The role of cyclic AMP signaling in promoting axonal regeneration after spinal cord injury

Beyond PDE4 inhibition: A comprehensive review on downstream cAMP signaling in the central nervous system

Cyclic adenosine monophosphate (cAMP) is a key second messenger that regulates signal transduction pathways pivotal for numerous biological functions. Intracellular cAMP levels are spatiotemporally regulated by their hydrolyzing enzymes called phosphodiesterases (PDEs). It has been shown that increased cAMP levels in the central nervous system (CNS) promote neuroplasticity, neurotransmission, neuronal survival, and myelination while suppressing neuroinflammation. Thus, elevating cAMP levels through PDE inhibition provides a therapeutic approach for multiple CNS disorders, including multiple sclerosis, stroke, spinal cord injury, amyotrophic lateral sclerosis, traumatic brain injury, and Alzheimer's disease. In particular, inhibition of the cAMP-specific PDE4 subfamily is widely studied because of its high expression in the CNS. So far, the clinical translation of full PDE4 inhibitors has been hampered because of dose-limiting side effects. Hence, focusing on signaling cascades downstream activated upon PDE4 inhibition presents a promising strategy, offering novel and pharmacologically safe targets for treating CNS disorders. Yet, the underlying downstream signaling pathways activated upon PDE(4) inhibition remain partially elusive. This review provides a comprehensive overview of the existing knowledge regarding downstream mediators of cAMP signaling induced by PDE4 inhibition or cAMP stimulators. Furthermore, we highlight existing gaps and future perspectives that may incentivize additional downstream research concerning PDE(4) inhibition, thereby providing novel therapeutic approaches for CNS disorders.

Amelioration of functional and histopathological consequences after spinal cord injury through phosphodiesterase 4D (PDE4D) inhibition

Spinal cord injury (SCI) is a life-changing event that severely impacts the patient's quality of life. Modulating neuroinflammation, which exacerbates the primary injury, and stimulating neuro-regenerative repair mechanisms are key strategies to improve functional recovery. Cyclic adenosine monophosphate (cAMP) is a second messenger crucially involved in both processes. Following SCI, intracellular levels of cAMP are known to decrease over time. Therefore, preventing cAMP degradation represents a promising strategy to suppress inflammation while stimulating regeneration. Intracellular cAMP levels are controlled by its hydrolyzing enzymes phosphodiesterases (PDEs). The PDE4 family is most abundantly expressed in the central nervous system (CNS) and its inhibition has been shown to be therapeutically relevant for managing SCI pathology. Unfortunately, the use of full PDE4 inhibitors at therapeutic doses is associated with severe emetic side effects, hampering their translation toward clinical applications. Therefore, in this study, we evaluated the effect of inhibiting specific PDE4 subtypes (PDE4B and PDE4D) on inflammatory and regenerative processes following SCI, as inhibitors selective for these subtypes have been demonstrated to be well-tolerated. We reveal that administration of the PDE4D inhibitor Gebr32a, even when starting 2 dpi, but not the PDE4B inhibitor A33, improved functional as well as histopathological outcomes after SCI, comparable to results obtained with the full PDE4 inhibitor roflumilast. Furthermore, using a luminescent human iPSC-derived neurospheroid model, we show that PDE4D inhibition stabilizes neural viability by preventing apoptosis and stimulating neuronal differentiation. These findings strongly suggest that specific PDE4D inhibition offers a novel therapeutic approach for SCI.

Cyclic AMP signaling promotes regeneration of cochlear synapses after excitotoxic or noise trauma

Introduction

Cochlear afferent synapses connecting inner hair cells to spiral ganglion neurons are susceptible to excitotoxic trauma on exposure to loud sound, resulting in a noise-induced cochlear synaptopathy (NICS). Here we assessed the ability of cyclic AMP-dependent protein kinase (PKA) signaling to promote cochlear synapse regeneration, inferred from its ability to promote axon regeneration in axotomized CNS neurons, another system refractory to regeneration.

Methods

We mimicked NICS in vitro by applying a glutamate receptor agonist, kainic acid (KA) to organotypic cochlear explant cultures and experimentally manipulated cAMP signaling to determine whether PKA could promote synapse regeneration. We then delivered the cAMP phosphodiesterase inhibitor rolipram via implanted subcutaneous minipumps in noise-exposed CBA/CaJ mice to test the hypothesis that cAMP signaling could promote cochlear synapse regeneration in vivo.

Results

We showed that the application of the cell membrane-permeable cAMP agonist 8-cpt-cAMP or the cAMP phosphodiesterase inhibitor rolipram promotes significant regeneration of synapses in vitro within twelve hours after their destruction by KA. This is independent of neurotrophin-3, which also promotes synapse regeneration. Moreover, of the two independent signaling effectors activated by cAMP - the cAMP Exchange Protein Activated by cAMP and the cAMP-dependent protein kinase - it is the latter that mediates synapse regeneration. Finally, we showed that systemic delivery of rolipram promotes synapse regeneration in vivo following NICS.

Discussion

In vitro experiments show that cAMP signaling promotes synapse regeneration after excitotoxic destruction of cochlear synapses and does so via PKA signaling. The cAMP phosphodiesterase inhibitor rolipram promotes synapse regeneration in vivo in noise-exposed mice. Systemic administration of rolipram or similar compounds appears to provide a minimally invasive therapeutic approach to reversing synaptopathy post-noise.

Brief Electrical Stimulation Promotes Recovery after Surgical Repair of Injured Peripheral Nerves

Injured peripheral nerves regenerate their axons in contrast to those in the central nervous system. Yet, functional recovery after surgical repair is often disappointing. The basis for poor recovery is progressive deterioration with time and distance of the growth capacity of the neurons that lose their contact with targets (chronic axotomy) and the growth support of the chronically denervated Schwann cells (SC) in the distal nerve stumps. Nonetheless, chronically denervated atrophic muscle retains the capacity for reinnervation. Declining electrical activity of motoneurons accompanies the progressive fall in axotomized neuronal and denervated SC expression of regeneration-associated-genes and declining regenerative success. Reduced motoneuronal activity is due to the withdrawal of synaptic contacts from the soma. Exogenous neurotrophic factors that promote nerve regeneration can replace the endogenous factors whose expression declines with time. But the profuse axonal outgrowth they provoke and the difficulties in their delivery hinder their efficacy. Brief (1 h) low-frequency (20 Hz) electrical stimulation (ES) proximal to the injury site promotes the expression of endogenous growth factors and, in turn, dramatically accelerates axon outgrowth and target reinnervation. The latter ES effect has been demonstrated in both rats and humans. A conditioning ES of intact nerve days prior to nerve injury increases axonal outgrowth and regeneration rate. Thereby, this form of ES is amenable for nerve transfer surgeries and end-to-side neurorrhaphies. However, additional surgery for applying the required electrodes may be a hurdle. ES is applicable in all surgeries with excellent outcomes.

Rolipram and pentoxifylline combination ameliorates the morphological abnormalities of dorsal root ganglion neurons in experimental diabetic neuropathy by reducing mitochondrial dysfunction and apoptosis

Diabetic neuropathy (DN) is the most prevalent complication of diabetes. Pharmacological treatments for DN are often limited in efficacy, so the development of new agents to alleviate DN is essential. The aim of this study was to evaluate the effects of rolipram, a selective phosphodiesterase-4 inhibitor (PDE-4I), and pentoxifylline, a general PDE inhibitor, using a rat model of DN. In this study, a diabetic rat model was established by i.p. injection of STZ (55 mg/kg). Rats were treated with rolipram (1 mg/kg), pentoxifylline (100 mg/kg), and combination of rolipram (0.5 mg/kg) and pentoxifylline (50 mg/kg), orally for 5 weeks. After treatments, sensory function was assessed by hot plate test. Then rats were anesthetized and dorsal root ganglion (DRG) neurons isolated. Cyclic adenosine monophosphate (cAMP), adenosine triphosphate (ATP, adenosine diphosphate and mitochondrial membrane potential (MMP) levels, Cytochrome c release, Bax, Bcl-2, caspase-3 proteins expression in DRG neurons were assessed by biochemical and ELISA methods, and western blot analysis. DRG neurons were histologically examined using hematoxylin and eosin (H&E) staining method. Rolipram and/or pentoxifylline significantly attenuated sensory dysfunction by modulating nociceptive threshold. Rolipram and/or pentoxifylline treatment dramatically increased the cAMP level, prevented mitochondrial dysfunction, apoptosis and degeneration of DRG neurons, which appears to be mediated by inducing ATP and MMP, improving cytochrome c release, as well as regulating the expression of Bax, Bcl-2, and caspase-3 proteins, and improving morphological abnormalities of DRG neurons. We found maximum effectiveness with rolipram and pentoxifylline combination on mentioned factors. These findings encourage the use of rolipram and pentoxifylline combination as a novel experimental evidence for further clinical investigations in the treatment of DN.

Regeneration of neuromuscular synapses after acute and chronic denervation by inhibiting the gerozyme 15-prostaglandin dehydrogenase

To date, there are no approved treatments for the diminished strength and paralysis that result from the loss of peripheral nerve function due to trauma, heritable neuromuscular diseases, or aging. Here, we showed that denervation resulting from transection of the sciatic nerve triggered a marked increase in the prostaglandin-degrading enzyme 15-hydroxyprostaglandin dehydrogenase (15-PGDH) in skeletal muscle in mice, providing evidence that injury drives early expression of this aging-associated enzyme or gerozyme. Treating mice with a small-molecule inhibitor of 15-PGDH promoted regeneration of motor axons and formation of neuromuscular synapses leading to an acceleration in recovery of force after an acute nerve crush injury. In aged mice with chronic denervation of muscles, treatment with the 15-PGDH inhibitor increased motor neuron viability and restored neuromuscular junctions and function. These presynaptic changes synergized with previously reported muscle tissue remodeling to result in a marked increase in the strength of aged muscles. We further found that 15-PGDH aggregates defined the target fibers that are histopathologic hallmarks of human neurogenic myopathies, suggesting that the gerozyme may be involved in their etiology. Our data suggest that inhibition of 15-PGDH may constitute a therapeutic strategy to physiologically boost prostaglandin E2, restore neuromuscular connectivity, and promote recovery of strength after acute or chronic denervation due to injury, disease, or aging.

Rolipram-loaded PgP nanoparticle reduces secondary injury and enhances motor function recovery in a rat moderate contusion SCI model

Spinal cord injury (SCI) results in immediate axonal damage and cell death, as well as a prolonged secondary injury consist of a cascade of pathophysiological processes. One important aspect of secondary injury is activation of phosphodiesterase 4 (PDE4) that leads to reduce cAMP levels in the injured spinal cord. We have developed an amphiphilic copolymer, poly (lactide-co-glycolide)-graft-polyethylenimine (PgP) that can deliver Rolipram, the PDE4 inhibitor. The objective of this work was to investigate the effect of rolipram loaded PgP (Rm-PgP) on secondary injury and motor functional recovery in a rat moderate contusion SCI model. We observed that Rm-PgP can increase cAMP level at the lesion site, and reduce secondary injury such as the inflammatory response by macrophages/microglia, astrogliosis by activated astrocytes and apoptosis as well as improve neuronal survival at 4 weeks post-injury (WPI). We also observed that Rm-PgP can improve motor functional recovery after SCI over 4 WPI.

Enhancing structural plasticity of PC12 neurons during differentiation and neurite regeneration with a catalytically inactive mutant version of the zRICH protein

BACKGROUND

Studies of the molecular mechanisms of nerve regeneration have led to the discovery of several proteins that are induced during successful nerve regeneration. RICH proteins were identified as proteins induced during the regeneration of the optic nerve of teleost fish. These proteins are 2',3'-cyclic nucleotide, 3'-phosphodiesterases that can bind to cellular membranes through a carboxy-terminal membrane localization domain. They interact with the tubulin cytoskeleton and are able to enhance neuronal structural plasticity by promoting the formation of neurite branches.

RESULTS

PC12 stable transfectant cells expressing a fusion protein combining a red fluorescent protein with a catalytically inactive mutant version of zebrafish RICH protein were generated. These cells were used as a model to analyze effects of the protein on neuritogenesis. Differentiation experiments showed a 2.9 fold increase in formation of secondary neurites and a 2.4 fold increase in branching points. A 2.2 fold increase in formation of secondary neurites was observed in neurite regeneration assays.

CONCLUSIONS

The use of a fluorescent fusion protein facilitated detection of expression levels. Two computer-assisted morphometric analysis methods indicated that the catalytically inactive RICH protein induced the formation of branching points and secondary neurites both during differentiation and neurite regeneration. A procedure based on analysis of random field images provided comparable results to classic neurite tracing methods.

The impact of phosphodiesterase inhibition on neurobehavioral outcomes in preclinical models of traumatic and non-traumatic spinal cord injury: a systematic review

Study design

Systematic review.

Objective

The objective of this study was to evaluate the impact of phosphodiesterase (PDE) inhibitors on neurobehavioral outcomes in preclinical models of traumatic and non-traumatic spinal cord injury (SCI).

Methods

A systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines and was registered with PROSPERO (CRD42019150639). Searches were performed in MEDLINE and Embase. Studies were included if they evaluated the impact of PDE inhibitors on neurobehavioral outcomes in preclinical models of traumatic or non-traumatic SCI. Data were extracted from relevant studies, including sample characteristics, injury model, and neurobehavioral assessment and outcomes. Risk of bias was assessed using the SYRCLE checklist.

Results

The search yielded a total of 1,679 studies, of which 22 met inclusion criteria. Sample sizes ranged from 11 to 144 animals. PDE inhibitors used include rolipram (n = 16), cilostazol (n = 4), roflumilast (n = 1), and PDE4-I (n = 1). The injury models used were traumatic SCI (n = 18), spinal cord ischemia (n = 3), and degenerative cervical myelopathy (n = 1). The most commonly assessed outcome measures were Basso, Beattie, Bresnahan (BBB) locomotor score (n = 13), and grid walking (n = 7). Of the 22 papers that met the final inclusion criteria, 12 showed a significant improvement in neurobehavioral outcomes following the use of PDE inhibitors, four papers had mixed findings and six found PDE inhibitors to be ineffective in improving neurobehavioral recovery following an SCI. Notably, these findings were broadly consistent across different PDE inhibitors and spinal cord injury models.

Conclusion

In preclinical models of traumatic and non-traumatic SCI, the administration of PDE inhibitors appeared to be associated with statistically significant improvements in neurobehavioral outcomes in a majority of included studies. However, the evidence was inconsistent with a high risk of bias. This review provides a foundation to aid the interpretation of subsequent clinical trials of PDE inhibitors in spinal cord injury.

Systematic review registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=150639, identifier: CRD42019150639.

Pre-Clinical Assessment of Roflumilast Therapy in a Thoracic Model of Spinal Cord Injury

The failure of axons to regenerate after a spinal cord injury (SCI) remains one of the greatest challenges in neuroscience. The initial mechanical trauma is followed by a secondary injury cascade, creating a hostile microenvironment, which not only is not permissive to regeneration but also leads to further damage. One of the most promising approaches for promoting axonal regeneration is to maintain the levels of cyclic adenosine monophosphate (cAMP), specifically by a phosphodiesterase-4 (PDE4) inhibitor expressed in neural tissues. Therefore, in our study, we evaluated the therapeutic effect of an FDA-approved PDE4 inhibitor, Roflumilast (Rof), in a thoracic contusion rat model. Results indicate that the treatment was effective in promoting functional recovery. Rof-treated animals showed improvements in both gross and fine motor function. Eight weeks post-injury, the animals significantly recovered by achieving occasional weight-supported plantar steps. Histological assessment revealed a significant decrease in cavity size, less reactive microglia, as well as higher axonal regeneration in treated animals. Molecular analysis revealed that IL-10 and IL-13 levels, as well as VEGF, were increased in the serum of Rof-treated animals. Overall, Roflumilast promotes functional recovery and supports neuroregeneration in a severe thoracic contusion injury model and may be important in SCI treatment.

Lampreys and spinal cord regeneration: "a very special claim on the interest of zoologists," 1830s-present

Employing history of science methods, including analyses of the scientific literature, archival documents, and interviews with scientists, this paper presents a history of lampreys in neurobiology from the 1830s to the present. We emphasize the lamprey's roles in helping to elucidate spinal cord regeneration mechanisms. Two attributes have long perpetuated studies of lampreys in neurobiology. First, they possess large neurons, including multiple classes of stereotypically located, 'identified' giant neurons in the brain, which project their large axons into the spinal cord. These giant neurons and their axonal fibers have facilitated electrophysiological recordings and imaging across biological scales, ranging from molecular to circuit-level analyses of nervous system structures and functions and including their roles in behavioral output. Second, lampreys have long been considered amongst the most basal extant vertebrates on the planet, so they have facilitated comparative studies pointing to conserved and derived characteristics of vertebrate nervous systems. These features attracted neurologists and zoologists to studies of lampreys between the 1830s and 1930s. But, the same two attributes also facilitated the rise of the lamprey in neural regeneration research after 1959, when biologists first wrote about the spontaneous, robust regeneration of some identified CNS axons in larvae after spinal cord injuries, coupled with recovery of normal swimming. Not only did large neurons promote fresh insights in the field, enabling studies incorporating multiple scales with existing and new technologies. But investigators also were able to attach a broad scope of relevance to their studies, interpreting them as suggesting conserved features of successful, and sometimes even unsuccessful, CNS regeneration. Lamprey research demonstrated that functional recovery takes place without the reformation of the original neuronal connections, for instance, by way of imperfect axonal regrowth and compensatory plasticity. Moreover, research performed in the lamprey model revealed that factors intrinsic to neurons are integral in promoting or hindering regeneration. As this work has helped illuminate why basal vertebrates accomplish CNS regeneration so well, whereas mammals do it so poorly, this history presents a case study in how biological and medical value have been, and could continue to be, gleaned from a non-traditional model organism for which molecular tools have been developed only relatively recently.

Unleashing Spinal Cord Repair: The Role of cAMP-Specific PDE Inhibition in Attenuating Neuroinflammation and Boosting Regeneration after Traumatic Spinal Cord Injury

Traumatic spinal cord injury (SCI) is characterized by severe neuroinflammation and hampered neuroregeneration, which often leads to permanent neurological deficits. Current therapies include decompression surgery, rehabilitation, and in some instances, the use of corticosteroids. However, the golden standard of corticosteroids still achieves minimal improvements in functional outcomes. Therefore, new strategies tackling the initial inflammatory reactions and stimulating endogenous repair in later stages are crucial to achieving functional repair in SCI patients. Cyclic adenosine monophosphate (cAMP) is an important second messenger in the central nervous system (CNS) that modulates these processes. A sustained drop in cAMP levels is observed during SCI, and elevating cAMP is associated with improved functional outcomes in experimental models. cAMP is regulated in a spatiotemporal manner by its hydrolyzing enzyme phosphodiesterase (PDE). Growing evidence suggests that inhibition of cAMP-specific PDEs (PDE4, PDE7, and PDE8) is an important strategy to orchestrate neuroinflammation and regeneration in the CNS. Therefore, this review focuses on the current evidence related to the immunomodulatory and neuroregenerative role of cAMP-specific PDE inhibition in the SCI pathophysiology.

A novel type of serum‑free medium for culturing human airway epithelium cells

The present study aimed to evaluate the ability of a novel serum-free medium (SFM) to culture human airway epithelium cells (hAECs). hAECs were cultured in the novel SFM as the experimental group in the PneumaCult-Ex medium and Dulbecco's modified eagle medium (DMEM) and fetal bovine serum (FBS) as the control groups. Cell morphology, proliferative capacity, differentiation capacity and expression levels of basal cell markers were assessed accordingly in both culture systems. Optical microscope photos of hAECs were collected for cell morphology assessment. Cell Counting Kit-8 assay was conducted to assess the proliferation ability, and an air-liquid interface (ALI) assay was conducted to assess the differentiation capacity. Markers for proliferating basal and differentiated cells were relatively identified by immunohistochemical and immunofluorescent analysis. The results show that whether grown in the novel SFM or Ex medium, hAECs exhibited similar morphology at every passage, whereas cells could hardly form colonies in the DMEM + FBS group. Cells typically exhibited cobblestone shape, while a proportion of them in the novel SFM at late passage exhibited a larger shape. White vesicles appeared in the cytoplasm of some control cells at the later stage of culture. Basal cell markers (P63⁺KRT5⁺KI67⁺CC10^-) for proliferating ability were found in the hAECs cultured by the novel SFM and Ex medium. hAECs at passage 3 cultured in the novel SFM and Ex medium both had the capacity to differentiate into ciliated cells (acetylated tubulin⁺), goblet cells (MUC5AC⁺) and club cells (CC10⁺) in the ALI culture assay. In conclusion, the novel SFM was capable of culturing hAECs. The hAECs cultured by the novel SFM could proliferate and differentiate in vitro. The novel SFM does not change the morphological characteristics or biomarkers of hAECs. The novel SFM has the potential for the amplification of hAECs for scientific research and clinical application.

Electrical stimulation for the treatment of spinal cord injuries: A review of the cellular and molecular mechanisms that drive functional improvements

Spinal cord injury (SCI) is a devastating condition that causes severe loss of motor, sensory and autonomic functions. Additionally, many individuals experience chronic neuropathic pain that is often refractory to interventions. While treatment options to improve outcomes for individuals with SCI remain limited, significant research efforts in the field of electrical stimulation have made promising advancements. Epidural electrical stimulation, peripheral nerve stimulation, and functional electrical stimulation have shown promising improvements for individuals with SCI, ranging from complete weight-bearing locomotion to the recovery of sexual function. Despite this, there is a paucity of mechanistic understanding, limiting our ability to optimize stimulation devices and parameters, or utilize combinatorial treatments to maximize efficacy. This review provides a background into SCI pathophysiology and electrical stimulation methods, before exploring cellular and molecular mechanisms suggested in the literature. We highlight several key mechanisms that contribute to functional improvements from electrical stimulation, identify gaps in current knowledge and highlight potential research avenues for future studies.

Rolipram and pentoxifylline combination ameliorates experimental diabetic neuropathy through inhibition of oxidative stress and inflammatory pathways in the dorsal root ganglion neurons

Diabetic neuropathy (DN) is the most challenging microvascular complication of diabetes and there is no suitable treatment for it, so the development of new agents to relieve DN is urgently needed. Since oxidative stress and inflammation play an essential role in the development of DN, clearance of these factors are good strategies for the treatment of this disease. According to key role of cyclic adenosine monophosphate (cAMP) in the regulation of oxidative stress and inflammatory pathways, it seems that phosphodiesterase inhibitors (PDEIs) can be as novel drug targets for improving DN through enhancement of cAMP level. The aim of this study was to evaluate the effects of rolipram, a selective PDE4 inhibitor, and pentoxifylline, a general PDE inhibitor on experimental model of DN and also to determine the possible mechanisms involved in the effectiveness of these agents. We investigated the effects of rolipram (1 mg/kg) and pentoxifylline (100 mg/kg) and also combination of rolipram (0.5 mg/kg) and pentoxifylline (50 mg/kg), orally for five weeks in rats that became diabetic by STZ (55 mg/kg, i.p.). After treatments, motor function was evaluated by open-field test, then rats were anesthetized and dorsal root ganglion (DRG) neurons isolated. Next, oxidative stress biomarkers and inflammatory factors were assessed by biochemical and ELISA methods, and RT-PCR analysis in DRG neurons. Rolipram and/or pentoxifylline treatment significantly attenuated DN - induced motor function deficiency by modulating distance moved and velocity. Rolipram and/or pentoxifylline treatment dramatically increased the cAMP level, as well as suppressed DN - induced oxidative stress which was associated with decrease in LPO and ROS and increase in TAC, total thiol, CAT and SOD in DRG neurons. On the other hand, the level of inflammatory factors (TNF-α, NF-kB and COX2) significantly decreased following rolipram and/or pentoxifylline administration. The maximum effectiveness was with rolipram and/or pentoxifylline combination on mentioned factors. These findings provide novel experimental evidence for further clinical investigations on rolipram and pentoxifylline combination for the treatment of DN.

Overlapping transcriptional programs promote survival and axonal regeneration of injured retinal ganglion cells

Injured neurons in the adult mammalian central nervous system often die and seldom regenerate axons. To uncover transcriptional pathways that could ameliorate these disappointing responses, we analyzed three interventions that increase survival and regeneration of mouse retinal ganglion cells (RGCs) following optic nerve crush (ONC) injury, albeit not to a clinically useful extent. We assessed gene expression in each of 46 RGC types by single-cell transcriptomics following ONC and treatment. We also compared RGCs that regenerated with those that survived but did not regenerate. Each intervention enhanced survival of most RGC types, but type-independent axon regeneration required manipulation of multiple pathways. Distinct computational methods converged on separate sets of genes selectively expressed by RGCs likely to be dying, surviving, or regenerating. Overexpression of genes associated with the regeneration program enhanced both survival and axon regeneration in vivo, indicating that mechanistic analysis can be used to identify novel therapeutic strategies.

NPC transplantation rescues sci-driven cAMP/EPAC2 alterations, leading to neuroprotection and microglial modulation

Neural progenitor cell (NPC) transplantation represents a promising treatment strategy for spinal cord injury (SCI); however, the underlying therapeutic mechanisms remain incompletely understood. We demonstrate that severe spinal contusion in adult rats causes transcriptional dysregulation, which persists from early subacute to chronic stages of SCI and affects nearly 20,000 genes in total tissue extracts. Functional analysis of this dysregulated transcriptome reveals the significant downregulation of cAMP signalling components immediately after SCI, involving genes such as EPAC2 (exchange protein directly activated by cAMP), PKA, BDNF, and CAMKK2. The ectopic transplantation of spinal cord-derived NPCs at acute or subacute stages of SCI induces a significant transcriptional impact in spinal tissue, as evidenced by the normalized expression of a large proportion of SCI-affected genes. The transcriptional modulation pattern driven by NPC transplantation includes the rescued expression of cAMP signalling genes, including EPAC2. We also explore how the sustained in vivo inhibition of EPAC2 downstream signalling via the intrathecal administration of ESI-05 for 1 week impacts therapeutic mechanisms involved in the NPC-mediated treatment of SCI. NPC transplantation in SCI rats in the presence and absence of ESI-05 administration prompts increased rostral cAMP levels; however, NPC and ESI-05 treated animals exhibit a significant reduction in EPAC2 mRNA levels compared to animals receiving only NPCs treatment. Compared with transplanted animals, NPCs + ESI-05 treatment increases the scar area (as shown by GFAP staining), polarizes microglia into an inflammatory phenotype, and increases the magnitude of the gap between NeuN + cells across the lesion. Overall, our results indicate that the NPC-associated therapeutic mechanisms in the context of SCI involve the cAMP pathway, which reduces inflammation and provides a more neuropermissive environment through an EPAC2-dependent mechanism.

Neuronal Redevelopment and the Regeneration of Neuromodulatory Axons in the Adult Mammalian Central Nervous System

One reason that many central nervous system injuries, including those arising from traumatic brain injury, spinal cord injury, and stroke, have limited recovery of function is that neurons within the adult mammalian CNS lack the ability to regenerate their axons following trauma. This stands in contrast to neurons of the adult mammalian peripheral nervous system (PNS). New evidence, provided by single-cell expression profiling, suggests that, following injury, both mammalian central and peripheral neurons can revert to an embryonic-like growth state which is permissive for axon regeneration. This “redevelopment” strategy could both facilitate a damage response necessary to isolate and repair the acute damage from injury and provide the intracellular machinery necessary for axon regrowth. Interestingly, serotonin neurons of the rostral group of raphe nuclei, which project their axons into the forebrain, display a robust ability to regenerate their axons unaided, counter to the widely held view that CNS axons cannot regenerate without experimental intervention after injury. Furthermore, initial evidence suggests that norepinephrine neurons within the locus coeruleus possess similar regenerative abilities. Several morphological characteristics of serotonin axon regeneration in adult mammals, observable using longitudinal in vivo imaging, are distinct from the known characteristics of unaided peripheral nerve regeneration, or of the regeneration seen in the spinal cord and optic nerve that occurs with experimental intervention. These results suggest that there is an alternative CNS program for axon regeneration that likely differs from that displayed by the PNS.

Multifaceted Roles of cAMP Signaling in the Repair Process of Spinal Cord Injury and Related Combination Treatments

Spinal cord injury (SCI) results in multiple pathophysiological processes, including blood–spinal cord barrier disruption, hemorrhage/ischemia, oxidative stress, neuroinflammation, scar formation, and demyelination. These responses eventually lead to severe tissue destruction and an inhibitory environment for neural regeneration.cAMP signaling is vital for neurite outgrowth and axonal guidance. Stimulating intracellular cAMP activity significantly promotes neuronal survival and axonal regrowth after SCI.However, neuronal cAMP levels in adult CNS are relatively low and will further decrease after injury. Targeting cAMP signaling has become a promising strategy for neural regeneration over the past two decades. Furthermore, studies have revealed that cAMP signaling is involved in the regulation of glial cell function in the microenvironment of SCI, including macrophages/microglia, reactive astrocytes, and oligodendrocytes. cAMP-elevating agents in the post-injury milieu increase the cAMP levels in both neurons and glial cells and facilitate injury repair through the interplay between neurons and glial cells and ultimately contribute to better morphological and functional outcomes. In recent years, combination treatments associated with cAMP signaling have been shown to exert synergistic effects on the recovery of SCI. Agents carried by nanoparticles exhibit increased water solubility and capacity to cross the blood–spinal cord barrier. Implanted bioscaffolds and injected hydrogels are potential carriers to release agents locally to avoid systemic side effects. Cell transplantation may provide permissive matrices to synergize with the cAMP-enhanced growth capacity of neurons. cAMP can also induce the oriented differentiation of transplanted neural stem/progenitor cells into neurons and increase the survival rate of cell grafts. Emerging progress focused on cAMP compartmentation provides researchers with new perspectives to understand the complexity of downstream signaling, which may facilitate the clinical translation of strategies targeting cAMP signaling for SCI repair.

Resilience of neural networks for locomotion

Locomotion is an essential behaviour for the survival of all animals. The neural circuitry underlying locomotion is therefore highly robust to a wide variety of perturbations, including injury and abrupt changes in the environment. In the short term, fault tolerance in neural networks allows locomotion to persist immediately after mild to moderate injury. In the longer term, in many invertebrates and vertebrates, neural reorganization including anatomical regeneration can restore locomotion after severe perturbations that initially caused paralysis. Despite decades of research, very little is known about the mechanisms underlying locomotor resilience at the level of the underlying neural circuits and coordination of central pattern generators (CPGs). Undulatory locomotion is an ideal behaviour for exploring principles of circuit organization, neural control and resilience of locomotion, offering a number of unique advantages including experimental accessibility and modelling tractability. In comparing three well-characterized undulatory swimmers, lampreys, larval zebrafish and Caenorhabditis elegans, we find similarities in the manifestation of locomotor resilience. To advance our understanding, we propose a comparative approach, integrating experimental and modelling studies, that will allow the field to begin identifying shared and distinct solutions for overcoming perturbations to persist in orchestrating this essential behaviour.

Design, synthesis, and pharmacological characterization of some 2-substituted-3-phenyl-quinazolin-4(3H)-one derivatives as phosphodiesterase inhibitors

Some 3-phenyl-quinazolin-4(3H)-one-2-thioethers (3a-e, 5a,b, 7a-e, 9a-d, 10a-d, and 12) along with 2-aminoquinazoline derivatives 13a-c were prepared and screened for their in vitro phosphodiesterase (PDE) inhibitory activity. Some compounds such as 7d,e, 9a,b,d, 10a,d, and 13b exhibited promising activity as compared with the non-selective PDE inhibitor IBMX. This inhibitory activity was validated by molecular docking in the active site of PDE7A and PDE4 to investigate their selectivity. Furthermore, the most active compound 10d (IC₅₀  = 1.15 μM) was tested in vivo using behavioral tests. Compound 10d was able to pass the blood-brain barrier and improve scopolamine-induced cognitive deficits. Therefore, this core can be considered as a promising scaffold for further optimization to obtain new compounds with better PDE7A selective inhibition.

Learning and Stroke Recovery: Parallelism of Biological Substrates

Stroke is a debilitating disease. Current effective therapies for stroke recovery are limited to neurorehabilitation. Most stroke recovery occurs in a limited and early time window. Many of the mechanisms of spontaneous recovery after stroke parallel mechanisms of normal learning and memory. While various efforts are in place to identify potential drug targets, an emerging approach is to understand biological correlates between learning and stroke recovery. This review assesses parallels between biological changes at the molecular, structural, and functional levels during learning and recovery after stroke, with a focus on drug and cellular targets for therapeutics.

BAY 73-6691 Alters Neuron Plasticity and Phosphorylation of Tau Through Regulation of Cyclic Guanosine Monophosphate/Protein Kinase G/Cyclic Adenosine Monophosphate Response Element-Binding Protein Pathway

Alzheimer’s disease (AD) is one of neurodegenerative diseases characterized by cognitive and memory decline, accompanying with neurofibrillary tangles (NFTs) made of hyperphosphorylated tau protein and senile plaques (SP) accumulated by β-amyloid protein (Aβ). BAY 73-6691, an inhibitor of phosphodiesterase-9 (PDE-9), can improve learning and memory of elderly rats. However, the effects of BAY 73-6691 on neuroapoptotic and neuroinflammatory events, as well as synaptic plasticity of differentiated PC12 cells are remain unclear. In this work, we screened apoptotic cells induced by Aβ25-35 via flow cytometry. TNF-α, IL-1β, IL-6 secreted by PC12 cells were estimated by ELISA kits. The levels of cGMP, PKG and CREB mediated by BAY 73-6691 were assessed. Moreover, we conducted western blots analysis to evaluate the phosphorylation of tau and synaptic related proteins. Results showed that BAY 73-6691 could reduce Aβ25-35-triggered neuroapoptosis and neuroinflammation. Phosphorylation of tau was inhibited by BAY 73-6691, whereas sildenafil citrate (SC, an inhibitor of cGMP) partially weakened the effect of BAY 73-6691. Additionally, synaptic plasticity restored by BAY 73-6691 was also suppressed via SC. Taken together, BAY 73-6691 exhibited neuro protective effects, and altered tau phosphorylation as well as synaptic related proteins through cGMP/PKG/CREB pathway.

Structural and Functional Plasticity in the Regenerating Olfactory System of the Migratory Locust

Regeneration after injury is accompanied by transient and lasting changes in the neuroarchitecture of the nervous system and, thus, a form of structural plasticity. In this review, we introduce the olfactory pathway of a particular insect as a convenient model to visualize neural regeneration at an anatomical level and study functional recovery at an electrophysiological level. The olfactory pathway of the locust (Locusta migratoria) is characterized by a multiglomerular innervation of the antennal lobe by olfactory receptor neurons. These olfactory afferents were axotomized by crushing the base of the antenna. The resulting degeneration and regeneration in the antennal lobe could be quantified by size measurements, dye labeling, and immunofluorescence staining of cell surface proteins implicated in axonal guidance during development. Within 3 days post lesion, the antennal lobe volume was reduced by 30% and from then onward regained size back to normal by 2 weeks post injury. The majority of regenerating olfactory receptor axons reinnervated the glomeruli of the antennal lobe. A few regenerating axons project erroneously into the mushroom body on a pathway that is normally chosen by second-order projection neurons. Based on intracellular responses of antennal lobe output neurons to odor stimulation, regenerated fibers establish functional synapses again. Following complete absence after nerve crush, responses to odor stimuli return to control level within 10–14 days. On average, regeneration of afferents, and re-established synaptic connections appear faster in younger fifth instar nymphs than in adults. The initial degeneration of olfactory receptor axons has a trans-synaptic effect on a second order brain center, leading to a transient size reduction of the mushroom body calyx. Odor-evoked oscillating field potentials, absent after nerve crush, were restored in the calyx, indicative of regenerative processes in the network architecture. We conclude that axonal regeneration in the locust olfactory system appears to be possible, precise, and fast, opening an avenue for future mechanistic studies. As a perspective of biomedical importance, the current evidence for nitric oxide/cGMP signaling as positive regulator of axon regeneration in connectives of the ventral nerve cord is considered in light of particular regeneration studies in vertebrate central nervous systems.

Inhibition of phosphodiesterase IV enzyme improves locomotor and sensory complications of spinal cord injury via altering microglial activity: Introduction of Roflumilast as an alternative therapy

Despite the great search for an effective approach to minimize secondary injury in spinal cord injury (SCI) setting, there have been limited advances. Roflumilast is a selective inhibitor of phosphodiesterase 4 with potent anti-inflammatory properties. Here, we sought to explore Roflumilast efficacy in the improvement of locomotor and sensory deficits of SCI. In an animal setting, 50 male rats were randomly assigned to five groups: an SCI group receiving Placebo, three SCI groups receiving Roflumilast at the doses of 0.25, 0.5, and 1 mg/kg prior to T9 vertebra laminectomy, and a sham-operated group. Locomotor, mechanical, and thermal activities were evaluated for 28 days. At the end of the study, spinal cord samples were taken to assess the relative ratio of microglial subtypes, including M1 and M2, histopathological changes, levels of pro-inflammatory (TNF-α and IL-1β) and anti-inflammatory (IL-10) biomarkers, and cAMP level. Repeated measure analysis revealed significant effect for time-treatment interaction on locomotion [F (24, 270) = 280.7, p < 0.001], thermal sensitivity [F (16, 180) = 4.35, p < 0.001], and mechanical sensitivity [F (16, 180) = 7.96, p < 0.001]. As expected, Roflumilast significantly increased the expression of spinal cAMP. H&E staining exhibited lesser histopathological disruptions in Roflumilast-treated rodents. We also observed a significant reduction in the M1/M2 ratio (p values < 0.001) as well as in pro-inflammatory biomarkers following the administration of Roflumilast to the injured rats. Furthermore, IL-10 level was increased in rodents receiving 1 mg/kg of the reagent. In conclusion, the increased spinal cAMP following Roflumilast therapy might attenuate neuroinflammation via altering microglial activity; therefore, it could be considered as an alternative therapeutic agent for SCI complications.

High-resolution mapping of injury-site dependent functional recovery in a single axon in zebrafish

In non-mammalian vertebrates, some neurons can regenerate after spinal cord injury. One of these, the giant Mauthner (M-) neuron shows a uniquely direct link to a robust survival-critical escape behavior but appears to regenerate poorly. Here we use two-photon microscopy in parallel with behavioral assays in zebrafish to show that the M-axon can regenerate very rapidly and that the recovery of functionality lags by just days. However, we also find that the site of the injury is critical: While regeneration is poor both close and far from the soma, rapid regeneration and recovery of function occurs for injuries between 10% and 50% of total axon length. Our findings show that rapid regeneration and the recovery of function can be studied at remarkable temporal resolution after targeted injury of one single M-axon and that the decision between poor and rapid regeneration can be studied in this one axon.

Alexander Hecker et al. study the regeneration potential of the axon of the giant Mauthner (M) neuron in zebrafish. Using two-photon microscopy and behavioral assays, they show that the M-axon can recover rapidly days after injury. They also characterize the optimal injury site that enables rapid regeneration and functional recovery.

The Cellular and Molecular Patterns Involved in the Neural Differentiation of Adipose-Derived Stem Cells

Injuries to the nervous system cause serious problems among affected patients by preventing them from the possibility of living a normal life. As this tissue possesses a reduced capacity of self-regeneration currently, lots of different strategies are being developed in order to make the regeneration in the nervous system possible. Among them, tissue engineering and stem cell-based therapies are to date very exploded fields and tremendous progress has been made in this direction. As the two main components of the nervous system, react differently to injuries and behave different during disease, it is clear that two separate regeneration approaches have been taken into consideration during development of treatment. Special attention is constantly given to the potential of adipose-derived stem cells for this kind of application. Due to the fact that they present remarkable properties, they can easily be obtained and have demonstrated that are capable of engaging in neural and glial lineages, adipose-derived stem cells are promising tools for the field of nervous system regeneration. Moreover, new insights into epigenetic control and modifications during the differentiation of adipose-derived stem cells towards the neural liege could provide new methods to maximize the regeneration process. In this review, we summarize the current applications of adipose-derived stem cells for neural regeneration and discuss in-depth molecular patterns involved in the differentiation of adipose-derived stem cells in neuron-like cells and Schwann-like cells.

Invasion of microglia/macrophages and granulocytes into the Mauthner axon myelin sheath following spinal cord injury of the adult goldfish, Carassius auratus

Rapid activation of resident glia occurs after spinal cord injury. Somewhat later, innate and adaptive immune responses occur with the invasion of peripheral immune cells into the wound site. The activation of resident and peripheral immune cells has been postulated to play harmful as well as beneficial roles in the regenerative process. Mauthner cells, large identifiable neurons located in the hindbrain of most fish and amphibians, provided the opportunity to study the morphological relationship between reactive cells and Mauthner axons (M-axons) severed by spinal cord crush or by selective axotomy. After crossing in the hindbrain, the M-axons of adult goldfish, Carassius auratus, extend the length of the spinal cord. Following injury, the M-axon undergoes retrograde degeneration within its myelin sheath creating an axon-free zone (proximal dieback zone). Reactive cells invade the wound site, enter the axon-free dieback zone and are observed in the vicinity of the retracted M-axon tip as early as 3 hr postinjury. Transmission electron microscopy allowed the detection of microglia/macrophages and granulocytes, some of which appear to be neutrophil-like, at each of these locations. We believe that this is the first report of the invasion of such cells within the myelin sheath of an identifiable axon in the vertebrate central nervous system (CNS). We speculate that microglia/macrophages and granulocytes that are attracted within a few hours to the damaged M-axon are part of an inflammatory response that allows phagocytosis of debris and plays a role in the regenerative process. Our results provide the baseline from which to utilize immunohistochemical and genetic approaches to elucidate the role of non-neuronal cells in the regenerative process of a single axon in the vertebrate CNS.

Brainstem-Evoked Transcription of Defensive Genes After Spinal Cord Injury

The spinal cord after injury shows altered transcription in numerous genes. We tested in a pilot study whether the nucleus raphé magnus, a descending serotonergic brainstem region whose stimulation improves recovery after incomplete spinal cord injury (SCI), can influence these transcriptional changes. Rats received 2 h of low-frequency electrical stimulation in the raphé magnus 3 days after an impact contusion at segment T8. Comparison groups lacked injuries or activated stimulators or both. Immediately following stimulation, spinal cords were extracted, their RNA transcriptome sequenced, and differential gene expression quantified. Confirming many previous studies, injury primarily increased inflammatory and immune transcripts and decreased those related to lipid and cholesterol synthesis and neuronal signaling. Stimulation plus injury, contrasted with injury alone, caused significant changes in 43 transcripts (39 increases, 4 decreases), all protein-coding. Injury itself decreased only four of these 43 transcripts, all reversed by stimulation, and increased none of them. The non-specific 5-HT7 receptor antagonist pimozide reversed 25 of the 43 changes. Stimulation in intact rats principally caused decreases in transcripts related to oxidative phosphorylation, none of which were altered by stimulation in injury. Gene ontology (biological process) annotations comparing stimulation with either no stimulation or pimozide treatment in injured rats highlighted defense responses to lipopolysaccharides and microorganisms, and also erythrocyte development and oxygen transport (possibly yielding cellular oxidant detoxification). Connectivity maps of human orthologous genes generated in the CLUE database of perturbagen-response transcriptional signatures showed that drug classes whose effects in injured rats most closely resembled stimulation without pimozide include peroxisome proliferator-activated receptor agonists and angiotensin receptor blockers, which are reportedly beneficial in SCI. Thus the initial transcriptional response of the injured spinal cord to raphé magnus stimulation is upregulation of genes that in various ways are mostly protective, some probably located in recently arrived myeloid cells.

Circ-Spidr enhances axon regeneration after peripheral nerve injury

Accumulating evidence suggests that circular RNAs (circRNAs) are abundant and play critical roles in the nervous system. However, their functions in axon regeneration after neuronal injury are unclear. Due to its robust regeneration capacity, peripheral nervous system is ideal for seeking the regulatory circRNAs in axon regeneration. In the present work, we obtained an expression profile of circRNAs in dorsal root ganglions (DRGs) after rat sciatic nerve crush injury by RNA sequencing (RNA-Seq) and found the expression level of circ-Spidr was obviously increased using quantitative real-time polymerase chain reaction (qRT-PCR). Furthermore, circ-Spidr was proved to be a circular RNA enriched in the cytoplasm of DRG neurons. Through in vitro and in vivo experiments, we determined that down-regulation of circ-Spidr could suppress axon regeneration of DRG neurons after sciatic nerve injury partially through modulating PI3K-Akt signaling pathway. Together, our results reveal a crucial role for circRNAs in regulating axon regeneration after neuronal injury which may further serve as a potential therapeutic avenue for neuronal injury repair.

Effect of Pentoxifylline on Staurosporine-Induced Neurite Elongation in PC12 Cells

Objective:

Pentoxifylline enhances neurite elongation in PC12 cells. This study investigated the effects of pentoxifylline on staurosporine-induced neurite elongation in PC12 cells.

Materials and Methods:

There were five treatment groups, including treatment group I (1 nM), treatment group II (10 nM), treatment group III (100 nM), treatment group IV (1uM), and treatment group V (10 mM of pentoxifylline), together with 214 nM staurosporine for a range of time (6, 12 and 24 hours). Cells only treated with staurosporine at a concentration of 214 nM were used as the control group. Cell proliferation, cell death, immunocytochemistry assay, and Total Neurite Length were assessed.

Results:

The results showed that pentoxifylline increased cell viability (p<0.05) in a dose- and time-dependent manner, and cell death assay showed that cell death decreased in a dose- and time-dependent manner (p<0.05). TNL increased significantly compared with control cells (p<0.05). Immunocytochemistry assay showed that pentoxifylline at low and high concentrations enhanced β-tubulin III and GFAP protein expression compared with control cells.

Conclusion:

It can be concluded that pentoxifylline has positive effects on the staurosporine-induced neurite outgrowth process in PC12 cells.

Epac2 Elevation Reverses Inhibition by Chondroitin Sulfate Proteoglycans In Vitro and Transforms Postlesion Inhibitory Environment to Promote Axonal Outgrowth in an Ex Vivo Model of Spinal Cord Injury

Millions of patients suffer from debilitating spinal cord injury (SCI) without effective treatments. Elevating cAMP promotes CNS neuron growth in the presence of growth-inhibiting molecules. cAMP's effects on neuron growth are partly mediated by Epac, comprising Epac1 and Epac2; the latter predominantly expresses in postnatal neural tissue. Here, we hypothesized that Epac2 activation would enhance axonal outgrowth after SCI. Using in vitro assays, we demonstrated, for the first time, that Epac2 activation using a specific soluble agonist (S-220) significantly enhanced neurite outgrowth of postnatal rat cortical neurons and markedly overcame the inhibition by chondroitin sulfate proteoglycans and mature astrocytes on neuron growth. We further investigated the novel potential of Epac2 activation in promoting axonal outgrowth by an ex vivo rat model of SCI mimicking post-SCI environment in vivo and by delivering S-220 via a self-assembling Fmoc-based hydrogel that has suitable properties for SCI repair. We demonstrated that S-220 significantly enhanced axonal outgrowth across the lesion gaps in the organotypic spinal cord slices, compared with controls. Furthermore, we elucidated, for the first time, that Epac2 activation profoundly modulated the lesion environment by reducing astrocyte/microglial activation and transforming astrocytes into elongated morphology that guided outgrowing axons. Finally, we showed that S-220, when delivered by the gel at 3 weeks after contusion SCI in male adult rats, resulted in significantly better locomotor performance for up to 4 weeks after treatment. Our data demonstrate a promising therapeutic potential of S-220 in SCI, via beneficial effects on neurons and glia after injury to facilitate axonal outgrowth.SIGNIFICANCE STATEMENT During development, neuronal cAMP levels decrease significantly compared with the embryonic stage when the nervous system is established. This has important consequences following spinal cord injury, as neurons fail to regrow. Elevating cAMP levels encourages injured CNS neurons to sprout and extend neurites. We have demonstrated that activating its downstream effector, Epac2, enhances neurite outgrowth in vitro, even in the presence of an inhibitory environment. Using a novel biomaterial-based drug delivery system in the form of a hydrogel to achieve local delivery of an Epac2 agonist, we further demonstrated that specific activation of Epac2 enhances axonal outgrowth and minimizes glial activation in an ex vivo model of spinal cord injury, suggesting a new strategy for spinal cord repair.

The influence of drugs on peripheral nerve regeneration

Currently, there are no established adjuvant drugs for the acceleration of peripheral nerve regeneration. In this paper, we reviewed the literature from the last 10 years and described the drugs proved to accelerate the functional and histological regeneration of the peripheral nerves, either after trauma or in neuropathy experimental models. The vast majority of the studies were experimental with very few small clinical studies, which indicates the need for prospective randomized studies to identify the best drugs to use as adjuvants for nerve regeneration.

Phosphodiesterase Inhibitors Revert Axonal Dystrophy in Friedreich's Ataxia Mouse Model

Friedreich's ataxia (FRDA) is a neurodegenerative disorder caused by an unstable GAA repeat expansion within intron 1 of the FXN gene and characterized by peripheral neuropathy. A major feature of FRDA is frataxin deficiency with the loss of large sensory neurons of the dorsal root ganglia (DRG), namely proprioceptive neurons, undergoing dying-back neurodegeneration with progression to posterior columns of the spinal cord and cerebellar ataxia. We used isolated DRGs from a YG8R FRDA mouse model and C57BL/6J control mice for a proteomic study and a primary culture of sensory neurons from DRG to test novel pharmacological strategies. We found a decreased expression of electron transport chain (ETC) proteins, the oxidative phosphorylation (OXPHOS) system and antioxidant enzymes, confirming a clear impairment in mitochondrial function and an oxidative stress-prone phenotype. The proteomic profile also showed a decreased expression in Ca2+ signaling related proteins and G protein-coupled receptors (GPCRs). These receptors modulate intracellular cAMP/cGMP and Ca2+ levels. Treatment of frataxin-deficient sensory neurons with phosphodiesterase (PDE) inhibitors was able to restore improper cytosolic Ca2+ levels and revert the axonal dystrophy found in DRG neurons of YG8R mice. In conclusion, the present study shows the effectiveness of PDE inhibitors against axonal degeneration of sensory neurons in YG8R mice. Our findings indicate that PDE inhibitors may become a future FRDA pharmacological treatment.

Wheel Running Improves Motor Function and Spinal Cord Plasticity in Mice With Genetic Absence of the Corticospinal Tract

Our previous studies showed that mutant mice with congenital absence of the corticospinal tract (CST) undergo spontaneous remodeling of motor networks to partially compensate for absent CST function. Here, we asked whether voluntary wheel running could further improve locomotor plasticity in CST-deficient mice. Adult mutant mice were randomly allocated to a “runners” group with free access to a wheel, or a “non-runners” group with no access to a wheel. In comparison with non-runners, there was a significant motor improvement including fine movement, grip strength, decreased footslip errors in runners after 8-week training, which was supported by the elevated amplitude of electromyography recording and increased neuromuscular junctions in the biceps. In runners, terminal ramifications of monoaminergic and rubrospinal descending axons were significantly increased in spinal segments after 12 weeks of exercise compared to non-runners. 5-ethynyl-2′-deoxyuridine (EDU) labeling showed that proliferating cells, 90% of which were Olig2-positive oligodendrocyte progenitors, were 4.8-fold more abundant in runners than in non-runners. In 8-week runners, RNAseq analysis of spinal samples identified 404 genes up-regulated and 398 genes down-regulated, and 69 differently expressed genes involved in signal transduction, among which the NF-κB, PI3K-Akt and cyclic AMP (cAMP) signaling were three top pathways. Twelve-week training induced a significant elevation of postsynaptic density protein 95 (PSD95), synaptophysin 38 and myelin basic protein (MBP), but not of brain derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF) and insulin like growth factor-1 (IGF-1). Thus, locomotor training activates multiple signaling pathways, contributes to neural plasticity and functional improvement, and might palliate locomotor deficits in patients.

Serotonin inhibits axonal regeneration of identifiable descending neurons after a complete spinal cord injury in lampreys

Classical neurotransmitters are mainly known for their roles as neuromodulators, but they also play important roles in the control of developmental and regenerative processes. Here, we used the lamprey model of spinal cord injury to study the effect of serotonin in axon regeneration at the level of individually identifiable descending neurons. Pharmacological and genetic manipulations after a complete spinal cord injury showed that endogenous serotonin inhibits axonal regeneration in identifiable descending neurons through the activation of serotonin 1A receptors and a subsequent decrease in cyclic adenosine monophosphate (cAMP) levels. RNA sequencing revealed that changes in the expression of genes that control axonal guidance could be a key factor determining the serotonin effects during regeneration. This study provides new targets of interest for research in non-regenerating mammalian models of traumatic central nervous system injuries and extends the known roles of serotonin signalling during neuronal regeneration.

This article has an associated First Person interview with the first author of the paper.

Summary: Pharmacological and genetic manipulations show that endogenous serotonin inhibits axonal regeneration of individually identifiable descending neurons of lampreys after a complete spinal cord injury.

Role of cAMP and phosphodiesterase signaling in liver health and disease

Liver disease is a significant health problem worldwide with mortality reaching around 2 million deaths a year. Non-alcoholic fatty liver disease (NAFLD) and alcoholic liver disease (ALD) are the major causes of chronic liver disease. Pathologically, NAFLD and ALD share similar patterns of hepatic disorders ranging from simple steatosis to steatohepatitis, fibrosis and cirrhosis. It is becoming increasingly important to identify new pharmacological targets, given that there is no FDA-approved therapy yet for either NAFLD or ALD. Since the evolution of liver diseases is a multifactorial process, several mechanisms involving parenchymal and non-parenchymal hepatic cells contribute to the initiation and progression of liver pathologies. Moreover, certain protective molecular pathways become repressed during liver injury including signaling pathways such as the cyclic adenosine monophosphate (cAMP) pathway. cAMP, a key second messenger molecule, regulates various cellular functions including lipid metabolism, inflammation, cell differentiation and injury by affecting gene/protein expression and function. This review addresses the current understanding of the role of cAMP metabolism and consequent cAMP signaling pathway(s) in the context of liver health and disease. The cAMP pathway is extremely sophisticated and complex with specific cellular functions dictated by numerous factors such abundance, localization and degradation by phosphodiesterases (PDEs). Furthermore, because of the distinct yet divergent roles of both of its effector molecules, the cAMP pathway is extensively targeted in liver injury to modify its role from physiological to therapeutic, depending on the hepatic condition. This review also examines the behavior of the cAMP-dependent pathway in NAFLD, ALD and in other liver diseases and focuses on PDE inhibition as an excellent therapeutic target in these conditions.

Dibutyryl Cyclic Adenosine Monophosphate Rescues the Neurons From Degeneration in Stab Wound and Excitotoxic Injury Models

Dibutyryl cyclic adenosine monophosphate (dBcAMP), a cell-permeable synthetic analog of cAMP, has been shown to induce astrogliosis in culture. However, the exact mechanism underlying how dBcAMP exerts its function in situ is not clear. The objective of this study was to examine the effects of dBcAMP on astrogliosis and survival of neurons in stab wound and kainic acid models of brain injury. Stab wound was done in cerebral cortex of BALB/c male mice. Kainic acid lesion was induced in hippocampus by injecting 1μl kainic acid into the lateral ventricle. Animals in both models of injury were divided into L+dBcAMP and L+PBS groups and treated with dBcAMP or PBS for 3, 5, and 7 days respectively. The brain sections were stained for Cresyl violet and Fluro jade-B to assess the degenerating neurons. Immunostaining for GFAP and Iba-1 was done for assessing the astrogliosis and microglial response respectively. Expression of GFAP and BDNF levels in the tissue were estimated by Western blotting and ELISA respectively. The results showed a gradual increase in the number of both astrocytes and microglia in both injuries with a significant increase in dBcAMP-treated groups. The number of degenerating neurons significantly decreased in dBcAMP treated groups. In addition, it was found that dBcAMP stimulated the expression of GFAP and BDNF in both stab wound and kainic acid injuries. Treatment with BDNF receptor inhibitor AZ-23, showed an increase in the degenerating neurons suggesting the role of BDNF in neuroprotection. This study indicates that dBcAMP protects neurons from degeneration by enhancing the production of BDNF and may be considered for use as therapeutic agent for treatment of brain injuries.

Phosphodiesterase 4 and 7 inhibitors produce protective effects against high glucose-induced neurotoxicity in PC12 cells via modulation of the oxidative stress, apoptosis and inflammation pathways

Diabetic neuropathy (DN) is the most common diabetic complication. It is estimated diabetic population will increase to 592 million by the year 2035. This is while at least 50-60% of all diabetic patients will suffer from neuropathy in their lifetime. Oxidative stress, mitochondrial dysfunction, apoptosis, and inflammation are crucial pathways in development and progression of DN. Since there is also no selective and effective therapeutic agent to prevent or treat high glucose (HG)-induced neuronal cell injury, it is crucial to explore tools by which one can reduce factors related to these pathways. Phosphodiesterase 4 and 7 (PDE 4 and 7) regulate oxidative damage, neurodegenaration, and inflammatory responses through modulation of cyclic adenosine monophosphate (cAMP) level, and thus can be as important drug targets for regulating DN. The aim of this study was to evaluate the protective effects of inhibitors of PDE 4 and 7, named rolipram and BRL5048, on HG-induced neurotoxicity in PC12 cells as an in vitro cellular model for DN and determine the possible mechanisms for theirs effects. We report that the PC12 cells pre-treatment with rolipram (2 μM) and/or BRL5048 (0.2 μM) for 60 min and then exposing the cells to HG (4.5 g/L for 72 h) or normal glucose (NG) (1 g/L for 72 h) condition show: (1) significant attenuation in ROS, MDA and TNF-a levels, Bax/Bcl-2 ratio, expression of caspase 3 and UCP2 proteins; (2) significant increase in viability, GSH/GSSG ratio, MMP and ATP levels. All these data together led us to propose PDE 4 and 7 inhibitors, and specifically, rolipram and BRL5048, as potential drugs candidate to be further studied for the prevention and treatment of DN.

Melanopsin (Opn4) utilizes Gαi and Gβγ as major signal transducers

Melanopsin (Opn4), a ubiquitously expressed photoreceptor in all classes of vertebrates, is crucial for both visual and non-visual signaling. Opn4 supports visual functions of the eye by sensing radiance levels and discriminating contrast and brightness. Non-image-forming functions of Opn4 not only regulate circadian behavior, but also control growth and development processes of the retina. It is unclear how a single photoreceptor could govern such a diverse range of physiological functions; a role in genetic hardwiring could be one explanation, but molecular and mechanistic evidence is lacking. In addition to its role in canonical G_q pathway activation, here we demonstrate that Opn4 efficiently activates G_i heterotrimers and signals through the G protein βγ. Compared with the low levels of G_i pathway activation observed for several G_q-coupled receptors, the robust Gα_i and Gβγ signaling of Opn4 led to both generation of PIP₃ and directional migration of RAW264.7 macrophages. We propose that the ability of Opn4 to signal through Gα_i and Gβγ subunits is a major contributor to its functional diversity.

The Dorsal Column Lesion Model of Spinal Cord Injury and Its Use in Deciphering the Neuron-Intrinsic Injury Response

The neuron‐intrinsic response to axonal injury differs markedly between neurons of the peripheral and central nervous system. Following a peripheral lesion, a robust axonal growth program is initiated, whereas neurons of the central nervous system do not mount an effective regenerative response. Increasing the neuron‐intrinsic regenerative response would therefore be one way to promote axonal regeneration in the injured central nervous system. The large‐diameter sensory neurons located in the dorsal root ganglia are pseudo‐unipolar neurons that project one axon branch into the spinal cord, and, via the dorsal column to the brain stem, and a peripheral process to the muscles and skin. Dorsal root ganglion neurons are ideally suited to study the neuron‐intrinsic injury response because they exhibit a successful growth response following peripheral axotomy, while they fail to do so after a lesion of the central branch in the dorsal column. The dorsal column injury model allows the neuron‐intrinsic regeneration response to be studied in the context of a spinal cord injury. Here we will discuss the advantages and disadvantages of this model. We describe the surgical methods used to implement a lesion of the ascending fibers, the anatomy of the sensory afferent pathways and anatomical, electrophysiological, and behavioral techniques to quantify regeneration and functional recovery. Subsequently we review the results of experimental interventions in the dorsal column lesion model, with an emphasis on the molecular mechanisms that govern the neuron‐intrinsic injury response and manipulations of these after central axotomy. Finally, we highlight a number of recent advances that will have an impact on the design of future studies in this spinal cord injury model, including the continued development of adeno‐associated viral vectors likely to improve the genetic manipulation of dorsal root ganglion neurons and the use of tissue clearing techniques enabling 3D reconstruction of regenerating axon tracts. © 2018 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 00: 000–000, 2018

Neuron-Macrophage Co-cultures to Activate Macrophages Secreting Molecular Factors with Neurite Outgrowth Activity

There is strong evidence that macrophages can participate in the regeneration or repair of injured nervous system. Here, we describe a protocol in which macrophages are induced to produce conditioned medium (CM) that promotes neurite outgrowth. Adult dorsal root ganglion (DRG) neurons are acutely dissociated and plated. After the neurons are stably attached, peritoneal macrophages are co-cultured on a cell culture insert overlaid on the same well. Dibutyryl cyclic AMP (db-cAMP) is applied to the co-cultures for 24 h, after which the cell culture insert containing the macrophages is moved to another well to collect CM for 72 h. The CM from the co-cultures treated with db-cAMP, when applied to a separate adult DRG neuron culture, exhibits robust neurite outgrowth activity. The CM obtained from the db-cAMP-treated cultures consisting of single cell type alone, either DRG neuron or peritoneal macrophage, did not exhibit neurite outgrowth activity. This indicates that the interaction between neurons and macrophages is indispensable for the activation of macrophages secreting molecular factors with neurite outgrowth activity into CM. Thus, our co-culture paradigm will also be useful to study intercellular signaling in the neuron-macrophage interaction to stimulate the macrophages to be endowed with a pro-regenerative phenotype.

Phosphodiesterase 1: A Unique Drug Target for Degenerative Diseases and Cognitive Dysfunction

The focus of this chapter is on the cyclic nucleotide phosphodiesterase 1 (PDE1) family. PDE1 is one member of the 11 PDE families (PDE 1-11). It is the only phosphodiesterase family that is calcium/calmodulin activated. As a result, whereas other families of PDEs 2-11 play a dominant role controlling basal levels of cyclic nucleotides, PDE1 is involved when intra-cellular calcium levels are elevated and, thus, has an "on demand" or activity-dependent involvement in the control of cyclic nucleotides in excitatory cells including neurons, cardiomyocytes and smooth muscle. As a Class 1 phosphodiesterase, PDE1 hydrolyzes the 3' bond of 3'-5'-cyclic nucleotides, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Here, we review evidence for this family of enzymes as drug targets for development of therapies aimed to address disorders of the central nervous system (CNS) and of degenerative diseases. The chapter includes sections on the potential for cognitive enhancement in mental disorders, as well as a review of PDE1 enzyme structure, enzymology, tissue distribution, genomics, inhibitors, pharmacology, clinical trials, and therapeutic indications. Information is taken from public databases. A number of excellent reviews of the phosphodiesterase family have been written as well as reviews of the PDE1 family. References cited here are not comprehensive, rather pointing to major reviews and key publications.

Cell therapy for spinal cord injury with olfactory ensheathing glia cells (OECs)

The prospects of achieving regeneration in the central nervous system (CNS) have changed, as most recent findings indicate that several species, including humans, can produce neurons in adulthood. Studies targeting this property may be considered as potential therapeutic strategies to respond to injury or the effects of demyelinating diseases in the CNS. While CNS trauma may interrupt the axonal tracts that connect neurons with their targets, some neurons remain alive, as seen in optic nerve and spinal cord (SC) injuries (SCIs). The devastating consequences of SCIs are due to the immediate and significant disruption of the ascending and descending spinal pathways, which result in varying degrees of motor and sensory impairment. Recent therapeutic studies for SCI have focused on cell transplantation in animal models, using cells capable of inducing axon regeneration like Schwann cells (SchCs), astrocytes, genetically modified fibroblasts and olfactory ensheathing glia cells (OECs). Nevertheless, and despite the improvements in such cell-based therapeutic strategies, there is still little information regarding the mechanisms underlying the success of transplantation and regarding any secondary effects. Therefore, further studies are needed to clarify these issues. In this review, we highlight the properties of OECs that make them suitable to achieve neuroplasticity/neuroregeneration in SCI. OECs can interact with the glial scar, stimulate angiogenesis, axon outgrowth and remyelination, improving functional outcomes following lesion. Furthermore, we present evidence of the utility of cell therapy with OECs to treat SCI, both from animal models and clinical studies performed on SCI patients, providing promising results for future treatments.

The protocadherin alpha cluster is required for axon extension and myelination in the developing central nervous system

In adult mammals, axon regeneration after central nervous system injury is very poor, resulting in persistent functional loss. Enhancing the ability of axonal outgrowth may be a potential treatment strategy because mature neurons of the adult central nervous system may retain the intrinsic ability to regrow axons after injury. The protocadherin (Pcdh) clusters are thought to function in neuronal morphogenesis and in the assembly of neural circuitry in the brain. We cultured primary hippocampal neurons from E17.5 Pcdhα deletion (del-α) mouse embryos. After culture for 1 day, axon length was obviously shorter in del-α neurons compared with wild-type neurons. RNA sequencing of hippocampal E17.5 RNA showed that expression levels of BDNF, Fmod, Nrp2, OGN, and Sema3d, which are associated with axon extension, were significantly down-regulated in the absence of the Pcdhα gene cluster. Using transmission electron microscopy, the ratio of myelinated nerve fibers in the axons of del-α hippocampal neurons was significantly decreased; myelin sheaths of P21 Pcdhα-del mice showed lamellar disorder, discrete appearance, and vacuoles. These results indicate that the Pcdhα cluster can promote the growth and myelination of axons in the neurodevelopmental stage.