The fibronectin domain-containing protein 5 (FNDC5), or irisin, is an adipo-myokine hormone produced during exercise, which shows therapeutic potential for conditions like metabolic disorders, osteoporosis, sarcopenia, obesity, type 2 diabetes, and neurodegenerative diseases, including Alzheimer’s disease (AD). This review explores its potential across various pathophysiological processes that are often considered independent. Elevated in healthy states but reduced in diseases, irisin improves muscle–adipose communication, insulin sensitivity, and metabolic balance by enhancing mitochondrial function and reducing oxidative stress. It promotes osteogenesis and mitigates bone loss in osteoporosis and sarcopenia. Irisin exhibits anti-inflammatory effects by inhibiting NF-κB signaling and countering insulin resistance. In the brain, it reduces amyloid-β toxicity, inflammation, and oxidative stress, enhancing brain-derived neurotrophic factor (BDNF) signaling, which improves cognition and synaptic health in AD models. It also regulates dopamine pathways, potentially alleviating neuropsychiatric symptoms like depression and apathy. By linking physical activity to systemic health, irisin emphasizes its role in the muscle–bone–brain axis. Its multifaceted benefits highlight its potential as a therapeutic target for AD and related disorders, with applications in prevention, in treatment, and as a complement to exercise strategies.

Background: Recent clinical and experimental studies have highlighted the involvement of Ventral Tegmental Area (VTA) dopamine (DA) neurons for the early pathogenesis of Alzheimer’s Disease (AD). We have previously described a progressive and selective degeneration of these neurons in the Tg2576 mouse model of AD, long before amyloid-beta plaque formation. The degenerative process in DA neurons is associated with an autophagy flux impairment, whose rescue can prevent neuronal loss. Impairments in autophagy can be the basis for accumulation of damaged mitochondria, leading to disturbance in calcium (Ca²⁺) homeostasis, and to functional and structural deterioration of DA neurons. Methods: In Tg2576 mice, we performed amperometric recordings of DA levels and analysis of dopaminergic fibers in the Nucleus Accumbens – a major component of the ventral striatum precociously affected in AD patients – together with retrograde tracing, to identify the most vulnerable DA neuron subpopulations in the VTA. Then, we focused on these neurons to analyze mitochondrial integrity and Apoptosis-inducing factor (AIF) localization by electron and confocal microscopy, respectively. Stereological cell count was also used to evaluate degeneration of DA neuron subpopulations containing the Ca²⁺-binding proteins Calbindin-D28K and Calretinin. The expression levels for these proteins were analyzed by western blot and confocal microscopy. Lastly, using electrophysiology and microfluorometry we analyzed VTA DA neuron intrinsic properties and cytosolic free Ca²⁺ levels. Results: We found a progressive degeneration of mesolimbic DA neurons projecting to the ventral striatum, located in the paranigral nucleus and parabrachial pigmented subnucleus of the VTA. At the onset of degeneration (3 months of age), the vulnerable DA neurons in the Tg2576 accumulate damaged mitochondria, while AIF translocates from the mitochondria to the nucleus. Although we describe an age-dependent loss of the DA neurons expressing Calbindin-D28K or Calretinin, we observed that the remaining cells upregulate the levels of Ca²⁺-binding proteins, and the free cytosolic levels of Ca²⁺ in these neurons are significantly decreased. Coherently, TUNEL-stained Tg2576 DA neurons express lower levels of Calbindin-D28K when compared with non-apoptotic cells. Conclusion: Overall, our results suggest that the overexpression of Ca²⁺-binding proteins in VTA DA neurons might be an attempt of cells to survive by increasing their ability to buffer free Ca²⁺. Exploring strategies to overexpress Ca²⁺-binding proteins could be fundamental to reduce neuronal suffering and improve cognitive and non-cognitive functions in AD.

Endocannabinoids have attracted great interest for their ability to counteract the neuroinflammation underlying Alzheimer’s disease (AD). Our study aimed at evaluating whether this activity was also due to a rebalance of autophagic mechanisms in cellular and animal models of AD. We supplied URB597, an inhibitor of Fatty-Acid Amide Hydrolase (FAAH), the degradation enzyme of anandamide, to microglial cultures treated with Aβ_25-35, and to Tg2576 transgenic mice, thus increasing the endocannabinoid tone. The addition of URB597 did not alter cell viability and induced microglia polarization toward an anti-inflammatory phenotype, as shown by the modulation of pro- and anti-inflammatory cytokines, as well as M1 and M2 markers; moreover microglia, after URB597 treatment released higher levels of Bdnf and Nrf2, confirming the protective role underlying endocannabinoids increase, as shown by RT-PCR and immunofluorescence experiments. We assessed the number and area of amyloid plaques in animals administered with URB597 compared to untreated animals and the expression of autophagy key markers in the hippocampus and prefrontal cortex from both groups of mice, via immunohistochemistry and ELISA. After URB597 supply, we detected a reduction in the number and areas of amyloid plaques, as detected by Congo Red staining and a reshaping of microglia activation as shown by M1 and M2 markers’ modulation. URB597 administration restored autophagy in Tg2576 mice via an increase in BECN1 (Beclin1), ATG7 (Autophagy Related 7), LC3 (light chain 3) and SQSTM1/p62 (sequestrome 1) as well as via the activation of the ULK1 (Unc-51 Like Autophagy Activating Kinase 1) signaling pathway, suggesting that it targets mTOR/ULK1-dependent autophagy pathway. The potential of endocannabinoids to rebalance autophagy machinery may be considered as a new perspective for therapeutic intervention in AD.

Understanding age-related neurophysiological changes is crucial for identifying brain aging biomarkers and developing strategies against motor and cognitive decline. To explore aging-related patterns across multiple domains, this study assessed motor and cognitive performance, functional connectivity, and synaptic organization in Young (4 months), Adult (14 months), and Old (24 months) mice. Adult mice exhibited reduced locomotor activity (−50.3%) and forelimb force (−38.3%) compared to Young mice, while Old mice showed decline in spatial (−30.4%) and recognition memory (−40.3%). Golgi–Cox staining revealed region-specific changes in spine density, including an increase in motor cortex layer II/III pyramidal neurons in Old versus Adult mice and reductions in the medial prefrontal cortex and CA1 hippocampal region. Immunofluorescence analysis indicated age-related alterations in VGLUT and VGAT expression across brain regions. Local field potential recordings revealed no significant changes in functional connectivity across age groups. Integration of behavioral and electrophysiological features using machine learning for an exploratory yielded a classification accuracy of 0.798. Although this represented only a modest and non-significant improvement over behavioral features alone, the highest pairwise discrimination was observed between Adult and Old mice (AUC = 0.861). Overall, these findings provide a multilevel descriptive analysis of brain aging, highlighting distinct behavioral and structural alterations alongside preserved functional connectivity.