Visualizing mitochondrial lipids and fusion events in Mammalian cells

Mitochondrial Dysfunction Underlies Cardiomyocyte Remodeling in Experimental and Clinical Atrial Fibrillation

Atrial fibrillation (AF), the most common progressive tachyarrhythmia, results in structural remodeling which impairs electrical activation of the atria, rendering them increasingly permissive to the arrhythmia. Previously, we reported on endoplasmic reticulum stress and NAD⁺ depletion in AF, suggesting a role for mitochondrial dysfunction in AF progression. Here, we examined mitochondrial function in experimental model systems for AF (tachypaced HL-1 atrial cardiomyocytes and Drosophila melanogaster) and validated findings in clinical AF. Tachypacing of HL-1 cardiomyocytes progressively induces mitochondrial dysfunction, evidenced by impairment of mitochondrial Ca²⁺-handling, upregulation of mitochondrial stress chaperones and a decrease in the mitochondrial membrane potential, respiration and ATP production. Atrial biopsies from AF patients display mitochondrial dysfunction, evidenced by aberrant ATP levels, upregulation of a mitochondrial stress chaperone and fragmentation of the mitochondrial network. The pathophysiological role of mitochondrial dysfunction is substantiated by the attenuation of AF remodeling by preventing an increased mitochondrial Ca²⁺-influx through partial blocking or downregulation of the mitochondrial calcium uniporter, and by SS31, a compound that improves bioenergetics in mitochondria. Together, these results show that conservation of the mitochondrial function protects against tachypacing-induced cardiomyocyte remodeling and identify this organelle as a potential novel therapeutic target.

A Futile Battle? Protein Quality Control and the Stress of Aging

There exists a phenomenon in aging research whereby early life stress can have positive impacts on longevity. The mechanisms underlying these observations suggest a robust, long-lasting induction of cellular defense mechanisms. These include the various unfolded protein responses of the endoplasmic reticulum (ER), cytosol, and mitochondria. Indeed, ectopic induction of these pathways, in the absence of stress, is sufficient to increase lifespan in organisms as diverse as yeast, worms, and flies. Here, we provide an overview of the protein quality control mechanisms that operate in the cytosol, mitochondria, and ER and discuss how they affect cellular health and viability during stress and aging.

Restoring Mitochondrial Function: A Small Molecule-mediated Approach to Enhance Glucose Stimulated Insulin Secretion in Cholesterol Accumulated Pancreatic beta cells

Dyslipidemia, particularly the elevated serum cholesterol levels, aggravate the pathophysiology of type 2 diabetes. In the present study we explored the relationship between fasting blood sugar and serum lipid parameters in human volunteers which revealed a significant linear effect of serum cholesterol on fasting blood glucose. Short term feeding of cholesterol enriched diet to rodent model resulted in elevated serum cholesterol levels, cholesterol accumulation in pancreatic islets and hyperinsulinemia with modest increase in plasma glucose level. To explore the mechanism, we treated cultured BRIN-BD11 pancreatic beta cells with soluble cholesterol. Our data shows that cholesterol treatment of cultured pancreatic beta cells enhances total cellular cholesterol. While one hour cholesterol exposure enhances insulin exocytosis, overnight cholesterol accumulation in cultured pancreatic beta cells affects cellular respiration, and inhibits Glucose stimulated insulin secretion. We further report that (E)-4-Chloro-2-(1-(2-(2,4,6-trichlorophenyl) hydrazono) ethyl) phenol (small molecule M1) prevents the cholesterol mediated blunting of cellular respiration and potentiates Glucose stimulated insulin secretion which was abolished in pancreatic beta cells on cholesterol accumulation.

Hydrophobic analogues of rhodamine B and rhodamine 101: potent fluorescent probes of mitochondria in living C. elegans

Mitochondria undergo dynamic fusion and fission events that affect the structure and function of these critical energy-producing cellular organelles. Defects in these dynamic processes have been implicated in a wide range of human diseases including ischemia, neurodegeneration, metabolic disease, and cancer. To provide new tools for imaging of mitochondria in vivo, we synthesized novel hydrophobic analogues of the red fluorescent dyes rhodamine B and rhodamine 101 that replace the carboxylate with a methyl group. Compared to the parent compounds, methyl analogues termed HRB and HR101 exhibit slightly red-shifted absorbance and emission spectra (5-9 nm), modest reductions in molar extinction coefficent and quantum yield, and enhanced partitioning into octanol compared with aqueous buffer of 10-fold or more. Comparison of living C. elegans (nematode roundworm) animals treated with the classic fluorescent mitochondrial stains rhodamine 123, rhodamine 6G, and rhodamine B, as well as the structurally related fluorophores rhodamine 101, and basic violet 11, revealed that HRB and HR101 are the most potent mitochondrial probes, enabling imaging of mitochondrial motility, fusion, and fission in the germline and other tissues by confocal laser scanning microscopy after treatment for 2 h at concentrations as low as 100 picomolar. Because transgenes are poorly expressed in the germline of these animals, these small molecules represent superior tools for labeling dynamic mitochondria in this tissue compared with the expression of mitochondria-targeted fluorescent proteins. The high bioavailabilty of these novel fluorescent probes may facilitate the identification of agents and factors that affect diverse aspects of mitochondrial biology in vivo.