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Merlin and expanded integrate cell signaling that regulates cyst stem cell proliferation in the Drosophila testis
Bryan Johnson1, Judith Leatherman1
1School of Biological Sciences, University of Northern Colorado, Greeley, CO, USA.
Abstract:
The Drosophila testis is a model organism stem cell niche in which two stem cell populations coordinate together to produce sperm; thus, these stem cells must be balanced in the niche. Merlin, a tumor-suppressor and human disease gene required for contact inhibition of proliferation, is known to limit the proliferation of the somatic cyst stem cells in the testis niche. Expanded encodes a protein that is structurally similar to Merlin in Drosophila, and is semi-redundant with Merlin in multiple tissues. We found that expanded depletion caused similar cyst lineage cell over-proliferation as observed with Merlin, and double mutants showed more severe phenotypes than either gene individually. Thus, these genes have partially redundant functions in the cyst lineage cells of this niche. We also expressed non-phosphorylatable constitutively "tumor suppressing" alleles of Merlin in cyst lineage cells, and surprisingly, we observed a similar cyst lineage over-proliferation phenotype. Merlin is known to impact multiple different signaling pathways to exert its effect on proliferation. We found that the Merlin loss of function phenotype was associated with an increase in MAPK/ERK signaling, consistent with Merlin's established role in transmembrane receptor inhibition. Constitutive Merlin displayed a reduction in both MAPK/ERK signaling and PI3K/Tor signaling. PI3K/Tor signaling is required for cyst cell differentiation, and inhibition of this pathway by Merlin activation phenocopied the Tor cyst lineage loss of function phenotype. Thus, Merlin impacts and integrates the activity of multiple signaling pathways in the testis niche. The ability of Merlin to dynamically change its activity via phosphorylation in response to local contact cues provides an intriguing mechanism whereby the signaling pathways that control these stem cells might be dynamically regulated in response to the division of a neighboring germ cell.
Insights
In Drosophila testes, the Merlin and Expanded proteins limit stem cell proliferation. Their combined loss causes severe overgrowth, revealing partially redundant tumor-suppressor functions crucial for niche balance.
Area of Science:
- Developmental Biology
- Stem Cell Biology
- Cancer Biology
Background:
- The Drosophila testis serves as a model for stem cell niche regulation.
- Merlin, a tumor suppressor, controls somatic cyst stem cell proliferation.
- Expanded is structurally similar to Merlin and shows functional redundancy.
Purpose of the Study:
- To investigate the roles of Merlin and Expanded in Drosophila testis stem cell niche regulation.
- To elucidate the signaling pathways influenced by Merlin in this niche.
Main Methods:
- Genetic manipulation of Merlin and Expanded in Drosophila.
- Analysis of stem cell proliferation and differentiation phenotypes.
- Assessment of MAPK/ERK and PI3K/Tor signaling pathway activity.
Main Results:
- Depletion of Expanded phenocopies Merlin's effect on cyst stem cell over-proliferation.
- Double mutants exhibit more severe over-proliferation, indicating partial functional redundancy.
- Constitutive Merlin alleles cause over-proliferation and alter MAPK/ERK and PI3K/Tor signaling.
Conclusions:
- Merlin and Expanded function redundantly to control stem cell proliferation in the Drosophila testis.
- Merlin integrates multiple signaling pathways, including MAPK/ERK and PI3K/Tor, to regulate stem cell behavior.
- Phosphorylation-dependent regulation of Merlin offers a mechanism for dynamic control of stem cell signaling in response to niche cues.