Neuroscience Letters 565 (2014) 59–64 Contents lists available at ScienceDirect Neuroscience Letters journal homepage: www.elsevier.com/locate/neulet Mini review Astrogliosis as a therapeutic target for neurodegenerative diseases夽 Anna Maria Colangelo a,c,∗ , Lilia Alberghina a,c , Michele Papa b,c a Laboratory of Neuroscience “R. Levi-Montalcini”, Department of Biotechnology and Bioscience, University of Milano-Bicocca, Milano, Italy Laboratory of Morfology of Neural Networks, Department of Medicina Pubblica Clinica e Preventiva, Second University of Napoli, Napoli, Italy c SYSBIO Centre of Systems Biology, University of Milano-Bicocca, Milano, Italy b h i g h l i g h t s • • • • We briefly review the role of astrocytes in brain function. We review the main pathways in astrocytic dysfunction. We focus on astroglial-specific targets for clinical management of neurodegenerative disorders. We describe the usefulness of a systems biology approach in drug discovery for neurodegenerative disorders. a r t i c l e i n f o Article history: Received 22 September 2013 Received in revised form 8 January 2014 Accepted 13 January 2014 Keywords: Reactive astrogliosis Brain homeostasis Neurotrophins Neuroprotection Systems biology a b s t r a c t Chronic neurodegenerative diseases represent major unmet needs for therapeutic interventions. Recently, the neurocentric view of brain function and disease has been challenged by a great number of evidence supporting the physiopathological potential of neuroglia. Astrocytes, in particular, play a pivotal role in brain homeostasis as they actively participate in neuronal metabolism, synaptic plasticity and neuroprotection. Furthermore, they are intrinsic components of brain responses to toxic and traumatic insults through complex processes involving several molecular and functional alterations that may lead to disruption of brain homeostasis and connectivity. This review provides a brief overview of current knowledge of astrocyte functions in the brain, and focuses on some glial-specific pathways involved in astrocytic dysfunction that might be effective therapeutic targets for clinical management of neurodegenerative disorders. © 2014 The Authors. Published by Elsevier Ireland Ltd. All rights reserved. Contents 1. 2. 3. 4. 5. 6. 7. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Astrocytes in synaptic function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Astrogliosis and neurological disorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactive astrocytes as potential therapeutic targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cell-based therapeutic strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Targeting inflammation-related signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glutamate metabolism and excitotoxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 60 60 60 61 61 62 Abbreviations: AD, Alzheimer’s disease; ALS, amyotrophic lateral sclerosis; ATP, adenosine triphosphate; BB14® , nerve growth factor-like peptide; BDNF, brain-derived neurotrophic factor; Ca2+ , calcium; CNS, central nervous system; COX-1/2, cyclooxygenase-1/2; EAAC1, excitatory amino acid carrier 1; EAAT1/2, excitatory amino acid transporter 1/2; EAE, experimental autoimmune encephalitis; GABA, ␥-aminobutyric acid; GFAP, glial fibrillary acidic protein; GLAST, glutamate-aspartate transporter; GLT-1, glutamate transporter; GlyT-1, glycine transporter; GSH, glutathione; HD, Huntington’s disease; Iba1, ionized calcium binding adaptor molecule 1; IP3, inositol triphosphate; i.t., intrathecal; JNK, jun N-terminal kinase; LPS, lipopolysaccharides; MAPK, mitogen-activated protein kinase; MMPs, metalloproteinases; MS, multiple sclerosis; NGF, nerve growth factor; NMDA, N-methyl-d-aspartate; NO, nitric oxide; PD, Parkinson’s disease; PI3K, phosphatidylinositol-3 kinase; PLC␥, phospholipase C-␥; SCI, spinal cord injury; STAT3, signal transducer and activator of transcription 3; TNF␣, tumor necrosis factor ␣; vGLUT1, vesicular glutamate transporter; vGAT, vesicular GABA transporter. 夽 This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ∗ Corresponding author at: Laboratory of Neuroscience “R. Levi-Montalcini”, Department of Biotechnology and Biosciences, University of Milano-Bicocca, piazza della Scienza 4, 20126 Milano, Italy. Tel.: +39 02 6448 3536; fax: +39 02 6448 3519. E-mail address: annamaria.colangelo@unimib.it (A.M. Colangelo). 0304-3940/$ – see front matter © 2014 The Authors. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.neulet.2014.01.014 60 A.M. Colangelo et al. / Neuroscience Letters 565 (2014) 59–64 8. Targeting astrogliosis by neurotrophins and neurotrophin-derived drugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A systems biology approach in drug discovery for neuro-glial network repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. 10. Conclusions and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 62 63 63 63 1. Introduction 3. Astrogliosis and neurological disorders Astrocytes are multifunctional cells that might well be considered the “cornerstone” of brain cytoarchitecture and function. In addition to their trophic and structural role, astrocytes are dynamic components of brain connectivity and function. The concept of “tripartite synapse” encompasses the fundamental role of astrocytes in synaptic function and plasticity, which involves maintenance of homeostatic balance of neurotransmitters and ions, as well as regulation of blood flow and neuronal energy metabolism. In addition, they participate to brain responses to toxic and traumatic insults through a complex process (reactive astrogliosis) that involves morphological and functional changes including hypertrophy, upregulation of intermediate filaments, such as GFAP, and increased proliferation [35,40]. Reactive astrocytes also release cytokines and many other factors that mediate inflammatory responses and remodeling processes [24], thus playing both beneficial and detrimental roles in brain pathology. In this review we will highlight how astrocytes alterations during neuroinflammation influence brain integrity, thus compromising synaptic homeostasis and function. We will then focus on molecular mechanisms of astrogliosis that can be potential targets for drug development to restore brain function in neurodegenerative diseases. Over the last two decades, the neurocentric view of brain function and disease has been challenged by the emerging evidence of the physiopathological potential of neuroglia [24,40]. Current knowledge of neuropathological processes indicates that early stages of disease are associated with the activation of common inflammatory pathways, involving microglia and astrocytes activation, and the release of pro-inflammatory cytokines and other inflammatory mediators that regulate astrocytic hypertrophy and proliferation. Evidence of neuroinflammatory processes has been found in several neurological disorders, such as ischemia, spinal cord injury (SCI) and psychiatric/mood disorders [34,42], as well as in neurodegenerative diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), spinocerebellar ataxia type 1 (SCA1), Huntington’s disease (HD) and multiple sclerosis (MS) [revised in Ref. [24]]. Our recent studies also indicate that glial activation after peripheral nerve injury is an important component of the neuropathic pain syndrome, as revealed by microglial activation and increased GFAP expression in the spinal cord [11–14] and supraspinal level [28]. Biochemical and structural changes of astrocytes during neuroinflammation represent a physiological response to CNS injury to minimize and repair the initial damage. Nevertheless, sustained inflammatory responses might be driven by positive feedback loops between microglia and astrocytes (Fig. 1) under conditions of severe and/or prolonged brain insults, thus providing detrimental signals that can compromise astrocytic and neuronal functions and lead to chronic neuroinflammation [24,40]. Accordingly, in our model of peripheral nerve injury, glial activation was paralleled by several modifications including: (i) decreased GLT-1 levels; (ii) alteration of vesicular transporters of glutamate (vGLUT) and GABA (vGAT); (iii) a functional block of the xCT system, most likely triggered by extracellular glutamate accumulation; (iv) decreased GSH content; and (v) alteration of nerve growth factor (NGF) and neurotrophin receptors levels [11–13]. Modifications of synaptic components appeared to be sustained by a calpain I-dependent process, while alteration of pro-NGF/NGF ratio was functionally related to activation of matrix metalloproteinases (MMPs) (Fig. 1). Similar biochemical alterations have been observed in other neurological models of diseases and suggest a general failure of normal astrocytic functions in brain homeostasis and neuroprotection, thus determining mechanisms of “maladaptive plasticity” [12,13,15,28]. 2. Astrocytes in synaptic function At synaptic level, astrocytes express functional neurotransmitter receptors (Fig. 1) and release several neurotransmitters, including glutamate, GABA, ATP and d-serine through Ca2+ dependent exocytosis, thereby modulating synaptic strength and efficacy at excitatory and inhibitory synapses. Moreover, astrocytes possess neurotransmitter transporters for glutamate (EAAT1 and EAAT2, in rodents known as GLAST and GLT-1) (Fig. 1), GABA and glycine, thus playing a crucial role in neurotransmitter homeostasis. At glutamatergic synapses, astrocytic glutamate uptake is the main route for glutamate removal from synaptic cleft to avoid its potential excitotoxicity [38]. Recovery of glutamate also occurs through vesicular glutamate transporters (vGLUT) [33] and the glutamate-glutamine shuttle system (Fig. 1): glutamate is converted to the non-toxic glutamine and shuttled back to presynaptic neurons to reconstitute neurotransmitter pools. The cystine-glutamate antiporter system (xCT) [26] is also functionally related to the role of astrocytes in regulating extracellular glutamate and supplying cystine for neuronal GSH homeostasis. Relevant to both synaptic transmission and neuroprotection is then the control of K+ homeostasis by inward rectifying K+ channels (Kir-channels) (Fig. 1). Other crucial functions of astrocytes include blood–brain barrier (BBB) formation and metabolic support: through their endfeet contacting capillary endothelial cells, astrocytes are active components of the BBB properties and regulate cerebral blood flow and metabolic supply in response to neuronal activity (neurometabolic coupling) [27]. Metabolic support to neurons is provided through the astrocyte-neuron lactate shuttle: lactate is released by astrocytes and taken up into neurons for their energy metabolism after its conversion to pyruvate (Fig. 1). 4. Reactive astrocytes as potential therapeutic targets Based on the pathogenetic role of astrocytic dysfunction, the strategy to restore or enhance normal astrocytic functions might be an appealing way to promote neuroprotection in a variety of brain disorders. Current understanding of the precise contribution of reactive astrogliosis in disease progression has been fostered by the development of transgenic animals enabling the manipulation of astrocyte-specific proteins and pathways. Interestingly, different types of transgenic models indicate that the overall impact of reactive astrogliosis is beneficial and neuroprotective, in particular A.M. Colangelo et al. / Neuroscience Letters 565 (2014) 59–64 61 Fig. 1. Schematic representation of a tripartite synapse showing some of the pharmacological targets in astrocytic dysfunction: molecular alterations induced by cytokines (antinflammatory drugs); excitotoxicity (riluzole); excessive purinergic receptors stimulation (oATP); alteration of NGF synthesis/degradation (BB14 and MMPs inhibition). Basic biochemical pathways of astrocytes in synaptic function are also displayed. Details about these mechanisms can be found in the text and in other reviews of this special issue. during early stages of disease. In fact, conditional ablation of proliferating astrocytes leads to increased inflammation and increased neuronal vulnerability in models of SCI and experimental autoimmune encephalitis (EAE) [40]. Nevertheless, astrocytic function in neuroprotection is greatly compromised during chronic neuroinflammation. New perspectives for therapeutic approaches include the replacement of dysfunctional astrocytes or pharmacological treatments that specifically target detrimental signaling pathways while preserving their neuroprotective functions. 5. Cell-based therapeutic strategies Different innovative approaches have been developed in regenerative medicine to promote neuroprotection. For instance, transplantation of mouse or human astrocytes derived from glial-restricted progenitors (GRP) was found to promote axonal regeneration and functional recovery following SCI. Other studies also showed that mesenchimal stem cells injected into the spinal cord were able to migrate to the lesioned area, differentiate into astrocytes and exert neuroprotection by reducing microglial activation and normalizing GLT-1 levels [25]. Cell grafting strategies have been successfully employed in different models of neurodegenerative diseases, such as PD, ALS and HD. For instance, grafting of astrocytes overexpressing brain-derived neurotrophic factor (BDNF) under control of the GFAP promoter was found to exert neuroprotection in a model of HD [23]. Recently, the possibility to obtain astrocytes by reprogramming induced pluripotent stem cells may also represent a new strategy for the replacement of damaged astrocytes, although further experimental studies are needed to evaluate therapeutic and side effects of cell therapy. 6. Targeting inflammation-related signaling Inflammatory processes are the hallmark of both acute and chronic neurodegenerative diseases. Glial reaction involves activation of receptors, such as Toll-like receptors (TLR), transcription factors (NF-kB, Nrf2, AP-1, etc.) and signaling molecules (p38MAPK, JNK, JAK/STAT3, etc.) of common inflammatory pathways (Fig. 1), as well as alteration of protein expression (GFAP, vimentin, aminoacid transporters, receptors, etc.) and enzymes, like cycloxygenases (COX2), nitric oxide synthase (NOS), MMPs, etc. [revised in Ref. [24]]. For instance, TLRs are associated with the activation of microglia and astrocytes by amyloid-␤ and the downstream inflammatory response in AD models. Therefore, molecules that antagonize or modulate sensors (TLR, etc.), transducers (NF-kB, AP-1, etc.) and effectors (TNF-␣, COX-2, etc.) of the inflammatory response may represent a new prospective for disease-modifying therapies. Besides the novel concept of a peripheral origin of neuroinflammation based on bi-directional gut-brain communication [29], evidence of the role of inflammation in neurodegenerative processes is provided by epidemiological studies showing that elevated plasma concentrations of TNF-␣, IL-6 and other cytokines correlate with increased PD risk [10]; whereas, chronic use of antiinflammatory drugs (like NSAIDs) reduces the risk of PD and AD [16]. Besides inhibiting COX-1/2 activity and prostaglandin production, other molecular targets of NSAIDs include NF-␬B, AP-1, NOS, PPAR and secretases [4]. Nuclear translocation of NF-␬B in reactive astrocytes was shown to mediate glial proliferation and inflammatory responses (Fig. 1). Indeed, mice expressing a constitutively active form of I␬B␣ under control of the GFAP promoter exhibit reduced inflammation and increased functional recovery following SCI or EAE [6]. Efficient inhibition of NF-␬B can also be achieved by antioxidant molecules, like flavonoids. Many experimental studies are currently testing the beneficial effects of antioxidants, based on the hypothesis that prooxidant environments, such as the inflammatory milieu, can favor the formation of toxic protein aggregates of amyloid-␤, SOD1, etc. The JAK/Stat pathway (Fig. 1) is another transducer of inflammatory signals mediated by growth factors and cytokines (IL-6, CNTF, EGF and TGF-␣): genetic deletion or inhibition of STAT3 was able to reduce all aspects of reactive astrogliosis (upregulation of GFAP, astrocytes proliferation and migration); however, as a result of impaired glial scar formation, mice with conditional 62 A.M. Colangelo et al. / Neuroscience Letters 565 (2014) 59–64 deletion (Cre–loxP system) of STAT3 in astrocytes displayed increased inflammation [34]. JNK and p38MAPK signaling pathways (Fig. 1) were also found to be relevant to reactive gliosis in response to a variety of cytokines and pathogenetic stimuli. Several MAPK inhibitors have been characterized in vitro and in animal models. Among them, the BIRB-796-BS molecule developed by Boehringer Ingelheim Pharmaceuticals was found to prevent LPS-induced cytokines production [7], whereas SB203580 reduced the expression of iNOS, TNF-␣, IL-1␤ and COX2 in a model of focal ischemia by middle cerebral artery occlusion resulting in reduced infarct volume and neurological deficits [36]. Molecular mediators and receptors of inflammatory processes are also the therapeutic target of all drugs currently used in MS (humanized antibodies against ␣4␤1 integrin, CD20 Blymphocytes and TNF-␣) to reduce neuroinflammation and the abnormal activity of the immune system. Interestingly, the antiinflammatory activity of ␤-lactam antibiotics also seems to involve the regulation of astrocytic function: ceftriaxone was found to exert neuroprotection in animal models of ALS by stimulating the expression of GLT-1 [39]. 7. Glutamate metabolism and excitotoxicity The scavenging activity of astrocytes is crucial in regulating excessive levels of glutamate, K+ and other ions. GLT-1 levels decline in different models of neurodegenerative diseases, such as AD, ALS and HD, suggesting a correlation between reactive gliosis and accumulation of excitotoxic levels of glutamate [39]. Loss or dysfunction of astrocytic glutamate transporters were also found in sporadic forms of ALS and HD patients at early stages of the disease [37]. Currently, riluzole is the only FDA-approved drug for the treatment of ALS, though it prolongs the life of ALS patients by only 3 months [30]. A major action of riluzole is the inhibition of glutamate release from presynaptic neurons, but it also blocks glutamate activity on postsynaptic terminals by non-competitive blockade of NMDA receptors (Fig. 1). Recently, riluzole was also found to enhance astrocytic glutamate uptake by upregulating GLT-1 levels and activity [9]. Interestingly, however, neuroprotection by riluzole seems to be also related to its capability to stimulate astrocytic synthesis of NGF, BDNF and GDNF [32]. Increased levels of BDNF and TGF-1␤ were also detected in the serum of patients treated with riluzole [41]. Excitotoxic levels of glutamate during neuroinflammation are also contributed by astrocytes through Ca2+ -dependent secretory pathways following intracellular [Ca2+ ] rise (Fig. 1). Several astrocytes receptors are coupled to Ca2+ -dependent signalings that can lead to glutamate release: prostaglandins (like PGE2 ), purinergic receptors (Fig. 1), bradykinin and other GPCRs acting through Ca2+ mobilization from IP3R-sensitive intracellular stores [1]. In addition to its role as energy source, ATP is important in modulating glia-glia and glia-neuron communication [revised in Ref. [21]]. Purinergic receptor stimulation by ATP (released from damaged cells) can modulate several mechanisms of neuroinflammatory pathways, such as synthesis of cytokines, upregulation of COX-2, production of PGE2 , proliferation, glutamate and GABA release, impairment of glutamate uptake, decreased expression/activity of glutamine synthetase (GS), increase of iNOS activity and NO production (Fig. 1). Therefore, these receptors might be well considered a functional link between inflammatory responses and altered glutamate and GABA transmission through the activation of multiple intracellular pathways including GPCRs/IP3R/DAG/PKC, GPCRs/AC/cAMP/PKA, MAPK, JNK or p38MAPK. Although the many activities of both ionotropic (P2X) and metabotropic (P2Y) receptors is not fully understood, purinergic receptors are important modulators of reactive gliosis, in particular in chronic neurodegenerative pathologies, thus representing a useful therapeutic target to modulate glial reaction and disease progression. 8. Targeting astrogliosis by neurotrophins and neurotrophin-derived drugs In addition to their crucial role in synaptic function and inflammatory responses, astrocytes provide trophic and metabolic support. Among neurotrophin family members, NGF and BDNF are the most relevant molecules in brain development and function, and it is generally recognized that their age-related decreased availability play a crucial role in the pathophysiology of neurodegenerative disorders. All neurotrophins are secreted on demand by astrocytes as proneurotrophins, proteolytically cleaved to mature neurotrophins in the synaptic cleft and rapidly degraded (Fig. 1). Precursors and mature neurotrophins can differently interact with tyrosine kinase (Trk) and p75NTR receptors, which can have opposing actions in modulating neuron death/survival, synaptic plasticity and glial proliferation [17,20]. Thus, neurotrophin activity is essentially linked to a dynamic balance between their maturation and degradation processes: NGF/pro-NGF ratio, for instance, is regulated by the tissue plasminogen activator (tPA)-plasmin-MMP-9 system (Fig. 1) [19]. Alteration of neurotrophin maturation/degradation processes and increased proNGF and proBDNF levels are believed to trigger neuronal vulnerability in several neurodegenerative diseases, such as AD and ALS, by a p75-mediated mechanism [19,20]. On the other hand, NGF interaction with p75 was shown to reduce astrocytic proliferation (Fig. 1) [17], thus exhibiting an anti-gliosis activity. We also found that following peripheral nerve injury, glial activation and synaptic changes were paralleled by increased MMPs activity and altered NGF/pro-NGF and receptors levels [11–13]. Interestingly, these alterations were reversed by i.t. administration of NGF or GM6001 (Fig. 1), a generic MMP inhibitor [11–14]. Overall, these findings are consistent with a model in which NGF and other neurotrophins are essential in maintaining synaptic homeostasis and neuroprotection by establishing a strict correlation between astrocytic dysfunction and aberrant balance between NGF synthesis and degradation [15]. Furthermore, it should be recalled the essential role of NGF in supporting neuronal survival by modulating mitochondrial function (Fig. 1) [5]. Recently, small molecules activating specific neurotrophin receptors represent a valid alternative to the poor pharmacokinetic properties of neurotrophin-based therapies. This strategy has been employed to construct small functional mimetics of NGF, BDNF and NT-3 endowed with agonist or antagonist activity for Trks or p75 receptors [8,43]. Some of these molecules were found to be effective in models of cholinergic dysfunction, mood disorders and glaucoma [8]. We recently developed the NGF-like peptide, BB14, which displayed a strong TrkA agonist activity both in vitro and in models of peripheral nerve injury. Moreover, BB14 reduced reactive gliosis and neuropathic behavior to same extent as the native NGF molecule, suggesting that it might be suitable for therapeutic applications in neurological conditions characterized by reactive gliosis [13–15]. 9. A systems biology approach in drug discovery for neuro-glial network repair The functional link between all components of reactive gliosis (excitotoxicity, decreased neuronal metabolism and antioxidant properties, alteration of neurotrophins metabolism/signaling, and A.M. Colangelo et al. / Neuroscience Letters 565 (2014) 59–64 impaired synaptic plasticity) is indicative of the impact that alterations of the complex neuro-glial network have on disease progression. This implies that effective neuroprotection of the whole system should take into account both the complex intracellular crosstalks (neuronal, astroglial, etc.) and how they influence proper reciprocal interactions, communication and function. A comprehensive understanding of mechanisms contributed by astrocytes appears to be relevant for development of targeted therapies for clinical management of neurodegenerative disorders. Given the complexity of neuro-glial networks in CNS homeostasis and function, a systems biology approach of neurodegeneration [2,18] might be necessary to obtain a whole picture of the glia-neuron interplay in synaptic function, and how changes of metabolic fluxes might influence the complex interactions between neuronal and glial compartments and lead to the structural/functional modifications underlying neuro-glial rearrangements during the degeneration process. As it is well known, systems biology considers the function of complex biological processes as system-level properties generated by the interactions of large molecular networks that underlie each given biological function, which is investigated by integration of molecular analyses and computational methods [3]. The first step, therefore, is to identify the molecular network that underlies neurodegeneration. Two recent reports have opened the way by making publicly available maps of pathways for AD and PD [22,31]. At the moment, the AD map is composed by 1347 molecules and 1070 reactions in neurons, BBB, pre-synaptic and post-synaptic neurons, astrocytes and microglial cells. The PD map currently has 2285 elements and 989 reactions: it is restricted to dopaminergic neurons in the sustantia nigra, whose progressive degeneration heavily contributes to the more relevant clinical motor symptoms of PD. Both AD and PD maps are open to the research community for utilization and improvement. Key questions raised by these maps are: (1) how to identify, among the intricate web of thousand molecules, the crucial steps that are essential in supporting the insurgence of the pathology; and (2) how to discriminate the one or few molecular targets that, when hit by a drug, will substantially and permanently ameliorate the clinical condition of the patients. A similar problem is faced for cancer and attempts are made to disassemble the phenotype of cancer cells in a number of systemlevel properties and then to apply multilevel modeling [18] to investigate for each property (or function) its general organizing principles. The next step is to focus on the molecular level trying to construct dynamic molecular mathematical models able to clarify the mechanism of any given specific feature [3]. The availability of AD and PD maps may facilitate this approach for a system-level analysis of neurodegeneration which, in the view presented in this paper, will consider as very relevant the role of astrogliosis both in preventing and, above a threshold level, in promoting neurodegeneration. 10. Conclusions and future perspectives Molecular dissection of neurological and neurodegenerative disorders strongly indicates that reactive astrogliosis is an intrinsic component of the complex mechanisms that, initially meant to be protective against toxic stimuli, above a threshold level turn into maladaptive responses that can compromise synaptic homeostasis and function, thus amplifying the pathology. Based on this knowledge, it is intuitive that effective neuroprotection might be achieved by preventing astrocytic dysfunction and their detrimental responses, at the same time preserving their protective and trophic/metabolic functions. Although many glial pathways have been identified as potential therapeutic targets, it is clear that more 63 work is needed to obtain a comprehensive understanding of the entire process, crucial steps and specific time windows for effective therapeutic interventions. The drug discovery process might be fostered by a systems biology approach to disassemble the astrogliosis phenotype in its components and apply multilevel modeling to identify key molecular targets. At the same time, given that celltherapy strategies to replace de-regulated/dysfunctional astrocytes by transplantation still hold great limitations, more efforts should be made to apply novel biotechnological approaches to translate current knowledge of astrocytes into effective therapeutic molecules. 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