Keywords

1 Neuroglia as a Central Element of Neuropathology

Understanding the healthy human brain and principles of its pathological remodelling represents the major challenge ever faced by biomedical sciences. The complexity of the brain networking is exceptional, as indeed >200 billions of cells connected with tens of trillions of synapses (chemical and electrical) create the organ with unparalleled computing power and memory capacity in excess of a petabyte [1]. The brain as an organ is exceptionally resilient to the extracellular environmental pressure; despite exceedingly high metabolism (massively generating highly toxic reactive oxygen species), the brain withstands ageing better than any other organ and system in the human body. Verily, human intellectual capacity remains high in advanced age, whereas the physical abilities start to decline already in the second or third decade of life.

The nervous system evolved through cell diversification and cell specialisation ; the brain contains hundreds of distinct types of neurones, which are highly specialised for fast signalling through propagating action potentials coupled to the synaptic machinery that represents the primary element of integration and information processing. Neurones , however, account only for a half of all cells in the brain; the other half being represented by neuroglia. Neuroglial cells, generally classified into astroglia, oligodendroglia, NG2 cells and microglia [2, 3], are no less heterogeneous than neurones. The neuroglial cells are the homeostatic and defensive arm of the nervous system, which ensures proper organ internal environment associated with brain function.

Prominent neurologists of the nineteenth century , including Virchow, Andriezen and Alzheimer, considered glia as the main site of morbid changes. Nonetheless, the neurono-centric doctrine, that associated disease with neurones, prevailed over the last century; neurogliopathology, however, started to be acknowledged rapidly in the most recent decade [4,5,6,7,8,9]. Conceptually, neurological diseases can be defined as homeostatic failure, often associated with the inability of neuroglia to provide the full homeostatic and neuroprotective support. Furthermore, the responses of neural cells to the insult are fundamentally different: neurones become stressed and lose their primary function of information transfer and information processing, whereas glial cells actively respond by mounting an evolutionary conserved defensive response, collectively known as reactive gliosis [10, 11]. Both astroglial reactivity and astroglial asthenia contribute to neuropathology.

2 Neuroglia in Neurodegeneration

Neurodegenerative diseases, which affect almost exclusively humans , are chronic neurological disorders that lead to a progressive loss of function, structure and number of neural cells, ultimately resulting in the atrophy of the brain and profound cognitive deficits. Underlying mechanisms remain largely unknown although neurodegeneration is often associated with aberrant protein synthesis with an accumulation of pathological proteins (such as β-amyloid or α-synuclein) either inside the cells or in the brain parenchyma. Extracellular protein aggregates form disease-specific histopathological lesions represented, for example, by senile plaques, Lewy bodies or Rosenthal fibres.

Neuroglial alterations in neurodegeneration are complex and include both glio-degeneration with a loss of glial function and glial reactivity (Table 17.1). In many neurodegenerative processes, asthenic and degenerative changes in astroglia precede astrogliosis, the latter being activated, most likely, by specific lesions and the appearance of damaged or dying neurones. In amyotrophic lateral sclerosis (ALS) for example, astrodegeneration and astroglial atrophy occur before clinical symptoms and neuronal death. In the animal model of ALS expressing human mutant superoxide dismutase 1 (Tg(SOD1*G93A)1Gur mice), the occurrence of atrophic astrocytes seems to be the earliest pathological signature [12, 14]. The ability of these atrophic astrocytes to remove l-glutamate is compromised , which is thought to result in the accumulation of extracellular l-glutamate with ensuing excitotoxicity [48]. At the later stages of ALS, astrocytes also become activated, albeit atrophic forms also remain. The importance of astrocytes in ALS pathogenesis is further corroborated by the observation that silencing of ALS-related mutant SOD1 gene in astrocytes delayed the appearance of clinical symptoms in the transgenic mouse model [15]. Astroglial degeneration and loss of function also assumes a pathological proportion in Wernicke encephalopathy, a thalamo-cortical neurodegeneration, which represents the substrate for Korsakoff syndrome; significant down-regulation of astroglial l-glutamate transporters results in prominent excitotoxicity [32, 33]. Similarly, in Huntington disease (HD), a decreased astroglial l-glutamate uptake as well as an aberrant release of L-glutamate from astrocytes contributes to neurotoxicity. Astroglial reactivity also contributes to HD. Suppression of astrogliotic response by inhibition of JAK/STAT3 signalling cascade increases the number of huntingtin aggregates [49], thus exacerbating pathological progression. In the context of Parkinson disease (PD), astrocytes are supposed to play a neuroprotective role [23, 24]. Astrocytes contribute to metabolism of dopamine, a transmitter being transported to astroglial cells by neutral amino acids transporter SLC7A5 as well as by dopamine transporter DAT1/SLC6A3. Astrocytes were also shown to convert L-DOPA to dopamine [50]. In the striatum, astrocytes act as a reservoir for L-DOPA , which they release to be subsequently transported to neurones [51]. The level of glial fibrillary acidic protein (GFAP) expression was decreased in astrocytes in PD human tissue [25], indicating astroglial atrophy and reduced astrogliotic response, which may reflect compromised astroglial neuroprotection.

Table 17.1 Neuroglia in major neurodegenerative diseases : an overview
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Neurodegenerative diseases are almost invariably accompanied by chronic neuroinflammation with activation of microglia . Again, the precise contribution of microglia in the neurodegenerative process remains controversial, because both microglial activation with generation of neuroinflammatory cellular phenotypes and microglial paralysis are considered [44, 46, 52, 53]. This dichotomy in particular may reflect the differences between human and laboratory animal brains; there is little evidence for microglial activation in AD human tissue [52, 54]. Furthermore, human ageing (in contrast to laboratory animals) is associated with significant atrophy of microglial cells, which may be a key factor for creating environment, permissive for neurodegenerative alterations [54]. Finally, oligodendrocytes also undergo degenerative changes in the context of neurodegeneration. The progression of AD, for example, is accompanied by substantial shrinkage of the white matter. Degenerative changes are also observed in oligodendroglial precursors/NG2 glial cells that may reflect a reduction in their remyelinating capacity [42].

3 Astrocytes in AD

The pathological potential of astroglia in the context of dementia was realised by Alois Alzheimer, who often observed activated glial cells in close contacts with pathologically altered neurones, and who also described glia as a cellular component of the senile plaque [55]. Subsequent studies frequently mentioned astroglial reactivity in the context of AD, although detailed analysis of astroglial pathology started to be investigated only very recently [38, 45, 56].

3.1 Astrodegeneration

Astrocytes undergo complex morphological changes in animal AD models (Figs. 17.1 and 17.2). For example, in the triple transgenic 3xTg-AD mouse, which harbours mutant genes for amyloid precursor protein (APPSwe), presenilin 1 (PS1M146V) and microtubule-associated protein Tau (TauP301L), at the early pre-symptomatic stages (i.e. before considerable accumulation of extracellular β-amyloid and formation of senile plaques) astrocytes in hippocampus, prefrontal and entorhinal cortices demonstrate signs of atrophy and astrodegeneration [34, 57, 59]. The above changes are manifested by a decrease in the GFAP-positive astroglial profiles (both in area and volume measurements), decreased somata volume as well as a decrease in the number and branching of cell processes (Fig. 17.1). The atrophic changes in astrocytes developed in a particular spatio-temporal pattern. Hence, the earliest signs of atrophy were observed in the entorhinal cortex (at 1 months of age), in the prefrontal cortex morphological atrophy developed from 3 months of age, and in the hippocampus from 9 to 12 months of age [34, 57, 59]. Atrophic astrocytes also showed some signs of loss of homeostatic function, i.e., in hippocampus a decrease in l-glutamine synthase (an enzyme central for l-glutamate turnover and l-glutamate-glutamine shuttle) have been detected [60]. There are, however, no significant changes in the overall number of GFAP-positive astrocytes either in the hippocampus or in the cortex.

Fig. 17.1
figure 1

Confocal micrographs illustrating atrophy of GFAP-positive astroglial profiles in the hippocampal dentate gyrus (DG), hippocampal cornu ammonis 1 (CA1) area, entorhinal cortex (EC) and prefrontal cortex (PFC) in 3xTg-AD mice compared with control animals. Modified from [35, 57,58,59]

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Fig. 17.2
figure 2

Confocal images of hippocampal preparations labelled by anti-GFAP (green) and anti-β amyloid (red) monoclonal antibodies illustrating accumulation of reactive astrocytes around senile plaques (a, b) and vascular β-amyloid deposits (c) Modified from [34]

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Astrodegenerative changes may contribute to the development of the early AD pathology. Astroglial atrophy is likely to result in a reduced synaptic coverage, which in turn may affect neurotransmission and connectivity of neuronal networks. In addition, atrophic astrocytes fail to provide an adequate homeostatic support, thus further exacerbating neuronal function. These changes may, therefore, account for the early cognitive impairment, which results from loss and weakening of synapses, rather than from neuronal death. Incidentally, morphological atrophy of astrocytes in transgenic AD animals can be restored by an exposure to enriched environment or physical activity environmental stimulation, which arguably may coincide with certain cognitive improvement [61,62,63].

3.2 Astroglial Reactivity

Emergence of parenchymal depositions of β-amyloid triggers astroglial reactivity , with reactive astrocytes mainly being associated with senile plaques and β-amyloid “infested” blood vessels [34, 36, 38] (Fig. 17.2). Astroglial reactivity is observed both in human post-mortem tissues and in the brains of AD animal models [37]. In the 3xTg-AD mice, for example, the reactivity in the hippocampus is characterised by a significant (up to 70%) increase in GFAP-positive astroglial profiles [34, 36]. The reactive astroglial response differs between brain regions. In the 3xTg-AD mice prominent reactivity is observed in the hippocampus whereas it is absent in entorhinal and prefrontal cortices [34, 57, 59]. Astroglial reactivity in the context of AD has been shown both in vitro [64] and in situ [65], and is triggered by the exposure to extracellular β-amyloid. Intracellular signalling cascades responsible for astroglial reactivity involve Ca2+ signalling, which is also affected in AD.

3.3 Aberrant Astroglial Ca2+ Signalling

Exposure of astrocytes to β-amyloid has complex effects on [Ca2+]i regulation and Ca2+ signalling [66, 67]. Treatment of astroglial cultures with oligomeric β-amyloid1–42 for several hours increased resting [Ca2+] i several fold [68, 69], which, however, was not confirmed in experiments with longer treatments and other amyloid peptides; 48 h exposure neither to 1–20 μM β-amyloid1–40 [70] nor to 200 nM β-amyloid25–35 [71] had any significant effect on [Ca2+]i. Treatment of astrocytes in dissociated or organotypic cultures with β-amyloid induced acute [Ca2+] i elevations or even [Ca2+] i oscillations [65, 72,73,74,75], although this finding was not confirmed by others [69,70,71]. These differences may reflect, for example, different concentrations of β-amyloid (higher concentration tend to induce [Ca2+] i elevations more reliably) or different species of β-amyloid that were used in various experiments. Of note, picomolar (200–300 pM) concentrations of β-amyloid were found to activate α7 nicotinic receptors that can mediate [Ca2+] i transients [76, 77]. Treatment with β-amyloid was also found to increase intracellular Ca2+ release and store-operated Ca2+ entry, SOCE [78, 79]. Overall it seems that β-amyloid generally increases astroglial Ca2+ excitability [66, 67].

An increased astroglial Ca2+ signalling has been observed in several AD animal models . For example, an increase in the resting [Ca2+] i as well as [Ca2+] i hyperactivity with aberrant long-spreading intercellular Ca2+ waves were detected in astrocytes surrounding senile plaques in APP/PS1 mice [80], expressing mutant human β-amyloid precursor protein (APPswe) and mutant PS1 (PS1ΔE9). High-frequency aberrant Ca2+ waves were also observed in astrocytes in APPSwe mice at the pre-plaque stages [81]. There is evidence that increased Ca2+ excitability of astrocytes from AD mice may result from an elevated release of ATP from reactive astrocytes, with subsequent over-activation of P2Y1 purinoceptors [82].

Aberrant Ca2+ signalling observed in transgenic AD animals can be also associated with an expression of pathologically mutated gene for PS1, rather than solely reflect effects of extracellular β-amyloid. Indeed, abnormal Ca2+ signalling was observed in primary cultures of astrocytes isolated from neonatal 3xTg-AD mice [69, 79]. Challenge with ATP triggered larger [Ca2+] i in cultured hippocampal astrocytes from 3xTg-AD mice [83] when compared to their respective controls. Likewise, the SOCE was also increased in cultured transgenic astrocytes [79]. Expression of mutant PS1 affected not only Ca2+ dynamics but also impaired vesicular trafficking and secretion of glio-signalling molecules [84], again suggesting that the M146V PS1 mutation perturbs several fundamental cellular functions. These abnormal functions result in early pathological remodelling of astroglia, which might contribute to evolution of pathology. Expression of APP also had complex effects on astroglial Ca2+ dynamics. The overexpression of APP did not affect [Ca2+] i transients and SOCE in primary cultured astroglial cells from Tg5469 AD mice. On the contrary, deletion of APP inhibited astroglial SOCE, possibly through down-regulation of expression of TRPC1 and Orai 1 Ca2+ channels [85]. In astrocytes obtained from Trisomy 16 mice (a mouse model of Down syndrome that shares several key features with AD), resting Ca2+ levels were elevated two-fold when compared to wild-type controls [86]. Challenge of these astrocytes with cyclopiazonic acid (CPA, a SERCA inhibitor which initiates unopposed leakage of Ca2+ from the endoplasmic reticulum, ER) triggered large [Ca2+]i elevations, which reflected higher ER Ca2+ content with a positive correlation between resting [Ca2+]i and the amplitude of CPA-induced [Ca2+] i transients [86].

The chronic exposure to β-amyloid also affects astroglial Ca2+ dynamics by modifying the calcium signalling toolkit, through changing expression of ionotropic and metabotropic receptors, intracellular Ca2+ channels, SOCE and Ca2+-dependent enzymes. These modifications have been observed both in vitro and in situ. Chronic (24–72 h) exposure of cultured astrocytes to oligomeric β-amyloid (100 nM to 20 μM) up-regulated expression of astroglial metabotropic l-glutamate receptors mGluR5 [69, 70, 83]. A rather similar up-regulation of mGluR5 expression was also found in the brains of patients with Down’s syndrome [87], as well as in cortical astrocytes associated with senile plaques in the APPswe/PS1ΔE9 mice [88], in post-mortem hippocampi of Braak V-VI stage AD patients [69] and in post-mortem hippocampi from late-stage sporadic AD cases [70]. Chronic treatment of astroglial cultures with low concentrations (0.1–100 nM) of β-amyloid1–42 [89] also increased expression of several nicotinic cholinoreceptors including highly Ca2+ permeable α7nAChRs. These latter receptors were found to be selectively expressed in astrocytes in the post-mortem brains of sporadic AD patients as well as in patients carrying the Swedish APP (KM670/671NL) mutation [90]. Another possible mechanism for aberrant Ca2+ signalling may be associated with direct activation of metabotropic Ca2+ sensing receptors by β-amyloid [91, 92].

Exposure of astrocytes to β-amyloid also affected expression of intracellular Ca2+ release channels. At mRNA and protein levels both inositol 1,4,5-trisphosphate receptors types 1 and 2 (InsP3R1 and InsP3R2) were up-regulated following 48 h treatment of rat hippocampal astrocytes with 100 nM oligomeric β-amyloid1–42 [69, 83]. This increase, however, was not observed in all regions of the brain; treatment with β-amyloid failed to increase expression of InsP3R1 protein in astrocytes isolated from the entorhinal cortex [83]. Even more interestingly, β-amyloid increased expression of InsP3R1 only in hippocampal astrocytes isolated from wild-type healthy mice, but not that in cells isolated from 3xTg-AD animals, suggesting that exogenous β-amyloid and genetic modifications of astroglia share common molecular mechanism in deregulating Ca2+ homeostasis [83]. Several reports showed overall decrease in InsP3Rs expression in post-mortem brains of AD patients in frontal, parietal and entorhinal cortices and hippocampus [93,94,95]; apparent contradiction to the in vitro/culture experiments need further clarifications. Expression of several other genes related to Ca2+ signalling was found to be modified in astrocytes in vitro or in AD animal models; these genes includes calsenilin [96], calpain-10 [97], NFAT [98], NF-kB [78], calcineurin [78, 99] as well as some subunits of L-type calcium channels [100]. Signalling through calcineurin (which is, a Ca2+/calmodulin-dependent protein phosphatase) may link [Ca2+] i fluctuations to astroglial reactivity or dysfunction. Expression of calcineurin is increased in β-amyloid treated astrocytes in vitro [78] and in vivo in APP/PS1AD model mice [101] and human AD hippocampus [69].

3.4 Calcium Signalling and Astroglial Reactivity

Regulation of astroglial reactivity is one of key components of defensive response of the CNS to all types of neuropathology. Although molecular cascades involved in the initiation of astrogliosis are far from being completely understood, there is growing evidence of the critical importance of cytosolic Ca2+ signalling and particularly that of Ca2+ release from the ER . Indeed, genetic deletion of InsP3R type II, an ER Ca2+ release channel, in astrocytes greatly diminishes astroglial reactive response to various lesions [102]. In the context of AD, the ER Ca2+ release is directly linked to initiation of reactive astrogliosis and inhibition of ER Ca2+ release channels (ryanodine receptors or InsP3Rs) suppressed astrogliotic response [65]. Exposure of astrocytes to β-amyloid not only triggers ER Ca2+ release and initiates astrogliotic remodelling but also induces ER stress (as indicated by substantial phosphorylation of eIF2α and an increased expression of the GRP78 ER-resident chaperones, both being the hallmark of a specific type of ER stress known as unfolded protein response (UPR). Inhibition of Ca2+ release channels in astrocytes effectively suppressed the UPR [65]. Incidentally, the ER stress may be instrumental in the initiation of reactive astrogliosis; depletion of the ER from releasable Ca2+ instigates the UPR, which in turn triggers biochemical remodelling that underlie reactive response of astroglia.

As has been mentioned before, astroglial reactivity in the AD animals differs between brain regions with strong reactive response in the hippocampus and negligible reactive remodelling in the entorhinal and prefrontal cortices. This difference also correlated with different sensitivity of Ca2+ signalling toolkits in astrocytes from these regions to β-amyloid; Ca2+ signalling components were up-regulated in the hippocampus but not in the entorhinal cortex [83]. This specific property of the entorhinal astroglia may possibly account for their inability to mount astrogliotic response to accumulating β-amyloid. This “functional paralysis” of astrocytes may be directly related to differences in vulnerability of brain regions to AD. Indeed, senile plaques appear in the entorhinal cortex much earlier than in the hippocampus. In this way Ca2+ signalling toolkits and their remodelling in the pathological context may be linked to the disease progression [66].

3.5 Vesicular Trafficking and Secretion in Astrocytes Are Altered in AD

Pathological changes in astroglia (for example, signs of astrogliotic activation) have been observed at the pre-symptomatic phase of AD before the formation of β-amyloid deposits [103] and hence changes in astroglial signalling may also occur early in the disease. Gliosignalling molecules, stored in membrane bound vesicles, are secreted by astrocytes through the stimulation-secretion coupling that involves exocytotic release [104]. In addition to molecules stored in vesicle lumen, ion channels, membrane receptors and transporters, such as major histocompatibility complex II (MHC-II [105]) and excitable amino acid transporter 2 (EAAT2 [106]) are delivered to the plasma membrane by vesicle traffic [107]. This brings vesicles from the Golgi complex, deep in the cytoplasm, to the cell surface. This traffic is maintained by an elaborated system regulated by increases in [Ca2+] i [107, 108]. The complexity of vesicle traffic regulation in astrocytes is characterised by two typical, yet opposing, properties of vesicles that contain peptides, such as atrial natriuretic peptide, and those that carry amino acid transmitters and are labelled by the vesicular l-glutamate transporter VGLUT1 [108, 109]. Namely, glutamatergic vesicle motility is accelerated by an increase in [Ca2+] i [110], whereas the same increase in [Ca2+] i slows down peptidergic vesicles and endolysosomes [111]. Similar regulation also applies to recycling peptidergic vesicles, which have merged with the plasma membrane and subsequently entered back the cytoplasm. The mobility of recycling peptidergic vesicles was studied in cultured astrocytes [112] and those residing in the intact brain slices [113]. At rest, peptidergic vesicles moved faster and more directionally, linked to cytoskeletal elements, than after the exposure of astrocytes to a calcium ionophore to increase [Ca2+] i [112]. The effect of increased [Ca2+] i was remarkable; the movement of vesicles was almost halted, with only a jitter remaining (that was associated with random diffusional movement). At least some of the peptidergic vesicles carry ATP and a similar attenuation was observed in their mobility when astrocytes were stimulated [114]. Proteolytic enzymes stored in endolysosomes may contribute to the development of AD. One of these proteases is represented by the insulin degrading enzyme (IDE), which, when secreted to the extracellular space, may degrade β-amyloid. While IDE is secreted from neurones [115], the main cell type secreting IDE in AD appears to be astrocytes [116, 117]. It has been proposed that in AD the capacity of secreting IDE is reduced, leading to an increase in β-amyloid, which involves a reduction in autophagy-based lysosomal secretion of IDE [117]. Why this reduction occurs is not clear, but it may relate to a general vesicle traffic impairment that has been observed in AD.

Astrocytes from 3xTg-AD mice isolated in the pre-symptomatic phase of the disease exhibit alterations in vesicle traffic (Fig. 17.3). Spontaneous mobility of peptidergic and endolysosomal vesicles as well as the ATP-evoked, Ca2+-dependent, vesicle mobility were all diminished in diseased astrocytes. Similar impairment of peptidergic vesicle trafficking was observed in healthy rat astrocytes transfected with familial AD-associated mutated presenilin 1 (PS1M146V). The stimulation-dependent peptide discharge from single vesicles was less efficient in 3xTg-AD and PS1M146V-expressing astrocytes than in respective controls. The impaired vesicle dynamics and reduced evoked secretion of the signalling peptides both may contribute to the development of AD [84]. Although in this study atrial natriuretic peptide-containing vesicles were examined, it is likely that all peptidergic vesicles exhibit similar changes.

Fig. 17.3
figure 3

Decreased spontaneous mobility of peptidergic vesicles in 3xTg-AD astrocytes . (a) Live cultured wild-type (wt) astrocyte under DIC optics and (b) the confocal image of the same cell expressing fluorescent peptide atrial natriuretic peptide-emerald green (ANP.emd), stored in individual vesicles, observed as bright fluorescent puncta; scale bars, 10 μm. (c) Vesicle tracks (N = 50) obtained in a 15-s epoch of imaging representative control (wt) and (d) 3xTg-AD astrocytes expressing ANP.emd, respectively. Note less elongated vesicle tracks in the 3xTg-AD astrocyte. (e, f) Frequency histogram of the step length in spontaneously moving vesicles in wt (N = 5025, e) and 3xTg-AD (N = 5072, f) astrocytes. The data were fitted with the function f = a × exp(−0.5 × (x/x0)/b)2/x, where a = 17.88 ± 0.00, b = 0.07 ± 0.00 μm − 0.5, x0 = 0.07 ± 0.00 μm (black curve) and a = 6.53 ± 0.13, b = 0.19 ± 0.01 μm − 0.5, x0 = 0.31 ± 0.01 μm (grey curve) in wt astrocyte, and with the function f = a × exp(−0.5 × (ln x/x0)/b)2/x, where a = 1.96 ± 0.04, b = 0.92 ± 0.02 μm − 0.5, x0 = 0.10 ± 0.00 μm (black curve) in 3xTg-AD astrocyte. The vertical dashed line indicates the step length of 0.2 μm obtained close to the intersection of distributions (black and grey curve) in wt astrocytes to discriminate small (<0.2 μm) from large (≥0.2 μm) steps. Note the higher proportion (%) of smaller steps lengths in the 3xTg-AD astrocyte indicated by the absence of the second mode distribution seen in wt astrocytes. (g) Track length (TL), (h) maximal displacement (MD), Note substantially diminished TL, MD in 3xTg-AD astrocytes. The numbers above the top of the bars (mean ± SEM) indicate the number of vesicles analysed; the numbers at the bottom of the bars indicate the number of cells analysed; “***”—indicates P values <0.001. Modified with permission from [84]

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4 Oligodendroglia in Alzheimer’s Disease

In the human brain white matter accounts for more than 50% of the total volume and is critical for assembling the CNS connectome [118]. The central myelination is provided by oligodendrocytes which are present in both white and grey matter; they are likely to be as numerous as astrocytes. Degeneration and death of oligodendrocytes with a subsequent decrease in CNS myelination and the shrinkage of the white matter are observed in the most (if not all) diseases of the brain and of the spinal cord including stroke, perinatal ischemia, multiple sclerosis, psychiatric disorders, traumatic injury and AD [119, 120]. The loss of myelin is a characteristic feature of the ageing CNS; in particular decreased myelination and oligodendroglial demise has been identified in the cerebral cortex, in areas related to cognition and memory including the frontal lobes [121]. In the course of human life the myelination of the CNS profile steadily increases during postnatal development, peaks at around 45 years and subsequently decreases in centenarians to levels comparable to those observed in infancy [121]. In the primary visual cortices of rhesus monkey , the age-dependent myelin deterioration has been characterised, and it appeared that the length of paranodes is decreased in ageing indicating some shortcomings in demyelisation [122]. These changes in the myelin developed in parallel with the decrease in the self-renewal capacity of oligodendroglial precursors/NG2 cells [123].

Oligodendroglial cell death and myelin shortages are associated with aberrant Ca2+ homeostasis and signalling , which is caused by either extracellular (neurotransmitter dyshomeostasis) or cellular (alterations in the Ca2+ homeostatic cascades such as channels, transporters and pumps) factors. Oligodendrocytes express several types of ionotropic receptors, including l-glutamate and P2X receptors, which are permeable to Ca2+. Prolonged or excessive activation of these receptors induces cytosolic Ca2+ overload, accumulation of Ca2+ within mitochondria, increased production of reactive oxygen species, and release of pro-apoptotic factors, which all, acting in concert, trigger oligodendrocyte death and myelin destruction [120, 124]. Excitotoxicity mediated by l-glutamate and ATP may also contribute to oligodendroglia death in the context of AD.

In AD, the white matter degenerates and the number of oligodendrocytes is decreased [39]. Experiments in vitro have demonstrated that the exposure to β-amyloid damages oligodendrocytes [43] possibly because the expression of mutant PS1 increases their sensitivity to l-glutamate toxicity [41]. Similar effect has been observed in vivo when injection of β-amyloid1–42 into white matter induced axon disruption and damaged myelin and triggered the death of oligodendrocytes [125]. The role for mutant PS1 has also been confirmed; exposure of PS1 mutant mice to the demyelinating agent cuprizone resulted in extended white matter damage and learning and memory deficits, which was not the case for healthy wild-type animals [41]. In the 3xTg-AD mice, gross deterioration of axonal morphology and compromised myelin integrity together with decreased expression of oligodendroglial markers was observed in the CA1 area of the hippocampus and in the layers IV/V of the entorhinal cortex at the early stages of the disease, much before the emergence of β-amyloid depositions [126]. Hence, myelin and oligodendrocyte defects in AD occur before the onset of symptoms and may be considered as early markers. White matter lesions are also quite prominent in the early-stage AD in periventricular and deep white matter [127]. In 3xTg-AD mice marked morphological atrophy and decreased numbers of NG2 glia were detected at the early stages [42]; at the later phase, the NG2 glia associate themselves with senile plaques and infiltrate the latter with processes [42]. Similar decrease in the NG2-positive profiles was observed in human AD post-mortem tissue [40]. All these alterations in myelin, oligodendroglia as well as degenerative changes in oligodendroglial precursors/NG2 cells may contribute to pathological remodelling of the connectome and hence to cognitive deficiency.

5 Microglia in Alzheimer’s Disease

Microglial changes, both reactive and degenerative, are now considered to be an important part of AD progression [44]. Activated microglial cells (together with astrocytes) are closely associated with senile plaques; they secrete numerous pro-inflammatory factors that may contribute to neuronal damage [44, 128]. At the same time the loss of microglial function has also been observed. In APP/PS1 mice, appearance of senile plaques coincided with the loss of microglial phagocytotic function (which, arguably, reduced β-amyloid clearance and facilitated plaque formation [46]). In the ageing human brain, degeneration of microglia can define neural tissue vulnerability to the AD pathology [54].

Activation of microglia can be triggered by β-amyloid [129], either soluble or oligomeric [45]. In vivo imaging of B6C3-YFP transgenic mice (harbouring APPswe and PS1d9x-yellow fluorescent protein genes) demonstrated that microglia are activated and recruited to Aβ plaques only after the plaques had been formed [130]. In another AD mice model (APP V717I transgenic mice), activation of microglia, however, occurred much earlier (at 3 month) than the formation of senile plaques (10–12) months [131]. Activation of microglia in AD context may also involve purinergic signalling. In particular, P2X7 receptors were found to be necessary for activation of microglia in response to β-amyloid injection [132]. Microglial activation in AD may also be regulated by TLR4 and TLR2 Toll receptors, which are up-regulated in AD animal models and in post-mortem AD brains [133, 134]. Of note, a spontaneous loss-of-function mutation in the TLR4 gene markedly decreased microglial activation induced by Aβ [135].

Microglial status does change in the progression of AD. In 3xTg-AD mice , a substantial increase in the density of resting microglia at both the early (i.e. pre-plaque) and late stages of the disease was identified [136]. At 9 month of age (before the emergence of senile plaques), the density of resting microglia in the hippocampus of 3xTg-AD was twice (by 105%) larger than in control mice. This increased density of resting microglia remained at older ages (54% higher at 12 months and 131% higher at 18 month when compared to the controls), when senile plaques became evident. Emergence of the microglial activation seen as a significant increase in the density of activated microglia in the CA1 hippocampal area of 3xTg-AD mice was detected at 12 and 18 months, a period that correlates with the appearance and development of Aβ plaques [136]. The early increase in the density of resting microglia may represent the generalised response of the brain defence system to the developing AD pathology. Exposure of 3xTg-AD mice to running and enriched environment prevented this increase in the density of both resting and activated microglia [47]. This finding indicates that environmental stress may affect microglial response to the AD pathology.

6 Conclusions

Pathological changes in neuroglia, including but not restricted to astrocytes, oligodendrocytes, NG2 cells and microglia, are omnipresent in neurodegenerative diseases. These neuroglial changes include cellular degeneration and asthenic responses at earlier stages of a disease, and, as disease progresses, evident by occurring neuronal damages, neuroglia turns to reactive phenotypes. The specific morpho-functional changes in glia not only occur during distinct temporal domains, but also are region-specific. Since the changes in neuroglia precede those in neurones, it is likely that the neuroglial cells failure to maintain CNS homeostasis is a malefactor causative to neuronal death. Neuroglia therefore may represent an opportunistic target for therapeutic intervention directed towards prevention and conceivably curing neurodegenerative diseases.