Gravity sensing in plant and animal cells

High-resolution, high-throughput analysis of Drosophila geotactic behavior

Drosophila innate response to gravity, geotaxis, has been previously used to assess the impact of aging and disease on motor performance. Despite its rich history, fly geotaxis continues to be largely measured manually and assessed through simplistic metrics. The manual nature of this assay introduces substantial experimental variability while simplistic metrics provide limited analytic insights into the behavior. To address these shortcomings, we have constructed a fully automated, programable apparatus, and developed a multi-object tracking software capable of following sub-second movements of individual flies, thus allowing reproducible, detailed, and quantitative analysis of geotactic behavior. The apparatus triggers and monitors geotaxis of 10 fly cohorts simultaneously, with each cohort consisting of up to 7 flies. The tracking program isolates cohorts and records individual fly coordinate outputs allowing for simultaneous multi-group, multi-fly tracks per experiment, greatly improving throughput and resolution. The algorithm tracks individual flies during the entire run with ~97% accuracy, yielding detailed climbing curve, speed, and movement direction with 1/30 second resolution. Our tracking also allows the construction of multi-variable metrics and the detection of transitory movement phenotypes, such as slips and falls, which have thus far been neglected in geotaxis studies due to limited spatio-temporal resolution. Through a combination of automation and robust tracking, the platform is therefore poised to advance Drosophila geotaxis assay into a comprehensive assessment of locomotor behavior.

Autophagy formation, microtubule disorientation, and alteration of ATG8 and tubulin gene expression under simulated microgravity in Arabidopsis thaliana

Autophagy plays an important role in plant growth and development, pathogen invasion and modulates plant response and adaptation to various abiotic stress stimuli. The biogenesis and trafficking of autophagosomes involve microtubules (MTs) as important actors in the autophagic process. However, initiation of autophagy in plants under microgravity has not been previously studied. Here we demonstrate how simulated microgravity induces autophagy development involving microtubular reorganization during period of autophagosome formation. It was shown that induction of autophagy with maximal autophagosome formation in root cells of Arabidopsis thaliana is observed after 6 days of clinostating, along with MT disorganization, which leads to visible changes in root morphology. Gradual decrease of autophagosome number was indicated on 9th and 12th days of the experiment as well as no significant re-orientation of MTs were identified. Respectively, analysis of α- and β-tubulins and ATG8 gene expression was carried out. In particular, the most pronounced increase of expression on both 6th and 9th days in response to simulated microgravity was detected for non-paralogous AtATG8b, AtATG8f, AtATG8i, and AtTUA2, AtTUA3 genes, as well as for the pair of β-tubulin duplicates, namely AtTUB2 and AtTUB3. Overall, the main autophagic response was observed after 6 and 9 days of exposure to simulated microgravity, followed by adaptive response after 12 days. These findings provide a key basis for further studies of cellular mechanisms of autophagy and involvement of cytoskeletal structures in autophagy biogenesis under microgravity, which would enable development of new approaches, aimed on enhancing plant adaptation to microgravity.

Rapid translocation of NGR proteins driving polarization of PIN-activating D6 protein kinase during root gravitropism

Root gravitropic bending represents a fundamental aspect of terrestrial plant physiology. Gravity is perceived by sedimentation of starch-rich plastids (statoliths) to the bottom of the central root cap cells. Following gravity perception, intercellular auxin transport is redirected downwards leading to an asymmetric auxin accumulation at the lower root side causing inhibition of cell expansion, ultimately resulting in downwards bending. How gravity-induced statoliths repositioning is translated into asymmetric auxin distribution remains unclear despite PIN auxin efflux carriers and the Negative Gravitropic Response of roots (NGR) proteins polarize along statolith sedimentation, thus providing a plausible mechanism for auxin flow redirection. In this study, using a functional NGR1-GFP construct, we visualized the NGR1 localization on the statolith surface and plasma membrane (PM) domains in close proximity to the statoliths, correlating with their movements. We determined that NGR1 binding to these PM domains is indispensable for NGR1 functionality and relies on cysteine acylation and adjacent polybasic regions as well as on lipid and sterol PM composition. Detailed timing of the early events following graviperception suggested that both NGR1 repolarization and initial auxin asymmetry precede the visible PIN3 polarization. This discrepancy motivated us to unveil a rapid, NGR-dependent translocation of PIN-activating AGCVIII kinase D6PK towards lower PMs of gravity-perceiving cells, thus providing an attractive model for rapid redirection of auxin fluxes following gravistimulation.

Hardware Development for Plant Cultivation Allowing In Situ Fluorescence Analysis of Calcium Fluxes in Plant Roots Under Microgravity and Ground-Control Conditions

Maintaining an optimal leaf and stem orientation to yield a maximum photosynthetic output is accomplished by terrestrial plants using sophisticated mechanisms to balance their orientation relative to the Earth's gravity vector and the direction of sunlight. Knowledge of the signal transduction chains of both gravity and light perception and how they influence each other is essential for understanding plant development on Earth and plant cultivation in space environments. However, in situ analyses of cellular signal transduction processes in weightlessness, such as live cell imaging of signaling molecules using confocal fluorescence microscopy, require an adapted experimental setup that meets the special requirements of a microgravity environment. In addition, investigations under prolonged microgravity conditions require extensive resources, are rarely accessible, and do not allow for immediate sample preparation for the actual microscopic analysis. Therefore, supply concepts are needed that ensure both the viability of the contained plants over a longer period of time and an unhindered microscopic analysis in microgravity. Here, we present a customized supply unit specifically designed to study gravity-induced Ca2+ mobilization in roots of Arabidopsis thaliana. The unit can be employed for ground-based experiments, in parabolic flights, on sounding rockets, and probably also aboard the International Space Station.

Morphological Changes of 3T3 Cells under Simulated Microgravity

BACKGROUND

Cells are sensitive to changes in gravity, especially the cytoskeletal structures that determine cell morphology. The aim of this study was to assess the effects of simulated microgravity (SMG) on 3T3 cell morphology, as demonstrated by a characterization of the morphology of cells and nuclei, alterations of microfilaments and microtubules, and changes in cycle progression.

METHODS

3T3 cells underwent induced SMG for 72 h with Gravite®, while the control group was under 1G. Fluorescent staining was applied to estimate the morphology of cells and nuclei and the cytoskeleton distribution of 3T3 cells. Cell cycle progression was assessed by using the cell cycle app of the Cytell microscope, and Western blot was conducted to determine the expression of the major structural proteins and main cell cycle regulators.

RESULTS

The results show that SMG led to decreased nuclear intensity, nuclear area, and nuclear shape and increased cell diameter in 3T3 cells. The 3T3 cells in the SMG group appeared to have a flat form and diminished microvillus formation, while cells in the control group displayed an apical shape and abundant microvilli. The 3T3 cells under SMG exhibited microtubule distribution surrounding the nucleus, compared to the perinuclear accumulation in control cells. Irregular forms of the contractile ring and polar spindle were observed in 3T3 cells under SMG. The changes in cytoskeleton structure were caused by alterations in the expression of major cytoskeletal proteins, including β-actin and α-tubulin 3. Moreover, SMG induced 3T3 cells into the arrest phase by reducing main cell cycle related genes, which also affected the formation of cytoskeleton structures such as microfilaments and microtubules.

CONCLUSIONS

These results reveal that SMG generated morphological changes in 3T3 cells by remodeling the cytoskeleton structure and downregulating major structural proteins and cell cycle regulators.

Cellular and physiological functions of SGR family in gravitropic response in higher plants

BACKGROUND

In plants, gravity directs bidirectional growth; it specifies upward growth of shoots and downward growth of roots. Due to gravity, roots establish robust anchorage and shoot, which enables to photosynthesize. It sets optimum posture and develops plant architecture to efficiently use resources like water, nutrients, CO2, and gaseous exchange. Hence, gravitropism is crucial for crop productivity as well as for the growth of plants in challenging climate. Some SGR members are known to affect tiller and shoot angle, organ size, and inflorescence stem in plants.

AIM OF REVIEW

Although the SHOOT GRAVITROPISM (SGR) family plays a key role in regulating the fate of shoot gravitropism, little is known about its function compared to other proteins involved in gravity response in plant cells and tissues. Moreover, less information on the SGR family's physiological activities and biochemical responses in shoot gravitropism is available. This review scrutinizes and highlights the recent developments in shoot gravitropism and provides an outlook for future crop development, multi-application scenarios, and translational research to improve agricultural productivity.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Plants have evolved multiple gene families specialized in gravitropic responses, of which the SGR family is highly significant. The SGR family regulates the plant's gravity response by regulating specific physiological and biochemical processes such as transcription, cell division, amyloplast sedimentation, endodermis development, and vacuole formation. Here, we analyze the latest discoveries in shoot gravitropism with particular attention to SGR proteins in plant cell biology, cellular physiology, and homeostasis. Plant cells detect gravity signals by sedimentation of amyloplast (starch granules) in the direction of gravity, and the signaling cascade begins. Gravity sensing, signaling, and auxin redistribution (organ curvature) are the three components of plant gravitropism. Eventually, we focus on the role of multiple SGR genes in shoot and present a complete update on the participation of SGR family members in gravity.

Fetal head-down posture may explain the rapid brain evolution in humans and other primates: An interpretative review

Evolutionary cerebrovascular consequences of upside-down postural verticality of the anthropoid fetus have been largely overlooked in the literature. This working hypothesis-based report provides a literature interpretation from an aspect that the rapid evolution of the human brain has been promoted by fetal head-down position due to maternal upright and semi-upright posture. Habitual vertical torso posture is a feature not only of humans, but also of monkeys and non-human apes that spend considerable time in a sitting position. Consequently, the head-down position of the fetus may have caused physiological craniovascular hypertension that stimulated expansion of the intracranial vessels and acted as an epigenetic physiological stress, which enhanced neurogenesis and eventually, along with other selective pressures, led to the progressive growth of the anthropoid brain and its organization. This article collaterally opens a new insight into the conundrum of high cephalopelvic proportions (i.e., the tight fit between the pelvic birth canal and fetal head) in phylogenetically distant lineages of monkeys, lesser apes, and humans. Low cephalopelvic proportions in non-human great apes could be accounted for by their energetically efficient horizontal nest-sleeping and consequently by their larger body mass compared to monkeys and lesser apes that sleep upright. One can further hypothesize that brain size varies in anthropoids according to the degree of exposure of the fetus to postural verticality. The supporting evidence for this postulation includes a finding that in fossil hominins cerebral blood flow rate increased faster than brain volume. This testable hypothesis opens a perspective for research on fetal postural cerebral hemodynamics.

A single amino acid substitution in MdLAZY1A dominantly impairs shoot gravitropism in Malus

Plant architecture is 1 of the most important factors that determines crop yield potential and productivity. In apple (Malus domestica), genetic improvement of tree architecture has been challenging due to a long juvenile phase and growth as complex trees composed of a distinct scion and a rootstock. To better understand the genetic control of apple tree architecture, the dominant weeping growth phenotype was investigated. We report the identification of MdLAZY1A (MD13G1122400) as the genetic determinant underpinning the Weeping (W) locus that largely controls weeping growth in Malus. MdLAZY1A is 1 of the 4 paralogs in apple that are most closely related to AtLAZY1 involved in gravitropism in Arabidopsis (Arabidopsis thaliana). The weeping allele (MdLAZY1A-W) contains a single nucleotide mutation c.584T>C that leads to a leucine to proline (L195P) substitution within a predicted transmembrane domain that colocalizes with Region III, 1 of the 5 conserved regions in LAZY1-like proteins. Subcellular localization revealed that MdLAZY1A localizes to the plasma membrane and nucleus in plant cells. Overexpressing the weeping allele in apple cultivar Royal Gala (RG) with standard growth habit impaired its gravitropic response and altered the growth to weeping-like. Suppressing the standard allele (MdLAZY1A-S) by RNA interference (RNAi) in RG similarly changed the branch growth direction to downward. Overall, the L195P mutation in MdLAZY1A is genetically causal for weeping growth, underscoring not only the crucial roles of residue L195 and Region III in MdLAZY1A-mediated gravitropic response but also a potential DNA base editing target for tree architecture improvement in Malus and other crops.

Perspectives on Computation in Plants

Plants thrive in virtually all natural and human-adapted environments and are becoming popular models for developing robotics systems because of their strategies of morphological and behavioral adaptation. Such adaptation and high plasticity offer new approaches for designing, modeling, and controlling artificial systems acting in unstructured scenarios. At the same time, the development of artifacts based on their working principles reveals how plants promote innovative approaches for preservation and management plans and opens new applications for engineering-driven plant science. Environmentally mediated growth patterns (e.g., tropisms) are clear examples of adaptive behaviors displayed through morphological phenotyping. Plants also create networks with other plants through subterranean roots-fungi symbiosis and use these networks to exchange resources or warning signals. This article discusses the functional behaviors of plants and shows the close similarities with a perceptron-like model that could act as a behavior-based control model in plants. We begin by analyzing communication rules and growth behaviors of plants; we then show how we translated plant behaviors into algorithmic solutions for bioinspired robot controllers; and finally, we discuss how those solutions can be extended to embrace original approaches to networking and robotics control architectures.

Gravity's effect on biology

Gravity is a fundamental interaction that permeates throughout our Universe. On Earth, gravity gives weight to physical objects, and has been a constant presence throughout terrestrial biological evolution. Thus, gravity has shaped all biological functions, some examples include the growth of plants (e.g., gravitropism), the structure and morphology of biological parts in multicellular organisms, to its effects on our physiological function when humans travel into space. Moreover, from an evolutionary perspective, gravity has been a constant force on biology, and life, to our understanding, should have no reason to not experience the effects of gravity. Interestingly, there appear to be specific biological mechanisms that activate in the absence of gravity, with the space environment the only location to study the effects of a lack of gravity on biological systems. Thus, in this perspective piece, biological adaptations from the cellular to the whole organism levels to the presence and absence of gravity will be organized and described, as well as outlining future areas of research for gravitational biological investigations to address. Up to now, we have observed and shown how gravity effects biology at different levels, with a few examples including genetic (e.g., cell cycle, metabolism, signal transduction associated pathways, etc.), biochemically (e.g., cytoskeleton, NADPH oxidase, Yes-associated protein, etc.), and functionally (e.g., astronauts experiencing musculoskeletal and cardiovascular deconditioning, immune dysfunction, etc., when traveling into space). Based from these observations, there appear to be gravity-sensitive and specific pathways across biological organisms, though knowledge gaps of the effects of gravity on biology remain, such as similarities and differences across species, reproduction, development, and evolutionary adaptations, sex-differences, etc. Thus, here an overview of the literature is provided for context of gravitational biology research to-date and consideration for future studies, as we prepare for long-term occupation of low-Earth Orbit and cis-Lunar space, and missions to the Moon and Mars, experiencing the effects of Lunar and Martian gravity on biology, respectively, through our Artemis program.

Understanding the role of starch sheath layer in graviception of Alternanthera philoxeroides: a biophysical and microscopical study

Plants' ability to sense and respond to gravity is a unique and fundamental process. When a plant organ is tilted, it adjusts its growth orientation relative to gravity direction, which is achieved by a curvature of the organ. In higher, multicellular plants, it is thought that the relative directional change of gravity is detected by starch-filled organelles that occur inside specialized cells called statocytes, and this is followed by signal conversion from physical information to physiological information within the statocytes. The classic starch statolith hypothesis, i.e., the starch accumulating amyloplasts movement along the gravity vector within gravity-sensing cells (statocytes) is the probable trigger of subsequent intracellular signaling, is widely accepted. Acharya Jagadish Chandra Bose through his pioneering research had investigated whether the fundamental reaction of geocurvature is contractile or expansive and whether the geo-sensing cells are diffusedly distributed in the organ or are present in the form of a definite layer. In this backdrop, a microscopy based experimental study was undertaken to understand the distribution pattern of the gravisensing layer, along the length (node-node) of the model plant Alternanthera philoxeroides and to study the microrheological property of the mobile starch-filled statocytes following inclination-induced graviception in the stem of the model plant. The study indicated a prominent difference in the pattern of distribution of the gravisensing layer along the length of the model plant. The study also indicated that upon changing the orientation of the plant from vertical position to horizontal position there was a characteristic change in orientation of the mobile starch granules within the statocytes. In the present study for the analysis of the microscopic images of the stem tissue cross sections, a specialized and modified microscopic illumination setup was developed in the laboratory in order to enhance the resolution and contrast of the starch granules.

"Force-From-Lipids" Dependence of the MscCG Mechanosensitive Channel Gating on Anionic Membranes

Mechanosensory transduction in Corynebacterium glutamicum plays a major role in glutamate efflux for industrial MSG, whose production depends on the activation of MscCG-type mechanosensitive channels. Dependence of the MscCG channel activation by membrane tension on the membrane lipid content has to date not been functionally characterized. Here, we report the MscCG channel patch clamp recording from liposomes fused with C. glutamicum membrane vesicles as well as from proteoliposomes containing the purified MscCG protein. Our recordings demonstrate that mechanosensitivity of MscCG channels depends significantly on the presence of negatively charged lipids in the proteoliposomes. MscCG channels in liposome preparations fused with native membrane vesicles exhibited the activation threshold similar to the channels recorded from C. glutamicum giant spheroplasts. In comparison, the activation threshold of the MscCG channels reconstituted into azolectin liposomes was higher than the activation threshold of E. coli MscL, which is gated by membrane tension close to the bilayer lytic tension. The spheroplast-like activation threshold was restored when the MscCG channels were reconstituted into liposomes made of E. coli polar lipid extract. In liposomes made of polar lipids mixed with synthetic phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin, the activation threshold of MscCG was significantly reduced compared to the activation threshold recorded in azolectin liposomes, which suggests the importance of anionic lipids for the channel mechanosensitivity. Moreover, the micropipette aspiration technique combined with patch fluorometry demonstrated that membranes containing anionic phosphatidylglycerol are softer than membranes containing only polar non-anionic phosphatidylcholine and phosphatidylethanolamine. The difference in mechanosensitivity between C. glutamicum MscCG and canonical MscS of E. coli observed in proteoliposomes explains the evolutionary tuning of the force from lipids sensing in various bacterial membrane environments.

Plant responses to hypergravity: a comprehensive review

Hypergravity is an effective novel stimulus to elucidate plant gravitational and mechanobiological behaviour. Here, we review the current understanding of phenotypic, physio-biochemical, and molecular plant responses to simulated hypergravity. Plants readily respond to altered gravity conditions, such as microgravity or hypergravity. Hypergravity-a gravitational force higher than that on the Earth's surface (> 1g)-can be simulated using centrifuges. Exposing seeds, seedlings, or plant cell cultures to hypergravity elicits characteristic morphological, physio-biochemical, and molecular changes. While several studies have provided insights into plant responses and underlying mechanisms, much is still elusive, including the interplay of hypergravity with gravitropism. Moreover, hypergravity is of great significance for mechano- and space/gravitational biologists to elucidate fundamental plant behaviour. In this review, we provide an overview of the phenotypic, physiological, biochemical, and molecular responses of plants to hypergravity. We then discuss the involvement of hypergravity in plant gravitropism-the directional growth along the gravity vector. Finally, we highlight future research directions to expand our understanding of hypergravity in plant biology.

Mechanosensing, from forces to structures

Sessile plants evolve diverse structures in response to complex environmental cues. These factors, in essence, involve mechanical stimuli, which must be sensed and coordinated properly by the plants to ensure effective growth and development. While we have accumulated substantial knowledge on plant mechanobiology, how plants translate mechanical information into three-dimensional structures is still an open question. In this review, we summarize our current understanding of plant mechanosensing at different levels, particularly using Arabidopsis as a model plant system. We also attempt to abstract the mechanosensing process and link the gaps from mechanical cues to the generation of complex plant structures. Here we review the recent advancements on mechanical response and transduction in plant morphogenesis, and we also raise several questions that interest us in different sections.

Genetic and cellular characterization of MscS-like putative channels in the filamentous fungus Aspergillus nidulans

Mechanosensitive ion channels are integral membrane proteins ubiquitously present in bacteria, archaea, and eukarya. They act as molecular sensors of mechanical stress to serve vital functions such as touch, hearing, osmotic pressure, proprioception and balance, while their malfunction is often associated with pathologies. Amongst them, the structurally distinct MscL and MscS channels from bacteria are the most extensively studied. MscS-like channels have been found in plants and Schizosaccharomyces pombe, where they regulate intracellular Ca²⁺ and cell volume under hypo-osmotic conditions. Here we characterize two MscS-like putative channels, named MscA and MscB, from the model filamentous fungus Aspergillus nidulans. Orthologues of MscA and MscB are present in most fungi, including relative plant and animal pathogens. MscA/MscB and other fungal MscS-like proteins share the three transmembrane helices and the extended C-terminal cytosolic domain that form the structural fingerprint of MscS-like channels with at least three additional transmembrane segments than Escherichia coli MscS. We show that MscA and MscB localize in Endoplasmic Reticulum and the Plasma Membrane, respectively, whereas their overexpression leads to increased CaCl₂ toxicity or/and reduction of asexual spore formation. Our findings contribute to understanding the role of MscS-like channels in filamentous fungi and relative pathogens.

Space Biomedicine: A Unique Opportunity to Rethink the Relationships between Physics and Biology

Space biomedicine has provided significant technological breakthroughs by developing new medical devices, diagnostic tools, and health-supporting systems. Many of these products are currently in use onboard the International Space Station and have been successfully translated into clinical practice on Earth. However, biomedical research performed in space has disclosed exciting, new perspectives regarding the relationships between physics and medicine, thus fostering the rethinking of the theoretical basis of biology. In particular, these studies have stressed the critical role that biophysical forces play in shaping the function and pattern formation of living structures. The experimental models investigated under microgravity conditions allow us to appreciate the complexity of living organisms through a very different perspective. Indeed, biological entities should be conceived as a unique magnification of physical laws driven by local energy and order states overlaid by selection history and constraints, in which the source of the inheritance, variation, and process of selection has expanded from the classical Darwinian definition. The very specific nature of the field in which living organisms behave and evolve in a space environment can be exploited to decipher the underlying, basic processes and mechanisms that are not apparent on Earth. In turn, these findings can provide novel opportunities for testing pharmacological countermeasures that can be instrumental for managing a wide array of health problems and diseases on Earth.

Impairment of 7F2 osteoblast function by simulated partial gravity in a Random Positioning Machine

The multifaceted adverse effects of reduced gravity pose a significant challenge to human spaceflight. Previous studies have shown that bone formation by osteoblasts decreases under microgravity conditions, both real and simulated. However, the effects of partial gravity on osteoblasts’ function are less well understood. Utilizing the software-driven newer version of the Random Positioning Machine (RPM^SW), we simulated levels of partial gravity relevant to future manned space missions: Mars (0.38 G), Moon (0.16 G), and microgravity (Micro, ~10⁻³ G). Short-term (6 days) culture yielded a dose-dependent reduction in proliferation and the enzymatic activity of alkaline phosphatase (ALP), while long-term studies (21 days) showed a distinct dose-dependent inhibition of mineralization. By contrast, expression levels of key osteogenic genes (Alkaline phosphatase, Runt-related Transcription Factor 2, Sparc/osteonectin) exhibited a threshold behavior: gene expression was significantly inhibited when the cells were exposed to Mars-simulating partial gravity, and this was not reduced further when the cells were cultured under simulated Moon or microgravity conditions. Our data suggest that impairment of cell function with decreasing simulated gravity levels is graded and that the threshold profile observed for reduced gene expression is distinct from the dose dependence observed for cell proliferation, ALP activity, and mineral deposition. Our study is of relevance, given the dearth of research into the effects of Lunar and Martian gravity for forthcoming space exploration.

Adaptation and Changes in Actin Dynamics and Cell Motility as Early Responses of Cultured Mammalian Cells to Altered Gravitational Vector

Cultured mammalian cells have been shown to respond to microgravity (μG), but the molecular mechanism is still unknown. The study we report here is focused on molecular and cellular events that occur within a short period of time, which may be related to gravity sensing by cells. Our assumption is that the gravity-sensing mechanism is activated as soon as cells are exposed to any new gravitational environment. To study the molecular events, we exposed cells to simulated μG (SμG) for 15 min, 30 min, 1 h, 2 h, 4 h, and 8 h using a three-dimensional clinostat and made cell lysates, which were then analyzed by reverse phase protein arrays (RPPAs) using a panel of 453 different antibodies. By comparing the RPPA data from cells cultured at 1G with those of cells under SμG, we identified a total of 35 proteomic changes in the SμG samples and found that 20 of these changes took place, mostly transiently, within 30 min. In the 4 h and 8 h samples, there were only two RPPA changes, suggesting that the physiology of these cells is practically indistinguishable from that of cells cultured at 1 G. Among the proteins involved in the early proteomic changes were those that regulate cell motility and cytoskeletal organization. To see whether changes in gravitational environment indeed activate cell motility, we flipped the culture dish upside down (directional change in gravity vector) and studied cell migration and actin cytoskeletal organization. We found that compared with cells grown right-side up, upside-down cells transiently lost stress fibers and rapidly developed lamellipodia, which was supported by increased activity of Ras-related C3 botulinum toxin substrate 1 (Rac1). The upside-down cells also increased their migratory activity. It is possible that these early molecular and cellular events play roles in gravity sensing by mammalian cells. Our study also indicated that these early responses are transient, suggesting that cells appear to adapt physiologically to a new gravitational environment.

Role of the Earth's Motions in Plant Orientation - Planetary Mechanism

According to the proposed theory, the starch-rich particles (statoliths) help the plant to convert the signals from Earth's motions into the signals necessary for the plant to perceive its orientation relative to the gravity vector while moving freely because of inertia in the sensory cells (statocytes) of roots and stems. Motions of the Earth are never constant, which, in particular, refers to the so-called polar motions and oscillations of the planet's rotation axis. Statoliths at any given moment move in the cytoplasmic liquid of statocytes due to inertial motion initiated by the action of the Earth's movements, maintaining the trajectory set by the previous movement of the oscillating planet. Unlike statoliths, the walls of a statocyte move in space along with the entire plant and with the Earth, in strict accordance with the current direction of motion of the planet's axis. This leads to the inevitable collision of statoliths with the statocytic wall/membrane. Cytoplasmic liquid, as a substance that is not able to maintain its shape, does not interfere with the inertial motions of the statoliths and collision with the wall of the statocyte. By striking the membrane, statoliths cause the release of ions and other factors at the impact site, which further participate in the gravitropic process. Pressure of the sediment of statoliths at the bottom of the statocyte, as well as position of this sediment, are not the defining factors of gravitropism.

Trabecular bone organoids: a micron-scale 'humanised' prototype designed to study the effects of microgravity and degeneration

Bone is a highly responsive organ, which continuously adapts to the environment it is subjected to in order to withstand metabolic demands. These events are difficult to study in this particular tissue in vivo, due to its rigid, mineralised structure and inaccessibility of the cellular component located within. This manuscript presents the development of a micron-scale bone organoid prototype, a concept that can allow the study of bone processes at the cell-tissue interface. The model is constructed with a combination of primary female osteoblastic and osteoclastic cells, seeded onto femoral head micro-trabeculae, where they recapitulate relevant phenotypes and functions. Subsequently, constructs are inserted into a simulated microgravity bioreactor (NASA-Synthecon) to model a pathological state of reduced mechanical stimulation. In these constructs, we detected osteoclastic bone resorption sites, which were different in morphology in the simulated microgravity group compared to static controls. Once encapsulated in human fibrin and exposed to analogue microgravity for 5 days, masses of bone can be observed being lost from the initial structure, allowing to simulate the bone loss process further. Constructs can function as multicellular, organotypic units. Large osteocytic projections and tubular structures develop from the initial construct into the matrix at the millimetre scale. Micron-level fragments from the initial bone structure are detected travelling along these tubules and carried to sites distant from the native structure, where new matrix formation is initiated. We believe this model allows the study of fine-level physiological processes, which can shed light into pathological bone loss and imbalances in bone remodelling.