Conference Agenda

The poorer clinical response and increased risk of amyloid-related imaging abnormalities (ARIA) observed in APOE4 carriers treated with novel amyloid-targeting antibodies highlight once again the intricate relationship between brain lipid metabolism and Alzheimer's disease (AD). This connection encompasses various aspects, including the impact on amyloid-beta and tau production and clearance, oxidative stress, synaptic and blood-brain barrier dysfunction, neuroinflammation, and mitochondrial dysregulation. The interplay between lipids and AD is complex and exerts a significant influence on the pathogenesis and progression of the disease. Gaining a comprehensive understanding of these lipid-related mechanisms is pivotal for a broader perspective on AD pathophysiology and for developing potential therapeutic strategies that target lipid dysregulation in AD.

An inverse association between plasma high density lipoprotein (HDL)-cholesterol levels and the risk of dementia and Alzheimer’s Disease (AD) has been reported, although with conflicting results, but an involvement of circulating HDLs in AD etiology and progression has never been proven. Few studies have focused on lipoproteins in the cerebrospinal fluid (CSF), mainly evaluating the cholesterol efflux capacity of CSF HDLs. Only a single study conducted in a very small number of subjects analyzed cholesterol esterification in the CSF of AD patients.

We have recently shown that cholesterol esterification, mainly depending on the activity of the lecithin:cholesterol acyltransferase (LCAT) enzyme, is hampered in plasma and CSF of AD patients. Interestingly, the cholesterol esterification biomarkers (unesterified cholesterol and unesterified/total cholesterol ratio) are significantly correlated to CSF AD biomarkers (i.e., Aβ1-42 and T-tau).

The observed dysregulation in cholesterol esterification in plasma and CSF of AD patients support the hypothesis that a modulation of the esterification process could be helpful in preventing/treating cognitive decline. The central role of LCAT in the cholesterol esterification process makes this enzyme a good target of potential novel therapeutic intervention.

The Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) has a well-established regulatory role on plasma cholesterol levels, due to the degradation of the hepatic LDL receptor. Interestingly, PCSK9 was firstly identified in the Central Nervous System (CNS) and named neural apoptosis-regulated convertase-1 (NARC-1), for its involvement in neuronal differentiation and apoptosis. More recently, PCSK9 has been hypothesized to modulate several processes known to be altered in Alzheimer’s disease (AD), including cerebral cholesterol handling and neuroinflammation, possibly contributing to neurodegeneration. We previously demonstrated increased PCSK9 levels in the cerebrospinal fluid (CSF) of AD patients, with the highest concentration observed in apoE4 carriers, clearly suggesting an involvement of this protein in AD pathogenesis. Interestingly, we detected increased PCSK9 CSF levels also in patients with mild cognitive impairment (MCI), indicating a dysregulation occurring since the early stages of the disease. Mechanistically, we have observed that PCSK9 in vitro is able to interfere with the cerebral cholesterol transport, an essential process providing astrocyte-derived cholesterol to neurons, to sustain neuronal physiological functions, including synaptogenesis and repairing of damaged membranes. In details, PCSK9 reduced the expression of the neuronal apoE-interacting receptors, leading to lower neuronal cholesterol content, a potentially deleterious effect. Consistently, we found that PCSK9 worsened Amyloid β fibrils (Aβ)-induced neurotoxicity. Moreover, PCSK9 incubation in cultured astrocytes enhanced the Aβ-induced expression of pro-inflammatory cytokines, suggesting a modulating role in neuroinflammation. Strongly supporting PCSK9 implication in AD, the genetic deletion of PCSK9 in an AD mouse model was associated to reduced Aβ burden, attenuated neuroinflammation and improved cognitive and memory performances. Altogether, this evidence unravels a pathological role for PCSK9, reinforcing the “lipid hypothesis” concept in AD, and providing the rationale for studying the pharmacological inhibition of PCSK9 as a potential valuable strategy to treat AD, with significant therapeutic perspectives.

Niemann-Pick type C1 disease (NP-C1) is a fatal and rare autosomal recessive lysosomal disorder resulting from mutation in npc1 gene. It is caused by an abnormal lipid transport and accumulation of unesterified cholesterol and other lipids in endosome-lysosome system. Patients show different visceral and neuropsychiatric symptoms that induce a severe cognitive impairment (Pallottini and Pfrieger, 2020) and to date there are no curative therapy. Most of the NPC1 protein mutations produce proteins which are immediately degraded but presenting residual activity (Ebrahimi-Fakhari, 2016). Therefore, increasing the availability of mutant NPC1 protein with residual activity, in the correct subcellular compartment, could be an emerging therapeutic approach to restore the physiologic lipid turnover in cells. We found out, in a hepatic-derived cell line, that an epigenetic pathway regulated by BET (Bromodomain and Extra-Terminal motif) proteins – amenable to pharmacologic manipulations – controls NPC1 protein expression, together with other proteins involved in cholesterol homeostasis (Tonini et al., 2020). Thus, the aim of our work is to investigate whether the modulation of BET proteins can increase NPC1 protein level and reduce cholesterol accumulation in both human-derived cells and transgenic mice carrying the most common NPC1 mutation. Therefore, fibroblasts derived from NP-C1 patients and mice carrying I1061T mutation on npc1 gene (Npc1(tm(I1061T)dso) have been treated with a specific BET protein inhibitor (JQ1).

Our results show that BET inhibition, modulates NPC1 protein expression, lysosomal size, and NPC1 localization in patient-derived fibroblasts. Moreover, JQ1 treatment seems to rescue some behavioral and locomotor parameters in mutated mice.

Altogether, our data suggest that BET proteins are regulators of cholesterol metabolism and could be used as a target for NP-C1.