Bioprinting 3D muscle tissue supplemented with endothelial-spheroids for neuromuscular junction model

The neuromuscular junction (NMJ) plays a critical role in muscle contraction, and its dysfunction can result in various neuromuscular disorders. In vitro models for studying NMJ are essential for understanding their functions and pathology. However, the engineering of muscle tissue presents challenges for the organization of myofiber-like oriented muscle bundles as well as the induction of vessel formation and innervation. To address these challenges, we fabricated a hybrid muscle construct comprising uniaxially aligned muscle struts and endothelial cell spheroids using a combination of in situ electric field-assisted bioprinting (E-printing) and microdroplet-based spheroid-forming bioprinting (MDS-printing) techniques. This resulted in self-aggregation of human umbilical vein endothelial cells (HUVECs) into spheroids without attachment to the structures. We tested various fabrication parameters, such as electric field and cross-linking conditions, for E-printing and the deposited cell density of MDS-printing, to stabilize the alignment of the human muscle progenitor cells (hMPCs) and HUVEC spheroids, respectively. The stimulated hMPCs efficiently formed fully aligned myofibers, and the incorporation of HUVEC spheroids induced highly upregulated crosstalk between different cell types compared to a simple E-printed hMPC/HUVEC mixture-loaded construct. This improved myogenesis and vessel formation in vitro. In addition, when co-cultured with a motor neuron-like cell (NSC-34) spheroid separated by a channel, we observed considerably improved neuromuscular junction formation compared to those formed with the normally mixed cell-bearing structures. Our findings suggest that this hybrid muscle construct has the potential to enhance muscle tissue engineering by improving biological activities through the incorporation of HUVEC-spheroids and facilitating neuromuscular junction formation through co-culture with NSC-34 spheroids.