Retrograde Signaling from Progranulin to Sort1 Counteracts Synapse Elimination in the Developing Cerebellum

Molecular and Anatomical Strengthening of "Winner" Climbing Fiber Synapses in Developing Mouse Purkinje Cells

Neural circuits are refined by strengthening frequently used or advantaged synapses while eliminating redundant connections. In neonatal mice, cerebellar Purkinje cells (PCs) are initially innervated by multiple climbing fibers (CFs) of similar strength. By postnatal day 7 (P7), one CF, the "winner," is selectively strengthened and begins dendritic translocation by P9, while both "winner" and "loser" CFs temporarily maintain somatic synapses. Although the functional differentiation of CF inputs is well understood, their structural differentiation is less clear. In this study, we examined "winner" CF synapses in dendrites and both "winner" and "loser" synapses in the soma using serial electron microscopy and immunohistochemistry in C57BL/6 mice. We found that "winner" CF synapses, both in the soma and dendrites, developed more complex pre- and postsynaptic structures than "loser" CFs, with an expanded area of postsynaptic density. Additionally, "winner" CF synapses expressed significantly higher levels of AMPA-type glutamate receptors. Notably, only dendritic "winner" synapses showed increased levels of Rab3-interacting molecule RIM, a key presynaptic regulator of neurotransmitter release. These findings reveal the molecular and structural features that enable "winner" CFs to reinforce their synaptic strength and innervation, allowing them to outcompete other inputs during early development.

Reduced GABAergic inhibition and impaired synapse elimination by neuroligin-2 deletion from Purkinje cells of the developing cerebellum

Functionally mature neural circuits are shaped during postnatal development by eliminating redundant synapses formed around birth. This process is known as synapse elimination and requires a proper balance of excitation and inhibition. Neuroligin-2 (NL2) is a postsynaptic cell adhesion molecule required for the formation, maintenance, and function of inhibitory synapses. However, how NL2 regulates synapse elimination during postnatal development is largely unknown. Here we report that the deletion of NL2 from Purkinje cells (PCs) in the cerebellum impairs the developmental elimination of redundant climbing fiber (CF) to PC synapses. In global NL2-knockout (KO) mice, GABAergic inhibition to PCs was attenuated and CF synapse elimination was impaired after postnatal day 10 (P10). These phenotypes were restored by the expression of NL2 into PCs of NL2-KO mice. Moreover, microRNA-mediated knockdown of NL2 specifically from PCs during development caused attenuated inhibition and impaired CF synapse elimination. In PCs innervated by "strong" and "weak" CFs, calcium transients elicited by "weak" CFs were enhanced in NL2-deficient PCs, suggesting that excess calcium signaling permits the survival of redundant "weak" CF synapses. We conclude that NL2 is crucial for maintaining inhibitory synaptic function and properly eliminating redundant CF synapses during postnatal development.

Neuronal Ceroid Lipofuscinosis-Concepts, Classification, and Avenues for Therapy

Neuronal ceroid lipofuscinosis (NCL) is a group of neurodegenerative lysosomal storage disorders characterized by excessive accumulation of lysosomal lipofuscin. Thirteen subtypes of NCL have been identified, each associated with distinct genes encoding various transmembrane proteins, secretory proteins, or lysosomal enzymes. Clinically, NCL manifests in infants through vision impairment, motor and cognitive dysfunctions, epilepsy, and premature death. The pathological complexity of NCL has hindered the development of effective clinical protocols. Current treatment modalities, including enzyme replacement therapy, pharmacological approaches, gene therapy, and stem cell therapy, have demonstrated limited efficacy. However, emerging evidence suggests a significant relationship between NCL and microglial cells, highlighting the potential of novel microglial cell replacement therapies. This review comprehensively examines the pathogenic genes associated with various NCL subtypes, elucidating their roles, clinical presentations, and corresponding mouse models. Especially, we thoroughly discuss the advances in the clinical study of potential therapeutics, which crucially calls for early diagnosis and treatment more than ever.

Glutamine metabolism prognostic index predicts tumour microenvironment characteristics and therapeutic efficacy in ovarian cancer

Mounting evidence has highlighted the multifunctional characteristics of glutamine metabolism (GM) in cancer initiation, progression and therapeutic regimens. However, the overall role of GM in the tumour microenvironment (TME), clinical stratification and therapeutic efficacy in patients with ovarian cancer (OC) has not been fully elucidated. Here, three distinct GM clusters were identified and exhibited different prognostic values, biological functions and immune infiltration in TME. Subsequently, glutamine metabolism prognostic index (GMPI) was constructed as a new scoring model to quantify the GM subtypes and was verified as an independent predictor of OC. Patients with low-GMPI exhibited favourable survival outcomes, lower enrichment of several oncogenic pathways, less immunosuppressive cell infiltration and better immunotherapy responses. Single-cell sequencing analysis revealed a unique evolutionary trajectory of OC cells from high-GMPI to low-GMPI, and OC cells with different GMPI might communicate with distinct cell populations through ligand-receptor interactions. Critically, the therapeutic efficacy of several drug candidates was validated based on patient-derived organoids (PDOs). The proposed GMPI could serve as a reliable signature for predicting patient prognosis and contribute to optimising therapeutic strategies for OC.

Neuronal DSCAM regulates the peri-synaptic localization of GLAST in Bergmann glia for functional synapse formation

In the central nervous system, astrocytes enable appropriate synapse function through glutamate clearance from the synaptic cleft; however, it remains unclear how astrocytic glutamate transporters function at peri-synaptic contact. Here, we report that Down syndrome cell adhesion molecule (DSCAM) in Purkinje cells controls synapse formation and function in the developing cerebellum. Dscam-mutant mice show defects in CF synapse translocation as is observed in loss of function mutations in the astrocytic glutamate transporter GLAST expressed in Bergmann glia. These mice show impaired glutamate clearance and the delocalization of GLAST away from the cleft of parallel fibre (PF) synapse. GLAST complexes with the extracellular domain of DSCAM. Riluzole, as an activator of GLAST-mediated uptake, rescues the proximal impairment in CF synapse formation in Purkinje cell-selective Dscam-deficient mice. DSCAM is required for motor learning, but not gross motor coordination. In conclusion, the intercellular association of synaptic and astrocyte proteins is important for synapse formation and function in neural transmission.

Molecular and cellular mechanisms of developmental synapse elimination in the cerebellum: Involvement of autism spectrum disorder-related genes

Neural circuits are initially created with excessive synapse formation until around birth and undergo massive reorganization until they mature. During postnatal development, necessary synapses strengthen and remain, whereas unnecessary ones are weakened and eventually eliminated. These events, collectively called "synapse elimination" or "synapse pruning", are thought to be fundamental for creating functionally mature neural circuits in adult animals. In the cerebellum of neonatal rodents, Purkinje cells (PCs) receive synaptic inputs from multiple climbing fibers (CFs). Then, inputs from a single CF are strengthened and those from the other CFs are eliminated, and most PCs become innervated by single CFs by the end of the third postnatal week. These events are regarded as a representative model of synapse elimination. This review examines the molecular and cellular mechanisms of CF synapse elimination in the developing cerebellum and argues how autism spectrum disorder (ASD)-related genes are involved in CF synapse development. We introduce recent studies to update our knowledge, incorporate new data into the known scheme, and discuss the remaining issues and future directions.

PTPδ is a presynaptic organizer for the formation and maintenance of climbing fiber to Purkinje cell synapses in the developing cerebellum

Functionally mature neural circuits are shaped during postnatal development by eliminating redundant synapses formed during the perinatal period. In the cerebellum of neonatal rodents, each Purkinje cell (PC) receives synaptic inputs from multiple (more than 4) climbing fibers (CFs). During the first 3 postnatal weeks, synaptic inputs from a single CF become markedly larger and those from the other CFs are eliminated in each PC, leading to mono-innervation of each PC by a strong CF in adulthood. While molecules involved in the strengthening and elimination of CF synapses during postnatal development are being elucidated, much less is known about the molecular mechanisms underlying CF synapse formation during the early postnatal period. Here, we show experimental evidence that suggests that a synapse organizer, PTPδ, is required for early postnatal CF synapse formation and the subsequent establishment of CF to PC synaptic wiring. We showed that PTPδ was localized at CF-PC synapses from postnatal day 0 (P0) irrespective of the expression of Aldolase C (Aldoc), a major marker of PC that distinguishes the cerebellar compartments. We found that the extension of a single strong CF along PC dendrites (CF translocation) was impaired in global PTPδ knockout (KO) mice from P12 to P29-31 predominantly in PCs that did not express Aldoc [Aldoc (-) PCs]. We also demonstrated via morphological and electrophysiological analyses that the number of CFs innervating individual PCs in PTPδ KO mice were fewer than in wild-type (WT) mice from P3 to P13 with a significant decrease in the strength of CF synaptic inputs in cerebellar anterior lobules where most PCs are Aldoc (-). Furthermore, CF-specific PTPδ-knockdown (KD) caused a reduction in the number of CFs innervating PCs with decreased CF synaptic inputs at P10-13 in anterior lobules. We found a mild impairment of motor performance in adult PTPδ KO mice. These results indicate that PTPδ acts as a presynaptic organizer for CF-PC formation and is required for normal CF-PC synaptic transmission, CF translocation, and presumably CF synapse maintenance predominantly in Aldoc (-) PCs. Furthermore, this study suggests that the impaired CF-PC synapse formation and development by the lack of PTPδ causes mild impairment of motor performance.

Building better brains: the pleiotropic function of neurotrophic factors in postnatal cerebellar development

The cerebellum is a multifunctional brain region that controls diverse motor and non-motor behaviors. As a result, impairments in the cerebellar architecture and circuitry lead to a vast array of neuropsychiatric and neurodevelopmental disorders. Neurotrophins and neurotrophic growth factors play essential roles in the development as well as maintenance of the central and peripheral nervous system which is crucial for normal brain function. Their timely expression throughout embryonic and postnatal stages is important for promoting growth and survival of both neurons and glial cells. During postnatal development, the cerebellum undergoes changes in its cellular organization, which is regulated by a variety of molecular factors, including neurotrophic factors. Studies have shown that these factors and their receptors promote proper formation of the cerebellar cytoarchitecture as well as maintenance of the cerebellar circuits. In this review, we will summarize what is known on the neurotrophic factors' role in cerebellar postnatal development and how their dysregulation assists in developing various neurological disorders. Understanding the expression patterns and signaling mechanisms of these factors and their receptors is crucial for elucidating their function within the cerebellum and for developing therapeutic strategies for cerebellar-related disorders.

Granulin loss of function in human mature brain organoids implicates astrocytes in TDP-43 pathology

Loss of function (LoF) of TAR-DNA binding protein 43 (TDP-43) and mis-localization, together with TDP-43-positive and hyperphosphorylated inclusions, are found in post-mortem tissue of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) patients, including those carrying LoF variants in the progranulin gene (GRN). Modeling TDP-43 pathology has been challenging in vivo and in vitro. We present a three-dimensional induced pluripotent stem cell (iPSC)-derived paradigm-mature brain organoids (mbOrg)-composed of cortical-like-astrocytes (iA) and neurons. When devoid of GRN, mbOrgs spontaneously recapitulate TDP-43 mis-localization, hyperphosphorylation, and LoF phenotypes. Mixing and matching genotypes in mbOrgs showed that GRN-/- iA are drivers for TDP-43 pathology. Finally, we rescued TDP-43 LoF by adding exogenous progranulin, demonstrating a link between TDP-43 LoF and progranulin expression. In conclusion, we present an iPSC-derived platform that shows striking features of human TDP-43 proteinopathy and provides a tool for the mechanistic modeling of TDP-43 pathology and patient-tailored therapeutic screening for FTD and ALS.

Ex vivo analysis platforms for monitoring amyloid precursor protein cleavage

Alzheimer's disease (AD) is a progressive neurodegenerative brain disorder and the most common cause of dementia in the elderly. The presence of large numbers of senile plaques, neurofibrillary tangles, and cerebral atrophy is the characteristic feature of AD. Amyloid β peptide (Aβ), derived from the amyloid precursor protein (APP), is the main component of senile plaques. AD has been extensively studied using methods involving cell lines, primary cultures of neural cells, and animal models; however, discrepancies have been observed between these methods. Dissociated cultures lose the brain's tissue architecture, including neural circuits, glial cells, and extracellular matrix. Experiments with animal models are lengthy and require laborious monitoring of multiple parameters. Therefore, it is necessary to combine these experimental models to understand the pathology of AD. An experimental platform amenable to continuous observation and experimental manipulation is required to analyze long-term neuronal development, plasticity, and progressive neurodegenerative diseases. In the current study, we provide a practical method to slice and cultivate rodent hippocampus to investigate the cleavage of APP and secretion of Aβ in an ex vivo model. Furthermore, we provide basic information on Aβ secretion using slice cultures. Using our optimized method, dozens to hundreds of long-term stable slice cultures can be coordinated simultaneously. Our findings are valuable for analyses of AD mouse models and senile plaque formation culture models.

Progranulin improves neural development via the PI3K/Akt/GSK-3β pathway in the cerebellum of a VPA-induced rat model of ASD

Autism spectrum disorder (ASD) is a neurodevelopmental disease featuring social interaction deficits and repetitive/stereotyped behaviours; the prevalence of this disorder has continuously increased. Progranulin (PGRN) is a neurotrophic factor that promotes neuronal survival and differentiation. However, there have not been sufficient studies investigating its effect in animal models of autism. This study investigated the effects of PGRN on autistic phenotypes in rats treated with valproic acid (VPA) and assessed the underlying molecular mechanisms. PGRN was significantly downregulated in the cerebellum at postnatal day 14 (PND14) and PND35 in VPA-exposed rats, which simultaneously showed defective social preference, increased repetitive behaviours, and uncoordinated movements. When human recombinant PGRN (r-PGRN) was injected into the cerebellum of newborn ASD model rats (PND10 and PND17), some of the behavioural defects were alleviated. r-PGRN supplementation also reduced cerebellar neuronal apoptosis and rescued synapse formation in ASD rats. Mechanistically, we confirmed that PGRN protects neurodevelopment via the PI3K/Akt/GSK-3β pathway in the cerebellum of a rat ASD model. Moreover, we found that prosaposin (PSAP) promoted the internalisation and neurotrophic activity of PGRN. These results experimentally demonstrate the therapeutic effects of PGRN on a rat model of ASD for the first time and provide a novel therapeutic strategy for autism.

Multiple Molecular Pathways Are Influenced by Progranulin in a Neuronal Cell Model-A Parallel Omics Approach

Progranulin (PGRN) is critical in supporting a healthy CNS. Its haploinsufficiency results in frontotemporal dementia, while in experimental models of age-related neurodegenerative diseases, the targeted expression of PGRN greatly slows the onset of disease phenotypes. Nevertheless, much remains unclear about how PGRN affects its target cells. In previous studies we found that PGRN showed a remarkable ability to support the survival of NSC-34 motor neuron cells under conditions that would otherwise lead to their apoptosis. Here we used the same model to investigate other phenotypes of PGRN expression in NSC-34 cells. PGRN significantly influenced morphological differentiation, resulting in cells with enlarged cell bodies and extended projections. At a molecular level this correlated with pathways associated with the cytoskeleton and synaptic differentiation. Depletion of PGRN led to increased expression of several neurotrophic receptors, which may represent a homeostatic mechanism to compensate for loss of neurotrophic support from PGRN. The exception was RET, a neurotrophic tyrosine receptor kinase, which, when PGRN levels are high, shows increased expression and enhanced tyrosine phosphorylation. Other receptor tyrosine kinases also showed higher tyrosine phosphorylation when PGRN was elevated, suggesting a generalized enhancement of receptor activity. PGRN was found to bind to multiple plasma membrane proteins, including RET, as well as proteins in the ER/Golgi apparatus/lysosome pathway. Understanding how these various pathways contribute to PGRN action may provide routes toward improving neuroprotective therapies.

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

An Autism-Associated Neuroligin-3 Mutation Affects Developmental Synapse Elimination in the Cerebellum

Neuroligin is a postsynaptic cell-adhesion molecule that is involved in synapse formation and maturation by interacting with presynaptic neurexin. Mutations in neuroligin genes, including the arginine to cystein substitution at the 451st amino acid residue (R451C) of neuroligin-3 (NLGN3), have been identified in patients with autism spectrum disorder (ASD). Functional magnetic resonance imaging and examination of post-mortem brain in ASD patients implicate alteration of cerebellar morphology and Purkinje cell (PC) loss. In the present study, we examined possible association between the R451C mutation in NLGN3 and synaptic development and function in the mouse cerebellum. In NLGN3-R451C mutant mice, the expression of NLGN3 protein in the cerebellum was reduced to about 10% of the level of wild-type mice. Elimination of redundant climbing fiber (CF) to PC synapses was impaired from postnatal day 10–15 (P10–15) in NLGN3-R451C mutant mice, but majority of PCs became mono-innervated as in wild-type mice after P16. In NLGN3-R451C mutant mice, selective strengthening of a single CF relative to the other CFs in each PC was impaired from P16, which persisted into juvenile stage. Furthermore, the inhibition to excitation (I/E) balance of synaptic inputs to PCs was elevated, and calcium transients in the soma induced by strong and weak CF inputs were reduced in NLGN3-R451C mutant mice. These results suggest that a single point mutation in NLGN3 significantly influences the synapse development and refinement in cerebellar circuitry, which might be related to the pathogenesis of ASD.

Abnormal spatiotemporal expression pattern of progranulin and neurodevelopment impairment in VPA-induced ASD rat model

Some environmental risk factors have been proven to contribute to the etiology of autism spectrum disorder (ASD). Exposure to the antiepileptic drug valproic acid (VPA) during pregnancy significantly increases the risk of ASD in humans, and consequently is utilized as a validated animal model of ASD in rodents; however, the precise molecular and cellular mechanisms remain ill-defined. In the present study, we investigated the effect of prenatal VPA exposure on the spatiotemporal dynamics of Progranulin (PGRN) expression, neuronal apoptosis, synapse density, and AKT/GSK-3β pathway activation in the brains of VPA-exposed offspring. Results from behavioral tests were consistent with prior studies showing impaired sociability, restricted interests and increased repetitive behaviors in VPA rats at postnatal days 28-32. Our data also indicated that VPA exposure resulted in abnormal dynamics of PGRN expression in different brain regions at the different development stages. The temporal and spatial patterns of PGRN expression were consistent with the spatiotemporal regularity of abnormalities, which observed in apoptosis-related protein levels, neuron numbers, dendritic spine density, synapse-related protein levels, and AKT/GSK-3β phosphorylation in VPA rats. It suggests that prenatal VPA exposure may affect the spatiotemporal regularity of neuronal apoptosis and synaptic development/regression via interfering with the spatiotemporal process of PGRN expression and downstream AKT/GSK-3β pathway activation. This may be a potential mechanism of the abnormal neuroanatomical changes and ASD-like behaviors in VPA-induced ASD.

BMPR-2 gates activity-dependent stabilization of primary dendrites during mitral cell remodeling

Developing neurons initially form excessive neurites and then remodel them based on molecular cues and neuronal activity. Developing mitral cells in the olfactory bulb initially extend multiple primary dendrites. They then stabilize single primary dendrites while eliminating others. However, the mechanisms underlying selective dendrite remodeling remain elusive. Using CRISPR-Cas9-based knockout screening combined with in utero electroporation, we identify BMPR-2 as a key regulator for selective dendrite stabilization. Bmpr2 knockout and its rescue experiments show that BMPR-2 inhibits LIMK without ligands and thereby permits dendrite destabilization. In contrast, the overexpression of antagonists and agonists indicates that ligand-bound BMPR-2 stabilizes dendrites, most likely by releasing LIMK. Using genetic and FRET imaging experiments, we demonstrate that free LIMK is activated by NMDARs via Rac1, facilitating dendrite stabilization through F-actin formation. Thus, the selective stabilization of primary dendrites is ensured by concomitant inputs of BMP ligands and neuronal activity.

Combining electrophysiology and optogenetics for functional screening of pyramidal neurons in the mouse prefrontal cortex

Here, we present a comprehensive protocol to analyze the roles of disease-related genes in synaptic transmission. We have developed a pipeline of electrophysiological techniques and combined these with optogenetics in the medial prefrontal cortex of mice. This methodology provides a cost-effective, faster, and easier screening approach to elucidate functional aspects of single genes in several regions in the mouse brain such as a specific layer of the mPFC. For complete details on the use and execution of this protocol, please refer to Nagahama et al. (2020) and Sacai et al. (2020).

Longitudinal Characterization of Transcriptomic, Functional, and Morphological Features in Human iPSC-Derived Neurons and Their Application to Investigate Translational Progranulin Disease Biology

The disease biology of frontotemporal lobe dementia (FTD) is complex and not fully understood, with limited translational value appreciated from animal models to date. Human cellular systems that can recapitulate phenotypic features of disease offer promise as translational tools to not only increase our understanding of disease processes but also increase the probability of success of translating novel treatment options to patients. However not all researchers may necessarily have access to well-characterized induced pluripotent stem cell (iPSC)-derived human neurons. As an example, we therefore comprehensively profiled phenotypic features over time in one commercially-available IPSC-derived human neuron cell line. This included systems-level assessments of neurite outgrowth dynamics, neuronal network function, and genome-wide gene expression. By investigating progranulin biology as an example we then demonstrated the utility of these cells as a tool to investigate human disease biology. For example, by using the siRNA-mediated knockdown of the progranulin (GRN) gene, we demonstrated the establishment of an isogenic human cellular model to facilitate translational FTD research. We reproduced findings from rodent neurons by demonstrating that recombinant progranulin (rPGRN) mediated neuroprotection. Contrary to previous rodent data, in our human cellular models, growth factor treatment showed no consistent sensitivity to modulate neurite outgrowth dynamics. Our study further provides the first evidence that rRPGRN modulated neuronal firing and synchrony in human neurons. Taken together, our datasets are a valuable systems-level resource demonstrating the utility of the tested commercially-available human iPSC neurons for investigating basic human neurobiology, translational neuroscience, and drug discovery applications in neurodegenerative and other CNS diseases.

Modelling frontotemporal dementia using patient-derived induced pluripotent stem cells

Frontotemporal dementia (FTD) describes a group of clinically heterogeneous conditions that frequently affect people under the age of 65 (Le Ber et al., 2013). There are multiple genetic causes of FTD, including coding or splice-site mutations in MAPT, GRN mutations that lead to haploinsufficiency of progranulin protein, and a hexanucleotide GGGGCC repeat expansion in C9ORF72. Pathologically, FTD is characterised by abnormal protein accumulations in neurons and glia. These aggregates can be composed of the microtubule-associated protein tau (observed in FTD with MAPT mutations), the DNA/RNA-binding protein TDP-43 (seen in FTD with mutations in GRN or C9ORF72 repeat expansions) or dipeptide proteins generated by repeat associated non-ATG translation of the C9ORF72 repeat expansion. There are currently no disease-modifying therapies for FTD and the availability of in vitro models that recapitulate pathologies in a disease-relevant cell type would accelerate the development of novel therapeutics. It is now possible to generate patient-specific stem cells through the reprogramming of somatic cells from a patient with a genotype/phenotype of interest into induced pluripotent stem cells (iPSCs). iPSCs can subsequently be differentiated into a plethora of cell types including neurons, astrocytes and microglia. Using this approach has allowed researchers to generate in vitro models of genetic FTD in human cell types that are largely inaccessible during life. In this review we explore the recent progress in the use of iPSCs to model FTD, and consider the merits, limitations and future prospects of this approach.

Setd1a Insufficiency in Mice Attenuates Excitatory Synaptic Function and Recapitulates Schizophrenia-Related Behavioral Abnormalities

SETD1A encodes a histone methyltransferase whose de novo mutations are identified in schizophrenia (SCZ) patients and confer a large increase in disease risk. Here, we generate Setd1a mutant mice carrying the frameshift mutation that closely mimics a loss-of-function variant of SCZ. Our Setd1a (+/-) mice display various behavioral abnormalities relevant to features of SCZ, impaired excitatory synaptic transmission in layer 2/3 (L2/3) pyramidal neurons of the medial prefrontal cortex (mPFC), and altered expression of diverse genes related to neurodevelopmental disorders and synaptic functions in the mPFC. RNAi-mediated Setd1a knockdown (KD) specifically in L2/3 pyramidal neurons of the mPFC only recapitulates impaired sociality among multiple behavioral abnormalities of Setd1a (+/-) mice. Optogenetics-assisted selective stimulation of presynaptic neurons combined with Setd1a KD reveals that Setd1a at postsynaptic site is essential for excitatory synaptic transmission. Our findings suggest that reduced SETD1A may attenuate excitatory synaptic function and contribute to the pathophysiology of SCZ.

Cerebellar Development and Circuit Maturation: A Common Framework for Spinocerebellar Ataxias

Spinocerebellar ataxias (SCAs) affect the cerebellum and its afferent and efferent systems that degenerate during disease progression. In the cerebellum, Purkinje cells (PCs) are the most vulnerable and their prominent loss in the late phase of the pathology is the main characteristic of these neurodegenerative diseases. Despite the constant advancement in the discovery of affected molecules and cellular pathways, a comprehensive description of the events leading to the development of motor impairment and degeneration is still lacking. However, in the last years the possible causal role for altered cerebellar development and neuronal circuit wiring in SCAs has been emerging. Not only wiring and synaptic transmission deficits are a common trait of SCAs, but also preventing the expression of the mutant protein during cerebellar development seems to exert a protective role. By discussing this tight relationship between cerebellar development and SCAs, in this review, we aim to highlight the importance of cerebellar circuitry for the investigation of SCAs.

Developmental synapse remodeling in the cerebellum and visual thalamus

Functional neural circuits of mature animals are shaped during postnatal development by eliminating early-formed redundant synapses and strengthening of necessary connections. In the nervous system of newborn animals, redundant synapses are only transient features of the circuit. During subsequent postnatal development, some synapses are strengthened whereas other redundant connections are weakened and eventually eliminated. In this review, we introduce recent studies on the mechanisms of developmental remodeling of climbing fiber-to-Purkinje cell synapses in the cerebellum and synapses from the retina to neurons in the dorsal lateral geniculate nucleus of the visual thalamus (retinogeniculate synapses). These are the two representative models of developmental synapse remodeling in the brain and they share basic principles, including dependency on neural activity. However, recent studies have disclosed that, in several respects, the two models use different molecules and strategies to establish mature synaptic connectivity. We describe similarities and differences between the two models and discuss remaining issues to be tackled in the future in order to understand the general schemes of developmental synapse remodeling.

Towards an Understanding of Synapse Formation

Synapses are intercellular junctions specialized for fast, point-to-point information transfer from a presynaptic neuron to a postsynaptic cell. At a synapse, a presynaptic terminal secretes neurotransmitters via a canonical release machinery, while a postsynaptic specialization senses neurotransmitters via diverse receptors. Synaptic junctions are likely organized by trans-synaptic cell-adhesion molecules (CAMs) that bidirectionally orchestrate synapse formation, restructuring, and elimination. Many candidate synaptic CAMs were described, but which CAMs are central actors and which are bystanders remains unclear. Moreover, multiple genes encoding synaptic CAMs were linked to neuropsychiatric disorders, but the mechanisms involved are unresolved. Here, I propose that engagement of multifarious synaptic CAMs produces parallel trans-synaptic signals that mediate the establishment, organization, and plasticity of synapses, thereby controlling information processing by neural circuits. Among others, this hypothesis implies that synapse formation can be understood in terms of inter- and intracellular signaling, and that neuropsychiatric disorders involve an impairment in such signaling.

Shaping Diversity Into the Brain's Form and Function

The brain contains a large diversity of unique cell types that use specific genetic programs to control development and instruct the intricate wiring of sensory, motor, and cognitive brain regions. In addition to their cellular diversity and specialized connectivity maps, each region's dedicated function is also expressed in their characteristic gross external morphologies. The folds on the surface of the cerebral cortex and cerebellum are classic examples. But, to what extent does structure relate to function and at what spatial scale? We discuss the mechanisms that sculpt functional brain maps and external morphologies. We also contrast the cryptic structural defects in conditions such as autism spectrum disorders to the overt microcephaly after Zika infections, taking into consideration that both diseases disrupt proper cognitive development. The data indicate that dynamic processes shape all brain areas to fit into jigsaw-like patterns. The patterns in each region reflect circuit connectivity, which ultimately supports local signal processing and accomplishes multi-areal integration of information processing to optimize brain functions.

The lysosomal function of progranulin, a guardian against neurodegeneration

Progranulin (PGRN), encoded by the GRN gene in humans, is a secreted growth factor implicated in a multitude of processes ranging from regulation of inflammation to wound healing and tumorigenesis. The clinical importance of PGRN became especially evident in 2006, when heterozygous mutations in the GRN gene, resulting in haploinsufficiency, were found to be one of the main causes of frontotemporal lobar degeneration (FTLD). FTLD is a clinically heterogenous disease that results in the progressive atrophy of the frontal and temporal lobes of the brain. Despite significant research, the exact function of PGRN and its mechanistic relationship to FTLD remain unclear. However, growing evidence suggests a role for PGRN in the lysosome-most striking being that homozygous GRN mutation leads to neuronal ceroid lipofuscinosis, a lysosomal storage disease. Since this discovery, several links between PGRN and the lysosome have been established, including the existence of two independent lysosomal trafficking pathways, intralysosomal processing of PGRN into discrete functional peptides, and direct and indirect regulation of lysosomal hydrolases. Here, we summarize the cellular functions of PGRN, its roles in the nervous system, and its link to multiple neurodegenerative diseases, with a particular focus dedicated to recent lysosome-related mechanistic developments.