Targeting myelin lipid metabolism as a potential therapeutic strategy in a model of CMT1A neuropathy

In patients with Charcot-Marie-Tooth disease 1A (CMT1A), peripheral nerves display aberrant myelination during postnatal development, followed by slowly progressive demyelination and axonal loss during adult life. Here, we show that myelinating Schwann cells in a rat model of CMT1A exhibit a developmental defect that includes reduced transcription of genes required for myelin lipid biosynthesis. Consequently, lipid incorporation into myelin is reduced, leading to an overall distorted stoichiometry of myelin proteins and lipids with ultrastructural changes of the myelin sheath. Substitution of phosphatidylcholine and phosphatidylethanolamine in the diet is sufficient to overcome the myelination deficit of affected Schwann cells in vivo. This treatment rescues the number of myelinated axons in the peripheral nerves of the CMT rats and leads to a marked amelioration of neuropathic symptoms. We propose that lipid supplementation is an easily translatable potential therapeutic approach in CMT1A and possibly other dysmyelinating neuropathies.

Autophagy in adipose tissue of patients with obesity and type 2 diabetes

BACKGROUND

Pathophysiology of obesity is closely associated with enhanced autophagy in adipose tissue (AT). Autophagic process can promote survival or activate cell death. Therefore, we examine the occurrence of autophagy in AT of type 2 diabetes (T2D) patients in comparison to obese and lean individuals without diabetes.

METHODOLOGY/PRINCIPAL FINDINGS

Numerous autophagosomes accumulated within adipocytes were visualized by electron transmission microscopy and by immunofluorescence staining for autophagy marker LC3 in obese and T2D patients. Increased autophagy was demonstrated by higher LC3-II/LC3-I ratio, up-regulated expression of LC3 and Atg5 mRNA, along with decreased p62 and mTOR protein levels. Increased autophagy occurred together with AT inflammation.

CONCLUSIONS

Our data suggest fat depot-related differences in autophagy regulation. In subcutaneous AT, increased autophagy is accompanied by increased markers of apoptosis in patients with obesity independently of T2D. In contrast, in visceral AT only in T2D patients increased autophagy was related to higher markers of apoptosis.

Resveratrol and desferoxamine protect human OxLDL-treated granulosa cell subtypes from degeneration

CONTEXT

Obese women suffer from anovulation and infertility, which are driven by oxidative stress caused by increased levels of lipid peroxides and circulating oxidized low-density lipoprotein (oxLDL). OxLDL binds to lectin-like oxLDL receptor 1 (LOX-1), cluster of differentiation 36 (CD36), and toll-like receptor 4 (TLR4) and causes cell death in human granulosa cells (GCs).

OBJECTIVE

Our objective was to reveal whether treatment with antioxidants resveratrol (RES) and/or desferoxamine (DFO) protect GCs from oxLDL-induced damage.

DESIGN AND SETTING

This basic research study was performed at the Institute of Anatomy and the Clinic of Reproductive Medicine.

PATIENTS

Patients were women undergoing in vitro fertilization therapy.

MAIN OUTCOME MEASURES

GC cultures were treated with oxLDL alone or with RES or DFO under serum-free conditions for up to 36 hours. Dead cells were determined by propidium iodide uptake, cleaved caspase-3 expression, and electron microscopy. Mitosis was detected by Ki-67 immunostaining. LOX-1, TLR4, CD36, and heat-shock protein 60 were examined by Western blot. Measurement of oxidative stress markers (8-iso-prostaglandin F2α, advanced glycation end products, and protein carbonyl content) was conducted with ELISA kits.

RESULTS

Different subtypes of human GCs exposed to RES or DFO were protected as evidenced by the lack of cell death, enhanced mitosis, induction of protective autophagy, reduction of oxidative stress markers, and reduced expression of LOX-1, TLR4, CD36, and heat-shock protein 60. Importantly, RES could restore steroid biosynthesis in cytokeratin-positive GCs, which exhibited significant induction of steroidogenic acute regulatory protein.

CONCLUSIONS

RES and DFO exert a protective effect on human GCs. Thus, RES and DFO may help improve the treatment of obese women or polycystic ovarian syndrome patients undergoing in vitro fertilization therapy.

Slice cultures from head and neck squamous cell carcinoma: a novel test system for drug susceptibility and mechanisms of resistance

Background:

Human head and neck squamous cell carcinoma (HNSCC) fundamentally vary in their susceptibility to different cytotoxic drugs and treatment modalities. There is at present no clinically accepted test system to predict the most effective therapy for an individual patient.

Methods:

Therefore, we established tumour-derived slice cultures which can be kept in vitro for at least 6 days. Upon treatment with cisplatin, docetaxel and cetuximab, slices were fixed and paraffin sections were cut for histopathological analysis.

Results:

Apoptotic fragmentation, activation of caspase 3, and cell loss were observed in treated tumour slices. Counts of nuclei per field in untreated compared with treated slices deriving from the same tumour allowed estimation of the anti-neoplastic activity of individual drugs on an individual tumour.

Conclusion:

HNSCC-derived slice cultures survive well in vitro and may serve not only to improve personalised therapies but also to detect mechanisms of tumour resistance by harvesting surviving tumour cells after treatment.

Increased autophagy in peripheral nerves may protect Wistar Ottawa Karlsburg W rats against neuropathy

OBJECTIVE

Wistar Ottawa Karlsburg W (RT1(u)) rats (WOKW) develop obesity, dyslipidemia, moderate hypertension, hyperinsulinemia and impaired glucose tolerance prone to induce peripheral neuropathy (PN). Autophagy has been shown to prevent neurodegeneration in the central and peripheral nervous system. We analyzed the potential protective role of autophagy in an established rat model in preventing PN.

METHODS

We examined electrophysiology (motor-and sensory/mixed afferent conduction velocities and the minimal F-wave latency) and morphology, including ultrathin sections, myelin sheath thickness (g-ratio) and immunohistochemical markers of autophagy and inflammation in the sciatic nerve of five-month-old, male WOKW as compared to Wistar derived, congenic LEW.1W control rats, characterized by the same major histocompatibility complex as WOKW rats (RT1(u)). Moreover, the expression of axonal and synaptic proteins (NF68, GAP43, MP0), autophagy- (Atg5, Atg7, LC3), and apoptosis (cleaved caspase-3)-related markers was measured using Western blot.

RESULTS

No abnormalities in nerve electrophysiology and morphology were found in WOKW compared to LEW.1W rats. However, autophagosomes were more frequently apparent in sciatic nerves of WOKW rats. In Western blot analyses no significant differences in expression of neuronal structural proteins were found, but autophagy markers were up-regulated in WOKW compared to LEW.1W sciatic nerves. Immunostaining revealed a greater infiltration of Iba1/ED-1-positive macrophages, CD-3-positive T-cells and LC3-expression in sciatic nerves of WOKW rats.

CONCLUSIONS

Our results indicate that WOKW rats show an up-regulated autophagy and a mild inflammatory response but do not develop overt neuropathy. We suggest that autophagy and inflammatory cells may exert a protective role in preventing neuropathy in this rat model of the metabolic syndrome but the mechanism of action is still unclear.

Axonal damage induced by invading T cells in organotypic central nervous system tissue in vitro: involvement of microglial cells

Neuroinflammation in the course of multiple sclerosis and experimental autoimmune encephalomyelitis results in demyelination and, recently demonstrated, axonal loss. Invading neuroantigen specific T cells are the crucial cellular elements in these processes. Here we demonstrate that invasion of activated T cells induces a massive microglial attack on myelinated axons in entorhinal-hippocampal slice cultures. Flow cytometry analysis of activation markers revealed that the activation state of invading MBP-specific T cells was significantly lower in comparison to PMA-activated T cells. Moreover, MBP-specific T cells showed a significantly lower secretion of IFN-gamma. Conversely, MBP-specific T cells displayed a significantly higher potential to trigger activation of microglial cells, i.e. upregulation of MHC class II and ICAM-1 expression, and, most importantly, microglial phagocytosis of pre-traced axons. Our data suggest that this was mediated via specific cellular interactions of T cells and microglial cells since IFN-gamma alone was not sufficient to induce axonal damage while such damage was apparent in response to TNF-alpha which is released by activated microglial cells. TNF-alpha secretion by both T cell populations was negligible. Thus, MBP-specific T cells which invade nervous tissue in the course of neuroinflammation are more effective in axon-damaging recruiting microglial cells than activated T cells of other specificities.

Self-tolerance in the immune privileged CNS: lessons from the entorhinal cortex lesion model

Upon peripheral immunization with myelin epitopes, susceptible rats and mice develop T cell-mediated demyelination similar to that observed in the human autoimmune disease multiple sclerosis (MS). In the same animals, brain injury does not induce autoimmune encephalomyelitis despite massive release of myelin antigens and early expansion of myelin specific T cells in local lymph nodes, indicating that the self-specific T cell clones are kept under control. Using entorhinal cortex lesion (ECL) to induce axonal degeneration in the hippocampus, we identified possible mechanisms of immune tolerance after brain trauma. Following ECL, astrocytes upregulate the death ligand CD95L, allowing apoptotic elimination of infiltrating activated T cells. Myelin-phagocytosing microglia express MHC-II and the costimulatory molecule CD86, but lack CD80, which is found only on activated antigen presenting cells (APCs). Restimulation of invading T cells by such immature APCs (e.g. CD80 negative microglia) may lead to T cell anergy and/or differentiation of regulatory/Th3-like cells due to insufficient costimulation and presence of high levels of TGF-beta and IL-10 in the CNS. Thus, T cell -apoptosis, -anergy, and -suppression apparently maintain immune tolerance after initial expansion of myelin-specific T lymphocytes following brain injury. This view is supported by a previous metastatistical analysis which rejected the hypothesis that brain trauma is causative of MS (Goddin et al., 1999). However, concomitant trauma-independent proinflammatory signals, e.g., those evoked by clinically quiescent infections, may trigger maturation of APCs, thus shifting a delicate balance from immune tolerance and protective immune responses to destructive autoimmunity.

Plasticity following lesion: help and harm from the immune system

In contrast to other organs where the tissue is capable of replacing lost cells and thus regaining tissue function, immune cell recruitment and activation is suppressed in the CNS in order to minimize secondary damage after lesion. This state of immune privilege has its cost because the injured tissue cannot fully benefit from growth-promoting effects accompanying inflammatory responses. These responses include phagocytosis of growth-inhibiting myelin debris by cells of the innate immune system and the recently described protection of surviving fibers by myelin-specifie T cells of the adaptive immune system. While the signals suppressing macrophage functions in the CNS are yet to be defined, it seems that help from T cells is diminished by apoptosis-induction via death-inducing ligands. Indeed, the death ligand CD95L (FasL, APO 1 L) is constitutively found on neurons, microglia and astrocytes. Its upregulation on astrocytes during axonal degeneration in the hippocampus after entorhinal lesion is accompanied by the appearance of apoptotic leukocytes. T cells also express CD95L and TNF-related apoptosis- inducing ligand (TRAIL), and the presence of CD95 (Fas, APOI) and TRAIL-receptors renders brain cells putative targets of T cell-induced apoptosis. Thus, blockade of death ligands could be helpful by simultaneously enhancing T cell survival and blocking T cell-mediated brain cell death. This is only one example of how boosting helpful immune cell functions and abrogating their destructive effects might help to overcome the brain's failure to regenerate after axonal lesions.

Differential expression of costimulatory molecules B7-1 and B7-2 on microglial cells induced by Th1 and Th2 cells in organotypic brain tissue

Autoreactive T-cells are involved in demyelination, neurodegeneration, and the recruitment of peripheral macrophages and nonspecific activated T-cells in autoimmune diseases such as multiple sclerosis (MS). The ligation of costimulatory B7 molecules on microglia with CD28/CTLA-4 on T-cells is thought to be crucial to the onset and course of MS and its rodent model experimental autoimmune encephalomyelitis (EAE). It is currently unclear as to how far the nature of infiltrating T-cells has an impact on the expression of the B7 molecules on microglia, the resident antigen-presenting cells (APCs) of the brain. We studied the expression of B7-1 and B7-2 on microglia after encounter with preactivated Th1 and Th2 cells from transgenic mice whose T-cells express a receptor (TCR) either specific to myelin basic protein (MBP) or ovalbumin (OVA) using murine organotypic entorhinal-hippocampal slice cultures (OEHSC). Our main finding was that Th1 cells downregulate the constitutive expression of B7-2 and induce B7-1 expression while Th2 cells do not induce this B7-1 upregulation. The main difference between MBP- and OVA-specific cells was seen in experiments were Th1 cells had direct contact to APCs but not to brain tissue. In contrast to MBP-specific Th1 cells, OVA-specific Th1 cells required the addition of antigen to upregulate B7-1 and downregulate B7-2. When the cells were allowed to have contact to brain tissue, no difference was seen in the pattern of B7 regulation between OVA- and MBP-specific T-cells. Our data suggest that T-cells are able to modulate B7 expression on microglial cells in the brain independent of antigen presentation through TCR/MHC-II ligation but presumably by soluble mediators.

Targeting gene-modified hematopoietic cells to the central nervous system: use of green fluorescent protein uncovers microglial engraftment

Gene therapy in the central nervous system (CNS) is hindered by the presence of the blood-brain barrier, which restricts access of serum constituents and peripheral cells to the brain parenchyma. Expression of exogenously administered genes in the CNS has been achieved in vivo using highly invasive routes, or ex vivo relying on the direct implantation of genetically modified cells into the brain. Here we provide evidence for a novel, noninvasive approach for targeting potential therapeutic factors to the CNS. Genetically-modified hematopoietic cells enter the CNS and differentiate into microglia after bone-marrow transplantation. Up to a quarter of the regional microglial population is donor-derived by four months after transplantation. Microglial engraftment is enhanced by neuropathology, and gene-modified myeloid cells are specifically attracted to the sites of neuronal damage. Thus, microglia may serve as vehicles for gene delivery to the nervous system.

Immune surveillance of mouse brain perivascular spaces by blood-borne macrophages

Virchow-Robin's perivascular spaces lie between the basement membrane around pericytes and the basement membrane at the surface of the glia limitans of the brain vessels. They are directly connected to the subpial space and harbour a population of cells distinct from pericytes, perivascular microglia and other cells within perivascular spaces (e.g. T cells and mast cells) in their ability to quickly phagocytose particles from the cerebrospinal fluid (CSF). Morphology, function, and cell surface proteins of these perivascular cells suggest an origin from the monocyte/macrophage lineage. It is currently unclear to what extent these brain perivascular cells represent a resident population of histiocytes or undergo continuous supplementation from blood monocytes. Using transplants of green-fluorescent-protein (GFP)-transfected bone marrow cells, we therefore investigated the replacement of perivascular cells by blood-borne macrophages in adult mice. GFP-positive cells in the perivascular spaces were found as early as 2 weeks post transplantation. The substitution of host perivascular cells by donor-derived macrophages was then evaluated using immunocytochemistry and intraventricular injection of hydrophilic rhodamine-fluorescent tracers. Such tracers diffuse along perivascular spaces and are subsequently phagocytosed by perivascular cells leading to stable phagocytosis-dependent labelling. Thus, the population of newly immigrated macrophages could be related to the total number of perivascular macrophages. This approach revealed a continuous increase of donor-derived perivascular cells. At 14 weeks post transplantation, all perivascular cells were donor-derived. These data show that brain perivascular cells are a population of migratory macrophages and not resident histiocytes.

Th2 cells support intrinsic anti-inflammatory properties of the brain

In experimental autoimmune encephalomyelitis (EAE), Th1 cells are responsible for disease induction while Th2 cells can be protective. To address the mechanisms of this differential behavior, we utilized organotypic murine entorhinal-hippocampal slice cultures to analyze interactions between myelin basic protein-specific Th1 and Th2 cells with microglial cells. While both Th1 and Th2 cells induced CD40 expression, only Th1 cells induced intercellular adhesion molecule-1 (ICAM-1) expression on microglia. Moreover, Th2 cells prevented or even reversed Th1-induced ICAM-1 upregulation. Evidently, Th2 cells could diminish Th1-induced inflammatory reactions and actively support the resting state of microglia, which could be one mechanism of Th2-mediated remission of neuroinflammation during EAE.

Early expression of glutamate transporter proteins in ramified microglia after controlled cortical impact injury in the rat

Traumatic brain injury is followed by increased extracellular glutamate concentration. Uptake of glutamate is mainly mediated by the glial glutamate transporters GLAST and GLT-1. Extent and distribution of GLAST and GLT-1 were studied in a rat model of controlled cortical impact injury (CCII). Western Blot analysis revealed lowest levels of GLAST and GLT-1 with a decrease by 40%-54% and 42%-49% between 24 and 72 h posttrauma. By 8 h after CCII, CSF glutamate levels were increased (10.5 microM vs. 2.56 microM in controls; P < 0.001), reaching maximum values by 48 h. A significant increase in de novo GLAST and GLT-1 expressing ramified microglia was observed within 4 h, reached a stable level by 48 h, and remained high up to 72 h after CCII. Furthermore, ramified microglia de novo expressed the neuronal glutamate transporter EAAC1 after CCII. Following CCII, GLAST/GLT-1 and GFAP coexpressing astrocytes were immediately reduced, reaching minimum levels within 8 h. This reduction of expression could be either due to protein downregulation or loss of astrocytes. At 72 h, a marked population of GLAST- and GLT-1-positive reactive astrocytes appeared. These results support the hypothesis that reduced astrocytic GLAST and GLT-1 protein levels following CCII contribute to evolving secondary injury. Microglia are capable of de novo expressing glutamate transporter proteins, indicating that the expression of glial and neuronal glutamate transporters is not restricted to a specific glial or neuronal lineage. Ramified microglia may play an important compensatory role in the early regulation of extracellular glutamate after CCII.

Turnover of rat brain perivascular cells

Brain perivascular spaces harbor a population of cells which exhibit high phagocytic capacity. Therefore, these cells can be labeled by intraventricular injection of tracers. Such perivascular cells at the interface between blood and brain are believed to belong to the monocyte/macrophage lineage and to be involved in antigen presentation. Currently, it is unclear whether these cells undergo a continuous turnover by entering and leaving the bloodstream. Using bone-marrow-chimeric animals, migration of donor macrophages into brain perivascular spaces has been reported. On the other hand, following intracerebral injection of india ink into nontransplanted animals, ink-labeled perivascular cells were still found 2 years after injection, suggesting a high stability of this cell pool. Thus, the turnover of perivascular cells observed in chimeras might be a result of bone marrow transplantation rather than a physiological occurrence. To address this issue, we monitored de novo invasion of macrophages into perivascular spaces of apparently healthy adult rats by applying techniques other than bone marrow transplantation, (i) consecutive injections of different tracers and (ii) ex vivo isolation of macrophages from the blood, cell labeling, and reinjection into the same animal to avoid MHC mismatch. Both approaches revealed vivid de novo invasion of macrophages into perivascular spaces, but not into brain parenchyma, rendering untenable the concept of perivascular cells forming a stable population of macrophages in the brain. Thus, brain perivascular spaces are under permanent immune surveillance of blood borne macrophages in normal adult rats.

Mitochondrial uncoupling protein 2 (UCP2) in the nonhuman primate brain and pituitary

Energy dissipating mechanisms and their regulatory components represent key elements of metabolism and may offer novel targets in the treatment of metabolic disorders, such as obesity and diabetes. Recent studies have shown that a mitochondrial uncoupling protein (UCP2), which uncouples mitochondrial oxidation from phosphorylation, is expressed in the rodent brain by neurons that are known to regulate autonomic, metabolic, and endocrine processes. To help establish the relevance of these rodent data to primate physiology, we now examined UCP2 messenger RNA and peptide expressions in the brain and pituitary gland of nonhuman primates. In situ hybridization histochemistry showed that UCP2 messenger RNA is expressed in the paraventricular, supraoptic, suprachiasmatic, and arcuate nuclei of the primate hypothalamus and also in the anterior lobe of the pituitary gland. Immunocytochemistry revealed abundant UCP2 expression in cell bodies and axonal processes in the aforementioned nuclei as well as in other hypothalamic and brain stem regions and all parts of the pituitary gland. In the hypothalamus, UCP2 was coexpressed with neuropeptide Y, CRH, oxytocin, and vasopressin. In the pituitary, vasopressin and oxytocin-producing axonal processes in the posterior lobe and POMC cells in the intermediate and anterior lobes expressed UCP2. On the other hand, none of the GH-producing cells of the anterior pituitary was found to produce UCP2. The abundance and distribution pattern of UCP2 in the primate brain and pituitary suggest that this protein is evolutionary conserved and may relate to central autonomic, endocrine and metabolic regulation.

Current concepts in neuroanatomical tracing

The development of new axonal tract tracing and cell labelling methods has revolutionised neurobiology in the last 30 years. The aim of this review is to consider some of the key methods of neuroanatomical tracing that are currently in use and have proved invaluable in charting the complex interconnections of the central nervous system. The review begins with a short overview of the most frequently used tracers, including enzymes, peptides, biocytin, latex beads, plant lectins and the ever-increasing number of fluorescent dyes. This is followed by a more detailed consideration of both well established and more recently introduced neuroanatomical tracing methods. Technical aspects of the application, uptake mechanisms, intracellular transport of tracers, and the problems of subsequent signal detection, are also discussed. The methods that are presented and discussed in detail include: (1) anterograde and retrograde neuroanatomical labelling with fluorescent dyes in vivo, (2) labelling of post mortem tissue, (3) developmental studies, (4) transcellular tracing (phagocytosis-dependent staining of glial cells), (5) electrophysiological mapping combined with neuronal tract tracing, and (6) simultaneous detection of more than one axonal tracer. (7) Versatile protocols for three-colour labelling have been developed to study complex patterns of connections. It is envisaged that this review will be used to guide the readers in their selection of the most appropriate techniques to apply to their own particular area of interest.

Reactive astrocytes upregulate Fas (CD95) and Fas ligand (CD95L) expression but do not undergo programmed cell death during the course of anterograde degeneration

Tissue homeostasis is determined by a balance between proliferation and apoptosis. Various lesions in the brain are accompanied by proliferation and subsequent death of glial cells, but the mechanisms that limit this expansion of glial populations remains unknown. One possible candidate is the death ligand, FasL, and its receptor Fas, because the expression of both proteins was reported on glial cells. To elucidate the expression and putative function of Fas and FasL on proliferative glial cells, we performed stereotactic lesion of the entorhinal cortex of adult rats. Such lesions induce proliferation of astrocytes and microglial cells in the hippocampal fields of anterograde degeneration. Subsequently, the total number of both cell types returns to pre-lesion counts. We found that Fas and FasL is strongly upregulated on astrocytes in the zone of anterograde degeneration with a peak 5 days postlesion (dpl) and a return to control levels at 10 dpl. However, evidence for astrocytic cell death was neither detected by TUNEL staining, immunocytochemistry for c-Jun, and apoptosis-specific protein (ASP), nor by staining for morphologic hallmarks of apoptotic or necrotic cell death at the light and electron microscopic level. Thus, increased expression of Fas and FasL is not accompanied by cell death of reactive astrocytes during anterograde degeneration.

Human brain-cell death induced by tumour-necrosis-factor-related apoptosis-inducing ligand (TRAIL)

Cell death induced by tumour-necrosis-factor-related apoptosis-inducing ligand (TRAIL) was believed to occur exclusively in tumour cells, suggesting that this drug is safe to use as an antitumour therapy. Concerns were raised, however, when cultured normal human hepatocytes were shown to be susceptible to TRAIL. Here we report that TRAIL induces apoptosis in the human brain. Our finding therefore argues against the use of TRAIL for therapy of human brain tumours. However, neuroinflammatory T cells that express TRAIL might induce apoptosis of brain tissue, indicating a potential target for treatment of multiple sclerosis.

Involvement of non-neuronal cells in entorhinal-hippocampal reorganization following lesions

Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main hippocampal termination zones. Subsequently, remaining fibers sprout and form new synapses on the denervated dendrites. This degeneration and reorganization is accompanied by sequential changes in glial morphology and function. Within a few hours following the lesion, amoeboid microglia migrate into the zone of denervation. Some hours later, signs of activation can be seen on astrocytes in the zone of denervation, where both cell types proliferate and remain in an activated state for more than two weeks. These activated glial cells might be involved in lesion-induced plasticity in at least two ways: (1) by releasing cytokines and growth factors which regulate layer-specific sprouting and (2) by phagocytosis of axonal debris, because myelin sheaths act as obstacles for sprouting fibers in the central nervous system. Whereas direct evidence for the former is still missing, the latter was investigated using phagocytosis-dependent labeling techniques. Both microglial cells and astrocytes incorporate axonal debris. Phagocytosing microglial cells develop the immune phenotype of antigen-presenting cells, whereas astrocytes strongly express FasL (CD95L), which induces apoptosis of activated lymphocytes. Thus, the interaction of glial cells with immune cells might be another, previously underestimated, aspect of reorganization following entorhinal lesion.

Estrogen and microglia: A regulatory system that affects the brain

Sex hormones are involved in the physiological regulation of several aspects of behavior and neuroendocrine events. It has been accepted that such effects are mediated directly by steroid actions on neurons; however, new studies have shown that the glial cells are also affected by gonadal steroids. The microglia are one specialized brain glial cell type, which is a target for estrogen actions. In fact, we believe that many of the immune and nonimmune regulatory functions of microglia in the brain are influenced directly by estrogen via expression and secretion of cytokines, and growth factors by the microglia. The present review details only a section of the known aspects of microglial function, focusing mainly on nonimmune regulatory actions in the brain and their functional relationship with sex hormones. Moreover, we present evidence for the presence of estrogen receptor-beta (ERbeta) in rat microglial cells.

Estrogen and microglia: A regulatory system that affects the brain

Sex hormones are involved in the physiological regulation of several aspects of behavior and neuroendocrine events. It has been accepted that such effects are mediated directly by steroid actions on neurons; however, new studies have shown that the glial cells are also affected by gonadal steroids. The microglia are one specialized brain glial cell type, which is a target for estrogen actions. In fact, we believe that many of the immune and nonimmune regulatory functions of microglia in the brain are influenced directly by estrogen via expression and secretion of cytokines, and growth factors by the microglia. The present review details only a section of the known aspects of microglial function, focusing mainly on nonimmune regulatory actions in the brain and their functional relationship with sex hormones. Moreover, we present evidence for the presence of estrogen receptor-beta (ERbeta) in rat microglial cells.

FasL (CD95L, Apo1L) is expressed in the normal rat and human brain: evidence for the existence of an immunological brain barrier

Despite the mechanical blood-brain barrier, activated T-cells can cross brain vessels. Thus, the CNS is routinely surveyed by immune competent cells; yet the healthy brain is not a target of antigen-specific immune reactions. Therefore, mechanisms must exist to prevent brain-antigen-specific T-cells from inducing immune responses. Data indicate that activated T-cells entering the CNS may undergo apoptosis rather than leaving the brain to induce immune responses. Applying RT-PCR, Western-blots, and immunocytochemistry, we have demonstrated expression of the apoptosis-inducing protein Fas ligand on astrocytes and neurons of apparently normal rat and human brains. FasL-positive astrocytes were often situated in close vicinity to cerebral blood vessels in vivo and induced apoptosis of Fas expressing Jurkat cells in vitro. We propose that similar to other immune privileged organs FasL-induced apoptosis of activated T-cells in the brain protects the tissue from self damaging immune attacks by forming an immunological brain barrier.

Tracing of the entorhinal-hippocampal pathway in vitro

In vitro tract tracing allowing for continuous observation of the perforant path is a crucial prerequisite for experimental studies on the entorhinal-hippocampal interaction in an organotypic slice culture containing the entorhinal cortex, the perforant path, and the dentate gyrus (OEHSC). We prepared horizontal slices of the temporal entorhinal-hippocampal region of the rat on a vibratome, and the perforant path axons were traced by application of the fluorescent tracer Mini Ruby on the entorhinal cortex. After 2 days in vitro (div), the perforant path became visible in most cultures. Entorhinal neurons and single perforant fibers could be followed to the outer molecular layers of the dentate gyrus by in vitro fluorescence microscopy and it was possible to monitor the perforant path directly over a period of 25 div. Moreover, ultrastructural analysis proved the existence of traced perforant path boutons forming synapses with spines and dendritic shafts in the outer molecular layers of the dentate gyrus. Transsection of the prelabelled perforant path in vitro resulted in anterograde degeneration and subsequent phagocytosis of axonal material by activated microglial cells in the zone of denervation. In conclusion, in vitro tracing demonstrates the maintenance of the entorhinal-hippocampal pathway in OEHSCs and permits monitoring of dynamic changes in the prelabeled perforant path after various lesion paradigms, e.g., transsection or neurotoxin treatment. This approach permits further studies on the efficacy of neuroprotectants, cytokines, and growth factors in the treatment of lesion-induced neuronal degeneration.

Expression of synaptophysin in sprouting neurons after entorhinal lesion in the rat

Expression of the synaptic vesicle protein synaptophysin was studied in lesion-induced sprouting neurons of the contralateral entorhinal cortex and in the contralateral dentate gyrus using immunocytochemistry at the light- and electron-microscopic level. Perikaryal immunoreactivity for synaptophysin was found between 8 and 10 days postlesion. Light microscopy revealed that synaptophysin immunostaining was present in almost all neurons of layers II and III of the contralateral medial entorhinal cortex. These neurons give rise to the sprouting, crossed temporodentate pathway. In addition, some hilar neurons of the contralateral dentate gyrus, which are the parent cells of sprouting commissural fibers, were immunostained for synaptophysin. Transient immunostaining for synaptophysin was observed within cell bodies and dendrites. Additionally, the cell bodies were outlined by immunoreactive puncta, identified by electron microscopy as nerve terminals. Our results revealed that sprouting neurons express the major synaptic vesicle protein synaptophysin during reactive synaptogenesis in a pattern that reflects biosynthesis and sorting of this protein as seen in developing neurons during synapse formation.

Astrocytes and microglial cells incorporate degenerating fibers following entorhinal lesion: a light, confocal, and electron microscopical study using a phagocytosis-dependent labeling technique

Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main termination zone in the outer molecular layers of the dentate gyrus. Concomitantly, astrocytes become hypertrophic, and microglial cells alter their phenotype, suggesting participation in anterograde degeneration. This study analyzes the involvement of these lesion-induced activated glial cells in the process of phagocytosis of degenerated axonal debris. We established a phagocytosis-dependent labeling technique that allows for direct and simultaneous visualization of both labeled incorporated axonal debris and incorporating glial cells. Stereotaxic application of small crystals of the biotin- and rhodamine-conjugated dextran amine Mini Ruby (MR) into the entorhinal cortex led to strong and stable axonal staining of perforant path axons. Following entorhinal lesion, labeled terminals and fibers condensed and formed small granules. Incorporation of these rhodamine-fluorescent granules resulted in a phagocytosis-dependent cell labeling. During the first 3 days, we were able to identify these cells as microglia by using double-fluorescence and confocal microscopy. The first unequivocally double-labeled astrocytes were found 6 days post lesion (dpl). Whereas in all stages a subpopulation of microglial cells remained devoid of MR-labeled granules, all astrocytes in the middle molecular layer were double-labeled after long survival times (20 dpl). On the ultrastructural level, labeled granules appeared to be perforant path axons containing the tracer. Both terminals and myelinated fibers could be seen inside the cytoplasm of microglial cells and astrocytes. Thus, anterograde degeneration is a sufficient stimulus to induce axon incorporation by both astrocytes and a subpopulation of microglial cells.

Heterogeneity in the neuropeptide Y-containing neurons of the rat arcuate nucleus: GABAergic and non-GABAergic subpopulations

Neuropeptide Y, produced in the arcuate nucleus of the hypothalamus, plays a key role in the central regulation of anterior pituitary and appetitive functions. The pleiotropic nature of neuropeptide Y in these mechanisms indicates the existence of heterogeneity in the hypothalamic neuronal population producing neuropeptide Y. In this study, we report the coexistence of neuropeptide Y and the amino acid transmitter, gamma-aminobutyric acid (GABA), in neuronal perikarya of the arcuate nucleus. Fluorescent double immunolabeling for neuropeptide Y and glutamic acid decarboxylase was carried out on vibratome sections collected through the hypothalamic arcuate nuclei of animals that were pretreated with colchicine. It was found that about one third of the neuropeptide Y-producing arcuate nucleus perikarya co-expressed glutamic acid decarboxylase. This population of neuropeptide Y-containing GABAergic neurons were distributed longitudinally within the arcuate nucleus located predominantly in its dorsomedial aspects. These results show that there are at least two distinct populations of neuropeptide Y-producing neurons in the arcuate nucleus: a subset of neuropeptide Y and GABA-co-producing neurons located in the dorsomedial arcuate nucleus and a subset of non-GABAergic neuropeptide Y cells located in the ventral arcuate nucleus. This heterogeneity in the neuropeptide Y-producing perikarya of the hypothalamus may help explain adverse neuroendocrine and behavioral effects of arcuate nucleus neuropeptide Y.

Identification of phagocytic glial cells after lesion-induced anterograde degeneration using double-fluorescence labeling: combination of axonal tracing and lectin or immunostaining

Retrograde and anterograde degeneration have been reported to be sufficient stimuli to activate glial cells, which, in turn, are involved in phagocytosis of degenerating material. Here we describe a double-fluorescence technique which allows for direct and simultaneous visualization of both labeled incorporated axonal debris and incorporating glial cells in the course of anterograde degeneration. Stereotaxic application of small crystals of biotinylated and tetramethylrhodamine (TRITC)-conjugated dextran amine Mini Ruby into the medial entorhinal cortex resulted in a stable rhodamine fluorescence confined to fibers and terminals in the middle molecular layer of the dentate gyrus, the stratum lacunosum-moleculare, and the crossed temporo-hippocampal pathway. Subsequent stereotaxic lesion of the entorhinal cortex induced transformation of rhodamine-fluorescent fibers and terminals into small granules. Incorporation of these granules by microglial cells [labeled by fluorescein isothiocyanate (FITC)-coupled Bandeiraea simplicifolia isolectin B4] or astrocytes (labeled by FITC-coupled glial fibrillary acidic protein antibodies) resulted in phagocytosis-dependent labeling of these non-neuronal cells, which could be identified by double-fluorescence microscopy. Electron microscopical analysis revealed that, following lesion, the tracer remained confined to entorhinal axons which were found to be incorporated by glial cells. Our data show that TRITC- and biotin-conjugated dextran amines are versatile tracers leading to Phaseolus vulgaris leucoagglutinin-like axonal staining. Lesion-induced phagocytosis of anterogradely degenerating axons by immunocytochemically identified glial cells can be directly observed by this technique on the light and electron microscopical levels.

Adhesion molecule expression on phagocytic microglial cells following anterograde degeneration of perforant path axons

Entorhinal cortex lesion (ECL) leads to anterograde degeneration of perforant path axons and is known to induce a rapid and intense reaction of astrocytes and microglial cells in the deafferented dentate gyrus. Phagocytosis of degenerating axons involves the establishment and maintenance of cell-matrix and cell-cell interactions by activated glial cells. It was thus our aim to investigate whether the process of axon phagocytosis is accompanied by the expression of adhesion molecules on activated microglial cells or reactive astrocytes, as such molecules mediate bot cell-matrix and cell-cell interactions. We found that the integrin adhesion molecules leukocyte function antigen-1 (LFA-1), very late antigen-4 (VLA-4), and the ligand for LFA-1, intercellular adhesion molecule-1 (ICAM-1), were expressed on microglial cells accumulating in the outer molecular layer of the deafferented dentate gyrus. This upregulation of adhesion molecule expression on microglial cells showing morphological criteria of activation occurred rapidly following ECL, reached its peak at 3 days post lesion (dpl), and gradually returned to control levels after 9 dpl. Astrocytes were never labeled by antibodies directed against these adhesion molecules. Prelabeling of the perforant path with a fluorescent tracer and subsequent ECL led to phagocytosis of fluorescent-labeled axonal debris by cells that were located in the outer molecular layer and showed typical microglial morphology. Double-fluorescence labeling demonstrated that microglial cells engaged in the phagocytosis of axonal debris expressed LFA-1, VLA-4, and the LFA-1-ligand ICAM-1. In conclusion, our results demonstrate that anterograde degeneration of perforant path axons results in adhesion molecule expression on activated microglial cells engaged in axon phagocytosis. The expression of such molecules could represent a mechanism that retains activated microglia in areas of axonal degeneration and perhaps enables the interaction of microglial cells with each other or with other immunocompetent cells.