Construct design - Think before you print

While think before you print has been a popular decade-old slogan to oppose the excessive waste of paper in offices, it is also 100% applicable to our approach of bioprinting, as the depth of thought that goes into construct planning is generally proportional to the construct's functionality. In the following, we are going to explore the thought processes that went into the design of our receptor that mediates cross-linking of cells after printing, as well as the design of other functional parts involved in our project.

The receptor - linking by inking

The receptor that we created is able to cross-link cells after injection into streptavidin solution by presenting biotin moieties or streptavidin on the cell surface. This allows the almost irreversible interaction of the cell surface with streptavidin - either directly or via a coninjected biotinylated linker peptide - which binds to several cell surfaces at once, mediating cross-linking. For this purpose, we have created receptors for both human cells as well as bacterial cells.

The functionality of the receptor is mediated by an extracellular biotin acceptor peptide (BAP) that is endogenously biotinylated by a coexpressed biotin ligase (BirA). The biotin ligase is hereby encoded by the same plasmid as the receptor, with an internal ribosome entry site - sequence (IRES) from the human eukaryotic translation initiation factor 4G1 gene (eIF4G1) allowing the polycistronic translation of two open reading frames (ORFs) from a single plasmid. BirA is targeted to the ER via an Igkappa signal peptide as well as an ER retention signal (also called KDEL sequence). Thus being permanently located in the ER, BirA is able to biotinylate the BAP of translocating proteins upon their translocation, i.e. to the cell surface.

For the receptor itself, it is of course of utmost importance that it is correctly anchored in the membrane, resulting in the correct localization of domains in either intercellular or extracellular space. For this purpose, a transmembrane domain of the human epidermal growth factor receptor (EGFR) was chosen. For correct trafficking of the receptor into the ER and, subsequently, its relocation on the cell surface, a properly functioning signal peptide had to be determined. Expression constructs containing three different and commonly used signal peptides from human transmembrane proteins were therefore tested via a secretion assay. These include the signal peptide from human EGFR itself, the signal peptide of the human immunoglobulin kappa-chain as well as the osteonectin (also known as basement-membrane protein 40, short BM40) signal peptide, of which the BM40 signal peptide proved to be the most functional. For quantification of the functionality of the signal peptide, secretion of a separately cloned pSB1C3 construct, being composed of only CMV promoter, signal peptide, nanoluciferase, Strep-tag II and a poly-A sequence, was quantified via a luciferase assay, allowing conclusions about the efficiency of the signal peptide.

Moreover, the receptor contains three functional elements for its detection: An intracellular red fluorescent protein (mRuby) for detection of the receptor via flu- orescence microscopy, an extracellular nanoluciferase for detection via luciferase assays and an extracellular epitope domain for detection via A3C5-monoclonal antibodies (which may be coupled to secondary conjugates for detection via immunofluorescence or which may be used for immunoprecipitation). For purifi- cation of the receptor, it furthermore contains a C-terminal Strep-tag II. The vector furthermore contains the poly-adenylation signal of human growth hormone (hGH) for functional polyadenylation of the transcribed mRNA. A schematic depiction of the vector is shown below. A description for the respective functional elements is given in the following sections.

Schematic depiction of the vector encoding the cell surface receptor for cross-linking of human cells.

The biotin acceptor peptide (BAP)

The biotin acceptor peptide is a short peptide sequence originating from E. coli. The sequence, being composed of 15 amino acid residues, is biotinylated specifi- cally at a lysine residue within the recognition sequence by a coexpressed biotin ligase (BirA) and thus mediates the functionality of the receptor.

1.3.3 EGFR transmembrane domain The N-terminal transmembrane alpha-helix of human EGFR (UniProt P00533, amino acids 622-653) was chosen as the transmembrane domain of the receptor. A stop-transfer sequence consisting of charged amino acids was added at the C-terminus, and the sequence was furthermore flanked by a (GGGGC)2-linker at the N- and C-terminus, re- spectively.13 As predicted by the TMHMM 2.0 server for the prediction of transmembrane helices, amino acid residues N-terminal of the transmembrane domain are positioned outside of the cell, while residues C-terminal of the domain are positioned on the inside of the cell. For terms of simplicity, the complete sequence including the linkers and the stop-transfer sequence is from here on termed ’EGFR-TMD’.

Schematic depiction of the vector encoding the cell surface receptor for cross-linking of human cells.