Increased cell proliferation and granule cell number in the dentate gyrus of protein repair-deficient mice

Biological effects of the loss of homochirality in a multicellular organism

Homochirality is a fundamental feature of all known forms of life, maintaining biomolecules (amino-acids, proteins, sugars, nucleic acids) in one specific chiral form. While this condition is central to biology, the mechanisms by which the adverse accumulation of non-L-α-amino-acids in proteins lead to pathophysiological consequences remain poorly understood. To address how heterochirality build-up impacts organism's health, we use chiral-selective in vivo assays to detect protein-bound non-L-α-amino acids (focusing on aspartate) and assess their functional significance in Drosophila. We find that altering the in vivo chiral balance creates a 'heterochirality syndrome' with impaired caspase activity, increased tumour formation, and premature death. Our work shows that preservation of homochirality is a key component of protein function that is essential to maintain homeostasis across the cell, tissue and organ level.

l-Isoaspartyl Methyltransferase Deficiency in Zebrafish Leads to Impaired Calcium Signaling in the Brain

Isomerization of l-aspartyl and l-asparaginyl residues to l-isoaspartyl residues is one type of protein damage that can occur under physiological conditions and leads to conformational changes, loss of function, and enhanced protein degradation. Protein l-isoaspartyl methyltransferase (PCMT) is a repair enzyme whose action initiates the reconversion of abnormal l-isoaspartyl residues to normal l-aspartyl residues in proteins. Many lines of evidence support a crucial role for PCMT in the brain, but the mechanisms involved remain poorly understood. Here, we investigated PCMT activity and function in zebrafish, a vertebrate model that is particularly well-suited to analyze brain function using a variety of techniques. We characterized the expression products of the zebrafish PCMT homologous genes pcmt and pcmtl. Both zebrafish proteins showed a robust l-isoaspartyl methyltransferase activity and highest mRNA transcript levels were found in brain and testes. Zebrafish morphant larvae with a knockdown in both the pcmt and pcmtl genes showed pronounced morphological abnormalities, decreased survival, and increased isoaspartyl levels. Interestingly, we identified a profound perturbation of brain calcium homeostasis in these morphants. An abnormal calcium response upon ATP stimulation was also observed in mouse hippocampal HT22 cells knocked out for Pcmt1. This work shows that zebrafish is a promising model to unravel further facets of PCMT function and demonstrates, for the first time in vivo, that PCMT plays a pivotal role in the regulation of calcium fluxes.

Inhibition of astrocyte FAK-JNK signaling promotes subventricular zone neurogenesis through CNTF

Astrocyte-derived ciliary neurotrophic factor (CNTF) promotes adult subventricular zone (SVZ) neurogenesis. We found that focal adhesion kinase (FAK) and JNK, but not ERK or P38, repress CNTF in vitro. Here, we defined the FAK-JNK pathway and its regulation of CNTF in mice, and the related leukemia inhibitory factor (LIF) and interleukin-6 (IL-6), which promote stem cell renewal at the expense of neurogenesis. Intrastriatal injection of FAK inhibitor, FAK14, in adult male C57BL/6 mice reduced pJNK and increased CNTF expression in the SVZ-containing periventricular region. Injection of a JNK inhibitor increased CNTF without affecting LIF and IL-6, and increased SVZ proliferation and neuroblast formation. The JNK inhibitor had no effect in CNTF-/- mice, suggesting that JNK inhibits SVZ neurogenesis by repressing CNTF. Inducible deletion of FAK in astrocytes increased SVZ CNTF and neurogenesis, but not LIF and IL-6. Intrastriatal injection of inhibitors suggested that P38 reduces LIF and IL-6 expression, whereas ERK induces CNTF and LIF. Intrastriatal FAK inhibition increased LIF, possibly through ERK, and IL-6 through another pathway that does not involve P38. Systemic injection of FAK14 also inhibited JNK while increasing CNTF, but did not affect P38 and ERK activation, or LIF and IL-6 expression. Importantly, systemic FAK14 increased SVZ neurogenesis in wild-type C57BL/6 and CNTF+/+ mice, but not in CNTF-/- littermates, indicating that it acts by upregulating CNTF. These data show a surprising differential regulation of related cytokines and identify the FAK-JNK-CNTF pathway as a specific target in astrocytes to promote neurogenesis and possibly neuroprotection in neurological disorders.

The impact of age on number and distribution of proliferating cells in subgranular zone in adult mouse brain

The mouse brain retains an ability to produce hippocampal granule neurons during the mouse’s entire lifespan. The neurons are produced in the subgranular zone (SGZ) located on the inner surface of the granule cell layer in the dentate gyrus (DG). In our study, we used a point cloud approach to characterize how the production and distribution of neural precursors for new hippocampal neurons change in the mouse brain relative to age. We found that the production of neural precursors decreases 64 fold from the age of 30 days to the age of 2.5 years. Within the SGZ the decline of cell proliferation continues during entire mouse life. We reconstructed the distribution of proliferating cells along the longitudinal axis of the SGZ and found that the highest number of proliferating cells are located approximately 0.75 mm from the dorsomedial end of the SGZ that corresponds to the most dorsal part of the DG in the mouse brain. The distribution of proliferating cells in the SGZ showed no apparent aggregations, periodicity or any other readily identifiable spatial characteristics. Proliferating cells in the SGZ tended to be located separately from other proliferating cells. About two thirds of them have no closely located other proliferating cells, and the remaining third is located in small clusters comprised of 2 or 3 proliferating cells. Based on our measurements, we calculated that from the age of 30 days to the age of 2.5 years 1.5 million neural precursors are produced in the SGZ.

Cell Densities in the Mouse Brain: A Systematic Review

The mouse brain is the most extensively studied brain of all species. We performed an exhaustive review of the literature to establish our current state of knowledge on cell numbers in mouse brain regions, arguably the most fundamental property to measure when attempting to understand a brain. The synthesized information, collected in one place, can be used by both theorists and experimentalists. Although for commonly-studied regions cell densities could be obtained for principal cell types, overall we know very little about how many cells are present in most brain regions and even less about cell-type specific densities. There is also substantial variation in cell density values obtained from different sources. This suggests that we need a new approach to obtain cell density datasets for the mouse brain.

Intrauterine inflammation reduces postnatal neurogenesis in the hippocampal subgranular zone and leads to accumulation of hilar ectopic granule cells

Prenatal inflammation is associated with poor neurobehavioral outcomes in exposed offspring. A common route of exposure for the fetus is intrauterine infection, which is often associated with preterm birth. Hippocampal development may be particularly vulnerable to an inflammatory insult during pregnancy as this region remains highly neurogenic both prenatally and postnatally. These studies sought to determine if intrauterine inflammation specifically altered hippocampal neurogenesis and migration of newly produced granule neurons during the early postnatal period. Microglial and astroglial cell populations known to play a role in the regulation of postnatal neurogenesis were also examined. We show that intrauterine inflammation significantly reduced hippocampal neurogenesis between postnatal days 7 (P7) and P14 as well as decreased granule cell density at P28. Ectopic migration of granule cells was observed in LPS-exposed mice at P14, but not at P28. Intrauterine inflammation had no effect on hippocampal astrocyte or microglia density or on apoptosis rate at the postnatal time points examined. Thus, exposure to intrauterine inflammation disrupts early postnatal neurogenesis and leads to aberrant migration of newly born granule cells.

Hilar granule cells of the mouse dentate gyrus: effects of age, septotemporal location, strain, and selective deletion of the proapoptotic gene BAX

The dentate gyrus (DG) principal cells are glutamatergic granule cells (GCs), and they are located in a compact cell layer. However, GCs are also present in the adjacent hilar region, but have been described in only a few studies. Therefore, we used the transcription factor prospero homeobox 1 (Prox1) to quantify GCs at postnatal day (PND) 16, 30, and 60 in a common mouse strain, C57BL/6J mice. At PND16, there was a large population of Prox1-immunoreactive (ir) hilar cells, with more in the septal than temporal hippocampus. At PND30 and 60, the size of the hilar Prox1-ir cell population was reduced. Similar numbers of hilar Prox1-expressing cells were observed in PND30 and 60 Swiss Webster mice. Prox1 is usually considered to be a marker of postmitotic GCs. However, many Prox1-ir hilar cells, especially at PND16, were not double-labeled with NeuN, a marker typically found in mature neurons. Most hilar Prox1-positive cells at PND16 co-expressed doublecortin (DCX) and calretinin, markers of immature GCs. Double-labeling with a marker of actively dividing cells, Ki67, was not detected. These results suggest that, surprisingly, a large population of cells in the hilus at PND16 are immature GCs (Type 2b and Type 3 cells). We also asked whether hilar Prox1-ir cell numbers are modifiable. To examine this issue, we conditionally deleted the proapoptotic gene BAX in Nestin-expressing cells at a time when there are numerous immature GCs in the hilus, PND2-8. When these mice were examined at PND60, the numbers of Prox1-ir hilar cells were significantly increased compared to control mice. However, deletion of BAX did not appear to change the proportion that co-expressed NeuN, suggesting that the size of the hilar Prox1-expressing population is modifiable. However, deleting BAX, a major developmental disruption, does not appear to change the proportion that ultimately becomes neurons.

Conditional Disabled-1 Deletion in Mice Alters Hippocampal Neurogenesis and Reduces Seizure Threshold

Many animal models of temporal lobe epilepsy (TLE) exhibit altered neurogenesis arising from progenitors within the dentate gyrus subgranular zone (SGZ). Aberrant integration of new neurons into the existing circuit is thought to contribute to epileptogenesis. In particular, adult-born neurons that exhibit ectopic migration and hilar basal dendrites (HBDs) are suggested to be pro-epileptogenic. Loss of reelin signaling may contribute to these morphological changes in patients with epilepsy. We previously demonstrated that conditional deletion of the reelin adaptor protein, disabled-1 (Dab1), from postnatal mouse SGZ progenitors generated dentate granule cells (DGCs) with abnormal dendritic development and ectopic placement. To determine whether the early postnatal loss of reelin signaling is epileptogenic, we conditionally deleted Dab1 in neural progenitors and their progeny on postnatal days 7–8 and performed chronic video-EEG recordings 8–10 weeks later. Dab1-deficient mice did not have spontaneous seizures but exhibited interictal epileptiform abnormalities and a significantly reduced latency to pilocarpine-induced status epilepticus. After chemoconvulsant treatment, over 90% of mice deficient for Dab1 developed generalized motor convulsions with tonic-clonic movements, rearing, and falling compared to <20% of wild-type mice. Recombination efficiency, measured by Cre reporter expression, inversely correlated with time to the first sustained seizure. These pro-epileptogenic changes were associated with decreased neurogenesis and increased numbers of hilar ectopic DGCs. Interestingly, neurons co-expressing the Cre reporter comprised a fraction of these hilar ectopic DGCs cells, suggesting a non-cell autonomous effect for the loss of reelin signaling. We also noted a dispersion of the CA1 pyramidal layer, likely due to hypomorphic effects of the conditional Dab1 allele, but this abnormality did not correlate with seizure susceptibility. These findings suggest that the misplacement or reduction of postnatally-generated DGCs contributes to aberrant circuit development and hyperexcitability, but aberrant neurogenesis after conditional Dab1 deletion alone is not sufficient to produce spontaneous seizures.

Expansion of the dentate mossy fiber-CA3 projection in the brain-derived neurotrophic factor-enriched mouse hippocampus

Structural changes that alter hippocampal functional circuitry are implicated in learning impairments, mood disorders and epilepsy. Reorganization of mossy fiber (MF) axons from dentate granule cells is one such form of plasticity. Increased neurotrophin signaling is proposed to underlie MF plasticity, and there is evidence to support a mechanistic role for brain-derived neurotrophic factor (BDNF) in this process. Transgenic mice overexpressing BDNF in the forebrain under the α-calcium/calmodulin-dependent protein kinase II promoter (TgBDNF mice) exhibit spatial learning deficits at 2-3months of age, followed by the emergence of spontaneous seizures at ∼6months. These behavioral changes suggest that chronic increases in BDNF progressively disrupt hippocampal functional organization. To determine if the dentate MF pathway is structurally altered in this strain, the present study employed Timm staining and design-based stereology to compare MF distribution and projection volumes in transgenic and wild-type mice at 2-3months, and at 6-7months. Mice in the latter age group were assessed for seizure vulnerability with a low dose of pilocarpine given 2h before euthanasia. At 2-3months, TgBDNF mice showed moderate expansion of CA3-projecting MFs (∼20%), with increased volumes measured in the suprapyramidal (SP-MF) and intra/infrapyramidal (IIP-MF) compartments. At 6-7months, a subset of transgenic mice exhibited increased seizure susceptibility, along with an increase in IIP-MF volume (∼30%). No evidence of MF sprouting was seen in the inner molecular layer. Additional stereological analyses demonstrated significant increases in molecular layer (ML) volume in TgBDNF mice at both ages, as well as an increase in granule cell number by 8months of age. Collectively, these results indicate that sustained increases in endogenous BDNF modify dentate structural organization over time, and may thereby contribute to the development of pro-epileptic circuitry.

Hippocampal GSK3β as a Molecular Link Between Obesity and Depression

Obesity is considered as a risk factor for mood disorders including depression. Nevertheless, the mechanisms underlying this association are not clearly understood. To address this issue, we investigated the impact of high-fat (HF)-diet-induced obesity on depressive-like behavior and on serotonin (5-HT)-dependent Akt/glycogen synthase kinase 3β (GSK3β) signaling in the dentate gyrus (DG) of the hippocampus, which has been associated with mood regulation. We first showed that a HF diet induced significant overweight and hyperglycemia as well as a depressive-like behavior in adult Wistar rats. By using an ex vivo approach on brain slices, we demonstrated that 5-HT activates the Akt/GSK3β cascade in the DG of control chow (C) diet-fed animals and that a 16-week HF diet feeding abolishes this activation, concurrently with a desensitization of leptin and insulin signaling in the same region. Furthermore, depressive-like behavior inversely correlated with 5-HT-induced phosphorylation of GSK3β in the subgranular neurons of the DG. Interestingly, a substitution of HF with C diet for 6 weeks induced a total loss of depressive symptoms, whereas body weight and glycemia remained significantly higher compared to control rats. In addition, food restoration led to a recovery of the Akt/GSK3β signaling pathway activation in the DG. In parallel, we observed a negative correlation between body weight and cell proliferation in the subgranular zone of the DG. To conclude, we provide evidence for a desensitization of 5-HT-induced Akt/GSK3β signaling and an impaired cell proliferation in the DG by HF diet, suggesting novel molecular mechanisms linking obesity to depression.

Molecular ageing of alpha- and Beta-synucleins: protein damage and repair mechanisms

Abnormal α-synuclein aggregates are hallmarks of a number of neurodegenerative diseases. Alpha synuclein and β-synucleins are susceptible to post-translational modification as isoaspartate protein damage, which is regulated in vivo by the action of the repair enzyme protein L-isoaspartyl O-methyltransferase (PIMT). We aged in vitro native α-synuclein, the α-synuclein familial mutants A30P and A53T that give rise to Parkinsonian phenotypes, and β-synuclein, at physiological pH and temperature for a time course of up to 20 days. Resolution of native α-synuclein and β-synuclein by two dimensional techniques showed the accumulation of a number of post-translationally modified forms of both proteins. The levels of isoaspartate formed over the 20 day time course were quantified by exogenous methylation with PIMT using S-Adenosyl-L-[³H-methyl]methionine as a methyl donor, and liquid scintillation counting of liberated ³H-methanol. All α-synuclein proteins accumulated isoaspartate at ∼1% of molecules/day, ∼20 times faster than for β-synuclein. This disparity between rates of isoaspartate was confirmed by exogenous methylation of synucleins by PIMT, protein resolution by one-dimensional denaturing gel electrophoresis, and visualisation of ³H-methyl esters by autoradiography. Protein silver staining and autoradiography also revealed that α-synucleins accumulated stable oligomers that were resistant to denaturing conditions, and which also contained isoaspartate. Co-incubation of approximately equimolar β-synuclein with α-synuclein resulted in a significant reduction of isoaspartate formed in all α-synucleins after 20 days of ageing. Co-incubated α- and β-synucleins, or α, or β synucleins alone, were resolved by non-denaturing size exclusion chromatography and all formed oligomers of ∼57.5 kDa; consistent with tetramerization. Direct association of α-synuclein with β-synuclein in column fractions or from in vitro ageing co-incubations was demonstrated by their co-immunoprecipitation. These results provide an insight into the molecular differences between α- and β-synucleins during ageing, and highlight the susceptibility of α-synuclein to protein damage, and the potential protective role of β-synuclein.

Wortmannin reduces insulin signaling and death in seizure-prone Pcmt1-/- mice

L-isoaspartyl (D-aspartyl) O-methyltransferase deficient mice (Pcmt1^−/−) accumulate isomerized aspartyl residues in intracellular proteins until their death due to seizures at approximately 45 days. Previous studies have shown that these mice have constitutively activated insulin signaling in their brains, and that these brains are 20–30% larger than those from age-matched wild-type animals. To determine whether insulin pathway activation and brain enlargement is responsible for the fatal seizures, we administered wortmannin, an inhibitor of the phosphoinositide 3-kinase that catalyzes an early step in the insulin pathway. Oral wortmannin reduced the average brain size in the Pcmt1^−/− animals to within 6% of the wild-type DMSO administered controls, and nearly doubled the lifespan of Pcmt1^−/− at 60% survival of the original population. Immunoblotting revealed significant decreases in phosphorylation of Akt, PDK1, and mTOR in Pcmt1^−/− mice and Akt and PDK1 in wild-type animals upon treatment with wortmannin. These data suggest activation of the insulin pathway and its resulting brain enlargement contributes to the early death of Pcmt1−/− mice, but is not solely responsible for the early death observed in these animals.

Phosphorylated Insulin Like Growth Factor-I Receptor Expression and Its Clinico-Pathological Significance in Histologic Subtypes of Human Thyroid Cancer

Overexpression of insulin-like growth factor-I receptor (IGF-IR) is seen in a multitude of human thyroid cancers and correlates with poor prognosis. However, recent studies suggest that low phospho-IGF-IR (pIGF-IR) expression rather than its overexpression may be an indicator of poorly differentiated disease. No previous study has evaluated the expression of pIGF-IR to determine if activation or loss of expression of this receptor is associated with thyroid tumor progression. Accordingly, a quantitative immunohistochemical (IHC) method was used to evaluate the clinico-pathological significance of pIGF-IR expression in archival samples of human thyroid carcinomas. Quantitative analysis of pIGF-IR levels revealed a significant difference in the median index of pIGF-IR between different histological subtypes of thyroid cancer ( P < 0.001). Specifically, the median pIGF-IR index of differentiated thyroid cancers was significantly higher than the median index of other poorly differentiated thyroid cancer ( P < 0.001). This was further confirmed in individual tumor sections of thyroid carcinoma where anaplastic and differentiated components co-existed. No significant difference was noted in the pIGF-IR index of tumors grouped by size or stage but a trend towards lower mean pIGF-IR index was noted in older patients. Our data indicates that pIGF-IR is upregulated in a majority of follicular thyroid carcinomas, suggesting it may be a potential target for therapy for patients with this disease. In addition, since low pIGF-IR expression was found to correlate with aggressive human thyroid carcinoma, it also suggests that IGF-IR may not be needed for progression of anaplastic thyroid carcinoma possibly because other cell signaling pathways are activated, obviating the need for IGF-IR signaling. However, more mechanistic studies would be necessary to substantiate the possibility that pIGF-IR may be important for differentiation of thyroid tissues and is lost with disease progression.

Isoaspartate-containing amyloid precursor protein-derived peptides alter efficacy and specificity of potential beta-secretases

Neuritic plaques of Alzheimer patients are composed of multiple protein components. Among them, the amyloid beta-peptides (Abeta) 1-40/42 and further N- and C-terminally modified fragments of Abeta are highly abundant. Most prominent are the isoaspartate (isoAsp)-Abeta peptides and pyroglutamyl (pGlu)-Abeta. While pGlu-Abeta can only be formed from an N-terminal glutamate by glutaminyl cyclase, spontaneous isoAsp-isomerization cannot occur at an N-terminal aspartate of peptides. This means that isoAsp-Abeta formation must precede proteolysis of the amyloid precursor protein (APP). Abeta generation from APP by beta- and gamma-secretases initiates the amyloid peptide aggregation and deposition process. Two aspartate proteases have been identified as secretases: BACE-1 (beta-site amyloid precursor protein cleaving enzyme) and the intramembrane gamma-secretase multiprotein complex. However, recent evidence supports more than one beta-secretase initiating this cascade. Formation of Abeta1-40/42 was predominantly studied by expression of mutated human APP sequences in cell culture and transgenic animals, generating Abeta fragments that did not contain such multiple posttranslational modifications as in Alzheimer's disease. This prompted us to investigate the catalytic turnover of Asp- or isoAsp-containing APP-derived peptide sequences by BACE-1 and cathepsin B, another potential beta-secretase. While cathepsin B is more effective than BACE-1 in processing the Asp-containing peptide derivatives, only cathepsin B can cleave the isoAsp-containing peptides, which occurs with high catalytic efficiency.

CD11c/EYFP transgene illuminates a discrete network of dendritic cells within the embryonic, neonatal, adult, and injured mouse brain

The CD11c enhanced yellow fluorescent protein (EYFP) transgenic mouse was constructed to identify dendritic cells in the periphery (Lindquist et al. [2004] Nat. Immunol. 5:1243-1250). In this study, we used this mouse to characterize dendritic cells within the CNS. Our anatomic results showed discrete populations of EYFP(+) brain dendritic cells (EYFP(+) bDC) that colocalized with a small fraction of microglia immunoreactive for Mac-1, Iba-1, CD45, and F4/80 but not for NeuN, Dcx, NG2 proteoglycan, or GFAP. EYFP(+) bDC, isolated by fluorescent activated cell sorting (FACS), expressed mRNA for the Itgax (CD11c) gene, whereas FACS anlaysis of EYFP(+) bDC cultures revealed the presence of CD11c protein. Light microscopy studies revealed that EYFP(+) bDC were present in the embryonic CNS when the blood-brain barrier is formed and postnatally when brain cells are amenable to culturing. In adult male mice, EYFP(+) bDC distribution was prominent within regions of the CNS that 1) are subject to structural plasticity and neurogenesis, 2) receive sensory and humoral input from the external environment, and 3) lack a blood-brain barrier. Ultrastructural analysis of EYFP(+) bDC in adult neurogenic niches showed their proximity to developing neurons and a morphology characteristic of immune/microglia cells. Kainic acid-induced seizures revealed that EYFP(+) bDC responded to damage of the hippocampus and displayed morphologies similar to those described for seizure-activated EGFP(+) microglia in the hippocampus of cfms (CSF-1R) EGFP mice. Collectively, these findings suggest a new member of the dendritic cell family residing among the heterogeneous microglia population.

Doublecortin (DCX) and doublecortin-like (DCL) are differentially expressed in the early but not late stages of murine neocortical development

During corticogenesis, radial glia-derived neural progenitors divide and migrate along radial fibers to their designated positions within the cortical plate. The microtubule-associated proteins doublecortin (DCX) and doublecortin-like (DCL) are critically involved in neuronal migration and division, and may function in a partially redundant pathway. Since little is known about the important early stages of corticogenesis, when neurogenesis is extensive, we addressed a possible differential role by examining spatiotemporal expression patterns of DCX, DCL, and the radial glia marker vimentin during murine development. We found expression patterns of DCL and DCX to differ remarkably prior to embryonic day (E)13. DCL was already expressed at E9 and largely overlapped with vimentin, whereas DCX expression started modestly from E10/E11 onward. DCL was mainly found in the ventricular zone, often in mitotic cells and in pial-oriented radial fibers. In contrast, DCX was expressed in tangential fibers in the outer cortical regions. After E13, DCX and DCL expression largely overlapped but DCL expression had disappeared from the ventricular zone. Also, DCL levels were attenuated, whereas DCX remained high beyond E17. In conclusion, DCX and DCL are differentially expressed, particularly during early corticogenesis, consistent with their different functional roles. Given its involvement in mitosis, DCL appears to have a unique role in the early neuroepithelium that is different from later developmental stages when DCX is coexpressed.

Ectopic granule cells of the rat dentate gyrus

Granule cells of the mammalian dentate gyrus normally form a discrete layer, and virtually all granule cells migrate to this location. Exceptional granule cells that are positioned incorrectly, in 'ectopic' locations, are rare. Although the characteristics of such ectopic granule cells appear similar in many respects to granule cells located in the granule cell layer, their rare occurrence has limited a full evaluation of their structure and function. More information about ectopic granule cells has been obtained by studying those that develop after experimental manipulations that increase their number. For example, after severe seizures, the number of ectopic granule cells located in the hilus increases dramatically. These experimentally-induced ectopic granule cells may not be equivalent to normal ectopic granule cells necessarily, but the vastly increased numbers have allowed much more information to be obtained. Remarkably, the granule cells that are positioned ectopically develop intrinsic properties and an axonal projection that are similar to granule cells that are located normally, i.e., in the granule cell layer. However, dendritic structure and synaptic structure/function appear to differ. These studies have provided new insight into a rare type of granule cell in the dentate gyrus, and the plastic characteristics of dentate granule cells that appear to depend on the location of the cell body.

Glial fibrillary acidic protein (GFAP)-positive radial-like cells are present in the vicinity of proliferative progenitors in the nucleus tractus solitarius of adult rat

The dorsal vagal complex (DVC), an integrative center of autonomic functions located dorsally in the caudal brainstem, comprises the nucleus tractus solitarius (NTS), the area postrema (AP), and the dorsal motor nucleus of the vagus nerve (DMNX). Recently, this area of the brainstem was shown to retain, during adulthood, the expression of developmental markers, which is consistent with several forms of morphological and functional plasticity. These data led us to attempt to determine the structural organization and phenotypical characteristics of the astroglial compartment in the adult DVC. We report a strikingly high density of glial fibrillary acidic protein (GFAP) immunoreactive cells in the NTS and the DMNX compared to other brainstem structures. Furthermore, we observed a subpopulation of atypical GFAP+ cells in the NTS. These cells expressed vimentin and nestin and displayed unbranched processes that radiate rostrocaudally from cuboid cell bodies located in the 4th ventricle wall. Interestingly, these radiating cells were found in close association with neural progenitors whose proliferation was stimulated by intracerebroventricular injection of epidermal growth factor/basic fibroblast growth factor or lesion of the vagus nerve. Newly born neurons in the NTS identified by doublecortin (DCX) immunolabeling were also preferentially found in the vicinity of the radiating cells. Altogether, these results indicate that the adult NTS retains, during adulthood, astroglial cells that display morphological and phenotypical features seen during development. The overlap in the distribution of proliferative neural progenitors, newborn neurons, and radiating GFAP-positive cells suggest a possible role of the glial compartment of the NTS in functional plasticity in this structure.

Proteomic Identification of Novel Substrates of a Protein Isoaspartyl Methyltransferase Repair Enzyme

We report the use of a proteomic strategy to identify hitherto unknown substrates for mammalian protein l-isoaspartate O-methyltransferase. This methyltransferase initiates the repair of isoaspartyl residues in aged or stress-damaged proteins in vivo. Tissues from mice lacking the methyltransferase (Pcmt1(-/-)) accumulate more isoaspartyl residues than their wild-type littermates, with the most "damaged" residues arising in the brain. To identify the proteins containing these residues, brain homogenates from Pcmt1(-/-) mice were methylated by exogenous repair enzyme and the radiolabeled methyl donor S-adenosyl-[methyl-(3)H]methionine. Methylated proteins in the homogenates were resolved by both one-dimensional and two-dimensional electrophoresis, and methyltransferase substrates were identified by their increased radiolabeling when isolated from Pcmt1(-/-) animals compared with Pcmt1(+/+) littermates. Mass spectrometric analyses of these isolated brain proteins reveal for the first time that microtubule-associated protein-2, calreticulin, clathrin light chains a and b, ubiquitin carboxyl-terminal hydrolase L1, phosphatidylethanolamine-binding protein, stathmin, beta-synuclein, and alpha-synuclein, are all substrates for the l-isoaspartate methyltransferase in vivo. Our methodology for methyltransferase substrate identification was further supplemented by demonstrating that one of these methyltransferase targets, microtubule-associated protein-2, could be radiolabeled within Pcmt1(-/-) brain extracts using radioactive methyl donor and exogenous methyltransferase enzyme and then specifically immunoprecipitated with microtubule-associated protein-2 antibodies to recover co-localized protein with radioactivity. We comment on the functional significance of accumulation of relatively high levels of isoaspartate within these methyltransferase targets in the context of the histological and phenotypical changes associated with the methyltransferase knock-out mice.

Protein repair in the brain, proteomic analysis of endogenous substrates for protein L-isoaspartyl methyltransferase in mouse brain

Protein L-isoaspartyl methyltransferase (PIMT) catalyzes repair of L-isoaspartyl peptide bonds, a major source of protein damage under physiological conditions. PIMT knock-out (KO) mice exhibit brain enlargement and fatal epileptic seizures. All organs accumulate isoaspartyl proteins, but only the brain manifests an overt pathology. To further explore the role of PIMT in brain function, we undertook a global analysis of endogenous substrates for PIMT in mouse brain. Extracts from PIMT-KO mice were subjected to two-dimensional gel electrophoresis and blotted onto membranes. Isoaspartyl proteins were radiolabeled on-blot using [methyl-(3)H]S-adenosyl-L-methionine and recombinant PIMT. Fluorography of the blot revealed 30-35 (3)H-labeled proteins, 22 of which were identified by peptide mass fingerprinting. These isoaspartate-prone proteins represent a wide range of cellular functions, including neuronal development, synaptic transmission, cytoskeletal structure and dynamics, energy metabolism, nitrogen metabolism, pH homeostasis, and protein folding. The following five proteins, all of which are rich in neurons, accumulated exceptional levels of isoaspartate: collapsin response mediator protein 2 (CRMP2/ULIP2/DRP-2), dynamin 1, synapsin I, synapsin II, and tubulin. Several of the proteins identified here are prone to age-dependent oxidation in vivo, and many have been identified as autoimmune antigens, of particular interest because isoaspartate can greatly enhance the antigenicity of self-peptides. We propose that the PIMT-KO phenotype results from the cumulative effect of isoaspartate-related damage to a number of the neuron-rich proteins detected in this study. Further study of the isoaspartate-prone proteins identified here may help elucidate the molecular basis of one or more developmental and/or age-related neurological diseases.

Estradiol enhances neurogenesis following ischemic stroke through estrogen receptors α and β

Neurogenesis persists throughout life under normal and degenerative conditions. The adult subventricular zone (SVZ) generates neural stem cells capable of differentiating to neuroblasts and migrating to the site of injury in response to brain insults. In the present study, we investigated whether estradiol increases neurogenesis in the SVZ in an animal model of stroke to potentially promote the ability of the brain to undergo repair. Ovariectomized C57BL/6J mice were implanted with capsules containing either vehicle or 17beta-estradiol, and 1 week later they underwent experimental ischemia. We utilized double-label immunocytochemistry to identify the phenotype of newborn cells (5-bromo-2'-deoxyuridine-labeled) with various cellular markers; doublecortin and PSA-NCAM as the early neuronal marker, NeuN to identify mature neurons, and glial fibrillary acidic protein to identify astrocytes. We report that low physiological levels of estradiol treatment, which exert no effect in the uninjured state, significantly increase the number of newborn neurons in the SVZ following stroke injury. This effect of estradiol is limited to the dorsal region of the SVZ and is absent from the ventral SVZ. The proliferative actions of estradiol are confined to neuronal precursors and do not influence gliosis. Furthermore, we show that both estrogen receptors alpha and beta play pivotal functional roles, insofar as knocking out either of these receptors blocks the ability of estradiol to increase neurogenesis. These findings clearly demonstrate that estradiol stimulates neurogenesis in the adult SVZ, thus potentially facilitating the brain to remodel and repair after injury.