Search

Reactive astrocytes are astrocytes undergoing morphological, molecular, and functional remodeling in response to injury, disease, or infection of the CNS. Although this remodeling was first described over a century ago, uncertainties and controversies remain regarding the contribution of reactive astrocytes to CNS diseases, repair, and aging. It is also unclear whether fixed categories of reactive astrocytes exist and, if so, how to identify them. We point out the shortcomings of binary divisions of reactive astrocytes into good-vs-bad, neurotoxic-vs-neuroprotective or A1-vs-A2. We advocate, instead, that research on reactive astrocytes include assessment of multiple molecular and functional parameters—preferably in vivo—plus multivariate statistics and determination of impact on pathological hallmarks in relevant models. These guidelines may spur the discovery of astrocyte-based biomarkers as well as astrocyte-targeting therapies that abrogate detrimental actions of reactive astrocytes, potentiate their neuro- and glioprotective actions, and restore or augment their homeostatic, modulatory, and defensive functions.

Glia in the central nervous system (CNS) express diverse inward rectifying potassium channels (Kir). The major function of Kir is in establishing the high potassium (K+) selectivity of the glial cell membrane and strongly negative resting membrane potential (RMP), which are characteristic physiological properties of glia. The classical property of Kir is that K+flows inwards when the RMP is negative to the equilibrium potential for K+(Ek), but at more positive potentials outward currents are inhibited. This provides the driving force for glial uptake of K+released during neuronal activity, by the processes of “K+spatial buffering” and “K+siphoning”, considered a key function of astrocytes, the main glial cell type in the CNS. Glia express multiple Kir channel subtypes, which are likely to have distinct functional roles related to their differences in conductance, and sensitivity to intracellular and extracellular factors, including pH, ATP, G‐proteins, neurotransmitters and hormones. A feature of CNS glia is their specific expression of the Kir4.1 subtype, which is a major K+conductance in glial cell membranes and has a key role in setting the glial RMP. It is proposed that Kir4.1 have a primary function in K+regulation, both as homomeric channels and as heteromeric channels by co‐assembley with Kir5.1 and probably Kir2.0 subtypes. Significantly, Kir4.1 are also expressed by oligodendrocytes, the myelin‐forming cells of the CNS, and the genetic ablation of Kir4.1 are also expressed by Oligodendrocytes, the myelin‐forming cells of the CNS, and the genetic ablation of Kir4.1 results in severe hypomyelination. Hence, Kir, and in particular Kir4.1, are key regulators of glial functions, which in turn determine neuronal excitability and axonal conduction.

A role for fibroblast growth factor‐2 (FGF‐2) has been proposed in mediating the glial response to injury in the central nervous system (CNS). We have tested this possibility in vivo, by injecting FGF‐2 into the cerebrospinal fluid (CSF) of the brain ventricles of young rats and analysing glial cells in the anterior medullary velum (AMV), which partly roofs the IVth ventricle. FGF‐2 was administered at two different doses, low FGF‐2 (500 ng mL−1CSF) and high FGF‐2 (10 µg mL−1CSF), and saline vehicle was injected in controls. Injections were performed twice daily for three days, commencing at postnatal day (P) 6, and AMV were analysed at P9, using immunohistochemistry and Western blotting. Glial cells were unaffected by treatment with saline or low FGF‐2, whereas high FGF‐2 induced reactive changes in glial cell types: (1) there was increased GFAP expression in astrocytes, demonstrated by Western blot and immunohistochemistry, and astrocytes appeared hypertrophic, with increased process thickness and number; (2) the number of ED1 labelled microglia/macrophages was doubled, from 47 ± 6 to 114 ± 17 cells per field (0.75 mm2; values are mean ± SEM), and microglia appeared activated, with a multipolar and granular appearance; (3) NG2 positive glial cells appeared more fibrous and there was increased density of processes, although there was no significant increase in their number; (4) oligodendrocyte somata were enlarged and there was a loss of myelin sheaths. The results show that at high CSF titres of FGF‐2 induce glial reactivity in vivoand support a role for FGF‐2 in the pathology of CNS injury and EAE.

Fibroblast growth factor-2 (FGF-2) controls in part the timely differentiation of oligodendrocytes into the myelin-producing cells of the CNS. However, although differentiated oligodendrocytes express FGF receptors (R), the effect of FGF-2 on myelin-producing oligodendrocytes in vivowas unknown. In the present study, we show that delivery of FGF-2 into the cerebrospinal fluid of anaesthetized rat pups, aged postnatal day (P) 6 to 9, induced a severe loss of myelin in the caudal anterior medullary velum (AMV). Furthermore, we show that the caudal AMV was myelinated at the time of treatment, so the effects of FGF-2 represent a loss of myelin and not delayed differentiation. This was confirmed by injection of platelet-derived growth factor-AA (PDGF-AA), a factor known to affect the differentiation of PDGF-αR expressing oligodendrocyte progenitors, but which did not induce myelin loss in the caudal AMV and did not affect differentiated oligodendrocytes, which do not express PDGF-αR. Compared to controls treated with saline or PDGF-AA, FGF-2 induced an accumulation of PLP protein and MBP mRNA within the somata of myelin-producing oligodendrocytes. The results indicate that FGF receptor signalling disrupts myelin production in differentiated oligodendrocytes in vivoand interrupted the transport of myelin-related gene products from the oligodendrocyte cell body to their myelin sheaths.

Although it has been established that immature glial cells express functional purinergic receptors, the responsiveness of mature glial cells in vivo had not been elucidated. This question was addressed using fura-2 ratiometric measurements of [Ca2+]iin the adult mouse optic nerve, a central nervous system (CNS) white matter tract, taking advantage of the facts that (i), the optic nerve contains glial cells but not neurons and (ii), that fura-2 loads primarily astrocytes in isolated intact optic nerves. We show that adenosine 5′ triphosphate (ATP) evoked an increase in [Ca2+]iin a concentration-dependent manner with a half-maximal effect at 3 μm ATP, and with a rank order of agonist potency of ATP>ADP>α,β-methyline-ATP>UDP>adenosine. The results indicate mainly P2Yand P2Xcomponents, consistent with the in vitro astroglial purinergic receptor profile. The in vivo response of mature glia to ATP may be important in their response to CNS damage.

Platelet-derived growth factor (PDGF) is a growth-regulatory dimer with A and B subunits. PDGF-AA, acting via PDGF receptors of the α-unit subtype (PDGF-αR), is implicated in the differentiation of oligodendrocyte precursors and in the survival of newly formed oligodendrocytes, which gradually lose expression of PDGF-αR. However, it is unclear whether terminally differentiated oligodendrocytes express PDGF-αRin vivo.To address this question, and to help clarify the role of PDGF-AA in late oligodendrocyte differentiation, we have used doublein situhybridization with digoxigenin- and fluorescein-labeled riboprobes to relate PDGF-αR mRNA and myelin basic protein (MBP) mRNA expression in the isolated intact anterior medullary velum (AMV) of rats ages Postnatal Day (P) 10–12 and P30–32. In parallel experiments, AMV were immunolabeled with the oligodendrocyte-specific monoclonal antibody Rip to provide information on oligodendrocyte development and the extent of myelination. At P10, the AMV contained tracts in which axons ranged from unmyelinated to fully myelinated, whereas myelination was complete in P30–32 AMV. The first oligodendrocytes to express MBP mRNA or Rip were promyelinating oligodendrocytes, which had a “star-burst” morphology and had not yet begun to form myelin sheaths. As myelination proceeded, MBP mRNA became dispersed throughout oligodendrocyte units, comprising cell somata, processes, and internodal myelin sheaths. By P30–32, MBP mRNA had been redistributed to the myelin sheaths only, reflecting a change in the site of protein synthesis in mature myelinated axon tracts. At no stage of oligodendrocyte differentiation did we observe cellular coexpression of mRNA for PDGFαR and MBP. Our results indicated that oligodendrocytes lost the expression of PDGFαR prior to gaining that of myelin gene products, and preclude an action of PDGF-AA on Rip+/MBP+star-burst promyelinating oligodendrocytes. The spatial and temporal expression of PDGF-αR mRNA in the AMV was inversely related to the pattern of maturation of both myelin and oligodendrocytes, and is consistent with PDGF-αR being expressed by pro-oligodendrocytes. A notable finding was the high level of expression of PDGF-αR mRNA in the AMV of juvenile rats, localized to cell bodies within the myelinated axon tracts, strongly suggesting that oligodendrocyte precursors persisted in the mature velum.

The effect of histamine, bradykinin and serotonin on blood-brain barrier permeability was investigated using in situ measurement of transendothelial electrical resistance in pial microvessels of anaesthetized rats. Mean resistance of vessels superfused with artificial cerebrospinal fluid was 1800 Ωcm2, indicating a tight barrier with extremely low ion permeability. In paired experiments from continuous measurements in single vessels, addition of 10−3 M serotonin to the solution bathing the brain had no marked effect on resistance, whereas both histamine and bradykinin, applied at a concentration of 10−4 M, caused a rapid and reversible decrease in resistance. Mean resistance was 408 and 505 Ωcm2 in 10−4 M histamine and bradykinin, respectively, and approximately 50% of vessels had a resistance less than 250 Ωcm2, compared to 12% in controls, indicating a leaky blood-brain barrier that is not capable of normal brain ion homeostasis. Histamine and bradykinin had similar dose-response relations, and a maximal effect was observed between 20 and 50 μM. Thus, histamine and bradykinin act at the abluminal (brain-facing) membranes of the cerebral endothelium to mediate blood-brain barrier opening. These results support a role for histamine and bradykinin in brain oedema formation.