Smart phone use changes the brain

One of the most remarkable things about our brains is how organized they are. Sensory information from our eyes, mouth, skin, nose and ears goes to different locations in the brain. For example, visual signals are processed first in the very back of the brain, whereas sensations of touch and pain activate the middle region of the brain called the somatosensory cortex.

Functional organization of brain cortex. (Source: imgarcade.com/1/sensorycortex)

Remarkably, the brain gets even more organized from there. Within the visual cortex, there are columns of neurons that only respond to light that is horizontal and others that only respond to lines that are tilted 45 degrees. The somatosensory cortex is also highly organized, with different parts of the body represented by specific sets of neurons. If you were to send electrical shocks into one specific area of the somatosensory cortex to activate those neurons, you may elicit feelings of touch from the right thumb, even though the subject is not being touched at all. Move those electrical signals over slightly to another area, and the subject may feel touch instead coming from the palm of their hand.

Wow, right? But here’s the real mind blower: this organization can change over time as the person experiences different sensory inputs. If you are a violin player, you feel the strings with your fingertips a lot, so the fingertip part of the somatosensory cortex is super active. This extra activity allows the fingertip representation in the brain to grow and recruit nearby neurons to also respond to touch in the fingertips. The cortical representations are “plastic” and always changing with use.

A violin player may practice this one particular skill a lot, but what about other activities we do everyday with less intensity, like using smart phones? Think about how often you are swiping the screen with your thumb. That’s a lot of sensory information being sent to the thumb part of your somatosensory cortex. Would this increase the thumb representation in your brain? A recent paper by Gindrat et al. addressed this exact question using EEG to record brain activity in smart phone users versus people with the old-style cell phones.

Electroencephalography

How can you actually measure the area of body representations in the somatosensory cortex? You could stick electrodes into people’s brains and record the activity in their neurons, but that’s a little invasive. You could put them into a MRI machine and measure brain activity when you touch their thumbs, but that is time consuming for so many subjects (37 total). Instead, the authors used a method known as electroencephalography, or EEG, which consists of 62 surface electrodes placed on the scalps of the subjects. Each electrode records the summed electrical activity from all the neurons positioned right under the electrode. Before an experiment, all the electrodes would be picking up a baseline of activity from lots of different neurons firing asynchronously. However, during an experiment, there is a single stimulus (like touching the subject’s thumb), which elicits activity in a lot of neurons all at the same time. This activity summates to give one large response called the event related potential (ERP), which is recorded by the nearest electrodes.

EEG electrodes record brain activity (source: Wikimedia commons)

Finger representations in smart phone users

The ERPs for the thumb, index finger and middle finger were larger for the smart phone users than for the non-touchscreen users. There was a correlation between the amount of phone use per hour and the ERP, so the more use, the greater the ERP, which is to say the more activity in the somatosensory cortex. The number of electrodes recording the ERP was greater in the touchscreen users, so when you touch the thumb of a touchscreen user, a larger part of the somatosensory cortex responds. In other words, the thumb representation was larger in smartphone users who use their thumbs more often.

The more recently the subjects had used their phones intensely, the larger the ERP for the thumb, which indicates that brain remodeling occurs on a very short time scale (within 10 days in this experiment). Interestingly, there was no correlation between ERPs and the age at which the subject started using a touchscreen. This is in contrast to the previous experiments done with trained violin players, which did show a correlation between the size of the finger representations and the age at which they first started playing. The authors suspect that a trained violinist develops a more stable sensory representation than touchscreen users who are casually using their phones (as opposed to years of disciplined practice).

So the take-home message is that normal day-to-day activities can influence brain plasticity and the way our sensory representations are organized in our brains. This could be a good thing, because subjects develop better touchscreen skills. On the other hand, the enlarged thumb representation could cause focal dystonia, which is characterized by involuntary muscle contractions and sometimes pain, as the various body part representations lose their distinct boundaries and start to overlap. This probably won’t be a problem for most phone users, but be forewarned all you smart phone addicts out there.

What big nuclei you have!

Eggs get ready for fertilization by producing and storing all the proteins necessary for early embryo development.  After fertilization, there are a series of rapid cell divisions without growth, producing a lot of small cells (here's a video).  At some point during this process, the embryo switches over from using the proteins from mom, to expressing their genome to make their own proteins.  This transition to embryonic transcription is known as the midblastula transition, or MBT.  How does the embryo know when it is time to turn on gene expression? 

One theory is that the ratio of nuclear to cytoplasmic volume (N/C volume) is the trigger for MBT.  The nucleus is where DNA is stored within a cell; this is where gene expression occurs.  The cytoplasm is the goo that the nucleus sits in.  During those rapid early cell divisions, nucleus size does not change much, while the cytoplasm in each cell keeps getting smaller and smaller.  The N/C volume increases, since the cytoplasmic volume is decreasing.  Is there a certain threshold of N/C volume, above which the embryo switches on gene expression?

Jevtic and Levy did a series of clever experiments, using frog embryos to address this question, which was published today in Current Biology.  In Xenopus laevis frogs, the midblastula transition always occurs after the 12^th cell division.  The researchers manipulated the nucleus size in the frog embryos to see if that would change the timing of MBT.

Changing nuclear volume

The authors were able to increase nucleus volume by injecting embryos with mRNA for importin and a type of lamin.  Importin acts as a shuttle that brings other proteins into the nucleus, including structural proteins that make up the nuclear envelope.  Lamins form the inside of the nuclear envelope, so by injecting the mRNA for these two proteins, they caused overexpression of nuclear proteins that will make the nucleus grow larger.  To decrease nuclear size, they instead injected mRNA for a protein that causes another cell structure to grow (the ER) at the expense of the nucleus.

They injected the mRNAs and a red dye into one cell in the two-cell stage.  When this cell divides, its daughter cells inherit the red dye and the mRNAs and proteins that change the nucleus size.  Thus, by the time a normal embryo is ready to undergo MBT (the midblastula transition to express their own genes), half of it will be red and have abnormally sized nuclei and the other half will be normal and act as an internal control. 

N/C volume triggers MBT

They looked at embryonic gene expression (as a readout of MBT) in the cells with abnormal nuclei at different developmental stages.  The cells that had larger nuclei reached the critical nucleus to cytoplasm (N/C) ratio earlier in development and began expressing embryonic genes earlier than the neighboring cells with normal nuclei.  Likewise, the cells with smaller nuclei took a little bit longer than normal to undergo MBT.  I love that the two halves of the embryo are out of sync with each other just because the sizes of the nuclei are different.  This really shows that there is a critical N/C volume and manipulating this ratio is sufficient to initiate the midblastula transition.  

How do the cells know the size of the nucleus and cytoplasm?  The authors suggest that the oocyte must have inhibitors in it that repress transcription, so the embryo’s genome is inhibited at first.  As the cells divide, these inhibitors are split among them, so the inhibitors become less and less concentrated in each cell.  Once they reach a certain low concentration, they no longer function, so the cells can begin transcription.  This would explain why increasing nucleus size would cause an earlier midblastula transition: the larger nuclear volume essentially dilutes the inhibitor further, so it reaches that low threshold concentration sooner.  It’s important to get the timing of gene expression just right during development and the N/C volume appears to be one way that cells manage to do this.

Improving reproductive cloning

Remember when Dolly the sheep was cloned in 1996?  That was the first cloned mammal and everyone freaked out thinking we would be cloning all our pets and even humans within a few years.  Well, nearly 20 years have passed since then and reproductive cloning is still a very difficult and inefficient procedure.  Most cloning has a 1-5% success rate.  Why is that?  Before we can answer that, we need to understand the procedure for reproductive cloning.

Somatic cell nuclear transfer

Our bodies are made up lots of different types of cells – neurons, skeletal muscle, intestinal cells, immune cells, etc.  Despite the different functions and structures of these cells, all of the cells in one organism have the same genome, the same set of genes.  What makes cells unique is that they express different genes at different times, so different proteins are made.  What this means is that the blueprint (DNA) for making a new organism is right there in every cell in your body.

The normal way of making an embryo is by taking half of the genome from a male (in sperm) and half from a female (in the egg) and combining them during fertilization.  In reproductive cloning, you already have a whole genome from any adult cell.  That nucleus from the adult can be inserted into an oocyte (or egg) that has had its DNA removed (“enucleated”).  The egg is necessary because it has lots of nutrients and signals in it that are important for the first few cell divisions during early development.

This process is shown in the diagram below and is called somatic cell nuclear transfer.  In the example, the adult genome is coming from a fibroblast cell and is transferred into an enucleated oocyte.  This is a way to get stem cells (ntES), which can be used for therapeutic purposes, like making more neurons that can be transplanted into someone with Parkinson’s.  Or you could let the cloned cells grow up into an embryo and then into a cloned organism.

Somatic nuclear cell transfer (from Stembooks.org)

Although it is possible to make cloned organisms using adult donor genomes, the efficiency is much higher when using genomes from embryos.  What happens to the adult genome that prevents it from directing the formation of a new organism?  This problem is addressed in a recent paper by Matoba et al., published in Cell.

Epigenetic changes

Although adult cells should have the same DNA sequences as their embryonic precursors, the genome can be organized differently, which can affect which genes are expressed.  DNA wraps around histone proteins as a way to organize the long DNA chains.  Histones can be modified in such a way that the DNA will wrap around more tightly or more loosely.  For example, if a particular amino acid in histone 3 is trimethylated (three CH3 groups are added), then that makes the DNA pack up closer together, so it is really hard to express those genes.  There are genes that may need to be expressed early on in development, so their histones will be modified to allow for loose packing, but then after they are expressed, they’ll get packed away, so they take up less space.  These kinds of modifications that affect gene expression are called epigenetics.

As an analogy, imagine you have had a child, so you have baby clothes, a crib, car seat and toys in your house.  Once that child grows up, you take all those baby things down to the basement.  You still have them, but you will probably never need to use them again, so you can pack them all up and store them so they are out of the way.  It may be hard to access them again, but they are still there.  So a gene that has been packed away into condensed chromatin is still present in a cell, but it is no longer giving instructions for making proteins, unless something comes along and unpacks it.

You can see now the problem with somatic cell nuclear cloning.  The adult cell already has some DNA packed away, so when the genome is put into an oocyte, it may be impossible to express the genes necessary to direct normal development.  In the paper by Matoba et al., they did indeed find that there are more trimethyl modifications (called H3K9me3) in mouse embryos derived from nuclear transfer than embryos from in vitro fertilization (using a sperm and egg).  These regions were associated with decreased gene expression and compact DNA.

Improving efficiency

Now we know one of the problems, but what can researchers do to improve the efficiency of reproductive cloning?  Somehow they need to decrease H3K9me3 modifications in the donor genome.  They do this two ways:

(1) There are too many methylated histones, so the authors injected an enzyme that removes methyl groups into the one-cell embryos.  The embryos expressed more genes and survived throughout development.  70% of these cloned embryos implanted into a surrogate mouse uterus and 8% survived to adulthood.  Those numbers are higher than before, but still not perfect.

(2) They also tried decreasing expression of the enzymes that put on the methyl groups.  This also improved development, so 50% of the embryos made it to later stages of development, but they did not see how many survived to adulthood.

There is an epigenetic barrier for nuclear transfer from adult cells in mouse oocytes.  Presumably a similar problem is preventing cloning in other organisms as well.  It makes sense that an adult cell would have a different pattern of epigenetic modifications than an embryonic genome.  The authors were able to improve cloning efficiency by decreasing the H3K9me3 modification, but there are probably other histone modifications that are also different in adults.  There is still a long way to go before cloning is a reliable procedure, but at least now we have some explanation of why it is so difficult.

Transgenerational inheritance of fear

A new semester has begun and I have no extra time to update this blog, so just a short entry today.  This paper was just too cool to pass up.  It was published earlier this year in Nature Neuroscience by Dias and Ressler.  They conducted a series of experiments which showed that learned fear can be passed on from generation to generation in the sperm DNA.  That’s Lamarckian evolution for all you evolution nerds out there.  A learned behavior that is inherited genetically -- totally crazy!

The DNA sequence itself isn’t changing, but instead the expression of genes is altered, so different amounts of proteins are being made.  This process is known as epigenetics (which I’ve discussed before with regard to histone modifications).  One way to change DNA expression is by methylating cytosines (the “C” in DNA sequences).  The methyl group (CH3) makes it harder for proteins to bind to the DNA and transcribe the genes into mRNA and subsequently into protein.  The general rule of thumb is: more methylation --> less gene expression, less methylation --> more gene expression.  This is a common way our cells regulate gene expression, and what’s really interesting is that many external influences can affect DNA methylation, like traumatic life experiences, smoking, exercise, environmental toxins, etc. 

Cytosine getting methylated (note the H3C addition on the the molecule on the right)

It is conceivable that DNA methylation patterns can be inherited through generations, so changes in gene expression that affected your great grandparents could still be maintained in your cells.  Most of these types of studies focus on changes that occur to the mom during pregnancy.  For example, lets say that researchers expose a pregnant mouse to a toxin that may affect DNA methylation.  The next generation (the F1 generation) is also being exposed in utero to the same toxin.  The eggs or sperm progenitor cells are also developing in the embryo, so the next next generation (F2) may also be exposed to the toxin.  Thus, if you see DNA methylation changes in the F1 and F2 offspring, this isn’t really so surprising since these cells were all exposed at the same time as the mom (F0 generation).  In order to really prove that DNA methylation patterns can be inherited across generations (transgenerationallly), you need to expose the parents to a stimulus before conception, before the F1 and F2 generation even exist, which is what they did in this paper.

The authors chose to initiate changes to gene expression by conducting odor conditioning in mice.  They paired a particular odor with a mild foot shock and conditioned F0 males to be afraid of the odor.  Then these males mated with naïve females (never exposed to the odor).  The F1 offspring showed excessive fear to the conditioned odor, even though they had never encountered it before.  It was their fathers who had been shocked, not them.  The authors found there were more cells in the olfactory region of their brains that expressed the olfactory receptor for the conditioned odor.  Furthermore, their dad’s sperm and their own sperm were less methylated in the gene that encodes for that particular odor receptor, so the offspring of F1 were also affected.

F0 dad learns fear for odor --> decreased methylation for receptor gene --> F1 offspring inherit methylation changes --> express more of the receptor --> increased sensitivity to odor --> F1 sperm have same decreased methylation --> sensitivity for odor passed on to F2 generation  

Grandpa (F0) had a bad experience with an odor and now his grandkids will be more afraid and sensitive to that odor.  And it’s all genetic.  The authors did a series of experiments to show that it isn’t behaviorally based (grandpa isn’t telling the grandkids about his horrible experience with this smell).  For instance, they took the sperm from the F0 mice and took it to a different mouse facility and did in vitro fertilization, so the father was nowhere near his offspring or the mother.  The F1 mice from in vitro fertilization were just as super sensitive to the odor.

This is nuts!

Here's a good blog post about this paper