Tumor necrosis factor mediates myelin and oligodendrocyte damage in vitro

Recombinant human tumor necrosis factor (rhTNF) has been tested for its effect on myelinated cultures of mouse spinal cord tissue. As controls, recombinant human interferon gamma (rhIFN) and interleukin-2 (rhIL-2) were tested, as well as T-cell supernatants, antigalactocerebroside serum, and normal culture medium. It was found that rhTNF induced delayed-onset (18-24 hr) oligodendrocyte necrosis and a type of myelin dilatation peculiar to this system. Some nerve fibers progressed to demyelination by 72 hours. The myelin dilatation was not reversible by return to normal feeding solution for 3 days. In contrast, rhIFN, rhIL-2, T-cell supernatants, and normal medium had little or no effect on cultures. This mechanism differs from other immune-mediated mechanisms in that it appears that a physiological (not structural) demyelination occurs initially without overt destruction of the myelin sheath. These observations are relevant to the evolution of the multiple sclerosis plaque: dysfunction of ionic channels might contribute to the eventual demise of oligodendrocytes and axons in the longstanding lesion.

Glutamatergic synapses on oligodendrocyte precursor cells in the hippocampus

Fast excitatory neurotransmission in the central nervous system occurs at specialized synaptic junctions between neurons, where a high concentration of glutamate directly activates receptor channels. Low-affinity AMPA (alpha-amino-3-hydroxy-5-methyl isoxazole propionic acid) and kainate glutamate receptors are also expressed by some glial cells, including oligodendrocyte precursor cells (OPCs). However, the conditions that result in activation of glutamate receptors on these non-neuronal cells are not known. Here we report that stimulation of excitatory axons in the hippocampus elicits inward currents in OPCs that are mediated by AMPA receptors. The quantal nature of these responses and their rapid kinetics indicate that they are produced by the exocytosis of vesicles filled with glutamate directly opposite these receptors. Some of these AMPA receptors are permeable to calcium ions, providing a link between axonal activity and internal calcium levels in OPCs. Electron microscopic analysis revealed that vesicle-filled axon terminals make synaptic junctions with the processes of OPCs in both the young and adult hippocampus. These results demonstrate the existence of a rapid signalling pathway from pyramidal neurons to OPCs in the mammalian hippocampus that is mediated by excitatory, glutamatergic synapses.

Embryonic stem cell-derived glial precursors: a source of myelinating transplants

Self-renewing, totipotent embryonic stem (ES) cells may provide a virtually unlimited donor source for transplantation. A protocol that permits the in vitro generation of precursors for oligodendrocytes and astrocytes from ES cells was devised. Transplantation in a rat model of a human myelin disease shows that these ES cell-derived precursors interact with host neurons and efficiently myelinate axons in brain and spinal cord. Thus, ES cells can serve as a valuable source of cell type-specific somatic precursors for neural transplantation.

NG2+ CNS glial progenitors remain committed to the oligodendrocyte lineage in postnatal life and following neurodegeneration

The mammalian CNS contains a ubiquitous population of glial progenitors known as NG2+ cells that have the ability to develop into oligodendrocytes and undergo dramatic changes in response to injury and demyelination. Although it has been reported that NG2+ cells are multipotent, their fate in health and disease remains controversial. Here, we generated PDGFαR-CreER transgenic mice and followed their fate in vivo in the developing and adult CNS. These studies revealed that NG2+ cells in the postnatal CNS generate myelinating oligodendrocytes, but not astrocytes or neurons. In regions of neurodegeneration in the spinal cord of ALS mice, NG2+ cells exhibited enhanced proliferation and accelerated differentiation into oligodendrocytes but remained committed to the oligodendrocyte lineage. These results indicate that NG2+ cells in the normal CNS are oligodendrocyte precursors with restricted lineage potential and that cell loss and gliosis are not sufficient to alter the lineage potential of these progenitors.

Myelin impairs CNS remyelination by inhibiting oligodendrocyte precursor cell differentiation

Demyelination in the adult CNS can be followed by extensive repair. However, in multiple sclerosis, the differentiation of oligodendrocyte lineage cells present in demyelinated lesions is often inhibited by unknown factors. In this study, we test whether myelin debris, a feature of demyelinated lesions and an in vitro inhibitor of oligodendrocyte precursor differentiation, affects remyelination efficiency. Focal demyelinating lesions were created in the adult rat brainstem, and the naturally generated myelin debris was augmented by the addition of purified myelin. After quantification of myelin basic protein mRNA expression from lesion material obtained by laser capture microdissection and supported by histological data, we found a significant impairment of remyelination, attributable to an arrest of the differentiation and not the recruitment of oligodendrocyte precursor cells. These data identify myelin as an inhibitor of remyelination as well as its well documented inhibition of axon regeneration.

High cholesterol level is essential for myelin membrane growth

Cholesterol in the mammalian brain is a risk factor for certain neurodegenerative diseases, raising the question of its normal function. In the mature brain, the highest cholesterol content is found in myelin. We therefore created mice that lack the ability to synthesize cholesterol in myelin-forming oligodendrocytes. Mutant oligodendrocytes survived, but CNS myelination was severely perturbed, and mutant mice showed ataxia and tremor. CNS myelination continued at a reduced rate for many months, and during this period, the cholesterol-deficient oligodendrocytes actively enriched cholesterol and assembled myelin with >70% of the cholesterol content of wild-type myelin. This shows that cholesterol is an indispensable component of myelin membranes and that cholesterol availability in oligodendrocytes is a rate-limiting factor for brain maturation.

LINGO-1 negatively regulates myelination by oligodendrocytes

The control of myelination by oligodendrocytes in the CNS is poorly understood. Here we show that LINGO-1 is an important negative regulator of this critical process. LINGO-1 is expressed in oligodendrocytes. Attenuation of its function by dominant-negative LINGO-1, LINGO-1 RNA-mediated interference (RNAi) or soluble human LINGO-1 (LINGO-1-Fc) leads to differentiation and increased myelination competence. Attenuation of LINGO-1 results in downregulation of RhoA activity, which has been implicated in oligodendrocyte differentiation. Conversely, overexpression of LINGO-1 leads to activation of RhoA and inhibition of oligodendrocyte differentiation and myelination. Treatment of oligodendrocyte and neuron cocultures with LINGO-1-Fc resulted in highly developed myelinated axons that have internodes and well-defined nodes of Ranvier. The contribution of LINGO-1 to myelination was verified in vivo through the analysis of LINGO-1 knockout mice. The ability to recapitulate CNS myelination in vitro using LINGO-1 antagonists and the in vivo effects seen in the LINGO-1 knockout indicate that LINGO-1 signaling may be critical for CNS myelination.

The effects of normal aging on myelin and nerve fibers: a review

It was believed that the cause of the cognitive decline exhibited by human and non-human primates during normal aging was a loss of cortical neurons. It is now known that significant numbers of cortical neurons are not lost and other bases for the cognitive decline have been sought. One contributing factor may be changes in nerve fibers. With age some myelin sheaths exhibit degenerative changes, such as the formation of splits containing electron dense cytoplasm, and the formation on myelin balloons. It is suggested that such degenerative changes lead to cognitive decline because they cause changes in conduction velocity, resulting in a disruption of the normal timing in neuronal circuits. Yet as degeneration occurs, other changes, such as the formation of redundant myelin and increasing thickness suggest of sheaths, suggest some myelin formation is continuing during aging. Another indication of this is that oligodendrocytes increase in number with age. In addition to the myelin changes, stereological studies have shown a loss of nerve fibers from the white matter of the cerebral hemispheres of humans, while other studies have shown a loss of nerve fibers from the optic nerves and anterior commissure in monkeys. It is likely that such nerve fiber loss also contributes to cognitive decline, because of the consequent decrease in connections between neurons. Degeneration of myelin itself does not seem to result in microglial cells undertaking phagocytosis. These cells are probably only activated when large numbers of nerve fibers are lost, as can occur in the optic nerve.

Electron microscopy of oligodendroglia in severe mental illness

Qualitative electron microscopy was performed to verify whether brain pathology in schizophrenia and bipolar disorder is associated with alterations of oligodendroglial cells and myelinated fibers. Ultrastructural signs of apoptosis and necrosis of oligodendroglial cells were found in the prefrontal area 10 and the caudate nucleus in both schizophrenia and bipolar disorder. Damage of myelin sheath lamellae, with the formation of concentric lamellar bodies, were detected in both brain structures in schizophrenia. There was also a significant decrease in the area of the nucleus and the volume density of mitochondria in oligodendrogliocytes in the caudate nucleus and in the prefrontal cortex in schizophrenia, as compared to normal controls. Volume density of heterochromatin was significantly increased (+14%) in the caudate nucleus in schizophrenia. The density of concentric lamellar bodies (as an indicator of damage of myelinated fibers) was dramatically increased (4.5-fold) in the caudate nucleus in schizophrenia, as compared to controls, and was positively correlated with volume density of heterochromatin. Multiple regression analysis and analysis of covariance demonstrated that these changes could not be explained by the effects of postmortem delay, age, neuroleptic medication, or gender. Pathology of oligodendroglia might be an essential feature of severe mental disorders.

Synaptic signaling between GABAergic interneurons and oligodendrocyte precursor cells in the hippocampus

Oligodendrocyte precursor cells (OPCs) express receptors for many neurotransmitters, but the mechanisms responsible for their activation are poorly understood. We have found that quantal release of GABA from interneurons elicits GABA(A) receptor currents with rapid rise times in hippocampal OPCs. These currents did not exhibit properties of spillover transmission or release by transporters, and immunofluorescence and electron microscopy suggest that interneuronal terminals are in direct contact with OPCs, indicating that these GABA currents are generated at direct interneuron-OPC synapses. The reversal potential of OPC GABA(A) currents was -43 mV, and interneuronal firing was correlated with transient depolarizations induced by GABA(A) receptors; however, GABA application induced a transient inhibition of currents mediated by AMPA receptors in OPCs. These results indicate that OPCs are a direct target of interneuronal collaterals and that the GABA-induced Cl(-) flux generated by these events may influence oligodendrocyte development by regulating the efficacy of glutamatergic signaling in OPCs.

P2X(7) receptor blockade prevents ATP excitotoxicity in oligodendrocytes and ameliorates experimental autoimmune encephalomyelitis

Oligodendrocyte death and demyelination are hallmarks of multiple sclerosis (MS). Here we show that ATP signaling can trigger oligodendrocyte excitotoxicity via activation of calcium-permeable P2X(7) purinergic receptors expressed by these cells. Sustained activation of P2X(7) receptors in vivo causes lesions that are reminiscent of the major features of MS plaques, i.e., demyelination, oligodendrocyte death, and axonal damage. In addition, treatment with P2X(7) antagonists of chronic experimental autoimmune encephalomyelitis (EAE), a model of MS, reduces demyelination and ameliorates the associated neurological symptoms. Together, these results indicate that ATP can kill oligodendrocytes via P2X(7) activation and that this cell death process contributes to EAE. Importantly, P2X(7) expression is elevated in normal-appearing axon tracts in MS patients, suggesting that signaling through this receptor in oligodendrocytes may be enhanced in this disease. Thus, P2X(7) receptor antagonists may be beneficial for the treatment of MS.

Remyelination in multiple sclerosis

Chronic plaques in central nervous system tissue fixed by in situ perfusion for electron microscopy were examined for evidence of remyelination in 2 patients with multiple sclerosis (MS). Fibers with abnormal central myelin sheaths of several types were found at the margins of most of the plaques studied. The most common of these were: (1) the presence of bare stretches of axon between contiguous internodes, (2) the presence of thin paranodes, (3) internodes which changed markedly in thickness along their length due to premature termination of superficial or deep myelin lamellae that ended as hypertrophic lateral loops, and (4) abnormally thin internodes which were of uniform thickness along their length, which were shorter than normal, and which terminated in the form of normal nodal complexes. The finding of internodes of the last type at the edges of many plaques indicates that remyelination by oligodendrocytes can occur in the adult human CNS and that it is common in some cases of MS, although limited in its extent.

Neurofilaments are obligate heteropolymers in vivo

Neurofilaments (NFs), composed of three distinct subunits NF-L, NF-M, and NF-H, are neuron-specific intermediate filaments present in most mature neurons. Using DNA transfection and mice expressing NF transgenes, we find that despite the ability of NF-L alone to assemble into short filaments in vitro NF-L cannot form filament arrays in vivo after expression either in cultured cells or in transgenic oligodendrocytes that otherwise do not contain a cytoplasmic intermediate filament (IF) array. Instead, NF-L aggregates into punctate or sheet like structures. Similar nonfilamentous structures are also formed when NF-M or NF-H is expressed alone. The competence of NF-L to assemble into filaments is fully restored by coexpression of NF-M or NF-H to a level approximately 10% of that of NF-L. Deletion of the head or tail domain of NF-M or substitution of the NF-H tail onto an NF-L subunit reveals that restoration of in vivo NF-L assembly competence requires an interaction provided by the NF-M or NF-H head domains. We conclude that, contrary to the expectation drawn from earlier in vitro assembly studies, NF-L is not sufficient to assemble an extended filament network in an in vivo context and that neurofilaments are obligate heteropolymers requiring NF-L and NF-M or NF-H.

Constitutively active Akt induces enhanced myelination in the CNS

The serine/threonine kinase Akt regulates multiple cellular functions. The current studies identify a new role for Akt in CNS myelination. In earlier studies on cultured oligodendrocytes, we showed that neuregulin signals through phosphatidylinositol-3'-OH kinase and Akt to enhance survival of oligodendrocytes. However, when transgenic animals were generated that overexpressed constitutively active Akt in oligodendrocytes and their progenitor cells, no enhanced survival of oligodendrocytes or progenitors was found. No alteration in the proliferation or death of progenitors was noted. In contrast, the major impact of Akt overexpression in oligodendrocytes was enhanced myelination. Most interestingly, oligodendrocytes in these mice continued actively myelinating throughout life. Thus, expression of constitutively active Akt in oligodendrocytes and their progenitor cells generated no more oligodendrocytes, but dramatically more myelin. The increased myelination continued as these mice aged, resulting in enlarged optic nerves and white matter areas. In older animals with enlarged white matter areas, the density of oligodendrocytes was reduced, but because of the increased area, the total number of oligodendrocytes remained comparable with wild-type controls. Interestingly, in these animals, overexpression of Akt in Schwann cells did not impact myelination. Thus, in vivo, constitutively active Akt enhances CNS myelination but not PNS myelination and has no impact developmentally on oligodendrocyte number. Understanding the unique aspects of Akt signal transduction in oligodendrocytes that lead to myelination rather than uncontrolled proliferation of oligodendrocyte progenitor cells may have important implications for understanding remyelination in the adult nervous system.

Macrophage depletion impairs oligodendrocyte remyelination following lysolecithin-induced demyelination

An association between macrophages and remyelination efficiency has been observed in a variety of different models of CNS demyelination. In order to test whether this association is causal or coincidental, we have examined the effects of macrophage depletion on the rate of remyelination of lysolecithin-induced demyelination in the spinal cord of young adult female rats. Macrophage depletion was achieved by reducing the monocyte contribution to the macrophages within the lesion using the clodronate-liposome technique. This technique not only resulted in a decrease in Ox-42-positive cells in the spleen of treated animals but also in the levels of macrophage scavenger receptor type B mRNA expression within the demyelinating lesion. In animals treated with clodronate-liposomes throughout the remyelination process, there was a significant decrease in the extent of oligodendrocyte remyelination at 3 weeks after lesion induction, but no effect on Schwann cell remyelination. If macrophage depletion was delayed until the second half of the remyelination phase, then there was no effect on the repair outcome, implying that macrophages are required for the early stages of CNS remyelination. The results of this study indicate that the macrophage response is an important component of successful CNS remyelination and that approaches to the treatment of demyelinating disease based on inhibition of the inflammatory response may also impair regenerative events that follow demyelination.

Functional implications for the microtubule-associated protein tau: localization in oligodendrocytes

We present evidence that the microtubule-associated protein tau is present in oligodendrocytes (OLGs), the central nervous system cells that make myelin. By showing that tau is distributed in a pattern similar to that of myelin basic protein, our results suggest a possible involvement of tau in some aspect of myelination. Tau protein has been identified in OLGs in situ and in vitro. In interfascicular OLGs, tau localization, revealed by monoclonal antibody Tau-5, was confined to the cell somata. However, in cultured ovine OLGs with an exuberant network of processes, tau was detected in cell somata, cellular processes, and membrane expansions at the tips of these processes. Moreover, in such cultures, tau appeared localized adjacent to or coincident with myelin basic protein in membrane expansions along and at the ends of the cellular processes. The presence of tau mRNA was documented using fluorescence in situ hybridization. The distribution of the tau mRNA was similar to that of the tau protein. Western blot analysis of cultured OLGs showed the presence of many tau isoforms. Together, these results demonstrate that tau is a genuine oligodendrocyte protein and pave the way for determining its functional role in these cells.

A functional role for EGFR signaling in myelination and remyelination

Cellular strategies for oligodendrocyte regeneration and remyelination involve characterizing endogenous neural progenitors that are capable of generating oligodendrocytes during normal development and after demyelination, and identifying the molecular signals that enhance oligodendrogenesis from these progenitors. Using both gain- and loss-of-function approaches, we explored the role of epidermal growth factor receptor (EGFR) signaling in adult myelin repair and in oligodendrogenesis. We show that 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) promoter-driven overexpression of human EGFR (hEGFR) accelerated remyelination and functional recovery following focal demyelination of mouse corpus callosum. Lesion repopulation by Cspg4+ (also known as NG2) Ascl1+ (also known as Mash1) Olig2+ progenitors and functional remyelination were accelerated in CNP-hEGFR mice compared with wild-type mice. EGFR overexpression in subventricular zone (SVZ) and corpus callosum during early postnatal development also expanded this NG2+Mash1+Olig2+ progenitor population and promoted SVZ-to-lesion migration, enhancing oligodendrocyte generation and axonal myelination. Analysis of hypomorphic EGFR-mutant mice confirmed that EGFR signaling regulates oligodendrogenesis and remyelination by NG2+Mash1+Olig2+ progenitors. EGFR targeting holds promise for enhancing oligodendrocyte regeneration and myelin repair.

Cortical neurons immunoreactive for the potassium channel Kv3.1b subunit are predominantly surrounded by perineuronal nets presumed as a buffering system for cations

Perineuronal nets (PNs) are known as chondroitin sulphate-rich, lattice-like coatings of the extracellular matrix. In the cortex of mammalian species investigated so far, they were mainly found around GABAergic neurons, but to a lesser degree also around pyramidal cells. Previous investigations in the rat revealed similar distribution patterns of fast-firing neurons expressing both the Kv3.1b subunit of voltage-gated potassium channels and the calcium-binding protein parvalbumin. In the present study, triple fluorescence labelling was applied for the simultaneous demonstration of PNs with the N-acetylgalactosamine-specific Wisteria floribunda agglutinin (WFA), parvalbumin-immunoreactivity (ir) with a monoclonal antibody and of Kv3.1b-ir with several rabbit antibodies. Subsets of non-pyramidal neurons - enwrapped by PNs and expressing parvalbumin and Kv3.1b - were detected in the rat and monkey neocortex and hippocampus. In the rat, faintly stained PNs were additionally found around several layer II/III and V pyramidal cells immunonegative for Kv3.1b, but contacted by Kv3.1b-containing boutons. In the monkey, more intensely labelled PNs frequently occurred around pyramidal cells which themselves appeared to be Kv3. 1b-immunopositive. We also observed minor Kv3.1b-ir and parvalbumin-ir cortical cell populations which were devoid of PNs; occasionally, nets were detected around neurons lacking both immunoreactivities. By confocal laser scanning microscopy, Kv3.1b-ir and WFA-binding sites were found adjoining at the soma and proximal dendritic surface, while lectin-binding sites usually extended on more distal dendritic segments and the axon initial segments which failed to express detectable Kv3.1b-ir. This spatial relationship of both markers was also confirmed by combined WFA-gold labelling and Kv3.1b-immunoperoxidase staining at the electron microscopic level. The data are used for a critical examination of current hypotheses concerning the functional role of PNs. We conclude that PNs may serve as rapid local buffers of excess cation changes in the extracellular space. Somatic membranes of fast-spiking neurons seem to be a main, but not the only source of such changes.

Edg8/S1P5: an oligodendroglial receptor with dual function on process retraction and cell survival

Endothelial differentiation gene (Edg) proteins are G-protein-coupled receptors activated by lysophospholipid mediators: sphingosine-1-phosphate (S1P) or lysophosphatidic acid. We show that in the CNS, expression of Edg8/S1P5, a high-affinity S1P receptor, is restricted to oligodendrocytes and expressed throughout development from the immature stages to the mature myelin-forming cell. S1P activation of Edg8/S1P5 on O4-positive pre-oligodendrocytes induced process retraction via a Rho kinase/collapsin response-mediated protein signaling pathway, whereas no retraction was elicited by S1P on these cells derived from Edg8/S1P5-deficient mice. Edg8/S1P5-mediated process retraction was restricted to immature cells and was no longer observed at later developmental stages. In contrast, S1P activation promoted the survival of mature oligodendrocytes but not of pre-oligodendrocytes. The S1P-induced survival of mature oligodendrocytes was mediated through a pertussis toxin-sensitive, Akt-dependent pathway. Our data demonstrate that Edg8/S1P5 activation on oligodendroglial cells modulates two distinct functional pathways mediating either process retraction or cell survival and that these effects depend on the developmental stage of the cell.

Connexin30 in rodent, cat and human brain: selective expression in gray matter astrocytes, co-localization with connexin43 at gap junctions and late developmental appearance

We previously presented evidence [Nagy et al. (1997) Neuroscience 78, 533-548] that, in addition to their ubiquitous expression of connexin43, astrocytes produce a second connexin suggested to be connexin30, a recently discovered member of the family of gap junction proteins. A connexin30 specific antibody was subsequently developed and utilized here to confirm and extend our earlier observations. On western blots, this antibody detected a 30,000 mol. wt protein in rat, mouse, cat and human brain, and exhibited no cross-reaction with connexin43, connexin26 or any other known connexins expressed in brain. Immunohistochemically, connexin30 was localized in astrocytes, at gap junctions between these cells and on the astrocyte side of gap junctions between astrocytes and oligodendrocytes. Double labelling revealed co-localization of connexin30 and connexin43 at astrocytic gap junctions. Punctate immunolabelling patterns for both connexins were qualitatively similar, but differences were evident. In contrast to regional connexin43 expression, diencephalic and hindbrain areas exhibited considerably greater expression than forebrain areas, subcortical perivascular astrocytic endfeet were more heavily labelled for connexin30, white matter tracts such as corpus callosum, internal capsule and anterior commissure were devoid of connexin30, and appreciable levels of connexin30 during development were not seen until about postnatal day 15. These results indicate that connexin30 is expressed by gray, but not white matter astrocytes, its distribution is highly heterogeneous in gray matter, it is co-localized with connexin43 at astrocytic gap junctions where it forms homotypic or heterotypic junctions, and its emergence is delayed until relatively late during brain maturation. Taken together, these results suggest that astrocytic connexin30 expression at both regional and cellular levels is subject to regulation in adult brain as well as during brain development.

Electron microscopic autoradiographic studies of gliogenesis in rat optic nerve. I. Cell proliferation

Electron microscopy and 3H-thymidine autoradiographic techniques were used to study the fine structure of proliferating cells in developing rat optic nerve. Before the closure of the optic canal almost all of the cells incorporating radioactive thymidine are ventricular cells, but after closure (16 days of gestation) the vast majority are differentiating astroblasts or oligodendroblasts. Labeled astroblasts show a range in their degree of differentiation; some cells lack 90 A cytoplasmic filaments while others have glial filaments and abundant cytoplasmic organelles. In contrast to astroblasts, all of the labeled oligodendroblasts are in the early stages of differentiation. The proliferation of oligodendroblasts starts at five days postnatal, approximately a day or two before the onset of myelination.During myelinogenesis a few of the labeled oligodendroblasts show presumptive connections to myelin sheaths. Microglial cells do not appear to play a major role in gliogenesis since they form less than 2% of all the labeled cells. The results of this study indicate that astroblasts and oligodendroblasts, rather than undifferentiated glioblasts, are the major source of macroglia. The finding that proliferating glia are in the processof differentiation agrees with recent studies which show that differentiated cells can divide.

Changes in prefrontal axons may disrupt the network in autism

Neural communication is disrupted in autism by unknown mechanisms. Here, we examined whether in autism there are changes in axons, which are the conduit for neural communication. We investigated single axons and their ultrastructure in the white matter of postmortem human brain tissue below the anterior cingulate cortex (ACC), orbitofrontal cortex (OFC), and lateral prefrontal cortex (LPFC), which are associated with attention, social interactions, and emotions, and have been consistently implicated in the pathology of autism. Area-specific changes below ACC (area 32) included a decrease in the largest axons that communicate over long distances. In addition, below ACC there was overexpression of the growth-associated protein 43 kDa accompanied by excessive number of thin axons that link neighboring areas. In OFC (area 11), axons had decreased myelin thickness. Axon features below LPFC (area 46) appeared to be unaffected, but the altered white matter composition below ACC and OFC changed the relationships among all prefrontal areas examined, and could indirectly affect LPFC function. These findings provide a mechanism for disconnection of long-distance pathways, excessive connections between neighboring areas, and inefficiency in pathways for emotions, and may help explain why individuals with autism do not adequately shift attention, engage in repetitive behavior, and avoid social interactions. These changes below specific prefrontal areas appear to be linked through a cascade of developmental events affecting axon growth and guidance, and suggest targeting the associated signaling pathways for therapeutic interventions in autism.

Electron microscopic autoradiographic studies of gliogenesis in rat optic nerve. II. Time of origin

The time of origin for astrocytes and oligodendrocytes in rat optic nerve was studied by 3H-thymidine autoradiographic techniques similar to those used in dating the time of origin for neurons. This study shows that astrocytes are formed throughout late embryonic and all of postnatal development, while oligodendrocytes are generated only during the postnatal period. A few astroglia undergo their final cell division as early as 15.5 days of gestation, but most astrocytes are not generated until the first week of postnatal development. Although the final cell division for more than half of the astrocytes takes place before the end of the first postnatal week, fully mature, fibrous astrocytes are not observed in electron micrographs until after 14 days of age. This time lag implies that the differentiation of these early generated cells takes place gradually over a 2-to 3-week interval. Oligodendroglia begin their final division a day or two before the onset of myelination (6-7 days postnatal), but the vast majority are produced during the period of myelinogenesis. After almost all of the axons have been myelinated, oligodendrocytes are still being generated in small numbers. These late forming cells are generally less differentiated in appearance than those formed earlier; this suggests that the degree of differentiated of oligodendrocytes may be dependent upon the number of axons available for myelination. As with astrocytes, oligodendrocytes show a lag of about two weeks from the time of final cell division until they transform into morphologically differentiated cells. In transverse sections of the optic nerve heavily labeled neuroglia are randomly distributed, indicating there are no temporal-radial gradients for the individual cell types. This observation taken together with the other information obtained from the present and the previous study (Stoff et al., '76) strongly suggest that the factors controlling gliogenesis are different from those governing neuronogenesis.

Global deprivation of brain-derived neurotrophic factor in the CNS reveals an area-specific requirement for dendritic growth

Although brain-derived neurotrophic factor (BDNF) is linked with an increasing number of conditions causing brain dysfunction, its role in the postnatal CNS has remained difficult to assess. This is because the bdnf-null mutation causes the death of the animals before BDNF levels have reached adult levels. In addition, the anterograde axonal transport of BDNF complicates the interpretation of area-specific gene deletion. The present study describes the generation of a new conditional mouse mutant essentially lacking BDNF throughout the CNS. It shows that BDNF is not essential for prolonged postnatal survival, but that the behavior of such mutant animals is markedly altered. It also reveals that BDNF is not a major survival factor for most CNS neurons and for myelination of their axons. However, it is required for the postnatal growth of the striatum, and single-cell analyses revealed a marked decreased in dendritic complexity and spine density. In contrast, BDNF is dispensable for the growth of the hippocampus and only minimal changes were observed in the dendrites of CA1 pyramidal neurons in mutant animals. Spine density remained unchanged, whereas the proportion of the mushroom-type spine was moderately decreased. In line with these in vivo observations, we found that BDNF markedly promotes the growth of cultured striatal neurons and of their dendrites, but not of those of hippocampal neurons, suggesting that the differential responsiveness to BDNF is part of a neuron-intrinsic program.

Demyelination and remyelination of the caudal cerebellar peduncle of adult rats following stereotaxic injections of lysolecithin, ethidium bromide, and complement/anti-galactocerebroside: a comparative study

Experimentally induced demyelination due to the direct injection of gliotoxic agents has provided powerful models for studying the biology of remyelination. For the most part, these models have involved injection into white matter tracts of the spinal cord. However, the spinal cord has a number of limitations, such as the size of lesions that it is possible to make and its unsuitability for long-term direct cannulation for the delivery of putative remyelination-enhancing agents. In this study, we describe the natural history of three new models of demyelination/remyelination based on the stereotaxic injection of three gliotoxins: lysolecithin, ethidium bromide, and a combination of anti-galactocerebroside antibody and complement (GalC-ab/comp) into the caudal cerebellar peduncle of adult rats. All three agents produced large areas of demyelination with minimal axonal damage, which undergo extensive remyelination. Ethidium bromide- and GalC-ab/comp-induced lesions remyelinated more slowly than those induced by lysolecithin. The contribution to the remyelination of the lesion by Schwann cells reflects the degree of astrocyte damage incurred within the demyelinated area and is greatest for ethidium bromide-induced demyelination. These new models not only provide further insights into the mechanisms of CNS remyelination but also provide a valuable new resource for addressing a series of key issues relevant to current efforts to promote CNS remyelination either by the enhancement of intrinsic processes or by the transplantation of myelinogenic cells.