Age-related changes in EGF and protease in submandibular glands of C57BL/6J mice

Structural and functional changes in salivary glands during aging

Various salivary glands in senescent humans and other animals have been examined extensively to characterize the structural and functional changes that occur during aging. Although a wide range of different structural changes, involving both the parenchymal and stromal tissues, have been described, it is unclear how any of these changes affects the function of the salivary glands. One major change in structure is the reduction in the volume of acini with a concomitant increase in the ductal volume. Despite this loss of functional acini, the salivary output and the contents seem to be unaltered, or minimally altered, due to aging. One consistent change observed in many salivary glands of aged animals is the decline in the rate of synthesis of proteins and their messenger RNA (mRNA). However, the salivary acinar cells from aged animals can synthesize secretory proteins at an elevated rate just as effectively as those from their younger counterparts in response to external stimuli, which are known to enhance the rate of protein synthesis. Thus, it appears that the salivary acinar cells, which remain structurally intact during aging, seem to retain their functional efficiency. Furthermore, these acinar cells, although reduced in number, are sufficient in quantity to carry out most of the salivary gland functions.

The granular convoluted tubule (GCT) cell of rodent submandibular glands

The granular convoluted tubule (GCT) is a segment of the duct system of all rodents, situated between the striated and intercalated ducts. It has the peculiar property of synthesizing a large variety of biologically active polypeptides whose role in saliva remains unknown. The literature on the fine structure of GCT cells is critically reviewed. Some recent developments on endocrine regulation of the structure and contents of rodent GCT cells are summarized, with emphasis on EGF, NGF, renin, and kallikrein proteases. A survey of the distribution of GCT cells in several vertebrate families is presented.

A biochemical analysis of parotid and submandibular salivary gland function with age after simultaneous stimulation with pilocarpine and isoproterenol in female NIA Fischer 344 rats

This analysis of physiological, biochemical and molecular changes related to aging was made in 3-, 12- and 24-month-old rats. The salivary gland weight/body weight ratio and the structural membrane proteins did not change with age for either gland, but a significant age-related decline in DNA synthesis for both glands was detected, unrelated to the hormonal responsiveness at the level of the plasma membrane. There was a marked increase in the concentration of soluble proteins in adolescent parotid gland and, for the two older age groups, in submandibular gland. The saliva flow rate was different when expressed as volume per time, as volume per time and g glandular wet weight, and/or kg body weight. The concentration of secreted proteins was not affected by age in either gland. The total amount of proteins secreted over 30 min revealed no age-related perturbation for the parotid gland, but showed a significant age-related increase in submandibular saliva. Sodium dodecyl sulphate-polyacrylamide gel analysis revealed changes in the protein bands between 39 and 50 kDa in the Coomassie blue-stained gels from 12-month-old animals. Amylase showed an initial increase (12 months), followed by a marked decline in its activity in parotid saliva. The glandular supernatant had low residual cellular amylase activity after stimulation. Therefore, secretory impairment with age after pilocarpine-isoproterenol stimulation was excluded. Analysis of total RNA showed a pronounced decrease of amylase mRNA in the parotid gland between 12 and 24 months of age. No amylase mRNA was expressed in any of the submandibular samples. For epidermal growth factor, total saliva showed a decrease with age. It seemed that the submandibular gland followed the same picture with age as the parotid gland, with a specific decline in the biosynthesis of single secretory proteins.

Changes with senescence in the fine structure of the granular convoluted tubule of the submandibular gland of the mouse

Cells of the granular convoluted tubules (GCTs) of the submandibular gland of senescent male mice show structural changes indicative of functional decline. In order to define the nature of these age-related changes more clearly, the fine structure of GCT cells of 12- and 28-month-old males was compared. In old mice, there was cell-to-cell variation in the extent of these changes, with some cells of senescent males appearing no different from those of young adults. In affected cells the most striking alterations were seen in secretion granules and lysosomal elements. Secretion granules varied greatly in size, with some GCT cells having only very fine apical granules. Secondary lysosomes and large lipofuscin granules were frequent in the basal cytoplasm. Very large dense bodies (3-5 micron) occurred in many cells. These possibly represent intracellular pools of released secretory materials, as they were occasionally seen in continuity with the luminal contents. Structures whose appearance was intermediate between the very large dense bodies and lipofuscin granules were common, suggesting crinophagic activity. There was an apparent decrease in numbers of polysomes and in the extent of the Golgi apparatus. These fine structural changes are consistent with impairments with advanced age in synthesis and posttranslational processing of secretory products by affected GCT cells. In addition to cell-to-cell variation in any one male, there was also interanimal variation in the degree and extent of these senescent changes.

Use of tissue culture models to study neuronal regulatory trophic and toxic factors in the aged brain

Dementia is believed to result from the loss of selective neurons within the brain, but approaches for systematic study of that degenerative process are hampered by the complexity of the neuronal milieu. Tissue culture models provide a means to reduce dramatically the variables inherent in the study of neuronal plasticity. Three levels of complexity can be described: cellular and molecular diversity; primary and secondary interconnections; and finally, the dynamics influenced by age. The following review discusses the advantages and disadvantages of tissue culture models for the detailed study of neuronal trophic and toxic factors. Our selection of factors is broadened to include ions, intermediate metabolites, antioxidants, steroids, neuropeptides, gangliosides, metals, neurotransmitters, brain extracts, and protein molecules. Most of these factors have been shown to be altered in the aged brain, to have a significant effect on cultured neurons, or both. This multilevel analysis provides the reader with an overview of the events regulating neuronal survival, differentiation and death. An understanding of these basic questions is necessary to sequence the molecular events resulting in neuronal death.

Effect of advanced age on the induction by androgen or thyroid hormone of epidermal growth factor and epidermal growth factor mRNA in the submandibular glands of C57BL/6 male mice

We have compared the responsiveness of the submandibular glands of mature (12 month old) and senescent (26-28 month old) male C57BL/6 mice to dihydrotestosterone (DHT) or triiodothyronine (T3) in terms of steady state levels of epidermal growth factor (EGF) protein and EGF mRNA. Northern blot analyses did not disclose any differences with age in the apparent sizes of EGF mRNA species. In untreated animals, submandibular glands of 26-28-month-old mice contained approximately 50% less EGF, and 75% less EGF mRNA than those of 12-month-old males. With advanced age, there was a 20% reduction in the absolute volume of the granular convoluted tubule (GCT) compartment, which is the exclusive site of EGF and EGF mRNA in the gland. In general, GCTs of old mice were composed of smaller cells with fewer secretion granules, but there was considerable cell-to-cell variation. In addition, there was greater variation in the intensity of immunocytochemical staining for EGF in senescent GCT cells, which also gave a lower and more variable in situ hybridization signal for EGF mRNA. After hormonal stimulation for 1 week with either tri-iodothyronine (T3) or dihydrotestosterone (DHT), EGF protein concentration in the glands was induced to the same level at both ages. However, EGF mRNA was 50% less abundant in old hormonally stimulated glands, compared to similarly treated young ones. Although many GCT cells in treated glands of senescent males respond to hormonal stimulation by increases in size and in content of secretion granules, there was cell-to-cell variation in responsiveness, especially after treatment with T3. These findings indicate that the decreases seen in the entire gland in EGF and EGF mRNA are caused by a wide-spread deterioration of the GCT cells themselves, which apparently can be reversed in many but not all GCT cells by stimulation with supraphysiologic doses of either T3 or DHT.

Changes during aging in rat lens endopeptidase activity

Protein degradation has been implicated in lens aging and cataract development. We propose that proteolytic enzymes are important in these processes. In this study, lens neutral proteinase activity and thermal stability have been measured as a function of cell age and animal age in rat lens. Epithelial, cortical, and nuclear lens regions from animals between 1 and 25 months were analyzed. Specific activity and thermal stability were found to decrease with lens cell age, that is, from epithelium to nucleus. Specific activity in the cortex increased with animal age, while specific activity in the nuclear region remained constant with animal age. In the epithelium, specific activity showed no correlation with animal age. In 15 day old rats, thermal stability curves for the cortical enzyme were linear, indicating only one form of activity hydrolyzing synthetic substrate was present. At 1.25 months of age, thermal stability curves for the cortical enzyme were non-linear indicating that multiple forms of the activity were present.