In an article published as the cover story of the journal Biochemistry, researchers Percillia Oliveira, Tiphany De Bessa, and Francisco Laurindo, from the Instituto do Coração of FMUSP and the RIDC Redoxoma, propose an innovative, systematized way of understanding the role of the endoplasmic reticulum in coordinating cellular redox processes. The work introduces the concept of the endoplasmic reticulum redoxome, defined as the integrated set of processes that determine the organelle’s redox state and organize intra- and intercellular redox communication and signaling.

The endoplasmic reticulum (ER) is the largest internal compartment of cells and acts as a production and quality control center for proteins. It also participates in lipid metabolism, calcium regulation, interorganelle communication, reorganization of the internal membrane system, and response to pathogens. Many of these functions depend on its redox organization, making the endoplasmic reticulum an increasingly evident player in cellular redox regulation.

“What is new in this review is precisely this systematization, the global conceptualization of these processes into a single integrated set, something that did not yet exist in the literature. We believe that’s why the article had an impact and was chosen for the cover of the journal,” said Laurindo.

According to the authors, the ER redoxome can be organized into three main aspects: the intra-reticulum axis, the contacts between the reticulum and other organelles, and a third axis, referred to as the ER-dependent outreach redoxome (ERDOR), which encompasses the ER redoxome that extends beyond the organelle and affects the cytosol and extracellular environment.

The concept of the ER redoxome arises directly from the group’s research, which for nearly two decades has investigated proteins of the protein disulfide isomerase (PDI) family, initially focusing on their classic role in catalyzing disulfide bond formation in nascent ER proteins. “When we started working with PDI, we didn’t think of the reticulum in those terms,” says Laurindo. “Our interest at the time was to study the functional convergence and physical interaction between PDIs and the NADPH oxidase system. But over time, it became clear that there was a whole world to be explored there.” While redox regulation mechanisms associated with mitochondria and NADPH oxidases were already well studied, the redox dynamics of the endoplasmic reticulum remained relatively unexplored for a long time.

Cellular Homeostasis

The correct folding of proteins is a central process for cellular homeostasis. “Evolutionarily, it is remarkable the existence of numerous mechanisms to prevent the incorrect folding of proteins. Nature really hates a misfolded protein,” says Laurindo. Misfolded proteins can acquire toxic functions, in addition to depriving the cell of the functional protein.

A fundamental step in protein folding involves the formation of disulfide bonds and depends directly on the redox environment of the ER, which is more oxidizing than other cellular compartments. The reticulum is responsible for the synthesis of approximately one-third of cellular proteins, particularly those more complex and rich in disulfide bonds, destined for secretion or for insertion into the plasma membrane. These proteins include collagen and other extracellular matrix proteins, which are subsequently secreted through the Golgi system.

Communication

The redox pathways of the ER interact with those of most other organelles and subcellular compartments outside the organelle. In particular, redox communication between the ER and the extracellular space is crucial for intercellular signaling.

From a redox perspective, the pro-oxidant environment in the ER lumen is similar to that of the extracellular milieu. “It is as if the lumen of the reticulum were an extracellular portion inside the cell, in redox terms,” ​​explains Laurindo, highlighting the elevated levels of hydrogen peroxide in these compartments, compared to other subcompartments such as mitochondria and cytosol.

This similarity reflects the fact that the ER maintains multiple forms of communication with the extracellular environment. From an evolutionary perspective, it is believed that the reticulum originated from an invagination of the plasma membrane; thus, what is now located in the ER lumen was ancestrally outside the cell. Therefore, the reticulum proteins that reach the cell surface and reflect the redox state of the ER likely play a significant role in redox communication between cells. “There is a crosstalk between the reticulum and the extracellular space, from which we created the concept of ERDOR, which is the third part of our review article.“

Cellular communication is also promoted by the reticulum’s contacts with other organelles. “The endoplasmic reticulum comprises about 50% of the cell membranes. Therefore, it is an important site of cell integration,” says the researcher. Many of these contacts involve calcium trafficking, a process influenced by redox reactions.

Mechanisms of ERDOR

According to the researchers, there are three main mechanisms by which the ER redoxome projects itself beyond the organelle. The first involves the release of small oxidizing molecules produced inside the ER, such as hydrogen peroxide. This process depends on specific channels, especially aquaporin 11, present in the ER membrane.

The second proposed mechanism involves the physical displacement of ER-derived structures. “These are pieces of the reticulum, compartments derived from the reticulum that translocate outwards,” explains the researcher. Experimental evidence supports this hypothesis. Laurindo highlights a recent study by RIDC Redoxoma researcher Flávia Meotti, who analyzed the proteome of endothelial cell surfaces and identified proteins typical of the reticulum there. “What caught our attention was that there were not only luminal proteins, but also structural proteins of the endoplasmic reticulum,” suggesting that entire fragments of the organelle can reach the cell surface.

The third ERDOR mechanism concerns the translocation of reticulum oxidoreductases, such as PDIs, to the cytosol and the extracellular environment. “The exit of PDI to the extracellular space is well characterized,” says Laurindo, highlighting that this process occurs through Golgi-independent pathways, as shown in an article published by the group in 2024.

Physiological implications of ERDOR

In general, processes associated with ERDOR participate in a wide range of cellular responses, including cell adhesion and migration, platelet activation, plasma membrane organization, redox signaling, inflammatory and immunological responses, as well as vascular remodeling and extracellular matrix reorganization.

In the extracellular environment, pecPDIs (cell-surface PDIs) are associated with different biological processes. Laurindo mentions examples such as thrombosis, viral infections, and vascular remodeling. “HIV internalization into the cell depends on pecPDI. For the dengue virus, this is also important,” he says.

In a different context, studies by the group demonstrated that extracellular PDI plays a direct role in cell migration and vascular mechanoadaptation. “When we stimulate a smooth muscle cell with growth factors, it displays directed migration. If we silence PDI, migration is completely lost: the cell organizes the cytoskeleton but cannot direct movement. But if we neutralize only pecPDI with an antibody, the cell migrates but loses persistence and follows an irregular trajectory.“

Finally, the authors emphasize the importance of ER oxidoreductases in mediating inflammatory processes and highlight the influence of the pericellular redox state on the immune response.

The article “Endoplasmic Reticulum Redoxome: Protein Folding and Beyond,” by Percillia V. S. Oliveira, Tiphany C. De Bessa, and Francisco R. M. Laurindo, can be accessed here