Date of Award
8-2024
Document Type
Campus Access Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Biology/Molecular, Cellular, and Organismal Biology
First Advisor
Jens Rister
Second Advisor
Alexey Veraksa
Third Advisor
Changmeng Cai, Linda Huang, Constance Cepko
Abstract
Signaling pathways are repurposed in different developmental contexts to elicit a wide variety of cellular responses. For instance, the conserved Hippo pathway was initially discovered in Drosophila as a tumor suppressor pathway in mitotically active tissue where it restrains tissue growth. However, in post-mitotic color photoreceptors (PRs), the pathway is repurposed to regulate a binary cell fate decision: activation of the pathway promotes expression of the green-sensitive Rhodopsin Rh6 (Rh6/Hippo ON fate), while inactivation of the pathway promotes the blue-sensitive Rhodopsin Rh5 (Rh5/Hippo OFF fate). The nexus of the pathway, the Warts (Wts) kinase, represses the transcriptional co-activator and oncoprotein Yorkie (Yki) through phosphorylation. When Yki is unphosphorylated, it enters the nucleus and regulates its target genes. In a growth context, Yki activates growth-promoting genes but also promotes the activity of its repressor Wts to achieve homeostasis; thus, Yki is controlled via negative feedback. In the post-mitotic PR context, Yki activates Rh5 and represses Rh6, and contrary to its role in growth, Yki represses its repressor wts, thus establishing a positive feedback loop. The underlying mechanism by which Yki represses wts and Rh6 in post-mitotic PRs remains poorly understood. Here, we elucidate the roles of both permissive (expressed in both PR subtypes) and Rh5/Hippo OFF PR-specific transcription factors in mediating Yki’s repression of both Rh6 and wts to establish a Yki positive feedback loop. This ensures the mutually exclusive expression of two color-sensing Rhodopsins in distinct subtypes of post-mitotic PRs, which is a prerequisite for color vision. First, we show that the conserved transcription factors, Blimp-1 and Hr3, are permissively required to establish a context wherein wts can be transcriptionally silenced to specify Rh5/Hippo OFF PR fate. Moreover, two conserved Blimp-1 motifs in an Rh6/Hippo ON subtype-specific intronic wts enhancer are required for wts repression in Rh5/Hippo OFF PRs, suggesting that Blimp-1 directly represses wts. Furthermore, we show that another pair of conserved permissive factors, Otd and Tj, act in parallel with Yki to activate the conserved homeodomain transcription factor Hmx exclusively in Rh5/Hippo OFF PRs. Hmx then represses Rh6 via a Q50 motif in the Rh6 promoter. Moreover, Hmx also represses Yki’s repressor wts downstream of Yki, thereby establishing the Yki positive feedback loop. Lastly, we show that a transient Activin signal during pupal development is necessary and sufficient to activate Hmx in developing Rh5/Hippo OFF fated PRs. Thus, the role of Hmx is to convert a transient developmental signal into a permanent and robust ‘OFF’ switch for the Hippo pathway by mediating a Yki positive feedback loop. These findings have relevance for human health, since the orthologs of the four permissive factors (Otd/OTX2, Tj/NRL, Hr3/RORβ, and Blimp-1/PRDM1) that promote Rh5/Hippo OFF PR fate in Drosophila, promote rod PR fate in mammals and their mutation causes severe retinopathies. Moreover, mutations in the Hmx orthologue, Hmx1, have been associated with a severe human eye disease known as Oculoauricular Syndrome. Taken together, five evolutionarily conserved transcription factors that have important functions in mammalian eye development rewire the Hippo pathway to control a fate decision in terminally differentiating color PRs in the Drosophila eye.
Recommended Citation
Bunker, Joseph Ralph, "Conserved Transcription Factors Rewire the Hippo Pathway to Regulate a Binary Cell Fate Decision in Color Photoreceptors" (2024). Graduate Doctoral Dissertations. 996.
https://scholarworks.umb.edu/doctoral_dissertations/996
Comments
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