Commentary on: Peptidylglycine ?-amidating Monooxygenase is Required for Atrial Secretory Granule Formation

Figures

• 

  Figure 1:. A. Transmission electron microscopic images highlight the secretory granules (SG) that accumulate near the Golgi (G) complex in atrial, but not in ventricular, myocytes. In both atrial and ventricular myocytes, the Golgi complex abuts the poles of the nucleus (N) and mitochondria (M) are prevalent. B. ARF proteins are active when GTP is bound and inactive when GDP is bound; ARF GDP/GTP Exchange Proteins (GEFs) and ARF-GTPase Activating Proteins (GAPS) control their activation/inactivation. Brefeldin A (BFA) and Golgicide A (GCA) inhibit selected Golgi-localized ARF-GEFs. C. GBF1, the only ARF-GEF localized to the cis-Golgi is inhibited by both BFA and GCA; BIG1 and BIG2, ARF-GEFs localized to the trans-Golgi and endocytic pathway are inhibited by BFA, but not by GCA.

• 

  Figure 2:. PAM knockout mice. The reaction catalyzed by PAM is outlined (A), along with the major form of PAM in humans (B) and its two enzymatic domains, PHM and PAL (peptidylglycine a-hydroxylating monooxygenase and peptidyl-a-hydroxyglycine a-amidating lyase; E.C. 1.4.17.3 and 4.3.2.5). C. Transmission electron micrographs of the PamMyh6-cKO/cKO adult atrium; secretory granules are rarely seen near the peri-nuclear Golgi or elsewhere. Quantitative data comparing the number of Golgi-localized secretory granules observed in control vs. PamMyh6-cKO/cKO atria revealed a nearly 20-fold decrease (Figure 1A). D. Immunoblot analyses of adult atria established near-total ablation of PAM and major loss of proANP and proBNP in PamMyh6-cKO/cKO atrium. Data replotted from [13].

• 

  Figure 3.. Necessary and sufficient. Experimental strategies used to demonstrate the essential role played by the PAM protein in proANP storage (A) and rescue of proANP storage in atrial myocytes lacking PAM (PamMyh6-cKO/cKO) in response to expression of exogenous PAM; surprisingly, even PAM in which the mutation of a single active site residue eliminated its monooxygenase activity (PAM/Met314Ile) was still capable of restoring proANP storage. (B). Data reported in [13].

• 

  Figure 4:. Proposed mechanism. A. The effects of BFA and GCA on the basal secretion of proANP by primary cultures of control and PamMyh6-cKO/cKO atrial myocytes was assessed. B. In our working model, the luminal domains of PAM bind newly synthesized proANP, facilitating its delivery to the cis-Golgi; PAM/Met314Ile can perform this same function. C. The delivery of proANP to nascent granules is facilitated by PAM; the return of PAM to the ER for re-use requires GBF1. In the absence of PAM, the basal secretion of proANP increases three-fold. Data from [13] depicted in diagrammatic form.