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Transcriptional specification and dendritic growth patterns of motion-sensing neurons in Drosophila during development
Transcriptional specification and dendritic growth patterns of motion-sensing neurons in Drosophila during development
The development of functional circuits in the brain requires neuronal morphogenesis combined with specificity in synapse formation. Diverse morphologies of neurons are generated by the precise interactions between transcription factors in gene regulatory networks. These networks specify the development of neurons, creating elaborate branching patterns of dendrites and axons. During this growth process, synaptic partners are located in close proximity to each other within the neuropil for synapse formation. The Drosophila motion vision circuit is well understood in the adult animal regarding morphology and function. Therefore, this system provides the necessary background to study the development of motion selective neurons. In combination with the genetic accessibility of Drosophila neurons and recent advances in transcriptomic profiling approaches, it is the ideal model system for the study the development of neuronal circuits. In my PhD, I focused on primary visual motion-sensing neurons in Drosophila. These neurons, T4 and T5, exist in four subtypes (a, b, c, d), each responding preferentially to visual motion in one of the four cardinal directions. Their dendrites are oriented according to their functional response and innervate the same neuropil layer, in the medulla or lobula for T4 and T5, respectively. Their axon terminals are located in one of the four layers of the lobula plate separated by their functional identity. These two morphological characteristics, the dendrite orientation and the layer of axonal arborisation, are the only difference between T4 and T5 subtypes. Therefore, it is possible to investigate how genes specify neuronal morphologies through transcriptomic profiling by studying the growth patterns of T4 and T5 dendrites. For the first manuscript of my PhD, I performed single-cell RNA sequencing of T4 and T5 neurons at different stages of dendritic development. I found multiple transcriptional differences between subtypes. In particular, I discovered that the transcription factor grain separates a/d from b/c subtypes. grain is additionally required to specify the morphological characteristics of b/c subtypes, while suppressing a/d morphologies. Through overexpression experiments a/d subtypes could be transformed to b/c and b/c could be transformed to a/d subtypes using RNA interference experiments, morphologically. Furthermore, overexpression experiments combined with recordings of neuronal activity, mainly showed preferred direction responses of b/c subtypes. It is, thus, a necessary component of the gene regulatory network to specify T4 and T5 subtype morphologies. Additionally, I identified many differentially expressed cell surface proteins that potentially play a role in guiding the growth of T4 and T5 dendrites. In the second manuscript of this thesis, I used an ex vivo brain preparation to image single T4 dendrites during their growth phase. I could distinguish horizontal, a/b, and vertical, c/d, subtypes of T4 neurons only by their dendritic growth pattern. Furthermore, horizontal subtypes could be split into a and b subtypes based on their direction of growth. Their growth pattern shows a sequential order growing from their proximal to their distal compartment. In conclusion, this work discovered the specification of T4 and T5 neurons through transcription factors, which, combined with their dendritic growth pattern, can help us understand how gene expression determines morphologies necessary to form neuronal circuits.
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Hoermann, Nikolai
2023
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Hoermann, Nikolai (2023): Transcriptional specification and dendritic growth patterns of motion-sensing neurons in Drosophila during development. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)
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Abstract

The development of functional circuits in the brain requires neuronal morphogenesis combined with specificity in synapse formation. Diverse morphologies of neurons are generated by the precise interactions between transcription factors in gene regulatory networks. These networks specify the development of neurons, creating elaborate branching patterns of dendrites and axons. During this growth process, synaptic partners are located in close proximity to each other within the neuropil for synapse formation. The Drosophila motion vision circuit is well understood in the adult animal regarding morphology and function. Therefore, this system provides the necessary background to study the development of motion selective neurons. In combination with the genetic accessibility of Drosophila neurons and recent advances in transcriptomic profiling approaches, it is the ideal model system for the study the development of neuronal circuits. In my PhD, I focused on primary visual motion-sensing neurons in Drosophila. These neurons, T4 and T5, exist in four subtypes (a, b, c, d), each responding preferentially to visual motion in one of the four cardinal directions. Their dendrites are oriented according to their functional response and innervate the same neuropil layer, in the medulla or lobula for T4 and T5, respectively. Their axon terminals are located in one of the four layers of the lobula plate separated by their functional identity. These two morphological characteristics, the dendrite orientation and the layer of axonal arborisation, are the only difference between T4 and T5 subtypes. Therefore, it is possible to investigate how genes specify neuronal morphologies through transcriptomic profiling by studying the growth patterns of T4 and T5 dendrites. For the first manuscript of my PhD, I performed single-cell RNA sequencing of T4 and T5 neurons at different stages of dendritic development. I found multiple transcriptional differences between subtypes. In particular, I discovered that the transcription factor grain separates a/d from b/c subtypes. grain is additionally required to specify the morphological characteristics of b/c subtypes, while suppressing a/d morphologies. Through overexpression experiments a/d subtypes could be transformed to b/c and b/c could be transformed to a/d subtypes using RNA interference experiments, morphologically. Furthermore, overexpression experiments combined with recordings of neuronal activity, mainly showed preferred direction responses of b/c subtypes. It is, thus, a necessary component of the gene regulatory network to specify T4 and T5 subtype morphologies. Additionally, I identified many differentially expressed cell surface proteins that potentially play a role in guiding the growth of T4 and T5 dendrites. In the second manuscript of this thesis, I used an ex vivo brain preparation to image single T4 dendrites during their growth phase. I could distinguish horizontal, a/b, and vertical, c/d, subtypes of T4 neurons only by their dendritic growth pattern. Furthermore, horizontal subtypes could be split into a and b subtypes based on their direction of growth. Their growth pattern shows a sequential order growing from their proximal to their distal compartment. In conclusion, this work discovered the specification of T4 and T5 neurons through transcription factors, which, combined with their dendritic growth pattern, can help us understand how gene expression determines morphologies necessary to form neuronal circuits.