Paul Hubbard

Synantocytes: New functions for novel NG2 expressing glia

In the adult CNS, antibodies to the NG2 chondroitin sulphate proteoglycan (CSPG) label a large population of glia that have the antigenic phenotype of oligodendrocyte progenitor cells (OPC). However, NG2 expressing glia have the morphological phenotype of astrocytes, not OPC. We propose adult NG2 expressing glia are a distinct mature glial type, which we have called syantocytes or synantoglia after the Greek ‘to contact’, because they specifically contact neurons and axons at synapses and nodes of Ranvier, respectively. Synantocytes are highly complex cells that elaborate multiple branching processes and are an equally significant population in both white and grey matter. We provide evidence that phenotypically distinct synantocytes develop postnatally and that neither postnatal nor adult synantocytes depend on axons for their survival, indicating they respond with markedly different behaviours to the environmental cues and axonal signals that control the differentiation of OPC into oligodendrocytes. The primary response of synantocytes to changes in the CNS environment is a rapid and localised reactive gliosis. Reactive synantocytes interact intimately with astrocytes and macrophages at lesion sites, consistent with them playing a key role in the orchestration of scar formation that protects the underlying neural tissue. It is our hypothesis that synantocytes are specialised to monitor and respond to changes in the integrity of the CNS, by way of their cellular contacts, repertoire of plasmalemmal receptors and the NG2 molecule itself. To paraphrase Del Rio Hortega, we propose that synantocytes are the fifth element in the CNS, in addition to neurons, astrocytes, oligodendrocytes and microglia.

Synantocytes: the fifth element

Classic studies have recognized neurons and three glial elements in the central nervous system (CNS) – astrocytes, oligodendrocytes and microglia. The identification of novel glia that specifically express the NG2 chondroitin sulphate proteoglycan (CSPG) raises the possibility of a fifth element. Until recently, all NG2‐expressing glia were considered to be oligodendrocyte precursor cells (OPCs) that persist in the adult CNS to generate oligodendrocytes throughout life. However, this narrow view of the function of ‘NG2‐glia’ is being challenged. The majority of NG2‐expressing glia in the adult CNS are a distinct class of cells that we have called ‘synantocytes’ (from the Greek synanto for contact). Synantocytes are stellate cells, with large process arborizations, and are exquisitely related to neurons. Individual cells traverse white and grey matter and form multiple contacts with neurons, astrocytes, oligodendrocytes and myelin. Synantocytes are an integral component of the ‘tetrapartite’ synapse, and provide a potential integrative neuron‐glial communications pathway. Neuronal activity, glutamate and adenosine triphosphate (ATP) act on synantocyte receptors and evoke raised intracellular calcium. It remains to be seen whether this serves a physiological function, but synantocytes may be specialized to monitor signals from neurons and glia, and to respond to changes in the integrity of the CNS via their specific contacts and ion channel and receptor profiles. The general consequences of synantocyte activation are proliferation and phenotypic changes, resulting in glial scar formation, or regeneration of oligodendrocytes, and possibly neurons.

Functions of optic nerve glia: axoglial signalling in physiology and pathology

AbstractAn early concept of glial function envisaged them as passive and unexcitable structural elements, much like the connective tissues of organs in the periphery. It is now known that glia have a widespread range of physiological roles and react to all forms of pathological insult. This paper reviews the major functions of oligodendrocytes and astrocytes, the main types of glia in the optic nerve, and examines novel NG2-glia, otherwise known as oligodendrocyte progenitor cells (OPCs). The major function of oligodendrocytes is to produce the myelin sheaths that insulate CNS axons, but they also have important roles in the establishment of nodes of Ranvier, the sites of action potential propagation, and axonal integrity. Astrocytes have multiple physiological and pathological functions, including potassium homeostasis and metabolism, and reactive astrogliosis in response to CNS insults. The bulk of NG2-glia are postmitotic complex cells, distinct from OPCs, and respond to any insult to the CNS by a rapid and stereotypic injury response. This may be their primary unction, but NG2-glia, or a subpopulation of NG2-expressing adult OPCs, also provide remyelinating oligodendrocytes following demyelination. Oligodendrocytes, astrocytes, and NG2-glia all contact axons at nodes of Ranvier and respond to glutamate, ATP, and potassium released during axonal electrical activity. Glutamate and ATP evoke calcium signalling in optic nerve glia and have dual roles in physiology and pathology, coupling glial functions to axonal activity during normal activity, but enhanced activation induces an injury response, as seen following injury, demyelination, and ischaemia.

Cytology and lineage of NG2-positive glia.

We present evidence that NG2+ glia are an integral part of an oligodendrocyte/synantocyte (OS) lineage stream the progenitors of which begin to produce both glial phenotypes at about birth. The NG2 CSPG is differentially distributed within the OS lineage, being expressed in progenitors and synantocytes but not in oligodendrocytes. All cells in the OS lineage, except the primordial stem cells, express O4. The oligodendrocyte line reacts with CD9, but synantocytes are CD9−. Nonetheless, synantocytes are morphologically complex and specialised glia which contact axolemma in myelinated fibres at nodes of Ranvier and synaptic terminals, and form >99% of all NG2+ glia in the adult CNS. Thus, the other NG2+ phenotype, the adult oligodendrocyte progenitor cell (AOPC), constitutes a small population of <1% of all NG2+ glia in the mature CNS. AOPC are a heterogeneous set of cells probably originating from multiple sources which, by definition, produce oligodendrocytes in the adult to replace loss after trauma, demyelination and normal ‘wear and tear’. The definitive functions of synantocytes remain undefined.

Alpha-synuclein pathology in the olfactory pathways of dementia patients.

Lewy‐type pathology is a characteristic of a number of neurodegenerative disorders, including Parkinsons disease and dementia with Lewy bodies. Thus far, the definitive diagnosis of these dementias can only be confirmed at post‐mortem. However, it is known that the loss of smell (anosmia) is an early symptom in patients who develop dementia, and the use of the smell test has been proposed as an early diagnostic procedure. The aim of this study was to understand further the extent of Lewy pathology in the olfactory system of patients with neurodegenerative disorders. Post‐mortem tissue from 250 subjects was obtained from the OPTIMA brain bank. Five areas of the olfactory pathway were examined by immunolabelling for alpha‐synuclein – a major component of Lewy pathology: the olfactory tract/bulb (n = 79), the anterior olfactory nucleus in the lateral olfactory gyrus (n = 193), the region of olfactory projection to the orbito‐frontal cortex (n = 225), the hippocampus (n = 236) and the amygdala (n = 201). Results show that Lewy pathology affects different parts of the olfactory pathways differentially, suggesting a specific pattern of development of pathology. Clinical Parkinsons disease is most likely to be identified if the orbito‐frontal cortex is affected, while the diagnosis is less likely if the pathology is restricted to the olfactory bulb or tract. These results suggest that pathology in the olfactory bulb and tract occurs prior to clinical signs of Parkinsons disease. Furthermore, the results presented here provide further evidence supporting the possible value of a smell test to aid the clinical diagnosis of neurodegenerative diseases.

Dysfunction of the mTOR pathway is a risk factor for Alzheimer's disease

BackgroundThe development of disease-modifying therapies for Alzheimer’s disease is hampered by our lack of understanding of the early pathogenic mechanisms and the lack of early biomarkers and risk factors.We have documented the expression pattern of mTOR regulated genes in the frontal cortex of Alzheimer’s disease patients. We have also examined the functional integrity of mTOR signaling in peripheral lymphocytes in Alzheimer’s disease patients relative to healthy controls.ResultsIn the brain mTOR is seen to control molecular functions related to cell cycle regulation, cell death and several metabolic pathways. These downstream elements of the mTOR signaling cascade are deregulated in the brain of Alzheimer’s disease patients well before the development of pathology. This dysregulation of the mTOR downstream signaling cascade is not restricted to the brain but appears to be systemic and can be detected in peripheral lymphocytes as a reduced Rapamycin response.ConclusionsThe dysfunction of the signaling pathways downstream of mTOR may represent a risk factor for Alzheimer’s disease and is independent of the ApoE status of the patients.We have also identified the molecular substrates of the beneficial effects of Rapamycin on the nervous system. We believe that these results can further inform the development of clinical predictive tests for the risk of Alzheimer’s disease in patients with mild cognitive impairment.

Effects of glutamate receptor activation on NG2‐glia in the rat optic nerve

NG2‐glia are a substantial population of cells in the central nervous system (CNS) that can be identified by their specific expression of the NG2 chondroitin sulphate (CSPG). NG2‐glia can generate oligodendrocytes, but it is unlikely this is their only function; indeed, they may be multipotent neural stem cells. Moreover, NG2‐glia are a highly reactive cell type and a major function is to help form the axon growth inhibitory glial scar in response to CNS injury. The factors that regulate these diverse behaviours of NG2‐glia are not fully resolved, but NG2‐glia express receptors to the neurotransmitter glutamate, which has known potent effects on other glia. Here, we have examined the actions of glutamate receptor activation on NG2‐glia in the rat optic nerve, a typical CNS white matter tract that does not contain neuronal cell bodies. Glutamate induces an increase in [Ca2+]i in immuno‐identified NG2‐glia in situ and in vitro. In addition, we examined the effects of glutamate receptor activation in vivo by focal injection of the glutamate receptor agonist kainate into the optic nerve; saline was injected in controls. Changes in glial and axonal function were determined at 7 days post injection (dpi), by immunohistochemistry and electrophysiological measurement of the compound action potential (CAP). Injection of kainate resulted in a highly localized ‘injury response’ in NG2‐glia, marked by dense labelling for NG2 at the lesion site, as compared to astrocytes, which displayed a more extensive reactive astrogliosis. Furthermore, injection of kainate resulted in an axonal conduction block. These glial and axonal changes were not observed following injection of saline vehicle. In addition, we provide evidence that endogenous glutamate induces calcium‐dependent phosphorylation of extracellular signal‐regulated kinases (ERK1/2), which may provide a potential mechanism by which glutamate‐mediated changes in raised intracellular calcium could regulate the observed gliosis. The results provide evidence that activation of AMPA‐kainate type ionotropic glutamate receptors evoke raised calcium in NG2‐glia and induces an injury response in NG2‐glia.

1: Novel NG2-glia (synantocytes): the fifth element

N G 2 is a novel chondroitin sulphate proteoglycan (CSPG) that is expressed in the adult CNS by novel NG2-glia. NG2-glia have largely been considered to be oligodendrocyte progenitor cells (OPCs). However, adult NG2-glia are not the same as simple OPCs described in culture. Instead, NG2-glia are stellate cells which make up approximately 5–8% of all glia in the adult CNS. They are equally numerous in grey and white matter, and are found closely associated with neurons in areas of the brain where oligodendrocytes never develop. Adult NG2glia form intimate contacts with neurons and axons at synapses and nodes of Ranvier, respectively. We now show that synantocytes respond to neurotransmitters by raised intracellular calcium, supporting a role for neuronal activity in activating calcium-dependent signalling in NG2-glia. The primary response of NG2-glia to the disruption of neuronal signalling appears to be a rapid and localized reactive gliosis. NG2-glia also become reactive in response to cytokines released following CNS injury, such as platelet-derived growth factor, fibroblast growth factor and transforming growth factor. Indeed, NG2-glia are an important component of the protective glial scar, and we now provide evidence that NG2-glia may provide guidance cues for regenerating axons. Moreover, enzymatic disruption of NG2-glia has adverse effects on neuronal function. In addition, there is evidence that adult NG2glia are able to regenerate oligodendrocytes in the adult CNS. Thus NG2-glia have multiple neuron-supportive functions, which are largely unresolved. To distinguish them from true OPCs, which give rise to oligodendrocytes during development, we have termed these NG2-glia synantocytes, after the Greek ‘to contact’, because they specifically contact neurons and appear to be specialized to monitor and respond to changes in neuronal integrity. In conclusion, to paraphrase Del Rio Hortega, we propose that synantocytes are the fifth element in the CNS after neurons, astrocytes, oligodendrocytes and microglia.