Abstract
CNS myelination is a well-known biological phenomenon related to increase in action potential conduction speed and synaptic integration. Demyelination or abnormal myelination has been related to seizures and epilepsy either in patients or animal models. Although its role in fastening action potential propagation has been confirmed, few additional biological functions of CNS myelin has been described. Among these, myelin proteins such as NogoA were demonstrated to block axon elongation and synaptic potentiation. In light of our novel data on NogoA role in synaptogenesis, we propose a mechanism by which CNS myelin might regulate epileptogenesis both in multiple sclerosis and polymicrogyria.
Introduction
Oligodendrocytes are the CNS cells responsible for the myelination of axons, a process in which the axon is ensheathed by a specialized oligodendrocyte-derived membrane. The consequent electrical isolation ensures saltatory conduction of the action potential and the rapid flow of information [1,2]. During the development of central connections, myelination seems to contribute to synaptic stabilization, since its emergence coincides with the period of reduction in the plasticity of connections, with stagnation in the formation of new synapses [3] and strengthening of pre-existing ones [4]. Although myelination-dependent strengthening of synapses has been associated with synchronization of the time of arrival of the electrical impulse at the postsynaptic neuron and the consequent synaptic integration [5], the mechanisms related to the influence on synaptogenesis have been less explored.
Different animal models that present demyelination have shown increased synaptogenesis. Studies with Long Evans Shaker rats and Shiverer mice, where myelin are decompacted and degenerated due to mutations in the myelin basic protein gene, showed that deficient myelination promotes sprouted axons and an abnormally high density of synaptic boutons [6-8]. Interestingly, the number of synapses may increase or decrease depending on the stage of pathological process in the experimental autoimmune encephalomyelitis mouse model [7]. Similarly, we observed an increased number of excitatory synapses in the supragranular layers of the demyelinated cerebral cortex as a result of innate immunity-mediated acute demyelination induced by cuprizone diet [9].
Oligodendrocyte-derived myelin has a set of proteins, including Nogo-A, that inhibit axonal growth, synaptic plasticity and regeneration after injury. Many of the effects of these myelin-associated inhibitors are associated with the Nogo receptor 1 (NgR1) pathway [10], expressed both by neuronal and glial cells [9-11]. NgR1 signaling involves the activation of the downstream proteins RhoA and Rock, resulting in actin cytoskeleton rearrangement [10,11]. Particularly in neurons, NgR1 receptor signaling has been shown to be relevant for inhibiting synapse formation in hippocampal development [11]. Additionally, our recent in-vitro data demonstrated that the NogoA pathway in astrocytes is also transduced by activation of RhoA, Rock and Fak, culminating in increased actin polymerization. Remarkably, we have shown that Nogo-A signaling results in repression of the synaptogenic secretome of the astrocyte. Through this mechanism, there is a reduction in the expression of pro-synaptogenic factors such as Hevin, and an increase in the expression of its antagonist Sparc. These effects might be mediated through the RhoA pathway, since the inhibition of Rock, the classical target of RhoA, reverses the effect of Nogo-A on the expression of Sparc. Consistently, the conditional medium secreted by Nogo-A activated astrocytes results in neurons with fewer excitatory synapses and less presynaptic activity [9]. Thus, the oligodendrocyte-myelin mediated inhibition of synapse formation must rely on Nogo-A signaling both in neurons and astrocytes.
Myelin-related Epileptogenic Mechanisms in Multiple Sclerosis
Although myelination modulation of conduction velocity is essential for synaptic integration and for the proper functioning of neural networks [12], it also has been shown to play an important role in excitability regulation, suggesting that there is a direct relationship between hyperexcitability and myelin abnormalities [13]. For example, patients with multiple sclerosis are 3 to 6 times more likely to develop epilepsy, indicating that reduced myelin content and/or CNS inflammation must be related to epileptogenesis and epileptic seizures [14,15]. However, Shiverer mice predisposition to seizures argues in favor of myelin defects sufficiency to predispose the brain to epileptiform activity [16]. There are correlates of myelin defects in MRI of epileptic patients with multiple sclerosis [17], where multiple demyelinating lesions with a large plaque adjacent to the cortex can be found. Kinnunem and Wikstrom related partial seizures with demyelinating lesions that would be acting as sensitization foci for triggering action potentials [18]. This hypothesis can be supported by histopathological studies of post-mortem multiple sclerosis patients that have shown that about 25% of demyelinating plaques are found adjacent to or extending into the grey matter of the cortex [19].
Although the myelin sheath optimizes action potential propagation and provides metabolic support along axonal fibers, the mechanisms that explain how myelin influences brain excitability are not well understood. In view of our data in an animal model of multiple sclerosis, demyelination accompanied by a reduction in Nogo-A and Hevin up-regulation could be leading to an increase in excitatory synapses in the cerebral cortex of subjects [9], leading to an increase in excitability. Indeed, our in vitro data showed that in the absence of NogoA, astrocyte-derived mediators promote the increase in the number of PSD95-positive excitatory postsynaptic sites, but does not modulate the number of non-specific synaptophysin presynaptic sites, suggesting an excitatory synaptogenic bias. Whether these events happen in patients with Multiple Sclerosis remains to be investigated.
Alternatively, J. Stedehouder and colleagues reported that there is a large fraction of myelinated GABAergic interneurons in the cerebral cortex of mice, reaching up to 80% in hippocampal regions [20]. Thus, demyelination of GABAergic axons can facilitate synaptic integration into excitatory neurons, which would fire excessively, causing hyperexcitability and seizures. Investigation of the differential demyelination in excitatory and inhibitory axons in the cerebral cortex of animal models or patients with Multiple Sclerosis will allow a better understanding in this area, enabling the identification of new therapeutic targets.
Myelin-related Epileptogenic Mechanisms in Polymicrogyria
Myelination deficiency has been related to epilepsy development not only in demyelinating diseases, but also in brain malformations [21]. Neocortical malformations investigation by magnetic resonance imaging (MRI) have shown poor myelination in the area adjacent to polymicrogyral zone [21-23]. Besides, histopathological investigations have shown increased astrocytic presence in layer V intracortical myelinated bundles of Baillarger from a Polymicrogyria patient [24]. If myelination abnormalities found in polymicrogyria reflects an inflammatory-mediated maldevelopment or if it occurs due to neuronal activity pattern alteration remains to be investigated. Additionally, animal model studies of microgyria have shown that the cortex adjacent to the microgyrus slowly develops epileptogenic activity, concomitant to increased spontaneous excitatory postsynaptic events [25,26]. Interestingly, the increase in the frequency of miniature and spontaneous excitatory postsynaptic currents precedes epileptiform activity, suggesting that increased synaptic activity drives epileptiform activity [27].
Our in vitro results provided a novel mechanism of synaptogenesis, where immature neocortical neurons are prone to excitatory synapses formation in the absence of Nogo-A-mediated inhibition of astrocytic synaptogenic factors release [9]. Thus, it is possible to speculate a permissive role for decreased content of myelin-derived Nogo-A adjacent to the microgyrus in the astrocytic-mediated synaptogenic process in polymicrogyria, creating an epileptogenic focus. If these synaptogenic and epileptogenic mechanisms occur in the region adjacent to the malformation remains to be investigated both in patients and animal models.
Conclusions
The present opinion article proposes a novel mechanism, where myelin deficiency or demyelination might trigger seizures, by regulation of synaptogenesis. The immunofluorescent quantification of excitatory and inhibitory synapses onto excitatory or inhibitory neurons in the neocortex of animal models of multiple sclerosis and polymicrogyria might reveal the network specificities that drives epileptogenesis. The mechanisms discussed here might be complementary to the extracellular potassium rise proposed by de Curtis and coworkers [28]. All in all, myelination deficiency seems to be an important mechanism of epileptogenesis that has been under investigated, requiring additional studies.
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