ER stress promoted MAMs formation and ER-Mito Ca2+ transfer: implication for fluoride-induced hippocampal neuronal injury and cognitive deficits in mice.

Fluoride is globally distributed in the natural environment, and its health effects in humans are well documented [1,2]. The consumption of fluoride in an adequate amount prevents the formation of dental caries and deficiencies in bone mineralization. However, excessive fluoride intake can result in deleterious effects on dental health as well as other soft tissues [[3], [4], [5]]. Excessive fluoride intake is capable of penetrating the blood-brain barrier (BBB), resulting in detrimental effects on various brain regions [6,7]. Numerous epidemiological investigations [8] have indicated a potential link between fluoride exposure and cognitive dysfunction [9]. Previous studies have disclosed that sustained fluoride exposure elicits mitochondrial oxidative stress [10], compromises mitochondrial function [11], and promotes chaperone-mediated autophagy-dependent neuronal ferroptosis [12].

The endoplasmic reticulum (ER) is a highly versatile organelle, with core functions including protein folding, lipid synthesis, and maintenance of intracellular calcium (Ca²⁺) homeostasis. Disruption of ER function by environmental or genetic perturbations results in the accumulation of misfolded proteins, triggering ER stress [13]. ER stress initiates the unfolded protein response (UPR) to resist the unfavorable external environment and restore ER homeostasis. However, sustained unresolved ER stress gives rise to cell death through multiple pathways [14,15]. Extensive literature suggests that continuous ER stress and disturbance in UPR signaling transduction are emerging as key drivers of neurological and neurobehavioral disorders [16]. Previous research has disclosed a close link between ER stress and fluoride toxicity [17]. Recent research has demonstrated the critical role of ER stress in fluoride exposure-provoked neurotoxicity in SH-SY5Y neurons and rat hippocampus and cortex [18]. However, the exact mechanism whereby ER stress mediates fluoride-induced hippocampal damage remains to be fully elucidated.

ER and mitochondria are physically coupled to constitute the dynamic stable membrane microdomains, referred to as mitochondria-associated membranes (MAMs) derived from the ER, which regulate a variety of fundamental biological processes, i.e., lipid homeostasis [19], Ca²⁺ dynamics/signaling [20], autophagy [21], mitochondrial shape and size [22], and apoptosis and energy metabolism [23,24]. Disruption of ER-mitochondria communication is associated with various pathophysiological conditions, including a variety of neurological disorders [25] and neurodegenerative diseases [26]. Recent studies have reported the MAMs abnormalities following fluoride exposure, as manifested by an obvious increase in the length of the ER-mitochondria contact sites and protein expression of MAMs-resident calcium channels in SH-SY5Y cells and primary neural cells [27]. The ER membrane inositol 1,4,5 triphosphate receptor (IP3R) couples with voltage-dependent anion channels (VDAC1) protein at the outer mitochondrial membrane via the chaperone glucose-regulated protein 75 (Grp75) to form an IP3R1/Grp75/VDAC1 macromolecular complex at MAMs, which modulates Ca²⁺ transfer from ER to mitochondria. Excessive MAMs formation causes Ca²⁺ transfer from ER to mitochondria via IP3R1/Grp75/VDAC1 and promotes Ca²⁺ overload in mitochondria, resulting in mitochondrial dysfunction. It is well-documented that mitochondrial dysfunction has been implicated in the pathological progression of fluoride neurotoxicity [28]. Nevertheless, the interplay among MAMs, ER stress and mitochondrial dysfunction underlying fluoride-related hippocampal neuronal damage and cognitive dysfunction has not been fully elucidated.