Mathias Beller

COPI complex is a regulator of lipid homeostasis.

Lipid droplets are ubiquitous triglyceride and sterol ester storage organelles required for energy storage homeostasis and biosynthesis. Although little is known about lipid droplet formation and regulation, it is clear that members of the PAT (perilipin, adipocyte differentiation related protein, tail interacting protein of 47 kDa) protein family coat the droplet surface and mediate interactions with lipases that remobilize the stored lipids. We identified key Drosophila candidate genes for lipid droplet regulation by RNA interference (RNAi) screening with an image segmentation-based optical read-out system, and show that these regulatory functions are conserved in the mouse. Those include the vesicle-mediated Coat Protein Complex I (COPI) transport complex, which is required for limiting lipid storage. We found that COPI components regulate the PAT protein composition at the lipid droplet surface, and promote the association of adipocyte triglyceride lipase (ATGL) with the lipid droplet surface to mediate lipolysis. Two compounds known to inhibit COPI function, Exo1 and Brefeldin A, phenocopy COPI knockdowns. Furthermore, RNAi inhibition of ATGL and simultaneous drug treatment indicate that COPI and ATGL function in the same pathway. These data indicate that the COPI complex is an evolutionarily conserved regulator of lipid homeostasis, and highlight an interaction between vesicle transport systems and lipid droplets.

Characterization of the Drosophila Lipid Droplet Subproteome

Lipid storage droplets are universal organelles essential for the cellular and organismal lipometabolism including energy homeostasis. Despite their apparently simple design they are proposed to participate in a growing number of cellular processes, raising the question to what extent the functional multifariousness is reflected by a complex organellar proteome composition. Here we present 248 proteins identified in a subproteome analysis using lipid storage droplets of Drosophila melanogaster fat body tissue. In addition to previously known lipid droplet-associated PAT (Perilipin, ADRP, and TIP47) domain proteins and homologues of several mammalian lipid droplet proteins, this study identified a number of proteins of diverse biological function, including intracellular trafficking supportive of the dynamic and multifaceted character of these organelles. We performed intracellular localization studies on selected newly identified subproteome members both in tissue culture cells and in fat body cells directly. The results suggest that the lipid droplets of fat body cells are of combinatorial protein composition. We propose that subsets of lipid droplets within single cells are characterized by a protein “zip code,” which reflects functional differences or specific metabolic states.

Control of Fat Storage by a Drosophila PAT Domain Protein

In Drosophila, the masses and sheets of adipose tissue that are distributed throughout the fly are collectively called the fat body. Like mammalian adipocytes, insect fat body cells provide the major energy reserve of the animal organism. Both cell types accumulate triacylglycerols (TAG) in intracellular lipid droplets; this finding suggests that the strategy of energy storage as well as the machinery and the control to achieve fat storage might be evolutionarily conserved. Studies addressing the control of lipid-based energy homeostasis of mammals identified proteins of the PAT domain family, such as Perilipin, which reside on lipid droplets. Perilipin knockout mice are lean and resistant to diet-induced obesity. Conversely, Perilipin expression in preadipocyte tissue culture increases lipid storage by reducing the rate of TAG hydrolysis. Factors that mediate corresponding processes in invertebrates are still unknown. We examined the function of Lsd2, one of only two PAT domain-encoding genes in the Drosophila genome. Lsd2 acts in a Perilipin-like manner, suggesting that components regulating homeostasis of lipid-based energy storage at the lipid droplet membrane are evolutionarily conserved.

Lipid droplets: A dynamic organelle moves into focus.

Lipid droplets (LDs) were perceived as static storage deposits, which passively participate in the energy homeostasis of both cells and entire organisms. However, this view has changed recently after the realization of a complex and highly dynamic LD proteome. The proteome contains key components of the fat mobilization system and proteins that suggest LD interactions with a variety of cell organelles, including the endoplasmic reticulum, mitochondria and peroxisomes. The study of LD cell biology, including cross‐talk with other organelles, the trafficking of LDs in the cell and regulatory events involving the LD coat proteins is now on the verge of leaving its infancy and unfolds that LDs are highly dynamic cellular organelles.

PERILIPIN-Dependent Control of Lipid Droplet Structure and Fat Storage in Drosophila

Lipid droplets are intracellular organelles enriched in adipose tissue that govern the body fat stores of animals. In mammals, members of the evolutionarily conserved PERILIPIN protein family are associated with the lipid droplet surface and participate in lipid homeostasis. Here, we show that Drosophila mutants lacking the PERILIPIN PLIN1 are hyperphagic and suffer from adult-onset obesity. PLIN1 is a central and Janus-faced component of fat metabolism. It provides barrier function to storage lipid breakdown and acts as a key factor of stimulated lipolysis by modulating the access of proteins to the lipid droplet surface. It also shapes lipid droplet structure, transforming unilocular into multilocular fat cells. We generated flies devoid of all PERILIPIN family members and show that they exhibit impaired yet functional body fat regulation. Our data reveal the existence of a basal and possibly ancient lipid homeostasis system.

Lipid droplets control the maternal histone supply of Drosophila embryos.

BACKGROUND Histones are essential for chromatin packing, yet free histones not incorporated into chromatin are toxic. While in most cells multiple regulatory mechanisms prevent accumulation of excess histones, early Drosophila embryos contain massive extranuclear histone stores, thought to be essential for development. Excess histones H2A, H2B, and H2Av are bound to lipid droplets, ubiquitous fat storage organelles especially abundant in embryos. It has been proposed that sequestration on lipid droplets allows safe transient storage of supernumerary histones. RESULTS Here, we critically test this sequestration hypothesis. We find that histones are anchored to lipid droplets via the previously uncharacterized protein Jabba: Jabba localizes to droplets, coimmunoprecipitates with histones, and is necessary to recruit histones to droplets. Jabba mutants lack the maternal H2A, H2B, and H2Av deposits altogether; presumably, these deposits are eliminated unless sequestered on droplets. Jabba mutant embryos compensate for this histone deficit by translating maternal histone mRNAs. However, when histone expression is mildly compromised, the maternal histone protein deposits are essential for proper early mitoses and for viability. CONCLUSIONS A growing number of proteins from other cellular compartments have been found to transiently associate with lipid droplets. Our studies provide the first insight into mechanism and functional relevance of this sequestration. We conclude that sequestration on lipid droplets allows embryos to build up extranuclear histone stores and provides histones for chromatin assembly during times of high demand. This work reveals a novel aspect of histone metabolism and establishes lipid droplets as functional storage sites for unstable or detrimental proteins.

Antagonistic action of Bicoid and the repressor Capicua determines the spatial limits of Drosophila head gene expression domains.

Bicoid (Bcd) is the anterior determinant in Drosophila. Accordingly, loss of Bcd causes loss of head and thorax and their replacement with posterior structures. bcd mRNA is maternally deposited at the anterior pole and Bcd forms an anterior-to-posterior (AP) concentration gradient. The expression of a series of zygotic head genes is thought to be differentially regulated by distinct threshold concentrations of the Bcd gradient. Thereby Bcd functions as a morphogen, instructing fields of cells to take on specific fates. Here, we show that spatial limits of anterior genes are also set in the absence of a Bcd gradient and depend on factors of the maternal terminal system. The receptor tyrosine kinase Torso (Tor), a key component of this system, is active in the pole regions of the embryo. Its activity downregulates the maternally deposited repressor Capicua (Cic), leaving high Cic activity in the central regions and decreasingly lower Cic activities toward the poles. We show that the positions of posterior boundaries of Bcd target genes are dependent not only on Bcd, but also on Tor-mediated Cic activity. The results indicate that Cic can mediate repression through distinct binding sites within a Bcd responsive enhancer and that gene activation by Bcd is antagonized by Cic. The activating and repressive effects of Bcd and Cic, respectively, are integrated by the Bcd target gene enhancer. We conclude that the spatial domains of head gene expression are determined by Bcd in concert with Tor-dependent repressors.

The why, when and how of lipid droplet diversity

ABSTRACT Lipid droplets are the universal cellular organelles for the transient or long-term storage of lipids. The number, size and composition of lipid droplets vary greatly within cells in a homogenous population as well as in different cell types. The variability of intracellular lipid-storage organelles reflects the diversification of lipid droplet composition and function. Lipid droplet diversification results, for example, in two cellular lipid droplet populations that are prone to diminish and grow, respectively. The aberrant accumulation or depletion of lipids are hallmarks or causes of various human pathologies. Thus, a better understanding of the origins of lipid droplet diversification is not only a fascinating cell biology question but also potentially serves to improve comprehension of pathologies that entail the accumulation of lipids. This Commentary covers the lipid droplet life cycle and highlights the early steps during lipid droplet biogenesis, which we propose to be the potential driving forces of lipid droplet diversification. Summary: We discuss the intriguing diversification of lipid droplets between and within cells and propose the early steps of lipid droplet biogenesis to determine the later lipid droplet fate.

The evolutionarily conserved protein CG9186 is associated with lipid droplets, required for their positioning and for fat storage.

Summary Lipid droplets (LDs) are specialized cell organelles for the storage of energy-rich lipids. Although lipid storage is a conserved feature of all cells and organisms, little is known about fundamental aspects of the cell biology of LDs, including their biogenesis, structural assembly and subcellular positioning, and the regulation of organismic energy homeostasis. We identified a novel LD-associated protein family, represented by the Drosophila protein CG9186 and its murine homolog MGI:1916082. In the absence of LDs, both proteins localize at the endoplasmic reticulum (ER). Upon lipid storage induction, they translocate to LDs using an evolutionarily conserved targeting mechanism that acts through a 60-amino-acid targeting motif in the center of the CG9186 protein. Overexpression of CG9186, and MGI:1916082, causes clustering of LDs in both tissue culture and salivary gland cells, whereas RNAi knockdown of CG9186 results in a reduction of LDs. Organismal RNAi knockdown of CG9186 results in a reduction in lipid storage levels of the fly. The results indicate that we identified the first members of a novel and evolutionarily conserved family of lipid storage regulators, which are also required to properly position LDs within cells.

One hundred years of high-throughput Drosophila research

From the beginning, Drosophila was a high-throughput model organism. Unbiased and genome-wide efforts ranging from Morgans search for spontaneous mutations and subsequent saturating loss-of-function and gain-of-function screens up to more recent techniques such as microarrays, proteomics and cellular assays have been and will continue to be the backbone of Drosophila research. Integrating these large datasets is one of the next challenges. However, once achieved, a plethora of information far exceeding the information content of the singular experiments will be revealed. Several high-throughput techniques and experimental strategies highlighting the unbiased and integrative nature of Drosophila research during the last century will be discussed.