Axonal regeneration of different tracts following transplants of human glial restricted progenitors into the injured spinal cord in rats

Glial-restricted progenitor cells: a cure for diseased brain?

The central nervous system (CNS) is home to neuronal and glial cells. Traditionally, glia was disregarded as just the structural support across the brain and spinal cord, in striking contrast to neurons, always considered critical players in CNS functioning. In modern times this outdated dogma is continuously repelled by new evidence unravelling the importance of glia in neuronal maintenance and function. Therefore, glia replacement has been considered a potentially powerful therapeutic strategy. Glial progenitors are at the center of this hope, as they are the source of new glial cells. Indeed, sophisticated experimental therapies and exciting clinical trials shed light on the utility of exogenous glia in disease treatment. Therefore, this review article will elaborate on glial-restricted progenitor cells (GRPs), their origin and characteristics, available sources, and adaptation to current therapeutic approaches aimed at various CNS diseases, with particular attention paid to myelin-related disorders with a focus on recent progress and emerging concepts. The landscape of GRP clinical applications is also comprehensively presented, and future perspectives on promising, GRP-based therapeutic strategies for brain and spinal cord diseases are described in detail.

Custom-engineered hydrogels for delivery of human iPSC-derived neurons into the injured cervical spinal cord

Cervical damage is the most prevalent type of spinal cord injury clinically, although few preclinical research studies focus on this anatomical region of injury. Here we present a combinatorial therapy composed of a custom-engineered, injectable hydrogel and human induced pluripotent stem cell (iPSC)-derived deep cortical neurons. The biomimetic hydrogel has a modular design that includes a protein-engineered component to allow customization of the cell-adhesive peptide sequence and a synthetic polymer component to allow customization of the gel mechanical properties. In vitro studies with encapsulated iPSC-neurons were used to select a bespoke hydrogel formulation that maintains cell viability and promotes neurite extension. Following injection into the injured cervical spinal cord in a rat contusion model, the hydrogel biodegraded over six weeks without causing any adverse reaction. Compared to cell delivery using saline, the hydrogel significantly improved the reproducibility of cell transplantation and integration into the host tissue. Across three metrics of animal behavior, this combinatorial therapy significantly improved sensorimotor function by six weeks post transplantation. Taken together, these findings demonstrate that design of a combinatorial therapy that includes a gel customized for a specific fate-restricted cell type can induce regeneration in the injured cervical spinal cord.

Perspectives in the Cell-Based Therapies of Various Aspects of the Spinal Cord Injury-Associated Pathologies: Lessons from the Animal Models

Traumatic injury of the spinal cord (SCI) is a devastating neurological condition often leading to severe dysfunctions, therefore an improvement in clinical treatment for SCI patients is urgently needed. The potential benefits of transplantation of various cell types into the injured spinal cord have been intensively investigated in preclinical SCI models and clinical trials. Despite the many challenges that are still ahead, cell transplantation alone or in combination with other factors, such as artificial matrices, seems to be the most promising perspective. Here, we reviewed recent advances in cell-based experimental strategies supporting or restoring the function of the injured spinal cord with a particular focus on the regenerative mechanisms that could define their clinical translation.

Recent insights on astrocyte mechanisms in CNS homeostasis, pathology, and repair

Astrocytes play essential roles in development, homeostasis, injury, and repair of the central nervous system (CNS). Their development is tightly regulated by distinct spatial and temporal cues during embryogenesis and into adulthood throughout the CNS. Astrocytes have several important responsibilities such as regulating blood flow and permeability of the blood-CNS barrier, glucose metabolism and storage, synapse formation and function, and axon myelination. In CNS pathologies, astrocytes also play critical parts in both injury and repair mechanisms. Upon injury, they undergo a robust phenotypic shift known as "reactive astrogliosis," which results in both constructive and deleterious outcomes. Astrocyte activation and migration at the site of injury provides an early defense mechanism to minimize the extent of injury by enveloping the lesion area. However, astrogliosis also contributes to the inhibitory microenvironment of CNS injury and potentiate secondary injury mechanisms, such as inflammation, oxidative stress, and glutamate excitotoxicity, which facilitate neurodegeneration in CNS pathologies. Intriguingly, reactive astrocytes are increasingly a focus in current therapeutic strategies as their activation can be modulated toward a neuroprotective and reparative phenotype. This review will discuss recent advancements in knowledge regarding the development and role of astrocytes in the healthy and pathological CNS. We will also review how astrocytes have been genetically modified to optimize their reparative potential after injury, and how they may be transdifferentiated into neurons and oligodendrocytes to promote repair after CNS injury and neurodegeneration.

Age-related loss of axonal regeneration is reflected by the level of local translation

Regeneration capacity is reduced as CNS axons mature. Using laser-mediated axotomy, proteomics and puromycin-based tagging of newly-synthesized proteins in a human embryonic stem cell-derived neuron culture system that allows isolation of axons from cell bodies, we show here that efficient regeneration in younger axons (d45 in culture) is associated with local axonal protein synthesis (local translation). Enhanced regeneration, promoted by co-culture with human glial precursor cells, is associated with increased axonal synthesis of proteins, including those constituting the translation machinery itself. Reduced regeneration, as occurs with the maturation of these axons by d65 in culture, correlates with reduced levels of axonal proteins involved in translation and an inability to respond by increased translation of regeneration promoting axonal mRNAs released from stress granules. Together, our results provide evidence that, as in development and in the PNS, local translation contributes to CNS axon regeneration.

Human Embryonic Stem Cell-derived Neural Crest Cells Promote Sprouting and Motor Recovery Following Spinal Cord Injury in Adult Rats

Spinal cord injury results in irreversible tissue damage and permanent sensorimotor impairment. The development of novel therapeutic strategies that improve the life quality of affected individuals is therefore of paramount importance. Cell transplantation is a promising approach for spinal cord injury treatment and the present study assesses the efficacy of human embryonic stem cell–derived neural crest cells as preclinical cell-based therapy candidates. The differentiated neural crest cells exhibited characteristic molecular signatures and produced a range of biologically active trophic factors that stimulated in vitro neurite outgrowth of rat primary dorsal root ganglia neurons. Transplantation of the neural crest cells into both acute and chronic rat cervical spinal cord injury models promoted remodeling of descending raphespinal projections and contributed to the partial recovery of forelimb motor function. The results achieved in this proof-of-concept study demonstrates that human embryonic stem cell–derived neural crest cells warrant further investigation as cell-based therapy candidates for the treatment of spinal cord injury.

Transplanting neural progenitor cells to restore connectivity after spinal cord injury

Spinal cord injury remains a scientific and therapeutic challenge with great cost to individuals and society. The goal of research in this field is to find a means of restoring lost function. Recently we have seen considerable progress in understanding the injury process and the capacity of CNS neurons to regenerate, as well as innovations in stem cell biology. This presents an opportunity to develop effective transplantation strategies to provide new neural cells to promote the formation of new neuronal networks and functional connectivity. Past and ongoing clinical studies have demonstrated the safety of cell therapy, and preclinical research has used models of spinal cord injury to better elucidate the underlying mechanisms through which donor cells interact with the host and thus increase long-term efficacy. While a variety of cell therapies have been explored, we focus here on the use of neural progenitor cells obtained or derived from different sources to promote connectivity in sensory, motor and autonomic systems.

Vascularization of self-assembled peptide scaffolds for spinal cord injury repair

The disruption of the blood-spinal cord barrier (BSCB) following spinal cord injury contributes to inflammation and glial scarring that inhibits axon growth and diminishes the effectiveness of conduits transplanted to the injury site to promote this growth. The purpose of this study is to evaluate whether scaffolds containing microvessels that exhibit BSCB integrity reduce inflammation and scar formation at the injury site and lead to increased axon growth. For these studies, a self-assembling peptide scaffold, RADA-16I, is used due to its established permissiveness to axon growth and ability to support vascularization. Immunocytochemistry and permeability transport assays verify the formation of tight-junction containing microvessels within the scaffold. Peptide scaffolds seeded with different concentrations of microvascular cells are then injected into a spinal contusion injury in rats to evaluate how microvessels affect axon growth and neurovascular interaction. The effect of the vascularized scaffold on inflammation and scar formation is evaluated by quantifying histological sections stained with ED-1 and GFAP, respectively. Our results indicate that the peptide scaffolds containing microvessels reduce inflammation and glial scar formation and increase the density of axons growing into the injury/transplant site. These results demonstrate the potential benefit of scaffold vascularization to treat spinal cord injury. STATEMENT OF SIGNIFICANCE: This study evaluates the benefit of transplanting microvascular cells within a self-assembling peptide scaffold, RADA-16I, that has shown promise for facilitating regeneration in the central nervous system in previous studies. Our results indicate that vasculature featuring tight junctions that give rise to the blood-spinal cord barrier can be formed within the peptide scaffold both in vitro and in a rat model of a subacute contusion spinal cord injury. Histological analysis indicates that the presence of the microvessels encourages axon infiltration into the site of injury and reduces the area of astrocyte activation and inflammation. Overall, these results demonstrate the potential of vascularizing scaffolds for the repair of spinal cord injury.