Summary of CMS Seminar Club presentation on Friday, June 23, 2023
Title: Ascidians, our primitive chordate cousins in the sea, teach us about heart formation, the dopaminergic system, and more
Speaker: Prof. Yasunori Sasakura, Ph.D., Professor at the Faculty of Life and Environmental Sciences, University of Tsukuba, Japan, and Director of the Shimoda Marine Research Center, University of Tsukuba, Japan
On Friday, June 23, Prof. Sasakura gave a presentation at Fujita Health University. He told us about ascidians, which are primitive chordates living in the sea, and how they can help us to understand chordate evolution.
Recording: For members of Fujita University, a recording of the meeting (without the discussion part) will be available at our Manabi system. Unfortunately, we cannot open the recording for a wider audience.
There were 25 participants. Remarkable was the happy, relaxed, even “cosey” atmosphere. This atmosphere was greatly contributed by the cheerful, pleasant character of Prof. Sasakura, and the beautiful images of the mysterious underwater world of ascidians. Probably also a bit by us, as Prof. Sasakura himself mentioned in an e-mail “I really enjoyed talking and especially having discussions with many audiences thanks to the comfortable atmosphere of loving science.” His story was easy to follow, with well-crafted slides and clear explanations (please watch the recording if you have access to it). As for reactions afterward, I received very positive mails from Professors Shosuke Ito and Kazumasa Wakamatsu of FHU, who mentioned the nice atmosphere as well, and a partly joking mail from Dr. Jerzy (Yurek) Kulski saying “..it was shocking to hear that my head and face evolved from Ascidian underwater siphon tubes. I will have nightmares about this. It was a masterful presentation!”
I was also impressed with several of the reports written by graduate students about Prof. Sasakura’s presentation. Although from the medical field, they were highly appreciative of learning about body plans and other biological processes from an evolutionary perspective.
Quite special was also that tyrosine hydroxylase (TH), the gene promoter of which Prof. Sasakura used for marking dopamine neurons, was discovered by Distinguished Professor Emeritus Toshiharu Nagatsu of FHU (Nagatsu et al. 1964), who is a loyal visitor of our seminar series. Professor Nagatsu asked Professor Sasakura questions about this.
The contents of the presentation
The below only describes a summary and selection of the topics presented by Professor Sasakura.
What are Ascidians?
Adult ascidians look a bit like sponges but actually are a group of chordate species that are the closest relatives of the vertebrates. They live in salt water, include >2000 species, have relatively tough outer layers (“ascidian” means “shelled animal”), and in their adult stages they are sessile (examples in Fig. 1), filtering food from the water that they circulate through their bodies. The best-studied model ascidian is Ciona intestinalis (Fig. 1).
Their chordate features are more apparent in their “tadpole-like” juvenile stages, in which they can swim, and in which they have a notochord in their tail with dorsally thereof a neural tube (Fig. 2). These juveniles do not eat, and they only are looking for a proper substrate (a rock or so) to attach themselves to, after which they undergo an extensive metamorphosis.
Compared to vertebrates, they have a uniquely dense genome which makes it very convenient to investigate the regulatory elements of their ⁓16,000 genes. Namely, on average, they have one gene per ⁓10,000 bp.
Their body plan is also simple, and the juvenile (tadpole) body comprises <3,000 cells including only 40 notochord cells, 36 muscle cells, 177 neurons in the central nervous system (CNS), and 54 neurons in the peripheral nervous system (PNS). Therefore, cell fates and lineages have been well analyzed and cataloged.
Professor Sasakura established ascidians as a transgenic system and is actively supporting/promoting the use of ascidians as a model system in Japan. So, if you have any questions in this regard, please contact him.
Head formation processes shared between ascidians and humans; were our facial muscles used for pooping?
Ascidians are urochordates and are closer to vertebrates than cephalochordates (lancelet/amphioxus), and one of their uniquely shared features is a concentration of sensory organs in the head region (Fig. 3) (e.g., Eposito et al. 2014).
Also in muscle development, there is a remarkable similarity between ascidians and vertebrates in regard to head formation. Namely, in vertebrates, the same cell population (in the embryonic cardiopharyngeal field) gives rise to muscle cells of the heart and the head/face (the branchiomeric muscles). Ascidians also have a heart, although it has no chambers and the bloodstream direction occasionally changes (Fig. 4). By following cell development and screening for genetic markers, it was discovered that the same muscle cell population that in vertebrates gives rise to heart and facial muscles, in ascidians gives also rise to muscles in two distinct organs, namely the heart and the atrial syphon (Fig. 5) (Raji-Krajka et al. 2010; Stolfi et al. 2010; Diogo et al. 2015). The atrial siphon is the location where the circulated water and waste products go out, thus used for a kind of “pooping,” so the evolutionary origin of our face muscles is not so glorious. The atrial siphon itself (the epithelium, not the muscles) is believed to correspond to our otic placode (the precursor of the inner ear), thus also corresponds with a head structure (e.g., Kourakis and Smith, 2007).
Dopamine (DA) in animals is used as a secreted signaling molecule between neurons and overall tends to promote actions and is associated with learning behavior. Even in the nematode C. elegans, dopamine is believed to play a role in neural plasticity and learning (e.g., Voglis and Tavernarakis, 2008) .
In C. intestinalis juveniles, there is only one single cluster of dopamine neurons (Fig. 6), which is thought to be homologous to our hypothalamus. However, so far, the function of dopamine in ascidians is not known, and Professor Sasakura described that C. intestinalis juveniles seemed to behave the same way, regardless if their dopamine expression was normal, was blocked, or was enhanced. The location of their dopamine neurons is quite close to the pigmented sensory organs the ocellus (for light perception) and the otolith (a gravity-sensory organ), so maybe it is about learning about how to adjust muscle movements to depth (light and water pressure; ascidians tend to grow in shallow waters)?
Even though the function of dopamine in ascidians is not known, exciting regulatory mechanisms were found. Horie et al. 2018 identified, by whole-embryo single-cell RNA sequencing (RNA-seq), the transcription factor Ptf1a as the most strongly expressed cell-specific transcription factor (TF) in dopamine cells. This agreed with fluorescent marker expression studies (Fig. 6). If Ptf1a was knocked down, TH gene expression was also silenced (Fig. 7 picture on the right), meaning the absence of dopamine, and if Ptf1a was expressed in all neurons many more but not all neurons were converted to TH-positive (dopamine-producing) cells (Fig. 7 bottom picture).
Then, the authors found that also the transcription factor Meis showed an expression pattern that agreed well with specificity for dopamine-releasing cells, and tried combinations of Ptf1a and Meis manipulation. They showed that Meis is necessary for the generation of the dopamine-releasing cell type (not shown here), and that the simultaneous expression of both Ptf1a and Meis in all neurons converted almost all of them to TH-positive cells (Fig. 8). By TH promoter analysis, they could find binding sites for both Ptf1a and Meis, and they concluded that both transcription factors are important (Fig. 8). The authors then hypothesized that the regulatory “cocktail” of Ptf1a and Meis might also control the development of DA neuronal cell types in vertebrates (Horie et al. 2018).
Professor Sasakura also explained about evolutionary developments unique to ascidians. I will only briefly mention one of them, namely the acquisition of the ability to synthesize cellulose for the extracellular matrix that surrounds their body and is called a “tunic.” Although (some) plants, fungi, and bacteria are known to synthesize cellulose, in animals this is very unique and only found in ascidians. The C. intestinalis cellulose synthase gene, Ci-CesA (Dehal et al. 2002) probably was acquired by horizontal transfer from bacteria (Sasakura et al. 2005).
So, we learned that, as all “primitive” species, ascidians are not only “primitive humans” but also established their own features after they separated in evolution from our ancestors.
