Twist1 in Infiltrating Macrophages Attenuates Kidney Fibrosis via Matrix Metallopeptidase 13–Mediated Matrix Degradation
Authors: Ren Jiafa , Zhang Jiandong , Rudemiller Nathan P. , Griffiths Robert , Wen Yi , Lu Xiaohan , Privratsky Jamie R. , Gunn Michael D. , Crowley Steven D. 
Published: July 2019

Significance Statement
Studies have shown profibrotic actions of the transcription factor Twist1 in intrinsic renal parenchymal cells. However, Twist1 expressed by immune cells can suppress inflammatory responses, and the role of macrophage-expressed Twist1 in kidney fibrosis has not been described. To study this, the authors used two conditional knockout mouse models in which Twist1 was specifically deleted from either infiltrating or resident myeloid cells. They found that Twist1 in infiltrating myeloid cells significantly induced matrix metallopeptidase 13 (collagenase 3) generation in CD11b+Ly6Clo macrophages, resulting in extracellular matrix degradation and attenuation of experimental kidney fibrosis. These findings elucidate paradoxical actions of myeloid Twist1 in renal fibrogenesis, which may help facilitate design of pharmacological interventions to precisely target Twist1 while limiting off-target side effects.

Background
Following an acute insult, macrophages regulate renal fibrogenesis through the release of various factors that either encourage the synthesis of extracellular matrix synthesis or the degradation of matrix via endocytosis, proteolysis, or both. However, the roles of infiltrating versus resident myeloid cells in these opposing processes require elucidation. The transcription factor Twist1 controls diverse essential cellular functions through induction of several downstream targets, including matrix metalloproteinases (MMPs). In macrophages, Twist1 can influence patterns of cytokine generation, but the role of macrophage Twist1 in renal fibrogenesis remains undefined.

Methods
To study Twist1 functions in different macrophage subsets during kidney scar formation, we used two conditional mutant mouse models in which Twist1 was selectively ablated either in infiltrating, inflammatory macrophages or in resident tissue macrophages. We assessed fibrosis-related parameters, matrix metallopeptidase 13 (MMP13, or collagen 3, which catalyzes collagen degradation), inflammatory cytokines, and other factors in these Twist1-deficient mice compared with wild-type controls after subjecting the animals to unilateral ureteral obstruction. We also treated wild-type and Twist1-deficient mice with an MMP13 inhibitor after unilateral ureteral obstruction.

Results
Twist1 in infiltrating inflammatory macrophages but not in resident macrophages limited kidney fibrosis after ureteral obstruction by driving extracellular matrix degradation. Moreover, deletion of Twist1 in infiltrating macrophages attenuated the expression of MMP13 in CD11b+Ly6Clo myeloid cells. Inhibition of MMP13 abrogated the protection from renal fibrosis afforded by macrophage Twist1.

Conclusions
Twist1 in infiltrating myeloid cells mitigates interstitial matrix accumulation in the injured kidney by promoting MMP13 production, which drives extracellular matrix degradation. These data highlight the complex cell-specific actions of Twist1 in the pathogenesis of kidney fibrosis.