Browsing by Author "Yao, TsoPang"

Primary cilia are evolutionarily conserved, acetylated microtubule-based organelles that transduce mechanical and chemical signals. Primary cilium assembly is tightly controlled and its deregulation causes a spectrum of human diseases. Formation of primary cilium is a collaborative effort of multiple cellular machineries, including microtubule, actin network and membrane trafficking. How cells coordinate these components to construct the primary cilia remains unclear. In this dissertation research, we utilized a combination of cell biology, biochemistry and light microscopy technologies to tackle the enigma of primary cilia formation, with particular focus on isoform-specific roles of non-muscle myosin II family members. We found that myosin IIB (Myh10) is required for cilium formation. In contrast, myosin IIA (Myh9) suppresses cilium formation. In Myh10 deficient cells, Myh9 inactivation significantly restores cilia formation. Myh10 antagonizes Myh9 and increases actin dynamics, permitting pericentrosomal preciliary complex formation required for cilium assembly. Importantly, Myh10 is upregulated upon serum starvation-induced ciliogenesis and this induction requires Atat1/Mec-17, the microtubule acetyltransferase. Our findings suggest that Atat1/Mec17-mediated microtubule acetylation is coupled to Myh10 induction, whose accumulation overcomes the Myh9-dependent actin cytoskeleton, thereby activating cilium formation. Thus, Atat1/Mec17 and myosin II coordinate microtubules and the actin cytoskeleton to control primary cilium biogenesis.

Cells thrive on sustaining order and balance to maintain proper homeostatic functions. However, the primary machinery involved in protein quality control including chaperones, ubiquitin proteasome system, and autophagy all decline in function and expression with age. Failures in protein quality control lead to enhanced protein misfolding and aggregation. Efficient elimination of misfolded proteins by the proteasome system is critical for cellular proteostasis. However, inadequate proteasome capacity can lead to aberrant aggregation of misfolded proteins and inclusion body formation, which is a hallmark of numerous neurodegenerative diseases. Due to the post-mitotic nature of neurons, they are more susceptible to the collapse in proteostasis correlated with age.

Here, we propose a cell based model of aggresome clearance using a reversible proteasome inhibitor, MG132, to identify the precise molecular machinery involved in proper processing of inclusions. It is known that once misfolded proteins are aggregated, the proteasome system can no longer degrade them. Furthermore, the continuous accumulation of aggregates often leads to aggresome formation, which results in amalgamated inclusion bodies that are simply too large for autophagosomes to engulf and degrade. Although, studies have shown that aggresomes can eventually be cleared by autophagy, the molecular mechanisms underlying this process remain unclear.

Our research reveals that regardless of impaired proteolysis, proteasomes can still stimulate autophagy-dependent aggresome clearance by producing unanchored lysine (K)63-linked ubiquitin chains via the deubiquitinating enzyme Poh1. Unanchored ubiquitin chains activate ubiquitin-binding histone deacetylase 6, which mediates actin-dependent disassembly of aggresomes. This crucial de-aggregation of aggresomes allows autophagosomes to efficiently engulf and eliminate the protein aggregates. Interestingly, the canonical function of Poh1 involves the cleavage of ubiquitin chains en bloc from proteasomal substrates prior to their degradation by the 20S core, which requires intact 26S proteasomes. In contrast, here we present evidence that during aggresome clearance, 20S proteasomes dissociate from protein aggregates, while Poh1 and selective subunits of 19S proteasomes are retained as an efficient K63 deubiquitinating enzyme complex. The dissociation of 20S proteasome components requires the molecular chaperone Hsp90. Hsp90 inhibition suppresses 26S proteasome remodeling, unanchored ubiquitin chain production, and aggresome clearance. Ultimately, we hope to apply these molecular markers of inclusion body processing to identify the underlying lesion in aggregate prone neurodegenerative disease.

Advances in modern medicine have helped to prolong human life. These advancements coupled with an ever-increasing population means that diseases associated with aging will become more prevalent in the coming years. As such, it is critical to understand the pathogenesis of disease where aging is the main risk factor. While not widely known, age is in fact a large risk factor in development of obesity and metabolic syndrome. More widely known and discussed are the neurodegenerative diseases that occur late in life. While age as a risk factor is a common point between these types of pathology, there are other similarities, such as the interaction between lipid metabolism and mitochondrial health.

To study the overlap between obesity and neurodegeneration, we investigated two pathways that regulate both. First, we find that loss of cytoplasmic deacetylase HDAC6 leads to aberrant accumulation of lipid in vitro and in vivo. HDAC6 knock-out (KO) mice gain more weight than WT counterparts after a high-fat diet regimen. Additionally, the intermediary metabolism of cells lacking HDAC6 is disrupted as they increase glucose uptake while downregulating fatty acid oxidation. HDAC6 not only plays a role in lipid metabolism, but regulates mitochondrial dynamics. Upon glucose-withdrawal, HDAC6 KO cells fail to elongate their mitochondria and display increased levels of mitochondrial toxic by-products. Therefore, HDAC6 has critical roles in lipid homeostasis and mitochondrial health.

The other pathway we investigated is critical in neurodegenerative disease, Parkinson's disease. Parkin, an E3 ubiquitin ligase, flags damaged mitochondria for destruction so they do not poison the other functional organelles. We found that Parkin promotes lipid remodeling at the surface of the mitochondria. Phosphatidic acid (PA) accumulates shortly after mitochondrial damage while diacylglycerol (DAG) appears several hours later. This lipid accumulation is dependent upon Parkin's translocation and E3 ligase activity. Additionally, we found that lipin-1, a PA phosphatase, and endophilin B1 (EndoB1) are critical for DAG accumulation and effective mitochondrial clearance.

Through this work, we show that two proteins critical in quality control mechanisms also play significant roles in energy homeostasis. We aim to highlight this overlap and posit that common diseases of aging, though presenting differently, might have disruptions in the same basic process.