Axotomy-induced target disconnection promotes an additional death mechanism involved in motoneuron degeneration in amyotrophic lateral sclerosis transgenic mice

Axonal regeneration and sprouting as a potential therapeutic target for nervous system disorders

Nervous system disorders are prevalent health issues that will only continue to increase in frequency as the population ages. Dying-back axonopathy is a hallmark of many neurologic diseases and leads to axonal disconnection from their targets, which in turn leads to functional impairment. During the course of many of neurologic diseases, axons can regenerate or sprout in an attempt to reconnect with the target and restore synapse function. In amyotrophic lateral sclerosis (ALS), distal motor axons retract from neuromuscular junctions early in the disease-course before significant motor neuron death. There is evidence of compensatory motor axon sprouting and reinnervation of neuromuscular junctions in ALS that is usually quickly overtaken by the disease course. Potential drugs that enhance compensatory sprouting and encourage reinnervation may slow symptom progression and retain muscle function for a longer period of time in ALS and in other diseases that exhibit dying-back axonopathy. There remain many outstanding questions as to the impact of distinct disease-causing mutations on axonal outgrowth and regeneration, especially in regards to motor neurons derived from patient induced pluripotent stem cells. Compartmentalized microfluidic chambers are powerful tools for studying the distal axons of human induced pluripotent stem cells-derived motor neurons, and have recently been used to demonstrate striking regeneration defects in human motor neurons harboring ALS disease-causing mutations. Modeling the human neuromuscular circuit with human induced pluripotent stem cells-derived motor neurons will be critical for developing drugs that enhance axonal regeneration, sprouting, and reinnervation of neuromuscular junctions. In this review we will discuss compensatory axonal sprouting as a potential therapeutic target for ALS, and the use of compartmentalized microfluidic devices to find drugs that enhance regeneration and axonal sprouting of motor axons.

CD4+ T cell expression of the IL-10 receptor is necessary for facial motoneuron survival after axotomy

Background

After peripheral nerve transection, facial motoneuron (FMN) survival depends on an intact CD4+ T cell population and a central source of interleukin-10 (IL-10). However, it has not been determined previously whether CD4+ T cells participate in the central neuroprotective IL-10 cascade after facial nerve axotomy (FNA).

Methods

Immunohistochemical labeling of CD4+ T cells, pontine vasculature, and central microglia was used to determine whether CD4+ T cells cross the blood-brain barrier and enter the facial motor nucleus (FMNuc) after FNA. The importance of IL-10 signaling in CD4+ T cells was assessed by performing adoptive transfer of IL-10 receptor beta (IL-10RB)-deficient CD4+ T cells into immunodeficient mice prior to injury. Histology and qPCR were utilized to determine the impact of IL-10RB-deficient T cells on FMN survival and central gene expression after FNA. Flow cytometry was used to determine whether IL-10 signaling in T cells was necessary for their differentiation into neuroprotective subsets.

Results

CD4+ T cells were capable of crossing the blood-brain barrier and associating with reactive microglial nodules in the axotomized FMNuc. Full induction of central IL-10R gene expression after FNA was dependent on CD4+ T cells, regardless of their own IL-10R signaling capability. Surprisingly, CD4+ T cells lacking IL-10RB were incapable of mediating neuroprotection after axotomy and promoted increased central expression of genes associated with microglial activation, antigen presentation, T cell co-stimulation, and complement deposition. There was reduced differentiation of IL-10RB-deficient CD4+ T cells into regulatory CD4+ T cells in vitro.

Conclusions

These findings support the interdependence of IL-10- and CD4+ T cell-mediated mechanisms of neuroprotection after axotomy. CD4+ T cells may potentiate central responsiveness to IL-10, while IL-10 signaling within CD4+ T cells is necessary for their ability to rescue axotomized motoneuron survival. We propose that loss of IL-10 signaling in CD4+ T cells promotes non-neuroprotective autoimmunity after FNA.

Identification of a resilient mouse facial motoneuron population following target disconnection by injury or disease

BACKGROUND

When nerve transection is performed on adult rodents, a substantial population of neurons survives short-term disconnection from target, and the immune system supports this neuronal survival, however long-term survival remains unknown. Understanding the effects of permanent axotomy on cell body survival is important as target disconnection is the first pathological occurrence in fatal motoneuron diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA).

OBJECTIVE

The goal of this study was to determine if facial motoneurons (FMN) could survive permanent target disconnection up to 26 weeks post-operation (wpo) after facial nerve axotomy (FNA). In addition, the potentially additive effects of immunodeficiency and motoneuron disease on post-axotomy FMN survival were examined.

METHODS

This study included three wild type (WT) mouse strains (C57BL/6J, B6SJL, and FVB/NJ) and three experimental models (RAG-2-/-: immunodeficiency; mSOD1: ALS; Smn-/-/SMN2+/+: SMA). All animals received a unilateral FNA, and FMN survival was quantified at early and extended post-operative timepoints.

RESULTS

In the C57BL/6J WT group, FMN survival significantly decreased at 10 wpo (55±6%), and then remained stable out to 26 wpo (47±6%). In the RAG-2-/- and mSOD1 groups, FMN death occurred much earlier at 4 wpo, and survival plateaued at approximately 50% at 10 wpo. The SMA model and other WT strains also exhibited approximately 50% FMN survival after FNA.

CONCLUSION

These results indicate that immunodeficiency and motoneuron disease accelerate axotomy-induced neuron death, but do not increase total neuron death in the context of permanent target disconnection. This consistent finding of a target disconnection-resilient motoneuron population is prevalent in other peripheral nerve injury models and in neurodegenerative disease models as well. Characterization of the distinct populations of vulnerable and resilient motoneurons may reveal new therapeutic approaches for injury and disease.

Modeling Environmentally-Induced Motor Neuron Degeneration in Zebrafish

Zebrafish have been used to investigate motor neuron degeneration, including as a model system to examine the pathogenesis of amyotrophic lateral sclerosis (ALS). The use of zebrafish for this purpose has some advantages over other in vivo model systems. In the current paper, we show that bisphenol A (BPA) exposure in zebrafish embryos results in motor neuron degeneration with affected motor function, reduced motor axon length and branching, reduced neuromuscular junction integrity, motor neuron cell death and the presence of activated microglia. In zebrafish, motor axon length is the conventional method for estimating motor neuron degeneration, yet this measurement has not been confirmed as a valid surrogate marker. We also show that reduced motor axon length as measured from the sagittal plane is correlated with increased motor neuron cell death. Our preliminary timeline studies suggest that axonopathy precedes motor cell death. This outcome may have implications for early phase treatments of motor neuron degeneration.

Impact of peripheral immune status on central molecular responses to facial nerve axotomy

When facial nerve axotomy (FNA) is performed on immunodeficient recombinase activating gene-2 knockout (RAG-2^-/-) mice, there is greater facial motoneuron (FMN) death relative to wild type (WT) mice. Reconstituting RAG-2^-/- mice with whole splenocytes rescues FMN survival after FNA, and CD4+ T cells specifically drive immune-mediated neuroprotection. Evidence suggests that immunodysregulation may contribute to motoneuron death in amyotrophic lateral sclerosis (ALS). Immunoreconstitution of RAG-2^-/- mice with lymphocytes from the mutant superoxide dismutase (mSOD1) mouse model of ALS revealed that the mSOD1 whole splenocyte environment suppresses mSOD1 CD4+ T cell-mediated neuroprotection after FNA. The objective of the current study was to characterize the effect of CD4+ T cells on the central molecular response to FNA and then identify if mSOD1 whole splenocytes blocked these regulatory pathways. Gene expression profiles of the axotomized facial motor nucleus were assessed from RAG-2^-/- mice immunoreconstituted with either CD4+ T cells or whole splenocytes from WT or mSOD1 donors. The findings indicate that immunodeficient mice have suppressed glial activation after axotomy, and cell transfer of WT CD4+ T cells rescues microenvironment responses. Additionally, mSOD1 whole splenocyte recipients exhibit an increased astrocyte activation response to FNA. In RAG-2^-/- + mSOD1 whole splenocyte mice, an elevation of motoneuron-specific Fas cell death pathways is also observed. Altogether, these findings suggest that mSOD1 whole splenocytes do not suppress mSOD1 CD4+ T cell regulation of the microenvironment, and instead, mSOD1 whole splenocytes may promote motoneuron death by either promoting a neurotoxic astrocyte phenotype or inducing Fas-mediated cell death pathways. This study demonstrates that peripheral immune status significantly affects central responses to nerve injury. Future studies will elucidate the mechanisms by which mSOD1 whole splenocytes promote cell death and if inhibiting this mechanism can preserve motoneuron survival in injury and disease.

Locomotor analysis identifies early compensatory changes during disease progression and subgroup classification in a mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a motoneuron degenerative disease that is challenging to diagnose and presents with considerable variability in survival. Early identification and enhanced understanding of symptomatic patterns could aid in diagnosis and provide an avenue for monitoring disease progression. Use of the mSOD1G93A mouse model provides control of the confounding environmental factors and genetic heterogeneity seen in amyotrophic lateral sclerosis patients, while investigating underlying disease-induced changes. In the present study, we performed a longitudinal behavioral assessment paradigm and identified an early hindlimb symptom, resembling the common gait abnormality foot drop, along with an accompanying forelimb compensatory mechanism in the mSOD1G93A mouse. Following these initial changes, mSOD1 mice displayed a temporary hindlimb compensatory mechanism resembling an exaggerated steppage gait. As the disease progressed, these compensatory mechanisms were not sufficient to sustain fundamental locomotor parameters and more severe deficits appeared. We next applied these initial findings to investigate the inherent variability in B6SJL mSOD1G93A survival. We identified four behavioral variables that, when combined in a cluster analysis, identified two subpopulations with different disease progression rates: a fast progression group and a slow progression group. This behavioral assessment paradigm, with its analytical approaches, provides a method for monitoring disease progression and detecting mSOD1 subgroups with different disease severities. This affords researchers an opportunity to search for genetic modifiers or other factors that likely enhance or slow disease progression. Such factors are possible therapeutic targets with the potential to slow disease progression and provide insight into the underlying pathology and disease mechanisms.

Necroptosis in amyotrophic lateral sclerosis and other neurological disorders

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by the progressive degeneration of upper and lower motor neurons. Cell death in ALS and in general was previously believed to exist as a dichotomy between apoptosis and necrosis. Most research investigating cell death mechanisms in ALS was conducted before the discovery of programmed necrosis thus did not use selective cell death pathway-specific markers. Recently, a new form of programmed cell death, termed "necroptosis", has been characterized and has been recently implicated in ALS as a primary mechanism driving motor neuron cell death in different forms of ALS. The present review is aimed at summarizing cell death pathways that are currently implicated in ALS and highlighting the emerging evidence on necroptosis as a major driver of motor neuron cell death.

CD4 + T Cells and Neuroprotection: Relevance to Motoneuron Injury and Disease

We have established a physiologically relevant mechanism of CD4+ T cell-mediated neuroprotection involving axotomized wildtype (WT) mouse facial motoneurons (FMN) with significance in the treatment of amyotrophic lateral sclerosis (ALS), a fatal MN disease. Use of the transgenic mouse model of ALS involving expression of human mutant superoxide dismutase genes (SOD1(G93A); abbreviated here as mSOD1) has accelerated basic ALS research. Superimposition of facial nerve axotomy (FNA) on the mSOD1 mouse during pre-symptomatic stages indicates that they behave like immunodeficient mice in terms of increased FMN loss and decreased functional recovery, through a mechanism that, paradoxically, is not inherent within the MN itself, but, instead, involves a defect in peripheral immune: CNS glial cell interactions. Our goal is to utilize our WT mouse model of immune-mediated neuroprotection after FNA as a template to elucidate how a malfunctioning peripheral immune system contributes to motoneuron cell loss in the mSOD1 mouse. This review will discuss potential immune defects in ALS, as well as provide an up-to-date understanding of how the CD4+ effector T cells provide neuroprotection to motoneurons through regulation of the central microglial and astrocytic response to injury. We will discuss an IL-10 cascade within the facial nucleus that requires a functional CD4+ T cell trigger for activation. The review will discuss the role of T cells in ALS, and our recent reconstitution experiments utilizing our model of T cell-mediated neuroprotection in WT vs mSOD1 mice after FNA. Identification of defects in neural:immune interactions could provide targets for therapeutic intervention in ALS.

Identification of B6SJL mSOD1(G93A) mouse subgroups with different disease progression rates

Disease progression rates among patients with amyotrophic lateral sclerosis (ALS) vary greatly. Although the majority of affected individuals survive 3-5 years following diagnosis, some subgroups experience a more rapidly progressing form, surviving less than 1 year, and other subgroups experience slowly progressing forms, surviving nearly 50 years. Genetic heterogeneity and environmental factors pose significant barriers in investigating patient progression rates. Similar to the case for humans, variation in survival within the mSOD1 mouse has been well documented, but different progression rates have not been investigated. The present study identifies two subgroups of B6SJL mSOD1(G93A) mice with different disease progression rates, a fast progression group (FPG) and slow progression group, as evidenced by differences in the rate of motor function decline. In addition, increased disease-associated gene expression within the FPG facial motor nucleus confirmed the presence of a more severe phenotype. We hypothesize that a more severe disease phenotype could be the result of 1) an earlier onset of axonal disconnection with a consistent degeneration rate or 2) a more severe or accelerated degenerative process. We performed a facial nerve transection axotomy in both mSOD1 subgroups prior to disease onset as a method to standardize the axonal disconnection. Instead of leading to comparable gene expression in both subgroups, this standardization did not eliminate the severe phenotype in the FPG facial nucleus, suggesting that the FPG phenotype is the result of a more severe or accelerated degenerative process. We theorize that these mSOD1 subgroups are representative of the rapid and slow disease phenotypes often experienced in ALS.

Facial nerve axotomy in mice: a model to study motoneuron response to injury

The goal of this surgical protocol is to expose the facial nerve, which innervates the facial musculature, at its exit from the stylomastoid foramen and either cut or crush it to induce peripheral nerve injury. Advantages of this surgery are its simplicity, high reproducibility, and the lack of effect on vital functions or mobility from the subsequent facial paralysis, thus resulting in a relatively mild surgical outcome compared to other nerve injury models. A major advantage of using a cranial nerve injury model is that the motoneurons reside in a relatively homogenous population in the facial motor nucleus in the pons, simplifying the study of the motoneuron cell bodies. Because of the symmetrical nature of facial nerve innervation and the lack of crosstalk between the facial motor nuclei, the operation can be performed unilaterally with the unaxotomized side serving as a paired internal control. A variety of analyses can be performed postoperatively to assess the physiologic response, details of which are beyond the scope of this article. For example, recovery of muscle function can serve as a behavioral marker for reinnervation, or the motoneurons can be quantified to measure cell survival. Additionally, the motoneurons can be accurately captured using laser microdissection for molecular analysis. Because the facial nerve axotomy is minimally invasive and well tolerated, it can be utilized on a wide variety of genetically modified mice. Also, this surgery model can be used to analyze the effectiveness of peripheral nerve injury treatments. Facial nerve injury provides a means for investigating not only motoneurons, but also the responses of the central and peripheral glial microenvironment, immune system, and target musculature. The facial nerve injury model is a widely accepted peripheral nerve injury model that serves as a powerful tool for studying nerve injury and regeneration.

Amyotrophic lateral sclerosis as a spatiotemporal mislocalization disease: location, location, location

Spatiotemporal localization of signals is a fundamental feature impacting cell survival and proper function. The cell needs to respond in an accurate manner in both space and time to both intra- and intercellular environment cues. The regulation of this comprehensive process involves the cytoskeleton and the trafficking machinery, as well as local protein synthesis and ligand-receptor mechanisms. Alterations in such mechanisms can lead to cell dysfunction and disease. Motor neurons that can extend over tens of centimeters are a classic example for the importance of such events. Changes in spatiotemporal localization mechanisms are thought to play a role in motor neuron degeneration that occurs in amyotrophic lateral sclerosis (ALS). In this review we will discuss these mechanisms and argue that possible misregulated factors can lead to motor neuron degeneration in ALS.

SOD1(G93A) transgenic mouse CD4(+) T cells mediate neuroprotection after facial nerve axotomy when removed from a suppressive peripheral microenvironment

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease involving motoneuron (MN) axonal withdrawal and cell death. Previously, we established that facial MN (FMN) survival levels in the SOD1(G93A) transgenic mouse model of ALS are reduced and nerve regeneration is delayed, similar to immunodeficient RAG2(-/-) mice, after facial nerve axotomy. The objective of this study was to examine the functionality of SOD1(G93A) splenic microenvironment, focusing on CD4(+) T cells, with regard to defects in immune-mediated neuroprotection of injured MN. We utilized the RAG2(-/-) and SOD1(G93A) mouse models, along with the facial nerve axotomy paradigm and a variety of cellular adoptive transfers, to assess immune-mediated neuroprotection of FMN survival levels. We determined that adoptively transferred SOD1(G93A) unfractionated splenocytes into RAG2(-/-) mice were unable to support FMN survival after axotomy, but that adoptive transfer of isolated SOD1(G93A) CD4(+) T cells could. Although WT unfractionated splenocytes adoptively transferred into SOD1(G93A) mice were able to maintain FMN survival levels, WT CD4(+) T cells alone could not. Importantly, these results suggest that SOD1(G93A) CD4(+) T cells retain neuroprotective functionality when removed from a dysfunctional SOD1(G93A) peripheral splenic microenvironment. These results also indicate that the SOD1(G93A) central nervous system microenvironment is able to re-activate CD4(+) T cells for immune-mediated neuroprotection when a permissive peripheral microenvironment exists. We hypothesize that a suppressive SOD1(G93A) peripheral splenic microenvironment may compromise neuroprotective CD4(+) T cell activation and/or differentiation, which, in turn, results in impaired immune-mediated neuroprotection for MN survival after peripheral axotomy in SOD1(G93A) mice.