What convinces a stem cell to determine its fate? It’s one of the most persistent questions in modern biology. Research over the last four decades has revealed there is no easy answer. For example, in the brain, stem cells in the embryo produce all of the different cell types at precise times and amounts. If stem cells are perturbed by altering their ability to make those cell types, this is thought to contribute to neuropsychiatric and developmental disorders.
To produce their progeny, stem cells receive signals from other cell types, blood vessels, and the cerebral spinal fluid, and even produce signals themselves. This in itself raises numerous questions. What are those signals? How many are there? How does a stem cell decide to respond to one signal and not another? More importantly, how can this all happen in a coordinated manner to ensure the proper development of the brain?
Finding answers is no easy task and requires knowledge of all of the signals that stem cells are exposed to, and the receptors on stem cells that recognize and transmit signals to the machinery within those cells that direct them to make neurons. Once those signals and receptors are identified, a communication network needs to be constructed that will enable researchers to predict and test which signals are used by stem cells to make the various cell types in the brain.
To construct a communication network, a team of researchers at the Hospital for Sick Children led by Drs. Freda Miller and David Kaplan identified signals produced by stem cells and neurons that may influence each other’s function in the mouse embryo. They used the approaches of gene expression analysis – also known as transcriptomics – as well as the protein equivalent, proteomics, to identify cell surface receptors. Then, using bioinformatics, they combined the data to construct the network and predict which signals are required for stem cells to produce neurons. The results, available in the journal Neuron revealed an unexpected diversity in signals and the involvement of a particular molecule normally associated with another bodily function.
The authors first found that stem cells are exposed to hundreds of signals in the developing brain, including those produced by neurons in their environment. They found, however, that stem cells had receptors for only about 32 of these signals, better known as ligands. After identifying these 32 proteins, they generated a communications network that enabled them to predict the ligands that potentially direct stem cells to make neurons.
With this information in hand, the team returned to the lab in order to test how many of the ligand-receptor combinations might be considered, in a word, proneurogenic. They didn’t test all of them as many had already been studied in other labs or were not functional during the stage of embryogenesis explored in the first experiments. This reduced the number to just eight possibilities.
The next steps were relatively straightforward. Lab cultures of NPCs were exposed to the various factors and then examined for any changes in the ability of stem cells to generate neurons. Three factors not previously known to instruct stem cells to make neurons were identified, neurturin, glial-derived neurotrophic factor, also known as GDNF, and interferon gamma, a modulator of immune responses.
Having reduced the number from hundreds to three, it was time to go back to the mouse model. This time, the team tested whether injection of the three ligands into the embryonic brain could promote neuron formation from stem cells. But this wasn’t the only task. They also wanted to know if they could block the function of those ligands using antibodies. As expected, when the ligands were introduced into embryonic mouse brains between day 13 and 14 – a critical time in mouse brain development – more neurons were produced, and the antibodies inhibited the generation of neurons.
To reinforce this observation, the team genetically removed the production of these receptors to see if they could prevent this effect. As expected, the rise was no longer seen. These three ligand-receptor combinations had proven to be proneurogenic.
In addition to revealing yet more information on how stem cells produce neurons, this study also offers a glimpse into the power of systems biology approaches to identify individual factors in cell biology and stem cell development. But perhaps the true value of this technique is an out-of-the-box approach to developmental neuroscience. The identification of interferon gamma is a surprise as it is primarily associated with immune responses. Yet, the molecule had a significant effect. This alone reveals the power of developing communications networks for all of the cell types in the developing and adult brain.
As we gain more information from communications networks, we may one day understand exactly what is happening at the microscopic level during development and provide insight into other brain activities such as learning. Perhaps more importantly, we may finally gain a grasp on what occurs during aging as well as when the brain suffers injury or undergoes neurodegeneration allowing for the developments of treatments and other means to heal.
Original research article:
Yuzwa SA, Yang G, Borrett MJ, Clarke G, Cancino GI, Zahr SK, Zandstra PW, Kaplan DR, Miller FD. Proneurogenic Ligands Defined by Modeling Developing Cortex
Growth Factor Communication Networks. Neuron. 2016 Sep 7;91(5):988-1004. doi: 10.1016/j.neuron.2016.07.037.