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Applications of 3D bioprinted iPSCs
sequences were used for the non-integrated molecules used for reprogramming fall under
expression of the transcription genes that enable the category of epigenetic events regulators,
episomal plasmid DNA replication in dividing mesenchymal-epithelial transition inhibitors,
cells . Polycistronic expression plasmids metabolic pathway modulators, wingless and
[36]
generates transgene-free and vector-free iPSCs integration site growth factor (WNT) signal
with limited genomic integration, but this method pathway modulators, regulators of cell death, and
requires multiple transfections [37,38] . senescence pathways . These small molecules
[48]
alone or in combination can substitute exogenous
2.3 mRNAs and miRNAs transcription factors. Using valproic acid, a
Human primary fibroblasts were reprogramed by Histone deacetylase inhibitor improved the
introducing synthetic modified mRNAs coding for reprogramming efficiency to 100-fold compared to
reprogramming proteins. MicroRNAs like miRNA- the transcription factor mediated reprogramming
[49]
367/302s are used to reprogram human primary method . Another histone methyltransferase
fibroblasts into iPSCs . The mRNA transfection inhibitor compound, BIX-01294 activated
[39]
is foot print free reprogramming. The capped calcium channels in the cell membrane, and
mRNAs coding for a 6-factor modified-mRNA improve reprogramming efficiency by increasing
referred to as 5fM O mod-mRNAs was used in the expression of October 4 and Klf4 [50,51] . In 2013,
3
fibroblasts to make iPSCs . Yamanaka factors and Hou et al. replaced all transcription factors and
[40]
the miRNA-367/302s act synergistically to increase made chemical induced iPSCs of mouse embryo
the efficiency of transfection . It has been reported fibroblasts using the small molecules VC6TFZ
[41]
[52]
that microRNAs such as miR-294, miR-291-3, and and 2i but the reprogramming was incomplete .
miR-295 can replace c-myc transcription factor Optimizing the use of small molecules to enhance
and help to generate homogeneous populations of reprogramming will definitely help to generate
iPSC colonies . Downregulation of let-7 miRNA safer and higher quality iPSCs for clinical use
[42]
upregulates the expression of target genes of c-myc without the risk of genomic integration and tumor
and Lin-28 to promote cell reprogramming [43,44] . induction.
The advantages and limitations of these
2.4 Reprogramming proteins reprogramming methods are summarized in
This method allows the direct introduction of Table 1.
the recombinantly expressed reprogramming 3 Sources of iPSCs for bioprinting and cell
factors to cells . This method mitigates the differentiation strategies of iPSCs to different
[49]
risks associated with the introduction of viral and cell lineages
external DNA and harmful chemicals into the
cells . The reprogramming proteins Oct4, Sox2, The iPSC technology allows the use of autologous
[46]
Klf4, and c-Myc were successfully delivered cells derived from the patients to be used in
into adult somatic cells with the help of cell regenerative medicine. The iPSC cell lines have
penetrating peptides (CPP). The cationic amino been derived from a variety of cells namely neuronal
acid rich CPPs are capable of penetrating the progenitor cells, keratinocytes, hepatocytes, B
cell membrane barrier and deliver the exogene- cells, fibroblasts, hepatocytes, gastric epithelial
free reprogramming proteins directly inside the cells, muscles, adipocytes, and adrenal glandular
cells . This method enables the production of cells. The reprograming efficiency varies
[47]
foot print-free iPSCs. among the type of cells used depending on the
2.5 Small molecules developmental origins and the epigenetic status.
Multiple studies showed that the efficiency of
Reprogramming can be achieved using small reprogramming of keratinocytes is better than
molecules by epigenetic modifications . Small fibroblasts [53-55] . The difference in the efficiency is
[48]
62 International Journal of Bioprinting (2020)–Volume 6, Issue 4
