Week 2 (1) - Glial Cells (Neuroglia) Neurobiology and Clinical Aspects PDF

Week 2 (1) Inherited Neurological Disorders (HMG 44110A ) Glial Cells (Neuroglia) Neurobiology and Clinical Aspects Dr. Merin Thomas [email protected] Office hours : Monday & Wednesday, 3.00pm to 5.00pm...

Week 2 (1) Inherited Neurological Disorders (HMG 44110A ) Glial Cells (Neuroglia) Neurobiology and Clinical Aspects Dr. Merin Thomas [email protected] Office hours : Monday & Wednesday, 3.00pm to 5.00pm 1  Learning Objectives Neuroglia – Types & their functions Neuroglia in neuropathology Neuroglia in neurodegeneration  Neuroglia The Nervous tissue comprises two types of cells Neurons - Highly specialized cells; connect all regions of body to brain & spinal cord; lost ability to undergo mitosis Neuroglia - support, nourish, and protect neurons; Much more in number than neurons; continue to divide throughout lifetime  Neuroglia The Neuroglia make up about half the volume of the CNS. Neuroglia are smaller than neurons, and they are 5 to 25 times more numerous. In contrast to neurons, glia do not generate or propagate nerve impulses They can multiply and divide in the mature nervous system. Brain tumors derived from glia, called gliomas  Neuroglia The Neuroglia in the Central Nervous System (CNS) Astrocytes Oligodendrocytes Microglial cells Ependymal cells The Neuroglia in the Peripheral Nervous System (PNS) Schwann Cells Satellite Cells  Neuroglia Astrocytes: Neuroglia Astrocytes are the most abundant glial cells in the central nervous system (CNS). Play a crucial role in regulating the chemical environment around neurons. Help maintain ion balance, control the blood-brain barrier, and provide nutrients and structural support to neurons.  Neuroglia Oligodendrocytes: Oligodendrocytes are found in the CNS Neuroglia Responsible for producing myelin, a fatty substance that wraps around the axons of neurons. Myelin acts as an insulator, increasing the speed and efficiency of nerve signal transmission.  Neuroglia Neuroglia Microglia: Microglia are the immune cells of the CNS. They function as macrophages, phagocytosing dead cells, and pathogens. Play a role in the immune defense and maintaining a healthy environment in the brain and spinal cord.  Neuroglia Neuroglia Ependymal Cells: Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. They produce cerebrospinal fluid (CSF), which surrounds and cushions the brain and spinal cord, providing buoyancy and protection.  Neuroglia – Why are we studying it? The most notable finding in neurodegenerative diseases is the Neuroglia progressive death of neurons. However, neuroglial changes can precede and facilitate neuronal loss. Astroglial cells maintain the brain homoeostasis, and are responsible for defense and regeneration, so that their malfunction manifest as degeneration or asthenia together with reactivity contribute to pathophysiology. Neuroglia may represent a novel target for therapeutic intervention, be that prevention, slowing progression of or possibly curing neurodegenerative diseases.  Neuroglia as a Central Element of Neuropathology The neuroglial cells are the homeostatic and defensive arm of the nervous system, which ensures proper organ internal environment associated with brain function. Conceptually, neurological diseases can be defined as homeostatic failure, often associated with the inability of neuroglia to provide full homeostatic and neuroprotective support.  Neuroglia as a Central Element of Neuropathology The responses of neural cells to the damage are fundamentally different: Neurons become stressed and lose their primary function of information transfer and information processing, Neuroglial cells actively respond by increasing an evolutionary conserved defensive response, collectively known as reactive gliosis.  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 abnormal protein synthesis with an accumulation of pathological proteins (such as β-amyloid or α-synuclein) either inside the cells or in the brain parenchyma.  Neuroglia in Neurodegeneration Extracellular protein aggregates form disease-specific histopathological lesions characterized (plaques and Lewy bodies). Neuroglial alterations in neurodegeneration are complex and include both glial-degeneration with a loss of glial function and glial reactivity  Neuroglia in Neurodegeneration In many neurodegenerative processes, asthenic (weakness) and degenerative changes in astroglia precede astrogliosis, most likely, by specific lesions and the appearance of damaged or dying neurons. In amyotrophic lateral sclerosis (ALS) for example, astrodegeneration and astroglial atrophy occur before clinical symptoms and neuronal death.  Neuroglia in Neurodegeneration In Huntington’s 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, thus worsening pathological progression.  Neuroglia in Neurodegeneration In Parkinson’s disease (PD), astrocytes are supposed to play a neuroprotective role. Astrocytes contribute to the metabolism of dopamine; Astrocytes were shown to convert L-DOPA to dopamine. In the striatum, astrocytes act as a reservoir for L-DOPA, which they release to be subsequently transported to neurons. The level of glial fibrillary acidic protein (GFAP) expression was decreased in astrocytes in PD human tissue, indicating astroglial atrophy and reduced astrogliotic response, which may reflect compromised astroglial neuroprotection.  Neuroglia in Neurodegeneration Neurodegenerative diseases are almost consistently accompanied by chronic neuroinflammation with activation of microglia. The exact contribution of microglia in the neurodegenerative process remains debatable because both microglial activation with generation of neuroinflammatory cellular phenotypes and microglial paralysis are to be considered. This contrast in particular may reflect the differences between human and laboratory animal brains; there is little evidence for microglial activation in AD human tissue.  Neuroglia in Neurodegeneration Furthermore, human aging (in contrast to laboratory animals) is associated with significant atrophy of microglial cells, which may be a key factor for creating an environment, permissive for neurodegenerative alterations.  Neuroglia in Neurodegeneration 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.  Astrocytes in Alzheimer's Disease The pathological potential of astroglia in the context of dementia was realized by Alois Alzheimer, who often observed activated glial cells in close contact with pathologically altered neurons. He also described glia as a cellular component of the senile plaque. Subsequent studies frequently mentioned astroglial reactivity in the context of AD, although detailed analysis of astroglial pathology started to be investigated only very recently.  Astrocytes in Alzheimer's Disease - Astrodegeneration Astrocytes undergo complex morphological changes in animal AD models For example, in the triple transgenic 3xTg-AD mouse, which harbours mutant genes for amyloid precursor protein, presenilin 1 and microtubule-associated protein Tau, 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.  Astrocytes in Alzheimer's Disease - Astrodegeneration These changes are expressed by a decrease in the GFAP-positive (Glial Fibrillary Acidic Protein) astroglial profiles (both in area and volume measurements), decreased somata volume as well as a decrease in the number and branching of cell processes. The atrophic changes in astrocytes developed in a particular spatiotemporal pattern. Hence: The earliest signs of atrophy were observed; In the entorhinal cortex (at 1 months of age), In the prefrontal cortex from 3 months of age, and In the hippocampus from 9 to 12 months of age.  Astrocytes in Alzheimer's Disease - Astrodegeneration Confocal micrographs illustrating atrophy of GFAP-positive astroglial profiles in 3xTg-AD mice compared with control animals: Hippocampal dentate gyrus (DG) Entorhinal cortex (EC) Hippocampal cornu ammonis 1 (CA1) area Prefrontal cortex (PFC)  Astrocytes in Alzheimer's Disease - Astrodegeneration 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. There are, however, no significant changes in the overall number of GFAP-positive astrocytes either in the hippocampus or in the cortex. Astroglial atrophy is likely to result in reduced synaptic coverage, which in turn may affect neurotransmission and connectivity of neuronal networks. Astrocytes in Alzheimer's Disease - Astrodegeneration Atrophic astrocytes fail to provide adequate homeostatic support, thus further worsening 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. Astrodegenerative changes may contribute to the development of early AD pathology.  Astrocytes in Disease - Astroglial Reactivity Astrocytes become reactive in response to injury and inflammation. There are at least two distinct categories of reactive astrocytes: hypertrophic reactive astrocytes and scar-forming astrocytes. Reactive astrocytes are found in many neurological diseases, such as Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), epilepsy, stroke and traumatic brain injury (TBI). Upregulation of glial fibrillary acidic protein (GFAP) is widely used as a marker of reactive astrocytes. Reactive astrocytes show hypertrophy with thicker processes.  Astrocytes in Disease - Astroglial Reactivity Roles of reactive astrocytes can be neuroprotective or neurotoxic, depending on the context. In brain or spinal cord injuries (SCIs), astrocytes become reactive astrocytes with morphological changes. At the site of injury, scar-forming astrocytes form glial scars. Preventing glial scar formation leads to infiltration of circulating immune cells and subsequent neuronal cell damage.  Astrocytes in Alzheimer's Disease - Astroglial Reactivity Astroglial reactivity is observed both in human post-mortem tissues and in the brains of AD animal models. The 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.  Astrocytes in Alzheimer's Disease - Astroglial Reactivity In the 3xTg-AD mice, for example, the reactivity in the hippocampus is characterized by a significant (up to 70%) increase in GFAP- positive astroglial profiles. 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 the entorhinal and prefrontal cortices. Astroglial reactivity in the context of AD has been shown both in vitro and in situ, 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.  Astrocytes in Alzheimer's Disease - Astroglial Reactivity - aberrant Ca2+ signals Astrocytes respond rapidly to injury and hyperexcitability to generate Ca2+ signals For example, astrocytes rapidly increase their Ca2+ in response to hyperexcitability in drug-induced seizure model. Aberrant Ca2+ signals are preferentially observed in the area where the tissue is strongly affected and where hypertrophic astrocytes are located. For example, in an in vivo adult mouse model of familial AD, reactive astrocytes displayed frequent Ca2+ signals near amyloid plaques and Ca2+ waves that originated from plaques  Astrocytes in Alzheimer's Disease - Astroglial Reactivity - aberrant Ca2+ signals In parallel with morphological changes, reactive astrocytes demonstrate dynamic, aberrant Ca2+ signals. In most cases, Ca2+ signals increase in terms of amplitude, duration and frequency. There is huge variation in the Ca2+ dynamics of reactive astrocytes in distinct pathological models, phases (acute or chronic) and regions, indicating diverse underlying mechanisms for aberrant Ca2+ signals that are dependent on the conditions. Severity of a disease relates to augmentation of aberrant Ca2+ signals in reactive astrocytes  Calcium Signaling and Astroglial Reactivity The regulation of astroglial reactivity is one of the key components of the 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 particularly that of Ca2+ release from the endoplasmic reticulum (ER). Genetic deletion of InsP3R type II, an ER Ca2+ release channel, in astrocytes greatly reduces astroglial reactive response to various lesions.  Calcium Signaling and Astroglial Reactivity In the context of AD, the ER Ca2+ release is directly linked to the initiation of reactive astrogliosis and inhibition of ER Ca2+ release channels (InsP3Rs) suppressed astrogliotic response. Exposure of astrocytes to β-amyloid not only triggers ER Ca2+ release and initiates astrogliotic remodelling but also induces ER stress, this type of ER stress known as unfolded protein response(UPR). Inhibition of Ca2+ release channels in astrocytes effectively suppressed the UPR.  Calcium Signaling and Astroglial Reactivity The ER stress may be involved in the initiation of reactive astrogliosis; depletion of the ER from releasable Ca2+ activates the UPR, which in turn triggers biochemical remodeling that underlie reactive response of astroglia. Astroglial reactivity in AD animals differs between brain regions with a 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.  Calcium Signaling and Astroglial Reactivity This specific property of the entorhinal astroglia may possibly account for their inability to support an astrogliotic response to accumulating β-amyloid. This “functional paralysis” of astrocytes may be directly related to differences in the susceptibility of brain regions to AD.  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 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. These molecules are delivered to the plasma membrane by vesicle traffic. This brings vesicles from the Golgi complex, deep in the cytoplasm, to the cell surface.  Vesicular Trafficking and Secretion in Astrocytes Are Altered in AD This traffic is maintained by an elaborated system regulated by increases in [Ca2+]i. The complexity of vesicle traffic regulation in astrocytes is characterized by two characteristic, 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.  Vesicular Trafficking and Secretion in Astrocytes Are Altered in AD Glutamatergic vesicle motility is accelerated by an increase in [Ca2+]i, whereas the same increase in [Ca2+]i slows down peptidergic vesicles and endolysosomes. Similar regulation also applies to recycling peptidergic vesicles, which have merged with the plasma membrane and subsequently entered the cytoplasm.  Vesicular Trafficking and Secretion in Astrocytes Are Altered in AD Astrocytes from 3xTg-AD mice isolated in the pre-symptomatic phase of the disease exhibit alterations in vesicle traffic. Spontaneous mobility of peptidergic and endolysosomal vesicles as well as the ATP-evoked, Ca2+-dependent, vesicle mobility were all diminished in diseased astrocytes.  Oligodendroglia in Disease In the human brain the central myelination is provided by oligodendrocytes which are present in both white and grey matter. 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. The loss of myelin is a characteristic feature of the aging 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.  Oligodendroglia in Disease In the progress 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. In the primary visual cortices of the rhesus monkey, age-dependent myelin deterioration has been characterized, and it appeared that the length of paranodes is decreased in aging indicating some shortcomings in demyelination. These changes in the myelin developed in parallel with the decrease in the self-renewal capacity of oligodendroglial precursors/NG2 cells.  Oligodendroglia in Alzheimer’s Disease Oligodendroglial cell death and myelin shortages are associated with abnormal Ca2+ homeostasis and signalling, which is caused by either extracellular (neurotransmitter dyshomeostasis) or cellular factors (alterations in the Ca2+ homeostatic cascades such as channels, transporters and pumps). Oligodendrocytes express several types of ionotropic receptors, including l-glutamate and P2X receptors, which are permeable to Ca2+.  Oligodendroglia in Alzheimer’s Disease 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. 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.  Oligodendroglia in Alzheimer’s Disease Experiments in-vitro have demonstrated that the exposure to β- amyloid damages oligodendrocytes possibly because the expression of mutant PS1 increases their sensitivity to l-glutamate toxicity. A similar effect has been observed in vivo when injection of β-amyloid into white matter induced axon disruption and damaged myelin and triggered the death of oligodendrocytes. The role of 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.  Oligodendroglia in Alzheimer’s Disease 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. In 3xTg-AD mice marked morphological atrophy and decreased numbers of NG2 glia were detected at the early stages; in the later phase, the NG2 glia associates themselves with senile plaques and infiltrate the latter with processes. A similar decrease in the NG2-positive profiles was observed in human AD post-mortem tissue.  Oligodendroglia in Alzheimer’s Disease All these alterations in myelin, oligodendroglia as well as degenerative changes in oligodendroglial precursors/NG2 cells may contribute to pathological remodeling of the connectome and hence to cognitive deficiency.  Microglia in Alzheimer’s Disease Microglial changes, both reactive and degenerative, are now considered to be an important part of AD progression. Activated microglial cells (together with astrocytes) are closely associated with senile plaques; they secrete numerous proinflammatory factors that may contribute to neuronal damage. 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).  Microglia in Alzheimer’s Disease In the ageing human brain, degeneration of microglia can define neural tissue vulnerability to the AD pathology. Activation of microglia can be triggered by β-amyloid, either soluble or oligomeric. In vivo imaging of transgenic mice demonstrated that microglia are activated and recruited to Aβ plaques only after the plaques had been formed. In another AD model of transgenic mice, activation of microglia, occurred much earlier (at 3 months) than the formation of senile plaques (10–12) months.  Microglia in Alzheimer’s Disease Activation of microglia in AD context may also involve purinergic signaling. In particular, P2X7 receptors were found to be necessary for activation of microglia in response to β-amyloid injection. Microglial activation in AD may also be regulated by TLR4 and TLR2 Toll receptors, which are upregulated in AD animal models and in post- mortem AD brains. A spontaneous loss-of-function mutation in the TLR4 gene markedly decreased microglial activation induced by Aβ.  Microglia in Alzheimer’s Disease 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. 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.  Microglia in Alzheimer’s Disease Appearance of the microglial activation seen as a significant increase in the density of activated microglia in the CA1 hippocampal area of 3xTgAD mice was detected at 12 and 18 months, a period that correlates with the appearance and development of Aβ plaques. The early increase in the density of resting microglia may represent the generalized response of the brain defense system to the developing AD pathology. Exposure of 3xTg-AD mice to running and enriched environments prevented this increase in the density of both resting and activated microglia. This finding indicates that environmental stress may affect microglial response to AD pathology.  In conclusion 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 morphofunctional changes in glia not only occur during distinct temporal domains, but also are region-specific. Since the changes in neuroglia precede those in neurons, 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.  REFERENCES Beart, P., Robinson, M., Rattray, M., & Maragakis, N. J. (2017). Neurodegenerative diseases: Pathology, Mechanisms, and Potential Therapeutic Targets. Springer. Tortora, Gerard J. And Bryan H Derrickson. Principles of Anatomy and Physiology. John Wiley & Sons, 2020