Individuality and maintenance of neuronal functionality : two projects in the fly eye.

M1 and M2 internships available. The team of Neurogenetics of Drosophila, headed by Daniel Vasiliauskas, has a number of Masters (M1 and M2) projects available, two of which are highlighted here.

Introduction :
How do closely related neuron types establish and then maintain their functional differences ? And, how do genetic programs differ between individuals to create unique brains ? Colour photoreceptor neurons of the fly retina have proven to be an excellent system for addressing these question. Our lab focuses on the R8 photoreceptor type, with its two subtypes that express distinct Rhodopsins (G-protein coupled receptor that acts as a photon detectors). The two Rhodopsins, Rh5 and Rh6, are sensitive to blue and green wavelengths respectively and thus define the colours that the photoreceptor neurons respond to. Normally, these two subtypes of R8 cells are present in about 30%:70% (Rh5:Rh6) ratio, they are distributed stochastically, and Rh5 and Rh6 are never co-expressed. (See the figures and the reference 1 for images of what this looks like in the eye and for more details about the system.) The differentiation program that leads to this Rhodospin expression pattern has been relatively well understood (ref.1), yet a number of interesting and important questions remain unanswered. Also, we recently discovered that in the natural populations, individual flies can have eyes which deveate substantially from this standard laboratory Rhodopsin pattern and that much of this is due to the genetic differences among the flies.

Project 1 : Fly individuality : Identification and characterization of a wild-derived genetic variant affecting photoreceptor neuron identities in the adult Drosophila retina.

The mutually exclusive Rh5/Rh6 expression is established through a bistable positive feedback-driven genetic switch which turns ON (for Rh6) or OFF (for Rh5) the Hippo tumor supressor pathway. The involvement of this pathway is curious, because in most other systems, it regulates growth and cell prolifferation through negative feedbacks that drive the pathway towards equilibrium (like a thermostat, i.e. in sharp contrast to the bistability observed in the photoreceptors). To identify new genes involved in establishment and maintenance of Rh5/Rh6 expression pattern we took an unusual approach : we examined 200 fly lines recently derived from wild-caught flies. Surprisingly, many of them have interesting phenotypes that affect differentiation, maintenance and function of the R8 photoreceptor neurons.

Left : normal Rh5 (immuno-stained Blue and Rh6 (Red) expression stochastically distributed p and y cells present in a 1:2 ratio.
Right : a natural variant showing a reversed ratio of two R8 photoreceptor subtypes.

One line in particular, has a reversed Rh5:Rh6 cell ratio (see figure) and is similar to the loss of the Hippo pathway phenotypes (which are normally lethal). Identification of the sequence change resulting in this phenotype will provide an interesting example of what type of natural genetic variation can affect a single function of this essential pathway without affecting its other roles. This is fundamental to understanding how pathways can independently evolve multiple functions.
The student will use powerful Drosophila genetics to identify the gene affected by the mutation and then will identify the mutation itself. In parallel, he or she will characterize the phenotype in order to understand the molecular mechanisms underlying it. The experiments will involve genetics, microdissection, immunohistochemistry (antibody stains), RNAscope-based in situ hybridization, confocal microscopy, and molecular biology.

Project 2 : Maintenance of neuronal functionality in an aging organism : molecular pathways that maintain the mutually exclusive Rhodopain5/Rhodopsin6 expression in the adult Drosophila retina.

How do terminally differentiated cells, particularly the long-living neurons, maintain their functionality throughout the life of the organism as it ages, is a fundamental biological question that is not well explored. We address it by asking what are the molecular mechanisms that maintain the mutually exclusive Rh5 vs. Rh6 expression in adult R8 neurons. We made a surprising discovery that a feedback signal from the Rh6 itself prevents transcription of Rh5 gene in the Rh6-expressing yR8 photoreceptors in adult flies (2). Furthermore, this signal does not involve key components of the phototransduction cascade, the molecular pathway by which light information is converted into an electrical signal. This implies a new unknown molecular signaling pathway downstream of Rh6.

Rh5 (immunostained in Blue) and Rh6 (Red) are never co-expressed in the same photoreceptor in the wild type fly retina (left). But, in our mutants (right) many photoreceptors that express Rh5 also co-express Rh6 (e. g. arrows).

What is also not known is how Rh6 expression is prevented in Rh5-expressing R8 photoreceptors. Recently we found a number of mutations in which, Rh6 is co-expressed with Rh5, the underlying genes normally function in repression of Rh6 in the “wrong” R8 photoreceptors.
This leads to a number of the “molecular pathway” questions which the student will address using powerful Drosophila genetics, microdissection, immunohistochemistry, RNAscope-based in situ hybridization, confocal microscopy, and molecular biology.

Relevant publications :
1. Rister, J., Desplan, C., and Vasiliauskas, D. (2013) Establishing and maintaining gene expression patterns : insights from sensory receptor patterning. Development 140, 493-503.
2. Vasiliauskas, D., Mazzoni, E.O., Sprecher, S.G., Brodetskiy, K., Johnston, R.J.Jr., Lidder, P., Vogt, N., Celik, A., and Desplan, C. (2011) Feedback from Rhodopsin controls rhodopsin exclusion in Drosophila photoreceptors. Nature, 479, 108-12.
3. Anderson, C., Reiss, I., Zhou, C., Cho, A., Siddiqi, H., Morman, B., Aviles, C.M., Deford, P., Bergland, A., Roberts, E., Taylor, J., Vasiliauskas, D., and Johnston, R.J., Jr. (2017)
Natural variation in stochastic photoreceptor specification and color preference in Drosophila. eLife, 6, e29593.

Daniel Vasiliauskas - Contact
Institut NeuroPSI à Saclay
Tél: 01 69 82 41 57

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