Utility of the visual system to monitor neurodegeneration in multiple sclerosis

Neurodegeneration occurs early in the multiple sclerosis (MS) disease course and is an important driver of permanent disability. Current immunomodulatory therapies do not directly target neuronal health; thus, there is a critical need to develop neuroprotective strategies in MS. Outcome measures in clinical trials primarily evaluate disease activity and clinical disability scores rather than measures of neurodegeneration. The visual system provides a noninvasive correlate of brain atrophy and neuronal function through structural and functional exams. Furthermore, optic nerve axons and their respective neuronal cell bodies in the retina, in addition to their synaptic input to the thalamus, provide a distinct anatomy to investigate neurodegenerative processes. This review discusses the utility of the visual system as an early output measure of neurodegeneration in MS as well as an important platform to evaluate neuroprotective strategies in preclinical models.

Neurodegeneration in multiple sclerosis

Axonal loss in multiple sclerosis (MS) is a key component of disease progression and permanent neurologic disability. MS is a heterogeneous demyelinating and neurodegenerative disease of the central nervous system (CNS) with varying presentation, disease courses, and prognosis. Immunomodulatory therapies reduce the frequency and severity of inflammatory demyelinating events that are a hallmark of MS, but there is minimal therapy to treat progressive disease and there is no cure. Data from patients with MS, post-mortem histological analysis, and animal models of demyelinating disease have elucidated patterns of MS pathogenesis and underlying mechanisms of neurodegeneration. MRI and molecular biomarkers have been proposed to identify predictors of neurodegeneration and risk factors for disease progression. Early signs of axonal dysfunction have come to light including impaired mitochondrial trafficking, structural axonal changes, and synaptic alterations. With sustained inflammation as well as impaired remyelination, axons succumb to degeneration contributing to CNS atrophy and worsening of disease. These studies highlight the role of chronic demyelination in the CNS in perpetuating axonal loss, and the difficulty in promoting remyelination and repair amidst persistent inflammatory insult. Regenerative and neuroprotective strategies are essential to overcome this barrier, with early intervention being critical to rescue axonal integrity and function. The clinical and basic research studies discussed in this review have set the stage for identifying key propagators of neurodegeneration in MS, leading the way for neuroprotective therapeutic development. This article is categorized under: Immune System Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology.

Hyperoxia Induced Hypomyelination

We asked whether hyperoxia might induce hypomyelination of the corpus callosum, clinically described as periventricular leukomalacia (PVL) of the severely preterm infant. Mouse pups and their nursing dams were placed in 80% oxygen from P4-P8, then removed to room air until P11. Corpus callosal sections were probed myelin immunofluorescence, tested for myelin basic protein concentration by Western blot, and both glial fibrillary acidic protein levels and apoptosis quantified. Density of corpus callosal capillaries were measured after lectin staining and hypoxia measured by Hypoxyprobe. Numbers of oligodendrocytes were quantified by immunohistochemistry. We next used hypoxiamimesis as a surrogate to hypoxia by comparing cerebral hypoxia inducible factor (HIF) stabilization to hepatic HIF stabilization. Hyperoxia induced hypomyelination and a reduction of corpus callosal capillaries. Hyperoxia decreased numbers of oligodendrocytes with an increase in corpus callosal fibrosis and apoptosis. Cerebral hypoxiamimesis induced hypomyelination whereas hepatic hypoxiamimesis alone increased myelination, oligodendrocyte numbers, and corpus callosal capillary density. Hepatic HIF-1 dependence on myelination was confirmed using the cre/lox hepatic HIF-1 knockout. These findings suggest that hyperoxia can induce hypomyelination through vasoobliteration and subsequent ischemia, adding a potential oxygen induced mechanism to the diverse causes of periventricular leukomalacia of the severely preterm infant. Targeting hepatic HIF-1 alone led to increased myelination.

Remyelinating strategies: What can be learned from normal brain development

Multiple sclerosis (MS) is a neuroinflammatory demyelinating and neurodegenerative disease of the central nervous system (CNS). Immunomodulatory therapies are effective in reducing relapses, however, there is no remedy for progressive disease emphasizing the need for regenerative strategies. Chronic demyelination causes axonal injury and loss which is a key component of neurodegeneration and permanent disability in MS. New oligodendrocyte progenitor cells (OPCs) proliferate in response to inflammatory demyelination representing the potential for remyelination to protect axons and preserve neuronal function. The majority of remyelinating therapies have targeted intrinsic signaling processes in oligodendrocytes to promote differentiation or utilized methods for transplantation of oligodendrocytes. Here, we discuss specific roles of microglia in contributing to normal myelin development and the significance of these functions for remyelinating strategies.

Visual imaging as a predictor of neurodegeneration in experimental autoimmune demyelination and multiple sclerosis

Thalamic volume is associated with clinical disability in multiple sclerosis (MS) and is vulnerable to secondary neurodegeneration due to its extensive connectivity throughout the central nervous system (CNS). Using a model of autoimmune demyelination that exhibits CNS-infiltrating immune cells in both spinal cord white matter and optic nerve, we sought to evaluate neurodegenerative changes due to lesions affecting the spino- and retino-thalamic pathways. We found comparable axonal loss in spinal cord white matter and optic nerve during the acute phase of disease consistent with synaptic loss, but not neuronal cell body loss in the thalamic nuclei that receive input from these discrete pathways. Loss of spinal cord neurons or retinal ganglion cells retrograde to their respective axons was not observed until the chronic phase of disease, where optical coherence tomography (OCT) documented reduced inner retinal thickness. In patients with relapsing–remitting MS without a history of optic neuritis, OCT measures of inner retinal volume correlated with retino-thalamic (lateral geniculate nucleus) and spino-thalamic (ventral posterior nucleus) volume as well as neuroperformance measures. These data suggest retinal imaging may serve as an important noninvasive predictor of neurodegeneration in MS.

Peripheral sTREM2-Related Inflammatory Activity Alterations in Early-Stage Alzheimer's Disease

Alzheimer's disease (AD) has been linked to multiple immune system-related genetic variants. Triggering receptor expressed on myeloid cells 2 (TREM2) genetic variants are risk factors for AD and other neurodegenerative diseases. In addition, soluble TREM2 (sTREM2) isoform is elevated in cerebrospinal fluid in the early stages of AD and is associated with slower cognitive decline in a disease stage-dependent manner. Multiple studies have reported an altered peripheral immune response in AD. However, less is known about the relationship between peripheral sTREM2 and an altered peripheral immune response in AD. The objective of this study was to explore the relationship between human plasma sTREM2 and inflammatory activity in AD. The hypothesis of this exploratory study was that sTREM2-related inflammatory activity differs by AD stage. We observed different patterns of inflammatory activity across AD stages that implicate early-stage alterations in peripheral sTREM2-related inflammatory activity in AD. Notably, fractalkine showed a significant relationship with sTREM2 across different analyses in the control groups that was lost in later AD-related stages with high levels in mild cognitive impairment. Although multiple other inflammatory factors either differed significantly between groups or were significantly correlated with sTREM2 within specific groups, three inflammatory factors (fibroblast growth factor-2, GM-CSF, and IL-1β) are notable because they exhibited both lower levels in AD, compared with mild cognitive impairment, and a change in the relationship with sTREM2. This evidence provides important support to the hypothesis that sTREM2-related inflammatory activity alterations are AD stage specific and provides critical information for therapeutic strategies focused on the immune response.

Sustained NPSLE development in the absence of systemic lupus-like disease in TLR7- deficient B6.Nba2

Neuropsychiatric Lupus (NPSLE) is a common manifestation of systemic lupus erythematosus (SLE) which includes presentations of cognitive learning and memory deficits and anxiety disorders. The B6.Nba2 mouse model of SLE presents with anxiety and depression. Toll-like receptor (TLR7), a known driver of type I interferons and SLE, drives systemic autoimmunity in the B6.Nba2 model, but whether TLR7-deficient B6.Nba2 mice develop NPSLE remains unknown.

Age-matched female B6, B6.Nba2, and B6.Nba2.TLR7 knockout mice (KO) were tested in open field (OFT), elevated plus maze (EPM), Y maze (YM) and novel object recognition (NOR) tests between the ages of 8–32 weeks. To determine levels of systemic autoimmunity and inflammation serum autoantibody and tumor necrosis factor-related of apoptosis (TWEAK) levels were determined by ELISA.

Both B6.Nba2 and B6.Nba2.TLR7 KO mice presented with anxiety (OFT, EPM) and dysfunctional spatial memory (YM). B6.Nba2 mice displayed a general memory defect (NOR), but this was not seen in TLR7-KO mice. Only B6.Nba2 mice presented with elevated levels of anti-dsDNA autoantibodies and corresponding DWEYS-reactive antibodies. In contrast, serum levels of TWEAK were elevated in both B6.Nba2 and B6.Nba2.TLR7 KO mice, suggesting that signals other than TLR7 may drive inflammation in this model.

In conclusion, both B6.Nba2 and B6.Nba2.TLR7KO presented with elevated serum TWEAK levels, anxiety and memory deficits, despite only B6.Nba2 mice displaying anti-dsDNA and DWEYS-reactive autoantibodies. Thus in this model, NPSLE appear to develop in an autoantibody-independent fashion. Studies are ongoing to determine the nature of TLR7/autoantibody-independent NPSLE.

Supported by a grant from the Department of Defense (LR 180040)

Microglia phagocytose oligodendrocyte progenitor cells and synapses during early postnatal development: implications for white versus gray matter maturation

Emerging roles for microglia in modifying normal brain development continue to provide new perspectives on the functions of this resident immune cell in the brain. While the molecular underpinnings driving microglia's position in regulating developmental programs remain largely an unchartered territory, innate immune signaling lies at the forefront. At least three innate immune receptors expressed on microglia-fractalkine, complement, and triggering receptor expressed on microglia (TREM2)-modulate developmental synaptic pruning to refine brain circuitry. Our laboratory recently published that microglia with a unique amoeboid morphology invade the corpus callosum and engulf oligodendrocyte progenitor cells (OPCs) during early postnatal development before myelination in a fractalkine receptor (CX3CR1)-dependent manner to modulate ensheathment of axons. Amoeboid microglia are observed in the corpus callosum but not cerebral cortex, and lose their amoeboid shape at the commencement of myelination assuming a resting phenotype. Furthermore, OPCs contacted or engulfed by microglia do not express markers of cell death suggesting a novel homeostatic mechanism facilitating an appropriate OPC:axon ratio for proper myelin ensheathment. The unique morphology of microglia and the restricted window for phagocytic engulfment of OPCs suggest a critical period for OPC engulfment important for action potential propagation during development when activity-dependent mechanisms regulate synaptic pruning. In this review, we summarize the role of activity-dependent mechanisms in sculpting brain circuitry, how myelin ensheathment influences action potential propagation, the spatial and temporal relationship of microglia-dependent elimination of OPCs and synapses, and implications for the synergistic role of microglial phagocytosis in shaping the architecture for neuronal function.

Fractalkine-Dependent Microglial Pruning of Viable Oligodendrocyte Progenitor Cells Regulates Myelination

Oligodendrogenesis occurs during early postnatal development, coincident with neurogenesis and synaptogenesis, raising the possibility that microglia-dependent pruning mechanisms that modulate neurons regulate myelin sheath formation. Here we show a population of ameboid microglia migrating from the ventricular zone into the corpus callosum during early postnatal development, termed “the fountain of microglia,” phagocytosing viable oligodendrocyte progenitor cells (OPCs) before onset of myelination. Fractalkine receptor-deficient mice exhibit a reduction in microglial engulfment of viable OPCs, increased numbers of oligodendrocytes, and reduced myelin thickness but no change in axon number. These data provide evidence that microglia phagocytose OPCs as a homeostatic mechanism for proper myelination. A hallmark of hypomyelinating developmental disorders such as periventricular leukomalacia and of adult demyelinating diseases such as multiple sclerosis is increased numbers of oligodendrocytes but failure to myelinate, suggesting that microglial pruning of OPCs may be impaired in pathological states and hinder myelination.

Nemes-Baran et al. show that ameboid microglia engulf living oligodendrocyte progenitor cells (OPCs) during brain development. Fractalkine receptor-deficient microglia exhibit a reduction in engulfment of OPCs, resulting in a surplus of oligodendrocytes and impaired myelination. These data provide evidence that microglia phagocytose OPCs as a homeostatic mechanism required for normal myelination.

Lupus-prone B6.Nba2 male and female mice display anti-DWEYS reactivity and a neuropsychiatric phenotype

OBJECTIVE

Neuropsychiatric lupus (NPSLE), a manifestation of the autoimmune disease systemic lupus erythematosus (SLE), is characterized by psychiatric symptoms including anxiety and depression and upregulated autoantibodies. The B6.Nba2 spontaneous mouse model develops SLE, but has not previously been tested for NPSLE.

METHODS

We investigated the NPSLE phenotype in male and female B6.Nba2 mice (n = 12 each) and age- and sex-matched B6 controls (n = 10 each) via behavioral assessments for anxiety, depression, and memory deficits. Serum anti-dsDNA, anti-nRNP, anti-DWEYS peptide reactive IgG autoantibody levels and soluble TWEAK levels were determined by ELISA. Hippocampal regions were stained for activated microglia and neurons.

RESULTS

Both male and female B6.Nba2 mice showed elevated anti-dsDNA IgG, anti-nRNP IgG and anti-DWEYS reactive antibodies, elevated serum soluble TWEAK levels, and a strong anxiety and depression phenotype (p < 0.05-0.0001). Male B6.Nba2 mice developed this phenotype at a slightly older age than females. Female B6.Nba2 mice displayed reduced numbers of neurons in the hippocampal region compared to female B6 controls (p < 0.05).

CONCLUSION

The B6.Nba2 mouse model recapitulates many known NPSLE phenotypes, making it a promising model to investigate the development of NPSLE in the context of SLE.

Quantification of microglial contact and engulfment of oligodendrocyte progenitor cells in the rodent brain

Microglia have emerged as essential regulators of neurodevelopment by phagocytosing synapses. Recently, we showed that microglia engulf viable oligodendrocyte progenitor cells (OPCs) during development to facilitate myelination. Here, we describe a protocol to quantify microglial engulfment of whole cells using 3D confocal microscopy to differentiate microglial contact. This protocol can be applied to assess whole-cell engulfment in a variety of contexts and cell types. For complete details on the use and execution of this protocol, please refer to Nemes-Baran et al. (2020).

Features of Neuropsychiatric Lupus in Nba2 mouse model

Background

Neuropsychiatric lupus (NPSLE) is a poorly understood manifestation of the autoimmune disease systemic lupus erythematosus (SLE). NPSLE is characterized by symptoms including anxiety and depression, and recent data have suggested a pathogenic role for interferon-alpha (IFNα). Additionally, IFNα can diminish the production of the neurotransmitter serotonin, suggesting a causative link between IFNα and NPSLE. B6.Nba2 mice develop SLE in an IFNα-dependent manner making it a valuable model for the study of NPSLE.

Methods

Age-matched female B6.Nba2 and B6 mice were tested. Behavioral studies included open field testing (OFT), elevated plus maze (EPM), and fear conditioning (FC) tests for detection of anxiety-like behavior, and forced swim test (FST) for detection of depression-like behavior. Serum serotonin and dopamine levels were measured by ELISA and serotonin receptors were enumerated by IHC.

Result

Results from both OFT (p=0.001), EPM (p&lt;0.05) and FC (p&lt;0.05) tests supported a strong anxiety phenotype in B6.Nba2 female mice, while data from FST (p&lt;0.05) suggested a depressive phenotype as well. B6.Nba2 females also showed significantly less serum serotonin (p&lt;0.0001) and dopamine (p&lt;0.05) levels than age-matched B6 females.

Conclusion

Our data suggests that the B6.Nba2 mouse model of SLE expresses an anxiety and depressive phenotype resembling NPSLE. Along with our previous data showing that IFNα levels are upregulated and required for disease development, our results suggest a positive correlation between IFNα, reduced serotonin, and an anxiety phenotype. Current studies are evaluating the effect of genetic IFNα-receptor manipulation in brain cells and the direct relationship between IFNα and NPSLE.

Reduction of AMPA receptor activity on mature oligodendrocytes attenuates loss of myelinated axons in autoimmune neuroinflammation

Glutamate dysregulation occurs in multiple sclerosis (MS), but whether excitotoxic mechanisms in mature oligodendrocytes contribute to demyelination and axonal injury is unexplored. Although current treatments modulate the immune system, long-term disability ensues, highlighting the need for neuroprotection. Glutamate is elevated before T2-visible white matter lesions appear in MS. We previously reported that myelin-reactive T cells provoke microglia to release glutamate from the system xc - transporter promoting myelin degradation in experimental autoimmune encephalomyelitis (EAE). Here, we explore the target for glutamate in mature oligodendrocytes. Most glutamate-stimulated calcium influx into oligodendrocyte somas is AMPA receptor (AMPAR)-mediated, and genetic deletion of AMPAR subunit GluA4 decreased intracellular calcium responses. Inducible deletion of GluA4 on mature oligodendrocytes attenuated EAE and loss of myelinated axons was selectively reduced compared to unmyelinated axons. These data link AMPAR signaling in mature oligodendrocytes to the pathophysiology of myelinated axons, demonstrating glutamate regulation as a potential neuroprotective strategy in MS.

Myocardial ischemia/reperfusion impairs neurogenesis and hippocampal-dependent learning and memory

The incidence of cognitive impairment in cardiovascular disease (CVD) patients has increased, adversely impacting quality of life and imposing a significant economic burden. Brain imaging of CVD patients has detected changes in the hippocampus, a brain region critical for normal learning and memory. However, it is not clear whether adverse cardiac events or other associated co-morbidities impair cognition. Here, using a murine model of acute myocardial ischemia/reperfusion (I/R), where the coronary artery was occluded for 30min followed by reperfusion, we tested the hypothesis that acute myocardial infarction triggers impairment in cognitive function. Two months following cardiac I/R, behavioral assessments specific for hippocampal cognitive function were performed. Mice subjected to cardiac I/R performed worse in the fear-conditioning paradigm as well as the object location memory behavioral test compared to sham-operated mice. Reactive gliosis was apparent in the hippocampal subregions CA1, CA3, and dentate gyrus 72h post-cardiac I/R as compared with sham, which was sustained two months post-cardiac I/R. Consistent with the inflammatory response, the abundance of doublecortin positive newborn neurons was decreased in the dentate gyrus 72h and 2months post-cardiac I/R as compared with sham. Therefore, we conclude that following acute myocardial infarction, rapid inflammatory responses negatively affect neurogenesis, which may underlie long-term changes in learning and memory.

Determining Immune System Suppression versus CNS Protection for Pharmacological Interventions in Autoimmune Demyelination

A major hallmark of the autoimmune demyelinating disease multiple sclerosis (MS) is immune cell infiltration into the brain and spinal cord resulting in myelin destruction, which not only slows conduction of nerve impulses, but causes axonal injury resulting in motor and cognitive decline. Current treatments for MS focus on attenuating immune cell infiltration into the central nervous system (CNS). These treatments decrease the number of relapses, improving quality of life, but do not completely eliminate relapses so long-term disability is not improved. Therefore, therapeutic agents that protect the CNS are warranted. In both animal models as well as human patients with MS, T cell entry into the CNS is generally considered the initiating inflammatory event. In order to assess if a drug protects the CNS, any potential effects on immune cell infiltration or proliferation in the periphery must be ruled out. This protocol describes how to determine whether CNS protection observed after drug intervention is a consequence of attenuating CNS-infiltrating immune cells or blocking death of CNS cells during inflammatory insults. The ability to examine MS treatments that are protective to the CNS during inflammatory insults is highly critical for the advancement of therapeutic strategies since current treatments reduce, but do not completely eliminate, relapses (i.e., immune cell infiltration), leaving the CNS vulnerable to degeneration.

T cell cytokine signatures: Biomarkers in pediatric multiple sclerosis

Although multiple sclerosis is predominantly regarded as a disease of young adulthood, up to 5% of MS patients are diagnosed prior to age eighteen. The predominant form of MS is relapsing-remitting characterized by exacerbations of symptoms followed by periods of remission. The majority of disease modifying drugs target T cell proliferation or block migration into the central nervous system. Although these treatments reduce relapses, disease progression still occurs, warranting therapeutic strategies that protect the CNS. Biomarkers to indicate relapses would facilitate a personalized approach for add-on therapies that protect the CNS. A multiplex cytokine bead array was performed to detect T cell associated cytokines in sera from patients 6-20years of age with pediatric onset MS clinically diagnosed in relapse or remission compared to healthy control patients. Of the 25 cytokines evaluated, 17 were increased in patients clinically diagnosed in relapse compared to sera from control patients in contrast to only 9 cytokines in the clinically diagnosed remission group. Furthermore, a linear regression analysis of cytokine levels in the remission population showed 12 cytokines to be statistically elevated as a function of disease duration, with no effect observed in the relapse population. To further explore this concept, we used a multivariable stepwise discriminate analysis and found that the following four cytokines (IL-10, IL-21, IL-23, and IL-27) are not only a significant predictor for MS, but have important predictive value in determining a relapse. Since IL-10 and IL-27 are considered anti-inflammatory and IL-21 and IL-23 are pro-inflammatory, ratios of these cytokines were evaluated using a Duncan's multiple range test. Of the six possible combinations, increased ratios of IL-10:IL-21, IL-10:IL-23, and IL-10:IL-27 were significant suggesting levels of IL-10 to be a driving force in predicting a relapse.

Motor and Sensory Deficits in the teetering Mice Result from Mutation of the ESCRT Component HGS

Neurons are particularly vulnerable to perturbations in endo-lysosomal transport, as several neurological disorders are caused by a primary deficit in this pathway. In this report, we used positional cloning to show that the spontaneously occurring neurological mutation teetering (tn) is a single nucleotide substitution in hepatocyte growth factor-regulated tyrosine kinase substrate (Hgs/Hrs), a component of the endosomal sorting complex required for transport (ESCRT). The tn mice exhibit hypokenesis, muscle weakness, reduced muscle size and early perinatal lethality by 5-weeks of age. Although HGS has been suggested to be essential for the sorting of ubiquitinated membrane proteins to the lysosome, there were no alterations in receptor tyrosine kinase levels in the central nervous system, and only a modest decrease in tropomyosin receptor kinase B (TrkB) in the sciatic nerves of the tn mice. Instead, loss of HGS resulted in structural alterations at the neuromuscular junction (NMJ), including swellings and ultra-terminal sprouting at motor axon terminals and an increase in the number of endosomes and multivesicular bodies. These structural changes were accompanied by a reduction in spontaneous and evoked release of acetylcholine, indicating a deficit in neurotransmitter release at the NMJ. These deficits in synaptic transmission were associated with elevated levels of ubiquitinated proteins in the synaptosome fraction. In addition to the deficits in neuronal function, mutation of Hgs resulted in both hypermyelinated and dysmyelinated axons in the tn mice, which supports a growing body of evidence that ESCRTs are required for proper myelination of peripheral nerves. Our results indicate that HGS has multiple roles in the nervous system and demonstrate a previously unanticipated requirement for ESCRTs in the maintenance of synaptic transmission.

Endocytic trafficking involves the internalization, endosomal sorting and lysosomal degradation of cell surface cargo. Many factors involved in endosomal sorting in mammalian cells have been identified, and mutations in these components are associated with a variety of neurological disorders. While the function of endosomal sorting components has been intensely studied in immortalized cell lines, it is not known what role these factors play in endosomal sorting in the nervous system. In this study, we show that the teetering (tn) gene encodes the hepatocytegrowth factor regulated tyrosine kinasesubstrate (Hgs), a core component of the endosomal sorting pathway. The tn mice exhibit several signs of motor neuron disease, including reduced muscle mass, muscle weakness and motor abnormalities. Although HGS is predicted to be required for the lysosomal degradation of receptor tyrosine kinases, there was no change in the levels of receptor tyrosine kinases in the spinal cords of the tn mice. Instead, we found that HGS is required for synaptic transmission at the neuromuscular junction and for the proper myelination of the peripheral nervous system.

Inhibition of System Xc(-) Transporter Attenuates Autoimmune Inflammatory Demyelination

T cell infiltration into the CNS is a significant underlying pathogenesis in autoimmune inflammatory demyelinating diseases. Several lines of evidence suggest that glutamate dysregulation in the CNS is an important consequence of immune cell infiltration in neuroinflammatory demyelinating diseases; yet, the causal link between inflammation and glutamate dysregulation is not well understood. A major source of glutamate release during oxidative stress is the system Xc(-) transporter; however, this mechanism has not been tested in animal models of autoimmune inflammatory demyelination. We find that pharmacological and genetic inhibition of system Xc(-) attenuates chronic and relapsing-remitting experimental autoimmune encephalomyelitis (EAE). Remarkably, pharmacological blockade of system Xc(-) 7 d after induction of EAE attenuated T cell infiltration into the CNS, but not T cell activation in the periphery. Mice harboring a Slc7a11 (xCT) mutation that inactivated system Xc(-) were resistant to EAE, corroborating a central role for system Xc(-) in mediating immune cell infiltration. We next examined the role of the system Xc(-) transporter in the CNS after immune cell infiltration. Pharmacological inhibitors of the system Xc(-) transporter administered during the first relapse in a SJL animal model of relapsing-remitting EAE abrogated clinical disease, inflammation, and myelin loss. Primary coculture studies demonstrate that myelin-specific CD4(+) Th1 cells provoke microglia to release glutamate via the system Xc(-) transporter, causing excitotoxic death to mature myelin-producing oligodendrocytes. Taken together, these studies support a novel role for the system Xc(-) transporter in mediating T cell infiltration into the CNS as well as promoting myelin destruction after immune cell infiltration in EAE.

Expression of EAAT2 in neurons and protoplasmic astrocytes during human cortical development

The major regulators of synaptic glutamate in the cerebral cortex are the excitatory amino acid transporters 1-3 (EAAT1-3). In this study, we determined the cellular and temporal expression of EAAT1-3 in the developing human cerebral cortex. We applied single- and double-label immunocytochemistry to normative frontal or parietal (associative) cortex samples from 14 cases ranging in age from 23 gestational weeks to 2.5 postnatal years. The most striking finding was the transient expression of EAAT2 in layer V pyramidal neuronal cell bodies up until 8 postnatal months prior to its expression in protoplasmic astrocytes at 41 postconceptional weeks onward. EAAT2 was also expressed in neurons in layer I (presumed Cajal-Retzius cells), and white matter (interstitial) neurons. This expression in neurons in the developing human cortex contrasts with findings by others of transient expression exclusively in axon tracts in the developing sheep and rodent brain. With western blotting, we found that EAAT2 was expressed as a single band until 2 postnatal months, after which it was expressed as two bands. The expression of EAAT2 in pyramidal neurons during human brain development may contribute to cortical vulnerability to excitotoxicity during the critical period for perinatal hypoxic-ischemic encephalopathy. In addition, by studying the expression of EAAT1 and EAAT2 glutamate transporters, it was possible to document the development of protoplasmic astrocytes.

17beta-estradiol protects against hypoxic/ischemic white matter damage in the neonatal rat brain

Developing oligodendrocytes (pre-OLs) are highly vulnerable to hypoxic-ischemic injury and associated excitotoxicity and oxidative stress. 17beta-Estradiol plays an important role in the development and function of the CNS and is neuroprotective. The sudden drop in circulating estrogen after birth may enhance the susceptibility of developing OLs to injury. Estrogen receptor (ER)-alpha and ER-beta are both expressed in OLs. We examined the effect of 17beta-estradiol on oxygen-glucose deprivation and oxidative stress-induced cell death in rat pre-OLs in vitro and on hypoxic-ischemic brain injury in vivo. Pre-OLs in culture were subjected to oxygen-glucose deprivation (OGD) or glutathione depletion in the presence or absence of 17beta-estradiol. LDH release, the Alamar blue assay, and phase-contrast microscopy were used to assess cell viability. Hypoxic-ischemic injury was generated in 6-day-old rats (P6) by unilateral carotid ligation and hypoxia (6% O(2) for 1 hr). Rat pups received one intraperitoneal injection of 300 or 600 microg/kg 17beta-estradiol or vehicle 12 hr prior to the surgical procedure. Injury was assessed by myelin basic protein (MBP) immunocytochemistry at P10. 17beta-Estradiol produced significant protection against OGD-induced cell death in primary OLs (EC(50) = 1.3 +/- 0.46 x 10(-9) M) and against oxidative stress. Moreover, 17beta-estradiol attenuated the loss of MBP labeling in P10 pups ipsilateral to the carotid ligation. These results suggest a potential role for estrogens in attenuation of hypoxic-ischemic and oxidative injury to developing OLs and in the prevention of periventricular leukomalacia.

Solution structures of the reduced and Cu(I) bound forms of the first metal binding sequence of ATP7A associated with Menkes disease

The coding sequence for the first N-terminal copper binding motif of the human Menkes disease protein (MNK1; residues 2-79) was synthesized, cloned, and expressed in bacteria for biochemical and structural studies. MNK1 adopts the betaalphabetabetaalphabeta fold common to all the metal binding sequences (MBS) found in other metal transport systems (e.g., the yeast copper chaperone for superoxide dismutase CCS, the yeast copper chaperone ATX1 bound to Hg(II), and most recently Cu(I), the bacterial copper binding protein, CopZ, and the bacterial Hg(II) binding protein MerP), although substantial differences were found in the metal binding loop. Similar to ATX1, MNK1 binds Cu(I) in a distorted linear bicoordinate geometry. As with MerP, MNK1 has a high affinity for both Hg(II) and Cu(I), although it displays a marked preference for Cu(I). In addition, we found that F71 is a key residue in the compact folding of MNK1, and its mutation to alanine results in an unfolded structure. The homologous residue in MerP has also been mutated with similar results. Finally, to understand the relationship between protein folding and metal affinity and specificity, we expressed a chimeric MBS with the MNK1 protein carrying the binding motif of MerP (CAAC-MNK1); this chimeric protein showed differences in structure and the dynamics of the binding site that may account for metal specificity.

Cellular and subcellular mRNA localization of glutamate transporter isoforms GLT1a and GLT1b in rat brain by in situ hybridization

GLT1, the predominant glutamate transporter of the forebrain, exists in two splice variant isoforms, i.e., GLT1a and GLT1b. Although GLT1 was originally detected only in astrocytes, we have recently demonstrated that GLT1a protein is expressed by neurons in the hippocampus as well. In the present study, the mRNA distribution patterns for the two isoforms were examined throughout the rat brain by using nonisotopic in situ hybridization and variant-specific RNA probes. Both isoforms were expressed in neuronal subgroups outside the hippocampus, such as in the cerebral cortex layer VI, or the neurons in the olfactory tubercle. As was the case in the hippocampus, GLT1a was the predominant transcript in neurons in these regions as well. Both GLT1 isoforms were widely expressed in astrocytes throughout the brain. GLT1a mRNA expression in astrocytes showed noticeable variation in labeling intensity in subregions of the hippocampus and other areas, whereas GLT1b expression in astrocytes was relatively homogeneous. On the subcellular level, GLT1a mRNA was expressed primarily in astrocyte processes, whereas GLT1b mRNA was more restricted to the astrocyte cell body. The two isoforms showed similar distributions in the subfornical organ and in tanycytes of the third ventricle. However, GLT1 expression in the pineal gland and the retina was due primarily to GLT1b, whereas GLT1a was more strongly expressed in Bergman glia in the cerebellum. These findings suggest that the expression of the two GLT1 isoforms is regulated by different mechanisms. Moreover, the function of the two isoforms may be subject to different regulatory processes.

Overexpression, purification, and characterization of recombinant Ca-ATPase regulators for high-resolution solution and solid-state NMR studies

Phospholamban (PLB) and Sarcolipin (SLN) are integral membrane proteins that regulate muscle contractility via direct interaction with the Ca-ATPase in cardiac and skeletal muscle, respectively. The molecular details of these protein-protein interactions are as yet undetermined. Solution and solid-state NMR spectroscopies have proven to be effective tools for deciphering such regulatory mechanisms to a high degree of resolution; however, large quantities of pure recombinant protein are required for these studies. Thus, recombinant PLB and SLN production in Escherichia coli was optimized for use in NMR experiments. Fusions of PLB and SLN to maltose binding protein (MBP) were constructed and optimal conditions for protein expression and purification were screened. This facilitated the large-scale production of highly pure protein. To confirm their functionality, the biological activities of recombinant PLB and SLN were compared to those of their synthetic counterparts. The regulation of Ca-ATPase activity by recombinant PLB and SLN was indistinguishable from the regulation by synthetic proteins, demonstrating the functional integrity of the recombinant constructs and ensuring the biological relevance of our future structural studies. Finally, NMR spectroscopic conditions were established and optimized for use in investigations of the mechanism of Ca-ATPase regulation by PLB and SLN.

Selectivity in heavy metal- binding to peptides and proteins

The metal-binding affinities and three-dimensional structures of three synthetic 18-residue peptides with sequences derived from that of the highly conserved metal-binding motif MXCXXC found in many heavy metal-binding proteins were determined. A change in register of the cysteines and alanines of the sequence from the periplasmic mercury-binding protein, MerP, i.e., CAAC, CACA, and CCAA, affects the specificity of metal binding, in particular, the peptide with vicinal cysteines binds only mercury. The three-dimensional structures of the mercury-bound forms of the three peptides determined in solution by NMR spectroscopy peptides differ considerably, even though they are all linear bicoordinate complexes. The three-dimensional structure of the peptide with CAAC bound to Cd(II) demonstrates that the metal-binding loop is malleable enough to accommodate modes of coordination other than linear bicoordinate.

Dipolar waves as NMR maps of protein structure

The anisotropy of nuclear spin interactions results in a unique mapping of structure to the resonance frequencies and split tings observed in NMR spectra, however, the determination of molecular structure from experimentally measured spectral parameters is complicated by angular ambiguities resulting from the symmetry properties of dipole-dipole and chemical shift interactions. This issue can be addressed through the periodicity inherent in secondary structure elements, which can be used as an index of topology. Distinctive wheel-like patterns are observed in two-dimensional 1H-15N heteronuclear dipolar/15N chemical shift PISEMA (polarization inversion spin-exchange at the magic angle) spectra of helical membrane proteins in highly aligned lipid bilayer samples. One-dimensional dipolar waves are an extension of two-dimensional PISA (polarity index slant angle) wheels to map protein structure in NMR spectra of both highly and weakly aligned samples. Dipolar waves describe the periodic wavelike variations of the magnitudes of the static heteronuclear dipolar couplings as a function of residue number in the absence of chemical shift effects. Weakly aligned samples of proteins display these same effects, primarily as residual dipolar couplings (RDCs), in solution NMR spectra. The corresponding properties of the RDCs in solution NMR spectra of weakly aligned helices represent a convergence of solid-state and solution NMR approaches to structure determination.

Structural biology of metal-binding sequences

NMR spectroscopy and X-ray crystallography in conjunction with extended X-ray absorption fine structure spectroscopy, have contributed to the elucidation of the structural biology of protein-mediated mechanisms of heavy metal homeostasis. Among the most striking aspects of these investigations are the remarkable similarity of metal-ion-transport and sequestering systems across different species, and the need to continue the research to confirm hypotheses for the molecular mechanisms of transfers of metal ions between proteins.

Physical properties of a single-motif erythrocyte spectrin peptide: a highly stable independently folding unit

Spectrin is a long flexible rod-like actin cross-linking protein mostly comprised of many tandem homologous 106-residue motifs. In this study, the conformational stability and physical properties of a single homologous motif peptide, alpha1, were evaluated and compared to intact spectrin monomers and alphabeta heterodimers. It is interesting that while spectrin dimers elongate by about 3-fold in low ionic strength buffers relative to their size in physiological buffers, the single-motif peptide does not show significant changes in secondary structure in 10 mM phosphate buffer compared with isotonic buffer. This single-motif peptide is monomeric in physiological buffer as demonstrated by equilibrium sedimentation studies, and its hydrodynamic radius determined by gel filtration and dynamic light scattering of about 2.2 nm is consistent with an elongated rod-like shape. Unfolding of the single-motif peptide in urea solutions was similar to unfolding of intact heterodimers. Differential scanning calorimetry analyses showed that this single motif undergoes a reversible two-state transition with a Tm of 53 degrees C and an enthalpy of 65 kcal/mol in physiological buffer. Thermal stability was unaffected by ionic strength changes, but was decreased below physiological pH. These data show that this 13 kDa spectrin motif is a monomeric, highly stable, triple-helical, independently folding protein building block with physical characteristics that define many of the structural properties of the 526 kDa spectrin heterodimer. In contrast, interactions between adjacent motifs are probably responsible for spectrin's molecular flexibility and elasticity.

Mapping the human erythrocyte beta-spectrin dimer initiation site using recombinant peptides and correlation of its phasing with the alpha-actinin dimer site

Human erythroid spectrin dimer assembly is initiated by the association of a specific region near the N-terminal of beta-spectrin with a complementary region near the C-terminal of alpha-spectrin (Speicher, D. W., Weglarz, L., and DeSilva, T. M. (1992) J. Biol. Chem. 267, 14775-14782). Both spectrin subunits consist primarily of tandem, 106-residue long, homologous, triple-helical motifs. In this study, the minimal region of beta-spectrin required for association with alpha-spectrin was determined using recombinant peptides. The start site (phasing) for construction of dimerization competent beta-spectrin peptides was particularly critical. The beginning of the first homologous motif for both beta-spectrin and the related dimerization site of alpha-actinin is approximately 8 residues earlier than most spectrin motifs. A four-motif beta-spectrin peptide (beta1-4+) with this earlier starting point bound to full-length alpha-spectrin with a Kd of about 10 nM, while deletion of these first 8 residues reduced binding nearly 10-fold. N- and C-terminal truncations of one or more motifs from beta1-4+ showed that the first motif was essential for dimerization since its deletion abolished binding, but beta1+ alone could not associate with alpha-monomers. The first two motifs (beta1 2+) represented the minimum lateral dimer assembly site with a Kd of about 230 nM for interaction with full-length alpha-spectrin or an alpha-spectrin nucleation site recombinant peptide, alpha18-21. Each additional motif increased the dimerization affinity by approximately 5-fold. In addition to this strong inter-subunit dimer association, interactions between the helices of a single triple-helical motif are frequently strong enough to maintain a noncovalent complex after internal protease cleavage similar to the interactions thought to be involved in tetramer formation. Analysis of hydrodynamic radii of recombinant peptides containing differing numbers of motifs showed that a single motif had a Stokes radius of 2.35 nM, while each additional motif added only 0.85 nM to the Stokes radius. This is the first direct demonstration that spectrin's flexibility arises from regions between each triple helical motif rather than from within the segment itself and suggests that current models of inter-motif connections may need to be revised.

Functional characterization of recombinant human red cell alpha-spectrin polypeptides containing the tetramer binding site

Spectrin, a heterodimer composed of alpha and beta subunits, interacts with itself head-to-head to form tetramers in the erythrocyte membrane cytoskeleton. The NH2-terminal region of alpha-spectrin, encompassing the alpha I 80-kDa domain, was expressed in Escherichia coli. In addition to the correctly initiated polypeptide, four smaller polypeptides were produced by initiation at internal codons. Only the full-length polypeptide was able to bind to spectrin dimers, beta monomers, and to a recombinant polypeptide containing the COOH terminus of beta-spectrin. The head-to-head interaction with beta-spectrin was also retained by a recombinant polypeptide containing the NH2-terminal 158 amino acids of the alpha subunit. Deletion of the first 27 or 49 NH2-terminal amino acids abolished binding of this polypeptide to the beta monomer. The phasing used to design these recombinant polypeptides was based on a conformational model recently refined by Speicher et al. (Speicher, D. W., DeSilva, T. M., Speicher, K. D., Ursitti, J. A., Hembach, P., and Weglarz, L. (1993) J. Biol. Chem. 268, 4227-4235), where the structural unit begins and terminates around residue 30 of the repeat unit. The binding properties, mobility on gel filtration, and circular dichroism data of the recombinant polypeptides indicated that most polypeptides were able to assume their native conformation.

Location of the human red cell spectrin tetramer binding site and detection of a related "closed" hairpin loop dimer using proteolytic footprinting

Head-to-head association of two spectrin alpha beta heterodimers to form tetramers involves the formation of two equivalent alpha-beta complexes. The sites on the alpha subunit N-terminal region and beta subunit C-terminal region that form these alpha beta complexes have been identified using protease footprinting and direct binding assays. The existence of a similar previously hypothesized internal head-to-head alpha beta interaction in dimers was also demonstrated. The discrete regions of both subunits that are protected from proteolysis in tetramers and dimers are not due to the laterally associated subunit since head-to-head complexes of a univalent alpha peptide with a univalent beta peptide show similar protection of the same sites. These sites are unshielded immediately after monomers assemble side-to-side to form heterodimers, demonstrating that reconstituted dimers are initially in an "open" conformation. Conversion of open dimers to a closed form through formation of the internal head-to-head alpha beta association, as demonstrated by restoration of protease protection, occurred on a time scale of hours at 0 degrees C. Analysis of peptide binding affinities as well as isolation and sequence analysis of head-to-head alpha beta noncovalent complexes further defined the regions required for association on both subunits. These regions are homologous to the 106-residue repetitive motif that comprises most of both chains. An algorithm designed to improve prediction accuracy of multiple homologous motifs was used to model the conformation of spectrin repetitive motifs as well as the contact regions. In this model, the separate alpha and beta binding sites are incomplete complementary parts of a triple stranded folding unit. Formation of the alpha beta head-to-head complex produces a triple stranded conformational unit that is slightly different from other homologous motifs in the protein. Most hemolytic anemia mutations that are known to disrupt tetramer association are located in the mapped regions, including several mutations that induce a conformational change in the paired subunit.

Analysis of human red cell spectrin tetramer (head-to-head) assembly using complementary univalent peptides

The mass-driven assembly of spectrin dimers to form tetramers involves two equal head-to-head alpha-beta associations and requires at least 30 degrees C for interconversion to occur readily. In this paper, the properties of tetramer formation were investigated using two complementary univalent peptides (the alpha I domain and beta monomers). Since the alpha I domain lacks an essential nucleation site required for side-to-side (lateral) heterodimer assembly [Speicher et al. (1992) J. Biol. Chem. 267, 14775-14782], these two peptides can only assemble head-to-head at a single site. This head-to-head assembly readily occurs at lower temperatures, indicating the temperature barrier for dimer-tetramer interconversion is caused by a conformational constraint of the dimer. This constraint, a closed hairpin loop, is released when the laterally associated partner is removed. The univalent alpha I-beta binding affinity at 37 degrees C (Ka = 1.4 x 10(5) M-1) is similar to the dimer-tetramer association constant at the same temperature. As the temperature is decreased from 37 to 0 degrees C, the alpha I-beta binding affinity increases about 32-fold. In contrast with head-to-head associations involving dimers, the second-order rate constants of two complementary univalent peptides (i.e., alpha I and beta) are dramatically higher, and the estimated activation energy (about 50 kJ mol-1) is about 5-fold lower. An open dimer conformation is an obligatory high-energy intermediate required for dimer-tetramer interconversion, and opening the dimer hairpin loop contributes about 190 kJ mol-1 to the activation energy for tetramer association.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of human red cell spectrin heterodimer (side-to-side) assembly and identification of an essential nucleation site

The antiparallel side-to-side association of spectrin alpha and beta monomers is a two-step process which occurs in seconds even at 0 degrees C and at low concentrations. Assembly involves initial contact of complementary nucleation sites on each subunit, which are located near the actin binding end of the long, flexible heterodimer rod. The minimum nucleation sites are comprised of approximately four contiguous 106-residue homologous segments or repeats. Three repeats in the nucleation site contain an 8-residue insertion and have the highest homology to the four spectrin-like repeats in alpha-actinin. The adjacent actin binding domain on the beta subunit and the adjacent EF hand motifs on the alpha subunit are not required for heterodimer assembly. The nucleation sites probably have a specific lock and key structure which defines the unique side-to-side pairing of the many homologous segments in both subunits. Assembly of spectrin heterodimers is probably most analogous to a zipper. After initial nucleation site binding, the remainder of the subunits quickly associate along their full lengths to reconstitute a normal dimer by supercoiling around each other to form a rope-like, flexible rod. Assembly is terminated if either polypeptide is interrupted by a protease cleavage. Heterozygotic mutations involving either nucleation site are predicted to affect allele incorporation into the mature membrane skeleton.