Transport Of Brain-derived Neurotrophic Factor Research Articles

Huntingtin-associated Protein-1 Interacts with Pro-brain-derived Neurotrophic Factor and Mediates Its Transport and Release

Brain-derived neurotrophic factor (BDNF) plays a pivotal role in brain development and synaptic plasticity. It is synthesized as a precursor (pro-BDNF), sorted into the secretory pathway, transported along dendrites and axons, and released in an activity-dependent manner. Mutant Huntingtin with expanded polyglutamine (polyQ) and the V66M polymorphism of BDNF reduce the dendritic distribution and axonal transport of BDNF. However, the mechanism underlying this defective transport remains unclear. Here, we report that Huntingtin-associated protein-1 (HAP1) interacts with the prodomain of BDNF and that the interaction was reduced in the presence of polyQ-expanded Huntingtin and BDNF V66M. Consistently, there was reduced coimmunoprecipitation of pro-BDNF with HAP1 in the brain homogenate of Huntington disease. Pro-BDNF distribution in the neuronal processes and its accumulation in the proximal and distal segments of crushed sciatic nerve and the activity-dependent release of pro-BDNF were abolished in HAP1(-/-) mice. These results suggest that HAP1 may participate in axonal transport and activity-dependent release of pro-BDNF by interacting with the BDNF prodomain. Accordingly, the decreased interaction between HAP1 and pro-BDNF in Huntington disease may reduce the release and transport of BDNF.

Genetic and pharmacological inhibition of calcineurin corrects the BDNF transport defect in Huntington's disease

BackgroundHuntington's disease (HD) is an inherited neurogenerative disease caused by an abnormal expansion of glutamine repeats in the huntingtin protein. There is currently no treatment to prevent the neurodegeneration caused by this devastating disorder. Huntingtin has been shown to be a positive regulator of vesicular transport, particularly for neurotrophins such as brain-derived neurotrophic factor (BDNF). This function is lost in patients with HD, resulting in a decrease in neurotrophic support and subsequent neuronal death. One promising line of treatment is therefore the restoration of huntingtin function in BDNF transport.ResultsThe phosphorylation of huntingtin at serine 421 (S421) restores its function in axonal transport. We therefore investigated whether inhibition of calcineurin, the bona fide huntingtin S421 phosphatase, restored the transport defects observed in HD. We found that pharmacological inhibition of calcineurin by FK506 led to sustained phosphorylation of mutant huntingtin at S421. FK506 restored BDNF transport in two complementary models: rat primary neuronal cultures expressing mutant huntingtin and mouse cortical neurons from HdhQ111/Q111 HD knock-in mice. This effect was the result of specific calcineurin inhibition, as calcineurin silencing restored both anterograde and retrograde transport in neurons from HdhQ111/Q111 mice. We also observed a specific increase in calcineurin activity in the brain of HdhQ111/Q111 mice potentially accounting for the selective loss of huntingtin phosphorylation and contributing to neuronal cell death in HD.ConclusionOur results validate calcineurin as a target for the treatment of HD and provide the first demonstration of the restoration of huntingtin function by an FDA-approved compound.

Dynactin regulates bidirectional transport of dense-core vesicles in the axon and dendrites of cultured hippocampal neurons

A critical aspect of nerve cell function is peptidergic secretion involving the packaging, transport, and processing of a large group of peptide hormones and other signaling molecules, e.g. brain-derived neurotrophic factor (BDNF). Dense-core vesicles (DCVs) are the organelles that transport these molecules to release sites in both the axon and dendrites of pyramidal neurons. DCVs exhibit complex transport behavior, where these organelles move bidirectionally, reverse direction, pause intermittently, and vary in velocities and run lengths. A key objective in the field of organelle transport is to define the molecules that mediate transport. This study investigated the role of dynactin, a putative opposite-polarity motor coordinator, in the microtubule-based transport of DCVs in primary cultured hippocampal neurons. First, by live cell imaging, we showed similar microtubule-based transport of BDNF, neuropeptide Y (NPY), and tissue plasminogen activator (tPA), consistent with the co-packaging of these DCV cargoes. However, we found higher DCV velocities in both the axon and dendrites than those of previous neuronal studies likely due to faster image acquisition times. Then, using well-characterized dynactin disruptors we demonstrate the need for dynactin in bidirectional transport where overexpression of both p50/dynamitin and the first coiled-coil domain of p150Glued (CC1) reduces the flux of DCVs in both directions in the axon and dendrites. We also observed that only CC1 reduces axonal and dendritic run lengths. These results suggest different functions for p50 and p150 in the dynactin complex in DCV transport. These findings are significant because they demonstrate that dynactin functions as a motor coordinator for the transport of DCVs in primary cultured rat hippocampal neurons.

Ca 2+-dependent activator protein for secretion 2 (CAPS2) and autism susceptibility

A critical aspect of nerve cell function is peptidergic secretion involving the packaging, transport, and processing of a large group of peptide hormones and other signaling molecules, e.g. brain-derived neurotrophic factor (BDNF). Dense-core vesicles (DCVs) are the organelles that transport these molecules to release sites in both the axon and dendrites of pyramidal neurons. DCVs exhibit complex transport behavior, where these organelles move bidirectionally, reverse direction, pause intermittently, and vary in velocities and run lengths. A key objective in the field of organelle transport is to define the molecules that mediate transport. This study investigated the role of dynactin, a putative opposite-polarity motor coordinator, in the microtubule-based transport of DCVs in primary cultured hippocampal neurons. First, by live cell imaging, we showed similar microtubule-based transport of BDNF, neuropeptide Y (NPY), and tissue plasminogen activator (tPA), consistent with the co-packaging of these DCV cargoes. However, we found higher DCV velocities in both the axon and dendrites than those of previous neuronal studies likely due to faster image acquisition times. Then, using well-characterized dynactin disruptors we demonstrate the need for dynactin in bidirectional transport where overexpression of both p50/dynamitin and the first coiled-coil domain of p150Glued (CC1) reduces the flux of DCVs in both directions in the axon and dendrites. We also observed that only CC1 reduces axonal and dendritic run lengths. These results suggest different functions for p50 and p150 in the dynactin complex in DCV transport. These findings are significant because they demonstrate that dynactin functions as a motor coordinator for the transport of DCVs in primary cultured rat hippocampal neurons.

Phosphorylation of mutant huntingtin at S421 restores anterograde and retrograde transport in neurons

Huntingtin (htt), the protein mutated in Huntington's disease, is a positive regulatory factor for vesicular transport whose function is lost in disease. Here, we demonstrate that phosphorylation of htt at serine 421 (S421) restores its function in axonal transport. Using a strategy involving RNA (ribonucleic acid) interference and re-expression of various constructs, we show that polyQ (polyglutamine)-htt is unable to promote transport of brain-derived neurotrophic factor (BDNF)-containing vesicles, but polyQ-htt constitutively phosphorylated at S421 is as effective as the wild-type (wt) as concerns transport of these vesicles. The S421 phosphorylated polyQ-htt displays the wt function of inducing BDNF release. Phosphorylation restores the interaction between htt and the p150(Glued) subunit of dynactin and their association with microtubules in vitro and in cells. We also show that the IGF-1 (insulin growth factor type I)/Akt pathway by promoting htt phosphorylation compensates for the transport defect. This is the first description of a mechanism that restores the htt function altered in disease.

Differential changes in brain‐derived neurotrophic factor and extracellular signal‐regulated kinase in rat primary afferent pathways with colitis

Brain-derived neurotrophic factor (BDNF) has been postulated to participate in inflammation-induced visceral hypersensitivity by modulating the sensitivity of visceral afferents through the activation of intracellular signalling pathways such as the extracellular signal-regulated kinase (ERK) pathway. In the current study, we assessed the expression levels of BDNF and phospho-ERK in lumbosacral dorsal root ganglia (DRG) and spinal cord before and during tri-nitrobenzene sulfonic acid (TNBS)-induced colitis in rats with real-time PCR, ELISA, western blot and immunohistochemical techniques. BDNF mRNA and protein levels were increased in L1 and S1 but not L6 DRG when compared with control (L1: two- to five-fold increases, P < 0.05; S1: two- to three-fold increases, P < 0.05); however, BDNF protein but not mRNA level was increased in L1 and S1 spinal cord when compared with control. In parallel, TNBS colitis significantly induced phospho-ERK1/2 expression in L1 (four- to five-fold, P < 0.05) and S1 (two- to three-fold, P < 0.05) but not in L6 spinal cord levels. Immunohistochemistry results showed that the increase in phospho-ERK1/2 expression occurred at the region of the superficial dorsal horn and grey commisure of the spinal cord. In contrast, there was no change in phospho-ERK5 in any level of the spinal cord examined during colitis. The regional and time-specific changes in the levels of BDNF mRNA, protein and phospho-ERK with colitis may be a result of increased transcription of BDNF in DRG and anterograde transport of BDNF from DRG to spinal cord where it activates intracellular signalling molecules such as ERK1/2.

A bi-directional carboxypeptidase E-driven transport mechanism controls BDNF vesicle homeostasis in hippocampal neurons

Anterograde transport of brain-derived neurotrophic factor (BDNF) vesicles from the soma to neurite terminals is necessary for activity-dependent secretion of BDNF to mediate synaptic plasticity, memory and learning, and retrograde BDNF transport back to the soma for recycling. In our study, overexpression of the cytoplasmic tail of the carboxypeptidase E (CPE) found in BDNF vesicles significantly reduced localization of BDNF in neurites of hippocampal neurons. Live-cell imaging showed that the velocity and distance of movement of fluorescent protein-tagged CPE- or BDNF-containing vesicles were reduced in both directions. In pulldown assays, the CPE tail interacted with dynactin along with kinesin-2 and kinesin-3, and cytoplasmic dynein. Competition assays using a CPE tail peptide verified specific interaction between the CPE tail and dynactin. Thus, the CPE cytoplasmic tail binds dynactin that recruits kinesins or dynein for driving bi-directional transport of BDNF vesicle to maintain vesicle homeostasis and secretion in hippocampal neurons.

RESEARCH ARTICLE: Systematic Assessment of BDNF and Its Receptor Levels in Human Cortices Affected by Huntington's Disease

AbstractOne cardinal feature of Huntington's disease (HD) is the degeneration of striatal neurons, whose survival greatly depends on the binding of cortical brain‐derived neurotrophic factor (BDNF) with high‐affinity (TrkB) and low‐affinity neurotrophin receptors [p75 pan‐neurotrophin receptor (p75NTR)]. With a few exceptions, results obtained in HD mouse models demonstrate a reduction in cortical BDNF mRNA and protein, although autopsy data from a limited number of human HD cortices are conflicting. These studies indicate the presence of defects in cortical BDNF gene transcription and transport to striatum. We provide new evidence indicating a significant reduction in BDNF mRNA and protein in the cortex of 20 HD subjects in comparison with 17 controls, which supports the hypothesis of impaired BDNF production in human HD cortex. Analyses of the BDNF isoforms show that transcription from BDNF promoter II and IV is down‐regulated in human HD cortex from an early symptomatic stage. We also found that TrkB mRNA levels are reduced in caudate tissue but not in the cortex, whereas the mRNA levels of T‐Shc (a truncated TrkB isoform) and p75NTR are increased in the caudate. This indicates that, in addition to the reduction in BDNF mRNA, there is also unbalanced neurotrophic receptor signaling in HD.

Impaired Secretion of Brain-Derived Neurotrophic Factor and Neuropsychiatric Diseases

Recent studies have elucidated mechanisms of brain-derived neurotrophic factor (BDNF) secretion, and impaired secretion of BDNF may be involved in the pathogenesis of several neuropsychiatric diseases. The huntingtin gene, for example, has been shown to regulate vesicular transport of BDNF, which may play a role in the neurodegeneration present in Huntington's disease. In animal studies, mice lacking calcium-dependent activator protein for secretion 2 (CADPS2), which is involved in the activity-dependent release of BDNF, showed several phenotypes including autistic behavior. A single nucleotide polymorphism that results in an amino-acid change (Val66Met) in the BDNF gene has been shown to cause a decline in the function of BDNF vesicular sorting and has been reported to be associated with behavioral and intermediate phenotypes (e.g., episodic memory) in humans. In this review, we introduce recent progress in the molecular mechanisms of BDNF secretion and discuss its possible role in the pathophysiology and treatment of neuropsy-

Brain‐derived neurotrophic factor over‐expression in the forebrain ameliorates Huntington’s disease phenotypes in mice

Huntington's disease (HD), a dominantly inherited neurodegenerative disorder characterized by relatively selective degeneration of striatal neurons, is caused by an expanded polyglutamine tract of the huntingtin (htt) protein. The htt mutation reduces levels of brain-derived neurotrophic factor (BDNF) in the striatum, likely by inhibiting cortical BDNF gene expression and anterograde transport of BDNF from cortex to striatum. However, roles of the BDNF reduction in HD pathogenesis have not been established conclusively. We reasoned that increasing striatal BDNF through over-expression would slow progression of the disease if BDNF reduction plays a pivotal role in HD pathogenesis. We employed a Bdnf transgene driven by the promoter for the alpha subunit of Ca(2+)/calmodulin-dependent kinase II to over-express BDNF in the forebrain of R6/1 mice which express a fragment of mutant htt with a 116-glutamine tract. The Bdnf transgene increased BDNF levels and TrkB signaling activity in the striatum, ameliorated motor dysfunction, and reversed brain weight loss in R6/1 mice. Furthermore, it normalized DARPP-32 expression of the 32 kDa dopamine and cAMP-regulated phosphoprotein, increased the number of enkephalin-containing boutons, and reduced formation of neuronal intranuclear inclusions in the striatum of R6/1 mice. These results demonstrate crucial roles of reduced striatal BDNF in HD pathogenesis and suggest potential therapeutic values of BDNF to HD.

Anterograde Transport and Secretion of Brain-Derived Neurotrophic Factor along Sensory Axons Promote Schwann Cell Myelination

The neurotrophin brain-derived neurotrophic factor (BDNF) inhibits Schwann cell (SC) migration and promotes myelination via the p75 neurotrophin receptor (NTR). Despite these recent findings, the expression, localization, and mechanism of BDNF action has yet to be determined. Here we demonstrate that the sensory neurons of the dorsal root ganglion (DRG) are a major source of BDNF during postnatal development. The expression of BDNF is initially elevated before myelination and decreases dramatically after the onset of myelination. BDNF expression is controlled in part by transcriptional regulation and the increased expression of the truncated TrkB receptor on SCs. To investigate the possible mechanism of BDNF transport and release, multicompartment Campenot chambers were used. DRG neurons transported and secreted endogenous BDNF along the surface of axons in anterograde fashion. In an attempt to enhance myelination by SCs, DRG neurons were transduced with an adenovirus to overexpress BDNF. BDNF was transported and secreted along the axons and enhanced myelination when compared with control cocultures. Together, the events surrounding the expression, localization, and mechanism of BDNF action in DRG neurons may hint at potential therapeutic implications to efficiently promote remyelination.

Histone Deacetylase 6 Inhibition Compensates for the Transport Deficit in Huntington's Disease by Increasing Tubulin Acetylation

A defect in microtubule (MT)-based transport contributes to the neuronal toxicity observed in Huntington's disease (HD). Histone deacetylase (HDAC) inhibitors show neuroprotective effects in this devastating neurodegenerative disorder. We report here that HDAC inhibitors, including trichostatin A (TSA), increase vesicular transport of brain-derived neurotrophic factor (BDNF) by inhibiting HDAC6, thereby increasing acetylation at lysine 40 of alpha-tubulin. MT acetylation in vitro and in cells causes the recruitment of the molecular motors dynein and kinesin-1 to MTs. In neurons, acetylation at lysine 40 of alpha-tubulin increases the flux of vesicles and the subsequent release of BDNF. We show that tubulin acetylation is reduced in HD brains and that TSA compensates for the transport- and release-defect phenotypes that are observed in disease. Our findings reveal that HDAC6 inhibition and acetylation at lysine 40 of alpha-tubulin may be therapeutic targets of interest in disorders such as HD in which intracellular transport is altered.

Mutant huntingtin Impairs the Post-Golgi Trafficking of Brain-Derived Neurotrophic Factor But Not Its Val66Met Polymorphism

Brain-derived neurotrophic factor (BDNF) polymorphism is associated with the pathophysiology of several neurodegenerative disorders, including Huntington's disease. In view of these data and the involvement of huntingtin in intracellular trafficking, we examined the intracellular transport and release of Val66Val BDNF (Val-BDNF) and Val66Met BDNF (Met-BDNF) in transfected striatal knock-in cells expressing wild-type or mutant full-length huntingtin. Colocalization studies with specific markers for endoplasmic reticulum showed no differences between the Val-BDNF and Met-BDNF and were not modified by mutant huntingtin. However, post-Golgi trafficking was altered by mutant huntingtin dependent on the BDNF form. Thus, fluorescence recovery after photobleaching (FRAP) and inverse FRAP analysis showed retention of Met-BDNF in the Golgi apparatus with respect to Val-BDNF in wild-type cells. Strikingly, mutant huntingtin diminished post-Golgi trafficking of Val-BDNF, whereas Met-BDNF was not modified. Accordingly, a reduction in the number of transport vesicles was only observed in mutant huntingtin cells transfected with Val-BDNF but not Met-BDNF. Moreover, mutant huntingtin severely affected the KCl-evoked release of Val-BDNF, although it had little effect on Met-BDNF regulated release. The constitutive release of Val-BDNF or Met-BDNF in mutant cells was only slightly reduced. Interestingly, mutant huntingtin only perturbed post-Golgi trafficking of proteins that follow the regulated secretory pathway (epidermal growth factor receptor or atrial natriuretic factor), whereas it did not change those that follow the constitutive pathway (p75(NTR)). We conclude that mutant huntingtin differently affects intracellular transport and release of Val-BDNF and Met-BDNF. In addition, our findings reveal a new role for huntingtin in the regulation of the post-Golgi trafficking of the regulated secretory pathway.

A Role for Retinal Brain-Derived Neurotrophic Factor in Ocular Dominance Plasticity

Visual deprivation is a classical tool to study the plasticity of visual cortical connections. After eyelid closure in young animals (monocular deprivation, MD), visual cortical neurons become dominated by the open eye, a phenomenon known as ocular dominance (OD) plasticity . It is commonly held that the molecular mediators of OD plasticity are cortically derived and that the retina is immune to the effects of MD . Recently, it has been reported that visual deprivation induces neurochemical, structural, and functional changes in the retina , but whether these retinal changes contribute to the effects of MD in the cortex is unknown. Here, we provide evidence that brain-derived neurotrophic factor (BDNF) produced in the retina influences OD plasticity. We found a reduction of BDNF expression in the deprived retina of young rats. We compensated this BDNF imbalance between the two eyes by either injecting exogenous BDNF in the deprived eye or reducing endogenous BDNF expression in the nondeprived eye. Both treatments were effective in counteracting the OD shift induced by MD. Retinal BDNF could also influence OD distribution in normal animals. These results show for the first time that OD plasticity is modulated by BDNF produced in the retina.

Anterograde axonal transport of BDNF and NT-3 by retinal ganglion cells: Roles of neurotrophin receptors

Retinal ganglion cells (RGCs) transport exogenous neurotrophins anterogradely to the midbrain tectum/superior colliculus with significant downstream effects. We determined contributions of neurotrophin receptors for anterograde transport of intraocularly injected radiolabeled neurotrophins. In adult rodents, anterograde transport of brain-derived neurotrophic factor (BDNF) was receptor-mediated, and transport of exogenous BDNF and neurotrophin-3 (NT-3) was more efficient, per RGC, in rodents than chicks. RT-PCR and Western blot analysis of purified murine RGCs showed that adult RGCs express the p75 receptor. Anterograde transport of BDNF or NT-3 was not diminished in p75 knock-out mice (with unaltered final numbers of RGCs), but BDNF transport was substantially reduced by co-injected trkB antibodies. In chick embryos, however, p75 antisense or co-injected p75 antibodies significantly attenuated anterograde transport of NT-3 by RGCs. Thus, neither BDNF nor NT-3 utilizes p75 for anterograde transport in adult rodent RGCs, while anterograde NT-3 transport requires the p75 receptor in embryonic chicken RGCs.

Maladie de Huntington : signalisation intracellulaire et mort neuronale

Huntington's disease (HD) is a mid-life onset neurodegenerative disorder characterized by unvoluntary movements (chorea), personality changes and dementia that progress to death within 10-20 years of onset. There are currently no treatment to delay or prevent appearance of the symptoms in the patients. The defective gene in HD contains a trinucleotide CAG repeat expansion within its coding region that is expressed as a polyglutamine (polyQ) repeat in the protein huntingtin. The exact molecular mechanims by which mutant huntingtin induces cell death as well as the function of huntingtin are not totally understood. Studying mechanisms by which polyQ-huntingtin induces neurodegeneration has shown that phosphorylation plays a key role in HD. The IGF-1/Akt/SGK pathway reduces polyQ-huntingtin induced toxicity. This anti-apopototic effect is mediated via the phosphorylation of serine 421 of huntingtin. Moreover, components of this pathway are altered in disease. What is the function of huntingtin? Several studies indicate that huntingtin is an anti-apoptotic protein that could regulate intracellular dynamic. We recently demonstrated, that huntingtin specifically enhances vesicular transport of brain-derived neurotrophic factor (BDNF) along microtubules. Huntingtin-mediated transport involves Huntingtin-Associated Protein-1 (HAP1) and the p150(Glued) subunit of dynactin, an essential component of molecular motors. BDNF transport is attenuated both in the disease context and by reducing the levels of wild-type huntingtin. The alteration of the huntingtin/HAP1/ p150(Glued) complex correlates with reduced association of motor proteins with microtubules. Finally, polyQ-huntingtin-induced transport deficit results in the loss of neurotrophic support and neuronal toxicity.

Regulation of Trophic Factor Expression by Innervating Target Regions in Intraocular Double Transplants.

Trophic factors have been found to play a significant role both in long-term survival processes and in more rapid and dynamic processes in the brain and spinal cord. However, little is known regarding the regulation of expression of growth factors, and how these proteins interact on a cell-to-cell basis. We have studied protein levels of one growth factor known to affect the noradrenergic innervation of the hippocampal formation, namely brain-derived neurotrophic factor (BDNF). The purpose of the present study was to determine if appropriate innervation or contact between the LC noradrenergic neurons and their target, the hippocampus, affects expression of this growth factor in either brain region. Fetal brain stem tissue, containing the LC, and hippocampal formation were dissected from embryonic day 17 rat fetuses and transplanted together or alone into the anterior chamber of the eye of adult Fisher 344 rats. The tissue was grown together for 6 weeks, after which the animals were sacrificed and ELISAs for BDNF were undertaken. Transplantation to the anterior chamber of the eye increased the expression of BDNF in the hippocampal but not the brain stem tissue, compared with levels observed in fetal and adult rats in vivo. In addition, double grafting with hippocampal tissue more than tripled BDNF levels in brain stem grafts and doubled BDNF levels in the hippocampal portion of double grafts compared with hippocampal single grafts. Triple grafts containing basal forebrain, hippocampus, and brain stem LC tissue increased brain stem and hippocampal BDNF levels even further. Colchicine treatment of LC-hippocampal double grafts gave rise to a significant decrease in hippocampal BDNF levels to levels seen in single hippocampal grafts, while only a partial reduction of BDNF levels was seen in the brain stem portion of the same double grafts treated with colchicine. The findings suggest that an appropriate hippocampal innervation or contact with its target tissues is essential for regulation of BDNF expression in the brain stem, and that retrograde transport of BDNF can occur between double grafted fetal tissues in oculo.

Mutant Huntingtin Blocks BDNF Transport

The mechanisms by which polyglutamine (polyQ) expansion in the huntingtin protein (htt) cause neuronal pathogenesis are unclear (see Ross). Gauthier et al. report that wild-type huntingtin is required for proper vesicular transport of brain-derived neurotrophic factor (BDNF) through its interactions with huntingtin-associated protein-1 (HAP1), which in turn interacts with the p150 Glued subunit of dynactin, a required regulator of the microtubule motor dynein. Vesicle movement was monitored by recording the movement of BDNF-eGFP (enhanced green fluorescent protein)-tagged vesicles by ultrafast 3D videomicroscopy in NG108-15 neuroblastoma cells expressing either wild-type htt or polyQ-htt. Only wild-type htt promoted the movement of vesicles, measured as either the proportion of moving vesicles or the velocity of vesicle movement. To determine the relevance to Huntington's disease, the authors analyzed BDNF vesicular transport rates in wild-type cells, cells with one mutant htt allele (polyQ-htt), and cells with two mutant htt alleles. Cells with polyQ-htt showed decreased vesicle velocity, and overexpression of wild-type htt increased the velocity of vesicle movement. HAP1 was required for the htt stimulation of vesicle movement, because reduction of HAP1 by small interfering RNA blocked htt's stimulation of transport, and htt truncation mutants that did not interact with HAP1 did not stimulate transport. Sucrose gradient sedimentation analysis of wild-type cells or polyQ-htt cells indicated that htt was present in the dynein and dynactin protein complex, whereas polyQ-htt was present in a lower density fraction. In cells from Huntington patients, a disruption in the dynein and dynactin complex was observed. PolyQ-htt inhibited the interaction of HAP1 and various motor proteins (components of both kinesin and dynein) with microtubules. Release of BDNF after an initial stimulation was compromised in cells with polyQ-htt. Thus, the disruption of vesicular transport by polyQ-htt appears to block the refilling of the releasable pool of BDNF, which results in loss of trophic support and cell death. C. A. Ross, Huntington's disease: New paths to pathogenesis. Cell 118 , 4-7 (2004). [Online Journal] L. R. Gauthier, B. C. Charrin, M. Borrell-Pagès, J. P. Dompierre, H. Rangone, F. P. Cordelières, J. de Mey, M. E. MacDonald, V. Leßmann, S. Humbert, F. Saudou, Huntingtin controls neurotrophic support and survival of neurons by enhancing BDNF vesicular transport along microtubules. Cell 118 , 127-138 (2004). [Online Journal]

Huntingtin Controls Neurotrophic Support and Survival of Neurons by Enhancing BDNF Vesicular Transport along Microtubules

Polyglutamine expansion (polyQ) in the protein huntingtin is pathogenic and responsible for the neuronal toxicity associated with Huntington's disease (HD). Although wild-type huntingtin possesses antiapoptotic properties, the relationship between the neuroprotective functions of huntingtin and pathogenesis of HD remains unclear. Here, we show that huntingtin specifically enhances vesicular transport of brain-derived neurotrophic factor (BDNF) along microtubules. Huntingtin-mediated transport involves huntingtin-associated protein-1 (HAP1) and the p150 Glued subunit of dynactin, an essential component of molecular motors. BDNF transport is attenuated both in the disease context and by reducing the levels of wild-type huntingtin. The alteration of the huntingtin/HAP1/p150 Glued complex correlates with reduced association of motor proteins with microtubules. Finally, we find that the polyQ-huntingtin-induced transport deficit results in the loss of neurotrophic support and neuronal toxicity. Our findings indicate that a key role of huntingtin is to promote BDNF transport and suggest that loss of this function might contribute to pathogenesis.

Spontaneous retinal activity modulates BDNF trafficking in the developing chick visual system

Both neuronal activity and neurotrophin signaling play critical roles in normal CNS development. This study examined whether spontaneous retinal activity (SRA) also governs the axonal transport of endogenous brain-derived neurotrophic factor (BDNF) protein within the developing chick visual system. In previous work, we have found that during the normal period of SRA, retinal BDNF protein levels decrease by about 50% while BDNF mRNA levels remain elevated. Here, we show that the blockade of SRA with tetrodotoxin (TTX), or the blockade of axonal transport with colchicine, both reversed the normal mismatch between retinal BDNF mRNA and protein. The axonal transport of retinal-derived BDNF in segments of the optic nerve as well as tectal-derived BDNF protein transported in segments of the optic tract were both significantly reduced after very brief periods of activity blockade. These results suggest that normal SRA plays a role in regulating the axonal transport of endogenous BDNF protein.