di Alberto Carrara, LC
Giovedì prossimo, 13 marzo 2014 (15:30-19), a Roma presso l’Aula Master dell’Ateneo Regina Apostolorum, si terrà il convegno “Memoria e Alzheimer” nell’ambito della Settimana Mondiale del Cervello (la BAW, Brain Awareness Week) promossa dalla DANA Foundation.
La rilevanza della tematica relativa alla memoria umana e ad una patologia multifattoriale come il morbo di Alzheimer, viene confermata dai numerosi studi scientifici che ogni settimana vengono pubblicati sulle diverse riviste di medicina e biologia. Uno degli ultimissimi studi d’avanguardia nel settore di questa patologia ancora prima di una soluzione terapeutica, riguarda il legame tra la patologia neurodegenerativa di Alzheimer e l’insonnia.
È noto in letteratura e nella prassi clinica, che uno dei segni precoci del morbo in questione è l’incapacità persistente di dormire di notte, come si esprimono gli autori dello studio intitolato The central molecular clock is robust in the face of behavioural arrhythmia in a Drosophila model of Alzheimer’s disease pubblicato online lo scorso 28 febbraio 2014 dalla rivista scientifica DMM, Disease Models & Mechanisms ():
“Being awake at night and dozing during the day can be a distressing early symptom of Alzheimer’s disease, but how the disease disrupts our biological clocks to cause these symptoms has remained elusive”.
Sia nei pazienti precoci, come nei pazienti con patologia conclamata, i periodi di sonno si riducono drasticamente, divenendo fragmentati e brevi, inducendo comportamenti impazienti, scompensi di vario genere, irritabilità, etc. il quadro prende il nome inglese di ‘sundowning’.
Il gruppo guidato da Damian Crowther dell’Università di Cambridge ha scoperto che nel modello patologico (transgenico) di moscerino della frutta (nome comune o volgare della Drosophila melanogaster), l’orologio biologico continua a funzionare, ma viene “sganciato” dal normale ciclo sonno-veglia normalmente regolato. Queste evidenze sperimentali, seppur limitate ad un modello non umano, possono certamente servire allo scopo di sviluppare strategie terapeutiche o lenitive degli effetti della malattia di Alzheimer più efficaci a risolvere o ridurre al minimo l’insonnia nei pazienti precoci o con la patologia conclamata.
L’articolo scientifico può essere interamente consultato online gratuitamente qui.
Qui si può trovare una breve rassegna a cura del portale NeuroscienceNews.com.
A mò di introduzione per il convegno del prossimo 13 marzo a Roma dedicato alla tematica, riprendo una parte significativa dell’abstract e dell’introduction di questo lavoro scientifico.
Circadian behavioural deficits, including sleep irregularity and restlessness in the evening, are a distressing early feature of Alzheimer’s disease (AD). We have investigated these phenomena by studying the circadian behaviour of transgenic Drosophila expressing the amyloid beta peptide (Aβ). We find that Aβ expression results in an age-related loss of circadian behavioural rhythms despite ongoing normal molecular oscillations in the central clock neurones. Even in the absence of any behavioural correlate, the synchronised activity of the central clock remains protective, prolonging lifespan, in Aβ flies just as it does in control flies. Confocal microscopy and bioluminescence measurements point to processes downstream of the molecular clock as the main site of Aβ toxicity. In addition there appears to be significant non-cell autonomous Aβ toxicity resulting in morphological and likely functional signalling deficits in central clock neurones.
Alzheimer’s disease (AD) is the most common cause of dementia in adults and is characterised at the microscopic level by extracellular amyloid plaques and intraneuronal tau tangles. Amyloid plaques are composed of fibrillar aggregates of a spectrum of amyloid (Aβ-peptides derived from the proteolytic cleavage of amyloid precursor protein (APP) (LaFerla et al., 2007). The significance of Aβ is underpinned by the numerous disease-linked mutations that dysregulate APP processing: mutations that result in a spectrum of Aβ peptides with a higher aggregation propensity have been linked to familial AD (Philipson et al., 2010) whereas sequence variation in APP that reduces Aβ production is protective (Jonsson et al., 2012). There is much evidence from cell culture and animal models systems (Iijima-Ando and Iijima, 2010; Philipson et al., 2010) that the conformers of Aβ that possess neurotoxic activity are likely to be soluble oligomeric species rather than the more easily detected amyloid plaques (Lesné et al., 2006; Shankar et al., 2008; Ono et al., 2009; Tomic et al., 2009; Brorsson et al., 2010; Jo et al., 2011; Tang et al., 2012).
Alongside the well-recognised memory and cognitive deficits that typify AD, a substantial proportion of AD patients also experience circadian abnormalities including increased day time napping, night time activity and fragmented sleep. Taken together these clinical features constitute a dampening of the variation in day-night activity (Volicer et al., 2001; Coogan et al., 2013); furthermore two thirds of patients living at home exhibit some degree of “sundowning” where restlessness and agitation increase late in the afternoon and early evening (Prinz et al., 1983; Volicer et al., 2001). It is readily apparent that such behavioural problems are a substantial burden for both AD patients and their caregivers.
Circadian timekeeping in animals is a cell-autonomous mechanism based on the intrinsic 24 hour-period oscillation of “clock gene” products (such as PER1 & 2, CRY1 & 2, CLOCK & BMAL1 in human) mediated by interlocked transcriptional-translational feedback and feedforward loops (TTFLs). Such cellular circadian oscillators are present throughout the body but those in suprachiasmatic nucleus (SCN, ~20,000 “clock neurons”) of the hypothalamus are considered to be the master pacemaker in humans (Mohawk et al., 2012). The SCN neurones are divided into a dorsal-shell, (arginine vasopressin, AVP positive) and ventral-core, (vasoactive intestinal polypeptide, VIP positive areas). Circadian oscillators in the SCN are entrained by light to keep them in synchrony with external light-dark cycle. The SCN then converts the entrained circadian signal into coordinated physiological and behavioural outputs via multiple humoural and neuronal pathways. (Mohawk et al., 2012). Importantly circadian oscillations are self-sustaining at both molecular and behavioural levels. Therefore “free-running” rhythms continue even in the absence of external cues (e.g., the constant darkness, DD).
The behavioural abnormalities linked to AD in the clinic have been substantiated by histological changes in the SCN in post-mortem brains, in particular the cell loss observed by Swaab and colleagues (Swaab et al., 1985; Swaab et al., 1988). Despite the cell loss seen in the SCN in AD brains, amyloid plaques here are sparse (Coogan et al., 2013), possibly indicating that Aβ toxicity may be largely non-cell autonomous, being derived from neighbouring cells. Concordant with this Tate and colleagues reported reduced amplitude of behavioural rhythms in rats carrying SCN grafts of PC12 cells expressing a disease-linked variant of APP as compared to animals grafted with control PC12 cells (Tate et al., 1992). However subsequent murine studies of AD-linked circadian locomotor abnormalities, using established model systems has Disease Models & Mechanisms DMM Accepted manuscript5 yielded a complex and sometimes contradictory picture. In particular mice expressing mutant APP in light-dark (LD) conditions exhibit normal circadian locomotor activity (Wisor et al., 2005; Ambrée et al., 2006; Gorman and Yellon, 2010). By contrast, increased locomotor activity during resting light hours was detected in transgenic animals expressing additional mutated human γ secretase (APPxPS1, Duncan et al., 2012) or the combination of mutant PS1 and tau (APPxPS1xtau, Sterniczuk et al., 2010). Furthermore, only minor deficits in free-running behaviour (DD) are detected in these AD model systems (Wisor et al., 2005; Gorman and Yellon, 2010; Sterniczuk et al., 2010). For these reasons, the role of toxic Aβ species in circadian deficits in AD remains elusive.
As a complement to murine models of AD we have generated a Drosophila system to study Aβ toxicity. Instead of replicating the proteolytic processing of APP, we and others, have fused the Aβ peptide with a secretion signal peptide and driven its expression in the nervous system (Finelli et al., 2004; Iijima et al., 2004; Crowther et al., 2005). Various Aβ species were expressed pan-neuronally in Drosophila using the Gal4-UAS expression system (Brand and Perrimon, 1993) and Aβ toxicity was detected using a range of biochemical, neuron-histological and behavioural assays (e.g., Jahn et al., 2011; Speretta et al., 2012; Huang et al., 2013). In this study we have combined the tools available to neurodegeneration modelling in the fly with the well-developed systems that are also available for studying circadian rhythms. The use of the fly as a model organism is justified by the many orthologies between Drosophila and human, in particular by the conserved circadian TTFLs, involving the clock genes period, timeless, clock and cycle (Allada and Chung, 2010). Circadian locomotor activity in Drosophila is controlled by approximately 150 clock gene expressing neurones (clock neurons) in the brain. As with SCN in humans, Drosophila Disease Models & Mechanisms DMM Accepted manuscript6 clock neurones can be divided into several groups (termed sLNvs, lLNvs, LNds, DN1s, DN2s, DN3s and LPNs in the fly) according to their ventral-dorsal anatomy and neuropeptide identity. Similar to the role of the neuropeptide VIP in synchronising among clock neurones in the SCN (Hastings and Herzog, 2004; Aton et al., 2005; Maywood et al., 2006), the neuropeptide PDF (pigment disperse factor) released from about 16 ventral neurones (sLNv and lLNvs) in Drosophila, maintains robust circadian behaviour by paracrinely synchronising the molecular oscillation of clock neurones (e.g., Renn et al., 1999; Peng et al., 2003; Cusumano et al., 2009). In addition, the majority of the axons from these clock neurones project to the dorsal protocerebrum (dorsal commissure, Helfrich‐Förster et al., 2007), where they communicate with each other and to their downstream targets. Normal free-running circadian behaviour in Drosophila also requires correct signalling at these synapses (Kaneko et al., 2000; Blanchardon et al., 2001; Nitabach et al., 2002). Rezával et al (2008) previously demonstrated that overexpression of wild-type human APP in PDF positive ventral clock neurons (pdf>hAPP) resulted in age-dependent loss of circadian rhythm. Although Drosophila do have the γ-secretase required to process APP they have little β-secretase-like (dBACE) activity and so the generation of Aβ peptides is inefficient (Fossgreen et al., 1998; Carmine-Simmen et al., 2009). Therefore the circadian abnormality in pdf>hAPP flies (Rezával et al., 2008) is likely unrelated to toxic Aβ peptides. In this study however, we have employed well-established tools for characterising the Drosophila clock system to investigate the mechanism of Aβ-mediated disruption of circadian rhythms.
Key words: Alzheimer's disease; circadian dysfunction; non-cell autonomous Aβ toxicity; Drosophila model; biological clock.