Transgenic mice with enhanced neuronal major histocompatibility complex class I expression recover locomotor function better after spinal cord injury

CD3ζ-Mediated Signaling Protects Retinal Ganglion Cells in Glutamate Excitotoxicity of the Retina

Excessive levels of glutamate activity could potentially damage and kill neurons. Glutamate excitotoxicity is thought to play a critical role in many CNS and retinal diseases. Accordingly, glutamate excitotoxicity has been used as a model to study neuronal diseases. Immune proteins, such as major histocompatibility complex (MHC) class I molecules and their receptors, play important roles in many neuronal diseases, while T-cell receptors (TCR) are the primary receptors of MHCI. We previously showed that a critical component of TCR, CD3ζ, is expressed by mouse retinal ganglion cells (RGCs). The mutation of CD3ζ or MHCI molecules compromises the development of RGC structure and function. In this study, we investigated whether CD3ζ-mediated molecular signaling regulates RGC death in glutamate excitotoxicity. We show that mutation of CD3ζ significantly increased RGC survival in NMDA-induced excitotoxicity. In addition, we found that several downstream molecules of TCR, including Src (proto-oncogene tyrosine-protein kinase) family kinases (SFKs) and spleen tyrosine kinase (Syk), are expressed by RGCs. Selective inhibition of an SFK member, Hck, or Syk members, Syk or Zap70, significantly increased RGC survival in NMDA-induced excitotoxicity. These results provide direct evidence to reveal the underlying molecular mechanisms that control RGC death under disease conditions.

Study of Effector CD8+ T Cell Interactions with Cortical Neurons in Response to Inflammation in Mouse Brain Slices and Neuronal Cultures

Cytotoxic CD8+ T cells contribute to neuronal damage in inflammatory and degenerative CNS disorders, such as multiple sclerosis (MS). The mechanism of cortical damage associated with CD8+ T cells is not well understood. We developed in vitro cell culture and ex vivo brain slice co-culture models of brain inflammation to study CD8+ T cell-neuron interactions. To induce inflammation, we applied T cell conditioned media, which contains a variety of cytokines, during CD8+ T cell polyclonal activation. Release of IFNγ and TNFα from co-cultures was verified by ELISA, confirming an inflammatory response. We also visualized the physical interactions between CD8+ T cells and cortical neurons using live-cell confocal imaging. The imaging revealed that T cells reduced their migration velocity and changed their migratory patterns under inflammatory conditions. CD8+ T cells increased their dwell time at neuronal soma and dendrites in response to added cytokines. These changes were seen in both the in vitro and ex vivo models. The results confirm that these in vitro and ex vivo models provide promising platforms for the study of the molecular details of neuron-immune cell interactions under inflammatory conditions, which allow high-resolution live microscopy and are readily amenable to experimental manipulation.

Reversible CD8 T cell-neuron cross-talk causes aging-dependent neuronal regenerative decline

Aging is associated with increased prevalence of axonal injuries characterized by poor regeneration and disability. However, the underlying mechanisms remain unclear. In our experiments, RNA sequencing of sciatic dorsal root ganglia (DRG) revealed significant aging-dependent enrichment in T cell signaling both before and after sciatic nerve injury (SNI) in mice. Lymphotoxin activated the transcription factor NF-κB, which induced expression of the chemokine CXCL13 by neurons. This in turn recruited CXCR5+CD8+ T cells to injured DRG neurons overexpressing major histocompatibility complex class I. CD8+ T cells repressed the axonal regeneration of DRG neurons via caspase 3 activation. CXCL13 neutralization prevented CXCR5+CD8+ T cell recruitment to the DRG and reversed aging-dependent regenerative decline, thereby promoting neurological recovery after SNI. Thus, axonal regeneration can be facilitated by antagonizing cross-talk between immune cells and neurons.

Role of MHC-I Expression on Spinal Motoneuron Survival and Glial Reactions Following Ventral Root Crush in Mice

Lesions to the CNS/PNS interface are especially severe, leading to elevated neuronal degeneration. In the present work, we establish the ventral root crush model for mice, and demonstrate the potential of such an approach, by analyzing injury evoked motoneuron loss, changes of synaptic coverage and concomitant glial responses in β2-microglobulin knockout mice (β2m KO). Young adult (8–12 weeks old) C57BL/6J (WT) and β2m KO mice were submitted to a L4–L6 ventral roots crush. Neuronal survival revealed a time-dependent motoneuron-like cell loss, both in WT and β2m KO mice. Along with neuronal loss, astrogliosis increased in WT mice, which was not observed in β2m KO mice. Microglial responses were more pronounced during the acute phase after lesion and decreased over time, in WT and KO mice. At 7 days after lesion β2m KO mice showed stronger Iba-1⁺ cell reaction. The synaptic inputs were reduced over time, but in β2m KO, the synaptic loss was more prominent between 7 and 28 days after lesion. Taken together, the results herein demonstrate that ventral root crushing in mice provides robust data regarding neuronal loss and glial reaction. The retrograde reactions after injury were altered in the absence of functional MHC-I surface expression.

Anastomotic stoma coated with chitosan film as a betamethasone dipropionate carrier for peripheral nerve regeneration

Scar hyperplasia at the suture site is an important reason for hindering the repair effect of peripheral nerve injury anastomosis. To address this issue, two repair methods are often used. Biological agents are used to block nerve sutures and the surrounding tissue to achieve physical anti-adhesion effects. Another agent is glucocorticosteroid, which can prevent scar growth by inhibiting inflammation. However, the overall effect of promoting regeneration of the injured nerve is not satisfactory. In this regard, we envision that these two methods can be combined and lead to shared understanding for achieving improved nerve repair. In this study, the right tibial nerve was transected 1 cm above the knee to establish a rat tibial nerve injury model. The incision was directly sutured after nerve transection. The anastomotic stoma was coated with 0.5 × 0.5 cm2 chitosan sheets with betamethasone dipropionate. At 12 weeks after injury, compared with the control and poly (D, L-lactic acid) groups, chitosan-betamethasone dipropionate film slowly degraded with the shape of the membrane still intact. Further, scar hyperplasia and the degree of adhesion at anastomotic stoma were obviously reduced, while the regenerated nerve fiber structure was complete and arranged in a good order in model rats. Electrophysiological study showed enhanced compound muscle action potential. Our results confirm that chitosan-betamethasone dipropionate film can effectively prevent local scar hyperplasia after tibial nerve repair and promote nerve regeneration.

The Emerging Role of the Major Histocompatibility Complex Class I in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting upper and lower motoneurons (MNs). The etiology of the disease is still unknown for most patients with sporadic ALS, while in 5–10% of the familial cases, several gene mutations have been linked to the disease. Mutations in the gene encoding Cu, Zn superoxide dismutase (SOD1), reproducing in animal models a pathological scenario similar to that found in ALS patients, have allowed for the identification of mechanisms relevant to the ALS pathogenesis. Among them, neuroinflammation mediated by glial cells and systemic immune activation play a key role in the progression of the disease, through mechanisms that can be either neuroprotective or neurodetrimental depending on the type of cells and the MN compartment involved. In this review, we will examine and discuss the involvement of major histocompatibility complex class I (MHCI) in ALS concerning its function in the adaptive immunity and its role in modulating the neural plasticity in the central and peripheral nervous system. The evidence indicates that the overexpression of MHCI into MNs protect them from astrocytes’ toxicity in the central nervous system (CNS) and promote the removal of degenerating motor axons accelerating collateral reinnervation of muscles.

The effects of immune protein CD3ζ development and degeneration of retinal neurons after optic nerve injury

Major histocompatibility complex (MHC) class I molecules and their receptors play fundamental roles in neuronal death during diseases. T-cell receptors (TCR) function as MHCI receptor on T-cells and both MHCI and a key component of TCR, CD3ζ, are expressed by mouse retinal ganglion cells (RGCs) and displaced amacrine cells. Mutation of these molecules compromises the development of RGCs. We investigated whether CD3ζ regulates the development and degeneration of amacrine cells after RGC death. Surprisingly, mutation of CD3ζ not only impairs the proper development of amacrine cells expressing CD3ζ but also those not expressing CD3ζ. In contrast to effects of MHCI and its receptor, PirB, on other neurons, mutation of CD3ζ has no effect on RGC death and starburst amacrine cells degeneration after optic nerve crush. Thus, unlike MHCI and PirB, CD3ζ regulates the development of RGCs and amacrine cells but not their degeneration after optic nerve crush.

Facial Nerve Recovery in KbDb and C1q Knockout Mice: A Role for Histocompatibility Complex 1

BACKGROUND

Understanding the mechanisms in nerve damage can lead to better outcomes for neuronal rehabilitation. The purpose of our study was to assess the effect of major histocompatibility complex I deficiency and inhibition of the classical complement pathway (C1q) on functional recovery and cell survival in the facial motor nucleus (FMN) after crush injury in adult and juvenile mice.

METHODS

A prospective blinded analysis of functional recovery and cell survival in the FMN after a unilateral facial nerve crush injury in juvenile and adult mice was undertaken between wild-type, C1q knockout (C1q-/-), and KbDb knockout (KbDb-/-) groups. Whisker function was quantified to assess functional recovery. Neuron counts were performed to determine neuron survival in the FMN after recovery.

RESULTS

After facial nerve injury, all adult wild-type mice fully recovered. Juvenile mice recovered incompletely corresponding to a greater neuron loss in the FMN of juveniles compared with adults. The C1q-/- juvenile and adult groups did not differ from wild type. The KbDb-/- adults demonstrated 50% recovery of whisker movement and decreased cell survival in FMN. The KbDb-/- juvenile group did not demonstrate any difference from control group.

CONCLUSION

Histocompatibility complex I plays a role for neuroprotection and enhanced facial nerve recovery in adult mice. Inhibition of the classical complement pathway alone does not affect functional recovery or neuronal survival. The alternative and mannose binding pathways pose alternative means for activating the final components of the pathway that may lead to acute nerve damage.

Major Histocompatibility Complex I Expression by Motor Neurons and Its Implication in Amyotrophic Lateral Sclerosis

Neuronal expression of major histocompatibility complex I (MHCI)-related molecules in adults and during CNS diseases is involved in the synaptic plasticity and axonal regeneration with mechanisms either dependent or independent of their immune functions. Motor neurons are highly responsive in triggering the expression of MHCI molecules during normal aging or following insults and diseases, and this has implications in the synaptic controls, axonal regeneration, and neuromuscular junction stability of these neurons. We recently reported that MHCI and immunoproteasome are strongly activated in spinal motor neurons and their peripheral motor axon in a mouse model of familial amyotrophic lateral sclerosis (ALS) during the course of the disease. This response was prominent in ALS mice with slower disease progression in which the axonal structure and function was better preserved than in fast-progressing mice. This review summarizes and discusses our observations in the light of knowledge about the possible role of MHCI in motor neurons providing additional insight into the pathophysiology of ALS.

New Insights on the Mechanisms of Disease Course Variability in ALS from Mutant SOD1 Mouse Models

Amyotrophic Lateral Sclerosis (ALS) is a heterogeneous disease in terms of progression rate and survival. This is probably one of the reasons for the failure of many clinical trials and the lack of effective therapies. Similar variability is also seen in SOD1(G93A) mouse models based on their genetic background. For example, when the SOD1(G93A) transgene is expressed in C57BL6 background the phenotype is mild with slower disease progression than in the 129Sv mice expressing the same amount of transgene but showing faster progression and shorter lifespan. This review summarizes and discusses data obtained from the analysis of these two mouse models under different aspects such as the motor phenotype, neuropathological alterations in the central nervous system (CNS) and peripheral nervous system (PNS) and the motor neuron autonomous and non-cell autonomous mechanisms with the aim of finding elements to explain the different rates of disease progression. We also discuss the identification of promising prognostic biomarkers by comparative analysis of the two ALS mouse models. This analysis might possibly suggest new strategies for effective therapeutic intervention in ALS to slow significantly or even block the course of the disease.

Neuronal MHC-I expression and its implications in synaptic function, axonal regeneration and Parkinson's and other brain diseases

Neuronal expression of major histocompatibility complex I (MHC-I) has been implicated in developmental synaptic plasticity and axonal regeneration in the central nervous system (CNS), but recent findings demonstrate that constitutive neuronal MHC-I can also be involved in neurodegenerative diseases by playing a neuroinflammtory role. Recent reports demonstrate its expression in vitro and in human postmortem samples and support a role in neurodegeneration involving proinflammatory cytokines, activated microglia and increased cytosolic oxidative stress. Major histocompatibility complex I may be important for both normal development and pathogenesis of some CNS diseases including Parkinson's.

From mechanosensitivity to inflammatory responses: new players in the pathology of glaucoma

PURPOSE OF THE STUDY

Many blinding diseases of the inner retina are associated with degeneration and loss of retinal ganglion cells (RGCs). Recent evidence implicates several new signaling mechanisms as causal agents associated with RGC injury and remodeling of the optic nerve head. Ion channels such as Transient receptor potential vanilloid isoform 4 (TRPV4), pannexin-1 (Panx1) and P2X7 receptor are localized to RGCs and act as potential sensors and effectors of mechanical strain, ischemia and inflammatory responses. Under normal conditions, TRPV4 may function as an osmosensor and a polymodal molecular integrator of diverse mechanical and chemical stimuli, whereas P2X7R and Panx1 respond to stretch- and/or swelling-induced adenosine triphosphate release from neurons and glia. Ca(2+) influx, induced by stimulation of mechanosensitive ion channels in glaucoma, is proposed to influence dendritic and axonal remodeling that may lead to RGC death while (at least initially) sparing other classes of retinal neuron. The secondary phase of the retinal glaucoma response is associated with microglial activation and an inflammatory response involving Toll-like receptors (TLRs), cluster of differentiation 3 (CD3) immune recognition molecules associated with the T-cell antigen receptor, complement molecules and cell type-specific release of neuroactive cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). The retinal response to mechanical stress thus involves a diversity of signaling pathways that sense and transduce mechanical strain and orchestrate both protective and destructive secondary responses.

CONCLUSIONS

Mechanistic understanding of the interaction between pressure-dependent and independent pathways is only beginning to emerge. This review focuses on the molecular basis of mechanical strain transduction as a primary mechanism that can damage RGCs. The damage occurs through Ca(2+)-dependent cellular remodeling and is associated with parallel activation of secondary ischemic and inflammatory signaling pathways. Molecules that mediate these mechanosensory and immune responses represent plausible targets for protecting ganglion cells in glaucoma, optic neuritis and retinal ischemia.

Opposing effects of Toll-like receptors 2 and 4 on synaptic stability in the spinal cord after peripheral nerve injury

Background

Glial cells are involved in the synaptic elimination process that follows neuronal lesions, and are also responsible for mediating the interaction between the nervous and immune systems. Neurons and glial cells express Toll-like receptors (TLRs), which may affect the plasticity of the central nervous system (CNS). Because TLRs might also have non-immune functions in spinal-cord injury (SCI), we aimed to investigate the influence of TLR2 and TLR4 on synaptic plasticity and glial reactivity after peripheral nerve axotomy.

Methods

The lumbar spinal cords of C3H/HePas wild-type (WT) mice, C3H/HeJ TLR4-mutant mice, C57BL/6J WT mice, and C57BL/6J TLR2 knockout (KO) mice were studied after unilateral sciatic nerve transection. The mice were killed via intracardiac perfusion, and the spinal cord was processed for immunohistochemistry, transmission electron microscopy (TEM), western blotting, cell culture, and reverse transcriptase PCR. Primary cultures of astrocytes from newborn mice were established to study the astrocyte response in the absence of TLR2 and the deficiency of TLR4 expression.

Results

The results showed that TLR4 and TLR2 expression in the CNS may have opposite effects on the stability of presynaptic terminals in the spinal cord. First, TLR4 contributed to synaptic preservation of terminals in apposition to lesioned motor neurons after peripheral injury, regardless of major histocompatibility complex class I (MHC I) expression. In addition, in the presence of TLR4, there was upregulation of glial cell-derived neurotrophic factor and downregulation of interleukin-6, but no morphological differences in glial reactivity were seen. By contrast, TLR2 expression led to greater synaptic loss, correlating with increased astrogliosis and upregulation of pro-inflammatory interleukins. Moreover, the absence of TLR2 resulted in the upregulation of neurotrophic factors and MHC I expression.

Conclusion

TLR4 and TLR2 in the CNS may have opposite effects on the stability of presynaptic terminals in the spinal cord and in astroglial reactions, indicating possible roles for these proteins in neuronal and glial responses to injury.

Major histocompatibility complex class I proteins in brain development and plasticity

Proper development of the central nervous system (CNS) requires the establishment of appropriate connections between neurons. Recent work suggests that this process is controlled by a balance between synaptogenic molecules and proteins that negatively regulate synapse formation and plasticity. Surprisingly, many of these newly identified synapse-limiting molecules are classic 'immune' proteins. In particular, major histocompatibility complex class I (MHCI) molecules regulate neurite outgrowth, the establishment and function of cortical connections, activity-dependent refinement in the visual system, and long-term and homeostatic plasticity. This review summarizes our current understanding of MHCI expression and function in the CNS, as well as the potential mechanisms used by MHCI to regulate brain development and plasticity.

Decreased MHC I expression in IFN γ mutant mice alters synaptic elimination in the spinal cord after peripheral injury

Background

The histocompatibility complex (MHC) class I expression in the central nervous system (CNS) regulates synaptic plasticity events during development and adult life. Its upregulation may be associated with events such as axotomy, cytokine exposition and changes in neuron electrical activity. Since IFNγ is a potent inducer of the MHC I expression, the present work investigated the importance of this pro-inflammatory cytokine in the synaptic elimination process in the spinal cord, as well as the motor recovery of IFN^−/−, following peripheral injury.

Methods

The lumbar spinal cords of C57BL/6J (wild type) and IFNγ^−/− (mutant) mice, subjected to unilateral sciatic nerve transection, were removed and processed for immunohistochemistry and real time RT-PCR, while the sciatic nerves from animals subjected to unilateral crush, were submitted to immunohistochemistry and electron microscopy for counting of the axons. Gait recovery was monitored using the Cat Walk system. Newborn mice astrocyte primary cultures were established in order to study the astrocytic respose in the absence of the IFNγ expression.

Results

IFNγ^−/− mutant mice showed a decreased expression of MHC I and β2-microglobulin mRNA coupled with reduced synaptophysin immunolabelling in the lesioned spinal cord segment. Following unilateral nerve transection, the Iba-1 (ionized calcium binding adaptor molecule 1) and glial fibrillary acid protein (GFAP) reactivities increased equally in both strains. In vitro, the astrocytes demonstrated similar GFAP levels, but the proliferation rate was higher in the wild type mice. In the crushed nerves (distal stump), neurofilaments and p75NTR immunolabeling were upregulated in the mutant mice as compared to the wild type and an improvement in locomotor recovery was observed.

Conclusion

The present results show that a lack of IFNγ affects the MHC I expression and the synaptic elimination process in the spinal cord. Such changes, however, do not delay peripheral nerve regeneration after nerve injury.

Major histocompatibility complex class I molecules modulate embryonic neuritogenesis and neuronal polarization

We studied cultured hippocampal neurons from embryonic wildtype, major histocompatibility complex class I (MHCI) heavy chain-deficient (K(b)D(b)-/-) and NSE-D(b) (which have elevated neuronal MHCI expression) C57BL/6 mice. K(b)D(b)-/- neurons displayed slower neuritogenesis and establishment of polarity, while NSE-D(b) neurons had faster neurite outgrowth, more primary neurites, and tended to have accelerated polarization. Additional studies with ß2M-/- neurons, exogenous ß2M, and a self-MHCI monomer suggest that free heavy chain cis interactions with other surface molecules can promote neuritogenesis while tripartite MHCI interactions with classical MHCI receptors can inhibit axon outgrowth. Together with the results of others, MHCI appears to differentially modulate neuritogenesis and synaptogenesis.

Allotransplanted neurons used to repair peripheral nerve injury do not elicit overt immunogenicity

A major problem hindering the development of autograft alternatives for repairing peripheral nerve injuries is immunogenicity. We have previously shown successful regeneration in transected rat sciatic nerves using conduits filled with allogeneic dorsal root ganglion (DRG) cells without any immunosuppression. In this study, we re-examined the immunogenicity of our DRG neuron implanted conduits as a potential strategy to overcome transplant rejection. A biodegradable NeuraGen® tube was infused with pure DRG neurons or Schwann cells cultured from a rat strain differing from the host rats and used to repair 8 mm gaps in the sciatic nerve. We observed enhanced regeneration with allogeneic cells compared to empty conduits 16 weeks post-surgery, but morphological analyses suggest recovery comparable to the healthy nerves was not achieved. The degree of regeneration was indistinguishable between DRG and Schwann cell allografts although immunogenicity assessments revealed substantially increased presence of Interferon gamma (IFN-γ) in Schwann cell allografts compared to the DRG allografts by two weeks post-surgery. Macrophage infiltration of the regenerated nerve graft in the DRG group 16 weeks post-surgery was below the level of the empty conduit (0.56 fold change from NG; p<0.05) while the Schwann cell group revealed significantly higher counts (1.29 fold change from NG; p<0.001). Major histocompatibility complex I (MHC I) molecules were present in significantly increased levels in the DRG and Schwann cell allograft groups compared to the hollow NG conduit and the Sham healthy nerve. Our results confirmed previous studies that have reported Schwann cells as being immunogenic, likely due to MHC I expression. Nerve gap injuries are difficult to repair; our data suggest that DRG neurons are superior medium to implant inside conduit tubes due to reduced immunogenicity and represent a potential treatment strategy that could be preferable to the current gold standard of autologous nerve transplant.