Intracellular protein and nucleic acid measured in eight cell types using deep-ultraviolet mass mapping.

We present measurements by deep-ultraviolet mass mapping of nucleic acid (NA) and protein for five commonly cultured and three primary cell types. The dry mass distribution at submicron resolution was determined on a single-cell basis for 250-500 cells from each of these types. Since the method carries a direct reference to a spectrophotometric standard (molar extinction coefficient), we are able to calibrate the absolute weight distributions both on a cell-to-cell basis within each type and across types. We also provide a calibration in absolute mass units for fluorescence-based measurements (flow cytometry and fluorescence microscopy). As might be expected the cultured cell lines show a high concentration of nucleic acids in the nuclear compartment, much larger than the genomic 2C number even in the G1 stage. The whole-cell nucleic-acid/protein ratio was found to be a characteristic of cell lines that persists independent of cell cycle and, as a result, this ratio has some value for phenotyping. Primary chicken red blood cells (cRBC), often used as a cytometry standard, were determined to have a nuclear-isolated nucleic acid content much closer to the genomic number than the cultured cell lines (cRBC: 3.00 pg total NA, 2.30 pg DNA, and 0.70 pg RNA). The individual blastomeres (n = 54) from mouse embryos at eight-cell stage were measured and found to vary by more than a factor or two in total protein and nucleic acid content (0.8-2.3 ng total protein, 70-150 pg total NA). The ratio of nucleic acid to protein was more nearly constant for each blastomere from a particular embryo and this ratio was found to be an identifying characteristic that varies from embryo to embryo obtained from a single flushing of a mouse.

Using molecular mechanics to predict bulk material properties of fibronectin fibers.

The structural proteins of the extracellular matrix (ECM) form fibers with finely tuned mechanical properties matched to the time scales of cell traction forces. Several proteins such as fibronectin (Fn) and fibrin undergo molecular conformational changes that extend the proteins and are believed to be a major contributor to the extensibility of bulk fibers. The dynamics of these conformational changes have been thoroughly explored since the advent of single molecule force spectroscopy and molecular dynamics simulations but remarkably, these data have not been rigorously applied to the understanding of the time dependent mechanics of bulk ECM fibers. Using measurements of protein density within fibers, we have examined the influence of dynamic molecular conformational changes and the intermolecular arrangement of Fn within fibers on the bulk mechanical properties of Fn fibers. Fibers were simulated as molecular strands with architectures that promote either equal or disparate molecular loading under conditions of constant extension rate. Measurements of protein concentration within micron scale fibers using deep ultraviolet transmission microscopy allowed the simulations to be scaled appropriately for comparison to in vitro measurements of fiber mechanics as well as providing estimates of fiber porosity and water content, suggesting Fn fibers are approximately 75% solute. Comparing the properties predicted by single molecule measurements to in vitro measurements of Fn fibers showed that domain unfolding is sufficient to predict the high extensibility and nonlinear stiffness of Fn fibers with surprising accuracy, with disparately loaded fibers providing the best fit to experiment. This work shows the promise of this microstructural modeling approach for understanding Fn fiber properties, which is generally applicable to other ECM fibers, and could be further expanded to tissue scale by incorporating these simulated fibers into three dimensional network models.

Deep ultraviolet mapping of intracellular protein and nucleic acid in femtograms per pixel.

By using imaging spectrophotometry with paired images in the 200- to 280-nm wavelength range, we have directly mapped intracellular nucleic acid and protein distributions across a population of Chinese hamster ovary (CHO-K1) cells. A broadband 100× objective with a numerical aperture of 1.2 NA (glycerin immersion) and a novel laser-induced-plasma point source generated high-contrast images with short (∼100 ms) exposures and a lateral resolution nearing 200 nm that easily resolves internal organelles. In a population of 420 CHO-K1 cells and 477 nuclei, we found a G1 whole-cell nucleic acid peak at 26.6 pg, a nuclear-isolated total nucleic acid peak at 11.4 pg, and, as inferred by RNase treatment, a G1 total DNA mass of 7.4 pg. At the G1 peak, we found a whole-cell protein mass of 95.6 pg, and a nuclear-isolated protein mass of 39.3 pg. An algorithm for protein quantification that senses peptide-bond (220-nm) absorbance was found to have a higher signal-to-noise ratio and to provide more reliable nucleic acid and protein determinations when compared to more classical 280/260-nm algorithms when used for intracellular mass mapping. Using simultaneous imaging with common nuclear stains (Hoechst 33342, Syto-14, and Sytox Orange), we have compared staining patterns to deep-UV images of condensed chromatin and have confirmed bias of these common nuclear stains related to nuclear packaging. The approach allows absolute mass measurements with no special sample preparation or staining. It can be used in conjunction with normal fluorescence microscopy and with relatively modest modification of the microscope.

Functional recovery of odor representations in regenerated sensory inputs to the olfactory bulb.

The olfactory system has a unique capacity for recovery from peripheral damage. After injury to the olfactory epithelium (OE), olfactory sensory neurons (OSNs) regenerate and re-converge on target glomeruli of the olfactory bulb (OB). Thus far, this process has been described anatomically for only a few defined populations of OSNs. Here we characterize this regeneration at a functional level by assessing how odor representations carried by OSN inputs to the OB recover after massive loss and regeneration of the sensory neuron population. We used chronic imaging of mice expressing synaptopHluorin in OSNs to monitor odor representations in the dorsal OB before lesion by the olfactotoxin methyl bromide and after a 12 week recovery period. Methyl bromide eliminated functional inputs to the OB, and these inputs recovered to near-normal levels of response magnitude within 12 weeks. We also found that the functional topography of odor representations recovered after lesion, with odorants evoking OSN input to glomerular foci within the same functional domains as before lesion. At a finer spatial scale, however, we found evidence for mistargeting of regenerated OSN axons onto OB targets, with odorants evoking synaptopHluorin signals in small foci that did not conform to a typical glomerular structure but whose distribution was nonetheless odorant-specific. These results indicate that OSNs have a robust ability to reestablish functional inputs to the OB and that the mechanisms underlying the topography of bulbar reinnervation during development persist in the adult and allow primary sensory representations to be largely restored after massive sensory neuron loss.