Seasonal cycle of lysosomal enzyme activities in the mantle tissue and isolated cells from the musselMytilus edulis

Abstract

InMytilus edulis L., gametogenesis takes place in the mantle at the expense of the connective storage tissue. There are two main types of storage cells: vesicular (VC) cells storing large amounts of glycogen and adipogranular (ADG) cells containing large numbers of protein granules, lipid droplets and lesser amounts of glycogen. One of the ways in which stored reserves can be mobilized for gamete formation is by controlled autophagy, in which the cellular constituents are degraded by lysosomes. Mussels were collected from the Menai Strait, North Wales, and monthly measurements made, over two years (1984–1986), of the activities of lysosomal acid hydrolases (acid phosphatase,β-N-acetyl-D-glucosaminidase andβ-glucuronidase) and Cathepsins B and L in the mantle tissue, isolated ADG cells, low-density cells and, during spawning, in the mature oocytes of female mussels. The lysosomal proteinases, Cathepsins B, L and H, were further characterised by activation with thiol compounds and inhibition with thiol blockers and by leupeptin. Because of the low activity in the mantle tissue ofM. edulis, Cathepsin H was not assayed on a seasonal basis. There was a general increase in lysosomal enzyme activity during the winter, which can be related to increased autolysis in the storage cells and to the process of maturation in the developing oocytes. The activity of Cathepsin B was highest in the ADG and low-density cells, implying an important role in proteolysis within the ADG cells. By contrast, Cathepsin L displayed the highest activity in the mature oocytes, suggesting a major function of Cathepsin L in the development and maturation of the oocytes. Two differentα-glucosidase activities were measured in the monthly assays, one with a pH optimum of 4.5 (acid) and the other at pH 7.5 (neutral). Highest activities of the acidα-glycosidase were found in the low-density cells, but there were no significant seasonal changes in the mantle tissue as a whole. Activities of the neutralα-glucosidase were low in the ADG cells and mature oocytes, but showed high activities in the mantle tissue, with marked seasonal changes that corresponded to the mobilization of glycogen reserves in the VC cells.