In humans, brain and testes have a lot in common

Every organ in the human body is made up of specialized cell tissue that performs the necessary physiological functions. These tissues mainly have different compositions due to the specialization of the cells found there. However, some organs that at first glance have absolutely nothing in common turn out to unexpectedly share a large number of their proteins. This is particularly the case with the brain and the testes, whose cells function in a similar way.

The new research shows amazing similarities between the brain and the testes in humans. ” The brain and testes have the highest number of common proteins, compared to other tissues in the human body », Writes the team led by biomedical researcher Bárbara Matos from the University of Aveiro in Portugal. While the brain has a very complex role, the human testes only have two main functions: the production of sperm and hormones.

Previous studies have suggested that there are links between sexual dysfunction and brain disorders, and even between intelligence and sperm quality. Of course, such links don’t mean much like that, but the team of researchers from Portugal and the UK have found an explanation for their existence.

They compared proteins from 33 types of tissues, including the heart, intestine, cervix, ovaries and placenta, and found that the testes and the brain share 13,442 proteins in common. This is corroborated by gene expression studies showing that these two distantly located organs share the highest number of genes among all organs in the body.

Cells with relatively similar functions

Taking a closer look at the shared proteins most highly expressed in these tissues, Matos and his colleagues found that they are primarily involved in tissue development and cellular communication. These shared proteins make sense when you consider how similar the two tissues are, the team explains.

The brain and testes both use a lot of energy to power very demanding processes like thinking and producing several million sperm per day. Thus, both organs have specialized cells to support neurons in the brain and germ cells in the testes; they contribute to their energy supply and provide structural support.

Diagram showing the number of proteins specific to the brain (blue), testes (light green) and common to both (dark green). © Bárbara Matos et al. 2021

Additionally, although these are cells for very different purposes, neurons function in many ways, such as sperm. Both cells have the important job of moving things from inside themselves to their outside environment – a process called exocytosis.

This is how brain cells transmit neurotransmitters to each other. In semen, the same process is used to release important fertilization factors. In neurons, exocytosis is also involved in the growth of branched extensions collectively reaching neurites (dendrites and axons), while in sperm, this process allows complete fusion with an egg.

The potential role of selection pressure

This is an under-explored subject, and the connection between these tissues needs to be clarified, which could help to understand the dysfunctions affecting the brain and testes. », Writes the team. These findings raise many questions, the most obvious being how did two such disparate organs come to share so much in common? Researchers suspect this is because they are both heavily influenced by the process of speciation.

Just as animals separated by millions of years of evolution can develop the same traits, so do different tissue groups within the human body. For example, unlike most other animals, koalas have confusingly similar fingerprints to ours – thanks to the obvious selection pressure of the need to grab trees – despite 70 million years of evolution between us. This process is called convergent evolution.

In this case, the researchers propose that the same selection pressures involved in maintaining species distinct from each other can be imposed on the two organs, causing them to evolve in a convergent fashion. They point to 60 genes encoding proteins, unique to humans, many of which are found in the brain and testes. ” The highest expression levels in the cerebral cortex and testes suggest that these genes may contribute phenotypic characteristics exclusive to humans, such as improved cognitive abilities. », Conclude the researchers.

Sources: Royal Society Open Biology

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