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which samples the entire affected tissue, it is plausible that
many cells in which Kir4.1 expression had been lost are not
included in our recordings. Thus to fully understand the com-
plex relationship between Kir4.1 expression, function, and
astrocyte pathology after SCI, it is crucial to analyze astrocytes
at several locations relative to the lesion and relative to the
degree of reactive astrogliosis as indicated by morphological
assessments.
1. Evaluation of transcriptional control of the GFAP gene .
One approach to evaluating the functional consequences of
reactive astrogliosis has been to produce GFAP-defi cient mice.
Although GFAP-null mice appear normal in development,
reproduction, and life span, subtle differences between GFAP-
knockout and wild type control mice have been observed,
including alterations in long-term potentiation and depression,
increased susceptibility to autoimmune encephalomyelitis
and age-induced defi cits in myelination ( 5, 26 ). Brenner
and colleagues induced acceleration/deceleration TBI using a
weight drop device in GFAP-null mice. They reported that
moderate weight drop injury (20 g weight from a height of
45 cm or 900 g-cm impact force) caused acute post-injury
death or paralysis in all GFAP-null mice as compared to no
death or overt defi cits in wild type controls. Evaluation of overt
pathophysiology in the brain and cervical spinal cord revealed
contusion and hemorrhage in the cervical spinal cord of GFAP-
null mice but not in wild type controls ( 27 ). Interestingly, the
spinal cord pathology was observed in the absence of overt
brain pathology and was not observed when the mouse head
was impacted with the weight but fi xed, suggesting that the
cervical spinal cord was most sensitive to the acceleration/
deceleration forces. An additional line of work by the Brenner
research team is exploring the transcriptional regulation of
the GFAP gene by evaluation of specifi c promoter regions in
transgenic mice. For example, the gfa2 promoter, which com-
prises of 2.2 kb of 5ยข-fl anking DNA of the human GFAP gene
was joined to an E. coli b-galactosidase reporter gene (lacZ) to
evaluate the role of this promoter in development and reactive
astrogliosis. They demonstrated up-regulation of the gfa2-lacZ
transgene in response to stab wound injury ( 28 ). These data
suggest that regulation of injury-induced reactive astrogliosis
could be achieved by greater understanding of the reactive sites
on regulatory elements altered by injury.
2. Transgenic mouse model for selective ablation of reactive
astrocytes.
In a series of pioneering studies, Sofroniew and colleagues
evaluated the effect of selective ablation of proliferating reactive
astrocytes after CNS injury ( 29-31 ). To do this, they generated
1.3.2. Novel Genetic
Approaches to Assess
Functional Role of Reactive
Astrogliosis
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