In response to neural pathologies, glia display reactive properties associated with wound healing including cellular hypertrophy, proliferation, migration and cytokine release [1–4]. In mammalian CNS and retina, reactive glia contribute to neural tissue repair [5–10] but also to neural dysfunction, scar formation, abberant neural rewiring, and vascular remodeling [1–3, 11, 12], ultimately exacerbating neuronal degenerations [11, 13]. Defining the components of reactive gliosis that are detrimental to neuronal survival and tissue integrity is an important goal but difficult to achieve. Animal models of reactive gliosis also induce neuronal cell death, microglial reactivity, inflammatory responses or tissue damage [1, 3, 4, 14]. An alternate approach to explore glial reactivity and neuronal metabolism, physiology and function is to develop genetically inducible models of reactivity in the absence of gross degenerative cues.
Two hallmarks of reactive glia are proliferation and enhanced intermediate filament expression. Both are associated with opposing properties: neuroprotection and degeneration. Experimental models and gene inactivation studies implicate upregulation of intermediate filament expression in the formation of hypertrophic glial processes. Glial hypertrophy helps maintain the structural integrity of the CNS by filling the space where neurons die and by restoring damaged protective barriers [9, 15, 16]. However, intermediate filaments are abundant in glial scars which are known to impede axonal regeneration [17, 18]. Chronic upregulation of intermediate filament expression is also correlated with glial metabolic dysfunction and altered neuronal electrophysiology [12, 19–21]. The role of glial proliferation is similarly perplexing. Genetic ablation of proliferating glia worsens neurodegeneration [5, 6] while pharmacological inhibition of glial proliferation enhances neuronal survival and function [14, 22]. Given these complexities, more precise dissections of the links between glial reactivity and progressive neurodegeneration are needed.
The cyclin-dependent kinase inhibitor p27 is one such link. It is expressed in many adult glial populations including Schwann cells, cortical astrocytes, spinal cord astrocytes, oligodendrocytes, and retinal Müller glia [23–27]. In germline p27-deficient mice (p27
), adult glia can display hallmarks of reactive gliosis [24–26, 28]. In the wild-type retina, quiescent Müller glia normally do not express the intermediate filament glial fibrillary acidic protein (GFAP), but Müller glia in p27
mice express high levels of GFAP and in some instances migrate into the subretinal space [24, 26]. This behavior is enhanced by the combinatorial inactivation of p27 and the cyclin-dependent kinase inhibitor p19
. Müller glial reactivity and abnormal retinal electrophysiology in p27
mice may partly arise from developmental dysregulation as p27 is critical for neural development and glial differentiation [24, 26, 30–33]. Even so, CNS and retinal trauma models support a role for p27 in maintaining mature glial cells in a quiescent, supportive state. After acute trauma, cortical astrocytes, spinal cord astrocytes and retinal Müller glia downregulate p27, upregulate GFAP and re-enter the cell cycle [14, 25, 27, 34]. Thus p27 appears to be a negative regulator of two classic indices of reactive glia: GFAP upregulation and proliferation. This implies that selective inactivation of p27 could trigger neural remodeling and reprogramming defects in an otherwise normal milieu.
To modulate discrete reactivity indices in the absence of other degenerative stimuli, we induced intermediate filament GFAP upregulation, migration, and proliferation in adult Müller glia by inactivating p27 using a tamoxifen-regulated, Cre-loxP system [35, 36]. This approach bypassed the developmental requirement for p27 [24, 26] as well as the complexities and broad effects of experimentally induced degeneration [1, 3, 4, 14]. To address the significance of enhanced discrete reactivity on neuronal survival and function, we surveyed metabolism, retinal electrophysiology, and visual acuity. Contrary to our expectations, proliferative and GFAP-expressing Müller glia did not significantly impair retinal metabolism, electrophysiology, or visual function. Thus, our genetic model and the p27 pathway offer a new platform to explore how environmental factors involved in neuronal cell stress, microglial activation, inflammatory responses, or blood barrier damage contribute to the transition of resident glia from a supportive to detrimental state.