“You are your synapses.” [Joseph LeDoux].
I first came across this quote while working in a neurophysiology lab after my freshman year of undergraduate studies. To this day, I am amazed by its simple eloquence and the grandeur of its implications.
Indeed, the idea that the essence of our experience of the world – our sensations, movements, and the ether of our consciousness – is somehow physically manifested as millions of interconnected neurons is awe-inspiring.
Given the complexity of the nervous system, it is hard to study individual pathways within it. Often the readouts we get, such as movement or “brain activity,” reflect the outputs of numerous, interconnected networks versus individual circuits. Furthermore, it is rarely easy or ethical to extract primary neural cells from humans, and even when they are put into cell culture, they lack a specific interconnectivity.
If we could somehow create miniature neural circuits in culture, by guiding the interactions of neurons, this could greatly enhance our ability to study disease pathophysiology and how drugs affect neural transmission. Fortunately, with combined improvements in biomaterial manipulation and tissue engineering, creating these kinds of ‘model systems’ is now becoming possible.
For example, in a recent publication by Dr. Michael McAlpine’s group, from the University of Minnesota, 3D printing is used to promote the formation of specific neural networks.
For those unfamiliar, the essence of 3D printing is to feed an industrial robot a computer generated design, which it builds by adding layer upon layer of material in a fixed 2D space. Since the machines are capable of fine movements, the 3D structure that is produced can have microscale features, which is useful for biological applications.
In their study, the authors sought to recreate core circuits of the nervous system containing several distinct types of neural/glial cells. To guide the interaction between these cells, the authors 3D printed a miniature cell-culture platform with three isolated compartments for plating distinct cell types, as well as channels to guide cell interaction.
To give you a mental image of how the platform was made (see what I did there?), consider the following steps:
3D printed layers (bottom to top); process takes ~1 hour
- A biocompatible polymer is printed onto a small, round petri dish, creating a region with parallel, unidirectional microchannels (350μm wide) that will ultimately guide/confine the direction of axon growth.
- Two single layer strips (along with a circular perimeter) of grease sealant are laid down, dividing the platform area into three chambers that are, at most, 6mm wide (grease prevents interchamber fluid exchange).
- A final layer of silicone is deposited along the outline of the sealant layer until a height of 10mm is obtained; with this depth, 3D growth chambers have been created.
A schematic can be seen in the graphical abstract.
With the microplatforms printed and ready to go, the authors could move on to seeding cells into the distinct chambers to facilitate the formation of simple neural circuits. In the body, neurons from the brain (CNS neurons) connect to those that extend to the periphery (PNS neurons) and the latter can be “wrapped up” by glial cells called Schwann cells to promote faster conduction (via electrical insulation).
They thus seeded CNS neurons, PNS neurons, and Schwann cells (from a rat) into their tri-chamber microplatform. True to design, they found that the PNS cell axons aligned within the microchannels, and that Schwann cells spontaneously wrapped around PNS cell axonal extensions into the third chamber.
A valid question to ask of any model system is: “What can we learn from a particular setup?” Indeed, most models aren’t meant to be perfect representations, but to rather recapitulate a particular behavior of a system. In this case, the simple circuit was amenable to studies of the spread of neurotrophic viruses between different neural cell types. Since the chambers were isolated, one could specifically infect the PNS neurons with neurotropic viruses and then track their spread to cells in the other chambers since the viruses encoded fluorescent reporter proteins.
By contrast, other recently published neural platforms have been designed to study different pathologies of the nervous system like dementia (tauopathies) and traumatic injury.
While this study used rat neurons, one can imagine that a similar system could be attempted using human stem cell-derived neurons – thus overcoming the many limitations of using primary human neural cells. In fact, with the growing number of stem cell lines and recent advances in gene editing technologies, it would even be possible to create circuits using cells with specific genetic defects. How cool would it be to study the effects of a genetic mutation in a simple, defined neural pathway!
For me, the brain will always hold an element of mystique. But as certain as I am of its complexity, I am confident in its ability to be creative. The rise of 3D printing and its ability to engineer miniature neural circuits only supports this latter conviction.
Other neural microplatforms:
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