A recent study study sheds light on how a protein called amyloid precursor protein (APP) affects the growth of nerve cells in the cortex — the human brain’s outer layer. The findings suggest that APP plays a crucial role in maintaining the delicate balance between neural stem cell proliferation and differentiation during the early stages of brain development.
The research, published in Science Advances, could have important implications for our understanding of neurodevelopmental processes and neurodegenerative diseases.
APP is a class I transmembrane protein that is widely expressed during nervous system development. It has been extensively studied due to its connection to Alzheimer’s disease (AD), where its fragmentation produces amyloid peptides that contribute to neuronal death. However, the physiological function of APP, especially in the context of human brain development, has remained unclear.
In their new study, the researchers aimed to understand whether APP played a crucial role in regulating the balance between stem cell proliferation and differentiation into various cell types during human neurogenesis. They wanted to determine whether the prolonged period of human neurogenesis, which is significantly longer than in other species, could be linked to vulnerabilities specific to our species, such as neurodegenerative diseases like Alzheimer’s.
“My lab has been working on the amyloid precursor protein for 20 years now, and from all the previous work we and others had done I had the intuition that there may be human specific aspects to its function that were not explored, so we decided to explore that,” said study author Bassem Hassan (@TheHassanLab), a team leader and scientific director at The Paris Brain Institute.
“At the same time, my lab has been studying neurogenesis all for more than 20 years, but no inkling that APP maybe involved in that. By examining the role of APP in human brain development, we realized it was involved in neurogenesis of the human cortex in ways that were apparent in other models, such as mice. In sense, this project ended up combining two of my major interests.”
To conduct the study, the researchers used various approaches. They first investigated the expression of APP during human brain development by analyzing cell sequencing data obtained from fetal brain tissue at different stages of gestation. This helped them track when and where APP was expressed in different cell types.
Hassan and his colleagues found that APP was initially expressed in six cell types and then later in up to 16 different cell types during fetal brain development. This suggested that APP’s role might change over time during neurogenesis.
“I never expected APP to be so strongly involved in the very early phases of human brain development,” Hassan told PsyPost. I was expecting more subtle and later effects, although I had an inclining that it was doing something specific in humans that we could not capture very well in other models.”
Next, they utilized the CRISPR-Cas9 gene editing technique to create neural stem cells derived from human induced pluripotent stem cells (iPSCs) in which APP expression was completely knocked out. This allowed them to compare the behavior of these genetically modified cells with normally developing neural cells obtained from in vivo sources.
The researchers found evidence that APP played a significant role in regulating the timing of neuronal generation from cortical progenitor cells. The absence of APP led to changes in the timing of neurogenesis. In the absence of APP, neural progenitor cells transitioned to a neurogenic state earlier than usual, resulting in accelerated neurogenesis.
APP was found to be intricately involved in regulating two finely-tuned genetic mechanisms that influence the process of neurogenesis in humans: canonical WNT signaling and AP-1 activation.
Canonical WNT signaling is a fundamental cellular pathway that plays a critical role in various developmental processes, including stem cell proliferation. In this context, WNT signaling is like a switch that can promote the division and multiplication of neural stem cells.
AP-1 stands for “Activator Protein 1,” which is a transcription factor complex that helps regulate the expression of specific genes in a cell. When AP-1 is activated, it triggers the genetic machinery to produce new neurons from neural stem cells.
By interacting with both the canonical WNT signaling pathway and the AP-1 activation process, APP can effectively orchestrate the balance between neural stem cell proliferation and the timing of neuron production.
“There are two take home messages,” Hassan explained. “The first is that APP, a protein known for its involvement in Alzheimer’s disease which is a disease of ageing and brain degeneration, is in fact also involved in the very early stages of the development of the human cortex, perhaps suggesting a link between how the brain develops and its potential vulnerability to age-dependent degeneration. The second, is that APP is involved in the slow pace of the development of the human cortex, one of the many features that allow our brain to grow much larger than most other species.”
Human brain development appeared to be particularly sensitive to the loss of APP. Genetic data from humans indicated a reduced tolerance to complete loss of APP, and both in vivo and in vitro studies on mouse neural progenitor cells lacking APP did not consistently show altered neurogenesis.
In other words, the absence of APP had a different effect on neurogenesis in humans compared to rodents. Unlike in humans, in rodents, the loss of APP did not lead to an acceleration of neurogenesis.
“In mouse models, neurogenesis is already very fast – too fast for APP deprivation to accelerate it further,” Hassan explained in a news release. “We can imagine that the regulatory role of this protein is negligible in mice, while it is essential in the neurodevelopment of our species: to acquire its final form, our brain needs to generate huge quantities of neurons over a very long period, and according to a definite plan.”
“APP-related abnormalities could cause premature neurogenesis and significant cellular stress, the consequences of which would be observable later. Moreover, the brain regions in which early signs of Alzheimer’s disease appear also take the longest to mature during childhood and adolescence.”
The study suggests that the absence of APP may mimic cellular stress, driving premature neurogenesis. This could have implications for neurodegenerative diseases like Alzheimer’s, potentially contributing to premature neurodegeneration.
“Of course, the major caveat with all studies that use ‘in vitro‘ human models, is exactly that: in vitro,” Hassan noted. “Although these models are a reasonable and practical approximation, we cannot be certain that the exact same effects occur in vivo in a developing human embryo. Nevertheless, it’s the only method we currently have, and, in this case, there are clinical reports that suggests APP may really be needed for early human brain development in vivo (Nguyen KV et al., doi: 10.1080/15257770.2016.1267361. PMID: 28102781; Klein S et al., doi: 10.1002/ana.24727. PMID: 27422356.).
“Many questions remain to be answered: how is it that a highly conserved protein found across species plays a role in human cortex development that it does not seem to play in mice for example? In other words, how do you get from genetic conservation to phenotypic divergence? Is the role of APP in timing human cortex development required across the entire development of the human cortex or only at specific stages/timepoints? Is this role specific to the cortex, or are other neural organs also affected?”
“To my knowledge this is the first example of a genetic factor that seems to be specifically involved in the overall timing of human cortex development, and I really wonder how this ties with its role in Alzheimer’s disease,” Hassan added.
The study, “The temporal balance between self-renewal and differentiation of human neural stem cells requires the amyloid precursor protein“, was authored by Khadijeh Shabani, Julien Pigeon, Marwan Benaissa Touil Zariouh, Tengyuan Liu, Azadeh Saffarian, Jun Komatsu, Elise Liu, Natasha Danda, Mathilde Becmeur-Lefebvre, Ridha Limame, Delphine Bohl, Carlos Parras, and Bassem A. Hassan.