Model for the neuromuscular complications of systemic lupus erythematosus

Emerging Molecular and Synaptic Targets for the Management of Chronic Pain Caused by Systemic Lupus Erythematosus

Patients with systemic lupus erythematosus (SLE) frequently experience chronic pain due to the limited effectiveness and safety profiles of current analgesics. Understanding the molecular and synaptic mechanisms underlying abnormal neuronal activation along the pain signaling pathway is essential for developing new analgesics to address SLE-induced chronic pain. Recent studies, including those conducted by our team and others using the SLE animal model (MRL/lpr lupus-prone mice), have unveiled heightened excitability in nociceptive primary sensory neurons within the dorsal root ganglia and increased glutamatergic synaptic activity in spinal dorsal horn neurons, contributing to the development of chronic pain in mice with SLE. Nociceptive primary sensory neurons in lupus animals exhibit elevated resting membrane potentials, and reduced thresholds and rheobases of action potentials. These changes coincide with the elevated production of TNFα and IL-1β, as well as increased ERK activity in the dorsal root ganglion, coupled with decreased AMPK activity in the same region. Dysregulated AMPK activity is linked to heightened excitability in nociceptive sensory neurons in lupus animals. Additionally, the increased glutamatergic synaptic activity in the spinal dorsal horn in lupus mice with chronic pain is characterized by enhanced presynaptic glutamate release and postsynaptic AMPA receptor activation, alongside the reduced activity of glial glutamate transporters. These alterations are caused by the elevated activities of IL-1β, IL-18, CSF-1, and thrombin, and reduced AMPK activities in the dorsal horn. Furthermore, the pharmacological activation of spinal GPR109A receptors in microglia in lupus mice suppresses chronic pain by inhibiting p38 MAPK activity and the production of both IL-1β and IL-18, as well as reducing glutamatergic synaptic activity in the spinal dorsal horn. These findings collectively unveil crucial signaling molecular and synaptic targets for modulating abnormal neuronal activation in both the periphery and spinal dorsal horn, offering insights into the development of analgesics for managing SLE-induced chronic pain.

The MRL Model: A Valuable Tool in Studies of Autoimmunity-Brain Interactions

The link between systemic autoimmunity, brain pathology, and aberrant behavior is still a largely unexplored field of biomedical science. Accumulating evidence points to causal relationships between immune factors, neurodegeneration, and neuropsychiatric manifestations. By documenting autoimmunity-associated neuronal degeneration and cytotoxicity of the cerebrospinal fluid from disease-affected subjects, the murine MRL model had shown high validity in revealing principal pathogenic circuits. In addition, unlike any other autoimmune strain, MRL mice produce antibodies commonly found in patients suffering from lupus and other autoimmune disorders. This review highlights importance of the MRL model as a useful preparation in understanding the links between immune system and brain function.

CNS dysfunction in the antiphospholipid syndrome

Though many neurological deficits have been described in the antiphospholipid syndrome (APS), only stroke is well establishedand accepted as a diagnosticcriterion in this disease. We review clinical data obtainedfrom a large series of cases regardingstroke, dementia, epilepsy, chorea, migraine, white matter disease and behavioralchangesin APS or linked to laboratory criteria such as antiphospholipid antibodies (aPL). The contribution of animal models to our understanding of these manifestations of APS is stressed, especially regarding the cognitive and behavioral aspects for which we have established model systems in the mouse. These models utilize immunization of mice with b2-glycoprotein I, a central autoantigen in APS, which induces persistent high levels of aPL. These mice develop hyperactive behavior after a period of four to five months as well as deficits in learning and memory and are potentiallyvaluableas a system in which to study the pathogenesisand treatment of cognitive and behavioral aspects of APS. Another model we have developed, in which IgG from APS patients induce depolarization of brain synaptoneurosomes, may serve as a model for the pathogenesis of epilepsy in APS.

The MRL model: an invaluable tool in studies of autoimmunity-brain interactions

The link between systemic autoimmunity, brain pathology, and aberrant behavior is still largely unexplored field of biomedical science. Accumulating evidence points to causal relationships between immune factors, neurodegeneration, and neuropsychiatric manifestations. By documenting autoimmunity-associated neuronal degeneration and cytotoxicity of the cerebrospinal fluid from disease-affected subjects, the murine MRL model had shown high validity in revealing principal pathogenic circuits. In addition, unlike any other autoimmune strain, MRL mice produce antibodies commonly found in patients suffering from lupus and other autoimmune disorders. This review highlights importance of the MRL model as an indispensible preparation in understanding the links between immune system and brain function.

Neuroimmunopathology in a murine model of neuropsychiatric lupus

Animal models are extremely useful tools in defining pathogenesis and treatment of human disease. For many years researchers believed that structural damage to the brain of neuropsychiatric (NP) patients lead to abnormal mental function, but this possibility was not extensively explored until recently. Imaging studies of NP-systemic lupus erythematosus (SLE) support the notion that brain cell death accounts for the emergence of neurologic and psychiatric symptoms, and evidence suggests that it is an autoimmunity-induced brain disorder characterized by profound metabolic alterations and progressive neuronal loss. While there are a number of murine models of SLE, this article reviews recent literature on the immunological connections to neurodegeneration and behavioral dysfunction in the Fas-deficient MRL model of NP-SLE. Probable links between spontaneous peripheral immune activation, the subsequent central autoimmune/inflammatory responses in MRL/MpJ-Tnfrsf6(lpr) (MRL-lpr) mice and the sequential mode of events leading to Fas-independent neurodegenerative autoimmune-induced encephalitis will be reviewed. The role of hormones, alternative mechanisms of cell death, the impact of central dopaminergic degeneration on behavior, and germinal layer lesions on developmental/regenerative capacity of MRL-lpr brains will also be explored. This model can provide direction for future therapeutic interventions in patients with this complex neuroimmunological syndrome.

Autoimmune-induced damage of the midbrain dopaminergic system in lupus-prone mice

Spontaneous development of lupus-like disease is accompanied by impaired dopamine catabolism and degenerating axon terminals in the mesencephalon of MRL-lpr mice. We presently examine the hypothesis that systemic autoimmunity affects the central dopaminergic system in behaviorally impaired animals. The functional damage of the nigrostriatal pathway was assessed from rotational behavior after a single injection of the D1/D2-receptor agonist apomorphine. Neurodegeneration in the midbrain was estimated by Fluoro Jade B (FJB) staining. The causal role of autoimmunity was tested by comparing asymptomatic and diseased MRL-lpr mice, and by employing the immunosuppressive drug cyclophosphamide. Damage of dopaminergic neurons was assessed by tyrosine-hydroxylase (TH) staining of the midbrain. Apomorphine induced significant asymmetry in limb use, which lead to increased circling in the diseased MRL-lpr group. While FJB-positive somas were not seen in the striatum, increased staining in the substantia nigra (SN) and ventral tegmental area (VTA) were detected in behaviorally impaired MRL-lpr mice, but not in age-matched controls. Reduced brain mass and increased levels of TNF-alpha in their cerebrospinal fluid (CSF) suggested cerebral atrophy and inflammation. In addition, CSF was neurotoxic to a dopaminergic progenitor cell line. Immunosuppression attenuated CSF cytotoxicity, TNF-alpha levels, and midbrain neurodegeneration. Supportive of the notion that dying neurons were dopaminergic, the SN of autoimmune mice showed approximately a 35% reduction in the number of TH-positive cells. A three-fold increase in serum brain-reactive antibodies accompanied this loss. Although the source of toxic mediator(s) remains unknown, present results are consistent with the hypothesis that autoimmunity-induced destruction of mesonigral and mesolimbic dopaminergic pathways contributes to the etiology of aberrant behavior in an animal model of neuropsychiatric lupus.

Behavioral and cognitive deficits occur only after prolonged exposure of mice to antiphospholipid antibodies

The antiphospholipid (Hughes) syndrome (APS) includes systemic and central nervous system (CNS) pathology associated with antibodies to a complex of phospholipids and beta2-glycoprotein I (beta2-GPI). Beta2-GPI immunized mice develop systemic manifestations of APS and we presently examined CNS manifestations in this APS model. Female BALB/c mice were immunized once with beta2-GPI in complete Freund's adjuvant (CFA) or with CFA alone (controls). A staircase test and a T-maze alternation test were performed to test behavior and cognition in independent groups of mice 6, 12 and 18 weeks following the immunization. The APS mice developed elevated levels of antibodies against negatively charged phospholipids and beta2-GPI. Neurological impairment was detected only 18 weeks after the induction of the APS and consisted of both cognitive (53 +/- 4 vs 71 +/- 3% correct choices in the T-maze alternation for APS vs control mice, P < 0.001) and behavioral changes (higher number of rears (18 +/- 2 vs 11 +/- 1, P < 0.006) and higher number of stairs climbed (12 +/- 2 vs 7 +/- 1, P < 0.02). This is the first report of cognitive deficits in this APS model and demonstrates the time course for the development of previously described behavioral changes. The mechanism involved in these CNS manifestations remains to be elucidated.

Hyperactivity in a mouse model of the antiphospholipid syndrome

In the antiphospholipid syndrome (APS), antibodies to a complex of phospholipids and beta2-glycoprotein I (beta2-GPI) are associated with recurrent thromboembolic events, spontaneous abortions, thrombocytopenia and central nervous system (CNS) disturbances. Animals immunized with beta2-GPI develop the systemic manifestations of APS but the involvement of the (CNS) in these animals has not been studied. The objective of the present study was to examine mice with induced experimental APS for behavioral changes. Female Balb/C mice were immunized once with beta2-GPI in complete Freund's adjuvant (CFA) or with CFA alone. Four months after immunization the mice were tested in the staircase apparatus and the following two variables were measured: (1) number of rears: and (2) number of stairs climbed by the mice. Immunization with beta2-GPI resulted in elevated levels of circulating anti-negatively charged phospholipids and anti-beta2-GPI antibodies. The APS mice exhibited hyperactive behavior as reflected by more frequent rears (P < 0.023) and higher number of stairs climbed (P < 0.019) by the mice in 3 min. This simple test demonstrated that experimental APS animals are significantly hyperactive and may serve as a marker for CNS involvement in this model.

beta(2)-Glycoprotein 1-dependent anticardiolipin antibodies and risk of ischemic stroke and myocardial infarction: the honolulu heart program

BACKGROUND

It has been hypothesized that immunoreactivity to beta(2)-glycoprotein 1 (beta2GP1)-dependent anticardiolipin antibody (aCL), but not beta2GP1-independent aCL, is associated with increased risk of ischemic stroke and myocardial infarction (MI).

METHODS

We performed a nested case-control study examining aCL as a risk factor for ischemic stroke and MI by using stored frozen sera obtained from subjects enrolled in the Honolulu Heart Program and followed for up for 20 years. We measured beta2GP1-dependent and beta2GP1-independent aCL and anti-beta2GP1 immunoreactivity in 259 men who developed an ischemic stroke, in 374 men who developed an MI, and in a control group of 1360 men who remained free of both conditions.

RESULTS

Only beta2GP1-dependent aCL of the IgG class was significantly associated with both incident ischemic stroke and MI. This association was attenuated in the last 5 years of the 20-year follow-up. For stroke, the risk factor-adjusted relative odds for men with a positive versus a negative beta2GP1-dependent aCL of the IgG class were 2.2 (95% CI 1.5 to 3.4) at 15 years and 1.5 (95% CI 1.0 to 2.3) at 20 years. For MI, the adjusted relative odds were 1.8 (95% CI 1.2 to 2.6) at 15 years and 1.5 (95% CI 1.1 to 2.1) at 20 years.

CONCLUSIONS

These data suggest that aCL IgG, particularly the beta2GP1-dependent variety, is an important predictor of future stroke and MI in men.

Antiphospholipid antibodies permeabilize and depolarize brain synaptoneurosomes

Antiphospholipid antibodies (aPL) are associated with neurological diseases such as stroke, migraine, epilepsy and dementia and are thus associated with both vascular and non-vascular neurological disease. We have therefore examined the possibility that these antibodies interact directly with neuronal tissue by studying the electrophysiological effects of aPL on a brain synaptosoneurosome preparation. IgG from patients with high levels of aPL and neurological involvement was purified by protein-G affinity chromatography as was control IgG pooled from ten sera with low levels of aPL. Synaptoneurosomes were purified from perfused rat brain stem. IgG from the patient with the highest level of aPL at a concentration equivalent to 1:5 serum dilution caused significant depolarization of the synaptoneurosomes as determined by accumulation of the lipophylic cation [3H]-tetraphenylphosphonium. IgG from this patient as well as IgG from two elderly patients with high levels of aPL were subsequently shown to permeabilize the synaptosomes to labeled nicotinamide adenine dinucleotide (NAD) and pertussis toxin-ADP-ribose transferase (PTX-A protein) as assayed by labeled ADP-ribosylation of G-proteins in the membranes. No such effects were seen with the control IgG. aPL may thus have the potential to disrupt neuronal function by direct action on nerve terminals. These results may explain some of the non-thromboembolic CNS manifestations of the antiphospholipid syndrome.

Reduced corticotropin-releasing factor and enhanced vasopressin gene expression in brains of mice with autoimmunity-induced behavioral dysfunction

The spontaneous development of autoimmune disease in MRL-lpr mice induces behavioral and endocrine changes that resemble effects of chronic stressors. To further examine the correspondence between autoimmune disease and chronic stress, we asked whether the brains of autoimmune mice show a shift in the corticotropin-releasing factor (CRF) to vasopressin (AVP) ratio. Using in situ hybridization histochemistry with 35S-labelled mouse riboprobes, the levels of mRNA transcripts encoding CRF and AVP were compared between autoimmune MRL-lpr and control MRL +/+ brains. CRF transcript levels were lower in the hypothalamic paraventricular nucleus and in the central nucleus of the amygdala in MRL-lpr mice. AVP transcript levels were higher in the paraventricular and the supraoptic nuclei in MRL-lpr mice compared to controls. CRF mRNA levels were inversely related to performance in stress-sensitive tasks and to measures of autoimmunity. As found previously for behavioral performance, immunosuppressive treatment with cyclophosphamide abolished the group difference in neuropeptide gene expression. These results indicate that an autoimmune disease process is necessary for the shift in the brain CRF:AVP ratio. Furthermore, they support the parallel between chronic stress and chronic autoimmunity/inflammation, and suggest common central mechanisms relevant to endocrine function and behavior.

Lessons from experimental APS models

Several animal models for antiphospholipid syndrome (APS) have been reported in the literature. These experimental models have contributed significantly in resolving enigmas in this multisystemic disease. We, and others have previously shown the pathogenicity of anticardiolipin (aCL) antibodies in pregnancy outcome. We have expanded our studies to show the pathogenicity of aCL antibodies in renal dysfunction and neurological and behavioral impairments in animals with experimental APS. Animals immunized with aCL or with the cofactor beta2GPI developed clinical manifestations of APS, including fetal loss, thrombocytopenia and neurological and behavioral dysfunction, along with elevated levels of aPL antibodies. In another animal model, peripheral blood lymphocytes (PBLs) derived from APS patients could initiate APS manifestations with renal dysfunction in SCID mice. A unique in vivo model for thrombus formation was recently established to show the pathogenicity of aPL in thrombosis associated with APS. Histological evaluation of affected tissues derived from animals or from patients with APS have pointed to common mechanisms underlying APS, showing mainly thrombotic changes accompanied by mild inflammatory reaction.

Skeletal muscle immune deposits in systemic lupus erythematosus. Correlation with histologic changes, autoantibodies, and clinical involvement

Skeletal muscle biopsy and autopsy samples of 132 SLE patients were studied by immunofluorescence and light microscopic techniques. Immune deposits were compared to histologic abnormalities and clinical and serologic findings. Immune deposits with a mainly granular pattern were observed at different locations in 49 patients (37%). They correlated significantly (p<0.001) with inflammatory myopathy, demonstrated in 33(25%), with noninflammatory myopathy which occurred in 38 (29%) vasculitis, observed in 13(10%), and with noninflammatory vasculopathy which was noted in 10 patients (8%). The correlation of muscle immune deposits with anti-DNA antibodies was significant at p= 0.016. Anti-Sm and anti-U1 RNP antibodies were associated significantly with the intensity of immune deposits. Furthermore, a correlation of immune deposits with increased creatine phosphokinase and myopathic electromyogram, but not with evidence of clinical involvement, was shown. A key pathogenetic role of immune deposits in the development of skeletal muscle histologic abnormalities in SLE was demonstrated.

Anti-intercellular adhesion molecule-1 (ICAM-1) antibody treatment prevents central and peripheral nervous system disease in autoimmune-prone mice

Abnormal neurological functioning similar to that seen in systemic lupus erythematosus (SLE) patients is detectable in an SLE-prone murine strain (MRL/lpr) by 8-10 weeks and is severe by 18 weeks of age. The purpose of this study was to evaluate the effectiveness of murine antiintercellular adhesion molecule-1 (ICAM-1) in suppressing neurological disease in MRL/lpr mice. Beginning at 6 weeks of age, five MRL/lpr mice received 5 weekly intraperitoneal injections of anti-ICAM-1-containing culture supernatant in phosphate-buffered saline (PBS) whereas four animals were treated with non-anti-ICAM-1 containing supernatant in PBS. A decline in neurological functioning began in control mice by 10 weeks, but anti-ICAM-1 treated mice remained normal throughout the study. All control mice had vasculitic skin lesions by 14 weeks of age whereas none of the anti-ICAM-1 treated mice ever developed skin lesions. Nerve conduction studies performed on all mice prior to sacrifice showed sciatic compound motor action potentials of anti-ICAM-1 treated mice that were of higher amplitude and shorter latency than those of controls. Inflammation in the sciatic nerve was more common in control mice. Brain histology revealed a similar degree of choroid plexus inflammation in both groups. Our study demonstrated that anti-ICAM-1 was effective in suppressing neurological abnormalities in MRL/lpr mice and may potentially be useful therapy in human SLE.

Animal models for nervous system disease in systemic lupus erythematosus

Animal models have much to teach us about nervous system dysfunction in SLE. It should be stressed that the murine strains described in this review have variable expression in the onset and severity of clinical and serological features, perhaps making them more like a heterogeneous human population with SLE. With this in mind, studies involving animal models like those involving human subjects should use a sample size that ensures adequate power. It is not surprising that studies that use sample sizes as low as four to five animals per group would find discrepant results, especially in outcomes that are measured prior to the terminal phases of the disease. Similar to human SLE patients, murine models have systemic autoimmune as well as neurological manifestations. Studies with murine models must continue to consider some type of SLE disease activity measures in order to control for the effects of systemic disease on nervous system dysfunction. Because of the short time window between the earliest evidence of neurologic dysfunction and severe autoimmune disease manifestations, especially in MRL/lpr mice, the disease acceleration model may allow a more careful dissection of how immunological events are related to nervous system dysfunction. Alternatively, the study of MRL/lpr mice ultraearly (e.g., 3 weeks of age) could also provide invaluable information about the first events leading to nervous system dysfunction in SLE. Both approaches promise to identify predictors of specific nervous system manifestations that may suggest novel and more specific therapeutic interventions.

Neurological dysfunction and hyperactive behavior associated with antiphospholipid antibodies. A mouse model

Antiphospholipid antibodies (aPL) have been associated with various neurological manifestations, but the underlying mechanism has not been elucidated. We assessed mice with induced experimental antiphospholipid syndrome (APS) for neurological and behavioral changes. After immunization with monoclonal human anticardiolipin antibody (H-3), female BALB/c mice developed elevated levels of circulating anti-negatively charged phospholipids (aPL), anti-beta2-glycoprotein I (abeta2GPI), and anti-endothelial cell antibodies (AECA), along with clinical manifestations of APS like thrombocytopenia and fetus resorption. APS mice were impaired neurologically and performed several reflexes less accurately compared to the controls, including placing reflex (P < 0.05), postural reflex (P < 0.05), and grip test (P = 0.05). The APS mice also exhibited hyperactive behavior in an open field, which tests spatial behavior (P < 0.03), and displayed impaired motor coordination on a rotating bar. aPL in combination with abeta2GPI and AECA is probably involved in the neurological and behavioral defects shown in mice with experimental APS.

Animal models for antiphospholipid syndrome in pregnancy

Experimental models for antipospholipid syndrome (APS) have been established recently in lupus-prone mice and induced in naive mice. The induction of APS is performed by passive infusion or active immunization of antiphospholipid antibodies (aPL) or the cofactor beta 2GP-1. High levels of diverse aPL develop in the animals in conjunction with clinical manifestations similar to the human disease, entailing low fecundity rate, fetal resorptions, thrombocytopenia, prolonged activated partial thromboplastin time, and neurological and behavioral impairments. The pathogenicity of aPL was confirmed in an in vivo thrombosis model. Immunomodulation of APS manifestations and treatment regimens in the experimental models are discussed.