Physical factors that elicit a response
•
•
•
•
•
•
Fluid dynamic shear stress (> 0.5 Pa)
Cyclic strain of cell substrate (> 1%)
Osmotic stress
Compression in a 3D matrix
Normal stress (> 500 Pa)
Mechanical perturbations via tethered
microbeads (> 1 nN)
1
Forces applied at one point in the cell, are
transmitted via the cytoskeletal network.
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Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/.
Fibroblast with fluorescent mitochondria forced by a
magnetic bead. D. Ingber, P. LeDuc
2
Mechanotransduction: Current theories
•
Changes in membrane fluidity
and the diffusivity of
transmembrane receptors -->
receptor clustering (Butler, 2002,
Wang, 2004)
© Cambridge University Press. All rights reserved. This content is
excluded from our Creative Commons license. For more information,
see http://ocw.mit.edu/help/faq-fair-use/.
Source: Mofrad, Mohammad RK, and Roger D. Kamm, eds. Cellular
Mechanotransduction: Diverse Perspectives from Molecules to Tissues.
Cambridge University Press, 2009.
•
Direct mechanical effects on the nuclear membrane, DNA,
and gene expression (Ingber)
•
Stretch-activated ion channels
•
Glycocalyx deformation coupling to the cortical
cytoskeleton (Weinbaum, 2003)
•
Force-induced changes in the conformation of load-bearing
proteins (Schwartz, 2001, Jiang, 2003, Bao, 2002)
•
Constrained autocrine signaling (Tschumperlin, et al., 2004)
(Gullinsgrud, 2003, 2004)
3
Early events in mechanotransduction: 1) Protein
activation progresses in a wave from the site of
bead forcing (Wang et al., 2005)
•  Response of a membrane-
targeted Src reporter. •  Phosphorylation of a domain taken from a c-Src substrate, P130cas, leads to a conformational change that reduces FRET. • A wave of activation propagates away from the site of forcing at a speed of ~18 nm/s
Neither mechanism -- of force
transduction or propagation of
activation wave -- are understood
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Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/.
4
Stretch-activated ion channels constitute one method of mechanotransduction
SEM of the
stereocilia on the
surface of a single
hair cell (Hudspeth)
Tension in the tip
link activates a
stretch-activated
ion channel, leading
to intracellular
calcium ion
fluctuations.
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Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/.
5
Leukocyte rolling and transient adhesion
http://labs.idi.harvard.edu/springer/pages/
multimedia
© www.visualphotos.com. All rights reserved. This content
is excluded from our Creative Commons license. For more
information, see http://ocw.mit.edu/help/faq-fair-use/.
Courtesy of Timothy A. Springer. Used with permission.
6
Courtesy of Timothy A. Springer. Used with permission.
htp://cbr.med.harvard.edu/investgators/springer/Iab/Iab goodies/
ROLLVIVO.MOV�
7
Leucocyte�adhesion/arrest�and�transmigraton�
Image removed due to copyright restrictions.
8
1D�nnergy�Landscape�hith�Tho�states:�Perturbed�by�ertrinsic�morce�
Transiton State T
(intermediate that must be crossed going from A to BJ or B to A; also called actvated state of GaO)
G�(kBT)�
GTo�
GB
o�
Go(χ
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ΔGBo�
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GB�e�GBo�G�>(χB - χ A )
GAo�
GB�
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Iandscape�about�pivot�point�at�state�A
χA�
State A
(bound or folded or.)
χT�
χB�
k12�
State B
(u
(unbound or unfolded or.)
χ�(nm)�
k21�
GAo�e� �
GA�e� �
GB�e� �
ΔGAo�e��
�
�mree�energy�om�state�Ae�in�absence�om�constant�appIied�morce�>�
�mree�energy�om�state�Ae�in�presence�om�constant�appIied�morce�>e�and�e�GAo�G�>(χA�G�χA)�e�GAo�
�mree�energy�om�state�Be�in�presence�om�constant�appIied�morce�>e�and�e�GBo�G�>(χB�G�χA)�
�actvaton�energy�that�must�be�overcome�to�get�mrom�state�A�to�transiton�state�T�under�no�>e�and�e�GTo�G�GAo�
pAo�e� �
Keuo�e� �
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Keu�e� �
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�probabiIity�om�being�in�state�A�in�absence�om�constant�>e�given�by�BoItZmann�distbn�e�1/��erp(nGAo/kBT)�hhere���is�partton�munct.�
�euuiIibrium�constant�in�absence�om�>s�dimensionIess�rato�om�probabiIites�om�being�in�state�B�to�state�Ae�
�denned�as�pBo/pAo�e�erp(GAo�G�GBo)/kBT�e�erp�G+(GBo�G�GAo)/kBT�e�nΔGABo/kBT�
�euuiIibrium�constant�in�presence�om�>s�dimensionIess�rato�om�probabiIites�om�being�in�state�B�to�state�Ae�
�denned�as�pB/pA�e�erp�G+(GB�G�GA)/kBT�e�erp[n+ΔGABo�G�>(χB�G�χA)]/kBT}�e�Keuo�erp[+F>(χB�G�χA)]/kBT}�
k12o�e� �
�
k12�e� �
�
�transiton�rate�mrom�A�to�B�governed�by�the�actvaton�energy�barrier�moving�mrom�A�to�Be�in�absence�om�constant�>e�in�+1/sec]�
�and�e�p�erp(nΔGAo/kBT)e�hhere�p�is�a�prenerponentaI�mreuuency�mactor�(Lecture���)��
�transiton�rate�mrom�A�to�B�in�the�presence�om�constant�>e�in�+1/sec]�
�and�e�p�erp�(n(ΔGAo�G�>(χT - χA)))�e��k12o�erp(F>(χT - χA)).�oote�k21�e�p�erp�(n(ΔGBo�G�>(χT - χB)))�e�k21o�erp(F>(χT - χB))�
�
9
state�probabiIitese�
rate�constants�and�
transiton�tmes�
Courtesy of Michael Woodside. Used with permission.
Source: Yu, Hao et al. "Energy Landscape Analysis of Native Folding of the
Prion Protein yields the Diffusion Constant, Transition Path Time, and Rates."
Proceedings of the National Academy of Sciences 109, no. 36 (2012): 14452-7.
⎡ ( Ga − G1 ) − F ( xa − x1 ) ⎤
k+ = C exp ⎢ −
⎥
k BT
⎣
⎦
10
Essential equations
⎡ ( G2 − G1 ) − F ( x2 − x1
) ⎤
p2
k+
=K =
= exp −
p1
eq
k−
⎢
⎣
k BT
⎥
⎦
⎡ ( Ga − G1 ) − F ( xa − x1 ) ⎤
k+ = C exp ⎢ −
⎥
k BT
⎣
⎦
⎡ ( Ga − G2 ) − F ( xa − x2 ) ⎤
k− = C exp ⎢ −
⎥
k BT
⎣
⎦
56
11
Measured�energy�Iandscape�mor�PrP�at�
>�e�1.1�po�
Courtesy of Michael Woodside. Used with permission.
Source: Yu, Hao et al. "Energy Landscape Analysis of Native Folding of the Prion Protein yields the Diffusion Constant,
Transition Path Time, and Rates." Proceedings of the National Academy of Sciences 109, no. 36 (2012): 14452-7.
12
MIT OpenCourseWare
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20.310J / 3.053J / 6.024J / 2.797J Molecular, Cellular, and Tissue Biomechanics
Spring 2015
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