Mucins were extracted from the epithelial surface and the submucosal tissue of human trachea in order to enrich glycoproteins from the goblet cells and the submucosal glands respectively. The macromolecules were purified using density-gradient centrifugation, and the presence of the MUC5AC mucin was investigated using an antiserum raised against a synthetic peptide based on the sequence of the MUC5AC apoprotein. Mucins from the surface epithelium showed a higher reactivity with the antiserum relative to carbohydrate than those from the submucosa, and ion-exchange HPLC of reduced subunits revealed the presence of two distinct mucin populations in the samples. The predominant species from the surface epithelium was more acidic than the major population from the submucosa and showed a strong reactivity with the anti-MUC5AC anti-serum. In contrast, the major portion of the submucosal mucins were less acidic and showed no MUC5AC reactivity, although a more acidic population did react with the antibody. Rate-zonal centrifugation showed that the MUC5AC mucin from the surface epithelium is smaller than the major submucosal mucin, and that both are composed of subunits. Immunolocalization confirmed that the MUC5AC mucin from human trachea originates from the goblet cells and that this glycoprotein is not a major product of the submucosal glands.

Airway mucus was collected from healthy and chronic bronchitic subjects. The chronic bronchitic sputum was separated into gel and sol phase by centrifugation and mucins were isolated using isopycnic density-gradient centrifugation in CsCl. The presence of the MUC5AC and MUC2 mucins was investigated with antisera raised against synthetic peptides with sequences from the respective apoproteins. The gel and sol phase of chronic bronchitic sputum as well as healthy respiratory secretions were shown to contain MUC5AC whereas the MUC2 mucin could not be detected. Rate-zonal centrifugation showed that the MUC5AC mucin was large, polydisperse in size and that reduction yielded subunits. Ion-exchange HPLC revealed the presence of two subunit populations in all secretions, the MUC5AC subunits always being the more acidic. MUC5AC is thus the first large, subunit-based, gel-forming respiratory mucin identified and this glycoprotein is biochemically distinct from at least one other population of large, gel-forming mucins also composed of subunits but lacking a genetic identity.

T h e mucosal surfaces represent an impressive barrier between the 'inside' of the body and the 'outside' world. This 'front line' is under constant threat from micro-organisms, degradative enzymes and other noxious agents and is, therefore, as a first line of defence, covered by a gellike secretionmucus. T h e polymer matrix of this biofilm is provided by high-M, glycoproteins referred to as mucus glycoproteins or mucins. The mucin superfamilyMucins constitute a family of glycoproteins traditionally regarded as secretory products from epithelial surfaces. T h e apoprotein is substituted with a large number of oligosaccharides attached via an 0-glycosidic linkage between serine and/or threonine and GalNAc. In a typical mucin, the 0-linked glycansoften referred to as mucintype oligosaccharidesare enriched within serine/threonine-rich domains. These often contain proline, which is believed to give the protein core an extended conformation, in particular after substitution with the link GalNAc [ l ] , and it is therefore likely that one major function of the serinelthreonine-rich regions is to provide a matrix for the presentation of carbohydrate structures. Similar sequences have been identified in, for example, some membrane-bound glycoproteins, and a 'mucin-like stretch' has become an expression used to denote such domains with multiple sites for substitution with $To whom correspondence should be addressed. 0-linked oligosaccharides in any protein. For example, glycoproteins carrying receptor structures for selectins have been referred to as endo-

Little is known about whether the properties of respiratory mucins are altered as a result of airway irritation, but histochemical studies of respiratory tract secretory cells show a more 'acidic' staining pattern after exposure to tobacco smoke. Furthermore it has been suggested that proteoglycans are the major glycoconjugates in 'normal' respiratory secretions, whereas mucins predominate in sputum. To investigate these observations further, mucins from secretions collected from the tracheal surface of healthy non-smoking 'normal' subjects and sputum from patients with chronic bronchitis were compared. All samples contained one major mucin population after density-gradient centrifugation, and a small amount of 'denser' mucin was present in some chronic bronchitic and one of the 'normal' samples. Proteoglycans were not a major component of 'normal' secretions. The major mucin population from chronic bronchitic samples had molecular masses between 10 and 30 MDa and behaved as random coils in solution. Whole mucins from 'normal' individuals and chronic bronchitic patients were excluded from Sepharose CL-2B, whereas reduced subunits were included. Proteolysis of subunits yielded two populations of high-molecular-mass glycopeptides differing in size, suggesting the presence of two different tandem repeat regions in the mucins. Finally, mucins from patients with chronic bronchitis are less, rather than more, acidic than those from 'normal' individuals. Mucins from bronchitic sputum and 'normal' secretions are thus similar in their macromolecular properties, but differ slightly in charge density.

Bovine trachea in organ culture secretes mucus containing a 'high-density' (1.46 g/ml) and a 'low-density' (1.37 g/ml) mucin similar to those identified previously in bovine respiratory secretions [Hovenberg, Carlstedt and Davies (1997) Biochem. J. 321, 117-123]. After pulse-labelling, autoradiography showed uptake of [35S]sulphate by both epithelial goblet cells and submucosal glands, while [3H]proline was mainly incorporated into the ciliated surface epithelial cells. After 24 h of radiolabelling, neither the high- nor the low-density mucin in the secreted mucus gel was heavily radiolabelled with the precursors. In contrast, a population of molecules banding at 1.50 g/ml was heavily radiolabelled with [35S]sulphate. This component was smaller than the high-density mucin from the mucus gel and was insensitive to reduction or digestion with chondroitin ABC lyase or heparan sulphate lyase. The molecules yielded two populations of high-Mr glycopeptides upon trypsin digestion, were sensitive to keratanase and endo-beta-galactosidase digestion and contained O-linked glycans. Extracts of the surface epithelium and submucosal tissue after radiolabelling showed that the high- and low-density mucins in the tissue were also poorly radiolabelled. Thus, under these conditions, the radiolabelled precursors were not effectively incorporated into the large oligomeric mucins but into a high-Mr monomeric species. This study suggests that data obtained in investigations where mucins are radiolabelled and studied without further separation into distinct components may rather reflect the turnover of this 'novel' monomeric species than the large oligomeric mucins.

Bovine respiratory secretions were separated into gel and sol phases to allow the identification of the gel-forming mucins. Mucins were subsequently isolated from the surface epithelium and submucosal tissue to investigate the tissue origins of the species in the secretions. Density-gradient centrifugation revealed 'high-density' and 'low-density' mucins in the gel phase of the secretions. The 'high-density' mucins were large, composed of subunits joined by disulphide bonds and contained two highly glycosylated domains of apparently different lengths, whereas the 'low-density' mucins were smaller and monomeric. The sol also contained both 'high-density' and 'low-density' species. A 'high-density' mucin similar to that in the gel was isolated from the surface epithelium, suggesting that the goblet cells produce large, gel-forming mucins. A second 'high-density' species was released from the submucosal tissue after reduction/alkylation, indicating that large mucins from the submucosal glands may also be a component of the mucus gel. In addition, two small, 'low-density' mucins were obtained from the submucosal tissue. One species was associated with the gel phase but was also present in the sol, whereas the other was present only in the sol. Bovine respiratory-tract secretions thus comprise a complex mixture of large gel-forming mucins originating from the goblet cells and submucosal glands, and smaller 'soluble' species from the submucosal glands which may interact with the gel.

A portfolio assessment system has been introduced into a biomedical science programme to promote both continuous learning and deep approaches to learning. Attention has been focused on creating harmony between the assessment system and the PBL curriculum of the programme. Biomedicine and laboratory work are central in the curriculum. The portfolio included evidence of laboratory work, personal reflections and certificates from the PBL tutor. The portfolio was assessed on three occasions over 20 weeks. The grades were 'pass' or 'fail'. The tutor certificate appeared to be a crucial part of the portfolio since a 'fail' in this part usually led to an overall 'fail'. Both students and teachers were concerned about ensuring that enough factual knowledge, as measured by a traditional test, had been achieved. The agreement was good enough for the pass or fail level but some expected differences were found at the detailed level. The course, including the portfolio, was evaluated orally during weekly whole-group meetings and using a questionnaire at the end. The students felt comfortable with the portfolio system and preferred it to a traditional test. The teachers felt that they needed to develop their teacher-student discussion skills and to improve their feedback on the reflections. Peer assessment between students is proposed as a line of action to enhance the credibility of the crucial tutor certificate. The portfolio might be an efficient tool for the students to concentrate their efforts on the most central concepts of medical laboratory work. The model will be developed through further discussions and better consensus among faculty.

In the human trachea, goblet cells in the surface epithelium and mucous cells in the submucosal glands synthesize and secrete mucus glycoproteins (mucins). Hybridization, in situ, has shown that the MUC2, MUC4 and MUCS genes are expressed in the airways [l] and recently it was shown that much subunits from sputum can be separated into two distinct populations with electrophoresis 121. It is not known, however, whether goblet cells and mucous gland cells produce the same or different mucins or, indeed, which mucin is the major constituent of normal airway secretion or of sputum produced in pathological conditions such as asthma, cystic fibrosis and chronic bronchitis. In order to assess to what extent the MUCZ and MUCS mucins are present in the airways, we have immunized rabbits with synthetic peptides with sequences from the non-glycosylated regions of these apoproteins [3,4]. Furthermore, to enrich the secretory product from the goblet cells and the submucosal glands, respectively, mucins were isolated separately from the surface epithelium and the submucosal tissue.Human tracheas were obtained at autopsy and the surface epithelium was removed by scraping with a spatula. Mucins were extracted from the surface epithelium and the remaining submucosal tissue with 6M guanidinium chloride supplemented with protease inhibitors and separated from proteins by using isopycnic density-gradient centrifugation in CsCY4M guanidinium chloride. Mucins were then subjected to a second density-gradient centrifugation step in CsCYOSM guanidinium chloride in order to remove DNA. Fractions were analysed for absorption at 280nm, carbohydrate (PAS assay), sialic acid, density and reactivity against the polyclonal antisera mentioned above. In CsCUOSM guanidinium chloride, mucus glycoproteins banded between 1.43 and 1.50 dml, well separated from material with a strong absorption at 280nm likely to be DNA. The ratio of MUCS reactivity to carbohydrate (PAS assay) or sialic acid was significantly higher for mucins from the surface epithelium than for those from the submucosal tissue. Rate-zonal centrifugation in a 6-8M guanidinium chloride gradient [5] showed that mucins from the surface epithelium and the submucosal tissue had similar size distributions ( Fig. 1 & 2). 0.4 . 8" $ 0.3 B r, 0.2 8 0.1 0.0 3 2 0.3 5 I ; , E 0.2 0.1 0.0 0 5 10 15 20 25 Fig2. Rate zonal centrifugation of whole mucins extracted from the submucosal tissue.Mucins at the smaller size end of the distributions (both from the surface epithelium and the submucosal tissue) showed much stronger reactivity with the MUCS antibody and those from the surface epithelium reacted much more avidly with the antibody than those from the submucosa confirming the observation from the density gradient. A significant population (PAS assay) of larger mucins from the submucosal tissue showed a relatively lower MUC5 reactivity. Little or no reactivity with the MUCZ antibody was found over either of the two size distributions suggesting that the MUCZ gene product is not a prominent secre...