Science Concentrates NEUROSCIENCE
Counting synapses in people When it comes to processing information in the human brain, much of the work happens at the organ’s 100 trillion or so synapses, the gaps between neurons through which the cells communicate. Researchers now report a way to measure the density of synapses in people using positron emission tomography, or PET (Sci. Transl. Med. 2016, DOI: 10.1126/scitranslmed.aaf6667). The method could help elucidate the mechanisms behind diseases in which people lose synapses, such as epilepsy, Alzheimer’s disease, and possibly depression, the scientists say. Measuring synaptic density in living people had not been possible previously, say Sjoerd J. Finnema and Richard E. Carson of Yale University, who led the team. Scientists have instead been making such measurements in tissue collected during brain surgery or from deceased patients.
F
F
F
N N
11CH
O
3
[11C] UCB-J A PET scan reveals synaptic density via a radiolabeled molecule (left) that binds a protein found at synapses. Density increases from black to red to white. Finnema, Carson, and colleagues designed a radiolabeled molecule that can be detected via PET and binds to synaptic vesicle glycoprotein 2A (SV2A), a protein found at synapses. The team confirmed that the PET tracer, dubbed [11C]UCB-J, targets SV2A in people’s brains, in part, by blocking its accumulation using an epilepsy drug known to bind the synaptic protein. And in three people with epilepsy, PET scans with the tracer revealed reduced synaptic density at the sites of known brain lesions in the patients. Kathryn A. Davis, a neurologist who studies epilepsy at the University of Pennsylvania, wonders if the technique could help locate brain lesions in epilepsy patients for whom standard imaging techniques have failed to identify such sites. And John Q. Trojanowski, also at UPenn, is excited by the method’s prospects for studying the loss of synapses in the progression of Alzheimer’s and other neurodegenerative diseases. “The communities studying these diseases,” he says, “have been awaiting a ligand like this for many years.”—MICHAEL TORRICE
8
C&EN | CEN.ACS.ORG | JULY 25, 2016
Removing the charges from a cellulose-binding protein left its structure and function unaffected. Hydrophilic residues are in blue; hydrophobic ones are in gray.
BIOCHEMISTRY
Protein stays stable without its charges A protein with its charged amino acids swapped for neutral ones remains soluble and functional Charge is a fundamental factor that helps dictate a protein’s structure and activity. Five out of the 20 amino acids commonly found in proteins are either positively or negatively charged under physiological conditions, and all known soluble proteins have at least a few of these residues. Now, in a surprising twist, researchers have mutated a protein to remove its charged amino acids and found that the protein retains its structure, solubility, and activity. The findings may one day help scientists build better algorithms for designing proteins with new functions (Biochemistry 2016, DOI: 10.1021/acs.biochem.6b00269). To make the uncharged protein, Jakob R. Winther of the University of Copenhagen and colleagues first searched a data bank of protein structures to find the least-charged protein or protein domain containing 100 or more residues. That turned out to be a cellulose-binding domain from the bacterium Cellulomonas fimi. Next, the researchers looked for ways to swap the domain’s four charged residues with neutral amino acids, while producing the most stable protein possible. The winning mutant replaced a lysine, aspartic acid, arginine, and histidine with a methionine, glutamine, methionine, and tryptophan, respectively. After expressing their mutant protein in Escherichia coli, the researchers tested its solubility and stability and assessed the protein’s structure with circular dichroism and nuclear magnetic resonance spectroscopy. On the basis of all these measures, the uncharged protein seemed largely the same as the charged version. Plus, it was actually more stable than the original. Finally, the researchers tested to see whether the protein still bound cellulose. “To our surprise, we got something that is fully functional” from pH 2 to 12, Winther says. Emil Alexov of Clemson University says the study provides important insight on the effect of charge on solubility. In an uncharged protein, “solubility should decrease, but this was not the case,” he says.—ERIKA GEBEL BERG, special to C&EN
CREDIT: SCI. TRANSL. MED. (BRAIN); JAKOB WINTHER (PROTEIN)
New radiotracer could help scientists study loss of synapses involved in disease
