Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

Influence of Janus Kinase Inhibitors on the Neuronal Activity as a Proof-of-Concept Model for Itch

BACKGROUND

Itching is considered to be a subjective symptom of the activation of neurosensory structures by different signal molecules and trigger factors. The signaling cascades responsible for it are closely linked to inflammatory processes. This explains why itching also occurs in many inflammatory diseases. One of these signaling cascades is mediated by Janus kinases (JAKs). Recently, it could be shown on a molecular level that Janus kinase 1 (JAK1) directly activates frontal cortex neurons and thus can cause chronic itching.

OBJECTIVES

This study deals with the influence of different JAK inhibitors (JAKi) on the activity of chip-based neural networks of cultured frontal cortex neurons by investigating neurophysiological activity parameters. This in vitro model provides information on dose-dependent effects of model substances with different specificity regarding the inhibition of different JAKs.

METHODS

Tofacitinib (pan-JAKi), baricitinib (JAK1/2i), and upadacitinib (JAK1i) in a concentration range from 10 nmol/L to 50 μmol/L were tested in a microelectrode array neurochip culture system.

RESULTS

The results show that the inhibition of the neuronal activity of frontal cortex neurons increases with JAK1 selectivity and is dependent on concentration.

CONCLUSION

These observations are supported by data from clinical studies in atopic dermatitis and psoriasis. The clinical relevance of these results must be proven by further clinical studies with subjective and objective parameters for itching.

Dorsal Root Ganglia Homeobox downregulation in primary sensory neurons contributes to neuropathic pain in rats

Transcriptional changes in primary sensory neurons are involved in initiation and maintenance of neuropathic pain. However, the transcription factors in primary sensory neurons responsible for neuropathic pain are not fully understood. Dorsal Root Ganglia Homeobox (DRGX) is a paired-like homeodomain transcription factor necessary for the development of nociceptive primary sensory neurons during the early postnatal period. However, roles for DRGX after development are largely unknown. Here, we report that DRGX downregulation in primary sensory neurons as a result of post-developmental nerve injury contributes to neuropathic pain in rats. DRGX expression was decreased in nuclei of small and medium primary sensory neurons after spinal nerve ligation. DRGX downregulation by transduction of a short hairpin RNA with an adeno-associated viral vector induced mechanical allodynia and thermal hyperalgesia. In contrast, DRGX overexpression in primary sensory neurons suppressed neuropathic pain. DRGX regulated matrix metalloproteinase-9 (MMP-9) and prostaglandin E receptor 2 mRNA expression in the DRG. MMP-9 inhibitor attenuated DRGX downregulation-induced pain. These results suggest that DRGX downregulation after development contributes to neuropathic pain through transcriptional modulation of pain-related genes in primary sensory neurons.

Transcriptomic differential lncRNA expression is involved in neuropathic pain in rat dorsal root ganglion after spared sciatic nerve injury

Dorsal root ganglia (DRG) neurons regenerate spontaneously after traumatic or surgical injury. Long noncoding RNAs (lncRNAs) are involved in various biological regulation processes. Conditions of lncRNAs in DRG neuron injury deserve to be further investigated. Transcriptomic analysis was performed by high-throughput Illumina HiSeq2500 sequencing to profile the differential genes in L4–L6 DRGs following rat sciatic nerve tying. A total of 1,228 genes were up-regulated and 1,415 down-regulated. By comparing to rat lncRNA database, 86 known and 26 novel lncRNA genes were found to be differential. The 86 known lncRNA genes modulated 866 target genes subject to gene ontology (GO) and KEGG enrichment analysis. The genes involved in the neurotransmitter status of neurons were downregulated and those involved in a neuronal regeneration were upregulated. Known lncRNA gene rno-Cntnap2 was downregulated. There were 13 credible GO terms for the rno-Cntnap2 gene, which had a putative function in cell component of voltage-gated potassium channel complex on the cell surface for neurites. In 26 novel lncRNA genes, 4 were related to 21 mRNA genes. A novel lncRNA gene AC111653.1 improved rno-Hypm synthesizing huntingtin during sciatic nerve regeneration. Real time qPCR results attested the down-regulation of rno-Cntnap lncRNA gene and the upregulation of AC111653.1 lncRNA gene. A total of 26 novel lncRNAs were found. Known lncRNA gene rno-Cntnap2 and novel lncRNA AC111653.1 were involved in neuropathic pain of DRGs after spared sciatic nerve injury. They contributed to peripheral nerve regeneration via the putative mechanisms.

The Glutamatergic System in Primary Somatosensory Neurons and Its Involvement in Sensory Input-Dependent Plasticity

Glutamate is the most common neurotransmitter in both the central and the peripheral nervous system. Glutamate is present in all types of neurons in sensory ganglia, and is released not only from their peripheral and central axon terminals but also from their cell bodies. Consistently, these neurons express ionotropic and metabotropic receptors, as well as other molecules involved in the synthesis, transport and release of the neurotransmitter. Primary sensory neurons are the first neurons in the sensory channels, which receive information from the periphery, and are thus key players in the sensory transduction and in the transmission of this information to higher centers in the pathway. These neurons are tightly enclosed by satellite glial cells, which also express several ionotropic and metabotropic glutamate receptors, and display increases in intracellular calcium accompanying the release of glutamate. One of the main interests in our group has been the study of the implication of the peripheral nervous system in sensory-dependent plasticity. Recently, we have provided novel evidence in favor of morphological changes in first- and second-order neurons of the trigeminal system after sustained alterations of the sensory input. Moreover, these anatomical changes are paralleled by several molecular changes, among which those related to glutamatergic neurotransmission are particularly relevant. In this review, we will describe the state of the art of the glutamatergic system in sensory ganglia and its involvement in input-dependent plasticity, a fundamental ground for advancing our knowledge of the neural mechanisms of learning and adaptation, reaction to injury, and chronic pain.

Increased fronto-hippocampal connectivity in the Prrxl1 knockout mouse model of congenital hypoalgesia

Despite the large number of studies addressing how prolonged painful stimulation affects brain functioning, there are only a handful of studies aimed at uncovering if persistent conditions of reduced pain perception would also result in brain plasticity. Permanent hypoalgesia induced by neonatal injection of capsaicin or carrageenan has already been shown to affect learning and memory and to induce alterations in brain gene expression. In this study, we used the Prrxl1 model of congenital mild hypoalgesia to conduct a detailed study of the neurophysiological and behavioral consequences of reduced pain experience. Prrxl1 knockout animals are characterized by selective depletion of small diameter primary afferents and abnormal development of the superficial dorsal laminae of the spinal cord, resulting in diminished pain perception but normal tactile and motor behaviour. Behavioral testing of Prrxl1 mice revealed that these animals have reduced anxiety levels, enhanced memory performance, and improved fear extinction. Neurophysiological recordings from awake behaving Prrxl1 mice show enhanced altered fronto-hippocampal connectivity in the theta- and gamma-bands. Importantly, although inflammatory pain by Complete Freund Adjuvant injection caused a decrease in fronto-hippocampal connectivity in the wild-type animals, Prrxl1 mice maintained the baseline levels. The onset of inflammatory pain also reverted the differences in forebrain expression of stress- and monoamine-related genes in Prrxl1 mice. Altogether our results suggest that congenital hypoalgesia may have an effect on brain plasticity that is the inverse of what is usually observed in animal models of chronic pain.

CaMKK-CaMK1a, a new post-traumatic signalling pathway induced in mouse somatosensory neurons

Neurons innervating peripheral tissues display complex responses to peripheral nerve injury. These include the activation and suppression of a variety of signalling pathways that together influence regenerative growth and result in more or less successful functional recovery. However, these responses can be offset by pathological consequences including neuropathic pain. Calcium signalling plays a major role in the different steps occurring after nerve damage. As part of our studies to unravel the roles of injury-induced molecular changes in dorsal root ganglia (DRG) neurons during their regeneration, we show that the calcium calmodulin kinase CaMK1a is markedly induced in mouse DRG neurons in several models of mechanical peripheral nerve injury, but not by inflammation. Intrathecal injection of NRTN or GDNF significantly prevents the post-traumatic induction of CaMK1a suggesting that interruption of target derived factors might be a starter signal in this de novo induction. Inhibition of CaMK signalling in injured DRG neurons by pharmacological means or treatment with CaMK1a siRNA resulted in decreased velocity of neurite growth in vitro. Altogether, the results suggest that CaMK1a induction is part of the intrinsic regenerative response of DRG neurons to peripheral nerve injury, and is thus a potential target for therapeutic intervention to improve peripheral nerve regeneration.

Collagen XVI in health and disease

Collagen XVI, by structural analogy a member of the FACIT- (fibril-associated collagens with interrupted triple helices) family of collagens, is described as a minor collagen component of connective tissues. Collagen XVI is expressed in various cells and tissues without known occurrence of splice variants or isoforms. For skin and cartilage tissues its suprastructure is known. Presumably, there it acts as an adaptor protein connecting and organizing large fibrillar networks and thus modulates integrity and stability of the extracellular matrix (ECM). Collagen XVI is produced by myofibroblasts in the normal intestine and its synthesis is increased in the inflamed bowel wall where myofibroblasts develop increased numbers of focal adhesion contacts on collagen XVI. Consequently, recruitment of α1 integrin into the focal adhesions at the tip of the cells is induced followed by increased cell spreading on collagen XVI. This presumably adds to the maintenance of myofibroblasts in the inflamed intestinal regions and thus promotes fibrotic responses of the tissue. Notably, α1/α2 integrins interact with collagen XVI through an α1/α2β1 integrin binding site located in the COL 1-3 domains. Collagen XVI may act as a substrate for adhesion and invasion of connective tissue tumor cells. In glioblastoma it induces tumor invasiveness by modification of the β1-integrin activation pattern. Thus, altering the cell-matrix interaction through collagen XVI might be a molecular mechanism to further augment the invasive phenotype of glioma cells. In this line, in oral squamous cell carcinoma collagen XVI expression is induced which results in an upregulation of Kindlin-1 followed by an increased interaction with beta1-integrin. Consequently, collagen XVI induces a proliferative tumor phenotype by promoting an early S-phase entry. In summary, collagen XVI plays a decisive role in the interaction of connective tissue cells with their ECM, which is impaired in pathological situations. Alteration of tissue location and expression level of collagen XVI appears to promote tumorigenesis and to perpetuate inflammatory reactions.

Experimental and clinical evidence for use of decellularized nerve allografts in peripheral nerve gap reconstruction

Despite the inherent capability for axonal regeneration, recovery following severe peripheral nerve injury remains unpredictable and often very poor. Surgeons typically use autologous nerve grafts taken from the patient's own body to bridge long nerve gaps. However, the amount of suitable nerve available from a given patient is limited, and using autologous grafts leaves the patient with scars, numbness, and other forms of donor-site morbidity. Therefore, surgeons and engineers have sought off-the-shelf alternatives to the current practice of autologous nerve grafting. Decellularized nerve allografts have recently become available as an alternative to traditional nerve autografting. In this review, we provide a critical analysis comparing the advantages and limitations of the three major experimental models of decellularized nerve allografts: cold preserved, freeze-thawed, and chemical detergent based. Current tissue engineering-based techniques to optimize decellularized nerve allografts are discussed. We also evaluate studies that supplement decellularized nerve grafts with exogenous factors such as Schwann cells, stem cells, and growth factors to both support and enhance axonal regeneration through the decellularized allografts. In examining the advantages and disadvantages of the studies of decellularized allografts, we suggest that experimental methods, including the animal model, graft length, follow-up time, and outcome measures of regenerative progress and success be consolidated. Finally, all clinical studies in which decellularized nerve allografts have been used to bridge nerve gaps in patients are reviewed.

Toxicogenomic analysis of a sustained release local anesthetic delivery system

Concerns over neurotoxicity have impeded the development of sustained release formulations providing prolonged duration local anesthesia (PDLA) from a single injection, for which there is an urgent clinical need. Here, we have used toxicogenomics to investigate whether nerve injury occurred during week-long continuous sciatic nerve blockade by microspheres containing bupivacaine, tetrodotoxin, and dexamethasone (TBD). Animals treated with amitriptyline solution (our positive control for local anesthetic-associated nerve injury) developed irreversible nerve blockade, had severely abnormal nerve histology, and the expression of hundreds of genes was altered in the dorsal root ganglia at 4 and 7 days after injection. In marked contrast, TBD-treated nerves reverted to normal function, were normal histologically and there were changes in the expression of a small number of genes. Toxicogenomic studies have great potential in delineating patterns of gene expression associated with specific patterns of tissue injury (e.g. amitriptyline neurotoxicity), and in identifying related changes in gene expression upon exposure to a drug, biomaterial, or drug delivery system.

Cortical gene expression in spinal cord injury and repair: insight into the functional complexity of the neural regeneration program

Traumatic spinal cord injury (SCI) results in the formation of a fibrous scar acting as a growth barrier for regenerating axons at the lesion site. We have previously shown (Klapka et al., 2005) that transient suppression of the inhibitory lesion scar in rat spinal cord leads to long distance axon regeneration, retrograde rescue of axotomized cortical motoneurons, and improvement of locomotor function. Here we applied a systemic approach to investigate for the first time specific and dynamic alterations in the cortical gene expression profile following both thoracic SCI and regeneration-promoting anti-scarring treatment (AST). In order to monitor cortical gene expression we carried out microarray analyses using total RNA isolated from layer V/VI of rat sensorimotor cortex at 1–60 days post-operation (dpo). We demonstrate that cortical neurons respond to injury by massive changes in gene expression, starting as early as 1 dpo. AST, in turn, results in profound modifications of the lesion-induced expression profile. The treatment attenuates SCI-triggered transcriptional changes of genes related to inhibition of axon growth and impairment of cell survival, while upregulating the expression of genes associated with axon outgrowth, cell protection, and neural development. Thus, AST not only modifies the local environment impeding spinal cord regeneration by reduction of fibrous scarring in the injured spinal cord, but, in addition, strikingly changes the intrinsic capacity of cortical pyramidal neurons toward enhanced cell maintenance and axonal regeneration.

Inhibition of Kinesin-5, a microtubule-based motor protein, as a strategy for enhancing regeneration of adult axons

Developing neurons express a motor protein called kinesin-5 (also called kif11 or Eg5) which acts as a 'brake' on the advance of the microtubule array during axonal growth. Pharmacological inhibition of kinesin-5 causes the developing axon to grow at a faster rate, retract less and grow past cues that would otherwise cause it to turn. Here we demonstrate that kinesin-5 is also expressed in adult neurons, albeit at lower levels than during development. We hypothesized that inhibiting kinesin-5 might enable adult axons to regenerate better and to overcome repulsive molecules associated with injury. Using adult mouse dorsal root ganglion neurons, we found that anti-kinesin-5 drugs cause axons to grow faster and to cross with higher frequency onto inhibitory chondroitin sulfate proteoglycans. These effects may be due in part to changes in the efficiency of microtubule transport along the axonal shaft as well as enhanced microtubule entry into the distal tip of the axon. Effects observed with the drugs are further enhanced in some cases when they are used in combination with other treatments known to enhance axonal regeneration. Collectively, these results indicate that anti-kinesin-5 drugs may be a useful addition to the arsenal of tools used to treat nerve injury.

Slowing of axonal regeneration is correlated with increased axonal viscosity during aging

Background

As we age, the speed of axonal regeneration declines. At the biophysical level, why this occurs is not well understood.

Results

To investigate we first measured the rate of axonal elongation of sensory neurons cultured from neonatal and adult rats. We found that neonatal axons grew 40% faster than adult axons (11.5 µm/hour vs. 8.2 µm/hour). To determine how the mechanical properties of axons change during maturation, we used force calibrated towing needles to measure the viscosity (stiffness) and strength of substrate adhesion of neonatal and adult sensory axons. We found no significant difference in the strength of adhesions, but did find that adult axons were 3 times intrinsically stiffer than neonatal axons.

Conclusions

Taken together, our results suggest decreasing axonal stiffness may be part of an effective strategy to accelerate the regeneration of axons in the adult peripheral nervous system.

Expression of the regeneration-associated protein SPRR1A in primary sensory neurons and spinal cord of the adult mouse following peripheral and central injury

Small proline-rich repeat protein 1A (SPRR1A) is expressed in dorsal root ganglion (DRG) neurons following peripheral nerve injury but it is not known whether SPRR1A is differentially expressed following injury to peripheral versus central DRG projections and a detailed characterization of expression in sensory neuron subpopulations and spinal cord has not been performed. Here we use immunocytochemical techniques to characterize SPRR1A expression following sciatic nerve, dorsal root, and dorsal column injury in adult mice. SPRR1A was not detected in naïve spinal cord, DRG, or peripheral nerves and there was minimal expression following injury to the centrally projecting branches of DRG neurons. However, following peripheral (sciatic) nerve injury, intense SPRR1A immunoreactivity was observed in the dorsal horn and motoneurons of the spinal cord, in L4/5 DRG neurons, and in the injured nerve. A time-course study comparing expression following sciatic nerve crush and transection revealed maximum SPRR1A levels at day 7 in both models. However, while SPRR1A was downregulated to baseline by 30 days postlesion following crush injury, it remained elevated 30 days after transection. Cell-size and double-labeling studies revealed that SPRR1A was expressed by DRG cells of all sizes and colocalized with classical markers of DRG subpopulations and their primary afferent terminals. High coexpression of SPRR1A with activating transcription factor-3 and growth-associated protein-43 was observed, indicating that it is expressed by injured and regenerating neurons. This study supports the hypothesis that SPRR1A is a regeneration-associated gene and that SPRR1A provides a valuable marker to assess the regenerative potential of injured neurons.

Gene expression analysis of zebrafish retinal ganglion cells during optic nerve regeneration identifies KLF6a and KLF7a as important regulators of axon regeneration

Unlike mammals, teleost fish are able to mount an efficient and robust regenerative response following optic nerve injury. Although it is clear that changes in gene expression accompany axonal regeneration, the extent of this genomic response is not known. To identify genes involved in successful nerve regeneration, we analyzed gene expression in zebrafish retinal ganglion cells (RGCs) regenerating their axons following optic nerve injury. Microarray analysis of RNA isolated by laser capture microdissection from uninjured and 3-day post-optic nerve injured RGCs identified 347 up-regulated and 29 down-regulated genes. Quantitative RT-PCR and in situ hybridization were used to verify the change in expression of 19 genes in this set. Gene ontological analysis of the data set suggests regenerating neurons up-regulate genes associated with RGC development. However, not all regeneration-associated genes are expressed in differentiating RGCs indicating the regeneration is not simply a recapitulation of development. Knockdown of six highly induced regeneration-associated genes identified two, KLF6a and KLF7a, that together were necessary for robust RGC axon re-growth. These results implicate KLF6a and KLF7a as important mediators of optic nerve regeneration and suggest that not all induced genes are essential to mount a regenerative response.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

Gene profiling during development and after a peripheral nerve traumatism reveals genes specifically induced by injury in dorsal root ganglia

In order to shed light on transcriptional networks involved in adult peripheral nerve repair program, we propose for the first time an organization of the transcriptional dynamics of the mouse dorsal root ganglia (DRG) following a sciatic nerve lesion. This was done by a non-hierarchical bioinformatical clustering of four Serial Analysis of Gene Expression libraries performed on DRG at embryonic day E13, neonatal day P0, adult and adult 3 days post-sciatic nerve section. Grouping genes according to their expression profiles shows that a combination of down-regulation of genes expressed at the adult stages, re-expression of embryonic genes and induction of a set of de novo genes takes place in injured neurons. Focusing on this latter event highlights Ddit3, Timm8b and Oazin as potential new injury-induced molecular actors involved in a stress response pathway. Their association with the traumatic state was confirmed by real-time PCR and in situ hybridization investigations. Clustering analysis allows us to distinguish developmental re-programming events from nerve-injury-induced processes and thus provides a basis for molecular understanding of transcriptional alterations taking place in the DRG after a sciatic nerve lesion.

Molecular mechanisms in successful peripheral regeneration

Peripheral nerve injury is normally followed by a robust regenerative response. Here we describe the early changes associated with injury from the initial rise in intracellular calcium and the subsequent activation of transcription factors and cytokines leading to an inflammatory reaction, and the expression of growth factors, cytokines, neuropeptides, and other secreted molecules involved in cell-to-cell communication promoting regeneration and neurite outgrowth. The aim of this review is to summarize the molecular mechanisms that play a part in executing successful regeneration.

Developmental regulation of homeobox gene expression in dorsal root ganglion neurons is not recapitulated during regeneration of the crushed sciatic nerve

The adult peripheral nervous system is able to regenerate after injury. Regeneration is associated with the expression of new genes and proteins. Proteins abundant in developing axons increase in expression after injury, whereas proteins involved in neurotransmission are downregulated. It has been hypothesized that molecular mechanisms underlying regeneration-associated alterations in gene expression may be a recapitulation of developmental processes. These gene expression changes are likely to be regulated by changes in the gene expression of transcription factors. As homeobox genes play important roles in embryonic development of the nervous system, it makes them candidates for a regulatory role in the process of regeneration. Here we show that the relative mRNA expression levels of Isl1 decreased shortly after crush, but those of DRG11, Lmx1b, and Pax3 did not change after crush. These data indicate that the developmental expression patterns of the homeobox genes studied here are not recapitulated during regeneration of the dorsal root ganglia neurons. We conclude that developmental gene expression programs controlled by these homeobox genes are not directly involved in sciatic nerve regeneration.