Faculty Bibliography

Neurotrophins, such as brain-derived neurotrophic factor (BDNF), are prominent regulators of neuronal survival, growth and differentiation during development. While trophic factors are viewed as well-understood but not innovative molecules, there are many lines of evidence indicating that BDNF plays an important role in the pathophysiology of many neurodegenerative disorders, depression, anxiety and other psychiatric disorders. In particular, lower levels of BDNF are associated with the aetiology of Alzheimer's and Huntington's diseases. A major challenge is to explain how neurotrophins are able to induce plasticity, improve learning and memory and prevent age-dependent cognitive decline through receptor signalling. This article will review the mechanism of action of neurotrophins and how BDNF/tropomyosin receptor kinase B (TrkB) receptor signaling can dictate trophic responses and change brain plasticity through activity-dependent stimulation. Alternative approaches for modulating BDNF/TrkB signalling to deliver relevant clinical outcomes in neurodegenerative and neuropsychiatric disorders will also be described.

Bor1p is a secondary transporter in yeast that is responsible for boron transport. Bor1p belongs to the SLC4 family which controls in bicarbonate exchange and pH regulation in animals as well as borate uptake in plants. The SLC4 family is more distantly related to members of the Amino acid-Polyamine-organoCation (APC) superfamily, which includes well studied transporters such as LeuT, Mhp1, AdiC, vSGLT, UraA, SLC26Dg. Their mechanism generally involve relative movements of two domains: a core domain that binds substrate and a gate domain that in many cases mediates dimerization. In order to shed light on conformational changes governing transport by the SLC4 family, we grew helical membrane crystals of Bor1p from Saccharomyces mikatae and determined a structure at approximately 6 A resolution using cryo-electron microscopy. In order to evaluate the conformation of Bor1p in these crystals, a homology model was built based on the related anion exchanger from red blood cells (AE1). This homology model was fitted to the cryo-EM density map using the Molecular Dynamics (MD) Flexible Fitting method and then relaxed by all-atom MD simulation in explicit solvent and membrane. Mapping of water accessibility indicates that the resulting structure represents an inward-facing conformation. Comparisons of the resulting Bor1p model with the X-ray structure of AE1 in an outward-facing conformation, together with MD simulations of inward-facing and outward-facing Bor1p models, suggest rigid body movements of the core domain relative to the gate domain. These movements are consistent with the rocking-bundle transport mechanism described for other members of the APC superfamily

Among mitochondrial lipids, cardiolipin occupies a unique place. It is the only phospholipid that is specific to mitochondria and although it is merely a minor component, accounting for 10-20% of the total phospholipid content, cardiolipin plays an important role in the molecular organization, and thus the function of the cristae. This review covers the formation of cardiolipin, a phospholipid dimer containing two phosphatidyl residues, and its assembly into mitochondrial membranes. While a large body of literature exists on this topic, the review focuses on papers that appeared in the past three years. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Store-operated Ca²⁺entry (SOCE) is a pathway for increasing intracellular Ca²⁺ levels regulated by stromal interaction molecule 1 (STIM1), STIM2, and the Ca²⁺ channel ORAI1. SOCE-deficient patients suffer from Calcium Release-Activated Calcium (CRAC) channelopathy characterized by immunodeficiency, autoimmunity, myopathy, and anhidrotic ectodermal dysplasia. Several mitochondrial enzymes/complexes depend on Ca²⁺ but the source of Ca²⁺ required for their function are not entirely clear. We recently showed a cell-intrinsic role of SOCE in human mitochondria (Maus M et al. Cell Metab. 2017;25(3):698- 712). MitoView Green showed reduced mitochondrial volume in fibroblasts of patients with ORAI1/STIM1 lossof- function mutations. mtDNA copy numbers and mRNAs expression of selected mitochondrial transcription factors were normal. SDS-PAGE/Western blot analysis showed reduced expression of NADH ubiquinone oxidoreductase subunit-B8, Cytochrome b-c1 complex subunit-2, Cytochrome c oxidase subunit-I, Cytochrome C, Mitochondrial porin and permeability transition pore, etc. Blue native PAGE of isolated mitochondria confirmed reduced expression of CI, CIV and supercomplex CICIII2. SOCE-deficient fibroblasts had reduced mRNA and protein expression of uncoupling protein 2, higher basal mitochondrial membrane potential (MMP) and higher numbers of damaged mitochondria as suggested by increased co-localization of mitochondria and lysosomes and increased MitoKeima reporter activity indicative of lysosomal mitophagy. Oligomycininduced ATP-synthase inhibition revealed decreased electron transport and proton pumping rates measured as MMP hyperpolarization rates and reduced superoxide production assessed by MitoSOX. Maximal O2 consumption rates in SOCE-deficient cells were decreased. Skeletal myocytes had reduced CI and CIV function in 2 out of 3 ORAI1-deficient patients. Gene expression of very long chain acyl-CoA dehydrogenase and long-chain fatty acid transporter carnitine palmitoyltransferase 1B was reduced in patient fibroblasts cultured in either high glucose medium or oleic acid (OA) medium followed by starvation in 2 mM glucose medium. Furthermore, SOCE-deficient fibroblasts were lacking a starvation-induced increase in etomoxir-sensitive mitochondrial respiration in OA medium and showed reduced rates of OA beta-oxidation when cultured in 14C-OA-medium with or without subsequent starvation. Our findings indicate an important new role of SOCE in mitochondrial function