Male NOD mouse external lacrimal glands exhibit profound changes in the exocytotic pathway early in postnatal development

The lacrimal glands of male NOD mice exhibit many of the features of the human lacrimal gland in patients afflicted with the autoimmune disease, Sjögren's syndrome, including loss of secretory functions and lymphocytic infiltration into the lacrimal gland. To elucidate the early changes in the secretory pathway associated with development of Sjögren's syndrome, we investigated the organization of the exocytotic pathway in lacrimal glands of age-matched male BALB/c and NOD mice. Cryosections from lacrimal glands from 1 and 4 month male BALB/c and NOD mice were processed for confocal fluorescence and electron microscopic evaluation of different participants in exocytosis. No changes in apical actin filaments were noted in glands from NOD mice, but these glands exhibited thickening of basolateral actin relative to that seen in the BALB/c mice. Rab3D immunofluorescence associated with mature secretory vesicles was distributed abundantly in a continuous vesicular network concentrated beneath the apical plasma membrane in glands from 1 and 4 month BALB/c mice. In glands from 1 month NOD mice, rab3D immunofluorescence exhibited marked discontinuity and irregularity in the vesicular labeling pattern. While this change was also detected in glands from 4 month NOD mice, many of these glands exhibited an additional extension of rab3D labeling through the cell to the basolateral membrane. Electron microscopic analysis confirmed the formation of irregularly shaped, unusually large secretory vesicles in lacrimal glands from NOD mice. Quantitation of multiple secretory vesicles from electron micrographs revealed a significant (p≤0.05) increase in the percentage of secretory vesicles incorporated into multivesicular aggregates in lacrimal glands from 1 and 4 month NOD mice compared to BALB/c mice. The M3 muscarinic receptor, a key signaling effector of exocytosis, was redistributed away from its normally basolateral locale in glands from BALB/c mice, with concomitant enrichment in intracellular aggregates in glands from NOD mice. These findings show that lacrimal glands in NOD mice as young as 1 month contain aberrant secretory vesicles with altered effector composition that undergo premature cytoplasmic fusion, and that changes in the distribution of the M3 muscarinic receptor occur within the same time frame.