Biology Reference
In-Depth Information
that is well suited for its specialized role as an efficient energy storage organelle. Over-
expansion of LDs in white adipocytes results in the development of obesity and insulin
resistance. Besides its central role in lipid storage and mobilization, LDs play crucial
roles in various cellular processes including virus packaging, host defense, protein
storage, and degradation. CIDE proteins, in particular Fsp27, initiates a unique LD fu-
sion process in adipocytes by clustering and enriching at LD contact site and promoting
neutral lipid exchange and transfer between contacted LDs. Here, we summarize our
approaches to quantitatively measure intracellular LD size and neutral lipid exchange
between LDs. Utilization of these methods has greatly facilitated our understanding of
molecular pathways governing LD growth in adipocytes.
INTRODUCTION
Biogenesis and cellular functions of lipid droplets
Lipid droplets (LDs) contain a neutral lipid core composed of triglycerides (TAG)
and cholesterol esters (CE) which are insulated from the cytosolic environment
by a monolayer of phospholipids and various proteins coated on the LD surface
(
Farese & Walther, 2009; Krahmer, Guo, Farese, & Walther, 2009
). Besides their
central roles in maintaining lipid homeostasis (
Walther & Farese, 2012
), LDs are also
shown to be involved in virus packaging (
Herker & Ott, 2011; Samsa et al., 2009
),
host defense (
Saka & Valdivia, 2012
), protein storage (
Anand et al., 2012; Li et al.,
2012
), protein quality control (
Suzuki et al., 2012
), and protein posttranslational
modification (
Krahmer et al., 2013
). It is generally believed that LDs are originated
from the endoplasmic reticulum (ER) where TAG is synthesized by DGAT1 and
DGAT2 and then budded into the cytosol (
Harris et al., 2011; Martin & Parton,
2006
). Some LD-associated proteins, for example, AAM-B (
Zehmer et al., 2009
),
localize on ER and are sorted to LD surface during LD biogenesis; other proteins
are shuttled between cytosol and LD surface (
Wolins, Brasaemle, & Bickel, 2006
).
Consistent with its central role inmaintaining lipid homeostasis, LD harbors crucial
enzymes involved in lipogenesis and lipolysis pathways. Two neutral lipid lipases, ad-
ipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL), are recruited to
LD surface to execute a consecutive lipolysis cascade in adipocytes. On the other hand,
key enzymes in TAG or phospholipid synthesis pathways are shown to be localized to
LDs to modulate LD growth (
Krahmer et al., 2011; Kuerschner, Moessinger, &Thiele,
2008; Stone et al., 2009; Wilfling et al., 2013
). Among all known LD-associated pro-
teins, perilipin family proteins play essential structural and regulatory roles for LDmor-
phology and function. Perilipin proteins include five members (Plin1/Perilipin, Plin2/
ADRP, Plin3/TIP47, Plin4/S3-12, and Plin5/LSDP5) that show different tissue expres-
sion patterns (
Bickel, Tansey, & Welte, 2009; Greenberg et al., 1991; Hickenbottom,
Kimmel, Londos, & Hurley, 2004; Kimmel, Brasaemle, McAndrews-Hill, Sztalryd,
& Londos, 2010
). Plin1 plays dual roles in lipid storage and mobilization. On the one
hand, it suppresses basal lipolysis at an unstimulated state by blocking association of
ATGL and HSL to the LD surface (
Bickel et al., 2009
). However, when Plin1 is phos-
phorylated byPKA, the phosphorylatedPlin1 recruitsATGLandHSL to theLDsurface
(
Brasaemle, 2007;Martinez-Botas et al., 2000;Miyoshi et al., 2007; Tanseyet al., 2001
).
