Effects of amyloid-beta on homeostatic network plasticity in human iPSC-derived neuronal networks
Alzheimer’s disease (AD) is a progressive, neurodegenerative disorder and it is the most common cause of dementia in elderly. The disease is characterized by memory loss, mood swings, and communication problems. This uniquely human disease has been investigated in various mouse models mimicking d...
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|Alzheimer’s disease (AD) is a progressive, neurodegenerative disorder and it is the most
common cause of dementia in elderly. The disease is characterized by memory loss,
mood swings, and communication problems. This uniquely human disease has been
investigated in various mouse models mimicking different pathological hallmarks of AD,
which supplied valuable insight into disease mechanisms; however, clinical trials based
on these models failed and current treatments are unsatisfactory. To overcome the
limitations of animal models of AD, the emerging induced pluripotent stem cell (iPSC)
technology promises great potential. It offers the possibility to investigate underlying
disease mechanisms, screen for drug targets and validate therapeutic effects in
disease-relevant cell types of human origin on a patient-specific background. Current
iPSC studies to model AD have been addressing the questions of the pathological
hallmarks such as an increase in amyloid beta (Aß) and hyperphosphorylated tau.
However, to investigate the AD-related phenomenon of neuronal hyperactivity, mature
human neuronal cultures with spontaneously active networks are necessary, and their
generation remains a challenge.
In this study, to achieve spontaneously firing mature neuronal networks, human iPS cells
were differentiated into neurons and were supported with endogenously differentiated
human astrocytes or primary cortical astrocytes (PCA) isolated from rat brains.
Neuronal activity was recorded by using multi electrode array (MEA) to detect single
spikes and network bursts. Calcium imaging of spontaneously firing networks was
performed to monitor synchronously active neurons in cultures. To trigger
hyperactivity-induced homeostatic plasticity in human networks, iPSC-derived cultures
were treated with 4-Aminopyridine (4AP), a non-selective inhibitor of voltage-gated K+
channels. It increased the network activity only in mature (burst firing) cultures. This
induced hyperactivity further led to activation of homeostatic plasticity dependent
mechanisms to reduce the firing rate. Single spike analysis suggested Na+ channel
removal from the axonal membrane as one of these compensatory mechanisms.
Moreover, repressor element-1 silencing transcription factor (REST) was identified as a
key player in this process.
To study AD-related impairments in the established model, cell-derived and synthetic Aß
oligomers were prepared and characterized by semi-native western blot. Synthetic Aß
oligomers were surprisingly stable when added to neuronal cultures and caused no cell death and no change in spontaneous network activity. However, upon 4AP treatment,
Aß-treated networks showed impaired homeostatic plasticity and were not able to
reduce the firing rate appropriately. According to the analysis of spike properties, the
plasticity-associated reduction of axonal Na+ channels was also impaired. In Aß-treated
cultures, nuclear REST expression was diminished at basal levels and after triggering
homeostatic plasticity by 4AP. Thus, AD-related hyperactivity may be caused by
dysfunctional homeostatic plasticity in a REST-dependent manner.
Taken together, the results of this study provided the first hint on a previously unknown
impairment of homeostatic plasticity mechanisms in AD and identified REST as a target
which might contribute to the hyperactivity phenomenon at early stages of AD. This
knowledge of plasticity impairment might expand our understanding of disease
development and REST manipulation might be a new target for potential therapeutic