2

Role of Metal Ions in Brain Function, Metal Transport, Storage and Homoeostasis

2.1 Introduction - The Importance of Metal Ions in Brain Function

Organic molecules, based on the elements C, H, N and O, such as amino acids, carbohydrates, lipids and their constituent fatty acids, purines and pyrimidine bases and the polymers made out of them, proteins, polysaccharides and the nucleic acids RNA and DNA, are extremely important for living organisms. However, in order to fulfil a series of important functions such as energy production, nerve transmission, muscle contraction and oxygen transport, metal ions are also absolutely essential. They include the alkali and alkaline earth metals potassium, sodium, calcium and magnesium together with the transition metals cobalt, copper, iron, manganese, zinc and a few others. Some of these metal ions are particularly important for brain function, and the term metalloneurochemistry has been proposed to describe the study of metal ions in the brain and the nervous system at the molecular level (Burdette and Lippard, 2003).

Na+ and K+ play a crucial role in the transmission of nervous impulses. The opening and closing of ion channels generates electrochemical gradients across the plasma membranes of neurons. As we will see shortly, nerve impulses are propagated by action potentials which result from a transient increase in the permeability of the membrane to Na+ rapidly followed by a transient increase in its permeability to K+. The determination of high-resolution X-ray structures of the K+ ion channel has allowed us to understand how it selectively filters completely dehydrated K+ ions, but not the smaller Na+ ions.

Within most cells, including nerve cells, fluxes of Ca2+ ions play an important role in signal transduction. Ca2+ channels mediate Ca2+ influx into neurons in response to membrane depolarization, mediating a wide range of intracellular processes such as activation of calcium-dependent enzymes, gene transcription and neurotransmitter exocytosis/secretion. Their activity is an essential requirement for the coupling of electric signals in the neuronal plasma membrane to physiological events within the cells.

Another metal ion that has been implicated considerably in brain function is Zn2+. In mammalian brain, the vast majority of zinc is tightly bound within zinc metalloproteins in neurons and glial cells as a structural or as a catalytic co-factor. However, approximately 10% of the total zinc, probably ionic zinc, is less tightly bound and is mainly localized within the synaptic vesicles of the forebrain, in a subset of glutamatergic axon terminals, as we will see shortly. When hippocampal fibres are stimulated, large amounts of Zn are co-released with glutamate and it could be that this synaptic pool of Zn functions as a neuromodulator.

Redox metal ions also play an important role in brain function. Metalloenzymes containing these metals play extremely important roles in a number of key metabolic pathways within nervous tissues, They are, for example, involved in neurotransmitter synthesis (the Fe enzyme, tyrosine hydroxylase in the formation of DOPA from tyrosine and the Cu enzyme dopamine ß-hydroxylase which transforms dopamine to nor-adrenaline) and in neuroprotection (the Cu/Zn superoxide dismutase in cytosol and the Mn superoxide dismutase in mitochondria). However, as we will see in later chapters, the presence of any of these redox-active metals in excess within localized regions of the central nervous system (CNS) frequently spells disaster, with associated neurodegeneration.

2.2 Sodium, Potassium and Calcium Channels and Pumps

Nerve impulses consist of a wave of transient membrane depolarization/re-polarization which traverses the nerve cell and is designated an action potential. In their Nobel Prize-winning studies1 to uncover the ionic mechanism of action potentials, Alan Hodgkin and Andrew Huxley carried out experiments on the giant axon of the squid (Hodgkin and Huxley, 1952). The large diameter of the axon (up to 1 mm, typically 0.5 mm) allowed them to insert voltage clamp electrodes inside the lumen of the axon and measure the action potential (Figure 2.1a). The depolarization is caused by the transient opening of Na+ channels resulting in an influx of 3.7 pmol/cm2 of Na+ (Figure 2.1b). This is offset by a more prolonged influx of 4.3 pmol/cm2 of K+ which begins a fraction of a millisecond later. The outcome is that in the first ~0.5 ms, the membrane potential increases from the resting potential of around -60 mV to about +30 mV. The Na+ channels now become refractory and no more Na+ enters the cell, while K+ channels remain open, causing a rapid repolarization, which allows the membrane potential to overshoot the resting potential (hyperpolarization) before the K+ channels also inactivate, allowing the membrane to recover to its initial value. The voltage-gated Na+ and K+ ion channels across the axonal membranes create the action potentials (essentially electrochemical gradients) which allow information transfer and also regulate cellular function.

Figure 2.1 The time course of an action potential. (a) The axon membrane undergoes rapid depolarization followed by a nearly as rapid hyperpolarization and then a slow recovery to its resting potential. (b) The depolarization is caused by a transient increase in Na+ permeability (conductance), whereas the hyperpolarization results from a more prolonged increase in K+ permeability that begins a fraction of a millisecond later. [From Voet, D. and Voet, J.G. (2011) Biochemistry, 4th edn. © John Wiley & Sons. This material is reproduced with permission of John Wiley & Sons, Inc.]

The axons of larger vertebrates are enveloped in a myelin sheath which electrically isolates the axon from the extracellular medium. The myelin sheath surrounding CNS axons is the plasma membrane of an oligodendrocyte [in the peripheral nervous system (PNS), this role is performed by Schwann cells], which, as it spirally grows around the axon, extrudes its cytoplasm from between the layers. The resulting double bilayer, which makes 10-150 turns about the axon, is an excellent insulator on account of its high lipid content. Nerve impulses in myelinated nerves propagate with velocities up to 100 m/s, as much as 100 times faster than in unmyelinated nerves. The myelin sheaths are interrupted every millimetre or so along the axon by unmyelinated gaps, known as the nodes of Ranvier (Figure 2.2) where the axon contacts the extracellular medium. Whereas the unmyelinated nerves have a uniform distribution of voltage-gated Na+ channels of ~20 channels/µm2, the Na+ channels of myelinated axons occur only at the nodes of Ranvier, concentrated at ~104 channels/µm2. The action potential hops between these nodes, a process known as saltatory conduction (Latin saltare, jump). Nerve impulse transmission between nodes occurs by passive conduction of an ionic current, a process inherently much more rapid than continuous propagation. The nodes also act as amplification stations maintaining the intensity of the electrical impulse as it travels down the axon. As we discuss in a later chapter, multiple sclerosis is an autoimmune disease which demyelinates nerve fibres in the brain and the spinal cord.

Figure 2.2 (a) An electron micrograph of myelinated nerve fibres in cross section. The myelin sheath surrounding the axon is the plasma membrane of an oligodendrocyte or a Schwann cell, which extrudes its cytoplasm from between the layers as it grows spirally around the axon. The resulting bilayer, which makes between 10 and 150 turns about the axon, is a good conductor of electricity because of its high lipid content (about 80%). (b) A schematic diagram of a myelinated axon in longitudinal cross section. The axonal membrane is in contact with the external medium at the nodes of Ranvier. A depolarization generated by an action potential at one node hops down the myelinated axon (arrows) at the neighbouring node, where it induces a new action potential. This results in saltatory conduction. [From Voet, D. and Voet, J.G. (2011) Biochemistry, 4th edn. © John Wiley & Sons. This material is reproduced with permission of John Wiley & Sons, Inc.]

Transport across membranes is carried out by two classes of membrane proteins, channels and pumps. Channels allow ions to flow down a concentration gradient by a process known as passive transport or facilitated diffusion. Channels cannot remain open all the time and so they are usually gated, which simply means that, like regular garden gates, they usually remain shut and can only be opened either by the binding of a ligand (ligand-gated) or by changes in the membrane potential (voltage-gated). Ligand-gated channels, like the acetylcholine receptors in postsynaptic membranes, are opened by the binding of the neurotransmitter acetylcholine, whereas the voltage-gated sodium and potassium channels, which mediate the action potentials in neuronal axons described below, are opened by membrane depolarization.

In contrast, pumps use energy in the form of either ATP or light to drive the unfavourable uphill transport of ions or molecules against a concentration gradient; in other words, they are involved in active transport. There are two types of ATP-driven pumps, the so-called P-type ATPases and...