Microscopic interactions control a structural transition in active mixtures of microtubules and molecular motors.

Microtubules and molecular motors are essential components of the cellular cytoskeleton, driving fundamental processes in vivo, including chromosome segregation and cargo transport. When reconstituted in vitro, these cytoskeletal proteins serve as energy-consuming building blocks to study the self-organization of active matter. Cytoskeletal active gels display rich emergent dynamics, including extensile flows, locally contractile asters, and bulk contraction.

Dissipation and energy propagation across scales in an active cytoskeletal material.

Living systems are intrinsically nonequilibrium: They use metabolically derived chemical energy to power their emergent dynamics and self-organization. A crucial driver of these dynamics is the cellular cytoskeleton, a defining example of an active material where the energy injected by molecular motors cascades across length scales, allowing the material to break the constraints of thermodynamic equilibrium and display emergent nonequilibrium dynamics only possible due to the constant influx of energy. Notwithstanding recent experimental advances in the use of local probes to quantify entropy production and the breaking of detailed balance, little is known about the energetics of active materials or how energy propagates from the molecular to emergent length scales.

Odd dynamics of living chiral crystals.

Active crystals are highly ordered structures that emerge from the self-organization of motile objects, and have been widely studied in synthetic and bacterial active matter. Whether persistent  crystalline order can emerge  in groups of autonomously developing multicellular organisms is currently unknown. Here we show that swimming starfish embryos spontaneously assemble into chiral crystals that span thousands of spinning organisms and persist for tens of hours.

Physical bioenergetics: Energy fluxes, budgets, and constraints in cells.

Cells are the basic units of all living matter which harness the flow of energy to drive the processes of life. While the biochemical networks involved in energy transduction are well-characterized, the energetic costs and constraints for specific cellular processes remain largely unknown. In particular, what are the energy budgets of cells? What are the constraints and limits energy flows impose on cellular processes? Do cells operate near these limits, and if so how do energetic constraints impact cellular functions? Physics has provided many tools to study nonequilibrium systems and to define physical limits, but applying these tools to cell biology remains a challenge.

A Micromachined Picocalorimeter Sensor for Liquid Samples with Application to Chemical Reactions and Biochemistry.

Calorimetry has long been used to probe the physical state of a system by measuring the heat exchanged with the environment as a result of chemical reactions or phase transitions. Application of calorimetry to microscale biological samples, however, is hampered by insufficient sensitivity and the difficulty of handling liquid samples at this scale. Here, a micromachined calorimeter sensor that is capable of resolving picowatt levels of power is described.

Self-straining of actively crosslinked microtubule networks.

Cytoskeletal networks are foundational examples of active matter and central to self-organized structures in the cell. In vivo, these networks are active and densely crosslinked. Relating their large-scale dynamics to the properties of their constituents remains an unsolved problem.

Measuring the Energetic Costs of Embryonic Development.

What are the thermodynamic costs of development? In this issue of Developmental Cell, Rodenfels et al. (2019) demonstrate that the high energetic cost of coordinated cell division that is regulated by phospho-signaling gives rise to a measurable periodicity in the heat dissipated during zebrafish embryogenesis.

From cytoskeletal assemblies to living materials.

Many subcellular structures contain large numbers of cytoskeletal filaments. Such assemblies underlie much of cell division, motility, signaling, metabolism, and growth. Thus, understanding cell biology requires understanding the properties of networks of cytoskeletal filaments.

Lipopolysaccharide is transported to the cell surface by a membrane-to-membrane protein bridge.

Gram-negative bacteria have an outer membrane that serves as a barrier to noxious agents in the environment. This protective function is dependent on lipopolysaccharide, a large glycolipid located in the outer leaflet of the outer membrane. Lipopolysaccharide is synthesized at the cytoplasmic membrane and must be transported to the cell surface.

Cooperative Accumulation of Dynein-Dynactin at Microtubule Minus-Ends Drives Microtubule Network Reorganization.

Cytoplasmic dynein-1 is a minus-end-directed motor protein that transports cargo over long distances and organizes the intracellular microtubule (MT) network. How dynein motor activity is harnessed for these diverse functions remains unknown. Here, we have uncovered a mechanism for how processive dynein-dynactin complexes drive MT-MT sliding, reorganization, and focusing, activities required for mitotic spindle assembly.

Bridging length scales to measure polymer assembly.

Time-resolvable quantitative measurements of polymer concentration are very useful to elucidate protein polymerization pathways. There are numerous techniques to measure polymer concentrations in purified protein solutions, but few are applicable in vivo. Here we develop a methodology combining microscopy and spectroscopy to overcome the limitations of both approaches for measuring polymer concentration in cells and cell extracts.

Developing and Testing a Bayesian Analysis of Fluorescence Lifetime Measurements.

FRET measurements can provide dynamic spatial information on length scales smaller than the diffraction limit of light. Several methods exist to measure FRET between fluorophores, including Fluorescence Lifetime Imaging Microscopy (FLIM), which relies on the reduction of fluorescence lifetime when a fluorophore is undergoing FRET. FLIM measurements take the form of histograms of photon arrival times, containing contributions from a mixed population of fluorophores both undergoing and not undergoing FRET, with the measured distribution being a mixture of exponentials of different lifetimes.

Active contraction of microtubule networks.

Many cellular processes are driven by cytoskeletal assemblies. It remains unclear how cytoskeletal filaments and motor proteins organize into cellular scale structures and how molecular properties of cytoskeletal components affect the large-scale behaviors of these systems. Here, we investigate the self-organization of stabilized microtubules in Xenopus oocyte extracts and find that they can form macroscopic networks that spontaneously contract.

Concentration of various trace elements in the rat retina and their distribution in different structures.

Inductively coupled plasma mass spectrometry (ICP-MS) was used to quantify the total amount of trace elements in retina from adult male Sprague-Dawley rats (n = 6). Concentration of trace elements within individual retinal areas in frozen sections of the fellow eye was established with the use of two methodologies: (1) particle-induced X-ray emission (PIXE) in combination with 3D depth profiling with Rutherford backscattering spectrometry (RBS) and (2) synchrotron X-ray fluorescence (SXRF) microscopy. The most abundant metal in the retina was zinc, followed by iron and copper.

Prevalence of short stature in Saudi children and adolescents.

Background And Objective: Data on stature in Saudi children and adolescents are limited. The objective of this report was to establish the national prevalence of short stature in Saudi children and adolescents.

Design And Setting: Community-based, cross-sectional study conducted over 2 years (2004, 2005)

Patients And Methods: The national data set of the Saudi reference was used to calculate the stature for age for children and adolescents 5 to 18 years of age.

Prevalence of malnutrition in Saudi children: a community-based study.

Background And Objective: There is no published information on the prevalence of malnutrition in Saudi Arabia. The objective of this study was to establish the prevalence data.

Methods: The prevalence of nutritional indicators in the form of underweight, stunting, and wasting in a national sample of children younger than 5 years of age was calculated using the new WHO standards as reference.

Regional disparity in prevalence of malnutrition in Saudi children.

Objective: To evaluate the regional difference in the prevalence of malnutrition in Saudi children.

Methods: Data for this study were collected over 2 years (2004 and 2005). A cross-sectional representative sample of the Saudi population of healthy children below 5 years of age was used to calculate the prevalence of malnutrition.

Prevalence of overweight and obesity in Saudi children and adolescents.

Background And Objective: There is limited information on overweight and obesity in Saudi children and adolescents. The objective of this study was to establish the national prevalence of overweight and obesity in Saudi children and adolescents.

Methods: The 2005 Saudi reference data set was used to calculate the body mass index (BMI) for children aged 5 to 18 years.

Pattern of sex differences in growth of Saudi children and adolescents.

Background: Although variations in growth between boys and girls have been reported, detailed descriptions according to age and growth parameters are not available.

Objective: The goal of this study was to determine the pattern and magnitude of differences in growth between boys and girls according to age that justify separate growth charts.

Methods: The data set was based on a cross-sectional representative sample of the Saudi population of healthy children and adolescents from birth to 19 years of age.

Body mass index in Saudi Arabian children and adolescents: a national reference and comparison with international standards.

Background And Objectives: Because there are no reference standards for body mass index (BMI) in Saudi children, we established BMI reference percentiles for normal Saudi Arabian children and adolescents and compared them with international standards.

Subjects And Methods: Data from a stratified multistage probability sample were collected from the 13 health regions in Saudi Arabia, as part of a nationwide health profile survey of Saudi Arabian children and adolescents conducted to establish normal physical growth references. Selected households were visited by a trained team.

Short-term growth in children with congenital adrenal hyperplasia.

Objective: To describe short-term growth patterns in children with congenital adrenal hyperplasia (CAH).

Methods: Height was measured daily in 5 children (1 boy) aged 3.9-9.

Comparison of the 2005 growth charts for Saudi children and adolescents to the 2000 CDC growth charts.

Background And Objectives: The 2000 CDC growth charts for the United States, a revision of the National Center for Health Statistics/World Health Organization (NCHS/WHO) growth charts, were released in 2002 to replace the NCHS/WHO charts. We evaluated the differences between the CDC growth charts and the Saudi 2005 reference to determine the implications of using the 2000 CDC growth charts in Saudi children and adolescents.

Subjects And Methods: The Saudi reference was based on a cross-sectional representative sample of the Saudi population of healthy children and adolescents from birth to 19 years of age.