# Peptidylarginine deiminase type 6 in rheumatoid arthritis pathogenesis

> **NIH NIH R21** · JOHNS HOPKINS UNIVERSITY · 2021 · $396,275

## Abstract

PROJECT SUMMARY/ABSTRACT
Citrullinated proteins are major targets of the immune system in rheumatoid arthritis (RA), with recent evidence
suggesting that mechanisms that enhance citrullination are associated with the worst disease outcome. Protein
citrullination is the enzymatic conversion of arginine residues to citrulline and is mediated by the family of calcium-
dependent peptidylarginine deiminase (PAD) enzymes. Five PAD members have been found in humans, PAD1-
4, and the catalytically inactive PAD6. While several mechanisms have been linked to abnormal citrullination in
RA, including hyperactivation of PADs (particularly PAD4) during neutrophil death and the production of
autoantibodies that enhance PAD4 activity, regulatory pathways that control citrullination are poorly understood.
Since PADs require supraphysiologic concentrations of calcium for activity in vitro, it is suspected that PAD
activation in vivo requires additional factors that modulate the enzyme’s sensitivity for calcium. Such co-factors,
however, have not been identified. Our preliminary data demonstrate that PAD6, which has no citrullinating
activity, and 3 novel PAD6 splicing variants described for the first time in this proposal, act as co-factors that
decrease the calcium requirement of PAD4 for catalysis. Moreover, we found that all PAD6 variants are abundant
in neutrophils, which are considered a major source of citrullinated proteins in the RA joint. Taken together, we
hypothesize that PAD6 and its variants may be responsible for modulating the magnitude of PAD4 activity both
in physiologic and pathologic conditions. We will examine this hypothesis directly in the human model in three
specific aims. In Aim 1, we will define how PAD6 variants regulate PAD4 activity with the goal of identifying the
biochemical mechanisms by which PAD6 variants decrease the calcium requirement of PAD4. Aim 2 will define
the expression and function of PAD6 and its splicing variants at the cellular level. Aim 3 will study the expression
of PAD6 isoforms in RA synovial fluid to address their relationship with the abnormal production of citrullinated
proteins in RA. Together, these studies will define novel interacting mechanisms of disease amplification relevant
to sustain the production of citrullinated proteins in the RA joints. Furthermore, these studies may provide
evidence for targeting PAD6 as a novel therapeutic strategy to modulate PAD4 activity, which is highly significant
for diseases in which citrullination has been pathogenically implicated.

## Key facts

- **NIH application ID:** 10317620
- **Project number:** 1R21AR079891-01
- **Recipient organization:** JOHNS HOPKINS UNIVERSITY
- **Principal Investigator:** Felipe Andrade
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $396,275
- **Award type:** 1
- **Project period:** 2021-09-23 → 2024-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10317620

## Citation

> US National Institutes of Health, RePORTER application 10317620, Peptidylarginine deiminase type 6 in rheumatoid arthritis pathogenesis (1R21AR079891-01). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10317620. Licensed CC0.

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