The main role of the nuclear envelope (NE) is to compartmentalize and protect the unfolded genomic DNA from the cytoplasm in eukaryote cells. It is composed of a lipid bilayer reinforced in its inner side with a sheet-like structure of proteins called the nuclear lamina. The outer nuclear membrane (ONM) shares a common border with the endoplasmic reticulum (ER). The nuclear envelope has also many nuclear pores that facilitate the transport of molecules between the cytosol and the nucleus (Figure 1A). The NE is a dynamic organelle that expends, disrupts and reconstitutes during mitosis. In the interphase, it constantly remodels to adapt to nuclear growth.
Figure 1. Nuclear envelope composition and organization. (A). The nucleus is surrounded by the nuclear envelope (NE). The Outer Nuclear Membrane (ONM) is continuous with the endoplasmic reticulum. The nuclear pore complex (NPC) regulates the export and import between the nucleus and the cytoplasm. The genome is organized in different compartments: euchromatin, heterochromatin, and nucleolus. (B). Structure of lamin layers in the Inner Nuclear Membrane (INM). (C). The NE is composed of a lipid bilayer anchored by several proteins forming the lamin-associated protein, the LINC complex, and by the lamins. The NE proteins regulate gene organization with the Lamin-Associated Domain (LAD).
1.1. Nuclear Lamina
The lamina conveys strength, flexibility, and rigidity as a function of its variable composition and ratio among the various lamins
[1][2]. In addition to providing mechanical support and being an anchorage platform, the nuclear lamina regulates important cellular events such as DNA replication
[3] and cell division
[4]. The lamina is primordial for gene regulation
[5][6] through the repression of genes in the Lamin-Associated Domain (LAD, Figure C), DNA repair
[7], organization of the nucleolus
[8], as well as chromosomal positioning
[9]. The Lamina contributes to the organization of the genome into its different compartments: (i) the heterochromatin, characterized by repressed DNA that is tethered into the LAD, (ii) the euchromatin, the active compartment with a loose chromatin structure that is active for transcription, and (iii) the nucleolus, the site of ribosome production and assembly (
Figure 1A).
The lamina also possesses mechano-responsivity in order to adapt to the cell’s environment. Lamin levels are dynamic, regulated by cell differentiation, and depend on the tissue stiffness
[2][10]. Lamins are interconnected to the cytoskeleton by intermediate proteins such as the Linker of the Nucleoskeleton and Cytoskeleton (LINC) complex composed by SUN1/2 and Nesprins proteins (
Figure 1C), which allow sensing and rapid cellular response via the complex post-translational control of its proteins
[2][11]. The NE can also adapt in response to stiffness
[2] or to function. For example, migrasive and invasive cells need to acquire nuclear elasticity and can decrease nuclear envelope proteins in order to compress through narrow space
[1][10][12].
The nuclear lamina is composed of lamins and nuclear lamin-associated membrane proteins (
Figure 1B,C). Lamins are type V intermediate filaments fibrous proteins that are divided into two major categories, the A and B type. The gene
LMNA can be spliced in two predominant isoforms, the longer version encoding the protein Lamin A and the shorter isoform generating the Lamin C protein. The type B lamins are expressed by two different genes:
LMNB1 and
LMNB2, encoding Lamin B1 and B2, respectively. While Lamin A is expressed in differentiated cells, type B lamins are ubiquitously expressed in all cells. Similar to other intermediate filament proteins, lamins self-assemble into complex structures. Lamins are highly dynamic and regulated proteins that assemble and disassemble pending stimuli
[13]. They are organized into distinct networks at the nuclear periphery
[14] (
Figure 1B). Lamin B1 forms an outer concentric ring, and its localization is curvature-dependent. This suggests a role of Lamin B1 in stabilizing nuclear shape by restraining outward protrusions of the Lamin A/C network
[15]. Lamins are subject to numerous post-translational modifications, most prominently phosphorylation, such as Lamin A that harbors more than 70 identified unique phosphorylation sites. Such phosphorylation regulates/coordinates the different structural state of lamins. For example, during interphase, Lamin A phosphorylation on Serine S22 and S390 promotes lamin degradation and nuclear softening in response to low cytoskeleton tension
[16][17]. Other post-translational modifications include farnesylation, sumoylation, and acetylation
[18]. Of particular interest is the farnesylation of the carboxyl terminal end cysteine (CaaX) that anchors lamins into the lipid layers. Type B lamins are permanently farnesylated (
Figure 1B), while type A lamins are only transiently farnesylated before the carboxy-terminal peptide is released by the cleavage of prelamin A by ZMPSTE24 to form the mature Lamin A protein. Lamin C proteins are not farnesylated due to their lack of the CaaX motif. Interestingly, the majority of premature aging diseases such as the Hutchinson–Gilford progeria syndrome aka progeria ensue from single point mutations within the
LMNA gene that give rise to a permanently farnesylated mutant Lamin A protein, which is termed progerin.
Other important constituents of the lamina are the lamin-associated proteins that mediate the attachment of lamins to the nuclear envelope. Their role is to assist lamins in regulating the chromatin, as exemplified by Lamin B Receptor (LBR) that interacts with several histone modifiers
[19], as well as facilitating mechanotransduction as for Emerin
[20] and regulating signaling pathways such as TGFβ by MAN1
[21] (
Figure 1C). The diversity of lamin-associated proteins justifies the various roles and functions played by the nuclear lamina.
1.3. Nuclear Lipid Bilayer
The nuclear envelope (NE) is composed of two phospholipid bilayers organized in an inner nuclear membrane (INM) and outer nuclear membrane (ONM) separated by a lumenal space (
Figure 1B,C). The layers are composed of several kind of lipids with different physical properties such as cylindrical lipids (phosphatidylcholine, PC) and conical lipids (phosphatidylethanolamine, PE, diacylglycerol, DAG). Eukaryotic cells maintain their membrane lipid composition within narrow limits with phosphatidylcholine (PC) being the most abundant phospholipid in their nuclear envelope
[24]. Despite this relative stable composition, studies in yeasts have shown that pending the environment, de novo specific lipid synthesis can take place in order to relieve the curvation elastic stress on the nuclear membrane protecting the nucleus from breakdown
[24]. Interestingly, PCYT1A, the rate-limiting enzyme of PC synthesis involved in this curvation compliance, is localized at the INM in mammalian cells, suggesting a similar adaptability in response to stress and cellular needs in higher organisms
[24]. Interestingly, it has also been shown that yeast cells keep lipid droplets at the INM, highlighting that INM has its own lipid metabolism and striking metabolic adaptability
[25]. The formation of those lipid droplets through nucleation is influenced by membrane proteins, lipids, and mechanical properties
[26]. Thus, multiple adaptability mechanisms may exist to selectively enrich and regulate specific lipid species at the INM in eukaryote cells
[27].