Post-embedding methods for immunolocalization of elastin and related components in tissues

Diabetes mellitus increases endothelin-1 gene transcription in rat kidney

BACKGROUND

Mesangial cell hypertrophy and increased extracellular matrix (ECM) contribute to mesangial expansion in early progressive diabetic nephropathy. Previous studies suggest that the growth factor endothelin-1 (ET-1) is not only up-regulated in diabetes, but may mediate the effects of hyperglycemia on mesangial cell hypertrophy and ECM synthesis. In models of diabetes mellitus, the mechanisms underlying increased ET-1 peptide and mRNA remain unknown. Therefore, our purpose is to determine whether ET-1 gene activity increases in kidneys of streptozotocin (SZT)-treated rats.

METHODS

Male Sprague-Dawley rats were injected with either SZT or vehicle. Parameters including glucose, body weight, 24-hour urine volume, urinary protein, and urinary ET-1 excretion were recorded. All rats were sacrificed at 12 weeks postinjection. Prepro-ET-1 mRNA from whole kidneys was determined using both RNase protection and reverse transcription-polymerase chain reaction (RT-PCR). The abundance of ET-1 peptide in primary cultured mesangial cells was detected by indirect immunofluorescence following treatment with 5.6, 11.2, or 22.5 mmol/L D-glucose for 24 hours. Cellular ET-1 mRNA was measured using RT-PCR in control cells at time 0 and also following exposure to increasing concentrations of glucose for 24 hours. Rat mesangial cells were transfected with a luciferase reporter construct containing the rat ET-1 promoter (pET1. Luc), and relative ET-1 promoter activity was measured after a 24-hour exposure to 5.6 and 22.5 mmol/L of D- or L-glucose.

RESULTS

After 12 weeks of hyperglycemia, diabetic rats gained less weight (344 +/- 23.9 vs. 548.75 +/- 15.08 g), had increased urinary volume (158.6 +/- 24.32 vs. 8.38 +/- 1.56 mL/day), and had marked proteinuria (101.7 +/- 12.2 vs. 14.1 +/- 2.8 mg/day) compared with controls. Total urinary ET-1 peptide increased 26.4-fold in diabetic versus control rats (17.5083 +/- 5.405 vs. 0.6635 +/- 0.343 ng/day). ET-1 mRNA extracted from whole rat kidneys was increased 2.1-fold in diabetic versus control animals. Primary cultured rat mesangial cells demonstrated a significant increase in immunofluorescence labeling of ET-1 peptide and ET-1 mRNA in response to increasing concentrations of glucose. Furthermore, transfected mesangial cells exposed to 22.5 mmol/L D-glucose showed a 1.6-fold increase in ET-1 promoter activity relative to those treated with 5.6 mmol/L glucose.

CONCLUSION

Glucose increases ET-1 gene expression in the kidney of the SZT-treated rat model of diabetes mellitus. Furthermore, high glucose induces ET-1 expression in primary cultured rat mesangial cells and directly enhances ET-1 promoter activity. The greater relative increase in peptide compared with transcription suggests the potential participation of other mechanisms such as increased mRNA stability, protein stability, and/or enhanced translational efficiency.

Immunohistochemical and ultrastructural localization of MP78/70 (betaig-h3) in extracellular matrix of developing and mature bovine tissues

MP78/70 is a matrix protein, with 78-kD and 70-kD isoforms, which was initially identified in bovine tissue extracts designed to solubilize elastin-associated microfibrils. Peptide analysis has shown that MP78/70 is closely related to the human protein, betaig-h3. In the present study an antibody raised to a synthetic betaig-h3 peptide was shown specifically to identify MP78/70 in purified form and in bovine tissue extracts. This is consistent with MP78/70 and betaig-h3 being the bovine and human forms, respectively, of the same protein. The antibody was further affinity-purified on MP78/70 bound to Sepharose and used to localize the protein in a range of bovine tissues. Immunofluorescence showed that MP78/70 was localized to collagen fibers in tissues such as developing nuchal ligament, aorta and lung, and mature cornea; to reticular fibers in fetal spleen; and to capsule and tubule basement membranes in developing kidney. No general localization to elastic fibers was observed. The staining pattern in most tissues more closely resembled that of Type VI collagen, which occurs as collagen fiber-associated microfibrils, than that of fibrillin-1, a component of elastin-associated microfibrils. However, MP78/70 appeared to be less widely distributed than Type VI collagen. Immunoelectron microscopy showed that MP78/70 was predominantly found in loose association with collagen fibers in most tissues examined and was also located on the surface of the capsule basement membrane in developing kidney. Double labeling experiments indicated that MP78/70 is co-distributed with Type VI collagen microfibrils located in these regions. In some elastic tissues significant immunolabel was detected in regions of interface between collagen fibers and fibrillin-containing microfibrils of adjacent elastic fibers, and at the outer margins of the latter structures. Overall, the evidence points to MP78/70 having a bridging function, perhaps in association with Type VI collagen microfibrils, linking or stabilizing the interaction between interstitial collagen fibrils and other matrix structures, including some basement membranes and elastin-associated microfibrils.

Further characterization of proteins associated with elastic fiber microfibrils including the molecular cloning of MAGP-2 (MP25)

Together with the 31-kDa microfibril-associated glycoprotein (MAGP), four polypeptides designated MP340 (340 kDa), MP78 (78 kDa), MP70 (70 kDa), and MP25 (25 kDa) have previously been identified in tissue extracts designed specifically to solubilize the microfibrillar component of elastic fibers. In the present study, both MP78 and MP70 were shown to be forms of a protein which is closely related to the human protein beta ig-h3, and MP340 was confirmed to be the bovine form of fibrillin-1. Peptide sequences from MP25 proved to be unique, and affinity-purified anti-MP25 antibodies were shown, by immunofluorescence and immunoelectron microscopy, to localize specifically to the elastin-associated microfibrils. This confirmed that MP25 was a distinct component of these structures. Expression screening of nuchal ligament cDNA libraries yielded a cDNA, cM10A (770 base pairs) which encodes amino acid sequences matching those of the MP25 peptides. Further library screening with cM10A identified cDNAs which encode the complete primary structures of bovine and human MP25. Bovine and human MP25 were found to be around 80% homologous and contain 170 and 173 amino acids, respectively. Data base searches revealed that MP25 had significant similarity of structure only with MAGP, indicating that the two proteins form a new family of microfibrillar proteins. In acknowledgment, MP25 has been formally renamed MAGP-2, and MAGP is referred to as MAGP-1. The close similarity between the two proteins (57%) is confined to a central region of 60 amino acids where there is precise alignment of 7 cysteine residues. Elsewhere the MAGP-2 molecule is rich in serine and threonine residues and contains an RGD motif. MAGP-2 lacks the proline-, glutamine-, and tyrosine-rich sequences and a hydrophobic carboxyl terminus, characteristic of MAGP-1. These structural differences suggest that MAGP-2 has some functions which are distinct from those of MAGP-1. The locus of the human MAGP-2 gene was identified on chromosome 12 in the region of 12p12.3-12p13.1.

Elastosis of the lamina cribrosa in pseudoexfoliation syndrome with glaucoma

BACKGROUND

Pseudoexfoliation syndrome is characterized by the presence of glycoprotein fibers in ocular and extraocular tissues, and often is associated with glaucoma. Pseudoexfoliation material may be associated closely with elastic microfibrillar-associated glycoprotein as well as elastin.

METHODS

Four optic nerve heads of two patients with pseudoexfoliation syndrome and glaucoma were examined using electron microscopy and immunogold detection of elastin. Optic nerve heads from healthy age-matched individuals and patients with primary open-angle glaucoma were used for comparisons.

RESULTS

In all eyes with pseudoexfoliation and glaucoma, there was marked and widespread elastosis in the connective tissue of the lamina cribrosa. Elastotic fibers appeared as large and irregular aggregates of electron-dense material labeled with anti-elastin antibody. Abundant microfibrils were interspersed in the elastotic aggregates, whereas no typical pseudoexfoliation fibers were observed. In contrast, there were less elastotic fibers in the lamina cribrosa from patients with primary open-angle glaucoma compared with pseudoexfoliation glaucoma. Other changes of extracellular matrix were similar to those observed in primary open-angle glaucoma: decreases in collagen fiber density, presence of basement membranes not associated with cell surfaces, and abundant bundles of microfibrils not labeled with elastin antibody. The elastic fibers appeared normal in other locations within the optic nerves of patients with pseudoexfoliation glaucoma, including in the pial septa and blood vessels of the retrolaminar myelinated optic nerve.

CONCLUSION

The authors' findings demonstrate marked and site-specific elastosis in the lamina cribrosa of patients with pseudoexfoliation syndrome with glaucoma, suggesting an abnormal regulation of elastin synthesis and/or degradation in the optic nerve of patients with this disease.

Human scleral elastic system: an immunoelectron microscopic study

An immunocytochemical study was conducted on elastic components in the sclera of seven aged human eyes. By conventional electron microscopy, elastic tissue consists of three distinct fibre types--elastic fibres, elaunin fibres, and oxytalan fibres. The distribution of six components associated with the elastic system (elastin, amyloid P component, laminin, fibronectin, gp 115, and vitronectin) were studied by immunogold transmission electron microscopy. The codistribution of amyloid P component and laminin was further studied by double immunolabelling. Both elastic and elaunin fibres contained elastin. The microfibrillar sheaths of elastic fibres labelled for amyloid P component, those of elaunin fibres for amyloid P and laminin, and those of oxytalan fibres for laminin only. No labelling was observed for fibronectin, gp 115, and vitronectin. In terms of the proteins investigated, the biochemical profile of the three fibre types was not completely identical and was manifest as different affinities in the binding of serum amyloid P component and an association with laminin.

Immunolocalization of microfibril and microfibril-associated proteins in the subendothelial matrix of the developing mouse aorta

In the developing aorta, endothelial cell connecting filaments extend from the abluminal surface of the endothelial cell to the subjacent elastic lamina. The connecting filaments are in alignment with intracellular stress fibers and are oriented parallel to the direction of blood flow. In the present study, the composition of the endothelial cell connecting filaments was investigated by indirect immunogold labeling with antibodies to the microfibril proteins, MP340 (fibrillin) and MAGP, and to fibronectin and heparan sulfate proteoglycan (HSPG). In the subendothelial matrix of both 15-day gestational and 5-day post-natal mouse aortae, the connecting filaments showed moderate immunoreactivity with anti-MP340; however, no significant immunoreaction was seen with anti-MAGP. Anti-fibronectin strongly labeled the connecting filaments and a weak immunoreaction was seen with anti-HSPG. In contrast, the adjacent ‘elastin-associated microfibrils’ showed a very strong immunoreaction with anti-MP340 and a moderate reaction with anti-MAGP. Little or no reaction was seen with anti-fibronectin or anti-HSPG. The filaments that connect endothelial cells to the subjacent elastic lamina during aortic development are thus microfibrillar in nature and related to elastin-associated microfibrils as evidenced by their positive immunoreaction with anti-MP340. The absence of labeling with anti-MAGP, however, suggests that either these fibrillin-containing filaments do not contain MAGP or that the immunoreactive epitopes are blocked by the proteins that coat the connecting filaments such as fibronectin. These results suggest that microfibrils not in association with elastin may play a role in cell anchorage and, more specifically, in the aorta may be involved in maintaining the structural integrity of the endothelial cell layer during early development of the vessel wall. Furthermore, the absence of immunoreactivity with anti-MAGP on the fibrillin-containing endothelial cell connecting filaments raises the possibility that microfibrils may consist of a family of related filaments rather than a single structural entity.

A re-evaluation of the distribution of the elastic meshwork within the periodontal ligament of the mouse

The elastic properties of the periodontal ligament have been attributed, in part, to oxytalan fibers, as no other types of elastic fibers are described there. It has been difficult to study the periodontal elastic meshwork by standard microscopic techniques because it is partially obscured by the adjacent periodontal ligament collagen fibers. Our study employed methods which either completely or partially removed mandibular molar periodontal ligament collagen fibers, exposing a previously undescribed periodontal elastic meshwork. The periodontal elastic meshwork was composed of many elastin lamellae containing both peripheral microfibrils of regular arrangement and central microfibrils of irregular arrangement, which could only be demonstrated in oxidized tissues. Peripheral, regularly arranged bundles of microfibrils resembled oxytalan fibers, which were often adherent to the border of the elastin lamella. Elastin lamellae containing irregular microfibrils resembled elaunin fibers. These fibers probably enclosed either blood vessels, nerves or collagen fiber bundles. Peripheral microfibrils attached elaunin to cementum, alveolar bone, blood vessels, and principal periodontal collagen fibers. Thus, the periodontal elastic meshwork is composed of both oxytalan and elaunin fibers. Microfibrils attach elaunin fibers to the adjacent non-elastic tissue and also form bundles which traverse the periodontal ligament space and are probably the oxytalan fibers demonstrable by light microscopic techniques. This meshwork of oxytalan and elaunin fibers probably contributes to tooth support and maintenance of periodontal homeostasis by dissipating chewing forces and maintaining patency of periodontal blood vessels.

Elastofibroma: disturbed elastic fibrillogenesis by periosteal-derived cells? An immunoelectron microscopic and in situ hybridization study

Monospecific antibodies to elastic tissue components have been used for immunoelectron microscopy of two examples of elastofibroma. The elastic-staining fibers typically seen in these lesions exhibited a variety of morphologies with differing ratios of the amorphous and microfibrillar components usually seen in elastic fibers. The amorphous elastic material in these fibers had variable affinity for ionic stains and exhibited several substructural morphologies. Despite this, each form reacted specifically with anti-elastin antibodies. Most of the elastic fibers were associated with relatively large numbers of 12-nm diameter microfibrils that were typical of those associated with normal elastic fibers, and were specifically reactive with monospecific antibodies to microfibril-associated glycoprotein. In situ hybridization studies with a cRNA probe for human elastin confirmed that active elastin biosynthesis was occurring patchily within the lesions. The appearances and staining characteristics of the elastic tissue elements, the morphology of the cells, and the structure of the collagen fibers in these lesions were shown to have many features in common with those of normal periosteum. It is proposed that elastofibromas arise from the periosteum as a result of chronic irritation and that the different elastic fiber morphologies represent disturbances of elastic fibrillogenesis by periosteal-derived cells.

Immunoelectron microscopic localization of elastic tissue components in archival tissue samples

Tissue samples that have been stored for many years, in different media and under a variety of conditions, have been examined by modern techniques of immunoelectron microscopy, using antibodies against elastic tissue components. A range of postembedding restorative procedures has been identified, which will allow reliable immunolocalization of antibodies against the elastic tissue component of such specimens. These methods have been applied successfully to autopsy-derived material, fixed in buffered formaldehyde, to archival material stored frozen at -70 or -20 degrees C, to specimens fixed for electron microscopy and stored for many years in buffer, and even to archival material from formaldehyde-fixed, paraffin-embedded blocks, reprocessed for electron microscopic examination. The successful restorative methods included pre-treatment of the sections with 6 M guanidine hydrochloride, or 1 M Tris/saline, each containing 100 mM dithiothreitol (a reducing agent) followed by alkylation with 220 mM iodoacetamide. The application of these techniques allowed reliable study of elastic tissue antibody distributions in archival tissues that could not be obtained again, as well as comparative studies with tissues processed many years previously.

Elastin in human, baboon, and mouse liver: an immunohistochemical and immunoelectron microscopic study

Light microscope histochemistry and immunohistochemistry, and routine electron microscopy techniques were performed to analyse elastin distribution and structure in the human liver compared with that in baboon and mouse. In man and baboon, elastic fibers stained by iron hematoxylin or orcinolnew fuchsin seemed to be solitary and were few in number; in the mouse they were thinner but abundant, both in the portal tract and in hepatic veins. Orcein or resorcin-fuchsin stains, employed after oxidation of tissue sections, revealed a network comprising elastic, elaunin, and oxytalan fibers, which was also demonstrated by immunofluorescence with anti-elastin antibody in man and baboon. At the ultrastructural level, the elastic fibers of the human portal tract corresponded to discontinuous patches of amorphous material intermingled with few microfibrils. These contrasted with the thinner elastic fibers of baboon and mouse liver which had a core of amorphous material. In man and baboon, these fibers meshed into slender bundles of microfibrils often exhibiting small spots of amorphous material (elaunin fibers) and terminated as isolated microfibrils (oxytalan fibers). Immunoelectron microscopy of elastin carried out on baboon liver tissue labelled the amorphous material and also its microfibrillar component. Immunoperoxidase deposits were also associated with isolated bundles of microfibrils in the baboon portal stroma. Immunolabelling and elastic stains disclosed an important elastin portal network located around vascular, biliary structures and interspaced with collagen bundles. The structural polymorphism of elastin, assembling different relative amounts of amorphous material and microfibrils, might have a relationship with the required elasticity in a given species.