Research reportSerine/threonine-kinase 33 (Stk33) - Component of the neuroendocrine network?

- Serine/threonine-kinase 33 (Stk33) is expressed in diencephalic neurons.
- Partially colocalised with ADH, OT or nNOS in neuroendocrine hypothalamus.
- Also expressed in tree shrew, baboon, and human brain.
- Stk33 is supposed component of the neuroendocrine network.

The present study was conducted to investigate the expression of serine/threonine-kinase 33 (Stk33) in neuronal structures of the central nervous system in rat and hamster as well as the presence of the protein in the brain of higher mammals, using a polyclonal antibody on cryosections of fixed brains. We found a distinct immunostaining pattern that included intense fluorescence of the ependymal lining of cerebral ventricles, and of hypothalamic tanycytes and their processes. We further observed intense staining of magnocellular neurons in the hypothalamic paraventricular, supraoptic and accessory neurosecretory nuclei, in particular the circular nuclei, and less intense stained neurons in other diencephalic regions. Double-immunostaining experiments showed a partial colocalization of Stk33 with arginine-vasopressin, oxytocin or neuronal nitric oxide-synthase in a large number of neurons of the hypothalamic nuclear regions. Colocalization of Stk33 with substance P or the catecholamine-synthesizing enzyme tyrosine-hydroxylase was not observed. Immunofluorescence was not found in autonomic regions of the lateral horn, suggesting that Stk33 does not contribute to hypothalamo-spinal connections. However, large Stk33-immunoreactive axonal projections from magnocellular hypothalamus to the neurohypophysis were evident. These functionally important connections provide the bridge from neuronal to humoral regulation of the endocrine system. Additionally, Western blots from mouse brain showed two distinct bands representing two Stk33 isoforms. We also present first evidence for the presence of Stk33/STK33 in neuronal structures, ependymal cells and tanycytes in tree shrew, baboon, and human brain.