At the surface: contact cell behavior interactions at the interface.
As cells strike artificial surfaces, what happens? Presently there is no exacting theory to explain such cell interactions as a predictive outcome following the contact event. The classical derivation is to proceed toward some series of sequential steps to illustrate the interplay of cells interacting at the interface. Three sequential steps of cell surface interactions are considered: “1) protein adsorption is a preliminary step liable to involve irreversible interaction between the surface and several hundreds of molecular species occurring in blood or plasma. 2) Formation of adhesive bonds, involving physical chemistry of surfaces, and formation of specific ligand receptor bonds. 3. Triggering of a specific cell program such as apoptosis, proliferation, migration, differentiation or activation. Recent evidence suggests that in addition to the nature and amount of stimulated surface receptors, additional cues such as substratum mechanical or topographical properties may significantly affect cell behavior.”
In order to understand any effort of predicting the outcome of a cell to artificial surface encounter some basic tenets have to be explored. If indeed we are to better understand the architecture of life the so-called simple cell to surface contact event needs to be understood in great detail. First image that tends to come to mind is the VELCRO® brand, the kind of one side hooks grabbed with loops of material, holding adhesion together. But despite the effectiveness of a VELCRO® brand attachment, this gets us thinking about the actual shape of attachment. In science the tried and true method is called the reductionist scheme, let’s look at the example of a single contact point to see what happens. The problem lies when we try to expand the contact point out to see if it matches the way proteins contact and interact, assembling. How do the shapes interact, are there rules? So to start, imagine just placing your finger in your mouth to smear some saliva onto a clean glass surface, like the side of a drinking glass, it sticks of course. Let’s zoom into a magnified view on that ‘stickiness.’
First, starting with biological fluids spilling onto a clean glass surface, the surface once exposed becomes coated with proteins within seconds, becoming a new bio-film surface. What do cells see, or what do cells feel or touch but some form of modified surface. But a surface with topographical features, the first structured layer is a adsorbed layer attached to the surface features. Within this reaction is a multitude of complexity for the variety of proteins that might be used with their individual conformations. The possibilities and outcomes are varied and very very numerous.
Second, a critical component of the adsorption reaction is the formation of adhesive bonds between the cells with the surface.
Third, once a cell has adhered to a surface the option of outcome in terms of a choice of which developmental cell line proceeding is the progression appropriate for? The cell may choose apoptosis and die or choose survival to proliferate, either remaining at the site of adhesion or to start migrating. Cell differentiation may follow in sequence. Finally the cell choice may opt to stay in a resting state or trigger active processes of synthesis or possibly secretion of active mediators. Knowing what influences the cell to decide its outcome is very much an enigma.
What is the macromolecular adsorption? Firstly soluble molecules have to encounter the surface. The order of encounter is determined by particular diffusion constants and concentrations from contributing species. Second is the dynamics that the actual binding is a reversible reaction of molecules to surfaces, within a reasonable time scale. Thirdly, once the adsorption has occurred progressive modification of the actual composition of the adsorbed layers to evolve with the most rapidly concentrated species being replaced with the more adhesive ones, the Vroman effect. Fourthly, adsorbed proteins will undergo progressive conformational changes, which usually increases adhesion, exposing new interaction sites to cells. Fifthly, the potential possibility will be formation of multiple layers due to continuous adsorption. ( The Vroman effect, named after Leo Vroman, is exhibited by protein adsorption to a surface by blood serum proteins. The highest mobility proteins generally arrive first and are later replaced by less motile proteins that have a higher affinity for the surface. A typical example of this occurs when fibrinogen displaces earlier adsorbed proteins on a biopolymer surface and is later replaced by high molecular weight kininogen. The process is delayed in narrow spaces and on hydrophobic surfaces fibrinogen is usually not displaced. Under stagnant conditions initial protein deposition takes place in the sequence: albumin; globulin; fibrinogen; fibronectin and HMWK )
What is the actual nature to relate the structure of uncoated compared to coated surfaces?
The first criterion is the low selectivity of the adsorption. Depending on the surface properties, which generally could be hydrophobic or hydrophilic and being either neutral or charged. Thus a general selection rule liable to occur for binding onto a bare surface getting exposed to biological materials hardly exists for the unlimited molecules that may attach themselves. Secondly when a surface is exposed to a mixture of macromolecules there will be competition between multiple adsorption processes. The individual components will not predict the behavior of a mixture. Thirdly, molecular mixtures are enormously complex with potentially hundreds of molecular species, plasma for example. Fourthly, irreversible processes occur as protein adsorption causes conformational changes that may change preventing efficient exchange between adsorbed and soluble phase with time is dependent on the whole history of progress of the adsorption process. Sixthly, the process depends on the conformational changes on underlying strata as well as adsorbed molecule layers, which may be reflecting features that are independent of each other. Lastly the ligand-receptor interaction energies are slight components of the folding energy of most proteins, so that minor conformational changes may strongly affect molecule to surface interactions.
What are the main theoretical frameworks best suited to predict cell surface adhesion?
Developed in the 1940’s the Derjaguin and Landau with Verwey and Overbeek (DLVOtheory) was developed to describe colloidal suspensions involving interaction energy as the sum of electrostatic repulsion to attraction between surface charges. “at large distances, exponential repulsion vanishes more rapidly than power-law attraction, resulting in overall attraction with a shallow minimum called the secondary minimum. When the distance is decreased, a repulsive barrier must be overcome to reach the so-called primary minimum, which is considered to result in irreversible adhesion.” What doomed the DLVO theory concerned the rough surface of cells when separated in proximity, approximately 20nm since the zone of integrins becomes engaged between 10-20nm. Thus cell to cell or cell surface interactions are dependent on the precise distribution of charges on cell membranes as well as the shape of the cell surface. Looking to thermodynamic interactions essentially assuming that in an aqueous environment a cell adheres to a substratum–water interface by replacing a cell-water and substratum-water interfaces if this involves a decrease of the systems free energy. Assuming interfacial energies actually exist and there is a combining rule the actual measurements between similar surface tensions can be quite unpredictable since physical chemical properties of cell surfaces can not always be measured. Specific ligand–receptor binding involves specific bond interactions by specific associations, cell-substratum interaction involves a small fraction of the cells molecular area at the contact. Rather than contact surface angle of interaction at specific interactions. “A prominent example is represented by so-called scavenger receptors, which are thought to play an important role in natural immunity and were shown to recognize a variety of ligands including bacterial structures or altered lipids (Pearson, 1996). Interestingly, these receptors seem to be involved in the recognition of plastic surfaces by macrophages, an interaction that was long considered as nonspecific.” Specfic topographical features are ones that fit on apposed surfaces with matching opposite charges, but interaction with opposite charge are non specific. Now each cell species is endowed with a high number of surface receptors, binding at the adhesion receptors reactive to immune binding sites. Many receptors are multi-specific acting as non specific. Studying the rupture of individual bonds has revealed some cell surface strength using three methods, atomic force microscopy, bio-membrane probe and laminar chamber flow. 1) Delayed formation of additional; bonds might occur after the initial binding event. 2) binding might involve the simultaneous formation of several bonds, 3) ligand-receptor association might behave as a multiphasic phenomenon, with initial formation of a transient complex leading to subsequent dissociation or a transition to a more stable state. This leads to bond topography as an important determinant of cell adhesion for the influence of receptor topography on functional capacity.
Physical contact interactions as a cell interacts with a surface arrange themselves around a variety of adhesion methods that implicate nonspecific physical states including electrostatic repulsion or steric stabilization. What is the shape of steric stabilization? Source: http://www4.ncsu.edu/~hubbe/Defnitns/StericSt.htm Steric stabilization is a form of suspension with additives that inhibit coagulation within the dispersion. The additives include hydrophilic (water loving) polymers and surfactants with hydrophilic chains. The extent of long loops and tails extend out like a loose rope erratically coiled around a hub. Even if the added concentration is increased or the surface conditions, referred to as zeta potential, which is a good predictor of the magnitude of electrical repulsive forces between particles of known size and shape as a function of distance, are reduced to near zero potential, the steric stabilized remains well dispersed, not clumping in its behavior.
The usual concept involves the common interpretation to assume a dominant phenomenon following the stimulation of cell membrane receptors, which triggers a cascade of biochemical events with second messenger generation. Scientists term this activity as: engaged membrane receptors, which will influence further events, hence the cascade of reactive chain of linked events. Let’s take an example: if a macrophage encounters an anti-body coated particle, the macrophage will engulf the foreign particle in the pattern of proper stimulation of immunoglobulin receptors. However in immune binding interactions if the same particle is bound through a lectin interacting with other membrane structures, no ingestion occurs. Lectins, sugar binding proteins, play a role during infection when viruses use lectins to attach themselves to the cells of the host organism. Lectins occur ubiquitously in nature regulating cell adhesion to glycoprotein synthesis and the control of protein levels in blood. Although it is tempting to get down to the nano detail of the various players involved as outcome pathways for cellular adhesion to occur the predictability of outcome knowing detailed identification of the nature and the number of receptors engaged in a given interaction is not a given at all. Perhaps the abundance of reports from the biochemical point of view, based on the number of published papers is perhaps a sign of convenience depending on the nature of free or bound ligands detected in the nearby environment. But the fact remains what is the oldest defined attachment perhaps that is the strategic setting for defining predictable completeness, like the mechanical physic-chemical and topographical cues. The oldest attachments on Earth appear to be between bacterial cell mats, which live in contact in a community comprising a strain/species specific extracellular substance, ( fancy terms for biofilm). It has been demonstrated that adherent cells exert forces (dare I say tension! ) on underlying surfaces, indicating a certain flexability dependency with mechanistic information (dare I say shape sensing? ). But the most intruiguing observation centers on ample evidence that the functions of an adherent cell are strongly influenced by the geometrical properties of contact areas. This was expressed as, cell proliferation to be highly correlated to available adhesion area. Specifically as the observer scales down to have compelling evidence that nanoscale surface features may strongly influence adherent cells. “A first approach might consist of trying to follow the lines of thought that were successful in the past. Thus, it might be argued that cell surface receptor stimulation is not only dependent on ligand recognition but also on topographical reorganization. Thus, a common mechanism of signal generation may consist of bringing a suitable kinase in close contact with a potential target, thus allowing tyrosine phosphorylation and generation of binding sites for scaffold proteins. Also, since it is well demonstrated that forces can change protein conformation, surface mechanical properties might affect the forces exerted on ligand proteins and influence the appearance of binding sites. According to this view, there might be a need to look for accurate relationships between ligand topography and receptor activation. A second approach might be to consider that biology must shift away from reductionism and aims at develop new methods to deal with biocomplexity. A notable example is the concept of tensegrity, suggested by Ingber (2003) as a means of overcoming difficulties,” toward resolving the adhesion tension geometrical interactions not only at the local neighborhood but at the larger neighborhood perhaps to the borough then city then province then country then continent then planet. ( I have paraphrased shamelessly from this brilliant review article, cited below).
J European Cells and Materials Vol. 7. 2004 (pages 52-63)
IS THERE A PREDICTABLE RELATIONSHIP BETWEEN SURFACE
PHYSICAL-CHEMICAL PROPERTIES AND CELL BEHAVIOUR AT THE
J. Vitte, A. M. Benoliel, A. Pierres and P. Bongrand*
INSERM UMR600-CNRS FRE2059, Laboratoire d’Immunologie,
Hôpital de Sainte-Marguerite, Marseille, France
So this essay is a excursion around the observation of attachment, the events that determine the contact event, as the physics people say, “It’s dense.” There are all kinds of forces, interactions, bonding, the lock and key of what is termed a protein ‘fit’. But as the observer trying to understand the process of attachment, the big picture is always present, ‘……how does it all work together in harmony?’ To get at the guts of that query, it’s all about how bacteria cling together, how they have clumped since the primordial goo that was the start of life on our planet. What is this ‘glue’ that binds things together? Here’s a parting thought. In the most recent Scientific American, June 2012 within the human gut microbiome there are approximately 3.3 million genes, compare that to the human total somewhere between 20,000-25,000 genes. Are we just a walking-talking biofilm? The heritage of the ancient communities of bacterial communities are not merely an ancient event they are very much a current event, these present day bacteria interact with our receptors in a way that we are only beginning to catch on to. And the bacteria are assembled into communities to interact within the rules of tensegrity.
The next cerebrovortex essay will be specifically on bacterial biofilm.