Co-storage and secretion of growth hormone and secretoneurin in retinal ganglion cells

• GH and secretoneurin (SN) are co-stored in retinal ganglion cells (RGCs).
• Intracellular colocalization of GH and SN was in electrodense bodies of the cells.
• Secretoneurin was constitutively released from QNR/D RGCs but not induced by GHRH.
• GH was also colocalized with SNAP-25 in QNR/Ds, supporting its vesicle secretion.

It is well established that growth hormone (GH) and granins are co-stored and co-secreted from pituitary somatotrophs. In this work we demonstrate for the first time that GH- and secretoneurin (SN) immunoreactivity (the secretogranin II (SgII) fragment) are similarly present in retinal ganglion cells (RGCs), which is an extrapituitary site of GH expression, and in quail QNR/D cells, which provide an experimental RGC model. The expression of SgII and chromogranin A in the pituitary gland, neuroretina and QNR/D cells was confirmed by RT-PCR analysis. Western blotting also showed that the SN-immunoreactivity in somatotrophs and QNR/D cells was associated with multiple protein bands (24, 35, 48, 72, 78, 93 and 148 kDa) of which the 72 kDa and 148 kDa bands were most abundant. Secretoneurin was constitutively secreted from QNR/D cells as 35 kDa and 37 kDa proteins and unlike GH, was not increased by exogenous GH-releasing hormone (GHRH). Intracellular analysis by EM showed co-localization of GH and SN in cell bodies and neurites in QNR/D cells. This co-localization was associated with small dark bodies in the neurites. In addition, co-localization of GH and SNAP-25 in the cell surface of QNR/D’s plasma membranes suggests GH-release involves specific vesicle-membrane recognition in QNR/D cells. As SN is a marker for secretory granules, GH secretion from RGCs is thus likely to be in secretory granules, as in somatotrophs.