Obesity is a growing worldwide health problem, affecting many people due to excessive saturated fat consumption, lack of exercise, or a sedentary lifestyle. Leptin is an adipokine secreted by adipose tissue that increases in obesity and has central actions not only at the hypothalamic level but also in other regions and nuclei of the central nervous system (CNS) such as the cerebral cortex and hippocampus. These regions express the long form of leptin receptor LepRb, which is the unique leptin receptor capable of transmitting complete leptin signaling, and are the first regions to be affected by chronic neurocognitive deficits, such as mild cognitive impairment (MCI) and Alzheimer’s Disease (AD). In this review, we discuss different leptin resistance mechanisms that could be implicated in increasing the risk of developing AD, as leptin resistance is frequently associated with obesity, which is a chronic low-grade inflammatory state, and obesity is considered a risk factor for AD. Key players of leptin resistance are SOCS3, PTP1B, and TCPTP whose signalling is related to inflammation and could be worsened in AD. However, some data are controversial, and it is necessary to further investigate the underlying mechanisms of the AD-causing pathological processes and how altered leptin signalling affects such processes.

Lysosomal dysfunction lies at the nexus of inflammaging, microglial dystrophy and synaptic fragility, making it an attractive target for brain rejuvenation. Here we demonstrate that a five-month oral course of ketotifen, an approved H1-antihistamine and mast-cell stabiliser, re-acidifies lysosomes in aged C57BL/6J male mice, restoring the quinacrine signal of peripheral macrophages and SIM-A9 microglia. This proton rebound is coupled to broad anti-cytokine effects: ketotifen attenuates lipopolysaccharide-evoked release of TNF-α, IL-1β and IL-10 in vitro and ex vivo. In the brain, the drug restores a highly ramified, homeostatic microglial morphology throughout the cortex and hippocampus. Ketotifen robustly elevates cortical synaptophysin and PSD-95 above age-matched levels. Behaviourally, ketotifen enhances spatial learning and object-location memory without altering locomotor activity or anxiety-like behaviour. Collectively, these findings identify lysosomal re-acidification as the initiating trigger of a multifaceted rejuvenation cascade that dampens multi-cytokine signalling, restores microglial morphology and preserves synaptic integrity. Because ketotifen is inexpensive, brain-permeable and already licensed for human use, our work unveils an immediately actionable geroprotective strategy to forestall early cognitive decline.

Mast cells (MCs) in the brain can respond to environmental cues and relay signals to neurons that may directly influence neuronal electrical activity, calcium signaling, and neurotransmission. MCs also express receptors for neurotransmitters and consequently can be activated by them. Here, we developed a coculture model of peritoneal MCs, incubated together with dissociated hippocampal neurons for the study of cellular mechanisms involved in the mast cell-neuron interactions. Calcium imaging was used to simultaneously record changes in intracellular calcium [Ca2+]i in neurons and MCs. To provide insight into the contribution of MCs on neurotransmitter release in rat hippocampal neurons, we used analysis of FM dye release, evoked by a cocktail of mediators from MCs stimulated by heat. Bidirectional communication is set up between MCs and hippocampal neurons. Neuronal depolarization caused intracellular calcium [Ca2+]i oscillations in MCs that produced a quick response in neurons. Furthermore, activation of MCs with antigen or the secretagogue compound 48/80 also resulted in a neuronal [Ca2+]i response. Moreover, local application onto neurons of the MC mediator cocktail elicited Ca2+ transients and a synaptic release associated with FM dye destaining. Neuronal response was partially blocked by D-APV, a N-methyl-D-aspartate receptor (NMDAR) antagonist, and was inhibited when the cocktail was pre-digested with chondroitinase ABC, which induces enzymatic removal of proteoglycans of chondroitin sulfate (CS). MC-hippocampal neuron interaction affects neuronal [Ca2+]i and exocytosis signaling through a NMDAR-dependent mechanism.

Mast-cell (MC) granules are secretory lysosomes whose function depends on a highly acidic lumen. We asked whether lysosomal pH drifts with age and whether this alteration is reversible. Lysosomal acidification was assessed by quinacrine imaging of peritoneal MCs, which revealed very low fluorescence in MCs from 2-month-old mice, consistent with immature granules. As MCs matured, quinacrine signal increased, reflecting expansion of the secretory lysosome pool; however, from 15 to 17 months, fluorescence progressively declined, indicating gradual deacidification. Chronic ketotifen treatment restored and amplified the quinacrine signal, enlarged granules, and reduced late-life MC expansion. Acute pharmacological assays revealed that ketotifen's effect requires V-ATPase activity and dynamin-dependent endocytosis. FM1-43 uptake confirmed enhanced endocytic activity with ketotifen. In the brain, ageing led to hypertrophy of toluidine blue-positive MCs without major changes in cell number; five months of ketotifen treatment reversed these morphometric alterations toward a youthful profile. These findings identify lysosomal deacidification as a hallmark of ageing MCs and demonstrate that ketotifen reacidifies secretory lysosomes via V-ATPase and endocytosis-dependent mechanisms, highlighting lysosomal pH control as a tractable strategy to mitigate MC-driven components of inflammaging.