Depletion of Calcium Stores in Injured Sensory Neurons

Abstract

Background Painful nerve injury leads to disrupted Ca2+ signaling in primary sensory neurons, including decreased endoplasmic reticulum (ER) Ca2+ storage. The present study examines potential causes and functional consequences of Ca2+ store limitation after injury. Methods Neurons were dissociated from axotomized fifth lumbar (L5) and the adjacent L4 dorsal root ganglia following L5 spinal nerve ligation that produced hyperalgesia, and were compared to neurons from control animals. Intracellular Ca2+ levels were measured with Fura-2 microfluorometry, and ER was labeled with probes or antibodies. Ultrastructural morphology was analyzed by electron microscopy of nondissociated dorsal root ganglia, and intracellular electrophysiological recordings were obtained from intact ganglia. Results Live neuron staining with BODIPY FL-X thapsigargin (Invitrogen, Carlsbad, CA) revealed a 40% decrease in sarco-endoplasmic reticulum Ca2+-ATPase binding in axotomized L5 neurons and a 34% decrease in L4 neurons. Immunocytochemical labeling for the ER Ca2+-binding protein calreticulin was unaffected by injury. Total length of ER profiles in electron micrographs was reduced by 53% in small axotomized L5 neurons, but increased in L4 neurons. Cisternal stacks of ER and aggregation of ribosomes occurred less frequently in axotomized neurons. Ca2+-induced Ca2+ release, examined by microfluorometry with dantrolene, was eliminated in axotomized neurons. Pharmacologic blockade of Ca2+-induced Ca2+ release with dantrolene produced hyperexcitability in control neurons, confirming its functional importance. Conclusions After axotomy, ER Ca2+ stores are reduced by anatomic loss and possibly diminished sarco-endoplasmic reticulum Ca2+-ATPase. The resulting disruption of Ca2+-induced Ca2+ release and protein synthesis may contribute to the generation of neuropathic pain.

Increased thrombospondin-4 after nerve injury mediates disruption of intracellular calcium signaling in primary sensory neurons

“…A consequence of ER stress is activation of extracellular signal-regulated kinase (ERK), JNK, and p38 MAPK, all of which are activated in sensory neurons after peripheral nerve injury (Jin et al, 2003; Zhuang et al, 2006), supporting the view that ER stress is a component of neuropathic pain. Furthermore, depleting ER Ca 2+ stores increases sensory neuron excitability (Gemes et al, 2009). These observations together suggest that the TSP4 signaling pathway may account for ER stress in neuropathic pain.…”

Divergent effects of painful nerve injury on mitochondrial Ca2+ buffering in axotomized and adjacent sensory neurons

“…This allows mitochondrial participation in Ca 2+ buffering despite relatively low Ca 2+ affinity of the uniporter intake path. We have observed extensively disordered ER ultrastructure after axotomy (Gemes et al, 2009), so a possible consequence is the loss of the particular physical interactions that underlie mitochondrial colocalization and the coordinated physiological function of these ultrastructural elements. In support of this, we have noted a decrease in ER profile lengths in SNL L5 neurons, but an increase in the L4 population (Gemes et al, 2009).…”

“…The cytoplasmic Ca 2+ signal initiated by Ca 2+ influx is further shaped in sensory neurons by the simultaneous processes of Ca 2+ extrusion, sequestration, and release from stores. We have identified dysfunction of these processes in axotomized neurons following painful nerve injury, including elevated function of the plasma membrane Ca 2+ -ATPase (PMCA) (Gemes et al, 2012b), accompanied by decreased resting cytoplasmic Ca 2+ ([Ca 2+ ] c ) (Fuchs et al, 2005), reduced function of the sarcoplasmic/endoplasmic reticulum Ca 2+ -ATPase (SERCA) (Duncan et al, 2013) that reduces ER Ca 2+ stores (Gemes et al, 2009; Rigaud et al, 2009), with resulting elevation of store-operated Ca 2+ entry (SOCE) (Gemes et al, 2011) and diminished release of Ca 2+ from stores upon neuronal activity through the process of Ca 2+ -induced Ca 2+ release. Together, these disturbances of Ca 2+ signaling contribute to elevated generation and transmission of high-frequency trains of action potentials in the injured sensory neurons (Gemes et al, 2009; Gemes et al, 2012a; Hogan et al, 2008; Lirk et al, 2008; Sapunar et al, 2005; Tang et al, 2012).…”

Labat Lecture: The Primary Sensory Neuron

“…Injury of the peripheral axon depletes the Ca 2+ stores, both by decreasing the size of the ER and by decreasing the concentration of Ca 2+ within the ER. 19,20 This has the effect of limiting the stored Ca 2+ available for release during neuronal activation. Because the Ca 2+ released by Ca 2+ -induced Ca 2+ release is par-ticularly important in activating K + channels, the result is exacerbation of neuronal hyperexcitability.…”

“…Because the Ca 2+ released by Ca 2+ -induced Ca 2+ release is par-ticularly important in activating K + channels, the result is exacerbation of neuronal hyperexcitability. 19 Depletion of Ca 2+ within the ER also impedes proper folding of newly minted proteins, which inhibits overall protein synthesis.…”