CRTC-CREB Axis as a Sensor of Proteotoxic Stress in Drosophila

Study Background and Research Question

Protein homeostasis is essential for cellular function and viability, especially under stress conditions where misfolded proteins accumulate. The ubiquitin-proteasome system (UPS) plays a pivotal role in degrading damaged or misfolded proteins. Dysregulation of UPS activity is associated with various pathologies, including neurodegenerative disorders and hematologic malignancies. The cAMP Response Element Binding Protein (CREB) is a highly conserved transcription factor that integrates diverse signaling cues, including those from protein kinase A (PKA), calcium influx, and stress-activated kinases, to regulate gene expression and cellular responses (paper).

The current study addresses a fundamental question: How do cells coordinate transcriptional responses to proteotoxic stress, and what role does the CRTC-CREB axis play in this adaptive process?

Key Innovation from the Reference Study

The central innovation of this research is the identification of the CRTC/CREB pathway as a transcriptional sensor that is robustly activated by proteasome inhibition in Drosophila. By employing a high-throughput screening approach, researchers found that all proteasome inhibitors tested—such as the proteasome β5 subunit inhibitor MLN2238—markedly increased CREB activity in adult flies. This activation is mechanistically linked to the generation of reactive oxygen species (ROS) and subsequent JNK pathway activation, positioning the CRTC-CREB axis as a node integrating redox and proteostatic signals (paper).

Methods and Experimental Design Insights

The study took advantage of a sustainable drug delivery platform (U-GLAD) to overcome solubility limitations in large-scale pharmacological screens in adult Drosophila. This enabled in vivo assessment of candidate compounds, including various reversible 20S proteasome inhibitors. The investigators used live imaging, CREB-responsive reporters, genetic manipulation of CRTC, and transcriptome analysis to map the downstream effects of proteasome inhibition—both in Drosophila and in mammalian 293T cells. The work also included functional studies in a Drosophila Huntington’s disease (HD) model to test the impact of CRTC overexpression on protein aggregation and muscle function (paper).

Protocol Parameters

• assay | MLN2238 treatment (in vivo) | 10-50 μM | activation of CREB in Drosophila adult flies | Based on dose-response in CREB reporter assays | paper
• assay | MLN2238 treatment (cellular) | 1-5 μM | CREB phosphorylation at Ser133 in 293T cells | Induces JNK-mediated CREB activation | paper
• assay | ROS scavenger co-treatment | 1-10 mM N-acetylcysteine | Blocks CREB activation by proteasome inhibitors | Mechanistic controls for ROS involvement | paper
• assay | CRTC overexpression | tissue-specific (muscle, intestine) | Restoration of protein folding/proteasome activity | Used in Drosophila HD model | paper
• assay | MLN2238 stock solution | 10 mM in DMSO | For cell-based and in vivo fly assays | Ensures compound solubility and stability | product_spec
• assay | Storage | -20°C, solid form | Long-term compound stability | Avoids activity loss from solution storage | product_spec

Core Findings and Why They Matter

Key findings of the study include:

• Proteasome inhibitors such as MLN2238 robustly induce CREB activity in Drosophila, as measured by reporter gene assays and confirmed across multiple compound classes (paper).
• ROS generation is both necessary and sufficient for CREB activation in response to proteasome inhibition. Co-treatment with ROS scavengers abolishes the effect, while exogenous ROS mimics it.
• JNK signaling mediates the link between ROS and CREB phosphorylation. In mammalian cells, MLN2238-induced CREB phosphorylation at Ser133 is also JNK-dependent, highlighting a conserved mechanism.
• Transcriptome profiling reveals that CRTC/CREB activation upregulates genes involved in redox homeostasis and protein quality control, suggesting an adaptive response to proteotoxic stress.
• Overexpression of CRTC in a Drosophila Huntington’s disease model alleviates protein aggregation, restores proteasome function, and improves organismal phenotypes such as motility and lifespan. This provides functional evidence that boosting the CRTC-CREB axis can ameliorate neurodegenerative pathology driven by protein misfolding.
• CREB activity naturally increases during aging, and further enhancement suppresses age-related protein aggregates in muscle.

Together, these results establish the CRTC/CREB axis as a critical transcriptional hub that senses and counteracts proteotoxic and oxidative stress, with implications for aging, neurodegeneration, and possibly hematologic malignancy research.

Comparison with Existing Internal Articles

Several internal resources expand on the application of MLN2238 and related proteasome β5 subunit inhibitors in research:

• "MLN2238 as a Proteasome β5 Subunit Inhibitor: Applied Workflows" details advanced assay workflows for dissecting protein homeostasis and drug resistance in multiple myeloma and lymphoma. This resource contextualizes the translational impact of CRTC-CREB axis discoveries, providing practical guidance for experimental design in oncology and neurodegeneration studies.
• "MLN2238: Reversible 20S Proteasome Inhibitor in Hematolog..." offers protocol optimization tips for leveraging chymotrypsin-like proteasome inhibition in both hematologic malignancy and bortezomib-resistant cell line studies, underscoring the importance of precise subunit targeting and workflow reproducibility.
• "MLN2238 (SKU A4008): Enhancing Assay Reproducibility in P..." discusses practical strategies for integrating MLN2238 in cell viability and cytotoxicity assays, directly addressing challenges identified in both the reference study and broader research contexts.

The current reference study bridges mechanistic insights in Drosophila with translational workflows described in these internal resources, particularly for workflows involving chymotrypsin-like proteasome inhibition, redox biology, and experimental models of neurodegeneration or hematologic malignancy.

Limitations and Transferability

The study is grounded in Drosophila models, with some validation in mammalian cell lines. While the ROS/JNK/CREB axis appears conserved, the functional consequences of boosting CRTC/CREB activity in mammalian systems, especially in the context of systemic aging or cancer, require further in vivo validation. Additionally, the use of high concentrations of proteasome inhibitors may not fully recapitulate clinical exposure scenarios. The transferability of findings to human disease models—such as multiple myeloma research or lymphoma research—should be approached with careful titration and context-specific controls (paper).

Research Support Resources

For researchers seeking to replicate or extend these findings, MLN2238 (SKU A4008) is a potent, reversible β5 proteasome inhibitor that has demonstrated robust activity in both Drosophila and mammalian systems (IC₅₀ = 3.4 nM for β5, Ki = 0.93 nM; product_spec). It is suitable for studies on chymotrypsin-like proteasome inhibition, redox signaling, and CREB pathway modulation. MLN2238 is available from APExBIO as a solid compound and should be stored at -20°C for optimal stability. For advanced workflows—such as those involving bortezomib-resistant cell lines or neurodegeneration models—see the referenced internal articles for further protocol guidance. Always consult primary literature and validated protocols for assay-specific details.