P2X7 receptor inhibition ameliorates ubiquitin-proteasome system dysfunction associated with Alzheimer's disease.

BACKGROUND: Over recent years, increasing evidence suggests a causal relationship between neurofibrillary tangles (NFTs) formation, the main histopathological hallmark of tauopathies, including Alzheimer's disease (AD), and the ubiquitin-proteasome system (UPS) dysfunction detected in these patients. Nevertheless, the mechanisms underlying UPS failure and the factors involved remain poorly understood. Given that AD and tauopathies are associated with chronic neuroinflammation, here, we explore if ATP, one of the danger-associated molecules patterns (DAMPs) associated with neuroinflammation, impacts on AD-associated UPS dysfunction. METHODS: To evaluate if ATP may modulate the UPS via its selective P2X7 receptor, we combined in vitro and in vivo approaches using both pharmacological and genetic tools. We analyze postmortem samples from human AD patients and P301S mice, a mouse model that mimics pathology observed in AD patients, and those from the new transgenic mouse lines generated, such as P301S mice expressing the UPS reporter UbG76V-YFP or P301S deficient of P2X7R. RESULTS: We describe for the first time that extracellular ATP-induced activation of the purinergic P2X7 receptor (P2X7R) downregulates the transcription of ß5 and ß1 proteasomal catalytic subunits via the PI3K/Akt/GSK3/Nfr2 pathway, leading to their deficient assembly into the 20S core proteasomal complex, resulting in a reduced proteasomal chymotrypsin-like and postglutamyl-like activities. Using UPS-reported mice (UbGFP mice), we identified neurons and microglial cells as the most sensitive cell linages to a P2X7R-mediated UPS regulation. In vivo pharmacological or genetic P2X7R blockade reverted the proteasomal impairment developed by P301S mice, which mimics that were detected in AD patients. Finally, the generation of P301S;UbGFP mice allowed us to identify those hippocampal cells more sensitive to UPS impairment and demonstrate that the pharmacological or genetic blockade of P2X7R promotes their survival. CONCLUSIONS: Our work demonstrates the sustained and aberrant activation of P2X7R caused by Tau-induced neuroinflammation contributes to the UPS dysfunction and subsequent neuronal death associated with AD, especially in the hippocampus.

Studying the Role of P2X7 Receptor in Axonal Growth Using In Utero Electroporation Technique.

The nervous system is formed by a complex network of neuronal connections. During development, neurons elongate their axons through highly stereotyped anatomical pathways to form precise connections. Defects in these mechanisms are related with neurological disorders. Previous studies have reported that inhibition of the P2X7 receptor, an ionotropic purinergic receptor, promotes axonal growth and branching in cultured neurons. However, little is known about the in vivo mechanism of axonal elongation regulated by P2X7. Here, we detailed a step-by-step method to perform in utero cortical electroporation and quantified the electroporated axons employing accessible and open-source image processing software. This effective surgical procedure manipulates in vivo the gene expression in a discrete population of callosal projection neuron. Thus, a better understanding of the involvement of P2X7 in the in vivo establishment of neuronal circuits might help to clarify the basic biology of several neurodevelopmental disorders and axonal regenerative processes.

TNAP upregulation is a critical factor in Tauopathies and its blockade ameliorates neurotoxicity and increases life-expectancy.

Tauopathies are a family of neurodegenerative diseases characterized by the presence of abnormally hyperphosphorylated Tau protein. Several studies have proposed that increased extracellular Tau (eTau) leads to the spread of cerebral tauopathy. However, the molecular mechanisms underlying eTau-induced neurotoxicity remain unclear. Previous in vitro studies reported that the ecto-enzyme tissue-nonspecific alkaline phosphatase (TNAP) dephosphorylate eTau at different sites increasing its neurotoxicity. Here, we confirm TNAP protein upregulation in the brains of Alzheimer's patients and found a similar TNAP increase in Pick's disease patients and P301S mice, a well-characterized mouse model of tauopathies. Interestingly, the conditional overexpression of TNAP causes intracellular Tau hyperphosphorylation and aggregation in cells neighbouring those overexpressing the ectoenzyme. Conversely, the genetic disruption of TNAP reduced the dephosphorylation of eTau and decreased neuronal hyperactivity, brain atrophy, and hippocampal neuronal death in P301S mice. TNAP haploinsufficiency in P301S mice prevents the decreased anxiety-like behaviour, motor deficiency, and increased memory capacity and life expectancy. Similar results were observed by the in vivo pharmacological blunting of TNAP activity. This study provides the first in vivo evidence demonstrating that raised TNAP activity is critical for Tau-induced neurotoxicity and suggest that TNAP blockade may be a novel and efficient therapy to treat tauopathies.

P2X7 receptor blockade reduces tau induced toxicity, therapeutic implications in tauopathies.

Tauopathies are neurodegenerative diseases characterized by the presence of aberrant intraneuronal aggregates of hyperphosphorylated Tau protein. Recent studies suggest that associated chronic neuroinflammation may contribute to the pathological Tau dissemination. However, the underlying molecular mechanisms remain unknown. Since purinergic P2X7 receptors (P2X7) can sense the rise of extracellular ATP levels associated with neuroinflammation, its involvement in neurodegeneration-associated inflammation was suggested. We found a P2X7 upregulation in patients diagnosed with different tauopathies and in a tauopathy mouse model, P301S mice. In vivo pharmacological or genetic blockade of P2X7 reverted microglial activation in P301S mice leading to a reduction in microglial migratory, secretory, and proliferative capacities, and promoting phagocytic function. Furthermore, it reduced the intraneuronal phosphorylated Tau levels in a GSK3-dependent way and increased extracellular phosphorylated Tau levels by reducing the expression of ectoenzyme TNAP. Accordingly, pharmacological or genetic blockade of P2X7 improved the cellular survival, motor and memory deficits and anxiolytic profile in P301S mice. Contrary, P2X7 overexpression caused a significant worsening of Tau-induced toxicity and aggravated the deteriorated motor and memory deficits in P301S mice. Our results indicate that P2X7 plays a deleterious role in tauopathies and suggest that its blockade may be a promising approach to treat Tauopathies.

Kidins220 deficiency causes ventriculomegaly via SNX27-retromer-dependent AQP4 degradation.

Several psychiatric, neurologic and neurodegenerative disorders present increased brain ventricles volume, being hydrocephalus the disease with the major manifestation of ventriculomegaly caused by the accumulation of high amounts of cerebrospinal fluid (CSF). The molecules and pathomechanisms underlying cerebral ventricular enlargement are widely unknown. Kinase D interacting substrate of 220 kDa (KIDINS220) gene has been recently associated with schizophrenia and with a novel syndrome characterized by spastic paraplegia, intellectual disability, nystagmus and obesity (SINO syndrome), diseases frequently occurring with ventriculomegaly. Here we show that Kidins220, a transmembrane protein effector of various key neuronal signalling pathways, is a critical regulator of CSF homeostasis. We observe that both KIDINS220 and the water channel aquaporin-4 (AQP4) are markedly downregulated at the ventricular ependymal lining of idiopathic normal pressure hydrocephalus (iNPH) patients. We also find that Kidins220 deficient mice develop ventriculomegaly accompanied by water dyshomeostasis and loss of AQP4 in the brain ventricular ependymal layer and astrocytes. Kidins220 is a known cargo of the SNX27-retromer, a complex that redirects endocytosed plasma membrane proteins (cargos) back to the cell surface, thus avoiding their targeting to lysosomes for degradation. Mechanistically, we show that AQP4 is a novel cargo of the SNX27-retromer and that Kidins220 deficiency promotes a striking and unexpected downregulation of the SNX27-retromer that results in AQP4 lysosomal degradation. Accordingly, SNX27 silencing decreases AQP4 levels in wild-type astrocytes whereas SNX27 overexpression restores AQP4 content in Kidins220 deficient astrocytes. Together our data suggest that the KIDINS220-SNX27-retromer-AQP4 pathway is involved in human ventriculomegaly and open novel therapeutic perspectives.

The Role of P2X7 Receptor in Alzheimer's Disease.

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease characterized by a progressive cognitive decline associated with global brain damage. Initially, intracellular paired helical filaments composed by hyperphosphorylated tau and extracellular deposits of amyloid-ß (Aß) were postulated as the causing factors of the synaptic dysfunction, neuroinflammation, oxidative stress, and neuronal death, detected in AD patients. Therefore, the vast majority of clinical trials were focused on targeting Aß and tau directly, but no effective treatment has been reported so far. Consequently, only palliative treatments are currently available for AD patients. Over recent years, several studies have suggested the involvement of the purinergic receptor P2X7 (P2X7R), a plasma membrane ionotropic ATP-gated receptor, in the AD brain pathology. In this line, altered expression levels and function of P2X7R were found both in AD patients and AD mouse models. Consequently, genetic depletion or pharmacological inhibition of P2X7R ameliorated the hallmarks and symptoms of different AD mouse models. In this review, we provide an overview of the current knowledge about the role of the P2X7R in AD.

Differential regulation of Kidins220 isoforms in Huntington's disease.

Huntington's disease (HD) is an inherited progressive neurodegenerative disease characterized by brain atrophy particularly in the striatum that produces motor impairment, and cognitive and psychiatric disturbances. Multiple pathogenic mechanisms have been proposed including dysfunctions in neurotrophic support and calpain-overactivation, among others. Kinase D-interacting substrate of 220 kDa (Kidins220), also known as ankyrin repeat-rich membrane spanning (ARMS), is an essential mediator of neurotrophin signaling. In adult brain, Kidins220 presents two main isoforms that differ in their carboxy-terminal length and critical protein-protein interaction domains. These variants are generated through alternative terminal exon splicing of the conventional exon 32 (Kidins220-C32) and the recently identified exon 33 (Kidins220-C33). The lack of domains encoded by exon 32 involved in key neuronal functions, including those controlling neurotrophin pathways, pointed to Kidins220-C33 as a form detrimental for neurons. However, the functional role of Kidins220-C33 in neurodegeneration or other pathologies, including HD, has not been explored. In the present work, we discover an unexpected selective downregulation of Kidins220-C33, in the striatum of HD patients, as well as in the R6/1 HD mouse model starting at early symptomatic stages. These changes are C33-specific as Kidins220-C32 variant remains unchanged. We also find the early decrease in Kidins220-C33 levels takes place in neurons, suggesting an unanticipated neuroprotective role for this isoform. Finally, using ex vivo assays and primary neurons, we demonstrate that Kidins220-C33 is downregulated by mechanisms that depend on the activation of the protease calpain. Altogether, these results strongly suggest that calpain-mediated Kidins220-C33 proteolysis modulates onset and/or progression of HD.

ATP Measurement in Cerebrospinal Fluid Using a Microplate Reader.

Imbalance in extracellular ATP levels in brain tissue has been suggested as a triggering factor for several neurological disorders. Here, we describe the most sensitive and reliable technique for monitoring the ATP levels in mice cerebrospinal samples collected by cisterna magna puncture technique and quantified using a microplate reader.

Amyloid Peptide Induced Neuroinflammation Increases the P2X7 Receptor Expression in Microglial Cells, Impacting on Its Functionality.

Alzheimer disease is a neurodegenerative disease characterized by the presence of senile plaques composed of amyloid-ß (Aß) peptide, neurofibrillary tangles, neuronal loss and neuroinflammation. Previous works have revealed that extracellular ATP, through its selective receptor P2X7 (P2X7R), is essential to neuroinflammation and neurotoxicity induced by Aß. P2X7R is upregulated on microglial cells around the senile plaques. This upregulation progressively rises with age and is parallel with an accumulation of senile plaques and also correlates with the synaptic toxicity detected both in animal models reproducing AD and human patients of AD. Furthermore, the late onset of the first AD-associated symptoms suggests that aging associated-changes may be relevant to the disease progression. Thus, microglia motility and its capacity to respond to exogenous ATP stimulus decrease with aging. To evaluate whether the P2X7R age related-changes on microglia cells may be relevant to the AD progression, we generated a new transgenic mouse model crossing an Aß peptide mouse model, J20 mice and the P2X7R reporter mice P2X7REGFP. Our results indicate that neuroinflammation induced by Aß peptide causes changes in the P2X7R distribution pattern, increasing it s expression in microglial cells at advanced and late stages, when microgliosis occurs, but not in the early stages, in the absence of microgliosis. In addition, we found that P2X7R activation promotes microglial cells migration to senile plaques but decreases their phagocytic capacity. Moreover, we found a significant reduction of P2X7R transcription on neuronal cells at the early and advanced stages, but not at the late stages. Since previous studies have reported that either pharmacological inhibition or selective downregulation of P2X7R significantly improve behavioral alterations and reduce the incidence and size of senile plaques in the early and advanced stages of AD, the results presented here provide new evidence, indicating that this therapeutic approach could be also efficient in the late stages of the disease.

Nucleotides regulate the common molecular mechanisms that underlie neurodegenerative diseases; Therapeutic implications.

Neurodegenerative diseases (ND) are a heterogeneous group of neurological disorders characterized by a progressive loss of neuronal function which results in neuronal death. Although a specific toxic factor has been identified for each ND, all of them share common pathological molecular mechanisms favouring the disease development. In the final stages of ND, patients become unable to take care of themselves and decline to a total functional incapacitation that leads to their death. Some of the main factors which contribute to the disease progression include proteasomal dysfunction, neuroinflammation, synaptic alterations, protein aggregation, and oxidative stress. Over recent years, evidence has been accumulated to suggest that purinergic signaling plays a key role in the aforementioned molecular pathways. In this review, we revise the implications of the purinergic signaling in the common molecular mechanism underlying the ND. In particular, we focus on the role of the purinergic receptors P2X7, P2Y2 and the ectoenzyme tissue-nonspecific alkaline phosphatase (TNAP).

Peripheral nervous system effects in the PS19 tau transgenic mouse model of tauopathy.

It is well known that transgenic mice overexpressing human tau protein with P301S mutation driven by the mouse prion protein promoter show clasping and limb retraction, hunched back and paralysis, followed by inability to feed that results in death around 12 months of age. To understand these motor deficits, we have carried out rotarod tests on PS19 line and demonstrated how they worsened during aging. Then, we have analyzed if these phenotypic characteristics correlate with sciatic nerve degeneration. We first demonstrated by western blot and immunohistochemistry that the sciatic nerve expresses the transgenic tau protein; then, electron microscopy studies showed alterations in myelin, mainly a detachment of myelin lamellae at Schmidt-Lanterman clefts. Similar motor deficits and myelin alterations have been previously reported in tau knockout and overexpressing transgenic mice; taking into account that PS19 model is widely used to study tauopathies, we suggest that analyzing the expression of transgenic tau protein and myelin abnormalities in the sciatic nerve should be considered when studying some features as motor performance or survival.

Haploinsufficient TNAP Mice Display Decreased Extracellular ATP Levels and Expression of Pannexin-1 Channels.

Hypophosphatasia (HPP) is a rare heritable metabolic bone disease caused by hypomorphic mutations in the ALPL (in human) or Akp2 (in mouse) gene, encoding the tissue-nonspecific alkaline phosphatase (TNAP) enzyme. In addition to skeletal and dental malformations, severe forms of HPP are also characterized by the presence of spontaneous seizures. Initially, these seizures were attributed to an impairment of GABAergic neurotransmission caused by altered vitamin B6 metabolism. However, recent work by our group using knockout mice null for TNAP (TNAP-/-), a well-described model of infantile HPP, has revealed a deregulation of purinergic signaling contributing to the seizure phenotype. In the present study, we report that adult heterozygous (TNAP+/-) transgenic mice with decreased TNAP activity in the brain are more susceptible to adenosine 5'-triphosphate (ATP)-induced seizures. Interestingly, when we analyzed the extracellular levels of ATP in the cerebrospinal fluid, we found that TNAP+/- mice present lower levels than control mice. To elucidate the underlying mechanism, we evaluated the expression levels of other ectonucleotidases, as well as different proteins involved in ATP release, such as pannexin, connexins, and vesicular nucleotide transporter. Among these, Pannexin-1 (Panx1) was the only one showing diminished levels in the brains of TNAP+/- mice. Altogether, these findings suggest that a physiological regulation of extracellular ATP levels and Panx1 changes may compensate for the reduced TNAP activity in this model of HPP.

The Neurotoxic Role of Extracellular Tau Protein.

Tauopathies are a class of neurodegenerative diseases associated with the microtubule-associated protein tau, with Alzheimer's disease (AD) being the most prevalent related disorder. Neurofibrillary tangles (NFTs) are one of the neuropathological hallmarks present in the brains of AD patients. Because NFTs are aberrant intracellular inclusions formed by hyperphosphorylated tau, it was initially proposed that phosphorylated and/or aggregated intracellular tau protein was causative of neuronal death. However, recent studies suggest a toxic role for non-phosphorylated and non-aggregated tau when it is located in the brain extracellular space. In this work, we will discuss the neurotoxic role of extracellular tau as well its involvement in the spreading of tau pathologies.

The regulation of proteostasis in glial cells by nucleotide receptors is key in acute neuroinflammation.

The disturbances of cellular proteostasis caused by the alteration in the ubiquitin-proteasome system (UPS) have been proposed as a common mechanism underlying several neural pathologies that involve a neuroinflammatory process. As we have previously reported that the nucleotide receptor P2Y purinoceptor 2 (P2Y2R) regulates the proteasomal catalytic activities, we wonder whether this receptor is involved in the UPS disturbances associated with the neuroinflammation process. With the use of mice expressing a UPS reporter [mice expressing the UPS reporter ubiquitinG76V-green fluorescent protein (UbGFP mice)], we found that LPS-induced acute neuroinflammation status causes a UPS impairment in astrocytes and microglial cells by a mechanism dependent on P2Y2R. In this line, LPS-treated double transgenic UbGFP; P2Y2R-/- mice did not present a UPS impairment in astrocytes or a social interaction deficit as severe as that observed in LPS-treated UbGFP mice. In vivo administration of selective P2Y2R agonist diuridine tetraphosphate reversed the UPS impairment completely in astrocytes and partially in microglial cells, promoting increased expression of the proteasomal ß5 subunit by a mechanism dependent on the Src/PI3K/ERK pathway. Altogether, our results suggest that LPS induces unbalanced proteostasis in astrocytes by blocking P2Y2R. Finally, our findings point to the design of selective P2Y2R agonist drugs as a new therapeutic approach to treat the neuroinflammatory status.-De Diego García, L., Sebastián-Serrano, Á., Hernández, I. H., Pintor, J., Lucas, J. J., Díaz-Hernández, M. The regulation of proteostasis in glial cells by nucleotide receptors is key in acute neuroinflammation.

Author Correction: Excitotoxic inactivation of constitutive oxidative stress detoxification pathway in neurons can be rescued by PKD1.

The original version of this Article contained an error in the spelling of the author Álvaro Sebastián-Serrano, which was incorrectly given as Álvaro Sebastián Serrano. This has now been corrected in both the PDF and HTML versions of the Article.

Excitotoxic inactivation of constitutive oxidative stress detoxification pathway in neurons can be rescued by PKD1.

Excitotoxicity, a critical process in neurodegeneration, induces oxidative stress and neuronal death through mechanisms largely unknown. Since oxidative stress activates protein kinase D1 (PKD1) in tumor cells, we investigated the effect of excitotoxicity on neuronal PKD1 activity. Unexpectedly, we find that excitotoxicity provokes an early inactivation of PKD1 through a dephosphorylation-dependent mechanism mediated by protein phosphatase-1 (PP1) and dual specificity phosphatase-1 (DUSP1). This step turns off the IKK/NF-κB/SOD2 antioxidant pathway. Neuronal PKD1 inactivation by pharmacological inhibition or lentiviral silencing in vitro, or by genetic inactivation in neurons in vivo, strongly enhances excitotoxic neuronal death. In contrast, expression of an active dephosphorylation-resistant PKD1 mutant potentiates the IKK/NF-κB/SOD2 oxidative stress detoxification pathway and confers neuroprotection from in vitro and in vivo excitotoxicity. Our results indicate that PKD1 inactivation underlies excitotoxicity-induced neuronal death and suggest that PKD1 inactivation may be critical for the accumulation of oxidation-induced neuronal damage during aging and in neurodegenerative disorders.

Neuronal P2X7 Receptor: Involvement in Neuronal Physiology and Pathology.

The proposed presence of P2X7 receptor (P2X7R) in neurons has been the source of some contention. Initial studies suggested an absence of P2X7R mRNA in neurons, and the apparent nonspecificity of the antibodies used to identify P2X7R raised further doubts. However, subsequent studies using new pharmacological and biomolecular tools provided conclusive evidence supporting the existence of functional P2X7Rs in neurons. The P2X7 receptor has since been shown to play a leading role in multiple aspects of neuronal physiology, including axonal elongation and branching and neurotransmitter release. P2X7R has also been implicated in neuronal pathologies, in which it may influence neuronal survival. Together, this body of research suggests that P2X7R may constitute an important therapeutic target for a variety of neurological disorders.

Regulation of proteasome activity by P2Y2 receptor underlies the neuroprotective effects of extracellular nucleotides.

The Ubiquitin-Proteasome System (UPS) is essential for the regulation of the cellular proteostasis. Indeed, it has been postulated that an UPS dysregulation is the common mechanism that underlies several neurological disorders. Considering that extracellular nucleotides, through their selective P2Y2 receptor (P2Y2R), play a neuroprotective role in various neurological disorders that course with an UPS impairment, we wonder if this neuroprotective capacity resulted from their ability to modulate the UPS. Using a cellular model expressing two different UPS reporters, we found that the stimulation of P2Y2R by its selective agonist Up4U induced a significant reduction of UPS reporter levels. This reduction was due to an increase in two of the three peptidase proteasome activities, chymotrypsin and postglutamyl, caused by an increased expression of proteasome constitutive catalytic subunits ß1 and ß5. The intracellular signaling pathway involved required the activation of IP3/MEK1/2/ERK but was independent of PKC or PKA. Interestingly, the P2Y2R activation was able to revert both UPS-reporter accumulation and the cell death induced by a prolonged inhibition of UPS. Finally, we also observed that intracerebroventricular administration of Up4U induced a significant increase both of chymotrypsin and postglutamyl activities as well as an increased expression of proteasome subunits ß1 and ß5 in the hippocampus of wild-type mice, but not in P2Y2R KO mice. All these results strongly suggest that the capacity to modulate the UPS activity via P2Y2R is the molecular mechanism which is how the nucleotides play a neuroprotective role in neurological disorders.

Neurodevelopmental alterations and seizures developed by mouse model of infantile hypophosphatasia are associated with purinergic signalling deregulation.

Hypomorphic mutations in the gene encoding the tissue-nonspecific alkaline phosphatase (TNAP) enzyme, ALPL in human or Akp2 in mice, cause hypophosphatasia (HPP), an inherited metabolic bone disease also characterized by spontaneous seizures. Initially, these seizures were attributed to the impairment of GABAergic neurotransmission caused by altered vitamin B6 (vit-B6) metabolism. However, clinical cases in human newborns and adults whose convulsions are refractory to pro-GABAergic drugs but controlled by the vit-B6 administration, suggest that other factors are involved. Here, to evaluate whether neurodevelopmental alterations are underlying the seizures associated to HPP, we performed morphological and functional characterization of postnatal homozygous TNAP null mice, a model of HPP. These analyses revealed that TNAP deficient mice present an increased proliferation of neural precursors, an altered neuronal morphology, and an augmented neuronal activity. We found that these alterations were associated with a partial downregulation of the purinergic P2X7 receptor (P2X7R). Even though deficient P2X7R mice present similar neurodevelopmental alterations, they do not develop neonatal seizures. Accordingly, we found that the additional blockage of P2X7R prevent convulsions and extend the lifespan of mice lacking TNAP. In agreement with these findings, we also found that exogenous administration of ATP or TNAP antagonists induced seizures in adult wild-type mice by activating P2X7R. Finally, our results also indicate that the anticonvulsive effects attributed to vit-B6 may be due to its capacity to block P2X7R. Altogether, these findings suggest that the purinergic signalling regulates the neurodevelopmental alteration and the neonatal seizures associated to HPP.

Cux1 Enables Interhemispheric Connections of Layer II/III Neurons by Regulating Kv1-Dependent Firing.

Neuronal subtype-specific transcription factors (TFs) instruct key features of neuronal function and connectivity. Activity-dependent mechanisms also contribute to wiring and circuit assembly, but whether and how they relate to TF-directed neuronal differentiation is poorly investigated. Here we demonstrate that the TF Cux1 controls the formation of the layer II/III corpus callosum (CC) projections through the developmental transcriptional regulation of Kv1 voltage-dependent potassium channels and the resulting postnatal switch to a Kv1-dependent firing mode. Loss of Cux1 function led to a decrease in the expression of Kv1 transcripts, aberrant firing responses, and selective loss of CC contralateral innervation. Firing and innervation were rescued by re-expression of Kv1 or postnatal reactivation of Cux1. Knocking down Kv1 mimicked Cux1-mediated CC axonal loss. These findings reveal that activity-dependent processes are central bona fide components of neuronal TF-differentiation programs and establish the importance of intrinsic firing modes in circuit assembly within the neocortex.