Heterogeneity of reactive astrocytes.

Astrocytes respond to injury and disease in the central nervous system (CNS) with a process referred to as reactive astrogliosis. Recent progress demonstrates that reactive astrogliosis is not a simple all-or-none phenomenon, but is a finely gradated continuum of changes that range from reversible alterations in gene expression and cell hypertrophy, to scar formation with permanent tissue rearrangement. There is now compelling evidence that reactive astrocytes exhibit a substantial potential for heterogeneity at multiple levels, including gene expression, cell morphology, topography (distance from lesions), CNS regions, local (among neighboring cells), cell signaling and cell function. Structural and functional changes are regulated in reactive astrocytes by many different potential signaling events that occur in a context dependent manner. It is noteworthy that different stimuli of astrocyte reactivity can lead to similar degrees of GFAP upregulation while causing substantially different changes in transcriptome profiles and cell function. Thus, it is not possible to equate simple and uniform measures such as cell hypertrophy and upregulation of GFAP expression with a single, uniform concept of astrocyte reactivity. Instead, it is necessary to recognize the considerable potential for heterogeneity and determine the functional implications of astrocyte reactivity in a context specific manner as regulated by specific signaling events.