorylating it.

T cell activation by processed antigen is equally blocked by I-E and I-A-restricted immunodominant peptides

The T cell response to a soluble protein requires the processing of the native antigen by an antigen-presenting cell (APC) to a peptide containing an antigenic determinant, which is transported to and bound on the antigen-presenting cell surface, where it is subsequently recognized by the specific T cell in the context of the appropriate Ia molecule. Investigating the response of a pigeon cytochrome c-specific, I-Ek-restricted T cell hybrid, which recognizes a determinant present within a 10-amino acid C-terminal fragment of the protein, it was previously demonstrated that peptides homologous to the peptide from pigeon cytochrome c, but which were not stimulatory, blocked the T cell response to pigeon cytochrome c as processed and presented by APC. In this report the ability of a series of fourteen, 20-amino acid overlapping peptides, representing the entire length of staphylococcal nuclease (Nase), were assessed for their ability to block the response of a pigeon cytochrome c-specific T cell hybrid to antigen-pulsed presenting cells. Only three Nase peptides blocked the I-Ek-restricted pigeon cytochrome c-specific T cell response. Two of these, Nase 61-80 and Nase 91-110, function as T cell antigens in the I-Ad and I-Ab-restricted response to Nase. The third blocking peptide, Nase 101-120, has not been shown to be a T cell antigen. Two other peptides, Nase 51-70 and Nase 81-100, which are recognized by Nase-specific T cells in the context of I-Ek, have no effect on the I-Ek-restricted cytochrome c-specific T cell response. None of these peptides block the higher affinity, heteroclitic response of pigeon cytochrome c-specific T cells to tobacco hornworm moth cytochrome c. Moreover, the response of an I-Ak-restricted T cell to ovalbumin was blocked by the I-Ek-restricted cytochrome c peptides from three different species. Thus, peptides with no obvious primary amino acid sequence homology, and which are not capable of being recognized in the context of the same Ia, compete with one another for the sites on the APC necessary for presentation of processed antigen to T cells. These results suggest that there are structures on the APC surface in addition to Ia, which are necessary for effective antigen presentation following processing. One suitable candidate for such a cell surface material is the recently identified peptide-binding protein, PBP72/74 (Lakey et al., Proc. Natl. Acad. Sci. USA 1987. 84: 1659).

Biosynthesis of lipid A precursors in Escherichia coli. A cytoplasmic acyltransferase that converts UDP-N-acetylglucosamine to UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine

Preliminary studies from our laboratory have suggested the existence of a novel set of fatty acyltransferases in extracts of Escherichia coli that attach two R-3-hydroxymyristoyl moieties to UDP-GlcNAc (Anderson, M.S., Bulawa, C.E., and Raetz, C.R.H. (1985) J. Biol. Chem. 260, 15536-15541). The resulting "glucosamine-derived" phospholipids appear to be crucial precursors for the biosynthesis of the lipid A component of lipopolysaccharide. We now describe an assay and a 1000-fold purification of the first enzyme in this pathway, which catalyzes the reaction: UDP-GlcNAc + R-3-hydroxymyristoyl-acyl carrier protein----UDP-3-O-(R-3-hydroxymyristoyl)-GlcNAc + acyl carrier protein. The covalent structure of the monoacylated UDP-GlcNAc product was established by fast atom bombardment mass spectrometry and 1H-NMR spectroscopy. The UDP-GlcNAc acyltransferase has a strict requirement for R-3-hydroxymyristoyl-acyl carrier protein, since R-3-hydroxymyristoyl coenzyme A and myristoyl-acyl carrier protein are not substrates. Of various NDP-GlcNAc preparations examined, only the uridine and thymidine derivatives were utilized to a significant extent. When the product of the reaction (UDP-3-O-(R-3-hydroxymyristoyl)-GlcNAc) was isolated and reincubated with crude E. coli extracts, it was rapidly converted to more hydrophobic products in the presence of R-3-hydroxymyristoyl-acyl carrier protein. We propose that the addition of an R-3-hydroxymyristoyl residue to the 3 position of the GlcNAc moiety of UDP-GlcNAc is the first committed step in lipid A biosynthesis and that UDP-GlcNAc is situated at a biosynthetic branchpoint in E. coli leading either to lipid A or to peptidoglycan.

Molecular cloning of the genes for lipid A disaccharide synthase and UDP-N-acetylglucosamine acyltransferase in Escherichia coli

Several enzymes have been discovered recently in crude extracts of Escherichia coli that appear to be involved in the biosynthesis of the lipid A component of lipopolysaccharide. Two of these are lipid A disaccharide synthase and UDP-N-acetylglucosamine acyltransferase. Lipid A disaccharide synthase activity is barely detectable in cells harboring a lesion in the lpxB (pgsB) gene. We subcloned the lpxB gene from plasmid pLC26-43 of the Clarke and Carbon collection (L. Clarke and J. Carbon, Cell 9:91-99, 1976) and localized it to a 1.7-kilobase-pair fragment of DNA counterclockwise of dnaE on the E. coli chromosome. Furthermore, we discovered a new gene (lpxA) located adjacent to and counterclockwise of lpxB that encodes or controls UDP-N-acetylglucosamine acyltransferase. Our data prove that lpxB and lpxA are transcribed in the clockwise direction and suggest that they may be cotranscribed.

The biosynthesis of gram-negative endotoxin. Formation of lipid A precursors from UDP-GlcNAc in extracts of Escherichia coli

The Gram-negative bacterium Escherichia coli has previously been shown to utilize two unique glucosamine (GlcN)-derived phospholipids in the biosynthesis of lipid A disaccharides (Bulawa, C.E., and Raetz, C. R.H. (1984) J. Biol. Chem. 259, 4846-4851; Ray, B. L., Painter, G.L., and Raetz, C.R.H. (1984) J. Biol. Chem. 259, 4852-4859. We now present evidence that these compounds, UDP-2,3-diacyl-GlcN and 2,3-diacyl-GlcN-1-phosphate (2,3-diacyl-GlcN-1-P), are generated in extracts of E. coli by fatty acylation of UDP-GlcNAc. The initial reaction is an O-acylation of the glucosamine ring, presumably of the 3-OH group, with (R)-beta-hydroxymyristate, followed by removal of the acetyl moiety, and further fatty acylation of the N atom with (R)-beta-hydroxymyristate to yield UDP-2,3-diacyl-GlcN. Hydrolysis of the pyrophosphate bridge in this molecule gives 2,3-diacyl-GlcN-1-P + UMP. In vivo pulse labeling with 32Pi supports this postulated pathway, since UDP-2,3-diacyl-GlcN is labeled prior to 2,3-diacyl-GlcN-1-P. UDP-glucosamine is inactive as a substrate in the initial acylation reaction. These acylations show an absolute specificity for fatty acyl moieties activated with acyl carrier protein. No reaction is detected with fatty acyl-CoA or free fatty acid. The fatty acylation of sugar nucleotides has not been reported previously in E. coli or any other organism.