CD44 in human glioma correlates with histopathological grade and cell migration

The role of CD44 in glioblastoma multiforme

A transmembrane molecule with several isoforms, CD44 is overexpressed in many tumors and promotes tumor formation through interactions with the tumor microenvironment. CD44 has been implicated in malignant processes including cell motility, tumor growth, and angiogenesis. The role of CD44 has been examined in many cancer types. This paper provides, to our knowledge, the first focused review of the role of CD44 in glioblastoma multiforme (GBM), the most common and fatal of primary brain cancers. We summarize research that describes how CD44 promotes GBM aggressiveness by increasing tumor cell invasion, proliferation and resistance to standard chemoradiation therapy. Effects of CD44 inhibition in GBM are also explored. Clinical trials investigating CD44 targeting in CD44-positive solid tumors are underway, and the evidence presented here suggests that CD44 inhibition in GBM may be a promising therapy.

Hyaluronic acid-conjugated liposome nanoparticles for targeted delivery to CD44 overexpressing glioblastoma cells

Glioblastoma Multiforme (GBM) is a highly prevalent and deadly brain malignancy characterized by poor prognosis and restricted disease management potential. Despite the success of nanocarrier systems to improve drug/gene therapy for cancer, active targeting specificity remains a major hurdle for GBM. Additionally, since the brain is a multi-cell type organ, there is a critical need to develop an approach to distinguish between GBM cells and healthy brain cells for safe and successful treatment. In this report, we have incorporated hyaluronic acid (HA) as an active targeting ligand for GBM. To do so, we employed HA conjugated liposomes (HALNPs) to study the uptake pathway in key cells in the brain including primary astrocytes, microglia, and human GBM cells. We observed that the HALNPs specifically target GBM cells over other brain cells due to higher expression of CD44 in tumor cells. Furthermore, CD44 driven HALNP uptake into GBM cells resulted in lysosomal evasion and increased efficacy of Doxorubicin, a model anti-neoplastic agent, while the astrocytes and microglia cells exhibited extensive HALNP-lysosome co-localization and decreased antineoplastic potency. In summary, novel CD44 targeted lipid based nanocarriers appear to be proficient in mediating site-specific delivery of drugs via CD44 receptors in GBM cells, with an improved therapeutic margin and safety.

A Unique Procedure to Identify Cell Surface Markers Through a Spherical Self-Organizing Map Applied to DNA Microarray Analysis

To identify cell-specific markers, we designed a DNA microarray platform with oligonucleotide probes for human membrane-anchored proteins. Human glioma cell lines were analyzed using microarray and compared with normal and fetal brain tissues. For the microarray analysis, we employed a spherical self-organizing map, which is a clustering method suitable for the conversion of multidimensional data into two-dimensional data and displays the relationship on a spherical surface. Based on the gene expression profile, the cell surface characteristics were successfully mirrored onto the spherical surface, thereby distinguishing normal brain tissue from the disease model based on the strength of gene expression. The clustered glioma-specific genes were further analyzed by polymerase chain reaction procedure and immunocytochemical staining of glioma cells. Our platform and the following procedure were successfully demonstrated to categorize the genes coding for cell surface proteins that are specific to glioma cells. Our assessment demonstrates that a spherical self-organizing map is a valuable tool for distinguishing cell surface markers and can be employed in marker discovery studies for the treatment of cancer.

miR-373 Inhibits Glioma Cell U251 Migration and Invasion by Down-Regulating CD44 and TGFBR2

Glioblastoma multiforme (GBM) is the most malignant glioma, unveiling the underlying mechanisms of its aggressiveness could promote the discovery of potential targets for effective treatment. MicroRNAs (miRNAs) are important participants in both development and disease, its involvement in cancers has long been recognized. In this study, we investigated the role of miRNA-373 (miR-373) in GBM cell line U251, demonstrated that although miR-373 does not affect cell growth of U251, it inhibits migration and invasion of U251. Forced expression of miR-373 down-regulates the expressions CD44 and TGFBR2, while knockdown of CD44 and TGFBR2 presents the similar phenotype as miR-373 overexpression, suggesting that CD44 and TGFBR2 are functional targets of miR-373, down-regulation of CD44 and TGFBR2 by miR-373 are partly responsible for the migration, and invasion suppressive role of miR-373 in U251.

Autoantibody Profiling of Glioma Serum Samples to Identify Biomarkers Using Human Proteome Arrays

The heterogeneity and poor prognosis associated with gliomas, makes biomarker identification imperative. Here, we report autoantibody signatures across various grades of glioma serum samples and sub-categories of glioblastoma multiforme using Human Proteome chips containing ~17000 full-length human proteins. The deduced sets of classifier proteins helped to distinguish Grade II, III and IV samples from the healthy subjects with 88, 89 and 94% sensitivity and 87, 100 and 73% specificity, respectively. Proteins namely, SNX1, EYA1, PQBP1 and IGHG1 showed dysregulation across various grades. Sub-classes of GBM, based on its proximity to the sub-ventricular zone, have been reported to have different prognostic outcomes. To this end, we identified dysregulation of NEDD9, a protein involved in cell migration, with probable prognostic potential. Another subcategory of patients where the IDH1 gene is mutated, are known to have better prognosis as compared to patients carrying the wild type gene. On a comparison of these two cohorts, we found STUB1 and YWHAH proteins dysregulated in Grade II glioma patients. In addition to common pathways associated with tumourigenesis, we found enrichment of immunoregulatory and cytoskeletal remodelling pathways, emphasizing the need to explore biochemical alterations arising due to autoimmune responses in glioma.

Targeting hyaluronic acid family for cancer chemoprevention and therapy

Hyaluronic acid or hyaluronan (HA) is perhaps one of the most uncomplicated large polymers that regulates several normal physiological processes and, at the same time, contributes to the manifestation of a variety of chronic and acute diseases, including cancer. Members of the HA signaling pathway (HA synthases, HA receptors, and HYAL-1 hyaluronidase) have been experimentally shown to promote tumor growth, metastasis, and angiogenesis, and hence each of them is a potential target for cancer therapy. Furthermore, as these members are also overexpressed in a variety of carcinomas, targeting of the HA family is clinically relevant. A variety of targeted approaches have been developed to target various HA family members, including small-molecule inhibitors and antibody and vaccine therapies. These treatment approaches inhibit HA-mediated intracellular signaling that promotes tumor cell proliferation, motility, and invasion, as well as induction of endothelial cell functions. Being nontoxic, nonimmunogenic, and versatile for modifications, HA has been used in nanoparticle preparations for the targeted delivery of chemotherapy drugs and other anticancer compounds to tumor cells through interaction with cell-surface HA receptors. This review discusses basic and clinical translational aspects of targeting each HA family member and respective treatment approaches that have been described in the literature.

Current perspectives concerning the multimodal therapy in Glioblastoma

GBM (Glioblastoma) is the most common, malignant type of primary brain tumor. It has a dismal prognosis, with an average life expectancy of less than 15 months. A better understanding of the tumor biology of GBM has been achieved in the past decade and set up new directions in the multimodal therapy by targeting the molecular paths involved in tumor initiation and progression. Invasion is a hallmark of GBM, and targeting the complex invasive mechanism of the tumor is mandatory in order to achieve a satisfactory result in GBM therapy. The goal of this review is to describe the tumor biology and key features of GBM and to provide an up-to-date overview of the current identified molecular alterations involved both in tumorigenesis and tumor progression.

Lentivirus-mediated silencing of spindle and kinetochore-associated protein 1 inhibits the proliferation and invasion of neuronal glioblastoma cells

Spindle and kinetochore‑associated protein 1 (SKA1) is an important component of the human kinetochore, which plays a key role in mitosis. The resent study was designed to investigate the role of SKA1 in human glioblastoma. The results of the present study demonstrated that SKA1 was expressed in human glioblastoma cells. In addition, the knockdown of SKA1 expression in the A172 and U251 human glioblastoma cell lines was accomplished using a lentivirus infection method. An MTT assay demonstrated that downregulation of SKA1 may inhibit cell proliferation, without affecting the cell cycle. Furthermore, knockdown of SKA1 expression resulted in reduced cell invasion. The results of the present study indicated that SKA1 may be a potential target protein for antiproliferative and anti‑invasive therapeutic strategies of human glioblastoma.

Recruited brain tumor-derived mesenchymal stem cells contribute to brain tumor progression

The identity of the cells that contribute to brain tumor structure and progression remains unclear. Mesenchymal stem cells (MSCs) have recently been isolated from normal mouse brain. Here, we report the infiltration of MSC-like cells into the GL261 murine glioma model. These brain tumor-derived mesenchymal stem cells (BT-MSCs) are defined with the phenotype (Lin-Sca-1+CD9+CD44+CD166+/-) and have multipotent differentiation capacity. We show that the infiltration of BT-MSCs correlates to tumor progression; furthermore, BT-MSCs increased the proliferation rate of GL261 cells in vitro. For the first time, we report that the majority of GL261 cells expressed mesenchymal phenotype under both adherent and sphere culture conditions in vitro and that the non-MSC population is nontumorigenic in vivo. Although the GL261 cell line expressed mesenchymal phenotype markers in vitro, most BT-MSCs are recruited cells from host origin in both wild-type GL261 inoculated into green fluorescent protein (GFP)-transgenic mice and GL261-GFP cells inoculated into wild-type mice. We show the expression of chemokine receptors CXCR4 and CXCR6 on different recruited cell populations. In vivo, the GL261 cells change marker profile and acquire a phenotype that is more similar to cells growing in sphere culture conditions. Finally, we identify a BT-MSC population in human glioblastoma that is CD44+CD9+CD166+ both in freshly isolated and culture-expanded cells. Our data indicate that cells with MSC-like phenotype infiltrate into the tumor stroma and play an important role in tumor cell growth in vitro and in vivo. Thus, we suggest that targeting BT-MSCs could be a possible strategy for treating glioblastoma patients.

How stemlike are sphere cultures from long-term cancer cell lines? Lessons from mouse glioma models

Cancer stem cells may mediate therapy resistance and recurrence in various types of cancer, including glioblastoma. Cancer stemlike cells can be isolated from long-term cancer cell lines, including glioma lines. Using sphere formation as a model for cancer cell stemness in vitro, we derived sphere cultures from SMA-497, SMA-540, SMA-560, and GL-261 glioma cells. Gene expression and proteomics profiling demonstrated that sphere cultures uniformly showed an elevated expression of stemness-associated genes, notably including CD44. Differences in neural lineage marker expression between nonsphere and sphere cultures were heterogeneous except for a uniform reduction of β-III-tubulin in sphere cultures. All sphere cultures showed slower growth. Self-renewal capacity was influenced by medium conditions but not nonsphere versus sphere culture phenotype. Sphere cultures were more resistant to irradiation, whereas both nonsphere and sphere cultures were highly resistant to temozolomide. Nonsphere cells formed more aggressive tumors in syngeneic mice than sphere cells in all models except SMA-560. There were no major differences in vascularization or infiltration by T cells or microglia/macrophages between nonsphere and sphere cell-derived tumors implanted in syngeneic hosts. Together, these data indicate that mouse glioma cell lines may be induced in vitro to form spheres that acquire features of stemness, but they do not exhibit a uniform biologic phenotype, thereby challenging the view that they represent a superior model system.

RRAD promotes EGFR-mediated STAT3 activation and induces temozolomide resistance of malignant glioblastoma

Glioblastoma multiforme (GBM) is an extremely aggressive brain cancer with a median survival of less than 2 years. GBM is characterized by abnormal activation of receptor tyrosine kinase and constitutively activated STAT3. Although EGFR phosphorylation and STAT3 activation are essential for the maintenance of GBM cancer stem cells, the molecular mechanism underlying endosome-mediated STAT3 activation is not fully understood. In the current study, we showed that GTP-binding protein RRAD (RAS associated with diabetes, RAD) physically associates with EGFR, and EEA1, enhancing the stability and endosome-associated nuclear translocation of EGFR. Functionally, RRAD contributes to the activation of STAT3 and expression of the stem cell factors OCT4, NANOG, and SOX2, thereby enhancing self-renewing ability, tumor sphere formation, EMT, and in vivo tumorigenesis. Most importantly, RRAD contributes to poor survival in patients with GBM. RRAD expression is correlated with temozolomide resistance, and, conversely, depletion of RRAD leads to sensitization of highly temozolomide-resistant GBM cells. Our data collectively support a novel function of RRAD in STAT3 activation and provide evidence that RRAD acts as a positive regulator in the EGFR signaling pathway. These results demonstrate a critical role for RRAD in GBM tumorigenesis and provide a rationale for the development of pharmacologic inhibitors of RRAD in GBM.

PTK7 regulates Id1 expression in CD44-high glioma cells

BACKGROUND

CD44 is a molecular marker associated with molecular subtype and treatment resistance in glioma. More effective therapies will result from approaches aimed at targeting the CD44-high gliomas.

METHODS

Protein tyrosine kinase 7 (PTK7) mRNA expression was analyzed based on The Cancer Genome Atlas glioblastoma dataset. PTK7 expression was depleted through lentivirus-mediated short hairpin RNA knockdown. Terminal deoxynucleotidyl transferase dUTP nick-end labeling was used to evaluate cell apoptosis following PTK7 knockdown. Gene expression analysis was performed on Affymetrix microarray. A nude mice orthotopic tumor model was used to evaluate the in vivo effect of PTK7 depletion.

RESULTS

PTK7 is highly expressed in CD44-high glioblastoma and predicts unfavorable prognosis. PTK7 knockdown attenuated cell proliferation, impaired tumorigenic potential, and induced apoptosis in CD44-high glioma cell lines. Gene expression analysis identified inhibitor of DNA Binding 1 (Id1) gene as a potential downstream effector for PTK7. Overexpression of Id1 mostly restored the cell proliferation and colony formation attenuated by PTK7 depletion. PTK7 enhanced anchorage-independent growth in normal human astrocytes, which was attenuated by Id1 knockdown. Furthermore, PTK7 regulated Id1 expression through modulating TGF-β/Smad signaling, while pharmacological inhibition on TGF-β/Smad signaling or PTK7/Id1 depletion attenuated TGF-β-stimulated cell proliferation. PTK7 depletion consistently reduced Id1 expression, suppressed tumor growth, and induced apoptosis in a murine orthotopic tumor model, which could be translated into prolonged survival in tumor-bearing mice.

CONCLUSIONS

PTK7 regulates Id1 expression in CD44-high glioma cell lines. Targeting PTK7 could be an effective strategy for treating glioma with high CD44 expression.

Transcription factors FOXG1 and Groucho/TLE promote glioblastoma growth

Glioblastoma (GBM) is the most common and deadly malignant brain cancer, with a median survival of less than two years. GBM displays a cellular complexity that includes brain tumour-initiating cells (BTICs), which are considered as potential key targets for GBM therapies. Here we show that the transcription factors FOXG1 and Groucho/TLE are expressed in poorly differentiated astroglial cells in human GBM specimens and in primary cultures of GBM-derived BTICs, where they form a complex. FOXG1 knockdown in BTICs causes downregulation of neural stem/progenitor and proliferation markers, increased replicative senescence, upregulation of astroglial differentiation genes, and decreased BTIC-initiated tumour growth upon intracranial transplantation into host mice. These effects are phenocopied by Groucho/TLE knockdown or dominant-inhibition of the FOXG1:Groucho/TLE complex. These results provide evidence that transcriptional programs regulated by FOXG1 and Groucho/TLE are important for BTIC-initiated brain tumour growth, implicating FOXG1 and Groucho/TLE in GBM tumorigenesis.

Cartilage differentiation in ependymoma: histogenetic considerations on a new case

The presence of cartilage in gliomas is a very unusual finding and has been mainly reported in ependymomas and in astrocytomas. A derivation of cartilage from neuroepithelial cells through a neuroepithelial-mesenchymal transition or directly from blood vessel-associated multipotent stromal elements has been proposed. We herein describe a further case of ependymoma with the presence of cartilage in a child affected by a tumor in the posterior fossa. In this case, only the last recurrence, characterized by focal areas of anaplasia, contained a nodule of cartilage. The immunohistochemical expression of fibronectin, tenascin-C, and CD44 was investigated, and the possible role of these molecules in the process of cartilage formation is discussed. Moreover, the literature on the subject is reviewed.

CD44-mediated adhesion to hyaluronic acid contributes to mechanosensing and invasive motility

The high-molecular-weight glycosaminoglycan, hyaluronic acid (HA), makes up a significant portion of the brain extracellular matrix. Glioblastoma multiforme (GBM), a highly invasive brain tumor, is associated with aberrant HA secretion, tissue stiffening, and overexpression of the HA receptor CD44. Here, transcriptomic analysis, engineered materials, and measurements of adhesion, migration, and invasion were used to investigate how HA/CD44 ligation contributes to the mechanosensing and invasive motility of GBM tumor cells, both intrinsically and in the context of Arg-Gly-Asp (RGD) peptide/integrin adhesion. Analysis of transcriptomic data from The Cancer Genome Atlas reveals upregulation of transcripts associated with HA/CD44 adhesion. CD44 suppression in culture reduces cell adhesion to HA on short time scales (0.5-hour postincubation) even if RGD is present, whereas maximal adhesion on longer time scales (3 hours) requires both CD44 and integrins. Moreover, time-lapse imaging demonstrates that cell adhesive structures formed during migration on bare HA matrices are more short lived than cellular protrusions formed on surfaces containing RGD. Interestingly, adhesion and migration speed were dependent on HA hydrogel stiffness, implying that CD44-based signaling is intrinsically mechanosensitive. Finally, CD44 expression paired with an HA-rich microenvironment maximized three-dimensional invasion, whereas CD44 suppression or abundant integrin-based adhesion limited it. These findings demonstrate that CD44 transduces HA-based stiffness cues, temporally precedes integrin-based adhesion maturation, and facilitates invasion.

IMPLICATIONS

This study reveals that the CD44 receptor, which is commonly overexpressed in GBM tumors, is critical for cell adhesion, invasion, and mechanosensing of an HA-based matrix.

Effects of tyrosine kinase inhibitors and CXCR4 antagonist on tumor growth and angiogenesis in rat glioma model: MRI and protein analysis study

The aim of the study was to determine the antiangiogenic efficacy of vatalanib, sunitinib, and AMD3100 in an animal model of human glioblastoma (GBM) by using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and tumor protein expression analysis. Orthotopic GBM-bearing animals were randomly assigned either to control group or vatalanib, sunitinib, and AMD3100 treatment groups. Following 2 weeks of drug treatment, tumor growth and vascular parameters were measured using DCE-MRI. Expression of different angiogenic factors in tumor extracts was measured using a membrane-based human antibody array kit. Tumor angiogenesis and invasion were determined by immunohistochemistry. DCE-MRI showed a significant increase in tumor size after vatalanib treatment. AMD3100-treated group showed a significant decrease in a number of vascular parameters determined by DCE-MRI. AMD3100 significantly decreased the expression of different angiogenic factors compared to sunitinib or vatalanib; however, there were no significant changes in vascular density among the groups. Sunitinib-treated animals showed significantly higher migration of the invasive cells, whereas in both vatalanib- and AMD3100-treated animals the invasive cell migration distance was significantly lower compared to that of control. Vatalanib and sunitinib resulted in suboptimal therapeutic effect, but AMD3100 treatment resulted in a significant reduction in tumor growth, permeability, interstitial space volume, and invasion of tumor cells in an animal model of GBM.

Cadherin-11 regulates motility in normal cortical neural precursors and glioblastoma

Metastasizing tumor cells undergo a transformation that resembles a process in normal development when non-migratory epithelial cells modulate the expression of cytoskeletal and adhesion proteins to promote cell motility. Here we find a mesenchymal cadherin, Cadherin-11 (CDH11), is increased in cells exiting the ventricular zone (VZ) neuroepithelium during normal cerebral cortical development. When overexpressed in cortical progenitors in vivo, CDH11 causes premature exit from the neuroepithelium and increased cell migration. CDH11 expression is elevated in human brain tumors, correlating with higher tumor grade and decreased patient survival. In glioblastoma, CDH11-expressing tumor cells can be found localized near tumor vasculature. Endothelial cells stimulate TGFβ signaling and CDH11 expression in glioblastoma cells. TGFβ promotes glioblastoma cell motility, and knockdown of CDH11 expression in primary human glioblastoma cells inhibits TGFβ-stimulated migration. Together, these findings show that Cadherin-11 can promote cell migration in neural precursors and glioblastoma cells and suggest that endothelial cells increase tumor aggressiveness by co-opting mechanisms that regulate normal neural development.

TGM2 inhibition attenuates ID1 expression in CD44-high glioma-initiating cells

BACKGROUND

CD44 is a molecular marker associated with cancer stem cell populations and treatment resistance in glioma. More effective therapies will result from approaches aimed at targeting glioma cells high in CD44.

METHODS

Glioma-initiating cell lines were derived from fresh surgical glioblastoma samples. Expression of tissue transglutaminase 2 (TGM2) was attenuated through lentivirus-mediated short hairpin RNA knockdown. MTT assay [(3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] was used to evaluate the growth inhibition induced by TGM2 inhibitor. Terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling was used to evaluate cell apoptosis following TGM2 inhibition. CD44(+) glioma stem cells were sorted by flow cytometry. A nude mice orthotopic xenograft model was used to evaluate the in vivo effect of TGM2 inhibitor.

RESULTS

TGM2 was highly expressed in CD44-high glioblastoma tissues and tumor-derived glioma-initiating cell lines. TGM2 knockdown impaired cell proliferation and induced apoptosis in CD44-high glioma-initiating cell lines. Further studies indicated that expression of inhibitor of DNA binding 1 protein (ID1) is regulated by TGM2 and might be an important mediator for TGM2-regulated cell proliferation in CD44-high glioma-initiating cell lines. TGM2 inhibitor reduces ID1 expression, suppresses cell proliferation, and induces apoptosis in CD44-high glioma-initiating cell lines. Furthermore, TGM2 is highly expressed in CD44(+) glioma stem cells, while pharmacological inhibition of TGM2 activity preferentially eliminates CD44(+) glioma stem cells. Consistently, TGM2 inhibitor treatment reduced ID1 expression and induced apoptosis in our orthotopic mice xenograft model, which can be translated into prolonged median survival in tumor-bearing mice.

CONCLUSIONS

TGM2 regulates ID1 expression in glioma-initiating cell lines high in CD44. Targeting TGM2 could be an effective strategy to treat gliomas with high CD44 expression.

Tumor Suppressor WWOX and p53 Alterations and Drug Resistance in Glioblastomas

Tumor suppressor p53 are frequently mutated in glioblastomas (GBMs) and appears to contribute, in part, to resistance to temozolomide (TMZ) and therapeutic drugs. WW domain-containing oxidoreductase WWOX (FOR or WOX1) is a proapoptotic protein and is considered as a tumor suppressor. Loss of WWOX gene expression is frequently seen in malignant cancer cells due to promoter hypermethylation, genetic alterations, and translational blockade. Intriguingly, ectopic expression of wild type WWOX preferentially induces apoptosis in human glioblastoma cells harboring mutant p53. WWOX is known to physically bind and stabilize wild type p53. Here, we provide an overview for the updated knowledge in p53 and WWOX, and postulate potential scenarios that wild type and mutant p53, or isoforms, modulate the apoptotic function of WWOX. We propose that triggering WWOX activation by therapeutic drugs under p53 functional deficiency is needed to overcome TMZ resistance and induce GBM cell death.

Dynamic changes of CD44 expression from progenitors to subpopulations of astrocytes and neurons in developing cerebellum

We previously reported that CD44-positive cells were candidates for astrocyte precursor cells in the developing cerebellum, because cells expressing high levels of CD44 selected by fluorescence-activated cell sorting (FACS) gave rise only to astrocytes in vitro. However, whether CD44 is a specific cell marker for cerebellar astrocyte precursor cells in vivo is unknown. In this study, we used immunohistochemistry, in situ hybridization, and FACS to analyze the spatial and temporal expression of CD44 and characterize the CD44-positive cells in the mouse cerebellum during development. CD44 expression was observed not only in astrocyte precursor cells but also in neural stem cells and oligodendrocyte precursor cells (OPCs) at early postnatal stages. CD44 expression in OPCs was shut off during oligodendrocyte differentiation. Interestingly, during development, CD44 expression was limited specifically to Bergmann glia and fibrous astrocytes among three types of astrocytes in cerebellum, and expression in astrocytes was shut off during postnatal development. CD44 expression was also detected in developing Purkinje and granule neurons but was limited to granule neurons in the adult cerebellum. Thus, at early developmental stages of the cerebellum, CD44 was widely expressed in several types of precursor cells, and over the course of development, the expression of CD44 became restricted to granule neurons in the adult.