Phase transitions towards global excitation at the end of pregnancy are modulated by heterogeneity in gap-­‐junctional conductance Rachel E. Sheldon, Chipo Mashayamombe, Shao-­‐Qin Shi, Robert E. Garfield, Anatoly Shmygol, Andrew M. Blanks, Hugo A. van den Berg The problem In the UK, 40,000 babies are born prematurely each year. To manage preterm labour effectively, we need to know much more about the way that the body regulates contractions of the uterus. Introduction •
•
•
•
•
The muscular layer of the uterus is called the myometrium The myometrium is made up of smooth muscle cells arranged in long, thin fibres These cells must contract to deliver a baby Cells are connected through gap junctions [1] The number of gap junctions increases dramatically as labour approaches [2] Mathematical model The location of the myometrium in the uterus Gap-­‐junctional conductance •  Each cell in the model has the ability to become excited (and so, contract) •  There are two distinct relationships describing the conductance between cells as a function of voltage difference [4] •  The two relationships match the conductance responses seen by gap junction proteins connexin 43 and connexin 45 •  Cells are connected by resistors which represent gap junctions 1.0
Ê
0.6
1.0
0.8
Ê
Ê
0.4
0.2
Ê
Ê
Ê
Ê
Ê
Ê
Ê
Ê
Ê
- 50
Ê
0
50
Ê
0.6
Ê
0.4
Ê
Ê
100
0.0
0.2
0.4
0.6
0.8
1.0
Ratio of Type I Gap Junctions to Type II Gap Junctions
The relationship between gap-­‐junctional conductance and the voltage difference between neighbouring cells A larger proportion of Cx43 relative to Cx45 is needed to excite the network •  Rat myometrial samples from a range of gestation lengths were analysed to compare Cx43 and Cx45 expression levels during pregnancy •  The expression of Cx45 decreases at the end of pregnancy, whereas Cx43 expression remains relatively constant Perturbation = 1.0
Perturbation = 1.5
Perturbation = 2.0
Perturbation = 2.5
0.6
0.8
Ê
0.8
Voltage di↵erence (mV)
25
0.2
0.4
0.6
0.8
1.0
Connectivity
Global excitation is only possible by removing 20% of inter-­‐cell connections An increase in spatial heterogeneity enhances the excitability of the network This work was published in the Royal Society Journal Interface in 2013 [3] Transcripts per million
0.0
0.0
Transcripts per million
0.4
1000
0.2
Relative Cluster Size
Ê
Ê
- 100
•  A current is applied to the central cell and we examine how the current spreads to the neighbouring cells •  Systems which are not able to achieve excitability when fully connected, are able to do so by removing ~20% of inter-­‐cell connections Ê
Ê
1.0
0.2
The model network Type II (Connexin 45) 0.0
Normalised conductance
Type I (Connexin 43) Relative Cluster Size
K 800
600
400
200
20
15
10
0
d19.25 d22 (LAB)
d19.25 d22 (LAB)
Cx43 (Inner)
Cx43 (Outer)
p < 0.05
5
0
d19.25 d22 (LAB)
d19.25 d22 (LAB)
Cx45 (Inner)
Cx45 (Outer)
Cx45 acts as a block on activity. It is removed at the end of pregnancy to allow contractions to spread effectively throughout the network This work has been submitted for publication in the Royal Society Journal Interface Conclusions •  The uterus is more readily excited when around 20% of connections between cells are removed •  Relative proportions of gap junction proteins are crucial to excite the network at the end of pregnancy •  There are fewer connexin 45 gap junctions at the end of pregnancy; we propose it is this that allows contractions to take place References [1] Kumar, N.M. et al. 1996 The gap junction communication channel. Cell [2] Challis, J.R.G. et al. 2000 Endocrine and paracrine regulation of birth at term and preterm. Endocrine Reviews [3] Sheldon, R.E. et al. 2013 Spatial heterogeneity enhances and modulates excitability in a mathematical model of the myometrium. J. R. Soc. Interface [4] Miyoshi, H. et al. 1996 Voltage-­‐clamp studies of gap junctions between uterine muscle cells during term and preterm labor. Biophysical Journal