7-dehydrocholesterol efficiently supports Ret signaling in a mouse model of Smith-Opitz-Lemli syndrome

Abstract

Smith-Lemli-Opitz syndrome (SLOS) is a rare disorder of cholesterol synthesis. Affected individuals exhibit growth failure, intellectual disability and a broad spectrum of developmental malformations. Among them, renal agenesis or hypoplasia, decreased innervation of the gut, and ptosis are consistent with impaired Ret signaling. Ret is a receptor tyrosine kinase that achieves full activity when recruited to lipid rafts. Mice mutant for Ret are born with no kidneys and enteric neurons, and display sympathetic nervous system defects causing ptosis. Since cholesterol is a critical component of lipid rafts, here we tested the hypothesis of whether the cause of the above malformations found in SLOS is defective Ret signaling owing to improper lipid raft composition or function. No defects consistent with decreased Ret signaling were found in newborn Dhcr7−/− mice, or in Dhcr7−/− mice lacking one copy of Ret. Although kidneys from Dhcr7−/− mice showed a mild branching defect in vitro, GDNF was able to support survival and downstream signaling of sympathetic neurons. Consistently, GFRα1 correctly partitioned to lipid rafts in brain tissue. Finally, replacement experiments demonstrated that 7-DHC efficiently supports Ret signaling in vitro. Taken together, our findings do not support a role of Ret signaling in the pathogenesis of SLOS.

7‐Dehydrocholesterol (7‐DHC), But Not Cholesterol, Causes Suppression of Canonical TGF‐β Signaling and Is Likely Involved in the Development of Atherosclerotic Cardiovascular Disease (ASCVD)

“…This results from the predominant localization of TβR‐I and TβR‐II in lipid rafts/caveolae in these cells [Ren et al, ]. We hypothesize that high‐level accumulation (10–2,000‐fold normal) of 7‐DHC, which varies in severity in SLOS patients and SLOS tissues, results in lipid raft/caveolae dysfunction, turnover, and down‐regulation of the many resident membrane proteins in lipid rafts/caveolae, including TGF‐β receptors [Huang and Huang, ; Chen et al, , , , 2011], neurotrophin receptors [Sebastião et al, ; Zhang et al, ; Gou‐Fàbregas et al, ], G protein‐coupled receptors (GPCRs) (neurological receptors) [Björk and Svenningsson, ], ion channels [Jaffrès et al, ], receptor protein kinases (RTKs) (e.g., growth factor receptors) [Basu et al, ] and growth hormone receptor [Yang et al, ]. Down‐regulation of these important membrane proteins in tissues during embryonic development may result in prenatal and postnatal growth retardation, microcephaly, intellectual disability, multiple external malformations (e.g., cleft palate), internal anomalies, and mental retardation characteristic of SLOS.…”

“…The 7-DHC-promoted newly formed (or stabilized) lipid rafts/caveolae are characterized by the co-localization of TbR-I and TbR-II with caveolin-1 (a lipid raft/cavelae marker) in the sucrose density gradient ultracentrifugation fractions of Mv1Lu cells treated with 7-DHC. 7-DHC-containing lipid rafts/caveolae exhibit various protein components and stability, dependent on the expression level of 7-DHC [Kovarova et al, 2006;Korade et al, 2009;Gou-F abregas et al, 2016]. We hypothesize that 7-DHC, at certain low concentrations ($0.4 mg/ml), enhances TGF-b canonical signaling by extrusion of cholesterol from resident lipid rafts/caveolae, resulting in destabilization of lipid rafts/caveolae and facilitating translocation of the TGF-b receptors from lipid rafts/caveolae to non-lipid raft microdomains (where canonical signaling occurs) .…”

Membrane cholesterol access into a G-protein-coupled receptor

“…This last step was repeated twice. The chloroform phase was evaporated in a Speed Vac (Thermo Fisher Scientific, Barcelona, Spain) and resuspended in 50 μl of methanol:chloroform (3:1) 53 54 . These lipid extracts (2 μl) were analysed by mean of mass-spectrometry using a HPLC 1290 series coupled to an ESI-Q-TOF MS/MS 6520 (Agilent Technologies, Barcelona, Spain).…”

Sterols lower energetic barriers of membrane bending and fission necessary for efficient clathrin-mediated endocytosis

“…The relative contribution of cholesterol depletion and/or accumulation of sterol precursors such as 7-dehydrocholesterol (7DHC) to disease pathogenesis also remains an open question (Wassif et al, 2017). To date, membrane defects within SLOS have primarily been attributed to the disruption of lipid rafts (Gou-Fa `bregas et al, 2016;Keller et al, 2004). Nevertheless, global effects on PM architecture may perturb lipid raft independent processes (Kwik et al, 2003;Sengupta et al, 2007;van Rheenen et al, 2005).…”