Viewing receptor clusters in a heart beat

In an article in the September issue of Nature Chemical Biology, researchers report the use of a high-resolution imaging technique to visualize heart cell receptor clusters for the first time. The receptors that control heart rate are known to cluster into distinct signalling zones. The density of receptors in these signalling zones defines the basis of the stimulatory signal.

The heart rate is controlled by beta-adrenergic receptors located on the surface of heart cells. Adrenergic receptors belong to a class of receptor called G protein-coupled receptors, which are common in many cell types and participate in many important biological functions. Although much is known about the structures and properties of this class of receptor, little is known about the receptors' actual cellular location and distribution during cell-signalling events.

Now, Pezacki and coworkers have used a state-of-the-art microscopy technique called near-field scanning optical microscopy (NSOM) to visualize cell-surface beta-adrenergic receptors of heart cells. Because NSOM has higher resolution than conventional light microscopy, it is possible to visualize individual molecules located on the cell surface. The authors showed that between 15% and 20% of the receptors located on the cell surface were clustered into distinct groups or signalling islands. Using a combination of NSOM and fluorescence microscopy, the authors were able to estimate the density of receptors clustered in the signalling zones. Receptor stimulation produced no change in receptor density, which suggests that the receptors are prearranged into signalling islands.

This work clearly demonstrates the power of NSOM as an imaging technique. Its application to other signalling receptors could provide valuable insights into the signalling events that occur in both healthy and diseased cells.

Seeing cathepsin

Cathepsins are cysteine proteases involved in a number of stages of tumor development. In the September issue of Nature Chemical Biology, cathepsin activity is visualized in a cellular model of a tumor microenvironment. As a cellular safeguard, proteases are often generated in an inactive form and then processed to the active form only when the protein cutting activity is needed. Although there are many ways for detecting proteases, small molecules called "activity-based probes" that only react with active enzymes offer an important method for monitoring active proteases. Until now, using these probes for in vivo imaging has been difficult.

Matthew Bogyo and colleagues report a new method of fluorescence labeling of active proteases using activity-based probes. The authors chemically synthesized a molecule in which the fluorescence generated by one part of the molecule is "quenched"—or turned off—by another part of the molecule. When the probe reacts with an active protease, the quenching part of the molecule breaks off and a fluorescent signal can be seen. These probes were effective in imaging cathepsin in living cells.

The probes may prove useful for monitoring cathepsin activity in live mice, and the design of similar quenched probes targeting other proteases will provide important tools for investigating proteolytic function in vivo.