Pontin/Tip49 acts as a novel regulator of JNK pathway

Pontin/Tip49, one of the superfamily members of the AAA+ ATPases, is involved in many functions in cell contexts from invertebrates to mammals. Pontin is reported to play a role in cancer^1,2 such as tumor invasion³, regulation of growth and proliferation⁴, acts as a cofactor for Oct4-dependent lincRNA expression in stem cells⁵, yet its role in cell death remains poorly understood. In this Comment, we will discuss our recently published article about Pontin as a negative regulator of Egr-induced JNK-mediated cell death, which highlights a possible relationship between ATPase and Egr/JNK⁶.

The c-jun N-terminal kinase (JNK) signaling pathway is highly conserved among species and governs diverse roles in animals, such as dorsal closure^7–9, cell death¹⁰, tumor metastasis^11–13, and progression of Alzheimer’s Disease¹⁴. To search additional regulators of the JNK pathway, we carried out a genetic screen in Drosophila for modulators of the tumor necrosis factor ortholog Eiger (Egr)-induced cell death¹⁰, and identified Pontin (Pont) as a negative regulator of the Egr–JNK pathway⁶. A mild expression of Egr in eye development induced weak cell death and produced a rough eye phenotype¹⁰, which could be enhanced to a small eye phenotype or suppressed to normal eye upon genetic modification, and thus, could serve as a powerful tool for genetic screen. We found that downregulation of pont by RNAi approach dramatically enhanced GMR>Egr-induced cell death and produced a small eye phenotype. This enhancement was confirmed in heterozygous pont mutants. Furthermore, depletion of pont induced JNK-dependent cell death and activated the expression of JNK target gene puc in wing development. As activation of JNK pathway in the developing thorax could induce cell death and generate a small notum phenotype, we wonder whether Pont also regulates JNK pathway in the thorax development. In line with this hypothesis, loss of pont in the developing thorax induced cell death and produced a small notum phenotype, which was suppressed in heterozygous mutants for bsk encoding the Drosophila JNK and fos encoding the AP-1 component, indicating that Pont is physiologically required to inhibit JNK-Fos-mediated cell death in thorax development. To probe how Pont regulates the JNK pathway, we checked the activity of JNK by its phosphorylation, finding that depletion of pont in the wing disc resulted in elevated JNK phosphorylation, which was fully suppressed by the expression of Puc, a JNK phosphatase. Thus, endogenous Pont negatively regulates JNK-mediated cell death by inhibiting the phosphorylation of JNK.

Next, we examined if increaseed Pont is sufficient to suppress ectopic Egr-induced JNK-mediated cell death in development. We found expression of Pont suffice to block ectopic Egr-induced puc expression, cell death and morphological defects in the adult eye, wing and thorax, suggesting Pont inhibits Egr-triggered JNK activation and cell death in a non-tissue-specific manner. To characterize the epistasis of Pont in the Egr–JNK pathway, we checked the genetic interaction between Hep, a JNK kinase, and Pont. We found that gain of Pont compromised Hep-induced cell death phenotypes in the eye, thorax and wing, indicating Pont may act downstream of Hep. Collectively, this study highlights a novel role of Pont ATPase in the Egr–JNK pathway (Fig. 1).

Fig. 1. The role of Pont in the Egr–JNK pathway.

An illustration of Pont in the Egr–JNK pathway is shown. Pont inhibits Hep-triggered JNK phosphorylation, which leads to cell death and target gene puc expression

Conflict of interest

The authors declare that they have no conflict of interest.