Primary cilia, A-kinase anchoring proteins and constitutive activity at the orphan G protein-coupled receptor GPR161: A tale about a tail

Prefrontal cortex modulation of stress by primary cilia

In this issue of Neuron, Yang et al.1 reveal that primary cilia in mouse prefrontal cortex excitatory neurons regulate stress responses via cAMP/PKA signaling. Stress induces ciliary elongation, enhancing corticosterone-mediated neuronal inhibition. Cilia loss reduces stress sensitivity, highlighting their role in stress adaptation, with potential therapeutic relevance.

Regulatory mechanisms governing GLI proteins in hedgehog signaling

The Hedgehog (Hh) signaling pathway is critical for regulating cell growth, survival, fate determination, and the overall patterning of both vertebrate and invertebrate body plans. Aberrations in Hh signaling are associated with congenital abnormalities and tumorigenesis. In vertebrates, Hh signaling depends uniquely on primary cilia, microtubule-based organelles that extend from the cell surface. Over the last 2 decades, studies have demonstrated that key molecules regulating Hh signaling dynamically accumulate in primary cilia via intraflagellar transport systems. Moreover, through the primary cilia, extracellular signals are converted to stabilize GLI2 and GLI3 that are transcription factors that play a central role in regulating Hh signaling at the post-translational modification level. Recent in vivo and anatomical studies have uncovered crucial molecules that facilitate the conversion of extracellular signals into the intracellular stabilization of GLI2/GLI3 via primary cilia, emphasizing their essential roles in tissue development and tumorigenesis. This review explores the regulatory mechanisms of GLI2/GLI3 with a focus on mammalian tissue development.

Primary cilia in Parkinson's disease: summative roles in signaling pathways, genes, defective mitochondrial function, and substantia nigra dopaminergic neurons

Primary cilia (PC) are microtubules-based, independent antennal-like sensory organelles, that are seen in most vertebrate cells of different types, including astrocytes and neurons. They send signals to cells to control many physiological and cellular processes by detecting changes in the extracellular environment. Parkinson's disease (PD), a neurodegenerative disease that progresses over time, is primarily caused by a gradual degradation of the dopaminergic pathway in the striatum nigra, which results in a large loss of neurons in the substantia nigra compact (SNpc) and a depletion of dopamine (DA). PD samples have abnormalities in the structure and function of PC. The alterations contribute to the cause, development, and recovery of PD via influencing signaling pathways (SHH, Wnt, Notch-1, α-syn, and TGFβ), genes (MYH10 and LRRK2), defective mitochondrial function, and substantia nigra dopaminergic neurons. Thus, restoring the normal structure and physiological function of PC and neurons in the brain are effective treatment for PD. This review summarizes the function of PC in neurodegenerative diseases and explores the pathological mechanisms caused by PC alterations in PD, in order to provide references and ideas for future research.

Highlights and hot topics in GPCR research from 'Down Under'

LINKED ARTICLES

This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

GPR161 structure uncovers the redundant role of sterol-regulated ciliary cAMP signaling in the Hedgehog pathway

The orphan G protein-coupled receptor (GPCR) GPR161 plays a central role in development by suppressing Hedgehog signaling. The fundamental basis of how GPR161 is activated remains unclear. Here, we determined a cryogenic-electron microscopy structure of active human GPR161 bound to heterotrimeric Gs. This structure revealed an extracellular loop 2 that occupies the canonical GPCR orthosteric ligand pocket. Furthermore, a sterol that binds adjacent to transmembrane helices 6 and 7 stabilizes a GPR161 conformation required for Gs coupling. Mutations that prevent sterol binding to GPR161 suppress Gs-mediated signaling. These mutants retain the ability to suppress GLI2 transcription factor accumulation in primary cilia, a key function of ciliary GPR161. By contrast, a protein kinase A-binding site in the GPR161 C terminus is critical in suppressing GLI2 ciliary accumulation. Our work highlights how structural features of GPR161 interface with the Hedgehog pathway and sets a foundation to understand the role of GPR161 function in other signaling pathways.

The Cholesterol-5,6-Epoxide Hydrolase: A Metabolic Checkpoint in Several Diseases

Cholesterol-5,6-epoxides (5,6-ECs) are oxysterols (OS) that have been linked to several pathologies including cancers and neurodegenerative diseases. 5,6-ECs can be produced from cholesterol by several mechanisms including reactive oxygen species, lipoperoxidation, and cytochrome P450 enzymes. 5,6-ECs exist as two different diastereoisomers: 5,6α-EC and 5,6β-EC with different metabolic fates. They can be produced as a mixture or as single products of epoxidation. The epoxide ring of 5,6α-EC and 5,6β-EC is very stable and 5,6-ECs are prone to hydration by the cholesterol-5,6-epoxide hydrolase (ChEH) to give cholestane-3β,5α,6β-triol, which can be further oxidized into oncosterone. 5,6α-EC is prone to chemical and enzymatic conjugation reactions leading to bioactive compounds such as dendrogenins, highlighting the existence of a new metabolic branch on the cholesterol pathway centered on 5,6α-EC. We will summarize in this chapter current knowledge on this pathway which is controlled by the ChEH.