Department of Comparative Biosciences
- B.S. in Biological Sciences, University of California, Irvine
- Ph.D. in Developmental and Cell Biology, University of California, Irvine
The primary focus of the Mitchell laboratory concerns mechanisms of neuroplasticity, specifically in the respiratory motor control system. We investigate fundamental mechanisms of compensatory plasticity elicited by alterations in respiratory gases (adult and developing animals), exercise, spinal injury or motor neuron disease (ie. ALS). An emergent theme from our work is that serotonin is a key molecule, initiating and orchestrating important forms of plasticity at the level of respiratory motor nuclei. One important role of serotonin is to regulate the synthesis of key proteins in the underlying plasticity, such as the neurotrophin brain derived neurotrophic factor (BDNF). Our basic studies of respiratory plasticity may yield novel insights concerning pathogenic mechanisms of respiratory control disorders, such as obstructive sleep apnea and sudden infant death syndrome. On the other hand, our basic research has suggested novel strategies in the treatment of sleep disordered breathing or respiratory insufficiency caused by cervical spinal injury or motor neuron disease.
Members of our group perform experiments using a wide range of techniques, allowing inferences at multiple levels of biological organization (cell and molecular approaches such as RNAi, neurophysiological recordings, neuroanatomical techniques and whole animal physiological measurements). Areas currently receiving major funding include:
MOTOR NEURON PLASTICTIY INDUCED BY INTERMITTENT HYPOXIA:
- Cellular/synaptic mechanisms of respiratory long-term facilitation following acute intermittent hypoxia. We continue to develop a comprehensive model of cellular/synaptic mechanisms that underlie phrenic long-term facilitation (pLTF) following brief exposures to intermittent hypoxia. pLTF is a pattern-sensitive, progressive increase in phrenic activity for hours following acute intermittent, but not acute sustained hypoxia. pLTF requires serotonin-receptor activation, new BDNF synthesis within the phrenic motor nucleus and the formation of reactive oxygen species. Okadaic acid-sensitive protein phosphatases constrain pLTF, preventing its expression following sustained hypoxia; when these phosphatases are inhibited in the cervical spinal cord, pLTF is revealed following sustained hypoxia. Our working hypothesis is that differential ROS formation between intermittent versus sustained hypoxia inhibits the relevant phosphatases, thereby accounting for pLTF pattern-sensitivity.
- Phrenic motor facilitation can also be induced by trans-activation of the high affinity BDNF receptor tyrosine kinase, TrkB. Activation of metabotropic, Gs protein coupled receptors (such as the adenosine 2A receptor) induces new TrkB synthesis, TrkB signaling and phrenic motor facilitation. This finding represents an important ability to harness intermittent hypoxia induced phrenic plasticity using small molecules that activate the fundamental mechanism without the need to apply intermittent hypoxia.
- Phrenic and hypoglossal long-term facilitation (pLTF and XII LTF) exhibit metaplasticity. Both phrenic and XII LTF exhibitmetaplasticity following repetitive exposures to acute intermittent hypoxia (10 episodes daily for one week, or three times a week for 10 weeks). Repetitive acute intermittent hypoxia enhances LTF and up-regulates key proteins in its underlying mechanism (serotonin receptors, BDNF, TrkB and downstream kinases).
- Age, gender and genetics influence pLTF expression. Phrenic and XII LTF exhibit profound age and sex specific patterns in rats, consistent with age and sex-specific alterations in serotonergic function. Futher, both pLTF and XII LTF vary widely among rat strains, or even sub-strains. Details of age/sex and genetic influences on serotonin-dependent LTF are under investigation.
- RNAi investigations of neuroplasticity. We were among the first to demonstrate that siRNA can be used effectively to control gene expression and protein synthesis (via translational regulation) in the mammalian nervous system in vivo.
- Neuron-glial interactions in respiratory plasticity. A new initiative concerns the supportive role of microglia and astroglia in phrenic motor plasticity. Both in vivo and in vitro experiments are underway, with current focus on the potential roles of microglia in motor neuron plasticity. In collaboration with J. Watters, cell co-culture experiments have been initiated using a model for motoneurons (NSC 34 cells) and microglia to investigate interactions among these cell types when confronted with episodic versus continuous serotonin and/or ATP receptor activation.
RESIRATORY PLASTICITY FOLLOWING SPINAL INJURY:
- Strategies to strengthen silent spinal synaptic pathways following cervical spinal injury. Our goal is to harness respiratory plasticity as a means of strengthening spared but ineffective (silent) synaptic pathways to respiratory motor neurons, thereby improving respiratory function following cervical spinal injury. For example, we have demonstrated that ineffective crossed-spinal synaptic pathways to phrenic motor neurons can be converted to effective synaptic pathways by application of repetitive acute intermittent hypoxia. Futher, strengthening these synaptic pathways is associated with nearly complete restoration of respiratory function in rats with cervical hemisection. Detailed mechanisms of these effects will extensively investigated in the next few years.
- Activation of Gs protein coupled receptors induces TrkB receptor trans-activation and strengthens spinal synaptic pathways to phrenic motor neurons, thereby restoring respiratory function in rats with cervical spinal hemisection. We are investigating the ability of Adenosine 2A receptor agonists (as well as agonists of other Gs protein coupled metabotropic receptors) to induce functional recovery via TrkB trans-activation.
- Microglial involvement in spontaneous and induced respiratory functional recovery following cervical spinal injury. We have begun investigations concerning the potential of microglial activation below cervical spinal injury to enable spontaneous and evoked functional recovery of respiratory motor output. For example, microglia are activated within the phrenic motor nucleus below C2-hemisection. However, we do not yet know whether they play a beneficial versus deleterious roles in functional recovery at this point.
COMPENSATORY PLASTICITY DURING MOTOR NEURON DISEASE (ALS):
- Respiratory plasticity in the SOD1 G93A rat. ALS is a devastating disease leading to progressive motor neuron degeneration and death. Most ALS patients develop severe respiratory insufficiency, and the most frequent cause of death is ventilatory failure. Our laboratory is investigating respiratory motor function in a rodent model of familial ALS, the transgenic rat over-expressing mutated superoxide dismutase-1 (SOD1 G93A rat). Our fundamental hypothesis is that compensatory spinal neuroplasticity offsets severe respiratory motor neuron degeneration, preserving the ability to breathe adequately until late in disease progression. We are investigating cellular mechanisms of spontaneous compensatory plasticity, and are investigating the potential to enhance plasticity with repetitive acute intermittent hypoxia. We have also begun investigations concerning the contributions of two trophic factors postulated to play key roles in respiratory plasticity or the pathogenesis of ALS: brain derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF).
- Do trophic factor secreting neural progenitor transplants enhance phrenic motor output and prolong motor neuron survival? In collaboration with the laboratory of C. Svendsen, we are using transplants of stem cells capable of producing and secreting relevant trophic factors (eg. BDNF, VEGF, GDNF) to protect and prolong survival of phrenic motor neurons during disease progression.
Primary teaching responsibilities are in the areas of respiratory, thermoregulatory and acid-base physiology in the veterinary medical curriculum (Veterinary Physiology A). Additional responsibilities include the training of graduate students and minor participation in a number of other courses on campus.
KEMP STATION REGULATION OF RESPIRATION RETREAT
July 19 - 21, 2007: PROGRAM
July 17 - 19, 2008: Program TBA
Golder, F.J., L. Ranganathan, I. Satriotomo, M. Hoffman, M.R. Lovett-Barr, J.J. Watters, T.L. Baker-Herman and G.S. Mitchell (2008). Spinal adenosine A2a receptor activation elicits long-lasting phrenic motor facilitation. J. Neuroscience. 28: 2033-2042. [Abstract]
Wilkerson, J.E.R., P.M. MacFarlane, M. Hoffman, I. Satriotomo, T.L. Baker-Herman, J.J. Watters and G.S. Mitchell (2008). Okadaic acid-sensitive protein phosphatases constrain phrenic long-term facilitation following sustained hypoxia. J. Neuroscience 28: 2949-2958. [Abstract]
Mahamed, S. and G.S. Mitchell (2007). Sleep Apnoea & Hypertension: Physiological bases for a causal relation: Is there a link between intermittent hypoxia-induced respiratory plasticity and obstructive sleep apnoea? Exp. Physiol. 92: 27-37. [Abstract]
Mitchell GS (2007) Respiratory plasticity following intermittent hypoxia: a guide for novel therapeutic approaches to ventilatory control disorders. In: Genetic Basis for Respiratory Control Disorders (Gaultier C, ed). New York: Springer Publishing Company.
Baker-Herman, T.L, D.D. Fuller, R.W. Bavis, A.G. Zabka, F.J. Golder, N.J. Doperalski, R.A. Johnson,m J.J. Watters and G.S. Mitchell. BDNF is necessary and sufficient for spinal respiratory plasticity following intermittent hypoxia. Nature Neurosci 7: 48-55, 2004.[Abstract]
Mitchell GS, Johnson SM. Invited Review: Neuroplasticity in respiratory motor control. J Appl Physiol. 94:358-74, 2003.[Abstract]
Feldman JL, Mitchell GS, Nattie EE. Breathing: Rhythmicity, Plasticity, Chemosensitivity. Annu Rev Neurosci. 26: 239-266, 2003.[Abstract]
Fuller DD, Johnson SM, Olson EB Jr, Mitchell GS. Synaptic pathways to phrenic motoneurons are enhanced by chronic intermittent hypoxia after cervical spinal cord injury. J Neurosci. 23:2993-3000, 2003.[Abstract]
Baker-Herman TL, Mitchell GS. Phrenic long-term facilitation requires spinal serotonin receptor activation and protein synthesis. J Neurosci. 22: 6239-46, 2002.[Abstract]