Scoping Out Colon Cancer's Varied Terrain
Colorectal cancer remains a leading cause of cancer deaths in the developed world despite the availability of a well-established screening procedure—colonoscopy—that can detect the disease at an early stage. Although patient noncompliance with screening recommendations is a major contributing factor to the disease's continued prevalence, questions have also been raised about whether conventional colonoscopy can reliably detect the full range of premalignant lesions that eventually progress to cancer. These lesions include not only adenomatous polyps but also morphologically subtle “flat” lesions. Soetikno et al. recently reported that flat lesions were present in nearly 10% of 1800 patients screened at a veterans' hospital and that these lesions were five times as likely as polyps to show malignant features.
These findings underscore the importance of ongoing efforts to improve the sensitivity of colonoscopy (the inner surface of a healthy colon is shown above). A progress report on one such strategy is provided by Hsiung et al., who used bacteriophage phage display libraries to identify a small peptide that binds preferentially to human premalignant colonic tissue. When a fluorescein-conjugated version of the peptide was sprayed onto the colon's inner surface and imaged by confocal microendoscopy during routine colonoscopy, it was found to bind to premalignant versus normal cells with 81% sensitivity and 82% selectivity. Although not directly tested in this study, such targeted peptides could in principle be optimized to detect the subtler lesions that are missed by current technologies. — PAK
J. Am. Med. Assoc. 299, 1027 (2008); Nat. Med. 14, 10.1038/nm1692 (2008).
A rotaxane is a molecular mimic of a wheel and axle—a large ring-shaped molecule that can translate and rotate along a second linear molecule, until its path is blocked bulky end groups. Frey et al. examined the application of rotaxanes bearing two macrocyclic groups as receptors in a functional arrangement resembling bookends. Each macrocycle was tethered through a long fused aromatic arm to a Zn porphyrin group, which could be used to bind pyridine molecules. The authors explored the binding of two different guest substrates—one comprising two pyridine rings bonded back-to-back at the 4 position and the other connecting two pyridines through a flexible 10-carbon-atom alkyl bridge—and found that both had nearly the same association constant for insertion into the gap between the Zn centers. Even when the macrocycles were anchored on the axle through copper coordination, the tethered Zn complexes retained sufficient flexibility to bind the guests with similar association constants. — PDS
J. Am. Chem. Soc. 130, 10.1021/ja7110493 (2008).
Monoclonal antibodies that block the binding of HIV to cellular receptors have been shown to neutralize the virus in vitro, to protect monkeys from HIV challenge, and to prevent viral transmission through mucosal tissue. But such antibodies can be produced only at high cost and low capacity through expression in mammalian cells and are therefore not optimal for commercial manufacture. Two studies describe the purification of the anti-HIV antibody 2G12 from genetically engineered maize on a large and cost-effective scale. Rademacher et al. used a fluorescent marker protein to identify and breed transgenic plants that accumulated a high amount of 2G12 in the seed endosperm, the plant's specialized storage tissue. Ramessar et al. purified the antibody from 2G12-expressing maize without using protein A-affinity chromatography, a step typically used for antibody isolation, but toxic if protein A leaches into the final product. Despite differences between mammalian and plant-specific processing of protein-linked carbohydrate, both studies found that glycan modification of maize-produced 2G12 antibodies did not alter antibody binding to the gp120 subunit of the envelope protein of HIV. The HIV-neutralizing properties of mammalian cell- and maize-produced 2G12 were comparable, with the latter being somewhat more potent. Maize-produced 2G12 could be an effective prophylactic mucosal microbicide, and large-scale plant cultivation and prolonged seed storage in the absence of cold temperatures make this method of antibody production economically attractive. — LC
Plant Biotechnol. J. 6, 189 (2008); Proc. Natl. Acad. Sci. U.S.A. 105, 3727 (2008).
A Fade-Away Injection
Although a number of drugs can be delivered via a transdermal patch, this pain-free method generally does not match the efficacy of a needle injection. In a hybrid approach, Lee et al. fabricated microneedle arrays out of two biocompatible polysaccharides, carboxymethylcellulose (CMC) and amylopectin. A key to making robust needles was pre-concentration of the aqueous polysaccharide solutions before casting the needles in a polymer mold. The strength of the microneedles depended on shape, because cylindrical CMC needles were not strong enough to penetrate skin, although pyramidal ones were. Drugs could be loaded into the needles themselves, into a backing layer, or both, depending on whether dosing required a bolus shot or a more prolonged delivery. On contact with skin, the microneedles dissolved, releasing the drug and creating pathways for transport from the backing layer. Using lysozyme as a model protein, the authors showed that it was possible to store the arrays for up to 2 months with almost no loss of enzymatic activity. — MSL
Biomaterials 29, 2113 (2008).
Inversion at Zinc
One of the first things taught in organic chemistry is that a carbon atom can form bonds to four other atoms. Furthermore, these bonds point toward the vertices of a tetrahedron, with the carbon atom located at its center. The consequent variety of carbon-based molecules is, of course, the basis for biochemistry and, in particular, the biological families of macromolecules—lipids, nucleic acids, carbohydrates, and proteins. Zinc can contribute either to the catalytic power or the structural integrity of proteins and usually binds in tetrahedral fashion to the sulfur or nitrogen/oxygen atoms of its four ligands, such as cysteine or histidine.
Koutmos et al. find that the zinc atoms in the cobalamin-dependent and -independent methionine synthases MetH and MetE undergo an inversion in their geometry as these enzymes mediate the transfer of a methyl group onto homocysteine (which reaction yields methionine). Like a tetrahedral carbon in a nucleophilic substitution reaction, the zinc atom releases one of its ligands (glutamate or aspara-gine, respectively) as it reaches to make contact with the sulfur in homocysteine; this motion resembles an umbrella turning inside out on a windy day. The unanticipated flexibility of an active-site metal fits nicely with recent thinking about the importance of intrinsic protein motions for enzyme catalysis. — GJC
Proc. Natl. Acad. Sci. U.S.A. 105, 3286 (2008).
- CELL BIOLOGY
Activity at the Pore
Throughout the nucleus, chromosomal regions localize to particular subcompartments, such as the nuclear periphery, that correspond with gene activation or repression. For example, genes positioned near the lamin-rich regions of the nuclear envelope in Drosophila generally correlate with transcription repression. Relatively less is known about spatial regulation in mammalian cells. Brown et al. have used biochemical mapping and visualization techniques in human cells to elucidate genomic organization at the nuclear pore complex. Upon treatment with a histone deacetylase inhibitor, extensive nuclear reorganization resulted, with an increased association of the nuclear pore protein nucleoporin 93 with genomic features and transcription factors characteristic of transcriptionally active genes. Hence, the nuclear pore plays an important part in the regulation of genes in the human genome. — BAP
Genes Dev. 22, 627 (2008).
Copper in the Crust
The concentration of most metals is extremely low in Earth's crust. For copper, the most important ores globally are the porphyry deposits, which form when copper and other metals are concentrated in fluids emanating from and flowing through shallow bodies of magma in the crust. Rapid cooling of these fluids, or a sudden change in their chemistry as they interact with rocks or other fluids, leads to precipitation of copious amounts of metals and metal sulfides. Their formation has been episodic throughout Earth's history, and many ore deposits, as they form at depths, have been eroded. Kesler and Wilkinson account for the formation, uplift, and erosion of porphyry copper deposits to provide a global accounting of these deposits through Earth's history and assess the remaining available resource. They estimate that about 0.25% of the copper in Earth's crust has actually been concentrated in ores, and that about two-thirds of the more than 100,000 ore deposits that have formed over Earth's history have been eroded and recycled. At current consumption rates, they estimate that there is about a 5000-year supply of copper remaining in the Earth. — BH
Geology 36, 255 (2008).