Copper signaling in the mammalian nervous system: synaptic effects

Abstract

Copper is an essential metal present at high levels in the CNS. Its role as a cofactor in mitochondrial ATP production and in essential cuproenzymes is well defined. Menkes and Wilson's diseases are severe neurodegenerative conditions that demonstrate the importance of Cu transport into the secretory pathway. In the brain, intracellular levels of Cu, which is almost entirely protein bound, exceed extracellular levels by more than 100-fold. Cu stored in the secretory pathway is released in a Ca(2+)-dependent manner and can transiently reach concentrations over 100 μM at synapses. The ability of low micromolar levels of Cu to bind to and modulate the function of γ-aminobutyric acid type A (GABA(A)) receptors, N-methyl-D-aspartate (NMDA) receptors, and voltage-gated Ca(2+) channels contributes to its effects on synaptic transmission. Cu also binds to amyloid precursor protein and prion protein; both proteins are found at synapses and brain Cu homeostasis is disrupted in mice lacking either protein. Especially intriguing is the ability of Cu to affect AMP-activated protein kinase (AMPK), a monitor of cellular energy status. Despite this, few investigators have examined the direct effects of Cu on synaptic transmission and plasticity. Although the variability of results demonstrates complex influences of Cu that are highly method sensitive, these studies nevertheless strongly support important roles for endogenous Cu and new roles for Cu-binding proteins in synaptic function/plasticity and behavior. Further study of the many roles of Cu in nervous system function will reveal targets for intervention in other diseases in which Cu homeostasis is disrupted.

Fig. 1

Key organelles and proteins involved in cellular Cu homeostasis. (Left) Cu enters cells through the Cu-specific transporter CTR1. Intracellular Cu is buffered by metallothioneins. Designated Cu chaperones deliver Cu to specific cuproenzymes in their respective organelles. Cox17 delivers Cu to COX in mitochondria. SOD1 is the only cytosolic cuproenzyme and receives Cu through CCS. Atox-1 delivers Cu to an ATPase (ATP7A or ATP7B) that transport Cu into the secretory pathway (Right). Cu is present in secretory granules along with cuproenzymes like DβM, extracellular SOD3 and PAM. Based on the localization of ATP7A and ATP7B, Cu is also present in constitutive vesicles and in endocytic compartments. APP and BDNF are also part of the secretory pathway in neurons (see Figs. 3,4). Abbreviations: APP, amyloid precursor protein; Atox-1, antioxidant-1 (a.k.a. HAH1); ATP7A, Menkes disease protein; CCS, Cu chaperone for SOD1; COX, cytochrome c oxidase; Cox17, Cu chaperone for COX; CTR1, Cu transporter 1; DβM, dopamine β-monooxygenase; ER, endoplasmic reticulum; LDCV, large dense-core vesicle; PAM, peptidylglycine α-amidating monooxygenase; SOD1, superoxide dismutase 1; SOD3, superoxide dismutase 3; TGN, trans Golgi network.

Fig. 2

GABA_A receptor structure and Cu binding. (A) Ionotropic receptors for γ-aminobutyric acid (GABA) are referred to as GABA_A receptors and are heteropentameric receptors that are permeable to Cl and thus generally serve an inhibitory function in the adult nervous system. GABA_A receptors typically comprise two α, two β, and one γ (or other) subunits, as depicted on the left. On the right, a schematic view of the top of the channel demonstrates the central ion pore (white), five subunits (each with four transmembrane domains; TMD1-4), and approximate binding sites for GABA, Cu, barbiturates and benzodiazepines. (B). Schematic of a single GABA_A receptor subunit. Specific residues important for Cu binding are labeled according to subunit type (α and β). Phosphorylation of the cytoplasmic loop between TMD3 and TMD4 regulates trafficking of the GABA_A receptor.

Fig. 3

Roles of Cu in synaptic function. Cu enters cells through CTR1 and binds to cytosolic chaperones including Atox-1, which delivers Cu to ATP7A. Cu gains entry into the secretory pathway through ATP7A and is stored in LDCVs along with secreted cuproenzymes DβM, SOD3 and PAM. Influx of Ca²⁺ through NMDARs and VGCCs results in LDCV release and Cu secretion. This calcium signal also facilitates long-term potentiation post-synaptically. Extracellularly, Cu has inhibitory effects on NMDARs, VGCCs and GABA_ARs. Cu binds PrP and APP, which are located at synapses. Upon binding of Cu, PrP can be internalized or can interact with the NR1 subunit of the NMDA receptor, attenuating ion flux. APP binds Cu at attomolar concentrations, facilitating the multiple cleavages that yield extracellular Aβ, which also binds Cu with high affinity. Aβ can abrogate the inhibitory effect of PrP on the NMDA receptor by sequestering Cu or by binding to PrP. Abbreviations: Aβ, beta amyloid peptide; AMPAR, 2-amino-3-(5-methyl-3-oxo-1,2- oxazol-4-yl)propanoic acid receptor; APP, amyloid precursor protein; Ca⁺⁺, calcium; CTR1, Cu transporter 1; GABA_AR, γ-aminobutyric acid A receptor; LDCV, large dense-core vesicle; NMDAR, N-methyl-D-aspartate receptor; PrP, prion protein; PSD, post-synaptic density; VGCC, voltage-gated Ca⁺⁺ channel.

Fig. 4

Potential mechanisms for Cu effects on synaptic transmission. After uptake, Cu bound to CCS is delivered to cytosolic SOD1 and Cu bound to Cox17 is delivered to mitochondrial COX. AMPK, a cellular energy sensor, is activated by superoxide and high AMP levels; SOD1 lowers superoxide levels and COX synthesizes ATP, reducing AMPK activity. AMPK phosphorylates GABA_B receptors, which facilitates the downstream GIRK channel activation that mediates slow inhibition. Prolonged activation of NMDARs has the opposite effect on slow inhibition by promoting internalization of GABA_B receptors via protein phosphatase 2 (not shown). Cu is also delivered to LDCVs via Atox-1 and ATP7A. Cu stored in vesicles is released through Ca²⁺-dependent mechanisms. Extracellular Cu stimulates MMP9, which facilitates the activation and release of BDNF. BDNF can bind to and activate TrkB receptors, which can enhance NMDAR-mediated currents and facilitate LTP. Abbreviations: AMP, adenosine monophosphate; AMPK, AMP-activated kinase; ATP, adenosine triphosphate; BDNF, brain-derived neurotrophic factor; COX, cytochrome c oxidase; GABA_BR, γ-amino butyric acid B receptor; GIRK, G protein-coupled inward rectifying potassium; MMP9, matrix metalloproteinase 9; SOD1, superoxide dismutase 1; TrkB, tropomyosin receptor kinase B.