Cultured melanocytes from dilute mutant mice exhibit dendritic morphology and altered melanosome distribution

Mutant alleles at the dilute unconventional myosin heavy chain locus cause diluted coat color, opisthotonic seizures, and death. The dilute coat color phenotype is caused by irregular clumping of pigment in the hair, but amounts of melanin are unchanged from wild-type controls. The melanocyte phenotype has been described as adendritic, since hair bulb and Harderian gland melanocytes appear to be rounded in tissue sections. These observations do not exclude the possibility that the processes lack pigment, since the melanocyte shape was judged by the distribution of melanin. We have tested this hypothesis by culturing primary melanocytes from dilute mutant and wild-type mice. The mutant melanocytes do not lack processes; instead, they exhibit a concentrated perinuclear distribution of melanosomes, while wild-type melanocytes have a very uniform cytoplasmic distribution of melanosomes. Electron micrographs show no detectable differences in melanosome morphology or maturation between dilute and wild-type melanocytes. Immunofluorescence experiments indicate that the dilute protein is concentrated in regions of the cytoplasm that contain melanosomes. These experiments show that the dilute myosin is necessary for the localization of melanosomes, either by active transport or tethering.

Length suppression in histone messenger RNA 3′-end maturation: Processing defects of insertion mutant premessenger RNAs can be compensated by insertions into the U7 small nuclear RNA

Efficient 3′-end processing of cell cycle-regulated mammalian histone premessenger RNAs (pre-mRNAs) requires an upstream stem–loop and a histone downstream element (HDE) that base pairs with the U7 small ribonuclearprotein. Insertions between these elements have two effects: the site of cleavage moves in concert with the HDE and processing efficiency declines. We used Xenopus oocytes to ask whether compensatory length insertions in the human U7 RNA could restore the fidelity and efficiency of processing of mouse histone insertion pre-mRNAs. An insertion of 5 nt into U7 RNA that extends its complementary to the HDE compensated for both defects in processing of a 5-nt insertion substrate; a noncomplementary insertion into U7 did not. Yet, the noncomplementary insertion mutant U7 was shown to be active on insertion substrates further mutated to allow base pairing. Our results suggest that the histone pre-mRNA becomes rigidified upstream of its HDE, allowing the bound U7 small ribonucleoprotein to measure from the HDE to the cleavage site. Such a mechanism may be common to other RNA measuring systems. To our knowledge, this is the first demonstration of length suppression in an RNA processing system.

Characterization of a replication-incompetent adenovirus type 5 mutant deleted for the preterminal protein gene

An adenovirus type 5 mutant deleted for the preterminal protein (pTP) gene was constructed using cell lines that express pTP. The pTP deletion mutant virus is incapable of replicating in the absence of complementation and does not express detectable levels of viral mRNAs that are expressed only after the onset of replication. Accumulation of early-region mRNAs, including that for E1A, exhibits a lag relative to that observed from the wild-type virus. However, E1A mRNA accumulation attains a steady-state level similar to the level of expression during the early phase of infection with the wild-type virus. In 293-pTP cells (human embryonic kidney cells that express pTP in addition to high levels of adenovirus E1A and E1B proteins), the pTP deletion mutant virus replicates efficiently and yields infectious titers within 5-fold of that of the wild-type virus. The deletion of 1.2 kb of pTP-encoding sequence increases the size of foreign DNA that can be introduced into the virus and, with an absolute block to replication, makes this virus an important tool for gene therapy.

Recombinant immunotoxins specific for a mutant epidermal growth factor receptor: Targeting with a single chain antibody variable domain isolated by phage display

EGFRvIII is a mutant epidermal growth factor receptor found in glioblastoma, and in carcinoma of the breast, ovary, and lung. The mutant receptor has a deletion in its extracellular domain that results in the formation of a new, tumor-specific extracellular sequence. Mice were immunized with a synthetic peptide corresponding to this sequence and purified EGFRvIII. A single chain antibody variable domain (scFv) phage display library of 8 × 106 members was made from the spleen of one immunized mouse. A scFv specific for EGFRvIII was isolated from this library by panning with successively decreasing amounts of synthetic peptide. This was used to make an immunotoxin by fusing the scFv DNA sequence to sequences coding for domains II and III of Pseudomonas exotoxin A. Purified immunotoxin had a Kd of 22 nM for peptide and a Kd of 11 nM for cell-surface EGFRvIII. The immunotoxin was very cytotoxic to cells expressing EGFRvIII, with an IC50 of 1 ng/ml (16 pM) on mouse fibroblasts transfected with EGFRvIII and an IC50 of 7–10 ng/ml (110–160 pM) on transfected glioblastoma cells. There was no cytotoxic activity at 1000 ng/ml on the untransfected parent glioblastoma cell line. The immunotoxin was completely stable upon incubation at 37°C for 24 h in human serum. The combination of good affinity, cytotoxicity and stability make this immunotoxin a candidate for further preclinical evaluation.

Assessment of normal and mutant human presenilin function in Caenorhabditis elegans

We provide evidence that normal human presenilins can substitute for Caenorhabditis elegans SEL-12 protein in functional assays in vivo. In addition, six familial Alzheimer disease-linked mutant human presenilins were tested and found to have reduced ability to rescue the sel-12 mutant phenotype, suggesting that they have lower than normal presenilin activity. A human presenilin 1 deletion variant that fails to be proteolytically processed and a mutant SEL-12 protein that lacks the C terminus display considerable activity in this assay, suggesting that neither presenilin proteolysis nor the C terminus is absolutely required for normal presenilin function. We also show that sel-12 is expressed in most neural and nonneural cell types in all developmental stages. The reduced activity of mutant presenilins and as yet unknown gain-of-function properties may be a contributing factor in the development of Alzheimer disease.

Kinetic and structural analysis of mutant CD4 receptors that are defective in HIV gp120 binding

The T-cell antigen coreceptor CD4 also serves as the receptor for the envelope glycoprotein gp120 of HIV. Extensive mutational analysis of CD4 has implicated residues from a portion of the extracellular amino-terminal domain (D1) in gp120 binding. However, none of these proteins has been fully characterized biophysically, and thus the precise effects on molecular structure and binding interactions are unknown. In the present study, we produced soluble versions of three mutant CD4 molecules (F43V, G47S, and A55F) and characterized their structural properties, thermostability, and ability to bind gp120. Crystallographic and thermodynamic analysis showed minimal structural alterations in the F43V and G47S mutant proteins, which have solvent-exposed mutant side chains. In contrast, some degree of disorder appears to exist in the folded state of A55F, as a result of mutating a buried side chain. Real time kinetic measurements of the interaction of the mutant proteins with gp120 showed affinity decreases of 5-fold for G47S, 50-fold for A55F, and 200-fold for F43V. Although both rate constants for the binding reaction were affected by these mutations, the loss in affinity was mainly due to a decrease in on rates, with less drastic changes occurring in the off rates. These observations suggest the involvement of conformational adaptation in the CD4–gp120 interaction. Together, the structural and kinetic data confirm that F43V is a critical residue in gp120 recognition site, which may also include main chain interactions at residue Gly-47.

Cell surface expression of the epithelial Na channel and a mutant causing Liddle syndrome: A quantitative approach

The epithelial amiloride-sensitive sodium channel (ENaC) controls transepithelial Na+ movement in Na+-transporting epithelia and is associated with Liddle syndrome, an autosomal dominant form of salt-sensitive hypertension. Detailed analysis of ENaC channel properties and the functional consequences of mutations causing Liddle syndrome has been, so far, limited by lack of a method allowing specific and quantitative detection of cell-surface-expressed ENaC. We have developed a quantitative assay based on the binding of 125I-labeled M2 anti-FLAG monoclonal antibody (M2Ab*) directed against a FLAG reporter epitope introduced in the extracellular loop of each of the α, β, and γ ENaC subunits. Insertion of the FLAG epitope into ENaC sequences did not change its functional and pharmacological properties. The binding specificity and affinity (Kd = 3 nM) allowed us to correlate in individual Xenopus oocytes the macroscopic amiloride-sensitive sodium current (INa) with the number of ENaC wild-type and mutant subunits expressed at the cell surface. These experiments demonstrate that: (i) only heteromultimeric channels made of α, β, and γ ENaC subunits are maximally and efficiently expressed at the cell surface; (ii) the overall ENaC open probability is one order of magnitude lower than previously observed in single-channel recordings; (iii) the mutation causing Liddle syndrome (β R564stop) enhances channel activity by two mechanisms, i.e., by increasing ENaC cell surface expression and by changing channel open probability. This quantitative approach provides new insights on the molecular mechanisms underlying one form of salt-sensitive hypertension.

Chemical chaperones mediate increased secretion of mutant α1-antitrypsin (α1-AT) Z: A potential pharmacological strategy for prevention of liver injury and emphysema in α1-AT deficiency

In α1-AT deficiency, a misfolded but functionally active mutant α1-ATZ (α1-ATZ) molecule is retained in the endoplasmic reticulum of liver cells rather than secreted into the blood and body fluids. Emphysema is thought to be caused by the lack of circulating α1-AT to inhibit neutrophil elastase in the lung. Liver injury is thought to be caused by the hepatotoxic effects of the retained α1-ATZ. In this study, we show that several “chemical chaperones,” which have been shown to reverse the cellular mislocalization or misfolding of other mutant plasma membrane, nuclear, and cytoplasmic proteins, mediate increased secretion of α1-ATZ. In particular, 4-phenylbutyric acid (PBA) mediated a marked increase in secretion of functionally active α1-ATZ in a model cell culture system. Moreover, oral administration of PBA was well tolerated by PiZ mice (transgenic for the human α1-ATZ gene) and consistently mediated an increase in blood levels of human α1-AT reaching 20–50% of the levels present in PiM mice and normal humans. Because clinical studies have suggested that only partial correction is needed for prevention of both liver and lung injury in α1-AT deficiency and PBA has been used safely in humans, it constitutes an excellent candidate for chemoprophylaxis of target organ injury in α1-AT deficiency.

Substrate sequestration by a proteolytically inactive Lon mutant

Lon protein of Escherichia coli is an ATP-dependent protease responsible for the rapid turnover of both abnormal and naturally unstable proteins, including SulA, a cell division inhibitor made after DNA damage, and RcsA, a positive regulator of transcription. Lon is a multimer of identical 94-kDa subunits, each containing a consensus ATPase motif and a serine active site. We found that overexpressing Lon, which is mutated for the serine active site (LonS679A) and is therefore devoid of proteolytic activity, unexpectedly led to complementation of the UV sensitivity and capsule overproduction of a lon deletion mutant. SulA was not degraded by LonS679A, but rather was completely protected by the Lon mutant from degradation by other cellular proteases. We interpret these results to mean that the mutant LonS679A binds but does not degrade Lon substrates, resulting in sequestration of the substrate proteins and interference with their activities, resulting in apparent complementation. Lon that carried a mutation in the consensus ATPase site, either with or without the active site serine, was no longer able to complement a Δlon mutant. These in vivo results suggest that the pathway of degradation by Lon couples ATP-dependent unfolding with movement of the substrate into protected chambers within Lon, where it is held until degradation proceeds. In the absence of degradation the substrate remains sequestered. Comparison of our results with those from a number of other systems suggest that proteins related to the regulatory portions of energy-dependent proteases act as energy-dependent sequestration proteins.

Induction of wild-type p53 activity in human cancer cells by ribozymes that repair mutant p53 transcripts

Several groups have attempted to develop gene therapy strategies to treat cancer via introduction of the wild-type (wt) p53 cDNA into cancer cells. Unfortunately, these approaches do not result in regulated expression of the p53 gene and do not reduce expression of the mutant p53 that is overexpressed in cancerous cells. These shortcomings may greatly limit the utility of this gene replacement approach. We describe an alternative strategy with trans-splicing ribozymes that can simultaneously reduce mutant p53 expression and restore wt p53 activity in various human cancers. The ribozyme accomplished such conversion by repairing defective p53 mRNAs with high fidelity and specificity. The corrected transcripts were translated to produce functional p53 that can transactivate p53-responsive promoters and down-modulate expression of the multidrug resistance (MDR1) gene promoter. The level of wt p53 activity generated was significant, resulting in a 23-fold induction of a p53-responsive promoter and a 3-fold reduction in MDR1 promoter expression in transfected cancer cells. Once efficient delivery systems are developed, this strategy should prove useful for making human cancers more responsive to p53 activity and more sensitive to chemotherapeutic agents.

The Arabidopsis male-sterile mutant, opr3, lacks the 12-oxophytodienoic acid reductase required for jasmonate synthesis

Jasmonic acid (JA) and its precursor 12-oxophytodienoic acid (OPDA) act as plant growth regulators and mediate responses to environmental cues. To investigate the role of these oxylipins in anther and pollen development, we characterized a T-DNA-tagged, male-sterile mutant of Arabidopsis, opr3. The opr3 mutant plants are sterile but can be rendered fertile by exogenous JA but not by OPDA. Cloning of the mutant locus indicates that it encodes an isozyme of 12-oxophytodienoate reductase, designated OPR3. All of the defects in opr3 are alleviated by transformation of the mutant with an OPR3 cDNA. Our results indicate that JA and not OPDA is the signaling molecule that induces and coordinates the elongation of the anther filament, the opening of the stomium at anthesis, and the production of viable pollen. Just as importantly, our data demonstrate that OPR3 is the only isoform of OPR capable of reducing the correct stereoisomer of OPDA to produce JA required for male gametophyte development.

A mutant allele of essential, general translation initiation factor DED1 selectively inhibits translation of a viral mRNA

Positive-strand RNA virus genomes are substrates for translation, RNA replication, and encapsidation. To identify host factors involved in these functions, we used the ability of brome mosaic virus (BMV) RNA to replicate in yeast. We report herein identification of a mutation in the essential yeast gene DED1 that inhibited BMV RNA replication but not yeast growth. DED1 encodes a DEAD (Asp-Glu-Ala-Asp)-box RNA helicase required for translation initiation of all yeast mRNAs. Inhibition of BMV RNA replication by the mutant DED1 allele (ded1–18) resulted from inhibited expression of viral polymerase-like protein 2a, encoded by BMV RNA2. Inhibition of RNA2 translation was selective, with no effect on general cellular translation or translation of BMV RNA1-encoded replication factor 1a, and was independent of p20, a cellular antagonist of DED1 function in translation. Inhibition of RNA2 translation in ded1–18 yeast required the RNA2 5′ noncoding region (NCR), which also conferred a ded1–18-specific reduction in expression on a reporter gene mRNA. Comparison of the similar RNA1 and RNA2 5′ NCRs identified a 31-nucleotide RNA2-specific region that was required for the ded1–18-specific RNA2 translation block and attenuated RNA2 translation in wild-type yeast. Further comparisons and RNA structure predictions suggest a modular arrangement of replication and translation signals in RNA1 and RNA2 5′ NCRs that appears conserved among bromoviruses. The 5′ attenuator and DED1 dependence of RNA2 suggest that, despite its divided genome, BMV regulates polymerase translation relative to other replication factors, just as many single-component RNA viruses use translational read-through and frameshift mechanisms to down-regulate polymerase. The results show that a DEAD-box helicase can selectively activate translation of a specific mRNA and may provide a paradigm for translational regulation by other members of the ubiquitous DEAD-box RNA helicase family.

Human sulfite oxidase R160Q: Identification of the mutation in a sulfite oxidase-deficient patient and expression and characterization of the mutant enzyme

Sulfite oxidase catalyzes the terminal reaction in the degradation of sulfur amino acids. Genetic deficiency of sulfite oxidase results in neurological abnormalities and often leads to death at an early age. The mutation in the sulfite oxidase gene responsible for sulfite oxidase deficiency in a 5-year-old girl was identified by sequence analysis of cDNA obtained from fibroblast mRNA to be a guanine to adenine transition at nucleotide 479 resulting in the amino acid substitution of Arg-160 to Gln. Recombinant protein containing the R160Q mutation was expressed in Escherichia coli, purified, and characterized. The mutant protein contained its full complement of molybdenum and heme, but exhibited 2% of native activity under standard assay conditions. Absorption spectroscopy of the isolated molybdenum domains of native sulfite oxidase and of the R160Q mutant showed significant differences in the 480- and 350-nm absorption bands, suggestive of altered geometry at the molybdenum center. Kinetic analysis of the R160Q protein showed an increase in Km for sulfite combined with a decrease in kcat resulting in a decrease of nearly 1,000-fold in the apparent second-order rate constant kcat/Km. Kinetic parameters for the in vitro generated R160K mutant were found to be intermediate in value between those of the native protein and the R160Q mutant. Native sulfite oxidase was rapidly inactivated by phenylglyoxal, yielding a modified protein with kinetic parameters mimicking those of the R160Q mutant. It is proposed that Arg-160 attracts the anionic substrate sulfite to the binding site near the molybdenum.

A mutant RNA polymerase that forms unusual open promoter complexes

We describe a mutant Escherichia coli RNA polymerase (RNAP) that forms stable open promoter complexes even at −20°C but with a shortened melted region that extends downstream to only position −7. In the presence of initiating transcription substrates, the mutant RNAP undergoes a temperature-dependent isomerization, resulting in a promoter complex that is indistinguishable from the wild-type RNAP–promoter complex, with the melted region extended downstream to position +4. We propose that the open complex formed by the mutant RNAP represents an intermediate on the normal promoter-opening pathway and that our results support earlier findings that initial promoter opening occurs in the upstream region of the −10 promoter consensus element and subsequently extends downstream to encompass the transcription start site.

Respiratory syncytial virus (RSV) SH and G proteins are not essential for viral replication in vitro: Clinical evaluation and molecular characterization of a cold-passaged, attenuated RSV subgroup B mutant

A live, cold-passaged (cp) candidate vaccine virus, designated respiratory syncytial virus (RSV) B1 cp-52/2B5 (cp-52), replicated efficiently in Vero cells, but was found to be overattenuated for RSV-seronegative infants and children. Sequence analysis of reverse-transcription–PCR-amplified fragments of this mutant revealed a large deletion spanning most of the coding sequences for the small hydrophobic (SH) and attachment (G) proteins. Northern blot analysis of cp-52 detected multiple unique read-through mRNAs containing SH and G sequences, consistent with a deletion mutation spanning the SH:G gene junction. Immunological studies confirmed that an intact G glycoprotein was not produced by the cp-52 virus. Nonetheless, cp-52 was infectious and replicated to high titer in tissue culture despite the absence of the viral surface SH and G glycoproteins. Thus, our characterization of this negative-strand RNA virus identified a novel replication-competent deletion mutant lacking two of its three surface glycoproteins. The requirement of SH and G for efficient replication in vivo suggests that selective deletion of one or both of these RSV genes may provide an alternative or additive strategy for developing an optimally attenuated vaccine candidate.