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Antibodies have been the workhorse reagents of protein capture and quantification since their 1959 debut in the RIAs developed by Yalow and Berson. However, there are technical challenges to the use of antibodies in highly multiplexed arrays aimed at measuring hundreds or even thousands of proteins at one time. We describe here a recently developed class of synthetic protein-binding reagents (slow off-rate modified aptamer). We discuss the chemical makeup and protein binding specifications of slow off-rate modified aptamer reagents, compare them to traditional aptamers and antibodies, briefly describe the novel proteomic assay that takes advantage of their unique properties, and provide several examples of their multiple applications to biomarker discovery and validation across a range of biomedical science questions.

Humanization of a potent neutralizing mouse anti-human IL-13 antibody (m836) using a method called human framework adaptation (HFA) is reported. HFA consists of two steps: human framework selection (HFS) and specificity-determining residue optimization (SDRO). The HFS step involved generation of a library of m836 antigen binding sites combined with diverse human germline framework regions (FRs), which were selected based on structural and sequence similarities between mouse variable domains and a repertoire of human antibody germline genes. SDRO consisted of diversifying specificity-determining residues and selecting variants with improved affinity using phage display. HFS of m836 resulted in a 5-fold loss of affinity, whereas SDRO increased the affinity up to 100-fold compared to the HFS antibody. Crystal structures of Fabs in complex with IL-13 were obtained for m836, the HFS variant chosen for SDRO, and one of the highest-affinity SDRO variants. Analysis of the structures revealed that major conformational changes in FR-H1 and FR-H3 occurred after FR replacement, but none of them had an evident direct impact on residues in contact with IL-13. Instead, subtle changes affected the V L /V H (variable-light domain/variable-heavy domain) interface and were likely responsible for the 5-fold decreased affinity. After SDRO, increased affinity resulted mainly from rearrangements in hydrogen-bonding pattern at the antibody/antigen interface. Comparison with m836 putative germline genes suggested interesting analogies between natural affinity maturation and the engineering process that led to the potent HFA anti-human IL-13 antibody.

At SomaLogic, we have embarked on an ambitious program of clinical studies using a novel aptamerbased proteomics technology to discover biomarkers and develop new tools to diagnose, understand, and treat human disease. As part of this program, we designed and implemented an automated assay for its highly multiplexed proteomics discovery platform. The performance of the automated assay was validated in a study that compared the automated assay to the specifications of an established manual method. Results showed that the automated method performed to the required specifications, and that the automation system improved the efficiency, productivity, and economics of our biomarker discovery program.

Summary Current understanding of microRNA (miRNA) biology is limited, and antisense oligonucleotide (ASO) inhibition of miRNAs is a powerful technique for their functionalization. To uncover the role of the liver-specific miR-122 in the adult liver, we inhibited it in mice with a 2′-O-methoxyethyl phosphorothioate ASO. miR-122 inhibition in normal mice resulted in reduced plasma cholesterol levels, increased hepatic fatty-acid oxidation, and a decrease in hepatic fatty-acid and cholesterol synthesis rates. Activation of the central metabolic sensor AMPK was also increased. miR-122 inhibition in a diet-induced obesity mouse model resulted in decreased plasma cholesterol levels and a significant improvement in liver steatosis, accompanied by reductions in several lipogenic genes. These results implicate miR-122 as a key regulator of cholesterol and fatty-acid metabolism in the adult liver and suggest that miR-122 may be an attractive therapeutic target for metabolic disease.

Interrogation of the human proteome in a highly multiplexed and efficient manner remains a coveted and challenging goal in biology. We present a new aptamer-based proteomic technology for biomarker discovery capable of simultaneously measuring thousands of proteins from small sample volumes (15 [mu]L of serum or plasma). Our current assay allows us to measure ~800 proteins with very low limits of detection (1 pM average), 7 logs of overall dynamic range, and 5% average coefficient of variation. This technology is enabled by a new generation of aptamers that contain chemically modified nucleotides, which greatly expand the physicochemical diversity of the large randomized nucleic acid libraries from which the aptamers are selected. Proteins in complex matrices such as plasma are measured with a process that transforms a signature of protein concentrations into a corresponding DNA aptamer concentration signature, which is then quantified with a DNA microarray. In essence, our assay takes advantage of the dual nature of aptamers as both folded binding entities with defined shapes and unique sequences recognizable by specific hybridization probes. To demonstrate the utility of our proteomics biomarker discovery technology, we applied it to a clinical study of chronic kidney disease (CKD). We identified two well known CKD biomarkers as well as an additional 58 potential CKD biomarkers. These results demonstrate the potential utility of our technology to discover unique protein signatures characteristic of various disease states. More generally, we describe a versatile and powerful tool that allows large-scale comparison of proteome profiles among discrete populations. This unbiased and highly multiplexed search engine will enable the discovery of novel biomarkers in a manner that is unencumbered by our incomplete knowledge of biology, thereby helping to advance the next generation of evidence-based medicine.

MicroRNAs (miRNAs) are a class of 20–24 nt noncoding RNAs that regulate target mRNAs post-transcriptionally by binding with imperfect complementarity in the 3′-untranslated region (3′-UTR) and inhibiting translation or RNA stability. Current understanding of miRNA biology is limited, and antisense oligonucleotide (ASO) inhibition is a powerful technique for miRNA functionalization in vitro and in vivo, and for therapeutic targeting of miRNAs. Identification of optimal ASO chemistries for targeting miRNAs is therefore of great interest. We evaluated a number of 2′-sugar and backbone ASO modifications for their ability to inhibit miR-21 activity on a luciferase reporter mRNA. ASO modifications that improved target affinity improved miRNA ASO activity, yet the positioning of high-affinity modifications also had dramatically different effects on miRNA activity, suggesting that more than affinity determined the effectiveness of the miRNA ASOs. We present data in which the activity of a modified miRNA ASO was inversely correlated to its tolerability as an siRNA passenger strand, suggesting that a similar mechanism could be involved in the dissociation of miRNA ASOs and siRNA passenger strands. These studies begin to define the factors important for designing improved miRNA ASOs, enabling more effective miRNA functionalization and therapeutic targeting.

In an effort to optimize the transfection of cell lines with antisense oligonucleotides, we examined cellular accumulation of a labeled oligonucleotide by flow cytometry. We were surprised to observe that a routinely used transfection protocol, a fixed lipid/oligonucleotide ratio, resulted in variable transfection efficiency depending on the concentration of oligonucleotide used. A significant population of cells, especially at lower doses of oligonucleotide and cationic lipid, were untransfected. We investigated lipid/oligonucleotide ratios, different lipid preparations, and different cell types and found that these variables did not alter the percentage of cells transfected at these lower doses of oligonucleotide. However, when lipid-oligonucleotide complexes were formed at the high dose and then diluted into a solution of lipid or a complex of lipid and unlabeled, negative control oligonucleotide, a constant percentage of cells was transfected. Under these conditions, mRNA target reduction dose-response curves were also shifted to lower doses. We hypothesize that poor transfection observed at a low concentration of lipid-oligonucleotide complex when diluted in medium is due to loss of active complexes, either by adsorption to the substrate or by changes in physical characteristics of complexes. By maintaining a constant lipid concentration, more consistent transfection was achieved.

MicroRNAs (miRNAs) are endogenously expressed 20-24 nucleotide RNAs thought to repress protein translation through binding to a target mRNA (1-3). Only a few of the more than 250 predicted human miRNAs have been assigned any biological function. In an effort to uncover miRNAs important during adipocyte differentiation, antisense oligonucleotides (ASOs) targeting 86 human miRNAs were transfected into cultured human pre-adipocytes, and their ability to modulate adipocyte differentiation was evaluated. Expression of 254 miRNAs in differentiating adipocytes was also examined on a miRNA microarray. Here we report that the combination of expression data and functional assay results identified a role for miR-143 in adipocyte differentiation. miR-143 levels increased in differentiating adipocytes, and inhibition of miR-143 effectively inhibited adipocyte differentiation. In addition, protein levels of the proposed miR-143 target ERK5 (4) were higher in ASO-treated adipocytes. These results demonstrate that miR-143 is involved in adipocyte differentiation and may act through target gene ERK5.