Cells transport substances by encapsulating them in membrane bubbles known as vesicles, which journey to different locations. These vesicles merge with other vesicles to release their contents, a process requiring two membranes to fuse without leakage. While scientists have long theorized that fused cell membranes enter a transient intermediate state, direct visualization of this process within intact cells has eluded researchers until now.

In a groundbreaking study, researchers from the NIH and the University of Virginia aimed to determine if the membranes of living cells form stable, observable structures that signify this intermediate state. They cultured various mammalian cells, including human, monkey, mouse, and rat cells, in nutrient-rich flasks, incubating them at 37°C (98.6°F) to ensure cell viability and growth.

The research team placed 80,000 to 100,000 cells on a specialized gold-coated platform designed for high-resolution imaging. To maintain the cell structure’s natural state, they quickly froze the cells to stabilize the membrane for observation. Utilizing a technique called cryogenic electron tomography, they generated detailed tomographic images of the specimens.

Using these cross-sectional images, the research team reconstructed a 3D view of the cell at the nanometer scale. This precision revealed the internal vesicle membranes and the external membrane, known as the plasma membrane. They successfully created around 300 3D reconstructions of areas near the cell edges, where membrane bubbles engage and interact.

Typically, cell membranes consist of two layers of lipid molecules forming a flexible barrier. However, these researchers discovered a novel membrane structure created when the outer layers of two membranes merge into a continuous sheet, while the inner layers remain apart. Where the two vesicles made contact, they identified a flat, circular area where the outer membrane layers fused, resembling a connection point between two soap bubbles. This configuration is termed “hemifsome.”

The team indicated that hemifsomes are significantly larger and more stable than the short-lived intermediate states previously suggested. This long-term stability suggests that hemifsomes are not merely transient fusion events but may persist long enough to fulfill cellular functions.

Additionally, some hemifsomes contained a single lens-shaped droplet within the membrane where the two vesicles were partially fused. These droplets appeared in approximately half of the 308 cross-sectional images analyzed, averaging 40 nanometers in diameter—roughly 100 times smaller than the surrounding vesicles—and were in contact with the membrane’s oily interior.

The unique droplets differed from surrounding membrane lipids, suggesting a mixture of lipids and proteins, referred to as proteolipid nanodroplets. The consistent one-to-one association between hemifsomes and these nanodroplets implies that they may assist in stabilizing hemifsomes or influence the shape and organization of the cell membrane.

To explore whether hemifsomes facilitate material movement within cells, researchers introduced 5- or 15-nanometer gold particles into cells. These particles are small enough to navigate through the internal transport systems responsible for moving nutrients and other molecules. The research team employed a powerful microscope to observe the gold particles as they traversed the cell’s transport compartments; however, the particles did not enter hemifusomes, suggesting they are not directly involved in cellular transport.

The findings concluded that hemifsomes emerge when cell membranes merge or alter shape, acting as temporary sites for building, repairing, or rearranging membrane structures. These results challenge existing models of membrane fusion and vesicle formation, suggesting that critical intermediate states can evolve into stable and functional cellular structures.

Future research should focus on identifying the molecular composition of proteolipid nanodroplets and elucidating the mechanisms through which cells manage the transition from hemifsomes to fully fused membranes. Investigating the roles of hemifsomes in vesicle formation, membrane recycling, and cellular stress responses across various cell types is also recommended.

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