Antibody surrogates

Xu et al. describe the potential of 'surrobodies'—protein scaffolds based on pre–B cell receptors—as alternatives to conventional antibodies and suggest ways in which they may have advantages for certain applications. Trimeric pre–B cell receptors differ from bona fide antibodies in that each heavy chain (green) is covalently associated with a surrogate light chain comprising noncovalently associated Vpreβ (blue) and λ5 (yellow) domains. Surrobody libraries prepared from individuals who survived exposure to H5N1 influenza virus yield several unique solutions for binding viral hemagglutinin with nanomolar affinities. Surrobodies express well in mammalian and Escherichia coli cells as well as on phage surfaces. The Vpreβ and λ5 domains have 21- and 50-amino-acid extensions on their C and N termini, respectively. These peptide tails could be engineered to increase the protein surface used for antigen binding or to confer unique properties such as catalysis, conditional lability or subcellular targeting. (Proc. Natl. Acad. Sci. USA 105, 10756–10761, 2008) PH

Brain creatine kinase and bone loss

Osteoclasts are important in bone remodeling and associated disorders such as osteoporosis. To discover new drug targets to treat bone loss, Kim and colleagues identify proteins important for osteoclast maturation and function by comparing the proteomes of precursor cells and differentiated osteoclasts. The comparison reveals that brain isoform of creatine kinase (Ckb) is highly upregulated upon osteoclast maturation, and reduction of Ckb levels by RNA interference or inhibition of creatine kinase activity by cyclocreatine impairs osteoclast-dependent bone resorption. Furthermore, Ckb appears to be important in osteoclastogenesis by regulating Rho GTPase activity and proton pumping by V-ATPase. The authors demonstrate that ovariectomy-induced bone loss is attenuated in Ckb−/− mice compared with that in wild-type mice. Cyclocreatine also reduces bone loss in ovariectomized rats. Furthermore, bone loss upon lipopolysaccharide challenge or collagen-induced arthritis is attenuated upon cyclocreatine treatment. In summary, these experiments show that Ckb is important in osteoclast-dependent bone resorption, but more work is needed to determine whether Ckb is a good target for treating bone-remodeling disorders. (Nat. Med. 14, 966–972, 2008) JWT

Controlling HIV with siRNA

Genetic therapies against HIV that appear promising in vitro often fail when tested in animals or in clinical trials. A recent report in Cell addresses one hurdle in some in vivo studies: effective delivery of the therapeutic agent to T cells, one of the main cell types infected by HIV. For targeting, Kumar et al. use a single-chain antibody (scFv) to CD7; the suitability of this antigen and antibody for delivery to human T cells was demonstrated previously in research on lymphoma and leukemia therapies. They conjugate the scFv to a peptide of nine arginine residues to enable binding to small interfering RNA (siRNA). As in earlier studies, the authors chose siRNAs directed against the HIV co-receptor CCR5 and conserved sequences in the viral Vif and Tat genes. The efficacy of the scFv-siRNAs for controlling HIV was tested in non-obese diabetic (NOD)/severe combined immunodeficiency (SCID)/IL2rγ −/− mice, which support long-term reconstitution with human immune cells. Treated mice had higher CD4+ T-cell counts and lower viral loads compared with control mice in several models of infection and therapy, including mice reconstituted with peripheral blood leukocytes from an HIV-positive donor or with human hematopoietic stem cells. (Cell 134, 577–586, 2008) KA

It takes two to be flexible

Nucleic acid aptamers are attractive alternatives to antibodies and in some cases are beginning to approach the latter in terms of affinity and functionality. In recent years, Sullenger and colleagues have produced bivalent aptamers that bind two targets in a similar manner to bivalent antibodies. Now they report a scaffold with a flexible linker between aptamer domains that may have advantages over previous formats. Using an aptamer that binds OX40 (a member of the tumor necrosis factor receptor family), they create a DNA scaffold that base pairs with the 3´ end of the aptamer and attach two such oligonucleotides together with a polyethelene spacer, the dimensions of which are tailored to fit between the ligand binding sites on OX40. This dimeric aptamer shows functionality both in vitro and in vivo: primed T cells, isolated from mice that had been injected with a bacterial antigen, are activated by re-exposure to the antigen in the presence of the dimeric aptamer, but not by a mutant aptamer. The dimeric aptamer also potentates the activity of an immunotherapeutic in melanoma-bearing mice, which had been vaccinated with dendritic cells pulsed with a melanoma antigen along with dimeric aptamers; mice receiving the aptamer remained tumor-free longer than control mice. The unparalleled flexibility of this platform should make bivalent aptamers applicable to a broader range of targets. (Chem. Biol. 15, 675–682, 2008) LD

Disease-specific iPS cells

Stable cell lines from individuals with genetic disease would open new avenues of medical research, including investigation of pathologic mechanisms and drug screening to develop therapies. Most cultured primary cells have a limited lifespan, and cell lines representing many diseases are lacking. Park et al. generate induced pluripotent stem (iPS) cells from the somatic cells of patients with ten different genetic diseases that encompass both single-gene disorders and conditions with a complex or unknown genetic basis. The diseases include adenosine deaminase deficiency–related severe combined immune deficiency, Down's syndrome, muscular dystrophy, Parkinson's disease, Huntington's disease and type-1 diabetes mellitus. The iPS cell lines provide a ready source of stem cells that bear the genetic mutations specific to the disease (and to the patient) and that could be differentiated in vitro into cell types relevant to the disease. In related work, Dimos et al. produced iPS cells from two elderly individuals with a familial form of amyotrophic lateral sclerosis and differentiated the cells toward neurons and glia. (Cell, published online 6 August 2008 (doi:10.1016/j.cell.2008.07.041); Science, published online 31 July 2008 (doi:10.1126/science.1158799)) KA

Written by Kathy Aschheim, Laura DeFrancesco, Peter Hare and Jan-Willem Theunissen