1
The Multifunctional Role of Filaggrin in Allergic Skin Disease 1
2
Authors 3
Maeve A McAleer, MRCP
1,2, 3
& Alan D Irvine, MD
1,2,3
4
5
Affiliations 6
1. National Children’s Research Centre 7
Our Lady’s Children’s Hospital, Crumlin, Dublin 8
2. Department of Pediatric Dermatology 9
Our Lady’s Children’s Hospital, Crumlin, Dublin 10
3. Department of Clinical Medicine, 11
Trinity College Dublin. 12
13
Corresponding Author: 14
Alan D Irvine, 15
The Department of Paediatric Dermatology 16
Our Lady’s Children’s Hospital, Crumlin 17
Dublin 12, IRELAND 18
Email: irvinea@tcd.ie 19
Telephone: +3531 428 2532 20
21
Sources of Funding: National Children’s Research Centre, Dublin 22
Conflicts of interest: none 23
24
2
25
Abstract 26
Filaggrin is a major structural protein in the stratum corneum of human epidermis. 27
Mutations in the filaggrin gene are the most significant known risk factor for the 28
development of atopic dermatitis. Mutations in
FLG
also confer risk for the associated 29
allergic diseases of food allergy, asthma, and allergic rhinitis. These discoveries have 30
highlighted the importance of skin barrier function in the pathogenesis of atopic 31
diseases and have motivated a surge in research characterizing the filaggrin deficient 32
skin barrier and its consequences. In this review we discuss the mechanisms through 33
which mutations in this protein contribute to the pathogenesis of atopic dermatitis 34
and associated atopic conditions. We focus on recent human and murine discoveries 35
characterizing the filaggrin deficient epidermis with respect to biophysical, 36
immunological and microbiome abnormalities. 37
38
Abstract word count: 12639
3
List of Key words 40
Filaggrin, atopy, dermatitis, eczema, IL-1β 41
Abbreviations 42
AD: atopic dermatitis, AMP: antimicrobial peptide, ASC: apoptosis-associated speck-43
like protein, CNV: copy number variation,
FLG
: human filaggrin gene,
Flg
: murine 44
filaggrin gene, IgE: immunoglobin E, IsdA: iron-regulated surface determinant A, 45
LEKTI: Lympho-epithelial Kazal-type-related inhibitor, NMF: natural moisturizing 46
factor. NLRP3: nucleotide-binding oligomerisation leucine-rich repeat and pyrin 47
domain containing 3, OR: odds ratio, OVA: ovalbumin, PAC: pyrroliodine-5-carboxylic 48
acid, PBMC: peripheral blood mononucleocyte, SEB: Staphlococcal enterotoxin B, 49
Staph aureus: Staphlococcus aureus
, SpA: Staphlococcus protein A, SC: stratum 50
corneum, Th: T helper, TEWL: transepidermal water loss, TJ: tight junction, UCA: 51
trans-urocanic acid, US: United States, UK: United Kingdom. 52
Following standard genetic practice, in this paper
FLG
-/- designates a patient 53
homozygous for null alleles (i.e. 2 null alleles);
FLG
+/- a heterozygote null 54
allele/ wildtype (i.e. one null allele) and
FLG
+/+ a homozygote wildtype (i.e. 55
0 null alleles). A further abbreviation describes AD patients with
FLG
56
mutations (
FLG
+/- and
FLG
-/-) as AD
FLG
and those without
FLG
mutations 57
(i.e.
FLG
+/+) as AD
NON-FLG
.
58
59
Main text word count: 5556 60
61
4
Introduction 62
Atopic dermatitis (AD) affects approximately 11% of children in the US (1) and up to 63
25% in the UK. (2, 3) It is the most common chronic inflammatory disease of early 64
childhood (4) and is associated with significant morbidity for the patient and their 65
families. (5) Atopic dermatitis is characterized by an epidermal barrier abnormality, 66
cutaneous inflammation, immune dysregulation with a systemic ‘allergic’ T helper 67
(Th2) cell response, and frequent
Staphylococcus aureus
colonization.(4) It is often 68
the initial step in the so called ‘atopic march’, with the subsequent development of 69
allergies, asthma and hay fever.(6) The critical importance of the epithelium in the 70
development of AD and allergic sensitisation has become apparent. Mutations in the 71
FLG
gene, which codes for the skin barrier protein filaggrin, have been shown to be 72
the most significant risk factor, to date, for developing AD.(4) The specific dynamic 73
interactions between an impaired skin barrier and the immune system remain to be 74
fully elucidated. Here we review recent insights into the role of filaggrin in the 75
pathomechanisms of AD and its associated diseases. 76
Epidermal Structure and Function: Role of Filaggrin 77
The epidermis, particularly the outermost stratum corneum (SC) layer, is the first line 78
of defense between the host organism and its environment. The SC also minimizes 79
water loss from the body and protects against both every day and extreme 80
environmental insults. (7) The SC is the end product of a highly organized 81
differentiation process in which keratinocytes in the basal layer of the epidermis 82
progress to form the spinous and granular layers, ultimately forming a tough 83
multilayer of corneocytes rich in intracellular lipids. (7) The SC matrix is an 84
extensively cross-linked lipid protein matrix organized into neutral, lipid enriched, 85
extracellular lamellar bilayers.(8) This hydrophobic extracellular matrix, together with 86
5
corneodesmosomes and tight junctions, specialized cohesive intercellular junctions in 87
the stratum corneum and stratum granulosum, forms a very effective barrier.(9, 10) 88
Filaggrin is a major structural protein in the SC. (4) The role of filaggrin in epidermal 89
structure and function has been reviewed in detail in recent papers. (4, 7, 11) 90
Filaggrin is produced as the precursor pro-protein profilaggrin. Profilaggrin is 91
expressed in terminally differentiating keratinocytes in the outmost layers of the 92
human epidermis and is the major constituent of keratohyalin granules in the 93
stratum granulosum. (7) Profilaggrin consists of multiple filaggrin repeats flanked by 94
an S100-type calcium-binding domain, A and B domains at the N-terminal, and a 95
unique tail sequence at the C terminal [Figure 1a](4)] 96
During terminal differentiation at the granular to cornified layer transition, 97
profilaggrin is rapidly dephosphorylayed and cleaved by several endoproteases to 98
generate 10,11 or 12 functional filaggrin monomers. (12) Extracellular proteases, 99
such as matriptase, may also influence the expression of filaggrin monomers.(13) 100
Filaggrin monomers aggregate and align keratin bundles,
in vitro
, and are thus 101
postulated to contribute to the mechanical strength and integrity of the SC
in vivo.
102
(14) [Figure 1a] Ultrastructural studies have shown that filaggrin deficiency results in 103
disorganized keratin filaments, impaired lamellar body loading, and abnormal 104
architecture of the lamellar bilayer.(10) [Figure 1b] It has been proposed that
FLG
N-105
terminal provides a feedback mechanism that controls epidermal homeostasis.(15) 106
The C-terminal domain’s exact function is unclear but it is necessary for profilaggrin 107
to filaggrin processing. Truncated profilaggrin, lacking a C-terminal, results in almost 108
a complete absence of filaggrin.(12) 109
In the upper layers of the SC, filaggrin monomers are deiminated and degraded by 110
proteases to release their component hygroscopic amino acids and their derivatives. 111