Peripheral nerve fibroblasts secrete neurotrophic factors to promote axon growth of motoneurons

MircoRNA-25-3p in skin precursor cell-induced Schwann cell-derived extracellular vesicles promotes axon regeneration by targeting Tgif1

Nerve injury often leads to severe dysfunction because of the lack of axon regeneration in adult mammal. Intriguingly a series of extracellular vesicles (EVs) have the obvious ability to accelerate the nerve repair. However, the detailed molecular mechanisms to describe that EVs switch neuron from a transmitter to a regenerative state have not been elucidated. This study elucidated the microRNA (miRNA) expression profiles of two types of EVs that promote nerve regeneration. The functions of these miRNAs were screened in vitro. Among the 12 overlapping miRNAs, miR-25-3p was selected for further analysis as it markedly promoted axon regeneration both in vivo and in vitro. Furthermore, knockdown experiments confirmed that PTEN and Klf4, which are the major inhibitors of axon regeneration, were the direct targets of miR-25-3p in dorsal root ganglion (DRG) neurons. The utilization of luciferase reporter assays and functional tests provided evidence that miR-25-3p enhances axon regeneration by targeting Tgif1. Additionally, miR-25-3p upregulated the phosphorylation of Erk. Furthermore, Rapamycin modulated the expression of miR-25-3p in DRG neurons. Finally, the pro-axon regeneration effects of EVs were confirmed by overexpressing miR-25-3p and Tgif1 knockdown in the optic nerve crush model. Thus, the enrichment of miR-25-3p in EVs suggests that it regulates axon regeneration, proving a potential cell-free treatment strategy for nerve injury.

Emerging role of extracellular vesicles and exogenous stimuli in molecular mechanisms of peripheral nerve regeneration

Peripheral nerve injuries lead to significant morbidity and adversely affect quality of life. The peripheral nervous system harbors the unique trait of autonomous regeneration; however, achieving successful regeneration remains uncertain. Research continues to augment and expedite successful peripheral nerve recovery, offering promising strategies for promoting peripheral nerve regeneration (PNR). These include leveraging extracellular vesicle (EV) communication and harnessing cellular activation through electrical and mechanical stimulation. Small extracellular vesicles (sEVs), 30-150 nm in diameter, play a pivotal role in regulating intercellular communication within the regenerative cascade, specifically among nerve cells, Schwann cells, macrophages, and fibroblasts. Furthermore, the utilization of exogenous stimuli, including electrical stimulation (ES), ultrasound stimulation (US), and extracorporeal shock wave therapy (ESWT), offers remarkable advantages in accelerating and augmenting PNR. Moreover, the application of mechanical and electrical stimuli can potentially affect the biogenesis and secretion of sEVs, consequently leading to potential improvements in PNR. In this review article, we comprehensively delve into the intricacies of cell-to-cell communication facilitated by sEVs and the key regulatory signaling pathways governing PNR. Additionally, we investigated the broad-ranging impacts of ES, US, and ESWT on PNR.

Transcription Factors in Brain Regeneration: A Potential Novel Therapeutic Target

Transcription factors play a crucial role in providing identity to each cell population. To maintain cell identity, it is essential to balance the expression of activator and inhibitor transcription factors. Cell plasticity and reprogramming offer great potential for future therapeutic applications, as they can regenerate damaged tissue. Specific niche factors can modify gene expression and differentiate or transdifferentiate the target cell to the required fate. Ongoing research is being carried out on the possibilities of transcription factors in regenerating neurons, with neural stem cells (NSCs) being considered the preferred cells for generating new neurons due to their epigenomic and transcriptome memory. NEUROD1/ASCL1, BRN2, MYTL1, and other transcription factors can induce direct reprogramming of somatic cells, such as fibroblasts, into neurons. However, the molecular biology of transcription factors in reprogramming and differentiation still needs to be fully understood.

Growth factors: Bioactive macromolecular drugs for peripheral nerve injury treatment - Molecular mechanisms and delivery platforms

Bioactive macromolecular drugs known as Growth Factors (GFs), approved by the Food and Drug Administration (FDA), have found successful application in clinical practice. They hold significant promise for addressing peripheral nerve injuries (PNIs). Peripheral nerve guidance conduits (NGCs) loaded with GFs, in the context of tissue engineering, can ensure sustained and efficient release of these bioactive compounds. This, in turn, maintains a stable, long-term, and effective GF concentration essential for treating damaged peripheral nerves. Peripheral nerve regeneration is a complex process that entails the secretion of various GFs. Following PNI, GFs play a pivotal role in promoting nerve cell growth and survival, axon and myelin sheath regeneration, cell differentiation, and angiogenesis. They also regulate the regenerative microenvironment, stimulate plasticity changes post-nerve injury, and, consequently, expedite nerve structure and function repair. Both exogenous and endogenous GFs, including NGF, BDNF, NT-3, GDNF, IGF-1, bFGF, and VEGF, have been successfully loaded onto NGCs using techniques like physical adsorption, blend doping, chemical covalent binding, and engineered transfection. These approaches have effectively promoted the repair of peripheral nerves. Numerous studies have demonstrated similar tissue functional therapeutic outcomes compared to autologous nerve transplantation. This evidence underscores the substantial clinical application potential of GFs in the domain of peripheral nerve repair. In this article, we provide an overview of GFs in the context of peripheral nerve regeneration and drug delivery systems utilizing NGCs. Looking ahead, commercial materials for peripheral nerve repair hold the potential to facilitate the effective regeneration of damaged peripheral nerves and maintain the functionality of distant target organs through the sustained release of GFs.

The Porous Structure of Peripheral Nerve Guidance Conduits: Features, Fabrication, and Implications for Peripheral Nerve Regeneration

Porous structure is an important three-dimensional morphological feature of the peripheral nerve guidance conduit (NGC), which permits the infiltration of cells, nutrients, and molecular signals and the discharge of metabolic waste. Porous structures with precisely customized pore sizes, porosities, and connectivities are being used to construct fully permeable, semi-permeable, and asymmetric peripheral NGCs for the replacement of traditional nerve autografts in the treatment of long-segment peripheral nerve injury. In this review, the features of porous structures and the classification of NGCs based on these characteristics are discussed. Common methods for constructing 3D porous NGCs in current research are described, as well as the pore characteristics and the parameters used to tune the pores. The effects of the porous structure on the physical properties of NGCs, including biodegradation, mechanical performance, and permeability, were analyzed. Pore structure affects the biological behavior of Schwann cells, macrophages, fibroblasts, and vascular endothelial cells during peripheral nerve regeneration. The construction of ideal porous structures is a significant advancement in the regeneration of peripheral nerve tissue engineering materials. The purpose of this review is to generalize, summarize, and analyze methods for the preparation of porous NGCs and their biological functions in promoting peripheral nerve regeneration to guide the development of medical nerve repair materials.

Peripheral nerve-derived fibroblasts promote neurite outgrowth in adult dorsal root ganglion neurons more effectively than skin-derived fibroblasts

NEW FINDINGS

What is the central question of this study? Although fibroblasts are involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. What is the main finding and its importance? Nerve-derived fibroblasts have a greater neurite-promoting effect than skin-derived fibroblasts, and epineurium-derived fibroblasts can promote neurite outgrowth more effectively than parenchyma-derived fibroblasts. The epineurium-derived fibroblasts and parenchyma-derived fibroblasts have distinctly different molecular profiles, including genes of soluble factors to promote axonal growth. Fibroblasts are molecularly and functionally different depending on their localization in nerve tissue, and epineurium-derived fibroblasts might be involved in axon regeneration after peripheral nerve injury more than previously thought.

ABSTRACT

Although fibroblasts (Fb) are components of a peripheral nerve involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. The objective of the present study was to investigate the capacity of Fb derived from peripheral nerves to stimulate the outgrowth of neurites from adult dorsal root ganglion neurons and to clarify their molecular characteristics. Fibroblasts were prepared from the epineurium and parenchyma of rat sciatic nerves and skin. The Fb derived from epineurium showed the greatest effect on neurite outgrowth, followed by the Fb derived from parenchyma, indicating that Fb derived from nerves promote neurite outgrowth more effectively than skin-derived Fb. Although both soluble and cell-surface factors contributed evenly to the neurite-promoting effect of nerve-derived Fb, in crush and transection injury models, Fb were not closely associated with regenerating axons, indicating that only soluble factors from Fb are available to regenerating axons. A transcriptome analysis revealed that the molecular profiles of these Fb were distinctly different and that the gene expression profiles of soluble factors that promote axonal growth are unique to each Fb. These findings indicate that Fb are molecularly and functionally different depending on their localization in nerve tissue and that Fb derived from epineurium might be involved more than was previously thought in axon regeneration after peripheral nerve injury.

Sensory and motor fibroblasts have different protein expression patterns and exert different growth promoting effects on sensory and motor neurons

Functional reconstruction after peripheral nerve injury depends on the ability of the regenerated sensory and motor axons to re-innervate the suitable target organs. Therefore, it is essential to explore the cellular mechanisms of peripheral nerve-specific regeneration. In a previous study, we found that sensory and motor fibroblasts can guide Schwann cells to migrate towards the same phenotype. In the present paper, we analyzed the different effects of sensory and motor fibroblasts on sensory or motor neurons. The fibroblasts and neurons co-culture assay showed that compared with motor fibroblasts, sensory fibroblasts promote the neurite outgrowth of sensory neurons on a larger scale, and vice versa. Furthermore, a higher proportion of sensory or motor fibroblasts migrated towards their respective (sensory or motor) neurons. Meanwhile, a comparative proteomic approach was applied to obtain the protein expression profiles of sensory and motor fibroblasts. Among a total of 2597 overlapping proteins identified, we counted 148 differentially expressed items, of those 116 had a significantly higher expression in sensory fibroblasts, and 32 had a significantly greater expression in motor fibroblasts. Functional categorization revealed that differentially expressed proteins were involved in regeneration, axon guidance and cytoskeleton organization, all of which might play a critical role in peripheral nerve-specific regeneration. After nerve crush injury, ITB1 protein expression decreased significantly in motor nerves and increased in sensory nerves. In vitro, ITB1 significantly promoted axonal regeneration of sensory neurons, but had no significant effect on motor neurons. Overall, sensory and motor fibroblasts express different proteins and exert different growth promoting effects on sensory and motor neurons. This comparative proteomic database of sensory and motor fibroblasts could provide future directions for in-depth research on peripheral nerve-specific regeneration. Data are available via ProteomeXchange with identifier PXD034827.

Trigeminal Sensory Supply Is Essential for Motor Recovery after Facial Nerve Injury

Recovery of mimic function after facial nerve transection is poor. The successful regrowth of regenerating motor nerve fibers to reinnervate their targets is compromised by (i) poor axonal navigation and excessive collateral branching, (ii) abnormal exchange of nerve impulses between adjacent regrowing axons, namely axonal crosstalk, and (iii) insufficient synaptic input to the axotomized facial motoneurons. As a result, axotomized motoneurons become hyperexcitable but unable to discharge. We review our findings, which have addressed the poor return of mimic function after facial nerve injuries, by testing the hypothesized detrimental component, and we propose that intensifying the trigeminal sensory input to axotomized and electrophysiologically silent facial motoneurons improves the specificity of the reinnervation of appropriate targets. We compared behavioral, functional, and morphological parameters after single reconstructive surgery of the facial nerve (or its buccal branch) with those obtained after identical facial nerve surgery, but combined with direct or indirect stimulation of the ipsilateral infraorbital nerve. We found that both methods of trigeminal sensory stimulation, i.e., stimulation of the vibrissal hairs and manual stimulation of the whisker pad, were beneficial for the outcome through improvement of the quality of target reinnervation and recovery of vibrissal motor performance.

Transcriptional Control of Peripheral Nerve Regeneration

Transcription factors are master regulators of various cellular processes under diverse physiological and pathological conditions. Many transcription factors that are differentially expressed after injury to peripheral nerves play important roles in nerve regeneration. Considering that rapid and timely regrowth of injured axons is a prerequisite for successful target reinnervation, here, we compile transcription factors that mediates axon elongation, including axon growth suppressor Klf4 and axon growth promoters c-Myc, Sox11, STAT3, Atf3, c-Jun, Smad1, C/EBPδ, and p53. Besides neuronal changes, Schwann cell phenotype modulation is also critical for nerve regeneration. The activation of Schwann cells at early time points post injury provides a permissive microenvironment whereas the re-differentiation of Schwann cells at later time points supports myelin sheath formation. Hence, c-Jun and Sox2, two critical drivers for Schwann cell reprogramming, as well as Krox-20 and Sox10, two essential regulators of Schwann cell myelination, are reviewed. These transcription factors may serve as promising targets for promoting the functional recovery of injured peripheral nerves.