Current Research Projects

Changes to synaptic connections driven by particular activity histories or other physiological cues are commonly believed to represent the main mechanism by which nervous systems learn new tasks or store new information. In recent years it has become apparent that synapses, excitatory and inhibitory alike, also change spontaneously in manners unrelated to overt, functionally relevant cues, and change even in the absence of activity or a history of prior activity. These spontaneous forms of synaptic remodeling, referred to as intrinsic dynamics, have fundamentally important consequences: They can produce the full range of synaptic sizes, give rise to skewed and heavy-tailed synaptic size distributions, set the scales of such distributions and constrain them, preventing ‘run-away’ potentiation. Finally, intrinsic dynamics drive both the ‘erosion’ of synaptic configurations (sets of input strengths, or ‘weights’) and synaptic loss, in particular of small ones, possibly giving rise to physiological and pathological forgetting. Given their influential consequences, understanding the processes that drive and regulate intrinsic dynamics would seem to be highly desirable. Somewhat surprisingly, not much is actually known. The aim of this project to unravel the fundamental processes that govern the intrinsic dynamics of synaptic specializations and uncover the identities of synaptic molecules that most strongly influence their tenacity.

Activity-induced modification of synaptic connections is widely believed to represent a major mechanism for modifying neuronal network function. In addition, however, recent studies show that synapses also change in manners that cannot be attributed to particular activity histories or even to activity per se. These findings raise questions on the exact relationships between synaptic change and neural system function; furthermore, they pose questions concerning the coexistence of functional invariance at the system level with substantial variance at the synaptic level. How do such synaptic changes translate to changes in network function in biological neuronal networks? To address this question, it would be desirable to study activity-dependent (and independent) changes in synaptic features, and changes (or invariance) in functional aspects of the same networks. Arguably, the aspect of network function most relevant to changes in synaptic properties relates to changes in signal propagation - spatiotemporal propagation patterns, firing times, firing rates and identities of neurons involved. To date, however, few experimental studies have concomitantly recorded synaptic remodeling and signal propagation within the same networks in manners that allow for quantitative comparisons between these two features over sufficiently long periods. 

To address these questions, we are  using networks of cortical neurons, multielectrode array (MEA) recordings, automated confocal microscopy, open/closed-loop stimulation and pharmacological manipulations to measure and quantitatively compare the remodeling of excitatory and inhibitory synapses to changes in signal propagation within the same networks using analytical approaches and metrics developed for this purpose.

Synaptic proteostasis is uniquely challenged by the exaggerated architecture of neurons, and the vast number of synapses they form. This is particularly true for presynaptic sites scattered along axons which can reach extraordinarily lengths. Quantitative information on presynaptic protein lifetimes can provide clues as to how these remote cellular specializations maintain their composition and function. Much of what is known on these lifetimes is based on proteomics and mass spectrometry, which while enabling comprehensive lifetime estimations for thousands of proteins, lack spatial resolution and the ability to measure turnover at presynaptic sites. In this project we are developing methods to measure the turnover of presynaptic molecules in-situ, examine the relationships between protein turnover and synaptic activation at individual synapses, and the relative importance of degradation pathways, including autophagy, in presynaptic protein turnover.

Huntington’s disease (HD) is caused by a glutamine repeat expansion in the protein huntingtin. Mutated huntingtin (mHtt) forms aggregates whose impacts on neuronal survival are still debated. In this project, a collaboration with the groups of Aaron Ciechanover (Technion) and Huu Phuc Nguyen (Ruhr-Universität Bochum), we have been studying the aggregation  dynamics of mutated huntingtin in neurons, and how these are affected by ubiquitination. Using a novel HD rat model, we identified two lysine residues, 6 and 9, in the first exon of mHtt that are specifically ubiquitinated in striatal and cortical brain tissues of mHtt-transgenic animals. Using very long term imaging we find that when these two lysines are left intact, mutated huntingtin fragments are gradually sequestrated into peripheral, mainly axonal aggregates, concomitant with dramatic reductions in cytosolic mHtt levels and enhanced neuronal survival. in-situ pulse-chase imaging reveals that aggregates continually gain and lose mHtt, in line with these acting as mHtt sinks at equilibrium with cytosolic pools. Mutating the two aforementioned N-terminal lysines in a manner that prohibits ubiquitination at these sites suppresses peripheral aggregate formation and reductions in cytosolic mHtt, promotes nuclear aggregate formation, stabilizes aggregates and leads to pervasive neuronal death. These studies, based in part on experimental approaches originally developed to study synaptic biology, demonstrate the capacity of aggregates of potentially toxic proteins forming at peripheral locations to sequester these away and support a crucial role for N-terminal ubiquitination in promoting aggregation and delaying neuronal death.