On the vital role of fluid movement in organisms and cells: a brief historical account from Harvey to Coulson, extending the hypothesis of circulation

Only one health, and so many omics

The development of new approaches based on wide profiling methods in studying biological and medical systems is bringing large amounts of data on a daily basis. The causes of complex diseases have been directed to the genome examination bringing formidable knowledge. We can study genome, but also proteome, exome, transcriptome, epigenome, metabolome, and newcomers too such as microbiome, connectome and exposome. The title of this editorial is paraphrasing the famous saying of Victor Schlichter from Buenos Aires children hospital in Argentina who said "How unfair! Only one health, and so many diseases". Today there is indeed a whole lot of omics. We think that we are lucky to have all the omics possible, but we also wanted to stress the importance of future holistic approach in integrating the knowledge omics has rewarded us.

Anomalous diffusion of brain metabolites evidenced by diffusion-weighted magnetic resonance spectroscopy in vivo

Translational displacement of molecules within cells is a key process in cellular biology. Molecular motion potentially depends on many factors, including active transport, cytosol viscosity and molecular crowding, tortuosity resulting from cytoskeleton and organelles, and restriction barriers. However, the relative contribution of these factors to molecular motion in the cytoplasm remains poorly understood. In this work, we designed an original diffusion-weighted magnetic resonance spectroscopy strategy to probe molecular motion at subcellular scales in vivo. This led to the first observation of anomalous diffusion, that is, dependence of the apparent diffusion coefficient (ADC) on the diffusion time, for endogenous intracellular metabolites in the brain. The observed increase of the ADC at short diffusion time yields evidence that metabolite motion is characteristic of hindered random diffusion rather than active transport, for time scales up to the dozen milliseconds. Armed with this knowledge, data modeling based on geometrically constrained diffusion was performed. Results suggest that metabolite diffusion occurs in a low-viscosity cytosol hindered by ∼2-μm structures, which is consistent with known intracellular organization.

Metabolic scaling: consensus or controversy?

Background

The relationship between body mass (M) and standard metabolic rate (B) among living organisms remains controversial, though it is widely accepted that in many cases B is approximately proportional to the three-quarters power of M.

Results

The biological significance of the straight-line plots obtained over wide ranges of species when B is plotted against log M remains a matter of debate. In this article we review the values ascribed to the gradients of such graphs (typically 0.75, according to the majority view), and we assess various attempts to explain the allometric power-law phenomenon, placing emphasis on the most recent publications.

Conclusion

Although many of the models that have been advanced have significant attractions, none can be accepted without serious reservations, and the possibility that no one model can fit all cases has to be more seriously entertained.

Diffusion, perfusion and the exclusion principles in the structural and functional organization of the living cell: reappraisal of the properties of the 'ground substance'

The thesis is presented that only within very small microdomains of the cell internum might diffusion operate in the sorting of molecular affinities. Much of cell metabolism is guided and controlled in rate by the speed with which molecules that have to interact encounter one another. What is clear, however, is that the cell does not have a single 'modus operandi' but has the choice of many different strategies, each of which can contribute in different proportion to the rate of ongoing activity. It is probably our own desire to simplify things and use the most (or more) probable strategy that confines our appreciation of the overall robustness of the cell as a 'survival machine'. The main operative process at any given time (perfusion, diffusion or whatever) has always to be considered very carefully in relation to the organisational structure of the cell, which can be transient and fickle but nevertheless has been seen as involving an extensive cytomatrix, a ground substance, within an aqueous environment in which the degree of water structuring is even more fickle.

Diffusion NMR spectroscopy

MR offers unique tools for measuring molecular diffusion. This review focuses on the use of diffusion-weighted MR spectroscopy (DW-MRS) to non-invasively quantitate the translational displacement of endogenous metabolites in intact mammalian tissues. Most of the metabolites that are observed by in vivo MRS are predominantly located in the intracellular compartment. DW-MRS is of fundamental interest because it enables one to probe the in situ status of the intracellular space from the diffusion characteristics of the metabolites, while at the same time providing information on the intrinsic diffusion properties of the metabolites themselves. Alternative techniques require the introduction of exogenous probe molecules, which involves invasive procedures, and are also unable to measure molecular diffusion in and throughout intact tissues. The length scale of the process(es) probed by MR is in the micrometer range which is of the same order as the dimensions of many intracellular entities. DW-MRS has been used to estimate the dimensions of the cellular elements that restrict intracellular metabolite diffusion in muscle and nerve tissue. In addition, it has been shown that DW-MRS can provide novel information on the cellular response to pathophysiological changes in relation to a range of disorders, including ischemia and excitotoxicity of the brain and cancer.

Random walks and cell size

For many years, it has been believed that diffusion is the principle motive force for distributing molecules within the cell. Yet, our current information about the cell makes this improbable. Furthermore, the argument that limitations responsible for the relative constancy of cell size--which seldom varies by more than a factor of 2, whereas organisms can vary in mass by up to 10(24)--are based on the limits of diffusion is questionable. This essay seeks to develop an alternative explanation based on transport of molecules along structural elements in the cytoplasm and nucleus. This mechanism can better account for cell size constancy, in light of modern biological knowledge of the complex microstructure of the cell, than simple diffusion.

Diffusion theory, the cell and the synapse

The possibility exists that the cell internum is far more highly organised right down to the molecular level than was hitherto appreciated, to the point where ideas of a relatively solid-state chemistry model have been entertained (Coulson, R.A., 1993. The flow theory of enzyme kinetics--a role of solid geometry in the control reaction velocity in live animals. Int. J. Biochem. 25, 1445-1474). This contrasts sharply with the traditional dogma that diffusion is the mechanism by which molecules interact within an aqueous solution of the cell internum, although it should have been clear from an early stage that diffusion could not play other than a very restricted role in metabolic regulation. When physicists began to question certain aspects of the fundamental Law of Heat Conductance formulated over 170 years ago by Fourier, Diffusion Theory was also implicated (Maddox, J., 1989. Heat conductance is a can of worms. Nature 338, 373), and application of Fick's Laws of Diffusion to living systems criticised (Agutter P.S., Malone, P.C., Wheatley, D.N., 1995. Intracellular transport mechanisms: a critique of diffusion theory. J. Theoret. Biol. 176, 261-272). While we have argued (Wheatley, D.N., Malone, P.C., 1993. Heat conductance, diffusion theory and intracellular metabolic regulation. Biol. Cell 79, 1-5) that diffusion cannot be prevented from occurring, we found that, irrespective of whether it was a valid theory, diffusion was of little relevance in most actively metabolising cell systems. However, diffusion is still perceived as essential for interacting molecules to demonstrate their specificities. Any new model of the internal state of the living cell has to resolve this dilemma. The question also relates to molecular movement and ligand-receptor interactions outside the cell. In looking at this situation, attention was paid to one site in the body in which diffusion has long been assumed to be essential, namely in the passage of the chemical transmitter between one neurone and the next across the synaptic cleft. A detailed examination of this assumption has helped to identify one possible place in which the importance of diffusion over a distance of no more than 20-30 nm occurs, although objections to diffusion being involved have been raised. The outcome, however, only re-enforces the conviction that diffusion has little role in metabolic activity and is normally 'assisted' in almost all aspects of cell physiology.