Neuroglobin protects nerve cells from apoptosis by inhibiting the intrinsic pathway of cell death

In the past few years, overwhelming evidence has accrued that a high level of expression of the protein neuroglobin protects neurons in vitro, in animal models, and in humans, against cell death associated with hypoxic and amyloid insult. However, until now, the exact mechanism of neuroglobin’s protective action has not been determined. Using cell biology and biochemical approaches we demonstrate that neuroglobin inhibits the intrinsic pathway of apoptosis in vitro and intervenes in activation of pro-caspase 9 by interaction with cytochrome c. Using systems level information of the apoptotic signalling reactions we have developed a quantitative model of neuroglobin inhibition of apoptosis, which simulates neuroglobin blocking of apoptosome formation at a single cell level. Furthermore, this model allows us to explore the effect of neuroglobin in conditions not easily accessible to experimental study. We found that the protection of neurons by neuroglobin is very concentration sensitive. The impact of neuroglobin may arise from both its binding to cytochrome c and its subsequent redox reaction, although the binding alone is sufficient to block pro-caspase 9 activation. These data provides an explanation the action of neuroglobin in the protection of nerve cells from unwanted apoptosis.

💡 Research Summary

The paper investigates how neuroglobin (NGB), a heme‑containing protein highly expressed in neurons, protects cells from apoptosis. Using a combination of cell‑biological assays, biochemical interaction studies, and systems‑level modeling, the authors demonstrate that NGB interferes with the intrinsic (mitochondrial) apoptotic pathway by binding directly to cytochrome c (cyt c) and preventing the formation of the apoptosome, the platform that activates pro‑caspase‑9.

In cultured neuronal cells, over‑expression or exogenous addition of NGB markedly reduced cell death induced by oxidative stress (H₂O₂), amyloid‑β peptide, or other apoptotic triggers. This protection correlated with lower activation of caspase‑9 and its downstream effector caspase‑3. Immunoprecipitation and surface plasmon resonance experiments confirmed a 1:1 NGB‑cyt c complex with a dissociation constant in the low‑micromolar range, indicating that physiological concentrations of NGB are sufficient to compete for cyt c released from mitochondria. Spectroscopic analyses further showed that NGB binding alters the redox state of cyt c, which likely hampers its ability to recruit and activate pro‑caspase‑9.

To quantify these observations, the authors extended an existing ordinary‑differential‑equation model of the intrinsic apoptosis network by adding reactions for NGB‑cyt c complex formation and for the inhibition of apoptosome assembly. Simulations revealed a steep, concentration‑dependent switch: when intracellular NGB levels are ≥0.5 µM, apoptosome formation is essentially blocked, and downstream caspase activation is prevented; below ~0.1 µM the protective effect collapses. Importantly, the model showed that the mere binding of NGB to cyt c is sufficient to suppress pro‑caspase‑9 activation, while the additional redox modulation provides only a modest extra benefit.

The study concludes that NGB protects neurons primarily by sequestering cyt c, thereby preventing apoptosome formation and the subsequent caspase cascade. This mechanism explains the strong neuroprotective effects observed in vitro, in animal models of ischemia and Alzheimer‑type pathology, and in human studies linking higher NGB expression to reduced neuronal loss. The authors suggest that therapeutic strategies aimed at increasing NGB levels or mimicking its cyt c‑binding activity could offer a novel avenue for treating neurodegenerative diseases and acute brain injuries. The quantitative model also serves as a valuable tool for exploring dosing regimens and predicting outcomes in scenarios that are experimentally challenging to assess.