Unveiling the sensory and interneuronal pathways of the neuroendocrine connectome in Drosophila
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Evaluation Summary:
This manuscript will be of broad interest to readers in the field of neuroscience. The authors use a serial section transmission electron microscopy data set to trace out the entire neuroendocrine system of a maggot from its sensory input to neuroendocrine cells. It highlights the complexity of brain circuits, describing how parallel processing systems can lead to a multitude of different input combinations for different neuroendocrine cell types and subcircuits. They provide interpretations about functionality of one of described neural circuits. While the analyses are generally rigorous, the functional interpretations need more supporting evidence.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 and Reviewer #3 agreed to share their names with the authors.)
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Abstract
Neuroendocrine systems in animals maintain organismal homeostasis and regulate stress response. Although a great deal of work has been done on the neuropeptides and hormones that are released and act on target organs in the periphery, the synaptic inputs onto these neuroendocrine outputs in the brain are less well understood. Here, we use the transmission electron microscopy reconstruction of a whole central nervous system in the Drosophila larva to elucidate the sensory pathways and the interneurons that provide synaptic input to the neurosecretory cells projecting to the endocrine organs. Predicted by network modeling, we also identify a new carbon dioxide-responsive network that acts on a specific set of neurosecretory cells and that includes those expressing corazonin (Crz) and diuretic hormone 44 (Dh44) neuropeptides. Our analysis reveals a neuronal network architecture for combinatorial action based on sensory and interneuronal pathways that converge onto distinct combinations of neuroendocrine outputs.
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Reviewer #3 (Public Review):
Developing animals must couple information about external and internal conditions with developmental programs to adapt to changing environments. In animals ranging from flies to mammals, growth and developmental progression is controlled by a neuroendocrine system that integrates environmental and developmental cues. In mammals, this system involves the reproductive axis (hypothalamic-pituitary-gonadal axis, HPG). In the fruit fly Drosophila, neurosecretory cells that project onto the ring gland, a composite endocrine organ that houses the corpora cardiaca (CC), the corpus allatum (CA), and the prothoracic gland (PG), serves analogous functions. Characterizing the neurosecretory cells that project to the ring gland and the inputs they receive is therefore key to a deeper understanding of how the …
Reviewer #3 (Public Review):
Developing animals must couple information about external and internal conditions with developmental programs to adapt to changing environments. In animals ranging from flies to mammals, growth and developmental progression is controlled by a neuroendocrine system that integrates environmental and developmental cues. In mammals, this system involves the reproductive axis (hypothalamic-pituitary-gonadal axis, HPG). In the fruit fly Drosophila, neurosecretory cells that project onto the ring gland, a composite endocrine organ that houses the corpora cardiaca (CC), the corpus allatum (CA), and the prothoracic gland (PG), serves analogous functions. Characterizing the neurosecretory cells that project to the ring gland and the inputs they receive is therefore key to a deeper understanding of how the neuroendocrine system receives and processes information about external and internal conditions, and in response, adjusts growth and development. Building on the electron-microscopic reconstruction of the Drosophila L1 larval brain, the authors perform a comprehensive analysis of the neurosecretory cells that target the larval ring gland and the neurons that form synaptic contacts with these neurosecretory cells. This work is truly impressive on its own, and more than that it will also be extremely important for the future characterization of inputs received by the neuroendocrine system to modulate its activity, thus coupling development with environmental conditions. The work is well-written, and I have no doubt that it will be of great value to the field.
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Reviewer #2 (Public Review):
Analyzing EM data from the Drosophila larva, Hueckesfeld et al. investigate and describe the synaptic connectivity of sensory neurons and interneurons that provide input into the neuroendocrine system in fly larvae. The output of neuroendocrine neurons projecting to the ring gland is mostly non-synaptic and identified by receptor expression analysis. Using a modelling approach, they provide a more detailed analysis on newly discovered CO2-responsive cells and their downstream network and also other possible processing pathways from sensory to endocrine neurons. To test some of their model predictions, they analyze the response of predicted CO2-downstream neurons to CO2 exposure.
Strengths of the paper:
The authors did a great job in visualizing the complex connectivity between sensory inputs, interneurons, …
Reviewer #2 (Public Review):
Analyzing EM data from the Drosophila larva, Hueckesfeld et al. investigate and describe the synaptic connectivity of sensory neurons and interneurons that provide input into the neuroendocrine system in fly larvae. The output of neuroendocrine neurons projecting to the ring gland is mostly non-synaptic and identified by receptor expression analysis. Using a modelling approach, they provide a more detailed analysis on newly discovered CO2-responsive cells and their downstream network and also other possible processing pathways from sensory to endocrine neurons. To test some of their model predictions, they analyze the response of predicted CO2-downstream neurons to CO2 exposure.
Strengths of the paper:
The authors did a great job in visualizing the complex connectivity between sensory inputs, interneurons, and endocrine neurons. Neuroendocrine neuron outputs, which are mostly non-synaptic, have been detected by identification of vesicle release regions. The authors went beyond the analysis of EM data and collected a lot of new data to confirm non-synaptic connectivity between neuroendocrine neurons and their downstream targets by performing antibody stainings and trans-tango experiments. This information will be highly valuable to the field.
Sensory inputs in the larvae have been attributed according to previous publications, but the authors also describe a new CO2 sensing function of tracheal TD neurons. Description of this new sensory function is also a valuable addition to the Drosophila field.
The authors used a modelling approach to describe and detect specific processing pathways, for example from a certain sensory modality, or to a specific endocrine neuron. This manuscript underlines that the use of a (simple) computational model framework to understand network motifs within an EM dataset is very powerful. Also, they can confirm that predicted CO2 downstream neurons indeed respond to CO2 in a certain way.
The authors discuss potential functional implications for faster and slower processing pathways (connections over interneurons or direct). Indeed there might be situations where the larva needs to respond in flexible ways that are however also easily reversable (fast pathways), but there might be also other situations where the larva needs to integrate more sensory evidence and which might induce non-reversible behaviors, such as pupation (slow pathways). I think this discussion suggests an interesting concept of the impact/cost of adaptive behavioral changes and the different timescales they can occur.
Weaknesses of the paper:
Data wise, this manuscript is a very descriptive study. The authors visualize the complex and diverse possible processing pathways; however, the function of the circuit remains unknown. To really understand the functional properties behind this complex architecture will require studies focused on single sensory modalities, single pathways and/or single peptidergic classes all in the context of a certain behavioral framework.
The authors try to provide a complete overview over the connectivity within the neuroendocrine system pathways. However, the authors should discuss that the connectivity data from the one EM dataset that they analyzed might be changing across individuals and development. Especially the vesicle release sites might be more variable across individual larvae than synaptic connections. Neuropeptide receptor expression might also change over development.
The authors investigate the TD CO2 sensing pathway in more detail. They show that the sensory neurons and the predicted downstream neurons respond to CO2. This shows that the neural connectivity might serve a functional purpose. There is however another type of sensory neurons that respond to CO2 in the larva (Gr21a receptor neurons- Faucher et al., 2006), which are required for an avoidance response to the stimulus. The authors should discuss and maybe analyze the EM data for possible circuit convergence between the two different CO2 sensory input neurons.
The authors discuss the CO2 response in the context of a stress response. However, the natural environment of larvae, rotten fruits, also emit CO2 as a by-product. Thus, sensing CO2 which converges together with information from Fructose/Glucose sensors might be used for finding or evaluating food sources.
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Reviewer #1 (Public Review):
The neuroendocrine system of the maggot has been mapped in parts at both the light and electron microscopic levels in earlier studies. In this manuscript, Hückesfeld et al map the entire endocrine system all the way from its sensory input neurons to the interneurons and secretory neurons and the glands. This is invaluable for many reasons, including because information about external stimuli are likely integrated at the level of interneurons.
The authors use this connectome to model how and to what extent each sensory modality might influence the different neurosecretory cells. They use the CO2 sensing pathway to functionally validate their model in vivo using CaMPARI. Through this they validate a circuitry where CO2 sensing neurons in the trachea influence 4 types of neurosecretory cells via 4 interneuron …
Reviewer #1 (Public Review):
The neuroendocrine system of the maggot has been mapped in parts at both the light and electron microscopic levels in earlier studies. In this manuscript, Hückesfeld et al map the entire endocrine system all the way from its sensory input neurons to the interneurons and secretory neurons and the glands. This is invaluable for many reasons, including because information about external stimuli are likely integrated at the level of interneurons.
The authors use this connectome to model how and to what extent each sensory modality might influence the different neurosecretory cells. They use the CO2 sensing pathway to functionally validate their model in vivo using CaMPARI. Through this they validate a circuitry where CO2 sensing neurons in the trachea influence 4 types of neurosecretory cells via 4 interneuron pathways. Interestingly, they find that the CO2 sensory information is not necessarily what dominates the sensory input onto some these neurons.
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Evaluation Summary:
This manuscript will be of broad interest to readers in the field of neuroscience. The authors use a serial section transmission electron microscopy data set to trace out the entire neuroendocrine system of a maggot from its sensory input to neuroendocrine cells. It highlights the complexity of brain circuits, describing how parallel processing systems can lead to a multitude of different input combinations for different neuroendocrine cell types and subcircuits. They provide interpretations about functionality of one of described neural circuits. While the analyses are generally rigorous, the functional interpretations need more supporting evidence.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the …
Evaluation Summary:
This manuscript will be of broad interest to readers in the field of neuroscience. The authors use a serial section transmission electron microscopy data set to trace out the entire neuroendocrine system of a maggot from its sensory input to neuroendocrine cells. It highlights the complexity of brain circuits, describing how parallel processing systems can lead to a multitude of different input combinations for different neuroendocrine cell types and subcircuits. They provide interpretations about functionality of one of described neural circuits. While the analyses are generally rigorous, the functional interpretations need more supporting evidence.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 and Reviewer #3 agreed to share their names with the authors.)
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