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Identification and characterization of sites of lipid droplet biogenesis in the ER

Periodic Reporting for period 1 - LD_Biogenesis (Identification and characterization of sites of lipid droplet biogenesis in the ER)

Reporting period: 2018-03-01 to 2020-02-29

The main scientific objective of this research project was to elucidate the molecular mechanisms of lipid droplet (LD) biogenesis. LDs are fat storage organelles found in all cell types and plays crucial role in cell physiology and lipid metabolism. Dysregulation of LD homeostasis often results in human pathologies such as obesity, type 2 diabetes, cardiovascular diseases, and lipodystrophy (defects in homeostasis of fat tissue). LDs originate from the endoplasmic reticulum (ER), however relatively little is known about how sites of LD formation are determined, and which proteins/lipids are necessary for the process. Studying how LD biogenesis is regulated has broad implication in deciphering the etiology of lipid-storage related disorders. Therefore, the aim of this research project was to study how and where lipid droplets originate inside the cells.
In addition, the overall objective of this research project was to impart training to the recruited researcher, by facilitating transfer of knowledge between the host institution and the recruited researcher, and aiding in his career development.
In this research project we determined if biogenesis of lipid droplets (LDs) is a random process that occurs stochastically in the endoplasmic reticulum (ER), or does it occur at some discrete ER subdomains. We used yeast as model eukaryote for our study. Here we show that induction of triacylglycerol (TAG)-synthesis in yeast by either of the two TAG-synthases, Lro1, or Dga1 results in formation of droplets at discrete ER sites. We have found that these discrete ER subdomains are marked by two ER proteins, Fld1, a homolog of mammalian seipin, and a regulator of diaclyglycerol (DAG) production, Nem1. Remarkably, Both Fld1 and Nem1 localize to ER sites, independent of each other, and of the presence of LDs, but both are required together to create functional sites of LD biogenesis. Fld1-Nem1 marked ER subdomains recruit TAG-synathses and other LD biogenesis factors: Pex30, an ER shaping protein; Yft2, a homolog of mammalian FIT2; Pet10, yeast perilipin homolog; Erg6, an LD marker protein of yeast. In addition these Fld1-Nem1 defined ER sites become enriched in diaclyglycerol (DAG), a neutral lipid precursor. We conclude that both Fld1 and Nem1 perform a crucial role in the first steps of LD biogenesis. Cells lacking either Fld1 or Nem1 show ectopic TAG synthesis and aberrant LD biogenesis.

Findings from this study has recently been accepted for publication in the Journal of Cell Biology with me as first and lead corresponding author (Choudhary et al., 2020, forthcoming, doi: 10.1083/jcb.201910177.).
During the tenure of this fellowship, the recruited researcher presented findings from this study as an oral presentation in a scientific conference and as a poster in a scientific workshop. The researcher also participated in an outreach activity within the scope of this fellowship.
Findings from this study has recently been accepted for publication in the Journal of Cell Biology with me as first and lead corresponding author (Choudhary et al., 2020, forthcoming, doi: 10.1083/jcb.201910177.).
This study open up new avenues for undertaking research about deciphering the pathophysiology of lipodystrophy diseases. One of the key factor that determines sites of LD formation in the ER is seipin, a protein implicated in lipodystrophy. Therefore, pursuing further research would shed light about the mechanisms of how lipid droplet biogenesis at wrong places in the ER results in diseases. Hence this study will pave the way forward for exploring potential therapeutic interventions of controlling excessive accumulation of LDs. Knowing about LD biogenesis also has potential implications in designing strategies for overproduction of Biofuels.
Lipid droplet (LD) biogenesis from discrete ER subdomains in yeast.