Cell division cycle 7 kinase is a negative regulator of cell-mediated collagen degradation

Genome-wide screens identify SEL1L as an intracellular rheostat controlling collagen turnover

Accumulating evidence has implicated impaired extracellular matrix (ECM) clearance as a key factor in fibrotic disease. Despite decades of research elucidating the effectors of ECM clearance, relatively little is understood regarding the upstream regulation of this process. Collagen is the most abundant constituent of normal and fibrotic ECM in mammalian tissues. Its catabolism occurs through extracellular proteolysis and cell-mediated uptake of collagen fragments for intracellular degradation. Given the paucity of information regarding the regulation of this latter process, here we execute unbiased genome-wide screens to understand the molecular underpinnings of cell-mediated collagen clearance. Using this approach, we discover a mechanism through which collagen biosynthesis is sensed by cells internally and directly regulates clearance of extracellular collagen. The sensing mechanism appears to be dependent on endoplasmic reticulum-resident protein SEL1L and occurs via a noncanonical function of this protein. This pathway functions as a homeostatic negative feedback loop that limits collagen accumulation in tissues. In human fibrotic lung disease, the induction of this collagen clearance pathway by collagen synthesis is impaired, thereby contributing to the pathological accumulation of collagen in lung tissue. Thus, we describe cell-autonomous, rheostatic collagen clearance as an important pathway of tissue homeostasis.

Genome-wide screens identify SEL1L as an intracellular rheostat controlling collagen turnover

Accumulating evidence has implicated impaired extracellular matrix (ECM) clearance as a key factor in fibrotic disease. Despite decades of research elucidating the effectors of ECM clearance, relatively little is understood regarding the upstream regulation of this process. Collagen is the most abundant constituent of normal and fibrotic ECM in mammalian tissues. Its catabolism occurs through extracellular proteolysis and cell-mediated uptake of collagen fragments for intracellular degradation. Given the paucity of information regarding the regulation of this latter process, we executed unbiased genome-wide screens to understand the molecular underpinnings of cell-mediated collagen clearance. Using this approach, we discovered a previously unappreciated mechanism through which collagen biosynthesis is sensed by cells internally and directly regulates clearance of extracellular collagen. The sensing mechanism is dependent on endoplasmic reticulum-resident protein SEL1L and occurs via a noncanonical function of SEL1L. This pathway functions as a homeostatic negative feedback loop that limits collagen accumulation in tissues. In human fibrotic lung disease, the induction of this collagen clearance pathway by collagen synthesis is impaired, thereby contributing to the pathological accumulation of collagen in lung tissue. Thus cell-autonomous, rheostatic collagen clearance is a previously unidentified pathway of tissue homeostasis.

Age-dependent regulation of cell-mediated collagen turnover

Although aging represents the most important epidemiologic risk factor for fibrotic disease, the reasons for this are incompletely understood. Excess collagen deposition in tissues is the sine qua non of tissue fibrosis and can be viewed as an imbalance between collagen production and collagen degradation. Yet we still lack a detailed understanding of the changes that take place during development, maturation, and aging in extracellular matrix (ECM) dynamics. Resolution of fibrosis is impaired in aging, and this impairment may explain why age is the most important risk factor for fibrotic diseases, such as idiopathic pulmonary fibrosis. However, ECM dynamics and impaired resolution of fibrosis in aging remain understudied. Here we show that cell-mediated collagen uptake and degradation are diminished in aged animals and this finding correlates with downregulation of the collagen endocytic receptor mannose receptor, C-type 2 (Mrc2). We identify myeloid zinc finger-1 as a potentially novel transcriptional regulator of Mrc2, and both this transcription factor and Mrc2 are downregulated in multiple tissues and organisms in an age-dependent manner. Thus, cell-mediated degradation of collagen is an essential process that promotes resolution of fibrosis, and impairment in this process contributes to age-related fibrosis.