Alzheimer disease (AD) is a progressive neurodegenerative disease that causes dementia in nearly 60 % of cases in elderly patients resulting in irreversible memory loss and cognitive dysfunctions(Xie, Romano et al. 2004, Pena, Gutiérrez-Lerma et al. 2006). The Alzheimer’s Association recently reported that after every 67second, a new person is diagnosed with AD only in the USA, affecting approximately near 27 million people globally (Echeverria, Yarkov et al. 2016). Extracellular tangles and intercellular plaques are the distinct features of AD. Besides these, different features like synaptic degeneration, hippocampal neuronal loss and aneuploidy are other histopathological features(Swerdlow 2007). In general, classification of AD based on their occurrence is done as early onset AD (EOAD) and late onset or sporadic AD (LOAD) (Kamer, Craig et al. 2008). Early onset is genetically determined whereas late-onset AD is mostly due to genetic as well as an environmental factor (Kamer, Craig et al. 2008). Late onset AD prevalence gradually increases after the age of 65 (Hickman, Faustin et al. 2016). Factors like age, gender (females have more tendency to develop AD than males), smoking, high blood pressure, obesity, diabetes and lack of physical activity contribute to the development of AD (Hickman, Faustin et al. 2016).
Alzheimer; exosomes; beta amyloid plaques; tau hyper phosphorylation; amyloid precursor gene; cholinergic hypothesis
Stages of Alzheimer
AD is a neurodegenerative disease that progresses from mild, moderate to severe form. Pathologically, AD is characterised by two hallmark protein aggregates i.e. amyloid beta plaques (Aβ) and neurofibrillary tangles (NFT) that are accompanied by neuroinflammation including microgliosis, elevated cytokine production and activation of complement pathway (Marsh, Abud et al. 2016). The characteristic features of tangles and plaques tend to spread different region of cortex in a predictable pattern. There is a shrinkage in the brain tissue observed with the progression of AD. Before the diagnosis of the disease, there is the formation of plaques and tangles in the area involved in learning, memory, thinking and planning.
With the progression of neurodegeneration, the plaques and tangles spread into the areas that are involved in speaking and understanding speech. As reported in early cases of AD, initially the cells of hippocampus begin to degenerate thereby affecting short memory and the patients also lose the ability to perform daily tasks. With the progression of diseases, the disease get worse and then spreads through the cerebral cortex i.e. outer layer of brain causing worsening of judgement, language impairment and emotional outburst (Zamolodchikov and Strickland 2016). Advancement of the disease leads to more death of nerve cells thereby causing outages in emotional behavior like agitation. In the final stage, people lose their ability to feed themselves, speak and control bodily function.
Diagnosis of AD patients occurs nearly after 2-3 years of its onset of symptoms. This can be considered as an early stage of AD where the patient shows no characteristic clinical symptoms. Average disease progression occurs within eight years and ultimately results in the death of the patient(Landreth and Reed-Geaghan 2009). As per the global clinical deterioration, apathy and agitation syndrome were mostly correlated with clinical deterioration followed by psychosis, affective and sleep syndromes in AD. Global deterioration scale for primary dementia has broadly classified seven stages of AD: wherein stage 1 shows no decline in the memory of the patient and appears to be clinically normal. In stage 2, there is a very mild cognitive decline, where the patients complain of forgetting familiar objects. In stage 3, a clear indication of the patient in AD can be diagnosed. As during this stage, the patient may get seriously lost during traveling to any unfamiliar location. Mild to moderate anxiety also accompany these symptoms. Stage 4 is the late confusional stage, where the patient no longer performs complex task accurately and efficiently. The moderately severe cognitive decline is related to stage 5, where the person demands a caregiver and is no longer able to recall a major relevant event of their present lives. Stage 6 is a middle phase of dementia where there is a gradual change in the personality and emotions(Reisberg, Ferris et al. 1982). Patients suffering from delusional behavior, obsessive symptoms, cognitive abulia can become violent in behavior.
It is studied that the entorhinal cortex is the first brain region to undergo neurodegeneration in initial stages of AD, owing to neuronal cell death there is early memory loss, after which the progression continues to other cortical and limbic system(Tuszynski and Nagahara 2016).
Pathogenesis of Alzheimer disease
Pathogenesis of Alzheimer Disease lies in the progression of amyloid plaques and tau protein hyperphosphorylation in various parts of the brain. Many hypotheses exist that explain the progression of AD however well-accepted Amyloid hypothesis and the cholinergic hypothesis is explained below. The role of tau hyperphosphorylation is also discussed below.
The amyloid hypothesis supports that the occurrence of AD is due to the excess formation of amyloid protein Aβ. Excessive Aβ accumulation is related to the rate at which it is formed compared to the rate it is clearance(Yasojima, McGeer et al. 2001). Cleavage of Amyloid precursor protein (APP) occurs in various fragments by three key enzymes that play a role in its cleavage: α, β and γ secretase(Yasojima, McGeer et al. 2001). Cleavage by α secretase prevents the formation of Aβ as the cleavage occurs within the Aβ region. Most of the reported reason for the cause of the familial onset of AD is associated with a mutation in APP (amyloid precursor protein) and presenilin gene (PSEN 1 and PSEN 2). In contrast the reason for sporadic AD remains largely unclear, however, it is postulated that polymorphism in exon 5 of BACE1, APP cleaving enzyme is associated with late-onset AD(Yoon and AhnJo 2012, Shi, Han et al. 2016).APP gene is located on chromosome 21, and is widely spread in the cells throughout the body and is highly conserved in evolution(Peng, Shi et al. 2016).
Mutation in APP gene correlates that there is the substitution of valine to isoleucine in the transmembrane protein from two residues at the carboxy terminus of the Beta-amyloid protein(Goate and Chartier-Harlin 1991). Substitution makes transmembrane domain more hydrophobic and thereby might anchor the protein firmly to transmembrane. Generation of beta amyloid protein occurs after the sequential cleavage by two protease enzyme called beta secretase and gamma secretase that sequentially cleaves carboxyl and amino terminal of the enzyme. Beta-secretase occurs in two isoforms BACE1 and BACE 2. In AD, cleavage of APP is reported mainly through the BACE 1. Cleavage by the amino terminal of Aβ peptide sequence between residues 671 and 672 of APP, leads to generation and extracellular release of APP soluble and a carboxy-terminal cell associated fragment. Cleavage of carboxy-terminal by gamma-secretase generates Aβ.
Cleavage of APP by BACE 1, generates a 99 length unit fragment that is then cleaved by the gamma-secretase enzyme into Aβ40 and Aβ44 unit length. Gamma secretase is multiprotein complex consisting of Presenilin, Aph-1, Nicastrin and Pen-2 and all these four protein are necessary for gamma-secretase activity(De Strooper 2003). Deposition of amyloid protein is also correlated with the decreased amyloid clearance and degradation in peripheral. It is also believed that decrease in Aβ clearance is a more causative factor for AD rather than an increase in Aβ synthesis(Yoon and AhnJo 2012).
Two approaches like enzymatic and non-enzymatic are the approach for clearance of Aβ. Non-enzymatic approaches include the bulk flow of the interstitial fluid (ISF) into the CSF followed by ISF drainage pathway through perivascular basement membranes, which will cause the uptake by microglia or astrocytic phagocytosis. Whereas, the transport across the blood vessel walls into the blood vessel is mediated by a series of clearance receptors such as low-density lipoprotein receptor-related protein 1 (LRP1), very low-density lipoprotein receptor (VLDLR) and P-glycoprotein localized predominantly on the abluminal side of the cerebral endothelium (Yoon and AhnJo 2012).The enzymatic clearance involves several proteases, including neprilysin (NEP), insulin-degrading enzyme (IDE), matrix metalloproteinase (MMP)-9 and glutamate carboxypeptidase II (GCPII), Myelin Basic Protein (MBP), cysteine proteases(Yoon and AhnJo 2012). In vitro transfection of NEP mRNA to primary neurons has shown to degrade APP superior to plasmid DNA delivery. NEP is capable of degrading soluble form of both Aβ40 and Aβ42 as well as monomeric Aβ. However, IDE is capable of degrading only the monomeric form of Aβs. Also, results suggest that simvastatin and atorvastatin promote the degradation of Aβ by the activation of ERK signaling pathway(Yamamoto, Fujii et al. 2016).
From most of the animal models, it is evident that Aβ clearance occurs at the brain blood barrier by the active transport mechanism in which low-density lipoprotein receptor-related protein (LRP1) transporter receptors play an important role (Kamer, Craig et al. 2008). Astrocytes are also considered in aiding clearance of Aβs through the regulation of Aβ clearance enzyme. However, recent research collective data demonstrate that astrocytes are responsible for causing the amyloid beta deposition in AD(Abdullah, Takase et al. 2016, Söllvander, Nikitidou et al. 2016).
Aβ degrading proteases involves pathological and therapeutic regulators. Pathological regulators are the Aβ degrading enzymes that are active only in the presence of overt AD pathology, while therapeutic regulators are the proteases that are useful in Aβ degradation even an engineered or the enzyme that is not expressed normally(Leissring 2016).These results suggest that if there is a mismanagement in any of the factor mentioned above, it may lead to development and progression of AD.
Neurofibrillary tangles (NFT) composed of hyperphosphorylated and microtubule-associated tau protein are another hallmarks of AD. Similar to amyloid Beta deposition, NFTs initially accustoms into entorhinal cortex region and then spreads to another region of the brain (Fu, Rodriguez et al. 2017). Tau phosphorylation is known to cause neuronal loss and progressive cognitive impairment (Rábano, Cuadros et al. 2016). Recent literature report suggests that amyloid plaques are found to induce tau protein hyperphosphorylation via alpha-7 nicotinic receptor (α7nh) activation, however, in contrast, α7nh is also found to involved in learning and memory formation and is neuroprotective (Oz, Petroianu et al. 2016). In the early pathogenesis of AD, microglia secretes exosomes containing hyperphosphorylated tau protein that play a role in the progression of tau hyperphosphorylated protein from enter-hinal cortex to hippocampus(Ikezu, Ikezu et al. 2016). In normal cases, tau protein binds to DNA and RNA and protect them from cellular stress, however, in neurodegeneration tau protein loses its ability to bind to DNA and protects it (Bukar Maina, Al-Hilaly et al. 2016).
Loss of cholinergic activity is largely seen in AD. Further, Acetylcholine holds its role in learning and memory formation(Francis, Palmer et al. 1999). FDA has approved three cholinergic drugs to treat mild to moderate type of AD, Donepezil, Rivastigmine, and Galantamine. These drugs are acetylcholinesterase enzyme inhibitors (ACHIs). Normally, in synaptic cleft after the synthesis of acetylcholine from the presynaptic cleft, an enzyme called acetylcholinesterase hydrolyze acetylcholine into acetyl group and choline which recycles back to form another molecule of acetylcholine(Martorana, Esposito et al. 2010). Thereby, inhibiting acetylcholine esterase enzyme will increase the acetylcholine level which may help in learning and memory formation.ACHI interferes with the formation of Aβ and cell death mechanism like glutamate excitotoxicity, mitochondrial dysfunctional and free radical production. (Francis, Nordberg et al. 2005). Despite the existing drugs as ACHI to treat Alzheimer disease, these theory fails to cure AD, and this suggests that amyloid beta deposition, tau protein hyperphosphorylation and mutation can be a key factor in AD.
Fig.1: Schematic representation of amyloid pathogenesis: APP( amyloid precursor protein) gene located on chromosome 21 undergoes a mutation during which isoleucine replaces valine. Mutated APP gene is cleaved by BACE 1, a beta secretase, that cleaves the APP into 99 chain length unit at carboxy terminal (C-99). Further, this C-99 is cleaved by gamma-secretase, which comprises of four unit (presenilin 1(1), presenilin 2(2), APH-1(3) and nicastrin (4)) into Aβ 1-40 andAβ1-44. These fragments being more hydrophobic aggregates themselves and form amyloid plaques that initiate various responses like recruitment of amyloid clearance protein (neprilysin), transport of amyloid plaques to different regions of the brain, initiation of the inflammatory response and ultimately causing neuronal cell death.
Reason for non-stop progression of disease
Progressive spread of protein aggregates is based on two theory direct cell to cell contact and non-cellular mechanism. The spread of aggregated protein generated in one part of brain neuron move from one neuron to another neuron and thus spreads into connected brain network. However, some hypotheses suggest that some neurons are highly vulnerable to the formation of aggregated protein and thus in response to some adverse stimuli, protein aggregation is initiated in a subset of neurons. Protein aggregates appear in first highly susceptible cells while as the time progresses the aggregation is also initiated in the less susceptible cells (Walsh and Selkoe 2016). Besides these some non-cellular mechanism like exosomes and tunneling nanotubes (TNT) are also responsible for the spread of the protein aggregates (Walsh and Selkoe 2016, Zheng, Pu et al. 2017). Exosomes seem to play a dual role in AD, firstly its seems to have a neuroprotective role by transferring neuro protective agents between the cells and on the other side it is also responsible to spread amyloid plaques and tau protein aggregates between cells and thereby inducing apoptosis and neuronal loss (Malm, Loppi et al. 2016). Stimulating the secretion of exosomes have resulted in the increased spread of protein aggregates while its inhibition has resulted in decreased transmission (Guo, Bellingham et al. 2016).
Clinical Trails failure in Alzheimer
Various agents have been tested to prevent the progression or to aid in providing a therapeutic relief to AD patients. However, unfortunately, several of these agents have been found to be clinically insignificant. As discussed earlier, increase in Ach level by inhibiting ACHE can prove to be beneficial for cognitive improvement. Phenserine, a derivative of physostigmine when tested in Randomized control trial (RCT) to reduce APP mRNA translation were found to be clinically insignificant. Several beta secretase inhibitors like rosiglitazone and pioglitazone, type 2 diabetes drug, when tested for APP degradation by promoting its ubiquitination have failed to improve cognition level and also the drugs have been withdrawn from RCT due to negative preliminary data.
The first antibody to target Aβ production was Bapineuzumab which failed as it was estimated that it is affecting the wrong isoform of Aβ (Mehta, Jackson et al. 2017). After that, despite spending US$3 billion in the past 27 years Eli Lilly company drug solanezumab, monoclonal antibody failed in the last clinical trials. While the results of third-phase clinical trials of another antibody, aducanumab are still awaited (Abbott and Dolgin 2016). Antibodies modulating gamma secretase enzymes like semagacestat have also been withdrawn due to their severe adverse effect (Mehta, Jackson et al. 2017).
There are several reasons suggested for the failure of these drugs in clinical trials like designing a drug candidate without any side effects or undesirable adverse effect is seem to be impossible. Further, problems associated with rates, inter-site variance, rating scale, biases in dose administration and its pharmacokinetic and pharmacodynamics parameters also seem to influence the drug development process in AD (Mehta, Jackson et al. 2017) (Becker, Greig et al. 2008)
Although Sir Alos Alzheimer invented this disease in 1906 A.D., and even after the excessive research on this area a promising treatment to treat the cause is not present. FDA approved drugs provide only symptomatic relief while a dire need arises to prevent the progression of the disease. The design of various agents such as inhibition of Beta-secretase, gamma secretase, and alpha-secretase modulators have clinically failed. This further directs us to treat the symptoms, and to prevent the cause, a need arises to treat the agents causing AD.
Extensive research in this area has revealed some unknown facts such as the role of mitochondrial dysfunction and inflammation in the progression of the disease. However, what remains unelucidated is how these beta amyloid protein aggregation remains stable in this environment and does this beta amyloid plaques modify the neuron environment favoring its progression. Further, future directional treatment should focus on preventing the progression by inhibiting the entry of amyloid plaques or solubilizing the insoluble protein aggregates.
Also, a large number of agents are tested to a therapeutic application to prevent the progression of AD ranging from Beta-secretase inhibitors, gamma secretase modulators and other agents. However, due to the complexity of the disease, a single agent may prove to be ineffective for the desired therapeutic achievement. Hence, the focus should be made for combinational therapy.
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Gopal AgarwalPh.D Research ScholarNational Institute of Pharmaceutical Education and Research-Ahmedabad,Palaj-Gandhinagar-382355.
- Dr. Akshay Srivastava
Assistant ProfessorNational Institute of Pharmaceutical Education and Research-Ahmedabad,Palaj-Gandhinagar-382355.