Tumour necrosis factor, or TNF, is one of the central messenger substances of the immune system. It helps regulate inflammatory processes and contributes to the defence against pathogens, but its dysregulation is also implicated in severe chronic inflammatory conditions, including rheumatoid arthritis, inflammatory bowel disease and psoriasis. TNF exerts its effects by binding to a receptor on the cell surface known as TNFR1, which then triggers two fundamentally different types of signals. The first is transmitted through an initial protein assembly called Complex I. When TNF engages TNFR1, Complex I forms at the receptor and stimulates the production of various signalling molecules that activate the immune system, promoting inflammation as part of a normal protective response.

The second signal is far more dramatic. If the physiological inflammatory signal transmitted through Complex I is disrupted, for example by viruses or bacteria interfering with the cell’s machinery, TNF switches tactics and triggers cell death through a second protein assembly known as Complex II. This switching process is not a malfunction; it is a deliberate strategy. Cell death triggered through Complex II acts as an alarm signal to the immune system, prompting a general mobilization. When an infection is present, this mechanism is essential for eliminating infected cells and giving the immune system the opportunity to fight off the invader. The dying cell effectively sacrifices itself to alert its neighbours and accelerate the immune response.

However, the same mechanism that protects the body during infection can become destructive when it goes wrong. If the TNF-driven cell death pathway is dysregulated, it can cause severe damage to, or even destroy, entire organs or organ systems through acute or persistent inflammation. This is precisely what happens in conditions such as rheumatism, inflammatory bowel disease and psoriasis. The clinical importance of this pathway is reflected in the pharmaceutical market: TNF inhibitors are among the most medically valuable and commercially successful medicines of the last three decades worldwide. Yet these drugs have a practical drawback. They are not available as tablets and must be administered to patients by injection, which limits convenience and accessibility for many patients.

Until now, a critical piece of the puzzle has been missing. Scientists knew that the TNF pathway could switch from a survival signal to a death signal, but the molecular mechanism governing that switch remained unclear. In their latest study, the research groups led by He, Walczak and Chen demonstrated that HERC4, a so-called E3 ubiquitin ligase, acts as this critical switching point. E3 ubiquitin ligases are enzymes that attach small protein tags called ubiquitin to target proteins, a process known as ubiquitination. These ubiquitin marks prepare proteins for further processing within the cell and can dramatically alter their behaviour, their interactions and their fate. In the case of the TNF pathway, HERC4’s ubiquitination activity turns out to be the decisive event that determines whether a cell lives or dies.

To identify HERC4, the team used a knockout screen, a technique in which individual genes are systematically disabled to reveal which proteins are involved in a given process. The researchers were specifically looking for proteins involved in the ubiquitination of RIPK1, the key protein found in both Complex I and Complex II. Among the candidates, HERC4 stood out as the only previously unknown hit whose knockout resulted in cell survival when the cells were treated with a cocktail of active ingredients that induce TNF-mediated cell death. In other words, when HERC4 was absent, the lethal signal could no longer be executed, and the cells lived on despite receiving death instructions from TNF.

The biochemical details of the discovery are particularly revealing. The researchers found that HERC4 binds to kinase-active RIPK1 within the survival signalling pathway of Complex I, ubiquitinates it, and thereby initiates the formation of Complex II. This second complex comprises RIPK1, RIPK3, FADD, caspase-8 and cFLIP, and it is this assembly that triggers cell death. Depending on the relative expression levels of these various component proteins, the affected cell dies either by apoptosis, a controlled and immunologically quiet form of cell death, or by necroptosis, a more explosive form that releases alarm signals and provokes inflammation. The balance between these two death modes is determined by the composition of Complex II itself, and HERC4 sits upstream of that decision, acting as the gatekeeper that licenses the entire process.

This finding explains why RIPK1 kinase activity has long been known to be required for both necroptosis and apoptosis, and it solves a decades-old mystery in TNF research: how RIPK1 can ensure the survival of the cell when it functions within Complex I, yet trigger cell death when it participates in Complex II. The answer lies in HERC4’s ubiquitination of kinase-active RIPK1, which converts a survival-promoting protein into the seed of a death-inducing complex. As Walczak explained, HERC4 is not only a previously missing piece of the puzzle in TNF signal transduction but also a promising therapeutic target, since many chronic inflammatory diseases arise precisely where this signalling pathway is dysregulated.

The therapeutic implications were tested in mouse models, which showed that mice lacking HERC4 are protected against TNF-induced systemic inflammatory responses and against TNF-mediated acute liver damage. These represent two distinct pathological processes, driven respectively by uncontrolled necroptosis and uncontrolled apoptosis, and the fact that HERC4 loss protects against both underscores its central role in the pathway. The discovery of HERC4 therefore opens new possibilities for developing treatments for rheumatoid arthritis, inflammatory bowel disease and psoriasis, as well as other TNF-associated conditions such as sepsis and certain types of cancer in which inflammation plays a driving role.

The research team’s next goal is to develop low-molecular-weight HERC4 inhibitors that could be administered orally, a significant advantage over the injectable TNF inhibitors that currently dominate the treatment landscape. The researchers speculate that HERC4 inhibitors could have a broader spectrum of activity than TNF inhibitors, an idea that is currently under investigation. Walczak described the discovery as a milestone in the understanding of inflammatory processes mediated by cell death, demonstrating how findings from basic science can have direct clinical relevance and underlining the real importance of basic research for clinical translation. If HERC4 inhibitors live up to their promise, a protein identified through fundamental curiosity-driven research could one day form the basis of oral medicines for millions of patients whose lives are shaped by chronic inflammation.

Subject of Research: The role of the E3 ubiquitin ligase HERC4 in regulating TNF-induced apoptosis and necroptosis signalling

Article Title: One of the final pieces in the cell death and inflammation signalling pathway puzzle has been revealed

Article References: One of the final pieces in the cell death and inflammation signalling pathway puzzle has been revealed. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: HERC4, TNF, RIPK1, necroptosis, apoptosis, ubiquitination, inflammation, rheumatoid arthritis, inflammatory bowel disease, psoriasis, cell death signalling, drug development