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pathology

VIEWS: 5 PAGES: 32

									  Center for Computational Medicine & Biology
                  http://www.bme.jhu.edu/ccmb
               Directors: R. L. Winslow & A. Popel


    Mission Statement
    To advance understanding of integrative function in complex biological
    systems, including human, through development and application of
    computational models, and to apply these models to the design of novel
    therapeutics


   Gene               Protein             Cellular             Tissue/Organ
Expression           Function             Dynamics              Dynamics


    Research Example:
    Mapping and Modeling Structure and Function of the Heart
    -- Heart Failure --
Mapping and Modeling Structure and Function of the Heart
                              Outline


          •   Overview of Heart Failure
               – Organ
               – Cell
               – Gene
          •   Modeling of normal and failing myocytes
               – The modeling process
               – Insights gained from cellular model
          •   Mapping the heart
               – Diffusion Tensor MR Imaging
          •   Bringing maps and models together
                            Heart Failure


•   Primary hospital discharge diagnosis in the US
      – 3-4 million per year
      – ~ 400,000 new cases per year
•   Diverse origins; common genotype & phenotype
•   Reduced cardiac contractility and output
•   Fatal without intervention (heart transplant/LV assist device)
•   Leading cause of Sudden Cardiac Death (50% of all deaths are sudden in the
    first 5 years)
         Heart Failure: Organ Level




                             Dilated
Normal                       Cardiomyopathy

                                                  McVeigh et al
           •   Structural changes
           •   Mechanical changes
           •   Abnormal electrical activation ?
                     Heart Failure: Cell Level
  Action Potentials                          Ca2+ Transients
   Epi          Endo

                       Normal



                       Failing



Beuckelmann et al (1992)
Circulation 85: 1046-1055

                                                 H. B. Nuss

  •   APD prolongation
  •   Reduced Ca2+ transient magnitude, increased relaxation time
  •   increased risk of arrhythmia
                           Heart Failure: Gene Level
                Ion Channels, Intracellular Ca2+ Handling Proteins

      Gene expression in end-stage heart failure
      (mRNA, protein level, channel densities)

      Kääb et al (1996) Circ. Res. 78(2):262

 Membrane
                Ito1       Kv4.3, Kv1.4           66 %      Whole-cell Recordings
                                                            Channel Density
 Currents
                IK1        Kir2.1, Kir2.3         32 %      mRNA




      O’Rourke et al (1999). Circ. Res. 84: 562        Western Blot Analysis

Ca2+ Handling   Iserca2a     SERCA2a
  Proteins
                INaCa        NCX1



  Temporal patterning and mechanism of altered gene expression are unknown
                                 Heart Failure
                                      Question


                    Does altered gene expression account for
                          • APD prolongation
                          • altered Ca2+ transients
                          • increased risk of arrhythmia
                    observed at the cellular level?




Differential Gene          Computational Models             Functional Consequences
   Expression                at Cellular Level             of Altered Gene Expression
Mapping and Modeling Structure and Function of the Heart
                              Outline


          •   Overview of Heart Failure
               – Organ
               – Cell
               – Gene
          •   Modeling of normal and failing myocytes
               – The modeling process
               – Insights gained from cellular model
          •   Mapping the heart
               – Diffusion Tensor MR Imaging
          •   Bringing maps and models together
           Modeling of Normal and Failing Myocytes
                        Cardiac Ventricular Cell Model

                                             40

                                             20
                                                              Model                     40          Exp
                                                                                        20
                                              0
                                                                                         0
                                             -20
                                                                                        -20
                                             -40
                                                                                        -40
                                             -60
                                                                                        -60
                                             -80
                                                                                        -80
                                            -100
                                                                                       -100
                                                   0    0.1    0.2   0.3   0.4   0.5          0
                                                                                              0   0.1
                                                                                                  100   0.2
                                                                                                        200   0.3
                                                                                                              300   0.4
                                                                                                                    400   0.5
                                                                                                                          500




                                             •         Mode-switching model of Ca-
                                                       mediated inactivation of ICa,L
                                             •         Ca subspace
                                             •         Keizer-Levine model of CICR
                                                       and RyR adaptation
Inward                          Outward      •         Isometric force generation
Currents                        Currents     •         Modified IKr, IKs IK1
  Winslow et al (1999) Circ. Res. 84: 571    •         Added Ito1
        Modeling of Normal and Failing Myocytes
         Example of “genetic characterization”: hKv4.3 and Ito1

• Hypothesize a model structure
• Measure whole-cell membrane
                                                             Experiment
  currents under voltage-clamp
  (isolated cells or in cell culture)
• Use minimization algorithms to fit
  model parameters (Kij) to data set
                                                 aij bijV
     hKv4.3 Channel Model              Kij  e



                                                               Model


              aij  bijV
   K ij  e

 Also see Irvine et al (1999). Biophys. J.
 76: 1868-1885, human SCN5a cardiac
 Na channel
       Modeling of Normal and Failing Myocytes
           “Genetically Characterized” Cardiac Cell Model


                                             A “snapshot” in time during
                                             end-stage HF

                                            Ito1       Kv4.3, Kv1.4


                                            IK1        Kir2.1, Kir2.3


                                            Iserca2a     serca2a


                                            INaCa         NCX1




Adjust membrane current magnitudes according to % changes in mRNA or
protein level
                                                     Modeling of Normal and Failing Myocytes
                                                                 Cellular APD Prolongation and Ca2+ Transients

                                                                   Model                                                                                Experiment
                                                                            Normal
                                                                            Ionic Currents                                                                            Normal
                                                                                                                                                                      CHF
       Membrane Potential (mV)




                                   50
                                                                            Ionic Currents &




                                                                                                            Membrane Potential (mV)
                                                                                                                                       40


                                       0
                                                                            Ca2+ Handling                                              20
                                                                                                                                        0
                                                                                                                                       -20
                                                                                                                                       -40
                                  -50
                                                                                                                                       -60
                                                                                                                                       -80
                                 -100                                                                                                 -100
                                           0       100     200     300   400     500     600    700                                          0   100    200    300   400   500     600   700
                                                                  Time (mSec)                                                                             Time (mS ec)




                                           B
                                                                                                                                                        Control Peak Ca2+ 695 uM
                                 600                               Model    Normal                                                    900
                                                                                                                                                        Failing Peak Ca2+ 294 uM
                                                                                                                                      800
                                 500
                                                                            CHF                                                       700
                                                                                                                                                                           Normal
                                                                                                      [Ca ] (nM)
[Ca ] (nM)




                                 400
                                                                                                                                      600
                                 300                                                                                                  500
                                                                                                                                                                           CHF
        i




                                                                                                                i




                                                                                                                                      400
                                 200
                                                                                                                                      300
                                 100
                                                                                                                                      200
                                  0                                                                                                   100
                                               0     200          400      600     800         1000                                          0    200         400    600         800     1000
                                                                 Time (mS ec)                                                                             Time (mS ec)
                                          Modeling of Normal and Failing Myocytes
                                                               Cellular Mechanism of APD Prolongation
                                        Model and Experimental
                                        Voltage-Clamp Responses
                    A                                                                                    B
             0.5                                                                                 0.5


             0.4
                                        Model                                                    0.4
                                                                                                                          Experiment
                                                                                                                                                   Normal

                                                                                                                                                   Heart Failure
[Ca ] (mM)




                                                                                   [Ca ] (uM)
             0.3                                                                                 0.3
       i




                                                                                          i
             0.2                                                                                 0.2


             0.1                                                                                 0.1
                                        Stimulus Duration                                                                 Stimulus Duration
               0                                                                                     0
                    0   0.1    0.2      0.3       0.4    0.5   0.6                                       0    0.1   0.2      0.3       0.4         0.5       0.6
                                     Time (sec)                                                                           Time (sec)




                    C                                                                                D
             0.5                        Model                                                   30                           Model
               0
                                                                                                                                                                          L-Type Ca2+ Channel
                                                                                                25

             -0.5
                                                                                                                                                                          Mode-Switching Model
                                                                                                20
(pA/pF)




                                                                      [Ca ] (mM)




              -1
                                                                                                15
                                                                             ss
       CaL




             -1.5
I




                                                                                                10
              -2

                                                                                                 5
             -2.5

              -3                                                                                 0                                                                 Jafri et al (1998) Biophys. J. 74: 1149-1168
                    0   0.05     0.1          0.15      0.2    0.25                                  0       0.05   0.1       0.15           0.2         0.25
                                     Time (sec)                                                                     Time (sec)
Modeling of Normal and Failing Myocytes
  Cellular Mechanism of APD Prolongation (Model)



        Pacing-Induced SR Loading and APD

   Model                                Experiment
Modeling of Normal and Failing Myocytes
        Cellular Mechanisms of Arrhythmia

 Experiment                                 Model

         EAD                                        EAD




 H. B. Nuss


     Can EADs trigger arrhythmias in the heart?
Mapping and Modeling Structure and Function of the Heart
                              Outline


          •   Overview of Heart Failure
               – Organ
               – Cell
               – Gene
          •   Modeling of normal and failing myocytes
               – The modeling process
               – Insights gained from cellular model
          •   Mapping the heart
               – Diffusion Tensor MR Imaging
          •   Bringing maps and models together
                 Mapping the Heart
                        Why Do It?




Integrative modeling of physiological systems at the tissue
and organ level requires:

     • accurate models of cellular dynamics
     • accurate models of tissue/organ micro-
       anatomical structure
     • an underlying set of dynamical equations
       describing tissue/organ behavior
                   Mapping the Heart
         Cardiac Anatomy - Shape and Fiber Structure




Auckland Heart
~ 2 month reconstruction time
low spatial resolution
  (500um, 5mm, 2mm)
                                Fox and Hutchins (1972). Johns Hopkins Med. J.
                                130(5): 289-299
                           Mapping the Heart
        Diffusion Tensor MR Imaging (DTMRI) of Cardiac Anatomy




                                                 x
• DTMRI yields a 3x3 diffusion tensor
  Mi(x) at each x
• Eigenvalues of DT eigenvectors yield
  relative rate of water diffusion at each x
• Hypothesis -rate of diffusion is greatest in
  fiber direction
                      Mapping the Heart
Fiber Angle - DTMRI (isolated Langendorf-perfused hearts) vs Histology




           Scollan et al 1998 Am. J. Physiol. 275: H2308-H2318
               Mapping the Heart
Fiber Angle - DTMRI (formalin fixed hearts) vs Histology




           Holmes et al (1999) MRM, in review
                       Mapping the Heart
              Diffusion Tensor MR Heart Reconstructions



A   Reconstruction of Cardiac Geometry      B Superposition of Fiber Angle




     •High spatial resolution (234 um, 234um, 470 um)
     •Geometry ~32 Mbyte data sets, diffusion images ~800 Mbyte data sets
     •relatively rapid reconstruction times (~40 hrs/heart)
Mapping and Modeling Structure and Function of the Heart
                              Outline


          •   Overview of Heart Failure
               – Organ
               – Cell
               – Gene
          •   Modeling of normal and failing myocytes
               – The modeling process
               – Insights gained from cellular model
          •   Mapping the heart
               – Diffusion Tensor MR Imaging
          •   Bringing maps and models together
                   Bringing Maps and Models Together
                                    Whole-Heart Models
  3D Heart Model

 Measure geometry H and micro-
 anatomical structure Mi(x) of the heart


 Parabolic (Elliptic) Reaction-Diffusion Equ.

v( x, t ) 1                                          1                              
                 I ion (v( x, t ))  I app ( x, t )          M i ( x)v( x, t ) , x  H
  t        Cm 
                                                         1                         
                Total Membrane                                 Coupling
                    Current                                    Current
 Discrete space steps         coupled system of ODEs
 Solve on parallel computers
 Large computations (13 hrs/1 Sec on 1 Gflop system)
 Large data sets (40 state variables/lattice pt, 1283 lattice pts, 4 Gbytes/Sec-state
 variable, 160 Gbytes/Sec, ~10 Sec simulations)
 Need for many parametric simulations
Bringing Maps and Models Together
           Normal Heart Model




   • Anatomically detailed heart model
   • Full biophysical equations
Bringing Maps and Models Together
             Heart Failure Model




• Anatomically detailed heart model
• Full biophysical equations for failing myocytes
• Local generation of EAD (in 5% muscle mass)
                                  Bringing Maps and Models Together
                                            Body Surface Potentials


                                    Model ECG                      Holter Recording of ECG
                 1.5
Normalized ECG




                   1

                 0.5

                   0

                 -0.5

                  -1
                       -500   0     500 1000 1500    2000   2500   Pak et al (1997). J Am Coll
                                       Time (mSec)
                                                                   Cardiol 30: 576, Fig. 1B


  Gene Expression                          Computational            Organ             Body
       Data                                   Models               Function          Surface
                                                                                     Potential
Mapping and Modeling Structure and Function of the Heart
                               Future Work



 • Database of DTMRI reconstructed hearts
     – diffusion image data, structural models
     – normal and pathological hearts
     – archive of important simulation data
     – computational anatomy/query tools
 • Use 3D heart models to study effects of disease-induced structural
   changes on electrical conduction and repolarization
 • Numerical screening of drugs, drug target identification
                     Collaborators
Modeling                                Experiment
J. Greenstein 1                         E. Marban 2
A. Holmes 1                             H. B. Nuss 2
L. Irvine 1                             B. O’Rourke 2
S. Jafri 1                              G. Tomaselli 2
J. J. Rice 1
D. Scollan 1
R. L. Winslow1,3
J. Zhang1

Department of Biomedical Engineering, Whitaker Biomedical Engineering
Institute, & Center for Computational Medicine and Biology
The Johns Hopkins University School of Medicine and Whiting School of
Engineering1

Division of Cardiology, Section of Molecular and Cellular Cardiology2, &
Institute for Molecular Cardiobiology3
The Johns Hopkins University School of Medicine
                Computational Model of Heart Failure
              Sheet Angle - DTMRI (formalin fixed hearts) vs Histology


               Perfused Rabbit DTMRI          Dog Histology (Hunter)
Tertiary                                                           Angles of the
eigenvector                                                        normals to the
angles                                                             laminae



                     Raw            Smoothed
                  Computational Model of Heart Failure
                  “Numerical Screening” of Drug and Gene Therapies


   Gene
                             Cell Modeling                  Tissue Modeling
 Expression


 Kv4.3, Kv1.4

 Kir2.1, Kir2.3


 serca2a
 NCX1
                            Inward     Outward
                                                                         ATP Modulated
                            Currents   Currents
                                                                        K-channel Openers

                                                  Drug & Gene Therapy

Nuss et al (1999) J. Clin. Invest. 103(6): 889-916 (Gene Therapy)
Irvine et al (1999b), in review (Drug Modeling)
               Computational Model of Heart Failure
          Extra-Systolic Restitution & Post Extra-Systolic Potentiation

Pulse Train S0      S1               S2
          S0 - S1 Variable
          S1 - S2 Fixed                       Experiment

                                                             Wier and Yue (1986)
                                                             J. Physiol. 376: 507




                                                Model      Winslow et al (1998)
                                                           Prog. Biophys. Mol. Biol.
                                                           69: 497

                                                           Rice et al (1999)
                                                           Am. J. Physiol. In press

								
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