Zhuo Hua Pan

Bio: Zhuo Hua Pan is an academic researcher from Harvard University. The author has contributed to research in topics: GABAA receptor & Retina. The author has an hindex of 9, co-authored 10 publications receiving 3869 citations. Previous affiliations of Zhuo Hua Pan include Boston Children's Hospital.

A redox-based mechanism for the neuroprotective and neurodestructive effects of nitric oxide and related nitroso-compounds

Abstract: Congeners of nitrogen monoxide (NO) are neuroprotective and neurodestructive. To address this apparent paradox, we considered the effects on neurons of compounds characterized by alternative redox states of NO: nitric oxide (NO.) and nitrosonium ion (NO+). Nitric oxide, generated from NO. donors or synthesized endogenously after NMDA (N-methyl-D-aspartate) receptor activation, can lead to neurotoxicity. Here, we report that NO.- mediated neurotoxicity is engendered, at least in part, by reaction with superoxide anion (O2.-), apparently leading to formation of peroxynitrite (ONOO-), and not by NO. alone. In contrast, the neuroprotective effects of NO result from downregulation of NMDA-receptor activity by reaction with thiol group(s) of the receptor's redox modulatory site. This reaction is not mediated by NO. itself, but occurs under conditions supporting S-nitrosylation of NMDA receptor thiol (reaction or transfer of NO+). Moreover, the redox versatility of NO allows for its interconversion from neuroprotective to neurotoxic species by a change in the ambient redox milieu. The details of this complex redox chemistry of NO may provide a mechanism for harnessing neuroprotective effects and avoiding neurotoxicity in the central nervous system.

Cloning of a gamma-aminobutyric acid type C receptor subunit in rat retina with a methionine residue critical for picrotoxinin channel block

Abstract: Ionotropic receptors for gamma-aminobutyric acid (GABA) are important to inhibitory neurotransmission in the mammalian retina, mediating GABAA and GABAC responses. In many species, these responses are blocked by the convulsant picrotoxinin (PTX), although the mechanism of block is not fully understood. In contrast, GABAC responses in the rat retina are extremely resistant to PTX. We hypothesized that this difference could be explained by molecular characterization of the receptors underlying the GABAC response. Here we report the cloning of two rat GABA receptor subunits, designated r rho 1 and r rho 2 after their previously identified human homologues. When coexpressed in Xenopus oocytes, r rho 1/r rho 2 heteromeric receptors mimicked PTX-resistant GABAC responses of the rat retina. PTX resistance is apparently conferred in native heteromeric receptors by r rho 2 subunits since homomeric r rho 1 receptors were sensitive to PTX; r rho 2 subunits alone were unable to form functional homomeric receptors. Site-directed mutagenesis confirmed that a single amino acid residue in the second membrane-spanning region (a methionine in r rho 2 in place of a threonine in r rho 1) is the predominant determinant of PTX resistance in the rat receptor. This study reveals not only the molecular mechanism underlying PTX blockade of GABA receptors but also the heteromeric nature of native receptors in the rat retina that underlie the PTX-resistant GABAC response.

Multiple GABA receptor subtypes mediate inhibition of calcium influx at rat retinal bipolar cell terminals

Abstract: Inhibitory effects of GABA on K(+)-evoked Ca2+ influx into rat retinal bipolar cell terminals were studied using calcium imaging methods. Application of high K+ evokes a sustained, reversible increase in [Ca2+]i at bipolar cell terminals, which occurs mainly via dihydropyridine-sensitive (L-type) Ca2+ channels. There are at least two GABA receptor subtypes coexisting at bipolar cell terminals: a conventional GABAA receptor and a bicuculline/baclofen-insensitive GABA receptor. Activation of either GABA receptor inhibited the K(+)-evoked Ca2+ response. However, these two GABA receptor subtypes have distinct properties. GABAA receptors suppress the Ca2+ response only at relatively high concentrations of agonist, and with fas kinetics and a narrow dynamic range. In contrast, the bicuculline/baclofen-insensitive GABA receptors produce inhibition on the Ca2+ response at a much lower concentration of agonist, and with slow onset and a wider dynamic range. The pharmacologic profile of the bicuculline/baclofen- insensitive GABA receptor at bipolar cell terminals is most similar to the GABAC receptor reported by Feigenspan et al. (1993). Unlike the GABAC receptors described in other species, it is extremely insensitive to picrotoxin. Therefore, it may be appropriate to refer to this receptor as a picrotoxin-insensitive GABAc receptor. 3- Aminopropyl(methyl)phosphinic acid (3-APMPA) and 3- aminopropylphosphonic (3-APA), two phosphate analogs of GABA, selectively antagonize the picrotoxin-insensitive GABAc receptors but not the GABAA receptors in this system. These results imply a functional role for multiple GABA receptors in regulating synaptic transmission at bipolar cell terminals.

Greater sensitivity of larger retinal ganglion cells to NMDA-mediated cell death.

Abstract: Glutamate toxicity in retinal ganglion cells has well documented both in vitro and in vivo, and has been suggested to play a role in the neuronal loss in glaucoma. Of note, glaucoma selectively damages larger retinal ganglion cells first, and we therefore sought to explore whether glutamate-mediated cell death was likewise more pronounced in larger retinal ganglion cells. We now report that glutamate--which exerts its toxic effect on neurons predominantly through the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor--is more toxic to larger retinal ganglion cells both in tissue culture and in the intact rat eye. Cells smaller than 10 microns were relatively unaffected by glutamate or NMDA. These agents are, however, markedly toxic to retinal ganglion cells larger than 10 microns. These observations indicate that glutamate-mediated loss is seen first in larger retinal ganglion cells, in a similar fashion to the pattern of loss seen in glaucoma.

Nitric Oxide and Peroxynitrite in Health and Disease

Abstract: The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

The glutamate receptor ion channels

Abstract: The ionotropic glutamate receptors are ligand-gated ion channels that mediate the vast majority of excitatory neurotransmission in the brain. The cloning of cDNAs encoding glutamate receptor subunits, which occurred mainly between 1989 and 1992 ([Hollmann and Heinemann, 1994][1]), stimulated this

Motor neuron degeneration in mice that express a human Cu,Zn superoxide dismutase mutation.

Abstract: Mutations of human Cu,Zn superoxide dismutase (SOD) are found in about 20 percent of patients with familial amyotrophic lateral sclerosis (ALS). Expression of high levels of human SOD containing a substitution of glycine to alanine at position 93--a change that has little effect on enzyme activity--caused motor neuron disease in transgenic mice. The mice became paralyzed in one or more limbs as a result of motor neuron loss from the spinal cord and died by 5 to 6 months of age. The results show that dominant, gain-of-function mutations in SOD contribute to the pathogenesis of familial ALS.

Oxidative stress, glutamate, and neurodegenerative disorders

Abstract: There is an increasing amount of experimental evidence that oxidative stress is a causal, or at least an ancillary, factor in the neuropathology of several adult neurodegenerative disorders, as well as in stroke, trauma, and seizures. At the same time, excessive or persistent activation of glutamate-gated ion channels may cause neuronal degeneration in these same conditions. Glutamate and related acidic amino acids are thought to be the major excitatory neurotransmitters in brain and may be utilized by 40 percent of the synapses. Thus, two broad mechanisms--oxidative stress and excessive activation of glutamate receptors--are converging and represent sequential as well as interacting processes that provide a final common pathway for cell vulnerability in the brain. The broad distribution in brain of the processes regulating oxidative stress and mediating glutamatergic neurotransmission may explain the wide range of disorders in which both have been implicated. Yet differential expression of components of the processes in particular neuronal systems may account for selective neurodegeneration in certain disorders.

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018.

Abstract: Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.