Neuroscience Fundamentals 112C Ian Parker Biophysics of intracellular neuronal signaling Ca2+ signaling

Ca2+ signaling Resting [Ca 2+ ] in cytosol maintained at ~ 50 nM by actions of pumps & exchangers. So, only a little Ca entering the cytosol will give big (a few mM) increase in concentration. Cytosolic Ca can…… Activate membrane ion channels Regulate enzyme activity Modulate protein function Regulate gene expression Kill cells! (necrotic & apoptotic death) It has functions in virtually all cells of the body. Christened a ‘life and death’ messenger

Calcium is the ONLY link between electrical activity of a neuron (or any other cell) and the end response of the cell: e.g. Ca2+ influx through voltage gated channels in presynaptic terminal evokes transmitter release Ca2+ liberation from SR triggers muscle contraction (skeletal or cardiac muscle) Ca2+ influx through ligand-gated channels involved in LTP Unlike other ions (Na+, K+, Cl-) it is the CHEMICAL signal carried by Ca2+ that is important, not the electrical charge of the ion.

Sources of Ca2+; and Ca2+ permeable channels Extracellular fluid (~ 2 mM) Plasma membrane channels- Voltage-gated channels (N, P, Q, L-type) Ligand-gated channels: e.g. several neurotransmitter- activated channels have appreciable Ca2+ permeability (nicotinic ACh, NMDA) Store-operated channels: open in response to depletion of e.r calcium stores Endoplasmic/sarcoplasmic reticulum (~ 1mM) SR membrane (skeletal muscle)- Ryanodine receptors; opening coupled to voltage sensors in plasma membrane SR membrane (cardiac muscle)- Ryanodine receptors; opening triggered by Ca2+ entering through plasma membrane voltage-gated channels ER membrane- Ryanodine receptors; opening triggered by cytosolic Ca2+ IP3 receptors; opening requires both IP3 and cytosolic Ca2+

Diffusion The only way calcium ions can transmit information from one place in a cell to another. A particle (ion, molecule, organelle, whatever…) undergoing diffusion follows a ‘random walk’ [run web movie] The mean distance L it will have moved (in 3-dimensions) from its starting point after time t is given by L = sqrt (6Dt) ; where D = diffusion coefficient (um2 sec-1) So, mean distance increases as square root of time E.g. a molecule with D = 17 um2 sec-1 will travel 100 nm in 0.1 ms; 1 mm in 10 ms; 10 mm in 1 s; 100 mm in ~2 min. Compare ‘chemical’ and electrical signaling in neurons…

Only a few % of the Ca2+ ions in the cytosol are ‘free’ Most bind to stationary Ca buffers (proteins), which slow their diffusion Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Diffusion coefficient for free Ca2+ in water is ~ 200 mm2 s-1, in cytosol it is about 10 mm2 s-1

Spatial and temporal aspects of Ca2+ signaling Because Ca2+ diffusion in the cell is hindered by immobile buffers, transient Ca 2+ elevations can be highly localized, and specifically regulate only nearby targets – e.g. neurotransmitter release. Or, Ca2+ ions can propagate as a wave throughout a cell: e.g. to communicate signals to nucleus Sustained Ca2+ elevations kill cells (e.g. glutamate neurotoxicity). So Ca waves are generated periodically. Frequency of repetitive waves encodes stimulus strength.

‘Digital’ vs. ‘Analog’ encoding of information Weak stimulus Strong stimulus Strength encoded by frequency or pattern of all-or none signals Strength encoded in a continuously-graded manner by signal amplitude

IP3/Ca2+ signaling pathway Cytosol Ca2+ Channel [Ca]Local>10mM Pump IP3R ER Cell Membrane Pump

Global Ca2+ signaling Ca2+ + IP3 [Ca2+]cyt IP3 receptor Ca2+ waves cytoplasm Ca2+ IP3 + IP3 receptor Ca2+ waves in a whole cell [Ca2+]cyt

- - + Global Ca2+ signaling Ca2+ + IP3 [Ca2+]cyt IP3 receptor Ca2+ cytoplasm Ca2+ IP3 + IP3 receptor - Ca2+ waves in a whole cell [Ca2+]cyt - +

Ca2+-induced Ca2+ release propagates Ca2+ waves y x Cytosol z Cytosol space ER membrane ER store Cytosol Membrane ER

Local & global Ca2+ signals 10 mM

Model of local & global IP3 - evoked Ca2+ signals Switched to other one.

IP3-dependent Ca2+ signals are ordered hierarchically local cellular molecular stochastic periodic