Genipin Crosslinks the Extracellular Matrix to Rescue Developmental and Degenerative Defects, and Accelerates Regeneration of Peripheral Neurons

The peripheral nervous system (PNS) is essential for proper body function. A high percentage of the population suffer nerve degeneration or peripheral damage. For example, over 40% of patients with diabetes or undergoing chemotherapy develop peripheral neuropathies. Despite this, there are major gaps in the knowledge of human PNS development and therefore, there are no available treatments. Familial Dysautonomia (FD) is a devastating disorder that specifically affects the PNS making it an ideal model to study PNS dysfunction. FD is caused by a homozygous point mutation in ELP1 leading to developmental and degenerative defects in the sensory and autonomic lineages. We previously employed human pluripotent stem cells (hPSCs) to show that peripheral sensory neurons (SNs) are not generated efficiently and degenerate over time in FD. Here, we conducted a chemical screen to identify compounds able to rescue this SN differentiation inefficiency. We identified that genipin, a compound prescribed in Traditional Chinese Medicine for neurodegenerative disorders, restores neural crest and SN development in FD, both in the hPSC model and in a FD mouse model. Additionally, genipin prevented FD neuronal degeneration, suggesting that it could be offered to patients suffering from PNS neurodegenerative disorders. We found that genipin crosslinks the extracellular matrix, increases the stiffness of the ECM, reorganizes the actin cytoskeleton, and promotes transcription of YAP-dependent genes. Finally, we show that genipin enhances axon regeneration in an in vitro axotomy model in healthy sensory and sympathetic neurons (part of the PNS) and in prefrontal cortical neurons (part of the central nervous system, CNS). Our results suggest genipin can be used as a promising drug candidate for treatment of neurodevelopmental and neurodegenerative diseases, and as a enhancer of neuronal regeneration.

Selective retinal ganglion cell loss and optic neuropathy in a humanized mouse model of familial dysautonomia

Familial dysautonomia (FD) is an autosomal recessive neurodegenerative disease caused by a splicing mutation in the gene encoding Elongator complex protein 1 (ELP1, also known as IKBKAP). This mutation results in tissue-specific skipping of exon 20 with a corresponding reduction of ELP1 protein, predominantly in the central and peripheral nervous system. Although FD patients have a complex neurological phenotype caused by continuous depletion of sensory and autonomic neurons, progressive visual decline leading to blindness is one of the most problematic aspects of the disease, as it severely affects their quality of life. To better understand the disease mechanism as well as to test the in vivo efficacy of targeted therapies for FD, we have recently generated a novel phenotypic mouse model, TgFD9; IkbkapΔ20/flox. This mouse exhibits most of the clinical features of the disease and accurately recapitulates the tissue-specific splicing defect observed in FD patients. Driven by the dire need to develop therapies targeting retinal degeneration in FD, herein, we comprehensively characterized the progression of the retinal phenotype in this mouse, and we demonstrated that it is possible to correct ELP1 splicing defect in the retina using the splicing modulator compound (SMC) BPN-15477.

Identification of cyclin D1 as a major modulator of 3-nitropropionic acid-induced striatal neurodegeneration

Mitochondria dysfunction occurs in the aging brain as well as in several neurodegenerative disorders and predisposes neuronal cells to enhanced sensitivity to neurotoxins. 3-nitropropionic acid (3-NP) is a naturally occurring plant and fungal neurotoxin that causes neurodegeneration predominantly in the striatum by irreversibly inhibiting the tricarboxylic acid respiratory chain enzyme, succinate dehydrogenase (SDH), the main constituent of the mitochondria respiratory chain complex II. Significantly, although 3-NP-induced inhibition of SDH occurs in all brain regions, neurodegeneration occurs primarily and almost exclusively in the striatum for reasons still not understood. In rodents, 3-NP-induced striatal neurodegeneration depends on the strain background suggesting that genetic differences among genotypes modulate toxicant variability and mechanisms that underlie 3-NP-induced neuronal cell death. Using the large BXD family of recombinant inbred (RI) strains we demonstrate that variants in Ccnd1 - the gene encoding cyclin D1 - of the DBA/2 J parent underlie the resistance to 3-NP-induced striatal neurodegeneration. In contrast, the Ccnd1 variant inherited from the widely used C57BL/6 J parental strain confers sensitivity. Given that cellular stress triggers induction of cyclin D1 expression followed by cell-cycle re-entry and consequent neuronal cell death, we sought to determine if the C57BL/6 J and DBA/2 J Ccnd1 variants are differentially modulated in response to 3-NP. We confirm that 3-NP induces cyclin D1 expression in striatal neuronal cells of C57BL/6 J, but this response is blunted in the DBA/2 J. We further show that striatal-specific alternative processing of a highly conserved 3'UTR negative regulatory region of Ccnd1 co-segregates with the C57BL/6 J parental Ccnd1 allele in BXD strains and that its differential processing accounts for sensitivity or resistance to 3-NP. Our results indicate that naturally occurring Ccnd1 variants may play a role in the variability observed in neurodegenerative disorders involving mitochondria complex II dysfunction and point to cyclin D1 as a possible therapeutic target.

Striatal Projection Neurons Require Huntingtin for Synaptic Connectivity and Survival

Huntington's disease (HD) is caused by an autosomal dominant polyglutamine expansion mutation of Huntingtin (HTT). HD patients suffer from progressive motor, cognitive, and psychiatric impairments, along with significant degeneration of the striatal projection neurons (SPNs) of the striatum. HD is widely accepted to be caused by a toxic gain-of-function of mutant HTT. However, whether loss of HTT function, because of dominant-negative effects of the mutant protein, plays a role in HD and whether HTT is required for SPN health and function are not known. Here, we delete Htt from specific subpopulations of SPNs using the Cre-Lox system and find that SPNs require HTT for motor regulation, synaptic development, cell health, and survival during aging. Our results suggest that loss of HTT function in SPNs could play a critical role in HD pathogenesis.

Huntington's Disease Pathogenesis Is Modified In Vivo by Alfy/Wdfy3 and Selective Macroautophagy

Despite being an autosomal dominant disorder caused by a known coding mutation in the gene HTT, Huntington's disease (HD) patients with similar trinucleotide repeat mutations can have an age of onset that varies by decades. One likely contributing factor is the genetic heterogeneity of patients that might modify their vulnerability to disease. We report that although the heterozygous depletion of the autophagy adaptor protein Alfy/Wdfy3 has no consequence in control mice, it significantly accelerates age of onset and progression of HD pathogenesis. Alfy is required in the adult brain for the autophagy-dependent clearance of proteinaceous deposits, and its depletion in mice and neurons derived from patient fibroblasts accelerates the aberrant accumulation of this pathological hallmark shared across adult-onset neurodegenerative diseases. These findings indicate that selectively compromising the ability to eliminate aggregated proteins is a pathogenic driver, and the selective elimination of aggregates may confer disease resistance.

ATAT1-enriched vesicles promote microtubule acetylation via axonal transport

Microtubules are polymerized dimers of α- and β-tubulin that underlie a broad range of cellular activities. Acetylation of α-tubulin by the acetyltransferase ATAT1 modulates microtubule dynamics and functions in neurons. However, it remains unclear how this enzyme acetylates microtubules over long distances in axons. Here, we show that loss of ATAT1 impairs axonal transport in neurons in vivo, and cell-free motility assays confirm a requirement of α-tubulin acetylation for proper bidirectional vesicular transport. Moreover, we demonstrate that the main cellular pool of ATAT1 is transported at the cytosolic side of neuronal vesicles that are moving along axons. Together, our data suggest that axonal transport of ATAT1-enriched vesicles is the predominant driver of α-tubulin acetylation in axons.

Lactobacillus plantarum prevents and mitigates alcohol-induced disruption of colonic epithelial tight junctions, endotoxemia, and liver damage by an EGF receptor-dependent mechanism

Pathogenesis of alcohol-related diseases such as alcoholic hepatitis involves gut barrier dysfunction, endotoxemia, and toxin-mediated cellular injury. Here we show that Lactobacillus plantarum not only blocks but also mitigates ethanol (EtOH)-induced gut and liver damage in mice. L. plantarum blocks EtOH-induced protein thiol oxidation, and down-regulation of antioxidant gene expression in colon L. plantarum also blocks EtOH-induced expression of TNF-α, IL-1β, IL-6, monocyte chemotactic protein 1 ( MCP1), C-X-C motif chemokine ligand ( CXCL)1, and CXCL2 genes in colon. Epidermal growth factor receptor (EGFR) signaling mediates the L. plantarum-mediated protection of tight junctions (TJs) and barrier function from acetaldehyde, the EtOH metabolite, in Caco-2 cell monolayers. In mice, doxycycline-mediated expression of dominant negative EGFR blocks L. plantarum-mediated prevention of EtOH-induced TJ disruption, mucosal barrier dysfunction, oxidative stress, and inflammatory response in colon. L. plantarum blocks EtOH-induced endotoxemia as well as EtOH-induced pathologic lesions, triglyceride deposition, oxidative stress, and inflammatory responses in the liver by an EGFR-dependent mechanism. L. plantarum treatment after injury accelerated recovery from EtOH-induced TJ, barrier dysfunction, oxidative stress, and inflammatory response in colon, endotoxemia, and liver damage. Results demonstrate that L. plantarum has both preventive and therapeutic values in treatment of alcohol-induced tissue injury, particularly in alcoholic hepatitis.-Shukla, P. K., Meena, A. S., Manda, B., Gomes-Solecki, M., Dietrich, P., Dragatsis, I., Rao, R. Lactobacillus plantarum prevents and mitigates alcohol-induced disruption of colonic epithelial tight junctions, endotoxemia, and liver damage by an EGF receptor-dependent mechanism.

Effect of early embryonic deletion of huntingtin from pyramidal neurons on the development and long-term survival of neurons in cerebral cortex and striatum

We evaluated the impact of early embryonic deletion of huntingtin (htt) from pyramidal neurons on cortical development, cortical neuron survival and motor behavior, using a cre-loxP strategy to inactivate the mouse htt gene (Hdh) in emx1-expressing cell lineages. Western blot confirmed substantial htt reduction in cerebral cortex of these Emx-htt^KO mice, with residual cortical htt in all likelihood restricted to cortical interneurons of the subpallial lineage and/or vascular endothelial cells. Despite the loss of htt early in development, cortical lamination was normal, as revealed by layer-specific markers. Cortical volume and neuron abundance were, however, significantly less than normal, and cortical neurons showed reduced brain-derived neurotrophic factor (BDNF) expression and reduced activation of BDNF signaling pathways. Nonetheless, cortical volume and neuron abundance did not show progressive age-related decline in Emx-htt^KO mice out to 24 months. Although striatal neurochemistry was normal, reductions in striatal volume and neuron abundance were seen in Emx-htt^KO mice, which were again not progressive. Weight maintenance was normal in Emx-htt^KO mice, but a slight rotarod deficit and persistent hyperactivity were observed throughout the lifespan. Our results show that embryonic deletion of htt from developing pallium does not substantially alter migration of cortical neurons to their correct laminar destinations, but does yield reduced cortical and striatal size and neuron numbers. The Emx-htt^KO mice were persistently hyperactive, possibly due to defects in corticostriatal development. Importantly, deletion of htt from cortical pyramidal neurons did not yield age-related progressive cortical or striatal pathology.

Elimination of huntingtin in the adult mouse leads to progressive behavioral deficits, bilateral thalamic calcification, and altered brain iron homeostasis

Huntington's Disease (HD) is an autosomal dominant progressive neurodegenerative disorder characterized by cognitive, behavioral and motor dysfunctions. HD is caused by a CAG repeat expansion in exon 1 of the HD gene that is translated into an expanded polyglutamine tract in the encoded protein, huntingtin (HTT). While the most significant neuropathology of HD occurs in the striatum, other brain regions are also affected and play an important role in HD pathology. To date there is no cure for HD, and recently strategies aiming at silencing HTT expression have been initiated as possible therapeutics for HD. However, the essential functions of HTT in the adult brain are currently unknown and hence the consequence of sustained suppression of HTT expression is unpredictable and can potentially be deleterious. Using the Cre-loxP system of recombination, we conditionally inactivated the mouse HD gene homologue at 3, 6 and 9 months of age. Here we show that elimination of Htt expression in the adult mouse results in behavioral deficits, progressive neuropathological changes including bilateral thalamic calcification, and altered brain iron homeostasis.

Role of major and brain-specific Sgce isoforms in the pathogenesis of myoclonus-dystonia syndrome

Loss-of-function mutations in SGCE, which encodes ε-sarcoglycan (ε-SG), cause myoclonus-dystonia syndrome (OMIM159900, DYT11). A "major" ε-SG protein derived from CCDS5637.1 (NM_003919.2) and a "brain-specific" protein, that includes sequence derived from alternative exon 11b (CCDS47642.1, NM_001099400.1), are reportedly localized in post- and pre-synaptic membrane fractions, respectively. Moreover, deficiency of the "brain-specific" isoform and other isoforms derived from exon 11b may be central to the pathogenesis of DYT11. However, no animal model supports this hypothesis. Gene-trapped ES cells (CMHD-GT_148G1-3, intron 9 of NM_011360) were used to generate a novel Sgce mouse model (C57BL/6J background) with markedly reduced expression of isoforms derived from exons 3' to exon 9 of NM_011360. Among those brain regions analyzed in adult (2month-old) wild-type (WT) mice, cerebellum showed the highest relative expression of isoforms incorporating exon 11b. Homozygotes (Sgce^{Gt(148G1)Cmhd/Gt(148G1)Cmhd} or Sgce^Gt/Gt) and paternal heterozygotes (Sgce^m+/pGt, m-maternal, p-paternal) showed 60 to 70% reductions in expression of total Sgce. Although expression of the major (NM_011360) and brain-specific (NM_001130189) isoforms was markedly reduced, expression of short isoforms was preserved and relatively small amounts of chimeric ε-SG/β-galactosidase fusion protein was produced by the Sgce gene-trap locus. Immunoaffinity purification followed by mass spectrometry assessments of Sgce^m+/pGt mouse brain using pan- or brain-specific ε-SG antibodies revealed significant reductions of ε-SG and other interacting sarcoglycans. Genome-wide gene-expression data using RNA derived from adult Sgce^m+/pGt mouse cerebellum showed that the top up-regulated genes were involved in cell cycle, cellular development, cell death and survival, while the top down-regulated genes were associated with protein synthesis, cellular development, and cell death and survival. In comparison to WT littermates, Sgce^m+/pGt mice exhibited "tiptoe" gait and stimulus-induced appendicular posturing between Postnatal Days 14 to 16. Abnormalities noted in older Sgce^m+/pGt mice included reduced body weight, altered gait dynamics, and reduced open-field activity. Overt spontaneous or stimulus-sensitive myoclonus was not apparent on the C57BL/6J background or mixed C57BL/6J-BALB/c and C57BL/6J-129S2 backgrounds. Our data confirm that mouse Sgce is a maternally imprinted gene and suggests that short Sgce isoforms may compensate, in part, for deficiency of major and brain-specific Sgce isoforms.

Familial Dysautonomia: Mechanisms and Models

Hereditary Sensory and Autonomic Neuropathies (HSANs) compose a heterogeneous group of genetic disorders characterized by sensory and autonomic dysfunctions. Familial Dysautonomia (FD), also known as HSAN III, is an autosomal recessive disorder that affects 1/3,600 live births in the Ashkenazi Jewish population. The major features of the disease are already present at birth and are attributed to abnormal development and progressive degeneration of the sensory and autonomic nervous systems. Despite clinical interventions, the disease is inevitably fatal. FD is caused by a point mutation in intron 20 of the IKBKAP gene that results in severe reduction in expression of IKAP, its encoded protein. In vitro and in vivo studies have shown that IKAP is involved in multiple intracellular processes, and suggest that failed target innervation and/or impaired neurotrophic retrograde transport are the primary causes of neuronal cell death in FD. However, FD is far more complex, and appears to affect several other organs and systems in addition to the peripheral nervous system. With the recent generation of mouse models that recapitulate the molecular and pathological features of the disease, it is now possible to further investigate the mechanisms underlying different aspects of the disorder, and to test novel therapeutic strategies.

Sensory and autonomic deficits in a new humanized mouse model of familial dysautonomia

Familial dysautonomia (FD) is an autosomal recessive neurodegenerative disease that affects the development and survival of sensory and autonomic neurons. FD is caused by an mRNA splicing mutation in intron 20 of the IKBKAP gene that results in a tissue-specific skipping of exon 20 and a corresponding reduction of the inhibitor of kappaB kinase complex-associated protein (IKAP), also known as Elongator complex protein 1. To date, several promising therapeutic candidates for FD have been identified that target the underlying mRNA splicing defect, and increase functional IKAP protein. Despite these remarkable advances in drug discovery for FD, we lacked a phenotypic mouse model in which we could manipulate IKBKAP mRNA splicing to evaluate potential efficacy. We have, therefore, engineered a new mouse model that, for the first time, will permit to evaluate the phenotypic effects of splicing modulators and provide a crucial platform for preclinical testing of new therapies. This new mouse model, TgFD9; Ikbkap(Δ20/flox) was created by introducing the complete human IKBKAP transgene with the major FD splice mutation (TgFD9) into a mouse that expresses extremely low levels of endogenous Ikbkap (Ikbkap(Δ20/flox)). The TgFD9; Ikbkap(Δ20/flox) mouse recapitulates many phenotypic features of the human disease, including reduced growth rate, reduced number of fungiform papillae, spinal abnormalities, and sensory and sympathetic impairments, and recreates the same tissue-specific mis-splicing defect seen in FD patients. This is the first mouse model that can be used to evaluate in vivo the therapeutic effect of increasing IKAP levels by correcting the underlying FD splicing defect.

Occludin deficiency promotes ethanol-induced disruption of colonic epithelial junctions, gut barrier dysfunction and liver damage in mice

BACKGROUND

Disruption of epithelial tight junctions (TJ), gut barrier dysfunction and endotoxemia play crucial role in the pathogenesis of alcoholic tissue injury. Occludin, a transmembrane protein of TJ, is depleted in colon by alcohol. However, it is unknown whether occludin depletion influences alcoholic gut and liver injury.

METHODS

Wild type (WT) and occludin deficient (Ocln(-/-)) mice were fed 1-6% ethanol in Lieber-DeCarli diet. Gut permeability was measured by vascular-to-luminal flux of FITC-inulin. Junctional integrity was analyzed by confocal microscopy. Liver injury was assessed by plasma transaminase, histopathology and triglyceride analyses. The effect of occludin depletion on acetaldehyde-induced TJ disruption was confirmed in Caco-2 cell monolayers.

RESULTS

Ethanol feeding significantly reduced body weight gain in Ocln(-/-) mice. Ethanol increased inulin permeability in colon of both WT and Ocln(-/-) mice, but the effect was 4-fold higher in Ocln(-/-) mice. The gross morphology of colonic mucosa was unaltered, but ethanol disrupted the actin cytoskeleton, induced redistribution of occludin, ZO-1, E-cadherin and β-catenin from the junctions and elevated TLR4, which was more severe in Ocln(-/-) mice. Occludin knockdown significantly enhanced acetaldehyde-induced TJ disruption and barrier dysfunction in Caco-2 cell monolayers. Ethanol significantly increased liver weight and plasma transaminase activity in Ocln(-/-) mice, but not in WT mice. Histological analysis indicated more severe lesions and fat deposition in the liver of ethanol-fed Ocln(-/-) mice. Ethanol-induced elevation of liver triglyceride was also higher in Ocln(-/-) mice.

CONCLUSION

This study indicates that occludin deficiency increases susceptibility to ethanol-induced colonic mucosal barrier dysfunction and liver damage in mice.

ALDH2 Deficiency Promotes Ethanol-Induced Gut Barrier Dysfunction and Fatty Liver in Mice

BACKGROUND

Acetaldehyde, the toxic ethanol (EtOH) metabolite, disrupts intestinal epithelial barrier function. Aldehyde dehydrogenase (ALDH) detoxifies acetaldehyde into acetate. Subpopulations of Asians and Native Americans show polymorphism with loss-of-function mutations in ALDH2. We evaluated the effect of ALDH2 deficiency on EtOH-induced disruption of intestinal epithelial tight junctions and adherens junctions, gut barrier dysfunction, and liver injury.

METHODS

Wild-type and ALDH2-deficient mice were fed EtOH (1 to 6%) in Lieber-DeCarli diet for 4 weeks. Gut permeability in vivo was measured by plasma-to-luminal flux of FITC-inulin, tight junction and adherens junction integrity was analyzed by confocal microscopy, and liver injury was assessed by the analysis of plasma transaminase activity, histopathology, and liver triglyceride.

RESULTS

EtOH feeding elevated colonic mucosal acetaldehyde, which was significantly greater in ALDH2-deficient mice. ALDH2(-/-) mice showed a drastic reduction in the EtOH diet intake. Therefore, this study was continued only in wild-type and ALDH2(+/-) mice. EtOH feeding elevated mucosal inulin permeability in distal colon, but not in proximal colon, ileum, or jejunum of wild-type mice. In ALDH2(+/-) mice, EtOH-induced inulin permeability in distal colon was not only higher than that in wild-type mice, but inulin permeability was also elevated in the proximal colon, ileum, and jejunum. Greater inulin permeability in distal colon of ALDH2(+/-) mice was associated with a more severe redistribution of tight junction and adherens junction proteins from the intercellular junctions. In ALDH2(+/-) mice, but not in wild-type mice, EtOH feeding caused a loss of junctional distribution of tight junction and adherens junction proteins in the ileum. Histopathology, plasma transaminases, and liver triglyceride analyses showed that EtOH-induced liver damage was significantly greater in ALDH2(+/-) mice compared to wild-type mice.

CONCLUSIONS

These data demonstrate that ALDH2 deficiency enhances EtOH-induced disruption of intestinal epithelial tight junctions, barrier dysfunction, and liver damage.

HIF1α is necessary for exercise-induced neuroprotection while HIF2α is needed for dopaminergic neuron survival in the substantia nigra pars compacta

Exercise reduces the risk of developing a number of neurological disorders and increases the efficiency of cellular energy production. However, overly strenuous exercise produces oxidative stress. Proper oxygenation is crucial for the health of all tissues, and tight regulation of cellular oxygen is critical to balance O2 levels and redox homeostasis in the brain. Hypoxia Inducible Factor (HIF)1α and HIF2α are transcription factors regulated by cellular oxygen concentration that initiate gene regulation of vascular development, redox homeostasis, and cell cycle control. HIF1α and HIF2α contribute to important adaptive mechanisms that occur when oxygen and ROS homeostasis become unbalanced. It has been shown that preconditioning by exposure to a stressor prior to a hypoxic event reduces damage that would otherwise occur. Previously we reported that 3 months of exercise protects SNpc dopaminergic (DA) neurons from toxicity caused by Complex I inhibition. Here, we identify the cells in the SNpc that express HIF1α and HIF2α and show that running exercise produces hypoxia in SNpc DA neurons, and alters the expression of HIF1α and HIF2α. In mice carrying a conditional knockout of Hif1α in postnatal neurons we observe that exercise alone produces SNpc TH+ DA neuron loss. Loss of HIF1α also abolishes exercise-induced neuroprotection. In mice lacking Hif2α in postnatal neurons, the number of TH+ DA neurons in the adult SNpc is diminished, but 3months of exercise rescues this loss. We conclude that HIF1α is necessary for exercise-induced neuroprotection and both HIF1α and HIF2α are necessary for the survival and function of adult SNpc DA neurons.

IKAP deficiency in an FD mouse model and in oligodendrocyte precursor cells results in downregulation of genes involved in oligodendrocyte differentiation and myelin formation

The splice site mutation in the IKBKAP gene coding for IKAP protein leads to the tissue-specific skipping of exon 20, with concomitant reduction in IKAP protein production. This causes the neurodevelopmental, autosomal-recessive genetic disorder - Familial Dysautonomia (FD). The molecular hallmark of FD is the severe reduction of IKAP protein in the nervous system that is believed to be the main reason for the devastating symptoms of this disease. Our recent studies showed that in the brain of two FD patients, genes linked to oligodendrocyte differentiation and/or myelin formation are significantly downregulated, implicating IKAP in the process of myelination. However, due to the scarcity of FD patient tissues, these results awaited further validation in other models. Recently, two FD mouse models that faithfully recapitulate FD were generated, with two types of mutations resulting in severely low levels of IKAP expression. Here we demonstrate that IKAP deficiency in these FD mouse models affects a similar set of genes as in FD patients' brains. In addition, we identified two new IKAP target genes involved in oligodendrocyte cells differentiation and myelination, further underscoring the essential role of IKAP in this process. We also provide proof that IKAP expression is needed cell-autonomously for the regulation of expression of genes involved in myelin formation since knockdown of IKAP in the Oli-neu oligodendrocyte precursor cell line results in similar deficiencies. Further analyses of these two experimental models will compensate for the lack of human postmortem tissues and will advance our understanding of the role of IKAP in myelination and the disease pathology.

Cutting edge: Conditional MHC class II expression reveals a limited role for B cell antigen presentation in primary and secondary CD4 T cell responses

The activation, differentiation, and subsequent effector functions of CD4 T cells depend on interactions with a multitude of MHC class II (MHCII)-expressing APCs. To evaluate the individual contribution of various APCs to CD4 T cell function, we have designed a new murine tool for selective in vivo expression of MHCII in subsets of APCs. Conditional expression of MHCII in B cells was achieved using a cre-loxP approach. After i.v. or s.c. priming, partial proliferation and activation of CD4 T cells was observed in mice expressing MHCII only by B cells. Restricting MHCII expression to B cells constrained secondary CD4 T cell responses in vivo, as demonstrated in a CD4 T cell-dependent model of autoimmunity, experimental autoimmune encephalomyelitis. These results highlight the limitations of B cell Ag presentation during initiation and propagation of CD4 T cell function in vivo using a novel system to study individual APCs by the conditional expression of MHCII.

IKAP expression levels modulate disease severity in a mouse model of familial dysautonomia

Hereditary sensory and autonomic neuropathies (HSANs) encompass a group of genetically inherited disorders characterized by sensory and autonomic dysfunctions. Familial dysautonomia (FD), also known as HSAN type III, is an autosomal recessive disorder that affects 1/3600 live births in the Ashkenazi Jewish population. The disease is caused by abnormal development and progressive degeneration of the sensory and autonomic nervous systems and is inevitably fatal, with only 50% of patients reaching the age of 40. FD is caused by a mutation in intron 20 of the Ikbkap gene that results in severe reduction in the expression of its encoded protein, inhibitor of kappaB kinase complex-associated protein (IKAP). Although the mutation that causes FD was identified in 2001, so far there is no appropriate animal model that recapitulates the disorder. Here, we report the generation and characterization of the first mouse models for FD that recapitulate the molecular and pathological features of the disease. Important for therapeutic interventions is also our finding that a slight increase in IKAP levels is enough to ameliorate the phenotype and increase the life span. Understanding the mechanisms underlying FD will provide insights for potential new therapeutic interventions not only for FD, but also for other peripheral neuropathies.

B cell antigen presentation plays a limited role in primary and secondary CD4 T cell responses (176.16)

The activation and differentiation of effector CD4 T cells depends on interactions with MHCII-expressing antigen presenting cells (APCs). While several APC populations coordinate CD4 T cell function, several lines of evidence indicate that antigen presentation by B cells is a critical element to the cellular immune response. We have sought to determine the contribution of B cell antigen presentation to CD4 T cell priming and secondary responses. To address the hypothesis that B cells are sufficient APCs for CD4 T cell responses, we have designed a new tool in which individual subsets of APCs conditionally express MHCII in vivo. We successfully targeted a loxP-flanked stop sequence to the murine IA-β chain locus to utilize the Cre/lox system for conditional expression of MHCII. Conditional manipulation was achieved using CD19-Cre mice, restricting expression of MHCII to B cells. After priming, sub-optimal proliferation and activation of CD4 T cells was observed in mice expressing MHCII only by B cells. Further, limiting MHCII expression to B cells constrained secondary CD4 T cell responses in vivo. In a CD4 T cell-dependent model of autoimmunity, EAE, B cell expression of MHCII was not sufficient to support secondary autoreactive CD4 T cell responses targeting the CNS. These results demonstrate a novel capacity to conditionally express MHCII in vivo and highlight the limitations of B cell antigen presentation during the initiation and propagation of CD4 T cell function

Ciliogenesis is regulated by a huntingtin-HAP1-PCM1 pathway and is altered in Huntington disease

Huntington disease (HD) is a devastating autosomal-dominant neurodegenerative disorder. It is caused by expansion of a CAG repeat in the first exon of the huntingtin (HTT) gene that encodes a mutant HTT protein with a polyglutamine (polyQ) expansion at the amino terminus. Here, we demonstrate that WT HTT regulates ciliogenesis by interacting through huntingtin-associated protein 1 (HAP1) with pericentriolar material 1 protein (PCM1). Loss of Htt in mouse cells impaired the retrograde trafficking of PCM1 and thereby reduced primary cilia formation. In mice, deletion of Htt in ependymal cells led to PCM1 mislocalization, alteration of the cilia layer, and hydrocephalus. Pathogenic polyQ expansion led to centrosomal accumulation of PCM1 and abnormally long primary cilia in mouse striatal cells. PCM1 accumulation in ependymal cells was associated with longer cilia and disorganized cilia layers in a mouse model of HD and in HD patients. Longer cilia resulted in alteration of the cerebrospinal fluid flow. Thus, our data indicate that WT HTT is essential for protein trafficking to the centrosome and normal ciliogenesis. In HD, hypermorphic ciliogenesis may affect signaling and neuroblast migration so as to dysregulate brain homeostasis and exacerbate disease progression.

Genetics and neuropathology of Huntington's disease

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder that prominently affects the basal ganglia, leading to affective, cognitive, behavioral and motor decline. The basis of HD is a CAG repeat expansion to >35 CAG in a gene that codes for a ubiquitous protein known as huntingtin, resulting in an expanded N-terminal polyglutamine tract. The size of the expansion is correlated with disease severity, with increasing CAG accelerating the age of onset. A variety of possibilities have been proposed as to the mechanism by which the mutation causes preferential injury to the basal ganglia. The present chapter provides a basic overview of the genetics and pathology of HD.

α8-integrins are required for hippocampal long-term potentiation but not for hippocampal-dependent learning

Integrins are heterodimeric transmembrane cell adhesion receptors that are essential for a wide range of biological functions via cell-matrix and cell-cell interactions. Recent studies have provided evidence that some of the subunits in the integrin family are involved in synaptic and behavioral plasticity. To further understand the role of integrins in the mammalian central nervous system, we generated a postnatal forebrain and excitatory neuron-specific knockout of alpha8-integrin in the mouse. Behavioral studies showed that the mutant mice are normal in multiple hippocampal-dependent learning tasks, including a T-maze, non-match-to-place working memory task for which other integrin subunits like alpha3- and beta1-integrin are required. In contrast, mice mutant for alpha8-integrin exhibited a specific impairment of long-term potentiation (LTP) at Schaffer collateral-CA1 synapses, whereas basal synaptic transmission, paired-pulse facilitation and long-term depression (LTD) remained unaffected. Because LTP is also impaired in the absence of alpha3-integrin, our results indicate that multiple integrin molecules are required for the normal expression of LTP, and different integrins display distinct roles in behavioral and neurophysiological processes like synaptic plasticity.

Presenilins are essential for regulating neurotransmitter release

Mutations in the presenilin genes are the major cause of familial Alzheimer's disease (AD). Loss of presenilin activity and/or accumulation of amyloid-β peptides have been proposed to mediate the pathogenesis of AD by impairing synaptic function1-5. However, the precise site and nature of the synaptic dysfunction remain unknown. Here we employ a genetic approach to inactivate presenilins conditionally in either presynaptic (CA3) or postsynaptic (CA1) neurons of the hippocampal Schaeffer-collateral pathway. We found that long-term potentiation (LTP) induced by theta burst stimulation is decreased after presynaptic but not postsynaptic deletion of presenilins. Moreover, presynaptic but not postsynaptic inactivation of presenilins alters short-term plasticity and synaptic facilitation. The probability of evoked glutamate release, measured with the open-channel NMDA receptor antagonist MK-801, is reduced by presynaptic inactivation of presenilins. Strikingly, depletion of endoplasmic reticulum Ca²⁺-stores by thapsigargin or blockade of Ca²⁺-release from these stores by ryanodine receptor inhibitors mimics and occludes the effects of presynaptic presenilin inactivation. Collectively, these results reveal a selective role for presenilins in the activity-dependent regulation of neurotransmitter release and LTP induction via modulation of intracellular Ca²⁺-release in presynaptic terminals, and further suggest that presynaptic dysfunction might be an early pathogenic event leading to dementia and neurodegeneration in AD.

CAG repeat lengths > or =335 attenuate the phenotype in the R6/2 Huntington's disease transgenic mouse

With spontaneous elongation of the CAG repeat in the R6/2 transgene to > or =335, resulting in a transgene protein too large for passive entry into nuclei via the nuclear pore, we observed an abrupt increase in lifespan to >20 weeks, compared to the 12 weeks common in R6/2 mice with 150 repeats. In the > or =335 CAG mice, large ubiquitinated aggregates of mutant protein were common in neuronal dendrites and perikaryal cytoplasm, but intranuclear aggregates were small and infrequent. Message and protein for the > or =335 CAG transgene were reduced to one-third that in 150 CAG R6/2 mice. Neurological and neurochemical abnormalities were delayed in onset and less severe than in 150 CAG R6/2 mice. These findings suggest that polyQ length and pathogenicity in Huntington's disease may not be linearly related, and pathogenicity may be less severe with extreme repeats. Both diminished mutant protein and reduced nuclear entry may contribute to phenotype attenuation.

Congenital hydrocephalus associated with abnormal subcommissural organ in mice lacking huntingtin in Wnt1 cell lineages

Huntingtin (htt) is a 350 kDa protein of unknown function, with no homologies with other known proteins. Expansion of a polyglutamine stretch at the N-terminus of htt causes Huntington's disease (HD), a dominant neurodegenerative disorder. Although it is generally accepted that HD is caused primarily by a gain-of-function mechanism, recent studies suggest that loss-of-function may also be part of HD pathogenesis. Huntingtin is an essential protein in the mouse since inactivation of the mouse HD homolog (Hdh) gene results in early embryonic lethality. Huntingtin is widely expressed in embryogenesis, and associated with a number of interacting proteins suggesting that htt may be involved in several processes including morphogenesis, neurogenesis and neuronal survival. To further investigate the role of htt in these processes, we have inactivated the Hdh gene in Wnt1 cell lineages using the Cre-loxP system of recombination. Here we show that conditional inactivation of the Hdh gene in Wnt1 cell lineages results in congenital hydrocephalus, implicating huntingtin for the first time in the regulation of cerebral spinal fluid (CSF) homeostasis. Our results show that hydrocephalus in mice lacking htt in Wnt1 cell lineages is associated with increase in CSF production by the choroid plexus, and abnormal subcommissural organ.

Elucidating a normal function of huntingtin by functional and microarray analysis of huntingtin-null mouse embryonic fibroblasts

Background

The polyglutamine expansion in huntingtin (Htt) protein is a cause of Huntington's disease (HD). Htt is an essential gene as deletion of the mouse Htt gene homolog (Hdh) is embryonic lethal in mice. Therefore, in addition to elucidating the mechanisms responsible for polyQ-mediated pathology, it is also important to understand the normal function of Htt protein for both basic biology and for HD.

Results

To systematically search for a mouse Htt function, we took advantage of the Hdh +/- and Hdh-floxed mice and generated four mouse embryonic fibroblast (MEF) cells lines which contain a single copy of the Hdh gene (Hdh-HET) and four MEF lines in which the Hdh gene was deleted (Hdh-KO). The function of Htt in calcium (Ca²⁺) signaling was analyzed in Ca²⁺ imaging experiments with generated cell lines. We found that the cytoplasmic Ca²⁺ spikes resulting from the activation of inositol 1,4,5-trisphosphate receptor (InsP₃R) and the ensuing mitochondrial Ca²⁺ signals were suppressed in the Hdh-KO cells when compared to Hdh-HET cells. Furthermore, in experiments with permeabilized cells we found that the InsP₃-sensitivity of Ca²⁺ mobilization from endoplasmic reticulum was reduced in Hdh-KO cells. These results indicated that Htt plays an important role in modulating InsP₃R-mediated Ca²⁺ signaling. To further evaluate function of Htt, we performed genome-wide transcription profiling of generated Hdh-HET and Hdh-KO cells by microarray. Our results revealed that 106 unique transcripts were downregulated by more than two-fold with p < 0.05 and 173 unique transcripts were upregulated at least two-fold with p < 0.05 in Hdh-KO cells when compared to Hdh-HET cells. The microarray results were confirmed by quantitative real-time PCR for a number of affected transcripts. Several signaling pathways affected by Hdh gene deletion were identified from annotation of the microarray results.

Conclusion

Functional analysis of generated Htt-null MEF cells revealed that Htt plays a direct role in Ca²⁺ signaling by modulating InsP₃R sensitivity to InsP₃. The genome-wide transcriptional profiling of Htt-null cells yielded novel and unique information about the normal function of Htt in cells, which may contribute to our understanding and treatment of HD.

A transgenic mouse class-III beta tubulin reporter using yellow fluorescent protein

A yellow fluorescence protein (YFP) reporter construct was cloned downstream of the beta-tubulin III promoter and injected to produce two founder lines of transgenic mice. YFP expression was observed in many regions of the developing peripheral and central nervous system. YFP expression was first observed in the peripheral and central nervous system as early as embryonic day 9.0. There was a dramatic increase in the number of neuronal systems expressing YFP through P0. Then as the animals reached adult age, the expression levels decreased, but many neurons still show YFP expression, notably in regions of the brain undergoing adult neurogenesis, i.e., the rostral migratory stream and subgranular layer of the dentate gyrus. This reporter-based staining was compared with anti-class-III beta-tubulin immunocytochemistry and shown to closely parallel the expression of the endogenous protein. These transgenic lines should provide unique models to study in vivo and in vitro neurodevelopment.

Neuron-specific inactivation of the hypoxia inducible factor 1 alpha increases brain injury in a mouse model of transient focal cerebral ischemia

In the present study, we show a biphasic activation of hypoxia inducible factor 1alpha (HIF-1) after stroke that lasts for up to 10 d, suggesting the involvement of the HIF pathway in several aspects of the pathophysiology of cerebral ischemia. We provide evidence that HIF-1-mediated responses have an overall beneficial role in the ischemic brain as indicated by increased tissue damage and reduced survival rate of mice with neuron-specific knockdown of HIF-1alpha that were subjected to transient focal cerebral ischemia. In addition, we demonstrated that drugs known to activate HIF-1 in cultured cells as well as in vivo had neuroprotective properties in this model of cerebral ischemia. This protective effect was significantly attenuated but not completely abolished in neuron-specific HIF-1alpha-deficient mice, suggesting that alternative mechanisms of neuroprotection are also implicated. Last, our study showed that hypoxia-induced tolerance to ischemia was preserved in neuron-specific HIF-1alpha-deficient mice, indicating that the neuroprotective effects of hypoxic preconditioning do not depend on neuronal HIF-1 activation.

Huntingtin inhibits caspase-3 activation

Huntington's disease results from a mutation in the HD gene encoding for the protein huntingtin. The function of huntingtin, although beginning to be elucidated, remains largely unclear. To probe the prosurvival function of huntingtin, we modulate levels of wild-type huntingtin in a number of cellular and in vivo models. Huntingtin depletion resulted in caspase-3 activation, and overexpression of huntingtin resulted in caspase-3 inhibition. Additionally, we demonstrate that huntingtin physically interacts with active caspase-3. Interestingly, mutant huntingtin binds active caspase-3 with a lower affinity and lower inhibitory effect on active caspase-3 than does wild-type huntingtin. Although reduction of huntingtin levels resulted in caspase-3 activation in all conditions examined, the cellular response was cell-type specific. Depletion of huntingtin resulted in either overt cell death, or in increased vulnerability to cell death. These data demonstrate that huntingtin inhibits caspase-3 activity, suggesting a mechanism whereby caspase-mediated huntingtin depletion results in a detrimental amplification cascade leading to further caspase-3 activation, resulting in cell dysfunction and cell death.

Brain-specific knock-out of hypoxia-inducible factor-1alpha reduces rather than increases hypoxic-ischemic damage

Hypoxia-inducible factor-1alpha (HIF-1alpha) plays an essential role in cellular and systemic O(2) homeostasis by regulating the expression of genes important in glycolysis, erythropoiesis, angiogenesis, and catecholamine metabolism. It is also believed to be a key component of the cellular response to hypoxia and ischemia under pathophysiological conditions, such as stroke. To clarify the function of HIF-1alpha in the brain, we exposed adult mice with late-stage brain deletion of HIF-1alpha to hypoxic injuries. Contrary to expectations, the brains from the HIF-1alpha-deficient mice were protected from hypoxia-induced cell death. These surprising findings suggest that decreasing the level of HIF-1alpha can be neuroprotective. Gene chip expression analysis revealed that, contrary to expectations, the majority of hypoxia-dependent gene-expression changes were unaltered, whereas a specific downregulation of apoptotic genes was observed in the HIF-1alpha-deficient mice. Although the role of HIF-1alpha has been extensively characterized in vitro, in cancer models, and in chronic preconditioning paradigms, this is the first study to evaluate the role of HIF-1alpha in vivo in the brain in response to acute hypoxia/ischemia. Our data suggest, that in acute hypoxia, the neuroprotection found in the HIF-1alpha-deficient mice is mechanistically consistent with a predominant role of HIF-1alpha as proapoptotic and loss of function leads to neuroprotection. Furthermore, our data suggest that functional redundancy develops after excluding HIF-1alpha, leading to the preservation of gene expression regulating the majority of other previously characterized HIF-dependent genes.

Neuronal deletion of Lepr elicits diabesity in mice without affecting cold tolerance or fertility

Leptin signaling in the brain regulates energy intake and expenditure. To test the degree of functional neuronal leptin signaling required for the maintenance of body composition, fertility, and cold tolerance, transgenic mice expressing Cre in neurons (CaMKIIalpha-Cre) were crossed to mice carrying a floxed leptin receptor (Lepr) allele to generate mice with neuron-specific deletion of Lepr in approximately 50% (C F/F mice) and approximately 75% (C Delta17/F mice) of hypothalamic neurons. Leptin receptor (LEPR)-deficient mice (Delta17/Delta17) with heat-shock-Cre-mediated global Lepr deletion served as obese controls. At 16 wk, male C F/F, C Delta17/F, and Delta17/Delta17 mice were 13.2 (P < 0.05), 45.0, and 55.9% (P < 0.001) heavier, respectively, than lean controls, whereas females showed 31.6, 68.8, and 160.7% increases in body mass (P < 0.001). Significant increases in total fat mass (C F/F: P < 0.01; C Delta17/F and Delta17/Delta17:P < 0.001 vs. sex-matched, lean controls), and serum leptin concentrations (P < 0.001 vs. controls) were present in proportion to Lepr deletion. Male C Delta17/F mice had significant elevations in basal serum insulin concentrations (P < 0.001 vs. controls) and were glucose intolerant, as measured by glucose tolerance test (AUC P < 0.01 vs. controls). In contrast with previous observations in mice null for LEPR signaling, C F/F and C Delta17/F mice were fertile and cold tolerant. These findings support the hypothesis that body weight, adiposity, serum leptin concentrations, and glucose intolerance are proportional to hypothalamic LEPR deficiency. However, fertility and cold tolerance remain intact unless hypothalamic LEPR deficiency is complete.

An allelic series for the leptin receptor gene generated by CRE and FLP recombinase

Body weight regulation is mediated through several major signaling pathways, some of which have been delineated by positional cloning of spontaneous genetic mutations in mice. Lepr(db/db) mice are obese due to a defect in the signaling portion of the leptin receptor, which has led to extensive study of this highly conserved system over the past several years. We have created an allelic series at Lepr for the further examination of LEPR signaling phenotypes using both the FLP /frt and CRE /loxP systems. By inserting a frt-PGK-neo-frt sequence in Lepr intron 16, we have generated a conditional gene repair Lepr allele ( Lepr-neo) that elicits morbid obesity, diabetes, and infertility in homozygous mice, recapitulating the obesity syndrome of Lepr(db/db) mice. Thus, in vivo excision of the PGK-neo cassette with a FLP recombinase transgene restores the lean and fertile phenotype to Lepr(flox/flox) mice. In the same construct, we have also inserted loxP sites that flank Lepr coding exon 17, a region that encodes a JAK docking site required for STAT3 signaling. CRE-mediated excision of Lepr coding exon 17 from Lepr with a frameshift in subsequent exons results in a syndrome of obesity, diabetes, and infertility in LeprDelta17/Delta17 mice, which is indistinguishable from Lepr(neo/neo) and Lepr(db/db) mice. We conclude that suppression of Lepr gene expression by PGK-neo is phenotypically equivalent to deletion of the Lepr signaling motifs, and therefore the Lepr(neo/neo) mouse may be used to investigate conditional gene repair of Lepr signaling deficiency.

Huntingtin-associated protein 1 (Hap1) mutant mice bypassing the early postnatal lethality are neuroanatomically normal and fertile but display growth retardation

Huntingtin-associated protein 1 (Hap1) is the first huntingtin interacting protein identified in a yeast two-hybrid screen. Although Hap1 expression has been demonstrated in neuronal and non-neuronal tissues, its molecular role is poorly understood. Recently, it has been shown that targeted disruption of Hap1 in mice results in early postnatal death as a result of depressed feeding behavior. Although this result clearly demonstrates an essential role of Hap1 in postnatal feeding, the mechanisms leading to this deficiency, as well as the role of Hap1 in adults, remain unclear. Here we show that Hap1 null mutants display suckling defects and die within the first days after birth due to starvation. Upon reduction of the litter size, some mutants survive into adulthood and display growth retardation with no apparent brain or behavioral abnormalities, suggesting that Hap1 function is essential only for early postnatal feeding behavior. Using a conditional gene repair strategy, we also show that the early lethality can be rescued if Hap1 expression is restored in neuronal cells before birth. Furthermore, no synergism was observed between Hap1 and huntingtin mutation during mouse development. Our results demonstrate that Hap1 has a fundamental role in regulating postnatal feeding in the first 2 weeks after birth and a non-essential role in the adult mouse.

Mutant huntingtin impairs axonal trafficking in mammalian neurons in vivo and in vitro

Recent data in invertebrates demonstrated that huntingtin (htt) is essential for fast axonal trafficking. Here, we provide direct and functional evidence that htt is involved in fast axonal trafficking in mammals. Moreover, expression of full-length mutant htt (mhtt) impairs vesicular and mitochondrial trafficking in mammalian neurons in vitro and in whole animals in vivo. Particularly, mitochondria become progressively immobilized and stop more frequently in neurons from transgenic animals. These defects occurred early in development prior to the onset of measurable neurological or mitochondrial abnormalities. Consistent with a progressive loss of function, wild-type htt, trafficking motors, and mitochondrial components were selectively sequestered by mhtt in human Huntington's disease-affected brain. Data provide a model for how loss of htt function causes toxicity; mhtt-mediated aggregation sequesters htt and components of trafficking machinery leading to loss of mitochondrial motility and eventual mitochondrial dysfunction.

Transgenic rescue of insulin receptor-deficient mice

The role of different tissues in insulin action and their contribution to the pathogenesis of diabetes remain unclear. To examine this question, we have used genetic reconstitution experiments in mice. Genetic ablation of insulin receptors causes early postnatal death from diabetic ketoacidosis. We show that combined restoration of insulin receptor function in brain, liver, and pancreatic beta cells rescues insulin receptor knockout mice from neonatal death, prevents diabetes in a majority of animals, and normalizes adipose tissue content, lifespan, and reproductive function. In contrast, mice with insulin receptor expression limited to brain or liver and pancreatic beta cells are rescued from neonatal death, but develop lipoatrophic diabetes and die prematurely. These data indicate, surprisingly, that insulin receptor signaling in noncanonical insulin target tissues is sufficient to maintain fuel homeostasis and prevent diabetes.

Wild-type huntingtin plays a role in brain development and neuronal survival

While the role of the mutated Huntington's disease (HD) protein in the pathogenesis of HD has been the focus of intensive investigation, the normal protein has received less attention. Nonetheless, the wild-type HD protein appears to be essential for embryogenesis, since deletion of the HD gene in mice results in early embryonic lethality. This early lethality is due to a critical role the HD protein, called huntingtin (Htt), plays in extraembryonic membrane function, presumably in vesicular transport of nutrients. Studies of mutant mice expressing low levels of Htt and of chimeric mice generated by blastocyst injection of Hdh-/- embryonic stem cells show that wildtype Htt plays an important role later in development as well, specifically in forebrain formation. Moreover, various lines of study suggest that normal Htt is also critical for survival of neurons in the adult forebrain. The observation that Htt plays its key developmental and survival roles in those brain areas most affected in HD raises the possibility that a subtle loss of function on the part of the mutant protein or a sequestering of wild-type Htt by mutant Htt may contribute to HD pathogenesis. Regardless of whether this is so, the prosurvival role of Htt suggests that HD therapies that block production of both wild-type and mutant Htt may themselves be harmful.

Time course of early motor and neuropathological anomalies in a knock-in mouse model of Huntington's disease with 140 CAG repeats

Huntington's disease (HD) is caused by an abnormal expansion of CAG repeats in the gene encoding huntingtin. The development of therapies for HD requires preclinical testing of drugs in animal models that reproduce the dysfunction and regionally specific pathology observed in HD. We have developed a new knock-in mouse model of HD with a chimeric mouse/human exon 1 containing 140 CAG repeats inserted in the murine huntingtin gene. These mice displayed an increased locomotor activity and rearing at 1 month of age, followed by hypoactivity at 4 months and gait anomalies at 1 year. Behavioral symptoms preceded neuropathological anomalies, which became intense and widespread only at 4 months of age. These consisted of nuclear staining for huntingtin and huntingtin-containing nuclear and neuropil aggregates that first appeared in the striatum, nucleus accumbens, and olfactory tubercle. Interestingly, regions with early pathology all receive dense dopaminergic inputs, supporting accumulating evidence for a role of dopamine in HD pathology. Nuclear staining and aggregates predominated in striatum and layer II/III and deep layer V of the cerebral cortex, whereas neuropil aggregates were found in the globus pallidus and layer IV/superficial layer V of the cerebral cortex. The olfactory system displayed early and marked aggregate accumulation, which may be relevant to the early deficit in odor discrimination observed in patients with HD. Because of their early behavioral anomalies and regionally specific pathology, these mice provide a powerful tool with which to evaluate the effectiveness of new therapies and to study the mechanisms involved in the neuropathology of HD.

Presynaptic BDNF required for a presynaptic but not postsynaptic component of LTP at hippocampal CA1-CA3 synapses

Brain-derived neurotrophic factor (BDNF) has been implicated in several forms of long-term potentiation (LTP) at different hippocampal synapses. Using two-photon imaging of FM 1-43, a fluorescent marker of synaptic vesicle cycling, we find that BDNF is selectively required for those forms of LTP at Schaffer collateral synapses that recruit a presynaptic component of expression. BDNF-dependent forms of LTP also require activation of L-type voltage-gated calcium channels. One form of LTP with presynaptic expression, theta burst LTP, is thought to be of particular behavioral importance. Using restricted genetic deletion to selectively disrupt BDNF production in either the entire forebrain (CA3 and CA1) or in only the postsynaptic CA1 neuron, we localize the source of BDNF required for LTP to presynaptic neurons. These results suggest that long-term synaptic plasticity has distinct presynaptic and postsynaptic modules. Release of BDNF from CA3 neurons is required to recruit the presynaptic, but not postsynaptic, module of plasticity.

Insulin-like growth factor (IGF) signaling through type 1 IGF receptor plays an important role in remyelination

We examined the role of IGF signaling in the remyelination process by disrupting the gene encoding the type 1 IGF receptor (IGF1R) specifically in the mouse brain by Cre-mediated recombination and then exposing these mutants and normal siblings to cuprizone. This neurotoxicant induces a demyelinating lesion in the corpus callosum that is reversible on termination of the insult. Acute demyelination and oligodendrocyte depletion were the same in mutants and controls, but the mutants did not remyelinate adequately. We observed that oligodendrocyte progenitors did not accumulate, proliferate, or survive within the mutant mice, compared with wild type, indicating that signaling through the IGF1R plays a critical role in remyelination via effects on oligodendrocyte progenitors.

Proliferation and growth factor expression in abnormally enlarged placentas of mouse interspecific hybrids

It has been shown previously that abnormal placental growth occurs in crosses and backcrosses between different mouse (Mus) species. In such crosses, late gestation placentas may weigh between 13 and 848 mg compared with a mean placental weight of approximately 100 mg in late gestation M. musculus intraspecific crosses. A locus on the X-chromosome was shown to segregate with placental dysplasia. Thus in the (M. musculus x M. spretus)F1 x M. musculus backcross, placental hyperplasia cosegregates with a M. spretus derived X-chromosome. Here we have investigated whether increased cell proliferation and aberrant expression of two genes that are involved in placental growth control, Igf2 and Esx1, may cause, or contribute to placental hyperplasia. Increased bromodeoxyuridine labeling of nuclei, reflecting enhanced proliferation, was indeed observed in hyperplastic placentas when compared with normal littermate placentas. Also, increased expression of Igf2 was seen in giant cells and spongiotrophoblast. However, when M. musculus x M. spretus F1 females were backcrossed with males that were heterozygous for a targeted mutation of the Igf2 gene, placentas that carried a M. spretus derived X-chromosome and were negative for a functional Igf2 allele exhibited an intermediate placental phenotype. Furthermore, in early developmental stages of placental hyperplasia, we observed a decreased expression of the X-chromosomal Esx1 gene. This finding suggests that abnormal expression of both Igf2 and Esx1 contributes to abnormal placental development in mouse interspecific hybrids. However, Esx1 is not regulated by IGF2.

Overgrowth of a mouse model of the Simpson-Golabi-Behmel syndrome is independent of IGF signaling

The type 1 Simpson-Golabi-Behmel overgrowth syndrome (SGBS1) is caused by loss-of-function mutations of the X-linked GPC3 gene encoding glypican-3, a cell-surface heparan sulfate proteoglycan that apparently plays a negative role in growth control by an unknown mechanism. Mice carrying a Gpc3 gene knockout exhibited several phenotypic features that resemble clinical hallmarks of SGBS1, including somatic overgrowth, renal dysplasia, accessory spleens, polydactyly, and placentomegaly. In Gpc3/DeltaH19 double mutants (lacking GPC3 and also carrying a deletion around the H19 gene region that causes bialellic expression of the closely linked Igf2 gene by imprint relaxation), the Gpc3-null phenotype was exacerbated, while additional SGBS1 features (omphalocele and skeletal defects) were manifested. However, results from a detailed comparative analysis of growth patterns in double mutants lacking GPC3 and also IGF2, IGF1, or the type 1 IGF receptor (IGF1R) provided conclusive genetic evidence inconsistent with the hypothesis that GPC3 acts as a growth suppressor by sequestering or downregulating an IGF ligand. Nevertheless, our data are compatible with a model positing that there is downstream convergence of the independent signaling pathways in which either IGFs or (indirectly) GPC3 participate.

Neurons lacking huntingtin differentially colonize brain and survive in chimeric mice

To determine whether neurons lacking huntingtin can participate in development and survive in postnatal brain, we used two approaches in an effort to create mice consisting of wild-type cells and cells without huntingtin. In one approach, chimeras were created by aggregating the 4-8 cell embryos from matings of Hdh (+/-) mice with wild-type 4-8 cell embryos. No chimeric offspring that possessed homozygous Hdh (-/-) cells were obtained thereby, although statistical considerations suggest that such chimeras should have been created. By contrast, Hdh (-/-) ES cells injected into blastocysts yielded offspring that were born and in adulthood were found to have Hdh (-/-) neurons throughout brain. The Hdh (-/-) cells were, however, 5-10 times more common in hypothalamus, midbrain, and hindbrain than in telencephalon and thalamus. Chimeric animals tended to be smaller than wild-type littermates, and chimeric mice rich in Hdh (-/-) cells tended to show motor abnormalities. Nonetheless, no brain malformations or pathologies were evident. The apparent failure of aggregation chimeras possessing Hdh (-/-) cells to survive to birth is likely attributable to the previously demonstrated critical role of huntingtin in extraembryonic membranes. That Hdh (-/-) cells in chimeric mice created by blastocyst injection are under-represented in adult telencephalon and thalamus implies a role for huntingtin in the development of these regions, whereas the neurological dysfunction in brains enriched in Hdh (-/-) cells suggests a role for huntingtin in adult brain. Nonetheless, the lengthy survival of Hdh (-/-) cells in adult chimeric mice indicates that individual neurons in many brain regions do not require huntingtin to participate in normal brain development and to survive.

A method for the generation of conditional gene repair mutations in mice

Conditional gene repair mutations in the mouse can assist in cell lineage analyses and provide a valuable complement to conditional gene inactivation strategies. We present a method for the generation of conditional gene repair mutations that employs a loxP-flanked (floxed) selectable marker and transcriptional/translational stop cassette (neostop) located within the first intron of a target gene. In the absence of Cre recombinase, expression of the targeted allele is suppressed generating a null allele, while in the presence of Cre, excision of neostop restores expression to wild-type levels. To test this strategy, we have generated a conditional gene repair allele of the mouse Huntington's disease gene homolog (HDH:). Insertion of neostop within the HDH: intron 1 generated a null allele and mice homozygous for this allele resembled nullizygous HDH: mutants and died after embryonic day 8.5. In the presence of a cre transgene expressed ubiquitously early in development, excision of neostop restored HDH: expression and rescued the early embryonic lethality. A simple modification of this strategy that permits the generation of conventional gene knockout, conditional gene knockout and conditional gene repair alleles using one targeting construct is discussed.

Inactivation of Hdh in the brain and testis results in progressive neurodegeneration and sterility in mice

Inactivation of the mouse homologue of the Huntington disease gene (Hdh) results in early embryonic lethality. To investigate the normal function of Hdh in the adult and to evaluate current models for Huntington disease (HD), we have used the Cre/loxP site-specific recombination strategy to inactivate Hdh expression in the forebrain and testis, resulting in a progressive degenerative neuronal phenotype and sterility. On the basis of these results, we propose that huntingtin is required for neuronal function and survival in the brain and that a loss-of-function mechanism may contribute to HD pathogenesis.

Expression of the Huntingtin-associated protein 1 gene in the developing and adult mouse

Huntingtin-associated protein 1 (HAP1) interacts with the product of the Huntington's disease gene. To investigate the function of Hap1 in development and in the adult mouse, we have examined the expression of Hap1 by northern analysis and in situ hybridization histochemistry. Hap1 expression is first detected in the embryonic day 8.5 (E8.5) neuroepithelium. Expression persists throughout development, predominantly in the brain and spinal cord, and to a lesser extent in enteric neurons and abdominal sympathetic ganglia. In the adult, Hap1 expression is detected not only in the brain but also in the ovary, testis, and the intermediate lobe of the pituitary.