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September 29, 2005


“Underground” tunnels provide communication between immune cells

Immune system cells are connected to each other by an extensive network of tiny tunnels that, like a building’s hidden pneumatic tube system, are used to shoot signals to distant cells. This discovery, reported by two School of Medicine researchers in the September issue of the journal Immunity, may explain how an immune response can be so swift. The research not only proves cells other than neurons are capable of long-distance communication, but it reveals a mechanism cells use for exchanging information.

Blood-derived dendritic cells and macrophages, both antigen-presenting cells, make use of these so-called tunneling nanotubules to relay molecular messages, report Simon C. Watkins and Russell D. Salter. Further research may show there are additional cell types with these microscopic tunnel connections. Thus far, their studies suggest the tunnels do not exist between commonly used fibroblast and tumor cell lines.

If not for a minor mishap while carrying out an experiment, the authors might not have discovered the existence of these physical structures or conducted the studies that revealed their role in intercellular communication.

Using a custom-built, multi-camera live cell microscopic imaging system, they report that, in a matter of seconds, dendritic cells and macrophages can send waves of calcium and other small molecules to cells hundreds of micrometers away. Each nanotubule measures between 35 and 200 nanometers across -— 5,000 times smaller than the width of a human hair — and at any given time, cells may have up to 75 of these extensions, each of varying lengths.

“Considering their scale, these nanotubules are allowing communication between fairly distant cells. If instead of a culture dish we were talking about a large metropolitan area, the distance would be about the equivalent of four or five city blocks. That’s nothing short of amazing,” remarked Salter, associate professor of immunology.

The authors are the first to explain the function of tunneling nanotubules, structures that initially were described in fruit flies in 1998, and subsequently identified in a handful of different types of animal and human cells.

“It’s one thing to find that this intricate physical network exists but quite astonishing to learn that immune system cells are using it to relay molecular signals to one another,” said Watkins, professor and vice chair of cell biology and physiology, and director of the Center for Biologic Imaging in the School of Medicine.

While gap junctions — interconnecting molecular bridges that conjoin tightly packed cells — are known to generate signals and transport other molecules between cells, the researchers say the tunneling nanotubules are something quite different.

“This is clearly a third form of intercellular communication, distinct from gap junctions and synapses used by nerve cells. And, it is possible that tunneling nanotubules are essential for the function of the immune system, just as gap junctions are critical for the function of cardiac muscle,” added Watkins, who also is a professor of immunology.

The authors’ discovery builds on their recent research showing how dendritic cells respond to stimuli, but, as they freely admit, it was due in large part to an accidental observation, that giving just the slightest poke to a single cell can set off a chain reaction whereby cell after cell discharges bursts of calcium.

In their earlier studies, they described how dendritic cells unfurl hidden veils — membranes that are so thin they can barely be imaged — and use these veils to move in on and capture their target. In the presence of E. coli, this occurs so rapidly and with such vigor that in accelerated time-lapse video, the cells appear more like a pack of wild animals feeding on a carcass.

But two things baffled the researchers. Dendritic cells extended their veils even before making physical contact with E. coli, yet macrophages, cells not normally picky about the antigens they engulf, were completely unresponsive to the bacteria. In order to understand how dendritic cells first sense the presence of an antigen and why the reaction is cell-specific, the authors decided to look at calcium flux, a well-recognized early measure of stimulation in numerous cell types. The use of a fluorescent dye, which allows direct measurement of calcium levels, would determine if calcium flux occurs before dendritic cells unfurl their veils.

With a microinjection tip, they squirted a mixture of E. coli fragments into a culture dish and one to two minutes before the appearance of the thin membranes, there were bursts of color indicative of calcium flux. Given their earlier results, the researchers anticipated that by repeating the experiment with macrophages there’d be no response. But the microscopic bacteria sample got clogged inside the tip, and before Watkins realized the need to pull away from the cell, he had already given it a jab.

“On the screen it looked like flash bulbs going off in a dark concert arena,” Salter recalled of that moment, when to their surprise the researchers witnessed how that little mishap had caused the macrophages to release bursts of calcium.

Returning to dendritic cells, they found that giving a deliberate poke with an empty microinjection tip caused the same reaction. But why some cells responded and others did not made Salter and Watkins wonder if there was some sort of physical structure connecting only those cells that discharged. A literature search turned up a handful of papers describing tunneling nanotubules, and further imaging using the highest magnification possible disclosed their presence in both the dendritic cells and macrophages.

In their most definitive experiment, the researchers placed dendritic cells, macrophages and a small amount of the E. coli mixture in the same culture dish. The dendritic cells, as would be expected, fluxed calcium in response to the E. coli. But a few seconds later, calcium also could be seen shooting through the tiny tunnels extending from dendritic cells to neighboring macrophages.

“This may solve some of the mystery of how a local stimulus directed at a very small number of cells can be amplified and result in a successful immune response,” explained Watkins.

“Quite possibly, the tunneling nanotubules enable a small number of dendritic cells with captured antigens to reach other dendritic cells in lymph nodes, increasing the number of these cells capable of stimulating T lymphocytes,” added Salter.

The finding that nanotubules play a role in sending molecular signals to other immune system cells calls into question the long-held belief that immune system cells talk to one another solely by secreting substances such as cytokines, the authors say.

It now seems clear that intercellular communication is much more complicated. While it would be fascinating to see this interplay inside living tissue, detecting the tiny tubules in such a complex environment may be nearly impossible with current technology.

The research was supported by the National Cancer Institute of the National Institutes of Health.


Late-life mood disorders center gets $6.8 million from NIH

One in five older Americans live with significant symptoms of depression; however, many do not seek treatment.

To develop and test new treatment methods for late-life depression, anxiety, grief and insomnia, and to bring these treatments to the people who need them the most, the National Institute of Mental Health (NIMH) has awarded a five-year, $6.8 million grant to the School of Medicine’s Department of Psychiatry to create the Advanced Center for Interventions and Services Research for Late Life Mood Disorders at Western Psychiatric Institute and Clinic (WPIC).

The center will provide an infrastructure to support research and programs with the goal of improving the care of elderly people living with depression and other severe mood disorders. The center’s research will focus on preventing depression and suicide in the elderly, especially those at high risk due to co-existing medical conditions; providing support and assistance to families of those with late-life mood disorders, and removing barriers to effective treatment in the community setting, specifically the African-American community.

“Depression and mood disorders affect the elderly in a much more complex way than they affect younger people, and many elderly have co-existing conditions that make the disorder harder on the patient and harder to treat,” said Charles F. Reynolds III, professor of psychiatry, neurology and neuroscience.

The Advanced Center for Interventions and Services Research builds on years of research conducted at the University to develop new approaches to treating this problem.

Related centers have been funded at similar amounts over the past 10 years, first as the Clinical Research Center in Late Life Mood Disorders (1995-2000) and later as the Intervention and Research Center for Late Life Mood Disorders (2000-2005). Each successive center grant builds on the work of the previous center.

These centers have supported research resulting in 17 grant awards from NIMH focused on treating mood disorders in the elderly, including depression, bipolar disorder and anxiety, and on preventing suicide in the elderly.

Additionally, research supported by the center has yielded findings that have had significant clinical applications:

• Demonstrating effectiveness of combined antidepressant medication and interpersonal psychotherapy in people up to age 75 with recurrent major depression and the effectiveness of antidepressant pharmacotherapy for people over age 75 with single episodes of major depression.

• Demonstrating the effectiveness of depression management strategies performed by non-physician clinicians for reducing suicidal thoughts and symptoms of depression in elderly people attending primary care clinics. In one study, researchers showed that these strategies were useful in real-world settings.

• Discovering that the speed of response to antidepressant medication is dependent on a person’s genetics, which led to the idea that a pharmacogenetic analysis can help physicians and their patients understand how a patient will respond to a medication.

• Establishing that most elderly people with depression also demonstrate some level of cognitive impairment. Treatment for depression can improve cognitive function to some extent, but rarely will it return to normal.

Under the new grant, investigators will continue to build on their 10 years of research, with a more concerted focus on delivering effective treatments to the patients in the community setting. They plan to do this by forming alliances among the caregivers, family and community organizations to offer support to those who live with late life mood disorders.

In addition to Reynolds, primary researchers in the center include Robert Bies, Charlotte Brown, Meryl Butters, Mario Cruz, Mary Amanda Dew, Linda Garand, Ariel Gildengers, Amy Kilbourne, Eric Lenze, Francis Lotrich, Lynn Martie, Sati Mazumdar, Benoit Mulsant, Harold Pincus, Bruce Pollock, Edward Post, Richard Schulz, Katalin Szanto and Ellen Whyte.

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