Type-2 astrocyte-like cells are more resistant than oligodendrocyte-like cells against non-N-methyl-D-aspartate glutamate receptor-mediated excitotoxicity

Technical approaches and challenges to study AMPA receptors in oligodendrocyte lineage cells: Past, present, and future

Receptors for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPARs) are ligand-gated ionotropic receptors for glutamate that is a major excitatory neurotransmitter in the central nervous system. AMPARs are located at postsynaptic sites of neuronal synapses where they mediate fast synaptic signaling and synaptic plasticity. Remarkably, AMPARs are also expressed by glial cells. Their expression by the oligodendrocyte (OL) lineage cells is of special interest because AMPARs mediate fast synaptic communication between neurons and oligodendrocyte progenitor cells (OPCs), modulate proliferation and differentiation of OPCs, and may also be involved in regulation of myelination. On the other hand, during pathological conditions, AMPARs may mediate damage of the OL lineage cells. In the present review, we focus on the technical approaches that have been used to study AMPARs in the OL lineage cells, and discuss future perspectives of AMPAR research in these glial cells.

Secondary Degeneration Impairs Myelin Ultrastructural Development in Adulthood following Adolescent Neurotrauma in the Rat Optic Nerve

Adolescence is a critical period of postnatal development characterized by social, emotional, and cognitive changes. These changes are increasingly understood to depend on white matter development. White matter is highly vulnerable to the effects of injury, including secondary degeneration in regions adjacent to the primary injury site which alters the myelin ultrastructure. However, the impact of such alterations on adolescent white matter maturation is yet to be investigated. To address this, female piebald-virol-glaxo rats underwent partial transection of the optic nerve during early adolescence (postnatal day (PND) 56) with tissue collection two weeks (PND 70) or three months later (PND 140). Axons and myelin in the transmission electron micrographs of tissue adjacent to the injury were classified and measured based on the appearance of the myelin laminae. Injury in adolescence impaired the myelin structure in adulthood, resulting in a lower percentage of axons with compact myelin and a higher percentage of axons with severe myelin decompaction. Myelin thickness did not increase as expected into adulthood after injury and the relationship between the axon diameter and myelin thickness in adulthood was altered. Notably, dysmyelination was not observed 2 weeks postinjury. In conclusion, injury in adolescence altered the developmental trajectory, resulting in impaired myelin maturation when assessed at the ultrastructural level in adulthood.

Comparing modes of delivery of a combination of ion channel inhibitors for limiting secondary degeneration following partial optic nerve transection

Injury to the central nervous system is exacerbated by secondary degeneration. Previous research has shown that a combination of orally and locally administered ion channel inhibitors following partial optic nerve injury protects the myelin sheath and preserves function in the ventral optic nerve, vulnerable to secondary degeneration. However, local administration is often not clinically appropriate. This study aimed to compare the efficacy of systemic and local delivery of the ion channel inhibitor combination of lomerizine, brilliant blue G (BBG) and YM872, which inhibits voltage-gated calcium channels, P2X7 receptors and Ca2+ permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors respectively. Following a partial optic nerve transection, adult female PVG rats were treated with BBG and YM872 delivered via osmotic mini pump directly to the injury site, or via intraperitoneal injection, both alongside oral administration of lomerizine. Myelin structure was preserved with both delivery modes of the ion channel inhibitor combination. However, there was no effect of treatment on inflammation, either peripherally or at the injury site, or on the density of oligodendroglial cells. Taken together, the data indicate that even at lower concentrations, the combinatorial treatment may be preserving myelin structure, and that systemic and local delivery are comparable at improving outcomes following neurotrauma.

Researching glutamate - induced cytotoxicity in different cell lines: a comparative/collective analysis/study

Although glutamate is one of the most important excitatory neurotransmitters of the central nervous system, its excessive extracellular concentration leads to uncontrolled continuous depolarization of neurons, a toxic process called, excitotoxicity. In excitotoxicity glutamate triggers the rise of intracellular Ca²⁺ levels, followed by up regulation of nNOS, dysfunction of mitochondria, ROS production, ER stress, and release of lysosomal enzymes. Excessive calcium concentration is the key mediator of glutamate toxicity through over activation of ionotropic and metabotropic receptors. In addition, glutamate accumulation can also inhibit cystine (CySS) uptake by reversing the action of the CySS/glutamate antiporter. Reversal of the antiporter action reinforces the aforementioned events by depleting neurons of cysteine and eventually glutathione’s reducing potential. Various cell lines have been employed in the pursuit to understand the mechanism(s) by which excitotoxicity affects the cells leading them ultimately to their demise. In some cell lines glutamate toxicity is exerted mainly through over activation of NMDA, AMPA, or kainate receptors whereas in other cell lines lacking such receptors, the toxicity is due to glutamate induced oxidative stress. However, in the greatest majority of the cell lines ionotropic glutamate receptors are present, co-existing to CySS/glutamate antiporters and metabotropic glutamate receptors, supporting the assumption that excitotoxicity effect in these cells is accumulative. Different cell lines differ in their responses when exposed to glutamate. In this review article the responses of PC12, SH-SY5Y, HT-22, NT-2, OLCs, C6, primary rat cortical neurons, RGC-5, and SCN2.2 cell systems are systematically collected and analyzed.

Dysmyelinated axons in shiverer mice are highly vulnerable to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated toxicity

Glutamate excitotoxicity plays a role in white matter injury in many neurological diseases. Oligodendrocytes in particular are highly vulnerable to excitotoxicity, mediated through activation of AMPA/kainate receptors. Myelin may also be injured independently via NMDA (N-methyl-D-aspartic acid) receptors located on peripheral oligodendroglial processes. Central axons are susceptible to glutamate receptor activation in vivo, but it is unclear whether this is mediated directly by activation of receptors expressed on axons, or indirectly through glutamate toxicity of myelin or neighboring glial cells. We examined axonal vulnerability in mice deficient in myelin basic protein (shiverer), also expressing yellow fluorescent protein (YFP) in a subset of axons. YFP fluorescence, EM, and mouse behavior were assessed 24 h after microstereotactical injections of S-AMPA or NMDA into lumbar dorsal columns. S-AMPA injection led to impaired rotarod performance and widespread axonal degeneration and was more pronounced in shiverer mice than controls. In contrast, NMDA injection did not cause axonal injury or behavioral changes in either group. These results indicate that spinal cord axons in vivo are vulnerable to toxicity mediated by AMPA but not NMDA receptors. The presence of compact myelin is not required for excitotoxic axon damage, and its absence may increase vulnerability. Further understanding of AMPA receptor-mediated axonal toxicity may provide new targets for neuroprotective therapy in WM diseases.

Developmental factors regulating susceptibility to perinatal brain injury and seizures

PURPOSE OF REVIEW

To review the unique pattern of developmentally regulated factors that govern the susceptibility of the brain during the preterm and term windows of development.

RECENT FINDINGS

The neonatal brain shows unique regional differences in susceptibility to injury. In response to the common insult of hypoxia/ischemia, the preterm brain exhibits regional white matter susceptibility, while gray matter is affected in the term brain. Developmental regulation of specific cellular factors is likely to underlie these age-specific differences.

SUMMARY

A better understanding of these factors could contribute to the development of new age-specific therapeutic strategies with clinical potential for disorders such as periventricular leukomalacia in the preterm and neonatal seizures in the term infant.

Developmental regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit expression in forebrain and relationship to regional susceptibility to hypoxic/ischemic injury. I. Rodent cerebral white matter and cortex

This is the first part of a two-part study to investigate the cellular distribution and temporal regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunits in the developing white matter and cortex in rat (part I) and human (part II). Western blot and immunocytochemistry were used to evaluate the differential expression of AMPAR subunits on glial and neuronal subtypes during the first 3 postnatal weeks in the Long Evans and Sprague Dawley rat strains. In Long Evans rats during the first postnatal week, GluR2-lacking AMPARs were expressed predominantly on white matter cells, including radial glia, premyelinating oligodendrocytes, and subplate neurons, whereas, during the second postnatal week, these AMPARs were highly expressed on cortical neurons, coincident with decreased expression on white matter cells. Immunocytochemical analysis revealed that cell-specific developmental changes in AMPAR expression occurred 2-3 days earlier by chronological age in Sprague Dawley rats compared with Long Evans rats, despite overall similar temporal sequencing. In both white and gray matter, the periods of high GluR2 deficiency correspond to those of regional susceptibility to hypoxic/ischemic injury in each of the two rat strains, supporting prior studies suggesting a critical role for Ca2+-permeable AMPARs in excitotoxic cellular injury and epileptogenesis. The developmental regulation of these receptor subunits strongly suggests that Ca2+ influx through GluR2-lacking AMPARs may play an important role in neuronal and glial development and injury in the immature brain. Moreover, as demonstrated in part II, there are striking similarities between rat and human in the regional and temporal maturational regulation of neuronal and glial AMPAR expression.

Developmental regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit expression in forebrain and relationship to regional susceptibility to hypoxic/ischemic injury. II. Human cerebral white matter and cortex

This report is the second of a two-part evaluation of developmental differences in alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunit expression in cell populations within white matter and cortex. In part I, we reported that, in rat, developmental expression of Ca2+-permeable (GluR2-lacking) AMPARs correlated at the regional and cellular level with increased susceptibility to hypoxia/ischemia (H/I), suggesting an age-specific role of these receptors in the pathogenesis of brain injury. Part II examines the regional and cellular progression of AMPAR subunits in human white matter and cortex from midgestation through early childhood. Similarly to the case in the rodent, there is a direct correlation between selective vulnerability to H/I and expression of GluR2-lacking AMPARs in human brain. For midgestational cases aged 20-24 postconceptional weeks (PCW) and for premature infants (25-37 PCW), we found that radial glia, premyelinating oligodendrocytes, and subplate neurons transiently expressed GluR2-lacking AMPARs. Notably, prematurity represents a developmental window of selective vulnerability for white matter injury, such as periventricular leukomalacia (PVL). During term (38-42 PCW) and postterm neonatal (43-46 PCW) periods, age windows characterized by increased susceptibility to cortical injury and seizures, GluR2 expression was low in the neocortex, specifically on cortical pyramidal and nonpyramidal neurons. This study indicates that Ca2+-permeable AMPAR blockade may represent an age-specific therapeutic strategy for potential use in humans. Furthermore, these data help to validate specific rodent maturational stages as appropriate models for evaluation of H/I pathophysiology.

Role of glutamate receptors in periventricular leukomalacia

Periventricular leukomalacia is a form of white-matter injury that occurs in the setting of either primary or secondary hypoxia-ischemia in the premature infant. Hypoxia-ischemia induces increases in cerebral extracellular glutamate levels, thereby activating glutamate receptors on a variety of cell types within the white matter. This review examines the evidence of a role for glutamate receptors in white-matter injury and periventricular leukomalacia. Multiple glutamate receptor subtypes exist, and these appear to play differential roles depending on cell type and time after injury. Glutamate receptors are developmentally regulated on neurons and glia, and certain subtypes are transiently overexpressed in developing rodent brain and are expressed on immature oligodendrocytes in human white matter in the premature period. Pharmacologic agents acting on glutamate receptors might represent age-specific therapeutic strategies for the treatment of periventricular leukomalacia.

Remyelination in multiple sclerosis: a new role for neurotrophins?

Multiple sclerosis (MS) is a common neurological disease, which affects young adults. Its course is unpredictable and runs over decades. It is considered as an autoimmune disease, and is neuropathologically characterized by demyelination, variable loss of oligodendroglial cells, and axonal degeneration. Demyelination provides a permitting condition for axonal degeneration, which seems to be causative of permanent neurological deficits. Hence, the current treatment, which works preferentially immunmodulatory, should be complemented by therapeutics, which improves remyelination not only for restoring conduction velocity but also for preventing an irreversible axonal damage. One strategy to achieve this aim would be to promote remyelination by stimulating oligodendroglial cells remaining in MS lesions. While central nervous system neurons were already known to respond to neurotrophins (NT), interactions with glial cells became apparent more recently. In vitro and in vivo studies have shown that NT influence proliferation, differentiation, survival, and regeneration of mature oligodendrocytes and oligodendroglial precursors in favor of a myelin repair. Two in vivo models provided direct evidence that NT can improve remyelination. In addition, their neuroprotective and anti-inflammatory role would support a repair. Hence, a wealth of data point to NT as promising therapeutical candidates.