Probing for Answers Improving Sensors for Diabetics by Re-directing Macrophages
It’s easy to measure glucose in a beaker.
But the simple sugar is hard to quantify directly in the human body, which is why long-term glucose monitoring for people with type 1 and type 2 diabetes through implantable devices has proven to be extremely difficult. Leland Clark of Cincinnati Children’s Hospital, considered the “Father of Biosensors,” first suggested the use of electrodes for measurement of glucose in humans with diabetes in 1962. But a half-century later, diabetics still do not have ways to better manage their diabetes other than finger sticks or implanted sensors that have to be replaced every five to seven days.
The challenges scientists have faced in developing reliable implantable glucose sensors are two-fold: the sensors must deal with the foreign-body reaction, where a person’s body attempts to wall off the implanted material from healthy tissue, and just as importantly, blood chemistry and tissue vary from person to person. After a week, these natural phenomena cause erratic glucose readings from sensor implants, said Julie Stenken, professor of chemistry and biochemistry.
“The whole foreign-body reaction drives a series of immune responses that then serve to encapsulate the sensor away from healthy tissue,” Stenken said. “This sensor is measuring glucose in an encapsulated ‘bag’ around the sensor rather than being able to sense what is out in healthy tissue. That becomes an extremely dangerous situation clinically, because if you dose with insulin when you don’t need to, the person can go into diabetic coma or eventually death due to an inaccurate reading from the sensor.”
Stenken is a leading expert in the area of in vivo (in the body) collection of proteins known as cytokines using a technique called microdialysis sampling. Her aim is to understand the inflammatory response caused by cells called macrophages to implanted foreign materials. But first she must ask the question: How can we change the way these cells communicate with each other?
The Host Response
Understanding the underlying biochemistry that occurs at the site of an implanted biomaterial is important in a wide range of clinical contexts, from reconstructive surgery to implantable glucose sensors. Many of the problems that scientists have encountered in the development of implanted sensors into living things have been due to the lack of understanding of the host response to implanted materials, said Stenken, the Twenty-First Century Chair in Proteomics in the J. William Fulbright College of Arts and Sciences.
“In this whole process, the macrophage cells that are ultimately encapsulating this device communicate with each other through protein signals,” Stenken said. “These messenger proteins, called cytokines, are part of a whole series of proteins that are involved in network responses within the immune system. We need to understand this communication process between all the cells so that we can direct the cells to the state where we want them to be.”
This chemical signaling response to an artificial material within a tissue includes large cells known as macrophages. In an immune response, it is the job of the macrophage to destroy a foreign object, typically a virus or bacteria. The term is literally translated from Greek as “big eater.” When the macrophages discover that they cannot destroy an object as large as a glucose sensor, they signal for fibroblast cells to lay down collagen, a group of proteins that form the main structural component of animal connective tissue. This “glue” eventually encapsulates an object, in this case a glucose sensor.
“There’s an enormous interest in trying to redirect or bioengineer this whole process to improve outcomes,” Stenken said. “These macrophages are very plastic. They have different states. We’re trying to direct the macrophages to a certain type of state, the M2. The M1 state is the classically activated macrophage where it is trying to destroy foreign material as quickly as possible. It’s pro-inflammatory. The M2 state is considered to promote a wound-healing state.”
To direct the macrophages, Stenken’s research group uses a microdialysis probe that is placed under the skin of rats in order to mimic an implanted glucose sensor. The researchers, in a process unique to this group, then infuse different agents through the probe that are hypothesized to direct the macrophages into the M2, or healing, state.
“Once they are driven into the healing state, the question is, ‘Do you see a longer lifetime for the implant?'” she said.
Stenken’s work has drawn the attention of researchers across the country, including William M. Reichert, a distinguished professor of biomedical engineering at Duke University who has worked on biosensors.
“Julie has almost single handedly made a finicky microdialysis system work for the monitoring of wound healing,” Reichert said. “I did some of this in the past but more or less gave up because of all of the experimental problems that she somehow overcomes. This can only be the mark of a careful and skilled experimentalist.”
Federally Funded Research
In 2007, Stenken came to the U of A to accept an appointment as the inaugural holder of the Twenty-First Century Chair in Proteomics. The chair was endowed with a $1.5 million gift raised by the university’s Campaign for the Twenty-First Century. She brought with her ongoing microdialysis research that included two grants totaling more than $1 million from the National Institutes of Health.
In addition to the current NIH grant, Stenken received a two-year, $375,000 grant from the national institutes to use the microdialysis probe to study cytokines in the brain. The Exploratory Developmental Research Grant (R21) will help to develop analytical chemistry methods to collect and detect cytokine proteins to allow for rapid translational medical treatments for humans. Cytokines as well as other neuropeptides are known to affect different human diseases related to the brain including, but not limited to, alcoholism, anxiety, appetite, depression, epilepsy, multiple sclerosis, pain, sleep, and various psychiatric disorders. These peptides and proteins are difficult to measure in the living brain.
“Cytokines are now considered the third-generation chemical communication system in the brain behind neurotransmitters and neuropeptides,” Stenken said. “People have been very interested in cytokines in the brain because there are known receptors and they show up in many neurodegenerative diseases.”
NIH R21 grants are meant to encourage high risk/high return research efforts. Stenken is enthusiastic about the possibilities.
“There are many biological areas that I know that these microdialysis probes will prove to be beneficial,” she said. “I’m not afraid to learn new things in biology.”
Stenken and Proteomics
Proteomics is the large-scale study of the structure and function of proteins. Julie Stenken is leading a research team that received a four-year, $1.3 million grant from the National Institutes of Health to study the “foreign body” response to implants.
Stenken is collaborating on the grant with Jeannine Durdik, professor of biological sciences and assistant dean for research in the Fulbright College of Arts and Sciences, and Liping Tang, professor of bioengineering at the University of Texas at Arlington.
In 2009, Stenken co-edited In Vivo Glucose Sensing with David D. Cunningham of Abbott Laboratories. The book, part of the Wiley Chemical Analysis series, is designed to provide state-of-the-art information on glucose monitoring to clinicians and medical educators.