Automated ELISA for potency measurements of therapeutic antibodies and antibody fragments.

Potency assays are essential for the development and quality control of biopharmaceutical drugs, but they are often a time limiting factor due to manual handling steps and consequently low analytical throughput. On the other hand, automation of potency assays can be challenging due to their complexity and the use of biological materials. ELISA (enzyme-linked immunosorbent assay) is widely used for potency determination and is a good candidate for automation as all ELISA types depend on the same basic steps: coating, blocking, sample incubation, detection, and signal measurement. Nevertheless, ELISA for relative potency measurements still require drug-specific development and assay validation thereby complicating automation efforts. To simplify potency testing by ELISA, we first developed a manual protocol generally applicable to different drugs and then adapted this protocol for automated measurements. We identified unexpected critical parameters which had to be adapted to transfer the manual ELISA to an automated liquid handling system and we demonstrated that gravimetric sample dilution is unnecessary with the automated protocol. Both manual and automated protocols were validated and compared using multiple biotherapeutics. The automated protocol showed similar or higher precision and accuracy when compared to the manual method.

Selective export of human GPI-anchored proteins from the endoplasmic reticulum.

Selective export of transmembrane proteins from the endoplasmic reticulum (ER) relies on recognition of cytosolic-domain-localized transport signals by the Sec24 subunit of the COPII vesicle coat. Human cells express four Sec24 isoforms, termed Sec24A, Sec24B, Sec24C and Sec24D that are differentially required for selective, signal-mediated ER export of transmembrane proteins. By contrast, luminally exposed glycosylphosphatidylinositol (GPI)-anchored membrane proteins cannot bind directly to Sec24 and must either use membrane-spanning cargo receptors or alternative mechanisms for ER export. Little is known about the mechanism underlying export of GPI-anchored proteins from the ER in higher eukaryotes. Using siRNA-based silencing, we identified that ER-to-Golgi transport of the human GPI-anchored protein CD59 requires Sec24, with preference for the Sec24C and Sec24D isoforms, and the recycling transmembrane protein complex p24-p23 that exhibited the same Sec24C-Sec24D isoform preference for ER export. Co-immunoprecipitation indicated unprecedented physical interaction of CD59 as well as a GFP-folate-receptor-GPI-anchor hybrid with a p24-p23 complex. Density gradient centrifugation revealed co-partitioning of CD59 and p24-p23 into biosynthetically early lipid raft fractions, and CD59 transport to the Golgi was cholesterol dependent. The results suggest that the 24p-23p complex acts as a cargo receptor for GPI-anchored proteins by facilitating their export from the ER in a Sec24-isoform-selective manner involving lipid rafts as early sorting platforms.

Intestinal LI-cadherin acts as a Ca2+-dependent adhesion switch.

Cadherins are Ca(2+)-dependent transmembrane glycoproteins that mediate cell-cell adhesion and are important for the structural integrity of epithelia. LI-cadherin and the classical E-cadherin are the predominant two cadherins in the intestinal epithelium. LI-cadherin consists of seven extracellular cadherin repeats and a short cytoplasmic part that does not interact with catenins. In contrast, E-cadherin is composed of five cadherin repeats and a large cytoplasmic domain that is linked via catenins to the actin cytoskeleton. Whereas E-cadherin is concentrated in adherens junctions, LI-cadherin is evenly distributed along the lateral contact area of intestinal epithelial cells. To investigate if the particular structural properties of LI-cadherin result in a divergent homotypic adhesion mechanism, we analyzed the binding parameters of LI-cadherin on the single molecule and the cellular level using atomic force microscopy, affinity chromatography and laser tweezer experiments. Homotypic trans-interaction of LI-cadherin exhibits low affinity binding with a short lifetime of only 1.4 s. Interestingly, LI-cadherin binding responds to small changes in extracellular Ca(2+) below the physiological plasma concentration with a high degree of cooperativity. Thus, LI-cadherin might serve as a Ca(2+)-regulated switch for the adhesive system on basolateral membranes of the intestinal epithelium.

Improved maturation of CFTR by an ER export signal.

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel in the plasma membrane of several epithelial cells. Maturation of CFTR is inefficient in most cells, with only a fraction of nascent chains being properly folded and transported to the cell surface. The most common mutation in CFTR, CFTR-deltaF508, leads to the genetic disease cystic fibrosis. CFTR-deltaF508 has a temperature-sensitive folding defect and is almost quantitatively degraded in the endoplasmic reticulum (ER). Here we tested whether a strong ER export signal appended to CFTR improves its transport and surface expression. We show that a single valine ER export signal at the C terminus of the cytoplasmic tail of CFTR improves maturation of wild-type CFTR by 2-fold. This conservative mutation interfered with neither plasma membrane localization nor stability of mature CFTR. In contrast, the valine signal was unable to rescue CFTR-deltaF508 from ER-associated degradation. Our finding of improved maturation of CFTR mediated by a valine signal may be of potential use in gene therapy of cystic fibrosis. Moreover, failure of the valine signal to rescue CFTR-deltaF508 from ER degradation indicates that the inability of CFTR-deltaF508 to leave the ER is unlikely to be due to a malfunctioning ER export signal.

Full automation and validation of a flexible ELISA platform for host cell protein and protein A impurity detection in biopharmaceuticals.

Monitoring host cell protein (HCP) and protein A impurities is important to ensure successful development of recombinant antibody drugs. Here, we report the full automation and validation of an ELISA platform on a robotic system that allows the detection of Chinese hamster ovary (CHO) HCPs and residual protein A of in-process control samples and final drug substance. The ELISA setup is designed to serve three main goals: high sample throughput, high quality of results, and sample handling flexibility. The processing of analysis requests, determination of optimal sample dilutions, and calculation of impurity content is performed automatically by a spreadsheet. Up to 48 samples in three unspiked and spiked dilutions each are processed within 24 h. The dilution of each sample is individually prepared based on the drug concentration and the expected impurity content. Adaptable dilution protocols allow the analysis of sample dilutions ranging from 1:2 to 1:2×10(7). The validity of results is assessed by automatic testing for dilutional linearity and spike recovery for each sample. This automated impurity ELISA facilitates multi-project process development, is easily adaptable to other impurity ELISA formats, and increases analytical capacity by combining flexible sample handling with high data quality.

MAPK signaling to the early secretory pathway revealed by kinase/phosphatase functional screening.

To what extent the secretory pathway is regulated by cellular signaling is unknown. In this study, we used RNA interference to explore the function of human kinases and phosphatases in controlling the organization of and trafficking within the secretory pathway. We identified 122 kinases/phosphatases that affect endoplasmic reticulum (ER) export, ER exit sites (ERESs), and/or the Golgi apparatus. Numerous kinases/phosphatases regulate the number of ERESs and ER to Golgi protein trafficking. Among the pathways identified, the Raf-MEK (MAPK/ERK [extracellular signal-regulated kinase] kinase)-ERK cascade, including its regulatory proteins CNK1 (connector enhancer of the kinase suppressor of Ras-1) and neurofibromin, controls the number of ERESs via ERK2, which targets Sec16, a key regulator of ERESs and COPII (coat protein II) vesicle biogenesis. Our analysis reveals an unanticipated complexity of kinase/phosphatase-mediated regulation of the secretory pathway, uncovering a link between growth factor signaling and ER export.

Heterotypic trans-interaction of LI- and E-cadherin and their localization in plasmalemmal microdomains.

Cadherins are calcium-dependent adhesion molecules important for tissue morphogenesis and integrity. LI-cadherin and E-cadherin are the two prominent cadherins in intestinal epithelial cells. Whereas LI-cadherin belongs to the subfamily of 7D (seven-domain)-cadherins defined by their seven extracellular cadherin repeats and short intracellular domain, E-cadherin is the prototype of classical cadherins with five extracellular domains and a highly conserved cytoplasmic part that interacts with catenins and thereby modulates the organization of the cytoskeleton. Here, we report a specific heterotypic trans-interaction of LI- with E-cadherin, two cadherins of distinct subfamilies. Using atomic force microscopy and laser tweezer experiments, the trans-interaction of LI- and E-cadherin was characterized on the single-molecule level and on the cellular level, respectively. This heterotypic interaction showed similar binding strength (20-52 pN at 200-4000 nm/s) and lifetime (0.8 s) as the respective homotypic interactions of LI- and E-cadherin. VE-cadherin, another classical cadherin, did not bind to LI-cadherin. In enterocytes, LI-cadherin and E-cadherin are located in different membrane regions. LI-cadherin is distributed along the basolateral membrane, whereas the majority of E-cadherin is concentrated in adherens junctions. This difference in membrane distribution was also reflected in Chinese hamster ovary cells stably expressing either LI- or E-cadherin. We found that LI-cadherin is localized almost exclusively in cholesterol-rich fractions, whereas E-cadherin is excluded from these membrane fractions. Given their different membrane localization in enterocytes, the heterotypic trans-interaction of LI- and E-cadherin might play a role during development of the intestinal epithelium when the cells do not yet have elaborate membrane specializations.

Identification of ERGIC-53 as an intracellular transport receptor of alpha1-antitrypsin.

Secretory proteins are exported from the endoplasmic reticulum (ER) by bulk flow and/or receptor-mediated transport. Our understanding of this process is limited because of the low number of identified transport receptors and cognate cargo proteins. In mammalian cells, the lectin ER Golgi intermediate compartment 53-kD protein (ERGIC-53) represents the best characterized cargo receptor. It assists ER export of a subset of glycoproteins including coagulation factors V and VIII and cathepsin C and Z. Here, we report a novel screening strategy to identify protein interactions in the lumen of the secretory pathway using a yellow fluorescent protein-based protein fragment complementation assay. By screening a human liver complementary DNA library, we identify alpha1-antitrypsin (alpha1-AT) as previously unrecognized cargo of ERGIC-53 and show that cargo capture is carbohydrate- and conformation-dependent. ERGIC-53 knockdown and knockout cells display a specific secretion defect of alpha1-AT that is corrected by reintroducing ERGIC-53. The results reveal ERGIC-53 to be an intracellular transport receptor of alpha1-AT and provide direct evidence for active receptor-mediated ER export of a soluble secretory protein in higher eukaryotes.

Role of Sec24 isoforms in selective export of membrane proteins from the endoplasmic reticulum.

Sec24 of the COPII (coat protein complex II) vesicle coat mediates the selective export of membrane proteins from the endoplasmic reticulum (ER) in yeast. Human cells express four Sec24 isoforms, but their role is unknown. Here, we report the differential effects of Sec24 isoform-specific silencing on the transport of the membrane reporter protein ERGIC-53 (ER-Golgi intermediate compartment-53) carrying the cytosolic ER export signals di-phenylalanine, di-tyrosine, di-leucine, di-isoleucine, di-valine or terminal valine. Knockdown of single Sec24 isoforms showed dependence of di-leucine-mediated transport on Sec24A, but transport mediated by the other signals was not affected. By contrast, double knockdown of Sec24A with one of the other three Sec24 isoforms impaired all aromatic/hydrophobic signal-dependent transport. Double knockdown of Sec24B/C or Sec24B/D preferentially affected di-leucine-mediated transport, whereas knockdown of Sec24C/D affected di-isoleucine- and valine-mediated transport. The isoform-selective transport correlated with binding preferences of the signals for the corresponding isoforms in vitro. Thus, human Sec24 isoforms expand the repertoire of cargo for signal-mediated ER export, but are in part functionally redundant.