Part of the Group: Glial-Neuronal Interactions

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Image from CNRS, http://www2.cnrs.fr/en/1412.htm


















Historically, glia were only seen as the “glue” in the brain, keeping neurons together in the form of a brain and served no other function. However, research into these cells has revealed that their roles go beyond simply being accessories for neurons, as they are highly involved in synaptogenesis during development as well as synaptic transmission (Suguira & Lin, 2011) . Many studies in this area have been performed at the neuromuscular junction, where perisynaptic Schwann cells reliably play a role in maintaining synaptic efficacy and repair after damage in this area. Glia such as astrocytes perform many crucial functions for neurons to regulate the neuronal environment. For example, since increased levels of extracellular K+ enhance depolarization of neurons, the astrocytic process of potassium spatial buffering allows the astrocyte to evenly disperse uptaken K+ to other areas of the environment (Bear et al, 2007). Microglia, as the inflammatory cells in the brain, also affect neuronal processes, as they have been shown to impair NMDA-dependent and independent forms of synaptic plasticity (Min et al, 2009). Receptors for various neurotransmitters have also been discovered on glial membranes, allowing them to be sensitive of surrounding neural activity. In addition to this, there has been evidence that glia are capable of synthesizing various neurotransmitters that influence synaptic activity. Armed with these capabilities, various glia can play the vital role of regulating and maintaining proper functioning of synapses throughout the nervous system.

The Tripartite Synapse



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Figure 1. Hypothesized astrocyte-neuronal interactions in the tripartite synapse. Figure from Agulhon et al, 2008
Astrocytes are highly abundant in the central nervous system (CNS), involved not only in regulating the neuronal environment but also modulating synaptic activity. In what has been coined the tripartite synapse, the canonical synapse is expanded to include the adjacent glial cell that extends its processes near the pre- and post-synaptic neuronal terminals (Araque et al, 1999). With more than 50% of the excitatory synapses in the hippocampus are associated with astrocytic processes (Ventura & Harris, 1999), one of the most studied tripartite synapses in the brain is the Schaffer collateral (SC) - CA1 synapse. Recent studies are beginning to examine the structures on the astrocytic membrane to investigate how G-protein coupled receptors (GPCR) sense neurotransmitters released into the synapse and cause intracellular changes (Perea et al, 2009). Furthermore, astrocytes can release gliotransmitters (signaling molecules similar to neurotransmitters but produced by glia) to influence synaptic activity. Miscommunication between neurons and glia may be implicated in conditions such as epilepsy and Alzheimer’s disease (Lalo et al, 2009), which makes these glial-neuronal interactions important to study.



Clearance of Neurotransmitters from the Synaptic Cleft


Similar to how astrocytes perform potassium spatial buffering to regulate the neuronal environment, glial clearance of neurotransmitters protects against excitotoxicity. Astrocytes express glutamate transporters that clear this neurotransmitter from the synaptic cleft (Oliet et al, 2001). If these channels are pharmacologically inhibited or antisense oligonucleotides specific for the transporter’s mRNA are injected, elevated glutamate levels are observed. This blockage resulted in glutamate buildup, culminating in excitotoxicity and paralysis in rats (Rothstein et al, 1994). Likewise, astrocytes also protect against excessive inhibition, through glial clearance of glycine, a major inhibitory neurotransmitter in the CNS. Deletion of the GlyT1 transporter gene caused severe deficits in motor and respiratory function in mice (Gomeza et al, 2003). Proper functioning in organisms is dependent on a fine balance between excitation and inhibition as an excess of either component can result in serious consequences. Thus, glial clearance of neurotransmitters is vital to regulating synaptic activity.


Sensing Synaptic Activity



The positioning of glial cells at synapses puts it in a perfect position not only to regulate the synaptic environment, but also allows it to contribute to synapses. Neurotransmitter receptors on astrocytes allow them to be sensitive to activity within the synapse (Panatier et al, 2011), and there is evidence that astrocytes express receptors for the most crucial neurotransmitters (glutamate, GABA, acetylcholine; Hatton, 2002). Intracellular levels of Ca2+ increase in response to synaptic activity in the tripartite synapse, which is not observed if neurotransmitter release is inhibited. In the neuromuscular junction where these studies were first conducted, the application of acetylcholine and ATP resulted in increased intracellular Ca2+ levels in perisynaptic Schwann cells, as well as during high frequency presynaptic stimulation (Jahromi et al, 1992). More recent studies of astrocytes near the SC-CA1 syanpse show that stimulation of the SC elicits intracellular Ca2+ increases (Honsek et al, 2010). Studies that use Ca2+ chelators to prevent Ca2+ increase reduce the glial impact on modulating synaptic transmission (Panatier et al, 2011).






With further study, it was discovered that intracellular Ca2+ changes were mediated by Gq-coupled GPCRs. Metabotropic glutamate receptors are likely to be involved, as studies have shown Group I receptors, especially mGluR5 (Cai et al, 2000) to be important in detecting levels of glutamate (Perea & Araque, 2006). It has been suggested that the phospholipase C (PLC) – inositol (1,4,5)-triphosphate (IP3) pathway is activated to release Ca2+ from its intracellular stores of the endoplasmic reticulum. This hypothesis has been supported by studies where knockouts of IP3 receptors in astrocytes (identified to be the IP­3R2 subtype) eliminates Ca2+ elevations, although spontaneous Ca2+ activity is not observed either (Agulhon et al, 2008). Although most studies suggest that these intracellular changes are observed more long term and occur more slowly, some studies done in situ (Di Castro et al, 2011; Santello et al, 2011) show evidence that astrocytic Ca2+ elevations occur within 500 ms of stimulation to provide fast modulatory action (Santello & Volterra, 2009). These fast responses are produced by endogenous synaptic activity (Henneberger & Rusakov, 2010) or metabotropic receptor activation (Santello et al, 2011). The field is currently limited by its physiological tools to use to study this phenomena, as a method that only activates Gq GPCR selectively in astrocytes has yet to be found. Thus, the mechanism by which the signal is transduced within the cell is not clear, although it is known that Ca2+ elevations are involved.
These interactions can be complex, where neurotransmitters can modulate uptake based on the presence or quantity of other neurotransmitters. For example, astrocytic uptake of GABA via GAT-1 and GAT-3 transporters are subject to modulation by varying levels of adenosine. The heterotetramer complex composed of A1R-A2R adenosine receptors can either enhance (via A2R homodimers on the Gs protein) or inhibit (via A1R homodimers acting on the Gi protein) GABA uptake (Cristóvão-Ferreira et al, 2011).


Secretion of Gliotransmitters into the Synaptic Cleft



Astrocytes are capable of synthesizing gliotransmitters (such as glutamate, Jourdain et al, 2007; and ATP, Blum et al, 2008), which allow them to directly influence synaptic activity and neuronal excitability (Sasaki et al, 2011). These gliotransmitters are stored in small synaptic-like microvesicles (Araque et al, 1998) that are similar to synaptic vesicles in neuronal terminals. There is substantial evidence of Ca2+ waves preceding astrocytic release of glutamate in CNS slices (Bezzi et al, 2004) and in vitro (Araque et al, 1998). The intracellular Ca2+ wave causes SNARE proteins (synaptobrevin II, synataxin and SNAP-23) in the cell to dock these microvesicles on the astrocytic membrane and release the contents of the vesicle to interact with the neighbouring neurons. In the SC-CA1 synapse, glutamate binds to extrasynaptic NR2B NMDA receptors on the dendrites of CA1 neuron, which is sufficient to cause the neuron to depolarize (Jourdain et al, 2007). The released glutamate may also affect the presynaptic neuron through the activation of Group 1 mGluRs (Perea et al, 2009), to increase the frequency of excitatory synaptic activity (Navarrete & Araque, 2010). In contrast, astrocytes can inhibit neuronal activity by releasing ATP, which is degraded into adenosine, to suppress neurons. This makes astrocytes a potent force in neuronal transmission (Araque et al, 1998).
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Apart from glutamate and ATP, astrocytes also seem to be capable of synthesizing acetylcholine due to the presence of the enzyme acetyltransferase (Lan et al, 1996), although less is known about its influence in vivo.


Role of Adenosine in Sleep and Sleep Deprivation
Due to the time scale during which glia-neuronal modifications take place, astrocytes may play a significant role in regulating mammalian sleep patterns and may be responsible for the cognitive consequences of sleep deprivation (Halassa et al, 2009). Since adenosine levels accumulate as a function of the amount of activity in the brain (Porkka-Heiskanen et al, 1997), astrocytic expression of adenosine receptors makes them an optimal monitor for mental fatigue. As a result, astrocytes can adjust the pressure to sleep appropriately by determining the intensity of sleep required for proper rest.

The astrocytic release of ATP can have both excitatory and inhibitory effects on neighbouring neurons. ATP directly activating neuronal P2X receptors can increase AMPA receptor expression for the potential of more excitation (Lalo et al, 2009). On the other hand, if ATP is hydrolyzed by extracellular enzymes into adenosine, it can activate A1 receptors presynaptically to attenuate neuronal activity (Basheer et al, 2004). Application of adenosine showed hippocampal activity suppression by nearby astrocytes. This opens up the possibility of using pharmacological substances to selectively control the inclination to sleep. Should prolonged awakeness be desired, selective inhibition of A1 receptors can reduce the pressure to sleep. Similarly, agonists for the A1 receptor can be used to induce sleep in the treatment of sleep disorders such as insomnia.


Controversy over Gliotransmitters Modulating Synaptic Activity In Vivo



However, many of the studies in this area involve artificial conditions, requiring reliable electrical stimulation of the SC or drugs that are not commonly in the neuronal environment. Although neuronal activity is definitively coupled with Ca2+ increases in astrocytes, gliotransmission has not been demonstrated in vivo. The importance of gliotransmission in neurophysiology remains unclear. Conflicting studies suggest that the process of gliotransmission does not reliably affect the pre- or post-synaptic membrane (Fiacco et al, 2007). Animals models with selective mutations that affect the Gq GPCR signaling pathway (eg. IP3R2 knockouts) have shown inconclusive evidence regarding the involvement of Ca2+ in glutamate release at the SC-CA1 synapse. Since it has yet to be demonstrated whether Ca2+ elicits release of gliotransmitters or affects synaptic activity in vivo is still a controversial issue (Agulhon et al, 2008).

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Figure from Agulhon et al, 2008.



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