*Spatiotemporal Regulation of Cell–Cell Adhesions DOI: http://dx.doi.org/10.5772/intechopen.97009*

cell membrane where the extracellular portion operates as a bioartificial cell adhesion receptor [51, 52, 57]. For instance, the proteins Cryptochrome 2 (CRY2) from *Arabidopsis thaliana* and its blue light-dependent binding partner cryptochromeinteracting basic helix–loop–helix (CIBN) protein, were expressed on the surfaces of MDA-MB-231 cells, which do no form native cell–cell adhesion. When cells expressing CRY2 and CIBN at their surface are mixed and cultured in the dark, no cell–cell adhesions form similar to the parent MDA-MB-231 cell line. However, if these cells are cultured under blue light, the cells grow in clusters indicating the formation of cell– cell adhesions (**Figure 3**). Moreover, the cell–cell interactions formed under blue light can be reversed in the dark, allowing for repeated deconstruction and reconstruction with light-dependent control [58]. This optogenetic approach has the advantage that the cell–cell adhesions can be triggered with visible blue light, which is non-toxic to the cells and the cell surface modifications are passed on to daughter cells following cell splitting.

The large repertoire of photoswitchable protein–protein interactions allows for the formation of bioartificial cell–cell adhesions with different properties in terms of cell–cell adhesion mode, the light of color the adhesions responds to, reversion kinetics in the dark, and cell–cell adhesion dynamics [53–55].

In biology, cells can either interact with cells of their own type forming homophilic interactions or cells of another type forming heterophilic interactions.

### **Figure 3.**

*Optogenetic proteins bind either in hetero or homophilic complexes. In heterophilic optogenetic systems an optogenetic protein undergoes conformational changes that enable the binding to a target protein. Homophilic optogenetic proteins also undergo conformation changes, but here a homomer is formed. iLID (improved light induced dimer), CRY2 (Cryptochrome 2), CIB1/N (cryptochrome-interacting basic helix–loop–helix/truncated), Cph1 (cyanobacterial phytochrome 1).*

To obtain light-responsive homophilic cell–cell adhesion, proteins that homodimerize under light are used as a mediator of cell–cell adhesion. For this purpose, the proteins Vivid, a member of the light oxygen voltage (LOV) domain from *Neurospora crassa*, and cyanobacterial phytochrome 1 (Cph1) from *Synechocysitis sp. PCC 6803* were used as these proteins homodimerize under blue and red light, respectively (**Figure 4**). Cells expressing Vivid at their plasma membrane form cell–cell adhesion exclusively when illuminated with blue light but not with red light. The reverse is true for cells expressing Cph1 at their cell surface, which only form cell–cell interactions under red light and not in the dark or under blue light. Similarly, the blue-green lightresponsive protein, CarH from *Thermus thermophilus*, has been used to mediate homophilic cell–cell interactions. The formation of a CarH homotetramer allows it to form cell–cell adhesions between cells expressing CarH on their surface in the dark [59].

### **Figure 4.**

*Co-culture of optogenetic proteins results in cluster segregation. When colloidial particles are labled with the iLID/Nano, nMag/pMag, or nMagHigh/pMagHigh clusters of particles can be seen to form with respect to the kinetics of the system (adapted from Müller et al. [62]). In cellular systems utilizing the vivid (VVD) and Cph1 systems descrete clusters are observed rather than any intermixing (adapted from Rasoulinejad et al. [57]).*
