Atypical protein kinase C expression in phrenic motor neurons of the rat

Inactivity-induced phrenic motor facilitation requires PKCζ activity within phrenic motor neurons

Prolonged inhibition of respiratory neural activity elicits a long-lasting increase in phrenic nerve amplitude once respiratory neural activity is restored. Such long-lasting facilitation represents a form of respiratory motor plasticity known as inactivity-induced phrenic motor facilitation (iPMF). Although facilitation also occurs in inspiratory intercostal nerve activity after diminished respiratory neural activity (iIMF), it is of shorter duration. Atypical PKC activity in the cervical spinal cord is necessary for iPMF and iIMF, but the site and specific isoform of the relevant atypical PKC are unknown. Here, we used RNA interference to test the hypothesis that the zeta atypical PKC isoform (PKCζ) within phrenic motor neurons is necessary for iPMF but PKCζ within intercostal motor neurons is unnecessary for transient iIMF. Intrapleural injections of siRNAs targeting PKCζ (siPKCζ) to knock down PKCζ mRNA within phrenic and intercostal motor neurons were made in rats. Control rats received a nontargeting siRNA (NTsi) or an active siRNA pool targeting a novel PKC isoform, PKCθ (siPKCθ), which is required for other forms of respiratory motor plasticity. Phrenic nerve burst amplitude and external intercostal (T2) electromyographic (EMG) activity were measured in anesthetized and mechanically ventilated rats exposed to 30 min of respiratory neural inactivity (i.e., neural apnea) created by modest hypocapnia (20 min) or a similar recording duration without neural apnea (time control). Phrenic burst amplitude was increased in rats treated with NTsi (68 ± 10% baseline) and siPKCθ (57 ± 8% baseline) 60 min after neural apnea vs. time control rats (-3 ± 3% baseline), demonstrating iPMF. In contrast, intrapleural siPKCζ virtually abolished iPMF (5 ± 4% baseline). iIMF was transient in all groups exposed to neural apnea; however, intrapleural siPKCζ attenuated iIMF 5 min after neural apnea (50 ± 21% baseline) vs. NTsi (97 ± 22% baseline) and siPKCθ (103 ± 20% baseline). Neural inactivity elevated the phrenic, but not intercostal, responses to hypercapnia, an effect that was blocked by siPKCζ. We conclude that PKCζ within phrenic motor neurons is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for, transient iIMF.NEW & NOTEWORTHY We report important new findings concerning the mechanisms regulating a form of spinal neuroplasticity elicited by prolonged inhibition of respiratory neural activity, inactivity-induced phrenic motor facilitation (iPMF). We demonstrate that the atypical PKC isoform PKCζ within phrenic motor neurons is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for, transient inspiratory intercostal facilitation. Our findings are novel and advance our understanding of mechanisms contributing to phrenic motor plasticity.

Serotonergic innervation of respiratory motor nuclei after cervical spinal injury: Impact of intermittent hypoxia

Although cervical spinal cord injury (cSCI) disrupts bulbo-spinal serotonergic projections, partial recovery of spinal serotonergic innervation below the injury site is observed after incomplete cSCI. Since serotonin contributes to functional recovery post-injury, treatments to restore or accelerate serotonergic reinnervation are of considerable interest. Intermittent hypoxia (IH) was reported to increase serotonin innervation near respiratory motor neurons in spinal intact rats, and to improve function after cSCI. Here, we tested the hypotheses that spontaneous serotonergic reinnervation of key respiratory (phrenic and intercostal) motor nuclei: 1) is partially restored 12 weeks post C2 hemisection (C2Hx); 2) is enhanced by IH; and 3) results from sprouting of spared crossed-spinal serotonergic projections below the site of injury. Serotonin was assessed via immunofluorescence in male Sprague Dawley rats with and without C2Hx (12 wks post-injury); individual groups were exposed to 28 days of: 1) normoxia; 2) daily acute IH (dAIH28: 10, 5 min 10.5% O2 episodes per day; 5 min normoxic intervals); 3) mild chronic IH (IH28-5/5: 5 min 10.5% O2 episodes; 5 min intervals; 8 h/day); or 4) moderate chronic IH (IH28-2/2: 2 min 10.5% O2 episodes; 2 min intervals; 8 h/day), simulating IH experienced during moderate sleep apnea. After C2Hx, the number of ipsilateral serotonergic structures was decreased in both motor nuclei, regardless of IH protocol. However, serotonergic structures were larger after C2Hx in both motor nuclei, and total serotonin immunolabeling area was increased in the phrenic motor nucleus but reduced in the intercostal motor nucleus. Both chronic IH protocols increased serotonin structure size and total area in the phrenic motor nuclei of uninjured rats, but had no detectable effects after C2Hx. Although the functional implications of fewer but larger serotonergic structures are unclear, we confirm that serotonergic reinnervation is substantial following injury, but IH does not affect the extent of reinnervation.

aPKC in neuronal differentiation, maturation and function

The atypical Protein Kinase Cs (aPKCs)—PRKCI, PRKCZ and PKMζ—form a subfamily within the Protein Kinase C (PKC) family. These kinases are expressed in the nervous system, including during its development and in adulthood. One of the aPKCs, PKMζ, appears to be restricted to the nervous system. aPKCs are known to play a role in a variety of cellular responses such as proliferation, differentiation, polarity, migration, survival and key metabolic functions such as glucose uptake, that are critical for nervous system development and function. Therefore, these kinases have garnered a lot of interest in terms of their functional role in the nervous system. Here we review the expression and function of aPKCs in neural development and in neuronal maturation and function. Despite seemingly paradoxical findings with genetic deletion versus gene silencing approaches, we posit that aPKCs are likely candidates for regulating many important neurodevelopmental and neuronal functions, and may be associated with a number of human neuropsychiatric diseases.

Spinal protein phosphatase 1 constrains respiratory plasticity after sustained hypoxia

Plasticity is an important aspect of the neural control of breathing. One well-studied form of respiratory plasticity is phrenic long-term facilitation (pLTF) induced by acute intermittent but not sustained hypoxia. Okadaic acid-sensitive protein phosphatases (PPs) differentially regulate phrenic nerve activity with intermittent vs. sustained hypoxia, at least partially accounting for pLTF pattern sensitivity. However, okadaic acid inhibits multiple serine/threonine phosphatases, and the relevant phosphatase (PP1, PP2A, PP5) for pLTF pattern sensitivity has not been identified. Here, we demonstrate that sustained hypoxia (25 min, 9-10.5% O₂) elicits phrenic motor facilitation in rats pretreated with bilateral intrapleural injections of small interfering RNAs (siRNAs; Accell-modified to preferentially transfect neurons, 3.33 μM, 3 days) targeting PP1 mRNA (48 ± 14% change from baseline, n = 6) but not PP2A (14 ± 9% baseline, n = 6) or nontargeting siRNAs (4 ± 10% baseline, n = 7). In time control rats (no hypoxia) treated with siRNAs ( n = 6), no facilitation was evident (-9 ± 9% baseline). siRNAs had no effect on the hypoxic phrenic response. Immunohistochemistry revealed PP1 and PP2A protein in identified phrenic motoneurons. Although PP1 and PP2A siRNAs significantly decreased PP1 and PP2A mRNA in PC12 cell cultures, we were not able to verify "knockdown" in vivo after siRNA treatment. On the other hand, PP1 and PP2A siRNAs significantly decreased PP1 and PP2A mRNA in PC12 cell cultures, verifying the intended siRNA effects. In conclusion, PP1 (not PP2A) is the relevant okadaic acid-sensitive phosphatase constraining phrenic motor facilitation after sustained hypoxia and likely contributing to pLTF pattern sensitivity. NEW & NOTEWORTHY This study demonstrates that the relevant okadaic acid-sensitive Ser/Thr protein phosphatase (PP) constraining facilitation after sustained hypoxia is PP1 and not PP2A. It suggests that PP1 may be critical in the pattern sensitivity of hypoxia-induced phrenic motor plasticity.

Cyclooxygenase enzyme activity does not impair respiratory motor plasticity after one night of intermittent hypoxia

Although inflammation is prevalent in many clinical disorders challenging breathing, we are only beginning to understand the impact of inflammation on neural mechanisms of respiratory control. We recently demonstrated one form of respiratory motor plasticity is extremely sensitive to even mild inflammation induced by a single night (8 h) of intermittent hypoxia (IH-1), mimicking aspects of obstructive sleep apnea. Specifically, phrenic long-term facilitation (pLTF) following moderate acute intermittent hypoxia (AIH) is abolished by IH-1, but restored by high doses of the non-steroidal anti-inflammatory drug, ketoprofen. Since a major target of ketoprofen is cyclooxygenase (COX) enzymes, we tested the involvement of COX in IH-1 suppression of pLTF using the selective COX inhibitor NS-398. Systemic COX inhibition (3 mg/kg, i.p., 3 h before AIH) had no effect on pLTF in normoxia treated rats (76 ± 40% change from baseline, n = 6), and did not restore pLTF in IH-1 treated rats (-9 ± 7% baseline, n = 6). Similarly, spinal COX inhibition (27 mM, 12 μl, i.t.) had no effect on pLTF in normoxic rats (76 ± 34% baseline, n = 7), and did not significantly restore pLTF after IH-1 (37 ± 18% baseline, n = 7). COX-2 protein is expressed in identified phrenic motor neurons of both normoxia and IH-1 exposed rats, but immunolabeling was minimal in surrounding microglia; IH-1 had no discernable effect on COX-2 immunoreactivity. We conclude that the inflammatory impairment of pLTF by IH-1 is independent of COX enzyme activity or upregulated COX-2 expression.

Plasticity in respiratory motor neurons in response to reduced synaptic inputs: A form of homeostatic plasticity in respiratory control?

For most individuals, the respiratory control system produces a remarkably stable and coordinated motor output-recognizable as a breath-from birth until death. Very little is understood regarding the processes by which the respiratory control system maintains network stability in the presence of changing physiological demands and network properties that occur throughout life. An emerging principle of neuroscience is that neural activity is sensed and adjusted locally to assure that neurons continue to operate in an optimal range, yet to date, it is unknown whether such homeostatic plasticity is a feature of the neurons controlling breathing. Here, we review the evidence that local mechanisms sense and respond to perturbations in respiratory neural activity, with a focus on plasticity in respiratory motor neurons. We discuss whether these forms of plasticity represent homeostatic plasticity in respiratory control. We present new analyses demonstrating that reductions in synaptic inputs to phrenic motor neurons elicit a compensatory enhancement of phrenic inspiratory motor output, a form of plasticity termed inactivity-induced phrenic motor facilitation (iPMF), that is proportional to the magnitude of activity deprivation. Although the physiological role of iPMF is not understood, we hypothesize that it has an important role in protecting the drive to breathe during conditions of prolonged or intermittent reductions in respiratory neural activity, such as following spinal cord injury or during central sleep apnea.

Phrenic long-term facilitation requires PKCθ activity within phrenic motor neurons

Acute intermittent hypoxia (AIH) induces a form of spinal motor plasticity known as phrenic long-term facilitation (pLTF); pLTF is a prolonged increase in phrenic motor output after AIH has ended. In anesthetized rats, we demonstrate that pLTF requires activity of the novel PKC isoform, PKCθ, and that the relevant PKCθ is within phrenic motor neurons. Whereas spinal PKCθ inhibitors block pLTF, inhibitors targeting other PKC isoforms do not. PKCθ is highly expressed in phrenic motor neurons, and PKCθ knockdown with intrapleural siRNAs abolishes pLTF. Intrapleural siRNAs targeting PKCζ, an atypical PKC isoform expressed in phrenic motor neurons that underlies a distinct form of phrenic motor plasticity, does not affect pLTF. Thus, PKCθ plays a critical role in spinal AIH-induced respiratory motor plasticity, and the relevant PKCθ is localized within phrenic motor neurons. Intrapleural siRNA delivery has considerable potential as a therapeutic tool to selectively manipulate plasticity in vital respiratory motor neurons.

Intermittent Hypoxia-Induced Spinal Inflammation Impairs Respiratory Motor Plasticity by a Spinal p38 MAP Kinase-Dependent Mechanism

Inflammation is characteristic of most clinical disorders that challenge the neural control of breathing. Since inflammation modulates neuroplasticity, we studied the impact of inflammation caused by prolonged intermittent hypoxia on an important form of respiratory plasticity, acute intermittent hypoxia (three, 5 min hypoxic episodes, 5 min normoxic intervals) induced phrenic long-term facilitation (pLTF). Because chronic intermittent hypoxia elicits neuroinflammation and pLTF is undermined by lipopolysaccharide-induced systemic inflammation, we hypothesized that one night of intermittent hypoxia (IH-1) elicits spinal inflammation, thereby impairing pLTF by a p38 MAP kinase-dependent mechanism. pLTF and spinal inflammation were assessed in anesthetized rats pretreated with IH-1 (2 min hypoxia, 2 min normoxia; 8 h) or sham normoxia and allowed 16 h for recovery. IH-1 (1) transiently increased IL-6 (1.5 ± 0.2-fold; p = 0.02) and inducible nitric oxide synthase (iNOS) (2.4 ± 0.4-fold; p = 0.01) mRNA in cervical spinal homogenates, (2) elicited a sustained increase in IL-1β mRNA (2.4 ± 0.2-fold; p < 0.001) in isolated cervical spinal microglia, and (3) abolished pLTF (-1 ± 5% vs 56 ± 10% in controls; p < 0.001). pLTF was restored after IH-1 by systemic NSAID administration (ketoprofen; 55 ± 9%; p < 0.001) or spinal p38 MAP kinase inhibition (58 ± 2%; p < 0.001). IH-1 increased phosphorylated (activated) p38 MAP kinase immunofluorescence in identified phrenic motoneurons and adjacent microglia. In conclusion, IH-1 elicits spinal inflammation and impairs pLTF by a spinal p38 MAP kinase-dependent mechanism. By targeting inflammation, we may develop strategies to manipulate respiratory motor plasticity for therapeutic advantage when the respiratory control system is compromised (e.g., sleep apnea, apnea of prematurity, spinal injury, or motor neuron disease).

Cellular localization of the atypical isoforms of protein kinase C (aPKCζ/PKMζ and aPKCλ/ι) on the neuromuscular synapse

Several classic and novel protein kinase C (PKC) isoforms are selectively distributed in specific cell types of the adult neuromuscular junction (NMJ), in the neuron, glia and muscle components, and are involved in many functions, including neurotransmission. Here, we investigate the presence in this paradigmatic synapse of atypical PKCs, full-length atypical PKC zeta (aPKCζ), its separated catalytic part (PKMζ) and atypical lambda-iota PKC (aPKCλ/ι). High resolution immunohistochemistry was performed using a pan-atypical PKC antibody. Our results show moderate immunolabeling on the three cells (presynaptic motor nerve terminal, teloglial Schwann cell and postsynaptic muscle cell) suggesting the complex involvement of atypical PKCs in synaptic function.

Spinal atypical protein kinase C activity is necessary to stabilize inactivity-induced phrenic motor facilitation

The neural network controlling breathing must establish rhythmic motor output at a level adequate to sustain life. Reduced respiratory neural activity elicits a novel form of plasticity in circuits driving the diaphragm known as inactivity-induced phrenic motor facilitation (iPMF), a rebound increase in phrenic inspiratory output observed once respiratory neural drive is restored. The mechanisms underlying iPMF are unknown. Here, we demonstrate in anesthetized rats that spinal mechanisms give rise to iPMF and that iPMF consists of at least two mechanistically distinct phases: (1) an early, labile phase that requires atypical PKC (PKCζ and/or PKCι/λ) activity to transition to a (2) late, stable phase. Early (but not late) iPMF is associated with increased interactions between PKCζ/ι and the scaffolding protein ZIP (PKCζ-interacting protein)/p62 in spinal regions associated with the phrenic motor pool. Although PKCζ/ι activity is necessary for iPMF, spinal atypical PKC activity is not necessary for phrenic long-term facilitation (pLTF) following acute intermittent hypoxia, an activity-independent form of spinal respiratory plasticity. Thus, while iPMF and pLTF both manifest as prolonged increases in phrenic burst amplitude, they arise from distinct spinal cellular pathways. Our data are consistent with the hypotheses that (1) local mechanisms sense and respond to reduced respiratory-related activity in the phrenic motor pool and (2) inactivity-induced increases in phrenic inspiratory output require local PKCζ/ι activity to stabilize into a long-lasting iPMF. Although the physiological role of iPMF is unknown, we suspect that iPMF represents a compensatory mechanism, assuring adequate motor output in a physiological system in which prolonged inactivity ends life.

Increased atypical PKC expression and activity in the phrenic motor nucleus following cervical spinal injury

Atypical protein kinase C (aPKC) isoforms are expressed in phrenic motor neurons, a group of motor neurons critical for breathing. Following C2 cervical hemisection (C2HS), spontaneous plasticity occurs in crossed-spinal synaptic pathways to phrenic motor neurons, at least partially restoring inspiratory phrenic activity below the injury. Since aPKCs are necessary for synaptic plasticity in other systems, we tested the hypothesis that C2HS increases aPKC expression and activity in spinal regions associated with the phrenic motor nucleus. C2 laminectomy (sham) or C2HS was performed on adult, male Lewis rats. Ventral spinal segments C3-5 were harvested 1, 3 or 28 days post-surgery, and prepared for aPKC enzyme activity assays and immunoblots. Ventral cervical aPKC activity was elevated 1 and 28, but not 3, days post-C2HS (1 day: 63% vs sham ipsilateral to injury; p<0.05; 28 day: 426% vs sham; p<0.05; no difference in ipsilateral vs contralateral response). Total PKCζ/ι protein expression was unchanged by C2HS, but total and phosphorylated PKMζ (constitutively active PKCζ isoform) increased ipsilateral to injury 28 days post-C2HS (p<0.05). Ipsilateral aPKC activity and expression were strongly correlated (r(2)=0.675, p<0.001). In a distinct group of rats, immunohistochemistry confirmed that aPKCs are expressed in neurons 28 days post-C2HS, including large, presumptive phrenic motor neurons; aPKCs were not detected in adjacent microglia (OX-42 positive cells) or astrocytes (GFAP positive cells). Changes in aPKC expression in the phrenic motor nucleus following C2HS suggests that aPKCs may contribute to functional recovery following cervical spinal injury.

Glial activation in the spinal ventral horn caudal to cervical injury

Microglia and astrocytes play complex roles following spinal cord injury (SCI), contributing to inflammatory processes that both exacerbate injury and promote functional recovery by supporting neuro-protection and neuroplasticity. The crossed phrenic phenomenon (CPP) is an example of respiratory plasticity in which C(2) cervical hemisection (C(2)HS) strengthens crossed-spinal synaptic pathways to phrenic motor neurons ipsilateral to injury. We hypothesized that microglia and astrocytes are activated in the phrenic motor nucleus caudal and ipsilateral to C(2)HS, suggesting their potential for involvement in the CPP. To test this hypothesis, an incomplete cervical spinal hemisection (C(2) lateral injury; C(2)LI) was performed, and rats were allowed to recover for 1, 3, 14 or 28 days before collecting perfused spinal tissues. Microglia (via OX42) and astrocytes [via glial fibrillary acidic protein (GFAP)] were visualized with immunofluorescence microscopy in the C(4)-C(5) ventral horn, the region encompassing most of the phrenic motor nucleus. OX42-occupied fractional area ipsilateral to injury increased with C(2)LI (vs. sham) at 1 (12.5±1.8%, p<0.001), 3 (29.0±1.9%, p<0.001), 14 (26.1±3.1%, p<0.001) and 28 (19.2±2.0%, p<0.001) days post-C(2)LI. GFAP-occupied fractional area also increased with C(2)LI at 3 (24.4±3.2%, p<0.001) and 14 (16.8±8.3%, p=0.012) days, but not at 1 (6.2±3.9%, p=0.262) or 28 (10.6±3.9%, p=0.059) days post-C(2)LI. Thus, microglia and astrocytes are activated in the phrenic motor nucleus caudal to C(2)LI, suggesting that they play a role in functional deficits and/or recovery following spinal injury.