Today we know the function and metabolism of any mammalian cell depends on its environment. Such cells have long been cultured under conditions consisting of a basal medium with added nutritional serum. This, however, is not an accurate replica of the cell’s environment within the human body.
Now, investigators are thinking about how to make such media more similar to the natural environment of the cells they want to grow. A new article in the journal Trends in Cell Biology on November 2019 describes how physiologic media, as they are called, will help to visualize the behavior of cells under natural body conditions when exposed to biological agents or drugs.
Jason Cantor is a metabolism investigator at the Morgridge Institute for Research in Madison and an assistant professor of biochemistry at the University of Wisconsin-Madison. Image Credit: Morgridge Institute for Research
The current study is run by Jason Cantor, a biochemist who helped to develop human plasma-like medium (HPLM) at Cambridge. This new medium was carefully crafted to be just like the plasma of a typical human adult, and is now being used in over 30 laboratories, to model the reaction of cells to various substances when introduced to them in the medium of blood.
The researchers want to examine how various types of blood cancer cells derived from humans grow and function when grown in HPLM as compared to a more conventional type of medium. The physiologic nature of HPLM means that the biology of these cells is more likely to be closely related to the way cells behave in the human body. Not only will this help us understand more about cell function under real conditions, but these insights help reveal how a drug really affects a cell, how its efficacy could be hindered under body conditions, and how it could be improved. Thus it could have a high impact on drug development and formulation.
Cell culture media
Frankly, until now, all scientists wanted of a cell culture is that it should grow a lot of cells in as little time as possible, and that the new cells should be normal and able to reproduce just as well as the parent cells. However, this setting doesn’t always allow them to simulate biological conditions, which means that experiments which focus on how cells behave under various exposures, for instance, aren’t always foolproof.
Much of biological knowledge has come from living cells grown in a flat petri dish, in cell culture media composed of selected chemicals and nutritive ingredients. This includes also mechanical support for cell attachment, as well as physical and chemical parameters like osmolar, pH and temperature conditions. This setup provides for rapid proliferation and growth of viable cells.
However, traditional culture media are made up of a base which is quite unlike anything in the human body, with nutrients added in the form of an undefined serum (usually from bovine fetuses or calves) that does not have any marked resemblance to human blood or extracellular fluid. The serum adds growth factors, hormones and trace elements to the mix, which are essential for proliferation. However, the fact is that we don’t know much about what else the serum could be adding to the culture medium. The result is a medium that is not much like human blood in its metabolite composition.
Avoiding the addition of serum, as an undefined ingredient, is also problematic because it prevents the use of the resulting medium, that must be built up with carefully added individual nutrients, for more than one or a very few cell types. In addition, when these elements, including trace elements, are added to simulate human plasma, there could be a possible overdose of trace metals leading to toxicity or oxidative stress because of the absence of binding or carrier proteins, which are found in serum.
Why not simply use serum? The cost is prohibitive, for one, coupled with the intrinsic differences between sera from different organisms, with the compositional complexity and the resistance to modulation of one or a few factors without having to change the whole thing around, which make this a less than ideal solution.
Physiologic cell culture media
Simulating natural physiology is one of the primary goals of many current technologies including 3D cultures, organs-on-a-chip, microfluidic cell culture systems, and hermetically sealed cultures that mimic the inside of a solid tumor, without access to oxygen.
The fundamental goals of classic culture media and physiologic media are quite different. With the former, the focus is rapid growth and multiplication of viable cells is, whereas physiologic media are designed to promote cell growth as close to the real thing as possible. The researchers insist that each type of physiologic medium should be developed in response to its need, to model a particular type of cell or application.
There are challenges, of course, with physiologic media. It is difficult to remove fats from this medium, for one, without simultaneously removing growth factors and hormones as well. Secondly, direct comparison of the metabolic profiles of cells grown in HPLM and cells extracted from the blood is impossible because it takes about 10 times longer to simply isolate the latter, as compared to the former. However, nowadays newer techniques are being developed to trace nutrient utilization patterns within tumors, and these could be tested using the same tumor cells grown in physiologic media. This comparison could yield interesting results.
Mouse models are often thought to be closer to human physiology than in vitro studies using cell cultures. However, physiologic media have already been shown to affect metabolism in a way that would have been impossible with mouse models owing to the ten-fold difference in concentration of a particular metabolite, uric acid. This should drive research to consider how best to exploit and incorporate cell culture in physiologic media as well as mouse models, to learn about how cells function in the body.
The implications are immense. Physiologic media could be used to generate cultured cells to produce biomolecules of interest, such as viruses or proteins; to develop new cell lines; to move cells in different directions of development; to understand how various metabolites regulate cell growth and function; and for high-throughput genomics, proteomics and metabolomics studies.
In Cantor’s words, “In most cases, people are using media to make something happen. In our case, we just want to study biology or drug efficacy in the conditions we are more likely to see in the human body. Modeling how a cell behaves is a different line of thinking.”
The Rise of Physiologic Media Cantor, Jason R. Trends in Cell Biology, Volume 29, Issue 11, 854 - 861, https://www.cell.com/trends/cell-biology/fulltext/S0962-8924(19)30145-X