Water and lipid MRI of the Xenopus oocyte

Lipid droplet motility and organelle contacts

Lipid droplets (LDs) are fat storage organelles integral to energy homeostasis and a wide range of cellular processes. LDs physically and functionally interact with many partner organelles, including the ER, mitochondria, lysosomes, and peroxisomes. Recent findings suggest that the dynamics of LD inter-organelle contacts is in part controlled by LD intracellular motility. LDs can be transported directly by motor proteins along either actin filaments or microtubules, via Kinesin-1, Cytoplasmic Dynein, and type V Myosins. LDs can also be propelled indirectly, by hitchhiking on other organelles, cytoplasmic flows, and potentially actin polymerization. Although the anchors that attach motors to LDs remain elusive, other regulators of LD motility have been identified, ranging from modification of the tracks to motor co-factors to members of the perilipin family of LD proteins. Manipulating these regulatory pathways provides a tool to probe whether altered motility affects organelle contacts and has revealed that LD motility can promote interactions with numerous partners, with profound consequences for metabolism. LD motility can cause dramatic redistribution of LDs between a clustered and a dispersed state, resulting in altered organelle contacts and LD turnover. We propose that LD motility can thus promote switches in the metabolic state of a cell. Finally, LD motility is also important for LD allocation during cell division. In a number of animal embryos, uneven allocation results in a large difference in LD content in distinct daughter cells, suggesting cell-type specific LD needs.

3D printed microchannels for sub-nL NMR spectroscopy

Nuclear magnetic resonance (NMR) experiments on subnanoliter (sub-nL) volumes are hindered by the limited sensitivity of the detector and the difficulties in positioning and holding such small samples in proximity of the detector. In this work, we report on NMR experiments on liquid and biological entities immersed in liquids having volumes down to 100 pL. These measurements are enabled by the fabrication of high spatial resolution 3D printed microfluidic structures, specifically conceived to guide and confine sub-nL samples in the sub-nL most sensitive volume of a single-chip integrated NMR probe. The microfluidic structures are fabricated using a two-photon polymerization 3D printing technique having a resolution better than 1 μm3. The high spatial resolution 3D printing approach adopted here allows to rapidly fabricate complex microfluidic structures tailored to position, hold, and feed biological samples, with a design that maximizes the NMR signals amplitude and minimizes the static magnetic field inhomogeneities. The layer separating the sample from the microcoil, crucial to exploit the volume of maximum sensitivity of the detector, has a thickness of 10 μm. To demonstrate the potential of this approach, we report NMR experiments on sub-nL intact biological entities in liquid media, specifically ova of the tardigrade Richtersius coronifer and sections of Caenorhabditis elegans nematodes. We show a sensitivity of 2.5x1013 spins/Hz1/2 on 1H nuclei at 7 T, sufficient to detect 6 pmol of 1H nuclei of endogenous compounds in active volumes down to 100 pL and in a measurement time of 3 hours. Spectral resolutions of 0.01 ppm in liquid samples and of 0.1 ppm in the investigated biological entities are also demonstrated. The obtained results may indicate a route for NMR studies at the single unit level of important biological entities having sub-nL volumes, such as living microscopic organisms and eggs of several mammalians, humans included.

NMR spectroscopy of single sub-nL ova with inductive ultra-compact single-chip probes

Nuclear magnetic resonance (NMR) spectroscopy enables non-invasive chemical studies of intact living matter. However, the use of NMR at the volume scale typical of microorganisms is hindered by sensitivity limitations, and experiments on single intact organisms have so far been limited to entities having volumes larger than 5 nL. Here we show NMR spectroscopy experiments conducted on single intact ova of 0.1 and 0.5 nL (i.e. 10 to 50 times smaller than previously achieved), thereby reaching the relevant volume scale where life development begins for a broad variety of organisms, humans included. Performing experiments with inductive ultra-compact (1 mm²) single-chip NMR probes, consisting of a low noise transceiver and a multilayer 150 μm planar microcoil, we demonstrate that the achieved limit of detection (about 5 pmol of ¹H nuclei) is sufficient to detect endogenous compounds. Our findings suggest that single-chip probes are promising candidates to enable NMR-based study and selection of microscopic entities at biologically relevant volume scales.

Magnetic Resonance Microscopy (MRM) of Single Mammalian Myofibers and Myonuclei

Recently, the first magnetic resonance microscopy (MRM) images at the cellular level in isolated mammalian brain tissues were obtained using microsurface coils. These methods can elucidate the cellular origins of MR signals and describe how these signals change over the course of disease progression and therapy. In this work, we explore the capability of these microimaging techniques to visualize mouse muscle fibers and their nuclei. Isolated myofibers expressing lacZ were imaged with and without a stain for β-galactosidase activity (S-Gal + ferric ammonium citrate) that produces both optical and MR contrast. We found that MRM can be used to image single myofibers with 6-μm resolution. The ability to image single myofibers will serve as a valuable tool to study MR properties attributed to healthy and myopathic cells. The ability to image nuclei tagged with MR/Optical gene markers may also find wide use in cell lineage MRI studies.

Dynamic properties of water in breast pathology depend on the histological compounds: distinguishing tissue malignancy by water diffusion coefficients

Background

The parameters that characterize the intricate water diffusion in tumors may also reveal their distinct pathology. Specifically, characterization of breast cancer could be aided by diffusion magnetic resonance.

The present in vitro study aimed to discover connections between the NMR biexponential diffusion parameters [fast diffusion phase (D_FDP ), slow diffusion phase (D_SDP ), and spin population of fast diffusion phase (P₁)] and the histological constituents of nonmalignant (control) and malignant human breast tissue. It also investigates whether the diffusion coefficients indicate tissue status.

Methods

Post-surgical specimens of control (mastopathy and peritumoral tissues) and malignant human breast tissue were placed in an NMR spectrometer and diffusion sequences were applied. The resulting decay curves were analyzed by a biexponential model, and slow and fast diffusion parameters as well as percentage signal were identified. The same samples were also histologically examined and their percentage composition of several tissue constituents were measured: parenchyma (P), stroma (St), adipose tissue (AT), vessels (V) , pericellular edema (PCE), and perivascular edema (PVE). Correlations between the biexponential model parameters and tissue types were evaluated for different specimens. The effects of tissue composition on the biexponential model parameters, and the effects of histological and model parameters on cancer probability, were determined by non-linear regression.

Results

Meaningful relationships were found among the in vitro data. The dynamic parameters of water in breast tissue are stipulated by the histological constituents of the tissues (P, St, AT, PCE, and V). High coefficients of determination (R²) were obtained in the non-linear regression analysis: D_FDP (R² = 0.92), D_SDP (R² = 0.81), and P₁(R² = 0.93).

In the cancer probability analysis, the informative value (R²) of the obtained equations of cancer probability in distinguishing tissue malignancy depended on the parameters input to the model. In order of increasing value, these equations were: cancer probability (P, St, AT, PCE, V) (R² = 0.66), cancer probability (D_FDP, D_SDP)(R² = 0.69), cancer probability (D_FDP, D_SDP, P₁) (R² = 0.85).

Conclusion

Histological tissue components are related to the diffusion biexponential model parameters. From these parameters, the relative probability of cancer in a given specimen can be determined with some certainty.

Effects of hypotonic stress and ouabain on the apparent diffusion coefficient of water at cellular and tissue levels in Aplysia

There is evidence that physiological or pathological cell swelling is associated with a decrease of the apparent diffusion coefficient (ADC) of water in tissues, as measured with MRI. However the mechanism remains unclear. Magnetic resonance microscopy, performed on small tissue samples, has the potential to distinguish effects occurring at cellular and tissue levels. A three-dimensional diffusion prepared fast imaging with steady-state free precession sequence for MR microscopy was implemented on a 17.2 T imaging system and used to investigate the effect of two biological challenges known to cause cell swelling, exposure to a hypotonic solution or to ouabain, on Aplysia nervous tissue. The ADC was measured inside isolated neuronal soma and in the region of cell bodies of the buccal ganglia. Both challenges resulted in an ADC increase inside isolated neuronal soma (+31 ± 24% and +30 ± 11%, respectively) and an ADC decrease at tissue level in the buccal ganglia (-12 ± 5% and -18 ± 8%, respectively). A scenario involving a layer of water molecules bound to the inflating cell membrane surface is proposed to reconcile this apparent discrepancy.

Single cell spectroscopy: noninvasive measures of small-scale structure and function

The advancement of spectroscopy methods attained through increases in sensitivity, and often with the coupling of complementary techniques, has enabled real-time structure and function measurements of single cells. The purpose of this review is to illustrate, in light of advances, the strengths and the weaknesses of these methods. Included also is an assessment of the impact of the experimental setup and conditions of each method on cellular function and integrity. A particular emphasis is placed on noninvasive and nondestructive techniques for achieving single cell detection, including nuclear magnetic resonance, in addition to physical, optical, and vibrational methods.

A time-domain nuclear magnetic resonance study of Mediterranean scleractinian corals reveals skeletal-porosity sensitivity to environmental changes

Mediterranean corals are a natural model for studying global warming, as the Mediterranean basin is expected to be one of the most affected regions and the increase in temperature is one of the greatest threats for coral survival. We have analyzed for the first time with time-domain nuclear magnetic resonance (TD-NMR) the porosity and pore-space structure, important aspects of coral skeletons, of two scleractinian corals, Balanophyllia europaea (zooxanthellate) and Leptopsammia pruvoti (nonzooxanthellate), taken from three different sites on the western Italian coast along a temperature gradient. Comparisons have been made with mercury intrusion porosimetry and scanning electron microscopy images. TD-NMR parameters are sensitive to changes in the pore structure of the two coral species. A parameter, related to the porosity, is larger for L. pruvoti than for B. europaea, confirming previous non-NMR results. Another parameter representing the fraction of the pore volume with pore sizes of less than 10-20 μm is inversely related, with a high degree of statistical significance, to the mass of the specimen and, for B. europaea, to the temperature of the growing site. This effect in the zooxanthellate species, which could reduce its resistance to mechanical stresses, may depend on an inhibition of the photosynthetic process at elevated temperatures and could have particular consequences in determining the effects of global warming on these species.

Formation of the dorsal marginal zone in Xenopus laevis analyzed by time-lapse microscopic magnetic resonance imaging

The dorsal marginal zone (DMZ) of the amphibian embryo is a key embryonic region involved in body axis organization and neural induction. Using time-lapse microscopic magnetic resonance imaging (MRI), we follow the pregastrula movements that lead to the formation of the DMZ of the stage 10 Xenopus embryo. 2D and 3D MRI time-lapse series reveal that pregastrular movements change the tissue architecture of the DMZ at earlier stages and in a different fashion than previously appreciated. Beginning at stage 9, epiboly of the animal cap moves tissue into the dorsal but not into the ventral marginal zone, resulting in an asymmetry between the dorsal and the ventral sides. Time-lapse imaging of labeled blastomeres shows that the animal cap tissue moves into the superficial DMZ overlying the deeper mesendoderm of the DMZ. The shearing of superficial tissue over the deeper mesendoderm creates the radial/vertical arrangement of ectoderm outside of mesendoderm within the DMZ, which is independent of involution and prior to the formation of the dorsal blastoporal lip. This tilting of the DMZ is distinct from, but occurs synchronously with, the vegetal rotation of the vegetal cell mass [R., Winklbauer, M., Schürfeld (1999). "Vegetal rotation, a new gastrulation movement involved in the internalization of the mesoderm and endoderm in Xenopus." Development. 126, 3703-3713.]. We present a revised model of gastrulation movements in Xenopus laevis.

In vivo magnetic resonance microscopy of differentiation in Xenopus laevis embryos from the first cleavage onwards

Differentiation inside a developing embryo can be observed by a variety of optical methods but hardly so in opaque organisms. Embryos of the frog Xenopus laevis--a popular model system--belong to the latter category and, for this reason, are predominantly being investigated by means of physical sectioning. Magnetic resonance imaging (MRI) is a noninvasive method independent of the optical opaqueness of the object. Starting out from clinical diagnostics, the technique has now developed into a branch of microscopy--MR microscopy--that provides spatial resolutions of tens of microns for small biological objects. Nondestructive three-dimensional images of various embryos have been obtained using this technique. They were, however, usually acquired by long scans of fixed embryos. Previously reported in vivo studies did not cover the very early embryonic stages, mainly for sensitivity reasons. Here, by applying high field MR microscopy to the X. laevis system, we achieved the temporal and spatial resolution required for observing subcellular dynamics during early cell divisions in vivo. We present image series of dividing cells and nuclei and of the whole embryonic development from the zygote onto the hatching of the tadpole. Additionally, biomechanical analyses from successive MR images are introduced. These results demonstrate that MR microscopy can provide unique contributions to investigations of differentiating cells and tissues in vivo.

Two-dimensional and three-dimensional time-lapse microscopic magnetic resonance imaging of Xenopus gastrulation movements using intrinsic tissue-specific contrast

The amphibian embryo undergoes radical tissue transformations during blastula and gastrula stages, but live observation of internal morphogenetic events by optical microscopy is not feasible due to the opacity of the early embryo. Here, we report on the use of microscopic magnetic resonance imaging (MRI) to directly follow morphogenetic movements during blastula and gastrula stages of the Xenopus laevis embryo. We compare three different MRI modalities that take advantage of the intrinsic contrast present in embryonic tissues: three-dimensional (3D) fat-imaging, 3D water-imaging, and 2D high-speed high-resolution imaging of early embryonic stages. We show that the features revealed by the intrinsic contrast correlate with the histological structure of the embryo. Using this tissue specific intrinsic contrast, the main embryonic tissues and internal tissue movements as well as archenteron invagination can be differentiated without cell labeling. We present 2D and 3D time-lapse sequences of early Xenopus embryonic development, spanning the stages from early blastula to the end of gastrula, which show the complex internal rearrangements of gastrulation in essentially real-time.

Nuclear magnetic resonance of mass-limited samples using small RF coils

Figure Schematic diagram of a typical arrangement used for hyphenating chemical microseparations (e.g. capillary HPLC, CE, or CEC) with microcoil NMR detection.

Time-lapse tracing of mitotic cell divisions in the early Xenopus embryo using microscopic MRI

Mitotic cell division is a highly regulated cellular event in all organisms, but its direct visualization in the vertebrates is limited to animals with transparent embryos. Here, we report on the use of microscopic magnetic resonance imaging (mMRI) to noninvasively observe mitotic cell division of early blastomeres in the optically opaque Xenopus laevis embryo. Due to intrinsic tissue contrast, cell nuclei can be directly visualized without the need for contrast enhancing labeling. By taking two-dimensional in vivo time-lapse image sequences, the karyokinesis of a blastomere is followed. Timing and orientation of the cleavages can be traced for five cell divisions to establish a cell lineage tree, including orientation and timing of the mitosis. This work demonstrates for the first time the use of MRI for the visualization of cell divisions and expands the experimental scope of the Xenopus embryo.

Osmotic and aging effects in caviar oocytes throughout water and lipid changes assessed by 1H NMR T1 and T2 relaxation and MRI

By combining NMR relaxation spectroscopy and magnetic resonance imaging techniques, unsalted (us) and salted (s) caviar (Acipenser transmontanus) oocytes were characterized over a storage period of up to 90 days. The aging and the salting effects on the two major cell constituents, water and lipids, were separately assessed. T1 and T2 decays were interpreted by assuming a two-site exchange model. At Day 0, two water compartments that were not in fast exchange were identified by the T1 relaxation measurements on the us oocytes. In the s samples, T1 decay was monoexponential. During the time of storage, an increment of the free water amount was found for the us oocytes, ascribed to an increased metabolism. T1 and T2 of the s oocytes shortened as a consequence of the osmotic stress produced by salting. Selective images showed the presence of water endowed with different regional mobility that severely changed during the storage. Lipid T1 relaxation decays collected on us and s samples were found to be biexponential, and the T1 values lengthened during storage. In us and s oocytes, the increased lipid mobility with the storage was ascribed to lipolysis. Selective images of us samples showed lipids that were confined to the cytoplasm for up to 60 days of storage.

Subcellular in vivo 1H MR spectroscopy of Xenopus laevis oocytes

In vivo magnetic resonance (MR) spectra are typically obtained from voxels whose spatial dimensions far exceed those of the cells they contain. This study was designed to evaluate the potential of localized MR spectroscopy to investigate subcellular phenomena. Using a high magnetic field and a home-built microscopy probe with large gradient field strengths, we achieved voxel sizes of (180 microm)3. In the large oocytes of the frog Xenopus laevis, this was small enough to allow the recording of the first compartment-selective in vivo MR spectra from the animal and vegetal cytoplasm as well as the nucleus. The two cytoplasmic regions differed in their lipid contents and NMR lineshape characteristics-differences that are not detectable with whole-cell NMR techniques. In the nucleus, the signal appeared to be dominated by water, whereas other contributions were negligible. We also used localized spectroscopy to monitor the uptake of diminazene acturate, an antitrypanosomal agent, into compartments of a single living oocyte. The resulting spectra from the nucleus and cytoplasm revealed different uptake kinetics for the two components of the drug and demonstrate that MR technology is on the verge of becoming a tool for cell biology.

Reversible and irreversible effects of chemical fixation on the NMR properties of single cells

The effects of chemical fixation are known to alter MR parameters, such as relaxation times and the apparent diffusion coefficient (ADC) of water. It is often assumed that such changes are reversible after samples have been reimmersed in a buffer solution for a sufficient period of time. In this study we characterize the changes associated with fixation of single Xenopus laevis oocytes and their subsequent reimmersion in buffer. Substantial reductions in both T(1) and T(2) values were measured for all compartments of the cell after fixation, with the cytoplasm showing larger changes than the nucleus. After reimmersion in buffer, there were small but statistically significant differences in MR parameters between fresh and reimmersed cells. Experiments with a gadolinium (Gd) contrast agent showed evidence of irreversible changes in the permeability of cellular membranes to small molecules.

Effects of physiologic challenge on the ADC of intracellular water in the Xenopus oocyte

The biophysical determinants of the intracellular water apparent diffusion coefficient (ADC) in mammalian tissues are poorly understood. Model systems that are more amenable to physical measurements may provide insights into the behavior of more complex systems. Toward that end, we used MRI to evaluate the effects of altered microtubule concentration, nuclear breakdown, and ATP depletion on intracellular water ADC in the Xenopus oocyte. Water ADC did not change in response to polymerization of microtubules with taxol or depolymerization with nocodazole. Water ADC did not change following the breakdown of the nucleus in healthy cells. Short-term depletion of ATP (approximately 20% of normal levels following 4 hr of exposure to sodium azide and 2-deoxy-D-glucose) was not associated with a change in intracellular ADC. Long-term depletion of ATP (approximately 20% of normal levels following 2 days of exposure to antimycin A) was associated with a significant decrease in intracellular water ADC. These findings suggest that intracellular water diffusion in oocytes is not dependent on the state of microtubule polymerization or short-term ATP depletion, although long-term ATP depletion is associated with changes that lead to a decrease in intracellular water ADC.

Importance of intracellular water apparent diffusion to the measurement of membrane permeability

The exchange of water across biological membranes is of fundamental significance to both animal and plant physiology. Diffusional membrane permeability (P(d)) for the Xenopus oocyte, an important model system for water channel investigation, is typically calculated from intracellular water pre-exchange lifetime, cell volume, and cell surface area. There is debate, however, whether intracellular water motion affects water lifetime, and thereby P(d). Mathematical modeling of water transport is problematic because the intracellular water diffusion rate constant (D) for cells is usually unknown. The measured permeability may be referred to as the apparent diffusional permeability, P, to acknowledge this potential error. Herein, we show that magnetic resonance (MR) spectroscopy can be used to measure oocyte water exchange with greater temporal resolution and higher signal-to-noise ratio than other methods. MR imaging can be used to assess both oocyte geometry and intracellular water diffusion for the same single cells. MR imaging is used to confirm the dependence of intracellular water lifetime on intracellular diffusion. A model is presented to relate intracellular lifetime to true membrane diffusional permeability. True water diffusional permeability (2.7 +/- 0.4 microm/s) is shown to be 39 +/- 6% greater than apparent diffusional permeability for 8 oocytes. This discrepancy increases with cell size and permeability (such as after water channel expression) and decreases with increasing intracellular water D.

Evidence that both fast and slow water ADC components arise from intracellular space

Evaluation of water diffusion in the brain has revealed both fast- and slow-diffusing water populations. It has been suggested that these populations represent extra- and intracellular water, respectively. We have identified and characterized both populations in the intracellular space of the Xenopus oocyte. We have also determined their T(1) and T(2) relaxation properties. The fast and slow intracellular populations have diffusion coefficients of 1.06 +/- 0.05 microm(2)/ms and 0.16 +/- 0.02 microm(2)/ms, respectively, with the fast fraction representing 89% +/- 1% of the total water signal. These values are quite similar to those for total water in brain and are observed in the absence of signal from the perfusate (extracellular) water population. Volumetric swelling (16% +/- 4%) of the oocyte in hypoosmotic media increased the diffusion coefficients of both intracellular populations (fast = 1.27 +/- 0.03 microm(2)/ms, slow = 0.22 +/- 0.02 microm(2)/ms), but did not change their relative signal fractions. This phenomenon runs counter to the effects observed in brain injury, following which the apparent diffusion coefficient (ADC) decreases 30-50%. The results presented herein suggest that this ADC decrease in brain occurs despite cell swelling, which by itself would be expected to induce an increase in intracellular diffusion coefficients.

Mobility of ions, sugar, and water in the cytoplasm of Xenopus oocytes expressing Na(+)-coupled sugar transporters (SGLT1)

A model was set up to study water transport in membrane proteins expressed in Xenopus oocytes. The model was tested experimentally using human and rabbit Na+-glucose cotransporters (SGLT1), and was used to explain controversies regarding unstirred layer effects. Cotransport of Na+, sugar and water was monitored by two-electrode voltage clamp and online measurements of oocyte volume. The specific resistance of the oocyte cytoplasm was found by means of microelectrodes to be 263 +/- 91 Omega cm (S.D., n = 52), or 2.5 times that of Kulori medium, in agreement with reported values of intracellular ion concentrations and diffusion constants. Osmotically induced volume and resistance changes were compatible with a model of the oocyte in which 37 +/- 17 % (S.D., n = 66) of the intracellular volume acts as a free solution while the remainder is inert, being occupied by organelles, etc. The model explains the results of several types of experiments: rapid changes in rates of water cotransport induced by changes in clamp voltage followed by osmotic equilibration in sugar-free conditions; volume changes induced by Na+ transport via the ionophore gramicidin; and uphill water transport. Ethanol (0.5 %) induced a marked swelling of the oocytes of about 16 pl x s(-1). If the specific inhibitor of SGLT1 phlorizin is added from stock solutions in ethanol, the effect of ethanol obfuscates the effects of the inhibitor. We conclude that the transport parameters derived for water cotransport by the SGLT1 can be attributed to the protein residing in the plasma membrane with no significant influences from unstirred layer effects.

Apparent diffusion of water, ions, and small molecules in the Xenopus oocyte is consistent with Brownian displacement

The incoherent displacement of water in living tissues is of considerable interest because of the widespread use of diffusion-weighted MRI, for which image contrast is based on the water apparent diffusion coefficient (ADC). It has been hypothesized that the decrease in water ADC associated with brain injury is primarily due to a reduction in the ADC of water in the intracellular space. Xenopus oocytes permit direct measurement of ADC values for intracellular molecules, thereby providing insight into the nature of intracellular motion. In this study, the measured ADC values of small molecules and ions are shown to be primarily size-dependent, indicating that intracellular water motion in the oocyte is mainly Brownian displacement with little or no role for cytoplasmic streaming. Further, intracellular water ADC values show no dependence on diffusion time over a broad range (3.4-100 ms), suggesting that barriers to displacement are finely spaced (< or = 2-3 microm). The water diffusion shows some small anisotropy, suggesting that the cell has structure, giving water displacement a directional preference. The calculated intracellular apparent viscosity, which reflects the combined effects of barriers to motion, intermolecular binding, and fluid phase viscosity was 2.07 +/- 0.09 cP.