Coordinated control of neuronal differentiation and wiring specificity by a sustained code of transcription factors

Mehmet Neşet Özel, PhD
Postdoctoral associate
Department of Biology
New York University
New York, NY, USA

This conference is part of the the IRCM Early-Career Scientist Seminar Series (ECS³), a groundbreaking initiative whose mission is to showcase early career scientists. This is a great opportunity to discover the exciting projects of these researchers in training in front of a multidisciplinary audience.

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In person: 
IRCM Auditorium
110, avenue des Pins O, H2W 1R7 Montreal
Wearing a mask is mandatory at all times

Online:
Zoom link : https://zoom.us/j/95269762104
ID : 952 6976 2104
Code : 476372

IRCM conferences are set to occur under a hybrid format. However, please note that last-minute changes to online-only lectures may occur due to unforeseen circumstances. We invite you to visit this webpage again a few days before attending.

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About the conference: 
Cascades of transcription factors (TF) that are transiently expressed in neural stem cells are responsible for generating the enormous diversity of cell types in nervous systems. However, the gene-regulatory mechanisms that establish and maintain these cell fates in postmitotic neurons and instruct their specific morphology, connectivity and physiology remain largely unclear. Using a large scRNA-seq atlas of the Drosophila optic lobes (Özel et al. 2020, Nature), we tracked ~200 neuronal types across 6 stages and found that each one stably expresses a unique combination of 95 TFs (~10 per cell type) that are maintained throughout development to adulthood. Through genetic gain- and loss-of-function experiments, we show that modification of these “selector” TF codes is sufficient to induce predictable switches of identity between various optic lobe neurons. For instance, pdm3 is necessary and sufficient to determine the fate choice between Tm2 and Tm4 neurons. In addition to their morphological identity, scRNA-seq of perturbed neurons revealed that such conversions are transcriptomically complete. Similarly, ectopic expression of Vsx genes in Mi15 neurons not only leads their morphological conversion to Dm2, but also a loss of their aminergic identity. Thus, continuously maintained TF codes that are activated in each neuron immediately after their birth function as top-level regulators of both the type-specific development and the adult terminal features of these neurons. Our results provide a unified framework of how specific fates are maintained in postmitotic neurons, and open up new avenues to understand neuronal development through gene regulatory networks.

About Mehmet Neşet Özel:
Dr. Neşet Özel obtained his PhD in Neuroscience at the University of Texas Southwestern Medical Centre. In 2018, he joined the team of Dr. Claude Desplan in the Department of Biology of New York University as a postdoctoral associate, where he studies neuronal diversity and circuitry and the transcriptional mechanisms that regulate them. Dr. Özel is the recipient of several prestigious prizes, including the NIH Pathway to Independence Award (2021-2026) and the Peter and Patricia Gruber International Research Award in Neuroscience (2021). His scientific discoveries have been published in top-level journals such as Nature and Developmental Cell.

Please tell us about your career path, leading up to your application to the ECS3 program:
I have been committed to a career in neuroscience since my early days as an undergraduate. Over the past 10 years as a PhD student and a postdoctoral fellow, my research has been focused on understanding how the genome encodes and instructs the formation of circuits containing thousands of different types of neurons connected in highly stereotyped patterns.

I started my PhD project by developing an imaging technique that enabled live monitoring of developing Drosophila brains at high-resolution and over long periods, which has since been widely adopted by fly labs around the world. Combining this technique with two-photon microscopy, I showed how molecules functioning as 'guidance cues' may in fact probabilistically affect stabilization through alteration of filopodial dynamics. I then explored the molecular and cellular mechanisms interlinking the axonal filopodial dynamics and synaptogenesis, and discovered a mechanism that allows a neuron to cell-autonomously make a specific number of synapses.

A fundamental gap in the field has been the mechanisms that translate distinct neuronal identities to differential cellular behaviors. Within the first two years of my postdoc in the Desplan lab, I generated a very large scRNA-seq atlas of the adult and developing neurons of the Drosophila optic lobes, accounting almost completely for their diversity. I developed a multitask machine learning framework to consistently classify these into over 200 cell types across all stages. Our annotation of this large resource revealed that a handful of neuronal types partition the visual circuits in dorsoventral axis with differential Wnt signaling, and led to the discovery of transient extrinsic neurons, which may be the invertebrate equivalent of the mammalian Cajal-Retzius cells. In addition, we showed that neurons display highest transcriptomic diversity during development; this is due to transient upregulation of cell-type specific cell-surface molecules that are involved in synapse formation, which explains how neurons with indistinguishable transcriptomes in adult brains could nevertheless have different connectivity and serve different functions as a result of their developmental history.

Having tracked all neuronal clusters throughout development in my scRNA-seq datasets enabled me to define the code of ‘selector’ transcription factors (TFs) across the entire visual circuit: I found that each neuron stably expresses a unique combination of 95 TFs (~10 per cell type) that are maintained throughout development to adulthood. Through genetic gain- and loss-of-function experiments, I have shown that modification of these selector TF codes is sufficient to induce predictable switches of identity between various optic lobe neurons, providing a unified framework of how specific fates are maintained in postmitotic neurons.

Please tell us about your passion for research. What motivates you most about your work?
I knew I wanted to be a neuroscientist as early as when I was 15. In high school, I was competing at the 'Biology Olympiads' that exposed me to college-level biology early on. In fact, Canada was the first foreign country I ever visited when I went to Saskatoon in 2007 to compete in the International Biology Olympiads as part of the Turkish national team. During that time, I became fascinated with the sheer complexity of the brain and how much we don't understand it. Frankly, nothing else seemed to be worth studying in comparison; and I still feel the same way.

I am a strong proponent of basic research. For sure, the taxpayers fund our work with the expectation that it will benefit public health. But decades of failed clinical trials to treat neurodegenerative disorders (for instance) is proof enough that we cannot begin to fix a system that we barely understand. There is still so much we do not know about the very fundamentals of how the brain develops and functions. And I feel lucky to be in a position to shed some light onto some of them.

Please tell us about your professional goals. What do you hope to accomplish as a scientist?
My goal is to lead a research program that integrates developmental neurobiology with systems biology and gene regulation with the goal of understanding brain wiring. Synaptic specificity has traditionally been studied from the "bottom", i.e. cell surface molecules that directly mediate the interactions between neurons. The problem has been that these processes are extremely robust, thus most mutants lead to no obvious phenotypes. I propose to study these processes "top-down": first identify TFs that cause broad changes, and then move down to the effector targets, which will provide much better candidates for genes that could implement specific cellular decisions compared to the screening-based approaches the field had so far relied on. This is now possible thanks to the advances in single-cell genomics. Molecular mechanisms of cell-type identity and synaptic specificity have long been studied independently; bringing these two fields together will dramatically expand our understanding of both processes. As someone who has been trained in both fields, I believe I am in a unique position to achieve that.