Sitting at a computer connected to a large microscope, Salman Khetani calls up a kaleidoscopic image: green islands of human liver cells in a hexagonal pattern, surrounded by a red sea of support cells. Sangeeta Bhatia, Khetani’s advisor, says that the cells have been carefully patterned to hit the liver “sweet spot.”Arranged just so – in 37 colonies about 1,200 micrometers from each other – the cells behave as though they were in the human body.
When grown in the lab using existing methods, liver cells can survive for a day or two, but over the course of a week, they lose the ability to perform their liver-specific functions and then die. Bhatia and Khetani’s cells, on the other hand, function for about a month. They secrete the blood protein albumin, synthesize urea, and make the enzymes necessary to break down drugs and toxins. Bhatia believes that the cells act enough like human tissue that they could be used to screen new drugs for liver toxicity or to study metabolism and, possibly, hepatitis C, a virus that grows only in human tissue. Indeed, the researchers have already developed a drug toxicity test that uses liver cells arranged in their signature hexagonal pattern.
[For images of this lab, its researchers, and their processes for growing liver tissues, click here.]
In addition to being a major health problem, liver toxicity is the primary reason pharmaceutical companies recall existing drugs or abandon new ones that are under development. Bhatia says that’s because “when you’re developing new [drugs], there aren’t really good models of human liver.”Instead, drug companies rely on cancer cells, dying liver cells, or rat tissue – poor substitutes for fully functioning human liver tissue. Bhatia and Khetani believe they can supply a better model.
Bhatia, an associate professor in the Department of Health Sciences and Technology and the Department of Electrical Engineering and Computer Science at MIT, developed her patterning technique using rat cells, when she was in graduate school in the mid-1990s. At the time, she was interested in using micropatterning, an emerging technique for physically arranging cells in culture, to build a dialysis-like device to support patients with liver disease. For her PhD, Bhatia worked on using the technique to bolster cell function and was particularly interested in finicky cells like liver cells (also called hepatocytes).
Inspired by the work of others in her lab who were growing multiple cell types in the same cultures and combining fibroblasts – supportive cells that normally live in connective tissue – with skin cells, she tried micropatterning fibroblasts alongside her hepatocytes. Micropatterning more than one cell type at a time and regulating the interaction that hepatocytes had with each other, and with the secondary cells, was an innovation. The fibroblasts Bhatia borrowed for her experiments turned out to be particularly good at bolstering liver functions. She describes her breakthrough as “a happy, lucky thing that I just stumbled upon.”
Even though there are no fibroblasts in the human liver, their presence in Bhatia’s cultures coddles the hepatocytes and keeps them functioning. Part of the reason that cells behave like liver, lung, or muscle cells is their environment: signals from neighboring cells, physical forces, and the matrix of supportive proteins stabilizing them. As successful as the method has proven to be, Bhatia is still investigating what exactly causes each patterned hepatocyte island to behave like liver tissue.
For his PhD, Khetani, now a postdoc at MIT, was able to apply Bhatia’s technique to human hepatocytes, making possible the development of the toxicity test. Bhatia says Khetani’s work “was a logical extension of mine, but again surprisingly, human hepatocytes turned out to be even more sensitive to clustering than rat hepatocytes.”