Dots dot tumors

Credit: Reprinted from Nature Bioechnology

Microscopists have been beside themselves since the invention of quantum dots, nano-size crystals that can label objects in a diverse palette of colors. Last year the little beads made their debut labeling parts of cells in microscope slides, and now they have moved on to larger targets—tumors. In the August issue of Nature Biotechnology (22, 969–976; 2004), Xiaohu Gao et al. use quantum dots to label human prostate tumor cells injected into mice. The technique, which seems safe, could some day enable cancer biologists to read rainbow-painted tumors in vivo to assess their molecular nature.

In many ways, quantum dots are superior to conventional organic dyes: they are brighter and give sharper signals, and their fluorescence is more stable. Moreover, the color of a quantum dot can easily be changed by changing its size, yet differently colored dots can all be excited by a single wavelength of light—an approach not possible with other labeling systems. The researchers optimized the dot's composition for whole-animal studies then attached an antibody that recognizes prostate-specific cell-surface antigen, PSMA. Cells expressing the antigen blazed orange under a whole-body imaging scheme, as shown here.

Joint healing

Drugs for rheumatoid arthritis such as dexamethasone generally target the inflammation that wears down the joints. In the 12 July online Proceedings of the National Academy of Sciences (doi:10.10.1073/ pnas.0404105101) Sylvie Bernier et al. investigate another approach, testing a compound that seems to inhibit the proliferation of synoviocytes, which form the lining of the joints and trigger the inflammation.

Rapid, uncontrolled proliferation of synovial cells drives joint destruction in rheumatoid arthritis. These cells, in turn, secrete a variety of mediators of inflammation and tissue degradation, as well as proangiogenic factors that contribute to harmful blood vessel formation. The compound, PPI-2458, potently inhibited proliferation of synoviocytes in culture, and alleviated paw swelling after the onset of chronic disease in a mouse model. PPI-2458 inhibits protein processing, and belongs to a class of compounds that inhibit angiogenesis and are in clinical trials for cancer. Somehow, PPI-2458 seems to have high specificity for synoviocytes.

Mast cells feel too

As many people with sensitive skin know, heat and other physical stimuli can cause hives and rashes. That is in part due to activation of mast cells, immune cells that reside in the connective tissue. Now it seems that mast cells might directly sense heat using the same types of receptors that sense physical stimuli in nerve cells. In the 12 July online Journal of Experimental Medicine (doi:10.1084/jem.20032082), Alexander Stokes et al. report that mast cells express transient receptor potential (vanilloid)-2 (TRPV2), a cation channel. The research suggests that TRPV2 could sense heat in these immune cells much as such channels sense stimuli in neurons.

TRPV1 has gained recognition as a neuronal receptor for heat and capsaicin, the 'hot' ingredient in peppers. But previous work has hinted at broader contexts for TRPV family members by demonstrating their expression outside the nervous system. After pinning TRPV2 expression to mast cells, Stokes et al. found that heat seemed to activate the channel. Pulses of heat caused caused transient intracellular calcium pulses and the release of immune mediators from mast cells; both effects were dampened with a broad inhibitor that affects TRPV channels. The channel seems to associate with the same signaling molecules as TRPV channels in neurons, including PKA.

Touching T cells

Credit: Rockefeller University Press

After T cells engage with antigen-presenting cells (APCs), numerous signaling molecules at the synapse kick into action. The work of Alex Chernyavsky et al. suggests that a scaffolding protein found at neural synapses, Discs large (Dlg1), might coordinate signaling events at the junction. In the 19 July Journal of Cell Biology (166, 1; 2004), they find that Dlg1 localizes to the synapse within 5 minutes after antigen recognition, complexing with proteins needed for T cell activation 9Dlg here in green; actin, red; APL, blue). After the assembly of the complex, Dlg1 leaves the synapse about 20 minutes later (bottom panel). The authors provide evidence that Dlg1 prevents overactivity of T cells, a factor in autoimmunity.

Picking out prenatal DNA

Fetal DNA accounts for about 3–6% of the total DNA in maternal plasma. This has led to techniques that test for abnormalities in fetal DNA in the maternal blood, to date limited largely to chromosomal abnormalities and deletions. In the July 20 Proceedings of the National Academy of Sciences (101, 10762; 2004), Chunming Ding et al. take the approach even further, detecting fetal single-nucleotide polymorphisms (SNPs) in the β-globin locus that lead to β-thalassemia.

The technique uses PCR and primer extension to amplify mutant alleles and mass spectrometry to assess the amplification products. Similar techniques were developed several years ago and used to identify fetuses with small deletions inherited from the father. But identifying SNPs in the fetus has been a particular challenge. Even more difficult has been detecting paternal contributions when these are the same alleles potentially contributed by the mother. Ding et al. honed and tweaked their technique to overcome the first hurdle, detecting minute amounts of paternally contributed SNPs. They overcame the second by using SNPs outside the β-globin gene to track which paternal chromosome had been transmitted to the fetus, thus measuring the fetal haplotype. The technique may speed up prenatal diagnosis and minimize the need for invasive techniques such as amniocentesis. It could also be used for other purposes, such as examining DNA shed into the bloodstream from tumors.

Written by Charlotte Schubert