Our goals are to understand the mechanism of growth regulation and utilize hormones/proteins involved in growth regulation as indices to evaluate growth status. Our focus is on insulin-like growth factor (IGF)-I and its binding proteins.
Growth of fish is influenced by various environmental factors such as photoperiod, water temperature, feeding ration and stress. Such stimuli are integrated by the brain and hormones regulate growth. Growth hormone (GH) and insulin-like growth factor (IGF)-I are two major hormones regulating growth. GH stimulate growth directly acting on target tissues such as muscle and bone or indirectly through stimulating hepatic production of IGF-I. This system is called “the GH-IGF-I system”. Circulating IGF-I mediates action of GH and has a negative feedback effect on the secretion of GH from the pituitary gland.
Most circulating IGF-I is bound to one of six IGF-binding proteins (IGFBPs). IGFBPs can inhibit or stimulate the activity of IGF-I depending on the type, post-translational modification and microenvironment. Thus, IGFBPs play a key role in modulating growth. Teleosts have up to 12 IGFBP subtypes due to the third-round whole genome duplication in this lineage. In addition, salmonids are tetraploid origin and thus have 22 subtypes.
Goal: We are interested in unraveling functions of IGF-I and IGFBPs in salmonids and utilize them as indices of growth. Availability of growth indices is useful to improve aquaculture and resource enhancement.
IGFBPs in salmon blood and their functions
Three major IGFBPs are detected in serum/plasma of salmonids. They are believed to inhibit or/and promote action of IGF-I.
Functional analysis using recombinant IGFBP-1a and -1b revealed that IGFBP-1a inhibited the action of IGF-I on GH release from the pituitary gland.
Relationships of circulating IGF-I, IGFBP-2b and IGFBP-1b with growth rate
Circulating IGF-I and IGFBP-2b positively correlated with individual growth rate whereas a negative correlation was seen between IGFBP-1b and growth rate. These results suggest that they are useful as growth indices.
Evaluation of growth status by IGF-I of juvenile chum salmon along the coast
Growth status of out-migrating juvenile chum salmon was monitored by using serum IGF-I as a growth index. The results suggest that fish under poor growth condition stayed in the estuary and might suffer growth-dependent mortality thereafter.
- Produce more recombinant IGFBP subtypes for functional analyses and unravel their roles in growth regulation.
- Establish a diagnosis method by using IGF-I and IGFBPs as growth/stress indices. Availability of such method is useful to evaluate rearing conditions of farmed fish and growth status of free-swimming fish.