Brain Infection by Group B Streptococcus Induces Inflammation and Affects Neurogenesis in the Adult Mouse Hippocampus

Central nervous system infections caused by pathogens crossing the blood-brain barrier are extremely damaging and trigger cellular alterations and neuroinflammation. Bacterial brain infection, in particular, is a major cause of hippocampal neuronal degeneration. Hippocampal neurogenesis, a continuous multistep process occurring throughout life in the adult brain, could compensate for such neuronal loss. However, the high rates of cognitive and other sequelae from bacterial meningitis/encephalitis suggest that endogenous repair mechanisms might be severely affected. In the current study, we used Group B Streptococcus (GBS) strain NEM316, to establish an adult mouse model of brain infection and determine its impact on adult neurogenesis. Experimental encephalitis elicited neurological deficits and death, induced inflammation, and affected neurogenesis in the dentate gyrus of the adult hippocampus by suppressing the proliferation of progenitor cells and the generation of newborn neurons. These effects were specifically associated with hippocampal neurogenesis while subventricular zone neurogenesis was not affected. Overall, our data provide new insights regarding the effect of GBS infection on adult brain neurogenesis.

Organoids: the third dimension of human brain development and disease

Stem cell technologies have opened up new avenues in the study of human biology and disease. In particular, the advent of human embryonic stem cells followed by reprograming technologies for generation of induced pluripotent stem cells have instigated studies into modeling human brain development and disease by providing a means to simulate a human tissue otherwise completely or largely inaccessible to researchers. Brain development is a complex process achieved in a remarkably controlled spatial and temporal manner through coordinated cellular and molecular events. In vitro models aim to mimic these processes and recapitulate brain organogenesis. Initially, two-dimensional neural cultures presented an innovative landmark for investigating human neuronal and, more recently, glial biology, as well as for modeling brain neurodevelopmental and neurodegenerative diseases. The establishment of three-dimensional cultures in the form of brain organoids was an equally important milestone in the field. Brain organoids mimic more closely the in vivo tissue composition and architecture and are more physiologically relevant than monolayer cultures. They therefore represent a more realistic cellular environment for modeling the cell biology and pathology of the nervous system. Here we highlight the journey towards recapitulating human brain development and disease in a dish, progressing from two-dimensional in vitro systems to the third dimension provided by brain organoids. We discuss the potential of these approaches for modeling human brain development and evolution, and their promising contribution towards understanding and treating brain disease.

Application of developmental principles for spinal cord repair after injury

The superiority of the mammalian central nervous system (CNS) among other vertebrates does not involve an advanced capacity for regeneration and any insult results to irreversible function loss. Spinal cord injury (SCI) is one example of CNS trauma affecting thousands of individuals, mostly young, each year. Despite enormous progress in our comprehension of the molecular and cellular mechanisms underlying the pathophysiology after SCI, also providing targets for therapeutic interventions, so far, no efficient therapy exists, emphasizing the necessity for further research. A breadth of studies have demonstrated that, after SCI, principles of development come at play either to promote or to prohibit spontaneous regeneration and their accurate manipulation holds promise toward functional recovery. In this overview, some of the most recent and important studies are discussed that offer explicitly novel input from the field of development to the field of CNS repair regarding the modification of the inhibitory environment of the injured spinal cord - majorly referring to the glial scar - the activation of endogenous cell populations such as ependymal stem cells and oligodendrocyte precursor cells, and the developmental transcriptional program that is transiently activated in neurons after injury. Furthermore, current advances in stem cell technology are highlighted in terms of refinement and precise design of the appropriate stem cell population to be transplanted not only for cell replacement but also for modulation of the host environment. As single-dimension applications were not yet clinically successful, combinatorial strategies tackling more than one targets are suggested as more auspicious.

An original infection model identifies host lipoprotein import as a route for blood-brain barrier crossing

Pathogens able to cross the blood-brain barrier (BBB) induce long-term neurological sequelae and death. Understanding how neurotropic pathogens bypass this strong physiological barrier is a prerequisite to devise therapeutic strategies. Here we propose an innovative model of infection in the developing Drosophila brain, combining whole brain explants with in vivo systemic infection. We find that several mammalian pathogens are able to cross the Drosophila BBB, including Group B Streptococcus (GBS). Amongst GBS surface components, lipoproteins, and in particular the B leucine-rich Blr, are important for BBB crossing and virulence in Drosophila. Further, we identify (V)LDL receptor LpR2, expressed in the BBB, as a host receptor for Blr, allowing GBS translocation through endocytosis. Finally, we show that Blr is required for BBB crossing and pathogenicity in a murine model of infection. Our results demonstrate the potential of Drosophila for studying BBB crossing by pathogens and identify a new mechanism by which pathogens exploit the machinery of host barriers to generate brain infection.

Neural stem/progenitor cells differentiate into oligodendrocytes, reduce inflammation, and ameliorate learning deficits after transplantation in a mouse model of traumatic brain injury

The central nervous system has limited capacity for regeneration after traumatic injury. Transplantation of neural stem/progenitor cells (NPCs) has been proposed as a potential therapeutic approach while insulin-like growth factor I (IGF-I) has neuroprotective properties following various experimental insults to the nervous system. We have previously shown that NPCs transduced with a lentiviral vector for IGF-I overexpression have an enhanced ability to give rise to neurons in vitro but also in vivo, upon transplantation in a mouse model of temporal lobe epilepsy. Here we studied the regenerative potential of NPCs, IGF-I-transduced or not, in a mouse model of hippocampal mechanical injury. NPC transplantation, with or without IGF-I transduction, rescued the injury-induced spatial learning deficits as revealed in the Morris Water Maze. Moreover, it had beneficial effects on the host tissue by reducing astroglial activation and microglial/macrophage accumulation while enhancing generation of endogenous oligodendrocyte precursor cells. One or two months after transplantation the grafted NPCs had migrated towards the lesion site and in the neighboring myelin-rich regions. Transplanted cells differentiated toward the oligodendroglial, but not the neuronal or astrocytic lineages, expressing the early and late oligodendrocyte markers NG2, Olig2, and CNPase. The newly generated oligodendrocytes reached maturity and formed myelin internodes. Our current and previous observations illustrate the high plasticity of transplanted NPCs which can acquire injury-dependent phenotypes within the host CNS, supporting the fact that reciprocal interactions between transplanted cells and the host tissue are an important factor to be considered when designing prospective cell-based therapies for CNS degenerative conditions.

Intraspinal Delivery of Polyethylene Glycol-coated Gold Nanoparticles Promotes Functional Recovery After Spinal Cord Injury

Failure of the mammalian central nervous system (CNS) to regenerate effectively after injury leads to mostly irreversible functional impairment. Gold nanoparticles (AuNPs) are promising candidates for drug delivery in combination with tissue-compatible reagents, such as polyethylene glycol (PEG). PEG administration in CNS injury models has received interest for potential therapy, but toxicity and low bioavailability prevents clinical application. Here we show that intraspinal delivery of PEG-functionalized 40-nm-AuNPs at early stages after mouse spinal cord injury is beneficial for recovery. Positive outcome of hind limb motor function was accompanied by attenuated inflammatory response, enhanced motor neuron survival, and increased myelination of spared or regrown/sprouted axons. No adverse effects, such as body weight loss, ill health, or increased mortality were observed. We propose that PEG-AuNPs represent a favorable drug-delivery platform with therapeutic potential that could be further enhanced if PEG-AuNPs are used as carriers of regeneration-promoting molecules.

From demyelination to remyelination: the road toward therapies for spinal cord injury

Myelin integrity is crucial for central nervous system (CNS) physiology while its preservation and regeneration after spinal cord injury (SCI) is key to functional restoration. Disturbance of nodal organization acutely after SCI exposes the axon and triggers conduction block in the absence of overt demyelination. Oligodendrocyte (OL) loss and myelin degradation follow as a consequence of secondary damage. Here, we provide an overview of the major biological events and underlying mechanisms leading to OL death and demyelination and discuss strategies to restrain these processes. Another aspect which is critical for SCI repair is the enhancement of endogenously occurring spontaneous remyelination. Recent findings have unveiled the complex roles of innate and adaptive immune responses in remyelination and the immunoregulatory potential of the glial scar. Moreover, the intimate crosstalk between neuronal activity, oligodendrogenesis and myelination emphasizes the contribution of rehabilitation to functional recovery. With a view toward clinical applications, several therapeutic strategies have been devised to target SCI pathology, including genetic manipulation, administration of small therapeutic molecules, immunomodulation, manipulation of the glial scar and cell transplantation. The implementation of new tools such as cellular reprogramming for conversion of one somatic cell type to another or the use of nanotechnology and tissue engineering products provides additional opportunities for SCI repair. Given the complexity of the spinal cord tissue after injury, it is becoming apparent that combinatorial strategies are needed to rescue OLs and myelin at early stages after SCI and support remyelination, paving the way toward clinical translation.

Functional cross-talk between the cellular prion protein and the neural cell adhesion molecule is critical for neuronal differentiation of neural stem/precursor cells

Cellular prion protein (PrP) is prominently expressed in brain, in differentiated neurons but also in neural stem/precursor cells (NPCs). The misfolding of PrP is a central event in prion diseases, yet the physiological function of PrP is insufficiently understood. Although PrP has been reported to associate with the neural cell adhesion molecule (NCAM), the consequences of concerted PrP-NCAM action in NPC physiology are unknown. Here, we generated NPCs from the subventricular zone (SVZ) of postnatal day 5 wild-type and PrP null (-/-) mice and observed that PrP is essential for proper NPC proliferation and neuronal differentiation. Moreover, we found that PrP is required for the NPC response to NCAM-induced neuronal differentiation. In the absence of PrP, NCAM not only fails to promote neuronal differentiation but also induces an accumulation of doublecortin-positive neuronal progenitors at the proliferation stage. In agreement, we noted an increase in cycling neuronal progenitors in the SVZ of PrP-/- mice compared with PrP+/+ mice, as evidenced by double labeling for the proliferation marker Ki67 and doublecortin as well as by 5-bromo-2'-deoxyuridine incorporation experiments. Additionally, fewer newly born neurons were detected in the rostral migratory stream of PrP-/- mice. Analysis of the migration of SVZ cells in microexplant cultures from wild-type and PrP-/- mice revealed no differences between genotypes or a role for NCAM in this process. Our data demonstrate that PrP plays a critical role in neuronal differentiation of NPCs and suggest that this function is, at least in part, NCAM-dependent.

Functional Interactions between BM88/Cend1, Ran-binding protein M and Dyrk1B kinase affect cyclin D1 levels and cell cycle progression/exit in mouse neuroblastoma cells

BM88/Cend1 is a neuronal-lineage specific modulator with a pivotal role in coordination of cell cycle exit and differentiation of neuronal precursors. In the current study we identified the signal transduction scaffolding protein Ran-binding protein M (RanBPM) as a BM88/Cend1 binding partner and showed that BM88/Cend1, RanBPM and the dual specificity tyrosine-phosphorylation regulated kinase 1B (Dyrk1B) are expressed in mouse brain as well as in cultured embryonic cortical neurons while RanBPM can form complexes with either of the two other proteins. To elucidate a potential mechanism involving BM88/Cend1, RanBPM and Dyrk1B in cell cycle progression/exit, we transiently co-expressed these proteins in mouse neuroblastoma Neuro 2a cells. We found that the BM88/Cend1-dependent or Dyrk1B-dependent down-regulation of cyclin D1 is reversed following their functional interaction with RanBPM. More specifically, functional interaction of RanBPM with either BM88/Cend1 or Dyrk1B stabilizes cyclin D1 in the nucleus and promotes 5-bromo-2'-deoxyuridine (BrdU) incorporation as a measure of enhanced cell proliferation. However, the RanBPM-dependent Dyrk1B cytosolic retention and degradation is reverted in the presence of Cend1 resulting in cyclin D1 destabilization. Co-expression of RanBPM with either BM88/Cend1 or Dyrk1B also had a negative effect on Neuro 2a cell differentiation. Our results suggest that functional interactions between BM88/Cend1, RanBPM and Dyrk1B affect the balance between cellular proliferation and differentiation in Neuro 2a cells and indicate that a potentially similar mechanism may influence cell cycle progression/exit and differentiation of neuronal precursors.

Cell adhesion molecules in gene and cell therapy approaches for nervous system repair

The inability of the central nervous system (CNS) to efficiently repair damages results in severe functional impairment after trauma or neurodegenerative/demyelinating diseases. Regeneration failure is attributed to inhibitory molecules creating a nonpermissive environment for axonal regrowth, and dictates the necessity for the development of novel therapeutic strategies. An emerging approach for improving regeneration is the use of gene therapy to manipulate cell adhesion molecule expression in experimental animal models of degeneration. Alternatively, cell transplantation to replace lost neurons and the grafting of myelinating cells to repair demyelinating lesions are promising approaches for treating CNS injuries and demyelination. Schwann cells (SCs), oligodendrocyte progenitors, olfactory ensheathing cells and embryonic and neural stem cells have been shown to form myelin after transplantation into the demyelinated CNS. The repair capacity of the peripheral nervous system (PNS) is much higher, but there is still a limit to the amount of nerve loss that can be bridged after injury, and longer nerve gaps call for the use of conduits populated with living cells. In both cases, the interaction of grafted cells with the host environment is of paramount importance for the incorporation and functional integration of these cells and the manipulation of cell adhesion molecules is an attractive approach towards achieving this goal. In this review we summarize data from the recent literature regarding the manipulation of cell adhesion molecule expression towards CNS and PNS repair and discuss the prospects for future therapeutic applications.

Soluble forms of the cell adhesion molecule L1 produced by insect and baculovirus-transduced mammalian cells enhance Schwann cell motility

For biotechnological applications, insect cell lines are primarily known as hosts for the baculovirus expression system that is capable to direct synthesis of high levels of recombinant proteins through use of powerful viral promoters. Here, we demonstrate the implementation of two alternative approaches based on the baculovirus system for production of a mammalian recombinant glycoprotein, comprising the extracellular part of the cell adhesion molecule L1, with potential important therapeutic applications in nervous system repair. In the first approach, the extracellular part of L1 bearing a myc tag is produced in permanently transformed insect cell lines and purified by affinity chromatography. In the second approach, recombinant baculoviruses that express L1-Fc chimeric protein, derived from fusion of the extracellular part of L1 with the Fc part of human IgG1, under the control of a mammalian promoter are used to infect mammalian HEK293 and primary Schwann cells. Both the extracellular part of L1 bearing a myc tag accumulating in the supernatants of insect cultures as well as L1-Fc secreted by transduced HEK293 or Schwann cells are capable of increasing the motility of Schwann cells with similar efficiency in a gap bridging bioassay. In addition, baculovirus-transduced Schwann cells show enhanced motility when grafted on organotypic cultures of neonatal brain slices while they retain their ability to myelinate CNS axons. This proof-of-concept that the migratory properties of myelin-forming cells can be modulated by recombinant protein produced in insect culture as well as by means of baculovirus-mediated adhesion molecule expression in mammalian cells may have beneficial applications in the field of CNS therapies.

BM88/Cend1 regulates stimuli-induced intracellular calcium mobilization

In neurogenesis, little is known about signal transduction pathways upstream of gene expression however, mounting evidence suggests that calcium release from internal stores plays a critical role. We have previously demonstrated that BM88 is a neuronal lineage-specific regulator of cell cycle exit and differentiation; we now report a link between BM88 and calcium signaling. Calcium imaging experiments revealed that P2Y-induced calcium mobilization is diminished in mouse neuroblastoma Neuro 2a cells stably transfected with BM88 (N2A-BM88 cells) as compared with N2A cells or N2A cells differentiated with retinoic acid. This effect is not restricted to N2A cells but is also observed in HeLa cells that are transiently transfected with BM88, indicating that cells of both neural and non-neural origin respond similarly. Further, activation of P2Y1 but not purinergic P2X receptors induces proliferation of N2A and to a lesser extent of N2A-BM88 cells. Conversely, knockdown of BM88 facilitates N2A cell proliferation both under stimulating and non-stimulating conditions. Importantly, N2A-BM88 cells are less susceptible to apoptosis triggered by C2-ceramide and exhibit reduced C2-ceramide-induced intracellular calcium release. Higher calcium uptake from mitochondria and/or lower calcium levels inside the endoplasmic reticulum may explain the reduced calcium mobilization in response to BM88. Overall, our data reveal a novel signaling mechanism by which BM88 interferes with calcium release from inositol 1,4,5-trisphosphate-sensitive stores and exerts anti-proliferative and anti-apoptotic functions.

Schwann cell transplantation for CNS repair

Demyelination occurs in several central nervous system (CNS) disorders, including multiple sclerosis, viral infection and spinal cord injury and can result in severe functional impairment. Therefore there is great interest in developing therapies promoting repair in CNS demyelinating diseases and trauma. Cell replacement therapy is an attractive approach for myelin repair, and experimental transplantation has provided convincing evidence of the repair potential of grafted myelin-forming cells. Schwann cells (SCs), oligodendrocyte progenitors, olfactory ensheathing cells and embryonic and neural stem cells have been shown to form myelin after transplantation into the demyelinated CNS. SCs are among the most promising candidates for autologous grafting. They can remyelinate spinal cord lesions after experimental demyelination, leading in some cases to functional recovery in rodent and primate models. However, SCs do not normally enter the CNS, and migration of SCs transplanted in CNS white matter is inhibited by astrocytes. As SC migration and myelination is mediated by interactions of sets of extracellular matrix molecules with cell surface molecules, genetic engineering of SCs to alter aspects of these interactions is a possible way forward. Thus efforts towards the development of SC-based therapies are focused in enhancing their migration and functional integration into the lesioned CNS. In addition, efforts are being made to use these cells as gene delivery vehicles for an array of molecules with repair potential. In this review we summarize data from the recent literature regarding the use of SCs in CNS repair and discuss the prospects for future therapeutic applications.

Grafts of Schwann cells engineered to express PSA-NCAM promote functional recovery after spinal cord injury

Schwann cells (SCs) are among the most attractive cellular candidates for the development of remyelination therapies for CNS lesions. Yet, their integration in the CNS is inhibited by astrocytes and therefore the use of genetically modified SCs with improved properties is an alternative promising approach. Our strategy for ameliorating the therapeutic potential of SCs has been to alter their adhesive properties by expressing on their surface the polysialylated (PSA) form of the neural cell adhesion molecule NCAM. In the present study, SCs from transgenic GFP-mice were transduced with a retroviral vector encoding sialyl-transferase X (STX), the enzyme responsible for transferring PSA on NCAM. Engineered STX-GFP-SCs with sustained PSA expression were thus generated and were found to have improved ability to associate with astrocytes in vitro. Importantly, when these cells were transplanted in vivo in a mouse model of spinal cord injury they promoted faster and significantly greater functional recovery as compared to using SCs transduced with a control retroviral vector or no cells at all. Morphological analysis indicated that the improved locomotor recovery correlated with earlier and enhanced remyelination by grafted STX-GFP-SCs, increased remyelination by host SCs as well as enhanced differentiation/remyelination by resident oligodendrocyte precursors. Moreover, sprouting of regenerating serotonergic nerve fibres, which are known to be important for locomotion and recovery after injury, was observed into and across the lesion site. These results underline the potential therapeutic benefit of early activation of myelin-forming cells to differentiate and remyelinate severed axons thus restoring functions in CNS trauma and/or demyelinating diseases.