The structure of human cells points the way towards left- and right-handedness

There is new evidence linking left- or right-handedness to microtubule formation in human cells. Microtubules are protein filaments that form the “framework” of the cell. These filaments are also involved in cell division and transporting materials through the cell.

A team of brain researchers in Nijmegen led by Clyde Franks (Max Planck Institute Nijmegen) has now discovered that some rare variants in the microtubule gene (TUBB4B) occur more often in left-handed people than in right-handed people. Some variants even occur only in left-handed people. The research was carried out with the help of the UK Biobank, using DNA data from nearly 40,000 left-handed people and more than 310,000 right-handed people. It was published on Tuesday in Nature Communications.

The genetic variants that exist now do not play a major role in the inheritance of left-handedness. In an email clarification, Franks wrote that the variants found only occur in one in every thousand left-handed people. These findings confirm that there is contact with microtubules, and that is what is important. These associations with microtubules have also appeared in previous genetic research on left-handedness. in The largest genetic study to date (with 200,000 left-handers and 1.1 million right-handers) showed that at least eight out of 41 genes related to left-handedness were related to microtubules. This finding is also important for research into other differences between the two hemispheres. “Different development of the cerebral hemispheres begins early in embryonic development, and chance plays a major role in this, but the mechanism behind this is still unknown,” says Franks.

Crossed nerves

Hand preference is the most well-known example of brain asymmetry. In left-handed people, the right hemisphere of the brain dominates hand control, while in right-handed people, the left hemisphere dominates. (This reflex occurs due to the intersection of nerves between the brain and the rest of the body’s organs, except for hearing.) Worldwide, about 10% of people are left-handed, and there aren't many differences between cultures (at least if we take into account things like forced right-handed writing, which was also common in the Netherlands for a long time). ). There is a relationship between autism, Parkinson's disease and schizophrenia, for example, which is slightly more common in left-handed people. Most left-handed people are perfectly healthy, but if there were no disadvantage to being left-handed, half the population would be expected to be left-handed, one researcher recently wrote. In a major review. “In some sports — like tennis — that can be an advantage, because the opponent is used to using the opponent's right hand,” Franks says. “There are stories that left-handed people are less or more creative or skilled than right-handed people, but I I haven't seen any real evidence of this.

The difference in hand preference is not very genetic. Twin research shows that approximately 25 percent of the difference in left- and right-handedness must have a genetic cause. Differences in left-right development in humans originate in the early embryo. In the 10th week of pregnancy, prenatal ultrasound scans show whether the baby prefers to move his left or right arm. Franks: “Right-handedness is standard development, which is genetically controlled. We believe that most left-handed people owe their preference for the hand to random variations during embryonic development, due to random fluctuations of certain substances at certain stages of brain development.

Interconnected organelles

It is still not clear how microtubules, those “framework proteins” of the cell, affect left-right growth, but their effect on asymmetry within the cell may play a role. This left-to-right division of the cell can in turn affect the composition of the tissue of which the cell is a part. The fact that microtubules in the cell are attached to cilia may also play a role. These are filamentous organelles located on the cell surface that can move extracellular fluid, which may influence asymmetry during fetal development.

It is not that left-handedness is irreversibly linked to all sorts of other symmetry differences in the body, although left-handed people have More variety In the distribution between left and right in the brain compared to those who use the right hand. For example, more often than right-handed people, the language center of left-handed people is not in the left hemisphere, but in the right hemisphere, but this is certainly not a norm.

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