Page 105 - IJB-7-1

Zheng, et al.
These autologous bone grafts have numerous biological clinical use in humans, surface modification ability, and
benefits over heterologous and synthetic bone substitutes the ease of export to other countries . In addition, HA15
[21]
due to their excellent combination of osteogenic, is an efficient inhibitor of endoplasmic reticulum (ER)
osteoinductive, and osteoconductive characteristics [3,4] . chaperone protein glucose regulatory protein 78 (GRP78,
Despite these advantages, bone transplantation can lead to also known as BiP), which can prohibit the ATPase
issues, such as bleeding, hematoma, infection, and chronic activity of BiP and trigger the ER stress. Through ER
pains. Furthermore, this form of treatment is restrained by stress, the cells activate unfolded protein response (UPR)
the donor sources and high costs of surgery [5,6] . Considering and other signaling pathways in reaction to misfolded
these issues and the great demand for treatment of bone and unfolded protein aggregation in the ER cavity and
defects, the need for modern designs and new strategies the disorder of calcium balance.
has been elevated to an urgent status, specifically for large- Many stress responses can activate ER stress, thus
area bone defects. In this relation, bone tissue engineering could be called UPR [22-24] . UPR is mediated by three major
is considered one of the most promising alternative signaling cascades that are triggered by the so-called ER
approach for bone defect repairment . One of the aims pressure sensors, such as double-stranded RNA activated
[7]
of bone tissue engineering is to fabricate osteoconductive protein kinase R (like endothelial reticulum kinase
scaffolds along with the successful delivery of osteogenic [PERK]), inositol-dependent enzyme 1 α (IRE1 α), and
cells and biological factors . activating transcription factor 6 (ATF6). PERK, IRE1 α,
[8]
The scaffold design should be able to accommodate living and ATF6 are ER membrane proteins, which are known to
cells and guide their growth, and assist tissue regeneration in form complexes with BiP in a steady-state condition. The
three dimensions [9,10] . The fabrication techniques also have a increased demand for protein folding leads to activation of
great impact on scaffold properties . Moreover, the material PERK, IRE1 α, and ATF6, subsequently activating their
[11]
and method selection must be designed according to specific downstream influencers to alleviate protein toxic stress on
demands of tissue (structural and metabolic) . According to ER and restore ER homeostasis [25,26] . In this regard, HA15
[12]
the biomimetic scaffold production protocols, the prepared can bind to the ATP enzyme of BiP and inhibit its activity,
scaffold must maintain a sufficient area for cell adhesion leading to BiP separation from PERK, IRE1 α, and ATF6
and proliferation, exchange of gaseous species, with the that elicits ER stress and UPR . The dissociation of BiP
[27]
optimized surface-to-volume ratio and the degradation rate can activate PERK that subsequently phosphorylates eIF2
that matches tissue formation rate . The scaffold’s porosity, α, leading to the inhibition of protein synthesis, which is
[13]
surface chemistry, morphology, three dimensional (3D) conducive to the folding of proteins in the ER. Unlike
structure, immunogenicity, and mechanical properties have most proteins, the phosphorylated eIF2 α selectively
an extensive impact on the matrix properties in the biological promotes the translation of ATF 4. Since the ATF4 has
artificial bone substitutes . As yet, a wide array of materials an upstream open reading frame in its fifth untranslated
[14]
has been used as matrix in bone tissue engineering, including region, under normal circumstances, these upstream
natural polymers and their monomers (elastin, chitosan, silk, open reading frames would prevent the translation of
collagen, gelatin, etc.) , synthetic biodegradable polymers ATF4. Phosphorylated eIF2 α can promote ribosome
[15]
(polylactides, polycaprolactone, polypropylene fumarate, binding to the open reading frame of ATF4, inducing the
polyethylene glycol, etc.) , inorganic compounds of bone expression of ATF4 mRNA and increasing the translation
[16]
extracellular matrix (calcium phosphates, β-tricalcium of ATF4. ATF4 and runt-related transcription factor
phosphate [β-TCP], hydroxyapatite [HA], calcium 2 (Runx2) can interact with osteoblast-specific actin
carbonate, etc.) , and signaling molecules (RGD proteins element 1 (OSE1) and osteoblast-specific actin element 2
[17]
and various growth factors) . It was clear that bone tissue (OSE2), respectively, in the osteocalcin (OCN) promoter
[18]
engineering scaffolds should imitate the composition region. This can induce the expression of osteoblast-
and structure of natural bone tissue mostly by engaging specific OCN along with bone matrix mineralization
biodegradable polymer matrix and inorganic bioactive fillers promotion, osteoblast differentiation regulation, and bone
in 3D composite porous scaffolds. formation [28,29] . Therefore, HA15 can promote osteoblast
The utilization of β-TCP and poly (lactic-co-glycolic differentiation through the PERK-eIf2 α-ATF4 pathway.
acid) (PLGA) is very common due to their beneficial It was known that the accumulation of misfolded protein
aspects. The β-TCP has found many applications in load- in the ER may lead to the secretion of HSPA5/BiP
bearing orthopedic implants because of its compositional (GRP78) from ERN1/IRE1, allowing homodimerization
analogy to natural bone, osteoconductivity, and and subsequent activation of ERN1/IRE1. This event
tailorable bioresorbability [19,20] . Furthermore, the PLGA has an auxiliary role in the post-translational transport of
is considered one of the base biomaterials because of its small presecretory proteins across the ER. Furthermore,
biocompatibility, potential for tailoring its biodegradation the HSPA5 gene is overexpressed due to UPR under the
rate, Food and Drug Administration certification for cellular stress conditions [30-32] ; therefore, the expression of

International Journal of Bioprinting (2021)–Volume 7, Issue 1 101