Macrophages Treated with VEGF and PDGF Exert Paracrine Effects on Olfactory Ensheathing Cell Function

Glial cell transplantation using olfactory ensheathing cells (OECs) holds a promising approach for treating spinal cord injury (SCI). However, integration of OECs into the hostile acute secondary injury site requires interaction and response to macrophages. Immunomodulation of macrophages to reduce their impact on OECs may improve the functionality of OECs. Vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF), known for their immunomodulatory and neuroprotective functions, have provided improved outcomes in SCI animal models. Thus, VEGF and PDGF modulation of the SCI microenvironment may be beneficial for OEC transplantation. In this in vitro study, the effect of VEGF and PDGF on macrophages in an inflammatory condition was tested. Combined VEGF + PDGF reduced translocation nuclear factor kappa B p65 in macrophages without altering pro-inflammatory cytokines. Further, the ability of OECs to phagocytose myelin debris was assessed using macrophage-conditioned medium. Conditioned medium from macrophages incubated with PDGF and combined VEGF + PDGF in inflammatory conditions promoted phagocytosis by OECs. The growth factor treated conditioned media also modulated the expression of genes associated with nerve repair and myelin expression in OECs. Overall, these results suggest that the use of growth factors together with OEC transplantation may be beneficial in SCI therapy.

Designing accessible educational resources for people living with spinal cord injury

Context/objective: To identify themes of interest for the production of educational resources for people with spinal cord injury (SCI).Design: A mixed-method study.Setting: Outpatient SCI community in Australia.Participants: Individuals with a SCI, or carers, family & friends of people who live with a SCI (n = 116).Interventions: Not applicable.Outcome measures: Quantify themes of interest perceived within the Australian SCI community as necessary for the development of SCI educational resources.Results: All seven individuals from the focus-group interviews suggested that educational resources on body physiology, secondary complications, injury pathophysiology, and health and wellbeing maintenance would be most pertinent for development. These themes (among others) were further explored and quantitatively evaluated via an online survey which demonstrated that interviewees ranked 'Your injury' as being of highest importance for the production of educational resources. Within each theme, the sub-categories; 'Bowel/bladder' and 'What equipment is covered in the National Disability Insurance Scheme (NDIS)' were ranked as being of highest importance for the production of educational resources.Conclusion: We have identified multiple areas of interest in the design and production of educational resources for individuals with SCI.

Streptococcus agalactiae Infects Glial Cells and Invades the Central Nervous System via the Olfactory and Trigeminal Nerves

Streptococcus agalactiae causes neonatal meningitis and can also infect the adult central nervous system (CNS). S. agalactiae can cross the blood-brain barrier but may also reach the CNS via other paths. Several species of bacteria can directly invade the CNS via the olfactory and trigeminal nerves, which extend between the nasal cavity and brain and injury to the nasal epithelium can increase the risk/severity of infection. Preterm birth is associated with increased risk of S. agalactiae infection and with nasogastric tube feeding. The tubes, also used in adults, can cause nasal injuries and may be contaminated with bacteria, including S. agalactiae. We here investigated whether S. agalactiae could invade the CNS after intranasal inoculation in mice. S. agalactiae rapidly infected the olfactory nerve and brain. Methimazole-mediated model of nasal epithelial injury led to increased bacterial load in these tissues, as well as trigeminal nerve infection. S. agalactiae infected and survived intracellularly in cultured olfactory/trigeminal nerve- and brain-derived glia, resulting in cytokine production, with some differences between glial types. Furthermore, a non-capsulated S. agalactiae was used to understand the role of capsule on glial cells interaction. Interestingly, we found that the S. agalactiae capsule significantly altered cytokine and chemokine responses and affected intracellular survival in trigeminal glia. In summary, this study shows that S. agalactiae can infect the CNS via the nose-to-brain path with increased load after epithelial injury, and that the bacteria can survive in glia.

Neuron-fibrous scaffold interfaces in the peripheral nervous system: a perspective on the structural requirements

The nerves of the peripheral nervous system are not able to effectively regenerate in cases of severe neural injury. This can result in debilitating consequences, including morbidity and lifelong impairments affecting the quality of the patient’s life. Recent findings in neural tissue engineering have opened promising avenues to apply fibrous tissue-engineered scaffolds to promote tissue regeneration and functional recovery. These scaffolds, known as neural scaffolds, are able to improve neural regeneration by playing two major roles, namely, by being a carrier for transplanted peripheral nervous system cells or biological cues and by providing structural support to direct growing nerve fibers towards the target area. However, successful implementation of scaffold-based therapeutic approaches calls for an appropriate design of the neural scaffold structure that is capable of up- and down-regulation of neuron-scaffold interactions in the extracellular matrix environment. This review discusses the main challenges that need to be addressed to develop and apply fibrous tissue-engineered scaffolds in clinical practice. It describes some promising solutions that, so far, have shown to promote neural cell adhesion and growth and a potential to repair peripheral nervous system injuries.

In vitro modulation of Schwann cell behavior by VEGF and PDGF in an inflammatory environment

Peripheral glial cell transplantation with Schwann cells (SCs) is a promising approach for treating spinal cord injury (SCI). However, improvements are needed and one avenue to enhance regenerative functional outcomes is to combine growth factors with cell transplantation. Vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) are neuroprotective, and a combination of these factors has improved outcomes in rat SCI models. Thus, transplantation of SCs combined with VEGF and PDGF may further improve regenerative outcomes. First, however, we must understand how the two factors modulate SCs. In this in vitro study, we show that an inflammatory environment decreased the rate of SC-mediated phagocytosis of myelin debris but the addition of VEGF and PDGF (alone and combined) improved phagocytosis. Cytokine expression by SCs in the inflammatory environment revealed that addition of PDGF led to significantly lower level of pro-inflammatory cytokine, TNF-α, but IL-6 and anti-inflammatory cytokines (TGF-β and IL-10), remained unaltered. Further, PDGF was able to decrease the expression of myelination associated gene Oct6 in the presence of inflammatory environment. Overall, these results suggest that the use of VEGF and/or PDGF combined with SC transplantation may be beneficial in SCI therapy.

Designing Olfactory Ensheathing Cell Transplantation Therapies: Influence of Cell Microenvironment

Olfactory ensheathing cell (OEC) transplantation is emerging as a promising treatment option for injuries of the nervous system. OECs can be obtained relatively easily from nasal biopsies, and exhibit several properties such as secretion of trophic factors, and phagocytosis of debris that facilitate neural regeneration and repair. But a major limitation of OEC-based cell therapies is the poor survival of transplanted cells which subsequently limit their therapeutic efficacy. There is an unmet need for approaches that enable the in vitro production of OECs in a state that will optimize their survival and integration after transplantation into the hostile injury site. Here, we present an overview of the strategies to modulate OECs focusing on oxygen levels, stimulating migratory, phagocytic, and secretory properties, and on bioengineering a suitable environment in vitro.

Optimizing Olfactory Ensheathing Cell Transplantation for Spinal Cord Injury Repair

Cell transplantation constitutes an important avenue for development of new treatments for spinal cord injury (SCI). These therapies are aimed at supporting neural repair and/or replacing lost cells at the injury site. To date, various cell types have been trialed, with most studies focusing on different types of stem cells or glial cells. Here, we review commonly used cell transplantation approaches for spinal cord injury (SCI) repair, with focus on transplantation of olfactory ensheathing cells (OECs), the glial cells of the primary olfactory nervous system. OECs are promising candidates for promotion of neural repair given that they support continuous regeneration of the olfactory nerve that occurs throughout life. Further, OECs can be accessed from the nasal mucosa (olfactory neuroepithelium) at the roof of the nasal cavity and can be autologously transplanted. OEC transplantation has been trialed in many animal models of SCI, as well as in human clinical trials. While several studies have been promising, outcomes are variable and the method needs improvement to enhance aspects such as cell survival, integration, and migration. As a case study, we include the approaches used by our team (the Clem Jones Centre for Neurobiology and Stem Cell Research, Griffith University, Nathan, QLD, Australia) to address the current problems with OEC transplantation and discuss how the therapeutic potential of OEC transplantation can be improved. Our approach includes discovery research to improve our knowledge of OEC biology, identifying natural and synthetic compounds to stimulate the neural repair properties of OECs, and designing three-dimensional cell constructs to create stable and transplantable cell structures.

Chlamydia muridarum Can Invade the Central Nervous System via the Olfactory and Trigeminal Nerves and Infect Peripheral Nerve Glial Cells

Chlamydia pneumoniae can infect the brain and has been linked to late-onset dementia. Chlamydia muridarum, which infects mice, is often used to model human chlamydial infections. While it has been suggested to be also important for modelling brain infection, nervous system infection by C. muridarum has not been reported in the literature. C. pneumoniae has been shown to infect the olfactory bulb in mice after intranasal inoculation, and has therefore been suggested to invade the brain via the olfactory nerve; however, nerve infection has not been shown to date. Another path by which certain bacteria can reach the brain is via the trigeminal nerve, but it remains unknown whether Chlamydia species can infect this nerve. Other bacteria that can invade the brain via the olfactory and/or trigeminal nerve can do so rapidly, however, whether Chlamydia spp. can reach the brain earlier than one-week post inoculation remains unknown. In the current study, we showed that C. muridarum can within 48 h invade the brain via the olfactory nerve, in addition to infecting the trigeminal nerve. We also cultured the glial cells of the olfactory and trigeminal nerves and showed that C. muridarum readily infected the cells, constituting a possible cellular mechanism explaining how the bacteria can invade the nerves without being eliminated by glial immune functions. Further, we demonstrated that olfactory and trigeminal glia differed in their responses to C. muridarum, with olfactory glia showing less infection and stronger immune response than trigeminal glia.

Basic insights into Zika virus infection of neuroglial and brain endothelial cells

Zika virus (ZIKV) has recently emerged as an important human pathogen due to the strong evidence that it causes disease of the central nervous system, particularly microcephaly and Guillain-Barré syndrome. The pathogenesis of disease, including mechanisms of neuroinvasion, may include both invasion via the blood-brain barrier and via peripheral (including cranial) nerves. Cellular responses to infection are also poorly understood. This study characterizes the in vitro infection of laboratory-adapted ZIKV African MR766 and two Asian strains of (1) brain endothelial cells (hCMEC/D3 cell line) and (2) olfactory ensheathing cells (OECs) (the neuroglia populating cranial nerve I and the olfactory bulb; both human and mouse OEC lines) in comparison to kidney epithelial cells (Vero cells, in which ZIKV infection is well characterized). Readouts included infection kinetics, intracellular virus localization, viral persistence and cytokine responses. Although not as high as in Vero cells, viral titres exceeded 104 plaque-forming units (p.f.u.) ml-1 in the endothelial/neuroglial cell types, except hOECs. Despite these substantial titres, a relatively small proportion of neuroglial cells were primarily infected. Immunolabelling of infected cells revealed localization of the ZIKV envelope and NS3 proteins in the cytoplasm; NS3 staining overlapped with that of dsRNA replication intermediate and the endoplasmic reticulum (ER). Infected OECs and endothelial cells produced high levels of pro-inflammatory chemokines. Nevertheless, ZIKV was also able to establish persistent infection in hOEC and hCMEC/D3 cells. Taken together, these results provide basic insights into ZIKV infection of endothelial and neuroglial cells and will form the basis for further study of ZIKV disease mechanisms.

Neisseria meningitidis Induces Pathology-Associated Cellular and Molecular Changes in Trigeminal Schwann Cells

Neisseria meningitidis, a common cause of sepsis and bacterial meningitis, infects the meninges and central nervous system (CNS), primarily via paracellular traversal across the blood-brain barrier (BBB) or blood-cerebrospinal fluid barrier. N. meningitidis is often present asymptomatically in the nasopharynx, and the nerves extending between the nasal cavity and the brain constitute an alternative route by which the meningococci may reach the CNS. To date, the cellular mechanisms involved in nerve infection are not fully understood. Peripheral nerve glial cells are phagocytic and are capable of eliminating microorganisms, but some pathogens may be able to overcome this protection mechanism and instead infect the glia, causing cell death or pathology. Here, we show that N. meningitidis readily infects trigeminal Schwann cells (the glial cells of the trigeminal nerve) in vitro in both two-dimensional and three-dimensional cell cultures. Infection of trigeminal Schwann cells may be one mechanism by which N. meningitidis is able to invade the CNS. Infection of the cells led to multinucleation and the appearance of atypical nuclei, with the presence of horseshoe nuclei and the budding of nuclei increasing over time. Using sequential window acquisition of all theoretical mass spectra (SWATH-MS) proteomics followed by bioinformatics pathway analysis, we showed that N. meningitidis induced protein alterations in the glia that were associated with altered intercellular signaling, cell-cell interactions, and cellular movement. The analysis also suggested that the alterations in protein levels were consistent with changes occurring in cancer. Thus, infection of the trigeminal nerve by N. meningitidis may have ongoing adverse effects on the biology of Schwann cells, which may lead to pathology.

Burkholderia pseudomallei invades the olfactory nerve and bulb after epithelial injury in mice and causes the formation of multinucleated giant glial cells in vitro

The infectious disease melioidosis is caused by the bacterium Burkholderia pseudomallei. Melioidosis is characterised by high mortality and morbidity and can involve the central nervous system (CNS). We have previously discovered that B. pseudomallei can infect the CNS via the olfactory and trigeminal nerves in mice. We have shown that the nerve path is dependent on mouse strain, with outbred mice showing resistance to olfactory nerve infection. Damage to the nasal epithelium by environmental factors is common, and we hypothesised that injury to the olfactory epithelium may increase the vulnerability of the olfactory nerve to microbial insult. We therefore investigated this, using outbred mice that were intranasally inoculated with B. pseudomallei, with or without methimazole-induced injury to the olfactory neuroepithelium. Methimazole-mediated injury resulted in increased B. pseudomallei invasion of the olfactory epithelium, and only in pre-injured animals were bacteria found in the olfactory nerve and bulb. In vitro assays demonstrated that B. pseudomallei readily infected glial cells isolated from the olfactory and trigeminal nerves (olfactory ensheathing cells and trigeminal Schwann cells, respectively). Bacteria were degraded by some cells but persisted in other cells, which led to the formation of multinucleated giant cells (MNGCs), with olfactory ensheathing cells less likely to form MNGCs than Schwann cells. Double Cap mutant bacteria, lacking the protein BimA, did not form MNGCs. These data suggest that injuries to the olfactory epithelium expose the primary olfactory nervous system to bacterial invasion, which can then result in CNS infection with potential pathogenic consequences for the glial cells.

Infections of the central nervous system (CNS), though uncommon, are associated with severe morbidity and mortality. Burkholderia pseudomallei, the causative agent of melioidosis, can infect the CNS. We have shown that B. pseudomallei can enter the CNS via peripheral nerves extending between the nasal cavity and the brain (bypassing the blood-brain/blood-cerebrospinal fluid barriers). In the current study, we show that prior injury to the olfactory epithelium can increase B. pseudomallei invasion of the olfactory nerve and bulb, highlighting a novel risk factor for CNS infections. We also demonstrate the ability of peripheral nerve glia to internalise B. pseudomallei, resulting in the formation of multinucleated giant cells (MNGCs), dependent on the bacterial protein BimA. These findings provide important new insights into the pathogenesis of B. pseudomallei.

Reliable cell purification and determination of cell purity: crucial aspects of olfactory ensheathing cell transplantation for spinal cord repair

Transplantation of olfactory ensheathing cells, the glia of the primary olfactory nervous system, has been trialed for spinal cord injury repair with promising but variable outcomes in animals and humans. Olfactory ensheathing cells can be harvested either from the lamina propria beneath the neuroepithelium in the nasal cavity, or from the olfactory bulb in the brain. As these areas contain several other cell types, isolating and purifying olfactory ensheathing cells is a critical part of the process. It is largely unknown how contaminating cells such as fibroblasts, other glial cell types and supporting cells affect olfactory ensheathing cell function post-transplantation; these cells may also cause unwanted side-effects. It is also, however, possible that the presence of some of the contaminant cells can improve outcomes. Here, we reviewed the last decade of olfactory ensheathing cell transplantation studies in rodents, with a focus on olfactory ensheathing cell purity. We analyzed how purification methods and resultant cell purity differed between olfactory mucosa- and olfactory bulb-derived cell preparations. We analyzed how the studies reported on olfactory ensheathing cell purity and which criteria were used to define cells as olfactory ensheathing cells. Finally, we analyzed the correlation between cell purity and transplantation outcomes. We found that olfactory bulb-derived olfactory ensheathing cell preparations are typically purer than mucosa-derived preparations. We concluded that there is an association between high olfactory ensheathing cell purity and favourable outcomes, but the lack of olfactory ensheathing cell-specific markers severely hampers the field.

3D-tips: user-friendly mesh barrier pipette tips for 3D-spheroid culture

Background

Three dimensional (3D) cell cultures have been an area of increasing interest and relevance across several research fields including drug discovery, developmental biology and stem cell-based therapies. However, handling 3D structures can be difficult. In particular, the replacement of liquid media and reagents in which liquid is removed using pipettes is difficult to perform as the 3D spheroids can be easily aspirated into the pipette tip.

Results

We have developed the 3D-tip, a novel tool that facilitates media change and washing procedures of 3D-spheroid cultures. The 3D-tip contains a mesh with 40-μm pores allowing the aspiration of liquids including media, drugs, buffers and reagents, with the mesh acting as a barrier preventing the spheroids being aspirated into the pipette tip. After aspiration of liquids, the spheroids are gently deposited back into the culture vessel. Our results demonstrate that the 3D-tips offer superior handling of 3D-spheroid cultures in comparison to commonly used methods. We showed that the 3D-tips can easily be used on both fixed and unfixed spheroids and on cancer cell, stem cell and glial cell spheroids.

In contrast with the 50/50 media exchange method, the 3D-tips allow a complete media change with minimal loss of spheroids and without damaging their morphology. Our results showed that 86.0% of spheroids remained in the chamber after changing the media using the 3D-tips. In contrast, only 45.0% of spheroids remained using the 50/50 media exchange strategy.

In comparison with the centrifugation technique, the 3D-tips preserved spheroids whereas centrifugation led to the loss of spheroids and/or the alteration of the size and shape of the 3D cellular structures. We observed that 87.6 and 84.6% of the fixed and unfixed spheroids remained using the 3D-tip, respectively. In contrast, only 66.3% of the fixed spheroids and 36.4% of the unfixed spheroids were left using the centrifugation method. From a time perspective, the 3D-tips dramatically reduce the time taken for replacing media.

Conclusions

This novel pipette tip is suitable for high throughput screening and automation and will revolutionise the techniques used for the production and analysis of 3D spheroids.

Why are olfactory ensheathing cell tumors so rare?

The glial cells of the primary olfactory nervous system, olfactory ensheathing cells (OECs), are unusual in that they rarely form tumors. Only 11 cases, all of which were benign, have been reported to date. In fact, the existence of OEC tumors has been debated as the tumors closely resemble schwannomas (Schwann cell tumors), and there is no definite method for distinguishing the two tumor types. OEC transplantation is a promising therapeutic approach for nervous system injuries, and the fact that OECs are not prone to tumorigenesis is therefore vital. However, why OECs are so resistant to neoplastic transformation remains unknown. The primary olfactory nervous system is a highly dynamic region which continuously undergoes regeneration and neurogenesis throughout life. OECs have key roles in this process, providing structural and neurotrophic support as well as phagocytosing the axonal debris resulting from turnover of neurons. The olfactory mucosa and underlying tissue is also frequently exposed to infectious agents, and OECs have key innate immune roles preventing microbes from invading the central nervous system. It is possible that the unique biological functions of OECs, as well as the dynamic nature of the primary olfactory nervous system, relate to the low incidence of OEC tumors. Here, we summarize the known case reports of OEC tumors, discuss the difficulties of correctly diagnosing them, and examine the possible reasons for their rare incidence. Understanding why OECs rarely form tumors may open avenues for new strategies to combat tumorigenesis in other regions of the nervous system.

Three-dimensional cell culture can be regulated by vibration: low-frequency vibration increases the size of olfactory ensheathing cell spheroids

Background

Olfactory ensheathing cell (OEC) transplantation is emerging as a promising therapy for spinal cord injuries. However, outcomes are inconsistent, and the method needs improvement. Currently, cells are injected into the injury site as a suspension, and often fail to form a three-dimensional (3D) network crucial for both survival of the transplanted cells, and for regeneration of severed axons. 3D culture systems are therefore likely to improve the method. Of the many 3D culture systems available, the spheroid-producing naked liquid marble (NLM) technique is particularly advantageous compared to other platforms as it rapidly generates cell spheroids which can easily be extracted for further handling. To improve production of the spheroids, we designed and tested a device which allows fine control over vibrational stimuli to liquid marble cell cultures. We applied vibrational frequencies of 20, 60, and 80 Hz with consistent amplitude to NLM containing OECs and assessed the size and number of the 3D cell spheroids generated as well as the migratory capacity of cells cultured in the vibrated spheroids.

Results

Vibrating the NLMs led to fewer and dramatically larger spheroids in comparison to non-vibrated NLMs. Of the frequencies tested, 60 Hz caused over 70-fold increase in spheroid volume. When transferred to a culture plate, the larger spheroids retained their structure after 72 h in culture, and cells that migrated out of the spheroids covered a significantly larger area compared to cells migrating out of spheroids formed at all the other frequencies tested.

Conclusions

We have shown that vibration can be used to regulate the formation of cell spheroids in NLM cultures. The ability to modulate the size of spheroids is useful for a range of 3D cell culture models and for preparing cells for in vivo transplantation.

Naked Liquid Marbles: A Robust Three-Dimensional Low-Volume Cell-Culturing System

Three-dimensional (3D) multicellular structures allow cells to behave and interact with each other in a manner that mimics the in vivo environment. In recent years, many 3D cell culture methods have been developed with the goal of producing the most in vivo-like structures possible. Whilst strongly preferable to  conventional cell culture, these approaches are often poorly reproducible, time-consuming, expensive, and labor-intensive and require specialized equipment. Here, we describe a novel 3D culture platform, which we have termed the naked liquid marble (NLM). Cells are cultured in a liquid drop (the NLM) in superhydrophobic-coated plates, which causes the cells to naturally form 3D structures. Inside the NLMs, cells are free to interact with each other, forming multiple 3D spheroids that are uniform in size and shape in less than 24 h. We showed that this system is highly reproducible, suitable for cell coculture, compound screening, and also compatible with laboratory automation systems. The low cost of production, small volume of each NLM, and production via automated liquid handling make this 3D cell-culturing system particularly suitable for high-throughput screening assays such as drug testing as well as numerous other cell-based research applications.

Novel insights into the glia limitans of the olfactory nervous system

Olfactory ensheathing cells (OECs) are often described as being present in both the peripheral and the central nervous systems (PNS and CNS). Furthermore, the olfactory nervous system glia limitans (the glial layer defining the PNS-CNS border) is considered unique as it consists of intermingling OECs and astrocytes. In contrast, the glia limitans of the rest of the nervous system consists solely of astrocytes which create a distinct barrier to Schwann cells (peripheral glia). The ability of OECs to interact with astrocytes is one reason why OECs are believed to be superior to Schwann cells for transplantation therapies to treat CNS injuries. We have used transgenic reporter mice in which glial cells express DsRed fluorescent protein to study the cellular constituents of the glia limitans. We found that the glia limitans layer of the olfactory nervous system is morphologically similar to elsewhere in the nervous system, with a similar low degree of intermingling between peripheral glia and astrocytes. We found that the astrocytic layer of the olfactory bulb is a distinct barrier to bacterial infection, suggesting that this layer constitutes the PNS-CNS immunological barrier. We also found that OECs interact with astrocytes in a similar fashion as Schwann cells in vitro. When cultured in three dimensions, however, there were subtle differences between OECs and Schwann cells in their interactions with astrocytes. We therefore suggest that glial fibrillary acidic protein-reactive astrocyte layer of the olfactory bulb constitutes the glia limitans of the olfactory nervous system and that OECs are primarily "PNS glia."

The serrulatane diterpenoid natural products RAD288 and RAD289 stimulate properties of olfactory ensheathing cells useful for neural repair therapies

Olfactory ensheathing cells (OECs) are being trialled for cell transplantation therapies for neural repair as they have unique properties which can enhance neuron regeneration. However, improvements in cell viability, proliferation and migration are needed to enhance therapeutic outcomes. Growth factors can enhance cell activity, but they can also induce side effects as they can act on numerous cell types. An alternative approach is to identify natural products (NPs) that more selectively activate specific cell functions. We have examined two pure NPs, 3-acetoxy-7,8-dihydroxyserrulat-14-en-19-oic acid (RAD288) and 3,7,8-trihydroxyserrulat-14-en-19-oic acid (RAD289) isolated from the Australian plant Eremophila microtheca. We determined that RAD288 and RAD289 stimulated the viability and proliferation of OECs in two-dimensional cultures and increased cell viability in three-dimensional spheroids. Both compounds also enhanced OEC-mediated phagocytosis of neural debris. However, only RAD288 stimulated migration of OECs, demonstrating that key structural changes to the compound can dramatically affect the resultant cellular action. In addition, cell-type specific action is highlighted by the result that neither compound stimulated the viability of Schwann cells which are a closely-related glial cell type. Therefore, these small molecules may have high potential for selective activation of specific therapeutically-useful activities of OECs for transplantation therapies to repair the nervous system.

Schwann cell lamellipodia regulate cell-cell interactions and phagocytosis

Lamellipodia in Schwann cells (SCs) are crucial for myelination, but their other biological functions remain largely uncharacterised. Two types of lamellipodia exist in SCs: axial lamellipodia at the outermost edge of the cell processes, and radial lamellipodia appearing peripherally along the entire cell. We have previously shown that radial lamellipodia on olfactory glia (olfactory ensheathing cells; OECs) promote cell-cell adhesion, contact-mediated migration and phagocytosis. Here we have investigated whether lamellipodia in SCs have similar roles. Using live-cell imaging, we show that the radial lamellipodia in SCs are highly motile, appear at multiple cellular sites and rapidly move in a wave-like manner. We found that axial and radial lamellipodia had strikingly different roles and are regulated by different intracellular pathways. Axial lamellipodia initiated interactions with other SCs and with neurons by contacting radial lamellipodia on SCs, and budding neurites/axons. Most SC-SC interactions resulted in repulsion, and, lamellipodial activity (unlike in OECs) did not promote contact-mediated migration. We show that lamellipodia are crucial for SC-mediated phagocytosis of both axonal debris and bacteria, and demonstrated that inhibition of lamellipodial activity by blocking the Rho/Rac pathways also inhibits phagocytosis. We also show that heregulin, which induces SC differentiation and maturation, alters lamellipodial behaviour but does not affect phagocytic activity. Overall, the results show that SC lamellipodia are important for cell interactions and phagocytosis.

Combined VEGF/PDGF improves olfactory regeneration after unilateral bulbectomy in mice

The olfactory receptor neurons lining the nasal cavity have a remarkable capacity to regenerate throughout life. They are replenished continuously and their axons make new connections within the olfactory bulb. However, some factors such as head trauma and skull base surgery damage the olfactory nerve which lead to olfactory dysfunction. Losing the sense of smell has considerable effects on quality of life and life-expectancy. Therefore, there is a clear need to find a treatment for olfactory dysfunction. One such potential treatment is growth factor therapy which showed promising results in the spinal cord and brain injuries. The aim of the present study was to investigate whether combined delivery of two growth factors, vascular endothelial growth factor and platelet-derived growth factor treatment can improve the olfactory neurons regeneration in mice. The degeneration of the olfactory neurons was induced by unilateral bulbectomy. The treatment group received 1.5 µg of the combined growth factors intranasally, while the control injured group received saline. Growth factor treatment significantly increased the number of immature neurons at 5 and 7 days post injury and also the number of mature olfactory neurons at 10 and 14 days post bulbectomy. Regenerating axons extended over a larger volume in the operated cavity in the treatment group compared to control group at 14 days post bulbectomy. The growth factor treatment also significantly reduced astrocytic glia scar in the operated cavity. The results indicate that the combined delivery of the growth factors has the potential to improve olfactory dysfunction.

Factors that modulate olfactory dysfunction

The olfactory system is one of a few areas in the nervous system which is capable of regeneration throughout the life. Olfactory sensory neurons reside in the nasal cavity are continuously replenished with new neurons arising from stem cells. Some factors such as aging, neurodegenerative diseases, head trauma, brain tumor extraction and infection cause olfactory dysfunction which significantly influences physical wellbeing, quality of life, mental health, nutritional status, memory processes, identifying danger and is associated with increased mortality. Therefore, finding a treatment to improve olfactory dysfunction is needed. Recent research efforts in the field have shown some very promising new approaches to treat olfactory dysfunction. This review explores the current studies that have addressed therapeutic approaches to improve olfactory neuron regeneration based on cell transplantation therapy, modulation of physiological olfactory dysfunction and drug treatments.

Impaired neurogenesis and associated gliosis in mouse brain with PEX13 deficiency

Zellweger syndrome (ZS), a neonatal lethal disorder arising from defective peroxisome biogenesis, features profound neuroanatomical abnormalities and brain dysfunction. Here we used mice with brain-restricted inactivation of the peroxisome biogenesis gene PEX13 to model the pathophysiological features of ZS, and determine the impact of peroxisome dysfunction on neurogenesis and cell maturation in ZS. In the embryonic and postnatal PEX13 mutant brain, we demonstrate key regions with altered brain anatomy, including enlarged lateral ventricles and aberrant cortical, hippocampal and hypothalamic organization. To characterize the underlying mechanisms, we show a significant reduction in proliferation, migration, differentiation, and maturation of neural progenitors in embryonic E12.5 through to P3 animals. An increasing reactive gliosis in the PEX13 mutant brain started at E14.5 in association with the pathology. Together with impaired neurogenesis and associated gliosis, our data demonstrate increased cell death contributing to the hallmark brain anatomy of ZS. We provide unique data where impaired neurogenesis and migration are shown as critical events underlying the neuropathology and altered brain function of mice with peroxisome deficiency.

A lab-on-a-chip device for investigating the fusion process of olfactory ensheathing cell spheroids

Understanding the process of fusion of olfactory ensheathing cell spheroids will lead to improvement of cell transplantation therapies to repair spinal cord injuries. The successful fusion of transplanted spheroids will enable alternative transplantation strategies to be developed for in vivo applications. This paper describes the use of a microfluidic device to trap and fuse olfactory ensheathing cell spheroids. The velocity, the pressure distribution in the device were simulated numerically to predict the trapping location. The simulation predicted the optimum flow rates for trapping the spheroids in the later experiments. Simulated particle trajectories were verified experimentally with tracing of fluorescent micro particles. The fusion process of the spheroids was investigated over a period of 48 hours. The microfluidic platform presented here can be used for testing potential drugs that can promote the fusion process and improve the transplantation therapy.

Floating mechanism of a small liquid marble

Flotation of small solid objects and liquid droplets on water is critical to natural and industrial activities. This paper reports the floating mechanism of liquid marbles, or liquid droplets coated with hydrophobic microparticles. We used X-ray computed tomography (XCT) to acquire cross-sectional images of the floating liquid marble and interface between the different phases. We then analysed the shape of the liquid marble and the angles at the three-phase contact line (TPCL). We found that the small floating liquid marbles follow the mechanism governing the flotation of solid objects in terms of surface tension forces. However, the contact angles formed and deformation of the liquid marble resemble that of a sessile liquid droplet on a thin, elastic solid. For small liquid marbles, the contact angle varies with volume due to the deformability of the interface.

Generation of three-dimensional multiple spheroid model of olfactory ensheathing cells using floating liquid marbles

We describe a novel protocol for three-dimensional culturing of olfactory ensheathing cells (OECs), which can be used to understand how OECs interact with other cells in three dimensions. Transplantation of OECs is being trialled for repair of the paralysed spinal cord, with promising but variable results and thus the therapy needs improving. To date, studies of OEC behaviour in a multicellular environment have been hampered by the lack of suitable three-dimensional cell culture models. Here, we exploit the floating liquid marble, a liquid droplet coated with hydrophobic powder and placed on a liquid bath. The presence of the liquid bath increases the humidity and minimises the effect of evaporation. Floating liquid marbles allow the OECs to freely associate and interact to produce OEC spheroids with uniform shapes and sizes. In contrast, a sessile liquid marble on a solid surface suffers from evaporation and the cells aggregate with irregular shapes. We used floating liquid marbles to co-culture OECs with Schwann cells and astrocytes which formed natural structures without the confines of gels or bounding layers. This protocol can be used to determine how OECs and other cell types associate and interact while forming complex cell structures.

Differing phagocytic capacities of accessory and main olfactory ensheathing cells and the implication for olfactory glia transplantation therapies

The rodent olfactory systems comprise the main olfactory system for the detection of odours and the accessory olfactory system which detects pheromones. In both systems, olfactory axon fascicles are ensheathed by olfactory glia, termed olfactory ensheathing cells (OECs), which are crucial for the growth and maintenance of the olfactory nerve. The growth-promoting and phagocytic characteristics of OECs make them potential candidates for neural repair therapies such as transplantation to repair the injured spinal cord. However, transplanting mixed populations of glia with unknown properties may lead to variations in outcomes for neural repair. As the phagocytic capacity of the accessory OECs has not yet been determined, we compared the phagocytic capacity of accessory and main OECs in vivo and in vitro. In normal healthy animals, the accessory OECs accumulated considerably less axon debris than main OECs in vivo. Analysis of freshly dissected OECs showed that accessory OECs contained 20% less fluorescent axon debris than main OECs. However, when assayed in vitro with exogenous axon debris added to the culture, the accessory OECs phagocytosed almost 20% more debris than main OECs. After surgical removal of one olfactory bulb which induced the degradation of main and accessory olfactory sensory axons, the accessory OECs responded by phagocytosing the axon debris. We conclude that while accessory OECs have the capacity to phagocytose axon debris, there are distinct differences in their phagocytic capacity compared to main OECs. These distinct differences may be of importance when preparing OECs for neural transplant repair therapies.

Olfactory ensheathing cells are the main phagocytic cells that remove axon debris during early development of the olfactory system

During development of the primary olfactory system, axon targeting is inaccurate and axons inappropriately project within the target layer or overproject into the deeper layers of the olfactory bulb. As a consequence there is considerable apoptosis of primary olfactory neurons during embryonic and postnatal development and axons of the degraded neurons need to be removed. Olfactory ensheathing cells (OECs) are the glia of the primary olfactory nerve and are known to phagocytose axon debris in the adult and postnatal animal. However, it is unclear when phagocytosis by OECs first commences. We investigated the onset of phagocytosis by OECs in the developing mouse olfactory system by utilizing two transgenic reporter lines: OMP-ZsGreen mice which express bright green fluorescent protein in primary olfactory neurons, and S100β-DsRed mice which express red fluorescent protein in OECs. In crosses of these mice, the fate of the degraded axon debris is easily visualized. We found evidence of axon degradation at embryonic day (E)13.5. Phagocytosis of the primary olfactory axon debris by OECs was first detected at E14.5. Phagocytosis of axon debris continued into the postnatal animal during the period when there was extensive mistargeting of olfactory axons. Macrophages were often present in close proximity to OECs but they contributed only a minor role to clearing the axon debris, even after widespread degeneration of olfactory neurons by unilateral bulbectomy and methimazole treatment. These results demonstrate that from early in embryonic development OECs are the primary phagocytic cells of the primary olfactory nerve.

Radial glia phagocytose axonal debris from degenerating overextending axons in the developing olfactory bulb

Axon targeting during the development of the olfactory system is not always accurate, and numerous axons overextend past the target layer into the deeper layers of the olfactory bulb. To date, the fate of the mis-targeted axons has not been determined. We hypothesized that following overextension, the axons degenerate, and cells within the deeper layers of the olfactory bulb phagocytose the axonal debris. We utilized a line of transgenic mice that expresses ZsGreen fluorescent protein in primary olfactory axons. We found that overextending axons closely followed the filaments of radial glia present in the olfactory bulb during embryonic development. Following overextension into deeper layers of the olfactory bulb, axons degenerated and radial glia responded by phagocytosing the resulting debris. We used in vitro analysis to confirm that the radial glia had phagocytosed debris from olfactory axons. We also investigated whether the fate of overextending axons was altered when the development of the olfactory bulb was perturbed. In mice that lacked Sox10, a transcription factor essential for normal olfactory bulb development, we observed a disruption to the morphology and positioning of radial glia and an accumulation of olfactory axon debris within the bulb. Our results demonstrate that during early development of the olfactory system, radial glia play an important role in removing overextended axons from the deeper layers of the olfactory bulb.

Low-dose curcumin stimulates proliferation, migration and phagocytic activity of olfactory ensheathing cells

One of the promising strategies for neural repair therapies is the transplantation of olfactory ensheathing cells (OECs) which are the glial cells of the olfactory system. We evaluated the effects of curcumin on the behaviour of mouse OECs to determine if it could be of use to further enhance the therapeutic potential of OECs. Curcumin, a natural polyphenol compound found in the spice turmeric, is known for its anti-cancer properties at doses over 10 µM, and often at 50 µM, and it exerts its effects on cancer cells in part by activation of MAP kinases. In contrast, we found that low-dose curcumin (0.5 µM) applied to OECs strikingly modulated the dynamic morphology, increased the rate of migration by up to 4-fold, and promoted significant proliferation of the OECs. Most dramatically, low-dose curcumin stimulated a 10-fold increase in the phagocytic activity of OECs. All of these potently stimulated behavioural characteristics of OECs are favourable for neural repair therapies. Importantly, low-dose curcumin gave a transient activation of p38 kinases, which is in contrast to the high dose curcumin effects on cancer cells in which these MAP kinases tend to undergo prolonged activation. Low-dose curcumin mediated effects on OECs demonstrate cell-type specific stimulation of p38 and ERK kinases. These results constitute the first evidence that low-dose curcumin can modulate the behaviour of olfactory glia into a phenotype potentially more favourable for neural repair and thereby improve the therapeutic use of OECs for neural repair therapies.

Pathogens penetrating the central nervous system: infection pathways and the cellular and molecular mechanisms of invasion

The brain is well protected against microbial invasion by cellular barriers, such as the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). In addition, cells within the central nervous system (CNS) are capable of producing an immune response against invading pathogens. Nonetheless, a range of pathogenic microbes make their way to the CNS, and the resulting infections can cause significant morbidity and mortality. Bacteria, amoebae, fungi, and viruses are capable of CNS invasion, with the latter using axonal transport as a common route of infection. In this review, we compare the mechanisms by which bacterial pathogens reach the CNS and infect the brain. In particular, we focus on recent data regarding mechanisms of bacterial translocation from the nasal mucosa to the brain, which represents a little explored pathway of bacterial invasion but has been proposed as being particularly important in explaining how infection with Burkholderia pseudomallei can result in melioidosis encephalomyelitis.

Crucial roles for olfactory ensheathing cells and olfactory mucosal cells in the repair of damaged neural tracts

Olfactory ensheathing cells, the glial cells of the olfactory nervous system, exhibit unique growth-promoting and migratory properties that make them interesting candidates for cell therapies targeting neuronal injuries such as spinal cord injury. Transplantation of olfactory cells is feasible and safe in humans; however, functional outcomes are highly variable with some studies showing dramatic improvements and some no improvements at all. We propose that the reason for this is that the identity and purity of the cells is different in each individual study. We have shown that olfactory ensheathing cells are not a uniform cell population and that individual subpopulations of OECs are present in different regions of the olfactory nervous system, with strikingly different behaviors. Furthermore, the presence of fibroblasts and other cell types in the transplant can dramatically alter the behavior of the transplanted glial cells. Thus, a thorough characterization of the differences between olfactory ensheathing cell subpopulations and how the behavior of these cells is affected by the presence of other cell types is highly warranted.

Olfactory ensheathing glia are required for embryonic olfactory axon targeting and the migration of gonadotropin-releasing hormone neurons

Kallmann's syndrome is caused by the failure of olfactory axons and gonadotropin-releasing hormone (GnRH) neurons to enter the embryonic forebrain, resulting in anosmia and sterility. Sox10 mutations have been associated with Kallmann's syndrome phenotypes, but their effect on olfactory system development is unknown. We recently showed that Sox10 is expressed by neural crest-derived olfactory ensheathing cells (OECs). Here, we demonstrate that in homozygous Sox10(lacZ/lacZ) mouse embryos, OEC differentiation is disrupted; olfactory axons accumulate in the ventromedial olfactory nerve layer and fewer olfactory receptor neurons express the maturation marker OMP (most likely owing to the failure of axonal targeting). Furthermore, GnRH neurons clump together in the periphery and a smaller proportion enters the forebrain. Our data suggest that human Sox10 mutations cause Kallmann's syndrome by disrupting the differentiation of OECs, which promote embryonic olfactory axon targeting and hence olfactory receptor neuron maturation, and GnRH neuron migration to the forebrain.

HuC-eGFP mosaic labelling of neurons in zebrafish enables in vivo live cell imaging of growth cones

The field of axon guidance is taking advantage of the powerful genetic and imaging tools that are now available to visualise growth behaviour in living cells, both in vivo and in real time. We have developed a method to visualise individual neurons within the living zebrafish embryo which provides exceptional cellular resolution of growth cones and their filopodia. We generated a DNA construct in which the HuC promoter drives expression of eGFP. Injection of the plasmid into single cell fertilised zebrafish egg resulted in mosaic expression of eGFP in neurons throughout the developing embryo. By manipulating the concentration of injected plasmid, it was possible to optimise the numbers of neurons that expressed the construct so that individual growth cones could be easily visualised. We then used time-lapse high magnification widefield epifluorescence microscopy to visualise the growth cones as they were exploring their environment. Growth cones both near the surface of the embryo as well as deep within the developing brain of embryos at 20 h post fertilisation were clearly imaged. With time-lapse sequence imaging with intervals between frames as frequent as 1 s there was minimal loss of fluorescence intensity and the dynamic nature of the growth cones became evident. This method therefore provides high magnification, high resolution time-lapse imaging of living neurons in vivo and by use of widefield epifluorescence rather than confocal it is a relatively inexpensive microscopy method.

Spatial and developmental heterogeneity of calcium signaling in olfactory ensheathing cells

Olfactory ensheathing cells (OECs) are specialized glial cells in the mammalian olfactory system supporting growth of axons from the olfactory epithelium into the olfactory bulb. OECs in the olfactory bulb can be subdivided into OECs of the outer nerve layer and the inner nerve layer according to the expression of marker proteins and their location in the nerve layer. In the present study, we have used confocal calcium imaging of OECs in acute mouse brain slices and olfactory bulbs in toto to investigate physiological differences between OEC subpopulations. OECs in the outer nerve layer, but not the inner nerve layer, responded to glutamate, ATP, serotonin, dopamine, carbachol, and phenylephrine with increases in the cytosolic calcium concentration. The calcium responses consisted of a transient and a tonic component, the latter being mediated by store-operated calcium entry. Calcium measurements in OECs during the first three postnatal weeks revealed a downregulation of mGluR(1) and P2Y(1) receptor-mediated calcium signaling within the first 2 weeks, suggesting that the expression of these receptors is developmentally controlled. In addition, electrical stimulation of sensory axons evoked calcium signaling via mGluR(1) and P2Y(1) only in outer nerve layer OECs. Downregulation of the receptor-mediated calcium responses in postnatal animals is reflected by a decrease in amplitude of stimulation-evoked calcium transients in OECs from postnatal days 3 to 21. In summary, the results presented reveal striking differences in receptor responses during development and in axon-OEC communication between the two subpopulations of OECs in the olfactory bulb.

The migration of olfactory ensheathing cells during development and regeneration

The primary olfactory nervous system is unique in that it continuously renews itself and regenerates after injury. These properties are attributed to the presence of olfactory glia, termed olfactory ensheathing cells (OECs). Evidence is now emerging that individual OEC populations exist with distinct anatomical localisations and physiological properties, but their differential roles have not been determined. Unlike other glia, OECs can migrate from the periphery into the central nervous system, and organised OEC migration can enhance axonal extension after injury. Despite this, the mechanisms regulating OEC migration are largely unknown. Here, we provide an overview of the roles of OECs in development and adulthood. We review the latest research describing the differences between individual OEC subpopulations and discuss potential regulatory mechanisms for OEC guidance and migration. Using advanced time lapse techniques, we have obtained novel insights into how OECs behave in a complex multicellular environment which we discuss here with particular focus on cell-cell interactions. Significantly, transplantation of OECs constitutes a promising novel therapy for nerve injuries, but results are highly variable and the method needs improvement. We here review the roles of transplanted OECs in neural repair of damaged neuronal tracts distinct from the primary olfactory nervous system.

Two phases of replacement replenish the olfactory ensheathing cell population after injury in postnatal mice

Olfactory ensheathing cells (OECs) support the regeneration of olfactory sensory neurons throughout life, however, it remains unclear how OECs respond to a major injury. We have examined the proliferation and migration of OECs following unilateral bulbectomy in postnatal mice. S100ß-DsRed and OMP-ZsGreen transgenic mice were used to visualize OECs and olfactory neurons, respectively, and we used the thymidine analogue ethynyl deoxyuridine (EdU) to identify cells that were proliferating at the time of administration. Following unilateral bulbectomy, there was an initial phase of OEC proliferation throughout the olfactory pathway with a peak of proliferation occurring 2 to 7 days after the injury. A second phase of proliferation also occurred in which precursors localized within the olfactory mucosa divided to replenish the OEC population. We then tracked the positions of OECs that had proliferated and found that there was a progressive increase in OECs in the cavity for at least 12 to 16 days after injury which could not be accounted for solely by local proliferation of OECs within the cavity. These results suggest that OECs migrated from the peripheral olfactory nerve to populate the mass of cells that filled cavity left by bulbectomy. Our results demonstrate that following injury to the olfactory nervous system, the OEC population is replenished by migration of cells that arise from both local proliferation of OECs throughout the olfactory nerve pathway as well as from precursor cells in the olfactory mucosa.

The carbohydrate CT1 is expressed in topographically fixed glomeruli in the mouse olfactory bulb

Cell surface carbohydrates define subpopulations of primary olfactory neurons whose axons terminate in select glomeruli in the olfactory bulb. The combination of carbohydrates present on axon subpopulations has been proposed to confer a unique identity that contributes to the establishment of the olfactory topographic map. We have identified a novel subpopulation of primary olfactory neurons in mice that express blood group carbohydrates with GalNAc-ß1,4[NeuAcα 2,3]Galß1 residues recognised by the CT1 antibody. The CT1 carbohydrate has been shown to modulate adhesion of nerve terminals to the extracellular matrix and to synaptic proteins. The axons of the CT1-positive primary olfactory neurons terminate in a subpopulation of glomeruli in the olfactory bulb. Four lines of evidence support the view that CT1 glomeruli are topographically fixed. First, CT1 glomeruli were restricted predominantly to the dorsomedial olfactory bulb and were absent from large patches of the ventrolateral bulb. Second, similar distributions were observed for CT1 glomeruli on both the left and right olfactory bulbs of each animal, and between animals. Third, CT1 glomeruli were typically present as small clusters of 2-4 glomeruli. Fourth, a single CT1 glomerulus was always apposed to the glomeruli innervated by axons expressing the M72 odorant receptor. We also show that the CT1 carbohydrate is lost in gain-of-function transgenic mice over-expressing the blood group A glycosyltransferase in which there is aberrant targeting of M72 axons. Taken together, these results suggest that the CT1 carbohydrate, together with other carbohydrates, contributes to axon guidance during the establishment of the olfactory topographic map.

OMP-ZsGreen fluorescent protein transgenic mice for visualisation of olfactory sensory neurons in vivo and in vitro

Research into the biology of the mammalian olfactory system would be greatly enhanced by transgenic reporter mice with cell-specific fluorescence. To this end we previously generated a mouse whose olfactory ensheathing cells (OECs) express DsRed driven by the S100ß promoter. We present here a transgenic reporter mouse whose olfactory sensory neurons express ZsGreen, driven by the olfactory marker protein (OMP) promoter. ZsGreen was very strongly expressed throughout the cytoplasm of olfactory sensory neurons labelling them in living cells and after fixation. Labelled sensory neurons were seen in all olfactory regions in the nose and fluorescent axons coursed through the lamina propria and into the main and accessory bulbs. We developed methods for culturing embryonic and postnatal olfactory sensory neurons using these mice to visualise living cells in vitro. ZsGreen was expressed along the length of axons providing exceptional detail of the growth cones. The ZsGreen fluorescence was very stable, without fading during frequent imaging. The combination of OMP-ZsGreen and S100ß-DsRed transgenic mice is ideal for developmental studies and neuron-glia assays and they can be bred with mutant mice to dissect the roles of various molecules in neurogenesis, differentiation, axon growth and targeting and other aspects of olfactory sensory neuron and glia biology.

Olfactory glia enhance neonatal axon regeneration

Olfactory ensheathing cells (OECs) migrate with olfactory axons that extend from the nasal epithelium into the olfactory bulb. Unlike other glia, OECs are thought to migrate ahead of growing axons instead of following defined axonal paths. However it remains unknown how the presence of axons and OECs influences the growth and migration of each other during regeneration. We have developed a regeneration model in neonatal mice to examine whether (i) the presence of OECs ahead of olfactory axons affects axonal growth and (ii) the presence of olfactory axons alters the distribution of OECs. We performed unilateral bulbectomy to ablate olfactory axons followed by methimazole administration to further delay neuronal growth. In this model OECs filled the cavity left by the bulbectomy before new axons extended into the cavity. We found that delaying axon growth increased the rate at which OECs filled the cavity. The axons subsequently grew over a significantly larger region and formed more distinct fascicles and glomeruli in comparison with growth in animals that had undergone only bulbectomy. In vitro, we confirmed (i) that olfactory axon growth was more rapid when OECs were more widely distributed than the axons and (ii) that OECs migrated faster in the absence of axons. These results demonstrate that the distribution of OECs can be increased by repressing by growth of olfactory axons and that olfactory axon growth is significantly enhanced if a permissive OEC environment is present prior to axon growth.

The cell surface carbohydrate blood group A regulates the selective fasciculation of regenerating accessory olfactory axons

Cell surface carbohydrates are differentially expressed by discrete subpopulations of primary sensory axons in the mammalian main and accessory olfactory systems. It has been proposed that these carbohydrates provide a glycocode which mediates the sorting of these sensory axons as they project from the olfactory neuroepithelium to their central targets in the main and accessory olfactory bulbs during development. As the differential expression of cell surface carbohydrates on olfactory axons persists in the adult we have now investigated their role during regeneration. We have recently generated a line of transgenic mice, BGAT-Tg, that mis-express the blood group A (BGA) carbohydrate on all primary olfactory axons rather than just on accessory olfactory axons as in wild-type mice. Following unilateral bulbectomy, accessory and main olfactory axons regenerate and grow into the frontal cortex where they fill the cavity which remains after the olfactory bulb ablation. In wild-type mice, the regenerating BGA-expressing accessory olfactory axons selectively aggregated with each other in large bundles but clearly separated from the BGA-negative main olfactory axons. In contrast, in the BGAT-Tg transgenic mice in which all main and accessory axons express the BGA carbohydrate, the accessory olfactory axons failed to correctly separate from the main olfactory axons. Instead, these axons formed numerous small bundles interspersed with main olfactory axons. These data provide strong evidence that the restricted expression of BGA is in part responsible for the selective segregation of accessory olfactory axons.

Motile membrane protrusions regulate cell-cell adhesion and migration of olfactory ensheathing glia

Olfactory ensheathing cells (OECs) are candidates for therapeutic approaches for neural regeneration due to their ability to assist axon regrowth in central nervous system lesion models. However, little is understood about the processes and mechanisms underlying migration of these cells. We report here that novel lamellipodial protrusions, termed lamellipodial waves, are integral to OEC migration. Time-lapse imaging of migrating OECs revealed that these highly dynamic waves progress along the shaft of the cells and are crucial for mediating cell-cell adhesion. Without these waves, cell-cell adhesion does not occur and migrational rates decline. The activity of waves is modulated by both glial cell line-derived neurotrophic factor and inhibitors of the JNK and SRC kinases. Furthermore, the activity of lamellipodial waves can be modulated by Mek1, independently of leading edge activity. The ability to selectively regulate cell migration via lamellipodial waves has implications for manipulating the migratory behavior of OECs during neural repair.

The shape of the olfactory bulb influences axon targeting

Each primary olfactory neuron in the mouse expresses a single type of odorant receptor. All neurons expressing the same odorant receptor gene typically project to two topographically fixed glomeruli, one each on the medial and lateral surfaces of the olfactory bulb. While topographic gradients of guidance receptors and their ligands help to establish the retinotectal projection, similar orthogonal distributions of cues have not yet been detected within the olfactory system. While odorant receptors are crucial for the final targeting of axons to glomeruli, it is unclear whether the olfactory bulb itself provides instructive cues for the establishment of the topographic map. To begin to understand the role of the olfactory bulb in the formation of the olfactory nerve pathway, we developed a model whereby the gross shape of the bulb in the P2-IRES-tau-LacZ line of mice was radically altered during postnatal development. We have shown here that the topography of axons expressing the P2 odorant receptor is dependent on the shape of the olfactory bulb. When the dorsoventral axis of the olfactory bulb was compressed during the early postnatal period, newly developing P2 axons projected to multiple inappropriate glomeruli surrounding their normal target site. These results suggest that the distribution of local guidance cues within the olfactory bulb is influenced by the shape of the olfactory bulb and that these cues contribute to the topographic positioning of glomeruli.

Genetic manipulation of blood group carbohydrates alters development and pathfinding of primary sensory axons of the olfactory systems

Primary sensory neurons in the vertebrate olfactory systems are characterised by the differential expression of distinct cell surface carbohydrates. We show here that the histo-blood group H carbohydrate is expressed by primary sensory neurons in both the main and accessory olfactory systems while the blood group A carbohydrate is expressed by a subset of vomeronasal neurons in the developing accessory olfactory system. We have used both loss-of-function and gain-of-function approaches to manipulate expression of these carbohydrates in the olfactory system. In null mutant mice lacking the alpha(1,2)fucosyltransferase FUT1, the absence of blood group H carbohydrate resulted in the delayed maturation of the glomerular layer of the main olfactory bulb. In addition, ubiquitous expression of blood group A on olfactory axons in gain-of-function transgenic mice caused mis-routing of axons in the glomerular layer of the main olfactory bulb and led to exuberant growth of vomeronasal axons in the accessory olfactory bulb. These results provide in vivo evidence for a role of specific cell surface carbohydrates during development of the olfactory nerve pathways.

Implantation of a scaffold following bulbectomy induces laminar organization of regenerating olfactory axons

Primary olfactory axons expressing different odorant receptors are interspersed within the olfactory nerve. However, upon reaching the outer nerve fiber layer of the olfactory bulb they defasciculate, sort out, and refasciculate prior to targeting glomeruli in fixed topographic positions. While odorant receptors are crucial for the final targeting of axons to glomeruli, it is unclear what directs the formation of the nerve fiber and glomerular layers of the olfactory bulb. While the olfactory bulb itself may provide instructive cues for the development of these layers, it is also possible that the incoming axons may simply require the presence of a physical scaffold to establish the outer laminar cytoarchitecture. In order to begin to understand the underlying role of the olfactory bulb in development of the outer layers of the olfactory bulb, we physically ablated the olfactory bulbs in OMP-IRES-LacZ and P2-IRES-tau-LacZ neonatal mice and replaced them with artificial biological scaffolds molded into the shape of an olfactory bulb. Regenerating axons projected around the edge of the cranial cavity at the periphery of the artificial scaffold and were able to form an olfactory nerve fiber layer and, to some extent, a glomerular layer. Our results reveal that olfactory axons are able to form rudimentary cytoarchitectonic layers if they are provided with an appropriately shaped biological scaffold. Thus, the olfactory bulb does not appear to provide any tropic substance that either attracts regenerating olfactory axons into the cranial cavity or induces these axons to form a plexus around its outer surface.