Type I collagen is the most abundant human protein that forms the structural scaffold of bone, skin and other tissues. Normal type I collagen is a heterotrimer of two alpha-1 and one alpha-2 chains. Homotrimers of alpha-1 chains can also be produced in fetal tissues and some disorders. We discovered that alpha-1 homotrimers are resistant to cleavage by all collagenases and characterized the mechanism of this resistance. In tumors, the homotrimers are synthesized by cancer but not normal cells. More rigid matrix made of the homotrimers supports faster proliferation and migration of cancer cells. Collagenase-resistant homotrimer fibers laid down by these cells may serve as tracks for outward cell migration and tumor growth. The homotrimers may thus present an appealing diagnostic and therapeutic target in cancer. The most prominent human development pathology associated with type I collagen is bone fragility and malformations caused by collagen mutations, which is referred to as osteogenesis imperfecta (OI). Skin, ligaments and other collagen-rich tissues are often affected as well. Sometimes the skin and ligament pathology (laxity and fragility) is significant enough to produce clinical features of Ehlers-Danlos syndrome. Over 70-80% of severe OI mutations are substitutions of glycine (Gly) required in every third position for maintaining the triple helical structure of collagen. By altering the triple helix folding and structure, Gly substitutions cause malfunction of bone producing cells (osteoblasts) and alter formation and function of the collagen scaffold of bone. Over the years, our studies revealed that osteoblast malfunction is a major and likely dominant pathogenic factor in Gly substitutions. The cause of this malfunction is cell stress resulting from accumulation of misfolded collagen precursor (procollagen) in osteoblast Endoplasmic Reticulum (ER). We characterized some of the key features of this cell stress response and identified potential therapeutic targets for its alleviation. To understand and target osteoblast cell stress, we created and characterized a novel G610C mouse OI model, which mimics a Gly610 to Cys substitution in the alpha-2 chain found in a large group of patients. We found that removal of excess misfolded mutant procollagen from the ER and its delivery to lysosomes for degradation is an important adaptation mechanism to cell stress in osteoblasts. The process of cellular cargo isolation and delivery to lysosomes for degradation is referred to as autophagy. We therefore created additional G610C mouse models, in which autophagy can be suppressed or enhanced by altering expression of an essential autophagy gene, Atg5. We demonstrated that the severity of bone pathology increases with increasing autophagy suppression. At the same time, we observed that perinatal lethality in the most affected animals is caused by deficient embryonic lung development, apparently due to abnormal collagen scaffold formation by lung fibroblasts. Given that lung malfunction is a common complication and cause of death in OI patients, we are currently pursuing the underlying causes and potential therapeutic targets. Our preliminary data suggest that the severity of both bone and lung pathology decreases with autophagy enhancement by Atg5 overexpression. Therefore, we are developing pharmacological and dietary approaches to autophagy enhancement, which would be more practical for therapeutic applications in humans. In addition to studies of cells stress associated with procollagen misfolding in mouse models of OI, we are also pursuing more fundamental cell biology studies of procollagen biosynthesis by osteoblasts and fibroblasts. The goal of these studies is better understanding of underlying molecular mechanisms, many of which are still unknown. For instance, observations made by us and others suggest that deficient procollagen trafficking and autophagy might be involved in a variety of pathologies spanning the entire lifespan, from skeletal dysplasia in early development to osteoporosis in aging. Better understanding of the trafficking and autophagy mechanisms might therefore reveal new therapeutic targets and approaches. To study procollagen trafficking and autophagy, we recently developed novel fluorescent constructs of procollagen chains for live cell imaging and correlative light and electron microscopy. We discovered that misfolded procollagen molecules are recognized and rerouted from normal secretory pathway to autophagy at ER exit sites (ERES). ERES that contain misfolded molecules are prevented from maturation into transport vesicles that would deliver procollagen from the ER to Golgi. Instead, they were modified by autophagic machinery and directly engulfed by lysosomes in a non-canonical process resembling micro-autophagy described for other cargo. We are currently investigating the mechanism of the lysosomal recruitment to ERES, which is an appealing target for enhancing the observed micro-ERES-phagy in therapeutic applications. At the same time, we are investigating whether micro-ERES-phagy is a more general protein quality control mechanism, which might be utilized by cells for many proteins and not just procollagen. We are also examining whether this process might be involved in rerouting of proteins from the secretory pathway to lysosomal degradation not only in the case of protein misfolding but under starvation conditions as well. To facilitate these studies, we are utilizing CRISPR/CAS gene editing technology for creating novel cell lines, in which endogenous procollagen is fluorescently tagged and can be manipulated by Flp-recombinase mediated cassette exchange to introduce mutations and change the fluorescent tags. Abnormal differentiation and function of collagen-producing cells also plays an important role in fibrosis and tumor formation. In collaboration with Dr. Stratakis, we are investigating bone pathology associated with osteoblast malfunction is caudal vertebrae tumors in mice with deficiencies in different catalytic and regulatory subunits of protein kinase A, which is a crucial enzyme for cAMP signaling. In these tumors, we found accelerated bone matrix formation and deficient mineralization reminiscent of the McCune-Albright syndrome as well as very unusual collagen matrix organization and bone structures, which appear to be associated with improper maturation and/or function of osteoblasts. We are characterizing the latter abnormalities and the origin of novel bone structures formed in these tumors. We hope that further studies of these animals will not only shed new light on the role of cAMP signaling in osteoblasts but also promote better general understanding of normal and pathological bone formation mechanisms. In addition to our own projects, we are assisting NIH and extramural researchers in studies of collagen-related pathology in a variety of human patients and animal models. For instance, we assisted Dr. Marini in discovering several novel forms of OI and characterizing underlying procollagen biosynthesis defects. In collaboration with Dr. Byers, we investigated OI caused by substitutions of arginine in type I collagen, demonstrating that such substitutions cause procollagen misfolding and accumulation in the ER similar to Gly substitutions. We assisted Dr. Bonnemann in characterization of a complex connective tissue disorder involving pathology of multiple tissues, which is caused by deficient function of prolyl 4 hydroxylase 1, an enzyme primarily responsible for hydroxylation of proline in type I collagen. We are collaborating with Dr. Otsuru on studies of growth plate pathology and resulting growth deficiency in the G610C mouse model of OI. We are also collaborating with Dr. Forlino on characterization of normal and abnormal type I collagen processing in zebra fish models of OI.
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