AZ Street in Vasopressin Synthesis.

Abstract

The antizyme (AZ) protein binds to and mediates the degradation of ornithine decarboxylase (ODC), the rate-limiting enzyme responsible for the conversion of Lornithine to putrescine. Biosynthetic polyamines (spermidine and spermine) formed from putrescine are present in all living cells. ODC degradation can be prevented by AZ inhibitors (Azin1 or Azin2), which bind to AZ to form a stable complex. Therefore, relationships between ODC, AZ, and Azin proteins are important in the regulation of polyamine synthesis. Polyamines are known to be multifunctional molecules that have also been implicated in the modulation of the gene expression (1). A recent study (2) has used a transcriptomic strategy and multiple other approaches including lentiviral vector techniques to show, for the first time, that polyamines are involved in the regulation of the arginine vasopressin (AVP) gene expression. Greenwood et al (2) demonstrated that Azin1, as well as AZ1, ODC, and polyamines (spermidine and spermine), are abundant in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) of the hypothalamus in rats. The hypothalamo-neurohypophyseal system is one of the most widely studied neuroendocrine model systems (3). The magnocellular neurosecretory cells of the SON and PVN project their axon terminals to the posterior pituitary from which they secrete their products, AVP and oxytocin (OT), into the systemic circulation. AVP, also called antidiuretic hormone, acts on AVP receptor 2 in the kidney to promote reabsorption of water via aquaporin 2. Greenwood et al (2) found that Azin1 mRNA levels were significantly increased in the SON and the PVN after dehydration and salt loading [drinking of 2% (wt/vol) NaCl solution], referred to as chronic hyperosmotic stressors. They also showed that acute osmotic challenge (ip administration of hypertonic saline) caused a rapid increase of Azin1 mRNA as well as AVP heteronuclear (hn) RNA levels in the SON and the PVN. AVP hnRNA levels reflect theactivationof AVPgene transcription in theSON and PVN after acute osmotic stimulation (4). These observations raised the possibility that polyamines may be involved in controlling AVP gene expression and hence body fluid balance in vivo. To test this hypothesis, Greenwood et al (2) designed in vivo experiments to study whether Azin1 overexpression or knockdown, using lentiviral vectors, could change the expression of the AVP gene in the SON and/or PVN. Thus, they showed that Azin1 overexpression in the SON caused a significant decrease in AVP mRNA levels. In contrast, short hairpin RNA-mediated knockdown of Azin1 significantly increased AVP mRNA levels in the SON and decreased plasma osmolality because of activated AVP synthesis and elevated secretion of AVP into the bloodstream. In an ex vivo study (organotypic hypothalamic culture), they clearly confirmed that ODC inhibition (blockade of putrescine synthesis from L-ornithine) caused a significant increase of AVP hnRNA but not OT hnRNA. It is very interesting that the effect of the ODC inhibitor on the AVP gene expression is so specific. This study used a transcriptomic strategy to identify significant genes from all of the expressed genes in the specific region of the brain after a physiological manipulation. This strategy is a very useful and effective way to find novel molecules that may regulate the target gene expression. The targetmoleculesdescribedbyMurphyandhis colleagues (2)are AVP and OT. Despite being cloned in early 1980s, it still is unclear which molecules regulate the expressions of these genes under physiological manipulations.