Fabrication of homotypic neural ribbons as a multiplex platform optimized for spinal cord delivery

Cell Transplantation for Repair of the Spinal Cord and Prospects for Generating Region-Specific Exogenic Neuronal Cells

Spinal cord injury (SCI) is associated with currently irreversible consequences in several functional components of the central nervous system. Despite the severity of injury, there remains no approved treatment to restore function. However, with a growing number of preclinical studies and clinical trials, cell transplantation has gained significant potential as a treatment for SCI. Researchers have identified several cell types as potential candidates for transplantation. To optimize successful functional outcomes after transplantation, one key factor concerns generating neuronal cells with regional and subtype specificity, thus calling on the developmental transcriptome patterning of spinal cord cells. A potential source of spinal cord cells for transplantation is the generation of exogenic neuronal progenitor cells via the emerging technologies of gene editing and blastocyst complementation. This review highlights the use of cell transplantation to treat SCI in the context of relevant developmental gene expression patterns useful for producing regionally specific exogenic spinal cells via in vitro differentiation and blastocyst complementation.

Embryonic Spinal Cord Innervation in Human Trunk Organogenesis Gastruloids: Cardiac Versus Enteric Customization and Beyond

Trunk-biased human gastruloids provide the ability to couple developmentally relevant spinal neurogenesis and organ morphogenesis via spatiotemporal self-organization events from derivatives of the three germ layers. The multi-lineage nature of gastruloids provides the full complexity of regulatory signaling cues that surpasses directed organoids and lays the foundation for an ex vivo self-evolving system. Here we detail two distinct protocols for trunk-biased gastruloids from an elongated, polarized structure with coordinated organ-specific neural patterning. Following an induction phase to caudalize iPSCs to trunk phenotype, divergent features of organogenesis and end-organ innervation yield separate models of enteric and cardiac nervous system formation. Both protocols are permissive to multi-lineage development and allow the study of neural integration events within a native, embryo-like context. We discuss the customizability of human gastruloids and the optimization of initial and extended conditions that maintain a permissive environment for multi-lineage differentiation and integration.

Accelerated differentiation of human induced pluripotent stem cells into regionally specific dorsal and ventral spinal neural progenitor cells for application in spinal cord therapeutics

Spinal cord injury can attenuate both motor and sensory function with minimal potential for full recovery. Research utilizing human induced pluripotent stem cell (hiPSC) -derived spinal cell types for in vivo remodeling and neuromodulation after spinal cord injury has grown substantially in recent years. However, the majority of protocols for the differentiation of spinal neurons are lengthy, lack the appropriate dorsoventral or rostrocaudal specification, and are not typically replicated in more than one cell line. Furthermore, most researchers currently utilize hiPSC-derived motor neurons for cell transplantation after injury, with very little exploration of spinal sensory neuron transplantation. The lack of studies that utilize sensory populations may be due in part to the relative scarcity of dorsal horn differentiation protocols. Building upon our previously published work that demonstrated the rapid establishment of a primitive ectoderm population from hiPSCs, we describe here the production of a diverse population of both ventral spinal and dorsal horn progenitor cells. Our work creates a novel system allowing dorsal and ventral spinal neurons to be differentiated from the same intermediate ectoderm population, making it possible to construct the dorsal and ventral domains of the spinal cord while decreasing variability. This technology can be used in tandem with biomaterials and pharmacology to improve cell transplantation for spinal cord injury, increasing the potential for neuroregeneration.

Generation of human elongating multi-lineage organized cardiac gastruloids

Human elongating multi-lineage organized (EMLOC) gastruloid technology captures key aspects of trunk neurodevelopment including neural integration with cardiogenesis. We generate multi-chambered, contractile EMLOC gastruloids with integrated central and peripheral neurons using defined culture conditions and signaling factors. hiPSC colonies are primed by activating FGF and Wnt signaling pathways for co-induced lineages. EMLOC gastruloids are then initialized with primed cells in suspension culture using timed exposure to FGF2, HGF, IGF1, and Y-27632. Cardiogenesis is stimulated by FGF2, VEGF, and ascorbic acid. For complete details on the use and execution of this protocol, please refer to Olmsted and Paluh (2022).¹.

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

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.

Alginate microfibers as therapeutic delivery scaffolds and tissue mimics

Alginate, a naturally occurring polysaccharide, has been widely used in cell encapsulation, 3D culture, cell therapy, tissue engineering, and regenerative medicine. Alginate's frequent use comes from its biocompatibility and ability to easily form hydrogel in a variety of forms (e.g. microcapsules, microfibers, and porous scaffolds), which can provide immunoprotection for cell therapy and mimic the extracellular matrix for tissue engineering. During the past 15 years, alginate hydrogel microfibers have attracted more and more attention due to its continuous thin tubular structures (diameter or shell thickness ⩽ 200 µm), high-density cell growth, high handleability and retrievability, and scalability. This review article provides a concise overview of alginate and its resultant hydrogel microfibers for the purpose of promoting multidisciplinary, collaborative, and convergent research in the field. It starts with a historical review of alginate as biomaterials and provides basics about alginate structure, properties, and mechanisms of hydrogel formation, followed by current challenges in effective cell delivery and functional tissue engineering. In particular, this work discusses how alginate microfiber technology could provide solutions to unmet needs with a focus on the current state of the art of alginate microfiber technology and its applications in 3D cell culture, cell delivery, and tissue engineering. At last, we discuss future directions in the perspective of alginate-based advanced technology development in biology and medicine.

Fully Characterized Mature Human iPS- and NMP-Derived Motor Neurons Thrive Without Neuroprotection in the Spinal Contusion Cavity

Neural cell interventions in spinal cord injury (SCI) have focused predominantly on transplanted multipotent neural stem/progenitor cells (NSPCs) for animal research and clinical use due to limited information on survival of spinal neurons. However, transplanted NSPC fate is unpredictable and largely governed by injury-derived matrix and cytokine factors that are often gliogenic and inflammatory. Here, using a rat cervical hemicontusion model, we evaluate the survival and integration of hiPSC-derived spinal motor neurons (SMNs) and oligodendrocyte progenitor cells (OPCs). SMNs and OPCs were differentiated in vitro through a neuromesodermal progenitor stage to mimic the natural origin of the spinal cord. We demonstrate robust survival and engraftment without additional injury site modifiers or neuroprotective biomaterials. Ex vivo differentiated neurons achieve cervical spinal cord matched transcriptomic and proteomic profiles, meeting functional electrophysiology parameters prior to transplantation. These data establish an approach for ex vivo developmentally accurate neuronal fate specification and subsequent transplantation for a more streamlined and predictable outcome in neural cell-based therapies of SCI.

Transplantable human motor networks as a neuron-directed strategy for spinal cord injury

To repair neural circuitry following spinal cord injury (SCI), neural stem cell (NSC) transplantation has held a primary focus; however, stochastic outcomes generate challenges driven in part by NSC differentiation and tumor formation. The recent ability to generate regionally specific neurons and their support cells now allows consideration of directed therapeutic approaches with pre-differentiated and networked spinal neural cells. Here, we form encapsulated, transplantable neuronal networks of regionally matched cervical spinal motor neurons, interneurons, and oligodendrocyte progenitor cells derived through trunk-biased neuromesodermal progenitors. We direct neurite formation in alginate-based neural ribbons to generate electrically active, synaptically connected networks, characterized by electrophysiology and calcium imaging before transplantation into rodent models of contused SCI for evaluation at 10-day and 6-week timepoints. The in vivo analyses demonstrate viability and retention of interconnected synaptic networks that readily integrate with the host parenchyma to advance goals of transplantable neural circuitry for SCI treatment.

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Neuromesodermal progenitor derivation of human spinal neurons as therapeutic cells

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Neural ribbons bridge in vitro network formation and in vivo host transplantation

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In vivo visualization of encapsulated graft placement with magnetic resonance imaging

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Six-week viability of human neuronal networks with OPCs in rat contusion SCI

Co-development of central and peripheral neurons with trunk mesendoderm in human elongating multi-lineage organized gastruloids

Stem cell technologies including self-assembling 3D tissue models provide access to early human neurodevelopment and fundamental insights into neuropathologies. Gastruloid models have not been used to investigate co-developing central and peripheral neuronal systems with trunk mesendoderm which we achieve here in elongating multi-lineage organized (EMLO) gastruloids. We evaluate EMLOs over a forty-day period, applying immunofluorescence of multi-lineage and functional biomarkers, including day 16 single-cell RNA-Seq, and evaluation of ectodermal and non-ectodermal neural crest cells (NCCs). We identify NCCs that differentiate to form peripheral neurons integrated with an upstream spinal cord region after day 8. This follows initial EMLO polarization events that coordinate with endoderm differentiation and primitive gut tube formation during multicellular spatial reorganization. This combined human central-peripheral nervous system model of early organogenesis highlights developmental events of mesendoderm and neuromuscular trunk regions and enables systemic studies of tissue interactions and innervation of neuromuscular, enteric and cardiac relevance.

Stem Cell Neurodevelopmental Solutions for Restorative Treatments of the Human Trunk and Spine

The ability to reliably repair spinal cord injuries (SCI) will be one of the greatest human achievements realized in regenerative medicine. Until recently, the cellular path to this goal has been challenging. However, as detailed developmental principles are revealed in mouse and human models, their application in the stem cell community brings trunk and spine embryology into efforts to advance human regenerative medicine. New models of posterior embryo development identify neuromesodermal progenitors (NMPs) as a major bifurcation point in generating the spinal cord and somites and is leading to production of cell types with the full range of axial identities critical for repair of trunk and spine disorders. This is coupled with organoid technologies including assembloids, circuitoids, and gastruloids. We describe a paradigm for applying developmental principles towards the goal of cell-based restorative therapies to enable reproducible and effective near-term clinical interventions.