Dissecting carboxypeptidase E: properties, functions and pathophysiological roles in disease

Abstract

Since discovery in 1982, carboxypeptidase E (CPE) has been shown to be involved in the biosynthesis of a wide range of neuropeptides and peptide hormones in endocrine tissues, and in the nervous system. This protein is produced from pro-CPE and exists in soluble and membrane forms. Membrane CPE mediates the targeting of prohormones to the regulated secretory pathway, while soluble CPE acts as an exopeptidase and cleaves C-terminal basic residues from peptide intermediates to generate bioactive peptides. CPE also participates in protein internalization, vesicle transport and regulation of signaling pathways. Therefore, in two types of CPE mutant mice, Cpe^fat/Cpe^fat and Cpe knockout, loss of normal CPE leads to a lot of disorders, including diabetes, hyperproinsulinemia, low bone mineral density and deficits in learning and memory. In addition, the potential roles of CPE and ΔN-CPE, an N-terminal truncated form, in tumorigenesis and diagnosis were also addressed. Herein, we focus on dissecting the pathophysiological roles of CPE in the endocrine and nervous systems, and related diseases.

Keywords: carboxypeptidase E; diabetes; exopeptidase; obesity; regulated secretory pathway.

Figure 1

CPE is a carboxypeptidase that removes C-terminal basic residues of prohormone. (A) The precursor protein of CPE contains a 27 amino acid hydrophobic signal peptide and a short propeptide that ends with five adjacent arginine residues at the N-terminus. The C-terminal 25 amino acid region has an amphiphilic α-helix domain that can be associated with membranes at acidic pH (5.5–6.5) and a cytoplasmic tail of about 6 amino acids that recycle CPE from the plasma membrane to the TGN. Species-conserved Ser₂₀₂ is required for CPE protein folding, while Arg₂₅₅ and Lys₂₆₀ are necessary for interacting with sorting signal domain of POMC. ΔN-CPE, an N-terminus truncated form of CPE, is expressed in mouse embryos from embryonic day 5.5 to postnatal day 1. Unlike full-length CPE, which is present in both embryonic and adult neurons, ΔN-CPE is expressed only in embryonic neurons and acts in the nucleus of cortical neurons to induce gene transcription. ΔN-CPE exhibits neuroprotection during the embryonic neurodevelopment. (B) CPE exists in membrane and soluble forms. 55 kDa membrane form functions as a sorting receptor for targeting prohormones and proneuropeptides to the regulated secretory pathway. However, the 53 kDa soluble form is a processing enzyme that removes C-terminal lysine or arginine residues from neuropeptide intermediates following endopeptidase cleavage to produce biological active peptide hormones and neuropeptides.

Figure 2

CPE serves as a regulated secretory pathway sorting receptor of many peptides including proinsulin, proenkephalin, POMC and BDNF. The process of sorting secretory proteins into the regulated secretory pathway is that the protein binds to a sorting receptor at the Golgi network TGN and is then packaged into dense core secretory granules for secretion. In the TGN, the pH decreases along with elevated Ca²⁺, CPE is activated and some of the CPEs bind to the membranes. Then in mature secretory granules, with an acidic pH of 5–6, CPE is maximally active and a majority of them tightly bind to membranes. Soluble CPE removes the C-terminal basic residues of prohormone for maturation. Finally, when secretory granules fuse with the plasma membrane, the extracellular neutral pH decreases the CPE activity and result in a release of both soluble and membrane CPE we well as hormones. After stimulated secretory granule exocytosis, membrane-bound CPE is recycled from the plasma membrane to the TGN facilitated by a small GTPase ARF6, where CPE is reused.

Figure 3

Both full-length CPE and potential ΔN-CPE regulates signaling pathways. CPE upregulates the expression of anti-apoptotic protein Bcl-2 and inhibits caspase-3 activation. This may be realize through binding of CPE to a specific receptor to initiate the ERK and AKT signaling. CPE also interacts with the Wnt3a and the frizzled receptor to form a complex, thereby disrupting disheveled-induced signalosomes of the Wnt signaling and leading to the degradation of β-catenin. However, ΔN-CPE was suggested to act in the nucleus of cortical neurons to induce gene transcription, increase the expression of β-catenin and FGF2, and activate the ERK/AKT signaling or Wnt pathways.