Migratory potential of transplanted glial progenitors as critical factor for successful translation of glia replacement therapy: The gap between mice and men

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 In modern times this outdated dogma is continuously repelled by new evidence unravelling the importance of glia in neuronal maintenance and function.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.

In Vivo Imaging of Allografted Glial-Restricted Progenitor Cell Survival and Hydrogel Scaffold Biodegradation

Transplanted glial-restricted progenitor (GRP) cells have potential to focally replace defunct astrocytes and produce remyelinating oligodendrocytes to avert neuronal death and dysfunction. However, most central nervous system cell therapeutic paradigms are hampered by high initial cell death and a host anti-graft immune response. We show here that composite hyaluronic acid-based hydrogels of tunable mechanical strengths can significantly improve transplanted GRP survival and differentiation. Allogeneic GRPs expressing green fluorescent protein and firefly luciferase were scaffolded in optimized hydrogel formulations and transplanted intracerebrally into immunocompetent BALB/c mice followed by serial in vivo bioluminescent imaging and chemical exchange saturation transfer magnetic resonance imaging (CEST MRI). We demonstrate that gelatin-sensitive CEST MRI can be exploited to monitor hydrogel scaffold degradation in vivo for ∼5 weeks post transplantation without necessitating exogenous labeling. Hydrogel scaffolding of GRPs resulted in a 4.5-fold increase in transplanted cell survival at day 32 post transplantation compared to naked cells. Histological analysis showed significant enhancement of cell proliferation as well as Olig2⁺ and GFAP⁺ cell differentiation for scaffolded cells compared to naked cells, with reduced host immunoreactivity. Hence, hydrogel scaffolding of transplanted GRPs in conjunction with serial in vivo imaging of cell survival and hydrogel degradation has potential for further advances in glial cell therapy.

Stem cell therapies as a therapeutic option to counter chemo brain: a negative effect of cancer treatment

Chemo brain, a constellation of cognitive deficiencies followed by chemotherapy drugs, used to treat different types of cancers and adversely impacts the quality of life of a cancer survivor. The underlying mechanism of chemo brain remains vague, thus delaying the advancement of efficient treatments. Unfortunately, there is no US FDA approved medicine for chemo brain and often medicines considered for chemo brain are already the ones approved for other diseases. Nevertheless, researches exploring stem cell transplantation in different neurodegenerative diseases demonstrate that cellular transplantation could reverse chemotherapy-induced chemo brain. This review talks about the mechanism behind the cognitive impairments instigated by different chemotherapy drugs used in cancer treatment, and how stem cell therapy could be advantageous to overcome this disease.

Glial restricted precursor delivery of dendrimer N-acetylcysteine promotes migration and differentiation following transplant in mouse white matter injury model

Oligodendrocyte replacement using glial restricted precursors (GRPs) is a promising avenue for the treatment of acquired or genetic white matter disorders; however, limited long-term survival of these cells post-transplant may impede maximal recovery. Nanotherapeutic approaches can facilitate stem cell delivery while simultaneously delivering factors aimed at enhancing and nourishing stem cells en route to, and at, the target site. Hydroxyl polyamidoamine (PAMAM) dendrimer nanoparticles have been used in a variety of models to deliver therapeutics in a targeted manner to injury sites at low doses. Here, survival and migration of GRPs was assessed in a mouse model of neonatal white matter injury with different methods of dendrimer nanoparticle support. Our findings demonstrate the ability of GRPs to take up nanoparticle-drug conjugates and for these conjugates to act beyond the injury site in vivo. Compared to GRPs alone, mice receiving dendrimer-drug in parallel to GRPs, or via GRPs as the delivery vector, showed improved migration and differentiation of cells 8 weeks post-transplant. These studies demonstrate that drug-conjugated nanoparticles can enhance transplanted progenitor cell survival and migration, and suggest that combination therapies may allow engraftment without overt immunosuppression.

Neuronal Stem Cell and Drug Interactions: A Systematic Review and Meta-Analysis: Concise Review

Stem cell therapy is a promising treatment option for neurodegenerative diseases that mostly affect geriatric patients who often suffer from comorbidities requiring multiple medications. However, not much is known about the interactions between stem cells and drugs. Here, we focus on the potential interactions between drugs used to treat the comorbidities or sequelae of neurodegenerative diseases and neuronal stem cells to reveal potential effects on drug safety and efficacy. To determine the potential effects of drugs frequently used in geriatric patients (analgesic, antibiotic, antidepressant, antidiabetic, antihyperlipidemic, and antihypertensive drugs) on neuronal stem cell differentiation and proliferation, we systematically searched PubMed to identify nonreview articles published in English in peer-reviewed journals between January 1, 1991, and June 7, 2018. We identified 5,954 publications, of which 214 were included. Only 62 publications provided the complete data sets required for meta-analysis. We found that antidepressants stimulated neuronal stem cell proliferation but not differentiation under physiologic conditions and increased the proliferation of stem cells in the context of stress. Several other potential interactions were identified, but the limited number of available data sets precludes robust conclusions. Although available data were in most cases insufficient to perform robust meta-analysis, a clear interaction between antidepressants and neuronal stem cells was identified. We reveal other potential interactions requiring further experimental investigation. We recommend that future research addresses such interactions and investigates the best combination of pharmacological interventions and neuronal stem cell treatments for more efficient and safer patient care. Stem Cells Translational Medicine 2019;8:1202-1211.

Proteolytic Rafts for Improving Intraparenchymal Migration of Minimally Invasively Administered Hydrogel-Embedded Stem Cells

The physiological spaces (lateral ventricles, intrathecal space) or pathological cavities (stroke lesion, syringomyelia) may serve as an attractive gateway for minimally invasive deployment of stem cells. Embedding stem cells in injectable scaffolds is essential when transplanting into the body cavities as they secure favorable microenvironment and keep cells localized, thereby preventing sedimentation. However, the limited migration of transplanted cells from scaffold to the host tissue is still a major obstacle, which prevents this approach from wider implementation for the rapidly growing field of regenerative medicine. Hyaluronan, a naturally occurring polymer, is frequently used as a basis of injectable scaffolds. We hypothesized that supplementation of hyaluronan with activated proteolytic enzymes could be a viable approach for dissolving the connective tissue barrier on the interface between the scaffold and the host, such as pia mater or scar tissue, thus demarcating lesion cavity. In a proof-of-concept study, we have found that collagenase and trypsin immobilized in hyaluronan-based hydrogel retain 60% and 28% of their proteolytic activity compared to their non-immobilized forms, respectively. We have also shown that immobilized enzymes do not have a negative effect on the viability of stem cells (glial progenitors and mesenchymal stem cells) in vitro. In conclusion, proteolytic rafts composed of hyaluronan-based hydrogels and immobilized enzymes may be an attractive strategy to facilitate migration of stem cells from injectable scaffolds into the parenchyma of surrounding tissue.

Biodistribution of Glial Progenitors in a Three Dimensional-Printed Model of the Piglet Cerebral Ventricular System

White matter damage persists in hypoxic-ischemic newborns even when treated with hypothermia. We have previously shown that intraventricular delivery of human glial progenitors (GPs) at the neonatal stage is capable of replacing abnormal host glia and rescuing the lifespan of dysmyelinated mice. However, such transplantation in the human brain poses significant challenges as related to high-volume ventricles and long cell migration distances. These challenges can only be studied in large animal model systems. In this study, we developed a three dimensional (3D)-printed model of the ventricular system sized to a newborn pig to investigate the parameters that can maximize a global biodistribution of injected GPs within the ventricular system, while minimizing outflow to the subarachnoid space. Bioluminescent imaging and magnetic resonance imaging were used to image the biodistribution of luciferase-transduced GPs in simple fluid containers and a custom-designed, 3D-printed model of the piglet ventricular system. Seven independent variables were investigated. The results demonstrated that a low volume (0.1 mL) of cell suspension is essential to keep cells within the ventricular system. If higher volumes (1 mL) are needed, a very slow infusion speed (0.01 mL/min) is necessary. Real-time magnetic resonance imaging demonstrated that superparamagnetic iron oxide (SPIO) labeling significantly alters the rheological properties of the GP suspension, such that, even at high speeds and high volumes, the outflow to the subarachnoid space is reduced. Several other factors, including GP species (human vs. mouse), type of catheter tip (end hole vs. side hole), catheter length (0.3 vs. 7.62 m), and cell concentration, had less effect on the overall distribution of GPs. We conclude that the use of a 3D-printed phantom model represents a robust, reproducible, and cost-saving alternative to in vivo large animal studies for determining optimal injection parameters.

Chemobrain as a Product of Growing Success in Chemotherapy - Focus on Glia as both a Victim and a Cure

Chemotherapy-induced cognitive impairment or chemobrain is a frequent consequence of cancer treatment with many psychiatric features. Ironically, the increasing efficacy of chemotherapy leaves growing number of patients alive with chemobrain. Therefore, there is an urgent need for strategies capable of returning cancer survivors back to their pre-morbid quality of life. Molecular mechanisms of chemobrain are largely unknown. Over the last decade there was a lot of emphasis in preclinical research on inflammatory consequences of chemotherapy and oxidative stress but so far none of these approaches were translated into clinical scenario. The co-administration of chemotherapy with protective agents was evaluated preclinically but it should be introduced with caution as potential interference was not yet studied and that could blunt therapeutic efficacy. Stem cell-based regenerative medicine approach has so far been exploited very sparsely in the context of chemobrain and the focus was on indirect mechanisms or neuronal replacement in the hippocampus. However, there is evidence for widespread white matter abnormalities in patients with chemobrain. This is quite logical considering life-long proliferation and turnover of glial cells, which makes them vulnerable to chemotherapeutic agents. Feasibility of glia replacement has been established in mice with global dysmyelination where profound therapeutic effect has been observed but only in case of global cell engraftment (across the entire brain). While global glia replacement has been achieved in mice translation to clinical setting might be challenging due to much larger brain size. Therefore, a lot of attention should be directed towards the route of administration to accomplish widespread cell delivery. Techniques facilitating that broad cell distribution including intra-arterial and intrathecal methods should be considered as very compelling options. Summarizing, chemobrain is a rapidly growing medical problem and global glia replacement should be considered as worthwhile therapeutic strategy.

Enforced microglial depletion and repopulation as a promising strategy for the treatment of neurological disorders

Microglia are prominent immune cells in the central nervous system (CNS) and are critical players in both neurological development and homeostasis, and in neurological diseases when dysfunctional. Our previous understanding of the phenotypes and functions of microglia has been greatly extended by a dearth of recent investigations. Distinct genetically defined subsets of microglia are now recognized to perform their own independent functions in specific conditions. The molecular profiling of single microglial cells indicates extensively heterogeneous reactions in different neurological disorders, resulting in multiple potentials for crosstalk with other kinds of CNS cells such as astrocytes and neurons. In settings of neurological diseases it could thus be prudent to establish effective cell‐based therapies by targeting entire microglial networks. Notably, activated microglial depletion through genetic targeting or pharmacological therapies within a suitable time window can stimulate replenishment of the CNS niche with new microglia. Additionally, enforced repopulation through provision of replacement cells also represents a potential means of exchanging dysfunctional with functional microglia. In each setting the newly repopulated microglia might have the potential to resolve ongoing neuroinflammation. In this review, we aim to summarize the most recent knowledge of microglia and to highlight microglial depletion and subsequent repopulation as a promising cell replacement therapy. Although glial cell replacement therapy is still in its infancy and future translational studies are still required, the approach is scientifically sound and provides new optimism for managing the neurotoxicity and neuroinflammation induced by activated microglia.