Smad proteins differentially regulate transforming growth factor-ß-mediated induction of chondroitin sulfate proteoglycans.

Traumatic injury to the CNS results in increased expression and deposition of chondroitin sulfate proteoglycans (CSPGs) that are inhibitory to axonal regeneration. Transforming growth factor-ß (TGF-ß) has been implicated as a major mediator of these changes, but the mechanisms through which TGF-ß regulates CSPG expression are not known. Using lentiviral expressed Smad-specific ShRNA we show that TGF-ß induction of CSPG expression in astrocytes is Smad-dependent. However, we find a differential dependence of the synthetic machinery on Smad2 and/or Smad3. TGF-ß induction of neurocan and xylosyl transferase 1 required both Smad2 and Smad3, whereas induction of phosphacan and chondroitin synthase 1 required Smad2 but not Smad3. Smad3 knockdown selectively reduced induction of chondroitin-4-sulfotransferase 1 and the amount of 4-sulfated CSPGs secreted by astrocytes. Additionally, Smad3 knockdown in astrocytes was more efficacious in promoting neurite outgrowth of neurons cultured on the TGF-ß-treated astrocytes. Our data implicate TGF-ß Smad3-mediated induction of 4-sulfation as a critical determinant of the permissiveness of astrocyte secreted CSPGs for axonal growth.

Internalization and degradation of the glutamate transporter GLT-1 in response to phorbol ester.

Activation of protein kinase C (PKC) decreases the activity and cell surface expression of the predominant forebrain glutamate transporter, GLT-1. In the present study, C6 glioma were used as a model system to define the mechanisms that contribute to this decrease in cell surface expression and to determine the fate of internalized transporter. As was previously observed, phorbol 12-myristate 13-acetate (PMA) caused a decrease in biotinylated GLT-1. This effect was blocked by sucrose or by co-expression with a dominant-negative variant of dynamin 1, and it was attenuated by co-expression with a dominant-negative variant of the clathrin heavy chain. Depletion of cholesterol with methyl-beta-cyclodextrin, co-expression with a dominant-negative caveolin-1 mutant (Cav1/S80E), co-expression with dominant-negative variants of Eps15 (epidermal-growth-factor receptor pathway substrate clone 15), or co-expression with dominant-negative Arf6 (T27N) had no effect on the PMA-induced loss of biotinylated GLT-1. Long-term treatment with PMA caused a time-dependent loss of biotinylated GLT-1 and decreased the levels of GLT-1 protein. Inhibitors of lysosomal degradation (chloroquine or ammonium chloride) or co-expression with a dominant-negative variant of a small GTPase implicated in trafficking to lysosomes (Rab7) prevented the PMA-induced decrease in protein and caused an intracellular accumulation of GLT-1. These results suggest that the PKC-induced redistribution of GLT-1 is dependent upon clathrin-mediated endocytosis. These studies identify a novel mechanism by which the levels of GLT-1 could be rapidly down-regulated via lysosomal degradation. The possibility that this mechanism may contribute to the loss of GLT-1 observed after acute insults to the CNS is discussed.

Ubiquitination-mediated internalization and degradation of the astroglial glutamate transporter, GLT-1.

Sodium-dependent glutamate uptake is essential for limiting excitotoxicity, and dysregulation of this process has been implicated in a wide array of neurological disorders. The majority of forebrain glutamate uptake is mediated by the astroglial glutamate transporter, GLT-1. We and others have shown that this transporter undergoes endocytosis and degradation in response to activation of protein kinase C (PKC), however, the mechanisms involved remain unclear. In the current study, transfected C6 glioma cells or primary cortical cultures were used to show that PKC activation results in incorporation of ubiquitin into GLT-1 immunoprecipitates. Mutation of all 11 lysine residues in the amino and carboxyl-terminal domains to arginine (11R) abolished this signal. Selective mutation of the seven lysine residues in the carboxyl terminus (C7K-R) did not eliminate ubiquitination, but it completely blocked PKC-dependent internalization and degradation. Two families of variants of GLT-1 were prepared with various lysine residues mutated to arginine. Analyses of these constructs indicated that redundant lysine residues in the carboxyl terminus were sufficient for the appearance of ubiquitinated product and degradation of GLT-1. Together these data define a novel mechanism by which the predominant forebrain glutamate transporter can be rapidly targeted for degradation.

Constitutive endocytosis and recycling of the neuronal glutamate transporter, excitatory amino acid carrier 1.

The neuronal glutamate transporter, excitatory amino acid carrier 1 (EAAC1), has a diverse array of physiologic and metabolic functions. There is evidence that there is a relatively large intracellular pool of EAAC1 both in vivo and in vitro, that EAAC1 cycles on and off the plasma membrane, and that EAAC1 cell surface expression can be rapidly regulated by intracellular signals. Despite the possible relevance of EAAC1 trafficking to both physiologic and pathologic processes, the cellular machinery involved has not been defined. In the present study, we found that agents that disrupt clathrin-dependent endocytosis or plasma membrane cholesterol increased steady-state levels of biotinylated EAAC1 in C6 glioma cells and primary neuronal cultures. Acute depletion of cholesterol increased the V(max) for EAAC1-mediated activity and had no effect on Na(+)-dependent glycine transport in the same system. These agents also impaired endocytosis as measured using a reversible biotinylating reagent. Co-expression with dominant-negative variants of dynamin or the clathrin adaptor, epidermal growth factor receptor pathway substrate clone 15, increased the steady-state levels of biotinylated myc-EAAC1. EAAC1 immunoreactivity was found in a subcellular fraction enriched in early endosome antigen 1 (EEA1) isolated by differential centrifugation and partially co-localized with EEA1. Co-expression of a dominant-negative variant of Rab11 (Rab11 S25N) reduced steady-state levels of biotinylated myc-EAAC1 and slowed constitutive delivery of myc-EAAC1 to the plasma membrane. Together, these observations suggest that EAAC1 is constitutively internalized via a clathrin- and dynamin-dependent pathway into early endosomes and that EAAC1 is trafficked back to the cell surface via the endocytic recycling compartment in a Rab11-dependent mechanism. As one defines the machinery required for constitutive trafficking of EAAC1, it may be possible to determine how intracellular signals regulate EAAC1 cell surface expression.

Temporal patterns of cortical proliferation of glial cell populations after traumatic brain injury in mice.

TBI (traumatic brain injury) triggers an inflammatory cascade, gliosis and cell proliferation following cell death in the pericontusional area and surrounding the site of injury. In order to better understand the proliferative response following CCI (controlled cortical impact) injury, we systematically analyzed the phenotype of dividing cells at several time points post-lesion. C57BL/6 mice were subjected to mild to moderate CCI over the left sensory motor cortex. At different time points following injury, mice were injected with BrdU (bromodeoxyuridine) four times at 3-h intervals and then killed. The greatest number of proliferating cells in the pericontusional region was detected at 3 dpi (days post-injury). At 1 dpi, NG2+ cells were the most proliferative population, and at 3 and 7 dpi the Iba-1+ microglial cells were proliferating more. A smaller, but significant number of GFAP+ (glial fibrillary acidic protein) astrocytes proliferated at all three time points. Interestingly, at 3 dpi we found a small number of proliferating neuroblasts [DCX+ (doublecortin)] in the injured cortex. To determine the cell fate of proliferative cells, mice were injected four times with BrdU at 3 dpi and killed at 28 dpi. Approximately 70% of proliferative cells observed at 28 dpi were GFAP+ astrocytes. In conclusion, our data suggest that the specific glial cell types respond differentially to injury, suggesting that each cell type responds to a specific pattern of growth factor stimulation at each time point after injury.

Evidence that protein kinase Calpha interacts with and regulates the glial glutamate transporter GLT-1.

Many of the sodium-dependent neurotransmitter transporters are rapidly (within minutes) regulated by protein kinase C (PKC), with changes in activity being correlated with changes in transporter trafficking to or from the plasma membrane. Our recent studies suggest that one of the classical subtypes of PKC, PKCalpha, may selectively mediate redistribution of the neuronal glutamate transporter, excitatory amino acid carrier (EAAC)1, and show that PKCalpha can be co-immunoprecipitated with EAAC1. When the glial glutamate transporter GLT-1a is transfected into C6 glioma cells, this transporter is internalized in response to activation of PKC, but the PKC subtype involved in this regulation is unknown. In the present study, expression of the phorbol ester-activated subtypes of PKC was examined in C6 glioma transfected with GLT-1. Of the classical subtypes, only PKCalpha was detected, and of the non-classical subtypes, PKCdelta and PKCepsilon were detected. In this system, phorbol ester-dependent internalization of GLT-1 was blocked by a general inhibitor of PKCs (bisindolylmaleimide II) and by concentrations of Gö6976 that selectively block classical PKCs, but not by an inhibitor of PKCdelta (rottlerin). PKCalpha immunoreactivity was found in GLT-1 immunoprecipitates obtained from transfected C6 cells and from crude rat brain synaptosomes, a milieu that better mimics in vivo conditions. The amount of PKCalpha in both types of immunoprecipitate was modestly increased by phorbol ester, and this increase was blocked by a PKC antagonist. These studies suggest that PKCalpha may be required for the regulated redistribution of GLT-1.

Rottlerin, an inhibitor of protein kinase Cdelta (PKCdelta), inhibits astrocytic glutamate transport activity and reduces GLAST immunoreactivity by a mechanism that appears to be PKCdelta-independent.

Protein kinase C (PKC) regulates the activity and/or cell surface expression of several different neurotransmitter transporters, including subtypes of glutamate transporters. In the present study, the effects of pharmacological inhibitors of PKC were studied in primary astrocyte cultures that express the glutamate aspartate transporter (GLAST) subtype of glutamate transporter. We found that general inhibitors of PKC, bisindolylmaleimide I (Bis I), bisindolylmaleimide II (Bis II), staurosporine and an inhibitor of classical PKCs, Gö6976, had no effect on Na+-dependent glutamate transport activity. However, rottlerin, a putative specific inhibitor of PKCdelta, decreased transport activity with an IC50 value (less than 10 micro m) that is comparable to that reported for inhibition of PKCdelta. The effect of rottlerin was very rapid (maximal effect within 5 min) and was due to a decrease in the capacity (Vmax) for transport. Rottlerin also caused a drastic loss of GLAST immunoreactivity within 5 min, suggesting that rottlerin accelerates GLAST degradation/proteolysis. Rottlerin had no effect on cell surface or total expression of the transferrin receptor, providing evidence that the effect on GLAST cannot be attributed to a non-specific internalization/degradation of plasma membrane proteins. Down-regulation of PKCdelta with chronic phorbol ester treatment did not block rottlerin-mediated inhibition of transport activity. These results suggest a novel mechanism for regulation of the GLAST subtype of glutamate transporter and indicate that there is a rottlerin target that is capable of controlling the levels of GLAST by controlling the rate of degradation or limited proteolysis. It appears that the target for rottlerin may not be PKCdelta.

Differential regulation of GLAST immunoreactivity and activity by protein kinase C: evidence for modification of amino and carboxyl termini.

Many neurotransmitter transporters, including the GLT-1 and EAAC1 subtypes of the glutamate transporter, are regulated by protein kinase C (PKC) and these effects are associated with changes in cell surface expression. In the present study, the effects of PKC activation on the glutamate aspartate transporter (GLAST) subtype of glutamate transporter were examined in primary astrocyte cultures. Acute (30 min) exposure to the phorbol 12-myristate 13-acetate (PMA) increased (approximately 20%) transport activity but had the opposite effect on both total and cell surface immunoreactivity. Chronic treatment (6 or 24 h) with PMA had no effect on transport activity but caused an even larger decrease in total and cell surface immunoreactivity. This loss of immunoreactivity was observed using antibodies directed against three different cytoplasmic epitopes, and was blocked by the PKC antagonist, bisindolylmaleimide II. We provide biochemical and pharmacological evidence that the activity observed after treatment with PMA is mediated by GLAST. Two different flag-tagged variants of the human homolog of GLAST were introduced into astrocytes using lentiviral vectors. Although treatment with PMA caused a loss of transporter immunoreactivity, flag immunoreactivity did not change in amount or size. Together, these studies suggest that activation of PKC acutely up-regulates GLAST activity, but also results in modification of several different intracellular epitopes so that they are no longer recognized by anti-GLAST antibodies. We found that exposure of primary cultures of neurons/astrocytes to transient hypoxia/glucose deprivation also caused a loss of GLAST immunoreactivity that was attenuated by the PKC antagonist, bisindolylmaleimide II, suggesting that some acute insults previously thought to cause a loss of GLAST protein may mimic the phenomenon observed in the present study.