Enhancement of Abeta-induced synaptotoxicity by adhesion protein CTFs in mouse and iPSC-derived human neurons

• Applicant: Professor Dr. Kurt Gottmann
• Subject Area: Experimental Models for the Understanding of Nervous System Diseases ; Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2014 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 263969882

Synapse impairment and synapse loss are crucial pathomechanisms in Alzheimer´s disease (AD). Although altered synaptic function and loss of dendritic spines have been described in mouse systems, the molecular mechanisms involved are not well understood. Oligomeric amyloid-beta (Abeta) peptides are thought to initiate a cascade of events leading to destabilization of synapses. The long-term stability of synapses depends on transsynaptic interactions of synaptic adhesion molecules such as N-cadherin and the Neurexin/Neuroligin system. However, how synaptic adhesion molecules influence Abeta-induced synapse destabilization has hardly been addressed. N-cadherin and Neurexins are proteolytically processed by alpha- and gamma-secretases similar to amyloid precursor protein. AD related dysfunction of gamma-secretase leads to accumulation of membrane-associated C-terminal fragments (CTFs) of synaptic adhesion molecules. These CTFs might interfere in a dominant-negative way with synapse stabilizing mechanisms such as N-cadherin mediated adhesion. In our previous work, we obtained first evidence that Abeta-induced synapse impairment is strongly accelerated by membrane-associated C-terminal fragments (CTF1) of N-cadherin. This might be of importance in AD, because the expression of N-cadherin-CTF1 was found to be increased in post-mortem brains of AD patients. In this project we will investigate the boosting of Abeta-induced synaptotoxicity by membrane-associated CTFs with special emphasis on N-cadherin-CTF1 and Neurexin3beta-CTF. To analyse Abeta-induced synapse impairment, we will record AMPA mEPSCs, stain pre- and postsynaptic marker proteins, and determine spine density in cultured mouse cortical neurons. An enhanced presence of CTFs will be accomplished by specific CTF expression and gamma-secretase inhibition, respectively. We will further develop a novel assay of Abeta-induced synapse damage by analysing the activity-dependent dynamics of synapse nanostructure with super-resolution microscopy, and use it to study the effects of CTFs. To further characterize an enhanced expression of membrane-associated CTFs in AD, we will study samples from post-mortem brains of AD patients by Western blot analysis of several synaptic adhesion proteins. We expect to learn, which specific CTFs are most relevant for the progression of AD. In a highly innovative approach, we will use human cortical neurons derived from induced pluripotent stem cells (iPSCs) to investigate CTF-dependent modulation of Abeta-induced synaptotoxicity. We will analyse both functional (AMPA mEPSCs) and structural (synaptic marker proteins, spines) damage of synapses by Abeta peptides and its boosting by enhanced presence of CTFs. In addition, we will study CTF-induced modulation of Abeta effects on synapse nanostructure using super-resolution microscopy. We expect to learn, whether human synapses exhibit similar Abeta-induced damage as detectable in cultured mouse neurons.

• DFG Programme: Research Grants