Newborn octopus neurons steadily march towards maturity from around the eyes into the brain

Anyone who watched the movie Arrival would not miss the conspicuous resemblance of the alien Heptapods to some of our own earthly beings – the octopuses. While serving as inspiration for alien creatures in movies, as clairvoyants in soccer World Cup or as savages in classic science fiction and mythology, cephalopods like octopuses and squids have been dubbed as one of the most intelligent creatures on the planet. There is a good reason for why cephalopods, particularly octopuses, have developed such a reputation. Octopuses have a striking organization of brain structure, different from that of any other studied organism. They have the largest nervous system among animals lacking a backbone, comprising a total of nine “brains”. Out of these, one is a major donut-shaped brain that contains ~200 million neurons surrounding the octopus’s food pipe, which is strangely located in the head! This brain communicates behavioral intricacies to the eight so-called mini-brains located within the arms, each containing ~40 million neurons. The central brain is responsible for executing complex behaviors like tool use, ability to plan for the future, shape-shift, camouflage, recognize individuals and solve complex puzzles. While the last common ancestor between octopuses and humans was about 680 million years ago, a recent surprising discovery showed that they both evolved to use the same molecules during development. Scientists discovered that genes that produced the camera-like eye in humans are the same ones that gave rise to the camera-like eye in octopuses. What’s more, these cephalopods have evolved complex brains that show behavioral innovation on par with a small primate.

Comparison between the number of neurons present in the octopus and human brains. The octopus has one major brain and eight “mini-brains” while humans have neurons in the head and the spinal cord.

Despite the potential that the octopus provides for understanding developmental biology, particularly the brain, the molecules that dictate how the mollusk’s brain is built are unknown. The common octopus, Octopus vulgaris, is specifically suited to address this question because it can produce thousands of small and transparent eggs in a single batch and scientists have recently mapped out most of its genes. Using O. vulgaris in studies led by Dr. Astrid Deryckere from Dr. Eve Seuntjens’s lab at KU Leuven in Belgium, the group set out to unravel these molecular mysteries. “If you would think of cephalopods as the primates of the sea, that have evolved a complex nervous system from a far more simple ancestral nervous system, surprisingly little is understood on the morphological and molecular mechanisms driving its development.”, said Dr. Deryckere. They approached the problem in two studies. In the first study, she established a system for controlled embryonic development, which enabled her to care for thousands of eggs without the need for the mother octopus. She used state of the art microscopy and recorded high-resolution images of octopus development from fertilization through hatching. This work from Dr. Deryckere and colleagues can now be used as an elaborate reference for cephalopod embryology. 

O. vulgaris hatchling imaged in 3D at high resolution after labelling DNA (cyan). The same embryo was imaged from different orientations: back view (left), side view (middle) and front view (right). The head is located on top and the arms are at the bottom. Images were produced by Dr. Astrid Deryckere in the lab of Dr. Eve Seuntjens, KU Leuven, Belgium.

In the second study, Dr. Deryckere dug deep into the origins of the octopus brain. She used precise staging to track the precursor cells of neurons, or “neuronal progenitors”, that generate specialized neurons. Intriguingly, these progenitor cells appeared in structures called the lateral lips, that are unrelated to the brain and are located around the eyes. So, it appeared that neurons were first born in these structures and eventually migrated into the central brain – a possibility that prompted the authors to investigate further. They found that hundreds of thousands of neurons were created within octopus embryos even before hatching. To find out what genes are required for this unique way of making neurons, Dr. Deryckere used molecular markers and showed for the first time that newborn neurons travel long-distances to reach their final location in the brain. The results showed that the genes were expressed in the same order that vertebrates like humans do. By closely observing entire embryos in three dimensions during their growth, she found that neurons proceed through maturation while migrating from the lateral lips to an intermediate transition zone that finally leads to the brain. 

Schematic of O. vulgaris embryo, indicating the location of the lateral lip in relation to the food pipe and the eye. Schematic adapted from images in Deryckere et al., 2021.

Using detailed molecular studies, the scientists now show support for the lateral lips harboring newly dividing neuronal cells in the embryo. This is unusual because unlike in human brains and many other organisms, the dividing cells are located outside the central brain. These dividing cells then unwaveringly make their way towards the final destination to maturity in the octopus central brain. “The migration is especially exceptional for invertebrates where neurons usually migrate only a few cell lengths”, noted Dr. Deryckere about the rarity of the observation.

This unique development in the octopus head and its interesting age-dependent arrangement of dividing cells and mature neurons only inspires further reverence for the cephalopod. While it continues to influence characters in pop culture, the glorious octopus and its brain hold even more promise in the real world. The octopus brain’s cognitive ability has galvanized a new age of artificial intelligence, leading to the construction of flexible robotics and prosthetics, but at the same time, is pushing scientists and philosophers to tackle the important question of how an intelligent life form is defined.

Dr. Astrid Deryckere is currently a postdoc in the lab of Maria Tosches in the Department of Biological Sciences at Columbia University. Her focus remains on brain development but she has transitioned to working on an animal with a backbone – the salamander.

New Technology allowing gene switch to study multiple sclerosis

Our genetic blueprint consists of thousands of genes (more than 30,000) with new genes being discovered and added to the growing list. Our genes provide DNA instructions to the protein-making machinery in our bodies. These instructions can influence our health and dictate if we will get debilitating diseases. Have you ever wondered how scientists unlock which genes are responsible for what? For example, does gene A control our hair colour or gene B dictates if we will develop an autoimmune disease such as multiple sclerosis? The answer lies in DNA recombination technology which allows scientists to delete, invert or replace DNA instructions. The technology called Cre-lox recombination relies on the use of an enzyme called Cre recombinase which can bind, cut and recombine DNA at specific sites that are inserted in pairs in the DNA. The Cre-binding site in DNA is called the LoxP sequence that consists of 34 nucleotides DNA sequence made up of two inverted repeats separated by a spacer.  Cre enzymes can recognize these LoxP sequences and edit the stretch of DNA resulting in gene deletion or inversion.

In a recent research article, Dr. Olaya Fernandez Gayol and colleagues use an advanced version of Cre-lox technology called DIO (Double Floxed Inverted Open reading frame) to understand the role of the Interleukin-6 (IL-6) gene in multiple sclerosis (MS). MS is a chronic disease of the brain and spinal cord in which our immune system eats away the myelin sheath around nerves disrupting the communication between the brain and the body. IL-6 is a proinflammatory cytokine known to promote MS. Gayol et al use an experimental mouse model of MS which acutely develops brain inflammation called encephalitis (Encephalo- “the brain” + itis “inflammation”) within 3 weeks of disease induction. This mouse is referred to as EAE (Experimental Acute Encephalomyelitis) which closely mimics human MS disease.  

Scientists have conventionally studied the role of IL6 in EAE mice by irreversibly deleting the IL6 gene in one cell type. However, the results were confounding due to the compensatory expression of IL6 from other cell types. Gayol et al circumvent this problem by wiping out IL6 from all the cells and then recover IL-6 expression specifically in the microglial cells. It is akin to entering a dark room and turning ON a light switch at one corner of the room to clearly see what’s lying there. 

Figure 1.  Cartoon depicting the genetic strategy used by Goyal et al to recover IL6 gene expression exclusively in microglial cells in the mouse brain. Created with Biorender.com.

Olaya and the team use the cutting edge DIO method to wipe out IL6 and introduce the inverted form of the IL6 gene which makes this gene non-functional (Figure 1A). This inverted form of the IL6 gene does not produce IL-6 protein and mice carrying the inverted IL-6 gene (referred to as IL6-DIO-KO) are healthy (Figure 1A). As shown in figure 1B, Cre mediated recombination flips the IL6 gene in the correct orientation to make it active. The IL6 gene flipping occurs exclusively in the microglial cells and only upon treatment of mice with tamoxifen (TAM) drug. Mice in which IL-6 expression is active (referred to as IL6-DIO-ON) develop EAE disease (Figure 1B).

The team carefully optimized the duration of tamoxifen treatment in mice. Just 5 days of TAM did not flip the IL6 gene, so they extended the drug treatment to 11 days and found the IL6 gene turned on in all IL6-DIO-ON mice. Olaya says it is important to validate when creating new mouse models. “We used EAE to validate the mouse because it was a model readily available in our lab and IL6KO [deficient] mice happen to be completely resistant to the disease.” Their interesting finding that IL6-DIO-ON with IL6 gene active exclusively in microglia indicate that IL6 made in the brain promotes disease in the EAE mouse model. 

As compared to more traditional methods of generating gene mutation which requires extensive mice breedings or continuous drug treatment, the strategy presented by Olaya and colleagues is labour and cost-effective. Their findings showed that in the absence of IL-6, EAE disease does not develop in mice. On the other hand, turning on the IL-6 gene (like a gene-switch) using DIO technology, mice develop the disease.  Overall, this technology is highly customizable to understand the role of different genes in specific cell types in the disease context. It paves the way to gain a deeper insight and more thorough analysis of different molecular blocks involved in disease.

 

Dr. Olaya Fernandez Gayol is a postdoctoral research scientist in the Department of Pediatrics and co-president of Columbia University Postdoc Society(CUPS).  She also manages the CUPS Press office that provides postdocs with a platform to publicize their science while improving their science communication skills.