Metabolic characteristics of transmembrane prolyl 4-hydroxylase (P4H-TM) deficient mice

Transmembrane prolyl 4-hydroxylase (P4H-TM) is an enigmatic enzyme whose cellular function and primary substrate remain to be identified. Its loss-of-function mutations cause a severe neurological HIDEA syndrome with hypotonia, intellectual disability, dysautonomia and hypoventilation. Previously, P4H-TM deficiency in mice was associated with reduced atherogenesis and lower serum triglyceride levels. Here, we characterized the glucose and lipid metabolism of P4h-tm-/- mice in physiological and tissue analyses. P4h-tm-/- mice showed variations in 24-h oscillations of energy expenditure, VO2 and VCO2 and locomotor activity compared to wild-type (WT) mice. Their rearing activity was reduced, and they showed significant muscle weakness and compromised coordination. Sedated P4h-tm-/- mice had better glucose tolerance, lower fasting insulin levels, higher fasting lactate levels and lower fasting free fatty acid levels compared to WT. These alterations were not present in conscious P4h-tm-/- mice. Fasted P4h-tm-/- mice presented with faster hepatic glycogenolysis. The respiratory rate of conscious P4h-tm-/- mice was significantly lower compared to the WT, the decrease being further exacerbated by sedation and associated with acidosis and a reduced ventilatory response to both hypoxia and hypercapnia. P4H-TM deficiency in mice is associated with alterations in whole-body energy metabolism, day-night rhythm of activity, glucose homeostasis and neuromuscular and respiratory functions. Although the underlying mechanism(s) are not yet fully understood, the phenotype appears to have neurological origins, controlled by brain and central nervous system circuits. The phenotype of P4h-tm-/- mice recapitulates some of the symptoms of HIDEA patients, making this mouse model a valuable tool to study and develop tailored therapies.

Collagen prolyl 4-hydroxylase isoenzymes I and II have sequence specificity towards different X-Pro-Gly triplets

Collagen biosynthesis requires several co- and post-translational modifications of lysine and proline residues to form structurally and functionally competent collagen molecules. Formation of 4-hydroxyproline (4Hyp) in Y-position prolines of the repetitive -X-Y-Gly- sequences provides thermal stability for the triple-helical collagen molecules. 4Hyp formation is catalyzed by a collagen prolyl 4-hydroxylase (C-P4H) family consisting of three isoenzymes. Here we identify specific roles for the two main C-P4H isoenzymes in collagen hydroxylation by a detailed 4Hyp analysis of type I and IV collagens derived from cell and tissue samples. Loss of C-P4H-I results in underhydroxylation of collagen where the affected prolines are not uniformly distributed, but mainly present in sites where the adjacent X-position amino acid has a positively charged or a polar uncharged side chain. In contrast, loss of C-P4H-II results in underhydroxylation of triplets where the X-position is occupied by a negatively charged amino acid glutamate or aspartate. Hydroxylation of these triplets was found to be important as loss of C-P4H-II alone resulted in reduced collagen melting temperature and altered assembly of collagen fibrils and basement membrane. The observed C-P4H isoenzyme differences in substrate specificity were explained by selective binding of the substrate to the active site resulting in distinct differences in Km and Vmax values. Furthermore, our results clearly show that the substrate proline selection is not dependent on the collagen type, but the main determinant is the X-position amino acid of the -X-Pro-Gly- triplet. Although our data clearly shows the necessity of both C-P4H-I and II for normal prolyl 4-hydroxylation and function of collagens, the mRNA expression of the isoenzymes with various procollagens was, surprisingly, not tightly coordinated, suggesting additional levels of control. In conclusion, this study provides a molecular level explanation for the need of multiple C-P4H isoenzymes to generate collagen molecules capable to assemble into intact extracellular matrix structures.

Contribution of collagen XIII to lung function and development of pulmonary fibrosis

BACKGROUND

Collagen XIII is a transmembrane collagen associated with neuromuscular junction development, and in humans its deficiency results in congenital myasthenic syndrome type 19 (CMS19), which leads to breathing difficulties. CMS19 patients usually have restricted lung capacity and one patient developed chronic lung disease. In single-cell RNA sequencing studies, collagen XIII has been identified as a marker for pulmonary lipofibroblasts, which have been implicated in the resolution of pulmonary fibrosis.

METHODS

We investigated the location and function of collagen XIII in the lung to understand the origin of pulmonary symptoms in human CMS19 patients. Additionally, we performed immunostainings on idiopathic pulmonary fibrosis (IPF) samples (N=5) and both normal and fibrotic mouse lung. To study whether the lack of collagen XIII predisposes to restrictive lung disease, we exposed Col13a1-modified mice to bleomycin-induced pulmonary fibrosis.

RESULTS

Apparently normal alveolar septum sections of IPF patients' lungs stained faintly for collagen XIII, and its expression was pinpointed to the septal fibroblasts in the mouse lung. Lung capacity was increased in mice lacking collagen XIII by over 10%. In IPF samples, collagen XIII was expressed by basal epithelial cells, hyperplastic alveolar epithelial cells and stromal cells in fibrotic areas, but the development of pulmonary fibrosis was unaffected in collagen XIII-deficient mice.

CONCLUSIONS

Changes in mouse lung function appear to represent a myasthenic manifestation of collagen XIII deficiency. We suggest that respiratory muscle myasthenia is the primary cause of the breathing problems suffered by CMS19 patients in addition to skeletal deformities. Induction of collagen XIII expression in the IPF patients' lungs warrants further studies to reveal collagen XIII-dependent disease mechanisms.

Reduced bone mass in collagen prolyl 4‐hydroxylase P4ha1 +/‐ ; P4ha2 ‐/‐ compound mutant mice

Proper deposition of the extracellular matrix and its major components, the collagens, is essential for endochondral ossification and bone mass accrual. Collagen prolyl 4‐hydroxylases (C‐P4Hs) hydroxylate proline residues in the ‐X‐Pro‐Gly‐ repeats of all known collagen types. Their product, 4‐hydroxyproline, is essential for correct folding and thermal stability of the triple‐helical collagen molecules in physiological body temperatures. We have previously shown that inactivation of the mouse P4ha1 gene, which codes for the catalytic α subunit of the major C‐P4H isoform, is embryonic lethal, whereas inactivation of the P4ha2 gene produced only a minor phenotype. Instead, mice with a haploinsufficiency of the P4ha1 gene combined with a homozygous deletion of the P4ha2 gene present with a moderate chondrodysplasia due to transient cell death of the growth plate chondrocytes. Here, to further characterize the bone phenotype of the P4ha1^+/−; P4ha2^−/− mice, we have carried out gene expression analyses at whole‐tissue and single‐cell levels, biochemical analyses, microcomputed tomography, histomorphometric analyses, and second harmonic generation microscopy to show that C‐P4H α subunit expression peaks early and that the C‐P4H deficiency leads to reduced collagen amount, a reduced rate of bone formation, and a loss of trabecular and cortical bone volume in the long bones. The total osteoblast number in the proximal P4ha1^+/−; P4ha2^−/− tibia and the C‐P4H activity in primary P4ha1^+/−; P4ha2^−/− osteoblasts were reduced, whereas the population of osteoprogenitor colony‐forming unit fibroblasts was increased in the P4ha1^+/−; P4ha2^−/− marrow. Thus, the P4ha1^+/−; P4ha2^−/− mouse model recapitulates key aspects of a recently recognized congenital connective tissue disorder with short stature and bone dysplasia caused by biallelic variants of the human P4HA1 gene. Altogether, the data demonstrate the allele dose‐dependent importance of the C‐P4Hs to the developing organism and a threshold effect of C‐P4H activity in the proper production of bone matrix. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Expression and Roles of Individual HIF Prolyl 4-Hydroxylase Isoenzymes in the Regulation of the Hypoxia Response Pathway along the Murine Gastrointestinal Epithelium

The HIF prolyl 4-hydroxylases (HIF-P4H) control hypoxia-inducible factor (HIF), a powerful mechanism regulating cellular adaptation to decreased oxygenation. The gastrointestinal epithelium subsists in "physiological hypoxia" and should therefore have an especially well-designed control over this adaptation. Thus, we assessed the absolute mRNA expression levels of the HIF pathway components, Hif1a, HIF2a, Hif-p4h-1, 2 and 3 and factor inhibiting HIF (Fih1) in murine jejunum, caecum and colon epithelium using droplet digital PCR. We found a higher expression of all these genes towards the distal end of the gastrointestinal tract. We detected mRNA for Hif-p4h-1, 2 and 3 in all parts of the gastrointestinal tract. Hif-p4h-2 had significantly higher expression levels compared to Hif-p4h-1 and 3 in colon and caecum epithelium. To test the roles each HIF-P4H isoform plays in the gut epithelium, we measured the gene expression of classical HIF target genes in Hif-p4h-1-/-, Hif-p4h-2 hypomorph and Hif-p4h-3-/- mice. Only Hif-p4h-2 hypomorphism led to an upregulation of HIF target genes, confirming a predominant role of HIF-P4H-2. However, the abundance of Hif-p4h-1 and 3 expression in the gastrointestinal epithelium implies that these isoforms may have specific functions as well. Thus, the development of selective inhibitors might be useful for diverging therapeutic needs.

Type I and type V procollagen triple helix uses different subsets of the molecular ensemble for lysine posttranslational modifications in the rER

Collagen is the most abundant protein in humans. It has a characteristic triple-helix structure and is heavily posttranslationally modified. The complex biosynthesis of collagen involves processing by many enzymes and chaperones in the rough endoplasmic reticulum. Lysyl hydroxylase 1 (LH1) is required to hydroxylate lysine for cross-linking and carbohydrate attachment within collagen triple helical sequences. Additionally, a recent study of prolyl 3-hydroxylase 3 (P3H3) demonstrated that this enzyme may be critical for LH1 activity; however, the details surrounding its involvement remain unclear. If P3H3 is an LH1 chaperone that is critical for LH1 activity, P3H3 and LH1 null mice should display a similar deficiency in lysyl hydroxylation. To test this hypothesis, we compared the amount and location of hydroxylysine in the triple helical domains of type V and I collagen from P3H3 null, LH1 null, and wild-type mice. The amount of hydroxylysine in type V collagen was reduced in P3H3 null mice, but surprisingly type V collagen from LH1 null mice contained as much hydroxylysine as type V collagen from wild-type mice. In type I collagen, our results indicate that LH1 plays a global enzymatic role in lysyl hydroxylation. P3H3 is also involved in lysyl hydroxylation, particularly at cross-link formation sites, but is not required for all lysyl hydroxylation sites. In summary, our study suggests that LH1 and P3H3 likely have two distinct mechanisms to recognize different collagen types and to distinguish cross-link formation sites from other sites in type I collagen.

The glucose-sensing transcription factor ChREBP is targeted by proline hydroxylation

Cellular energy demands are met by uptake and metabolism of nutrients like glucose. The principal transcriptional regulator for adapting glycolytic flux and downstream pathways like de novo lipogenesis to glucose availability in many cell types is carbohydrate response element-binding protein (ChREBP). ChREBP is activated by glucose metabolites and post-translational modifications, inducing nuclear accumulation and regulation of target genes. Here we report that ChREBP is modified by proline hydroxylation at several residues. Proline hydroxylation targets both ectopically expressed ChREBP in cells and endogenous ChREBP in mouse liver. Functionally, we found that specific hydroxylated prolines were dispensable for protein stability but required for the adequate activation of ChREBP upon exposure to high glucose. Accordingly, ChREBP target gene expression was rescued by re-expressing WT but not ChREBP that lacks hydroxylated prolines in ChREBP-deleted hepatocytes. Thus, proline hydroxylation of ChREBP is a novel post-translational modification that may allow for therapeutic interference in metabolic diseases.

Prolyl and lysyl hydroxylases in collagen synthesis

Collagens are the most abundant proteins in the extracellular matrix. They provide a framework to build organs and tissues and give structural support to make them resistant to mechanical load and forces. Several intra- and extracellular modifications are needed to make functional collagen molecules, intracellular post-translational modifications of proline and lysine residues having key roles in this. In this article, we provide a review on the enzymes responsible for the proline and lysine modifications, that is collagen prolyl 4-hydroxylases, 3-hydroxylases and lysyl hydroxylases, and discuss their biological functions and involvement in diseases.

Null mutation in P4h-tm leads to decreased fear and anxiety and increased social behavior in mice

HIF prolyl 4-hydroxylases (HIF-P4Hs, also known as PHDs and EGLNs) are crucial enzymes that modulate the hypoxia inducible factor (HIF) response and help to maintain cellular oxygen homeostasis. This function is especially well-known for cytoplasmic or nuclear enzymes HIF-P4H-1-3 (PHDs 1-3, EGLNs 2, 1 and 3, respectively), but the physiological role is still obscure for a fourth suggested HIF-P4H, P4H-TM that is a transmembrane protein and resides in the endoplasmic reticulum. Recently however, both experimental and clinical evidence of the P4H-TM involvement in CNS physiology has emerged. In this study, we first investigated the expression pattern of P4H-TM in the mouse brain and found a remarkably selective abundance in brains areas that are involved in social behaviors and anxiety including amygdala, lateral septum and bed nucleus of stria terminalis. Next, we performed behavioral assays in P4h-tm^-/- mice to investigate a possible phenotype associated to these brain areas. In locomotor activity tests, we found that P4h-tm^-/- mice were significantly more active than their wild-type (WT) littermate mice, and habituation to test environment did not abolish this effect. Instead, spatial learning and memory seemed normal in P4h-tm^-/- mice as assessed by Morris swim task. In several tests assessing anxiety and fear responses, P4h-tm^-/- mice showed distinct courageousness, and they presented increased interaction towards fellow mice in social behavior tests. Most strikingly, P4h-tm^-/- mice practically lacked behavioral despair response, a surrogate marker of depression, in forced swim and tail suspension tests. Instead, mutant mice of all other Hif-p4h isoforms lacked such a behavioral phenotype. In summary, this study presents a remarkable anatomy-physiology association between the brain expression of P4H-TM and the behavioral phenotype in P4h-tm^-/- mice. Future studies will reveal whether P4H-TM may serve as a novel target for anti-depressant and anti-anxiety pharmacotherapy.

Proline hydroxylation in collagen supports integrin binding by two distinct mechanisms

Collagens are the most abundant extracellular matrix proteins in vertebrates and have a characteristic triple-helix structure. Hydroxylation of proline residues is critical for helix stability, and diminished prolyl hydroxylase activity causes wide-spread defects in connective tissues. Still, the role of proline hydroxylation in the binding of collagen receptors such as integrins is unclear. Here, we isolated skin collagen from genetically modified mice having reduced prolyl 4-hydroxylase activity. At room temperature, the reduced proline hydroxylation did not affect interactions with the recombinant integrin α2I domain, but at 37 °C, collagen hydroxylation correlated with the avidity of α2I domain binding. Of note, LC-MS/MS analysis of isolated skin collagens revealed no major changes in the hydroxyproline content of the main integrin-binding sites. Thus, the disrupted α2I domain binding at physiological temperatures was most likely due to structural destabilization of the collagenous helix. Integrin α2I binding to the triple-helical GFPGER motif was slightly weaker than to GFOGER (O = hydroxyproline). This phenomenon was more prominent when α1 integrin was tested. Integrin α1β1 expressed on CHO cells and recombinant α1I domain showed remarkably slower binding velocity and weaker avidity to GFPGER when compared with GFOGER. Structural modeling revealed the critical interaction between Arg-218 in α1I and the hydroxyproline residue in the integrin-binding motif. The role of Arg-218 was further validated by testing a variant R218D α1I domain in solid-phase binding assays. Thus, our results show that the lack of proline hydroxylation in collagen can affect integrin binding by a direct mechanism and via structural destabilization of the triple helix.

P4HA1 mutations cause a unique congenital disorder of connective tissue involving tendon, bone, muscle and the eye

Collagen prolyl 4-hydroxylases (C-P4Hs) play a central role in the formation and stabilization of the triple helical domain of collagens. P4HA1 encodes the catalytic α(I) subunit of the main C-P4H isoenzyme (C-P4H-I). We now report human bi-allelic P4HA1 mutations in a family with a congenital-onset disorder of connective tissue, manifesting as early-onset joint hypermobility, joint contractures, muscle weakness and bone dysplasia as well as high myopia, with evidence of clinical improvement of motor function over time in the surviving patient. Similar to P4ha1 null mice, which die prenatally, the muscle tissue from P1 and P2 was found to have reduced collagen IV immunoreactivity at the muscle basement membrane. Patients were compound heterozygous for frameshift and splice site mutations leading to reduced, but not absent, P4HA1 protein level and C-P4H activity in dermal fibroblasts compared to age-matched control samples. Differential scanning calorimetry revealed reduced thermal stability of collagen in patient-derived dermal fibroblasts versus age-matched control samples. Mutations affecting the family of C-P4Hs, and in particular C-P4H-I, should be considered in patients presenting with congenital connective tissue/myopathy overlap disorders with joint hypermobility, contractures, mild skeletal dysplasia and high myopia.

Wnt5a Deficiency Leads to Anomalies in Ureteric Tree Development, Tubular Epithelial Cell Organization and Basement Membrane Integrity Pointing to a Role in Kidney Collecting Duct Patterning

The Wnts can be considered as candidates for the Congenital Anomaly of Kidney and Urinary Tract, CAKUT diseases since they take part in the control of kidney organogenesis. Of them Wnt5a is expressed in ureteric bud (UB) and its deficiency leads to duplex collecting system (13/90) uni- or bilateral kidney agenesis (10/90), hypoplasia with altered pattern of ureteric tree organization (42/90) and lobularization defects with partly fused ureter trunks (25/90) unlike in controls. The UB had also notably less tips due to Wnt5a deficiency being at E15.5 306 and at E16.5 765 corresponding to 428 and 1022 in control (p<0.02; p<0.03) respectively. These changes due to Wnt5a knock out associated with anomalies in the ultrastructure of the UB daughter epithelial cells. The basement membrane (BM) was malformed so that the BM thickness increased from 46.3 nm to 71.2 nm (p<0.01) at E16.5 in the Wnt5a knock out when compared to control. Expression of a panel of BM components such as laminin and of type IV collagen was also reduced due to the Wnt5a knock out. The P4ha1 gene that encodes a catalytic subunit of collagen prolyl 4-hydroxylase I (C-P4H-I) in collagen synthesis expression and the overall C-P4H enzyme activity were elevated by around 26% due to impairment in Wnt5a function from control. The compound Wnt5a^+/-;P4ha1^+/- embryos demonstrated Wnt5a^-/- related defects, for example local hyperplasia in the UB tree. A R260H WNT5A variant was identified from renal human disease cohort. Functional studies of the consequence of the corresponding mouse variant in comparison to normal ligand reduced Wnt5a-signalling in vitro. Together Wnt5a has a novel function in kidney organogenesis by contributing to patterning of UB derived collecting duct development contributing putatively to congenital disease.

Severe Extracellular Matrix Abnormalities and Chondrodysplasia in Mice Lacking Collagen Prolyl 4-Hydroxylase Isoenzyme II in Combination with a Reduced Amount of Isoenzyme I

Collagen prolyl 4-hydroxylases (C-P4H-I, C-P4H-II, and C-P4H-III) catalyze formation of 4-hydroxyproline residues required to form triple-helical collagen molecules. Vertebrate C-P4Hs are α2β2 tetramers differing in their catalytic α subunits. C-P4H-I is the major isoenzyme in most cells, and inactivation of its catalytic subunit (P4ha1(-/-)) leads to embryonic lethality in mouse, whereas P4ha1(+/-) mice have no abnormalities. To study the role of C-P4H-II, which predominates in chondrocytes, we generated P4ha2(-/-) mice. Surprisingly, they had no apparent phenotypic abnormalities. To assess possible functional complementarity, we established P4ha1(+/-);P4ha2(-/-) mice. They were smaller than their littermates, had moderate chondrodysplasia, and developed kyphosis. A transient inner cell death phenotype was detected in their developing growth plates. The columnar arrangement of proliferative chondrocytes was impaired, the amount of 4-hydroxyproline and the Tm of collagen II were reduced, and the extracellular matrix was softer in the growth plates of newborn P4ha1(+/-);P4ha2(-/-) mice. No signs of uncompensated ER stress were detected in the mutant growth plate chondrocytes. Some of these defects were also found in P4ha2(-/-) mice, although in a much milder form. Our data show that C-P4H-I can to a large extent compensate for the lack of C-P4H-II in proper endochondral bone development, but their combined partial and complete inactivation, respectively, leads to biomechanically impaired extracellular matrix, moderate chondrodysplasia, and kyphosis. Our mouse data suggest that inactivating mutations in human P4HA2 are not likely to lead to skeletal disorders, and a simultaneous decrease in P4HA1 function would most probably be required to generate such a disease phenotype.

Reduction of lysyl hydroxylase 3 causes deleterious changes in the deposition and organization of extracellular matrix

Lysyl hydroxylase 3 (LH3) is a multifunctional enzyme possessing lysyl hydroxylase, collagen galactosyltransferase, and glucosyltransferase (GGT) activities. We report here an important role for LH3 in the organization of the extracellular matrix (ECM) and cytoskeleton. Deposition of ECM was affected in heterozygous LH3 knock-out mouse embryonic fibroblasts (MEF(+/-)) and in skin fibroblasts collected from a member of a Finnish epidermolysis bullosa simplex (EBS) family known to be deficient in GGT activity. We show the GGT deficiency to be due to a transcriptional defect in one LH3 allele. The ECM abnormalities also lead to defects in the arrangement of the cytoskeleton in both cell lines. Ultrastructural abnormalities were observed in the skin of heterozygous LH3 knock-out mice indicating that even a moderate decrease in LH3 has deleterious consequences in vivo. The LH3 null allele in the EBS family member and the resulting abnormalities in the organization of the extracellular matrix, similar to those found in MEF(+/-), may explain the correlation between the severity of the phenotype and the decrease in GGT activity reported in this family.

A connective tissue disorder caused by mutations of the lysyl hydroxylase 3 gene

Lysyl hydroxylase 3 (LH3, encoded by PLOD3) is a multifunctional enzyme capable of catalyzing hydroxylation of lysyl residues and O-glycosylation of hydroxylysyl residues producing either monosaccharide (Gal) or disaccharide (Glc-Gal) derivatives, reactions that form part of the many posttranslational modifications required during collagen biosynthesis. Animal studies have confirmed the importance of LH3, particularly in biosynthesis of the highly glycosylated type IV and VI collagens, but to date, the functional significance in vivo of this enzyme in man is predominantly unknown. We report here a human disorder of LH3 presenting as a compound heterozygote with recessive inheritance. One mutation dramatically reduced the sugar-transfer activity of LH3, whereas another abrogated lysyl hydroxylase activity; these changes were accompanied by reduced LH3 protein levels in cells. The disorder has a unique phenotype causing severe morbidity as a result of features that overlap with a number of known collagen disorders.

The lysyl hydroxylase isoforms are widely expressed during mouse embryogenesis, but obtain tissue- and cell-specific patterns in the adult

Lysyl hydroxylase catalyzes the hydroxylation of lysine residues in collagenous sequences. Three isoforms (LH1, LH2 and LH3) of lysyl hydroxylase have been characterized, and LH2 is present as two alternatively spliced forms. In order to better understand the functional differences between the isoforms in vivo, the expression of the different isoforms was studied in mouse embryos and adult tissues. Our data indicate a widespread expression of all isoforms during embryogenesis, whereas the expression profiles become more specialized in adult tissues. The expression of LH2 was more tissue-specific, whereas a uniform and housekeeping like behavior was observed for LH3. Some cells express both LH2 and LH3, while a clear cell specificity was seen in some tissues. Moreover, immunoelectron microscopy revealed differences in the localization of LH2 and LH3. LH2 was localized intracellularly in the ER in all tissues studied, whereas the localization of LH3 was either intracellular or extracellular or both, depending on the tissue. Furthermore, our data indicate that the alternative splicing of LH2 is developmentally regulated. The short form of LH2 (LH2a) is the predominant form until E11.5; the long form (LH2b) dominates thereafter and is the major form in many adult tissues. Interestingly, however, adult mouse kidney and testis express exclusively the short form, LH2a. The results reveal a specific regulation for the expression of LH isoforms as well as for alternative splicing of LH2 during embryogenesis and in different tissues.

Glycosylation catalyzed by lysyl hydroxylase 3 is essential for basement membranes

Lysyl hydroxylase 3 (LH3) is a multifunctional enzyme possessing lysyl hydroxylase (LH), hydroxylysyl galactosyltransferase (GT) and galactosylhydroxylysyl glucosyltransferase (GGT) activities in vitro. To investigate the in vivo importance of LH3-catalyzed lysine hydroxylation and hydroxylysine-linked glycosylations, three different LH3-manipulated mouse lines were generated. Mice with a mutation that blocked only the LH activity of LH3 developed normally, but showed defects in the structure of the basement membrane and in collagen fibril organization in newborn skin and lung. Analysis of a hypomorphic LH3 mouse line with the same mutation, however, demonstrated that the reduction of the GGT activity of LH3 disrupts the localization of type IV collagen, and thus the formation of basement membranes during mouse embryogenesis leading to lethality at embryonic day (E) 9.5-14.5. Strikingly, survival of hypomorphic embryos and the formation of the basement membrane were directly correlated with the level of GGT activity. In addition, an LH3-knockout mouse lacked GGT activity leading to lethality at E9.5. The results confirm that LH3 has LH and GGT activities in vivo, LH3 is the main molecule responsible for GGT activity and that the GGT activity, not the LH activity of LH3, is essential for the formation of the basement membrane. Together our results demonstrate for the first time the importance of hydroxylysine-linked glycosylation for collagens.

Lysyl hydroxylase 3 (LH3) modifies proteins in the extracellular space, a novel mechanism for matrix remodeling

Lysyl hydroxylase 3 (LH3), the multifunctional enzyme associated with collagen biosynthesis that possesses lysyl hydroxylase and collagen glycosyltransferase activities, has been characterized in the extracellular space in this study. Lysine modifications are known to occur in the endoplasmic reticulum (ER) prior to collagen triple-helix formation, but in this study we show that LH3 is also present and active in the extracellular space. Studies with in vitro cultured cells indicate that LH3, in addition to being an ER resident, is secreted from the cells and is found both in the medium and on the cell surface associated with collagens or other proteins with collagenous sequences. Furthermore, in vivo, LH3 is present in serum. LH3 protein levels correlate with the galactosylhydroxylysine glucosyltransferase (GGT) activity of mouse tissues. This, together with other data, indicates that LH3 is responsible for GGT activity in the tissues and that GGT activity assays can be used to quantify LH3 in tissues. LH3 in vivo is located in two compartments, in the ER and in the extracellular space, and the partitioning varies with tissue type. In mouse kidney the enzyme is located mainly intracellularly, whereas in mouse liver it is located solely in the extracellular space. The extracellular localization and the ability of LH3 to modify lysyl residues of extracellular proteins in their native, nondenaturated conformation reveals a new dynamic in extracellular matrix remodeling, suggesting a novel mechanism for adjusting the amount of hydroxylysine and hydroxylysine-linked carbohydrates in collagenous proteins.

Clinical significance of circulating immune complexes in patients with metastatic breast cancer

Serum circulating immune complexes (CIC) were repeatedly measured by means of the CIq binding assay (Cba) and the Raji cell assay (Rca) in 158 patients with metastatic breast cancer (mbc). Frequency of occurrence and levels of CIC were only slightly increased in mbc when compared to age-matched healthy women. They were identical to those of patients with localized breast cancer prior to mastectomy and to those of post mastectomy patients without evidence of recurrent disease. In mbc the results of the Cba and the Rca showed a poor correlation, whereas in a control group of patients with rheumatoid arthritis both tests showed significantly elevated levels of CIC. Patients with mbc were followed up clinically and biochemically by serially measuring CIC for an average of 10 months. Patients with positive CIC did not prove to be an unfavorable group regarding progression of disease and response to chemotherapy. When CEA and CIC levels were compared, CIC, unlike CEA, was a poor tumor marker. In conclusion, CIC as measured by CIq binding assay and Raji cell assay are not clinically significant tumor markers or prognostic indicators in patients with metastatic breast cancer.