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  • br Conclusion Until recently astrocytes were recognized

    2021-09-11


    Conclusion Until recently, astrocytes were recognized solely for their maintenance role in the regulation of Compound C homeostasis. Nowadays, this idea has changed, it is clear that these cells play a crucial role in the regulation of neuronal activity and signal transmission. In particular, glial Glu transporters are responsible for the vast majority of the removal of this neurotransmitter from the synaptic cleft, highlighting the importance of these cells in brain physiology. Currently, the number of diseases related to the imbalance of the glutamatergic system, is increasing. Therefore, the interest and importance on the regulation of Glu transporters gene expression and function has also increased. Glu transporters are tightly regulated at different levels including gene expression, transporter protein targeting and trafficking (Fig. 1). Posttranslational modifications of the transporter protein are also involved the regulatory mechanisms. It is clear that the establishment of the biochemical transactions involved in the regulation of Glu transporters is fundamental for the development of new therapeutic strategies for a significant number of neurological diseases associated to glutamatergic neurotransmission.
    Conflicts of interest
    Acknowledgments The work in the laboratory is supported by Conacyt-Mexico (255087) and Soluciones para un México Verde S.A. de C.V. grants to A.O.
    Introduction Glutamate (glutamic acid) is one of the most common amino acids, and its free form (free glutamate) presents naturally in various foods. Specific food items, such as tomatoes and cheese, are high in free glutamate content [1]. Free glutamate has a function as Umami substance [2] and its sodium salt, monosodium glutamate (MSG), is widely used as a flavor enhancer during food processing as well as home cooking [1], [3]. When added to foods, MSG dissociates in the neutral area and presents as free glutamate. The naturally-occurring glutamate and the one derived from MSG are chemically indistinguishable and human bodies metabolize them in the same way, regardless of their source of origin [3], [4]. Glutamate, only in its free form but not in protein-bound form, activates umami taste receptors such as T1R1 and T1R3, and this function is thought to mediate appetitive responses to protein-rich foods [5]. According to the US Food and Drug Administration (FDA), the mean glutamate intake (both free and protein-bound) estimated based on protein intake was 13 g/d, while MSG intake was 0.55 g/d [4]. Over the years, many studies have investigated the metabolic functions and health effects of glutamate in animals, experimental, or randomized controlled studies [6], [7], [8], [9], [10]. Use of MSG has been suggested to cause several health outcomes, including headache and nausea, in addition to a risk factor for obesity [11]. However, a systematic review of intervention trials studying a causal relationship between MSG intake and adverse health effects concluded that this relationship has not been proven due to the absence of proper blinded experiments and consistent findings [10], [12]. In actuality, the FDA classifies MSG as “Generally Recognized as Safe [4].” The possible mechanism of glutamate for health effects in humans has been poorly explained in many previous studies [6], [7], [8], [9], [10], [11], [12] or only suggested without clinical data [3]. The results from animal studies using mice should be interpreted with caution due to their vulnerability to MSG, compared to other animal species [13]. Thus, the relationship between MSG intake and health outcomes in humans remains unclear at present. To investigate the relationship between MSG intake and health outcomes, total free glutamate intake (including naturally-occurring free glutamate and added free glutamate as MSG) should be taken into consideration. While a limited number of studies have reported estimates of MSG intake in free-living populations [14], [15], [16], [17], [18], [19], the quality of the assessment methods has been limited by the intake calculation being solely dependent on MSG that is added as food additives or present in seasonings (eg, soy source) and without a consideration of naturally-occurring free glutamate. To our knowledge, there have not been any studies conducted that comprehensively evaluate dietary intake and major sources of free glutamate naturally-occurring in a free-living setting. This is mainly due to the unavailability of sufficient data on free glutamate naturally-contained in foods.