Increased polyamines alter chromatin and stabilize autoantigens in autoimmune diseases

Abstract

Polyamines are small cations with unique combinations of charge and length that give them many putative interactions in cells. Polyamines are essential since they are involved in replication, transcription, translation, and stabilization of macro-molecular complexes. However, polyamine synthesis competes with cellular methylation for S-adenosylmethionine, the methyl donor. Also, polyamine degradation can generate reactive molecules like acrolein. Therefore, polyamine levels are tightly controlled. This control may be compromised in autoimmune diseases since elevated polyamine levels are seen in autoimmune diseases. Here a hypothesis is presented explaining how polyamines can stabilize autoantigens. In addition, the hypothesis explains how polyamines can inappropriately activate enzymes involved in NETosis, a process in which chromatin is modified and extruded from cells as extracellular traps that bind pathogens during an immune response. This polyamine-induced enzymatic activity can lead to an increase in NETosis resulting in release of autoantigenic material and tissue damage.

Keywords: NETosis; autoimmune disease; neutrophil extracellular TRAPs; nuclear aggregates of polyamines; peptidylarginine deiminase; polyamines.

Figure 1

The polyamine pathway. (A) Polyamines at physiological pH: Putrescine (+2, ∼8Å), Spermidine (+3, ∼12Å), Spermine (+4, ∼16Å). (B) Polyamine synthesis and recycling. S-adenosylmethionine decarboxylase (AMD1) and ornithine decarboxylase (ODC) are rate limiting steps in polyamine synthesis. AMD1 decarboxylates S-adenosylmethionine (SAM) to decarboxylated SAM (dcSAM) so that dcSAM can provide aminopropyl groups added to putrescine to make spermidine by spermidine synthase (SRM) and added to spermidine to make spermine by spermine synthase (SMS). Spermine can be recycled to spermidine directly by spermine oxidase (SMOX). Spermine and spermidine can be recycled to spermidine and putrescine, respectively, by spermidine/spermine-N1-acetyltransferase (SAT1) followed by oxidation by polyamine oxidase (PAO).

Figure 2

Polyamines and NAPs. (A) Components of NAPs. (B) Small nuclear aggregates of polyamines (s-NAP) proposed by D’Agostino et al. (2005) consists of phosphate ions with one putrescine, one spermidine, and two spermines. (C) Medium NAP (m-NAP), as proposed, consists of pentamers of s-NAPs. (D) Z-DNA stabilization by spermine versus NAPs. Left: Z-DNA can be co-crystallized with spermine [based on 2DCG.pdb (Wang et al., 1979) deposited in the Protein Data Bank, www.rcsb.org (Berman et al., 2000)]. Note how spermine molecules interact individually with DNA and could be displaced easily by nucleases. Right: proposed interaction of NAPs with Z-DNA. An s-NAP could bind in the Z-DNA minor groove, aligning other polyamines in the NAP to unroll into a growing stretch of Z-DNA. Polyamines from NAPs would reinforce each other in binding to DNA, making the DNA less vulnerable to nucleases.

Figure 3

NAPs and chromatin. NAPs bound to chromatin capture negative supercoiling stress released from nucleosomes during NETosis. In this depiction, NAPs stabilize Z-DNA and cruciforms which are usually transient forms of negative supercoiling. These could become autoantigens when released as part of NETs.

Figure 4

Peptidylarginine deiminase 4 (PAD4) and polyamines. (A) Inactive PAD4 without calcium (top, 1WD8.pdb; Arita et al., 2004) and active PAD4 (bottom, 1WD9.pdb; Arita et al., 2004) with bound calcium ions (pink spheres). Note the active site, stabilized helix and loop in active PAD4. (B) Calcium ions bound in PAD4 primarily by aspartic acid residues. Faint red dash lines indicate electrostatic interactions. (C) Putrescine superimposed over calcium ions. (D) Spermidine superimposed over calcium ions. (E) Spermine superimposed over calcium ions.