G-quadruplex folding in Xist RNA antagonizes PRC2 activity for stepwise regulation of X chromosome inactivation

How Polycomb repressive complex 2 (PRC2) is regulated by RNA remains an unsolved problem. Although PRC2 binds G-tracts with the potential to form RNA G-quadruplexes (rG4s), whether rG4s fold extensively in vivo and whether PRC2 binds folded or unfolded rG4 are unknown. Using the X-inactivation model in mouse embryonic stem cells, here we identify multiple folded rG4s in Xist RNA and demonstrate that PRC2 preferentially binds folded rG4s. High-affinity rG4 binding inhibits PRC2's histone methyltransferase activity, and stabilizing rG4 in vivo antagonizes H3 at lysine 27 (H3K27me3) enrichment on the inactive X chromosome. Surprisingly, mutagenizing the rG4 does not affect PRC2 recruitment but promotes its release and catalytic activation on chromatin. H3K27me3 marks are misplaced, however, and gene silencing is compromised. Xist-PRC2 complexes become entrapped in the S1 chromosome compartment, precluding the required translocation into the S2 compartment. Thus, Xist rG4 folding controls PRC2 activity, H3K27me3 enrichment, and the stepwise regulation of chromosome-wide gene silencing.

Imprinted X chromosome inactivation at the gamete-to-embryo transition

In mammals, dosage compensation involves two parallel processes: (1) X inactivation, which equalizes X chromosome dosage between males and females, and (2) X hyperactivation, which upregulates the active X for X-autosome balance. The field currently favors models whereby dosage compensation initiates "de novo" during mouse development. Here, we develop "So-Smart-seq" to revisit the question and interrogate a comprehensive transcriptome including noncoding genes and repeats in mice. Intriguingly, de novo silencing pertains only to a subset of Xp genes. Evolutionarily older genes and repetitive elements demonstrate constitutive Xp silencing, adopt distinct signatures, and do not require Xist to initiate silencing. We trace Xp silencing backward in developmental time to meiotic sex chromosome inactivation in the male germ line and observe that Xm hyperactivation is timed to Xp silencing on a gene-by-gene basis. Thus, during the gamete-to-embryo transition, older Xp genes are transmitted in a "pre-inactivated" state. These findings have implications for the evolution of imprinting.

Human Coronavirus Infection Reorganizes Spatial Genomic Architecture in Permissive Lung Cells

Chromatin conformation capture followed by next-generation sequencing in combination with large-scale polymer simulations (4DHiC) produces detailed information on genomic loci interactions, allowing for the interrogation of 3D spatial genomic structures. Here, Hi-C data was acquired from the infection of fetal lung fibroblast (MRC5) cells with α-coronavirus 229E (CoV229E). Experimental Hi-C contact maps were used to determine viral-induced changes in genomic architecture over a 48-hour time period following viral infection, revealing substantial alterations in contacts within chromosomes and in contacts between different chromosomes. To gain further structural insight and quantify the underlying changes, we applied the 4DHiC polymer simulation method to reconstruct the 3D genomic structures and dynamics corresponding to the Hi-C maps. The models successfully reproduced experimental Hi-C data, including the changes in contacts induced by viral infection. Our 3D spatial simulations uncovered widespread chromatin restructuring, including increased chromosome compactness and A-B compartment mixing arising from infection. Our model also suggests increased spatial accessibility to regions containing interferon-stimulated genes upon infection with CoV229E, followed by chromatin restructuring at later time points, potentially inducing the migration of chromatin into more compact regions. This is consistent with previously observed suppression of gene expression. Our spatial genomics study provides a mechanistic structural basis for changes in chromosome architecture induced by coronavirus infection in lung cells.

Variable expression of MECP2, CDKL5, and FMR1 in the human brain: Implications for gene restorative therapies

MECP2, CDKL5, and FMR1 are three X-linked neurodevelopmental genes associated with Rett, CDKL5-, and fragile-X syndrome, respectively. These syndromes are characterized by distinct constellations of severe cognitive and neurobehavioral anomalies, reflecting the broad but unique expression patterns of each of the genes in the brain. As these disorders are not thought to be neurodegenerative and may be reversible, a major goal has been to restore expression of the functional proteins in the patient's brain. Strategies have included gene therapy, gene editing, and selective Xi-reactivation methodologies. However, tissue penetration and overall delivery to various regions of the brain remain challenging for each strategy. Thus, gaining insights into how much restoration would be required and what regions/cell types in the brain must be targeted for meaningful physiological improvement would be valuable. As a step toward addressing these questions, here we perform a meta-analysis of single-cell transcriptomics data from the human brain across multiple developmental stages, in various brain regions, and in multiple donors. We observe a substantial degree of expression variability for MECP2, CDKL5, and FMR1 not only across cell types but also between donors. The wide range of expression may help define a therapeutic window, with the low end delineating a minimum level required to restore physiological function and the high end informing toxicology margin. Finally, the inter-cellular and inter-individual variability enable identification of co-varying genes and will facilitate future identification of biomarkers.

Site-specific R-loops induce CGG repeat contraction and fragile X gene reactivation

Here, we describe an approach to correct the genetic defect in fragile X syndrome (FXS) via recruitment of endogenous repair mechanisms. A leading cause of autism spectrum disorders, FXS results from epigenetic silencing of FMR1 due to a congenital trinucleotide (CGG) repeat expansion. By investigating conditions favorable to FMR1 reactivation, we find MEK and BRAF inhibitors that induce a strong repeat contraction and full FMR1 reactivation in cellular models. We trace the mechanism to DNA demethylation and site-specific R-loops, which are necessary and sufficient for repeat contraction. A positive feedback cycle comprising demethylation, de novo FMR1 transcription, and R-loop formation results in the recruitment of endogenous DNA repair mechanisms that then drive excision of the long CGG repeat. Repeat contraction is specific to FMR1 and restores the production of FMRP protein. Our study therefore identifies a potential method of treating FXS in the future.

Long non-coding RNAs: definitions, functions, challenges and recommendations

Genes specifying long non-coding RNAs (lncRNAs) occupy a large fraction of the genomes of complex organisms. The term 'lncRNAs' encompasses RNA polymerase I (Pol I), Pol II and Pol III transcribed RNAs, and RNAs from processed introns. The various functions of lncRNAs and their many isoforms and interleaved relationships with other genes make lncRNA classification and annotation difficult. Most lncRNAs evolve more rapidly than protein-coding sequences, are cell type specific and regulate many aspects of cell differentiation and development and other physiological processes. Many lncRNAs associate with chromatin-modifying complexes, are transcribed from enhancers and nucleate phase separation of nuclear condensates and domains, indicating an intimate link between lncRNA expression and the spatial control of gene expression during development. lncRNAs also have important roles in the cytoplasm and beyond, including in the regulation of translation, metabolism and signalling. lncRNAs often have a modular structure and are rich in repeats, which are increasingly being shown to be relevant to their function. In this Consensus Statement, we address the definition and nomenclature of lncRNAs and their conservation, expression, phenotypic visibility, structure and functions. We also discuss research challenges and provide recommendations to advance the understanding of the roles of lncRNAs in development, cell biology and disease.

Targeting RNA with small molecules: lessons learned from Xist RNA

Although more than 98% of the human genome is noncoding, nearly all drugs on the market target one of about 700 disease-related proteins. However, an increasing number of diseases are now being attributed to noncoding RNA and the ability to target them would vastly expand the chemical space for drug development. We recently devised a screening strategy based upon affinity-selection mass spectrometry and succeeded in identifying bioactive compounds for the noncoding RNA prototype, Xist. One such compound, termed X1, has drug-like properties and binds specifically to the RepA motif of Xist in vitro and in vivo. Small-angle X-ray scattering analysis reveals that X1 changes the conformation of RepA in solution, thereby explaining the displacement of cognate interacting protein factors (PRC2 and SPEN) and inhibition of X-chromosome inactivation. In this Perspective, we discuss lessons learned from these proof-of-concept experiments and suggest that RNA can be systematically targeted by drug-like compounds to disrupt RNA structure and function.

The EUFOREA pocket guide for chronic rhinosinusitis

Chronic rhinosinusitis (CRS) is known to affect around 5 % of the total population, with major impact on the quality of life of those severely affected (1). Despite a substantial burden on individuals, society and health economies, CRS often remains underdiagnosed, under-estimated and under-treated (2). International guidelines like the European Position Paper on Rhinosinusitis and Nasal Polyps (EPOS) (3) and the International Consensus statement on Allergy and Rhinology: Rhinosinusitis 2021 (ICAR) (4) offer physicians insight into the recommended treatment options for CRS, with an overview of effective strategies and guidance of diagnosis and care throughout the disease journey of CRS.

Selective Xi reactivation and alternative methods to restore MECP2 function in Rett syndrome

The human X-chromosome harbors only 4% of our genome but carries over 20% of genes associated with intellectual disability. Given that they inherit only one X-chromosome, males are more frequently affected by X-linked neurodevelopmental genetic disorders than females. However, despite inheriting two X-chromosomes, females can also be affected because X-chromosome inactivation enables only one of two X-chromosomes to be expressed per cell. For Rett syndrome and similar X-linked disorders affecting females, disease-specific treatments have remained elusive. However, a cure may be found within their own cells because every sick cell carries a healthy copy of the affected gene on the inactive X (Xi). Therefore, selective Xi reactivation may be a viable approach that would address the root cause of various X-linked disorders. Here, we discuss Rett syndrome and compare current approaches in the pharmaceutical pipeline to restore MECP2 function. We then focus on Xi reactivation and review available methods, lessons learned, and future directions.

Denaturing cross-linking immunoprecipitation to identify footprints for RNA-binding proteins

The isolation of protein-RNA complexes in the “denaturing cross-linked RNA immunoprecipitation” (dCLIP) protocol is based on biotin-tagging proteins of interest, UV cross-linking RNA to protein in vivo, RNase protection assay, and isolating RNA-protein complexes under denaturing conditions over a streptavidin column. Insofar as conventional antibody-based CLIP assays have been challenging to apply to Polycomb complexes, dCLIP has been applied successfully and yields small RNA footprints from which de novo motif analysis can be performed to identify RNA binding motifs.

For complete details on the use and execution of this protocol, please refer to Rosenberg et al. (2017).

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dCLIP biotags a protein of interest to identify cross-linked RNA interactors in vivo

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Biotin-streptavidin purification system enables denaturing washing conditions

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dCLIP is successfully applied to chromatin-modifying protein complexes

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dCLIP allows high-resolution mapping of RNA binding sites and de novo motif analysis

Jpx RNA regulates CTCF anchor site selection and formation of chromosome loops

Chromosome loops shift dynamically during development, homeostasis, and disease. CCCTC-binding factor (CTCF) is known to anchor loops and construct 3D genomes, but how anchor sites are selected is not yet understood. Here, we unveil Jpx RNA as a determinant of anchor selectivity. Jpx RNA targets thousands of genomic sites, preferentially binding promoters of active genes. Depleting Jpx RNA causes ectopic CTCF binding, massive shifts in chromosome looping, and downregulation of >700 Jpx target genes. Without Jpx, thousands of lost loops are replaced by de novo loops anchored by ectopic CTCF sites. Although Jpx controls CTCF binding on a genome-wide basis, it acts selectively at the subset of developmentally sensitive CTCF sites. Specifically, Jpx targets low-affinity CTCF motifs and displaces CTCF protein through competitive inhibition. We conclude that Jpx acts as a CTCF release factor and shapes the 3D genome by regulating anchor site usage.

Revealing RCOR2 as a regulatory component of nuclear speckles

Background

Nuclear processes such as transcription and RNA maturation can be impacted by subnuclear compartmentalization in condensates and nuclear bodies. Here, we characterize the nature of nuclear granules formed by REST corepressor 2 (RCOR2), a nuclear protein essential for pluripotency maintenance and central nervous system development.

Results

Using biochemical approaches and high-resolution microscopy, we reveal that RCOR2 is localized in nuclear speckles across multiple cell types, including neurons in the brain. RCOR2 forms complexes with nuclear speckle components such as SON, SRSF7, and SRRM2. When cells are exposed to chemical stress, RCOR2 behaves as a core component of the nuclear speckle and is stabilized by RNA. In turn, nuclear speckle morphology appears to depend on RCOR2. Specifically, RCOR2 knockdown results larger nuclear speckles, whereas overexpressing RCOR2 leads to smaller and rounder nuclear speckles.

Conclusion

Our study suggests that RCOR2 is a regulatory component of the nuclear speckle bodies, setting this co-repressor protein as a factor that controls nuclear speckles behavior.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-021-00425-4.

Effect of a Swing-Assist Knee Prosthesis on Stair Ambulation

This paper describes stair ambulation control and functionality of a semi-powered knee prosthesis that supplements nominally passive prosthesis behavior with swing-phase assistance. A set of stair ascent and descent controllers are described. The controllers were implemented in a semi-powered prosthesis prototype, and the prospective benefits of swing assist in stair ambulation were assessed on a group of three participants with unilateral, transfemoral amputation, relative to their respective daily-use prostheses. Results indicate that ambulation with the semi-powered knee resulted in improved stair ascent gait symmetry when compared to the participants' passive daily-use devices, and increased similitude to healthy stair ascent movement.

A disproportionate impact of G9a methyltransferase deficiency on the X chromosome

In this study from Szanto et al., the authors investigated the role of G9a, a histone methyltransferase responsible for the dimethylation of histone H3 at lysine 9 (H3K9me2) that plays key roles in transcriptional silencing of developmentally regulated genes, in X-chromosome inactivation (XCI). They found a female-specific function of G9a and demonstrate that deleting G9a has a disproportionate impact on the X chromosome relative to the rest of the genome, and show RNA tethers G9a for allele-specific targeting of the H3K9me2 modification and the G9a–RNA interaction is essential for XCI.

G9a is a histone methyltransferase responsible for the dimethylation of histone H3 at lysine 9 (H3K9me2). G9a plays key roles in transcriptional silencing of developmentally regulated genes, but its role in X-chromosome inactivation (XCI) has been under debate. Here, we uncover a female-specific function of G9a and demonstrate that deleting G9a has a disproportionate impact on the X chromosome relative to the rest of the genome. G9a deficiency causes a failure of XCI and female-specific hypersensitivity to drug inhibition of H3K9me2. We show that G9a interacts with Tsix and Xist RNAs, and that competitive inhibition of the G9a-RNA interaction recapitulates the XCI defect. During XCI, Xist recruits G9a to silence X-linked genes on the future inactive X. In parallel on the future Xa, Tsix recruits G9a to silence Xist in cis. Thus, RNA tethers G9a for allele-specific targeting of the H3K9me2 modification and the G9a-RNA interaction is essential for XCI.

The combined action of CTCF and its testis-specific paralog BORIS is essential for spermatogenesis

CTCF is a key organizer of the 3D genome. Its specialized paralog, BORIS, heterodimerizes with CTCF but is expressed only in male germ cells and in cancer states. Unexpectedly, BORIS-null mice have only minimal germ cell defects. To understand the CTCF-BORIS relationship, mouse models with varied CTCF and BORIS levels were generated. Whereas Ctcf+/+Boris+/+, Ctcf+/-Boris+/+, and Ctcf+/+Boris-/- males are fertile, Ctcf+/-Boris-/- (Compound Mutant; CM) males are sterile. Testes with combined depletion of both CTCF and BORIS show reduced size, defective meiotic recombination, increased apoptosis, and malformed spermatozoa. Although CM germ cells exhibit only 25% of CTCF WT expression, chromatin binding of CTCF is preferentially lost from CTCF-BORIS heterodimeric sites. Furthermore, CM testes lose the expression of a large number of spermatogenesis genes and gain the expression of developmentally inappropriate genes that are "toxic" to fertility. Thus, a combined action of CTCF and BORIS is required to both repress pre-meiotic genes and activate post-meiotic genes for a complete spermatogenesis program.

Balancing cohesin eviction and retention prevents aberrant chromosomal interactions, Polycomb-mediated repression, and X-inactivation

Depletion of architectural factors globally alters chromatin structure but only modestly affects gene expression. We revisit the structure-function relationship using the inactive X chromosome (Xi) as a model. We investigate cohesin imbalances by forcing its depletion or retention using degron-tagged RAD21 (cohesin subunit) or WAPL (cohesin release factor). Cohesin loss disrupts the Xi superstructure, unveiling superloops between escapee genes with minimal effect on gene repression. By contrast, forced cohesin retention markedly affects Xi superstructure, compromises spreading of Xist RNA-Polycomb complexes, and attenuates Xi silencing. Effects are greatest at distal chromosomal ends, where looping contacts with the Xist locus are weakened. Surprisingly, cohesin loss creates an Xi superloop, and cohesin retention creates Xi megadomains on the active X chromosome. Across the genome, a proper cohesin balance protects against aberrant inter-chromosomal interactions and tempers Polycomb-mediated repression. We conclude that a balance of cohesin eviction and retention regulates X inactivation and inter-chromosomal interactions across the genome.

Tryptophan requirement of first-cycle commercial laying hens in peak egg production

An experiment was conducted to evaluate the digestible tryptophan (Trp) requirement of laying hens from 22 to 34 wk of age. A total of 252 Hy-line W-36 laying hens were selected at 16 wk of age and allocated by weight (P = 0.90) to 7 dietary treatments resulting in 12 replicate cages of 3 birds for each treatment. A Trp-deficient basal diet was formulated using corn, corn gluten meal, and soybean meal for each of the 3 dietary phases and supplemented with synthetic L-Trp to provide 105, 119, 133, 147, 162, 176, and 190 mg digestible Trp on a daily basis over the experimental period. To adapt the hens to experimental diets, pullets were fed complete diets that contained increasing amounts of corn gluten meal. Hens received a controlled amount of feed daily based on feed intake expected under commercial conditions. Linear and quadratic broken-line, and quadratic polynomial models were used to estimate digestible Trp requirements based on hen-housed egg production (HHEP), egg mass (EM), and feed efficiency (FE). FE was calculated using EM and feed intake. Digestible Trp requirements were estimated to be 137, 183, and 192 mg/d for HHEP; 133, 180, and 183 for EM and 133, 177, and 173 for FE using linear broken-line, quadratic broken-line, and quadratic polynomial analysis, respectively. The quadratic broken line model in this experiment resulted in the best fit (R²) for all parameters measured. Linear broken line estimates resulted in lower estimates that the other models, and HHEP resulted in higher estimated digestible Trp requirement than EM and FE.

Motif-driven interactions between RNA and PRC2 are rheostats that regulate transcription elongation

Although Polycomb repressive complex 2 (PRC2) is now recognized as an RNA-binding complex, the full range of binding motifs and why PRC2-RNA complexes often associate with active genes have not been elucidated. Here we identify high-affinity RNA motifs whose mutations weaken PRC2 binding and attenuate its repressive function in mouse embryonic stem cells. Interactions occur at promoter-proximal regions and frequently coincide with pausing of RNA Polymerase II (POL-II). Surprisingly, while PRC2-associated nascent transcripts are highly expressed, ablating PRC2 further upregulates expression via loss of pausing and enhanced transcription elongation. Thus, PRC2-nascent RNA complexes operate as rheostats to fine-tune transcription by regulating transitions between pausing and elongation, explaining why PRC2-RNA complexes frequently occur within active genes. Nascent RNA also targets PRC2 in cis and downregulates neighboring genes. We propose a unifying model in which RNA specifically recruits PRC2 to repress genes through POL-II pausing and, more classically, H3K27-trimethylation.

Determining the 4th limiting amino acid in low crude protein diets for male and female Cobb MV × 500 broilers

1. Four experiments were conducted to determine the 4th limiting amino acid (AA) in maize-soybean meal-based diets. 2. Deletion assay methodology was used to quantify performance and carcase trait responses to potential deficiencies in essential and conditionally essential AA caused by reductions in dietary crude protein of maize-soybean meal-based diets from 202.9 to 186.5 g/kg. 3. The deletion of Val, Phe and Gly + Pro resulted in negative effects on live performance and carcase traits for male broilers, whereas AA deletion only affected wing weights for females with no response on live performance. 4. Further experimentation could not duplicate a response to Phe or Pro in male broilers. 5. Valine was identified as the potential 4th limiting AA in maize-soybean meal-based diets and was not found to be co-limiting with Ile.

Decapping enzyme 1A breaks X-chromosome symmetry by controlling Tsix elongation and RNA turnover

How allelic asymmetry is generated remains a major unsolved problem in epigenetics. Here we model the problem using X-chromosome inactivation by developing "BioRBP", an enzymatic RNA-proteomic method that enables probing of low-abundance interactions and an allelic RNA-depletion and -tagging system. We identify messenger RNA-decapping enzyme 1A (DCP1A) as a key regulator of Tsix, a noncoding RNA implicated in allelic choice through X-chromosome pairing. DCP1A controls Tsix half-life and transcription elongation. Depleting DCP1A causes accumulation of X-X pairs and perturbs the transition to monoallelic Tsix expression required for Xist upregulation. While ablating DCP1A causes hyperpairing, forcing Tsix degradation resolves pairing and enables Xist upregulation. We link pairing to allelic partitioning of CCCTC-binding factor (CTCF) and show that tethering DCP1A to one Tsix allele is sufficient to drive monoallelic Xist expression. Thus, DCP1A flips a bistable switch for the mutually exclusive determination of active and inactive Xs.

Aberrant mitochondrial function in patient-derived neural cells from CDKL5 deficiency disorder and Rett syndrome

The X-linked neurodevelopmental diseases CDKL5 deficiency disorder (CDD) and Rett syndrome (RTT) are associated with intellectual disability, infantile spasms and seizures. Although mitochondrial dysfunction has been suggested in RTT, less is understood about mitochondrial function in CDD. A comparison of bioenergetics and mitochondrial function between isogenic wild-type and mutant neural progenitor cell (NPC) lines revealed increased oxygen consumption in CDD mutant lines, which is associated with altered mitochondrial function and structure. Transcriptomic analysis revealed differential expression of genes related to mitochondrial and REDOX function in NPCs expressing the mutant CDKL5. Furthermore, a similar increase in oxygen consumption specific to RTT patient-derived isogenic mutant NPCs was observed, though the pattern of mitochondrial functional alterations was distinct from CDKL5 mutant-expressing NPCs. We propose that aberrant neural bioenergetics is a common feature between CDD and RTT disorders. The observed changes in oxidative stress and mitochondrial function may facilitate the development of therapeutic agents for CDD and related disorders.

Nucleoporin 153 links nuclear pore complex to chromatin architecture by mediating CTCF and cohesin binding

Nucleoporin proteins (Nups) have been proposed to mediate spatial and temporal chromatin organization during gene regulation. Nevertheless, the molecular mechanisms in mammalian cells are not well understood. Here, we report that Nucleoporin 153 (NUP153) interacts with the chromatin architectural proteins, CTCF and cohesin, and mediates their binding across cis-regulatory elements and TAD boundaries in mouse embryonic stem (ES) cells. NUP153 depletion results in altered CTCF and cohesin binding and differential gene expression - specifically at the bivalent developmental genes. To investigate the molecular mechanism, we utilize epidermal growth factor (EGF)-inducible immediate early genes (IEGs). We find that NUP153 controls CTCF and cohesin binding at the cis-regulatory elements and POL II pausing during the basal state. Furthermore, efficient IEG transcription relies on NUP153. We propose that NUP153 links the nuclear pore complex (NPC) to chromatin architecture allowing genes that are poised to respond rapidly to developmental cues to be properly modulated.

Two- and three-color STORM analysis reveals higher-order assembly of leukotriene synthetic complexes on the nuclear envelope of murine neutrophils

Over the last several years it has become clear that higher order assemblies on membranes, exemplified by signalosomes, are a paradigm for the regulation of many membrane signaling processes. We have recently combined two-color direct stochastic optical reconstruction microscopy (dSTORM) with the (Clus-DoC) algorithm that combines cluster detection and colocalization analysis to observe the organization of 5-lipoxygenase (5-LO) and 5-lipoxygenase-activating protein (FLAP) into higher order assemblies on the nuclear envelope of mast cells; these assemblies were linked to leukotriene (LT) C4 production. In this study we investigated whether higher order assemblies of 5-LO and FLAP included cytosolic phospholipase A2 (cPLA2) and were linked to LTB4 production in murine neutrophils. Using two- and three-color dSTORM supported by fluorescence lifetime imaging microscopy we identified higher order assemblies containing 40 molecules (median) (IQR: 23, 87) of 5-LO, and 53 molecules (62, 156) of FLAP monomer. 98 (18, 154) molecules of cPLA2 were clustered with 5-LO, and 77 (33, 114) molecules of cPLA2 were associated with FLAP. These assemblies were tightly linked to LTB4 formation. The activation-dependent close associations of cPLA2, FLAP, and 5-LO in higher order assemblies on the nuclear envelope support a model in which arachidonic acid is generated by cPLA2 in apposition to FLAP, facilitating its transfer to 5-LO to initiate LT synthesis.