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      Neuroinflammation as Fuel for Axonal Regeneration in the Injured Vertebrate Central Nervous System

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          Abstract

          Damage to the central nervous system (CNS) is one of the leading causes of morbidity and mortality in elderly, as repair after lesions or neurodegenerative disease usually fails because of the limited capacity of CNS regeneration. The causes underlying this limited regenerative potential are multifactorial, but one critical aspect is neuroinflammation. Although classically considered as harmful, it is now becoming increasingly clear that inflammation can also promote regeneration, if the appropriate context is provided. Here, we review the current knowledge on how acute inflammation is intertwined with axonal regeneration, an important component of CNS repair. After optic nerve or spinal cord injury, inflammatory stimulation and/or modification greatly improve the regenerative outcome in rodents. Moreover, the hypothesis of a beneficial role of inflammation is further supported by evidence from adult zebrafish, which possess the remarkable capability to repair CNS lesions and even restore functionality. Lastly, we shed light on the impact of aging processes on the regenerative capacity in the CNS of mammals and zebrafish. As aging not only affects the CNS, but also the immune system, the regeneration potential is expected to further decline in aged individuals, an element that should definitely be considered in the search for novel therapeutic strategies.

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          Most cited references122

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          Inflamm-aging. An evolutionary perspective on immunosenescence.

          In this paper we extend the "network theory of aging," and we argue that a global reduction in the capacity to cope with a variety of stressors and a concomitant progressive increase in proinflammatory status are major characteristics of the aging process. This phenomenon, which we will refer to as "inflamm-aging," is provoked by a continuous antigenic load and stress. On the basis of evolutionary studies, we also argue that the immune and the stress responses are equivalent and that antigens are nothing other than particular types of stressors. We also propose to return macrophage to its rightful place as central actor not only in the inflammatory response and immunity, but also in the stress response. The rate of reaching the threshold of proinflammatory status over which diseases/disabilities ensue and the individual capacity to cope with and adapt to stressors are assumed to be complex traits with a genetic component. Finally, we argue that the persistence of inflammatory stimuli over time represents the biologic background (first hit) favoring the susceptibility to age-related diseases/disabilities. A second hit (absence of robust gene variants and/or presence of frail gene variants) is likely necessary to develop overt organ-specific age-related diseases having an inflammatory pathogenesis, such as atherosclerosis, Alzheimer's disease, osteoporosis, and diabetes. Following this perspective, several paradoxes of healthy centenarians (increase of plasma levels of inflammatory cytokines, acute phase proteins, and coagulation factors) are illustrated and explained. In conclusion, the beneficial effects of inflammation devoted to the neutralization of dangerous/harmful agents early in life and in adulthood become detrimental late in life in a period largely not foreseen by evolution, according to the antagonistic pleiotropy theory of aging.
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            Microglial and macrophage polarization—new prospects for brain repair.

            The traditional view of the adult brain as a static organ has changed in the past three decades, with the emergence of evidence that it remains plastic and has some regenerative capacity after injury. In the injured brain, microglia and macrophages clear cellular debris and orchestrate neuronal restorative processes. However, activation of these cells can also hinder CNS repair and expand tissue damage. Polarization of macrophage populations toward different phenotypes at different stages of injury might account for this dual role. This Perspectives article highlights the specific roles of polarized microglial and macrophage populations in CNS repair after acute injury, and argues that therapeutic approaches targeting cerebral inflammation should shift from broad suppression of microglia and macrophages towards subtle adjustment of the balance between their phenotypes. Breakthroughs in the identification of regulatory molecules that control these phenotypic shifts could ultimately accelerate research towards curing brain disorders.
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              Promoting axon regeneration in the adult CNS by modulation of the PTEN/mTOR pathway.

              The failure of axons to regenerate is a major obstacle for functional recovery after central nervous system (CNS) injury. Removing extracellular inhibitory molecules results in limited axon regeneration in vivo. To test for the role of intrinsic impediments to axon regrowth, we analyzed cell growth control genes using a virus-assisted in vivo conditional knockout approach. Deletion of PTEN (phosphatase and tensin homolog), a negative regulator of the mammalian target of rapamycin (mTOR) pathway, in adult retinal ganglion cells (RGCs) promotes robust axon regeneration after optic nerve injury. In wild-type adult mice, the mTOR activity was suppressed and new protein synthesis was impaired in axotomized RGCs, which may contribute to the regeneration failure. Reactivating this pathway by conditional knockout of tuberous sclerosis complex 1, another negative regulator of the mTOR pathway, also leads to axon regeneration. Thus, our results suggest the manipulation of intrinsic growth control pathways as a therapeutic approach to promote axon regeneration after CNS injury.
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                Author and article information

                Journal
                Mediators Inflamm
                Mediators Inflamm
                MI
                Mediators of Inflammation
                Hindawi Publishing Corporation
                0962-9351
                1466-1861
                2017
                19 January 2017
                : 2017
                : 9478542
                Affiliations
                Laboratory of Neural Circuit Development and Regeneration, Animal Physiology and Neurobiology Section, Department of Biology, KU Leuven, Leuven, Belgium
                Author notes

                Academic Editor: Marta Agudo-Barriuso

                Author information
                http://orcid.org/0000-0002-3313-6022
                http://orcid.org/0000-0003-0779-0570
                http://orcid.org/0000-0002-3329-3474
                http://orcid.org/0000-0003-0186-1411
                Article
                10.1155/2017/9478542
                5288536
                28203046
                48f94876-7f52-4153-8535-8c4166f721fc
                Copyright © 2017 Ilse Bollaerts et al.

                This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 23 September 2016
                : 5 December 2016
                : 25 December 2016
                Funding
                Funded by: Hercules
                Award ID: AKUL/09/038
                Funded by: Research Council of KU Leuven
                Award ID: BOF-OT/10/033
                Funded by: FWO
                Funded by: VLAIO
                Categories
                Review Article

                Immunology
                Immunology

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