Cajal-Retzius (CR) cells are transient neurons that control cortical lamination during development. Although most CR cells disappear before birth, a small population persists in the hippocampus postnatally for several months. In a new study, Giulia Quattrocolo and colleagues investigate the role of postnatal CR cells in establishing the hippocampal network. To find out more about the story, we caught up with first authors Ingvild Lynneberg Glærum and Keagan Dunville, and corresponding author Giulia Quattrocolo, Associate Professor at Norwegian University of Science and Technology.

From left to right: Keagan Dunville, Giulia Quattrocolo and Ingvild Lynneberg Glærum

Giulia, can you give us your scientific biography and the questions your lab is trying to answer?

GQ: I started to be interested in developmental neuroscience during my Masters in neurobiology, at the University of Pavia, in Italy. It was during my PhD at Northwestern University, in Chicago, that I discovered Cajal-Retzius cells, in Gianmaria Maccaferri's lab, and I started my long, complicated relationship with these cells. I then had two postdoctoral experiences, the first at NYU with Gord Fishell, and the second with Edvard Moser, at the Kavli Institute for Systems Neuroscience (KISN), at the NTNU in Norway. Since 2021, I have been heading the Circuit Development group at the KISN, where we focus on understanding the role of different cell types in the development of the circuits in the hippocampal formation.

Ingvild and Keagan, how did you come to work in Giulia's lab and what drives your research today?

ILG: I did my Masters in neuroscience with Giulia in her transition period from post-doc to PI and found that I liked the project and that we worked very well together. Therefore, I decided to stay on to do my PhD with her. Those early days are in many ways what shaped this paper, originating the questions we tried to answer, with new lab members joining and contributing with interesting ideas. I like working on a project like this, as development is a good framework for understanding the emerging properties of the brain.

KD: When Giulia started her lab, I was finishing my PhD at Scuola Normale Superiore in Italy, and I started following her on Twitter (now called X) after finding some of her previous work on Rbfox2. She posted an advertisement for a postdoc position in single-cell transcriptomic work. Since my research focuses on genetic drivers of hippocampal formation development, I thought the opportunity was too good to pass up, so I applied, and the rest is history!

What was known about the role of CR cells in hippocampal development before your work?

GQ: Not much. We knew they are integrated in the hippocampal network, and some very recent works have shown that manipulating the output of CR cells or the survival of CR cells can affect spine and dendritic development, as well as induce behavioural deficits. But our question has always been a bit different. All the studies so far have used manipulation not restricted to the hippocampus or to postnatal development, while we really wanted to concentrate on their role in the postnatal hippocampus. This is why we spent several years narrowing down the strategy we used in this paper to manipulate CR cell survival.

Can you give us the key results of the paper in a paragraph?

GQ: After specifically ablating CR cells from the postnatal hippocampus, we found alterations in dendrite growth and a spine reduction during development. We further confirmed synaptic loss through RNA sequencing and proteomics. This led to the identification of an interesting expression pattern of latrophilin 2, a synaptic anchoring protein known to regulate perforant axon pathfinding and to mitigate synaptic anchoring. At P15, latrophilin 2 is upregulated in the CR cell-ablated mouse but is then downregulated at P30, matching the developmental trajectory of thin spines in the experimental group. Because the major postsynaptic components were lost here, it is possible that disruption to the CR cell network leads to disrupted integration of perforant path axons to the entorhinal-hippocampal circuit.

Were you surprised to find from your transcriptomics and proteomics analysis that latrophilin 2 is significantly affected after the ablation of CR cells?

GQ: Yes and no. It was surprising that we had such a clear result which was easy to replicate with other approaches, such as immunohistochemistry and qRT-PCR. However, we suspected that we had an alteration in the entorhinal-hippocampal pathway, as previous literature had suggested that CR cells could contribute to the formation of this connection.

How do you think CR cells that persist postnatally play a role in hippocampal circuit development?

GQ: At this point it is really hard to understand the mechanisms involved in the deficits we observe. CR cells release both reelin and glutamate, and both molecules have a critical role in plasticity and development. We expect CR cells to regulate multiple pathways, so their removal will severely affect them, as well as other downstream pathways. What is certain is that CR cells are important for the postnatal maturation of the hippocampal circuit, so we need to keep them in mind when we study the development of the hippocampal formation, in physiological and pathophysiological conditions.

A coronal section of the hippocampus of a Pde1c-Cre;flox-tdTomato P30 mouse. NeuN (blue) labels the nuclei of all neurons, reelin (green) labels different types of interneurons and Cajal-Retzius cells, and tdTomato (red) is specifically expressed in Cajal-Retzius cells.

A coronal section of the hippocampus of a Pde1c-Cre;flox-tdTomato P30 mouse. NeuN (blue) labels the nuclei of all neurons, reelin (green) labels different types of interneurons and Cajal-Retzius cells, and tdTomato (red) is specifically expressed in Cajal-Retzius cells.

Ingvild and Keagan, was there any particular result or eureka moment that has stuck with you?

ILG: I remember the first time I ran the spine analysis during my Masters. It was the first result I ever acquired, and it was significant. I remember crying out ‘Yelp’ and contacting Giulia right away. It definitely was a moment that sparked a scientific drive and curiosity to understand what is going on.

KD: For me, the latrophilin 2 results were serendipitous as Giulia had actually just finished reading a paper from the Südhof laboratory about latrophilin 2 specificity for CA1-entorhinal trans-synaptic anchoring. So when I checked it in our dataset and it was downregulated, the rest of the experiments for the paper felt very clear. That result was also coupled with getting to mentor an incredibly promising young scientist, Hinako Kirikae, on how to perform qRT-PCR, which she then used to corroborate the latrophilin 2 expression pattern in Fig. 8. It was a rewarding paper overall that I'm grateful to have contributed to.

It definitely was a moment that sparked a scientific drive and curiosity to understand what is going on

How about the flipside: any moments of frustration or despair?

ILG: There definitely are a lot of frustrating moments when working with transgenic animals. Mostly relating to extra rounds of experiments and waiting because you either don't get the right genetic distribution or, when you do, you missed the target with the injection. Which, of course, we don't know until we finish the experiment and extract the brain. But that makes it all the more satisfying when things work out.

KD: I think any bioinformatician will tell you that the perceived limitlessness of analysing a dataset can be daunting and overwhelming. So the prospect of starting to sift through the RNA-Seq data and ensuring high-quality analyses was scary. I will say though, in general, we have a good team, and we are all able to rely on each other when we get stuck, so the despair and frustration don't last too long.

Why did you choose to submit your paper to Development?

GQ: This was a bit of a tactical choice. I am a young PI and I am really interested in increasing my visibility in the developmental neuroscience community. Development is a great journal publishing really solid work in this field, so we felt our story could fit well here and be visible to the community we are interested in reaching. I knew that having my first senior author paper published in Development would give me confidence and show that we do good science. I discussed about this with Ingvild, Keagan and the other authors, and we all agreed Development would be a perfect fit. So we are really happy we were right!

Ingvild and Keagan, what's next for you after the paper?

ILG: I will be continuing to work on CR cells, but from a behavioural and electrophysiological perspective. Looking forward, I am starting to look for post-doctoral positions with the primary interest of expanding my experience in electrophysiology. This will be relevant for 2025 and onwards.

KD: On the project side, I'll be finishing up my primary study in the lab where we have been looking at what's genetically regulating medial entorhinal cortex formation in the developing mouse brain. On the professional side, I'll be taking the next steps to start my own neurodevelopmental laboratory with a focus on transcription factor divergence across cortical evolution.

Where will this story take your lab next, Giulia?

GQ: As all the good scientific works should, I feel this story gave us more questions than answers! We are following up in different directions, actually. We are looking at the role of these cells in hippocampal function and what circuits they are involved in, and then we are really interested in digging into the mechanisms leading to all the observed deficits. I think this will be my lifelong quest, but I hope with every experiment we can get closer to understanding what CR cells do!

As all the good scientific works should, I feel this story gave us more questions than answers!

Finally, let's move outside the lab: what do you like to do in your spare time?

GQ: I love to travel with my husband and our son. My husband tells me I am always planning a vacation, which is actually true! I also love reading about history, especially from the Middle Ages.

ILG: I like to spend time with family and friends, but I also enjoy reading and training, specifically strength training, cycling and skiing.

KD: I like to paint, play tabletop games with my friends, travel with my wife, skateboard, cook and read horror novels.