Starvation-Driven ER-Ca2+-Calpain Axis Switches Autophagy to Apoptosis in Bombyx mori

Study Background and Research Question

Nutritional stress represents a universal environmental challenge for animals, prompting adaptive responses at the cellular level. In insects, the fat body is a pivotal metabolic organ, orchestrating energy homeostasis during periods of nutrient deprivation. While autophagy and apoptosis are both recognized as forms of programmed cell death (PCD) that can be triggered by starvation, the precise regulatory mechanisms that govern the switch between these two processes in insects have remained incompletely understood. The reference study (Cheng et al., 2026) interrogates how sustained energy depletion in Bombyx mori (the silkworm) fat body induces a functional transition from autophagy to apoptosis, focusing on the molecular role played by endoplasmic reticulum (ER) calcium signaling and calpain activation.

Key Innovation from the Reference Study

The central innovation in this research lies in delineating the ER-Ca²⁺-calpain signaling axis as the critical mediator of cell fate under starvation. The study links starvation-induced metabolic depletion to a cascade involving ER calcium efflux, cytoplasmic calcium overload, and subsequent activation of calpain proteases. This pathway orchestrates the conversion of autophagy (a survival mechanism) to apoptosis (a cell elimination process) via regulated cleavage of autophagy-related proteins. Importantly, the authors demonstrate that inhibiting the inositol 1,4,5-trisphosphate receptor (IP3R)—the main ER calcium release channel—using 2-aminoethoxydiphenyl borate (2-APB) markedly suppresses both calcium signaling and starvation-induced PCD. This provides a mechanistic framework for how nutritional stress modulates cell fate decisions in insect physiology.

Methods and Experimental Design Insights

The researchers utilized the B. mori fat body as an in vivo model for dissecting starvation-induced signaling events. The experimental design incorporated acute and prolonged starvation protocols to differentiate between early autophagic and late apoptotic responses. Key molecular markers were quantified, including LC3-II and ATG5 (autophagy), NtATG5 (the N-terminal fragment generated during ATG5 cleavage), and cleaved caspase-3 (apoptosis). Intracellular ATP, glycogen, and triglyceride levels were measured to assess metabolic depletion. Calcium dynamics were monitored using cytosolic Ca²⁺ assays, while the activities and expression levels of the ER Ca²⁺ pump (SERCA) and the IP3R channel were evaluated to characterize ER calcium mobilization.

Pharmacological intervention was achieved using 2-APB, a well-characterized IP3R antagonist. This compound was employed to block IP3R-mediated calcium release, allowing for direct assessment of the role of ER calcium signaling in starvation-induced autophagy and apoptosis. The effect of 2-APB on downstream calpain activation and caspase-3 cleavage was also examined to establish pathway specificity.

Core Findings and Why They Matter

The study establishes a detailed timeline of molecular events during starvation in the B. mori fat body:

• Metabolic Depletion: Starvation rapidly reduced ATP, glycogen, and triglyceride levels, confirming an acute energy crisis.
• ER Calcium Release: SERCA activity (responsible for pumping Ca²⁺ into the ER) was suppressed, while IP3R expression was upregulated, resulting in heightened Ca²⁺ efflux from the ER and a transient increase in cytoplasmic Ca²⁺.
• Autophagy Initiation: Early starvation promoted increased expression of LC3-II and ATG5, markers of autophagic activity.
• Transition to Apoptosis: Prolonged starvation led to calpain-mediated cleavage of ATG5, generating the proapoptotic NtATG5 fragment and activating caspase-3, hallmarks of the apoptotic process.
• Role of Calcium Signaling: The spike in cytoplasmic Ca²⁺ was critical for calpain activation and the autophagy-to-apoptosis transition.
• 2-APB Intervention: Pharmacological inhibition of IP3R with 2-APB suppressed both calcium mobilization and downstream PCD events, confirming the centrality of ER Ca²⁺ signaling in these processes (Cheng et al., 2026).

This evidence positions the ER-Ca²⁺-calpain axis as a universal regulatory node for PCD transitions in response to energy stress, with significant implications for understanding insect physiology, stress adaptation, and calcium signaling mechanisms more broadly.

Comparison with Existing Internal Articles

Several recent reviews and experimental reports have explored the mechanistic functions of 2-APB and related calcium signaling inhibitors. For instance, the article “2-APB (2-aminoethoxydiphenyl borate): Mechanism & Research Use” summarizes the use of 2-APB as a benchmark tool in dissecting intracellular calcium dynamics and PCD pathways, highlighting its reproducibility and specificity for IP3R inhibition. The present reference study extends these findings by providing a live-animal model and revealing the sequential timeline through which calcium signals switch autophagic to apoptotic responses.

Additionally, “Starvation Triggers ER-Ca2+-Calpain Switch in Bombyx mori Fat Body” offers an accessible summary of the same experimental system, underlining the mechanistic role of 2-APB in suppressing starvation-induced PCD. Meanwhile, more focused reviews such as “2-APB in Calcium Channel Modulation: Advanced Applications and Mechanistic Insights” discuss the broader utility of 2-APB in modulating not only IP3R but also TRPC channels, reinforcing the compound’s versatility for calcium oscillation and SOCE inhibition studies.

Limitations and Transferability

While the findings in B. mori provide a robust model for studying starvation-induced PCD transitions, several limitations should be acknowledged:

• Taxonomic Specificity: The fat body of insects is functionally analogous to mammalian liver and adipose tissue, but direct extrapolation to vertebrate systems should be approached with caution.
• Pharmacological Specificity: 2-APB, while effective as an IP3R antagonist in this study, can also modulate other calcium channels at higher concentrations, including TRPC channels. The precise selectivity profile should be considered when designing experiments for different cell types or species (product information).
• Mechanistic Breadth: The study focuses primarily on the ER-Ca²⁺-calpain-caspase axis. Other signaling pathways and stress responses may also contribute to PCD regulation under starvation, warranting future investigation.

Despite these caveats, the data provide a mechanistic blueprint for investigating similar transitions in other models, especially where calcium signaling and ER stress are implicated in cell fate decisions.

Protocol Parameters

• Starvation protocol (Bombyx mori): Acute (short-term) and prolonged (long-term, ≥24h) deprivation of nutrients to differentiate autophagic and apoptotic responses.
• 2-APB administration: In cell/tissue assays, concentrations of 10–100 μM are commonly employed for IP3R inhibition and calcium signaling studies (product information); use of freshly prepared solutions in ethanol or DMSO is recommended.
• Calcium signaling assays: Monitor cytoplasmic Ca²⁺ using fluorescent indicators before and after 2-APB treatment.
• Autophagy and apoptosis markers: Assess LC3-II, ATG5, NtATG5, and cleaved caspase-3 expression by immunoblotting or immunostaining to distinguish between PCD modalities.

Research Support Resources

For researchers aiming to replicate or extend these findings, 2-APB (2-aminoethoxydiphenyl borate) (SKU B6643, APExBIO) is a widely used and well-characterized IP3R antagonist suitable for store-operated calcium entry (SOCE) inhibition, calcium oscillation studies, and programmed cell death pathway analysis. Its efficacy and solubility characteristics support its use in diverse cell-based and animal models, as described in both the reference study and internal reviews. When designing experiments, consider the recommended concentration ranges and solvent compatibility for optimal activity.