Targeted mutation of the gene encoding the low affinity NGF receptor p75 leads to deficits in the peripheral sensory nervous system

We have generated mice carrying a mutation of the gene encoding the low affinity NGF receptor p75NGFR by targeted mutation in embryonic stem cells. Mice homozygous for the mutation were viable and fertile. Immunohistochemical analyses of the footpad skin of mutant mice revealed markedly decreased sensory innervation by calcitonin gene-related peptide- and substance P-immunoreactive fibers. The defective innervation was correlated with loss of heat sensitivity and associated with the development of ulcers in the distal extremities. Complicated by secondary bacterial infection, the ulcers progressed to toenail and hair loss. Crossing a human transgene encoding p75NGFR into the mutant animals rescued the absent heat sensitivity and the occurrence of skin ulcers and increased the density of neuropeptide-immunoreactive sensory innervation of footpad skin. The mutation in the gene encoding p75NGFR did not decrease the size of sympathetic ganglia or the density of sympathetic innervation of the iris or salivary gland. Our results suggest that p75NGFR has an important role in the development and function of sensory neurons.

Fluorescence studies of rat cellular retinol binding protein II produced in Escherichia coli: an analysis of four tryptophan substitution mutants

Rat intestinal cellular retinol binding protein II (CRBP II) is an abundant 134-residue protein that binds all-trans-retinol which contains 4 tryptophans in positions 9, 89, 107, and 110. Our ability to express CRBP II in Escherichia coli and to construct individual tryptophan substitution mutants by site-directed mutagenesis has provided a useful model system for studying the fluorescence of a multi-tryptophan protein. Each of the four mutant proteins binds all-trans-retinol with high affinity, although their affinities are less than that of the wild-type protein. Steady-state and time-resolved fluorescence analyses of these proteins indicate that W107 is at the hydrophobic binding site, W110 is in a polar environment, and the remaining two tryptophans are in a hydrophobic environment. Time-resolved fluorescence study indicates that excited-state energy transfer occurs from the hydrophobic tryptophans to W110. The Stern-Volmer analysis with acrylamide of these proteins reveals that static quenching occurs in the W9F mutant protein while others do not. The fluorescence of rat intestinal fatty acid binding protein (I-FABP), a related protein of known X-ray structure, was also studied for comparison. The results of these findings, coupled with those derived from NMR studies and molecular graphics, suggest that CRBP II undergoes minor structural changes in all of the mutant proteins. Since these effects may be cumulative on the protein structure and function, any conclusions derived from higher mutants in this family of proteins must be treated with caution.

Isolation and partial characterization of a surface glycoconjugate of Entamoeba histolytica

To study surface molecules of Entamoeba histolytica we produced monoclonal antibodies from mice immunized with lysates from the pathogenic amebic strain HM1:IMSS, and screened them for the ability to inhibit E. histolytica adhesion. One monoclonal antibody, CC 8.6, was a potent inhibitor of amebic adhesion to a Chinese hamster ovary cell line, and was capable of inhibiting HM1:IMSS mediated cytotoxicity by 50%. We found that monoclonal antibody CC 8.6 bound to an amebic glycoconjugate. The glycoconjugate is present only in E. histolytica and not in other Entamoeba sp. It migrates as a polydisperse band on SDS-PAGE, and can be metabolically radiolabeled with [14C]glucose, [32P]phosphate, and [3H]palmitate. The glycoconjugate can be purified by hydrophobic interaction chromatography on octyl-Sepharose; enzymatic hydrolysis with phosphatidylinositol-specific phospholipase C alters the hydrophobic properties of the molecule. HPLC analysis of [14C]glucose-labeled glycoconjugate saccharides revealed that approximately 82% of the incorporated label was in glucose and 12% in galactose. Our studies demonstrate that one of the immunogenic surface molecules of E. histolytica is a phosphorylated, lipid-containing, glycoconjugate, and that antibodies to this antigen may have the potential to protect against E. histolytica adhesion and cytotoxicity.

Alteration of the binding specificity of cellular retinol-binding protein II by site-directed mutagenesis

Rat cellular retinol-binding protein II (CRBP II) is an abundant 134-residue intestinal protein that binds all-trans-retinol and all-trans-retinal. It belongs to a family of homologous, 15-kDa cytoplasmic proteins that bind hydrophobic ligands in a noncovalent fashion. These binding proteins include a number of proteins that bind long chain fatty acids. X-ray analyses of the structure of two family members, rat intestinal fatty acid-binding protein and bovine myelin P2 protein, indicate that they have a high degree of conformational similarity and that the carboxylate group of their bound fatty acid interacts with a delta-guanidium group of at least 1 of 2 "buried" arginine residues. These 2 Arg residues are conserved in other family members that bind long chain fatty acids and in cellular retinoic acid-binding protein, but are replaced by Gln109 and Gln129 in CRBP II. We have genetically engineered two amino acid substitutions in CRBP II: 1) Gln109 to Arg and 2) Gln129 to Arg. The purified Escherichia coli-derived CRBP II mutant proteins were analyzed by fluorescence and nuclear magnetic resonance spectroscopy. Both mutants exhibit markedly decreased binding of all-trans-retinol and all-trans-retinaldehyde, but no increased binding of all-trans-retinoic acid. Arg substitution for Gln109 but not for Gln129 produces a dramatic increase in palmitate binding activity. Analysis of the endogenous fatty acids associated with the purified E. coli-derived proteins revealed that E. coli-derived intestinal fatty acid binding protein and the Arg109 CRBP II mutant are complexed with endogenous fatty acids in a qualitatively and quantitatively similar manner. These results provide evidence that this internal Arg may play an important role in the binding of long chain fatty acids by members of this protein family.

Serodiagnosis of invasive amebiasis using a recombinant Entamoeba histolytica protein

One hundred eight serum samples from 106 patients were examined by Western blot analysis for the presence of antibodies to a recombinant fusion protein containing the sequence of the newly described serine-rich Entamoeba histolytica protein (SREHP). Among patients with invasive amebiasis from Durban, Republic of South Africa; San Diego, Calif; Mexico City, Mexico; and St Louis, Mo, 53 (82%) of 65 had antibodies to SREHP. In contrast, only one patient (2%) of 43 without acute invasive amebiasis had antibodies to SREHP. The predictive value of a positive test for anti-SREHP antibodies in the detection of acute invasive amebiasis was most marked when analyzed in the patients from Durban, where 11 (92%) of 12 patients who were seropositive for SREHP had acute invasive amebiasis vs 17 (65%) of 26 patients who had a positive serologic diagnosis as determined by agar gel diffusion. The use of a serologic test based on the recombinant SREHP fusion protein may be a useful adjunct to the diagnosis of acute invasive amebiasis in endemic regions.

Pulmonary surfactant will secure free airflow through a narrow tube

Well functioning pulmonary surfactant is necessary to ensure alveolar stability. It is proposed that surfactant is also required to keep the finest cylindrical airways open, thereby securing an unrestricted flow of air to and from the alveoli. If the surfactant is inadequate in quality or quality there is a risk that liquid will accumulate in the most marrow section of the airway and form a blocking column. To study that possibility special glass capillaries were used. The glass capillaries were heated and extended to make a short section very narrow. In the lumen of that section a minute volume (1 microliter) of liquid was deposited, which formed a blocking column. When pressure was raised on one side of the column, it forced the liquid to move away from the narrow section. Pressure dropped to zero as air could pass, and if the liquid column consisted of calf lung surfactant extract (CLSE), pressure remained at zero because a new liquid column did not form. If, on the other hand, the liquid column consisted of saline solution it would repeatedly reform as soon as it had been pressed out of the capillary's narrow section. The same occurred if the CLSE suspension forming the liquid column was very dilute or contained inhibiting proteins. These observations did not require that the capillary consisted of the material glass; they were also noted when the narrow tube was outlined by epithelium.

Isolation of a strain-specific Entamoeba histolytica cDNA clone

Entamoeba histolytica is an intestinal parasite causing significant morbidity and mortality worldwide. More tools are needed to understand the epidemiology and molecular pathogenesis of amebiasis. A cDNA library was constructed by using poly(A)+ RNA isolated from an axenic strain of E. histolytica, HM1:IMSS, which expresses a pathogenic isoenzyme pattern (zymodeme). Differential screening of the library yielded a strain-specific 3' polyadenylated cDNA clone, C2, possessing nine 26-nucleotide tandem repeats. RNA and DNA transfer blot analysis of four axenic strains of E. histolytica possessing the same pathogenic zymodeme revealed that the gene is present and expressed in pathogenic E. histolytica HM1:IMSS and 200:NIH but is not present in pathogenic strains HK-9 and Rahman. In addition, Southern blot analysis using the C2 clone showed heterogeneity of genomic organization between HM1:IMSS and 200:NIH. DNA dot blot hybridization analysis demonstrated that cDNA clone C2 was also able to distinguish axenically cultured E. histolytica strains possessing pathogenic zymodemes from those possessing nonpathogenic zymodemes and could detect as few as 100 amebic trophozoites. We conclude that C2 is a strain-specific E. histolytica cDNA clone that, in conjunction with other E. histolytica-specific probes, could serve as a useful epidemiologic tool.

Fluorine nuclear magnetic resonance analysis of the ligand binding properties of two homologous rat cellular retinol-binding proteins expressed in Escherichia coli

Comparative 19F NMR studies were performed on rat cellular retinol-binding protein (CRBP) and cellular retinol-binding protein II (CRBPII) to better understand their role in intracellular retinol metabolism within the polarized absorptive epithelial cells (enterocytes) of the intestine. Efficient incorporation of 6-fluorotryptophan (6-FTrp) into these homologous proteins was achieved by growing a tryptophan auxotroph of Escherichia coli, harboring prokaryotic expression vectors with either a full-length rat CRBPII or CRBP cDNA on defined medium supplemented with the analog. It is possible to easily distinguish resonances corresponding to 6-FTrp-apoCRBP, 6-FTrp-CRBP-retinol (or retinal), 6-FTrp-apoCRBPII, and 6-FTrp-CRBPII-retinol (or retinal). We were thus able to use 19F NMR spectroscopy to monitor transfer of all-trans-retinol and all-trans-retinal between CRBPII and CRBP in vitro. Retinol complexed to CRBPII is readily transferred to CRBP, whereas retinol complexed to CRBP is not readily transferred to CRBPII. We estimated that the Kd for CRBP-retinol is approximately 100-fold less than the Kd for CRBPII-retinol. Transfer of all-trans-retinal occurs readily from CRBPII to CRBP and from CRBP to CRBPII. Results from competitive binding studies with retinol and retinal indicated that there is a much larger difference between the affinities of CRBP for retinol and retinal than between the affinities of CRBPII for these two ligands. However, the differences in binding specificities reflect differences in how the two proteins interact with retinol, rather than with retinal. 19F NMR analysis of recombinant isotopically labeled proteins represents a sensitive new and useful method for monitoring retinoid flux between the CRBPs in vitro.

19F nuclear magnetic resonance studies of 6-fluorotryptophan-substituted rat cellular retinol binding protein II produced in Escherichia coli. An analysis of four tryptophan substitution mutants and their interactions with all-trans-retinol

Rat cellular retinol binding protein (CRBP II) is a 134-amino acid intracellular protein synthesized in the polarized absorptive cells of the intestine. We have previously used 19F nuclear magnetic resonance (NMR) spectroscopy to survey the structural effects of ligand binding on the apoprotein. For these studies, all 4 Trp residues of rat CRBP II were efficiently labeled with 6-fluorotryptophan (6-F-Trp) by inducing its expression in a tryptophan auxotroph of Escherichia coli. Resonances corresponding to 2 of its Trp residues underwent large downfield shifts upon binding of all-trans-retinol and retinal, while resonances corresponding to the other 2 Trp residues underwent only minor perturbations in chemical shifts. To identify which Trp residues undergo changes in their environment upon ligand binding, we have constructed four CRBP II mutants where Trp9, Trp89, Trp107, or Trp110 have been replaced by another hydrophobic amino acid. By comparing the 19F NMR spectrum of each 6-F-Trp-labeled mutant with that of wild type 6-F-Trp CRBP II, we demonstrate that the 19F resonance corresponding to Trp107 undergoes the largest change in chemical shift upon ligand binding (2.0 ppm downfield). This is consistent with the position of this residue predicted from molecular modeling studies. The 19F resonance corresponding to Trp9 also undergoes a downfield change in chemical shift of 0.5 ppm associated with retinol binding even though it is predicted to be removed from the ligand binding site. By contrast, the resonances assigned to Trp89 and Trp110 undergo only minor perturbations in chemical shifts. These results have allowed us to identify residue-specific probes for evaluating the interactions of all-trans-retinol (and other retinoids) with this intracellular binding protein.

Cloning and expression of a membrane antigen of Entamoeba histolytica possessing multiple tandem repeats

Entamoeba histolytica causes amebic dysentery and amebic liver abscess, major causes of morbidity and mortality worldwide. We have used differential hybridization screening to isolate an E. histolytica-specific cDNA clone. The cDNA was found to encode a serine-rich E. histolytica protein (SREHP) containing multiple tandem repeats. The structural motif of SREHP resembles some of the repetitive antigens of malarial species, especially the circumsporozoite proteins. A recombinant trpE fusion protein containing the tandem repeats of SREHP was recognized by immune serum from a patient with amebiasis, demonstrating that SREHP is a naturally immunogenic protein. An antiserum raised against the recombinant fusion protein specifically bound to two distinct bands with apparent molecular masses of 46 and 52 kDa in a crude preparation of E. histolytica trophozoite membranes. This antiserum also inhibited E. histolytica trophozoite adhesion to Chinese hamster ovary cells in vitro. The ability to isolate E. histolytica-specific genes, and to express those genes in Escherichia coli, may be important in studying the molecular basis of E. histolytica pathogenesis and for the future development of vaccines.

Cardiac involvement in systemic lupus erythematosus detected by echocardiography

Cardiac involvement in patients with systemic lupus erythematosus (SLE) was assessed by full echocardiography and continuous wave Doppler in 50 consecutive patients and 50 age- and sex-matched control subjects in a prospective, blinded study. The left ventricular ejection fraction was decreased in patients compared to control subjects (61 +/- 9 vs 68 +/- 7%, p less than 0.001), whereas interventricular septum (12 +/- 3 vs 9 +/- 1 mm, p less than 0.001), and posterior wall dimension (9 +/- 2 vs 8 +/- 1 mm, p less than 0.001), left ventricular mass (186 +/- 54 vs 130 +/- 32 g, p less than 0.001) and mitral valve Doppler A:E ratio (0.8 +/- 0.2 vs 0.7 +/- 0.1, p less than 0.01) were increased. Pericardial effusion was detected in 27 patients and 5 control subjects, and valvular regurgitation was more frequent in the patients (aortic 2 vs 0; mitral 23 vs 5, p less than 0.001; tricuspid 34 vs 22, p less than 0.01 and pulmonary 28 vs 17, p less than 0.05). Mitral or aortic regurgitation was more common in patients with active SLE (60 vs 40%, difference not significant) but was not related to the duration of SLE (r = 0.02), duration of prednisone therapy (r = -0.13) or current dosage of prednisone (r = 0.01). This study demonstrates that pericardial effusion, valvular regurgitation and myocardial abnormalities are frequently present in patients with SLE.

Germ-line transmission of a disrupted beta 2-microglobulin gene produced by homologous recombination in embryonic stem cells

Major histocompatibility complex (MHC) class I molecules are integral membrane proteins present on virtually all vertebrate cells and consist of a heterodimer between the highly polymorphic alpha-chain and the beta 2-microglobulin (beta 2-m) protein of relative molecular mass 12,000 (ref. 1). These cell-surface molecules play a pivotal part in the recognition of antigens, the cytotoxic response of T cells, and the induction of self tolerance. It is possible, however, that the function of MHC class I molecules is not restricted to the immune system, but extends to a wide variety of biological reactions including cell-cell interactions. For example, MHC class I molecules seem to be associated with various cell-surface proteins, including the receptors for insulin, epidermal growth factor, luteinizing hormone and the beta-adrenergic receptor. In mice, class I molecules are secreted in the urine and act as highly specific olfactory cues which influence mating preference. The beta 2-m protein has also been identified as the smaller component of the Fc receptor in neonatal intestinal cells, and it has been suggested that the protein induces collagenase in fibroblasts. Cells lacking beta 2-m are deficient in the expression of MHC class I molecules, indicating that the association with beta 2-m is crucial for the transport of MHC class I molecules to the cell surface. The most direct means of unravelling the many biological functions of beta 2-m is to create a mutant mouse with a defective beta 2-m gene. We have now used the technique of homologous recombination to disrupt the beta 2-m gene. We report here that introduction of a targeting vector into embryonic stem cells resulted in beta 2-m gene disruption with high frequency. Chimaeric mice derived from blastocysts injected with mutant embryonic stem cell clones transmit the mutant allele to their offspring.

Nuclear magnetic resonance studies of 6-fluorotryptophan-substituted rat cellular retinol-binding protein II produced in Escherichia coli. Analysis of the apoprotein and the holoprotein containing bound all-trans-retinol and all-trans-retinal

Rat cellular retinol-binding protein II (CRBP II) is a 15.6-kDa intestinal protein which binds all-trans-retinol and all-trans-retinal but not all-trans-retinoic acid. We have previously analyzed the interaction of Escherichia coli-derived rat apoCRBP II with several retinoids using fluorescence spectroscopic techniques. Interpretation of these experiments is complicated, because the protein has 4 tryptophan residues. To further investigate ligand-protein interactions, we have utilized 19F nuclear magnetic resonance (NMR) spectroscopy of CRBP II labeled at its 4 tryptophan residues with 6-fluorotryptophan. Efficient incorporation of 6-fluorotryptophan (93%) was achieved by growing a tryptophan auxotroph of E. coli harboring a prokaryotic expression vector with a full-length rat CRBP II cDNA on defined medium supplemented with the analog. Comparison of the 19F NMR spectra of 6-fluorotryptophan-substituted CRBP II with and without bound all-trans-retinol revealed that resonances corresponding to 2 tryptophan residues (designated WA and WB) undergo large downfield changes in chemical shifts (2.0 and 0.5 ppm, respectively) associated with ligand binding. In contrast, 19F resonances corresponding to two other tryptophan residues (WC and WD) undergo only minor perturbations in chemical shifts. The 19F NMR spectra of 6-fluorotryptophan-substituted CRBP II complexed with all-trans-retinal and all-trans-retinol were very similar, suggesting that the interactions of these two ligands with the protein are similar. Molecular model building, based on the crystalline structures of two homologous proteins was used to predict the positions of the 4 tryptophan residues of CRBP II and to make tentative resonance assignments. The fact that ligand binding produced residue-specific changes in the chemical shifts of resonances in CRBP II suggests that NMR analysis of isotopically labeled retinoid-binding proteins expressed in E. coli will provide an alternate, albeit it complementary, approach to fluorescence spectroscopy for examining the structural consequences of their association with ligand.

An unusual case of systemic lupus erythematosus with isolated hypoglossal nerve palsy, fulminant acute pneumonitis, and pulmonary amyloidosis

A 53 year old Chinese man with systemic lupus erythematosus (SLE) had an isolated 12th nerve palsy and acute pneumonitis. He died of respiratory failure despite intensive treatment. A limited necropsy was performed, and amyloid deposits were identified in both lung and kidney tissue. This case is highly unusual because (a) to our knowledge an isolated hypoglossal nerve palsy associated with active SLE has never been reported; (b) only one of nine reported cases of amyloidosis in patients with SLE had amyloid deposits in the lung.

Chinese hamster ovary cells deficient in N-acetylglucosaminyltransferase I activity are resistant to Entamoeba histolytica-mediated cytotoxicity

To study the relationship between carbohydrate-specific amebic cytoadherence and ameba-mediated cytotoxicity, we measured Entamoeba histolytica trophozoite-mediated cytolysis directed against a panel of four Chinese hamster ovary (CHO) cell lines that have defined alterations in their glycosylation patterns. We recently measured amebic trophozoite adherence to this panel of CHO cells and showed that trophozoites bind variant cells (RICR 15B), which are deficient in Asn-linked N-acetyllactosamine units, at 12% of the level observed for wild-type cells (E. Li, A. Becker, and S. L. Stanley, J. Exp. Med 167:1725-1730, 1988). Using a 51Cr release assay to measure trophozoite-mediated cytolysis, we demonstrate in this study that RICR 15B cells are less susceptible to trophozoite-mediated cytolysis than are wild-type cells. In addition, we found that N-acetyllactosamine, which inhibits trophozoite adherence to CHO cells, also inhibited trophozoite-mediated cytolysis of wild-type cells. These studies indicate that surface carbohydrates on target cells can influence susceptibility to ameba-mediated cytotoxicity. This panel of CHO cells provides a useful model system for investigating the role of glycoconjugates in mediating amebic interactions with mammalian cells.

Comparison of the ligand binding properties of two homologous rat apocellular retinol-binding proteins expressed in Escherichia coli

Cellular retinol-binding protein (CRBP) and cellular retinol-binding protein II (CRBP II) are 132-residue cytosolic proteins which have 56% amino acid sequence identity and bind all-trans-retinol as their endogenous ligand. They belong to a family of cytoplasmic proteins which have evolved to bind distinct hydrophobic ligands. Their patterns of tissue-specific and developmental regulation are distinct. We have compared the ligand binding properties of rat apo-CRBP and apo-CRBP II that have been expressed in Escherichia coli. Several observations indicate that the E. coli-derived apoproteins are structurally similar to the native rat proteins: they co-migrate on isoelectric focusing gels; and when complexed with all-trans-retinol, their absorption and excitation/emission spectra are nearly identical to those of the authentic rat holoproteins. Comparative lifetime and acrylamide quenching studies suggest that there are differences in the conformations of apo-CRBP and apo-CRBP II. The interaction of E. coli-derived apo-CRBP and apo-CRBP II with a variety of retinoids was analyzed using spectroscopic techniques. Both apoproteins formed high affinity complexes with all-trans-retinol (K'd approximately 10 nM). In direct binding assays, all-trans-retinal bound to both apoproteins (K'd approximately 50 nM for CRBP; K'd approximately 90 nM for CRBP II). However, all-trans-retinal could displace all-trans-retinol bound to CRBP II but not to CRBP. These observations suggests that there is a specific yet distinct interaction between these two proteins and all-trans-retinal. Apo-CRBP and apo-CRBP II did not demonstrate significant binding to either retinoic acid or methyl retinoate, an uncharged derivative of all-trans-retinoic acid. This indicates that the carboxymethyl group of methyl retinoate cannot be sterically accommodated in their binding pockets and that failure to bind retinoic acid probably is not simply due to the negative charge of its C-15 carboxylate group. Finally, neither all-trans-retinol nor retinoic acid bound to E. coli-derived rat intestinal fatty acid-binding protein, a homologous protein whose tertiary structure is known. Together, the data suggest that these three family members have acquired unique functional capabilities.

Use of Chinese hamster ovary cells with altered glycosylation patterns to define the carbohydrate specificity of Entamoeba histolytica adhesion

We compared the adherence of E. histolytica trophozoites with a panel of lectin-resistant CHO mutants with altered glycosylation patterns. Our results coupled with data from saccharide inhibition studies indicate that terminal N-acetyllactosamine units on Asn-linked complex type oligosaccharides provide the major determinants on the cellular receptor for E. histolytica adhesion.