Cell guidance systems: Topotaxis

Cell guidance systems: Topotaxis

In addition to substrate elasticity (durotaxis) and chemical gradients (chemotaxis), which we explored in previous blog articles, surface topography also impacts cell movement and behavior. Cells develop and function embedded within in a highly complex, and evolving, extracellular matrix (ECM) environment. Various biochemical and biophysical ECM cellular cues and their subsequent cell responses shape the development and homeostasis of tissues. An important component of this extracellular environment, governing cell function and behaviour, is the differing micro-/nanotopographical features. Also known as contact guidance, this phenomenon is capable of orienting and directing cell phenotype and migratory behavior, both in vitro and in vivo.

Cellular Responses

Throughout morphogenesis, cells undergo changes in shape, protrusive activity and polarity whilst also exerting force on cells/tissues to generate various structures. The effect of compositional topography is most notable during embryogenesis and branching morphogenesis. Due to differences in architectural cues on each layer of the cells cause differences in cell polarity and other intracellular signals, different signaling pathways responsible for segmentation and dynamic rearrangements are activated.

Similar effects are also observed during the formation of cell adhesion complexes. When confined to areas of high compactness (long and thin rectangles) cells will experience high physiological forces and tend to differentiate towards tissue types that physiologically experience high intracellular stress, such as the cartilage and bone. Conversely, cells confined to areas of low compactness (circle) will experience less force and differentiate towards tissues that experience lower intracellular stress, such as adipose tissue.

Topographical influence on wound healing and disease progression

Over the years, a variety of cell migration mechanisms influenced by the local microenvironmental topographical cues have also been identified. Various unicellular and multicellular migration processes, such as occur during the formation of biofilm, primordial germ cell migration, endothelial cell migration, highlight the different ways that topography can influence cell anchorage and migration. The importance of topographical cues in regulating cell migration mechanisms during immune surveillance, tissue repair and regeneration (wound healing) is becoming increasingly clear. Medical interventions in wound healing are being developed.

Several key observations have also established the role of ECM topography in disease progression and development, especially for cells that have evolved to use adhesion and migration as part of their pathogenic strategy. For example, cancer cells utilise various topographical anisotropy features to their advantage to proliferate and migrate away from the primary tumour more efficiently and persistently. Additionally, abnormal ECM topographies have also been shown to dysregulate stromal cells' behaviour, facilitate tumour-associated angiogenesis and inflammation, and thus lead to the generation of a tumorigenic microenvironment.

The intriguing phenomenon of topography-mediated differentiation and migration naturally begs the question: how do cells sense, adapt and respond to topographical features? There are strong indications that the biological mechanism responsible for cell-topographical responses is based on the adhesion-mediated sensing of cells with the topographical elements upon coating with focal adhesions.

Focal Adhesions

Focal adhesions (FAs) are dynamic plasma membrane-associated complexes that engage the intracellular actin bundles with the ECM receptors of the integrin family to mediate robust cell-matrix adhesion. During cell migration and spreading, FAs act as holding points that suppress membrane contraction and promote protrusion at the leading edge. In stationary cells, they serve as anchorage devices that maintain the cell morphology and function.

The different stages of the FAs lifecycle and corresponding force-dependent morphological changes are regulated by various biochemical/biophysical stimuli of the ECMs. Recently, a growing number of reports have reported that nanoscale surface topography highly affects FA protein formation, maturation and disassembly, which eventually regulate the cell-topography adhesive interactions.

Applications for Regenerative Medicine

This knowledge is appealing in the field of biomaterials design where topographical surface modifications offer a simple and cost-effective alternative to traditional differentiation techniques. A variety of cell culture systems on engineered tissues have been established to recapitulate the complex architecture of native ECM that cells encounter in vivo, leading to advancements in reproducing various pathophysiological scenarios for applications in cell studies, regenerative medicine and drug discovery.

Certain patterns have also been shown to induce stem cell differentiation down specific pathways, providing a valuable platform for studies in regenerative medicine. By using a topographically patterned substrate of variable local wavelength and amplitude, one study also illustrated the crucial role of wavelength/amplitude-decoupled guidance effect in dermal wound healing and exemplify these influences on the speed of fibroblast migration. Inconsistencies between the studies and the variation of cell responses according to different topographic features remain to be ironed out.

3D Studies

Another obvious question is the influence of these topographical cues on cell fate in vivo where interactions occur in 3D? To date, most studies surrounding cell-topography interactions were conducted in vitro, with the vast majority relying on flat 2D monolayers on polymers.

While these 2D studies offer the fundamental knowledge of the mechanobiology of various biological phenomena, the correlation between cell functions and some components of the microenvironment lack most of the interactions occurring in the native tissues—raising the question of whether analogous effects can also be observed in vitro on engineered intestinal tissue models. Nevertheless, there is compelling evidence suggesting that these cues are important in pathophysiological environments in vivo.

As more novel bioengineering tools, techniques and biomaterials emerge, significant efforts are currently focused on the development of more complex 3D systems to recreate a more realistic microenvironment that recapitulates the hierarchical and dynamic architectural nature in vivo.

The results presented from these studies are in good agreement with the data reported for in vivo experiments on the native tissues. Although many challenges remain, the exciting potential of such an approach paves the way to gain better insights into the mechanisms by which cells respond to ECM cues and aid efforts in various biomedical applications.

IMAGE:  MG63 cells grown on flat and slotted surfaces. From: Sun L, Pereira D, Wang Q, Barata DB, Truckenmüller R, Li Z, et al. (2016) Controlling Growth and Osteogenic Differentiation of Osteoblasts on Microgrooved Polystyrene Surfaces. PLoS ONE 11(8): e0161466. https://doi.org/10.1371/journal.pone.0161466

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