Starvation-Induced Autophagy–Apoptosis Transition Mediated by ER-Ca2+-Calpain Signaling in Bombyx mori

Study Background and Research Question

Programmed cell death (PCD), encompassing autophagy and apoptosis, is fundamental to organismal adaptation under stress. In insects, the fat body acts as a metabolic hub, orchestrating physiological responses to nutritional deprivation. While mammalian systems have established crosstalk between calcium signaling, autophagy, and apoptosis, the regulatory mechanisms driving PCD transitions in insect tissues under starvation remain inadequately defined. The reference study (Cheng et al., 2026) addresses how sustained energy depletion governs the switch from autophagy to apoptosis in the Bombyx mori fat body, centering on the ER-Ca2+-calpain axis.

Key Innovation from the Reference Study

The core innovation lies in delineating a sequentially regulated process whereby starvation-induced metabolic stress triggers ER calcium release, cytoplasmic Ca2+ overload, and calpain activation, culminating in a shift from autophagy to apoptosis. The study provides direct evidence that inhibition of IP3 receptor (IP3R)-mediated calcium release via 2-aminoethoxydiphenyl borate (2-APB) can suppress both autophagic and apoptotic pathways, establishing a causal link between ER calcium dynamics and PCD fate decisions in an insect model. This mechanistic insight extends and specifies the role of calcium signaling in stress adaptation beyond what is known from mammalian systems.

Methods and Experimental Design Insights

Cheng et al. employed a combination of biochemical, molecular, and pharmacological approaches in Bombyx mori larvae subjected to starvation. Physiological indicators—ATP, glycogen, and triglyceride levels—were measured to confirm metabolic stress. Calcium dynamics were evaluated by assessing cytosolic Ca2+ concentrations and the activity of the SERCA pump and IP3R channel. Autophagy and apoptosis were tracked using markers such as LC3-II, ATG5/NtATG5, and cleaved caspase-3. Pharmacological intervention with 2-APB, a selective IP3R antagonist, allowed dissection of the calcium signaling contribution to cell fate transitions. The study also quantified calpain activity and examined the temporal relationship between calcium signaling, calpain-mediated cleavage of ATG5, and subsequent activation of apoptotic pathways.

Core Findings and Why They Matter

The study demonstrated that starvation rapidly depleted energy reserves in the fat body, inhibiting SERCA activity and upregulating IP3R expression. This dual modulation precipitated ER Ca2+ efflux, resulting in cytosolic Ca2+ overload, which peaked early and declined with prolonged starvation. Elevated intracellular Ca2+ levels activated calpain, which in turn cleaved ATG5 to produce NtATG5, a proapoptotic fragment. The temporal sequence showed that short-term starvation upregulated autophagy (elevated LC3-II and ATG5), whereas persistent stress favored apoptosis via NtATG5 production and caspase-3 activation (Cheng et al., 2026).

Pharmacological inhibition with 2-APB significantly suppressed starvation-induced Ca2+ signaling, autophagy, and apoptosis, confirming the pivotal role of ER-mediated calcium release in orchestrating the autophagy–apoptosis switch. These findings position the ER-Ca2+-calpain axis as a central integrator of stress signaling and cell fate, with direct relevance for studies of oxidative stress-related cell injury and calcium oscillations and waves in both invertebrate and vertebrate systems.

Comparison with Existing Internal Articles

Several internal resources offer context and mechanistic parallels. For instance, the article “2-APB: A Selective IP3R Antagonist for Calcium Signaling” highlights the established use of 2-APB for dissecting ER-mediated calcium dynamics, including its benchmark values for IP3R and TRPC inhibition. The study by Cheng et al. goes further by mapping the downstream consequences of ER-Ca2+ release in a physiological stress model, connecting 2-APB’s mode of action to cell fate outcomes. Similarly, “Decoding ER-Ca2+ Signaling: 2-APB in Autophagy–Apoptosis Research” discusses the translational utility of 2-APB in mapping the autophagy–apoptosis axis, citing Bombyx mori as a model system. These articles collectively emphasize that precise pharmacological inhibition of IP3R with 2-APB is indispensable for mechanistic dissection of calcium-driven cell death pathways in experimental contexts.

Limitations and Transferability

While the study establishes a robust mechanistic framework in Bombyx mori, several limitations are noted. The insect fat body, though functionally similar to mammalian liver and adipose tissue, may exhibit distinct regulatory nuances. Transferability to vertebrate systems or other cell types should be approached with caution, as calcium signaling effectors and thresholds for autophagy–apoptosis transitions can vary. Furthermore, in vivo pharmacological specificity of 2-APB may be influenced by off-target effects at higher concentrations, and the study’s findings are limited to the acute to subacute window of starvation stress.

Protocol Parameters

• 2-APB treatment for ER-Ca2+ modulation: Typically 10–100 μM in cell culture, as supported by product information and corroborated by application in Bombyx mori fat body studies.
• Starvation induction: Withhold nutrients for durations sufficient to deplete glycogen and triglyceride stores (time course optimization recommended for each model system).
• Calcium imaging: Use fluorescent Ca2+ indicators to track cytosolic changes before and after 2-APB administration.
• Autophagy and apoptosis assays: Evaluate LC3-II/ATG5 (autophagy) and cleaved caspase-3/NtATG5 (apoptosis) via immunoblotting or immunofluorescence.
• Calpain activity measurement: Employ specific fluorogenic substrates to quantify enzyme activation in response to Ca2+ shifts.

Research Support Resources

Researchers aiming to investigate ER-mediated calcium signaling, autophagy–apoptosis transitions, or oxidative stress-related cell injury can utilize 2-APB (2-aminoethoxydiphenyl borate, SKU B6643) as a validated IP3 receptor antagonist. The compound’s specificity for ER Ca2+ release and additional effects on TRPC channels make it suitable for dissecting calcium-dependent signaling events. For detailed application guidelines, see the cited product documentation and workflow discussions in related internal articles. APExBIO’s 2-APB is widely reported in the literature for use in studies of store-operated calcium entry (SOCE) inhibition and cell fate determination under stress.