Axonal degeneration in multiple sclerosis: the mitochondrial hypothesis

CXCR7 antagonism prevents axonal injury during experimental autoimmune encephalomyelitis as revealed by in vivo axial diffusivity

Background

Multiple Sclerosis (MS) is characterized by the pathological trafficking of leukocytes into the central nervous system (CNS). Using the murine MS model, experimental autoimmune encephalomyelitis (EAE), we previously demonstrated that antagonism of the chemokine receptor CXCR7 blocks endothelial cell sequestration of CXCL12, thereby enhancing the abluminal localization of CXCR4-expressing leukocytes. CXCR7 antagonism led to decreased parenchymal entry of leukocytes and amelioration of ongoing disease during EAE. Of note, animals that received high doses of CXCR7 antagonist recovered to baseline function, as assessed by standard clinical scoring. Because functional recovery reflects axonal integrity, we utilized diffusion tensor imaging (DTI) to evaluate axonal injury in CXCR7 antagonist- versus vehicle-treated mice after recovery from EAE.

Methods

C57BL6/J mice underwent adoptive transfer of MOG-reactive Th1 cells and were treated daily with either CXCR7 antagonist or vehicle for 28 days; and then evaluated by DTI to assess for axonal injury. After imaging, spinal cords underwent histological analysis of myelin and oligodendrocytes via staining with luxol fast blue (LFB), and immunofluorescence for myelin basic protein (MBP) and glutathione S-transferase-π (GST-π). Detection of non-phosphorylated neurofilament H (NH-F) was also performed to detect injured axons. Statistical analysis for EAE scores, DTI parameters and non-phosphorylated NH-F immunofluorescence were done by ANOVA followed by Bonferroni post-hoc test. For all statistical analysis a p < 0.05 was considered significant.

Results

In vivo DTI maps of spinal cord ventrolateral white matter (VLWM) axial diffusivities of naïve and CXCR7 antagonist-treated mice were indistinguishable, while vehicle-treated animals exhibited decreased axial diffusivities. Quantitative differences in injured axons, as assessed via detection of non-phosphorylated NH-F, were consistent with axial diffusivity measurements. Overall, qualitative myelin content and presence of oligodendrocytes were similar in all treatment groups, as expected by their radial diffusivity values. Quantitative assessment of persistent inflammatory infiltrates revealed significant decreases within the parenchyma of CXCR7 antagonist-treated mice versus controls.

Conclusions

These data suggest that CXCR7 antagonism not only prevents persistent inflammation but also preserves axonal integrity. Thus, targeting CXCR7 modifies both disease severity and recovery during EAE, suggesting a role for this molecule in both phases of disease.

Polymorphic locus rs10492972 of the KIF1B gene association with multiple sclerosis in Russia: case control study

Axonal degeneration is responsible for the progression of the irreversible destruction caused by multiple sclerosis (MS) resulting ultimately in permanent disability. The KIF1B protein, a member of the kinesin family, is necessary for axon growth and myelination in vertebrates. In the recent paper, Aulchenko et al. suggested that the rs10492972[C] variant of KIF1B increases susceptibility to MS, but three following replication study didn't confirm this association. We studied the association of the polymorphic locus rs10492972 present in the KIF1B gene with genetic predisposition and its occurrence in clinical presentations of MS patients resident in western Siberia and the Sakha Republic (Yakutia), Russia. rs10492972 has been genotype in 833 samples of MS patient and 689 healthy controls. Distribution of rs10492972 genotypes corresponded with a Hardy-Weinberg distribution in both the MS patient and control groups, with the frequency of the C allele being the same in both groups (33%). Frequencies of occurrence of the genotypes were not shown to be associated with different disease courses or other characteristics of the disease, such as age at onset or duration. A complete meta-analysis of all analogous studies published to date showed that the protective effect of the rs10492972[C] allele is statistically significant (OR=0.95, C.I.95% [0.90-0.99], p=0.02).

Chronic progressive multiple sclerosis - pathogenesis of neurodegeneration and therapeutic strategies

Multiple sclerosis (MS) is an inflammatory, autoimmune, demyelinating disease of the central nervous system (CNS) that usually starts as a relapsing-remitting disease. In most patients the disease evolves into a chronic progressive phase characterized by continuous accumulation of neurological deficits. While treatment of relapsing-remitting MS (RRMS) has improved dramatically over the last decade, the therapeutic options for chronic progressive MS, both primary and secondary, are still limited. In order to find new pharmacological targets for the treatment of chronic progressive MS, the mechanisms of the underlying neurodegenerative process that becomes apparent as the disease progresses need to be elucidated. New animal models with prominent and widespread progressive degenerative components of MS have to be established to study both inflammatory and non-inflammatory mechanisms of neurodegeneration. Here, we discuss disease mechanisms and treatment strategies for chronic progressive MS.

Beyond the serotonin hypothesis: mitochondria, inflammation and neurodegeneration in major depression and affective spectrum disorders

For many years, a deficiency of monoamines including serotonin has been the prevailing hypothesis on depression, yet research has failed to confirm consistent relations between brain serotonin and depression. High degrees of overlapping comorbidities and common drug efficacies suggest that depression is one of a family of related conditions sometimes referred to as the "affective spectrum disorders", and variably including migraine, irritable bowel syndrome, chronic fatigue syndrome, fibromyalgia and generalized anxiety disorder, among many others. Herein, we present data from many different experimental modalities that strongly suggest components of mitochondrial dysfunction and inflammation in the pathogenesis of depression and other affective spectrum disorders. The three concepts of monoamines, energy metabolism and inflammatory pathways are inter-related in many complex manners. For example, the major categories of drugs used to treat depression have been demonstrated to exert effects on mitochondria and inflammation, as well as on monoamines. Furthermore, commonly-used mitochondrial-targeted treatments exert effects on mitochondria and inflammation, and are increasingly being shown to demonstrate efficacy in the affective spectrum disorders. We propose that interactions among monoamines, mitochondrial dysfunction and inflammation can inspire explanatory, rather than mere descriptive, models of these disorders.

Role of gender in multiple sclerosis: clinical effects and potential molecular mechanisms

Multiple sclerosis (MS) is more prevalent in females than males, and this female predominance is increasing as time goes by. Additionally, gender appears to play critical roles in development, progression and treatment of MS, and is therefore an aspect that should always be considered in the design and interpretation of research and clinical trials for MS. In this review, factors that could potentially explain the gender-biased observations in MS are discussed. These include sex-specific differences between the male and female immune systems and nervous systems, genetic and epigenetic or environmental-related effects, the effects of gonadal hormones, and materno-fetal interactions.

Activation of the mitochondrial permeability transition pore modulates Ca2+ responses to physiological stimuli in adult neurons

The participation of mitochondria in cellular and neuronal Ca(2+) homeostatic networks is now well accepted. Yet, critical tests of specific mitochondrial pathways in neuronal Ca(2+) responses have been hampered because the identity of mitochondrial proteins that must be integrated within this dynamic system remain uncertain. One putative pathway for Ca(2+) efflux from mitochondria exists through the formation of the permeability transition pore (PTP) that is often associated with cellular and neuronal death. Here, we have evaluated neuronal Ca(2+) dynamics and the PTP in single adult neurons in wild-type mice and those missing cyclophilin D (CyPD), a key regulator of the PTP. Using high-resolution time-lapse imaging, we demonstrate that PTP opening only follows simultaneous activation with two physiological stimuli that generate critical threshold levels of cytosolic and mitochondrial Ca(2+) . Our results are the first to demonstrate CyPD-dependent PTP opening in normal neuronal Ca(2+) homeostatic mechanisms not leading to activation of cell death pathways. As neurons in mice lacking CyPD are protected in a number of neurodegenerative disease models, the results suggest that improved viability of CyPD-knockout animals in these pathological states may be due to the transient, rather than persistent, activation of the PTP in mutant mitochondria, thereby shielding neurons from cytoplasmic Ca(2+) overload.

Autophagy, inflammation and neurodegenerative disease

Autophagy is emerging as a central regulator of cellular health and disease and, in the central nervous system (CNS), this homeostatic process appears to influence synaptic growth and plasticity. Herein, we review the evidence that dysregulation of autophagy may contribute to several neurodegenerative diseases of the CNS. Up-regulation of autophagy may prevent, delay or ameliorate at least some of these disorders, and - based on recent findings from our laboratory - we speculate that this goal may be achieved using a safe, simple and inexpensive approach.

Is mPTP the gatekeeper for necrosis, apoptosis, or both?

Permeabilization of the mitochondrial membranes is a crucial step in apoptosis and necrosis. This phenomenon allows the release of mitochondrial death factors, which trigger or facilitate different signaling cascades ultimately causing the execution of the cell. The mitochondrial permeability transition pore (mPTP) has long been known as one of the main regulators of mitochondria during cell death. mPTP opening can lead to matrix swelling, subsequent rupture of the outer membrane, and a nonspecific release of intermembrane space proteins into the cytosol. While mPTP was purportedly associated with early apoptosis, recent observations suggest that mitochondrial permeabilization mediated by mPTP is generally more closely linked to events of late apoptosis and necrosis. Mechanisms of mitochondrial membrane permeabilization during cell death, involving three different mitochondrial channels, have been postulated. These include the mPTP in the inner membrane, and the mitochondrial apoptosis-induced channel (MAC) and voltage-dependent anion-selective channel (VDAC) in the outer membrane. New developments on mPTP structure and function, and the involvement of mPTP, MAC, and VDAC in permeabilization of mitochondrial membranes during cell death are explored. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Cerebrospinal fluid ATP metabolites in multiple sclerosis

Increased axonal energy demand and mitochondrial failure have been suggested as possible causes for axonal degeneration and disability in multiple sclerosis. Our objective was to test whether ATP depletion precedes clinical, imaging and biomarker evidence for axonal degeneration in multiple sclerosis. The method consisted of a longitudinal study which included 21 patients with multiple sclerosis. High performance liquid chromatography was used to quantify biomarkers of the ATP metabolism (oxypurines and purines) from the cerebrospinal fluid at baseline. The Expanded Disability Status Scale, MRI brain imaging measures for brain atrophy (ventricular and parenchymal fractions), and cerebrospinal fluid biomarkers for axonal damage (phosphorylated and hyperphosphorylated neurofilaments) were quantified at baseline and 3-year follow-up. Central ATP depletion (sum of ATP metabolites >19.7 micromol/litre) was followed by more severe progression of disability if compared to normal ATP metabolites (median 1.5 versus 0, p< 0.05). Baseline ATP metabolite levels correlated with change of Expanded Disability Status Scale in the pooled cohort (r= 0.66, p= 0.001) and subgroups (relapsing-remitting patients: r= 0.79, p< 0.05 and secondary progressive/primary progressive patients: r= 0.69, p< 0.01). There was no relationship between central ATP metabolites and either biomarker or MRI evidence for axonal degeneration. The data suggests that an increased energy demand in multiple sclerosis may cause a quantifiable degree of central ATP depletion. We speculate that the observed clinical disability may be related to depolarisation associated conduction block.