INTRODUCTION
Arterial diseases including hypertension and coronary artery disease
(CAD) display a degree of dysregulation in the contractile behaviour of
the smooth muscle. Vascular tone is regulated by the relative
contractile state of vascular smooth muscle cells
(VSMCs)1,2. VSMC contraction provides force generation
through the phosphorylation of myosin light chain kinase (MLCK) which
facilitates interaction between actin and myosin filaments. MLCK is
directly phosphorylated by calcium-bound calmodulin. Increases in
intracellular calcium concentrations
([Ca2+]i) occur through
PKC-mediated activation of L-type voltage-gated calcium channels (VGCCs)
leading to an influx of extracellular Ca2+ ions, or
via activation of Gαq-Protein Coupled Receptors (GPCRs)
leading to PLCβ-mediated Ca2+ ion release from the
sarcoplasmic reticulum. Whilst these central signalling pathways
controlling contraction are widely characterised, modulation of these
pathways remains ill-defined.
Several integrins can directly modulate vascular smooth muscle cell
contraction by regulating calcium influx through
VGCCs3-7. Within the airway, integrin α9β1 has been
specifically identified as preventing exaggerated airway smooth muscle
contraction, where conditional knockout of the α9 subunit in airway
smooth muscle causes a spontaneous increase in pulmonary resistance in
response to multiple GPCR agonists8. SVEP1, a high
affinity ligand for integrin α9β19, is a 390 kDa
secreted extracellular matrix (ECM) protein comprised of sushi
(complement control protein (CCP)), von Willebrand factor type A,
epidermal growth factor-like, and pentraxin domains10.
The function of SVEP1 remains unclear but it has reported roles in cell
adhesion9-12, lymphatic vessel
formation13,14, epidermal
differentiation15, and cancer
progression16,17. A low frequency coding variant
rs111245230 (p.D2702G) within SVEP1 associates with elevated blood
pressure (BP)18 and CAD19.
rs111245230 is situated adjacent to the binding motif through which
SVEP1 binds to integrin α9β19. Genetic variants
associated with reduced expression of ITGA9 , which encodes the α9
subunit of integrin α9β1, have also been reported to associate with
increased BP20,21. Neither SVEP1 nor integrin α9β1
have a reported role in vasoregulation, however direct activation of
integrin α4β1, with which integrin α9β1 forms an integrin
subfamily22, can induce VSMC
contraction4.
The role of SVEP1 in cardiovascular disease is only now beginning to be
realised with two recent in vivo studies showing SVEP1 to be
primarily expressed within blood vessels23 and SVEP1
knockout affecting atherosclerosis development23,24.
Due to the genetic association between variants in SVEP1 andITGA9 with BP, the role of integrin α9β1 in airway contraction,
and SVEP1 expression, we examined the role of these proteins in
regulating VSMC contractility.