Calcium handling: a strategy to fight neurodegeneration in Alzheimer’s disease

In the last few years, preclinical and clinical studies identified the ventral tegmental area (VTA) as one of the first brain regions to be affected in the prodromal phase of Alzheimer’s disease (AD). The VTA is a deep midbrain nucleus rich in dopaminergic neurons that innervates and releases dopamine (DA) in several cortical and subcortical brain regions. DA plays a crucial role in the modulation of both cognitive and non-cognitive functions and failure of its release underlies both cognitive and neuropsychiatric symptoms in patients with AD. DA neurons in the VTA are characterized by distinctive electrophysiological properties. Typically, VTA DA neurons exhibit phasic bursts of action potentials during novelty seeking and reward-related behaviors, while the low-frequency tonic spontaneous firing, called “pacemaking” activity, may contribute to maintaining awareness during learning and working memory functions. DA neuron pacemaking activity requires high energy levels and a consequent efficient mitochondrial turnover; this is possible thanks to an efficient autophagy(definition) mechanism to remove damaged organelles including exhausted mitochondria. All these cellular processes rely on the tight regulation of intracellular calcium levels. Calcium homeostasis mechanisms: The cytosolic calcium concentration depends on calcium influx from the extracellular space and efflux from intracellular stores, two finely regulated mechanisms. The extracellular calcium influx is mediated by different types of calcium channels as the voltage-gated calcium channels, N-methyl-D-aspartic acid, and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors. Calcium is also released into the cytosol from endoplasmic reticulum (ER) stores through the ryanodine receptors and the inositol 1,4,5-triphosphate receptors, and from the mitochondria via the sodium/calcium exchanger. This cytosolic calcium increase is counteracted by the sarcoendoplasmic reticulum calcium transport ATPase pump and the mitochondrial calcium uniporter. Neurons display also intrinsic calcium buffering ways to control calcium levels, thanks to the expression of calcium-binding proteins (i.e., calbindin and calretinin), calcium sensors (such as calmodulin), and a tied interplay between the ER and mitochondria. The functional consequence of this intricate machinery is the tight control of physiological calcium oscillations, crucial for the activity of DA neurons in VTA. Calcium homeostasis dysregulation: from aging to neurodegenerative diseases: During aging, neurons progressively lose their ability to handle calcium homeostasis due to the dysfunction of calcium-regulating systems. This causes an increase in intracellular free calcium and impaired calcium-dependent cellular processes. Indeed, altered activity of calcium channels, decreased mitochondria ability to buffer calcium and the reduction of calcium-binding protein and sensor levels, linked to oxidative and metabolic stress, were observed in aged neurons. The cytosolic calcium overload can also modulate the amyloid precursor protein (APP) processing, promoting the accumulation of amyloid-β (Aβ) peptide and the Aβ-related neurotoxicity, typical hallmarks of AD. Symmetrically, Aβ oligomers contribute to the calcium homeostasis perturbation, resulting in an excessive intracellular calcium influx from the extracellular space and intracellular stores. Aβ oligomers can induce the increased calcium influx via different mechanisms: i) they interact with the cellular membrane and disrupt its permeability by forming Aβ pore-channels; ii) they can stimulate the activity of calcium channels located in the neuronal membrane, as the voltage-gated calcium channels or the N-methyl-D-aspartic acid receptor; iii) Aβ peptides interfere with ER calcium channels, inducing a high release of calcium from ER stores into the cytoplasm, or iv) facilitating mitochondrial calcium uptake with the consequent calcium overload. High levels of intracellular calcium impair the activity of mitochondria, leading to their membrane permeabilization and the cytochrome c release. The abnormal calcium increase also induces the activation of calpain, a calcium-sensitive protease. Active calpain cleaves the apoptosis-inducing factor (AIF) from the mitochondrial membrane and elicits AIF translocation to the nucleus. Eventually, all these pathways, together with Aβ toxicity, result in the death of neurons. Many of these age-related calcium deficits are described in AD animal models and patients, supporting the hypothesis that calcium homeostasis dysregulation could be responsible for neuronal loss in neurodegenerative diseases. In this context, the recent work (La Barbera et al., 2022) found that DA neurons in the VTA accumulate AIF in the nuclei and damaged mitochondria in their cytosol; this is linked to an impaired autophagy flux (La Barbera et al., 2021). These events result in early DA neurodegeneration of a validated AD mouse model (Tg2576), overexpressing a mutated form of the human APP (APPswe); the overexpression of APPswe in all neurons, including the DA ones of the VTA, leads to the accumulation of intracellular Aβ oligomers. However, a previous work in the same mouse model provided evidence that DA neurons in VTA are more susceptible to cell death, and probably this is determined by DA neuron intrinsic features underlying their functions (Nobili et al., 2017). In Tg2576 mice, VTA DA neurons undergoing apoptosis are characterized by low levels of calcium-binding proteins, suggesting a global defective calcium-regulating system. By contrast, surviving VTA DA neurons show the overexpression of calcium-binding proteins, calbindin, and calretinin, to probably counteract the observed mitochondrial dysfunctions and potential calcium alterations. This explains why survived neurons display lower levels of cytosolic free calcium compared to controls (La Barbera et al., 2022; Figure 1). Taken together, the results suggest that pres