For this blog post, I tried to turn my weakness into strength. The weakness is that, even though I write for this neuroscience-focused blog, I know little about neuroscience, as my PhD research is in the cell cycle, a completely different field of biology. Of course, PhD training in any field teaches you to learn quickly. It’s been fun to sit down once a month, learn some bits of neuroscience, and synthesize them into a blog post.
This month, however, I challenged myself to relate my PhD research about the cell cycle to neuroscience. It’s a challenge because the cell cycle is about how cells divide, and cells of the brain mostly don’t divide. When they do divide, like during the embryonic development that forms the brain, or during the formation of brain tumors, so much more is going on apart from cell division, that I would not be able to separate my discussion of the cell cycle from developmental biology and oncology. I am no more an expert in those areas than I am an expert in neuroscience, and even if I were, I cannot do them justice in just one blog post.
So I took a different, less content-heavy tack. As I promised in the title, I will present three concepts from the cell-cycle field and explain their potential usefulness for neuroscience. One concept is that systems that include a positive feedback loop can undergo an irreversible transition. “Irreversible transition” and “feedback loop” are, of course, familiar terms for neuroscientists; the action potential has them. What may be less familiar are the exact characteristics of a positive feedback loop that gives rise to irreversible transitions as well as the possible diversity of components (voltage-gated ion channels in the action potential, protein kinases in the cell cycle, etc.) that are capable of triggering the transition. Another concept is that a cell can have mechanisms for monitoring completion of important events. Cell biologists discovered these mechanisms, known as checkpoints, when studying the cell cycle, but the concept is broad, and I’ll speculate about its implications for neuroscience. The third concept is that a protein can have more than one unrelated function. I’ll focus on several proteins that function both in the cell cycle and in neurons. I chose these concepts in the hope that they are novel and stimulating. But before I get to them, allow me to briefly define the cell cycle.
A Quick Overview of the Cell Cycle
A newly born cell can enter the cell cycle or quiescence. Most cells of the human body enter quiescence. They can still grow in size, if their function in the body requires it, but they do not divide.
In some cell types, like smooth muscle, the quiescence is reversible. If these cells receive a signal to divide they will divide. Biomedical researchers recently took advantage of this reversibility to feed smooth muscle cells division-promoting signals and make functional lab-grown vaginas for women deprived of that organ by a rare genetic disease.
Other cell types, notably those of the adult brain, are in irreversible quiescence. If a brain cell dies, no known signal can make its neighboring brain cells divide to fill up the gap, though the adult human brain may be able to make a few new brain cells from stem cells and though there must be signals that make brain tumor cells divide.
Once a cell receives division-promoting signals, there is a signaling pathway that coordinates the preparations for division. From studies of knock-out mice we have a list of components of this signaling pathway that are strictly essential for the earliest embryonic development: Cyclin-dependent kinase 1 (Cdk1), cyclin A2, cyclin B1, Cdk-activating kinase (CAK), and Wee1. Many more proteins are important, but these five are known to be indispensable.
We can go further and reduce the cell cycle to the activity of Cdk1. At low Cdk1 activity, cells are quiescent, at intermediate Cdk1 activity cells replicate their DNA to have a copy to pass on to each daughter cell, and at high Cdk1 activity cells enter mitosis. The cell has a constant number of Cdk1 molecules throughout the cell cycle, but they are not active unless they bind a cyclin, either A2 or B1, and get phosphorylated by CAK. Wee1 is a kinase that can inactivate Cdk1 and get inactivated by Cdk1.
When Cdk1 binds cyclin A2 it gains specific activity toward substrates that promote DNA replication and the beginning of mitosis. Even though cyclin A2-Cdk1 promotes both DNA replication and mitosis, the cell manages, by a still-unknown mechanism, to keep these processes from occurring at the same time. One possible mechanism is that the tug-of-war between Cdk1 and Wee1 maintains a level of Cdk1 activity that is sufficient for DNA replication but insufficient for mitosis. When Cdk1 binds cyclin B1, it has activity toward substrates that maintain mitosis.
One important feature of mitosis is that entry into it should be irreversible. It wouldn’t make sense for a cell to begin dividing and then stop half-way through and start growing. How the cell makes sure that it enters mitosis irreversibly leads us to our first concept.
How To Make An Irreversible Transition
Entry into mitosis is an irreversible transition; so too is an action potential in a neuron. Classic work by A.L. Hodgkin and A.F. Huxley on the action potential in the giant axon of the squid inspired the elucidation of the irreversible transition at mitotic entry. What the action potential and mitotic entry have in common is both a positive feedback loop and a negative feedback loop. During the action potential, depolarization opens more sodium channels and causes further depolarization in a positive feedback loop, but eventually potassium channels open and repolarize the neuron in a negative feedback loop. For mitotic entry, once Cdk1 activity passes a threshold, Cdk1 overwhelms the inhibition by Wee1 and becomes even more active, but once the cell is in mitosis cyclin A2 and cyclin B1 get destroyed, inactivating Cdk1.
Both the action potential and mitotic entry have been modeled mathematically. However, the mathematical models of the cell cycle (see here and here) are more detailed now than the Hodgkin-Huxley model, and they provide a more detailed recipe for how to make an irreversible transition. For instance, they show that a negative feedback loop is not important for irreversibility but is important to make sure that the system reacts to genuine signal and not to low-level noise. To my knowledge, no one has applied the cell-cycle models to action potentials and doing so may yield new insight.
Checking for Completion
When in the 1970s researchers looked for mutations that interfered with the cell cycle, they were surprised to find not just proteins that coordinate division or proteins that do the work (replicate DNA, pinch off two cells from each other, etc.), but also signaling components that seemed to have no direct role in the cell cycle. Further work elucidated that these proteins check for completion of important cell-cycle events. In my brief overview of the cell cycle above, I glossed over the proteins that check DNA for damage and the proteins that make sure the two sets of chromosomes can separate. Nobel Prize winner Lee Hartwell coined the term “checkpoint” to describe these processes. In retrospect, it makes complete sense that the cell would check that its DNA is in good condition before trying to give it to daughter cells. Could it be that important processes within neurons are also monitored by checkpoints and nobody has found them yet?
Biologists tend to categorize proteins by function. What is often overlooked is that proteins can have more than one function. A recent review gives several examples of proteins with a function both in the cell cycle and in neurons. For some of these proteins the functions are related. For instance, the Retinoblastoma (RB) protein indirectly prevents activation of Cdk1 by inhibiting transcription of cyclin A2 and cyclin B1, and helps maintain irreversible quiescence of neurons. However, the RB protein also has a distinct function in promoting neuronal maturation. And in most of the examples, the functions are bizarrely unrelated. The proteins that degrade cyclin A2 and cyclin B1 in mitosis also regulate presynaptic differentiation. One of the components of the checkpoint that monitors for DNA damage in the cell cycle also participates in synaptic vesicle trafficking and is required in hippocampal neurons for long-term potentiation.
Multi-purpose proteins are probably a consequence of the evolutionary process, of repurposing something that already exists instead of coming up with something new. However, especially for proteins whose multiple functions are related, awareness of them can lead to new insights.
There and Back Again
Through this blog post, I have gained an appreciation for the difficulty of interdisciplinary communication. I hope I have also given you an appealing, neuroscience-friendly introduction to the rather distant discipline of the cell cycle. If you want to learn more about the cell cycle this is a good starting point.