Peripheral Serotonin Controls Dietary Fat Absorption and Chylomicron Secretion via 5-HT4 Receptor in Males.

Postprandial dyslipidemia is commonly present in people with type 2 diabetes and obesity and is characterized by overproduction of apolipoprotein B48-containing chylomicron particles from the intestine. Peripheral serotonin is emerging as a regulator of energy homeostasis with profound implications for obesity; however, its role in dietary fat absorption and chylomicron production is unknown. Chylomicron production was assessed in Syrian golden hamsters by administering an olive oil gavage and IP poloxamer to inhibit lipoprotein clearance. Administration of serotonin or selective serotonin reuptake inhibitor, fluoxetine, increased postprandial plasma triglyceride (TG) and TG-rich lipoproteins. Conversely, inhibiting serotonin synthesis pharmacologically by p-chlorophenylalanine (PCPA) led to a reduction in both the size and number of TG-rich lipoprotein particles, resulting in lower plasma TG and apolipoprotein B48 levels. The effects of PCPA occurred independently of gastric emptying and vagal afferent signaling. Inhibiting serotonin synthesis by PCPA led to increased TG within the intestinal lumen and elevated levels of TG and cholesterol in the stool when exposed to a high-fat/high-cholesterol diet. These findings imply compromised fat absorption, as evidenced by reduced lipase activity in the duodenum and lower levels of serum bile acids, which are indicative of intestinal bile acids. During the postprandial state, mRNA levels for serotonin receptors (5-HTRs) were upregulated in the proximal intestine. Administration of cisapride, a 5-HT4 receptor agonist, alleviated reductions in postprandial lipemia caused by serotonin synthesis inhibition, indicating that serotonin controls dietary fat absorption and chylomicron secretion via 5-HT4 receptor.

Structural studies of a serum amyloid A octamer that is primed to scaffold lipid nanodiscs.

Serum amyloid A (SAA) is a highly conserved acute-phase protein that plays roles in activating multiple pro-inflammatory pathways during the acute inflammatory response and is commonly used as a biomarker of inflammation. It has been linked to beneficial roles in tissue repair through improved clearance of lipids and cholesterol from sites of damage. In patients with chronic inflammatory diseases, elevated levels of SAA may contribute to increased severity of the underlying condition. The majority of circulating SAA is bound to lipoproteins, primarily high-density lipoprotein (HDL). Interaction with HDL not only stabilizes SAA but also alters its functional properties, likely through altered accessibility of protein-protein interaction sites on SAA. While high-resolution structures for lipid-free, or apo-, forms of SAA have been reported, their relationship with the HDL-bound form of the protein, and with other possible mechanisms of SAA binding to lipids, has not been established. Here, we have used multiple biophysical techniques, including SAXS, TEM, SEC-MALS, native gel electrophoresis, glutaraldehyde crosslinking, and trypsin digestion to characterize the lipid-free and lipid-bound forms of SAA. The SAXS and TEM data show the presence of soluble octamers of SAA with structural similarity to the ring-like structures reported for lipid-free ApoA-I. These SAA octamers represent a previously uncharacterized structure for lipid-free SAA and are capable of scaffolding lipid nanodiscs with similar morphology to those formed by ApoA-I. The SAA-lipid nanodiscs contain four SAA molecules and have similar exterior dimensions as the lipid-free SAA octamer, suggesting that relatively few conformational rearrangements may be required to allow SAA interactions with lipid-containing particles such as HDL. This study suggests a new model for SAA-lipid interactions and provides new insight into how SAA might stabilize protein-lipid nanodiscs or even replace ApoA-I as a scaffold for HDL particles during inflammation.

A microfluidic device for partial immobilization, chemical exposure and behavioural screening of zebrafish larvae.

The zebrafish larva is an important vertebrate model for sensory-motor integration studies, genetic screening, and drug discovery because of its excellent characteristics such as optical transparency, genetic manipulability, and genetic similarity to humans. Operations such as precise manipulation of zebrafish larvae, controlled exposure to chemicals, and behavioural monitoring are of utmost importance to the abovementioned studies. In this work, a novel microfluidic device is presented to easily stabilize an individual larva's head using a microfluidic trap while leaving the majority of the body and the tail unhindered to move freely in a downstream chamber. The device is equipped with a microvalve to prevent the larva's escape from the trap and a microchannel beside the larva's head to expose it to chemicals at desired concentrations and times, while investigating multiple behaviours such as the tail, eye, and mouth movement frequencies. An in situ air bubble removal module was also incorporated to increase the yield of experiments. The functionality of our device in comparison to a conventional droplet-based technique was tested using l-arginine exposure and viability assays. We found that the larvae in the device and the droplet exhibit similar tail and eye response trends to nM-mM concentrations of l-arginine, and that the survival of the larvae is not affected by the device. However, the tail responses in the device were numerically higher than the droplet-tested larvae at nM-mM l-arginine concentrations. In the future, our device has the potential to be used for conducting simultaneous whole-brain functional imaging, upon optimized immobilization of the brain, and behavioural analysis to uncover differences between diseased and healthy states in zebrafish.