Loss of TAX1BP1-Directed Autophagy Results in Protein Aggregate Accumulation in the Brain

Protein aggregates disrupt cellular homeostasis, causing toxicity linked to neurodegeneration. Selective autophagic elimination of aggregates is critical to protein quality control, but how aggregates are selectively targeted for degradation is unclear. We compared the requirements for autophagy receptor proteins: OPTN, NBR1, p62, NDP52, and TAX1BP1 in clearance of proteotoxic aggregates. Endogenous TAX1BP1 is recruited to and required for the clearance of stress-induced aggregates, whereas ectopic expression of TAX1BP1 increases clearance through autophagy, promoting viability of human induced pluripotent stem cell-derived neurons. In contrast, TAX1BP1 depletion sensitizes cells to several forms of aggregate-induced proteotoxicity. Furthermore, TAX1BP1 is more specifically expressed in the brain compared to other autophagy receptor proteins. In vivo, loss of TAX1BP1 results in accumulation of high molecular weight ubiquitin conjugates and premature lipofuscin accumulation in brains of young TAX1BP1 knockout mice. TAX1BP1 mediates clearance of a broad range of cytotoxic proteins indicating therapeutic potential in neurodegenerative diseases.

Neurolastin, a dynamin family GTPase, translocates to mitochondria upon neuronal stress and alters mitochondrial morphology in vivo

Neurolastin is a dynamin family GTPase that also contains a RING domain and exhibits both GTPase and E3 ligase activities. It is specifically expressed in the brain and is important for synaptic transmission, as neurolastin knockout animals have fewer dendritic spines and exhibit a reduction in functional synapses. Our initial study of neurolastin revealed that it is membrane-associated and partially co-localizes with endosomes. Using various biochemical and cell-culture approaches, we now show that neurolastin also localizes to mitochondria in HeLa cells, cultured neurons, and brain tissue. We found that the mitochondrial localization of neurolastin depends upon an N-terminal mitochondrial targeting sequence and that neurolastin is imported into the mitochondrial intermembrane space. Although neurolastin was only partially mitochondrially localized at steady state, it displayed increased translocation to mitochondria in response to neuronal stress and mitochondrial fragmentation. Interestingly, inactivation or deletion of neurolastin's RING domain also increased its mitochondrial localization. Using EM, we observed that neurolastin knockout animals have smaller but more numerous mitochondria in cerebellar Purkinje neurons, indicating that neurolastin regulates mitochondrial morphology. We conclude that the brain-specific dynamin GTPase neurolastin exhibits stress-responsive localization to mitochondria and is required for proper mitochondrial morphology.

Using double pulsed-field gradient MRI to study tissue microstructure in traumatic brain injury (TBI)

Double pulsed-field gradient (dPFG) MRI is proposed as a new sensitive tool to detect and characterize tissue microstructure following diffuse axonal injury. In this study dPFG MRI was used to estimate apparent mean axon diameter in a diffuse axonal injury animal model and in healthy fixed mouse brain. Histological analysis was used to verify the presence of the injury detected by MRI.

White matter microstructure from nonparametric axon diameter distribution mapping

We report the development of a double diffusion encoding (DDE) MRI method to estimate and map the axon diameter distribution (ADD) within an imaging volume. A variety of biological processes, ranging from development to disease and trauma, may lead to changes in the ADD in the central and peripheral nervous systems. Unlike previously proposed methods, this ADD experimental design and estimation framework employs a more general, nonparametric approach, without a priori assumptions about the underlying form of the ADD, making it suitable to analyze abnormal tissue. In the current study, this framework was used on an ex vivo ferret spinal cord, while emphasizing the way in which the ADD can be weighted by either the number or the volume of the axons. The different weightings, which result in different spatial contrasts, were considered throughout this work. DDE data were analyzed to derive spatially resolved maps of average axon diameter, ADD variance, and extra-axonal volume fraction, along with a novel sub-micron restricted structures map. The morphological information contained in these maps was then used to segment white matter into distinct domains by using a proposed k-means clustering algorithm with spatial contiguity and left-right symmetry constraints, resulting in identifiable white matter tracks. The method was validated by comparing histological measures to the estimated ADDs using a quantitative similarity metric, resulting in good agreement. With further acquisition acceleration and experimental parameters adjustments, this ADD estimation framework could be first used preclinically, and eventually clinically, enabling a wide range of neuroimaging applications for improved understanding of neurodegenerative pathologies and assessing microstructural changes resulting from trauma.

Activation of Rho GTPases in Smith-Lemli-Opitz syndrome: pathophysiological and clinical implications

Smith-Lemli-Opitz syndrome (SLOS) is a malformation syndrome with neurocognitive deficits due to mutations of DHCR7 that impair the reduction of 7-dehydrocholesterol to cholesterol. To investigate the pathological processes underlying the neurocognitive deficits, we compared protein expression in Dhcr7(+/+) and Dhcr7(Delta3-5/Delta3-5) brain tissue. One of the proteins identified was cofilin-1, an actin depolymerizing factor which regulates neuronal dendrite and axon formation. Differential expression of cofilin-1 was due to increased phosphorylation. Phosphorylation of cofilin-1 is regulated by Rho GTPases through Rho-Rock-Limk-Cofilin-1 and Rac/Cdc42-Pak-Limk-Cofilin-1 pathways. Pull-down assays were used to demonstrate increased activation of RhoA, Rac1 and Cdc42 in Dhcr7(Delta3-5/Delta3-5) brains. Consistent with increased activation of these Rho GTPases, we observed increased phosphorylation of both Limk and Pak in mutant brain tissue. Altered Rho/Rac signaling impairs normal dendritic and axonal formation, and mutations in genes encoding regulators and effectors of the Rho GTPases underlie other human mental retardation syndromes. Thus, we hypothesized that aberrant activation of Rho/Rac could have functional consequences for dendrite and axonal growth. In vitro analysis of Dhcr7(Delta3-5/Delta3-5) hippocampal neurons demonstrated both axonal and dendritic abnormalities. Developmental abnormalities of neuronal process formation may contribute to the neurocognitive deficits found in SLOS and may represent a potential target for therapeutic intervention.

Transgenic mice expressing a cameleon fluorescent Ca2+ indicator in astrocytes and Schwann cells allow study of glial cell Ca2+ signals in situ and in vivo

Glial cell Ca2+ signals play a key role in glial-neuronal and glial-glial network communication. Numerous studies have thus far utilized cell-permeant and injected Ca2+ indicator dyes to investigate glial Ca2+ signals in vitro and in situ. Genetically encoded fluorescent Ca2+ indicators have emerged as novel probes for investigating cellular Ca2+ signals. We have expressed one such indicator protein, the YC 3.60 cameleon, under the control of the S100beta promoter and directed its expression predominantly in astrocytes and Schwann cells. Expression of YC 3.60 extended into the entire cellular cytoplasmic compartment and the fine terminal processes of protoplasmic astrocytes and Schwann cell Cajal bands. In the brain, all the cells known to express S100beta in the adult or during development, expressed YC 3.60. While expression was most extensive in astrocytes, other glial cell types that express S100beta, such as NG2 and CNP-positive oligodendrocyte progenitor cells (OP cells), microglia, and some of the large motor neurons in the brain stem, also contained YC 3.60 fluorescence. Using a variety of known in situ and in vivo assays, we found that stimuli known to elicit Ca2+ signals in astrocytes caused substantial and rapid Ca2+ signals in the YC 3.60-expressing astrocytes. In addition, forepaw stimulation while imaging astrocytes through a cranial window in the somatosensory cortex in live mice, revealed robust evoked and spontaneous Ca2+ signals. These results, for the first time, show that genetically encoded reporter is capable of recording activity-dependent Ca2+ signals in the astrocyte processes, and networks.

Deletion at ITPR1 underlies ataxia in mice and spinocerebellar ataxia 15 in humans

We observed a severe autosomal recessive movement disorder in mice used within our laboratory. We pursued a series of experiments to define the genetic lesion underlying this disorder and to identify a cognate disease in humans with mutation at the same locus. Through linkage and sequence analysis we show here that this disorder is caused by a homozygous in-frame 18-bp deletion in Itpr1 (Itpr1^Δ18/Δ18), encoding inositol 1,4,5-triphosphate receptor 1. A previously reported spontaneous Itpr1 mutation in mice causes a phenotype identical to that observed here. In both models in-frame deletion within Itpr1 leads to a decrease in the normally high level of Itpr1 expression in cerebellar Purkinje cells. Spinocerebellar ataxia 15 (SCA15), a human autosomal dominant disorder, maps to the genomic region containing ITPR1; however, to date no causal mutations had been identified. Because ataxia is a prominent feature in Itpr1 mutant mice, we performed a series of experiments to test the hypothesis that mutation at ITPR1 may be the cause of SCA15. We show here that heterozygous deletion of the 5′ part of the ITPR1 gene, encompassing exons 1–10, 1–40, and 1–44 in three studied families, underlies SCA15 in humans.

We have identified a spontaneous in-frame deletion mutation in the gene Itpr1 that causes a recessive movement disorder in mice. In an attempt to define whether any similar disease occurs in humans we performed a literature search for diseases linked to the human chromosomal region containing ITPR1. We identified the disease spinocerebellar ataxia 15 as linked to this region. High-density genomic analysis of affected members from three families revealed that disease in these patients was caused by deletion of a large portion of the region containing ITPR1. We show here that this mutation results in a dramatic reduction in ITPR1 in cells from these patients. These data show convincingly that ITPR1 deletion underlies spinocerebellar ataxia 15 in humans.

Signaling proteins in raft-like microdomains are essential for Ca2+ wave propagation in glial cells

The hypothesis that calcium signaling proteins segregate into lipid raft-like microdomains was tested in isolated membranes of rat oligodendrocyte progenitor (OP) cells and astrocytes using Triton X-100 solubilization and density gradient centrifugation. Western blot analysis of gradient fractions showed co-localization of caveolin-1 with proteins involved in the Ca2+ signaling cascade. These included agonist receptors, P2Y1, and M1, TRPC1, IP3R2, ryanodine receptor, as well as the G protein Galphaq and Homer. Membranes isolated from agonist-stimulated astrocytes showed an enhanced recruitment of phospholipase C (PLCbeta1), IP3R2 and protein kinase C (PKC-alpha) into lipid raft fractions. IP3R2, TRPC1 and Homer co-immunoprecipitated, suggesting protein-protein interactions. Disruption of rafts by cholesterol depletion using methyl-beta-cyclodextrin (beta-MCD) altered the distribution of caveolin-1 and GM1 to non-raft fractions with higher densities. beta-MCD-induced disruption of rafts inhibited agonist-evoked Ca2+ wave propagation in astrocytes and attenuated wave speeds. These results indicate that in glial cells, Ca2+ signaling proteins might exist in organized membrane microdomains, and these complexes may include proteins from different cellular membrane systems. Such an organization is essential for Ca2+ wave propagation.

Astrocytes in adult rat brain express type 2 inositol 1,4,5-trisphosphate receptors

Astrocytes respond to neuronal activity by propagating Ca(2+) waves elicited through the inositol 1,4,5-trisphosphate pathway. We have previously shown that wave propagation is supported by specialized Ca(2+) release sites, where a number of proteins, including inositol 1,4,5-trisphosphate receptors (IP(3)R), occur together in patches. The specific IP(3)R isoform expressed by astrocytes in situ in rat brain is unknown. In the present report, we use isoform-specific antibodies to localize immunohistochemically the IP(3)R subtype expressed in astrocytes in rat brain sections. Astrocytes were identified using antibodies against the astrocyte-specific markers, S-100 beta, or GFAP. Dual indirect immunohistochemistry showed that astrocytes in all regions of adult rat brain express only IP(3)R2. High-resolution analysis showed that hippocampal astrocytes are endowed with a highly branched network of processes that bear fine hair-like extensions containing punctate patches of IP(3)R2 staining in intimate contact with synapses. Such an organization is reminiscent of signaling microdomains found in cultured glial cells. Similarly, Bergmann glial cell processes in the cerebellum also contained fine hair-like processes containing IP(3)R2 staining. The IP(3)R2-containing fine terminal branches of astrocyte processes in both brain regions were found juxtaposed to presynaptic terminals containing synaptophysin as well as PSD 95-containing postsynaptic densities. Corpus callosum astrocytes had an elongated morphology with IP(3)R2 studded processes extending along fiber tracts. Our data suggest that PLC-mediated Ca(2+) signaling in astrocytes in rat brain occurs predominantly through IP(3)R2 ion channels. Furthermore, the anatomical arrangement of the terminal astrocytic branches containing IP(3)R2 ensheathing synapses is ideal for supporting glial monitoring of neuronal activity.

Müller cell Ca2+ waves evoked by purinergic receptor agonists in slices of rat retina

We have measured agonist evoked Ca2+ waves in Müller cells in situ within freshly isolated retinal slices. Using an eye cup dye loading procedure we were able to preferentially fill Müller glial cells in retinal slices with calcium green. Fluorescence microscopy revealed that bath perfusion of slices with purinergic agonists elicits Ca2+ waves in Müller cells, which propagate along their processes. These Ca2+ signals were insensitive to tetrodotoxin (TTX, 1.0 microM) pretreatment. Cells were readily identified as Müller cells by their unique morphology and by subsequent immunocytochemical labeling with glial fibrillary acidic protein antibodies. While cells never exhibited spontaneous Ca2+ oscillations, purinoreceptor agonists, ATP, 2 MeSATP, ADP, 2 MeSADP, and adenosine readily elicited Ca2+ waves. These waves persisted in the absence of [Ca2+]o but were abolished by thapsigargin pretreatment, suggesting that the purinergic agonists tested act by releasing Ca2+ from intracellular Ca2+ stores. The rank order of potency of different purines and pyrimidines for inducing Ca2+ signals was 2 MeSATP = 2MeSADP > ADP > ATP >> alphabetameATP = uridine triphosphate (UTP) > uridine diphosphate (UDP). The Ca2+ signals evoked by ATP, ADP, and 2 MeSATP were inhibited by reactive blue (100 microM) and suramin (200 microM), and the adenosine induced signals were abolished only by 3,7-dimethyl-1-propargylxanthine (200 microM) and not by 1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine) or 8-cyclopentyl-1,3-dipropylxanthine at the same concentration. Based on these pharmacological characteristics and the dose-response relationships for ATP, 2 MeSATP, 2 MeSADP, ADP, and adenosine, we concluded that Müller cells express the P1A2 and P2Y1 subtypes of purinoceptors. Analysis of Ca2+ responses showed that, similar to glial cells in culture, wave propagation occurred by regenerative amplification at specialized Ca2+ release sites (wave amplification sites), where the rate of Ca2+ release was significantly enhanced. These data suggest that Müller cells in the retina may participate in signaling, and this may serve as an extra-neuronal signaling pathway.

Characterization of ryanodine receptors in oligodendrocytes, type 2 astrocytes, and O-2A progenitors

In this study we have investigated the expression of ryanodine receptors (RyRs), and the ability of caffeine to evoke RyR-mediated elevation of intracellular Ca2+ levels ([Ca2+]i) in glial cells of the oligodendrocyte/type 2 astrocyte lineage. Immunocytochemistry with specific antibodies identified ryanodine receptors in cultured oligodendrocytes, type 2 astrocytes, and O-2A progenitor cells, at high levels in the perinuclear region and in a variegated pattern along processes. Glia acutely isolated from rat brain and in aldehydefixed sections of cortex were similarly found to express RyRs. Caffeine (5-50 mM) caused an increase in [Ca2+]i in most cultured type 2 astrocytes and in 50% of oligodendrocytes. Responses elicited by caffeine were inhibited by pretreatment with ryanodine (10 microM) or thapsigargin (1 microM), and the peak response was unaffected by removal of [Ca2+]o. O-2A progenitor cells, in contrast, were largely unresponsive to caffeine treatment. Pretreatment with kainate (200 microM) to activate Ca2+ entry increased the magnitude of caffeine-evoked [Ca2+]i elevations in type 2 astrocytes and oligodendrocytes, and caused caffeine to activate responses in a significant proportion of previously non-responding O-2A progenitors. In both type 2 astrocytes and oligodendrocytes, caffeine evoked Ca2+ changes which propagated as wavefronts from several initiation sites. These wave amplification sites were characterized by significantly higher local Ca2+ release kinetics. Our results indicate that several glial cell types express RyRs, and that their functionality differs within different cell types of the oligodendrocyte lineage. In addition, ionotropic glutamate receptor activation fills the caffeine-sensitive Ca2+ stores in these cells.

AMPA receptors shape Ca2+ responses in cortical oligodendrocyte progenitors and CG-4 cells

Intracellular calcium signals triggered by glutamate receptor activation were studied in primary cortical oligodendrocyte lineage cells and in the oligodendrocyte cell line CG-4. Glutamate, kainate, and AMPA (30-300 microM) increased [Ca2+]i in both types of cells at the stage of oligodendrocyte progenitors (O-2A; GD3+) or pro-oligodendroblasts (O4+). The peak amplitude of Ca2+ responses to glutamate receptor agonists was significantly larger in cortical cells. In CG-4 and in cortical cells, the majority (more than 90%) of bipolar GD3+ or multipolar O4+ cells responded to kainate. In all the cells analyzed, kainate was more efficacious than AMPA and glutamate. The percentage of bipolar or multipolar cells responding to glutamate was significantly lower in the CG-4 cell line than in primary cultures. Cellular responses typical of metabotropic glutamate receptor activation were observed in 20% of the cortical O-2A progenitors, but in none of the CG-4 cells. The AMPA-selective antagonist GYKI 52466 blocked kainate-induced Ca2+ responses in cortical O-2A cells. The selective AMPA receptor modulator cyclothiazide (30 microM) greatly potentiated the effects of AMPA (30-100 microM) on [Ca2+]i in cortical and CG-4 cells. Our findings indicate that Ca2+ responses in cells of the oligodendrocyte lineage are primarily shaped by functional AMPA receptors.

Subcellular calcium oscillators and calcium influx support agonist-induced calcium waves in cultured astrocytes

We have analysed Ca2+ waves induced by norepinephrine in rat cortical astrocytes in primary culture using fluorescent indicators fura-2 or fluo-3. The temporal pattern of the average [Ca2+]i responses were heterogeneous from cell to cell and most cells showed an oscillatory response at concentrations of agonist around EC50 (200 nM). Upon receptor activation, [Ca2+]i signals originated from a single cellular locus and propagated throughout the cell as a wave. Wave propagation was supported by specialized regenerative calcium release loci along the length of the cell. The periods of oscillations, amplitudes, and the rates of [Ca2+]i rise of these subcellular oscillators differ from each other. These intrinsic kinetic properties of the regenerative loci support local waves when stimulation is continued over long periods of time. The presence of local waves at specific, invariant cellular sites and their inherent kinetic properties provide for the unique and reproducible pattern of response seen in a given cell. We hypothesize that these loci are local specializations in the endoplasmic reticulum where the magnitude of the regenerative Ca2+ release is higher than other regions of the cell. Removal of extracellular Ca2+ or blockade of Ca2+ channels by inorganic cations (Cd2+ and Ni2+) during stimulation of adrenergic receptors alter the sustained plateau component of the [Ca2+]i response. In the absence of Ca2+ release, due to store depletion with thapsigargin, agonist occupation alone does not induce Ca2+ influx in astrocytes. This finding suggests that, under these conditions, receptor-operated Ca2+ entry is not operative. Furthermore, our experiments provide evidence for local Ca2+ oscillations in cells which can support both wave propagation as well as spatially discrete Ca2+ signalling.

Glutamate regulates intracellular calcium and gene expression in oligodendrocyte progenitors through the activation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Oligodendrocytes and their progenitors (O-2A) express functional kainate- and DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring glutamate receptors. The physiological consequences of activation of these receptors were studied in purified rat cortical O-2A progenitors and in the primary oligodendrocyte cell line CG-4. Changes in the mRNA levels of a set of immediate early genes were studied and were correlated to intracellular Ca2+ concentration, as measured by fura-2 Ca2+ imaging. Both in CG-4 and in cortical O-2A progenitors, basal mRNA levels of NGFI-A were much higher than c-fos, c-jun, or jun-b. Glutamate, kainate, and AMPA greatly increased NGFI-A mRNA and protein by activation of membrane receptors in a Ca(2+)-dependent fashion. Agonists at non-N-methyl-D-aspartate receptors promoted transmembrane Ca2+ influx through voltage-dependent channels as well as kainate and/or AMPA channels. The influx of Ca2+ ions occurring through glutamate-gated channels was sufficient by itself to increase the expression of NGFI-A mRNA. AMPA receptors were found to be directly involved in intracellular Ca2+ and NGFI-A mRNA regulation, because the effects of kainate were greatly enhanced by cyclothiazide, an allosteric modulator that selectively suppresses desensitization of AMPA but not kainate receptors. Our results indicate that glutamate acting at AMPA receptors regulates immediate early gene expression in cells of the oligodendrocyte lineage by increasing intracellular calcium. Consequently, modulation of these receptor channels may have immediate effects at the genomic level and regulate oligodendrocyte development at critical stages.

Vasoactive intestinal peptide increases intracellular calcium in astroglia: synergism with alpha-adrenergic receptors

In type I astrocytes from rat cerebral cortex, vasoactive intestinal peptide (VIP) at concentrations below 1 nM evoked an increase in intracellular calcium ion concentration. This response, however, was observed in only 18% of the astrocytes examined. alpha-Adrenergic stimulation with phenylephrine or norepinephrine also resulted in an intracellular calcium response in these cells and the threshold sensitivity of astrocytes to phenylephrine was vastly different from cell to cell. Treatment of these astrocytes with VIP (0.1 nM) together with phenylephrine at subthreshold concentrations produced large increases in intracellular Ca2+ concentration ([Ca2+]i) and oscillations. The continued occupation of the alpha-adrenergic receptor was required for sustained synergism. Both alpha-receptor stimulation and stimulation with the mixture of agonists induced the cellular calcium response by triggering release of calcium from cellular stores, since the response persisted in the absence of extracellular calcium. Furthermore, thapsigargin pretreatment, which depletes intracellular stores, abolished the agonist-induced [Ca2+]i response. VIP (0.1 nM) and phenylephrine were found to increase cellular levels of inositol phosphates; however, there was no apparent additivity in this response when the agonists were added together. These observations suggest a calcium-mediated second messenger system for the high-affinity VIP receptor in astrocytes and that alpha-adrenergic receptors act synergistically with the VIP receptor to augment an intracellular calcium signal. The synergism between diverse receptor types may constitute an important mode of cellular signaling in astroglia.