Recent advances in endogenous neural stem/progenitor cell manipulation for spinal cord injury repair

Abstract

Traumatic spinal cord injury (SCI) can cause severe neurological impairments. Clinically available treatments are quite limited, with unsatisfactory remediation effects. Residing endogenous neural stem/progenitor cells (eNSPCs) tend to differentiate towards astrocytes, leaving only a small fraction towards oligodendrocytes and even fewer towards neurons; this has been suggested as one of the reasons for the failure of autonomous neuronal regeneration. Thus, finding ways to recruit and facilitate the differentiation of eNSPCs towards neurons has been considered a promising strategy for the noninvasive and immune-compatible treatment of SCI. The present manuscript first introduces the responses of eNSPCs after exogenous interventions to boost endogenous neurogenesis in various SCI models. Then, we focus on state-of-art manipulation approaches that enhance the intrinsic neurogenesis capacity and reconstruct the hostile microenvironment, mainly consisting of pharmacological treatments, stem cell-derived exosome administration, gene therapy, functional scaffold implantation, inflammation regulation, and inhibitory element delineation. Facing the extremely complex situation of SCI, combined treatments are also highlighted to provide more clues for future relevant investigations.

Keywords: Endogenous neural stem/progenitor cells; combined treatment; neurogenesis; spinal cord injury repair.

Figure 1

Responses of EpCs after SCI. Post SCI, 95% of the EpCs differentiated into astrocytes, accounting for 53% of newly generated astrocytes (GFAP^-), with the remaining 47% of new astrocytes being derived from the proliferation of resident astrocytes (GFAP⁺). These cells promote glial scar formation which can prevent secondary injury but restrict neural regeneration. Less than 5% of the activated EpCs differentiated into myelin-forming oligodendrocytes, accounting for 3% of newly generated oligodendrocytes, with the remaining 97% of newly generated oligodendrocytes being derived from the differentiation of OPCs. Unfortunately, almost none of the EpCs differentiated into neurons which leads to the lossof neural circuit regeneration.

Figure 2

Schematic illustration of typical strategies modulating eNSPCs for the treatment of SCI. Pharmacological treatments, stem cell-derived exosome administration, gene therapy, and functional scaffold implantation can boost eNSPCs' intrinsic proliferation and differentiation capacity. Inflammatory regulation and inhibitory element delineation can remold the hostile microenvironment. All these strategies trigger eNSPCs to differentiate into neurons and oligodendrocytes but not astrocytes.

Figure 3

Lentivirus-mediated expression of neurogenic TFs increased neurogenesis of specific interneurons. (A) Lateral hemisection SCI was performed on 8- to 12-week-old mice at the T9-T10 level immediately followed by the injection of lentivirus encoding Gsx1 along with a RFP reporter (lenti-Gsx1-RFP). Lentivirus encoding, only the reporter RFP was used as a control (lenti-Ctrl-RFP). (B) Confocal images of sagittal sections of spinal cord tissues at 56 DPI show the expression of the viral reporter RFP and the mature neuron marker NeuN, glutamatergic neuron marker vGlut2, cholinergic neuron marker ChAT, and GABAergic neuron marker GABA, with quantification (n = 4). (C) Representative images of walking posture at 56 DPI and a plot of the BMS scores of the left hindlimb over 56 DPI. (D) Representative confocal images of the sagittal section of spinal cord tissue samples stained for the expression of mature neural markers: NeuN, ChAT, vGlut2, and GABA at 56 DPI and quantification of virally transduced cells co-labeled with a cell-specific marker. (A-C) Adapted with permission from , Copyright 2021 Elsevier. (D) Adapted with permission from , Copyright 2021 Elsevier.

Figure 4

(A) Engineering a nanofiber-hydrogel composite with interfacial bonding between the fiber surface and hydrogel network. Left column: Schematic of the synthesis and structure of the PCL nanofiber-HA hydrogel composite (NHC). Right column: SEM images of the NHC (left) and porcine native spinal cord ECM (right), and the oscillatory strain sweep tests of the HA hydrogel and composites with and without interfacial bonding. (B) The NHC limits the collapse of the contused spinal cord and facilitates tissue formation in the injury. (C) Upper panel: The NHC stimulated neurogenesis in the injury. Microphotographs showing immature neurons stained with antibodies against βIII-tubulin and around the injury in representative horizontal sections of each treatment group at 3 (a), 7 (b), and 28 (c) days post-injury (dpi) and a representative area for each group at 28 dpi shown in high magnification. Bottom panel: Bar graph showing the average density of immature neurons per mm² in the injury for each experimental group at 3, 7, and 28 dpi, and the microphotographs showing neural precursor cells stained with antibodies against doublecortin (DCX, red) in and around the injury in horizontal sections of each treatment group at 28 dpi. Adapted with permission from , Copyright 2020 Elsevier.

Figure 5

(A) Illustration of the fabrication of a cross-linked CPH following the addition of TA, Py, and Fe³⁺, as well as the physical and electrical properties of CPHs with different TA concentrations. (B) Upper three panels: Immunohistofluorescence images of transverse spinal cord sections obtained from animals in the sham, SCI, and hydrogel groups at 6 weeks. Bottom panel: NF staining of the regenerated nerve fibers in the hydrogel group, and the Western blot analysis of the spinal cord protein extracts showing the Tuj1, GFAP, and NF protein bands in the sham, SCI, and hydrogel groups. Adapted with permission from , Copyright 2018 American Chemical Society.

Figure 6

(A) Schematic of injury-induced activation and migration of NSPCs and cetuximab-promoted neuronal differentiation of NSPCs. Endogenous NSPCs remain quiescent in the intact spinal cord, but severe SCI can activate NSPCs and induce their migration into the lesion site. Blocking EGF signaling can promote the injury-activated NSPCs to differentiate into neurons. These newborn neurons can then build neuronal relays to bridge injury gaps and lead to functional recovery after SCI. (B) Endogenous neurogenesis and CSPG deposition in lesion sites of dogs with long-term T8 removal injury. Upper panels: Tuj-1-positive newborn neurons, MAP2⁺ mature neurons, and NeuN⁺ cells in the lesion sites of dogs in the control, scaffolds implantation and cetuximab-modified scaffolds implantation (LOCS+Cetuximab) groups at 9 months after injury. Bottom panels: enlargements of areas indicated in AeC (yellow boxes), and quantification of newborn neurons, mature neurons, and neuronal nuclei in lesion centers of dogs among each group. (C) Locomotion recovery, assessed with Olby scores, at each time point examined. Adapted with permission from , Copyright 2017 Elsevier.

Figure 7

Illustration of the multilayered extrinsic and intrinsic mechanisms of regulating eNSPCs behavior post SCI. Delivering extrinsic niche signals (e.g. PTX, Curcumin, Epo, Substance P, CNTF, etc.) activates intracellular signaling pathways that induce changes in neurogenic gene expression. In addition, manipulating intrinsic transcriptional programs (e.g. transcription factors, epigenetic information, non-coding RNA, etc.) can direct neurogenic gene expression in eNSPCs. These strategies trigger eNSPC neurogenesis which leads to neural connection reconstruction and glial scar reduction. Arrows with dotted lines mean some steps of the signal pathway are omitted. Created with BioRender.com.