ENCALADA LAB RESEARCH

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Endolysosomal trafficking pathways in neurons: clear misfolded mutant PrP in the soma, but promote formation of endoggresomes in axons (From Chassefeyre, Chaiamarit, Verhelle et al. Sci. Adv 2021)

MECHANISMS OF NEURODEGENERATION

 

Endolysosomal and Trafficking Pathways in Prion and Alzheimer’s diseases

 

Neuroaxonal pathology is a frequent, early, and major neurological characteristic of neurodegenerative disorders, including in prion diseases and Alzheimer’s disease. The endolysosomal-autophagosomal system has emerged as a key and early signature feature of Alzheimer’s disease, and deficits in endolysosomal sorting and trafficking are directly linked by distinct mechanisms to neurodegeneration.

Our lab recently identified a new endolysosomal pathway that accounts for the formation of mutant prion protein (PrP) aggregates inside axons of mammalian neurons, a common feature of virtually all neurodegenerative diseases. We call this the axonal rapid endosomal sorting and transport-dependent aggregation (ARESTA) pathway, which encompasses proteins and pathways involved directly in the intracellular transport and endolysosomal trafficking and sorting of mutant PrP vesicles inside neurons. ARESTA is directly involved in the generation of endolysosomal membrane-delimited aggregates uniquely in axons, that we call endoggresomes. We have shown that endoggresomes are neurotoxic, and that reducing the function of ARESTA components inhibits endoggresome formation in axons, and circumvents neuronal toxicity and neuronal death. We are actively dissecting the regulation of the ARESTA and other endolysosomal pathways, and investigating the generality of these mechanisms to the development of intracellular aggregates and toxicity in tauopathies including Alzheimer’s disease.

INTERCELLULAR SPREAD MECHANISMS OF MISFOLDED PRIONS AND TAU

 

Misfolded protein aggregates including tau and prions can spread from cell-to-cell by a prion-like seeding mechanism, which catalyzes the transfer of tau/prion-related pathological lesions throughout the brain. Intercellular spreading is a process strongly associated with the onset and progression of neurodegeneration. How misfolded protein aggregates spread within the mammalian brain has remained one of the key unanswered questions in neurodegeneration. Insights into spreading mechanisms are critical for designing therapies to delay tauopathy disease progression. We use genetics, high- and super-resolution and single-particle and microscopy, biochemistry, and molecular biophysical approaches in mammalian neurons, in mice, and in vitro systems to elucidate the mechanisms of axonal transport and the spread of prions and prion-like proteins including Tau.

Fluorescently-labeled infectious mouse prions actively mobilizing in hippocampal neurons.

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REGULATION OF AXONAL TRANSPORT

 

Two important questions in motor biology are: (1) what is the identity of all the different cargo-motor complexes traveling in axons? and (2) how is bidirectional transport regulated? We are studying these questions by focusing on identifying the structural and regulatory transport complexes that move vesicles carrying various cargoes including the normal mammalian prion protein (PrPC) and synaptic vesicle precursors in axons, and have identified the motor proteins involved in transport of these vesicles in neurons. We use computational particle tracking and quantitative image analyses, as well as biochemical, proteomic, and genetic approaches, to identify and characterize the dynamics of cargo transport in neurons in mammalian and Caenorhabditis elegans systems. Our lab is interested in identifying and characterizing signaling and other components that regulate the movement of vesicles and of misfolded protein aggregates involved in disease. With this knowledge we build models of intra-axonal transport regulation.

Model of the mechanism of axonal transport of vesicles carrying the normal mammalian prion protein (PrP ) in neurons (Reprinted from Encalada et al. Cell 2011).

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PHARMACOLOGICAL ACTIVATION OF NEURONAL DEGRADATIVE PATHWAYS TO AMELIORATE PRIONOPATHY AND TAUOPATHY DEFICITS

 

Lysosomal-based degradation and autophagy are markedly impaired with aging and in neurodegeneration, including in prion diseases and in Alzheimer's disease. Axons are particularly vulnerable to the formation of misfolded protein aggregates, which accumulate inside lysosome-like compartments that lack degradative capacity. Indeed, axonal dystrophies featuring enlarged non-acidic endolysosomes occur early in disease in virtually all neurodegenerative disorders, and compelling evidence shows that these aggregates impair neuronal function by driving the accumulation of organelles, cytoskeletal elements, and vesicles, and poisoning axonal transport. Pharmacological targeting of toxic axonal aggregates by compounds that activate lysosomal- and autophagy-based clearance could be a key strategy for preventing or ameliorating neuronal dysfunction in the proteinopathies. In collaboration with Jeff Kelly's lab at Scripps Research, we have identified  various lysosomal flux activators (LFAs) compounds that hasten the clearance of misfolded mutant PrP endoggresomes via the upregulation of macroautophagy. We are actively characterizing the actions of these compounds in mouse models of prion and tau disease, as well as their targets of action. Our goal is to determine the pathways that lead to toxic misfolded PrP aggregates in neurons, and to develop therapies to inhibit their formation in prion diseases and in Alzheimer’s Disease and prion diseases. These goals are relevant because there are currently no disease-modifying drugs to treat prion diseases and tauopathies, nor are there agreed-upon drug targets.

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Lysosomal flux activators (LFAs) promote clearance of mutant PrP endoggresomes in axons (Yoon et al bioRxiv 2022)

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In vivo labeling of TTR expressed in C. elegans, with fluorogenic CMPD5, a probe that binds specifically to natively folded TTR. This compound remains dark until it selectively reacts with TTR (Madhivanan et al PNAS 2018).

MECHANISMS OF TRANSTHYRETIN (TTR) NEUROTOXICITY

 

The transthyretin amyloidoses are a related group of systemic degenerative diseases, wherein secretion of the transthyretin (TTR) tetramer from the liver followed by its dissociation, aberrant misfolding and aggregation causes proteotoxicity and degeneration of post-mitotic tissues including neurons. These diseases include Senile Systemic Amyloidosis (SSA), a highly under-diagnosed but common cardiovascular disease of the elderly, and Familial Amyloid Polyneuropathy (FAP).  The peripheral and autonomic nervous systems, as well as the heart are compromised by TTR aggregation in humans, even though these tissues do not synthesize TTR. The mechanistic basis for this cell non-autonomous cytotoxicity as well as the TTR conformations involved, are unknown. Our laboratory has generated C. elegans TTR proteotoxicity models that exhibit TTR aggregation and compelling neuronal phenotypes relevant to human disease, and that point to specific cell non-autonomous targets of TTR toxicity. We use genetic, genomics, biochemistry, high-resolution in vivo live microscopy, and small molecule approaches to characterize the mechanisms of TTR neuronal toxicity. 

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