CRTC–CREB Sensing of Proteotoxic Stress in Drosophila

Proteotoxic stress arises when cells accumulate misfolded, damaged, or aggregation-prone proteins. The ubiquitin–proteasome system is central to controlling this burden, but proteasome inhibition can itself generate oxidative and endoplasmic reticulum stress. The study by Yin and colleagues examines how cells convert this potentially damaging signal into a protective transcriptional response. Its central result is that the CRTC–CREB axis functions as a sensor that couples proteasome inhibition to ROS–JNK signaling and adaptive regulation of protein quality control.

The work is particularly informative because it does not treat proteasome inhibition only as a mechanism of protein accumulation. Instead, it investigates how a controlled proteasome perturbation activates a transcriptional program that can improve folding capacity, proteasomal activity, aggregate handling, and organismal resilience.

Study Background and Research Question

CREB is an evolutionarily conserved leucine-zipper transcription factor involved in metabolic regulation, development, growth, and neuronal function. In mammalian cells, phosphorylation at Ser133 facilitates recruitment of CBP or p300, whereas Drosophila CREB2 is regulated differently: its relevant Ser231 site is predominantly phosphorylated under basal conditions, making nuclear abundance an important determinant of activity. CRTC is a second conserved regulatory component. Following stress-related signaling, CRTC can enter the nucleus, bind CREB, and enhance transcription from cAMP response elements. These distinctions make the fly system useful for testing how CREB activity is regulated in vivo.

The research question was whether an in vivo chemical screen could identify compounds that activate CREB in adult flies, and, if so, what upstream stress pathway accounts for that activation. The authors also asked whether CRTC–CREB activity is merely a marker of proteotoxic stress or whether it actively protects tissues from protein misfolding and aggregation. The reference study addresses both questions through pharmacological, biochemical, genetic, transcriptomic, and disease-model experiments.

Key Innovation from the Reference Study

The major innovation is the identification of CRTC–CREB as a transcriptional sensor for proteotoxic stress rather than simply a downstream effector of classical cAMP or calcium signaling. A large-scale compound screen unexpectedly showed that proteasome inhibitors, including MLN2238, robustly increase CREB activity in adult flies. This observation connects a disturbance in protein degradation to a defined transcriptional response.

Mechanistically, the study places reactive oxygen species upstream of c-Jun N-terminal kinase. ROS generated during proteasome inhibition were reported to be both necessary and sufficient for CREB activation through JNK. Experiments in 293T cells further showed that JNK activation by MLN2238 is required for increased phosphorylation of CREB at Ser133. Thus, the proposed pathway is not limited to a fly-specific response: it includes a stress-signaling module with evidence of conservation in mammalian cells.

The conceptual advance is the distinction between proteasome damage and proteasome-induced adaptation. Proteasome inhibition increases the burden of misfolded proteins, but the resulting ROS–JNK–CRTC/CREB response can increase cellular capacity to manage that burden. This framework helps explain how a stress signal can produce either toxicity or protection depending on its intensity, duration, and tissue context.

Methods and Experimental Design Insights

A central technical challenge was compound delivery in adult flies. Poor aqueous solubility can limit whole-animal screening and create false negatives when compounds fail to reach relevant tissues. The authors used the U-GLAD delivery system to improve administration of poorly soluble compounds and then screened compounds for effects on CREB activity. This design allowed the investigators to study transcriptional responses in an intact organism rather than relying only on cultured cells.

The mechanistic experiments combined several complementary approaches. Pharmacological proteasome inhibition established the initiating perturbation, while measurements and manipulations of ROS and JNK tested pathway order and causality. Genetic alteration of CRTC was used to assess the transcriptional coactivator’s role in downstream protection. Transcriptome analysis of the fly intestine then identified gene classes altered by CRTC overexpression, with particular representation of redox control and proteostatic functions.

The disease-relevance experiments used a Drosophila Huntington’s disease model with muscle expression of aggregation-prone huntingtin. CRTC was overexpressed in muscle, and the investigators evaluated protein folding, proteasomal activity, aggregate burden, motility, and lifespan. The study also examined aged muscle, where CREB activity naturally increased and further enhancement of CREB activity reduced protein aggregation. This progression from screening to pathway validation and then to disease phenotypes is a strong feature of the experimental design.

Protocol Parameters

• Compound-screening context: Use an in vivo delivery strategy capable of handling poorly soluble compounds when testing CREB responses in adult flies; the study used U-GLAD for this purpose.
• Pathway validation: Pair proteasome perturbation with ROS and JNK measurements or pathway manipulations to distinguish correlation from pathway dependence.
• Transcriptional analysis: Compare control and CRTC-overexpressing tissues, with attention to redox regulation, protein folding, degradation, and transport programs.
• Disease-model assessment: Evaluate aggregate load together with proteasomal activity and functional phenotypes such as motility and survival rather than using aggregate staining alone.
• Mammalian follow-up: In cultured cells, monitor JNK activation and CREB Ser133 phosphorylation while maintaining appropriate vehicle and proteasome-inhibition controls.

Core Findings and Why They Matter

First, proteasome inhibitors activated CREB in adult flies. This result is important because it places CREB within the cellular response to impaired protein turnover. The effect was not presented as a nonspecific consequence of compound exposure: ROS generation and JNK activity were functionally required for the increase in CREB activity. The paper’s mechanistic data therefore support a directional ROS-to-JNK-to-CREB model.

Second, CRTC overexpression changed the intestinal transcriptional landscape. Genes associated with redox balance and proteostasis were increased, suggesting that CRTC–CREB activation can prepare cells to manage oxidative and protein-folding stress. This finding broadens the role of CREB beyond growth and neuronal signaling and links it to tissue-level maintenance of protein quality.

Third, muscle-specific CRTC overexpression improved multiple phenotypes in the Huntington’s disease model. The reported effects included restoration of protein-folding and proteasomal activity, reduction of huntingtin-related aggregates, improved motility, and extension of lifespan. Because these outcomes span molecular, cellular, and organismal levels, they provide stronger evidence for a protective function than a transcriptional readout alone.

Finally, the aging experiments suggest that CREB activity is dynamically engaged during age-associated proteotoxic stress. Further enhancing this activity reduced aggregates in aged muscle. The result supports a model in which endogenous stress sensing increases with age but may not be sufficient to maintain proteostasis fully. Importantly, the findings justify investigation of CRTC–CREB as a research target for protein aggregation disorders, while not establishing a therapeutic intervention in humans.

Comparison with Existing Internal Articles

The internal overview CRTC–CREB Sensing of Proteotoxic Stress in Drosophila presents the same study as a broad map of ROS/JNK signaling, transcriptional adaptation, and Huntington’s disease phenotypes. The present analysis adds methodological emphasis, particularly the value of combining whole-animal screening with tissue transcriptomics and genetic rescue experiments.

A second related resource, The CRTC-CREB Axis and Proteotoxic Stress: Insights from MLN2238, foregrounds the connection between proteasome inhibition and stress-responsive transcription. In comparison, the reference paper supports a more cautious interpretation: MLN2238 is a useful perturbation for studying the pathway, but the protective conclusions depend on the independent CRTC gain-of-function and Huntington’s disease experiments rather than on compound exposure alone.

Limitations and Transferability

The main limitation is biological context. Most of the protective evidence comes from Drosophila, including the intestine, muscle, aging tissues, and a fly Huntington’s disease model. Fly physiology is valuable for organism-level genetics and screening, but it does not reproduce the full cellular diversity, immune environment, pharmacokinetics, or disease progression of humans. The 293T experiments strengthen mechanistic conservation by linking JNK to CREB Ser133 phosphorylation, yet they do not establish that the complete protective program operates identically in human tissues.

CRTC overexpression is also a strong genetic intervention and may produce effects that exceed normal physiological signaling. Transcriptomic enrichment supports changes in redox and proteostatic regulation, but individual target genes and their necessity for aggregate clearance were not all resolved by the summarized experiments. In addition, proteasome inhibitors impose broad stress, so ROS and JNK responses may vary with dose, exposure time, cell type, and baseline mitochondrial function.

Why this cross-domain matters, maturity, and limitations

The study bridges basic stress biology and protein aggregation disease research. That bridge matters because it proposes a general principle: a transcriptional sensor can convert a degradation defect into an adaptive increase in protein-quality capacity. Its maturity is strongest at the level of pathway discovery and genetic validation in Drosophila, with partial mammalian support at the signaling level. Translation to clinical disease remains preliminary. The work should therefore guide mechanistic experiments on proteostasis and aggregation, not be interpreted as evidence that pharmacological activation of CRTC–CREB is already safe or effective in patients.

Research Support Resources

For experiments that model the proteasome perturbation used in the reference study, researchers can use MLN2238 (SKU A4008). The product information describes this reversible 20S proteasome inhibitor as a β5 subunit inhibitor with chymotrypsin-like proteasome inhibition, reporting an IC50 of 3.4 nM and a Ki of 0.93 nM at that site. At higher concentrations, it also reports proteasome β1 and β2 subunit inhibition, with IC50 values of 31 nM and 3500 nM, respectively. These concentration-dependent activities should be considered when designing proteasome-stress experiments relevant to multiple myeloma research or lymphoma research. The compound is supplied as a solid, is water-insoluble, and should be prepared and stored according to the product guidance.