Abstract. It is thought that the high protein density in the mitochondrial matrix results in severely restricted solute diffusion and metabolite channeling from one enzyme to another without free aqueous-phase diffusion. To test this hypothesis, we measured the diffusion of green fluorescent protein (GFP) expressed in the mitochondrial matrix of fibroblast, liver, skeletal muscle, and epithelial cell lines. Spot photobleaching of GFP with a 100× objective (0.8-μm spot diam) gave half-times for fluorescence recovery of 15–19 ms with >90% of the GFP mobile. As predicted for aqueous-phase diffusion in a confined compartment, fluorescence recovery was slowed or abolished by increased laser spot size or bleach time, and by paraformaldehyde fixation. Quantitative analysis of bleach data using a mathematical model of matrix diffusion gave GFP diffusion coefficients of 2–3 × 10−7 cm2/s, only three to fourfold less than that for GFP diffusion in water. In contrast, little recovery was found for bleaching of GFP in fusion with subunits of the fatty acid β-oxidation multienzyme complex that are normally present in the matrix. Measurement of the rotation of unconjugated GFP by time-resolved anisotropy gave a rotational correlation time of 23.3 ± 1 ns, similar to that of 20 ns for GFP rotation in water. A rapid rotational correlation time of 325 ps was also found for a small fluorescent probe (BCECF, ∼0.5 kD) in the matrix of isolated liver mitochondria. The rapid and unrestricted diffusion of solutes in the mitochondrial matrix suggests that metabolite channeling may not be required to overcome diffusive barriers. We propose that the clustering of matrix enzymes in membrane-associated complexes might serve to establish a relatively uncrowded aqueous space in which solutes can freely diffuse.

Pathogenic de novo variants in the X-linked gene SLC35A2 encoding the major Golgi-localized UDP-galactose transporter required for proper protein and lipid glycosylation cause a rare type of congenital disorder of glycosylation known as SLC35A2congenital disorders of glycosylation (CDG; formerly CDG-IIm). To date, 29 unique de novo NG ET AL. | 909

Studies have suggested disparate variables affecting long-term outcomes in patients with infantile spasms. Using a retrospective chart review, the authors identified 109 patients who had follow-up data for at least 1 year since the onset of spasms. Patient and treatment variables were recorded, in addition to neurodevelopmental and seizure outcomes. Etiology was strongly associated with motor and cognitive status but not with long-term seizure control. Lag time to initiation of treatment was not predictive of any outcome, nor for need to use a second agent to resolve spasms, even when controlling for etiology. However, patients who responded to the first medication achieved superior seizure and cognitive outcomes. The delayed impact of individual medications could not be analyzed because many patients received multiple agents. While etiology and response to first medication predict better outcomes, the majority of patients with infantile spasms continue to have epilepsy with long-term motor and cognitive disabilities.

Few studies have focused on tolerability and adverse events associated with natural adrenocorticotropic hormone injections for treatment of infantile spasms. Using a retrospective chart review of 130 patients, the authors compare major adverse events, weight and blood pressure changes, and unplanned medication changes associated with adrenocorticotropic hormone (ACTH) injections versus other antiepileptic drugs. Children treated with adrenocorticotropic hormone injections experienced significant short-term weight gain and blood pressure elevations, which were readily reversible with weaning off the drug. Twenty-three percent of patients treated with adrenocorticotropic hormone (14 of 60) and 17% of patients treated with other antiepileptic drugs (11 of 65) experienced a major adverse event during treatment. Few patients overall required a change in medication due to intolerable side effects. Despite early changes in weight and blood pressure, short courses of high-dose natural adrenocorticotropic hormone are generally well tolerated with no increased major adverse events when compared to antiepileptic drugs in the treatment of infantile spasms.