Ying-jian Zhud and Mu-jun Luan from the Shanghai Jiao Tong University in Shanghai, China teamed up to examine a new way to regenerate the bladder.
Several different synthetic and natural biomaterials have been pretty widely used in tissue regeneration experiments, particularly in the regeneration of the urinary bladder. The vast majority of this work has been done in rat model systems, which are fairly good animals to model bladder pathology and regeneration.
To date, the attempted reconstructive procedures don’t seem to work all that well, and this is due to the lack of appropriate scaffolding upon which cells can attach, grow and spread to form the new bladder tissue. Any scaffolding material for the bladder has to provide a waterproof barrier and it has to be able to support several different cell types. While this might not sound difficult on paper, it is in fact rather difficult. Some biomaterials might be well tolerated by the body, but cannot be fashioned into the shape of the organ. Others might support the growth of cells quite well, but are not tolerated by the body.
Zhud and Luan addressed these issues by turning to two different compounds that would compose a two-layered structure. Such a two-layered structure would support the cell types of the bladder. The outside layer was composed of silk fibroin, which is very moldable and usually well tolerated by cells. The inner layer consisted of a natural, acellular matrix (or BAMG for bladder acellular matrix graft). They used this two-layered structure to regenerate an injured bladder in rats.
First of all, it was clear that this material was relatively easy to make and it also could be nicely molded and sewn into the existing bladder. Tissue stains showed something even more interesting: the bilayer scaffold promoted the growth and recruitment of smooth muscles, blood vessels, and even nerves in a time-dependent manner. So by 12 weeks after implantation, bladders reconstructed with the bilayered matrix displayed superior structural and functional properties without significant local tissue responses or systemic toxicity.
Thus, the silk/BAMG scaffold could potentially be a promising scaffold for bladder regeneration. It shows good tissue compatibility, and allows the growth of cells on it. More work is required to take this to the next step, and the scaffold will undoubtedly undergo some changes. But this work represents a terrific start to what might be a superior scaffold for bladder regeneration.
Patients who suffer from malformation of the spinal cord or have suffered a severe spinal cord injury sometimes have bladder malfunction as well. Replacing a poorly functioning bladder is a goal of regenerative medicine, but it is not an easy goal. The bladder is lined with a special cell population called “urothelium.” Urothelium is found throughout the urinary tract and it is highly elastic. Persuading stem cells to form a proper urothelium has proved difficult.
Now scientists from the University of California, Davis (my alma mater), have succeeded in devising a protocol for differentiating human pluripotent stem cells into urothelium. The laboratory of Eric Kurzock, chief of the division of pediatric urologic surgery at UC Davis Children’s Hospital, published this work in the journal Stem Cells Translational Medicine. This work is quite exciting, since it provides a way to potentially replace bladder tissue for patients whose bladders are too small or do not function properly.
Kurzock explained: “Our goal is to use human stem cells to regenerate tissue in the lab that can be transplanted into patients to augment or replace their malfunctioning bladders,”
In order to make bladder cells in the laboratory, Kurzrock and his coworkers used two different types of human pluripotent stem cells. First, they used two types of induced pluripotent stem cells (iPS cells). The first came from laboratory cultures of human skin cells that were genetically engineered and cultured to form iPS cultures. The second iPS line was derived from umbilical cord blood cells that had been genetically reprogrammed into an embryonic stem cell-like state.
Even though further work is needed to establish that bladder tissues made from such stem cells are safe or effective for human patients, Kurzrock thinks that iPS cell–derived bladder grafts made from a from a patient’s own skin or umbilical cord blood cells represent the ideal tissue source for regenerative bladder treatments. This type of tissue would be optimal, he said, because it lowers the risk of immunological rejection that typifies most transplants.
One of the truly milestone developments in this research is the protocol Kurzrock and his colleagues developed to direct pluripotent stem cells to differentiate into bladder cells. This protocol was efficient and, most importantly, allowed the stem cells to proliferate in culture over a long period of time. This is crucial in order to have enough material for therapeutic purposes.
“What’s exciting about this discovery is that it also opens up an array of opportunities using pluripotent cells,” said Jan Nolta, professor and director of the UC Davis Stem Cell program and a co-author on the new study. “When we can reliably direct and differentiate pluripotent stem cells, we have more options to develop new and effective regenerative medicine therapies. The protocols we used to create bladder tissue also provide insight into other types of tissue regeneration.”
To hone their urothelium-differentiation protocol, Kurzrock and his colleagues used human embryonic stem cells obtained from the National Institutes of Health’s human stem cell repository. These cells were successfully differentiated into bladder cells. Afterwards, the Kurzrock group used the same protocol to coax iPS cells made from skin and umbilical cord blood into urothelium. Not only did these cells look like urothelium, but they also expressed the protein “uroplakin,” which is unique to the bladder and helps make it impermeable to toxins in urine.
In order to bring this protocol to the clinic, the cells must proliferate, differentiate and express bladder-specific proteins without depending on any animal or human products. They must do all these things independent of signals from other human cells, said Kurzrock. Therefore, for future research, Kurzrock and his colleagues plan to modify their laboratory cultures so that they will not require any animal and human products, which will allow use of the cells in patients.
Kurzrock’s primary goal as a physician is with children who suffer from spina bifida and other pediatric congenital disorders. Currently, when he surgically reconstructs a child’s defective bladder, he must use a segment of their own intestine. Because the function of intestine, which absorbs food, is almost the opposite of bladder, bladder reconstruction with intestinal tissue may lead to serious complications, including urinary stone formation, electrolyte abnormalities and cancer. According to Kurzrock, developing a stem cell alternative not only will be less invasive, but should prove to be more effective, too, he said.
Another patient group who might benefit from this research is bladder cancer patients. More than 70,000 Americans each year are diagnosed with bladder cancer, according to the National Cancer Institute. “Our study may provide important data for basic research in determining the deviations from normal biological processes that trigger malignancies in developing bladder cells,” said Nolta. More than 90 percent of patients who need replacement bladder tissue are adults with bladder cancer. Kurzrock said “cells from these patients’ bladders cannot be used to generate tissue grafts because the implanted tissue could carry a high risk of becoming cancerous. On the other hand, using bladder cells derived from patients’ skin may alleviate that risk. Our next experiments will seek to prove that these cells are safer.”