Tau in Oligodendrocytes Takes Neurons in Sickness and in Health.

Oligodendrocytes (OLGs), the myelin-forming cells of the central nervous system (CNS), are lifelong partners of neurons. They adjust to the functional demands of neurons over the course of a lifetime to meet the functional needs of a healthy CNS. When this functional interplay breaks down, CNS degeneration follows. OLG processes are essential features for OLGs being able to connect with the neurons. As many as fifty cellular processes from a single OLG reach and wrap an equal number of axonal segments. The cellular processes extend to meet and wrap axonal segments with myelin. Further, transport regulation, which is critical for myelination, takes place within the cellular processes. Because the microtubule-associated protein tau plays a crucial role in cellular process extension and myelination, alterations of tau in OLGs have deleterious effects, resulting in neuronal malfunction and CNS degeneration. Here, we review current concepts on the lifelong role of OLGs and myelin for brain health and plasticity. We present key studies of tau in OLGs and select important studies of tau in neurons. The extensive work on tau in neurons has considerably advanced our understanding of how tau promotes either health or disease. Because OLGs are crucial to neuronal health at any age, an understanding of the functions and regulation of tau in OLGs could uncover new therapeutics for selective CNS neurodegenerative diseases.

Inducible Expression of a Truncated Form of Tau in Oligodendrocytes Elicits Gait Abnormalities and a Decrease in Myelin: Implications for Selective CNS Degenerative Diseases.

The cytoskeleton protein Tau present in oligodendrocytes (OLGs) promotes cellular process outgrowth and myelination; whereas abnormally hyperphosphorylated Tau has been shown to be present in the most debilitating form of multiple sclerosis and in selective dementias. This research examined the functional consequences of expressing a truncated form of Tau in OLGs during the second postnatal life. In particular, this truncated form of Tau (∆Tau) retains the Fyn-binding domain but lacks the microtubule-binding domain. Similar to hyperphosphorylated Tau, ∆Tau cannot bind the cytoskeleton and is missorted. The Cre/loxP recombination system was used to generate transgenic (TG) founder lines, which contain a Floxed LacZ-STOP cassette to prevent expression of enhanced green fluorescence protein (EGFP)-∆Tau. The founder lines were then crossed with a Tamoxifen (TM)-inducible proteolipid protein (PLP)-dependent Cre driver line. Myelin PLP is the major myelin protein in the central nervous system (CNS). TM was given at postnatal day (p) 12 for 3 days, and CNS tissues were collected at p22. Only TG mice with both EGFP-∆Tau and Cre manifested an overt phenotype of loss of balance and stumbles starting around p18. CNS tissues obtained from TM-treated EGFP-∆Tau/Cre double transgenic mice had recombined PCR products, GFP, and diminished brain myelin. GFP was expressed in OLGs, but not in neurons or astrocytes. On the contrary, TM-treated TG mice with only one of the two transgenes, i.e., Cre or Tau, did not have recombinant PCR products, GFP, diminished myelin, or abnormal phenotype. Thus, this inducible model shows for the first time that a non-microtubule-associated Tau protein in OLGs elicits both myelin decrease and gait abnormalities, similar to the occurrence in selective demyelinating and neurodegenerative diseases.

Glatiramer acetate guards against rapid memory decline during relapsing-remitting experimental autoimmune encephalomyelitis.

Cognitive decline presents a therapeutic challenge for patients with multiple sclerosis (MS), a disease characterized by recurrent autoimmune demyelination and by progressive CNS degeneration. Glatiramer acetate (GA, also known as Copolymer 1, Cop-1, or Copaxone), commonly used to treat MS, reduces the frequency of relapses; it has both anti-inflammatory and neuroprotective properties. However, clinical trials have not definitively shown that GA improves cognitive impairment during MS. Using an in vivo animal model of autoimmune demyelination, i.e., relapsing-remitting experimental autoimmune encephalomyelitis (EAE), we tested short-term memory in EAE mice (EAE), in EAE mice treated with GA for 10 days starting at the time of immunization (EAE + GA), and in age-matched healthy, naïve mice (Naïve). Short-term memory was assessed using the cross-maze test at 10, 20, and 30 days post-immunization (d.p.i.); data were analyzed at each time point and over time. At 10 d.p.i., EAE and EAE + GA mice had better memory function than Naïve mice. However, at the later time points, EAE mice had a steep negative slope of memory function (indicating decline), whereas EAE + GA mice had a flatter, less-negative slope of memory function. Notably, the memory function of EAE mice significantly decreased over time compared with that of Naïve mice, indicating that EAE had a negative impact on cognitive ability. In contrast, there was no statistically significant difference between the slopes of memory function in mice with EAE treated with GA versus Naïve mice, which revealed effective, albeit partial, protection by GA treatment against progressive memory decline during EAE disease. Of particular interest, although EAE mice had memory decline over 30 d.p.i., their clinical disease scores improved during that time. Thus, our results suggest that EAE mice had a significant progressive memory decline and that GA, administered at the time of immunization, partially guards against rapid memory decline.

Oligodendrogenesis and myelinogenesis during postnatal development effect of glatiramer acetate.

Myelinogenesis in the mammal nervous system occurs predominantly postnatally. Glatiramer acetate (GA), a drug for the treatment for multiple sclerosis (MS), has been shown to induce immunomodulation and neuroprotection in the inflamed CNS in MS and in experimental autoimmune encephalomyelitis (EAE). Here we investigated whether GA can affect myelinogenesis and oligodendrogenesis in the developing nervous system under nonpathological conditions. Towards this end we studied myelination in mice injected daily by GA, at postnatal Days 7-21. Immunohistological and ultrastructural analyses revealed significant elevation in the number of myelinated axons as well as in the thickness of the myelin encircling them and their resulting g-ratios, in spinal cords of GA-injected mice compared with their PBS-injected littermates, at postnatal Day 14. Elevation in myelinated axons was detected also in the peripheral ventral roots of the motor nerves. GA induced also an increase in axonal diameter, implying an effect on the overall development of the nervous system. A prominent elevation in the amount of progenitor oligodendrocytes and their BrdU incorporation, as well as in mature oligodendrocytes indicated that the effect of GA is linked to increased proliferation and differentiation along the oligodendroglial maturation cascade. In addition, elevation in insulin-like growth factor (IGF-1) and brain-derived neurotrophic factor (BDNF) was found in the white matter of the GA-injected mice. Furthermore, a functional advantage in rotating rod test was exhibited by GA-injected mice over their littermates at postnatal Day 21. These cumulative findings corroborate the beneficial effect of GA on oligodendrogenesis and myelination.

Serum-Based Biomarkers in Neurodegeneration and Multiple Sclerosis.

Multiple Sclerosis (MS) is a debilitating disease with typical onset between 20 and 40 years of age, so the disability associated with this disease, unfortunately, occurs in the prime of life. At a very early stage of MS, the relapsing-remitting mobility impairment occurs in parallel with a progressive decline in cognition, which is subclinical. This stage of the disease is considered the beginning of progressive MS. Understanding where a patient is along such a subclinical phase could be critical for therapeutic efficacy and enrollment in clinical trials to test drugs targeted at neurodegeneration. Since the disease course is uneven among patients, biomarkers are needed to provide insights into pathogenesis, diagnosis, and prognosis of events that affect neurons during this subclinical phase that shapes neurodegeneration and disability. Thus, subclinical cognitive decline must be better understood. One approach to this problem is to follow known biomarkers of neurodegeneration over time. These biomarkers include Neurofilament, Tau and phosphotau protein, amyloid-peptide-ß, Brl2 and Brl2-23, N-Acetylaspartate, and 14-3-3 family proteins. A composite set of these serum-based biomarkers of neurodegeneration might provide a distinct signature in early vs. late subclinical cognitive decline, thus offering additional diagnostic criteria for progressive neurodegeneration and response to treatment. Studies on serum-based biomarkers are described together with selective studies on CSF-based biomarkers and MRI-based biomarkers.

HDAC6 in Diseases of Cognition and of Neurons.

Central nervous system (CNS) neurodegenerative diseases are characterized by faulty intracellular transport, cognition, and aggregate regulation. Traditionally, neuroprotection exerted by histone deacetylase (HDAC) inhibitors (HDACi) has been attributed to the ability of this drug class to promote histone acetylation. However, HDAC6 in the healthy CNS functions via distinct mechanisms, due largely to its cytoplasmic localization. Indeed, in healthy neurons, cytoplasmic HDAC6 regulates the acetylation of a variety of non-histone proteins that are linked to separate functions, i.e., intracellular transport, neurotransmitter release, and aggregate formation. These three HDAC6 activities could work independently or in synergy. Of particular interest, HDAC6 targets the synaptic protein Bruchpilot and neurotransmitter release. In pathological conditions, HDAC6 becomes abundant in the nucleus, with deleterious consequences for transcription regulation and synapses. Thus, HDAC6 plays a leading role in neuronal health or dysfunction. Here, we review recent findings and novel conclusions on the role of HDAC6 in neurodegeneration. Selective studies with pan-HDACi are also included. We propose that an early alteration of HDAC6 undermines synaptic transmission, while altering transport and aggregation, eventually leading to neurodegeneration.

QKI binds MAP1B mRNA and enhances MAP1B expression during oligodendrocyte development.

Microtubule-associated protein 1B (MAP1B) is essential for neural development. Besides the abundant expression in neurons, MAP1B recently was found in myelinating oligodendroglia. Moreover, MAP1B deficiency causes delayed myelin development, suggesting the functional importance of MAP1B in oligodendroglia. However, molecular mechanisms that control MAP1B expression in oligodendroglia remain elusive. We report here that MAP1B mRNA is markedly up-regulated in the oligodendroglia cell line CG4 upon induced differentiation, leading to elevated MAP1B protein production. A coordinated regulation of homeoprotein transcription factors was observed during CG4 cell differentiation, which recapitulates the regulation in neurons that promotes MAP1B transcription. Hence, transcriptional regulation of MAP1B appears to be a common mechanism in both neurons and oligodendroglia. In addition, we found posttranscriptional regulation of MAP1B mRNA by the selective RNA-binding protein QKI in oligodendroglia. The 3'UTR of MAP1B mRNA interacts with QKI, and oligodendroglia-specific QKI-deficiency in the quakingviable mutant mice resulted in reduced MAP1B mRNA expression. Moreover, RNAi-mediated QKI-knockdown caused destabilization of the MAP1B mRNA in CG4 cells. Furthermore, forced expression of exogenous QKI was sufficient for promoting MAP1B expression. Because QKI is absent in neurons, QKI-dependent stabilization of MAP1B mRNA provides a novel mechanism for advancing MAP1B expression specifically in oligodendroglia during brain development.

The Selective HDAC6 Inhibitor ACY-738 Impacts Memory and Disease Regulation in an Animal Model of Multiple Sclerosis.

Multiple sclerosis (MS) is a complex disease characterized by autoimmune demyelination and progressive neurodegeneration. Pathogenetic mechanisms of the disease remain largely unknown. Changes in synaptic functions have been reported; however, the significance of such alterations in the disease course remains unclear. Furthermore, the therapeutic potential of targeting synapses is not well-established. Synapses have key signaling elements that regulate intracellular transport and overall neuronal health. Histone deacetylase (HDAC)6 is a microtubule-associated deacetylase. The interaction between HDAC6 and microtubules is augmented by HDAC6 inhibitors. In this study, experimental autoimmune encephalomyelitis (EAE) mice, an animal model of MS, were treated with the HDAC6 inhibitor drug ACY-738 (20 mg/kg) on day 9 and day 10 post-immunization. Mice were assessed for working memory using the cross-maze test at 10 days post-immunization (d.p.i.), whereas disease scores were recorded over approximately 4 weeks post-immunization. We observed that ACY-738 delayed disease onset and reduced disease severity. Most importantly, ACY-738 increased short-term memory in a manner sensitive to disease severity. We induced EAE disease with various amounts of myelin oligodendrocyte glycoprotein (MOG35-55). EAE mice receiving 100 µg of MOG35-55 and treated with ACY-738 had a statistically significant increase in short term-memory compared to naive mice. Additionally, EAE mice receiving 50 µg MOG35-55 and treated with ACY-738 had a statistically significant increase in short term-memory when compared to EAE mice without drug treatment. In contrast, ACY-738 did not change short-term memory in EAE mice immunized with 200 µg of MOG35-55. Because ACY-738 increases short-term memory only with lower amounts of EAE-inducing reagents, we hypothesize that the inflammatory-demyelinating environment induced by higher amount of EAE-inducing reagents overpowers (at day 10 post-immunization) the synaptic molecules targeted by ACY-738. These studies pave the way for developing ACY-738-like compounds for MS patients and for using ACY-738 as a probe to elucidate disease-sensitive changes at the synapses occurring early in the disease course.

Truncated Tau with the Fyn-binding domain and without the microtubule-binding domain hinders the myelinating capacity of an oligodendrocyte cell line.

The mechanisms underlying developmental myelination have therapeutic potential following CNS injury and degeneration. We report that transplanted central glial (CG)-4 cells had a diminished myelinating capacity in myelin-deficient (md) rats when cells express a mutated form of Tau (Tau [688]), which binds Fyn but not the microtubules. In the brain of the md rats, Tau [688]-transfected CG-4 cells displayed a decrease in cellular process outgrowth and myelination; in the spinal cord the extent of myelination rostral and caudal to the injection site was decreased. In contrast, control Tau [605]-transfected CG-4 cells formed long cellular processes and substantial areas of myelin both in the brain and spinal cord. In culture, Tau [688]-transfected CG-4 cells displayed a decrease in cellular process outgrowth, and Fyn localized largely in the cell body, not the processes. Thus, Tau in oligodendrocytes plays a key role in myelination, and a functional Tau-Fyn interaction might have therapeutic potential during demyelination and myelin repair following CNS injury and degeneration.

Silent Free Fall at Disease Onset: A Perspective on Therapeutics for Progressive Multiple Sclerosis.

Central nervous system (CNS) degeneration occurs during multiple sclerosis (MS) following several years of reversible autoimmune demyelination. Progressive CNS degeneration appears later during the course of relapsing-remitting MS (RRMS), although it starts insidiously at disease onset. We propose that there is an early subclinical phase also for primary-progressive (PP) MS. Consensus exists that many different cell types are involved during disease onset. Furthermore, the response to the initial damage, which is specific for each individual, would result in distinct pathological pathways that add complexity to the disease and the mechanisms underlying progressive CNS degeneration. Progressive MS is classified as either active or not active, as well as with or without progression. Different forms of progressive MS might reflect distinct or overlapping pathogenetic pathways. Disease mechanisms should be determined for each patient at diagnosis and the time of treatment. Until individualized and time-sensitive treatments that specifically target the molecular mechanisms of the progressive aspect of the disease are identified, combined therapies directed at anti-inflammation, regeneration, and neuroprotection are the most effective for preventing MS progression. This review presents selected therapeutics in support of the overall idea of a multidimensional therapy applied early in the disease. This approach could limit damage and increase CNS repair. By targeting several cellular populations (i.e., microglia, astrocytes, neurons, oligodendrocytes, and lymphocytes) and multiple pathological processes (e.g., inflammation, demyelination, synaptopathy, and excitatory/inhibitory imbalance) progressive MS could be attenuated. Early timing for such multidimensional therapy is proposed as the prerequisite for effectively halting progressive MS.

Decreased oligodendrocyte nuclear diameter in Alzheimer's disease and Lewy body dementia.

To better understand the pathogenesis of dementia, it is important to understand histopathologic changes in neurodegenerative diseases because they might highlight key aspects of the degenerative process. In this study, the nuclear diameter of neurons and oligodendrocytes in selected temporal lobe areas were determined in autopsy tissue sections from patients with Alzheimer's disease (AD), Lewy body dementia (LBD) and controls. Our morphometric studies targeted neurons in the CA4 region of the pyramidal cell layer of the hippocampus, neurons in the granular layer of the dentate gyrus and oligodendrocytes in parahippocampal white matter. Mean neuronal nuclear diameters were not different among the studied groups. However, our studies revealed a statistically significant reduction of mean oligodendrocyte nuclear diameter in AD and LBD relative to controls. The reduction of the mean nucleus diameter of oligodendrocytes in LBD was independent of the presence of associated AD pathology in LBD. These findings for the first time identify decreased oligodendrocyte nucleus diameter as a morphologic feature of AD and LBD and may lead to a better understanding of the role of oligodendrocytes in AD and LBD pathogenesis.

Regulation and differential expression of tau mRNA isoforms as oligodendrocytes mature in vivo: implications for myelination.

Oligodendrocytes and neurons derive from the same cell type but develop distinct morphologic and functional properties as they mature in vivo. Both cells express tau protein, a developmentally regulated protein in the central nervous system. The regulation of tau has been investigated extensively in neurons but not in oligodendrocytes, so we studied regulation of tau in oligodendrocytes in vivo. The amino-derived tau isoforms consist of isoforms with zero (A0), one (A1), or two (A2) inserts. We examined the developmental regulation of tau mRNA isoforms at the amino domain by comparing tau expression in oligodendrocytes (OLGs) isolated from 1- and 20-day-old rat brain and in age-matched cortex, which abounds in neurons. In the rat brain, myelination peaks at 20 days. By using semiquantitative RT-PCR, we found that OLGs and cortex from 1-day-old rat brain largely had amino-derived tau isoforms with no insert, whereas OLGs from 20-day-old rat brain had similar levels of amino-derived tau isoforms with no insert or with one insert. We also found that 20-day-old OLGs had twofold more tau mRNA levels than younger OLGs. In contrast to OLGs from 20-day-old rat brain, age-matched cortex had comparable levels of A0, A1, and A2 tau amino-derived isoforms. Further, younger and older OLGs had a reciprocal pattern of expression of both carboxy-derived tau mRNA isoforms with either three (3R) or four (4R) repeats. In contrast, younger and older cortex expressed either 3R or 4R tau. This study showed an upregulation of tau mRNA and cell-specific tau mRNA isoform expression in OLGs forming myelin.

Tau protein expression in adult bovine oligodendrocytes: functional and pathological significance.

In tauopathies, overexpression of tau exon 10 is linked to degeneration and abnormal tau deposition in neurons and oligodendroglia (OLGs). To compare exon 10 expression in normal neurons and OLGs, adult bovine brain was examined for the expression of tau in gray matter and cultured OLGs isolated from white matter. Using exon-specific antibodies, we found that both types of tissues abundantly expressed exon 2 but isolated OLGs had a lower expression of exons 3 and 10 when compared to gray matter. Relative expression of exons 3 and 10 did not change significantly during the in vitro maturation of OLGs for 39 days. Using a panel of well-characterized antibodies against tau, we determined that isolated OLGs contained tau phosphorylated at the Tau-1, 12E8, and PHF-1 but not the AT8, AT100, AT180, and AT270 epitopes. Tau phosphorylation status diminished during in vitro maturation, suggesting that healthy OLG processes require regulated phosphorylation of tau at specific sites. We propose that the tau isoform profile and phosphorylation status contribute to the vulnerability of OLGs in degenerative diseases linked to overexpression of exon 10.