Durotaxis and its role in directing engineering tissue development by Tyler Kimball

Durotaxis

Figure 1.Durotaxis [12]

Durotaxis is the migration of cells based on the structural gradients of the rigidity of the substrate (referring to ECM, bone, or any scaffold construct)(figure1).The elastic modulus is used to measure the stiffness of the substrate which changes with each cell type.[1] The cells typically migrate in the direction of higher stiffness (figure 2).[2] The rigidity of the substrate is known to affect migration, immunology, morphology, motility, and various other processes due to the interactions between the extracellular matrix and the cytoskeleton.[1](figure 3)

The stiffness of the substrate allows cells to generate forces using focal actin-myosin complexes onto the the substrate. These forces produce focal adhesion kinases which controls the overall structure of the cytoskeleton. The cytoskeleton is highly dependent on the stiffness of the substrate which is important because the cytoskeleton determines the morphology of the cell as well as the chemical responses in the signaling pathways. Controlling the stiffness is therefore key in generating the type of cell that is needed.[3]

Figure 2 shows the elongation that is experienced by the cells when they sense stiffness

Figure 2.Elongation[2]

Figure 3 shows the affects of different cellular cross linkages on migration.  ]

History

▪1892: Ramón y Cajal predicted axons navigate in response to gradients of cues

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▪1914: Migration occurs due to the substrate in the microenvironment (Harrison)

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▪1934: Contact guidance – migration directed by substrate topology during embryonic development (Weiss)

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▪1990's: Cytoskeletal stiffness, morphology, motility, and fibronectin fibril assembly was proven to influence migration

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▪ 20th century: It was discovered that cells could detect and respond to substrate stiffness

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Motivation

Understanding how to stimulate or inhibit the migration of cells is crucial for cellular regeneration and treatment. The medical field is driving the need for this research to potentially solve problems like host rejection makes durotaxis in engineering a hot topic.

Stem Cell Differentiation

There have been several different developments among researchers in the use of Haptotaxis and its properties in tissue engineering and regenerative medicine efforts. Notably, the use of stem cells seeded certain grafts to induce tissue and differentiation proliferation [5] In these studies, there is an overarching goal to create tissue derived from human cells to be incorporated in vivo. These studies can affect a vast range of people, including those who with bone deficiencies and tissue deformations and/or deficiencies. [5].

As an example, there has been development in the creation of a human mesenchymal stem cell (hMSC) seeded polymer bone grafts. The grafts are designed to be porous to allow access to certain growth factors. In doing so, a chemical gradient is created, in which draws certain cells to certain areas to proliferate and differentiate. Further, stiff, rod-like fibers are incorporated into the graft to act as guides for the cell. By doing so, the hMSCs can differentiate into osteoblasts, allowing for incorporation in vivo [7].

Bone Scaffolds

For years, doctors have been using allograft bone transplants and other replacement methods that are cost inefficient, cause inflammation, extensive pain, and are prone to rejection or replacement after so many years of use. Recent studies in the mechanical properties of bone have produced a bone scaffold that has similar rigidity and porosity to real bone and would allow for the integration of cells completely in less than four weeks. This is possible because the scaffold mimics the stiffness of the mesenchymal stem cells. As said before, the mimicry of the rigidity of the ECM allows for the bound stem cells to create focal adhesion forces which give the the cytoskeletal organization.

Durotaxis in Cancer

Cancer cells use durotaxis extensively affecting the lives of more than 200,000 people every year. One of the key properties that enable cancer to succeed is its stiffness. Tumors can be found to be as much as 10 times stiffer than the normal breast tissue this dramatic gradient causes the pull of endothelial cells to its mass allowing for tumor growth and for metastasis. [3] By finding ways to counteract these properties doctors could slow down the progression of cancer and make treatment more successful.

Conclusions

Work in tissue engineering and regenerative medicine by using Haptotaxis as a vehicle to create specific tissue regeneration is promising. With many different types of biocompatible grafts and the use of stem cell differentiation, and with promising results, it would seem in vivo generation of tissue might happen. However, there are pitfalls to this approach - first of which, specific stem cell differentiation is difficult and can often lead to different desired products. Further, the creation and maintenance of a chemical gradient is difficult in vivo. Additionally, the 3D microenvironment is incredibly influential to the properties of Haptotaxis and can be difficult to replicate [6]. However, technology is advancing in the creation of 3D hydrogels. Lastly, the use of people’s cells is expensive and difficult to procure.

Scientists have just begun to unveil the hidden importance of durotaxis to regenerative medicine and tissue engineering. Hydrogels have been key in proving the importance of the rigidity of the extracellular matrix and the influence it holds over the morphology, migration, etc. of cells in the microenvironment.

References

[1] Ulrich, Theresa A., Elena M. de Juan Pardo, and Sanjay Kumar. “The Mechanical Rigidity of the Extracellular Matrix Regulates the Structure, Motility, and Proliferation of Glioma Cells.” Cancer research 69.10 (2009): 4167–4174.

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[2]Friedl, Peter, and Bettina Weigelin. "Interstitial leukocyte migration and immune function." Nature immunology 9.9 (2008): 960-969.

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[3]Sant, Shilpa et al. “BIOMIMETIC GRADIENT HYDROGELS FOR TISSUE ENGINEERING.” The Canadian journal of chemical engineering 88.6 (2010): 899–911.

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[4]Breuls, Roel G.M, Timothy U Jiya, and Theo H Smit. “Scaffold Stiffness Influences Cell Behavior: Opportunities for Skeletal Tissue Engineering.” The Open Orthopaedics Journal 2 (2008): 103–109.

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[5]Quintero, Andres J, et al. “Stem Cells for the Treatment of Skeletal Muscle Injury.” Clinics in sports medicine 28.1 (2009): 1–11. PMC. Web. 21 Mar. 2017.

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[6]Isenberg, Brett C. et al. “Vascular Smooth Muscle Cell Durotaxis Depends on Substrate Stiffness Gradient Strength.” Biophysical Journal 97.5 (2009): 1313–1322. PMC.

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[7]Wang, N; Butler, JP; Ingber, DE (21 May 1993). "Mechanotransduction across the cell surface and through the cytoskeleton.". Science. 260 (5111): 1124–7.

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[8] Halliday, NL; Tomasek, JJ (March 1995). "Mechanical properties of the extracellular matrix influence fibronectin fibril assembly in vitro.". Experimental Cell Research. 217 (1): 109–17.

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[9]DiMilla, Paul A., et al. "Maximal migration of human smooth muscle cells on fibronectin and type IV collagen occurs at an intermediate attachment strength." The Journal of cell biology 122.3 (1993): 729-737.

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[10] Plotnikov, SV; Pasapera, AM; Sabass, B; Waterman, CM (21 December 2012). "Force fluctuations within focal adhesions mediate ECM-rigidity sensing to guide directed cell migration.". Cell. 151 (7): 1513–27.

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