The Next Step in Tissue Culture
(October 20th, 2016) When 3D cell culture doesn't satisfy your expectations anymore, growing organoids could be a better option. In a new paper, scientists managed to fool stem cells into making lung alveoli look-alikes.
Remember when we used to believe that cultured cells would actually be of some use in drug discovery? Sadly, it didn't take long to realise that what works on single cells floating around aimlessly in a flask often fails to have the desired effect when applied to a fireman, a footballer or a train driver. The problem is that cells are very sensitive to their mechanical environment, altering their physiology depending on what is around them and what they are in contact with. Hence, the current interest in 3D cell culture.
But unless you are modelling a tumour, even 3D cell culture is still a long way from the steric complexity of real tissues. Take the lung, for instance. Research into the very common lung diseases, such as chronic obstructive pulmonary disease and asthma, are hampered by the inability to preserve the exquisite structure in cell culture.
There is, however, a half-way house between cultured and real organs: Organoids - a special kind of cell culture that mimics tissue structure. They rely on the convenient fact that when cells find a structure similar to the ones they expect to find in real tissue, they develop and differentiate almost like normal.
A recent paper from Brigitte Gomperts' group at UCLA illustrates this exquisitely. Bemoaning the slow pace of research into therapies targeting the lung, Gomperts' group has developed a way of making something that very much looks like, and even behaves like (if you screw your eyes up and don't look too closely), the alveoli of the lung.
The basic idea is to use alginate beads to play the part of alveolar sacs, and fool cells into growing around them. Gomperts had to get quite a lot right for the deception to work. First off, cells don't like to grow on alginate. So Gomperts coated them with an "adlayer" - a complex layering of different chemicals, in this case collagen and dopamine. Why dopamine? Next time you are at the seaside, look for a bed of mussels and see how much effort it takes to pull them off their substrate. The reason for this was found by Phillip Messersmith nearly ten years ago - the glue used by mussels is none other than DOPA, a precursor of dopamine, the neurotransmitter sadly lacking in Parkinson's Disease. This magic ingredient enables mussels to stick even onto PTFE, the lining on non-stick saucepans, leading Messersmith to call them "promiscuous foulers".
Anyway, back to organoids. The collagen/dopamine layer proved enough to fool the cells into adhering to the alginate beads. To get the cells to stick, the beads and cells are incubated in a rotating bioreactor. Once the cells have stuck on, you can add other stem cells to build up the tissue. As cells and beads roll around like washing in a tumble drier, the beads and cells compact together to roughly reproduce the foam-like structure of lung alveolar tissue.
As proof of concept, Gomperts added some transforming growth factor-β1 (TGF-β1), known to play a key causative role in the development of idiopathic pulmonary fibrosis (IPF). The organoids then developed something very much like the scarring seen in IPF, and showed some of the molecular signatures of IPF, such as local patches of αSMA (indicating that myofibroblasts had been activated) and increased numbers of pAKT-positive cells. The organoids can be kept going for two weeks and can be scaled up. Just what you want to hear if you have high-throughput drug screening in mind.
Gomperts' technique is a bottom-up version of the artificial organ approaches using, for example, colonisation of decellularised tissue. It also complements the 3D tissue-printing approach, which we reviewed in a recent editorial here at Lab Times. It adds to the growing consensus that once you provide the right mechanical structure, it is surprisingly easy to get cells to fall into place and start behaving as if they were "at home".
Together, these approaches quite literally add another dimension to models of disease.
Picture: Intestinal organoid grown from Lgr5+ stem cells (Meritxell Huch)