Su Lab Decodes the Chemical Code of Extracellular Communication — How Sugars and Metabolic Small Molecules Construct Active Signaling Granules

For a long time, the formation and function of intracellular condensates (such as the nucleolus and stress granules) have attracted extensive research. These structures, which self-assemble through phase separation of proteins or protein/RNA mixtures, represent an important mechanism for cellular compartmentalization and regulation. However, does this type of phase-separated condensate structure also exist outside the cell and perform similarly important biological functions? Recently, this question has found a decent answer in the classic model of immune cell communication: mast cell extracellular granules.

Mast cells play critical roles in allergies, infection, and tissue homeostasis. Upon activation, they not only release soluble mediators but also secrete a class of stable, membrane-less "granule remnants" known as mast cell extracellular granules (MCEGs). These granules can enter the circulatory system and remotely regulate immune responses. Previously, it was widely believed that MCEGs were inert storage structures formed on a glycosaminoglycan matrix, used to transport various proteases and cytokines between cells.

However, a recent publication in Nature Chemical Biology has rewritten this understanding. This work was led by Dr. Yiwei Xiong in Xiaolei Su’s Lab, with a joint effort from Rohit Pappu’s Lab in Washington University in St. Louis. The scientists discovered that MCEGs are essentially functional biomolecular condensates formed by the spontaneous assembly of polyamines (such as spermine) and glycosaminoglycans (such as heparin). This is the first experimental confirmation that metabolic small molecules and polysaccharides can directly drive phase separation, forming structurally stable, membrane-less extracellular structures, thereby expanding the chemical basis of condensates from a "protein-centric" view to a broader realm including small molecule metabolites and sugars.

The assembly of MCEGs does not rely on traditional protein-protein interactions but on electrostatic interactions between positively charged polyamines and negatively charged heparin. The study's experiments showed that inhibiting intracellular polyamine synthesis specifically blocked the formation of MCEGs and the storage of their contents (such as proteases, TNFα), without affecting the secretion of soluble mediators, demonstrating that polyamines are the critical factor for granule assembly.

The research further revealed that these heparin-spermine condensates can spontaneously form and maintain a chemical microenvironment in physiological buffer that is distinct from the extracellular fluid: their internal pH is higher (approximately 8.4, alkaline), and they significantly enrich divalent cations like calcium and zinc.

The activity of CPA3, a major protease within the granules, is significantly higher inside the condensate compared to its soluble state. This enhanced activity can be directly attributed to the internal alkaline conditions and enriched metal ions. This indicates that MCEGs are not passive "storage sacs" but rather "catalytic microreactors" that actively regulate the function of their contents.

Using the important inflammatory cytokine TNFα as an example, the study demonstrated how the "delivery form" determines the "signal strength." When TNFα was delivered to vascular endothelial cells encapsulated within heparin-spermine condensates, compared to soluble TNFα, its retention time on the cell surface was significantly prolonged, leading to a stronger and more sustained activation response (e.g., ICAM-1 upregulation).

This work not only provides a new framework for understanding the remote regulatory mechanisms of mast cells in immunity and inflammation but also opens avenues for exploring whether other cell types (such as platelets, neurons, etc.) utilize similar mechanisms for extracellular organization and communication. It suggests that in intercellular dialogue, the efficacy of information depends not only on the "messenger" (mediator molecule) itself but also on the "signal carrier" (the condensate) possessing specific chemical properties that bears it.