MIT-Harvard study yields insight into cell growth and division

Findings offers a possible explanation for how cells determine when they start dividing and ultimately could help researchers gather new types of information that could not be obtained from conventional proliferation assays

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CAMBRIDGE, Mass.—An article recently authored by scientistsat the Massachusetts Institute of Technology (MIT) and Harvard Medical School (HMS)sheds light on the causal relationship between cell growth and size regulation.The finding offers a possible explanation for how cells determine when theystart dividing, according to the research team.
Previously, technical problems or limited precisionassociated with measuring mammalian cell growth over time have hampered thestudy of cell growth. That all began to change in 2007, when the laboratory of ScottManalis, a professor at MIT's Koch Institute for Integrative Cancer Researchand departments of biological and mechanical engineering, developed amicrofluidic system for simultaneously measuring single-cell mass and cellcycle progression over multiple generations. Known as a suspended microchannelresonator (SMR), this technology pumps cells in fluid through a microchannel thatruns across a tiny silicon cantilever. That cantilever vibrates within avacuum. When a cell flows through the channel, the frequency of thecantilever's vibration changes, and the cell's buoyant mass can be calculatedfrom that change in frequency.
Since then, Manalis and his colleagues have used the SMR tomeasure a wide range of physical properties with a precision and throughputthat would have been possible with existing technologies.
"In the beginning, we never anticipated the idea of weighingcells," Manalis says. "And when we stumbled upon this, we had no clear idea onhow it would be useful." 
But because the original SMR offered limited control overthe motion of cells in the channel, Manalis' lab tweaked it so the scientistscould trap cells over a much longer period of time, track cell growth andrelate it to the timing of cell division by measuring the cells' mass every 60seconds throughout their lifespans.
The result, as described in an article that was published inthe Aug. 5 online edition of NatureMethods, was the observation that mammalian cells divide not when theyreach a critical size, but when their growth rate hits a specific threshold. Acell devotes itself to growth in a phase called "G1." A critical transitionoccurs when the cell enters the "S" phase, during which DNA is replicated inpreparation for division. The researchers found that growth rate increasesrapidly during the G1 phase. This rate varies a great deal from cell to cellduring G1, but converges as cells approach the S phase. Once cells complete thetransition into S phase, growth rates diverge again.
"The rapid increase in growth rate per mass in G1 indicatesthat the growth rate is not simply proportional to cell mass and suggests thatthere may be unique regulation mechanisms established during the G1 phase ofthe cell cycle," the scientists concluded.
Manalis' company, Affinity Biosensors of Santa Barbara,Calif., has sold instruments to many research labs in both academia andindustry. Affinity Biosensors has turned the SMR into a benchtop instrumentcalled Archimedes, which is named after the Greek mathematician, physicist,engineer, inventor and astronomer.
"Right now, the commercial unit is used to facilitate thedevelopment of antibody-based drugs by monitoring protein aggregates," Manalissays.
Next, Manalis' lab will measure the cell's response on shorttimescales to various perturbations, such as depleting a particular nutrient oradding a drug, he says.
"We believe this could offer new types of information thatcould not be obtained from conventional proliferation assays," he notes.
Manalis' co-authors were former MIT grad student YaochungWeng; Amit Tzur, a former research fellow at HMS; Paul Jorgensen, a former HMSpostdoctoral student; Jisoo Kim, a former undergraduate student at MIT; andMarc Kirschner, a professor of systems biology at HMS.

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