Human language in dog brains

Spoken language conveys meaning in two ways: the meaning of the words (semantics or lexical knowledge) and the intonation that the speaker uses. We can sense questions by the rising pitch at the end of the sentence. Likewise, we can tell if someone is upset or being sarcastic based on how they say the words. The patterns of intonation in language is known as prosody. There are areas of the brain that are specialized for decoding the semantic meaning of language and different areas for interpreting prosody. In fact, you can have damage to one area during a stroke, while the other area remains intact. There are great examples of this in “The President’s Speech” in Oliver Sacks' book The Man who Mistook his Wife for a Hat.

In most people, word meanings are processed by the left side of the brain and prosody is localized to the right side of the brain. Some animals also use the left side of their brains to understand meaningful and familiar sounds of their species (like alert calls or bird songs). What about for animals, like dogs, which can understand the sounds of another species (i.e. commands from humans). Is the dog brain really processing the intonations of praise “good dog!” or are they responding to the words? Do they process meaning and intonation separately like humans do?

Dogs in MRI machines

In the latest issue of Science, Andies et al. published their studies of language processing in dog brains. My first thought when I read the abstract was “how do you get a dog into an MRI machine?” We commonly study which areas of human brains are active during different tasks using a technique called functional MRI (or fMRI). fMRI was done on these dogs while they listened to their trainers speak. If you have ever had an MRI scan, you know they strap you in and you cannot move your head at all. Same thing with these dogs. Needless to say, they were very well trained dogs. If you still can’t believe it, check out this video the researchers made and the cute photo of dogs in an MRI machine below.

Really well trained dogs lying still before their MRIs. (Image from phys.org)

Dogs process language like humans

Okay, so they got the dogs in the MRI machine and scanned their brains while they heard their trainer say different things. The trainer would either say words of praise, like “good boy” (in Hungarian), or neutral words. And they used either a neutral, flat intonation or they raised the pitch of their voice to create a praising intonation. This created four possibilities:

• Praise words with praising intonation
• Praise words with neutral intonation
• Neutral words with praising intonation
• Neutral words with neutral intonation

They compared the brain responses to each combination and found that the left side of the brain responded to words of praise regardless of the intonation. This is amazing, right? The dogs have heard “good boy” enough times that their brains responded specifically to that phrase regardless of how it was said. It’s like they sort of know what it means. It would be interesting to see if they respond to the same phrase spoken by a stranger.

The researchers also found that the right side of the brain had active areas when praising intonation was used, regardless of the word meaning. So dogs also understand how our voices change when we praise them.

Finally, the researchers looked at areas of the brain associated with reward. These areas are active in a variety of animals when they receive natural rewards like food or during sex, but the reward pathways are also active if the animal is given an addictive drug like cocaine. Alternatively, you can put an electrode into a mouse brain that stimulates the reward pathway and the mouse will push a lever to receive an electrical shock in this area of the brain over and over until it starves.

Andies et al. found that praising words spoken in a praising intonation activated the reward pathway in the dogs. Praise words alone and praise intonation alone had no effect. So dogs really do feel good when you say “good dog” in a high pitched voice.

Notice the organization of language processing in the dog brain. Just like in humans, language semantics (praise vs neutral words) was processed on the left side and prosody (praise vs neutral intonation) was processed on the right side. What does this tell us about the evolution of language? Language lateralization has likely been around a long time and is not uniquely human. The authors end the article with this gem: “What makes lexical items uniquely human is thus not the neural capacity to process them, but the invention of using them.”

No Y genes? No problem, bro.

The Y-chromosome is one of the smallest chromosomes in the human genome and contains genes involved in male development and production of sperm. Previous research has shown that just two genes on the Y chromosome are necessary to make male mice who can sort of produce sperm. By “sort of” I mean that the mice make things called “round spermatids”, which genetically are the same as sperm, but are underdeveloped, so they can’t naturally fertilize an egg. A lab in Hawaii took these round spermatids and injected them into oocytes to demonstrate that the resulting zygotes are viable and develop into normal mice. In other words, the experimental mice have only one X chromosome and the two Y genes, and they develop into males who can reproduce with a little help from scientists.  That is pretty amazing that only two genes can make a male.

The necessary Y genes

So what are these two genes? One of them is called Sry, which encodes for a transcription factor that regulates expression of other genes important for the development of the male reproductive system (see the figure below). The other necessary gene is Eif2s3y, which is involved in protein synthesis and somehow necessary for the production of sperm. There is a similar gene on the X-chromosome, which may serve the same function. Normal XY males express both Eif2s3y and Eif2s3x, the version on the X-chromosome.

 

In a paper that came out earlier this year in Science, Yamauchi et al. asked whether they could replace the function of Sry and Eif2s3y with other genes that are found on other chromosomes. Instead of a male mouse with Eif2s3y, what if you made a mouse that was overexpressing Eif2s3x?  Could the X version compensate for the Y version? And instead of Sry, could you overexpress one of its target genes to replace its function?

Through the power of mouse genetics, the researchers created a mouse line with one X-chromosome and no Y-chromosome, which overexpressed Eif2s3x and Sox9, one of the Sry targets. In other words, these mice do not have any genes that are normally found on the Y-chromosome.

A male mouse with no Y

The mice with no Y-chromosomes and no Y genes, but overexpression of Sox9, developed into males, with male reproductive systems (though smaller and less developed). When Eif2s3x was overexpressed along with Sox9, the males were able to produce the round spermatids (precursors for sperm). The researchers did their artificial fertilization with these round spermatids and were able to produce healthy offspring. 

 

So just to repeat: the mice without a single gene from the Y-chromosome developed into males and produced sperm that are good enough for successful in vitro fertilization. Just by overexpressing two genes found on other chromosomes. That’s amazing!

Pandas are lazy!

Pandas are closely related to carnivorous mammals (like all the other bears), but they consume mostly bamboo.  Their digestive tracts are short and adapted for digesting meat, not cellulose that is found in plants.  In fact, they only digest about 20% of all the bamboo they eat, and they eat a lot of bamboo (30-60 pounds a day)!  How are these large, adorable bears able to get enough energy to function from their inefficient digestion of bamboo?  Researchers in China and Scotland addressed this question by studying captive and wild pandas, described in a recent Science article.

Low energy expenditure

Nie et al. measured the daily energy expenditure of the pandas and found that they used an unusually low amount of energy, only 37.7% of the predicted value based on their body mass.  In fact, pandas are expending energy at levels similar to the three-toed sloth, the epitome of a low-energy mammal.  The measly amount of nutrients they get from all that bamboo would be able to sustain such a low energy expenditure, so that’s how the panda is able to get by with such a maladapted digestive system.

How do the pandas manage to spend so little energy?  There must be some adaptations that are allowing the panda to survive without expending so much energy.  The authors found a number of these adaptations:

1) Pandas have a thick layer of fur, so they can maintain their internal body temperature with less heat loss through the skin.  The researchers measured temperature at the surface of various animals and the pandas consistently were cooler than other mammals (like a cow or dog).  Their internal body temperature would be considerably warmer because the fur helps insulate them, so they don’t have to spend as much energy on maintaining their body temperature.

2) Pandas are lazy.  No surprise: pandas spend more time inactive and when they do move, it is slowly.  So that is less energy needed for muscle contractions.

3) Pandas have small brains, livers and kidneys, so their organs need less energy.

4) Pandas have a low resting metabolic rate, which is driven by the thyroid hormones, T3 and T4.  In fact, levels of these two hormones were considerably lower than for other mammals of the same body mass, even lower than a hibernating bear.  The thyroid hormones regulate protein, carbohydrate and fat metabolism, as well as growth and development.  If the pandas don’t need to produce as much heat or energy, then there is no reason to have a high metabolic rate.

Interestingly, pandas have a single mutation in a gene called DUOX2, which is not found in any other mammals.  DUOX2 encodes for a protein that is necessary for the production of T3 and T4.  The mutation causes a premature “stop” in the protein, so it likely affects the function of DUOX2. 

In other words, pandas cannot synthesize T3 and T4 as well because of this mutation, so they have a reduced metabolic rate.  But that’s okay, because they are good at maintaining their body temperature and they have developed an enjoyable lifestyle of relaxing and eating.  The fact that their digestive tracts have not evolved for plant digestion is alright given the fact that they don’t really need that much energy from their food.  So it all works out: pandas are able to survive on their diet of bamboo and we can watch them sit around.

Bigger brains with Frizzled HARE

We have all heard that the sequence of human DNA differs from chimpanzee DNA by only about 1%.  Yet humans are capable of building complex civilizations while the chimps are still eating bugs in the forest.  If you compare the human brain to the brain of any other primate, it’s easy to see where our sophisticated cognitive abilities come from. 

From thebrain.mcgill.ca

DNA is the blueprint for making proteins, cells and organs, so is there something special hidden in that 1% sequence difference that gives humans bigger brains?  In particular, scientists have focused on regions in the human genome that have undergone rapid sequence changes in the human lineage, but not in other primates.  Besides looking for differences in genes that make proteins, we can also look for changes in regulatory regions, like enhancers, that control when and where the genes are expressed.

A recent paper in Current Biology by Boyd et al. explores these questions by studying a human-accelerated regulatory enhancer (HARE5), which differs significantly between humans and chimps.

Enhancer activity of HARE5

How do you study enhancers?  One way is to use a reporter gene.  Enhancers drive expression of nearby genes, so what if you swapped out a nearby gene and replaced it with a gene for a fluorescent protein?  Then you can look at your organism and wherever you see the fluorescent protein, the enhancer is active, meaning that the normal “nearby gene” is normally expressed in those cells.  Instead of doing these experiments with chimps and humans, which would take forever and be unethical in some cases, the authors put these reporter constructs into mice.  The enhancers from the chimps and humans drove expression of the reporter gene in the embryonic mouse brains.  The gene adjacent to the human enhancer was expressed earlier in development and more strongly than when placed next to the chimp enhancer (in other words, a lot more protein is being made).

Reporter gene experiment.  The mouse brain images are actual results from Figure 2 in Boyd et al. (2015).

This tells us that whatever normal gene is near HARE5, it is probably expressed earlier and way more in humans than in chimps.  There are just 10 sequence differences in the human HARE5 (i.e. mutations), which is enough to affect the way the enhancer functions and activates expression of genes. 

Frizzled expression is regulated by HARE5

So which genes are near the HARE5 sequence?  The closest gene is called Frizzled 8 and it is a receptor that responds to signals sent by other cells.  Frizzled 8 (FZD8) is a well known component of the Wnt signaling pathway that regulates many aspects of embryonic development, including neurogenesis (formation of new neurons).  The authors demonstrate that the mouse HARE5 physically interacts with Fzd8, which is a necessary  first step of gene expression, so Fzd8 is likely affected by the HARE5 sequence differences in humans and chimps.

The authors wanted to see what would happen to development of the mouse brain when Fzd8 is expressed in the same pattern as in humans or chimps.  They repeated the earlier experiments, but this time instead of using a reporter gene, they put the mouse Fzd8 gene next to the chimp or human HARE5 sequence.  They injected these DNA constructs into mice and waited to see what would happen to embryonic brain development.  When the chimp-HARE5 was driving expression of Fzd8, not much changed in terms of mouse brain development.  However, when the human-HARE5 sequence was activating the mouse Fzd8 gene, the mouse brain grew 12% bigger!! 

Let me be clear here-- they are not expressing the human Fzd gene in mice.  No, they are using the human enhancer to drive expression of the mouse Fzd8 gene, so presumably it is expressed more and earlier in development (like they saw in the reporter gene experiment).  The neural progenitor cells (pre-neurons) divided faster than in a normal mouse, leading to formation of more neurons, and a bigger brain! 

10 sequence changes in an enhancer may be one reason why I am able to write and you are able to read and understand this blog.  Whoa.  No news yet about whether these mice with bigger brains are also able to read and write… I’m sure they’re saving that for another paper.

I should say too, that there are probably a number of other similar changes to other enhancers and genes that all led to the rapid development of the big ol’ human brain.

Here's another blogger's take on this paper