Michael S. Beattie

Microglia are key mediators of the immune response in the central nervous system (CNS). They are closely related to macrophages and undergo dramatic morphological and functional changes after CNS trauma or excitotoxic lesions. Microglia can be directly stimulated by excitatory neurotransmitters and are known to express many neurotransmitter receptors. The role of these receptors, however, is not clear. This study describes the microglial response to the glutamate receptor agonist kainate (KA) and shows via immunochemistry that the α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionate (AMPA)‐type glutamate receptor subunit GluR1 is present on cultured microglia. In the presence of 100 μM or 1 mM KA, cultured microglia underwent dramatic morphological and cytoskeletal changes as observed by time‐lapse photography and quantitative confocal analysis of phalloidin labeling. KA‐stimulated microglia showed condensation of cytoplasmic actin filaments, rapid de‐ and repolymerization, and cytoplasmic redistribution of condensed actin bundles. Rearrangement of actin filaments—thought to be involved in locomotion and phagocytosis and to indicate an increased level of activation (for reviews see Greenberg [ 1995] Trends Cell Biol. 5:93–99; Imai and Kohsaka [ 2002] Glia 40:164–174)—was significantly increased in treated vs. control cultures. Morphological plasticity and membrane ruffling were also seen. These findings suggest direct microglial excitation via glutamate receptor pathways. Thus, neurotransmitter release after brain or spinal cord injury might directly modulate the inflammatory response.

The Basso, Beattie, Bresnahan (BBB) open field locomotor scale is a popular measure of functional recovery following spinal cord injury (SCI). To examine the metric properties of the scale, we performed detailed analyses of BBB scores from 643 rats with moderate and severe SCI (12.5, 25, or 50 mm MASCIS) from two different laboratories. The analyses revealed that the BBB scale is ordinal in the most frequently used portion of the scale. Higher scores (14 and greater) were not frequently assigned in the dataset as animals with mild injuries were not sampled, making the ordinal nature of the upper end of the scale difficult to assess. The rare scores assigned in this range disproportionately increased variance. Under these conditions collapsing scores above 14 into one category increased effect size. Analysis of the lower region of the scale revealed that some scores (2 and 3) were rarely assigned, implying a discontinuity in the scale. The discontinuous nature of the lower portion of the scale presents a problem for both parametric and nonparametric statistical analyses. Pooling scores 2/3/4 eliminated the gap, enhancing the metric properties of the scale. Under the injury conditions evaluated, the transformation helped assure that the data were continuous and ordered. Further, interval durations were comparable across the entire range of the transformed scale, allowing application of parametric statistical techniques. The transformation should be applied in a post hoc fashion to reduce variability and increase power in cases where few scores fall in upper portion of the scale.

Activity-dependent modulation of synaptic efficacy in the brain contributes to neural circuit development and experience-dependent plasticity. Although glia are affected by activity and ensheathe synapses, their influence on synaptic strength has largely been ignored. Here, we show that a protein produced by glia, tumor necrosis factor α (TNFα), enhances synaptic efficacy by increasing surface expression of AMPA receptors. Preventing the actions of endogenous TNFα has the opposite effects. Thus, the continual presence of TNFα is required for preservation of synaptic strength at excitatory synapses. Through its effects on AMPA receptor trafficking, TNFα may play roles in synaptic plasticity and modulating responses to neural injury.

The initial mechanical tissue disruption of spinal cord injury (SCI) is followed by a period of secondary injury that increases the size of the lesion. The secondary injury has long been thought to be due to the continuation of cellular destruction through necrotic (or passive) cell death. Recent evidence from brain injury and ischemia suggested that cellular apoptosis, an active form of programmed cell death seen during development, could play a role in CNS injury in adulthood. Here, we review the evidence that apoptosis may be important in the pathophysiology of SCI. There is now strong morphological and biochemical evidence from a number of laboratories demonstrating the presence of apoptosis after SCI. Apoptosis occurs in populations of neurons, oligodendrocytes, microglia, and, perhaps, astrocytes. The death of oligodendrocytes in white matter tracts continues for many weeks after injury and may contribute to post-injury demyelination. The mediators of apoptosis after SCI are not well understood, but there is a close relationship between microglia and dying oligodendrocytes, suggesting that microglial activation may be involved. There is also evidence for the activation of important intracellular pathways known to be involved in apoptosis in other cells and systems. For example, some members of the caspase family of cysteine proteases are activated after SCI. It appears that the evolution of the lesion after SCI involves both necrosis and apoptosis. It is likely that better understanding of apoptosis after SCI will lead to novel strategies for therapeutic interventions that can diminish secondary injury.

Following spinal cord contusion in the rat, apoptosis has been observed in the white matter for long distances remote from the center of the lesion and is primarily associated with degenerating fiber tracts. We have previously reported that many of the apoptotic cells are oligodendrocytes. Here we show that the oligodendrocyte death is maximal at 8 days postinjury and suggest that loss of oligodendrocytes may result in demyelination of axons that have survived the initial trauma. There are two mechanisms that may account for the observed oligodendrocyte apoptosis. The apoptotic cell death may result from the loss of trophic support after axonal degeneration or it may be the consequence of microglial activation. The hypothesis that oligodendrocyte apoptosis is secondary to microglial activation is supported by our observations of microglia with an activated morphology in the same regions as apoptosis and apparent contact between some of the apoptotic oligodendrocytes and microglial processes. In addition to oligodendrocyte apoptosis, a subpopulation of microglia appears to be susceptible to apoptotic cell death as well, as evidenced by the presence of apoptotic bodies in OX42 immunopositive profiles. Thus, the population of apoptotic cells following spinal cord contusion is comprised of oligodendrocytes and putative phagocytic microglia or macrophages. Given the delayed time course of oligodendrocyte death, the apoptotic death of oligodendrocytes may be amenable to pharmacological intervention with subsequent improvement in functional recovery. J. Neurosci. Res. 50:798–808, 1997.