Mitochondrial quality control in cardiac cells: Mechanisms and role in cardiac cell injury and disease.

Mitochondria play an important role in maintaining cardiac homeostasis by supplying the major energy required for cardiac excitation-contraction coupling as well as controlling the key intracellular survival and death pathways. Healthy mitochondria generate ATP molecules through an aerobic process known as oxidative phosphorylation (OXPHOS). Mitochondrial injury during myocardial infarction (MI) impairs OXPHOS and results in the excessive production of reactive oxygen species (ROS), bioenergetic insufficiency, and contributes to the development of cardiovascular diseases. Therefore, mitochondrial biogenesis along with proper mitochondrial quality control machinery, which removes unhealthy mitochondria is pivotal for mitochondrial homeostasis and cardiac health. Upon damage to the mitochondrial network, mitochondrial quality control components are recruited to segregate the unhealthy mitochondria and target aberrant mitochondrial proteins for degradation and elimination. Impairment of mitochondrial quality control and accumulation of abnormal mitochondria have been reported in the pathogenesis of various cardiac disorders and heart failure. Here, we provide an overview of the recent studies describing various mechanistic pathways underlying mitochondrial homeostasis with the main focus on cardiac cells. In addition, this review demonstrates the potential effects of mitochondrial quality control dysregulation in the development of cardiovascular disease.

Evidence for the impact of BAG3 on electrophysiological activity of primary culture of neonatal cardiomyocytes.

Homeostasis of proteins involved in contractility of individual cardiomyocytes and those coupling adjacent cells is of critical importance as any abnormalities in cardiac electrical conduction may result in cardiac irregular activity and heart failure. Bcl2-associated athanogene 3 (BAG3) is a stress-induced protein whose role in stabilizing myofibril proteins as well as protein quality control pathways, especially in the cardiac tissue, has captured much attention. Mutations of BAG3 have been implicated in the pathogenesis of cardiac complications such as dilated cardiomyopathy. In this study, we have used an in vitro model of neonatal rat ventricular cardiomyocytes to investigate potential impacts of BAG3 on electrophysiological activity by employing the microelectrode array (MEA) technology. Our MEA data showed that BAG3 plays an important role in the cardiac signal generation as reduced levels of BAG3 led to lower signal frequency and amplitude. Our analysis also revealed that BAG3 is essential to the signal propagation throughout the myocardium, as the MEA data-based conduction velocity, connectivity degree, activation time, and synchrony were adversely affected by BAG3 knockdown. Moreover, BAG3 deficiency was demonstrated to be connected with the emergence of independently beating clusters of cardiomyocytes. On the other hand, BAG3 overexpression improved the activity of cardiomyocytes in terms of electrical signal amplitude and connectivity degree. Overall, by providing more in-depth analyses and characterization of electrophysiological parameters, this study reveals that BAG3 is of critical importance for electrical activity of neonatal cardiomyocytes.

Network analysis of hippocampal neurons by microelectrode array in the presence of HIV-1 Tat and cocaine.

HIV-associated neurocognitive disorders affecting greater than 30% of patients are caused by HIV-1 infection of the CNS, and in part, include neurotoxic effects of the viral transactivator of transcription, Tat protein. In addition to increasing the risk for becoming HIV infected, cocaine abuse enhances the neuropathogenic impacts of HIV-1. To investigate the outcome of Tat and cocaine interference in the hippocampal neuronal network, cross-rank-corrlation was employed to develop a systematic framework to assess hippocampal neurons behavior cultured on multielectrode arrays. Tat and cocaine differentially disturbed neuronal spiking rates, amplitude, synchronous activity, and oscillations within the hippocampal neuronal network via potentiation of inhibitory neurotransmission. The Tat-mediated impairment of neuronal spiking was reversible by removal of Tat, which restored neuronal activity. The presence of astrocytes co-cultured with neuronal networks diminished the effects of Tat and cocaine on neuron function suggesting a role for astrocytes in stabilizing neuronal behavior and increasing neuronal spontaneous activities such as bursting amplitude, frequency, and wave propagation rate. Taken together, our studies indicate that the HIV protein Tat and cocaine impair hippocampal neuronal network functioning and that the presence of astrocytes alleviates network dysfunction pointing to a newly discovered pathway through which ionic homeostasis is maintained by neuron-glial crosstalk in the CNS.

Neuromodulation of BAG co-chaperones by HIV-1 viral proteins and H2O2: implications for HIV-associated neurological disorders.

Despite increasing numbers of aged individuals living with HIV, the mechanisms underlying HIV-associated neurological disorders (HANDs) remain elusive. As HIV-1 pathogenesis and aging are characterized by oxidative stress as well as altered protein quality control (PQC), reactive oxygen species (ROS) themselves might constitute a molecular mediator of neuronal PQC by modulating BCL-2 associated athanogene (BAG) family members. Present results reveal H2O2 replicated and exacerbated a reduction in neuronal BAG3 induced by the expression of HIV-1 viral proteins (i.e., Tat and Nef), while also causing an upregulation of BAG1. Such a reciprocal regulation of BAG3 and BAG1 levels was also indicated in two animal models of HIV, the doxycycline-inducible Tat (iTat) and the Tg26 mouse. Inhibiting oxidative stress via antioxidants in primary culture was capable of partially preserving neuronal BAG3 levels as well as electrophysiological functioning otherwise altered by HIV-1 viral proteins. Current findings indicate HIV-1 viral proteins and H2O2 may mediate neuronal PQC by exerting synergistic effects on complementary BAG family members, and suggest novel therapeutic targets for the aging HIV-1 population.

Cross-talk between lipid homeostasis and endoplasmic reticulum stress in neurodegeneration: Insights for HIV-1 associated neurocognitive disorders (HAND).

The dysregulation of lipid homeostasis is emerging as a hallmark of many CNS diseases. As aberrant protein regulation is suggested to be a shared pathological feature amongst many neurodegenerative conditions, such as Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD), disruptions in neuronal lipid processing may contribute to disease progression in the CNS. Specifically, given the endoplasmic reticulum (ER) dual role in lipid homeostasis as well as protein quality control (PQC) via unfolded protein response (UPR), lipid dysregulation in the CNS may converge on ER functioning and constitute a crucial mechanism underlying aberrant protein aggregation. In the current review, we discuss the diverse roles of lipid species as essential components of the CNS. Moreover, given the importance of both lipid dysregulation and protein aggregation in pathology of CNS diseases, we attempt to assess the potential downstream cross-talk between lipid dysregulation and ER dependent PQC mechanisms, with special focus on HIV-associated neurodegenerative disorders (HAND).

The lncRNA LOC102549805 (U1) modulates neurotoxicity of HIV-1 Tat protein.

HIV-1 Tat is a potent neurotoxic protein that is released by HIV-1 infected cells in the brain and perturbs neuronal homeostasis, causing a broad range of neurological disorders in people living with HIV-1. Furthermore, the effects of Tat have been addressed in numerous studies to investigate the molecular events associated with neuronal cells survival and death. Here, we discovered that exposure of rat primary neurons to Tat resulted in the up-regulation of an uncharacterized long non-coding RNA (lncRNA), LOC102549805 (lncRNA-U1). Our observations showed that increased expression of lncRNA-U1 in neurons disrupts bioenergetic pathways by dysregulating homeostasis of Ca2+, mitigating mitochondrial oxygen reduction, and decreasing ATP production, all of which point mitochondrial impairment in neurons via the Tat-mediated lncRNA-U1 induction. These changes were associated with imbalances in autophagy and apoptosis pathways. Additionally, this study showed the ability of Tat to modulate expression of the neuropeptide B/W receptor 1 (NPBWR1) gene via up-regulation of lncRNA-U1. Collectively, our results identified Tat-mediated lncRNA-U1 upregulation resulting in disruption of neuronal homeostasis.

Perturbation of synapsins homeostasis through HIV-1 Tat-mediated suppression of BAG3 in primary neuronal cells.

HIV-1 Tat is known to be released by HIV infected non-neuronal cells in the brain, and after entering neurons, compromises brain homeostasis by impairing pro-survival pathways, thus contributing to the development of HIV-associated CNS disorders commonly observed in individuals living with HIV. Here, we demonstrate that synapsins, phosphoproteins that are predominantly expressed in neuronal cells and play a vital role in modulating neurotransmitter release at the pre-synaptic terminal, and neuronal differentiation become targets for Tat through autophagy and protein quality control pathways. We demonstrate that the presence of Tat in neurons results in downregulation of BAG3, a co-chaperone for heat shock proteins (Hsp70/Hsc70) that is implicated in protein quality control (PQC) processes by eliminating mis-folded and damaged proteins, and selective macroautophagy. Our results show that treatment of cells with Tat or suppression of BAG3 expression by siRNA in neuronal cells disturbs subcellular distribution of synapsins and synaptotagmin 1 (Syt1) leading to their accumulation in the neuronal soma and along axons in a punctate pattern, rather than being properly distributed at axon-terminals. Further, our results revealed that synapsins partially lost their stability and their removal via lysosomal autophagy was noticeably impaired in cells with low levels of BAG3. The observed impairment of lysosomal autophagy, under this condition, is likely caused by cells losing their ability to process LC3-I to LC3-II, in part due to a decrease in the ATG5 levels upon BAG3 knockdown. These observations ascribe a new function for BAG3 in controlling synaptic communications and illuminate a new downstream target for Tat to elicit its pathogenic effect in impacting neuronal cell function and behavior.

Dysregulation of Neuronal Cholesterol Homeostasis upon Exposure to HIV-1 Tat and Cocaine Revealed by RNA-Sequencing.

HIV-1 Tat protein is released from HIV-1-infected cells and can enter non-permissive cells including neurons. Tat disrupts neuronal homeostasis and may contribute to the neuropathogenesis in people living with HIV (PLWH). The use of cocaine by PLWH exacerbates neuronal dysfunction. Here, we examined the mechanisms by which Tat and cocaine facilitate alterations in neuronal homeostatic processes. Bioinformatic interrogation of the results from RNA deep sequencing of rat hippocampal neurons exposed to Tat alone indicated the dysregulation of several genes involved in lipid and cholesterol metabolism. Following exposure to Tat and cocaine, the activation of cholesterol biosynthesis genes led to increased levels of free cholesterol and cholesteryl esters in rat neurons. Results from lipid metabolism arrays validated upregulation of several processes implicated in the biogenesis of ß-amyloid and Alzheimer's disease (AD), including sterol o-acyltransferase 1/acetyl-coenzyme A acyltransferase 1 (SOAT1/ACAT1), sortilin-related receptor L1 (SORL1) and low-density lipoprotein receptor-related protein 12 (LRP12). Further studies in Tat-treated primary neuronal cultures and brain tissues from HIV-1 transgenic mice as well as SIV-infected macaques confirmed elevated levels of SOAT1/ACAT 1 proteins. Our results offer novel insights into the molecular events involved in HIV and cocaine-mediated neuronal dysfunction that may also contribute to neuropathogenic events associated with the development of AD.