Focal malformations of cortical development: new vistas for molecular pathogenesis.

Focal malformations of cortical development (FMCD) are highly associated with several neurological disorders including intractable epilepsy and neurocognitive disabilities. Over the past decade, several FMCD subtypes have been linked to hyperactivation of the mammalian target of rapamycin (mTOR) signaling cascade. In view of the roles that mTOR plays in cell proliferation, size, motility, and stem cell phenotype, many of the features of FMCD such as cytomegaly, disorganized lamination, and expression of stem cell markers can be explained by enhanced mTOR signaling. FMCD result from several distinct and fascinating molecular mechanisms including biallelic gene inactivation, somatic mutation, and potentially, viral infection. These mechanisms have been directly linked to mTOR activation. Perhaps most compelling, pharmacological inhibition of mTOR has been implemented successfully in clinical trials for select FMCD and provides a new vista for treatment.

Inflammatory processes in cortical tubers and subependymal giant cell tumors of tuberous sclerosis complex.

Cortical tubers and subependymal giant cell tumors (SGCT) are two major cerebral lesions associated with tuberous sclerosis complex (TSC). In the present study, we investigated immunocytochemically the inflammatory cell components and the induction of two major pro-inflammatory pathways (the interleukin (IL)-1beta and complement pathways) in tubers and SGCT resected from TSC patients. All lesions were characterized by the prominent presence of microglial cells expressing class II-antigens (HLA-DR) and, to a lesser extent, the presence of CD68-positive macrophages. We also observed perivascular and parenchymal T lymphocytes (CD3(+)) with a predominance of CD8(+) T-cytotoxic/suppressor lymphoid cells. Activated microglia and reactive astrocytes expressed IL-1beta and its signaling receptor IL-1RI, as well as components of the complement cascade, such as C1q, C3c and C3d. Albumin extravasation, with uptake in astrocytes, was observed in both tubers and SGCT, suggesting that alterations in blood brain barrier permeability are associated with inflammation in TSC-associated lesions. Our findings demonstrate a persistent and complex activation of inflammatory pathways in cortical tubers and SGCT.

A neuropathological study of two autopsy cases of syndromic hemimegalencephaly.

Hemimegalencephaly (HMEG) is a malformation of cortical development characterized by unilateral enlargement of the cerebral hemisphere, severe architectural and cellular abnormalities and association with intractable epilepsy. HMEG may represent an isolated lesion of the central nervous system, but may also be associated with several neurocutaneous syndromes. In the present study we discuss the neuropathological findings of two autopsy cases of HMEG associated with linear naevus sebaceous syndrome. Both cases showed the presence of linear naevus sebaceous on extensive areas of the face. The neurochemical profile of the glial and neuronal components in the affected hemisphere was determined using immunocytochemical markers and was compared with the unaffected contralateral hemisphere and normal control tissue. The observed cytomegalic neurones expressed receptors for distinct neurotransmitters, neuropeptides and growth factors. Analysis of components of the phosphoinositide 3-kinase pathway revealed expression of phospho-S6 ribosomal protein in cytomegalic neurones. Autopsy findings confirm the complexity of the histologic phenotypic manifestations in HMEG and proved useful in determining the spectrum of cytoarchitectural and neurochemical abnormalities, underlying the molecular pathogenesis and epileptogenesis of this brain malformation.

Mechanisms of epileptogenicity in cortical dysplasias.

Cortical dysplasias (CDs) increasingly are recognized as pathologic substrates in patients with medically intractable epilepsy. Several studies have demonstrated the intrinsic epileptogenicity of these lesions, but the cellular and molecular mechanisms responsible for seizure initiation remain unknown. The increased availability of surgically resected neocortical tissue has provided the opportunity for direct histopathologic and electrocorticographic correlations. Moreover, the description of various animal models of CDs allowed the testing of various mechanistic hypotheses. It is likely that the mechanisms of epileptogenicity in CDs are multifactorial. In this article, the authors summarize current knowledge of the molecular and cellular mechanisms of epileptogenicity in focal CDs based on human and animal data. In particular, they focus on the roles of glutamate (NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) and gamma-aminobutyric acid receptors identified in animal models and resected human neocortex.

Differential cellular expression of neurotrophins in cortical tubers of the tuberous sclerosis complex.

Neurotrophins and their receptors modulate cerebral cortical development. Tubers in the tuberous sclerosis complex (TSC) are characterized histologically by disorganized cortical cytoarchitecture and thus, we hypothesized that expression of neurotrophin mRNAs and proteins might be altered in tubers. Using in situ transcription and mRNA amplification to probe cDNA arrays, we found that neurotrophin-3 (NT3) and trkB mRNA expression were reduced whereas neurotrophin-4 (NT4) and trkC mRNA expression were increased in whole tuber sections. Alterations in mRNA abundance were defined in single microdissected dysplastic neurons (DNs) and giant cells (GCs). NT3 mRNA expression was reduced in GCs and trkB mRNA expression was reduced in DNs. NT4 mRNA expression was increased in DNs and trkC mRNA expression was increased in both DNs and GCs. In three patients, TSC2 locus mutations were confirmed and the mean tuberin mRNA expression levels was reduced across all nine cases. Consistent with these observations, NT3 mRNA expression was reduced but trkC mRNA expression was increased in vitro in human NTera2 neurons (NT2N) transfected with a tuberin antisense construct that reduced tuberin expression. Western analysis of tuber homogenates and computer-assisted densitometry of immunolabeled sections confirmed the neurotrophin mRNA expression data in whole sections and single neurotrophin immunoreactive cells. We conclude that alterations in NT4/trkB and NT3/trkC expression may contribute to tuber formation during brain development as downstream effects of the hamartin and tuberin pathway in TSC.

Transcription of intermediate filament genes is enhanced in focal cortical dysplasia.

Focal cortical dysplasia (FCD) is characterized by disorganized cerebral cortical cytoarchitecture. Increased expression of several intermediate filament (IF) proteins such as neurofilament, vimentin, alpha-internexin, and nestin observed in dysplastic "balloon" neurons (DN) may contribute to disrupted cortical lamination. We hypothesized that increased IF protein expression results from enhanced IF gene transcription within dysplastic neurons. We used a novel strategy to evaluate IF mRNA expression in three FCD specimens from medically intractable epilepsy patients. Poly(A) mRNA was amplified (aRNA) from single microdissected DN, morphologically normal neurons at the margin of the FCD resection, morphologically normal neurons in non-FCD cortex from epilepsy patients, and normal control neurons. Radiolabeled aRNA from single neurons was used to probe cDNA arrays containing the low (NFL), medium (NFM) and high (NFH) molecular weight neurofilament isoform, alpha-internexin, desmin, vimentin, peripherin (PRPH), nestin, and glial fibrillary acidic protein (GFAP) cDNAs. Hybridization intensity of aRNA-cDNA hybrids was used to quantify relative IF abundance. Increased expression of nestin, alpha-internexin, PRPH, vimentin, NFL, NFM, and NFH mRNAs was found in DN when compared with the three control neuronal subtypes. Desmin and GFAP mRNAs were not detected in any cell types. Expression of PRPH mRNA and protein in select DN was confirmed by reverse transcription-polymerase chain reaction and immunohistochemistry. We conclude that aberrant expression of IF proteins in FCD likely results from enhanced transcription of IF genes in dysplastic neurons and propose that future analysis of transcriptional elements that regulate IF expression be evaluated in FCD.

Postoperative epilepsy in patients undergoing craniotomy for glioblastoma multiforme.

Glioblastoma multiforme (GBM) has associated with it one of the poorest prognoses among brain tumors. Postoperative seizures and the side effects of anticonvulsants, routinely given for prophylactic purposes, add to patient morbidity. The primary goal of this study was to determine who, of those undergoing craniotomy for GBM resection, is at risk for epilepsy. We studied 72 consecutive patients who underwent craniotomy and palliative resection for GBM. Twenty-nine presented with seizures and 17 had postoperative seizures. All patients were treated with a postoperative anticonvulsant for at least six months; anticonvulsants were continued longer if there was a postoperative seizure. Patient factors examined for an association with risk for postoperative seizure included age, sex, tumor size, tumor location, adjuvant therapy, postoperative complications and history of preoperative seizures. The majority of patients with no prior seizure history and who seized postoperatively had their first seizure after withdrawal from their anticonvulsant medication. All, but one, of the patients with both pre- and postoperative seizures had their first postoperative seizure while still on anticonvulsants. Smaller tumor size and frontal resection were associated with an increased risk of postoperative seizures. Our data suggests that those who do not present with seizures and undergo GBM resection may still be prone to seize but more easily protected from postoperative seizures with anticonvulsant therapy than patients who present with seizures; resection of frontal tumors and smaller tumors seemed to indicate an increased risk for postoperative seizures.

Differential expression of glutamate and GABA-A receptor subunit mRNA in cortical dysplasia.

OBJECTIVE: Focal cortical dysplasia is characterized by disorganized cortical lamination, dysplastic and heterotopic neurons, and an association with epilepsy. The contribution that dysplastic and heterotopic neurons make to epileptogenesis in focal cortical dysplasia is unknown and the phenotype of these cells may be distinct. The authors hypothesized that the expression of genes encoding glutamatergic (glutamate [GluR] and N-methyl-D-aspartate NMDA receptors [NR]) and gamma-aminobutyric acid A receptor (GABA(A)R) subunits is distinct in dysplastic and heterotopic neurons and that changes in receptor gene expression could be defined in a cell-specific pattern. METHODS: Single immunohistochemically labeled dysplastic and heterotopic neurons were microdissected from human focal cortical dysplasia specimens obtained during epilepsy surgery. Pyramidal neurons were microdissected from postmortem control cortex and from temporal cortex without dysplasia resected during temporal lobectomy. Poly (A) messenger RNA (mRNA) from single neurons was amplified, radiolabeled, and used to probe complementary DNA (cDNA) arrays containing GluR(1-6), NR(1A,1B), NR(2A-D), and GABA(A)Ralpha(1-6), and -Rbeta(1-3) subunit cDNAS: The relative hybridization intensities of each mRNA-cDNA hybrid were quantified by phosphorimaging. RESULTS: GluR, NR, and GABA(A)R subunit mRNA expression did not differ between control neurons and nondysplastic epilepsy specimens. Expression of GluR(4), NR(2B), and NR(2C) subunit mRNA was increased, and NR(2A) and GABA(A)Rbeta(1) subunit mRNA was decreased in dysplastic compared with pyramidal and heterotopic neurons. In contrast, GABA(A)Ralpha(1), -Ralpha(2), and -Rbeta(2) as well as GluR(1) mRNA levels were reduced in both dysplastic and heterotopic neurons. CONCLUSIONS: Differential expression of GluR, NR, and GABA(A)R mRNA in dysplastic and heterotopic neurons demonstrates cell specific gene transcription changes in focal cortical dysplasia. These results suggest that dysplastic and heterotopic neurons may be pharmacologically distinct and make differential contributions epileptogenesis in focal cortical dysplasia.

Selective alterations in glutamate and GABA receptor subunit mRNA expression in dysplastic neurons and giant cells of cortical tubers.

The molecular pharmacologic basis of epileptogenesis in cortical tubers in the tuberous sclerosis complex is unknown. Altered transcription of genes encoding glutamatergic and gamma-aminobutyric acid (GABA)-ergic receptors and uptake sites may contribute to seizure initiation and may occur selectively in dysplastic neurons and giant cells. Arrays containing GABA A (GABAAR), GluR, NMDA receptor (NR) subunits, GAD65, the vesicular GABA transporter (VGAT), and the neuronal glutamate transporter (EAAC1) cDNAs were probed with amplified poly (A) mRNA from tubers or normal neocortex to identify changes in gene expression. Increased levels of EAAC1, and NR2B and 2D subunit mRNAs and diminished levels of GAD65, VGAT, GluR1, and GABAAR alpha1 and alpha2 were observed in tubers. Ligand-binding experiments in frozen tuber homogenates demonstrated an increase in functional NR2B-containing receptors. Arrays were then probed with poly (A) mRNA from single, microdissected dysplastic neurons, giant cells, or normal neurons (n = 30 each). Enhanced expression of GluR 3, 4, and 6 and NR2B and 2C subunit mRNAs was noted in the dysplastic neurons, whereas only the NR2D mRNA was upregulated in giant cells. GABAAR alpha1 and alpha2 mRNA levels were reduced in both dysplastic neurons and giant cells compared to control neurons. Differential expression of GluR, NR, and GABAAR mRNAs in tubers reflects cell-specific changes in gene transcription that argue for a distinct molecular phenotype of dysplastic neurons and giant cells and suggests that dysplastic neurons and giant cells make differential contributions to epileptogenesis in the tuberous sclerosis complex.

Traumatic brain injury alters the molecular fingerprint of TUNEL-positive cortical neurons In vivo: A single-cell analysis.

The cerebral cortex is selectively vulnerable to cell death after traumatic brain injury (TBI). We hypothesized that the ratio of mRNAs encoding proteins important for cell survival and/or cell death is altered in individual damaged neurons after injury that may contribute to the cell's fate. To investigate this possibility, we used amplified antisense mRNA (aRNA) amplification to examine the relative abundance of 31 selected candidate mRNAs in individual cortical neurons with fragmented DNA at 12 or 24 hr after lateral fluid percussion brain injury in anesthetized rats. Only pyramidal neurons characterized by nuclear terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling (TUNEL) reactivity with little cytoplasmic staining were analyzed. For controls, non-TUNEL-positive neurons from the cortex of sham-injured animals were obtained and subjected to aRNA amplification. At 12 hr after injury, injured neurons exhibited a decrease in the relative abundance of specific mRNAs including those encoding for endogenous neuroprotective proteins. By 24 hr after injury, many of the mRNAs altered at 12 hr after injury had returned to baseline (sham-injured) levels except for increases in caspase-2 and bax mRNAs. These data suggest that TBI induces a temporal and selective alteration in the gene expression profiles or "molecular fingerprints" of TUNEL-positive neurons in the cerebral cortex. These patterns of gene expression may provide information about the molecular basis of cell death in this region after TBI and may suggest multiple avenues for therapeutic intervention.

New developments in the neurobiology of the tuberous sclerosis complex.

OBJECTIVE: To outline recent developments in the neurobiology of the tuberous sclerosis complex (TSC). BACKGROUND: TSC may be associated with neuropsychiatric disorders including epilepsy, mental retardation, and autism. The uncontrolled growth of subependymal giant cell astrocytomas may lead to hydrocephalus and death. The recent identification of mutations in two genes (TSC1 and TSC2) that cause TSC has led to rapid progress in understanding the molecular and cellular pathogenesis of this disorder. How distinct mutations lead to the varied clinical phenotype of TSC is under intense investigation. RESULTS: We report the recent diagnostic criteria for TSC and provide an overview of the molecular genetics, molecular pathophysiology, and neuropathology of TSC. Important diagnostic criteria for TSC include facial angiofibromas, ungual fibromas, retinal hamartomas, and cortical tubers. Both familial and sporadic TSC cases occur. Approximately 50% of TSC families show genetic linkage to TSC1 and 50% to TSC2. Among sporadic TSC cases, mutations in TSC2 are more frequent and often accompanied by more severe neurologic deficits. Multiple mutational subtypes have been identified in the TSC1 and TSC2 genes. The TSC1 (chromosome 9) and TSC2 (chromosome 16) genes encode distinct proteins, hamartin and tuberin, respectively, which are widely expressed in the brain and may interact as part of a cascade pathway that modulates cellular differentiation, tumor suppression, and intracellular signaling. Tuberin has a GTPase activating protein-related domain that may contribute to a role in cell cycle passage and intracellular vesicular trafficking. CONCLUSION: Identification of tuberous sclerosis complex (TSC) gene mutations has fostered understanding of how brain lesions in TSC are formed. Further characterization of the roles of hamartin and tuberin will provide potential therapeutic avenues to treat seizures, mental retardation, and tumor growth in TSC.

Preparation of cDNA from single cells and subcellular regions.

Phenotypic characterization of cells in conjunction with single-cell mRNA analysis, which yields information regarding expression of multiple genes in individual neurons, facilitates a detailed and comprehensive view of neuronal cell biology. More specifically, the aRNA amplification method has provided an approach to analyze mRNA levels in single cells that have been phenotypically characterized on the basis of electrophysiology, morphology, and/or protein expression. In this way, relative mRNA abundances can be directly assayed from a well-defined population of neurons. The concept of expression profiling led to the development of robotics methods for arraying thousands of cDNAs on microarrays. These cDNA arrays can be screened with labeled aRNA or cDNA to generate a molecular fingerprint of a specific cell type, disease state, or therapeutic efficacy. A broad view of how gene expression is altered in single neurons affected by a particular disease process may provide clues to pathogenetic disease mechanisms or avenues for therapeutic interventions. The use of mRNA profiles to produce diagnostics and therapeutics is called transcript-aided drug design (TADD). When coupled with single-cell resolution, TADD promises to be an important tool in diagnosis of disease states, as well as provide a blueprint on which to develop therapeutic strategies. For example, mRNA abundances in an individual diseased cell may increase, decrease, or remain constant, and thus it is possible that a pharmaceutical alone or in combination with other drugs may be specifically designed to restore mRNA abundances to a normal state. Alternatively, if functional protein levels parallel the mRNA level changes, then drugs targeting the function of the proteins translated from these altered mRNAs may prove to be therapeutic. One promise of such an approach is that information about mRNA abundances that are altered in a diseased cell may provide new therapeutic indications for existing drugs. For example, if the abundance of mRNA for the beta-adrenergic receptor is altered as shown by the microarrays for a particular disease, already available adrenergic receptor agonists or antagonists that had not previously been used in this particular disease paradigm may prove to be therapeutically efficacious. The expression profile of a given cell is a measure of the potential for protein expression. Proteins are generally the functional entities within cells and differences in protein function often result in disease. The ability to monitor the coordinate changes in gene expression, in single phenotypically identified cells, that correlate with disease will provide unique insight into the expressed genetic variability of cells and will likely furnish unforeseen insight into the underlying cellular mechanisms that produce disease etiology.

Predominance of neuronal mRNAs in individual Alzheimer's disease senile plaques.

The sequestration of RNA in Alzheimer's disease (AD) senile plaques (SPs) and the production of intraneuronal amyloid-beta peptides (Abeta) prompted analysis of the mRNA profile in single immunocytochemically identified SPs in sections of AD hippocampus. By using amplified RNA expression profiling, polymerase chain reaction, and in situ hybridization, we assessed the presence and abundance of 51 mRNAs that encode proteins implicated in the pathogenesis of AD. The mRNAs in SPs were compared with those in individual CA1 neurons and the surrounding neuropil of control subjects. The remarkable demonstration here, that neuronal mRNAs predominate in SPs, implies that these mRNAs are nonproteinaceous components of SPs, and, moreover, that mRNAs may interact with Abeta protein and that SPs form at sites where neurons degenerate in the AD brain.

Internexin, MAP1B, and nestin in cortical dysplasia as markers of developmental maturity.

Cortical dysplasias (CD) are characterized histologically by disorganized cortical lamination and abnormally shaped neurons. We hypothesized that neurons within CD have failed to differentiate fully and may express proteins such as cytoskeletal elements characteristic of immature cells. Disrupted expression of certain cytoskeletal proteins, which have been implicated in neuronal polarity, process outgrowth, and migration, could result in disorganized cortical lamination. Thus, we probed two CD subtypes, focal CD (FCD) and hemimegalencephaly (HME), with antibodies specific for cytoskeletal proteins that are developmentally regulated in neural progenitor cells and neurons to define more fully the developmental phenotype of neurons within CD. Microtubule-associated protein 1B (MAP1B) and the intermediate filament (IF) protein nestin are enriched in neural progenitors, whereas MAP2B, phosphorylated and non-phosphorylated forms of medium (NFM) and high (NFH) molecular weight neurofilament (NF) proteins, as well as the light NF subunit (NFL) and IF protein alpha internexin are expressed in developing and mature neurons. Immunolabeling for internexin and MAP1B was more abundant in the most abnormally shaped neurons that populated dysplastic regions than in adjacent regions exhibiting milder cytoarchitectural abnormalities or control cortex. Nestin immunoreactivity was noted in large dysplastic and heterotopic neurons within the deeper cortical layers of CD specimens but not in normal cortex. In contrast, neurons in CD specimens also expressed cytoskeletal markers characteristic of differentiated neurons such as NF subunits and MAP2B. These findings suggest that the cytoarchitectural abnormalities in CD may reflect pathophysiological changes in the developing brain that disrupt expression of several key components of the neuronal cytoskeleton and may contribute to impaired migration of cortical neurons.

Machado-Joseph disease gene product is a cytoplasmic protein widely expressed in brain.

Machado-Joseph disease (MJD) is one of at least six neurodegenerative diseases caused by expansion of a CAG repeat encoding a polyglutamine tract in the disease protein. To study the molecular mechanism of disease, we isolated both normal and expanded repeat MJD1 cDNAs, and generated antiserum against the recombinant gene product, called ataxin-3. Using this antiserum, we demonstrate that in disease tissue, both the normal and mutant ataxin-3 protein are expressed throughout the body and in all regions of the brain examined, including areas generally spared by disease. In brain, certain regions (the striatum, for example) express ataxin-3 in only a limited subset of neurons. Immunolocalization studies in normal and disease brain, and in transfected cells, indicate that ataxin-3 is predominantly a cytoplasmic protein that localizes to neuronal processes as well. We conclude that in MJD, as in other polyglutamine repeat diseases, cellular expression of the disease gene is not itself sufficient to cause neuronal degeneration; other cell-specific factors must be invoked to explain the restricted neuropathology seen in MJD. The restricted expression of ataxin-3 in certain regions, however, may influence the pattern of neurodegeneration and provide clues to the protein's function.

Sequestration of RNA in Alzheimer's disease neurofibrillary tangles and senile plaques.

The polypeptide composition of neurofibrillary tangles (NFTs) and senile plaques (SPs) has been characterized extensively within the Alzheimer's disease (AD) brain. Because few data exist on the nonproteinaceous components of these lesions, we sought to determine if NFTs, neuropil threads (NTs), and SPs contain RNA species. To accomplish this, acridine orange (AO) histofluorescence was employed, alone or in combination with thioflavine S (TS) staining and immunohistochemistry to identify RNAs in paraffin-embedded tissue sections of hippocampus and entorhinal cortex. Postmortem brain samples came from 32 subjects including AD and elderly Down's syndrome (DS) patients, age-matched normal controls, and non-AD diseased controls. AO stained the cytoplasm of normal hippocampal and entorhinal neurons in all of the cases, while NFTs, NTs, and SPs were AO-positive in the same regions of AD and DS brains. Cytoplasmic AO histofluorescence was abolished with RNase, but not DNase or proteinase K, indicating the relative specificity of AO for RNA species. Quantitative analysis of double-labeled sections demonstrated that approximately 80% of TS-positive NFTs also were AO-positive, whereas approximately 55% of TS-stained SPs contained AO labeling. These novel observations demonstrate the presence of RNAs in NFTs, NTs, and SPs.

Cellular and molecular basis of cerebral dysgenesis.

Maldevelopment of the cerebral cortex, cortical dysgenesis (CD), may be associated with epilepsy, mental retardation (MR), and focal or widespread neurologic deficits. The histologic hallmark of CD is disrupted cytoarchitecture, including disorganized lamination, malpositioned neurons with respect to their normal radial orientation, abnormal dendritic arborization, and heterotopic neurons within the white matter. Seizures in these patients are particularly difficult to control with conventional anti-epileptic drugs (AEDs) and may require epilepsy surgery to remove these abnormal foci. Focal CD has been reported in up to 30% of epilepsy surgery specimens and are believed to provide the central pathologic substrate responsible for seizures in these patients. How and why CD results in epileptiform activity is unknown. Advances in understanding the pathogenesis of some types of CD have occurred recently with the cloning genes responsible for a few types of X-linked and autosomal CD. This review will outline the major subtypes of CD, the pathologic findings, and the molecular etiologies for a variety of CD. We will also address recent experimental advances in studying the pathogenesis of CD.

Molecular characterization of the dendritic growth cone: regulated mRNA transport and local protein synthesis.

The molecular mechanisms that regulate growth cone guidance of dendrite outgrowth remain to be elucidated. We hypothesized that mRNA localization in dendritic growth cones and their local protein synthesis may be important for growth cone functioning. The appearance of 23 of 31 growth cone mRNAs was developmentally regulated. Also, alteration of growth cone morphology affected the relative levels of three mRNAs. Finally, using single dendrite transfection, it was shown that local protein synthesis occurs in dendrites and growth cones. A heterogeneous population of mRNAs exists in dendritic growth cones of cultured hippocampal neurons whose relative abundances are developmentally regulated and can vary with changes in growth cone physiology. The demonstration of protein synthesis in growth cones suggests that translation of the localized mRNAs may contribute to regulation of growth cone motility and dendrite outgrowth.