Not Just Fat: The Structure and Function of the Lipid DropletToyoshi Fujimoto & Robert G. Parton

Presented by Bang Tran, Dharma Varapula, and Edward Waddell

Introduction

• Known as mere deposits of lipid esters for many years

• Redefined as authentic organelles with multiple functions

• Lipid metabolism

• Lipid storage function in white adipocytes

• Various new functions

• Diseases related to LDs

Structure and Biogenesis of the Lipid Droplet (LD)

LD’s Structure

• Diameter

• 0.1 to 5 µm in non-adipocyte

• 100 µm in white adipocyte

• Outside

• Phospholipid monolayer

• Inside

• Lipid Esters

• Amphiphilic proteins

Structure• Phospholipid monolayer

• Phosphatidylchloline (PC)

• Phosphatidylethanolamin

e (PE)

• Phosphatidylinositol

• lysoPC and lysoPE

• LD Core

• Triglycerides

• Cholesterol ester

• Amphiphilic proteinGuo, et al. (2009)

Biogenesis• 3 proposed models

1. Budding from the ER

covered by the

cytoplasmic leaflet

2. “Hatching” from a

bicellular structure

3. Budding from a vesicle

• Not spontaneous form but require some active mechanism

Guo, et al. (2009)

Functions of LDs

Functions of LDs

Can be classified into:

Most of these functions can be ascribed to:

• Surface proteins, commonly PAT proteins

• Availability of organic/hydrophobic phase within the cell

Canonical (or lipid-related)

Non-canonical

1 Storage of lipid esters Capturing faulty proteins, histones

2 Lipid metabolism regulation

Signaling

PAT Proteins

Regulate the lipid storage and metabolism

● PLIN 1 (perilipin) - in adipocytes and steroidogenic

cells

● PLIN 2 (ADRP), PLIN 3 (TIP47) - almost everywhere

● PLIN 4 (S3-12) - largely adipocytes

● PLIN 5 (OXPAT/MLDP/LSDP5) - fatty acid oxidation

sites (liver, muscle, brown adipocytes)

Lipolysis of lipid esters

CA catecholamines (dopamine,

isoproterenol, etc)

βAR β-adrenergic receptor

(GPCR)

cAMP cyclic-AMP (secondary

messenger)

PKA Protein kinase A

HSL Hormone-sensitive lipase

ATGL Adipose triglyceride lipase

CGI-58 a co-activator of ATGL

Bickel, et al. (2009)

Lipid Metabolism Regulation

Mutant lacking neutral lipids displayed delayed growth and morphological defects.

Loss of ATGL (lipase) ortholog induced lethality in embryos of Drosophila melanogaster - Gronke, et al. (2005). - de novo fatty acid synthesis

pathways were undisturbed- this implies lipids from LDs

are in some way preferred

Petschnigg, et al. (2009)

Non-canonical functions

Hydrophobic proteins (ApoB, α-synuclein) temporarily

stored in LDs for degradation

- how large lipidated-ApoB moves from ER to LD

through the degrading cytosol is not clear

Free histones (toxic and not hydrophobic) are

sequestered in LD of Drosophila embryo for release at

a later stage

- why cell chooses LD (fluctuating surface area and

numbers) over conventional membranes not clear

LDs in a Cellular Context

LD interactions with Other Organelles• LD-ER interaction may be a

mechanism which allows stored lipids in LDs to become mobilized for use in other cellular sites.

• LDs perform a “kiss-and-run” contact with phagosomes in order to supply them with arachidonic acid for NADPH oxidase activation.

• LDs form a close relationship with mitochondria and peroxisomes. This is to allow fatty acids liberated by lipolysis to enter into β-oxidation.

Sturmey, R. G., et al. (2006)

LD interaction with Caveolae and Caveolins• Caveolins are a family of integral membrane proteins that

associate with LDs in fatty-acid loaded cells and regenerating hepatocytes.

• Caveolins form the framework of caveolae, which are specialized types of lipid rafts rich in cholesterol, sphingolipids, and proteins. Caveolae are important for several functions in signal transduction.

• Caveolin-1 is translocated to LDs through a hemi-fusion interaction between the vesicular membrane and LD. Caveolin-1 is associated with LD function and lipid storage in adipocytes.

Fernandez, et al. (2006)

LD Motility• Motility of LDs is essential to

regulate their distribution within the cell and how they interact with other organelles.

• LDs show both microtubule-based directional long-distance and random short-distance types of movement.

• Dynein and kinesin-1 were found to be associated with LDs

• LSD2, a PAT protein homolog in Drosophila, regulates LD movement by coordinating dynein and kinesin-1.

• Manipulation of LSD2 shows that LSD2 is required for normal lipid storage by allowing for the formation of larger LDs through microtubule based movements.

Welte, et al. (2005)

LD Motility

Welte, et al. (2005)

LDs and Disease

LDs and Motor Neuron Disease

• Spartin/SPG20, a LD protein, binds to TIP47 and competes with ADRP for LD association.

• An E3 ligase, WWP1, regulates the amount of Spartin/SPG20 thus regulating lypolysis by adjusting the TIP47:ADRP ratio.

• Spartin/SPG20 mutants that cannot compete with ADRP cause Troyer syndrome, a motor neuron disease.

• This defect appears to be caused by aberrant turnover of LD lipids.

References• Fujimoto, T., Parton, R. G. (2011) Not Just Fat: The Structure and Function of the Lipid Droplet. Cold

Spring Harb Perpect Biol. 3:a004838.

• Guo, Y., Cordes, K., Farese, R., and Walther, T. (2009). Lipid Droplets at a glance. Journal of Cell Science. 122:749-752.

• Bickel, P. E., Tansey, J. T., Welte, M. A. (2009) PAT proteins, an ancient family of lipid droplet proteins that regulate cellular lipid stores. Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids. 1791 (6): 419-440.

• Petschnigg, J., Wolinski, H., Kolb, D., Zellnig, G., Kurat, C. F., Natter, K., Kohlwein, S. D. (2009) Lipids and Lipoproteins: Metabolism, Regulation and Signaling. J. Biol. Chem. 284: 30981-30993.

• Gronke, S., Mildner, A., Fellert, S., Tennagels, N., Petry, S., Muller, G., Jackle, H., Kuhnlein, R. P. (2005) Brummer lipase is an evolutionarily conserved fat storage regulator in Drosophila. Cell Metab. 1:323:330.

• Fernandez, M., Albor, C., Ingelmo-Torres, M., Nixon, S., Ferguson, C., Kurzchalia, T., Tebar, F., Enrich, C., Parton, R., and Pol, A. (2006) Caveolin-1 is essential for liver regeneration. Science. 313: 1628–1632.

• Sturmey, R., O’Toole, P., and Leese, H. (2006) Fluorescence resonance energy transfer analysis of mitochondrial:Lipid association in the porcine oocyte. Reproduction. 132: 829–837.

• Welte, M., Cermelli, S., Griner, J., Viera, A., Guo, Y., Kim, D., Gindhart, J., and Gross, S. (2005) Regulation of lipid-droplet transport by the perilipin homolog lsd2. Curr Biol. 15: 1266–1275.