Ngan Pan Bennett Au

Pleiotropic molecules in axon regeneration and neuroinflammation

Neuroinflammation is the foremost defense reaction of the nervous system to most if not all insults. Injuries to the central and peripheral nervous system (CNS and PNS) are followed by immediate activation of innate immune cells and infiltration of peripheral immune cells, amid waves of upregulation of numerous inflammatory mediators. Prolonged inflammation can lead to secondary tissue damage and prohibit regeneration of the injured nervous system. The regulation of inflammation and neuroregeneration are orchestrated through a complex network of signal transduction. Interestingly, many molecules play pleiotropic roles in both processes. Growing evidence implicates a handful of axon regeneration regulators in the processes of neuroinflammation, among which are the myelin and glial scar associated axon growth inhibitors and their axonal receptors. In this article, we will review the roles of these canonical axon regeneration regulators in neuroinflammation.

Probing for chemotherapy-induced peripheral neuropathy in live dorsal root ganglion neurons with atomic force microscopy

UNLABELLED Chemotherapy-induced peripheral neuropathy (CIPN) remains a major reason for cancer patients to withdraw from their lifesaving therapy. CIPN results in irreversible sensory and motor impairments; however, the epidemiology is largely unknown. Here, we report for the first time that chemotherapy drug vincristine not only reduced axonal regeneration in primary dorsal root ganglion neuron but also induced substantial changes in cell mechanical properties detected by atomic force microscopy (AFM). Confocal imaging analysis revealed vincristine-induced microtubule depolymerization. By using AFM for high-resolution live cell imaging and quantitative analysis, we observed significant changes in cell surface roughness and stiffness of vincristine-treated neurons. Elastic modulus was decreased (21-45%) with increasing dosage of vincristine. Further study with paclitaxel, another well-known CIPN drug, confirmed the link between cell mechanics and cytoskeleton organization. These data support that our system can be used for probing potential CIPN drugs that are of enormous benefit to new chemotherapy drug development. FROM THE CLINICAL EDITOR This study concludes that reduced cell elasticity in dorsal root ganglion neurons accompanies the development of chemotherapy-induced peripheral neuropathy, providing a model system that enables testing of upcoming chemotherapy agents for this particularly inconvenient and often treatment-limiting complication.

The association between laminin and microglial morphology in vitro

Microglia are immune cells in the central nervous system (CNS) that contribute to primary innate immune responses. The morphology of microglia is closely associated with their functional activities. The majority of microglial studies have focused on the ramified or amoeboid morphology; however, bipolar/rod-shaped microglia have recently received much attention. Bipolar/rod-shaped microglia form trains with end-to-end alignment in injured brains and retinae, which is proposed as an important mechanism in CNS repair. We previously established a cell culture model system to enrich bipolar/rod-shaped microglia simply by growing primary microglia on scratched poly-D-lysine (PDL)/laminin-coated surfaces. Here, we investigated the role of laminin in morphological changes of microglia. Bipolar/rod-shaped microglia trains were transiently formed on scratched surfaces without PDL/laminin coating, but the microglia alignment disappeared after 3 days in culture. Amoeboid microglia digested the surrounding laminin, and the gene and protein expression of laminin-cleaving genes Adam9 and Ctss was up-regulated. Interestingly, lipopolysaccharide (LPS)-induced transformation from bipolar/rod-shaped into amoeboid microglia increased the expression of Adam9 and Ctss, and the expression of these genes in LPS-treated amoeboid-enriched cultures remained unchanged. These results indicate a strong association between laminin and morphological transformation of microglia, shedding new light on the role of bipolar/rod-shaped microglia in CNS repair.

Ciguatoxin reduces regenerative capacity of axotomized peripheral neurons and delays functional recovery in pre-exposed mice after peripheral nerve injury

Ciguatera fish poisoning (CFP) results from consumption of tropical reef fish containing ciguatoxins (CTXs). Pacific (P)-CTX-1 is among the most potent known CTXs and the predominant source of CFP in the endemic region responsible for the majority of neurological symptoms in patients. Chronic and persistent neurological symptoms occur in some CFP patients, which often result in incomplete functional recovery for years. However, the direct effects of exposure to CTXs remain largely unknown. In present study, we exposed mice to CTX purified from ciguatera fish sourced from the Pacific region. P-CTX-1 was detected in peripheral nerves within hours and persisted for two months after exposure. P-CTX-1 inhibited axonal regrowth from axotomized peripheral neurons in culture. P-CTX-1 exposure reduced motor function in mice within the first two weeks of exposure before returning to baseline levels. These pre-exposed animals exhibited delayed sensory and motor functional recovery, and irreversible motor deficits after peripheral nerve injury in which formation of functional synapses was impaired. These findings are consistent with reduced muscle function, as assessed by electromyography recordings. Our study provides strong evidence that the persistence of P-CTX-1 in peripheral nerves reduces the intrinsic growth capacity of peripheral neurons, resulting in delayed functional recovery after injury.

Recent Advances in the Study of Bipolar/Rod-Shaped Microglia and their Roles in Neurodegeneration

Microglia are the resident immune cells of the central nervous system (CNS) and they contribute to primary inflammatory responses following CNS injuries. The morphology of microglia is closely associated with their functional activities. Most previous research efforts have attempted to delineate the role of ramified and amoeboid microglia in the pathogenesis of neurodegenerative diseases. In addition to ramified and amoeboid microglia, bipolar/rod-shaped microglia were first described by Franz Nissl in 1899 and their presence in the brain was closely associated with the pathology of infectious diseases and sleeping disorders. However, studies relating to bipolar/rod-shaped microglia are very limited, largely due to the lack of appropriate in vitro and in vivo experimental models. Recent studies have reported the formation of bipolar/rod-shaped microglia trains in in vivo models of CNS injury, including diffuse brain injury, focal transient ischemia, optic nerve transection and laser-induced ocular hypertension (OHT). These bipolar/rod-shaped microglia formed end-to-end alignments in close proximity to the adjacent injured axons, but they showed no interactions with blood vessels or other types of glial cell. Recent studies have also reported on a highly reproducible in vitro culture model system to enrich bipolar/rod-shaped microglia that acts as a powerful tool with which to characterize this form of microglia. The molecular aspects of bipolar/rod-shaped microglia are of great interest in the field of CNS repair. This review article focuses on studies relating to the morphology and transformation of microglia into the bipolar/rod-shaped form, along with the differential gene expression and spatial distribution of bipolar/rod-shaped microglia in normal and pathological CNSs. The spatial arrangement of bipolar/rod-shaped microglia is crucial in the reorganization and remodeling of neuronal and synaptic circuitry following CNS injuries. Finally, we discuss the potential neuroprotective roles of bipolar/rod-shaped microglia, and the possibility of transforming ramified/amoeboid microglia into bipolar/rod-shaped microglia. This will be of considerable clinical benefit in the development of novel therapeutic strategies for treating various neurodegenerative diseases and promoting CNS repair after injury.

Commensal microflora-induced T cell responses mediate progressive neurodegeneration in glaucoma

Glaucoma is the most prevalent neurodegenerative disease and a leading cause of blindness worldwide. The mechanisms causing glaucomatous neurodegeneration are not fully understood. Here we show, using mice deficient in T and/or B cells and adoptive cell transfer, that transient elevation of intraocular pressure (IOP) is sufficient to induce T-cell infiltration into the retina. This T-cell infiltration leads to a prolonged phase of retinal ganglion cell degeneration that persists after IOP returns to a normal level. Heat shock proteins (HSP) are identified as target antigens of T-cell responses in glaucomatous mice and human glaucoma patients. Furthermore, retina-infiltrating T cells cross-react with human and bacterial HSPs; mice raised in the absence of commensal microflora do not develop glaucomatous T-cell responses or the associated neurodegeneration. These results provide compelling evidence that glaucomatous neurodegeneration is mediated in part by T cells that are pre-sensitized by exposure to commensal microflora.Glaucoma is a neurodegenerative disease of which the etiology is still unclear. Here the authors show that elevation of intraocular pressure induces T cell infiltration in the eyes. Furthermore, they show that T cell cross-reactivity between endogenous and commensal antigens contributes to disease onset in mice.

Author Correction: Commensal microflora-induced T cell responses mediate progressive neurodegeneration in glaucoma

The originally published version of this Article contained an error in Figure 4. The bar chart in panel f was inadvertently replaced with a duplicate of the bar chart in panel e. This error has now corrected in both the PDF and HTML versions of the Article.

Targeting Axon Integrity to Prevent Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is an irreversible off-target adverse effect of many chemotherapeutic agents such as paclitaxel, yet its mechanism is poorly understood and no preventative measure is available. CIPN is characterized by peripheral nerve damages resulting in permanent sensory function deficits. Our recent unbiased genome-wide analysis revealed that heat shock protein (Hsp) 27 is part of a transcriptional network induced by axonal injury and highly enriched for genes involved in adaptive neuronal responses, particularly axonal regeneration. To examine if Hsp27 could prevent the occurrence of CIPN, we first demonstrated that paclitaxel-induced allodynia was associated directly with axonal degeneration in sensory neurons in a mouse model of CIPN. We therefore hypothesize that by preventing axonal degeneration could prevent the development of CIPN. We drove expression of human Hsp27 (hHsp27) specifically in neurons. Development of mechanical and thermal allodynia was prevented completely in paclitaxel-treated hHsp27 transgenic mice. Strikingly, hHsp27 protected against paclitaxel-induced neurotoxicity in vivo including degeneration of afferent nerve fibers, demyelination, mitochondrial swelling, apoptosis, and restored sensory nerve action potential. Finally, we delineated signaling cascades that link CIPN development to caspase 3 and RhoA/cofilin activation in sensory neurons and peripheral nerves. hHsp27 exerted anti-apoptotic effect and maintained axon integrity by restoring caspase 3 and RhoA expression to basal levels. Taken together, our data suggest that by preventing axonal degeneration might prove beneficial as anti-CIPN drugs, which represents an emerging research area for therapeutic development.