Reduced lysosomal density in neuronal dendrites mediates deficits in synaptic plasticity in Huntington's disease.

Huntington's disease (HD) usually causes cognitive disorders, including learning difficulties, that emerge before motor symptoms. Mutations related to lysosomal trafficking are linked to the pathogenesis of neurological diseases, whereas the cellular mechanisms remain elusive. Here, we discover a reduction in the dendritic density of lysosomes in the hippocampus that correlates with deficits in synaptic plasticity and spatial learning in early CAG-140 HD model mice. We directly manipulate intraneuronal lysosomal positioning with light-induced CRY2:CIB1 dimerization and demonstrate that lysosomal abundance in dendrites positively modulates long-term potentiation of glutamatergic synapses onto the neuron. This modulation depends on lysosomal Ca2+ release, which further promotes endoplasmic reticulum (ER) entry into spines. Importantly, optogenetically restoring lysosomal density in dendrites rescues the synaptic plasticity deficit in hippocampal slices of CAG-140 mice. Our data reveal dendritic lysosomal density as a modulator of synaptic plasticity and suggest a role of lysosomal mispositioning in cognitive decline in HD.

Facial nerve axotomy induces morphological changes in hippocampal pyramidal neurons.

Facial nerve injury in rats have been widely used to study functional and structural changes that occur in the injured motoneurons and other central nervous system structures related with sensorimotor processing. A decrease in long-term potentiation of hippocampal CA3-to-CA1 commissural synapse has recently been reported related to this peripheral injury. Additionally, it has been found increased corticosterone plasmatic levels, impairment in spatial memory consolidation, and hippocampal microglial activation in animals with facial nerve axotomy. In this work, we analyzed the neuronal morphology of hippocampal CA1 and CA3 pyramidal neurons in animals with either reversible or irreversible facial nerve injury. For this purpose, brain tissues of injured animals sacrificed at different postlesion times, were stained with the Golgi-Cox method and compared with control brains. It was found that both reversible and irreversible facial nerve injury-induced significant decreases in dendritic tree complexity, dendritic length, branch points, and spine density of hippocampal neurons. However, such changes' timing varied according to hippocampal area (CA1 vs. CA3), dendritic area (apical vs. basal), and lesion type (reversible vs. irreversible). In general, the observed changes were transient when animals had the possibility of motor recovery (reversible injury), but perdurable if the recovery from the lesion was impeded (irreversible injury). CA1 apical and CA3 basal dendritic tree morphology were more sensible to irreversible injury. It is concluded that facial nerve injury induced significant changes in hippocampal CA1 and CA3 pyramidal neurons morphology, which could be related to LTP impairments and microglial activation in the hippocampal formation, previously described.

Dendrite remodeling according to GARP.

Disruptions in membrane trafficking are associated with neurodevelopmental disorders, but underlying pathological mechanisms remain largely unknown. In this issue, O'Brien et al. (2023. J. Cell Biol.https://doi.org/10.1083/jcb.202112108) show how GARP regulates sterol transfer critical for remodeling of dendrites in flies.

Changes in dendritic complexity and spine morphology following BCG immunization in APP/PS1 mice.

Bacillus Calmette - Guerin (BCG) is an immune regulator that can enhance hippocampal synaptic plasticity in rats; however, it is unclear whether it can improve synaptic function in a mouse model with Alzheimer's disease (AD). We hypothesized that BCG plays a protective role in AD mice and investigated its effect on dendritic morphology. The results obtained show that BCG immunization significantly increases dendritic complexity, as indicated by the increased number of dendritic intersections and branch points, as well as the increase in the fractal dimension. Furthermore, the number of primary neurites and dendritic length also increased following BCG immunization, which increased the number of spines and promoted maturation. IFN-γ and IL-4 levels increased, while TNF-α levels decreased following BCG immunization; expression levels of p-JAK2, P-STAT3, SYN, and PSD-95 also increased. Therefore, this study demonstrates that BCG immunization in APP/PS1 mice mitigated hippocampal dendritic spine pathology, especially after the third round of immunization. This effect could possibly be attributed to; changes in dendritic arborization and spine morphology or increases in SYN and PSD-95 expression levels. It could also be related to mechanisms of BCG-induced increases in IFN-γ or IL-4/JAK2/STAT3 levels.

BCG immunization in a mouse model for Alzheimer's disease significantly increased dendritic complexity, as indicated by an increase in the number of dendritic intersections and branch points, as well as an increase in the fractal dimension of hippocampal CA1 neurons.

Estrogen Enhances Dendrite Spine Function and Recovers Deficits in Neuroplasticity in the prpTDP-43A315T Mouse Model of Amyotrophic Lateral Sclerosis.

Amyotrophic lateral sclerosis (ALS) attacks the corticomotor system, with motor cortex function affected early in disease. Younger females have a lower relative risk of succumbing to ALS than males and older females, implicating a role for female sex hormones in disease progression. However, the mechanisms driving this dimorphic incidence are still largely unknown. We endeavoured to determine if estrogen mitigates disease progression and pathogenesis, focussing upon the dendritic spine as a site of action. Using two-photon live imaging we identify, in the prpTDP-43A315T mouse model of ALS, that dendritic spines in the male motor cortex have a reduced capacity for remodelling than their wild-type controls. In contrast, females show higher capacity for remodelling, with peak plasticity corresponding to highest estrogen levels during the estrous cycle. Estrogen manipulation through ovariectomies and estrogen replacement with 17ß estradiol in vivo was found to significantly alter spine density and mitigate disease severity. Collectively, these findings reveal that synpatic plasticity is reduced in ALS, which can be amelioriated with estrogen, in conjuction with improved disease outcomes.

Myotubularin-related protein protects against neuronal degeneration mediated by oxidative stress or infection.

Microbial infections have been linked to the onset and severity of neurodegenerative diseases such as amyotrophic lateral sclerosis, multiple sclerosis, Alzheimer's disease, but the underlying mechanisms remain largely unknown. Here, we used a genetic screen for genes involved in protection from infection-associated neurodegeneration and identified the gene mtm-10. We then validated the role of the encoded myotubularin-related protein, MTM-10, in protecting the dendrites of Caenorhabditis elegans from degeneration mediated by oxidative stress or Pseudomonas aeruginosa infection. Further experiments indicated that mtm-10 is expressed in the AWC neurons of C. elegans, where it functions in a cell-autonomous manner to protect the dendrite degeneration caused by pathogen infection. We also confirm that the changes observed in the dendrites of the animals were not because of premature death or overall sickness. Finally, our studies indicated that mtm-10 functions in AWC neurons to preserve chemosensation after pathogen infection. These results reveal an essential role for myotubularin-related protein 10 in the protection of dendrite morphology and function against the deleterious effects of oxidative stress or infection.

Physiological significance of WDR45, a responsible gene for ß-propeller protein associated neurodegeneration (BPAN), in brain development.

WDR45 plays an essential role in the early stage of autophagy. De novo heterozygous mutations in WDR45 have been known to cause ß-propeller protein-associated neurodegeneration (BPAN), a subtype of neurodegeneration with brain iron accumulation (NBIA). Although BPAN patients display global developmental delay with intellectual disability, the neurodevelopmental pathophysiology of BPAN remains largely unknown. In the present study, we analyzed the physiological role of Wdr45 and pathophysiological significance of the gene abnormality during mouse brain development. Morphological and biochemical analyses revealed that Wdr45 is expressed in a developmental stage-dependent manner in mouse brain. Wdr45 was also found to be located in excitatory synapses by biochemical fractionation. Since WDR45 mutations are thought to cause protein degradation, we conducted acute knockdown experiments by in utero electroporation in mice to recapitulate the pathophysiological conditions of BPAN. Knockdown of Wdr45 caused abnormal dendritic development and synaptogenesis during corticogenesis, both of which were significantly rescued by co-expression with RNAi-resistant version of Wdr45. In addition, terminal arbors of callosal axons were less developed in Wdr45-deficient cortical neurons of adult mouse when compared to control cells. These results strongly suggest a pathophysiological significance of WDR45 gene abnormalities in neurodevelopmental aspects of BPAN.

Disruption of mitochondrial complex I induces progressive parkinsonism.

Loss of functional mitochondrial complex I (MCI) in the dopaminergic neurons of the substantia nigra is a hallmark of Parkinson's disease1. Yet, whether this change contributes to Parkinson's disease pathogenesis is unclear2. Here we used intersectional genetics to disrupt the function of MCI in mouse dopaminergic neurons. Disruption of MCI induced a Warburg-like shift in metabolism that enabled neuronal survival, but triggered a progressive loss of the dopaminergic phenotype that was first evident in nigrostriatal axons. This axonal deficit was accompanied by motor learning and fine motor deficits, but not by clear levodopa-responsive parkinsonism-which emerged only after the later loss of dopamine release in the substantia nigra. Thus, MCI dysfunction alone is sufficient to cause progressive, human-like parkinsonism in which the loss of nigral dopamine release makes a critical contribution to motor dysfunction, contrary to the current Parkinson's disease paradigm3,4.

The sympathetic nervous system in development and disease.

The sympathetic nervous system prepares the body for 'fight or flight' responses and maintains homeostasis during daily activities such as exercise, eating a meal or regulation of body temperature. Sympathetic regulation of bodily functions requires the establishment and refinement of anatomically and functionally precise connections between postganglionic sympathetic neurons and peripheral organs distributed widely throughout the body. Mechanistic studies of key events in the formation of postganglionic sympathetic neurons during embryonic and early postnatal life, including axon growth, target innervation, neuron survival, and dendrite growth and synapse formation, have advanced the understanding of how neuronal development is shaped by interactions with peripheral tissues and organs. Recent progress has also been made in identifying how the cellular and molecular diversity of sympathetic neurons is established to meet the functional demands of peripheral organs. In this Review, we summarize current knowledge of signalling pathways underlying the development of the sympathetic nervous system. These findings have implications for unravelling the contribution of sympathetic dysfunction stemming, in part, from developmental perturbations to the pathophysiology of peripheral neuropathies and cardiovascular and metabolic disorders.

Temporal manipulation of Cdkl5 reveals essential postdevelopmental functions and reversible CDKL5 deficiency disorder-related deficits.

CDKL5 deficiency disorder (CDD) is an early onset, neurodevelopmental syndrome associated with pathogenic variants in the X-linked gene encoding cyclin-dependent kinase-like 5 (CDKL5). CDKL5 has been implicated in neuronal synapse maturation, yet its postdevelopmental necessity and the reversibility of CDD-associated impairments remain unknown. We temporally manipulated endogenous Cdkl5 expression in male mice and found that postdevelopmental loss of CDKL5 disrupts numerous behavioral domains, hippocampal circuit communication, and dendritic spine morphology, demonstrating an indispensable role for CDKL5 in the adult brain. Accordingly, restoration of Cdkl5 after the early stages of brain development using a conditional rescue mouse model ameliorated CDD-related behavioral impairments and aberrant NMDA receptor signaling. These findings highlight the requirement of CDKL5 beyond early development, underscore the potential for disease reversal in CDD, and suggest that a broad therapeutic time window exists for potential treatment of CDD-related deficits.

Connecting the Neurobiology of Developmental Brain Injury: Neuronal Arborisation as a Regulator of Dysfunction and Potential Therapeutic Target.

Neurodevelopmental disorders can derive from a complex combination of genetic variation and environmental pressures on key developmental processes. Despite this complex aetiology, and the equally complex array of syndromes and conditions diagnosed under the heading of neurodevelopmental disorder, there are parallels in the neuropathology of these conditions that suggest overlapping mechanisms of cellular injury and dysfunction. Neuronal arborisation is a process of dendrite and axon extension that is essential for the connectivity between neurons that underlies normal brain function. Disrupted arborisation and synapse formation are commonly reported in neurodevelopmental disorders. Here, we summarise the evidence for disrupted neuronal arborisation in these conditions, focusing primarily on the cortex and hippocampus. In addition, we explore the developmentally specific mechanisms by which neuronal arborisation is regulated. Finally, we discuss key regulators of neuronal arborisation that could link to neurodevelopmental disease and the potential for pharmacological modification of arborisation and the formation of synaptic connections that may provide therapeutic benefit in the future.

Ste20-like Kinase Is Critical for Inhibitory Synapse Maintenance and Its Deficiency Confers a Developmental Dendritopathy.

The size and structure of the dendritic arbor play important roles in determining how synaptic inputs of neurons are converted to action potential output. The regulatory mechanisms governing the development of dendrites, however, are insufficiently understood. The evolutionary conserved Ste20/Hippo kinase pathway has been proposed to play an important role in regulating the formation and maintenance of dendritic architecture. A key element of this pathway, Ste20-like kinase (SLK), regulates cytoskeletal dynamics in non-neuronal cells and is strongly expressed throughout neuronal development. However, its function in neurons is unknown. We show that, during development of mouse cortical neurons, SLK has a surprisingly specific role for proper elaboration of higher, ≥ third-order dendrites both in male and in female mice. Moreover, we demonstrate that SLK is required to maintain excitation-inhibition balance. Specifically, SLK knockdown caused a selective loss of inhibitory synapses and functional inhibition after postnatal day 15, whereas excitatory neurotransmission was unaffected. Finally, we show that this mechanism may be relevant for human disease, as dysmorphic neurons within human cortical malformations revealed significant loss of SLK expression. Overall, the present data identify SLK as a key regulator of both dendritic complexity during development and inhibitory synapse maintenance.SIGNIFICANCE STATEMENT We show that dysmorphic neurons of human epileptogenic brain lesions have decreased levels of the Ste20-like kinase (SLK). Decreasing SLK expression in mouse neurons revealed that SLK has essential functions in forming the neuronal dendritic tree and in maintaining inhibitory connections with neighboring neurons.

Patient-derived iPSC-cerebral organoid modeling of the 17q11.2 microdeletion syndrome establishes CRLF3 as a critical regulator of neurogenesis.

Neurodevelopmental disorders are often caused by chromosomal microdeletions comprising numerous contiguous genes. A subset of neurofibromatosis type 1 (NF1) patients with severe developmental delays and intellectual disability harbors such a microdeletion event on chromosome 17q11.2, involving the NF1 gene and flanking regions (NF1 total gene deletion [NF1-TGD]). Using patient-derived human induced pluripotent stem cell (hiPSC)-forebrain cerebral organoids (hCOs), we identify both neural stem cell (NSC) proliferation and neuronal maturation abnormalities in NF1-TGD hCOs. While increased NSC proliferation results from decreased NF1/RAS regulation, the neuronal differentiation, survival, and maturation defects are caused by reduced cytokine receptor-like factor 3 (CRLF3) expression and impaired RhoA signaling. Furthermore, we demonstrate a higher autistic trait burden in NF1 patients harboring a deleterious germline mutation in the CRLF3 gene (c.1166T>C, p.Leu389Pro). Collectively, these findings identify a causative gene within the NF1-TGD locus responsible for hCO neuronal abnormalities and autism in children with NF1.

AMPK hyperactivation promotes dendrite retraction, synaptic loss, and neuronal dysfunction in glaucoma.

BACKGROUND: The maintenance of complex dendritic arbors and synaptic transmission are processes that require a substantial amount of energy. Bioenergetic decline is a prominent feature of chronic neurodegenerative diseases, yet the signaling mechanisms that link energy stress with neuronal dysfunction are poorly understood. Recent work has implicated energy deficits in glaucoma, and retinal ganglion cell (RGC) dendritic pathology and synapse disassembly are key features of ocular hypertension damage. RESULTS: We show that adenosine monophosphate-activated protein kinase (AMPK), a conserved energy biosensor, is strongly activated in RGC from mice with ocular hypertension and patients with primary open angle glaucoma. Our data demonstrate that AMPK triggers RGC dendrite retraction and synapse elimination. We show that the harmful effect of AMPK is exerted through inhibition of the mammalian target of rapamycin complex 1 (mTORC1). Attenuation of AMPK activity restores mTORC1 function and rescues dendrites and synaptic contacts. Strikingly, AMPK depletion promotes recovery of light-evoked retinal responses, improves axonal transport, and extends RGC survival. CONCLUSIONS: This study identifies AMPK as a critical nexus between bioenergetic decline and RGC dysfunction during pressure-induced stress, and highlights the importance of targeting energy homeostasis in glaucoma and other neurodegenerative diseases.

Microglia provide structural resolution to injured dendrites after severe seizures.

Although an imbalance between neuronal excitation and inhibition underlies seizures, clinical approaches that target these mechanisms are insufficient in containing seizures in patients with epilepsy, raising the need for alternative approaches. Brain-resident microglia contribute to the development and stability of neuronal structure and functional networks that are perturbed during seizures. However, the extent of microglial contributions in response to seizures in vivo remain to be elucidated. Using two-photon in vivo imaging to visualize microglial dynamics, we show that severe seizures induce formation of microglial process pouches that target but rarely engulf beaded neuronal dendrites. Microglial process pouches are stable for hours, although they often shrink in size. We further find that microglial process pouches are associated with a better structural resolution of beaded dendrites. These findings provide evidence for the structural resolution of injured dendrites by microglia as a form of neuroprotection.

Treadmill Exercise Reverses the Change of Dendritic Morphology and Activates BNDF-mTOR Signaling Pathway in the Hippocampus and Cerebral Cortex of Ovariectomized Mice.

A decline of estrogen level leads to spatial learning and memory impairments, which mediated by hippocampus and cortex. Accumulating evidences demonstrated that aerobic exercise improved memory of postmenopausal women and ovariectomized (OVX) mice. However, the molecular mechanisms for this protection of exercise are not completely clear. Accordingly, the present study was designed to examine the effect of aerobic exercise on the dendritic morphology in the hippocampus and cerebral cortex, as well as the BNDF-mTOR signaling pathway of OVX mice. Adult female C57BL/6 mice were divided into four groups (n = 10/group): sham-operated (SHAM/CON), sham-operated with 8-week treadmill exercise (SHAM/EX), ovariectomized operated (OVX/CON), and ovariectomized operated with exercise (OVX/EX). Aerobic exercise improved the impairment of dendritic morphology significantly induced by OVX that was tested by Golgi staining, and it also upregulated the synaptic plasticity-related protein expression of PSD95 and GluR1 as well as activated BDNF-mTOR signaling pathway in the hippocampus and cerebral cortex. In conclusion, aerobic exercise reversed the change of dendritic morphology and increased the synaptic plasticity-related protein expression in the hippocampus and cerebral cortex of OVX mice. The positive effects induced by exercise might be mediated through the BDNF-mTOR signaling pathway.

Chronic SSRI treatment reverses HIV-1 protein-mediated synaptodendritic damage.

HIV-1 infection affects approximately 37 million individuals, and approximately 50% of seropositive individuals will develop symptoms of clinical depression and/or apathy. Dysfunctions of both serotonergic and dopaminergic neurotransmission have been implicated in the pathogenesis of motivational alterations. The present study evaluated the efficacy of a SSRI (escitalopram) in the HIV-1 transgenic (Tg) rat. Behavioral, neurochemical, and neuroanatomical outcomes with respect to HIV-1 and sex were evaluated to determine the efficacy of chronic escitalopram treatment. Escitalopram treatment restored function in each of the behavioral tasks that were sensitive to HIV-1-induced impairments. Further, escitalopram treatment restored HIV-1-mediated synaptodendritic damage in the nucleus accumbens; treatment with escitalopram significantly increased dendritic proliferation in HIV-1 Tg rats. However, restoration did not consistently occur with the neurochemical analysis in the HIV-1 rat. Taken together, these results suggest a role for SSRI therapies in repairing long-term HIV-1 protein-mediated neuronal damage and restoring function.

Exendin-4 improves long-term potentiation and neuronal dendritic growth in vivo and in vitro obesity condition.

Metabolic syndrome, which increases the risk of obesity and type 2 diabetes has emerged as a significant issue worldwide. Recent studies have highlighted the relationship between metabolic imbalance and neurological pathologies such as memory loss. Glucagon-like peptide 1 (GLP-1) secreted from gut L-cells and specific brain nuclei plays multiple roles including regulation of insulin sensitivity, inflammation and synaptic plasticity. Although GLP-1 and GLP-1 receptor agonists appear to have neuroprotective function, the specific mechanism of their action in brain remains unclear. We investigated whether exendin-4, as a GLP-1RA, improves cognitive function and brain insulin resistance in metabolic-imbalanced mice fed a high-fat diet. Considering the result of electrophysiological experiments, exendin-4 inhibits the reduction of long term potentiation (LTP) in high fat diet mouse brain. Further, we identified the neuroprotective effect of exendin-4 in primary cultured hippocampal and cortical neurons in in vitro metabolic imbalanced condition. Our results showed the improvement of IRS-1 phosphorylation, neuronal complexity, and the mature of dendritic spine shape by exendin-4 treatment in metabolic imbalanced in vitro condition. Here, we provides significant evidences on the effect of exendin-4 on synaptic plasticity, long-term potentiation, and neural structure. We suggest that GLP-1 is important to treat neuropathology caused by metabolic syndrome.

A new reduced-morphology model for CA1 pyramidal cells and its validation and comparison with other models using HippoUnit.

Modeling long-term neuronal dynamics may require running long-lasting simulations. Such simulations are computationally expensive, and therefore it is advantageous to use simplified models that sufficiently reproduce the real neuronal properties. Reducing the complexity of the neuronal dendritic tree is one option. Therefore, we have developed a new reduced-morphology model of the rat CA1 pyramidal cell which retains major dendritic branch classes. To validate our model with experimental data, we used HippoUnit, a recently established standardized test suite for CA1 pyramidal cell models. The HippoUnit allowed us to systematically evaluate the somatic and dendritic properties of the model and compare them to models publicly available in the ModelDB database. Our model reproduced (1) somatic spiking properties, (2) somatic depolarization block, (3) EPSP attenuation, (4) action potential backpropagation, and (5) synaptic integration at oblique dendrites of CA1 neurons. The overall performance of the model in these tests achieved higher biological accuracy compared to other tested models. We conclude that, due to its realistic biophysics and low morphological complexity, our model captures key physiological features of CA1 pyramidal neurons and shortens computational time, respectively. Thus, the validated reduced-morphology model can be used for computationally demanding simulations as a substitute for more complex models.

Corneal confocal microscopy differentiates inflammatory from diabetic neuropathy.

BACKGROUND: Immune-mediated neuropathies, such as chronic inflammatory demyelinating polyneuropathy (CIDP) are treatable neuropathies. Among individuals with diabetic neuropathy, it remains a challenge to identify those individuals who develop CIDP. Corneal confocal microscopy (CCM) has been shown to detect corneal nerve fiber loss and cellular infiltrates in the sub-basal layer of the cornea. The objective of the study was to determine whether CCM can distinguish diabetic neuropathy from CIDP and whether CCM can detect CIDP in persons with coexisting diabetes. METHODS: In this multicenter, case-control study, participants with CIDP (n = 55) with (n = 10) and without (n = 45) diabetes; participants with diabetes (n = 58) with (n = 28) and without (n = 30) diabetic neuropathy, and healthy controls (n = 58) underwent CCM. Corneal nerve fiber density (CNFD), corneal nerve fiber length (CNFL), corneal nerve branch density (CNBD), and dendritic and non-dendritic cell density, with or without nerve fiber contact were quantified. RESULTS: Dendritic cell density in proximity to corneal nerve fibers was significantly higher in participants with CIDP with and without diabetes compared to participants with diabetic neuropathy and controls. CNFD, CNFL, and CNBD were equally reduced in participants with CIDP, diabetic neuropathy, and CIDP with diabetes. CONCLUSIONS: An increase in dendritic cell density identifies persons with CIDP. CCM may, therefore, be useful to differentiate inflammatory from non-inflammatory diabetic neuropathy.