Elsevier

Neurobiology of Disease

Volume 85, January 2016, Pages 234-244
Neurobiology of Disease

Review
Reactive astrocytes and therapeutic potential in focal ischemic stroke

https://doi.org/10.1016/j.nbd.2015.05.003Get rights and content

Abstract

Astrocytes are specialized and the most abundant cell type in the central nervous system (CNS). They play important roles in the physiology of the brain. Astrocytes are also critically involved in many CNS disorders including focal ischemic stroke, the leading cause of brain injury and death in patients. One of the prominent pathological features of a focal ischemic stroke is reactive astrogliosis and glial scar formation. Reactive astrogliosis is accompanied with changes in morphology, proliferation, and gene expression in the reactive astrocytes. This study provides an overview of the most recent advances in astrocytic Ca2 + signaling, spatial, and temporal dynamics of the morphology and proliferation of reactive astrocytes as well as signaling pathways involved in the reactive astrogliosis after ischemic stroke based on results from experimental studies performed in various animal models. This review also discusses the therapeutic potential of reactive astrocytes in focal ischemic stroke. As reactive astrocytes exhibit high plasticity, we suggest that modulation of local reactive astrocytes is a promising strategy for cell-based stroke therapy.

Introduction

Astrocytes are the most numerous glial cell type in the central nervous system (CNS). In a normal brain, there are two major types of astrocytes: fibrous and protoplasmic astrocytes. They can be found in white matter such as the corpus callosum and in grey matter such as the cortex. Glial fibrillary acidic protein (GFAP) is primarily expressed in the thick main processes and has been considered as a ‘pan-astrocyte’ marker (Brenner, 2014), but its expression levels are higher in the fibrous astrocytes than in the protoplasmic astrocytes. Transcriptome analysis found that the Aldh1L1gene is most widely and homogenously expressed in the astrocytes, while immunostaining with an anti-Aldh1L1 antibody revealed that the Aldh1L1 protein is highly expressed in the astrocyte cell body and its extensive processes (Cahoy et al., 2008). Thus, Aldh1L1 is now considered as a new ‘pan-astrocyte’ marker.

It has been long recognized that astrocytes play a critical role in the physiology and are very important for overall brain architecture as well as function. Astrocytes can maintain ionic homeostasis by acting as a potassium (K+) sink (Djukic et al., 2007). Astrocytes can remove synaptically released glutamate by their glutamate transporters to avoid glutamate excitotoxicity (Huang et al., 2004, Bergles et al., 1999). Astrocytes also mediate Ca2 + signaling and intercellular waves through the stimulation of different G-protein coupled receptors (GPCRs) via phospholipase-C/inositol 1,4,5-triphosphate (PLC/IP3) pathway in vivo. These GPCRs include metabotropic glutamate receptors (mGluRs) (Ding et al., 2007, Fellin et al., 2004, Sun et al., 2013), P2Y receptors (Ding et al., 2009, Thrane et al., 2012, Sun et al., 2013, Wang et al., 2006, Nizar et al., 2013), GABAB receptors (GABABRs) (Ding et al., 2009, Meier et al., 2008), noradrenergic receptors (Bekar et al., 2008, Ding et al., 2013, Paukert et al., 2014). Due to the intimate physical contact with synapses, astrocytes are considered as a part of the ‘tripartite’ synapse where they can listen and talk to the synapse by regulating Ca2 + increase in response to neuronally released transmitters and by gliotransmitter release (Haydon, 2001). We have also known that astrocytes and the blood vessels have intimate anatomic relationship. This was further confirmed by studies using fluorescence imaging and electron microscopy showing that astrocyte endfeet almost completely cover the cerebral vascular surface (Simard et al., 2003, Petzold et al., 2008, Mathiisen et al., 2010, Kacem et al., 1998). Astrocytic Ca2 + signaling is involved in functional hyperemia from in vitro studies of brain slice preparations (Zonta et al., 2003, Mulligan and MacVicar, 2004, Gordon et al., 2008); however, its role in the regulation of cerebral blood flow (CBF) in vivo is controversial as suggested by results from studies using type 2 IP3 receptor knockout mice (Jego et al., 2014, Takata et al., 2013, Nizar et al., 2013, Bonder and McCarthy, 2014).

Growing evidence indicates that astrocytes are heterogeneous in morphology, molecular expression, and physiological function under normal conditions (Zhang and Barres, 2010, Matyash and Kettenmann, 2010). Morphologically, protoplasmic astrocytes, and fibrous astrocytes are different. Protoplasmic astrocytes are complex (sponge like) and highly branched with numerous fine processes and their endfeet wrap around blood vessels, while fibrous astrocytes are less complex and have thicker and less branched processes (Wilhelmsson et al., 2006, Bushong et al., 2002). Numerous studies have found that different genes are expressed among different subsets of astrocytes in vivo (Zhang and Barres, 2010). GFAP expression is higher in the astrocytes in corpus callosum, but it is expressed in astrocytes in the cortex at lower levels (Xie et al., 2010). Electrophysiologically, astrocytes exhibit a different current–voltage relationship with one type of astrocytes, known as outward rectifying astrocytes, when compared to the other known as variably rectifying astrocytes (Zhou and Kimelberg, 2000). Astrocytes also exhibit different properties of Ca2 + signaling in vivo. Two-photon (2-P) in vivo Ca2 + imaging has shown that astrocytes in the cortical layer 1 (L1) nearly doubled the Ca2 + activity compared to the astrocytes in L2/3 in anaesthetized rats; moreover, Ca2 + signals in the processes in the same astrocyte were asynchronous in L1 while those in L2/3 were more synchronous (Takata and Hirase, 2008). The morphological, molecular, and functional heterogeneity of astrocytes indicates a diversity among astrocytes and the complex physiological and pathological roles that astrocytes play in the CNS.

Astrocytes respond to different neurological diseases through a common phenomenon of GFAP upregulation, a process termed as reactive astrogliosis. Severe CNS injuries such as stroke, traumatic brain injury (TBI), and spinal cord injury (SCI), as well as neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease, and amyotrophic lateral sclerosis (ALS) all cause a massive upregulation of GFAP. Therefore, GFAP is considered a reliable marker to characterize reactive astrocytes. However, given the different causes and onsets of diseases, the temporal and spatial changes of the reactive astrocytes are different. For example, in the AD brain, due to slow disease progression, the reactive astrocytes are more evenly distributed and do not form glial scars. While after ischemic stroke or SCI, reactive astrocytes in the peri-infarct region express higher GFAP and eventually form glial scar, which establishes both a physical and biochemical barrier that separates dead and vital tissues. Thus, the properties of reactive astrocytes in chronic neurodegenerative diseases are different from those seen in acute conditions like focal ischemia and SCI. Although similar phenomena, such as glial scar formation, is observed in both focal ischemia and SCI, in experimental animal models of SCI, the injury occurs in the large area of white matter rather than in grey matter as seen in ischemic strokes. The functions and role of reactive astrocytes have been much more extensively studied in SCI than in the focal ischemic stroke (for reviews, see Burda and Sofroniew, 2014, Sofroniew, 2009, Sofroniew and Vinters, 2010, Silver and Miller, 2004, Rolls et al., 2009). Thus, this article will review the dynamic changes in astrocytic Ca2 + signaling, morphology, and proliferation of reactive astrocytes. The article also examines the distribution of reactive astrocytes surrounding the ischemic core, i.e., in the penumbra, in experimental animal models of focal ischemic stroke. Discussion then focuses on the signaling pathways involved in reactive astrogliosis after focal ischemia followed by the therapeutic potential of reactive astrocytes in ischemic stroke. For extensive reviews of reactive astrocytes in various aspects in different neurological diseases, readers are advised to consult a few detailed reviews (Burda and Sofroniew, 2014, Sofroniew and Vinters, 2010, Escartin and Bonvento, 2008, Anderson et al., 2014).

Section snippets

Dynamics of reactive astrocytes in the penumbra after focal ischemia

Focal ischemic stroke, resulting from the blockage of cerebral blood vessels, leads to cell death and brain damage and causes human disability and death (Stapf and Mohr, 2002). After the onset of ischemia, astrocytes undergo numerous pathological alterations over time, including rapid swelling (Nedergaard and Dirnagl, 2005, Barber and Demchuk, 2003, Swanson et al., 2004, Li et al., 2014) and enhanced Ca2 + signaling (Ding et al., 2009). Astrocytes can also become reactive following ischemia. The

Reactive astrogliosis and behavioral recovery

After ischemia, the brain undergoes spontaneous recovery with improvement in behavioral deficits over time (Badan et al., 2003, Li et al., 2004, Clarkson et al., 2013). Focal ischemia-induced reactive astrocytes exhibit heterogeneity in morphology, GFAP expression, and proliferation capability in a spatiotemporally dependent manner. This can be seen in Fig. 2 where GFAP expression levels and proliferation capability eventually decrease after the first 4-day acute phase in a spatiotemporally

Signaling pathways of reactive astrogliosis after ischemia

Over the last two decades, advancements in genetics and molecular biology have equipped researchers with unique tools, which have resulted in an extensive characterization of reactive astrocytes following CNS injury. Studies have identified a plethora of genes and molecular markers for reactive astrocytes, and the list still continues to grow (Ridet et al., 1997, Zamanian et al., 2012, Colangelo et al., 2014). Moreover, many studies have provided clues that have shed some light on key signaling

Therapeutic potential of the reactive astrocytes in stroke

Despite tremendous efforts and advances in translational research, the treatment strategies for stroke are still limited. Tissue plasminogen activator (tPA) is the only FDA approved drug currently available for acute stroke treatment, but it is only effective within a narrow therapeutic window, that is, within a few hours after a stroke. Beyond this window, treatment of a stroke is primarily dependent on supportive care, secondary prevention, and rehabilitation. Manipulation of the functional

Conclusions

Current neuron-centric strategies have not resulted in major breakthroughs in stroke therapy. Reactive astrogliosis is one of the most prominent pathological features in strokes and is an adaptive defense response that is both beneficial and detrimental to the injured CNS. Especially during the early time after ischemic injury, the main function of reactive astrocytes is to preserve integrity of the nervous tissue. However, with time, the process becomes increasingly unregulated and

Acknowledgments

This work was supported by the National Institutes of Health [R01NS069726] and the American Heart Association Grant in Aid [13GRNT17020004] to SD.

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