Microinjection of mRNA coding for an anti-Golgi antibody inhibits intracellular transport of a viral membrane protein

Pharmaceutical strategies for optimized mRNA expression

Messenger RNA (mRNA)-based immunotherapies and protein in situ production therapies hold great promise for addressing theoretically all the diseases characterized by aberrant protein levels. The safe, stable, and precise delivery of mRNA to target cells via appropriate pharmaceutical strategies is a prerequisite for its optimal efficacy. In this review, we summarize the structural characteristics, mode of action, development prospects, and limitations of existing mRNA delivery systems from a pharmaceutical perspective, with an emphasis on the impacts from formulation adjustments and preparation techniques of non-viral vectors on mRNA stability, target site accumulation and transfection efficiency. In addition, we introduce strategies for synergistical combination of mRNA and small molecules to augment the potency or mitigate the adverse effects of mRNA therapeutics. Lastly, we delve into the challenges impeding the development of mRNA drugs while exploring promising avenues for future advancements.

Prolactin-induced Subcellular Targeting of GLUT1 Glucose Transporter in Living Mammary Epithelial Cells

BACKGROUND

Studying the biological pathways involved in mammalian milk production during lactation could have many clinical implications. The mammary gland is unique in its requirement for transport of free glucose into the cell for the synthesis of lactose, the primary carbohydrate in milk.

OBJECTIVE

To study GLUT1 trafficking and subcellular targeting in living mammary epithelial cells (MEC) in culture.

METHODS

Immunocytochemistry was used to study GLUT1 hormonally regulated subcellular targeting in human MEC (HMEC). To study GLUT1 targeting and recycling in living mouse MEC (MMEC) in culture, we constructed fusion proteins of GLUT1 and green fluorescent protein (GFP) and expressed them in CIT3 MMEC. Cells were maintained in growth medium (GM), or exposed to secretion medium (SM), containing prolactin.

RESULTS

GLUT1 in HMEC localized primarily to the plasma membrane in GM. After exposure to prolactin for 4 days, GLUT1 was targeted intracellularly and demonstrated a perinuclear distribution, co-localizing with lactose synthetase. The dynamic trafficking of GFP-GLUT1 fusion proteins in CIT3 MMEC suggested a basal constitutive GLUT1 recycling pathway between an intracellular pool and the cell surface that targets most GLUT1 to the plasma membrane in GM. Upon exposure to prolactin in SM, GLUT1 was specifically targeted intracellularly within 90-110 minutes.

CONCLUSIONS

Our studies suggest intracellular targeting of GLUT1 to the central vesicular transport system upon exposure to prolactin. The existence of a dynamic prolactin-induced sorting machinery for GLUT1 could be important for transport of free glucose into the Golgi for lactose synthesis during lactation.

Specific in vivo knockdown of protein function by intrabodies

Intracellular antibodies (intrabodies) are recombinant antibody fragments that bind to target proteins expressed inside of the same living cell producing the antibodies. The molecules are commonly used to study the function of the target proteins (i.e., their antigens). The intrabody technology is an attractive alternative to the generation of gene-targeted knockout animals, and complements knockdown techniques such as RNAi, miRNA and small molecule inhibitors, by-passing various limitations and disadvantages of these methods. The advantages of intrabodies include very high specificity for the target, the possibility to knock down several protein isoforms by one intrabody and targeting of specific splice variants or even post-translational modifications. Different types of intrabodies must be designed to target proteins at different locations, typically either in the cytoplasm, in the nucleus or in the endoplasmic reticulum (ER). Most straightforward is the use of intrabodies retained in the ER (ER intrabodies) to knock down the function of proteins passing the ER, which disturbs the function of members of the membrane or plasma proteomes. More effort is needed to functionally knock down cytoplasmic or nuclear proteins because in this case antibodies need to provide an inhibitory effect and must be able to fold in the reducing milieu of the cytoplasm. In this review, we present a broad overview of intrabody technology, as well as applications both of ER and cytoplasmic intrabodies, which have yielded valuable insights in the biology of many targets relevant for drug development, including α-synuclein, TAU, BCR-ABL, ErbB-2, EGFR, HIV gp120, CCR5, IL-2, IL-6, β-amyloid protein and p75NTR. Strategies for the generation of intrabodies and various designs of their applications are also reviewed.

The intrabody targeting of hTERT attenuates the immortality of cancer cells

hTERT (human telomerase reverse transcriptase) plays a key role in the process of cell immortalization. Overexpression of hTERT has been implicated in 85% of malignant tumors and offers a specific target for cancer therapy. In this paper, we describe an effective approach using a single-chain variable fragment (scFv) intrabody derived from monoclonal hybridoma directed against hTERT to attenuate the immortalization of human uterine cervix and hepatoma cells. The scFv we constructed had a high affinity to hTERT, and specifically neutralized over 70% of telomere synthesis activity, thereby inhibiting the viability and proliferation of the cancer cells. Our results indicate that this anti-hTERT intrabody is a promising tool to target hTERT and intervene in the immortalization process of cancer cells.

Inhibition of phosphatidic acid synthesis alters the structure of the Golgi apparatus and inhibits secretion in endocrine cells

In mammalian cells, activation of a Golgi-associated phospholipase D by ADP-ribosylation factor results in the hydrolysis of phosphatidylcholine to form phosphatidic acid (PA). This reaction stimulates the release of nascent secretory vesicles from the trans-Golgi network of endocrine cells. To understand the role of PA in mediating secretion, we have exploited the transphosphatidylation activity of phospholipase D. Rat anterior pituitary GH3 cells, which secrete growth hormone and prolactin, were treated with 1-butanol resulting in the synthesis of phosphatidylbutanol rather than PA. Under these conditions transport from the ER through the Golgi apparatus and secretion of polypeptide hormones were inhibited quantitatively. Furthermore, the in vitro synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) by Golgi membranes was inhibited quantitatively. Most significantly, in the presence of 1-butanol the architecture of the Golgi apparatus was disrupted, resulting in its disassembly and fragmentation. Removal of the alcohol resulted in the rapid restoration of Golgi structure and secretion of growth hormone and prolactin. Our results suggest that PA stimulation of PtdIns(4,5)P(2) synthesis is required for maintaining the structural integrity and function of the Golgi apparatus.

Expression of antibody interferes with disulfide bond formation and intracellular transport of antigen in the secretory pathway

To determine whether antibodies would interfere with the folding of glycoprotein antigens in the endoplasmic reticulum lumen of living cells, hybridoma cells producing monoclonal anti-hemagglutinin (HA) antibodies were infected with influenza virus. The fate of the newly synthesized HA was determined using an established pulse-chase approach. When the monoclonal antibodies were against epitopes present on early folding intermediates, folding and intracellular transport of HA to the Golgi complex were severely disturbed. On the other hand, when the antibodies were specific for the native HA trimers, immune complexes were formed, but folding or transport of HA was not affected. The use of antibodies in this way provided in situ information about the protein folding process inside the endoplasmic reticulum lumen of cells without external perturbation of the folding chains or the folding compartment.

Antibody recognition of epitopes on wild-type and mutant beta-(1-->4)-galactosyltransferase-1

The epitopes present on beta-(1-->4)-galactosyltransferase-1 (beta 4Gal-T1) have been explored using a panel of monoclonal antibodies (mAbs) raised against the soluble form of the human enzyme. Reactivity of the antibodies with site-specific and truncated mutants of human beta 4Gal-T1 suggests the presence of a major immunogenic epitope cluster consisting of four epitopes within the stem region and mapping between amino acids 42 and 115. The catalytic activity of the enzyme is increased in the presence of stem region-specific antibody. Two of the epitopes were further localized to a region between amino acids 42 and 77, sequences which are not shared with the recently cloned beta 4Gal-T2 and beta 4Gal-T3 enzymes. An epitope located close to or within the catalytic domain is also identified, and the mAb to this region binds synergistically with antibodies to the stem region.

Immuno-atomic force microscopy of purple membrane

The atomic force microscope is a useful tool for imaging native biological structures at high resolution. In analogy to conventional immunolabeling techniques, we have used antibodies directed against the C-terminus of bacteriorhodopsin to distinguish the cytoplasmic and extracellular surface of purple membrane while imaging in buffer solution. At forces > or = 0.8 nN the antibodies were removed by the scanning stylus and the molecular topography of the cytoplasmic purple membrane surface was revealed. When the stylus was retracted, the scanned membrane area was relabeled with antibodies within 10 min. The extracellular surface of purple membrane was imaged at 0.7 nm resolution, exhibiting a major and a minor protrusion per bacteriorhodopsin monomer. As confirmed by immuno-dot blot analysis and sodium dodecyl sulfate-gel electrophoresis, labeling of the purple membrane was not observed if the C-terminus of bacteriorhodopsin was cleaved off by papain.

The propeptide of anglerfish preprosomatostatin-I rescues prosomatostatin-II from intracellular degradation

Polypeptide hormones and neuropeptides are initially synthesized as precursors possessing one or several domains that constitute the propeptide. Previous work from our laboratory demonstrated that expression of anglerfish prosomatostatin-I (proSRIF-I) in rat anterior pituitary GH3 cells resulted in efficient and accurate cleavage of the prohormone to generate the mature 14-amino acid peptide, SRIF-I. We also implicated the propeptide in mediating intracellular sorting to the trans Golgi network where proteolytic processing is initiated. In contrast, expression of a second form of the precursor, proSRIF-II in GH3 cells resulted in its intracellular degradation in an acidic, post-trans Golgi network compartment, most probably lysosomes. To further investigate the positive sorting signal present in proSRIF-I, we constructed a chimera comprising the signal peptide and proregion of SRIF-I fused to proSRIF-II and expressed the cDNA in GH3 cells. Here we demonstrate that the propeptide of SRIF-I rescued proSRIF-II from intracellular degradation quantitatively and diverted it to secretory vesicles. Furthermore, the chimera was processed to SRIF-28, an amino-terminally extended form of the hormone that is the physiological cleavage product of proSRIF-II processing in vivo. Most significantly, the SRIF-I propeptide functioned only in cis as part of the fusion protein and not in trans when expressed as a separate polypeptide. These data suggest that the SRIF-I propeptide may possess a sorting signal for sequestration into the secretory pathway rather than functioning as an intramolecular chaperone to promote protein folding.

Nuclear protein import is inhibited by an antibody to a lumenal epitope of a nuclear pore complex glycoprotein

Gp210 is a major transmembrane glycoprotein associated with the nuclear pore complex that is suggested to be important for organizing pore complex architecture and assembly. A mouse monoclonal IgG directed against an epitope in the lumenal domain of rat gp210 was expressed in cultured rat cells by microinjection of mRNA prepared from a hybridoma cell line. The expressed IgG, which becomes assembled into a functional antibody in the lumen of the endoplasmic reticulum, bound to the nuclear envelope in vivo. Expression of anti-gp210 antibody in interphase cells specifically reduced approximately fourfold the mediated nuclear import of a microinjected nuclear protein (nucleoplasmin) coupled to gold particles. The antibody also significantly decreased nuclear influx of a 10-kD dextran by passive diffusion. This transport inhibition did not result from removal of pore complexes from nuclear membranes or from gross alterations in pore complex structure, as shown by EM and immunocytochemistry. A physiological consequence of this transport inhibition was inhibition of cell progression from G2 into M phase. Hence, binding of this antibody to the lumenal side of gp210 must have a transmembrane effect on the structure and functions of the pore complex. These data argue that gp210 is directly or indirectly connected to pore complex constituents involved in mediated import and passive diffusion.

Brefeldin A's effects on endosomes, lysosomes, and the TGN suggest a general mechanism for regulating organelle structure and membrane traffic

Addition of brefeldin A (BFA) to most cells results in both the formation of extensive, uncoated membrane tubules through which Golgi components redistribute into the ER and the failure to transport molecules out of this mixed ER/Golgi system. In this study we provide evidence that suggests BFA's effects are not limited to the Golgi apparatus but are reiterated throughout the central vacuolar system. Addition of BFA to cells resulted in the tubulation of the endosomal system, the trans-Golgi network (TGN), and lysosomes. Tubule formation of these organelles was specific to BFA, shared near identical pharmacologic characteristics as Golgi tubules and resulted in targeted membrane fusion. Analogous to the mixing of the Golgi with the ER during BFA treatment, the TGN mixed with the recycling endosomal system. This mixed system remained functional with normal cycling between plasma membrane and endosomes, but traffic between endosomes and lysosomes was impaired.

Identification of a Golgi-associated protein that undergoes mitosis dependent phosphorylation and relocation

By means of a monoclonal antibody (BH3), we have identified a 57-kD protein (p57) that in interphase is restricted largely to the perinuclear region of the cell. Double label immunofluorescence microscopy suggests localization of p57 to the Golgi complex and associated membranous structures. Protease protection experiments and chemical extractability indicate that p57 is a peripheral membrane protein exposed to the cytoplasm. p57 displays unique behavior during mitosis. At the end of G2 or in early prophase, p57 leaves the perinuclear region and accumulates very rapidly within the nucleus, at a time when the nuclear envelope is still intact and before nuclear lamina disassembly. This relocation of p57 coincides with its hyperphosphorylation on serine and threonine residues. After nuclear envelope breakdown p57 becomes uniformly distributed throughout the mitotic cytoplasm until in late telophase when it returns to its perinuclear location and is once again excluded from the nucleus. The behavior of p57 during mitosis suggests that it may play a role in the cellular reorganization evident during mitotic prophase.

An antibody against secretogranin I (chromogranin B) is packaged into secretory granules

We have investigated the sorting and packaging of secretory proteins into secretory granules by an immunological approach. An mAb against secretogranin I (chromogranin B), a secretory protein costored with various peptide hormones and neuropeptides in secretory granules of many endocrine cells and neurons, was expressed by microinjection of its mRNA into the secretogranin I-producing cell line PC12. An mAb against the G protein of vesicular stomatitis virus--i.e., against an antigen not present in PC12 cells--was expressed as a control. The intracellular localization and the secretion of the antibodies was studied by double-labeling immunofluorescence using the conventional and the confocal microscope, as well as by pulse-chase experiments. The secretogranin I antibody, like the control antibody, was transported along the secretory pathway to the Golgi complex. However, in contrast to the control antibody, which was secreted via the constitutive pathway, the secretogranin I antibody formed an immunocomplex with secretogranin I, was packaged into secretory granules, and was released by regulated exocytosis. Our results show that a constitutive secretory protein, unaltered by genetic engineering, can be diverted to the regulated pathway of secretion by its protein-protein interaction with a regulated secretory protein. The data also provide the basis for immunologically studying the role of luminally exposed protein domains in the biogenesis and function of regulated secretory vesicles.

Effect of anti-ER antibodies within the ER lumen of living cells

We describe the production and partial characterization of 12 monoclonal antibodies raised against a preparation of endoplasmic reticulum membranes obtained from Xenopus laevis liver. Four of the antibodies cross-react with liver melanocytes; two of the antibodies recognize extracellular antigens, whilst the remaining six recognize antigens present in hepatocytes. The concentrations of these latter antigens increase markedly in livers stimulated by estrogen. Western blotting analysis revealed that the six anti-hepatocyte monoclonal antibodies recognize at least five different antigens whose molecular weights are 14K, 18K, 19K, 43K, and 125K. The possible functional involvement of the various antigens in the secretory pathway was investigated using Xenopus oocytes as a surrogate secretory system. The mRNAs coding for the monoclonal antibodies were injected into oocytes and the resulting immunoglobulin chains were translated and assembled into active anti-ER antibodies inside the lumen of the ER. The effect on secretion was then observed. Our data indicate that the binding of antibodies to most antigens of the endoplasmic reticulum membrane may result in a blockage of secretion.

Polypeptides of the Golgi apparatus of neurons from rat brain

An antiserum was raised against fractions of the Golgi apparatus of neurons from rat brain. Immunoblots of these fractions with the antiserum showed two principal bands of 185 and 150 kilodaltons (kd) in apparent molecular mass. The antiserum reacted with five or six bands of 200, 150, 130, 100-110, 64, and 40 kd in apparent molecular mass in immunoblots of several crude brain membrane fractions. Affinity-purified antibodies from the different gel bands transferred to nitrocellulose paper were used in immunoblot and immunocytochemical studies. Antibodies eluted from the 200-, 150-, 100-110-, and 64-kd bands reacted not only with the corresponding band but also with the other three bands. Antibodies eluted from the 40-kd band stained only the corresponding band. On light and/or electron microscopic immunocytochemistry, the antiserum stained the Golgi apparatus of rat neurons, glia, liver, and kidney tubule cells. Weaker, segmented, and less consistent staining was observed in nuclear envelopes, rough endoplasmic reticulum, and plasma membranes of neurons. Antibodies eluted from the bands at 200, 150, 100-110, and 64 kd stained intermediate cisterns of the Golgi apparatus of neurons. These findings suggest that a group of related polypeptides of brain membranes is preferentially expressed or enriched in the Golgi apparatus of neurons. Polypeptides with apparent molecular masses of 185 and 150 kd probably represent moieties endogenous to membranes of the neuronal Golgi apparatus.

The Golgi apparatus-complex of neurons and astrocytes studied with an anti-organelle antibody

An antiserum reacting with a 135-kDa antigen of rat liver Golgi apparatus-complex was used to stain, by light microscopic and ultrastructural immunocytochemistry, sections of rat cerebellum and by immunoblot homogenates of whole brain, isolated neurons and a fraction of enriched neuronal Golgi apparatus. In sections of rat cerebellum fixed with periodate-lysine-paraformaldehyde and immunostained with the direct peroxidase or peroxidase-antiperoxidase methods, the Golgi apparatus-complex in perikarya of neurons and glia was stained. Occasionally, nuclear envelopes and cisternae of the rough endoplasmic reticulum of neurons and glia were stained. Immunostain was not observed in peripheral dendrites, axons and presynaptic terminals. In striking contrast, peripheral smooth cisternae of astrocytic perikarya and processes were stained. Immunoblots of whole-brain membrane fractions, homogenates of isolated neurons and an enriched neuronal fraction of the Golgi apparatus-complex showed a principal single band of 64-kDa apparent mol. wt. We have concluded that the putative 64-kDa antigen(s) is distributed in cisternae of the Golgi apparatus-complex and occasionally in the nuclear envelope and rough reticulum, within the perikarya of neurons and glia. A second important distribution of the 64-kDa antigen(s), involving peripheral cisternae in perikarya and processes of astrocytes, is consistent with the hypothesis that the Golgi apparatus-complex of these cells extends to the periphery of these cells. The functional implications of the peripheral localization of the 64-kDa antigen(s) in astrocytes are discussed.

Expression of biologically active bovine luteinizing hormone in Chinese hamster ovary cells

Biologically active bovine luteinizing hormone (LH) has been obtained through expression of the alpha- and LH beta-subunit genes in stably transformed clones of DUXB11, a Chinese hamster ovary cell line deficient in dihydrofolate reductase (DHFR). Expression of alpha-and LH beta-subunit mRNAs of the expected sizes (approximately 910 and 770 nucleotides, respectively) were revealed by blot analysis after electrophoresis of total cellular RNA. Furthermore, presence or absence of the gonadotropin mRNAs in several clonal lines was directly correlated with the appearance of one or both bovine LH subunits in the culture medium. Media from three clones secreting significant immunoreactive levels of both subunits also stimulated the release of progesterone in ovine luteal cells, suggesting that the secreted LH was assembled into a biologically active and glycosylated dimer. Immunoprecipitation and NaDodSO4/PAGE of [35S]methionine-labeled proteins secreted from one of the clones, CHODLH20, further confirmed the presence of an alpha/beta dimer with apparent subunit molecular weights of 20,500 and 16,000, only slightly higher than those of pituitary alpha and LH beta subunits.

Highly specific antibody to Rous sarcoma virus src gene product recognizes a novel population of pp60v-src and pp60c-src molecules

Antiserum to the Rous sarcoma virus (RSV)-transforming protein, pp60v-src, was produced in rabbits immunized with p60 expressed in Escherichia coli. alpha p60 serum immunoprecipitated quantitatively more pp60v-src than did tumor-bearing rabbit (TBR) sera. When RSV-transformed cell lysates were preadsorbed with TBR serum, the remaining lysate contained additional pp60v-src, which was recognized only by reimmunoprecipitation with alpha p60 serum and not by TBR serum. In subcellular fractions of RSV-infected chicken embryo fibroblasts (RSV-CEFs) and field vole cells probed with TBR serum, the majority of the pp60v-src was associated with the plasma membrane-enriched P100 fraction. However, alpha p60 serum revealed equal distribution of pp60v-src and its kinase activity between the P1 (nuclear) and P100 fractions. The same results were obtained for pp60c-src in uninfected CEFs. On discontinuous sucrose gradients nearly 50% of the P1-pp60v-src sedimented with nuclei, in fractions where no plasma membrane was detected. Indirect immunofluorescence microscopy of RSV-CEFs with alpha p60 serum revealed a distinct pattern of perinuclear fluorescence, in addition to staining at the cell periphery. Thus the use of a highly specific antibody reveals that enzymatically active pp60v-src and pp60c-src molecules are present in other intracellular structures, probably juxtareticular nuclear membranes, in addition to the plasma membrane in normal, uninfected, and wild-type RSV-infected cells.