Tuesday, May 28, 2013

Stop seizures with a brain graft

There are two types of neurons in the brain: excitatory and inhibitory neurons.  They do exactly what you think they would.  Excitatory neurons release chemical messengers, which activate other neurons, which may eventually lead to some sort of perception or action.  Inhibitory neurons release chemicals that silence other neurons.  Why would you want inhibitory neurons in your brain?  Well, if all your neurons were excitatory and interconnected, all your neurons would be active all the time and the signals would be meaningless.  In fact, this sort of overactivation in the brain can lead to seizures.  It’s been shown in numerous cases of epilepsy that there is some sort of dysfunction of the inhibitory neurons.  The excitatory neurons have free reign and go crazy, leading to a seizure.

How is epilepsy treated?  Medications that potentiate the inhibitory neurons can help, but they activate all inhibitory neurons throughout the brain, when maybe the problem is more localized to one spot.  Just as all excitatory neurons is a bad thing, too much inhibition is also bad and can lead to cognitive side effects.  Another treatment is to open up the patient’s head, try to find the overactive area and cut it out or zap those neurons with a laser.  Destroying brain cells is always a last resort, though.

In a recent paper published in Nature Neuroscience by Hunt et al., the authors propose another potential treatment: adding new inhibitory neurons into the epileptic brain.  Like all new medical ideas, the story starts with mice.  They can create a model of human epilepsy in these mice by treating them with a potent drug.  These epileptic mice have seizures just like humans do.

Where do you get new inhibitory neurons?

The researchers obtained progenitor cells from mice embryos.  In other words, these weren’t inhibitory neurons yet, but they were destined to turn into them as the mice developed.  They grafted these progenitors into adult epileptic mice in the hippocampal region of the brain (a common area for seizures).   Amazingly, these pre-neurons migrated throughout the brain region, as far as 1.5 mm (that’s a lot… think about how small a mouse brain is).  Then the progenitors differentiated into inhibitory neurons, as if they were in a normal developing brain.  One week later, the epileptic mice with extra inhibitory neurons had hardly any seizures, whereas the untreated mice were having about 2 a day.  Not only that, but the treated mice showed cognitive improvements compared to the untreated epileptic mice. 

So they seemed to “cure” the epileptic mice by giving them some new inhibitory neurons that were able to make functional connections with the existing neurons.  This isn’t as invasive as brain surgery and it’s much more localized than medication.  If the epilepsy were focused in a different part of the brain, then they could transplant the cells there instead.

Is this possible to try in humans?  Maybe so, but the first problem is that we can’t take inhibitory progenitor cells from human embryos.  There are some ethical issues with growing clones to harvest parts from them.  However, you could use embryonic stem cells, or induced pluripotent stem cells.  Pluri-what?  Recent technology allows researchers to take a skin biopsy, do some genetic engineering to these cells and push them back in developmental time to a stem cell.  Pluripotent means that these stem cells have the potential to become any type of cell, like an inhibitory neuron.  All it takes is turning on the right genes in these cells to push them to a particular fate, and if that isn’t already known for inhibitory neurons, I bet it’s not too far off.  Plus there’s the benefit that the transplanted cells will have the same genome as all the patient’s other cells, because they originated from their skin cells.  Just wait, regenerative medicine is moving ahead at lightning speed.

Friday, May 17, 2013

Go go gadget extendo filopodia

I’m back from an intense semester of learning and teaching Developmental Biology.  One theme that emerged from my studies was that the development of organisms is centered around gene expression and cell to cell signaling.  Often times, one cell will differentiate into its mature form, and then release a signaling protein that tells neighboring cells what to develop into.  For instance, the nervous system is induced by signals released from the embryonic backbone.  There are a number of common signals that are used over and over throughout development, like BMP, Wnt and Shh.

A recent paper by Sanders et al., published in Nature, looked at how distant cells can signal to each other via the Shh pathway.  Unfortunately for Developmental Biology teachers everywhere, Shh stands for Sonic Hedgehog.  Oftentimes, strange or humorous gene names like this can be blamed on the fruit fly researchers who first discovered the gene, but in this case everyone is to blame.  This gene was originally discovered by researchers studying fruit fly embryonic development; they named the gene hedgehog because the mutant embryos had lots of tiny bristles all over, kind of like a hedgehog.  The mammalian researchers took it to the next ridiculous level, by naming the mammalian version of this gene Sonic Hedgehog.  The Shh protein is a secreted signal that binds receptors on other cells, which activate gene expression in the receiving cell.  Shh signaling is important for specifying many different cell fates, such as the different neurons in the spinal cord, the cells that become the vertebrae, as well as the formation of the digits of the hand.

Although Shh is secreted from the cell, it has chemical modifications that make it stick to the plasma membrane that surrounds the cell that released Shh.  How then can Shh induce the development of cells that are located at a distance?  Well, the answer is by stretching out long cellular extensions with Shh localized at the tip.

Shh Filopodia
Sanders et al. did live imaging of cells in the developing limb of the chicken using fluorescent proteins.  They did some genetic trickery so only a few cells were labeled in red and others in green.  This way they could detect individual cells in a sea of unlabeled cells and examine their structure in real time.  They observed individual cells extending long protrusions, called filopodia, from the cell bodies.  These filopodia could stretch long distances (150 micrometers, like 3-5 cell widths) and were dynamic-- retracting and growing over time. 

How to think about filopodia?  Imagine a stretchy balloon with a stick inside of it.  If you could push that stick into the wall of the balloon, the balloon would protrude from that one spot as the stick pushes it out.  That is like a filopodia, where the balloon wall is the plasma membrane and the stick is a protein called Actin.  Actin forms long chains that can grow, pushing out the membrane in front of it. 

The thin, string-like extensions from this cell are filopodia and are filled with Actin.  Image from proteopedia.org

The authors then labeled the Shh protein with another fluorescent marker and saw that it localized to the tips of filopodia.  Not only that, but the filopodia expressing Shh were more stable and did not retract as often.  In order for Shh to act as a signaling molecule, it has to bind a receptor on another cell.  Using a different color, the authors observed two co-receptors for Shh localized to filopodia from other cells.  They even saw filopodia from two different cells make contact with each other, where one cell expressed Shh and the other expressed the receptors.

This is amazing!  Instead of releasing a signal out of the cell with the hope that it goes to the right place and isn’t degraded, the cells literally grow to the right place with the signal on their membranes.  This is like hand delivering a note to your coworker, rather than making the note into a paper airplane and throwing it in the direction of their desk.

This is how I imagine this working.  Two cells that are located at a distance, reach out extensions and meet somewhere in the middle. The Shh signal would bind the receptor, causing changes to the pink cell.

A study like this could not have been done before recent innovations in live imaging and molecular biology to introduce the fluorescent proteins into the cells.  The filopodia are not preserved during the more traditional, static method of fixing cells with formaldehyde and then staining them.  Who knows what other tricks live cells use during embryonic development.  I suspect this is only the beginning.