Human stem cell-derived neurons and neural circuitry therapeutics: Next frontier in spinal cord injury repair

Abstract

Spinal cord injury (SCI) remains a life-altering event that devastates those injured and the families that support them. Numerous laboratories are engaged in preclinical and clinical trials to repair the injured spinal cord with stem cell-derived therapeutics. A new developmental paradigm reveals early bifurcation of brain or trunk neurons in mammals via neuromesodermal progenitors (NMPs) relevant to therapies requiring homotypic spinal cord neural populations. Human-induced pluripotent stem cell (hiPSC) NMP-derived spinal motor neurons generated ex vivo following this natural developmental route demonstrate robust survival in vivo when delivered as suspension grafts or as in vitro preformed encapsulated neuronal circuitry when transplanted into a rat C4-C5 hemicontusion injury site. Use of in vitro matured neurons avoids in vivo differentiation challenges of using pluripotent hiPSC or multipotent neural stem cell (NSC) or mesenchymal stem cell therapeutics. In this review, we provide an injury to therapeutics overview focusing on how stem cell and developmental fields are merging to generate exquisitely matched spinal motor neurons for SCI therapeutic studies. The complexity of the SCI microenvironment generated by trauma to neurons and vasculature, along with infiltrating inflammatory cells and scarring, underlies the challenging cytokine microenvironment that therapeutic cells encounter. An overview of evolving but limited stem cell-based SCI therapies that have progressed from preclinical to clinical trials illustrates the challenges and need for additional stem cell-based therapeutic approaches. The focus here on neurons describes how NMP-based neurotechnologies are advancing parallel strategies such as transplantation of preformed neuronal circuitry as well as human in vitro gastruloid multicellular models of trunk central and peripheral nervous system integration with organs. NMP-derived neurons are expected to be powerful drivers of the next generation of SCI therapeutics and integrate well with combination therapies that may utilize alternate biomimetic scaffolds for bridging injuries or flexible biodegradable electronics for electrostimulation.

Keywords: Neuromesodermal progenitor; bioengineering; clinical trial; gastruloid; human-induced pluripotent stem cell spinal motor neuron subtype; microenvironment.

Figure 1.

Complexity of cell types in the SCI microenvironment and injury impact. (A) Timeline of microenvironment pathological changes following SCI. A wave of inflammatory response is followed by decreased inflammation and long-term glial scar formation. Immediate therapeutic intervention into the post-traumatic inflammatory microenvironment is challenging regarding survival of grafted cells and the presence of complex cytokine regulators. Human clinical trials have shown transplantation of cell therapy as early as 10 days. (B) Diagram of the spinal column. Shown in increasing detail are the complexity of the SCI microenvironment, cell types involved, and functional impacts. Perturbations to vascular, neural circuitry and metabolism, and myelination functions are critical to survival and regeneration of neuronal circuits following SCI.

Figure 2.

hiPSC NMP-trunk specification and neural ribbons. (A) The anterior–posterior neuraxis in the human embryo is specified from NSCs that generate brain neurons or NMP-derived cells that form the trunk and spine, directing somites and neural crest cells (NCCs) for CNS–PNS integration. (B) Application of NMPs to achieve trunk regionally matched CNS, PNS, and mesodermal lineage cells is showing promise in transplantation circuitry. Grafted NMP-derived regionally matched spinal motor neurons and support cells in neural ribbons as preformed synaptically connected networks facilitates host–graft interactions and neural circuitry integration, as alginate-based encapsulation provides temporary support for networks before dissolution. Advantages of neural ribbon circuitry and prospective assisting technologies for improved clinical translation.

Figure 3.

Preformed neural circuitry ribbons for SCI. Encapsulated hiPSC NMP-derived spinal motor neurons and oligodendrocytes in a 1:5 ratio in RGD-alginate-type I collagen hydrogel as 150-micron diameter neural ribbons. Images left to right: images 1 and 2, DAPI, nuclei (blue), antibodies: SMI312, pan-axonal neurofilament (red), β-III tubulin, Tuj1 (green); image 3, DAPI, nuclei (blue), pan-cadherin (green); image 4, mitochondria (mitotracker). Technology is published in Olmsted et al.^, Scale bars are 50 microns.

Figure 4.

Current clinical trial landscape and forefront of SCI therapeutic approaches. (A) Brief overview of current stem cell therapeutic sources for SCI and future landscape for the application of hiPSC NMP neurotechnology and preformed neuronal circuitry in clinical trials. (B to D) Technologies at the forefront of SCI therapeutic approaches. (B) Gastruloid models based on NMP protocols can mimic human multilineage development of the trunk and spine regions in a dish and are ideal models to understand integration of CNS and PNS neurons. Shown center in the diagram is a human embryo near 28 days. Gastruloid EMLO and EMLOC technologies model neurons of the multichambered embryonic heart (left) or enteric gut (right). (C) Implementation of electrical stimulus and exercise as a synergistic and complementing therapeutic approach for improved results is expected to be a required step to optimize clinical translational recovery of SCI therapy. (D) The future of SCI therapeutic approaches is expected to advance transplantable neural circuitry considerations applying novel platforms with hiPSC NMP-derived subtype-specific networks.