CRTC-CREB Axis Senses Proteasome Inhibition via ROS/JNK in Drosophila

Study Background and Research Question

Proteotoxic stress, resulting from the accumulation of misfolded proteins, is a hallmark of both aging and numerous neurodegenerative diseases. The ubiquitin-proteasome system (UPS) serves as a central mechanism for maintaining protein homeostasis (proteostasis) by degrading damaged or misfolded proteins. However, the cellular transcriptional networks that detect and respond to disruptions in proteasome activity remain incompletely understood. While the cAMP Response Element Binding Protein (CREB) is a well-established regulator of cell growth, metabolism, and synaptic plasticity, its role in sensing proteasome inhibition had not been fully characterized. The referenced study (Yin et al., 2022) addresses this knowledge gap by investigating how the CRTC-CREB axis responds to proteasome inhibition and protects against proteotoxic stress in Drosophila.

Key Innovation from the Reference Study

The principal innovation of this work is the discovery that the CRTC-CREB transcriptional axis functions as a conserved sensor for proteasome inhibition-driven stress. Specifically, the study demonstrates that inhibition of the 20S proteasome, as achieved with MLN2238 and other inhibitors, robustly increases CREB activity in adult flies. This activation is not a direct consequence of proteasome inhibition per se, but is mediated through a cascade involving reactive oxygen species (ROS) and c-Jun N-terminal kinase (JNK) signaling. Furthermore, the downstream effects of this axis extend to the upregulation of genes involved in redox balance and proteostasis, highlighting a transcriptional feedback mechanism that mitigates proteotoxic damage.

Methods and Experimental Design Insights

To systematically investigate CREB regulation in vivo, the authors employed a large-scale compound screening in adult Drosophila, facilitated by a novel U-shape Gum Arabic Liquid Assisted Drug (U-GLAD) delivery system. This approach enabled efficient administration of poorly water-soluble compounds, such as MLN2238—a reversible proteasome β5 subunit inhibitor with high specificity for the chymotrypsin-like activity of the 20S proteasome. The study combined genetic manipulation (overexpression and mutants of CRTC), transcriptome profiling, and biochemical assays to map the signaling events linking proteasome inhibition to CREB activation. In parallel, human 293T cells were used to verify the role of JNK in CREB phosphorylation at Ser133, underlining the evolutionary conservation of this axis.

Protocol Parameters

• Compound administration (Drosophila): Use the U-GLAD system for solubilizing and delivering MLN2238 or other proteasome inhibitors in adult flies; optimize for compound stability and bioavailability.
• MLN2238 dosage and timing: For robust CREB activation, titrate MLN2238 concentrations to achieve effective proteasome inhibition while minimizing toxicity, referencing IC₅₀ values for the β5 subunit as reported in the product information.
• Monitoring CREB activity: Employ CRE-luciferase reporter lines or quantitative RT-PCR of CREB target genes in tissue-specific contexts.
• Oxidative signaling manipulation: Use genetic or pharmacological approaches (e.g., ROS scavengers or JNK inhibitors) to dissect pathway components as done in the reference workflow.

Core Findings and Why They Matter

The study’s findings comprise several mechanistic advances:

• Proteasome inhibitors activate CREB: All FDA-approved proteasome inhibitors tested, including MLN2238, markedly increased CREB activity in adult Drosophila.
• ROS/JNK-dependent signaling: Proteasome inhibition led to mitochondrial dysfunction and elevated ROS, which in turn activated the JNK pathway. JNK activation was both necessary and sufficient for CREB phosphorylation and nuclear translocation.
• Transcriptional feedback enhances proteostasis: Transcriptome analysis revealed that CRTC overexpression upregulated genes involved in redox control and protein folding, supporting a protective feedback loop.
• Therapeutic relevance: Overexpression of CRTC in muscle tissue of a Huntington’s disease (HD) fly model restored folding capacity, suppressed protein aggregates, improved motility, and extended lifespan. Enhanced CREB activity also reduced protein aggregates in aged muscle, pointing to broad relevance for aging and neurodegeneration research.

Collectively, these results identify the CRTC-CREB axis as a central transcriptional sensor and effector in the cellular response to proteotoxic and oxidative stress, building a mechanistic bridge between proteasome inhibition (e.g., by MLN2238), redox signaling, and adaptive gene expression.

Comparison with Existing Internal Articles

Several recent reviews and research-focused commentaries provide complementary context and expand upon the implications of these findings. For example, the article "MLN2238: Reversible Proteasome β5 Inhibition as a Next-Ge..." offers a translational perspective on the application of MLN2238 in multiple myeloma and lymphoma research, highlighting its capacity to overcome resistance mechanisms in bortezomib-refractory settings. This review integrates the molecular insights provided by the CRTC-CREB axis study, especially regarding chymotrypsin-like proteasome inhibition and its downstream effects on stress signaling pathways.

Similarly, "MLN2238 and CREB Pathways: Unveiling New Frontiers in Proteasome β5 Inhibition" delves into the intersection of proteasome inhibition and CREB-mediated transcriptional responses, reinforcing the notion that targeting the CREB axis may be beneficial in models of oncologic and neurodegenerative disease. These internal resources contextualize the reference study’s findings within broader translational research strategies, underscoring the importance of understanding signaling crosstalk and adaptive stress responses in drug development.

Limitations and Transferability

There are several considerations when extrapolating the results of this Drosophila-based study to mammalian and clinical systems. First, while the CREB and CRTC signaling components are evolutionarily conserved, species-specific differences in post-translational regulation (e.g., the phosphorylation sites involved) may impact pathway dynamics. Second, the use of overexpression models (e.g., CRTC in muscle) may not fully recapitulate physiological regulation in complex tissues. Third, the systemic delivery of proteasome inhibitors in flies may not directly mirror pharmacokinetics and biodistribution in higher organisms. Finally, while the link between proteasome inhibition, ROS/JNK signaling, and CREB activation is robustly demonstrated, the long-term effects and potential off-target consequences remain to be fully elucidated.

Why this cross-domain matters, maturity, and limitations

The relevance of these findings extends beyond neurodegeneration to oncology, as proteasome inhibitors like MLN2238 are widely used in hematologic malignancy research. Understanding the transcriptional adaptations triggered by proteasome inhibition may inform strategies to optimize therapeutic efficacy and manage resistance in multiple myeloma and lymphoma. However, the maturity of the cross-domain bridge remains at the preclinical stage; further validation in mammalian disease models is warranted before clinical translation.

Research Support Resources

Researchers aiming to replicate or extend the workflows described in the reference study can utilize MLN2238 (SKU A4008), a potent and reversible proteasome β5 subunit inhibitor, for both chymotrypsin-like proteasome inhibition and mechanistic studies involving the CRTC-CREB axis. The compound’s well-characterized selectivity and utility in multiple myeloma and lymphoma research, as well as in models of proteotoxic stress, make it a valuable tool for dissecting proteasome-mediated signaling pathways. For optimal use, consult the product guidelines for solubility and storage. Additional mechanistic and translational insights can be found in related internal articles and the APExBIO product documentation.