Amyloid-beta signaling in physiology and pathology: from astrocytes to SUMO

Amyloidβ signaling in physiology and pathology: from astroc ytes to SUMO Linda Lee Alzheimer’s disease (AD) is a neurodegenerative disorder that is characterized clinically by progressive dementia and histopathologically by amyloid plaques and neurofibrillary tangles. The primary molecular culprit in AD is the amyloid-beta (Aβ) peptide, aggregates of which are the main components of the plaques. Numerous studies have implicated soluble Aβ oligomers as the predominant neurotoxic species, although the underlying mechanisms that lead to cognitive failure are not fully understood. In this thesis, I demonstrate that post-translational modification with the small ubiquitin-like modifier (SUMO) is required for normal synaptic and cognitive function but can be impaired by Aβ oligomers. I discovered that SUMOylation was significantly reduced in brain tissue from AD patients and a transgenic mouse model of AD. While neuronal activation normally induced upregulation of SUMOylation, this effect was impaired by Aβ and in the transgenic mice. Aβ is also a known potent disruptor of synaptic function. However, enhancing SUMOylation via transduction of its conjugating enzyme, Ubc9, rescued Aβ-induced deficits in synaptic plasticity and memory. I further demonstrate that inhibition of SUMOylation can directly cause such deficits, similar to Aβ. Overall, the data establish SUMO as a novel regulator of synaptic plasticity and cognition and point to SUMOylation impairments as an underlying factor in AD pathology. In addition to the pathological effects of Aβ, the normal physiological functions of this peptide, which is produced in the brain throughout life, remain unclear. A previous study in our lab demonstrated that physiologically-relevant (low picomolar) amounts of Aβ can enhance synaptic plasticity and memory. Astrocytes, as crucial glial support cells with roles in modulating synaptic transmission, are likely cellular candidates for participating in this type of physiological Aβ signaling. To test this hypothesis, primary cultures of murine astrocytes were exposed to exogenous picomolar Aβ peptides while undergoing calcium imaging. Upon addition of 200 pM Aβ peptides, the percentage of astrocytes exhibiting spontaneous oscillatory calcium transients increased significantly. The periodicities of these transients were analyzed, and it was found that both the frequency and amplitude of the transients were enhanced after Aβ exposure. These effects were dependent on calcium influx and α7 nicotinic acetylcholine receptors (α7-nAChRs), as the potentiation was blocked by a pharmacological α7 inhibitor and in cultures from an α7 knockout mouse strain. In addition to spontaneous signaling, evoked intercellular calcium waves were also analyzed. After picomolar Aβ exposure, no significant changes were found in several wave parameters, including spatial and temporal spread, propagation speed and maximum signal intensity. These results indicate that at physiologically-relevant concentrations, Aβ peptides enhance spontaneous astrocyte calcium signaling via astrocytic α7-nAChRs. Since astrocyte-mediated "gliotransmission" has been found to have multiple neuromodulatory roles, Aβ peptides may have a normal physiological function in regulating this type of neuron-glia signaling. These studies illustrate the diverse effects of Aβ peptides, which are dependent on the concentration and conformation state. Ultimately, knowledge of both normal Aβ physiology as well as Aβ pathology are necessary to truly understand Alzheimer’s disease and enable development of effective therapeutics.