Mass measurements during lymphocytic leukemia cell polyploidization decouple cell cycle- and cell size-dependent growth

Cell size is believed to influence cell growth and metabolism. Consistently, several studies have revealed that large cells have lower mass accumulation rates per unit mass (i.e., growth efficiency) than intermediate-sized cells in the same population. Sizedependent growth is commonly attributed to transport limitations, such as increased diffusion timescales and decreased surface-to-volume ratio. However, separating cell size- and cell cycle-dependent growth is challenging. To address this, we monitored growth efficiency of pseudodiploid mouse lymphocytic leukemia cells during normal proliferation and polyploidization. This was enabled by the development of large-channel suspended microchannel resonators that allow us to monitor buoyant mass of single cells ranging from 40 pg (small pseudodiploid cell) to over 4,000 pg, with a resolution ranging from ∼1% to ∼0.05%. We find that cell growth efficiency increases, plateaus, and then decreases as cell cycle proceeds. This growth behavior repeats with every endomitotic cycle as cells grow into polyploidy. Overall, growth efficiency changes 33% throughout the cell cycle. In contrast, increasing cell mass by over 100-fold during polyploidization did not change growth efficiency, indicating exponential growth. Consistently, growth efficiency remained constant when cell cycle was arrested in G2. Thus, cell cycle is a primary determinant of growth efficiency. As growth remains exponential over large size scales, our work finds no evidence for transport limitations that would decrease growth efficiency

Nanoscale Tweezers for Single Cell Biopsies

Much of the functionality of multi-cellular systems arises from the spatial organisation and dynamic behaviours, both within and between cells. Current single-cell genomic methods only provide a transcriptional “snapshot” of individual cells. The real-time analysis and perturbation of living cells would generate a step-change in the way single-cell analysis is approached. To address this challenge, we report on the development of minimally invasive nanotweezers that can be spatially controlled to extract and manipulate samples from living cells with single-molecule precision. These nanotweezers consists of two closely spaced electrodes with gaps as small as 10-20 nm, which can be used for the dielectrophoretic trapping of DNA and proteins under physiological conditions. This is in part due to the high electric field gradients being generated and the low operating voltages. Aside from trapping single molecules, we show that it is possible to extract nucleic acids from living cells including the nucleus and cytoplasm for gene expression analysis without affecting cell viability. We also report on the trapping, manipulation and extraction of a single mitochondrion. This work bridges the gap between single-molecule/organelle manipulation and cell biology and can ultimately enable a better understanding of living cells