Astrocyte Reactivity Characterized with Monoclonal Antibody J1-31: An Evaluation of cAMP Effectors
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Apart from the embryonic stage, regeneration of the central nervous system (CNS) following injury in higher vertebrates is limited if not precluded entirely. This inability of neurons in the CNS to regenerate is at least partially attributed to the transition of astrocytes, a supportive cell type in the CNS, to a reactive state in response to injury, which may lead to the formation of a glial scar. The most well characterized indicator of reactive astrocytes is increased labeling of glial fibrillary acidic protein (GFAP), a structural component of the astrocytic cytoskeleton (Faulkner et al. 2004, Hausmann et al. 2000, Geisert et al. 1990, Tetzlaff 1988, among others). Interestingly, when GFAP and another cytoskeletal protein, vimentin, were knocked out in a mouse model, regenerative capabilities were evident after spinal cord injury (Menet et al. 2003). This study has given researchers the notion that cytoskeletal elements may be one of the components responsible for the lack of regeneration in mammals after CNS insults.
Astrocytes have a broad range of functions in the CNS, although the role of reactive astrocytes, whether positive or negative with respect to neuronal growth/regrowth, has been debated among researchers for years. Selectively abolishing proliferative reactive astrocytes altogether has a negative effect (Sofroniew et al. 2005, Myer et al. 2006), with degeneration seen far past the injury site. Reactive astrocytes normally demarcate the injury site, ensuring degeneration only occurs within the restricted area. However, highly reactive astrocytes begin forming a glial scar, which presents a fibrous network impeding axonal regeneration, remyelination, or both, following insults to the CNS.
The dual role, both friend and foe, of these reactive cells piques an interest not in determining how to rid systems of reactive astrocytes entirely, but rather in targeting molecules and proteins involved in the scar-producing aspect of reactivity without inhibiting the positive role that reactive astrocytes may play. Cyclic adenosine monophosphate (cAMP) is a secondary messenger that has been shown to increase GFAP expression and stellated morphology in several studies of astrocytes (Lee et al. 1997, Fedoroff et al. 1984). In order to determine the downstream effector(s) of cAMP involved in the transition to reactivity, specific activators and/or inhibitors of downstream pathways were utilized in this study. The marker of reactivity was increased immunolabeling of monoclonal antibody J1-31 (mAb J1-31), an antibody raised against multiple sclerotic plaques, which recognizes cytoskeletal elements that are upregulated in reactive astrocytes (Malhotra et al. 1989, Malhotra et al. 1993, Predy et al. 1988).
Treatments activating the guanine nucleotide exchange factor (GEF) directly activated by cAMP, and not the protein kinase A pathway, revealed phenotypes and immunolabeling patterns of mAb J1-31 characteristic of reactive astrocytes. The GEF activated by cAMP is named Exchange protein activated by cAMP (Epac). Additionally, we observed partial, but not complete, colocalization of the J1-31 antigen and GFAP. We thus conclude that labeling of the J1-31 antigen increases in response to the cAMP dependent GEF stimulation, and that the J1-31 antigen is most likely a variant of GFAP as well as other cytoskeletal components.