Assessments of Reactive Astrogliosis Following CNS Injuries - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 56

role of Kir4.1 in reactive astrogliosis come from the relationship
between channel function and cell proliferation/differentiation.
Previous work has shown that dividing and immature cells have
a relatively positive resting membrane potential that becomes
more negative as cells mature and terminally differentiate and
this transition is associated with the increased expression of
Kir4.1 channels ( 18 ). Several lines of evidence substantiate this
relationship between Kir4.1 expression and cell division. For
example, blockade of the Kir4.1 channel delays differentiation
and exit from the cell cycle ( 19 ), and glia in Kir4.1 knock-out
mice display depolarized resting membrane potentials and
immature morphologies ( 20, 21 ). Also, malignant human
glioma cells maintain a relatively depolarized membrane poten-
tial and have mislocalized/nonfunctional Kir4.1 channels, but
transfection with functional Kir4.1 channels hyperpolarizes the
cell membrane and arrests cell division ( 22 ). With regard to
injury, in vitro injury to spinal cord astrocytes decreased Kir4.1
activity and induced proliferation ( 23 ). TBI was shown to
induce abnormal K + accumulation that was similar to that
achieved by blockade of Kir4.1 channels ( 24 ). Taken together,
these data suggest that mature astrocytes express Kir4.1 which
is crucial for K + spatial buffering, extracellular K + homeostasis,
glutamate uptake, and maintaining astrocyte-neuronal inter-
actions in normal fi ring. Furthermore, injury decreases Kir4.1
expression and function which shifts cells to an immature state
and induces hypertrophy and cell division which are character-
istics of reactive astrocytosis.
We recently evaluated the expression and function of
Kir4.1 after SCI in rats and found that Kir4.1 protein expres-
sion was signifi cantly down-regulated following SCI. To exam-
ine whether these changes in Kir4.1 expression are also refl ected
in the biophysical properties of astrocytes, we performed
whole-cell voltage-clamp recording from control and injured
animals at the epicenter of the lesion at 7 days post-SCI. Cell
type was determined by responses to linear voltage ramps and
voltage steps with astrocytes displaying time and voltage sensi-
tive currents, relatively low input resistances (25-300 MW),
and a lack of spontaneous electrical activity and spiked in
responses as previously described ( 25 ). Kir4.1 mediated cur-
rents in spinal cord astrocytes were isolated and we found that
Kir4.1 currents density values fell within a narrow range of −10
to −45 pA/pF in control animals. Surprisingly however, injured
animals showed a large degree of scatter in current density val-
ues with some cells showing barely recordable currents yet
other expressing currents at densities exceeding that of control
cells by over fourfold. These data suggest that injured cells do
not show a homogenous reduction in Kir4.1. In light of the
overall reduction in Kir4.1 protein observed by Western blot