Anyone who has ever built a structure will have wanted to see it last, and preferably in the intended configuration. In some cases, one’s pride is on the line: a collapsed cake at a dinner party will prompt “don’t quit your day job” one-liners. In more serious cases, like that of a badly built bridge, lives can be at risk. We’ve been studying the assembly of a structure that features most prominently during the development of oocytes and sperm (gametes); here too, it’s preferable that the structure be built right.

Cells are little machines that can make duplicates of themselves, and although most biology textbooks draw daughter cells as two separate circles, estranged siblings, it is becoming increasingly clear that cells sometimes remain intimately connected to their siblings—sometimes for the remainder of their short lives. This cellular connection is aptly called an intercellular bridge (ICB) and is large enough to allow the sloshing of most cytoplasmic components back and forth, or their directed transport from one cell to another.

Perhaps unsurprisingly, some very important cells in the body, like those that carry genetic information from one generation to the next, do not do things on their own; rather, they rely on other cells, making use of the advantages that cellular connections confer. In numerous animals, gametes develop within clusters of cells that remain connected to their sister cells through ICBs. The formation of these clusters and the ensuing intercellular transport and cell fate specification (oocyte or ”not oocyte”) are probably best understood in the fruit fly. Here, ICBs are the means through which proteins, mRNAs, and organelles are transported to the cell destined to become the fertilizable oocyte. The cargo that the oocyte acquires from its sister cells is necessary for its proper development, and after fertilization, for supporting the early life of the embryo.

So how does a cell build a bridge that lasts?

Typically, a dividing cell grows to roughly twice its size, duplicates its insides, then assembles a contractile actomyosin belt-like ring that constricts its equator; subsequent remodeling of the membrane severs the daughter cells, and off they go. To build a cluster of connected cells, however, the action of a dividing cell’s contractile belt must be halted somewhere along the way, and the resulting dumbbell configuration of the daughter cells must be stabilized—think incomplete cytokinesis, but not in an idiopathic sense. Our goal was to figure out the design principles that enable the construction and assembly of stable ICBs, starting with a contractile ring template and other cytoskeletal components the cell has at its disposal.

Our group gets a lot of mileage out of using the fruit fly egg chamber as an experimental system, alongside some theory, to study interesting and perhaps fundamental biological questions. The egg chamber is a multicellular cluster that eventually gives rise to a single egg cell; however, for most of its life before that (~4 days), it exists as a cluster of 16 germline cells that are connected by 15 ICBs. Surrounding that 16-cell cluster is a layer of hundreds of somatic cells that are also connected by hundreds of ICBs. Both tissue types therefore form through cells that divide incompletely and remain connected to their siblings. The egg chamber is therefore a good system for studying ICB formation: they’re everywhere, and whereas germline cells make ICBs with diameters on the order of microns, somatic cells make ICBs with diameters of a few hundred nanometers. Our recent work has shown that these ICBs can affect cell packing as well as tissue growth dynamics in the germline and the soma.

The cover image of the August 16 issue of Biophysical Journal shows ICBs in the germline cluster and the overlying soma, appearing as small puncta and as proper rings, respectively. To acquire this image, we took a z-stack of a relatively young egg chamber expressing fluorescently labeled Pavarotti (red)—a key component of the ICBs (myosin is also fluorescently labeled, shown here in cyan)—on a confocal microscope.  

What our recent study showed was that one way to construct an ICB is to stiffen the ring as it constricts. Do that too early, and the cell ends up with too large a ring; do that too late, and the ring is too small. The point is that there’s competition between constricting and stiffening, and both processes must occur at the right time, even overlap, to build ICBs of the proportions we see in living things. Timing was such that we got interested in this problem during the pandemic, and so the closest experimental setup at hand was a collection of water balloons and rubber bands. Using these materials, we were able to reproduce the stable configurations predicted by the mathematical model we developed.  Our plan now is to test some of the predictions made by theory, through genetic perturbations, and more excitingly through imaging.  Little can replace catching cells in the act.

- Jaspreet Singh, Jasmin Imran Alsous, Krishna Garikipati, and Stanislav Y. Shvartsman