Implications of regional identity for neural stem and progenitor cell transplantation in the injured or diseased nervous system

Prakruthi Amar Kumar,Jennifer N.Dulin

Neural stem and progenitor cell (NSPC) transplantation has emerged as a promising therapeutic strategy for replacing lost neuronal populations and repairing damaged neural circuits following nervous system injury and disease.A great deal of experimental work has investigated the biology of NSPC grafting in preclinical animal models;more recently,NSPC transplantation has advanced to clinical trials.However,there are fundamental questions regarding the biology of NSPC grafting that warrant further investigation.Here,we focus on the importance of the regional identity of donor cells for determining outcomes following transplantation.We discuss key findings in models of traumatic brain injury and Parkinson’s disease,focusing on how concepts learned from this work may have potential applications for transplantation following spinal cord injury (SCI).

SCI typically results in immediate and permanent deficits in neurological function,due to massive loss of neurons and disruption of neural circuits.The adult central nervous system has a limited ability for regeneration after injury,so the replacement of lost neurons via cell and tissue transplantation approaches has long been a focus of scientific research.Neural progenitor cells are an attractive cell source for transplantation for multiple reasons: they differentiate into diverse neuronal cell types,astrocytes,and oligodendrocytes;graft-derived neurons extend throughout the host nervous system and form functional synaptic connections onto host neurons;and grafts support the regeneration of functionally important host axon projections;together,these phenomena support new neural relay formation (Fischer et al.,2020).There is evidence of modest gains in forelimb,hindlimb,sensory,and bladder function following NSPC transplantation in preclinical models of spinal cord injury;however,there remains much room for improvement.

In the last two decades,a mounting body of evidence in rodent models has shown that fundamental characteristics of NSPCs such as their sex (Pitonak et al.,2022),phenotypic identity (Zholudeva et al.,2018),and regional identity (Michelsen et al.,2015;Espuny-Camacho et al.,2018;Kumamaru et al.,2018) can all play pivotal roles in determining outcomes following transplantation into the injured or diseased host nervous system (Figure 1).Such outcomes include graft survival and maturation,graftderived cell phenotypes,graft axon extension,synaptic connectivity,functional integration into host sensory and motor circuits,and recovery of neurological functions.For example,we have recently shown that male NSPCs transplanted into cervical dorsal column lesion SCI in female host animals induce an immune response characterized by hypervascularization and cytotoxic T-cell infiltration into grafts (Pitonak et al.,2022).In this Perspective article,we will explore the importance of phenotypic identity and regional identity,two characteristics that can be modulated to improve the efficacy of stem cell transplantation for neural repair and regeneration.By focusing on key transplantation studies in the injured and diseased brain,we will discuss how previous findings in neural regeneration research highlight the importance of regional identity and cell phenotype in determining the formation of appropriate neural relays between the host and graft and promoting better integration of grafts into host circuitry.

Figure 1｜Key factors to consider in the optimization of neural stem and progenitor cell (NSPC) transplantation for nervous system injury and disease.

Transplantation studies in the brain were the first to highlight the importance of regional identity in the efficacy of NPSC transplantation (Michelsen et al.,2015;Espuny-Camacho et al.,2018;Kawata et al.,2022).Kawata et al.(2022) showed that orthotopic transplantation of mouse embryonic stem cell derived hypothalamic neurons into the supraoptic nuclei resulted in graft axon extension along the hypothalamic bundle in the host brain as well as graft axonal projections into the posterior pituitary region.However,when the same population of mouse embryonic stem cellderived neurons was ectopically transplanted into the substantia nigra reticular part,no axonal projections were found in the posterior pituitary region.This suggests that orthotopic/regional identity-matched transplantation is essential for better regeneration and integration into the host brain in hypothalamic or pituitary disorders (Kawata et al.,2022).In another study by Michelsen et al.(2015),mouse embryonic stem cell-derived neurons of visual cortical identity transplanted into the visual cortex restored damaged pathways including longrange and reciprocal axonal projections and synaptic connections with targets of the damaged cortex.A proportion of the grafted neurons were also functionally responsive to visual stimuli as observed through electrophysiological recordings.However,no appropriate neural relay or integration was observed following transplantation of the same visual cortical cell grafts into the motor cortex or grafting of motor cortical cell grafts into the visual cortex,suggesting that a regional identity match between the grafted and lesioned host neurons is necessary for successful transplantation (Michelsen et al.,2015).Additionally,the same group showed that transplantation of human pluripotent stem cell derived visual cortical cell grafts into the visual cortex yielded established axonal pathways and extended axons to specific visual cortical targets,expressed the accurate range of markers of all six cortical layers,and were identified to be synaptically active within the host brain;whereas the same cell grafts transplanted into the motor cortex failed to integrate (Espuny-Camacho et al.,2018).Collectively,these findings demonstrate that regional identity matching of the graft to host tissue may be critical for the successful reconstruction of damaged adult neural circuits.

As for the role of cell phenotype,Adler et al.(2019) examined factors governing targetappropriate innervation and circuit integration of human embryonic stem cell (hESC)-derived grafts following transplantation into the adult rat brain.The authors showed that cell-intrinsic factors determined the efficiency of graft-derived axonal innervation.They identified that ventral midbrainpatterned hESC transplants displayed similar axonal projections and innervated appropriate targets for dopaminergic neurons,namely the dorsolateral striatum,nucleus accumbens,and ventromedial prefrontal cortex,when grafted either orthotopically or ectopically (Adler et al.,2019).Another recent study showed that following transplantation into the same location of the substantia nigra in a mouse model of Parkinson’s disease,two distinct phenotypes of hESC-derived neurons,midbrain dopaminergic and glutamatergic neurons,projected axons to different brain regions (Xiong et al.,2021).These studies showed that the nature of integration and innervation by engrafted neurons was significantly influenced by their cell phenotype.Taken together,the above findings underscore the importance of both cell phenotype and graft location matching for the formation of appropriate and specific neural relays and synaptic integration,further enhancing functional outcomes.

A handful of studies have also emphasized the importance of cell and regional identity for neural transplantation in spinal cord injury models.Dell’Anno et al.(2018) found that human neuroepithelial stem cell grafts of the spinal cord but not neocortical identity could form a relay through the lesioned spinal cord,reconnecting spared host neural elements and establishing functional connections with local host neurons.In contrast to this,neocortical identity grafts failed to acquire a mature neuronal phenotype and failed to integrate and extend neurites.Neocortical cell integration was found to be dramatically enhanced only in the cerebral cortex,clearly demonstrating that anatomical matching of the graft with recipient tissue is critical for functional neuronal networks (Dell’Anno et al.,2018).A 2016 study by Kadoya and colleagues assessed the ability of neural progenitor cell grafts to facilitate regeneration of the injured corticospinal tract,a functionally important tract responsible for voluntary movement in humans.They observed that NPCs were required to possess caudal (spinal cord) and not rostral (telencephalon) fates to support robust corticospinal axon regeneration,synapse formation,and improvements in skilled forelimb function after SCI.Indeed,corticospinal axons largely failed to penetrate telencephalic grafts in this study.Additionally,it has been observed that hESC-derived NSPCs are required to have a caudalized SC identity to support hindlimb motor functional improvements in a clinically relevant rat contusion model of SCI (Kumamaru et al.,2018).These results highlight the need for homologous (spinalized) NSPCs for neuronal replacement after injury to enable robust regeneration of corticospinal through and beyond lesion sites.

With regard to specific regional identities within the spinal cord,White and colleagues were the first to show that engrafted dorsal or ventral rat fetal spinal cord tissue into sites of high cervical SCI were able to anatomically and functionally integrate with injured host phrenic circuitry (White et al.,2010).However,phrenic nerve recordings revealed differential effects of dorsalvs.ventral grafts on phrenic nerve recovery and activity,thus suggesting that recovery of function and respiratory outcomes are influenced by the phenotypic identities of the donor cells used (White et al.,2010).Dulin et al.(2018) later demonstrated that the regeneration of host corticospinal and CGRP+axons in the injured adult spinal cord was enhanced if spinal cord NPC grafts were dorsally restricted compared to ventrally-restricted grafts.In a recent study,regionally specific human iPSC-derived NPCs were used as a cell therapy approach following a thoracic contusion SCI in mice.The authors achieved this by regulating Wnt and retinoic acid signaling during neural induction.They found that only the spinal cord-type cells (not forebrain type) enhanced the motor recovery of mice with spinal cord injury,especially in those with mildto-moderate lesions (Kajikawa et al.,2020).In this study,they showed that the SC-type identity without sonic hedgehog (Shh) activation exhibited primarily a dorsal identity,but two other previous studies had reported that iPSC-derived NPCs with a ventral spinal cord identity (not dorsal cord identity) improved locomotor function in spinally injured animals (Kawabata et al.,2016).Together,these studies underscore the effects of not only anterior/posterior,but dorsoventral identity of transplanted spinal cord NPCs and the impact of their phenotypes on host axon regeneration.

In light of the growing body of evidence indicating that homotypic tissue identity and dorsoventral spinal identity will be important factors to consider in the development of clinically effective human cell therapies,future work is needed to understand the cellular substrates that support the recovery of neurological functions.For example,if caudalized,ventralized grafts are required to promote recovery of locomotor function in mouse models,identification of the specific cell types within these grafts that possess the ability to synaptically and functionally integrate into locomotor circuits is the next step toward engineering human cell grafts designed to repair locomotor circuits.In this way,the clinical translation of NSPC transplantation following spinal cord injury can be accelerated.

This work was supported by the National Institutes of Health (R01NS116404,to JND);MissionConnect,a program of TIRR Foundation (021-101,to JND).

Prakruthi Amar Kumar,Jennifer N.Dulin*

Department of Biology,Texas A&M University,College Station,TX,USA (Amar Kumar P,Dulin JN) Texas A&M Institute for Neuroscience,Texas A&M University,College Station,TX,USA (Dulin JN)

*Correspondence to:Jennifer N.Dulin,PhD,jdulin@bio.tamu.edu.

https://orcid.org/0000-0001-5767-4290(Jennifer N.Dulin)

Date of submission:May 5,2023

Date of decision:June 8,2023

Date of acceptance:June 29,2023

Date of web publication:August 14,2023

https://doi.org/10.4103/1673-5374.382236

How to cite this article:Amar Kumar P,Dulin JN (2024) Implications of regional identity for neural stem and progenitor cell transplantation in the injured or diseased nervous system.Neural Regen Res 19(4):715-716.

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Open peer reviewer:Ying Jin,Drexel University College of Medicine,USA.

Additional file:Open peer review report 1.