Retinoic acid signaling in axonal regeneration

Progress and challenges in directing the differentiation of human iPSCs into spinal motor neurons

Motor neuron (MN) diseases, including amyotrophic lateral sclerosis, progressive bulbar palsy, primary lateral sclerosis and spinal muscular atrophy, cause progressive paralysis and, in many cases, death. A better understanding of the molecular mechanisms of pathogenesis is urgently needed to identify more effective therapies. However, studying MNs has been extremely difficult because they are inaccessible in the spinal cord. Induced pluripotent stem cells (iPSCs) can generate a theoretically limitless number of MNs from a specific patient, making them powerful tools for studying MN diseases. However, to reach their potential, iPSCs need to be directed to efficiently differentiate into functional MNs. Here, we review the reported differentiation protocols for spinal MNs, including induction with small molecules, expression of lineage-specific transcription factors, 2-dimensional and 3-dimensional cultures, as well as the implementation of microfluidics devices and co-cultures with other cell types, including skeletal muscle. We will summarize the advantages and disadvantages of each strategy. In addition, we will provide insights into how to address some of the remaining challenges, including reproducibly obtaining mature and aged MNs.

Effect of vitamin A on recovery from the acute phase of multiple sclerosis-related optic neuritis, double-blind, randomized, placebo-controlled trial

Background

Optic neuritis (ON) is one of the main neuro-ophthalmic presentations of multiple sclerosis (MS), and it causes optic nerve atrophy and axonal loss. However, so far, there is no effective treatment to improve long-term outcomes.

Methods

In a double-blind placebo-controlled randomized clinical trial, 50 patients with MS-related ON were allocated into two arms (24 in the control group and 26 in the intervention group) receiving either 25000IU retinyl palmitate or an identical placebo for six months. Visual evoked potential (VEP), visual acuity, and the retinal nerve fiber layer (RNFL) thickness were evaluated and compared before and after the treatment.

Results

RNFL thickness reduction in the affected eyes at sixth month compared to the baseline were 14.81 and 19.46 μm, in the intervention and control groups, respectively (P=0.017). However, VitA therapy did not affect visual acuity and VEP.

Conclusion

Vitamin A supplementation in the patients with acute ON in MS could lessen optic nerve axonal loss.

Regulatory significance of CULLIN2 in neuronal differentiation and regeneration

BACKGROUND

Scaffold proteins coordinate multiple signalling pathways by integrating various proteins but the role of these proteins in neuronal pathways remains to be elucidated. The present study focused to evaluate the expression of the scaffold protein CULLIN2 in neuronal cells.

METHODS

The neuronal precursor cell line N2A was differentiated to neurons in-vitro with retinoic acid and biochemical assays were used to understand the gene expression profiling of CULLIN2. Moreover, neddylation inhibitor MLN4924 was used to inhibit the activity of CULLIN2 and the downstream substrates were validated. Finally, the role of CULLIN2 in nerve regeneration was evaluated in an in vivo zebrafish model.

RESULTS

Experimental data showed that the neuronal cells N2A have lower expression of CULLIN2 compared to skin cell lines (HaCaT and A431) and inactivation with the neddylation inhibitor resulted in cell death. Furthermore differentiating the neural precursor cell line into neurons with retinoic acid enhanced the expression of CULLIN2. Examining downstream signalling molecules with the neddylation inhibitor illuminates that MLN4924 treatment influences the cytokine signalling cascade (JAK-STAT) in neuronal cells. Moreover, for the first time, we show that the ubiquitin ligase protein CULLIN2 is perturbed in neural regeneration. Expression profile of CULLIN2 was significantly decreased in response to a nerve injury in Zebra fish and as the nerve regenerates there is corresponding reduction in the mRNA levels.

CONCLUSION

During differentiation CULLIN2 is upregulated whereas during regeneration there is significant downregulation. Thus, our findings reveal a crucial role of the scaffold protein CULLIN2 in nerve differentiation and regeneration which can be vital for the treatment of nerve injury.

MRP5 and MRP9 play a concerted role in male reproduction and mitochondrial function

Multidrug Resistance Proteins (MRPs) are typically implicated in cancer biology. Here, we show that MRP9 and MRP5 localize to mitochondrial-associated membranes and play a concerted role in maintaining mitochondrial homeostasis and male reproductive fitness. Our work fills in significant gaps in our understanding of MRP9 and MRP5 with wider implications in male fertility. It is plausible that variants in these transporters are associated with male reproductive dysfunction.

Multidrug Resistance Proteins (MRPs) are transporters that play critical roles in cancer even though the physiological substrates of these enigmatic transporters are poorly elucidated. In Caenorhabditis elegans, MRP5/ABCC5 is an essential heme exporter because mrp-5 mutants are unviable due to their inability to export heme from the intestine to extraintestinal tissues. Heme supplementation restores viability of these mutants but fails to restore male reproductive deficits. Correspondingly, cell biological studies show that MRP5 regulates heme levels in the mammalian secretory pathway even though MRP5 knockout (KO) mice do not show reproductive phenotypes. The closest homolog of MRP5 is MRP9/ABCC12, which is absent in C. elegans, raising the possibility that MRP9 may genetically compensate for MRP5. Here, we show that MRP5 and MRP9 double KO (DKO) mice are viable but reveal significant male reproductive deficits. Although MRP9 is highly expressed in sperm, MRP9 KO mice show reproductive phenotypes only when MRP5 is absent. Both ABCC transporters localize to mitochondrial-associated membranes, dynamic scaffolds that associate the mitochondria and endoplasmic reticulum. Consequently, DKO mice reveal abnormal sperm mitochondria with reduced mitochondrial membrane potential and fertilization rates. Metabolomics show striking differences in metabolite profiles in the DKO testes, and RNA sequencing shows significant alterations in genes related to mitochondrial function and retinoic acid metabolism. Targeted functional metabolomics reveal lower retinoic acid levels in the DKO testes and higher levels of triglycerides in the mitochondria. These findings establish a model in which MRP5 and MRP9 play a concerted role in regulating male reproductive functions and mitochondrial sufficiency.

RXRα Blocks Nerve Regeneration after Spinal Cord Injury by Targeting p66shc

Nerve regeneration after spinal cord injury is regulated by many factors. Studies have found that the expression of retinoid X receptor α (RXRα) does not change significantly after spinal cord injury but that the distribution of RXRα in cells changes significantly. In undamaged tissues, RXRα is distributed in motor neurons and the cytoplasm of glial cells. RXRα migrates to the nucleus of surviving neurons after injury, indicating that RXRα is involved in the regulation of gene expression after spinal cord injury. p66shc is an important protein that regulates cell senescence and oxidative stress. It can induce the apoptosis and necrosis of many cell types, promoting body aging. The absence of p66shc enhances the resistance of cells to reactive oxygen species (ROS) and thus prolongs life. It has been found that p66shc deletion can promote hippocampal neurogenesis and play a neuroprotective role in mice with multiple sclerosis. To verify the function of RXRα after spinal cord injury, we established a rat T9 spinal cord transection model. After RXRα agonist or antagonist administration, we found that RXRα agonists inhibited nerve regeneration after spinal cord injury, while RXRα antagonists promoted the regeneration of injured neurites and the recovery of motor function in rats. The results showed that RXRα played an impeding role in repair after spinal cord injury. Immunofluorescence staining showed that p66shc expression was upregulated in neurons after spinal cord injury (in vivo and in vitro) and colocalized with RXRα. RXRα overexpression in cultured neurons promoted the expression of p66shc, while RXRα interference inhibited the expression of p66shc. Using a luciferase assay, we found that RXRα could bind to the promoter region of p66shc and regulate the expression of p66shc, thereby regulating nerve regeneration after spinal cord injury. The above results showed that RXRα inhibited nerve regeneration after spinal cord injury by promoting p66shc expression, and interference with RXRα or p66shc promoted functional recovery after spinal cord injury.

Schizophrenia risk ZNF804A interacts with its associated proteins to modulate dendritic morphology and synaptic development

Schizophrenia (SZ) is a devastating brain disease that affects about 1% of world population. Among the top genetic associations, zinc finger protein 804A (ZNF804A) gene encodes a zinc finger protein, associated with SZ and biolar disorder (BD). Copy number variants (CNVs) of ZNF804A have been observed in patients with autism spectrum disorders (ASDs), anxiety disorder, and BD, suggesting that ZNF804A is a dosage sensitive gene for brain development. However, its molecular functions have not been fully determined. Our previous interactomic study revealed that ZNF804A interacts with multiple proteins to control protein translation and neural development. ZNF804A is localized in the cytoplasm and neurites in the human cortex and is expressed in various types of neurons, including pyramidal, dopaminergic, GABAergic, and Purkinje neurons in mouse brain. To further examine the effect of gene dosage of ZNF804A on neurite morphology, both knockdown and overexpression of ZNF804A in primary neuronal cells significantly attenuate dendritic complex and spine formation. To determine the factors mediating these phenotypes, interestingly, three binding proteins of ZNF804A, galectin 1 (LGALS1), fasciculation and elongation protein zeta 1 (FEZ1) and ribosomal protein SA (RPSA), show different effects on reversing the deficits. LGALS1 and FEZ1 stimulate neurite outgrowth at basal level but RPSA shows no effect. Intriguingly, LGALS1 but not FEZ1, reverses the neurite outgrowth deficits induced by ZNF804A knockdown. However, FEZ1 and RPSA but not LGALS1, can ameliorate ZNF804A overexpression-mediated dendritic abnormalities. Thus, our results uncover a critical post-mitotic role of ZNF804A in neurite and synaptic development relevant to neurodevelopmental pathologies.

Complex functions of Gcn5 and Pcaf in development and disease

A wealth of biochemical and cellular data, accumulated over several years by multiple groups, has provided a great degree of insight into the molecular mechanisms of actions of GCN5 and PCAF in gene activation. Studies of these lysine acetyltransferases (KATs) in vitro, in cultured cells, have revealed general mechanisms for their recruitment by sequence-specific binding factors and their molecular functions as transcriptional co-activators. Genetic studies indicate that GCN5 and PCAF are involved in multiple developmental processes in vertebrates, yet our understanding of their molecular functions in these contexts remains somewhat rudimentary. Understanding the functions of GCN5/PCAF in developmental processes provides clues to the roles of these KATs in disease states. Here we will review what is currently known about the developmental roles of GCN5 and PCAF, as well as emerging role of these KATs in oncogenesis.

Cytotoxic Immunity in Peripheral Nerve Injury and Pain

Cytotoxicity and consequent cell death pathways are a critical component of the immune response to infection, disease or injury. While numerous examples of inflammation causing neuronal sensitization and pain have been described, there is a growing appreciation of the role of cytotoxic immunity in response to painful nerve injury. In this review we highlight the functions of cytotoxic immune effector cells, focusing in particular on natural killer (NK) cells, and describe the consequent action of these cells in the injured nerve as well as other chronic pain conditions and peripheral neuropathies. We describe how targeted delivery of cytotoxic factors via the immune synapse operates alongside Wallerian degeneration to allow local axon degeneration in the absence of cell death and is well-placed to support the restoration of homeostasis within the nerve. We also summarize the evidence for the expression of endogenous ligands and receptors on injured nerve targets and infiltrating immune cells that facilitate direct neuro-immune interactions, as well as modulation of the surrounding immune milieu. A number of chronic pain and peripheral neuropathies appear comorbid with a loss of function of cellular cytotoxicity suggesting such mechanisms may actually help to resolve neuropathic pain. Thus while the immune response to peripheral nerve injury is a major driver of maladaptive pain, it is simultaneously capable of directing resolution of injury in part through the pathways of cellular cytotoxicity. Our growing knowledge in tuning immune function away from inflammation toward recovery from nerve injury therefore holds promise for interventions aimed at preventing the transition from acute to chronic pain.

Mechanism of delayed encephalopathy after acute carbon monoxide poisoning

Many hypotheses exist regarding the mechanism underlying delayed encephalopathy after acute carbon monoxide poisoning (DEACMP), including the inflammation and immune-mediated damage hypothesis and the cellular apoptosis and direct neuronal toxicity hypothesis; however, no existing hypothesis provides a satisfactory explanation for the complex clinical processes observed in DEACMP. Leucine-rich repeat and immunoglobulin-like domain-containing protein-1 (LINGO-1) activates the Ras homolog gene family member A (RhoA)/Rho-associated coiled-coil containing protein kinase 2 (ROCK2) signaling pathway, which negatively regulates oligodendrocyte myelination, axonal growth, and neuronal survival, causing myelin damage and participating in the pathophysiological processes associated with many central nervous system diseases. However, whether LINGO-1 is involved in DEACMP remains unclear. A DEACMP model was established in rats by allowing them to inhale 1000 ppm carbon monoxide gas for 40 minutes, followed by 3000 ppm carbon monoxide gas for an additional 20 minutes. The results showed that compared with control rats, DEACMP rats showed significantly increased water maze latency and increased protein and mRNA expression levels of LINGO-1, RhoA, and ROCK2 in the brain. Compared with normal rats, significant increases in injured neurons in the hippocampus and myelin sheath damage in the lateral geniculate body were observed in DEACMP rats. From days 1 to 21 after DEACMP, the intraperitoneal injection of retinoic acid (10 mg/kg), which can inhibit LINGO-1 expression, was able to improve the above changes observed in the DEACMP model. Therefore, the overexpression of LINGO-1 appeared to increase following carbon monoxide poisoning, activating the RhoA/ROCK2 signaling pathway, which may be an important pathophysiological mechanism underlying DEACMP. This study was reviewed and approved by the Medical Ethics Committee of Xiangya Hospital of Central South Hospital (approval No. 201612684) on December 26, 2016.

Epigenetic inhibitor zebularine activates ear pinna wound closure in the mouse

BACKGROUND

Most studies on regenerative medicine focus on cell-based therapies and transplantations. Small-molecule therapeutics, though proved effective in different medical conditions, have not been extensively investigated in regenerative research. It is known that healing potential decreases with development and developmental changes are driven by epigenetic mechanisms, which suggests epigenetic repression of regenerative capacity.

METHODS

We applied zebularine, a nucleoside inhibitor of DNA methyltransferases, to stimulate the regenerative response in a model of ear pinna injury in mice.

FINDINGS

We observed the regeneration of complex tissue that was manifested as improved ear hole repair in mice that received intraperitoneal injections of zebularine. Six weeks after injury, the mean hole area decreased by 83.2 ± 9.4% in zebularine-treated and by 43.6 ± 15.4% in control mice (p < 10^-30). Combined delivery of zebularine and retinoic acid potentiated and accelerated this effect, resulting in complete ear hole closure within three weeks after injury. We found a decrease in DNA methylation and transcriptional activation of neurodevelopmental and pluripotency genes in the regenerating tissues.

INTERPRETATION

This study is the first to demonstrate an effective induction of complex tissue regeneration in adult mammals using zebularine. We showed that the synergistic action of an epigenetic drug (zebularine) and a transcriptional activator (retinoic acid) could be effectively utilized to induce the regenerative response, thus delineating a novel pharmacological strategy for regeneration. The strategy was effective in the model of ear pinna regeneration in mice, but zebularine acts on different cell types, therefore, a similar approach can be tested in other tissues and organs.

Potential therapeutic roles of retinoids for prevention of neuroinflammation and neurodegeneration in Alzheimer's disease

All retinoids, which can be natural and synthetic, are chemically related to vitamin A. Both natural and synthetic retinoids use specific nuclear receptors such as retinoic acid receptors and retinoid X receptors to activate specific signaling pathways in the cells. Retinoic acid signaling is extremely important in the central nervous system. Impairment of retinoic acid signaling pathways causes severe pathological processes in the central nervous system, especially in the adult brain. Retinoids have major roles in neural patterning, differentiation, axon outgrowth in normal development, and function of the brain. Impaired retinoic acid signaling results in neuroinflammation, oxidative stress, mitochondrial malfunction, and neurodegeneration leading to progressive Alzheimer's disease, which is pathologically characterized by extra-neuronal accumulation of amyloid plaques (aggregated amyloid-beta) and intra-neurofibrillary tangles (hyperphosphorylated tau protein) in the temporal lobe of the brain. Alzheimer's disease is the most common cause of dementia and loss of memory in old adults. Inactive cholinergic neurotransmission is responsible for cognitive deficits in Alzheimer's disease patients. Deficiency or deprivation of retinoic acid in mice is associated with loss of spatial learning and memory. Retinoids inhibit expression of chemokines and neuroinflammatory cytokines in microglia and astrocytes, which are activated in Alzheimer's disease. Stimulation of retinoic acid receptors and retinoid X receptors slows down accumulation of amyloids, reduces neurodegeneration, and thereby prevents pathogenesis of Alzheimer's disease in mice. In this review, we described chemistry and biochemistry of some natural and synthetic retinoids and potentials of retinoids for prevention of neuroinflammation and neurodegeneration in Alzheimer's disease.

Retinoic acid synthesis by NG2 expressing cells promotes a permissive environment for axonal outgrowth

Stimulation of retinoic acid (RA) mediated signalling pathways following neural injury leads to regeneration in the adult nervous system and numerous studies have shown that the specific activation of the retinoic acid receptor β (RARβ) is required for this process. Here we identify a novel mechanism by which neuronal RARβ activation results in the endogenous synthesis of RA which is released in association with exosomes and acts as a positive cue to axonal/neurite outgrowth. Using an established rodent model of RARβ induced axonal regeneration, we show that neuronal RARβ activation upregulates the enzymes involved in RA synthesis in a cell specific manner; alcohol dehydrogenase7 (ADH7) in neurons and aldehyde dehydrogenase 2 (Raldh2) in NG2 expressing cells (NG2 + cells). These release RA in association with exosomes providing a permissive substrate to neurite outgrowth. Conversely, deletion of Raldh2 in the NG2 + cells in our in vivo regeneration model is sufficient to compromise axonal outgrowth. This hitherto unidentified RA paracrine signalling is required for axonal/neurite outgrowth and is initiated by the activation of neuronal RARβ signalling.

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Raldh2, the enzyme for retinoic acid synthesis, is upregulated in NG2 + cells during axonal regeneration.

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Deletion of Raldh2 in NG2 + cells prevents regeneration.

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RA signalling modulates axonal pathfinding.

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Fine-tuned regulation of RA distribution via exosome transport

Retinoic acid-loaded polymeric nanoparticles induce neuroprotection in a mouse model for Parkinson's disease

Retinoic acid (RA) plays an important role in the commitment, maturation and survival of neural cells. Recently, RA was pointed as a therapeutic option for some neurodegenerative diseases, including Parkinson's disease (PD). The administration of RA has been defying, and in this sense we have previously developed novel RA-loaded polymeric nanoparticles (RA-NPs) that ensure the efficient intracellular transport and controlled release of RA. Herein, we show that nanoformulation as an efficient neuroprotective effect on dopaminergic (DA) neurons in the 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) induced mouse model for PD. The results showed that the RA-NPs administration induced a significant reduction of DA neuron loss in the substantia nigra (SN) as well as their neuronal fiber/axonal innervations in the striatum. Furthermore, we observed an increase in the expression levels of the transcription factors Pitx3 and Nurr1 induced by RA-NPs, showing its supportive effect on the development and functional maintenance of DA neurons in PD. This is the first study showing that RA-NPs can be an innovative strategy to halt the progression of PD pathogenesis, suggesting that this nanoformulation could be of particular interest for the development of new approaches for PD therapeutics.

Behavioral improvement and regulation of molecules related to neuroplasticity in ischemic rat spinal cord treated with PEDF

Pigment epithelium derived factor (PEDF) exerts trophic actions to motoneurons and modulates nonneuronal restorative events, but its effects on neuroplasticity responses after spinal cord (SC) injury are unknown. Rats received a low thoracic SC photothrombotic ischemia and local injection of PEDF and were evaluated behaviorally six weeks later. PEDF actions were detailed in SC ventral horn (motor) in the levels of the lumbar central pattern generator (CPG), far from the injury site. Molecules related to neuroplasticity (MAP-2), those that are able to modulate such event, for instance, neurotrophic factors (NT-3, GDNF, BDNF, and FGF-2), chondroitin sulfate proteoglycans (CSPG), and those associated with angiogenesis and antiapoptosis (laminin and Bcl-2) and Eph (receptor)/ephrin system were evaluated at cellular or molecular levels. PEDF injection improved motor behavioral performance and increased MAP-2 levels and dendritic processes in the region of lumbar CPG. Treatment also elevated GDNF and decreased NT-3, laminin, and CSPG. Injury elevated EphA4 and ephrin-B1 levels, and PEDF treatment increased ephrin A2 and ephrins B1, B2, and B3. Eph receptors and ephrins were found in specific populations of neurons and astrocytes. PEDF treatment to SC injury triggered neuroplasticity in lumbar CPG and regulation of neurotrophic factors, extracellular matrix molecules, and ephrins.

Retinoids activate the irritant receptor TRPV1 and produce sensory hypersensitivity

Retinoids are structurally related derivatives of vitamin A and are required for normal vision as well as cell proliferation and differentiation. Clinically, retinoids are effective in treating many skin disorders and cancers. Application of retinoids evokes substantial irritating side effects, including pain and inflammation; however, the precise mechanisms accounting for the sensory hypersensitivity are not understood. Here we show that both naturally occurring and synthetic retinoids activate recombinant or native transient receptor potential channel vanilloid subtype 1 (TRPV1), an irritant receptor for capsaicin, the pungent ingredient of chili peppers. In vivo, retinoids produced pain-related behaviors that were either eliminated or significantly reduced by genetic or pharmacological inhibition of TRPV1 function. These findings identify TRPV1 as an ionotropic receptor for retinoids and provide cellular and molecular insights into retinoid-evoked hypersensitivity. These findings also suggest that selective TRPV1 antagonists are potential therapeutic drugs for treating retinoid-induced sensory hypersensitivity.