Research Interests

Endothelin Signaling

The theme of our research is function and dysfunction of the endothelin signaling system in cardiovascular tissues.  Endothelin is a 21 amino acid stress hormone synthesized and released locally in diverse tissues where it maintains the homeostatic balance of cellular growth, differentiation and tissue energy consumption.  In healthy cardiovascular tissues, endothelin modulates cardiac contractility, excitability, blood pressure and responses to hypoxia (and other forms of oxygen stress), metabolic stress and mechanical load.  Endothelin also plays a central role in development of the enteric nervous system and of the electrical conduction system of the heart.  Endothelin functions primarily by activating two G-protein coupled receptors (GPCRs), ET_A and ET_B, often co-expressed in target tissues.  Excessive activation and subsequent up-regulation of the endothelin signaling pathway is associated with chronic disease states as diverse as cardiac hypertrophy, chronic heart failure, hypertension, atherosclerosis, insulin resistance and type 2 diabetes. 

Importantly, this signaling system can be distinguished from the actions of the b-adrenergic system (the fight-or-flight response) which promotes short term powerful bursts of cardiac performance.  In contrast, the endothelin system promotes efficient heart muscle contraction under conditions of chronic stress.  Establishing precisely how ET_A coupling to Gaq and phospholipase C-b1, and subsequent activation of protein kinase C-e and -d targets L-type Ca²⁺ channels and troponin (a heterotrimer that mediates Ca²⁺ activation of the contractile apparatus) to enhance cardiac efficiency remains a primary goal.

Growth Signaling and Receptor Cross-Talk

Endothelin promotes hypertrophy of ventricular myocytes but the mechanisms underlying this phenomenon are poorly understood.  The hypothesis that endothelin causes cellular hypertrophy in adult ventricular myocytes via the phenomenon of transactivation of receptor tyrosine kinases is under investigation.  Using an immunoprecipitation and proteomics strategy, we are characterizing a physical association between the ET_A GPCR and receptor tyrosine kinases erbB2/4 in a macromolecular complex that also contains the growth signaling molecules PI3 kinase, Akt and MAP kinases MEK and Erk1/2.  Recognition of a new form of cross-talk between endothelin and erbB2/4 receptors has emerged from these studies that may be analogous to inhibitory cross-talk between endothelin and insulin receptors in adipocytes.  The links between endothelin receptor activation, insulin resistance and heart failure highlight the importance of understanding the mechanisms and significance of inhibitory cross-talk between GPCR and receptor tyrosine kinase pathways.  The role played by endothelin receptor oligomerization in creating novel intracellular signals with unique pharmacological profiles is also under active investigation.

The impact of cellular remodeling of ventricular myocytes placed under stress is of interest, especially the loss of a characteristic membrane compartment known as transverse tubules (T-tubules).  It is in the T-tubules where L-type Ca²⁺ channels are in close communication with ryanodine receptors of the sarcoplasmic reticulum, and also where functional ET_A signaling complexes are localized.  Loss of T-tubules profoundly alters the response of ventricular myocytes to ET-1 and to erbB2/4 receptor ligands, and presumably disrupts crucial communication between these receptors.  Interestingly, the monoclonal antibody directed against erbB2, also known as herceptin, has emerged as an effective treatment for breast tumors that overexpress the erbB2 oncogene.  However, acquired cardiomyopathy is a significant side effect in patients on herceptin therapy.  Presumably, it is the T-tubule associated erbB2 that is targeted by herceptin in female patients with this acquired cardiomyopathy, and therapy directed at ET-1 signaling may ameliorate this side effect.  Mechanisms of stabilizing T-tubules in culture and in disease are being explored and cytoskeletal scaffolding molecules are being identified that control the formation and stability of this critical subcellular specialization.  A detailed proteomic analysis in conjunction with high resolution imaging of myocytes is being carried out to identify candidate scaffolding and regulatory proteins that dictate T-tubule structure and function.

Myofilament Protein Phosphorylation

A leading hypothesis for what triggers the onset of heart failure is hyperphosphorylation of key cardiac regulatory proteins.  Indeed, overstimulation of ET_A by endothelin causes up-regulation of and over-activity of PKC isoforms in the heart.  We are using a combined transgenic mouse and proteomics approach to identify phosphoproteins that contribute to a detrimental hyperphosphorylated state during heart failure.  This work is continuing with the goal of demonstrating the physiological consequences of phosphorylation of these sites on myofilament Ca²⁺ sensitivity, ATP consumption and contractile efficiency. 

Graduate Program Affiliations

Physiological Sciences

Biochemistry & Molecular Biophysics

Publications

Sancho Solis R, Ge Y, Walker JW. Jan 2009. Single amino acid sequence polymorphisms in rat cardiac troponin revealed by top-down tandem mass spectrometry. J Muscle Res Cell Motil, [Epub]:open access

Zabrouskov V, Ge Y, Schwartz J, Walker JW. Oct 2008. Unraveling molecular complexity of phosphorylated human cardiac troponin I by top down electron capture dissociation/electron transfer dissociation mass spectrometry. Molecular and Cellular Proteomics, 7(10):1838-1849

Walker JW. Sep 2008. Protein scaffolds, lipid domains and substrate recognition in protein kinase C function: implications for rational drug design. Handbook of Experimental Pharmacology, 186:185-203

Evans NJ, Walker JW. Aug 2008. Sustained calcium signaling and delayed internalization associated with endothelin receptor heterodimers linked through a PDZ finger. Canadian Journal of Physiology and Pharmacology, 86(8):526-535

Druckenbrod NR, Powers PA, Bartley CR, Walker JW, Epstein ML. Aug 2008. Targeting of endothelin receptor-B to the neural crest. Genesis, 46(8):396-400

Evans NJ, Walker JW. Jul 2008. Endothelin receptor dimers evaluated by FRET, ligand binding and calcium mobilization. Biophysical Journal, 95(1):483-492

Chung KY, Kang M, Walker JW. May 2008. Contractile regulation by overexpressed ETA requires intact T tubules in adult rat ventricular myocytes. American Journal of Physiology, 294(5):H2391-H2399

Sarkisov DV, Gelber SE, Walker JW, Wang SS. Aug 2007. Synapse specificity of calcium release probed by chemical two photon uncaging of inositol 1,4,5-trisphophate. Journal of Biological Chemistry, 282(35):25517-25526

Displaying 1 - 8 of 33     << first < prev | [1] | 2 | 3 | 4 | 5 | next > last >>

Faculty information is maintained by the BIO5 Institute.

Faculty may edit their information by logging into BioGate.