The adult cardiac extracellular matrix (ECM) is largely comprised of
type I collagen. In addition to serving as the primary structural support component of the cardiac ECM,
type I collagen also provides an organizational platform for other ECM
proteins, matricellular
proteins, and signaling components that impact cellular stress sensing in vivo.
OBJECTIVE: We generated and characterized
Col1a2-/- mice using standard gene targeting.
Col1a2-/- mice were viable, although by young adulthood their hearts showed alterations in ECM mechanical properties, as well as an unanticipated activation of cardiac fibroblasts and induction of a progressive fibrotic response. This included augmented TGFβ activity, increases in fibroblast number, and progressive
cardiac hypertrophy, with reduced functional performance by 9 months of age. Col1a2-loxP-targeted mice were also generated and crossed with the
tamoxifen-inducible Postn-MerCreMer mice to delete the
Col1a2 gene in myofibroblasts with pressure overload injury. Interestingly, while germline
Col1a2-/- mice showed gradual pathologic
hypertrophy and
fibrosis with aging, the acute deletion of
Col1a2 from activated adult myofibroblasts showed a loss of total
collagen deposition with acute cardiac injury and an acute reduction in pressure overload-induce
cardiac hypertrophy. However, this reduction in
hypertrophy due to myofibroblast-specific
Col1a2 deletion was lost after 2 and 6 weeks of pressure overload, as fibrotic deposition accumulated.
CONCLUSIONS: Defective
type I collagen in the heart alters the structural integrity of the ECM and leads to
cardiomyopathy in adulthood, with fibroblast expansion, activation, and alternate fibrotic ECM deposition. However, acute inhibition of
type I collagen production can have an anti-fibrotic and anti-hypertrophic effect.