Research at the Institute of Neurosciences CSIC-UMH, in Alicante (Spain), led by Dr. Eloisa Herrera, in collaboration with Dr. Ángel Barco, has identified by multiomics analysis several dozen new regulators involved in the guidance of neural axons to reach their targets in the brain, a key process during the development of the nervous system for the formation of neural circuits. The work has been published in Advanced Science .
For the perfect development and functioning of the adult brain, it is essential that the axons of the different types of neurons that make up the nervous system grow and move towards the places where they will establish synapses with other neurons. Until now, most of the molecules known to be involved in this process were signaling proteins that tell axons where they can and cannot navigate in the developing brain, or when to turn on their way to connect with other neurons. However, the transcription factors that regulate and orchestrate the expression of these signaling molecules that mark the path of axons to their final destination had hardly been identified.
Now researchers at the Instituto de Neurociencias of Alicante (SPAIN) have expanded the number of regulatory molecules involved in this process by analyzing two subpopulations of retinal cells, called ganglion cells, which, although they have equivalent functions in the processing of visual information, differ in the trajectory that their axons follow on their journey to brain structures such as the thalamus or the superior colliculus. Thanks to these different paths, the brain can process the images received from each eye and generate 3D vision.
Crossing paths
Retinal ganglion cells project their axons to two different routes: to the cerebral hemisphere on the same side of the eye from which they start (ipsilateral ganglion cells), or to the opposite hemisphere (contralateral ganglion cells) crossing in this case an X-shaped structure, which serves as a crossroads for the visual axons, called the optic chiasm. The axons of the neurons located in the area of the retina closest to the nose cross the midline through the optic chiasm projecting to the opposite hemisphere, while the rest of the axons avoid the midline at the level of the optic chiasm to project to the same side of the brain from which they start.
"This binary decision of visual axons to cross or not to cross the midline at the optic chiasm is essential for perceiving the world in 3D and represents an excellent paradigm to investigate the mechanisms that enable the connection of visual neurons with other distant neurons in the brain during late embryonic development. To find new regulatory mechanisms involved in defining the axonal trajectory, we performed a multi-omics analysis comparing gene expression profiles (the transcriptome) and chromatin occupancy in retinal neurons projecting to the ipsilateral and contralateral cerebral hemispheres," said Eloísa Herrera.
Although numerous membrane and cytoplasmic proteins that regulate axonal guidance have been identified over the past three decades, the epigenetic and transcriptional mechanisms that control their expression remained poorly understood and very few transcription factors directly involved in the regulation of axonal guidance decisions had been described.
This new research has made it possible to find new genes that code for proteins not previously implicated in the axonal guidance process or in the formation of vision circuits. Some of these candidates were analyzed through gain- and loss-of-function assays. "Our results demonstrate that the novel regulators of axon path guidance identified operate in different contexts and open new avenues of research," highlighted Dr. Herrera.
Multiomics analysis
The multiomics analysis of the two subpopulations of retinal neurons used in this research, differing only in the trajectory followed by their axons, has been key to finding new genes encoding proteins not previously implicated in axon guidance. Particularly interesting is the identification of new transcription factors involved in this process, since it is these proteins that control the expression of other genes by binding to specific DNA sequences and determining where and when they should be activated or repressed.
Among the genes identified, gamma-synuclein is identified as essential for inducing midline crossing in the optic chiasm, further revealing that its expression is controlled by the transcription factor Pou4f1. This work also demonstrates that the transcription factor Lhx2/9 is specifically expressed in ipsilateral ganglion cells and controls a program that partially overlaps with that regulated by Zic2, previously described as essential for determining ipsilateral projection.
"In summary, our analyses have led to the identification of dozens of new genes potentially involved in axonal trajectory selection. These results open the door to innovative therapeutic approaches aimed at restoring damaged neuronal circuits that will benefit from a better understanding of the mechanisms controlling the wiring and assembly of the circuits that enable binocular vision and probably other bilateral circuits," Dr. Herrera concluded.
Advanced Science
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Multiomic Analysis of Neurons with Divergent Projection Patterns Identifies Novel Regulators of Axon Pathfinding
21-Aug-2022