Home | Pregnancy Timeline | News Alerts |News Archive Jul 10, 2015 

As seen above as the bright GREEN in the amoeba Dictyostelium discoideum, the protein Callipygian determinines which area of a cell remains in back when a cell begins to move. CLICK ON IMAGE TO VIEW VIDEO: "The Front and Back of Migrating Cells" Image Credit: Kristen Swaney, Johns Hopkins Medicine

Determining the front and back of a cell

A new protein is identified that helps cells determine their polarity and direction. The direction a cell moves in impacts wound healing, cancer metastasis and many other cell outcomes.

Where Greek mythology and cell biology meet, researchers have found the protein Callipygian. Named after the statue of Venus Callipygea, it was discovered and named by scientists at The Johns Hopkins University. Callipygnea determines which area of a cell will remain at the back when a cell begins to move.

The findings were made in the amoeba Dictyostelium discoideum and have shed light on how symmetrical, round cells become "polarized," or asymmetric and thus directional. A summary of these findings was published in the Proceedings of the National Academy of Sciences.

"Cells have to have a front and a back to migrate. Callipygian shuts off proteins that work at the front edge of cells, therefore helping to create the back of a cell. So we named the protein after the statue Venus Callipygea."

Peter Devreotes PhD, Professor and Director of the Department of Cell Biology at the Johns Hopkins University School of Medicine.

During cell migration, the front and back of a cell have to coordinate as if in a dance. When a chemical "move-this-way" signal reaches a cell edge, it initiates a pseudopod to form made up of rodlike actin filaments that grow toward that signal, stretching the cell's membrane. The back of the cell is then pulled forward. This process repeats itself with every new "move-this-way" signal. Both the extension of the front and retraction of the back of a cell require proteins to localize to specific areas and thus propel movement.

Kristen Swaney PhD, a former graduate student in Devreotes' laboratory,  decided to mark proteins by genetically attaching a fluorescent tag to each to find which were involved in cell movement. Based on her research, two dozen proteins were predicted to interact with and bind to the well-known front molecule PIP3. This observation suggested these proteins would also move to the front of the cell in response to the migration signal. Most of them did, but Callipygian went to the back of the cell instead, said Swaney. It didn't bind to PIP3.

"We already knew a lot about front proteins and how pseudopods are generated, but the formation of the back of the cell is more of a mystery, so we decided to follow Callipygian's lead."

Eric Devreotes PhD

Migrating cells must prevent pseudopods from forming anywhere other than the front of the cell. Researchers found that Callipygian assists in this process by accumulating at the back of the cell — preventing actin rods from growing into a pseudopod. "Callipygian essentially turns off the back of the cell," adds Swaney.

"[Callipygian] responds to cell polarity by moving to the back of the cell creating more polarity, thus drawing more Callipygian," explains Devreotes. The team also found that taking a key segment from Callipygian and adding it to another protein will make that protein also move to the back of the cell. According to Swaney, this ability to steer proteins will be a useful tool in the continued study of cell polarity.

The research is continuing to understand the dynamics of "front-and-back proteins" and how they contribute to cell polarity and migration. The hope is by understanding all cell migration processes, we will be able to mitigate wound healing and cancer cell metastasis.

Significance
Though the asymmetric distribution of proteins is a crucial first step in establishing polarity and guiding cell migration, the molecular mechanisms regulating many of these localizations are unknown. Our study reports on the novel protein Callipygian (CynA), which localizes to the rear of cells during symmetry breaking, thereby promoting polarity and increasing migration efficiency. Our data indicate that CynA localization is mediated by two distinct mechanisms, which may be important for segregating proteins in other polarized cell types including epithelial cells, neurons, and immune cells. Thus, our findings have implications for tissue formation during embryonic development, the migration of immune cells during wound healing and infection, and the aberrant migrations associated with arthritis, asthma, atherosclerosis, cancer metastasis, and other diseases.

Abstract
Asymmetric protein localization is essential for cell polarity and migration. We report a novel protein, Callipygian (CynA), which localizes to the lagging edge before other proteins and becomes more tightly restricted as cells polarize; additionally, it accumulates in the cleavage furrow during cytokinesis. CynA protein that is tightly localized, or “clustered,” to the cell rear is immobile, but when polarity is disrupted, it disperses throughout the membrane and responds to uniform chemoattractant stimulation by transiently localizing to the cytosol. These behaviors require a pleckstrin homology-domain membrane tether and a WD40 clustering domain, which can also direct other membrane proteins to the back. Fragments of CynA lacking the pleckstrin homology domain, which are normally found in the cytosol, localize to the lagging edge membrane when coexpressed with full-length protein, showing that CynA clustering is mediated by oligomerization. Cells lacking CynA have aberrant lateral protrusions, altered leading-edge morphology, and decreased directional persistence, whereas those overexpressing the protein display exaggerated features of polarity. Consistently, actin polymerization is inhibited at sites of CynA accumulation, thereby restricting protrusions to the opposite edge. We suggest that the mutual antagonism between CynA and regions of responsiveness creates a positive feedback loop that restricts CynA to the rear and contributes to the establishment of the cell axis.

Other authors of the report include Jane Borleis and Pablo Iglesias of The Johns Hopkins University.

This work was supported by grants from the National Institute of General Medical Sciences (GM28007, GM34933) and the American Heart Association (09PRE2300057).

Return to top of page