Diagnosed by the German psychiatrist and neuropathologist, Dr. Alois Alzheimer in 1906, Alzheimer’s disease (AD) is the most prevalent form of dementia in the ageing population (Korolev et al., 2014). Recently declared as the sixth major cause of death in the world; patients affected with AD suffer a gradual decline of cognitive abilities and memory functions till the disease renders them incapable of performing daily activities. Some of these traits, which are typical of neurodegenerative diseases are also shared by other forms of dementia.
Age is the biggest risk factor for AD . Ironically, with the tremendous advancement in medicine and healthcare, as life expectancy continues to increase, so does the prevalence of neurodegenerative diseases (Korolev et al., 2014). Undeniably, research has provided significant insights into many aspects of the disease mechanism; however AD being a multifactorial disease presents formidable challenges towards finding a cure. Statistical data reveals that over 30 million people are suffering from AD worldwide and this number is estimated to double every 20 years to reach 66 million in 2030 and about 115 million by 2050 (ARUK Dementia Facts).
About 95% of AD patients are aged 65 years or older and are diagnosed with “late-onset” or “sporadic AD” while 5% of AD patients carry rare genetic mutations associated with “early-onset” or “familial AD” (FAD) that causes the development of AD before age 65. While the mutations in the presenilin or amyloid precursor protein (APP) genes accounts for most of the early-onset cases, the genetics of sporadic AD is more complex and less understood. Genome-wide association studies (GWAS) reveal that the epsilon four allele of the apolipoprotein E (APOE) gene is a risk factor for the development of sporadic AD (Bertram et al., 2009). However, a large body of recent evidence suggests that cerebrovascular risk factors, family history of diabetes, hypertension and obesity play a significant role in the development and disease progression (Felice et al., 2014).
The two hallmarks of AD pathology are extracellular plaques of β-amyloid and intracellular tangles of hyperphosphorylated tau . The AD pathogenesis is explained by the “β-amyloid hypothesis” that proposes that excessive accumulation of amyloid-β (Aβ) triggers a pathological cascade leading to synaptic dysfunction, tau hyperphosphorylation, neurofibrillary tangles (NFTs) and neuronal death. Due to the multifactorial nature and heterogeneity of AD, the disease pathogenesis is not well understood. Hence, the approved medications for AD, usually directed at improving neurotransmission help control the symptoms but cannot halt the progression or reverse the disease.
Although the neuroscience community has extensively researched the progression of AD pathology from the animal models, until now, there are no preclinical models available that faithfully replicate amyloid-plaque and tau tangle pathologies and correlate these hallmarks with progressive memory impairment. Hence, many of the drugs that succeed in preclinical studies fail in the clinical trials.
Induced pluripotent stem cells (iPSCs) are emerging as powerful translational models for investigation of complex disease pathways and high throughput drug screening. Recent advances in reprogramming technology have led the scientists to generate the first FAD neurons from the fibroblasts of AD patients (Yagi et al., 2011). These iPSCs carrying the FAD mutations along with iPSCs derived from sporadic AD patients have an AD genetic background and are able to replicate the hallmarks of early-onset pathogenesis including accumulation of toxic Aβ species and an increase in endoplasmic reticulum (ER) and oxidative stress (Israel et al., 2012). However, the 2D iPSC cultures , unable to replicate the physiological environment of the human brain, expressed significantly lower levels of Aβ and 4R tau isoforms in comparison to AD brains.
In collaboration with Dr Sandrine Willaime-Morawaek , we at Mudher and Vargas-Caballero laboratories at the University of Southampton , have characterized a 3D neural model of iPSCs derived from age-matched control and AD patients with a FAD mutation (L286V) that causes an early-onset form of AD . In this methodology, the Human iPSC-Derived Neural Stem Cells (hiPSC-NSCs) (Axol Bioscience, ax0112) were combined with the MatrigelTM to develop a brain tissue in a pathophysiology simulating environment in a closed 3D matrix.
The hiPSCs-NSCs were cultured for 18 weeks before immunohistochemistry (IHC) was carried out using antibodies, NeuN, MAP2 and GFAP. IHC staining revealed a population of mature cortical neurons and astrocytes . In these populations, we have been able to detect the 4R tau isoform and electrophysiological readouts signifying neuronal maturity . In addition, we have been able to demonstrate significant levels of tau hyperphosphorylation in patient-derived 3D cultures in comparison to controls .
Our initial observations suggest that the 3D scaffold provides a microenvironment that manages to reconstruct the pathophysiological alterations in the diseased neurons . The early trigger of tau pathology in the mature diseased neurons coupled with the generation of AD-relevant proteins render this iPSC-derived model a valuable translational platform for therapeutic drug screening purposes.
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Israel M.A, Yuan S.H, Bardy C, Reyna S.M, Mu Y et al. Probing sporadic and familial Alzheimer's disease using induced pluripotent stem cells . Nature. 2012 Jan 25; 482(7384):216-20.