Physiological phenotype and vulnerability in Parkinson's disease

Abstract

This review will focus on the principles underlying the hypothesis that neuronal physiological phenotype-how a neuron generates and regulates action potentials-makes a significant contribution to its vulnerability in Parkinson's disease (PD) and aging. A cornerstone of this hypothesis is that the maintenance of ionic gradients underlying excitability can pose a significant energetic burden for neurons, particularly those that have sustained residence times at depolarized membrane potentials, broad action potentials, prominent Ca(2+) entry, and modest intrinsic Ca(2+) buffering capacity. This energetic burden is shouldered in neurons primarily by mitochondria, the sites of cellular respiration. Mitochondrial respiration increases the production of damaging superoxide and other reactive oxygen species (ROS) that have widely been postulated to contribute to cellular aging and PD. Many of the genetic mutations and toxins associated with PD compromise mitochondrial function, providing a mechanistic linkage between known risk factors and cellular physiology that could explain the pattern of pathology in PD. Because much of the mitochondrial burden created by this at-risk phenotype is created by Ca(2+) entry through L-type voltage-dependent channels for which there are antagonists approved for human use, a neuroprotective strategy to reduce this burden is feasible.

Figure 1.

Schematic representation of the plasma proteins that regulate ionic gradients following neurotransmitter release or during the time course of an action potential spike. Synaptic input and spiking dissipate ionic gradients maintained by pumps and exchangers creating a metabolic burden. Illustrated in the model are the classic influx and efflux of ions favored by their electrochemical gradients. Particularly, the electrochemical gradient for Ca²⁺ (∼ +128 mV under physiological conditions) favors a strong influx of Ca²⁺ ions through NMDA receptors (NR) following glutamate release or activation of Ca_v channels during pacemaking spiking activity. The tight regulation of intracellular Ca²⁺ is routed into the endoplasmic reticulum (ER) via sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pumps. However, Ca²⁺ influx into the ER can trigger the process of Ca²⁺-induced Ca²⁺ release (CICR), displayed here as Ca²⁺ released through the IP3R/RyR pathway. Another venue of regulation of Ca²⁺ depends on whether the cell expresses Ca²⁺-binding proteins (CBP), which help buffer intracellular Ca²⁺. Whether CBP is present or not, Ca²⁺ can also be extruded out of the cell via plasma membrane Ca²⁺-ATPase (PMCA) or Na⁺/Ca²⁺ exchanger (NCX) proteins. SERCA, PMCA, NCX, and the Na/K ATPase all require ATP as the energy source to maintain these gradients, and this ATP consumption can have a strong impact on the bioenergetics of the cell to maintain normal levels of intracellular Ca²⁺.

Figure 2.

Spike width shapes Ca²⁺ entry. (A) Spikes recorded from a substantia nigra pars reticulata (SNr) GABAergic neuron and a substantia nigra pars compacta (SNc) dopaminergic neuron (DA). SNc DA neurons have wider spikes compared to SNr GABAergic neurons. (B) In the top panel, simulated spikes of varying width using the computer program Neuron are shown. In the bottom panel, Ca²⁺ currents were generated in the model in response to the simulated spikes. Simulated Ca²⁺ currents were evoked by opening of Ca_v1.3 and Ca_v2.1 model channels in Neuron. The color-coding of the simulated spikes and respective Ca²⁺ currents show that the amount of Ca²⁺ charges that enter the cell is higher in simulated spikes with largest width. This model supports our hypothesis that SNc DA neurons that have large spike width will allow more Ca²⁺ entry compared to an SNr GABAergic neuron, which then can have a strong impact on the undergoing Ca²⁺ signaling, handling, and metabolic demands for neuronal populations that can differ based on spike waveform.

Figure 3.

Ca_v1.3 L-type Ca²⁺ channels contribute to the excitability of SNc DA neurons. (A) Pacemaking firing of SNc DA neurons recorded in current-clamp whole cell configuration in mouse midbrain slices. Pharmacological blockade of sodium channels with tetrodotoxin (TTX) leaves an ongoing membrane potential oscillation. (B) Another example of SNc DA neuron in pacemaking mode synchronized to dendritic Ca²⁺ oscillations imaged from a distal dendrite using Fluo4 Ca²⁺ indicator. Following application of TTX, the membrane potential oscillation is associated with an ongoing Ca²⁺ oscillation attenuated following antagonism of the DHP-sensitive L-type channels with isradipine. (C) Representative pacemaking SNc DA neuron displaying Ca²⁺ transients from a proximal dendrite (∼30 µm away from the soma, blue trace), and following bath application of isradipine to block L-type channels. (D) The same as part C, with the difference that Ca²⁺ imaging was performed on a distal dendrite (∼80 µm away from the soma, red trace). Antagonism of L-type channels attenuates the underlying Ca²⁺ oscillation in distal dendrites, while in a proximal dendrite; there is a residual Ca²⁺ transient that might be dependent on other voltage-gated Ca²⁺ channels. These results can suggest a potential gradient of L-type channel expression that increases as you move farther away from the soma. Parts C and D also show that isradipine did not affect pacemaking automaticity, suggesting that Ca²⁺ influx through L-type channels can be attenuated without affecting the pacemaking firing of SNc DA neurons. (Data used in this figure have been adapted from Chan et al. 2007 and Guzman et al. 2010, with permission from the authors.)

Figure 4.

Neurons with the pacemaking phenotype might have a diminished respiratory reserve, putting them at risk of bioenergetics failure. (A) Schematic model of the respiratory capacity of a quiescent neuron, an active neuron, and an active Ca²⁺-regulated neuron. Respiratory capacity here is defined as the sum of the basal respiration of sustained cellular function and the respiratory reserve, which represents the “back-up” extra fuel for the cell that is normally used under active conditions (e.g., an excitable neuron). Active neurons with a Ca²⁺ handling phenotype like SNc DA neurons, reach a bioenergetics cliff owing to the enhanced stimulation of the tricarboxylic Krebs cycle (TCA) by Ca²⁺. This boost increases respiratory capacity of the cell to generate and supply more ATP necessary to support further Ca²⁺ extrusion. (B) Hypothetical plot of ROS production by the electron transport chain (ETC) as a function of inner mitochondrial membrane potential. Also plotted are the effects of Ca²⁺ stimulation of the TCA cycle and the effects of complex V and uncoupling proteins (UCPs). (C) Metabolic demand expressed as an increase in respiratory capacity comes at a trade-off of generating reactive oxygen species (ROS) (shown in the graph as the increase in oxidant stress), which will feedback to negatively affect proteostatic function and compromise cellular respiration until it reaches the point of a bioenergetics failure, in which a cell can no longer meet the metabolic demand and will fail to continuously supply the cell with energy fuel to extrude Ca²⁺.