Biomedical Engineering Reference
In-Depth Information
50 CHAPTER 3.
IN VITRO
TISSUE ENGINEERING
modification, materials that are otherwise unattractive to cells can now be successfully seeded and
used in tissue engineering applications. Peptide sequences are also proposed as a means to modify
cellular gene expression and protein synthesis, not just for controlling cell attachment. For example,
if only the functional sequence associated with a growth factor binds to a cell, then the resulting
response is expected to be similar to binding the entire molecule since the same pathways are activated
in both cases.
Peptides are typically grafted to a material by covalent bonding, which securely attaches the
sequence to a location and prevents diffusion through the construct or disassociation from the
surface. However, as noted previously, chemical bonding can have the negative effect of reducing
the biological activity of attached peptides. Tethering the molecules via a linker chain can help
prevent this by moving the peptide away from the surface, allowing more flexibility in its binding
configuration with cells [
420
]. Because some base scaffold materials can sterically hinder biological
reactions, the linker chain must be long enough to allow cell integrin-peptide binding away from the
surface. Typically, peptides are grafted to biomaterials that otherwise prevent the attachment of cells
and proteins. Therefore, scaffolds can be designed that exhibit attachment characteristics entirely
dependent on the types, concentration, and location of peptides bound to their surface.
The peptide density on a material controls not only cell attachment, but also cell motility.
While increasing the concentration of peptides can increase cellular attachment, it can also decrease
the ability of cells to migrate since they are adhered to the surface at numerous binding sites [
420
].
Balancing these two parameters is difficult and is highly dependent on cell type and application. If
migration of cells into a scaffold is desired, then high densities of peptides cannot be used. While
this could reduce cell attachment during seeding, other parameters such as the duration of seeding
or total cell numbers used might compensate for the lower seeding efficiency. Alternatively, integrin
clustering can be used to help facilitate migration without totally restricting cell motility [
422
].
This approach is especially important for applications that require ingrowth of surrounding cell
populations into the construct.
The most common peptide sequence used for cell attachment is Arg-Gly-Asp (RGD), which
was originally identified as a recognition sequence located on the larger fibronectin molecule [
420
].
Further investigation has shown that it is a ubiquitous peptide found in many species of both plants
and animals, indicating its importance and longevity in evolution [
435
]. RGD exhibits an ability
to bind with 8-12 integrins (out of 20+ currently identified), making it incredibly useful for tissue
engineering applications [
426
]. In chondrocytes, RGD binds strongly to
α
5
β
1
,
α
V
β
5
, and
α
V
β
3
and
weakly to
α
3
β
1
, making it an attractive sequence for cartilage engineering studies.
Many different peptides have been investigated for their use in tissue engineering and drug
delivery applications. Peptide binding is associated with cellular integrins, so some sequences are
only useful for specific cell types. Besides RGD, other peptides that have known activities are:
KGD (
α
IIb
β
3
), PECAM (
α
V
β
3
), KQAGDV (
α
IIb
β
3
), LDV (
α
4
β
1
and
α
4
β
7
), YGYYGDALR and
FYFDLR (
α
2
β
1
), and RLD/KRLDGS (
α
V
β
3
and
α
M
β
2
)[
426
,
435
,
436
]. Other sequences have also
shown functionality although researchers are still investigating the binding relationships involved in
