The role of the tripartite glutamatergic synapse in the pathophysiology of Alzheimer's disease

Dysfunction of NMDA receptors in Alzheimer's disease

N-methyl-D-aspartate receptors (NMDARs) play a pivotal role in the synaptic transmission and synaptic plasticity thought to underlie learning and memory. NMDARs activation has been recently implicated in Alzheimer's disease (AD) related to synaptic dysfunction. Synaptic NMDARs are neuroprotective, whereas overactivation of NMDARs located outside of the synapse cause loss of mitochondrial membrane potential and cell death. NMDARs dysfunction in the glutamatergic tripartite synapse, comprising presynaptic and postsynaptic neurons and glial cells, is directly involved in AD. This review discusses that both beta-amyloid (Aβ) and tau perturb synaptic functioning of the tripartite synapse, including alterations in glutamate release, astrocytic uptake, and receptor signaling. Particular emphasis is given to the role of NMDARs as a possible convergence point for Aβ and tau toxicity and possible reversible stages of the AD through preventive and/or disease-modifying therapeutic strategies.

Signaling pathways regulating neuron-glia interaction and their implications in Alzheimer's disease

Astrocytes are the most abundant cells in the central nervous system. They play critical roles in neuronal homeostasis through their physical properties and neuron-glia signaling pathways. Astrocytes become reactive in response to neuronal injury and this process, referred to as reactive astrogliosis, is a common feature accompanying neurodegenerative conditions, particularly Alzheimer's disease. Reactive astrogliosis represents a continuum of pathobiological processes and is associated with morphological, functional, and gene expression changes of varying degrees. There has been a substantial growth of knowledge regarding the signaling pathways regulating glial biology and pathophysiology in recent years. Here, we attempt to provide an unbiased review of some of the well-known players, namely calcium, proteoglycan, transforming growth factor β, NFκB, and complement, in mediating neuron-glia interaction under physiological conditions as well as in Alzheimer's disease. This review discusses the role of astrocytic NFκB and calcium as well as astroglial secreted factors, including proteoglycans, TGFβ, and complement in mediating neuronal function and AD pathogenesis through direct interaction with neurons and through cooperation with microglia.

Danggui-Shaoyao-San: New Hope for Alzheimer's Disease

Danggui-Shaoyao-San (DSS), also called Toki-shakuyaku-san (TJ-23) or Dangguijakyak-san (DJS), is a well-known herbal formula (Angelica sinensis (Oliv.) Diels., Ligusticum chuanxiong Hort., Paeonia lactiflora pall., Poria cocos (Schw.) Wolf, Alisma orientalis (Sam.) Juzep., Atractylodes macrocephala Koidz.), which has been widely used in oriental countries for the treatment of various gynecological diseases. Recent studies show that DSS has an effect on free radical-mediated neurological diseases and exhibits anti-inflammatory and antioxidant activities and reduces cell apoptosis in the hippocampus. In addition, DSS mediates the modulation of central monoamine neurotransmitter systems and ameliorates dysfunction of the central cholinergic nervous system and scopolamine-induced decrease in ACh levels. DSS improves the function of the dopaminergic, adrenergic, and serotonergic nervous systems. Interestingly, DSS can alleviate cognitive dysfunction of Alzheimer's disease (AD) patients, suggesting that it is a useful therapeutic agent for AD. This paper reviews the mechanism of DSS for the treatment of AD.

Long live the axon. Parallels between ageing and pathology from a presynaptic point of view

All animals have to find the right balance between investing resources into their reproductive cycle and protecting their tissues from age-related damage. In higher order organisms the brain is particularly vulnerable to ageing, as the great majority of post-mitotic neurons are there to stay for an entire life. While ageing is unavoidable, it may progress at different rates in different individuals of the same species depending on a variety of genetic and environmental factors. Inevitably though, ageing results in a cognitive and sensory-motor decline caused by changes in neuronal structure and function. Besides normal ageing, age-related pathological conditions can develop in a sizeable proportion of the population. While this wide array of diseases are considerably different compared to physiological ageing, the two processes share many similarities and are likely to interact. At the subcellular level, two key structures are involved in brain ageing: axons and their synapses. Here I highlight how the ageing process affects these structures in normal and neurodegenerative states in different brain areas.

Is riluzole a new drug for Alzheimer's disease?

This Editorial highlights a study by Hunsberger et al. (2015) in the current issue of Journal of Neurochemistry, in which the authors explore the effects of riluzole (R) treatment on tau-P301L transgenic mice. The authors employed a comprehensive analysis of possible restorative effects of the drug by examining glutamate levels in subregions of the hippocampus, expression of tau and its hyper-phosphorylated forms, and memory function using behavioral tests. The authors report a simultaneous reduction in glutamate reuptake and an increase in glutamate release in the tau-P301L model, both of which are ameliorated with riluzole treatment. The authors' findings have implications for our understanding of synaptic transmission mechanisms also associated with Alzheimer's disease pathology.

The multiple roles of the α7 nicotinic acetylcholine receptor in modulating glutamatergic systems in the normal and diseased nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) play an important role in a variety of modulatory and regulatory processes including neurotransmitter release and synaptic transmission in various brain regions of the central nervous system (CNS). Glutamate is the principal excitatory neurotransmitter in the brain and the glutamatergic system participates in the pathophysiology of several neuropsychiatric disorders. Underpinning the importance of nAChRs, many studies demonstrated that nAChRs containing the α7 subunit facilitate glutamate release. Here, we review the currently available body of experimental evidence pertaining to α7 subunit containing nAChRs in their contribution to the modulation of glutamatergic neurotransmission, and we highlight the role of α7 in synaptic plasticity, the morphological and functional maturation of the glutamatergic system and therefore its important contribution in the modulation of neural circuits of the CNS.

Gintonin stimulates gliotransmitter release in cortical primary astrocytes

Lysophosphatidic acid (LPA) is a simple and minor phospholipid, but serves as a lipid-derived neurotransmitter via activation of G protein-coupled LPA receptors. Astrocytes abundantly express LPA receptors and contain gliotransmitters that modulate astrocyte-neuron interactions. Gintonin is a novel ginseng-derived G protein-coupled LPA receptor ligand. Gintonin induces [Ca(2+)]i transients in neuronal and non-neuronal cells via activation of LPA receptors, which regulate calcium-dependent ion channels and receptors. A line of evidence shows that neurotransmitter-mediated [Ca(2+)]i elevations in astrocytes are coupled with gliotransmitter release. However, little is known about whether gintonin-mediated [Ca(2+)]i transients are coupled to gliotransmitter release in astrocytes. In the present study, we examined the effects of gintonin on adenosine triphosphate (ATP) and glutamate release in mouse cortical primary astrocytes. Application of gintonin to astrocytes induced [Ca(2+)]i transients in a concentration-dependent and reversible manner. However, ginsenosides, other active ingredients in ginseng, had no effect on [Ca(2+)]i transients. The induction of gintonin-mediated [Ca(2+)]i transients was attenuated/blocked by the LPA1/3 receptor antagonist Ki16425, a phospholipase C inhibitor, an inositol 1,4,5-triphosphate receptor antagonist, and an intracellular Ca(2+) chelator. Gintonin treatment on astrocytes increased ATP and glutamate release in a concentration- and time-dependent manner. BAPTA and Ki16425 attenuated gintonin-mediated ATP and glutamate release in astrocytes. The present study shows that gintonin-mediated [Ca(2+)]i transients are coupled to gliotransmitter release via LPA receptor activation. Finally, gintonin-mediated [Ca(2+)]i transients and gliotransmitter release from astrocytes via LPA receptor activation might explain one mechanism of gintonin-mediated neuromodulation in the central nervous system.