Development and Characterisation of an in vitro Model of Wallerian Degeneration

Schwann cell‐specific RhoA knockout accelerates peripheral nerve regeneration via promoting Schwann cell dedifferentiation

Our previous studies indicated that RhoA knockdown or inhibition could alleviate the proliferation, migration, and differentiation of Schwann cells. However, the role of RhoA in Schwann cells during nerve injury and repair is still unknown. Herein, we developed two lines of Schwann cells conditional RhoA knockout (cKO) mice by breeding RhoA^{flox / flox} mice with Plp^{Cre -ERT2} or Dhh^Cre mice. Our results indicate that RhoA cKO in Schwann cells accelerates axonal regrowth and remyelination after sciatic nerve injury, which enhances the recovery of nerve conduction and hindlimb gait, and alleviates the amyotrophy in gastrocnemius muscle. Mechanistic studies in both in vivo and in vitro models revealed that RhoA cKO could facilitate Schwann cell dedifferentiation via JNK pathway. Schwann cell dedifferentiation subsequently promotes Wallerian degeneration by enhancing phagocytosis and myelinophagy, as well as stimulating the production of neurotrophins (NT-3, NGF, BDNF, and GDNF). These findings shed light on the role of RhoA in Schwann cells during nerve injury and repair, indicating that cell type-specific RhoA targeting could serve as a promising molecular therapeutic strategy for peripheral nerve injury.

Morphometric Analysis of Retinal Ganglion Cell Axons in Normal and Glaucomatous Brown Norway Rats Optic Nerves

Purpose

A reference atlas of optic nerve (ON) retinal ganglion cell (RGC) axons could facilitate studies of neuro-ophthalmic diseases by detecting subtle RGC axonal changes. Here we construct an RGC axonal atlas for normotensive eyes in Brown Norway rats, widely used in glaucoma research, and also develop/evaluate several novel metrics of axonal damage in hypertensive eyes.

Methods

Light micrographs of entire ON cross-sections from hypertensive and normotensive eyes were processed through a deep learning-based algorithm, AxoNet2.0, to determine axonal morphological properties and were semiquantitatively scored using the Morrison grading scale (MGS) to provide a damage score independent of AxoNet2.0 outcomes. To construct atlases, ONs were conformally mapped onto an ON "template," and axonal morphometric data was computed for each region. We also developed damage metrics based on myelin morphometry.

Results

In normotensive eyes, average axon density was ∼0.3 axons/µm2 (i.e., ∼80,000 axons in an ON). We measured axoplasm diameter, eccentricity, cross-sectional area, and myelin g-ratio and thickness. Most morphological parameters exhibited a wide range of coefficients of variation (CoV); however, myelin thickness CoV was only ∼2% in normotensive eyes. In hypertensive eyes, increased myelin thickness correlated strongly with MGS (P < 0.0001).

Conclusions

We present the first comprehensive normative RGC axon morphometric atlas for Brown Norway rat eyes. We suggest objective, repeatable damage metrics based on RGC axon myelin thickness for hypertensive eyes.

Translational Relevance

These tools can evaluate regional changes in RGCs and overall levels of damage in glaucoma studies using Brown Norway rats.

Inhibition of Schwann Cell Pyroptosis Promotes Nerve Regeneration in Peripheral Nerve Injury in Rats

Background

Peripheral nerve injury (PNI) is one of the most debilitating injuries, but therapies for PNI are still far from satisfactory. Pyroptosis, a recently identified form of cell death, has been demonstrated to participate in different diseases. However, the role of pyroptosis of Schwann cells in PNI remains unclear.

Methods

We established a rat PNI model, and western blotting, transmission electron microscopy, and immunofluorescence staining were used to confirm pyroptosis of Schwann cells in PNI in vivo. In vitro, pyroptosis of Schwann cells was induced by lipopolysaccharides (LPS)+adenosine triphosphate disodium (ATP). An irreversible inhibitor of pyroptosis, acetyl (Ac)-Tyr-Val-Ala-Asp-chloromethyl ketone (Ac-YVAD-cmk), was used to attenuate Schwann cell pyroptosis. Moreover, the influence of pyroptotic Schwann cells on the function of dorsal root ganglion neurons (DRGns) was analyzed by a coculture system. Finally, the rat PNI model was intraperitoneally treated with Ac-YVAD-cmk to observe the effect of pyroptosis on nerve regeneration and motor function.

Results

Schwann cell pyroptosis was notably observed in the injured sciatic nerve. LPS+ATP treatment effectively induced Schwann cell pyroptosis, which was largely attenuated by Ac-YVAD-cmk. Additionally, pyroptotic Schwann cells inhibited the function of DRGns by secreting inflammatory factors. A decrease in pyroptosis in Schwann cells promoted regeneration of the sciatic nerve and recovery of motor function in rats.

Conclusion

Given the role of Schwann cell pyroptosis in PNI progression, inhibition of Schwann cell pyroptosis might be a potential therapeutic strategy for PNI in the future.

Advancing Our Understanding of the Chronically Denervated Schwann Cell: A Potential Therapeutic Target?

Outcomes for patients following major peripheral nerve injury are extremely poor. Despite advanced microsurgical techniques, the recovery of function is limited by an inherently slow rate of axonal regeneration. In particular, a time-dependent deterioration in the ability of the distal stump to support axonal growth is a major determinant to the failure of reinnervation. Schwann cells (SC) are crucial in the orchestration of nerve regeneration; their plasticity permits the adoption of a repair phenotype following nerve injury. The repair SC modulates the initial immune response, directs myelin clearance, provides neurotrophic support and remodels the distal nerve. These functions are critical for regeneration; yet the repair phenotype is unstable in the setting of chronic denervation. This phenotypic instability accounts for the deteriorating regenerative support offered by the distal nerve stump. Over the past 10 years, our understanding of the cellular machinery behind this repair phenotype, in particular the role of c-Jun, has increased exponentially, creating opportunities for therapeutic intervention. This review will cover the activation of the repair phenotype in SC, the effects of chronic denervation on SC and current strategies to 'hack' these cellular pathways toward supporting more prolonged periods of neural regeneration.

A Brief Review of In Vitro Models for Injury and Regeneration in the Peripheral Nervous System

Nerve axonal injury and associated cellular mechanisms leading to peripheral nerve damage are important topics of research necessary for reducing disability and enhancing quality of life. Model systems that mimic the biological changes that occur during human nerve injury are crucial for the identification of cellular responses, screening of novel therapeutic molecules, and design of neural regeneration strategies. In addition to in vivo and mathematical models, in vitro axonal injury models provide a simple, robust, and reductionist platform to partially understand nerve injury pathogenesis and regeneration. In recent years, there have been several advances related to in vitro techniques that focus on the utilization of custom-fabricated cell culture chambers, microfluidic chamber systems, and injury techniques such as laser ablation and axonal stretching. These developments seem to reflect a gradual and natural progression towards understanding molecular and signaling events at an individual axon and neuronal-soma level. In this review, we attempt to categorize and discuss various in vitro models of injury relevant to the peripheral nervous system and highlight their strengths, weaknesses, and opportunities. Such models will help to recreate the post-injury microenvironment and aid in the development of therapeutic strategies that can accelerate nerve repair.

SIRT6 Negatively Regulates Schwann Cells Dedifferentiation via Targeting c-Jun During Wallerian Degeneration After Peripheral Nerve Injury

Silent information regulator 6 (SIRT6) is a mammalian homolog of the nicotinamide adenine dinucleotide (NAD)-dependent deacetylase sirtuin family. Previous studies have been reported a pro-regenerative role of SIRT6 in central nervous system injury. However, the role of SIRT6 in peripheral nerve injury is still unknown. Given the importance and necessity of Schwann cell dedifferentiation response to peripheral nerve injury, we aim to investigate the molecular mechanism of SIRT6 steering Schwann cell dedifferentiation during Wallerian degeneration in injured peripheral nerve. Herein, we first examined the expression pattern of SIRT6 after peripheral nerve injury. Using the explants of sciatic nerve, an ex vivo model of nerve degeneration, we provided evidences indicating that SIRT6 inhibitor accelerates Schwann cell dedifferentiation as well as axonal and myelin degeneration, while SIRT6 activator attenuates this process. Moreover, in an in vitro Schwann cell dedifferentiation model, we found SIRT6 inhibitor promotes Schwann cell dedifferentiation through upregulating the expression of c-Jun. In addition, downregulation of c-Jun reverse the effects of SIRT6 inhibition on the Schwann cells dedifferentiation and axonal and myelin degeneration. In summary, we first described SIRT6 acts as a negative regulator for Schwann cells dedifferentiation during Wallerian degeneration and c-Jun worked as a direct downstream partner of SIRT6 in injured peripheral nerve.