alpaca antibodies

When Great is not Good Enough—VHH Antibodies Engineered for 10 Fold Affinity Increase

Single Domain antibodies (VHH fragments, nanobodies, or as we call them, nAbs) have been generated by injecting llamas with ligand-bound GPCR for the purpose of obtaining crystals of active-state structures. Such structural information could be critical in understanding drug functions and screening for new drugs. The unique ability of VHH fragments to fit into protein-protein complex crevices and hold proteins together was demonstrated by two Nature publications from Brian Kobilka’s group at Stanford ([1, 2], also see Allele Newsletter of Sep 4th, 2013). The nano antibody used in those studies, Nb80, showed affinity towards only the active state of the target GPCR.

However, even with an antibody as great as Nb80, the authors were only able to co-crystal GPCR beta2-adrenoceptor (b2AR) with high affinity agonists, not its natural agonists such as adrenaline. In yet another Nature paper published just now, the Kobilka lab showed that Nb80 could be further improved by 10 times in affinity, through in vitro evolution [3]. They presented Nb80 on the surface of yeast using an existing yeast display system, then applied standard limited mutagenesis and magnetic separation technologies for screening. After about 5 rounds of selection, a new version of VHH Nb6B9 was isolated that bound to ligand-loaded GPCR with a kD of 6.4 nM. For the first time, a co-crystal of b2AR-adrenoline was made.

Rasmussen et al. Nature, 2011 Structure of a nanobody-stabilized active state of the b2 adrenoceptor
Rasmussen et al. Nature, 2011 Crystal structure of the b2 adrenergic receptor–Gs protein complex
Ring et al. Nature, 2013 Adrenaline-activated structure of b2-adrenoceptor stabilized by an engineered nanobody

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Visualizing Endogenous Synaptic Proteins in Living Neurons

The recently published method is based on the generation of disulfide-free “intrabodies”, a structure from the 10th fibronectin type III domain known as FingRs. These affinity molecules were fused to GFP for direct fluorescence miscroscopy. The FingRs do not need di-sulfite bonds and are therefore better folders in mammalian cells. Specifically, a library was screened with in vitro display to identify FingRs that bind two synaptic proteins, Gephyrin and PSD95. After the initial selection, the researchers from USC secondarily screened binders using a cellular localization assay to identify potential FingRs that bind at high affinity in an intracellular environment. As it turned out, only 10-20% of the original positive clones bind well inside the cells, suggesting this type of further screening was a critical step.

The expression of intrabody is transcriptionally regulated by the target protein through a ZFN-repressor fusion. This transcriptional control system matches the expression of the intrabody to that of the target protein regardless of the target’s expression level. This design virtually eliminates unbound FingR, resulting in very low background that allows unobstructed visualization of the target proteins. As result, the FingRs presented in this study enabled live cell visualization of excitatory and inhibitory synapses, and apparently without affecting neuronal function.

Technically, the reason to use in vitro mRNA display was required by the need to use a large library (>10exp12, beyond the limit of the more commonly used phase display) to find good binders. A similar visualization system can be established using more potent affinity domains such as the VHH single-domain antibodies that have only one, sometimes dispensable, di-sulfite bond. The VHH domain nanobodies can be more easily isolated from camelid animals. Another improvement to the visualization system can be made by using stronger, superresolution-ready FPs such as mNeonGreen or mMaple to enable single molecule imaging, which is particularly interesting for studying synapses and applied to the BRAIN initiative.

Gross et al. Neuron, June 2013,

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New Frontiers for Research Tool Development in the New Year

Looking into the future of technologies in biology research

Allele Biotech's Green Crystal Ball

Chosen as the Method of the Year 2010 by Nature Method and mentioned in a number of year-end recaps, this is a technology that allows the use of light to precisely (at least in a temporal sense) control engineered proteins within a targeted cell population. For example, by introducing light-activated channelrhodopsins into neurons, one can use a pulse of light to initiate a movement of ion across the cell membrane. The technology, first reported in 2005 then made headlines as a major impact on neurosciences since 2007, is now being combined with other components in controlling a broader array of biological events, such as DNA binding, enzyme activities, etc. Looking forward, a few areas will be more than likely the frontlines of moving optogenetics into more labs:

Additional combinations: The few known channelrhodopsins and their fast growing variations will be combined with more “effecter” domains to control different events. The challenge will be to find ways to use the structural changes or any responses channelrhodopsins have to stimulating lights in order to trigger a reaction in the associated effecter domain.

Tracking mechanisms: A platter of fluorescent proteins (FPs) will be used as an independent tracking method to follow cells being targeted. FPs that have optical spectra that do not interfere with the optogenetic molecules will be tested and established. In addition, FPs with less toxicity, narrower excitation and emission peaks, and more tolerance to different cellular environment will be preferred and eventually set up as standards.

Delivery tools: To bring the optogenetic reagents into cells like neurons researchers will most likely rely on lentiviral vectors in most cases. Other vehicles such as baculovirus, MMLV-based retrovirus, even herpes virus may find broader applications in this field. Pre-packaged lentiviruses and MMLV-retroviruses already contain optogenetic constructs will become popular products.

VHH Antibodies
The small capture polypeptides based on single-domain Camelid antibodies (nanobodies, nano antbodies or nAbs) and similar VHH domains will become much dramatically more popular this year, judging from the significant increase in demands of the only camelid reagent products, GFP-Trap and RFP-Trap, in 2010. There are a number of NIH initiated programs that aim to find capture reagents that eventually target the complete human proteome. One of the key criteria for the current phase of the relevant NIH Director’s Initiative is ability to co-immunoprecipitate. The Human Proteome Organization (HUPO) recently expressed frustration due to the lack of high quality capture reagents necessary to isolate and identify most proteins. HUPO promotes global research on proteins in order to decode the human proteome. From what we have learned from dozens of publications showing the use of GFP-Trap, VHH molecules pulls down GFP-tagged proteins with unprecedented efficiency and purity. VHH antibodies show strong affinity and specificity, at a level superior or comparable to monoclonal antibodies. In addition, VHH antibodies are increasingly appreciated for their capabilities to recognize concave epitopes by their relatively convex-shaped paratopes. VHH nanobodies are small (~12-15 kD), with a limited number of functionally important disulfide bonds, can be expressed very well in E. coli, and are amazingly stable in extreme denaturing conditions such as heat and acid. They have been shown to be better suited for in vivo and trans-cellular membrane delivery than other antibodies. It should not be surprising that one day in the coming years VHH antibodies will be more dominant than monoclonal antibodies.

Super-Resolution Imaging
One of the goals of developing technologies such as photoactivated localization microscopy (PALM) and related super-resolution imaging (SRI) techniques was to achieve electron microscopy (EM) level resolution without using EM. Now new developments show that maybe combining EM and photoactivable FPs would provide more specific and more detailed morphology. It would be anticipated that more photoconvertible FPs will prove to work well for one type of SRI or another. The event that will bring this technology to nearly every cell biology lab is the improvement and availability of necessary instruments that some companies have already begun to commercialize.

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Choosing siRNA, shRNA, and miRNA for Gene Silencing

RNAi refers to dsRNA-induced gene silencing, a cellular process that degrades RNA homologous to one strand of the dsRNA [1, 2]. The intermediates of long dsRNA-initiated RNAi are double-stranded small interfering RNAs (siRNA), typically 21-23 nucleotide (nt) long. The siRNAs, when introduced into cells, can be used to silence genes in mammalian systems where long dsRNAs prompt protein kinase R (PKR), RNase L, and interferon activities that result in non-specific RNA degradation and general shutdown of protein synthesis [3]. siRNAs can either be chemically synthesized then directly transfected into cells or can be generated inside the cell by introducing vectors that express short-hairpin RNA (shRNA) precursors of siRNAs. The process of shRNA into functional siRNA involves cellular RNAi machinery that naturally process genome encoded microRNAs (miRNA) that are responsible for cellular regulation of gene expression by modulating mRNA stability, translation, and chromatin structures [4].

Chemically synthesized siRNA is the simplest format for RNAi. One of the biggest hurdles for achieving effective RNAi with siRNA is that many cells are difficult to transfect. An RNAi experiment is typically considered successful when the target gene expression is reduced by >70%, a threshold not reachable by many types of cells due to their low transfection efficiency. Another drawback of using synthetic siRNA is the limited duration of post-transfection effects, typically with gene silencing activities peaking around 24 hours, and diminishing within 48 hours [5]. Chemical synthesis of siRNA, which is a service Allele Biotech and Orbigen (now merged under the Allele brand) pioneered and still provides, is expensive on a per transfection basis relative to DNA vector based reagents.

shRNA can be introduced by DNA plasmid, linear template, or packaged retroviral/lentiviral vectors. Using any form of DNA construct, except the PCR template format such as Allele’s LineSilence platform, requires creating DNA constructs and sequence verification; a taxing work load if multiple genes need to be studied. However, once the constructs are made, they can be reproduced easily and inexpensively. It is difficult to directly compare the effectiveness of siRNA versus shRNA on a per molecule basis because RNA polymerase III (Pol III) promoters such as U6 or H1 commonly used to express shRNAs can make thousands of copies of shRNA from a single DNA template. However when both siRNA and shRNA are produced the same way, e.g. synthesized chemically, shRNA is reported to be somewhat more effective [6, 7]. For the goals of this research, the most important advantage using shRNA can provide over siRNA is that it can be carried on a lentiviral vector and introduced into a wide variety of cells.

Similar to the comparison between siRNA versus shRNA, it is also difficult to rank the efficiency of shRNA versus miRNA from published data, partly due to different results from different experimental systems. There have been several reports that showed shRNA can cause significant cell toxicity, especially in vivo such as after injection into mouse brain. It was originally reasoned that highly efficient expression from Pol III promoters might overwhelm the cellular machinery that is needed to execute endogenous RNAi functions such as transporting miRNA from the nucleus to the cytoplasm. It was later found out that even using Pol III promoter to create miRNA could still mitigate the toxic effects of shRNA [8]. Since shRNA and miRNA are processed by endonuclease Dicer before being incorporated into RNA induced silencing complex (RISC), the exact identity of siRNAs produced from a given shRNA or miRNA targeting the same region on the mRNA are not known in most of the earlier studies. By designing shRNA and miRNA to give exactly the same processed siRNAs, Boudreau et al. showed that shRNA is actually more potent than miRNA in various systems [9].

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Currently Trendy Product Line: Camelid antibody group against fluorescent proteins as precipitation tag for co-IP (replacing formerly GFP-Trap line)–GFP-nAb, promotion ongoing now.

1. Fire, A., S. Xu, M.K. Montgomery, S.A. Kostas, S.E. Driver, and C.C. Mello, Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. Nature, 1998. 391(6669): p. 806-11.
2. Hannon, G.J., RNA interference. Nature, 2002. 418(6894): p. 244-51.
3. McManus, M.T. and P.A. Sharp, Gene silencing in mammals by small interfering RNAs. Nat Rev Genet, 2002. 3(10): p. 737-47.
4. Hutvagner, G. and P.D. Zamore, A microRNA in a multiple-turnover RNAi enzyme complex. Science, 2002. 297(5589): p. 2056-60.
5. Rao, D.D., J.S. Vorhies, N. Senzer, and J. Nemunaitis, siRNA vs. shRNA: similarities and differences. Adv Drug Deliv Rev, 2009. 61(9): p. 746-59.
6. Vlassov, A.V., B. Korba, K. Farrar, S. Mukerjee, A.A. Seyhan, H. Ilves, R.L. Kaspar, D. Leake, S.A. Kazakov, and B.H. Johnston, shRNAs targeting hepatitis C: effects of sequence and structural features, and comparision with siRNA. Oligonucleotides, 2007. 17(2): p. 223-36.
7. Siolas, D., C. Lerner, J. Burchard, W. Ge, P.S. Linsley, P.J. Paddison, G.J. Hannon, and M.A. Cleary, Synthetic shRNAs as potent RNAi triggers. Nat Biotechnol, 2005. 23(2): p. 227-31.
8. McBride, J.L., R.L. Boudreau, S.Q. Harper, P.D. Staber, A.M. Monteys, I. Martins, B.L. Gilmore, H. Burstein, R.W. Peluso, B. Polisky, B.J. Carter, and B.L. Davidson, Artificial miRNAs mitigate shRNA-mediated toxicity in the brain: implications for the therapeutic development of RNAi. Proc Natl Acad Sci U S A, 2008. 105(15): p. 5868-73.
9. Boudreau, R.L., A.M. Monteys, and B.L. Davidson, Minimizing variables among hairpin-based RNAi vectors reveals the potency of shRNAs. Rna, 2008. 14(9): p. 1834-44.

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Wednesday, February 3rd, 2010 RNAi patent landscape, Viruses and cells No Comments

Immunoprecipitation Tags

Immunoprecipitation is a process of isolating a protein as an antigen by using antibodies against it. It is a powerful tool for studying proteins in biological samples and, in case of Co-IP (meaning immunoprecipitation of complexes containing a known antigen), for analyzing protein-protein interactions. Similar technologies such as chromatin immunoprecipitation (ChIP), RNA immunoprecipitation (RIP), or crosslinked and iImmunoprecipitation of RNA-protein complexes (CLIP) aid analysis of protein-DNA or protein-RNA interactions.

The major obstacle for achieving effective immunoprecipitation is the difficulty of finding usable antibodies against a target of interest. A common practice is to use tags that are fused to the C- or N-terminus of the target protein, thereby any validated, commercially available antibody can be used for co-IP in different experimental systems. However, caution must be exercised against potential interference of biological functions from the added tags. In general, one should choose tags that have been tested in many situations and proven non-interfering; still, each biological system is different. Independent validation or supporting data should be used when interpreting results from tag-based co-IP.

Tags are often selected based on high quality and commercially available antibodies. Most commonly used tags include: FLAG, Myc, HA, V5, T7, and His, which are quite small in size and in theory less likely to interfere. GST and GFP are in between 20-30kDa, but they are well documented to form self-contained and stable structures independent of their fusion partners and proved to not interfere in many cases. GST can bind to glutathione beads directly, therefore a top choice for pulldown experiments. GFP or other FPs as tags have the advantages of being also a visualization module to follow the protein both inside cells and during pulldown. However, previously available anti-GFP antibodies, either polyclonal or monoclonal, are not comparable to those against other tags, thereby limiting the use of GFP as fusion tag in pulldown experiments.

GFP-Trap, a recent addition to anti-tag antibodies, is an E. coli expressed, single domain fragment derived from camelid heavy chain antibodies (VHH antibodies) with much higher stability, specificity, and affinity, making GFP based pulldown quantitative. This recent advancement should make GFP in line to become the most suitable tags for many aforementioned precipitation experiments.

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