Mobilization of fat body glycogen and haemolymph trehalose under nutritional stress in Bombyx mori larvae in relation to their physiological age and the duration of food deprivation

Abstract

In insects, fat body glycogen and haemolymph trehalose are principal carbohydrate reserves. Here, in Bombyx mori (Linnaeus, 1758), we report an accelerated fat body glycogen accumulation in feeding 5th instar larvae, when they reach the terminal growth period (TGP). Haemolymph trehalose and glucose levels, however, fluctuated between 7–21 mM and 0.8–10 mM, respectively, depending on the larval physiological age. Food deprivation triggered a dramatic increase in mobilization of glycogen in larvae at TGP than at active growth period (AGP), mobilized almost 78% of stored glycogen at an initial 12 h of food deprivation, possibly by increasing the percentage of active glycogen phosphorylase, as observed. This rapid glycogen mobilization, along with meeting the energy demand of starving tissues, might also have provided glucose for trehalose synthesis, as evident by observed hypertrehalosemia. A prolonged food deprivation (24–48 h) triggered hypotrehalosemia along with an increase in the enzyme activity of trehalase, and the transcript level of trehalase-2, indicating the utilization of trehalose as an energy source for the demanding tissues. Further, in TGP larvae, a prolonged food deprivation increased the level of haemolymph glucose (1.8–3.5 mM) as well as glucose/trehalose ratio, and this suggests that the fat body might have released the glucose molecules directly into the haemolymph, instead of utilizing it for trehalose synthesis, possibly to minimize the expenditure of energy. Thus, short-term food deprivation triggered an initial glycogen mobilization whereas, prolonged food deprivation led to the utilization of trehalose as an energy source. Hence, mobilization of carbohydrate reserves varied depending on the larval physiological age and the extent of food deprivation period. The future investigation can expand our understanding on the role of glucose as an important haemolymph sugar along with trehalose.