Pro-opiomelanocortin (POMC) neurons form an integral part of the central melanocortin system regulating food intake and energy expenditure. Genetic and pharmacological studies have revealed that defects in POMC synthesis, processing, and receptor signaling lead to obesity. It is well established that POMC is extensively processed by a series of enzymes, including prohormone convertases PC1/3 and PC2, and that genetic insufficiency of both PC1/3 and POMC is strongly associated with obesity risk. However, whether PC1/3-mediated POMC processing is absolutely tied to body weight regulation is not known. To investigate this question, we generated a Pomc-CreER  T2; Pcsk1  lox/lox mouse model in which Pcsk1 is specifically and temporally knocked out in POMC-expressing cells of adult mice by injecting tamoxifen at eight weeks of age. We then measured the impact of Pcsk1 deletion on POMC cleavage to ACTH and α-MSH, and on body weight. In whole pituitary, POMC cleavage was significantly impacted by the loss of Pcsk1, while hypothalamic POMC-derived peptide levels remained similar in all genotypes. However, intact POMC levels were greatly elevated in Pomc-CreER  T2; Pcsk1  lox/lox mice. Males expressed two-fold greater levels of pituitary PC1/3 protein than females, consistent with their increased POMC cleavage. Past studies show that mice with germline removal of PC1/3 do not develop obesity, while mice expressing mutant PC1/3 forms do develop obesity. We conclude that obesity pathways are not disrupted by PC1/3 loss solely in POMC-expressing cells, further disfavoring the idea that alterations in POMC processing underlie obesity in PCSK1 deficiency.

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A striking number of SNPs and rare mutations have been identified in PCSK1, the gene that codes for the enzyme proprotein convertase 1/3 (PC1/3) which proteolytically activates prohormones within the secretory pathway. All infants bearing two copies of catalytically inactivating mutations, including G209R, exhibit severe neonatal malabsorption requiring parenteral nutrition for months and subsequently develop additional endocrinopathies, often including diabetes and obesity. In order to create a mouse model to explore the underlying mechanism of the malabsorption phenomenon and the endocrinopathies, a G209R point mutation was introduced into exon 6 of mouse Pcsk1 using CRISPR-Cas9 genome editing. Fifty-six live pups were collected at postnatal days one or two; however, most homozygous G209R mutant pups succumbed by day 2, and surviving pups were severely dwarfed. In homozygous, but not heterozygous pups, blood glucose levels were significantly lower with elevated plasma insulin-like immunoreactivity and accumulation of unprocessed proinsulin in G209R pancreas compared to the wild type pups from the same litters. The POMC product α-MSH (produced by PC2 from PC1/3-generated ACTH) has been strongly implicated in obesity mechanisms. We found pituitary POMC processing to ACTH was also affected by the G209R mutation in combined anterior and intermediate pituitary lobes. ACTH was markedly reduced in homozygote pituitary, with significant accumulation of POMC. Using Western blotting, we observed a significant reduction in PC1/3 protein in homozygote brains, while PC2 protein levels remained unaffected. Most likely due to the continued presence of PC2, pituitary and brain levels of α-MSH were not impaired, suggesting that α-MSH itself is not involved in the phenotype. Prior studies have shown that G209R PC1/3 is not efficiently trafficked out of the ER; further studies will examine the contribution of misfolded G209R PC1/3 to possible cellular ER stress, as well as determine peptide hormone levels in brain and peripheral tissues.

PCSK1 encodes an enzyme required for prohormone maturation into bioactive peptides. A striking number of SNPs and rare mutations in PCSK1 are associated with a range of clinical phenotypes. Infants bearing two copies of a catalytically inactivating mutation, such as G209R, exhibit life-threatening chronic diarrhea and subsequently develop systemic endocrinopathies. Using CRISPR/Cas9 technology, we have engineered a mouse model bearing a G209R missense mutation in exon 6 of the murine Pcsk1 locus. Most pups homozygous for the G209R mutation succumbed by day 2, and surviving pups were severely dwarfed. In homozygous (but not heterozygous) pups, blood glucose levels were significantly lower, accompanied by elevated plasma insulin-like immunoreactivity and accumulation of large quantities of unprocessed proinsulin in the pancreas. Peptide hormone processing was also aberrant in G209R mouse pituitary, with mature ACTH levels markedly reduced in homozygotes, accompanied by a significant accumulation of POMC. We also observed a significant reduction in PC1/3 protein in the brains of G209R homozygous mice by Western blotting, while PC2 levels remained unaffected. Most likely due to the continued presence of PC2, pituitary and brain levels of α-MSH were not impaired. Analysis of intestinal cell types indicated a modest reduction of enteroendocrine cells in G209R homozygotes. We suggest that the G209R Pcsk1 mouse model recapitulates many of the dramatic neonatal deficiencies of human patients with this homozygous mutation.