Running 'LAPS' Around nLD: Nuclear Lipid Droplet Form and Function

Abstract

The nucleus harbours numerous protein subdomains and condensates that regulate chromatin organization, gene expression and genomic stress. A novel nuclear subdomain that is formed following exposure of cells to excess fatty acids is the nuclear lipid droplet (nLD), which is composed of a neutral lipid core surrounded by a phospholipid monolayer and associated regulatory and lipid biosynthetic enzymes. While structurally resembling cytoplasmic LDs, nLDs are formed by distinct but poorly understood mechanisms that involve the emergence of lipid droplets from the lumen of the nucleoplasmic reticulum and de novo lipid synthesis. Luminal lipid droplets that emerge into the nucleoplasm do so at regions of the inner nuclear membrane that become enriched in promyelocytic leukemia (PML) protein. The resulting nLDs that retain PML on their surface are termed lipid-associated PML structures (LAPS), and are distinct from canonical PML nuclear bodies (NB) as they lack key proteins and modifications associated with these NBs. PML is a key regulator of nuclear signaling events and PML NBs are sites of gene regulation and post-translational modification of transcription factors. Therefore, the subfraction of nLDs that form LAPS could regulate lipid stress responses through their recruitment and retention of the PML protein. Both nLDs and LAPS have lipid biosynthetic enzymes on their surface suggesting they are active sites for nuclear phospholipid and triacylglycerol synthesis as well as global lipid regulation. In this review we have summarized the current understanding of nLD and LAPS biogenesis in different cell types, their structure and composition relative to other PML-associated cellular structures, and their role in coordinating a nuclear response to cellular overload of fatty acids.

Keywords: CCTalpha; PML; fatty acid; lipin; nuclear lipid droplets; phosphatidylcholine.

FIGURE 1

nLD biogenesis in mammalian cells. Two mechanisms have been identified for nLD formation. Mechanism 1: Much like cLD biogenesis, nLD biogenesis in U2OS cells, S. cerevisiae, and C. elegans involves in situ TAG synthesis at the INM facilitated by lipid biosynthetic enzymes. nLD biogenesis is seipin-dependent in S. cerevisiae, whereas the process is seipin-independent in U2OS cells. Mechanism 2: In specialized lipoprotein-exporting mammalian cells like hepatocytes and intestinal epithelial cells, ApoB-free eLDs form in an MTP-dependent manner and subsequently migrate through the lumen of the ER into the lumen of the NE. Next, eLDs enter into type-I NR invaginations of the INM that extend into the nucleoplasm. PML-II localizes to INM at lamin-deficient regions, possibly facilitating translocation of the LD through ruptures of the INM into the nucleoplasm. In mammalian cells more generally, lipid biosynthetic enzymes DGAT2, Lipin-1 and CCTα, LD coat protein perilipin-3, and PML are all present at the surface of nLDs. The binding of lipid biosynthetic enzymes and the formation of LAPS are two commonalities of nLDs irrespective of their biogenesis in mammalian cells, suggesting a possible conserved function for these structures.

FIGURE 2

The Kennedy pathway for TAG synthesis and LD biogenesis in mammalian cells. (A) Biosynthetic enzymes of the Kennedy pathway act sequentially to synthesize triacylglycerol (TAG) at the ER membrane; GPAT3/4 synthesizes lysophosphatidic acid (LPA) from glycerol 3-phosphate (G-3-P) and fatty acids (FA), AGPAT2 synthesizes phosphatidic acid (PA) from LPA and FA, Lipin-1 hydrolyzes PA to diacylglycerol (DAG), and DGAT2 catalyzes the final acylation to form TAG. (B) TAG nucleates between the two leaflets of the ER membrane bilayer, which is partly facilitated by a complex of LDAF and seipin at distinct domains throughout the ER. These points of TAG nucleation develop into lens-like structures that proceed to bud into the cytoplasm as a budding LD coated with LDAF as it dissociates from seipin. As seipin funnels TAG and DAG into nascent LDs, lipid biosynthetic enzymes (class I LD proteins) like GPAT3/4, AGPAT2, and DGAT2 transfer across membrane bridges to the surface monolayer, further facilitating the maturation of LDs. Once the mature LD separates from the ER, it recruits class II LD proteins like perilipin-2/3, which coat the surface to regulate access of LDs to lipases and autophagy proteins. This graphic was created with Biorender.com.

FIGURE 3

Overview of nuclear PML structures and their interactors. The formation of canonical phase-separated spherical PML NB is driven by protein oligomerization and SUMO-SIM interactions, which also recruit other proteins such as DAXX and SP100. However, other novel PML structures form under specific stimuli. These include ALT-associated PML bodies (APB), PML-I nucleolar caps and PML clastosomes. SUMO-independent LAPS form on nLDs and host lipid biosynthetic enzymes such as Lipin1 and CCTα. During mitosis, SUMO-independent structure known as mitotic accumulations of PML protein (MAPP) form and are tethered to endosomes. In certain disease states such as with Hutchinson-Gilford progeria syndrome, PML-II lamin threads are formed. These aforementioned PML structures uniquely interact with a number of proteins, such as RNF4, CRAG, TRF2, and the BTR complex, to modulate normal cellular functions and the cell’s response to stress states. This graphic was. created with Biorender.com.