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Background: In fast firing, parvalbumin (PV)-expressing (Pvalb) interneurons, PV acts as an intracellular Ca 2+ signal modulator with slow-onset kinetics. In Purkinje cells of PV -/- mice, adaptive/homeostatic mechanisms lead to an increase in mitochondria, organelles equally capable of delayed Ca 2+ sequestering/buffering. An inverse regulation of PV and mitochondria likewise operates in cell model systems in vitro including myotubes, epithelial cells, and oligodendrocyte-like cells overexpressing PV. Whether such opposite regulation pertains to all Pvalb neurons is currently unknown. In oligodendrocyte-like cells, PV additionally decreases growth and branching of processes in a cell-autonomous manner., Methods: The in vivo effects of absence of PV were investigated in inhibitory Pvalb neurons expressing EGFP, present in the somatosensory and medial prefrontal cortex, striatum, thalamic reticular nucleus, hippocampal regions DG, CA3, and CA1 and cerebellum of mice either wild-type or knockout (PV -/- ) for the Pvalb gene. Changes in Pvalb neuron morphology and PV concentrations were determined using immunofluorescence, followed by 3D-reconstruction and quantitative image analyses., Results: PV deficiency led to an increase in mitochondria volume and density in the soma; the magnitude of the effect was positively correlated with the estimated PV concentrations in the various Pvalb neuron subpopulations in wild-type neurons. The increase in dendrite length and branching, as well as thickness of proximal dendrites of selected PV -/- Pvalb neurons is likely the result of the observed increased density and length of mitochondria in these PV -/- Pvalb neuron dendrites. The increased branching and soma size directly linked to the absence of PV is assumed to contribute to the increased volume of the neocortex present in juvenile PV -/- mice. The extended dendritic branching is in line with the hypothesis of local hyperconnectivity in autism spectrum disorder (ASD) and ASD mouse models including PV -/- mice, which display all ASD core symptoms and several comorbidities including cortical macrocephaly at juvenile age., Conclusion: PV is involved in most proposed mechanisms implicated in ASD etiology: alterations in Ca 2+ signaling affecting E/I balance, changes in mitochondria structure/function, and increased dendritic length and branching, possibly resulting in local hyperconnectivity, all in a likely cell autonomous way.

The clearance of mitochondria by autophagy, mitophagy, is important for cell and organism health [1], and known to be regulated by ubiquitin. During Drosophila intestine development, cells undergo a dramatic reduction in cell size and clearance of mitochondria that depends on autophagy, the E1 ubiquitin-activating enzyme Uba1, and ubiquitin [2]. Here we screen a collection of putative ubiquitin-binding domain-encoding genes for cell size reduction and autophagy phenotypes. We identify the endosomal sorting complex required for transport (ESCRT) components TSG101 and Vps36, as well as the novel gene Vps13D. Vps13D is an essential gene that is necessary for autophagy, mitochondrial size, and mitochondrial clearance in Drosophila. Interestingly, a similar mitochondrial phenotype is observed in VPS13D mutant human cells. The ubiquitin-associated (UBA) domain of Vps13D binds K63 ubiquitin chains, and mutants lacking the UBA domain have defects in mitochondrial size and clearance and exhibit semi-lethality, highlighting the importance of Vps13D ubiquitin binding in both mitochondrial health and development. VPS13D mutant cells possess phosphorylated DRP1 and mitochondrial fission factor (MFF) as well as DRP1 association with mitochondria, suggesting that VPS13D functions downstream of these known regulators of mitochondrial fission. In addition, the large Vps13D mitochondrial and cell size phenotypes are suppressed by decreased mitochondrial fusion gene function. Thus, these results provide a previously unknown link between ubiquitin, mitochondrial size regulation, and autophagy., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Mitochondria are dynamic organelles that alter their morphology in response to cellular signaling and differentiation through balanced fusion and fission. In this study, we found that the mitochondrial inner membrane ATPase ATAD3A interacted with ccdc56/MITRAC12/COA3, a subunit of the cytochrome oxidase (COX)-assembly complex. Overproduction of ccdc56 in HeLa cells resulted in fragmented mitochondrial morphology, while mitochondria were highly elongated in ccdc56-repressed cells by the defective recruitment of the fission factor Drp1. We also found that mild and chronic inhibition of COX led to mitochondrial elongation, as seen in ccdc56-repressed cells. These results indicate that ccdc56 positively regulates mitochondrial fission via regulation of COX activity and the mitochondrial recruitment of Drp1, and thus, suggest a novel relationship between COX assembly and mitochondrial morphology., (Copyright © 2015 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

Hypoxia is a major cause of radiation resistance, which may predispose to local recurrence after radiation therapy. While hypoxia increases tumor cell survival after radiation exposure because there is less oxygen to oxidize damaged DNA, it remains unclear whether signaling pathways triggered by hypoxia contribute to radiation resistance. For example, intratumoral hypoxia can increase hypoxia inducible factor 1 alpha (HIF-1α), which may regulate pathways that contribute to radiation sensitization or radiation resistance. To clarify the role of HIF-1α in regulating tumor response to radiation, we generated a novel genetically engineered mouse model of soft tissue sarcoma with an intact or deleted HIF-1α. Deletion of HIF-1α sensitized primary sarcomas to radiation exposure in vivo. Moreover, cell lines derived from primary sarcomas lacking HIF-1α, or in which HIF-1α was knocked down, had decreased clonogenic survival in vitro, demonstrating that HIF-1α can promote radiation resistance in a cell autonomous manner. In HIF-1α-intact and -deleted sarcoma cells, radiation-induced reactive oxygen species, DNA damage repair and activation of autophagy were similar. However, sarcoma cells lacking HIF-1α had impaired mitochondrial biogenesis and metabolic response after irradiation, which might contribute to radiation resistance. These results show that HIF-1α promotes radiation resistance in a cell autonomous manner.