Instability in NAD+ metabolism leads to impaired cardiac mitochondrial function and communication.

Poly(ADP-ribose) polymerase (PARP) enzymes initiate (mt)DNA repair mechanisms and use nicotinamide adenine dinucleotide (NAD+) as energy source. Prolonged PARP activity can drain cellular NAD+ reserves, leading to de-regulation of important molecular processes. Here, we provide evidence of a pathophysiological mechanism that connects mtDNA damage to cardiac dysfunction via reduced NAD+ levels and loss of mitochondrial function and communication. Using a transgenic model, we demonstrate that high levels of mice cardiomyocyte mtDNA damage cause a reduction in NAD+ levels due to extreme DNA repair activity, causing impaired activation of NAD+-dependent SIRT3. In addition, we show that myocardial mtDNA damage in combination with high dosages of nicotinamideriboside (NR) causes an inhibition of sirtuin activity due to accumulation of nicotinamide (NAM), in addition to irregular cardiac mitochondrial morphology. Consequently, high doses of NR should be used with caution, especially when cardiomyopathic symptoms are caused by mitochondrial dysfunction and instability of mtDNA.

Essential Roles of Lactate in Müller Cell Survival and Function.

Müller cells are pivotal in sustaining retinal ganglion cells, and an intact energy metabolism is essential for upholding Müller cell functions. The present study aimed to investigate the impact of lactate on Müller cell survival and function. Primary mice Müller cells and human Müller cell lines (MIO-M1) were treated with or without lactate (10 or 20 mM) for 2 and 24 hours. Simultaneously, Müller cells were incubated with or without 6 mM of glucose. L-lactate exposure increased Müller cell survival independently of the presence of glucose. This effect was abolished by the addition of the monocarboxylate inhibitor 4-cinnamic acid to the treatment media, whereas survival continued to increase in response to addition of D-lactate during glucose restriction. ATP levels decreased over time in MIO-M1 cells and remained stable over time in primary Müller cells. Lactate was preferably metabolized in MIO-M1 cells compared to glucose, and 10 mM of L-Lactate exposure prevented complete glycogen depletion in MIO-M1 cells. Glutamate uptake increased after 2 hours and decreased after 24 hours in glucose-restricted Müller cells compared to cells with glucose supplement. The addition of 10 mM of lactate to the treatment media increased glutamate uptake in glucose supplemented and restricted cells. In conclusion, lactate is a key component in maintaining Müller cell survival and function. Hence, lactate administration may be of great future interest, ultimately leading to novel therapies to rescue retinal ganglion cells.

Expression of the vesicular glutamate transporters during development indicates the widespread corelease of multiple neurotransmitters.

Three closely related proteins transport glutamate into synaptic vesicles for release by exocytosis. Complementary patterns of expression in glutamatergic terminals have been reported for VGLUT1 and VGLUT2. VGLUT3 shows expression by many cells not considered to be glutamatergic. Here we describe the changes in VGLUT expression that occur during development. VGLUT1 expression increases gradually after birth and eventually predominates over the other isoforms in telencephalic regions. Expressed at high levels shortly after birth, VGLUT2 declines with age in multiple regions, in the cerebellum by 14-fold. In contrast, Coexpression of the two isoforms occurs transiently during development as well as permanently in a restricted subset of glutamatergic terminals in the adult. VGLUT3 is transiently expressed at high levels by select neuronal populations, including terminals in the cerebellar nuclei, scattered neurons in the cortex, and progenitor-like cells, implicating exocytotic glutamate release in morphogenesis and development. VGLUT3 also colocalizes extensively during development with the neuronal vesicular monoamine transporter VMAT2, with the vesicular acetylcholine transporter VAChT, and with the vesicular gamma-aminobutyric acid transporter VGAT. Such coexpression occurs particularly at some specific developmental stages and is restricted to certain sets of cells. In skeletal muscle, VGLUT3 localizes to granular organelles in the axon terminal as well as in the muscle sarcoplasm. The results suggest novel mechanisms and roles for regulated transmitter release.