Mechanisms of Enhanced Phrenic Long-Term Facilitation in SOD1^G93A Rats

Abstract

Amyotrophic lateral sclerosis (ALS) is a degenerative motor neuron disease, causing muscle paralysis and death from respiratory failure. Effective means to preserve/restore ventilation are necessary to increase the quality and duration of life in ALS patients. At disease end-stage in a rat ALS model (SOD1 G93A), acute intermittent hypoxia (AIH) restores phrenic nerve activity to normal levels via enhanced phrenic long-term facilitation (pLTF). Mechanisms enhancing pLTF in end-stage SOD1 G93A rats are not known. Moderate AIH-induced pLTF is normally elicited via cellular mechanisms that require the following: G q -protein-coupled 5-HT 2 receptor activation, new BDNF synthesis, and MEK/ERK signaling (the Q pathway). In contrast, severe AIH elicits pLTF via a distinct mechanism that requires the following: G s -protein-coupled adenosine 2A receptor activation, new TrkB synthesis, and PI3K/Akt signaling (the S pathway). In endstage male SOD1G93A rats and wild-type littermates, we investigated relative Q versus S pathway contributions to enhanced pLTF via intrathecal (C4) delivery of small interfering RNAs targeting BDNF or TrkB mRNA, and MEK/ERK (U0126) or PI3 kinase/Akt (PI828) inhibitors. In anesthetized, paralyzed and ventilated rats, moderate AIH-induced pLTF was abolished by siBDNF and UO126, but not siTrkB or PI828, demonstrating that enhanced pLTF occurs via the Q pathway. Although phrenic motor neuron numbers were decreased in end-stage SOD1 G93A rats (ϳ30% survival; p Ͻ 0.001), BDNF and phosphorylated ERK expression were increased in spared phrenic motor neurons (p Ͻ 0.05), consistent with increased Q-pathway contributions to pLTF. Our results increase understanding of respiratory plasticity and its potential to preserve/restore breathing capacity in ALS.

Phrenic long-term facilitation following intrapleural CTB-SAP-induced respiratory motor neuron death

“…Phrenic motor neuron death is relatively stable once it is induced by CTB-SAP ( i.e. comparison of 7 and 28 day CTB-SAP treated rats indicate no statistical difference in phrenic motor neuron death; Nichols et al, 2015b; Figure 1), and is similar to phrenic motor neuron death that has been observed in end-stage SOD1 G93A rats (Nichols et al, 2013a; 2013b; 2015a; 2017). Thus, differential phrenic motor neuron death alone (Figure 1) does not account for the differential pLTF that is exhibited following 7 and 28 days of CTB-SAP (Figures 3–5).…”

“…This latter pathway either constrains pLTF during moderate AIH, or elicits enhanced A 2A receptor dependent pLTF in response to severe AIH (Hoffman et al, 2010; Nichols et al, 2012). Since 7 day CTB-SAP treated rats exhibit enhanced pLTF (Figures 2 and 4A), it is possible that the underlying mechanism: 1) is similar to that utilized in SOD1 G93A rats (BDNF synthesis and MEK/ERK dependent pLTF; Nichols et al, 2017); 2) switches to the A 2A receptor dependent pathway to express enhanced pLTF; or 3) requires both the G q and G s pathways as has been observed following chronic intermittent hypoxia (Ling et al, 2001; McGuire et al, 2004). In contrast, pLTF is still present but enhancement is lost in 28 day CTB-SAP treated rats (Figures 4 and 5A), and it remains unknown if pLTF simply returns to normal and requires the G q and/or the G s pathway or if pLTF is being constrained by peripheral and/or local influences such as inflammation.…”

“…There were no differences in phrenic motor neuron survival for TC treated animals vs. AIH treated animals, and there were no differences for controls regardless of time-point; thus, TC and AIH treated animals were combined together and all controls were grouped together. The number of phrenic motor neurons in the C 4 segment was extrapolated from the 6 sections (length of the entire phrenic motor nucleus is ~2000μm; 40μm sections), as described previously (Nichols et al, 2013a, 2015a, 2015b, 2017).…”

The Impact of Kinases in Amyotrophic Lateral Sclerosis at the Neuromuscular Synapse: Insights into BDNF/TrkB and PKC Signaling

“…Thus, recovering the BDNF signaling is a good option for ALS therapy, due to its strong pro-survival effects through TrkB and p75 NTR in developing and injured MN [122][123][124]. However, because intrathecally administered exogenous BDNF does not improve motor function and survival in ALS patients [108] or autonomic nervous system function [109,125], it is essential to target the altered alternative splicing of the TrkB, as it is strongly related to the maintenance of NMJs done by the bidirectional synaptic and neurotrophic controls, as has already been pointed out.…”

Mechanisms of compensatory plasticity for respiratory motor neuron death

“…Although central serotonergic neurons degenerate in rodents models (87) and patients with ALS (88), mAIH-induced pLTF is enhanced in late-stage SOD1 G93A rats, and the underlying mechanisms remain 5-HT-2 receptor and BDNF-dependent, suggesting that it arises from amplification of the same fundamental mechanism (89). Enhanced pLTF is also observed following intrapleural CTB-Saporin-induced phrenic motor neuron death (90), suggesting that motor neuron death per se is sufficient to elicit certain forms of spinal respiratory motor plasticity.…”