Nuclear phosphoinositides: a signaling enigma wrapped in a compartmental conundrum

Abstract

While the presence of phosphoinositides in the nuclei of eukaryotes and the identity of the enzymes responsible for their metabolism have been known for some time, their functions in the nucleus are only now emerging. This is illustrated by the recent identification of effectors for nuclear phosphoinositides. Like the cytosolic phosphoinositide signaling pathway, nuclear phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) is at the center of the pathway and acts both as a messenger and as a precursor for many additional messengers. Here, recent advances in the understanding of nuclear phosphoinositide signaling and its functions are reviewed with an emphasis on PI4,5P(2) and its role in gene expression. The compartmentalization of nuclear phosphoinositide phosphates (PIP(n)) remains a mystery, but emerging evidence suggests that phosphoinositides occupy several functionally distinct compartments.

Figure 1

Phosphoinositide kinases, phosphatases, and phospholipases. A. Canonical PI cycle. Inositol phospholipids are named according to the number and position of phosphate groups on the inositol headgroup. The singly phosphorylated PIPs (PI3P, PI4P and PI5P) are created by the phosphorylation of PI at the 3, 4 and possibly 5 positions. PI5P can also be generated by the type I PI4,5P₂ 4-phosphatase (type I 4-pptase). Theses PIPs act as intermediates for the synthesis of inositol bis- and tris-phosphates (PIP₂ and PIP₃). PIP₂ is generated by phosphorylation of PI3P, PI4P or PI5P by the indicated PIP kinases and PI4,5P₂ is hydrolyzed by PLC to form IP₃ and DAG. PIP₃ is generated by phosphorylation of PI4,5P₂ by PI3K. The classical PI cycle is highlighted with bold arrows and enzymes. B. A graphic representation of PI and the kinases, phosphatases, and phospholipases that to date have been shown to be localized in the nucleus according to the position the on inositol head group where they act. Nuclear speckle targeted enzymes are indicated in bold. C. PIPKIα colocalizes with components of the mRNA-processing machinery in nuclear speckles and PIP₂. Top panel: Cells were double labeled with an anti-PIPKIα polyclonal antibody and anti-PIP₂ monoclonal antibody. Bottom panel: Cells were double-labeled with an anti-PIPKIα polyclonal antibody and human Sm antiserum (a nuclear speckle marker).

Figure 2

A nuclear phosphoinositide-mediated stress response pathway. Under resting conditions, PIPKIIβ controls PI5P levels by its synthesis of PI4,5P₂. Upon cellular stress, PIPKIIβ activity is attenuated via type 1 PI4,5P₂ 4-phosphatase and p38 MAPK activity resulting in the accumulation of PI5P. Specifically, in response to cellular stress, such as oxidative stress or UV irradiation, type 1 PI4,5P₂ 4-phosphatase translocates to the nucleus where it hydrolyzes PI4,5P₂ into PI5P. Concurrently, PIPKIIβ is phosphorylated by activated p38 MAPK, inhibiting its lipid kinase activity and resulting in increased nuclear levels of PI5P. The accumulation of PI5P recruits ING2 to chromatin and promotes ING2-dependent p53 acetylation. Acetylation of p53 enhances its activity and stability and therefore increases apoptotic death. PI5P also may modulate an upstream activator of p38 MAPK, resulting in the activation of the Cul3-SPOP ubiquitin ligase complex toward multiple substrates, including PIPKIIβ. Represented are the defined functions of PI5P and PI4,5P₂; however, both PI5P and PI4,5P₂ may bind as of yet unidentified effectors (green boxes), which could play diverse roles in nuclear signaling. Ub = ubiquitin; A = acetylation

Figure 3

Phosphoinositides in eukaryotic mRNA transcription and processing. A diagram depicting the events of mRNA generation in eukaryotes, including chromatin remodeling and transcription, mRNA processing (5′-end capping, splicing, and cleavage and polyadenylation), and mRNA export and translation, and the PIP_n or IP_n molecules that have been implicated in each step. Phosphoinositides have been implicated in most aspects of mRNA synthesis except for 5' capping. Phosphoinositide species and the process regulated are indicated. This schematic also highlights the processing of mRNA at the 3′-end modulated by Star-PAP, the only nuclear effector identified to date that is directly activated by PI4,5P₂. Star-PAP is a nuclear poly(A) polymerase that is required for the expression of select mRNAs. Star-PAP assembles into a complex with RNA polymerase II (RNAPII) and known 3′-processing factors, but the complex is notably devoid of canonical PAPα. The Star-PAP complex contains unique components, such as PIPKIα and the PI4,5P₂ sensitive protein kinase CKIα. Star-PAP is necessary for the 3′-processing of its target mRNAs and functionally both PIPKIα and CKIα are required for the maturation of a subset of Star-PAP target mRNAs.

Figure 4

A model illustrating the compartmentalization of phosphoinositide signaling in the nucleus. Current data suggests two compartments for the nuclear phosphoinositide cycle: One associated with the nuclear envelope and another in a subnuclear compartment separate from known membrane structures. In both compartments, PI is sequentially phosphorylated by PI kinases (PIK) and PIP kinases (PIPK) to generate PIP₂, which could then be metabolized by PLC to generate IP₃ and then higher inositol phosphates (IP_n) or phosphorylated by PI 3-kinase creating PIP₃. In subnuclear compartments, phosphoinositides are hypothesized to be associated with carrier or effector proteins. Such proteins could be specific for certain functions and/or could present phosphoinositides to other effectors. In addition, regulation of nuclear actin polymerization and actin binding proteins, such as N-WASP, CapZ and ADF (actin/cofilin depolymerising factor), either from envelope-bound or endonuclear phosphoinositides (shown by dashed arrows) has been shown to affect many aspects of gene expression.