Signaling Mechanisms Detailed for Cell’s Primary Cilia

By sampling the electrical currents flowing through primary cilia, scientists have taken another step toward deciphering the biological function of these tiny, fingerlike projections, which emanate from the surface of nearly all eukaryotic cells. A research team led by Horacio Cantiello, HMS associate professor of medicine at Massachusetts General Hospital, describes the discovery and characterization of electrical profiles that reflect ion channel activity on the surface of primary cilia. The report, which appears in the Oct. 14 Journal of Biological Chemistry, provides details about the molecular mechanisms that underlie ciliary function and offers the first direct electrophysiological evidence of ion flow at the ciliary membrane.

Historically, primary cilia were dismissed as mere vestigial appendages with no apparent purpose, but now they stand at center stage, spotlighted by current studies that suggest prominent roles in embryonic development and human disease. Scientists appreciate that these organelles in particular are fundamental players in polycystic kidney disease, a prevalent human disorder in which fluid-filled cysts are formed in several organs, including the kidney. Normally, bending of primary cilia by physical forces, such as the flow of fluid within the kidney’s tubules, sparks the relay of signals from the extracellular environment into cells. “The question is how that cell signaling event that starts in the cilia takes place,” explained Cantiello. Since membrane-spanning ion channels are important signaling components of other, more specialized sensory cilia, his group set out to explore their potential roles in primary cilia.

To examine the organelles independently, free from the cell membrane, the researchers first developed a method for isolating cilia from kidney cells. Next, they employed a patch-clamping technique—latching directly onto the membrane of the microscopic cilia with a miniature electrode—to detect and record the flow of ions through channel proteins. By bathing the cilia in solutions with different ionic compositions, Cantiello and his colleagues noted corresponding changes in their electrical recordings, which helped them make informed hypotheses about the identities of the channels. Then they confirmed these ideas visually, by labeling cilia with antibodies directed against specific ion channel proteins.

Using this functional approach, the investigators detected a handful of ion channels within the primary cilia of the kidney, including polycystin-2, a calcium-permeable channel, and ENaC, an epithelial sodium channel. Moreover, Cantiello’s group observed that the ciliary membrane has a greater overall density of ion channels than the cell membrane, underscoring its enhanced electrical activity. Their work points to the influx of positively charged ions, like calcium and sodium, as a key event in the signaling capacity of primary cilia. It also suggests that in the kidney, primary cilia may act as the cells’ feelers, sensitive not only to mechanical stimulation but also to osmotic changes.

—Nicole Davis

Genetic Variant Tied to Posttraumatic Stress Disorder

Most people will face a severely traumatic event in their lifetime, whether it is a war, natural disaster, rape, car accident, domestic abuse, or the unexpected death of a loved one. Psychologically, most people can bounce back surprisingly quickly. Only about 20 percent of those who experience the trauma go on to develop posttraumatic stress disorder (PTSD), a defined cluster of disabling symptoms with few proven therapies.

Scientists have only recently begun to study the biology of trauma and resilience in hopes of intervening earlier and developing effective treatments. Studies have shown that PTSD can run in families. Other research has implicated different brain regions
and neurotransmitters.

A new study of how injured children experience serious accidents strongly links dissociation during the event, a well-established risk factor for PTSD, to a variation of a gene involved in the stress response.

“If a bad thing happens, we expect people to cry or be emotional,” said clinical psychologist Karestan Koenen, HSPH assistant professor of society, human development, and health and first author of the paper. “What’s striking with kids who are dissociating is that they seem totally detached. They are so overwhelmed that they may feel numb. They describe the accident as being like watching themselves in a movie and as if the film was slowed down.”

The researchers interviewed 46 children admitted to the Boston University Medical Center for acute medical injuries. More than half had been hurt in motor vehicle accidents. The rest had been shot, stabbed, or involved in falls. Researchers also genotyped the young people’s FKBP5 genes, a glucocorticoid receptor-regulating cochaperone of stress proteins, looking for a variation implicated in a rapid and hyperactive stress hormone response.

Children in the high-risk genotype group had twice as many symptoms of dissociation, or a mean of 8.6 symptoms compared to four symptoms in the others. The genotype explained seven to 14 percent of the difference in dissociation response. “If you have the risk genotype, our study says you are more likely to dissociate during trauma,” Koenen said.

The findings will appear in the December Molecular Psychology and are published in the journal’s advance online edition.

—Carol Cruzan Morton

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