The mysterious "water droplets" in cells have been discovered! Is this the "key" to conquering many difficult diseases?

Author: Duan Yuechu and Huang Xianghong

According to a report by Scientific American on August 14, 2024: In the microscopic world of life, cells are the basic units of all life activities. Inside the cells, there are some mysterious water droplets, namely biomolecular condensates, which are gradually unveiling the mysteries of life and bringing new perspectives and breakthroughs to biological research.

All living things are made up of cells, which have complex and delicate structures inside. In addition to the membrane-enclosed organelles we are familiar with, such as mitochondria and cell nuclei, there is also a type of membrane-free organelles - biomolecular condensates. In the past decade, biologists have gradually realized that these seemingly tiny droplets play a more important role in cell function than previously known.

Textbook descriptions of cell biology often focus on organelles with clear membrane structures, however, this description is incomplete. The discovery of biomolecular condensates has given us a more comprehensive understanding of the internal world of cells. They are like "small communities" within cells, concentrating proteins or other biomolecules and performing specific tasks. The existence of these droplets is not accidental, but an efficient way of organization and regulation formed by cells during a long evolutionary process.

The physics of biomolecular condensates is full of mystery. Thermodynamics, the branch of physics that studies the relationship between heat and other forms of energy, provides a theoretical basis for our understanding of liquid-liquid phase separation. Take the common example of oil and water. When the two are mixed, they will automatically separate into two different phases under certain conditions. Similarly, when molecules such as proteins, DNA or RNA in cells aggregate at high concentrations, phase separation may also occur to form biomolecular condensates. But unlike simple oil and water stratification, many condensates in cells are dynamic and changeable. They exist temporarily and form or disappear as the needs of the cell and the environment change.

The history of biomolecular condensates research has been marked by many pioneers. As early as 1782, Danish naturalist Otto Frederick Muller observed and mapped nucleoid proteins in green algae, one of the earliest known biomolecular condensates. Later, in the 1830s, German physiologists Rudolf Wagner and Gabriel Valentin observed tiny structures within the nucleus of nerve cells. In 1899, American biologist Edmund B. Wilson proposed the idea that the cytoplasm of cells is not a uniform liquid. And in 2009, a landmark study directly demonstrated for the first time that biomolecular condensates can form in living cells.

For researchers like Trevor Grandpre, their research journey is full of challenges and surprises. Grandpre entered the university as a biology major, but was attracted by the quantitative nature of physics. He is committed to studying the phase separation phenomenon in cells, especially the condensates formed in T cells. Through the joint efforts of collaborators, they revealed the complex process of the formation of special types of condensates in cells, providing important clues for a deeper understanding of the immune regulation mechanism of cells.

The formation mechanisms of biomolecular condensates are diverse. On the one hand, suitable conditions can promote the formation of chemical bonds, allowing individual protein molecules to bond to each other and form condensates. On the other hand, proteins containing intrinsically disordered regions (IDRs), due to the high repetitiveness of their amino acid sequences and the characteristics of their charge distribution, can also form separate phases through complex interactions with water and attraction or repulsion with other amino acids. In addition, some condensates are formed through energy-consuming "active" processes. For example, Sharon Glotzer's research shows that under specific chemical reaction conditions, centrosomes, liquid cell structures that help coordinate cell division, can exist stably, and their size and number are closely related to the energy used in the chemical reaction.

Mysterious droplets inside cells play an important role in a variety of diseases. In neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease and Parkinson's disease, abnormal protein phase transitions and condensate formation are important causes of neuronal dysfunction and death. Taking ALS as an example, mutations in the gene encoding FUS cause hereditary ALS, and its variant forms of condensates in test tube experiments are similar to FUS protein clusters in patient brain tissue. Over time, the properties of these droplets change, becoming more dense and requiring greater force to deform. Similar processes may also occur in other neurodegenerative diseases, such as amyloid fiber formation in Alzheimer's disease and ribonucleoprotein deposition in Parkinson's disease. This suggests that normal physiological conditions support the liquid state of these proteins, and the occurrence of diseases may be related to the transition from liquid to solid aggregates.

In cancer research, some cancer-related proteins may regulate cell proliferation, differentiation, apoptosis and other processes by forming aggregates. For example, YAP, a key pathway protein involved in tumor cell tolerance, undergoes phase transition and generates biomolecular aggregates. The formation of excessive YAP aggregates is closely related to tolerance. By inhibiting their formation, it is expected to provide new strategies for cancer treatment.

In terms of cardiovascular disease, abnormalities in some protein aggregates related to cardiovascular function may affect the normal physiological functions of the heart and blood vessels, but the specific mechanism still needs further study.

In order to further study the role of biomolecular condensates in diseases, scientists have adopted a variety of advanced technical means. Gene editing technologies, such as CRISPR-Cas9, can accurately change genes related to condensate formation, thereby observing the impact of gene changes on disease models. Proteomics analysis uses technologies such as mass spectrometry to comprehensively detect the composition and changes of intracellular proteins under disease conditions, especially proteins related to condensates. High-resolution microscopy techniques, including confocal microscopy and super-resolution microscopy, allow us to directly observe the subtle morphology, distribution and dynamic changes of intracellular condensates.

Live cell imaging technology combined with fluorescent labeling allows us to observe biomolecular condensates in real time for a long time while the cells remain physiologically active. Fluorescence resonance energy transfer (FRET) technology reveals the interactions between molecules in condensates by detecting energy transfer between fluorescent molecules. Total internal reflection fluorescence microscopy (TIRF) can selectively excite fluorescent molecules close to the cell membrane surface, which helps to observe condensates near the cell surface. Fluorescence lifetime imaging (FLIM) reflects changes in the surrounding environment and interactions by measuring changes in the lifetime of fluorescent molecules. Two-photon excitation fluorescence microscopy uses long-wavelength lasers to excite fluorescent molecules, reducing light damage to cells and is suitable for long-term real-time observation. Light sheet fluorescence microscopy can quickly obtain high-resolution three-dimensional images, which has unique advantages for studying the spatial distribution and dynamic changes of condensates in cells.

In addition to its importance in disease research, the new physics of biomolecular condensates also has broad application prospects in other fields. In the study of chromatin assembly, it was found that the acetylation modification of nucleosomes can recruit specific proteins to form a new liquid phase, thereby dynamically regulating the assembly of chromatin. During the formation of the cytoskeleton, some proteins bound to actin can undergo liquid-liquid phase separation, self-assemble to form a planar f-actin network, and serve as a nucleation center to adhere multiple microfilaments into bundles. In the study of the individual development of Drosophila, biomolecular condensates involved in the asymmetric division of neuroblasts can be asymmetrically distributed at the two poles of the cell, playing a key role in the differentiation of nerve cells. In the formation and change of synapses, the interaction between proteins triggers phase separation, "depositing" on the surface of lipid membranes to participate in the signal transduction of neurotransmitters, and is even closely related to complex physiological processes such as human sleep and consciousness.

In general, the new physics of biomolecular condensates has opened a new window for us to the microscopic world inside cells. Although our understanding of it is still in the stage of continuous development and improvement, with the deepening of research and the advancement of technology, we have reason to believe that research in this field will bring unprecedented opportunities and breakthroughs for human beings to understand the mysteries of life, overcome various diseases, and promote the development of biological sciences. In the future, we look forward to more mysteries about biomolecular condensates being unveiled, contributing more to human health and scientific progress.