Brian D Ackley

Synaptic Development

Synapses are the gateways between neurons and their targets. Synapses are specialized subcellular domains where proteins that specialize in the transmission or reception of signals are organized into functional complexes. Despite being foundational to the function of brains, and the memories we all have, synapses can be highly dynamic. Synapse formation and elimination occur throughout the day in our brains, and it is the net balance of those two opposing forces that give rise to things like learning and memory.

Our lab has demonstrated that, at synapses, adhesion proteins do more than just hold the structures together. That is, while they do help align the pre- and post-synaptic domains in close register, they also regulate the processes of synapse formation and elimination. For example, a protein called nidogen is located between neurons and muscles and is a ligand for the LAR-like receptor, PTP-3A. Genetic mutations in these proteins result in increased synaptic growth, as evidenced by markers of the presynaptic region. While we first assumed that this change was due to a loss of organizational structure, we later showed we could genetically suppress the effects of nidogen or LAR loss-of-function with mutations in the synaptically associated calcium channel. More specifically, even when the adhesion protein was gone, if the calcium channel was also gone, synapses looked normal. Blocking synaptic transmission with other mutations did not suppress nidogen or LAR, and mutations that increased calcium channel activity resulted in synaptic overgrowth. We are continuing to study how adhesion and calcium-dependent synapse dynamics are contributing to normal synapse formation and elimination.

We have also shown that the Flamingo-like receptor can act cell non-autonomously to control synapse development. Specifically, it appears that within excitatory neurons, Flamingo, called FMI-1 in C. elegans, acts to instruct inhibitory neurons where synapses should be made and maintained. This feedback appears to be important for the excitatory-to-inhibitory balance (E:I) that all nervous systems must maintain. We are continuing to understand how FMI-1 functions in this regard.

Invertebrate Models of Neurodegenerative Diseases

Neurodegenerative diseases, e.g., Alzheimer's disease, etc., are a significant burden in human health. The incidence of these diseases is increasing as human lifespans are increasing. They cause enormous social, economic, and familial hardship as aging family members can no longer care for themselves. One of the main challenges in studying these diseases is the time it takes for the phenotypes to manifest. Mouse models can take months or years to develop the kinds of neuronal defects and cognitive loss that we equate with human dementia. However, we have developed models of C. elegans that demonstrate synaptic degeneration within the first week of adulthood. Synaptic loss occurs early in the disease progression, before the more notable cellular death and neuronal loss. We believe that by studying this early stage we may find a way to intervene while there is still time to salvage cognitive function.

Recently, in collaboration with the Wolfe lab at KU, we have provided evidence for a non-amyloid mechanism in Familial Alzheimer’s Disease (FAD). In our model, mutations in the Presenilin protein, PSEN1, could induce synapse degeneration even in the absence of the Amyloid Precursor Protein. Our model predicts that FAD mutations create a stabilized enzyme-substrate complex that is inducing phenotypes, rather than a model whereby the production of the aggregating Ab42 is inducing cellular degeneration. Going forward we are working to better understand how neurons are sensing these FAD mutations, and why they are then demonstrating the synaptic degeneration characteristic of AD patients.

Separately, in collaboration with the Gamblin lab at UT San Antonio we have made novel models of tauopathies. The tau protein normally functions to protect microtubules inside neurons, but in disease states it can oligomerize and form neurotoxic species. We have found that mutations associated with disorders like Frontotemporal Dementia or Progressive Supranuclear Palsy, result in the progression degeneration of synapses during aging. We will use genetic tools to elucidate the mechanisms of this degeneration.

Specification of Neuronal Fates and Axon Outgrowth

Neurons can have a variety of diverse morphologies and functions. How do they acquire those during development? Environmental cues and transcription factors combine to produce both generalized neuronal fates and very specific fates. We have created markers that allow us to probe those events genetically in early development. Because C. elegans are transparent, we can use fluorescent proteins, expressed under the control of cell-specific promoters, to understand the genetic networks that guide fate decisions. We have found that the highly conserved Wnt signaling pathway functions with Hox-family transcription factors to specify fates in individual neurons. Importantly, our recent results suggest that this occurs much earlier than previously thought. In fact, our data suggest that this signaling cascade may inform fates several cell divisions prior to the formation of the neurons being observed. How are those instructions maintained through time and space, allowing other developmental events to occur normally? Those questions are still being investigated.

We have also found that some of the same proteins regulate the pathfinding of neurons once they are developed. We can use cell biological tools and genetic analysis to define how signaling pathways are different in cell fate decisions or synaptic development compared to axon guidance. For example, loss of function in FMI-1 leads to errors in the pathfinding of GABAergic neurons, in a manner that is independent of how it contributes to synaptic development. Recently, we have found that downstream of FMI-1 there are enzymes called flavin monooxygenases that are important for axon pathfinding. Our data suggest that these proteins are novel regulators of the flamingo pathway.

Host Pathogen Interactions

All organisms face threats from microorganisms such as viruses, bacteria, or fungi. However, our bodies also have beneficial interactions with related microbes. How are some interactions pathogenic, while others are beneficial? Part of this has to do with the development of our innate immune system. Pathogenic interactions can activate defenses that, in addition to fighting the pathogen can injure the host as well. Thus, it is important that the defenses be agile, both in their activation, but also in their cessation. We are interested in the genetic and genomic networks that regulate host-pathogen interactions.

C. elegans isolates have been found and submitted to repositories from all over the world. Within these diverse ecosystems it is reasonable to assume that they have adapted to local microbial threats. This may be in the form of genetic adaptations in specific defense proteins, or in the regulation of those proteins. We have developed a framework to test the differences in immune activation in different isolates as a way to map the changes in the genome. Importantly, even in cases where we see very little difference in the outcome of the different interactions, i.e., the effects on lifespan are equivalent between isolates, the transcriptional response after infection can be very different. This suggests that the historical adaptations within these isolates has arrived at the same “functional” space, but does so in a different way. We can cross these isolates together to map the genomic changes that underlie these differences to map them to functional loci.