Knowing Neurons
Brain DevelopmentCognitionNeuroscience Technologies

If You Give a Mouse a Cookie … and Rat Olfactory Neurons

By Paige Nicklas

Think of some of the most famous superheroes we know and love like Spider-Man, Ant-Man, Catwoman, Black Panther, Wolverine, Batman, and the list could go on. These fantastical characters all possess qualities of another species that allow them to follow their superhuman pursuits.

This idea that a quality from one species can be transplanted into another species to create some sort of enhanced hybrid may seem like it could only exist in comic books and whimsical stories, but a study recently published in Cell by a team of scientists has shown for the first time that functional hybrid brains are possible. The goal of this project was rooted in regenerative neuroscience, and investigated if neurons from two different species could work together in the same brain. To do this, the authors used rat stem cells in mice and tested whether the rat neurons in the olfactory, or smell, sensory system would still let the mice smell and perform their normal odor-driven behaviors like seeking food. (Throesch et al., 2024).

… a study recently published in Cell by a team of scientists has shown for the first time that functional hybrid brains are possible

The ultimate goal of regenerative neuroscience is not to create superheroes, but to create tools that allow scientists and medical professionals to form synthetic, functional neural circuits in brains that have lost some functions due to disease or injury. For example, in Parkinson’s Disease, the loss of dopaminergic neurons in the substantia nigra of the brain leads to the characteristic motor symptoms  (Lo Bianco et al,. 2002). Hybrid brains will provide researchers with enhanced insights into the mechanisms underlying the deterioration and death of brain cells, as well as a better understanding of how we may repair or replace neurons in people with Parkinson’s, epilepsy, and other conditions.

In order to accomplish this, researchers need to initially understand the flexibility, also known as plasticity, of the brain and its ability to adapt to various inputs, such as those from human-machine interfaces or transplanted stem cells (Wu et al., 2017). The current study involved injecting pluripotent stem cells (PSCs), or stem cells that can develop into cells of all tissues of the body (Reik & Surani, 2015; Romito & Cobellis, 2016), from one species into the developing blastocyst of another species, in a method called “blastocyst complementation”. Rat PSCs were injected into the host species, mouse blastocysts, and allowed to develop into brain cells in synchrony with and form active synaptic connections with mouse neurons resulting in a chimera brain, with more than one distinct genotype (Reik & Surani, 2015).

The current study involved injecting pluripotent stem cells (PSCs), or stem cells that can develop into cells of all tissues of the body, from one species into the developing blastocyst of another species.

The researchers analyzed results across different levels: anatomically via whole brain imaging, developmentally by tracking the rat cells with mouse neural brains, functionally with electrophysiology and synapse characterization, and finally, behaviorally via a cookie-finding task.

ANATOMY

The team first wanted to see where the rat neural cells ended up in the brain after development. They used a fluorescent protein that glows red-orange under a microscope to pinpoint the rat neural cells that were derived from the rat PSCs. They found that rat cells were present in various brain regions, including those involved in smell, memory, movement, and coordination, although the amount and distribution of rat cells varied across different brain regions and even between the left and right sides of the same brain. They also showed that the rat PSCs contributed to the development of non-neural cells, astrocytes and microglia, too. Further, some areas of the brain were more receptive to rat cells than others. However, the exact distribution varied from chimera to chimera.

DEVELOPMENT

One key difference in the brains of different species is the exact timing of when different types of brain cells develop. In rats, certain types of brain cells mature later than in mice and the entire process takes longer because rats have more complex brains (Throesch et al., 2024).  This difference is important when considering using brain models from different species for research like this, and it is necessary to understand how cells from one species react to being in the brain environment of another.

The researchers wanted to see if rat brain cells, growing alongside mouse brain cells from early in development, would adopt the timing of mouse brain development or if they would stick to their own timing. To test this idea, they marked the “birth dates” of rat brain cells and used another fluorescent marker to track developing cells during days when rats and mice show distinct developmental timings. Knowing that the cortical layers of the brain develop from deeper to superficial, the researchers could see which cells were in which layers as development progressed. 

This adjustment suggests that the environment of the mouse brain is capable of influencing the development of the transplanted rat cells

They found that, regardless of when the rat cells were born, they integrated into the mouse brain and then followed the mouse’s developmental timeline as they ended up in the same layers of the brain as mouse cells born on the same day. This adjustment suggests that the environment of the mouse brain is capable of influencing the development of the transplanted rat cells, demonstrating that the host environment plays a significant role in shaping the development of foreign cells, even from the earliest stages. This aligns with findings from other studies involving mixed-species organs, where the host’s environment influences the development of the combined organ (Oberst et al., 2019; Kobayashi et al., 2010).

FUNCTION

The authors then wanted to show that the rat neural cells were actually functional within the mouse brain, and utilized optogenetics, a method that allows cells to be activated with light to release electrical signals, to target the rat neural cells. When they stimulated mouse and rat neurons with light, they observed the expected electrical responses, meaning both mouse and rat neurons functioned normally. When they stimulated the rat neurons after administering certain drugs that block signals in mouse neurons, they were not affected. This suggests that rat neurons can indeed form functional connections with mouse neurons in the brain, even though they come from different species and retain species specific qualities.

The next experiment asked if the rat neurons being able to form functional connections in a healthy brain would still be effective in a brain that had damaged circuits, specifically in the olfactory system. In the olfactory system, sensory information begins at specialized cells called olfactory sensory neurons (OSNs) located in the nose. These neurons project their axons to specific regions in the olfactory bulb depending on which odor they detect (Glezer & Malnic, 2019). The researchers created mouse models where OSNs were either genetically disabled (Silence) or killed (Ablate) to simulate conditions where neurons are dysfunctional and examined the rat OSNs’ ability to restore olfactory circuit anatomy and function using these two models. In the Ablate model, mouse OSNs are missing and rat cells contributed to nearly all cells in the olfactory epithelium. In the Silence model, rat neurons were mixed with silenced, but still present, mouse neurons. 

When examining the communication between rat OSNs and circuits within the olfactory bulb, the authors observed that the expression of tyrosine hydroxylase (TH), a marker of synaptic activity, in inhibitory neurons surrounding glomeruli was strongly rescued in the Silence model and not in the Ablate model. This indicated neurons at these synapses were not actively communicating with each other. 

… despite evolutionary differences, rat OSNs maintain enough plasticity to integrate into and partly restore the structure of the olfactory circuit when the mouse neurons were silenced, but still present.

These findings demonstrate that despite evolutionary differences, rat OSNs maintain enough plasticity to integrate into and partly restore the structure of the olfactory circuit when the mouse neurons were silenced, but still present. More research is needed to fully understand this, but it is possible that silenced mouse OSNs were still able to provide some sort of cues to the rat neurons, allowing them to rescue the olfactory circuitry.

BEHAVIOR

Finally, the authors conducted the buried cookie test (Machado et al., 2018) to characterize these chimeras at the highest level of functioning, behavioral performance. This involved placing the animals in a space with typical rodent cage bedding, with a cookie (specifically, an Oreo mini) buried under the bedding in a random spot and measuring how quickly the animals are able to smell and uncover the piece of food. This food-seeking behavior is natural for mice, but relies on a functioning olfactory system.

Surprisingly, rat OSNs successfully restored food-seeking behavior in the Ablate models but not in the Silence models. The unexpected difference in rescue capability between the Ablate and Silence models suggests that there may be competition from resident mouse OSNs in the Silence model. Even though they are dysfunctional, their mere presence might interfere with rat sensory signals getting from the olfactory circuitry to cortical circuits. Nonetheless, other factors could also contribute to this observation and more research is needed.

This study is the first to show an animal successfully using sensory system structures of another species.

CONCLUSION

This study is the first to show an animal successfully using sensory system structures of another species. While some of the results need further experimentation to fully understand the mechanisms behind them, such as the different distribution of rat neurons in different chimeras or the divergent results of the Ablate and Silence models depending on if the assessment is of behavior or synaptic connectivity, it does create a myriad of new potential avenues of research. It also serves as a major step forward in developing approaches for researching regenerative medicine. This work paves the way for researchers to begin meticulously analyzing these processes across various species and behaviors and holds promise for enhancing the effectiveness of human cell transplantation and advancing insights into degenerative neurological diseases and disorders.

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Written by Paige Nicklas
Illustrated by Johanna Mayer
Edited by Liza Chartampila, Faustina J, & May Rudd

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References

Glezer, I., & Malnic, B. (2019). Olfactory receptor function. Handbook of clinical neurology, 164, 67–78. https://doi.org/10.1016/B978-0-444-63855-7.00005-8

Kobayashi, T., Yamaguchi, T., Hamanaka, S., Kato-Itoh, M., Yamazaki, Y., Ibata, M., Sato, H., Lee, Y. S., Usui, J., Knisely, A. S., Hirabayashi, M., & Nakauchi, H. (2010). Generation of rat pancreas in mouse by interspecific blastocyst injection of pluripotent stem cells. Cell, 142(5), 787–799. 

Lo Bianco, C., Ridet, J. L., Schneider, B. L., Deglon, N., & Aebischer, P. (2002). alpha -Synucleinopathy and selective dopaminergic neuron loss in a rat lentiviral-based model of Parkinson’s disease. Proceedings of the National Academy of Sciences of the United States of America, 99(16), 10813–10818. https://doi.org/10.1073/pnas.152339799

Machado, C. F., Reis-Silva, T. M., Lyra, C. S., Felicio, L. F., & Malnic, B. (2018). Buried Food-seeking Test for the Assessment of Olfactory Detection in Mice. Bio-protocol, 8(12), e2897. 

Oberst, P., Fièvre, S., Baumann, N., Concetti, C., Bartolini, G., & Jabaudon, D. (2019). Temporal plasticity of apical progenitors in the developing mouse neocortex. Nature, 573(7774), 370–374. 

Reik, W., & Surani, M. A. (2015). Germline and Pluripotent Stem Cells. Cold Spring Harbor perspectives in biology, 7(11), a019422. 

Romito, A., & Cobellis, G. (2016). Pluripotent Stem Cells: Current Understanding and Future Directions. Stem cells international, 2016, 9451492

Throesch, B. T., bin Imtiaz, M. K., Muñoz-Castañeda, R., Sakurai, M., Hartzell, A. L., James, K. N., Rodriguez, A. R., Martin, G., Lippi, G., Kupriyanov, S., Wu, Z., Osten, P., Izpisua Belmonte, J. C., Wu, J., & Baldwin, K. K. (2024). Functional sensory circuits built from neurons of two species. Cell, 187(9), 2143-2157.

Wu, J., Platero-Luengo, A., Sakurai, M., Sugawara, A., Gil, M. A., Yamauchi, T., Suzuki, K., Bogliotti, Y. S., Cuello, C., Morales Valencia, M., Okumura, D., Luo, J., Vilariño, M., Parrilla, I., Soto, D. A., Martinez, C. A., Hishida, T., Sánchez-Bautista, S., Martinez-Martinez, M. L., Wang, H., … Izpisua Belmonte, J. C. (2017). Interspecies Chimerism with Mammalian Pluripotent Stem Cells. Cell, 168(3), 473–486.e15. https://doi.org/10.1016/j.cell.2016.12.036

Author

  • Paige Nicklas

    Paige is a PhD student at the University of Rochester School of Medicine and Dentistry, studying neuroscience. She has a BS in Psychology and an MS in Neuroscience, and is interested in researching the blend of these two disciplines. Her current research investigates cognitive-motor interactions in typically and neurodivergently developing children and young adults, exploring the impact of movement on cognitive performance in early life. Outside of the lab, she is passionate about expanding science education and communication for all, and encouraging public engagement with science. In her free time, she enjoys reading, drawing, and caring for her many houseplants.

Paige Nicklas

Paige is a PhD student at the University of Rochester School of Medicine and Dentistry, studying neuroscience. She has a BS in Psychology and an MS in Neuroscience, and is interested in researching the blend of these two disciplines. Her current research investigates cognitive-motor interactions in typically and neurodivergently developing children and young adults, exploring the impact of movement on cognitive performance in early life. Outside of the lab, she is passionate about expanding science education and communication for all, and encouraging public engagement with science. In her free time, she enjoys reading, drawing, and caring for her many houseplants.