By Saundra Amrhein
As a graduate student David Birk, PhD, developed an interest in connective tissues and the cells that produced these molecular building blocks. He became fascinated by the question of how the self-assembly of collagen fibrils and fibers outside of cells was regulated as well as by how these extracellular matrix components (ECM) influenced the cells residing in them.
Dr. David Birk’s ongoing work provides the biological foundation needed for understanding connective tissue diseases and potential therapeutic interventions.
Thirty-five years later, Dr. David Birk’s decades of research and ongoing work with collagens and a family of proteins known as small leucine rich proteoglycans have led to a groundbreaking understanding of the extracellular matrix or framework involved in the development of connective tissue. His work on the regulation of ECM assembly has major implications for current and future treatments of connective tissue disorders ranging from poor wound healing and scarring, to progressive fibrotic diseases, to corneal blindness to congenital diseases such as Ehlers-Danlos syndrome (EDS).
“There’s probably not a system in the body not affected by connective tissue,” said Dr. Birk, a professor of molecular pharmacology and physiology at the University of South Florida.
Dr. Birk’s work influences not only the understanding of, and treatments for connective tissue disorders, but also future treatments for common tendon and ligament injuries, such as the tearing of the anterior cruciate ligament, or the ACL.
All told, more than 200 types of connective tissue disorders impact millions of people worldwide. The connective tissue – which is the glue or cement holding the body together, from ligaments to tendons, to skin, corneal stroma, cartilage and bone – can be altered in numerous ways. This results from injuries and scarring. And many stem from genetic mutations inherited from parents. This includes osteogenesis imperfecta (where bones break easily); Marfan syndrome (often affecting the heart, blood vessels, lungs); and Ehlers-Danlos syndrome, or EDS (marked by loose joints, stretchy skin, small blood vessels and abnormal wound healing).
Using electron microscopy, Dr. Birk has mapped out the process of collagen fibril assembly and developed a model for regulation of matrix assembly. He worked with human geneticists to create mouse models for diseases of human connective tissues.
Last year, Dr. Birk – who came to USF in 2007 from Thomas Jefferson University in Philadelphia – was the principal investigator on a study that examined the roles of tendon-proper and peritenon-derived progenitor cells of the tendon in order to understand their unique capabilities in response to an injury. Using an in vitro model, his team found that the tendon-proper progenitors showed greater levels of tendon gene markers and matrix assembly genes for rebuilding or generating new functional tendon tissue in comparison to the peritenon-derived progenitor cells. However, the latter also showed great potential, as well, when stimulated with growth factors. His team’s findings highlighted the “synergistic” potential of both types of cell populations that could be plugged into engineering models for restoring damaged tendons.
“My philosophy has always been if you understand the process of development, this can serve as a paradigm for regeneration,” Dr. Birk said. (To read the research, see: Stem Cell Research & Therapy 2014; 5(4): 86.)
Showing just that was another recent study in which Dr. Birk was involved and that may one day impact millions of people with corneal scarring and vision loss. Building upon Dr. Birk’s expertise in analysis of connective tissue structure and function, he worked with Dr. James Funderburgh and his group at the University of Pittsburgh who used human ocular stem cells derived from the corneal limbus in scarred and injured mouse corneas. Not only did the stem cells reestablish normal connective tissue architecture necessary for transparency at the scarred site, but healing in nearby tissue was promoted as well. This, according to the Pittsburgh group, suggested that the cells were not just replacing lost tissue, but were promoting regeneration.
The implications could one day be enormous for hundreds of millions of people suffering from and blinded by corneal infections, burns and other types of trauma and scarring to the cornea. It could possibly lead to the growth and use of stem cells from patients’ uninjured eye to heal the other – reducing the need for corneal transplants, which carry high risks of rejections. (See the full research in Science Translational Medicine, 10 Dec. 2014, Vol. 6, Issue 266, p. 266ra172.)
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“Repairing anything with stem cells is a hot topic,” Dr. Birk said. “But I think what we have to recognize with stem cells, you can get them to differentiate along different paths, but if you want them to regenerate tissue structure and function, we need to define the context.”
That context, or the study and knowledge of how tissues develop in the first place, has been at the heart of Dr. Birk’s career.
Among his accomplishments, he is considered a “pioneer” in understanding the role of the ECM in tissue development and other biological processes, said Louis J. Soslowsky, who is the Fairhill Professor of orthopaedic surgery; professor of bioengineering; founding director for the Penn Center for Musculoskeletal Disorders; and director of the McKay Orthopaedic Research Laboratory at the University of Pennsylvania.
Specifically, the model systems that Dr. Birk developed have provided “paradigm-shifting” research for the field, Dr. Soslowsky said.
“For example, his work crossing multiple tissues including tendon, ligament and cornea has shown that even small constituents – small by percentage of material – can play dramatically important regulatory roles in tissue development, function, aging and response to injury,” Dr. Soslowsky said. “His work is considered true ‘foundation work’ upon which he and others continue to build.”
That “regulatory role” of a protein-like collagen was one of his major discoveries. When Dr. Birk started his research back in the 1970s and 1980s, it was understood that collagen helped assemble or form the structure of connective tissue, the glue or cement holding the body together. What was not known – and what Dr. Birk intuited from his research – was that collagen was instructional as well as structural. In other words, collagen was not just the cement holding the building of our bodies together. It also played the roles of architect and construction foreman.
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“What’s been the biggest change in the past 30 years is that we’re not talking just about structural components, we’re talking about dynamic tissues,” Dr. Birk said.
Using electron microscopy, Dr. Birk proceeded to map out the process of collagen fibril assembly and developed a model for regulation of matrix assembly. He worked with human geneticists like Dr. Richard Wenstrup to create mouse models of diseases of human connective tissues.
In doing so, Dr. Birk found the essential instructional role of various collagens and the diseases that result not only in their absence or mutation but also from changes in collagens relative to other collagens in temporal or developmental stages of an embryo.
For instance, Dr. Birk and his team found that the absence of collagen V in development is lethal and reduced expression or mutation of collagen V results in the classic Ehlers-Danlos syndrome. Certain combinations revealed the variety of ways disorders can express themselves. Dr. Birk and his fellow researchers found that the absence of collagen XI with a reduction in collagen V were associated with the most severe fibril phenotype. Dr. Birk’s more recent studies examined how a mutation in the collagen XII gene creates skeletal abnormalities. He has also studied the role of key ECM regulatory molecules like decorin in the matrix assembly of tissue, from tendons to the cornea.
The animal models created by Dr. Birk and his researchers have become crucial tools used by other specialists to develop therapeutic interventions for patients with EDS and joint laxity. His discoveries have informed everything from genetic counseling to the diagnosis of mysterious multiple fractures in children seen in emergency departments.
Essential to this work and the advancements made in the field was Dr. Birk’s talent in building research collaborations across disciplines.
“In collaboration with myself and others, David has used that mouse model to really understand how human extracellular connective tissue develops,” said Dr. Wenstrup, who is currently the chief medical officer at Myriad Genetic Laboratories in Salt Lake City and who has collaborated with Dr. Birk from the 1990s to the present.
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“Some collaborations work well and some don’t. It has a lot to do with the personality and the generosity of the individual collaborators, and working with David is such that he makes it desirable for different types of people with different types of skills to work together,” Dr. Wenstrup said. “He is an excellent research partner.”
For his part, Dr. Birk feels confident that though there is no known cure for many of the connective tissue disorders he studies, great strides have been made in education, awareness and treatment based on the foundational work carried out by his team and other researchers through the years.
“I think we are making progress,” Dr. Birk said. “My interest is in providing a biological foundation necessary for understanding connective tissue diseases and potential therapeutic interventions, and that hopefully will provide a springboard for others with more clinical interests to pursue.”
It’s a matter, he said, of continuing the research and integrating it with research conducted in clinical settings and in other disciplines.
“Looking back 30 plus years, we have made significant progress, the field has developed and the questions have changed,” he said. “We don’t understand the whole process, but a solid foundation is in place.”
Dr. Birk and his research team collaborate nationally with other scientists in various disciplines — a key to advancement in the field.
Photos by Eric Younghans, USF Health Communications and Marketing
