γ-Secretase-mediated growth hormone receptor proteolysis: Mapping of the intramembranous cleavage site

Molecular mechanism of substrate recognition and cleavage by human γ-secretase

Successive cleavages of amyloid precursor protein C-terminal fragment with 99 residues (APP-C99) by γ-secretase result in amyloid-β (Aβ) peptides of varying lengths. Most cleavages have a step size of three residues. To elucidate the underlying mechanism, we determined the atomic structures of human γ-secretase bound individually to APP-C99, Aβ49, Aβ46, and Aβ43. In all cases, the substrate displays the same structural features: a transmembrane α-helix, a three-residue linker, and a β-strand that forms a hybrid β-sheet with presenilin 1 (PS1). Proteolytic cleavage occurs just ahead of the substrate β-strand. Each cleavage is followed by unwinding and translocation of the substrate α-helix by one turn and the formation of a new β-strand. This mechanism is consistent with existing biochemical data and may explain the cleavages of other substrates by γ-secretase.

GHR signalling: Receptor activation and degradation mechanisms

Growth hormone (GH) actions via initiating cell signalling through the GH receptor (GHR) are important for many physiological processes, in addition to its well-known role in regulating growth. The activation of JAK-STAT signalling by GH is well characterized, however knowledge on GH activation of SRC family kinases (SFKs) is still limited. In this review we summarise the collective knowledge on the activation, regulation, and downstream signalling of GHR. We highlight studies on GH activation of SFKs and the important outcome of this signalling pathway with a focus on the different degradation mechanisms that can regulate GHR availability since this is an area that warrants further study considering its role in tumour progression.

The substrate repertoire of γ-secretase/presenilin

The intramembrane protease γ-secretase is a hetero-tetrameric protein complex with presenilin as the catalytic subunit and cleaves its membrane protein substrates within their single transmembrane domains. γ-Secretase is well known for its role in Notch signalling and in Alzheimer's disease, where it catalyzes the formation of the pathogenic amyloid β (Aβ) peptide. However, in the 21 years since its discovery many more substrates and substrate candidates of γ-secretase were identified. Although the physiological relevance of the cleavage of many substrates remains to be studied in more detail, the substrates demonstrate a broad role for γ-secretase in embryonic development, adult tissue homeostasis, signal transduction and protein degradation. Consequently, chronic γ-secretase inhibition may cause significant side effects due to inhibition of cleavage of multiple substrates. This review provides a list of 149 γ-secretase substrates identified to date and highlights by which expeirmental approach substrate cleavage was validated. Additionally, the review lists the cleavage sites where they are known and discusses the functional implications of γ-secretase cleavage with a focus on substrates identified in the recent past, such as CHL1, TREM2 and TNFR1. A comparative analysis demonstrates that γ-secretase substrates mostly have a long extracellular domain and require ectodomain shedding before γ-secretase cleavage, but that γ-secretase is also able to cleave naturally short substrates, such as the B cell maturation antigen. Taken together, the list of substrates provides a resource that may help in the future development of drugs inhibiting or modulating γ-secretase activity in a substrate-specific manner.

Receptor tyrosine kinases in the nucleus

To date, 18 distinct receptor tyrosine kinases (RTKs) are reported to be trafficked from the cell surface to the nucleus in response to ligand binding or heterologous agonist exposure. In most cases, an intracellular domain (ICD) fragment of the receptor is generated at the cell surface and translocated to the nucleus, whereas for a few others the intact receptor is translocated to the nucleus. ICD fragments are generated by several mechanisms, including proteolysis, internal translation initiation, and messenger RNA (mRNA) splicing. The most prevalent mechanism is intramembrane cleavage by γ-secretase. In some cases, more than one mechanism has been reported for the nuclear localization of a specific RTK. The generation and use of RTK ICD fragments to directly communicate with the nucleus and influence gene expression parallels the production of ICD fragments by a number of non-RTK cell-surface molecules that also influence cell proliferation. This review will be focused on the individual RTKs and to a lesser extent on other growth-related cell-surface transmembrane proteins.

Growth hormone-induced JAK2 signaling and GH receptor down-regulation: role of GH receptor intracellular domain tyrosine residues

GH receptor (GHR) mediates important somatogenic and metabolic effects of GH. A thorough understanding of GH action requires intimate knowledge of GHR activation mechanisms, as well as determinants of GH-induced receptor down-regulation. We previously demonstrated that a GHR mutant in which all intracellular tyrosine residues were changed to phenylalanine was defective in its ability to activate signal transducer and activator of transcription (STAT)5 and deficient in GH-induced down-regulation, but able to allow GH-induced Janus family of tyrosine kinase 2 (JAK2) activation. We now further characterize the signaling and trafficking characteristics of this receptor mutant. We find that the mutant receptor's extracellular domain conformation and its interaction with GH are indistinguishable from the wild-type receptor. Yet the mutant differs greatly from the wild-type in that GH-induced JAK2 activation is augmented and far more persistent in cells bearing the mutant receptor. Notably, unlike STAT5 tyrosine phosphorylation, GH-induced STAT1 tyrosine phosphorylation is retained and augmented in mutant GHR-expressing cells. The defective receptor down-regulation and persistent JAK2 activation of the mutant receptor do not depend on the sustained presence of GH or on the cell's ability to carry out new protein synthesis. Mutant receptors that exhibit resistance to GH-induced down-regulation are enriched in the disulfide-linked form of the receptor, which reflects the receptor's activated conformation. Furthermore, acute GH-induced internalization, a proximal step in down-regulation, is markedly impaired in the mutant receptor compared to the wild-type receptor. These findings are discussed in the context of determinants and mechanisms of regulation of GHR down-regulation.