Journal of Neuroscience, Vol 14, 3195-3207, Copyright © 1994 by Society for Neuroscience
Astroglial differentiation is required for support of neurite outgrowth
LC Wang, DH Baird, ME Hatten and CA Mason
Department of Pathology, Columbia University, College of Physicians and Surgeons, New York, New York 10032.
Models of astrocyte differentiation stress a lineage program that involves
a progressive loss of astroglial support of neuronal differentiation. These
models predict that astroglial promotion of neurite extension declines with
the "age" of the astrocyte. An alternative view is that astroglial support
of neurite growth is regulated by epigenetic factors that induce the cells
either to differentiate and support neuronal functions or to undergo cell
proliferation and fail to support neurons. To compare the contribution of
astroglial cell "age" to astroglial support of neurite extension, mouse
cerebellar astroglia were maintained in vitro for 3-90 d, and assayed for
their ability to support neurite formation. When cultured in isolation,
astroglial support of neurite extension declined with time in vitro, as
assayed by quantifying outgrowth from explants of pontine nuclei, falling
from a robust level just after the astroglia were harvested to negligible
levels 21-90 d later. Since previous studies have shown that neurons can
change the state of astroglial cells (Hatten, 1985), we tested the neurite
promoting activity of astroglia that were cultured for 21-90 d in vitro and
subsequently induced to differentiate by the addition of neurons. When
granule neurons were added to aged astroglia and pontine explants plated 2
d later, neurite growth from the explants was exuberant, regardless of the
time astroglia spent in vitro prior to the addition of neurons. The state
of astroglia that were growth promoting or growth inhibiting was examined
by bromodeoxyuridine staining and with antisera to glial filament protein.
Aged astroglia cultured alone and thus inhibitory to axon growth,
proliferated at high rates and had polygonal shapes. In contrast, aged
astroglia to which neurons had been added, proliferated at low rates and
developed process-bearing stellate shapes. To test further whether
proliferation levels related to the growth-supporting properties of
astroglia, astroglia were plated alone in medium without serum, or with the
addition of transforming growth factor-beta 1, each treatment known to
arrest proliferation. In both cases, promotion of neurite growth was
restored in aged astroglia, but the morphology of astroglia did not
correlate with the ability to support neurite growth. Finally, the
growth-inhibiting properties of aged astroglia do not appear to be mediated
by diffusible factors, and require close apposition with living astroglial
cells. We conclude that astroglial support of neurite extension depends on
the state of differentiation of astroglial cells, and that these properties
can be modified by coculture with neurons or conditions that arrest of
astroglial proliferation, irrespective of astroglial "age".
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