Rhabdomyosarcomas are oncogene addicted to the activation of AVIL.

Rhabdomyosarcoma (RMS) is one of the most common pediatric soft-tissue cancer. Previously, we discovered a gene fusion, MARS-AVIL formed by chromosomal inversion in RMS. Suspecting that forming a fusion with a housekeeping gene may be one of the mechanisms to dysregulate an oncogene, we investigated AVIL expression and its role in RMS. We first showed that MARS-AVIL translates into an in-frame fusion protein, which is critical for RMS cell tumorigenesis. Besides forming a gene fusion with the housekeeping gene, MARS, the AVIL locus is often amplified, and its RNA and protein expression are overexpressed in the majority of RMSs. Tumors with AVIL dysregulation exhibit evidence of oncogene addiction: Silencing MARS-AVIL in cells harboring the fusion, or silencing AVIL in cells with AVIL overexpression, nearly eradicated the cells in culture, as well as inhibited in vivo xenograft growth in mice. Conversely, gain-of-function manipulations of AVIL led to increased cell growth and migration, enhanced foci formation in mouse fibroblasts, and most importantly transformed mesenchymal stem cells in vitro and in vivo. Mechanistically, AVIL seems to serve as a converging node functioning upstream of two oncogenic pathways, PAX3-FOXO1 and RAS, thus connecting two types of RMS associated with these pathways. Interestingly, AVIL is overexpressed in other sarcoma cells as well, and its expression correlates with clinical outcomes, with higher levels of AVIL expression being associated with worse prognosis. AVIL is a bona fide oncogene in RMS, and RMS cells are addicted to its activity.

Structural and functional damage to neuronal nuclei caused by extracellular tau oligomers.

INTRODUCTION: Neuronal nuclei are normally smoothly surfaced. In Alzheimer's disease (AD) and other tauopathies, though, they often develop invaginations. We investigated mechanisms and functional consequences of neuronal nuclear invagination in tauopathies. METHODS: Nuclear invagination was assayed by immunofluorescence in the brain, and in cultured neurons before and after extracellular tau oligomer (xcTauO) exposure. Nucleocytoplasmic transport was assayed in cultured neurons. Gene expression was investigated using nanoString nCounter technology and quantitative reverse transcription polymerase chain reaction. RESULTS: Invaginated nuclei were twice as abundant in human AD as in cognitively normal adults, and were increased in mouse neurodegeneration models. In cultured neurons, nuclear invagination was induced by xcTauOs by an intracellular tau-dependent mechanism. xcTauOs impaired nucleocytoplasmic transport, increased histone H3 trimethylation at lysine 9, and altered gene expression, especially by increasing tau mRNA. DISCUSSION: xcTauOs may be a primary cause of nuclear invagination in vivo, and by extension, impair nucleocytoplasmic transport and induce pathogenic gene expression changes. HIGHLIGHTS: Extracellular tau oligomers (xcTauOs) cause neuronal nuclei to invaginate. xcTauOs alter nucleocytoplasmic transport, chromatin structure, and gene expression. The most upregulated gene is MAPT, which encodes tau. xcTauOs may thus drive a positive feedback loop for production of toxic tau.

Localization, induction, and cellular effects of tau phosphorylated at threonine 217.

INTRODUCTION: Tau phosphorylation at T217 is a promising Alzheimer's disease (AD) biomarker, but its functional consequences were unknown. METHODS: Human brain and cultured mouse neurons were analyzed by immunoblotting and immunofluorescence for total tau, taupT217 , taupT181 , taupT231 , and taupS396/pS404 . Direct stochastic optical reconstruction microscopy (dSTORM) super resolution microscopy was used to localize taupT217 in cultured neurons. Enhanced green fluorescent protein (EGFP)-tau was expressed in fibroblasts as wild type and T217E pseudo-phosphorylated tau, and fluorescence recovery after photobleaching (FRAP) reported tau turnover rates on microtubules. RESULTS: In the brain, taupT217 appears in neurons at Braak stages I and II, becomes more prevalent later, and co-localizes partially with other phospho-tau epitopes. In cultured neurons, taupT217 is increased by extracellular tau oligomers (xcTauOs) and is associated with developing post-synaptic sites. FRAP recovery was fastest for EGFP-tauT217E . CONCLUSION: TaupT217 increases in the brain as AD progresses and is induced by xcTauOs. Post-synaptic taupT217 suggests a role for T217 phosphorylation in synapse impairment. T217 phosphorylation reduces tau's affinity for microtubules. HIGHLIGHTS: Validation of anti-tau phosphorylated at threonine-217 (taupT217 ) specificity is essential due to epitope redundancy. taupT217 increases as Alzheimer's disease progresses and is found throughout diseased neurons. taupT217 is associated with developing post-synaptic sites in cultured neurons. Extracellular oligomers of tau, but not amyloid beta, increase intracellular taupT217 . T217E pseudo-phosphorylation reduces tau's affinity for microtubules.

Structural and functional features of central nervous system lymphatic vessels.

One of the characteristics of the central nervous system is the lack of a classical lymphatic drainage system. Although it is now accepted that the central nervous system undergoes constant immune surveillance that takes place within the meningeal compartment, the mechanisms governing the entrance and exit of immune cells from the central nervous system remain poorly understood. In searching for T-cell gateways into and out of the meninges, we discovered functional lymphatic vessels lining the dural sinuses. These structures express all of the molecular hallmarks of lymphatic endothelial cells, are able to carry both fluid and immune cells from the cerebrospinal fluid, and are connected to the deep cervical lymph nodes. The unique location of these vessels may have impeded their discovery to date, thereby contributing to the long-held concept of the absence of lymphatic vasculature in the central nervous system. The discovery of the central nervous system lymphatic system may call for a reassessment of basic assumptions in neuroimmunology and sheds new light on the aetiology of neuroinflammatory and neurodegenerative diseases associated with immune system dysfunction.

Focused Ultrasound Preconditioning for Augmented Nanoparticle Penetration and Efficacy in the Central Nervous System.

Microbubble activation with focused ultrasound (FUS) facilitates the noninvasive and spatially-targeted delivery of systemically administered therapeutics across the blood-brain barrier (BBB). FUS also augments the penetration of nanoscale therapeutics through brain tissue; however, this secondary effect has not been leveraged. Here, 1 MHz FUS sequences that increase the volume of transfected brain tissue after convection-enhanced delivery of gene-vector "brain-penetrating" nanoparticles were first identified. Next, FUS preconditioning is applied prior to trans-BBB nanoparticle delivery, yielding up to a fivefold increase in subsequent transgene expression. Magnetic resonance imaging (MRI) analyses of tissue temperature and Ktrans confirm that augmented transfection occurs through modulation of parenchymal tissue with FUS. FUS preconditioning represents a simple and effective strategy for markedly improving the efficacy of gene vector nanoparticles in the central nervous system.

Phosphatidylserine receptor BAI1 and apoptotic cells as new promoters of myoblast fusion.

Skeletal muscle arises from the fusion of precursor myoblasts into multinucleated myofibres. Although conserved transcription factors and signalling proteins involved in myogenesis have been identified, upstream regulators are less well understood. Here we report an unexpected discovery that the membrane protein BAI1, previously linked to recognition of apoptotic cells by phagocytes, promotes myoblast fusion. Endogenous BAI1 expression increased during myoblast fusion, and BAI1 overexpression enhanced myoblast fusion by means of signalling through ELMO/Dock180/Rac1 proteins. During myoblast fusion, a fraction of myoblasts within the population underwent apoptosis and exposed phosphatidylserine, an established ligand for BAI1 (ref. 3). Blocking apoptosis potently impaired myoblast fusion, and adding back apoptotic myoblasts restored fusion. Furthermore, primary human myoblasts could be induced to form myotubes by adding apoptotic myoblasts, even under normal growth conditions. Mechanistically, apoptotic cells did not directly fuse with the healthy myoblasts, rather the apoptotic cells induced a contact-dependent signalling with neighbours to promote fusion among the healthy myoblasts. In vivo, myofibres from Bai1(-/-) mice are smaller than those from wild-type littermates. Muscle regeneration after injury was also impaired in Bai1(-/-)mice, highlighting a role for BAI1 in mammalian myogenesis. Collectively, these data identify apoptotic cells as a new type of cue that induces signalling via the phosphatidylserine receptor BAI1 to promote fusion of healthy myoblasts, with important implications for muscle development and repair.

Proteomic identification of synaptic caspase substrates.

The dismantling and elimination of excess neurons and their connections (pruning) is essential for brain development and may be aberrantly reactivated in some neurodegenerative diseases. Growing evidence implicates caspase-mediated apoptotic and nonapoptotic cascades in the dysfunction and death of neurons in neurodegenerative disorders such as Alzheimer's, Parkinson, and Huntington's diseases. It is the cleaved caspase substrates that are the effectors of synapse elimination. However, their identities, specific cleavage sites, and functional consequences of cleavage are largely unknown. An important gap in our knowledge is a comprehensive catalog of synapse-specific or synapse-enriched caspase targets. Traditional biochemical approaches have revealed only a small number of neuronal caspase targets. Instead, we utilized a gel-based proteomics approach to enable the first global analysis of caspase-mediated cleavage events in mammalian brain synapses, employing both an in vitro system with recombinant activated caspases and an in vivo model of ethanol-induced neuronal apoptosis. Of the more than 70 putative cleavage substrates that were identified, 22 were previously known caspase substrates. Among the novel targets identified and validated by Western blot were the proton pump ATPase subunit ATP6V1B2 and the N-ethylmaleimide-sensitive fusion protein (NSF). Our work represents the first comprehensive, proteome-wide screen for proteolytic targets of caspases in neuronal synapses. Our discoveries will have significance for both furthering basic understanding of roles of caspases in synaptic plasticity and synaptic loss in neurodegeneration, and on a more immediately practical level, may provide candidate biomarkers for measuring synapse loss in human disease states.

Novel Focused Ultrasound Gene Therapy Approach Noninvasively Restores Dopaminergic Neuron Function in a Rat Parkinson's Disease Model.

Therapies capable of decelerating, or perhaps even halting, neurodegeneration in Parkinson's disease (PD) remain elusive. Clinical trials of PD gene therapy testing the delivery of neurotrophic factors, such as the glial cell-line derived neurotrophic factor (GDNF), have been largely ineffective due to poor vector distribution throughout the diseased regions in the brain. In addition, current delivery strategies involve invasive procedures that obviate the inclusion of early stage patients who are most likely to benefit from GDNF-based gene therapy. Here, we introduce a two-pronged treatment strategy, composed of MR image-guided focused ultrasound (FUS) and brain-penetrating nanoparticles (BPN), that provides widespread but targeted GDNF transgene expression in the brain following systemic administration. MR image-guided FUS allows circulating gene vectors to partition into the brain tissue by noninvasive and transient opening of the blood-brain barrier (BBB) within the areas where FUS is applied. Once beyond the BBB, BPN provide widespread and uniform GDNF expression throughout the targeted brain tissue. After only a single treatment, our strategy led to therapeutically relevant levels of GDNF protein content in the FUS-targeted regions in the striatum of the 6-OHDA-induced rat model of PD, which lasted at least up to 10 weeks. Importantly, our strategy restored both dopamine levels and dopaminergic neuron density and reversed behavioral indicators of PD-associated motor dysfunction with no evidence of local or systemic toxicity. Our combinatorial approach overcomes limitations of current delivery strategies, thereby potentially providing a novel means to treat PD.

mTOR and neuronal cell cycle reentry: How impaired brain insulin signaling promotes Alzheimer's disease.

A major obstacle to presymptomatic diagnosis and disease-modifying therapy for Alzheimer's disease (AD) is inadequate understanding of molecular mechanisms of AD pathogenesis. For example, impaired brain insulin signaling is an AD hallmark, but whether and how it might contribute to the synaptic dysfunction and neuron death that underlie memory and cognitive impairment has been mysterious. Neuron death in AD is often caused by cell cycle reentry (CCR) mediated by amyloid-ß oligomers (AßOs) and tau, the precursors of plaques and tangles. We now report that CCR results from AßO-induced activation of the protein kinase complex, mTORC1, at the plasma membrane and mTORC1-dependent tau phosphorylation, and that CCR can be prevented by insulin-stimulated activation of lysosomal mTORC1. AßOs were also shown previously to reduce neuronal insulin signaling. Our data therefore indicate that the decreased insulin signaling provoked by AßOs unleashes their toxic potential to cause neuronal CCR, and by extension, neuron death.

Expression of lncRNAs in Low-Grade Gliomas and Glioblastoma Multiforme: An In Silico Analysis.

BACKGROUND: Each year, over 16,000 patients die from malignant brain cancer in the US. Long noncoding RNAs (lncRNAs) have recently been shown to play critical roles in regulating neurogenesis and brain tumor progression. To better understand the role of lncRNAs in brain cancer, we performed a global analysis to identify and characterize all annotated and novel lncRNAs in both grade II and III gliomas as well as grade IV glioblastomas (glioblastoma multiforme [GBM]). METHODS AND FINDINGS: We determined the expression of all lncRNAs in over 650 brain cancer and 70 normal brain tissue RNA sequencing datasets from The Cancer Genome Atlas (TCGA) and other publicly available datasets. We identified 611 induced and 677 repressed lncRNAs in glial tumors relative to normal brains. Hundreds of lncRNAs were specifically expressed in each of the three lower grade glioma (LGG) subtypes (IDH1/2 wt, IDH1/2 mut, and IDH1/2 mut 1p19q codeletion) and the four subtypes of GBMs (classical, mesenchymal, neural, and proneural). Overlap between the subtype-specific lncRNAs in GBMs and LGGs demonstrated similarities between mesenchymal GBMs and IDH1/2 wt LGGs, with 2-fold higher overlap than would be expected by random chance. Using a multivariate Cox regression survival model, we identified 584 and 282 lncRNAs that were associated with a poor and good prognosis, respectively, in GBM patients. We developed a survival algorithm for LGGs based on the expression of 64 lncRNAs that was associated with patient prognosis in a test set (hazard ratio [HR] = 2.168, 95% CI = 1.765-2.807, p < 0.001) and validation set (HR = 1.921, 95% CI = 1.333-2.767, p < 0.001) of patients from TCGA. The main limitations of this study are that further work is needed to investigate the clinical relevance of our findings, and that validation in an independent dataset is needed to determine the robustness of our survival algorithm. CONCLUSIONS: This work identifies a panel of lncRNAs that appear to be prognostic in gliomas and provides a critical resource for future studies examining the role of lncRNAs in brain cancers.

Disseminated Balamuthia mandrillaris Infection.

Balamuthia mandrillaris is a rare cause of human infection, but when infections do occur, they result in high rates of morbidity and mortality. A case of disseminated Balamuthia infection is presented. Early diagnosis and initiation of recommended therapy are essential for increased chances of successful outcomes.

BAI1 is an engulfment receptor for apoptotic cells upstream of the ELMO/Dock180/Rac module.

Engulfment and subsequent degradation of apoptotic cells is an essential step that occurs throughout life in all multicellular organisms. ELMO/Dock180/Rac proteins are a conserved signalling module for promoting the internalization of apoptotic cell corpses; ELMO and Dock180 function together as a guanine nucleotide exchange factor (GEF) for the small GTPase Rac, and thereby regulate the phagocyte actin cytoskeleton during engulfment. However, the receptor(s) upstream of the ELMO/Dock180/Rac module are still unknown. Here we identify brain-specific angiogenesis inhibitor 1 (BAI1) as a receptor upstream of ELMO and as a receptor that can bind phosphatidylserine on apoptotic cells. BAI1 is a seven-transmembrane protein belonging to the adhesion-type G-protein-coupled receptor family, with an extended extracellular region and no known ligands. We show that BAI1 functions as an engulfment receptor in both the recognition and subsequent internalization of apoptotic cells. Through multiple lines of investigation, we identify phosphatidylserine, a key 'eat-me' signal exposed on apoptotic cells, as a ligand for BAI1. The thrombospondin type 1 repeats within the extracellular region of BAI1 mediate direct binding to phosphatidylserine. As with intracellular signalling, BAI1 forms a trimeric complex with ELMO and Dock180, and functional studies suggest that BAI1 cooperates with ELMO/Dock180/Rac to promote maximal engulfment of apoptotic cells. Last, decreased BAI1 expression or interference with BAI1 function inhibits the engulfment of apoptotic targets ex vivo and in vivo. Thus, BAI1 is a phosphatidylserine recognition receptor that can directly recruit a Rac-GEF complex to mediate the uptake of apoptotic cells.

Structural and functional damage to neuronal nuclei caused by extracellular tau oligomers.

INTRODUCTION: Neuronal nuclei are normally smoothly surfaced. In Alzheimer's disease (AD) and other tauopathies, though, they often develop invaginations. We investigated mechanisms and functional consequences of neuronal nuclear invagination in tauopathies. METHODS: Nuclear invagination was assayed by immunofluorescence in brain, and in cultured neurons before and after extracellular tau oligomers (xcTauO) exposure. Nucleocytoplasmic transport was assayed in cultured neurons. Gene expression was investigated using nanoString nCounter technology and qRT-PCR. RESULTS: Invaginated nuclei were twice as abundant in human AD as in cognitively normal adults, and were increased in mouse neurodegeneration models. In cultured neurons, nuclear invagination was induced by xcTauOs by an intracellular tau-dependent mechanism. xcTauOs impaired nucleocytoplasmic transport, increased histone H3 trimethylation at lysine 9 and altered gene expression, especially by increasing tau mRNA. DISCUSSION: xcTauOs may be a primary cause of nuclear invagination in vivo, and by extension, impair nucleocytoplasmic transport and induce pathogenic gene expression changes.

Extracellular Tau Oligomers Damage the Axon Initial Segment.

BACKGROUND: In Alzheimer's disease (AD) brain, neuronal polarity and synaptic connectivity are compromised. A key structure for regulating polarity and functions of neurons is the axon initial segment (AIS), which segregates somatodendritic from axonal proteins and initiates action potentials. Toxic tau species, including extracellular oligomers (xcTauOs), spread tau pathology from neuron to neuron by a prion-like process, but few other cell biological effects of xcTauOs have been described. OBJECTIVE: Test the hypothesis that AIS structure is sensitive to xcTauOs. METHODS: Cultured wild type (WT) and tau knockout (KO) mouse cortical neurons were exposed to xcTauOs, and quantitative western blotting and immunofluorescence microscopy with anti-TRIM46 monitored effects on the AIS. The same methods were used to compare TRIM46 and two other resident AIS proteins in human hippocampal tissue obtained from AD and age-matched non-AD donors. RESULTS: Without affecting total TRIM46 levels, xcTauOs reduce the concentration of TRIM46 within the AIS and cause AIS shortening in cultured WT, but not TKO neurons. Lentiviral-driven tau expression in tau KO neurons rescues AIS length sensitivity to xcTauOs. In human AD hippocampus, the overall protein levels of multiple resident AIS proteins are unchanged compared to non-AD brain, but TRIM46 concentration within the AIS and AIS length are reduced in neurons containing neurofibrillary tangles. CONCLUSION: xcTauOs cause partial AIS damage in cultured neurons by a mechanism dependent on intracellular tau, thereby raising the possibility that the observed AIS reduction in AD neurons in vivo is caused by xcTauOs working in concert with endogenous neuronal tau.

Phagocytic clearance in neurodegeneration.

The cellular and molecular mechanisms of phagocytic clearance of apoptotic cells and debris have been intensely studied in invertebrate model organisms and in the mammalian immune system. This evolutionarily conserved process serves multiple purposes. Uncleared debris from dying cells or aggregated proteins can be toxic and may trigger exaggerated inflammatory responses. Even though apoptotic cell death and debris accumulation are key features of neurodegenerative diseases, relatively little attention has been paid to this important homeostatic function in the central nervous system (CNS). This review attempts to summarize our knowledge of phagocytic clearance in the CNS, with a focus on retinal degeneration, forms of which are caused by mutations in genes within known phagocytic pathways, and on Alzheimer's disease (AD). Interest in phagocytic clearance mechanisms in AD was stimulated by the discovery that immunization could promote phagocytic clearance of amyloid-ß; however, much less is known about clearance of neuronal and synaptic corpses in AD and other neurodegenerative diseases. Because the regulation of phagocytic activity is intertwined with cytokine signaling, this review also addresses the relationships among CNS inflammation, glial responses, and phagocytic clearance.

Brain-specific angiogenesis inhibitor-1 expression in astrocytes and neurons: implications for its dual function as an apoptotic engulfment receptor.

Brain-specific angiogenesis inhibitor-1 (BAI1) is a transmembrane protein highly expressed in normal brain that has been ascribed two apparently distinct functions: inhibition of angiogenesis and recognition and engulfment of apoptotic cells by phagocytes. A previous localization study reported BAI1 expression only in neurons. Because a phagocytic function of BAI1 could be important for neuroglial antigen processing and presentation, we performed immunolocalization studies in adult mouse brain and cultured neural cells, using a pair of antibodies directed against N- and C-terminal epitopes. BAI1 immunoreactivity is enriched in gray matter structures and largely excluded from myelinated axon tracts. Neuronal BAI1 expression was readily detectable in the cerebellar molecular layer as well as in primary hippocampal cultures. In some brain regions, especially olfactory bulb glomeruli, BAI1 was expressed by GFAP-positive astrocytes. Cultured cortical astrocytes show small (∼0.4µm(2)) BAI1 immunoreactive membrane puncta as well as prominent focal adhesion localization in a subset of cells. In mixed neuronal-glial cultures, BAI1-expressing astrocytes frequently contained engulfed apoptotic debris. Cultured astrocytes engulfed apoptotic targets, and BAI1 showed accumulation within the phagocytic cup. We hypothesize that glial BAI1 may subserve an engulfment function in adult brain regions such as olfactory bulb with ongoing apoptotic turnover, whereas neuronal-derived BAI1 may serve primarily as an anti-angiogenic factor in the mature neuropil.

Serotonergic dystrophy induced by excess serotonin.

Administration of certain serotonin-releasing amphetamine derivatives (fenfluramine and/or 3,4-methylenedioxymethamphetamine, MDMA, 'ecstasy') results in dystrophic serotonergic morphology in the mammalian brain. In addition to drug administration, dystrophic serotonergic neurites are also associated with neurodegenerative disorders. We demonstrate here that endogenously elevated serotonin in the Drosophila CNS induces aberrant enlarged varicosities, or spheroids, that are morphologically similar to dystrophic mammalian serotonergic fibers. In Drosophila these spheroids are specific to serotonergic neurons, distinct from typical varicosities, and form only after prolonged increases in cytoplasmic serotonin. Our results also suggest that serotonin levels during early development determine later sensitivity of spheroid formation to manipulations of the serotonin transporter (SERT). Elevated serotonin also interacts with canonical protein aggregation and autophagic pathways to form spheroids. The data presented here support a model in which excess cytoplasmic neurotransmitter triggers a cell-specific pathway inducing aberrant morphology in fly serotonergic neurons that may be shared in certain mammalian pathologies.

FGFR1-TACC1 fusion associated with malignant transformation in a primary spinal cord glioma: a case report.

Molecular mechanisms of malignant transformation in spinal cord gliomas are not well-understood. Our objective was to investigate genetic causes of malignant transformation in a primary spinal cord glioma. A 32-year-old female patient presented with bilateral lower extremity weakness and was diagnosed with a primary spinal cord glioma from T9 to T12, with a syrinx extending from the craniocervical junction to the conus. She underwent resection in 2006. Pathology showed an abundance of Rosenthal fibers, calcification and degenerative features consistent with a low-grade pilocytic astrocytoma. She presented in 2020 with tumor recurrence and underwent re-resection. Whole exome sequencing, DNA methylation profiling and immunohistochemistry were performed on her initial and recurrent tumor samples. Immunohistochemical profiling of her recurrent tumor showed pleomorphic cells with extensive necrosis consistent with a high-grade glioma. DNA methylation profiling showed that the initial tumor clustered with pilocytic astrocytomas, whereas the recurrent lesion clustered with anaplastic astrocytomas, confirming malignant transformation. Whole-exome sequencing showed interim acquisition of a rare fibroblast growth factor receptor-transforming acidic coiled-coil (FGFR1-TACC1) gene fusion. We report an FGFR1-TACC1 fusion associated with malignant transformation in a primary spinal cord glioma. Our study adds to growing reports of FGFR-TACC fusions, which are amenable to receptor tyrosine kinase inhibition.

Brain-specific deletion of extracellular signal-regulated kinase 2 mitogen-activated protein kinase leads to aberrant cortical collagen deposition.

The mitogen-activated protein kinases extracellular signal-regulated kinase (ERK)1 and 2 are essential intracellular mediators of numerous transmembrane signals. To investigate neural-specific functions of ERK2 in the brain, we used a Cre/lox strategy using Nestin:Cre to drive recombination in neural precursor cells. Nestin:Cre;ERK2(fl/fl) conditional knockout (cKO) mice have architecturally normal brains and no gross behavioral deficits. However, all cKO mice developed early-onset (postnatal day 35 to 40) frontal cortical astrogliosis, without evidence of neuronal degeneration. Frontoparietal cortical gray matter, but not underlying white matter, was found to contain abundant pericapillary and parenchymal reticulin fibrils, which were shown by immunohistochemistry to contain fibrillar collagens, including type I collagen. ERK1 general KO mice showed neither fibrils nor astrogliosis, indicating a specific role for ERK2 in the regulation of brain collagen. Collagen fibrils were also observed to a lesser extent in GFAP:Cre;ERK2(fl/fl) mice but not in CamKII-Cre;ERK2(fl/fl) mice (pyramidal neuron specific), consistent with a possible astroglial origin. Primary astroglial cultures from cKO mice expressed elevated fibrillar collagen levels, providing further evidence that the phenotype may be cell autonomous for astroglia. Unlike most other tissues, brain and spinal cord parenchyma do not normally contain fibrillar collagens, except in disease states. Determining mechanisms of ERK2-mediated collagen regulation may enable targeted suppression of glial scar formation in diverse neurological disorders.