Sorting and secretion of salivary proteins

Electron microscopic immunogold localization of statherin in human minor salivary glands

In this study, which supplements a recent article on the localization of statherin in human major salivary glands, we investigated the intracellular distribution of this peptide in minor salivary glands by immunogold cytochemistry at the electron microscopy level. In the lingual serous glands of von Ebner, gold particles were found in serous granules of all secreting cells, indicating that statherin is released through granule exocytosis. In buccal and labial glands, mostly composed of mucous tubuli, statherin reactivity was detected in the serous element, which represents only a small population of the glandular parenchyma. In these serous cells, however, statherin labeling was absent in secretory granules and restricted to small cytoplasmic vesicles near or partially fused with granules. Vesicle labeling could be related to the occurrence of an alternative secretory pathway for statherin in buccal and labial glands.

Hypo-phosphorylation of salivary peptidome as a clue to the molecular pathogenesis of autism spectrum disorders

RP-HPLC-ESI-MS profile of naturally occurring salivary peptides of subjects with autistic spectrum disorder [ASD; N = 27:12 with diagnosis of autism, 1 with diagnosis of Asperger, 14 with diagnosis of pervasive developmental disorders not otherwise specified (PDD-NOS)] was compared to that of age-matched controls with the goal of identifying differences that could turn out to become hallmarks of at least a subgroup of ASD individuals. Phosphorylation level of four specific salivary phospho-peptides, namely statherin, histatin 1 (both, p < 0.0001) and acidic proline-rich proteins (both entire and truncated isoforms) (p < 0.005) was found significantly lower in autistic patients, with hypo-phosphorylation of at least one peptide observed in 18 ASD subjects (66%). Developmental scale assessment (Griffith or WISC-R) carried out on 14 ASD subjects highlighted a normal to borderline cognitive development in 10 of them, all included in the hypo-phosphorylated group. Phosphorylation of salivary peptides involves a Golgi casein kinase common to many organs and tissues, CNS included, whose expression seems to be synchronized during fetal development. Hypo-phosphorylation of salivary peptides suggests potential asynchronies in the phosphorylation of other secretory proteins, which could be relevant in CNS development either during embryonic development or in early infancy. These results suggest that analysis of salivary phospho-peptides might help to discriminate a considerable subgroup of ASD patients.

Electron microscopic detection of statherin in secretory granules of human major salivary glands

In order to increase current knowledge regarding statherin secretion into the oral cavity, ultrastructural localization of this peptide was investigated in human salivary glands by using a post-embedding immunogold staining technique. Statherin reactivity was found inside the granules of serous cells of parotid and submandibular glands. In parotid granules immunostaining was preferentially present in the less electron-dense region, whereas in submandibular serous granules the reactivity was uniform and the dense core always stained. By contrast, none or weak reactivity was observed in serous cells of major sublingual glands. These findings reveal for the first time the subcellular localization of statherin by electron transmission microscopy and confirm that of the three major types of salivary glands, the parotid and submandibular glands are the greatest source of salivary statherin. Moreover, they suggest that more than one packaging mechanism may be involved in the storage of statherin within serous granules of salivary glands.

Isoproterenol increases sorting of parotid gland cargo proteins to the basolateral pathway

Exocrine cells have an essential function of sorting secreted proteins into the correct secretory pathway. A clear understanding of sorting in salivary glands would contribute to the correct targeting of therapeutic transgenes. The present work investigated whether there is a change in the relative proportions of basic proline-rich protein (PRP) and acidic PRPs in secretory granules in response to chronic isoproterenol treatment, and whether this alters the sorting of endogenous cargo proteins. Immunoblot analysis of secretory granules from rat parotids found a large increase of basic PRP over acidic PRPs in response to chronic isoproterenol treatment. Pulse chase experiments demonstrated that isoproterenol also decreased regulated secretion of newly synthesized secretory proteins, including PRPs, amylase and parotid secretory protein. This decreased efficiency of the apical regulated pathway may be mediated by alkalization of the secretory granules since it was reversed by treatment with mild acid. We also investigated changes in secretion through the basolateral (endocrine) pathways. A significant increase in parotid secretory protein and salivary amylase was detected in sera of isoproterenol-treated animals, suggesting increased routing of the regulated secretory proteins to the basolateral pathway. These studies demonstrate that shifts of endogenous proteins can modulate regulated secretion and sorting of cargo proteins.

XBP-1 is required for biogenesis of cellular secretory machinery of exocrine glands

The secretory function of cells relies on the capacity of the endoplasmic reticulum (ER) to fold and modify nascent polypeptides and to synthesize phospholipids for the subsequent trafficking of secretory proteins through the ER-Golgi network. We have previously demonstrated that the transcription factor XBP-1 activates the expression of certain ER chaperone genes and initiates ER biogenesis. Here, we have rescued the embryonic lethality of XBP-1 deficient fetuses by targeting an XBP-1 transgene selectively to hepatocytes (XBP-1-/-;LivXBP1). XBP-1-/-;LivXBP1 mice displayed abnormalities exclusively in secretory organs such as exocrine pancreas and salivary gland that led to early postnatal lethality from impaired production of pancreatic digestive enzymes. The ER was poorly developed in pancreatic and salivary gland acinar cells, accompanied by decreased expression of ER chaperone genes. Marked apoptosis of pancreatic acinar cells was observed during embryogenesis. Thus, the absence of XBP-1 results in an imbalance between the cargo load on the ER and its capacity to handle it, leading to the activation of ER stress-mediated proapoptotic pathways. These data lead us to propose that XBP-1 is both necessary and sufficient for the full biogenesis of the secretory machinery in exocrine cells.

Effect of human vasoactive intestinal peptide gene transfer in a murine model of Sjogren's syndrome

BACKGROUND

Sjögren's syndrome (SS), an autoimmune exocrinopathy mainly affecting lachrymal and salivary glands, results in ocular and oral dryness (keratoconjunctivitis sicca and xerostomia). The aetiology and pathogenesis are largely unknown; currently, only palliative treatment is available.

OBJECTIVE

To determine whether gene transfer of vasoactive intestinal peptide (VIP), based on its immunomodulatory properties, might be useful in the management of SS.

METHODS

A recombinant serotype 2 adeno-associated virus encoding the human VIP transgene (rAAV2hVIP) was constructed and its efficacy tested in the female non-obese diabetic (NOD) mouse model for SS after retrograde instillation in submandibular glands (SMGs). 10(10) particles/gland of rAAV2hVIP or rAAV2LacZ (encoding beta-galactosidase; control vector) were administered at 8 weeks of age (before sialadenitis onset). Salivary flow rates were determined before vector delivery and at time of death (16 weeks). After death, saliva, serum, and SMGs were harvested. Salivary output, inflammatory infiltrates (focus scores), VIP protein expression, cytokine profile, and serum anti-VIP antibodies were analysed.

RESULTS

rAAV2hVIP significantly improved the salivary flow, increased SMG and serum expression of VIP, and reduced SMG cytokines interleukin (IL) 2, IL10, IL12 (p70), and tumour necrosis factor alpha, and serum RANTES, compared with the control vector. No difference in focus scores or apoptotic rates was found; neutralising antibodies were not detected.

CONCLUSIONS

Local delivery of rAAV2hVIP can have disease modifying and immunosuppressive effects in SMGs of the NOD mouse model of SS. The new strategy of employing VIP prophylactically may be useful for both understanding and managing the salivary component of SS.

Sorting of an exocrine secretory protein to the regulated secretory pathway in endocrine cells

Regulated secretory proteins are stored in secretory granules. While the sorting and storage process appears similar in endocrine and exocrine cells, the extent of overlap of sorting between endocrine and exocrine cell types is not clear. It is predicted that exocrine regulated secretory proteins that are stored with high efficiency in exocrine granules would also be stored efficiently in endocrine granules. To test this hypothesis, parotid secretory protein (PSP), which is stored efficiently in parotid acinar cells, was expressed in the endocrine cell lines GH4C1 and PC12. PSP undergoes stimulated secretion in both cell types. Secretion is similar to that of the endocrine regulated secretory protein chromogranin A but distinct from secreted alkaline phosphatase, a marker for the constitutive secretory pathway in endocrine cells. Subcellular fractionation of GH4C1 cells revealed that PSP co-fractionates with chromogranin A but not with secreted alkaline phosphatase.

A sulfated proteoglycan is necessary for storage of exocrine secretory proteins in the rat parotid gland

Sulfated proteoglycans have been proposed to play a role in the sorting and storage of secretory proteins in exocrine secretory granules. Rat parotid acinar cells expressed a 40- to 60-kDa proteoglycan that was stored in secretory granules. Treatment of the tissue with the proteoglycan synthesis inhibitor paranitrophenyl xyloside resulted in the complete abrogation of the sulfated proteoglycan. Pulse-chase experiments in the presence of the xyloside analog showed a significant reduction in the stimulated secretion and granule storage of the newly synthesized regulated secretory proteins amylase and parotid secretory protein. Inhibition of proteoglycan sulfation by chlorate did not affect the sorting of these proteins. The effect of proteoglycan synthesis inhibition on protein sorting was completely reversed upon treatment with a weak acid. These results suggest that the sulfated proteoglycan is necessary for sorting and storage of regulated secretory proteins in the exocrine parotid gland. Preliminary evidence suggests that the mechanism involves the modulation of granule pH by the proteoglycan rather than a direct interaction with other granule components.

High levels of GM(1)-ganglioside and GM(1)-ganglioside beta-galactosidase in the parotid gland: a new model for secretory mechanisms of the parotid gland

A new model for the subcellular basis of parotid secretion is presented in this article. GM(1)-ganglioside, typically found in neural tissues, is shown to be abundant in the parotid gland. This ganglioside may play a central role in membrane turnover mechanisms underlying exocytosis/endocytosis in its role as a promoter of membrane fusion or a fusogen. The lysosome and lysosomal hydrolases also play a central role in this model in catabolism of GM(1)-ganglioside. Consequently, high levels of the lysosomal hydrolase acidic beta-galactosidase are demonstrated in the salivary gland. GM(1)-gangliosidosis of the parotid glands, as described in mice, appears to be the first single-gene heritable disease found so far in the salivary glands.

Localization of salivary proteins in granules of human parotid and submandibular acinar cells

Electron microscopic immunocytochemistry was used to investigate the distribution of four secretory proteins within the granules of human parotid and submandibular gland acinar cells. The results demonstrate that the distribution of the individual proteins differs significantly. Amylase is concentrated within the electron-dense regions of the granules, whereas agglutinin is present mainly in the regions of lower density. In contrast, the proline-rich proteins and histatins are distributed throughout the content of the granules and do not appear to be concentrated in either the high- or low-density regions. The results also suggest that different mechanisms, such as self-aggregation or ionic complexes with metal ions and charged macromolecules, may be involved in the packaging of individual proteins within the same granule. Further, they also show that among the salivary glands of various species, the intragranular distribution of similar proteins (e.g., amylase and proline-rich proteins) may differ.