Synaptogenesis and Neural Cholesterol

Current guidelines encourage ambitious long term cholesterol lowering with statins, in order to decrease cardiovascular disease events. However, by regulating the biosynthesis of cholesterol we potentially change the form and function of every cell membrane from the head to the toe. As research…

Nowhere is the impact of cholesterol depletion more keenly studied than in the neurologic arena.

The work of Pfrieger et al. described the functional role of cholesterol in memory through synaptogenesis [24]. Mauch et al. [25] reported evidence that cholesterol is vital to the formation and correct operation of neurons to such an extent that neurons require additional sources of cholesterol to be secreted by glial cells. A recent mini-review by Jang et al. describes the synaptic vesicle secretion in neurons and its dependence upon cholesterol-rich membrane areas of the synaptic membrane [26]. Furthermore, working on rat brain synaptosomes, Waseem [23] demonstrated that a mere 9.3% decrease in the cholesterol level of the synaptosomal plasma membrane could inhibit exocytosis. These data might be particularly worrisome for lovastatin and simvastatin which are known to cross the blood brain barrier [27].

In fact, the proposed use of statins as a therapeutic agent in Alzheimer’s Disease (AD) [28] counters Pfrieger’s evidence [24]. Indeed, a reduction in cholesterol synthesis leads to depletion of cholesterol in the lipid rafts – i.e. the de-novo cholesterol is required in the neurons for synaptic function and also in the neuronal membrane fusion pores [29].

Cognitive problems are the second most frequent type of adverse events, after muscle complaints, to be reported with statin therapy [30] and this has speculatively been attributed to mitochondrial effects. The central nervous sytem (CNS) cholesterol is synthesised in situ and CNS neurons only produce enough cholesterol to survive. The substantial amounts needed for synaptogenesis have to be supplemented by the glia cells. Having previously shown that in rat retinal ganglion cells without glia cells fewer and less efficient synapses could form, Göritz et al. [31] indicate that limiting cholesterol availability from glia directly affects the ability of CNS neurons to create synapses. They note that synthesis, uptake and transport of cholesterol directly impacts the development and plasticity of the synaptic circuitry. We note their very strong implication that local de-novo cholesterol synthesis in situ is essential in the creation and maintenance of memory..

There should be further consideration of cholesterol depletion on synaptogenesis, behaviours and memory loss for patients undergoing long-term statin therapy. This is particularly important with lipophilic statins which easily cross the blood brain barrier [32].

The effects of statins on cognitive function and the therapeutic potential of statins in Alzheimer´s disease are not clearly understood [28]. Two randomised trials of statins versus placebo in relatively younger healthier samples (lovastatin in one, simvastatin in other) showed significant worsening of cognitive indices relative to placebo [33, 34]. On the other hand, two trials in Alzheimer samples (with atorvastatin and simvastatin respectively) suggested possible trends to cognitive benefit, although these appeared to dissipate at 1 year [35, 36]. A recent Cochrane review concluded that there is good evidence from randomised trials that statins given in late life to individuals at risk of vascular disease have no effect in preventing Alzheimer´s disease or dementia [37]. However, case reports and case series from clinical practice in the real world reported cognitive loss on statins that resolved with discontinuation and recurred with rechallenge [30].

Evidence from observational data and prestatin hypolipidemic randomised trials showed higher hemorrhagic stroke risk with low cholesterol [30]. In fact, in the Stroke Prevention with Aggressive Reductions in Cholesterol Levels (SPARCL) trial as compared with placebo, the use of high-dose atorvastatin was associated with a 66% increase in the relative risk of hemorrhagic stroke among the patients receiving the statin drug [38]. In addition to treatment with atorvastatin, an exploratory analysis of the SPARCL trial found that having hemorrhagic stroke as an entry event, male sex, and advancing age at baseline accounted for the great majority of the increased risk of hemorrhagic strokes [39]. However, a sensitivity analysis excluding all patients with a hemorrhagic stroke as an entry event in the SPARCL trial found that statin treatment was still associated with an increased risk of hemorrhagic stroke [40]. Furthermore, in a subgroup of patients with a history of cerebrovascular disease enrolled in the Heart Protection Study [41] which did not include patients with hemorrhagic stroke, a similar increased risk of hemorrhagic stroke during follow-up was demonstrated [40].

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