Cholecystokinin

Discovery

CCK is released from I-cells in the small intestine. CCK stimulates gall bladder contraction and pancreatic enzyme secretion. In the central nervous system, CCK is implicated in anxiogenesis, satiety, appetite, nociception, memory, and learning. In 1928, CCK was first described as an inducer of gall bladder contractions derived from the small intestines of dogs and cats. Pig intestine extracts were known to contain a substance called pancreozymin, which stimulated the secretion of pancreatic enzymes. In 1962, a 33-aa peptide isolated from pig intestines was reported to activate both gall bladder contractions (CCK action) and pancreatic enzyme secretion (pancreozymin action). The sequence of porcine CCK/pancreozymin was determined in 1968.

Structure

Human preproCCK consists of 115 aa residues. Bioactive peptides of varying lengths with different N-terminal extensions (CCK-58, CCK-33, CCK-22, CCK-12, CCK-8, and CCK-4) are produced by enzymatic processing. CCK possesses a sulfated tyrosine at the seventh residue from the C-terminus. The C-terminal phenylalanine residue is amidated. In vertebrates, the amino acid sequences of CCKs are highly conserved, particularly in the C-terminal sequence that retains biological activity .

Gene, mRNA, and precursor

The human preproCCK gene, CCK, location 3p22.1, contains five exons. CCK mRNA is 1511 bases in length and encodes a precursor of 115 aa residues. The CCK gene is expressed in the I-cells of the duodenum and small intestine. In rats, Cck is expressed in the brain, heart, lung, kidney, and small intestine.

Synthesis and release

In intestinal I-cells, dietary lipids and proteins are sensed by GPR40 and the calcium-sensing receptor, respectively, resulting in the stimulation of CCK release.

Receptors

The CCK1 receptor (CCK1R) and CCK2 receptor (CCK2R) are GPCRs that have seven transmembrane domains. Human CCK1R consists of 428 aa residues. CCK binds to CCK1R with 500 to 1000 times the affinity and potency of gastrin and nonsulfated CCK. The sulfation of CCK is essential for receptor activation. The dissociation constants (Kd) of CCK-8 and nonsulfated CCK-8 with CCK1R are 1 and 500 nM, respectively.

Agonists and Antagonists

CCK, A-71623, GW-5823. Devazepide, Lintitript, Lorglumide, PD-140548, T-0632.

Biological functions

CCK stimulates contraction of the gall bladder and secretion of pancreatic enzymes via CCK1R. CCK affects the vagal afferent neurons in paracrine and endocrine manners via CCK1R and inhibits gastric emptying and food intake. Insulin secretion is induced by CCK stimulation. In the central nervous system, CCK is implicated in anxiogenesis, satiety, appetite, nociception, memory, and learning.

Description

Cholecystokinin is the most widespread and abundant peptide in the brain, with only the mature cerebellum possibly devoid of representation. Interest in this peptide also stems from its potency, prominent colocalization with dopamine, nonsynaptic associations influencing neuronal excitability and cerebral blood flow, and putative links to several neuropsychiatric disorders (Rehfeld 1992a, 1992b).

General Description

Receptor interacting serine/threonine kinase 2 (RIPK2) or RICK is a serine/threonine kinase. It contains a kinase domain at the amino-terminal, an intermediate domain and a caspase activation and recruitment domain (CARD) at the carboxy-terminal. The gene encoding RIPK2 is localized on human chromosome 8q21.3.

Biochem/physiol Actions

Receptor interacting serine/threonine kinase 2 (RIPK2) or RICK is activated during innate immune responses and acts as a scaffold for downstream effectors. It functions in nuclear factor-κB (NFκB) activation pathways. The protein also has roles in toll-like receptor (TLR)-signaling pathways and in the production of inflammatory cytokines.