Spatial and temporal changes in promoter activity of the astrocyte glutamate transporter GLT1 following traumatic spinal cord injury

Abstract

After traumatic spinal cord injury (SCI), there is an opportunity for preserving function by attenuating secondary cell loss. Astrocytes play crucial roles in the adult CNS and are responsible for the vast majority of glutamate buffering, potentially preventing excitotoxic loss of neurons and oligodendrocytes. We examined spatial and temporal changes in gene expression of the major astrocyte glutamate transporter GLT1 following moderate thoracic contusion SCI using transgenic BAC-GLT1-eGFP promoter reporter mice. In dorsal column white matter, total intensity of GLT1-eGFP expression per region was significantly reduced following SCI at both lesion epicenter and at rostral and caudal areas where no tissue loss occurred. This regional decrease in GLT1 expression was due to significant loss of GLT1-eGFP(+) cells, partially accounted for by apoptosis of eGFP(+) /GFAP(+) astrocytes in both white and gray matter. There were also decreased numbers of GLT1-eGFP-expressing cells in multiple gray matter regions following injury; nevertheless, there was sustained or even increased regional GLT1-eGFP expression in gray matter as a result of up-regulation in astrocytes that continued to express GLT1-eGFP. Although there were increased numbers of GFAP(+) cells both at the lesion site and in surrounding intact spinal cord following SCI, the majority of proliferating Ki67(+) /GFAP(+) astrocytes did not express GLT1-eGFP. These findings demonstrate that spatial and temporal alterations in GLT1 expression observed after SCI result from both astrocyte death and gene expression changes in surviving astrocytes. Results also suggest that following SCI a significant portion of astrocytes lacks GLT1 expression, possibly compromising the important role of astrocytes in glutamate homeostasis.

Heterogeneous astrocytes: Active players in CNS

“…In addition, the spatial heterogeneity of reactive astrocytes is also observed following CNS injury (Lepore et al 2011;Rusnakova et al 2013). After acute injury, live imaging of cerebral cortex revealed a marked heterogeneity in the reaction of individual astrocytes, with one subpopulation retaining their initial morphology, another directing their processes toward the lesion, and a distinct subpopulation located at juxtavascular sites proliferating (Bardehle et al 2013).…”

Human iPS cell-derived astrocyte transplants preserve respiratory function after spinal cord injury

“…Transplantation also provides for long-term astrocyte integration and therapeutic replacement. For example, the lasting nature of dysregulation of extracellular glutamate homeostasis after SCI (Lepore et al, 2011a; Lepore et al, 2011c) calls for longer-term maintenance of therapeutic effects, both with respect to early cell loss occurring during secondary degeneration and outcomes of SCI associated with more persistent pathophysiology of glutamate signaling such as chronic neuropathic pain (Gwak et al, 2012; Hulsebosch, 2008). …”

Transplantation of stem cell-derived astrocytes for the treatment of amyotrophic lateral sclerosis and spinal cord injury

“…Noteworthy, we and others demonstrated that astrocyte GLT1 was chronically lost at the injury epicenter following SCI but also downregulated in spinal cord regions distant from the lesion core [103][104][105][106] . Furthermore, experimental data showed that the newlygenerated astrocytes arising during the SCI repair phase lacked GLT1 expression, possibly compromising long-term astrocyte glutamate homeostasis [107] . Other consequences of astrocyte dysfunction or loss should be considered in ALS and SCI: shortage of neurotrophic factors important for neuronal survival, overwhelming of anti-oxidative defenses, lack of support to maintain endothelial BBB integrity, impaired water, ionic and metabolic transport, release of harmful proinflammatory cytokines and synthesis of glial scar-related ECM proteins that block axonal regrowth [1] .…”