The temporal sequence in which stem cells form different neurons in the brain is regulated by the successive expression of regulatory proteins called temporal transcription factors (tTFs).

Using single-cell mRNA sequencing, an international team of investigators led by Claude Desplan, PhD, at the department of biology at the New York University (NYU), has identified a complete series of ten tTFs that determine the precise order in which neurons develop in the optic lobe of the fruit fly. The study on the simpler model system of the fruit fly makes new inroads into explaining the development of complex neural networks in the human brain, which is a central question in developmental neurobiology and regenerative medicine.

“Knowing how the human brain develops could allow us in the future to repeat these developmental processes in the lab to generate specific types of neurons in a Petri dish—and potentially transplant them in patients—or to trigger neuronal stem cells in living organisms to generate and replace missing neurons,” said Desplan who is a professor of biology at NYU and the study’s senior author.

The findings, published in an article in Nature, (“A complete temporal transcription factor series in the fly visual system“), on April 6, 2022, provides insights into the evolution of brain development across species and clues for regenerative therapeutics.

“Our findings suggest that understanding the mechanisms of neuron development in flies can generate insight for the equivalent process in humans,” said co-first author Anthony Rossi, PhD, who is a postdoctoral fellow at Harvard and was a graduate student in the Desplan lab.

For obvious reasons, temporal patterns of neural development in the brain cannot be studied in humans, causing scientists to rely on model organisms with evolutionarily conserved mechanisms that regulate the sequential development of diverse neurons from the same progenitor stem cell.

“We verify that tTFs regulate the progression of the series by activating the next tTF(s) and repressing the previous one(s), and also identify more complex mechanisms of regulation,” the authors note. This time-dependent expression of tTFs creates temporal windows of neural development marked by the expression of a specific set of genes.

The researchers uncovered the complete set of tTFs needed to generate the nearly 120 types of neurons in the medulla, a hindbrain structure that houses the visual system. Analyzing the pool of mRNAs in each of more than 50,000 cells in the developing fly medulla, the researchers grouped the cell types by their transcriptomic profiles. Analyzing transcriptomes of neural stem cells in the medulla, the researchers not only identified the tTFs that define the developmental windows but also the genetic network that regulates their progressive expression.

“Several tTFs had been previously identified in the brain’s visual system using available antibodies. We have now identified the comprehensive series of ten tTFs that can specify all the neuron types in this brain region,” said one of the study’s lead authors, Nikolaos Konstantinides, PhD, who was a postdoctoral fellow in Desplan’s lab.

The researchers have also determined how the genetic progression relates to the “birth order” of all neurons in the medulla, linking specific temporal windows with the generation of specific types of neurons. This genetic regulatory mechanism produces the entire set of optic lobe neurons in a stereotypic order.

“Impairment of the temporal cascade progression leads to the generation of reduced neuronal diversity, hence altering brain development,” said Isabel Holguera, PhD, a postdoctoral fellow in NYU’s Department of Biology and one of the co-first authors of the study.

The team also clarified the first steps in the maturation and specialization of neural stem cells into neurons (neuronal differentiation) and found that the process is remarkably alike for fly and human cortical neurons with overlapping patterns of genes expressed during different developmental stages.