The sorting receptor Rer1 controls Purkinje cell function via voltage gated sodium channels

PTRH2 is Necessary for Purkinje Cell Differentiation and Survival and its Loss Recapitulates Progressive Cerebellar Atrophy and Ataxia Seen in IMNEPD Patients

Hom ozygous variants in the peptidyl-tRNA hydrolase 2 gene (PTRH2) cause infantile-onset multisystem neurologic, endocrine, and pancreatic disease. The objective is to delineate the mechanisms underlying the core cerebellar phenotype in this disease. For this, we generated constitutive (Ptrh2LoxPxhCMVCre, Ptrh2-/- mice) and Purkinje cell (PC) specific (Ptrh2LoxPxPcp2Cre, Ptrh2ΔPCmice) Ptrh2 mutant mouse models and investigated the effect of the loss of Ptrh2 on cerebellar development. We show that Ptrh2-/- knockout mice had severe postnatal runting and lethality by postnatal day 14. Ptrh2ΔPC PC specific knockout mice survived until adult age; however, they showed progressive cerebellar atrophy and functional cerebellar deficits with abnormal gait and ataxia. PCs of Ptrh2ΔPC mice had reduced cell size and density, stunted dendrites, and lower levels of ribosomal protein S6, a readout of the mammalian target of rapamycin pathway. By adulthood, there was a marked loss of PCs. Thus, we identify a cell autonomous requirement for PTRH2 in PC maturation and survival. Loss of PTRH2 in PCs leads to downregulation of the mTOR pathway and PC atrophy. This suggests a molecular mechanism underlying the ataxia and cerebellar atrophy seen in patients with PTRH2 mutations leading to infantile-onset multisystem neurologic, endocrine, and pancreatic disease.

Nedd4-2 Haploinsufficiency in Mice Impairs the Ubiquitination of Rer1 and Increases the Susceptibility to Endoplasmic Reticulum Stress and Seizures

Neural precursor cell expressed developmentally downregulated gene 4-like (NEDD4-2) is an epilepsy-associated gene encoding an E3 ligase that ubiquitinates neuroactive substrates. An involvement of NEDD4-2 in endoplasmic reticulum (ER) stress has been recently found with mechanisms needing further investigations. Herein, Nedd4-2+/− mice were found intolerant to thapsigargin (Tg) to develop ER stress in the brain. Pretreatment of Tg aggravated the pentylenetetrazole (PTZ)-induced seizures. Retention in endoplasmic reticulum 1 (Rer1), an ER retrieval receptor, was upregulated through impaired ubiquitination in Nedd4-2+/− mouse brain. Nedd4-2 interacted with Rer1 more strongly in mice with Tg administration. The negative regulation and NEDD4-2-mediated ubiquitination on RER1 were evaluated in cultured neurocytes and gliacytes by NEDD4-2 knockdown and overexpression. NEDD4-2 interacted with RER1 at higher levels in the cells with Tg treatment. Disruption of the 36STPY39 motif of RER1 attenuated the interaction with NEDD4-2, and the ubiquitinated RER1 underwent proteasomal degradation. Furthermore, the interactome of Rer1 was screened by immunoprecipitation-mass spectrometry in PTZ-induced mouse hippocampus, showing multiple potential ER retrieval cargoes that mediate neuroexcitability. The α1 subunit of the GABAA receptor was validated to interact with Rer1 and retain in ER more heavily in Nedd4-2+/− mouse brain by Endo-H digestion. In conclusion, Nedd4-2 deficiency in mice showed impaired ubiquitination of Rer1 and increased ER stress and seizures. These data indicate a protective effect of NEDD4-2 in ER stress and seizures possibly via RER1. We also provided potential ER retention cargoes of Rer1 awaiting further investigation.

Protein quality control at the Golgi

The majority of the proteome in eukaryotic cells is targeted to organelles. To maintain protein homeostasis (proteostasis), distinct protein quality control (PQC) machineries operate on organelles, where they detect misfolded proteins, orphaned and mis-localized proteins and selectively target these proteins into different ubiquitin-dependent or -independent degradation pathways. Thereby, PQC prevents proteotoxic effects that would disrupt organelle integrity and cause cellular damage that leads to diseases. Here, we will discuss emerging mechanisms for PQC machineries at the Golgi apparatus, the central station for the sorting and the modification of proteins that traffic to the endo-lysosomal system, or along the secretory pathway to the PM and to the extracellular space. We will focus on Golgi PQC pathways that (1) retrieve misfolded and orphaned proteins from the Golgi back to the endoplasmic reticulum, (2) extract these proteins from Golgi membranes for proteasomal degradation, (3) or selectively target these proteins to lysosomes for degradation.

Adult neurogenesis and the molecular signalling pathways in brain: the role of stem cells in adult hippocampal neurogenesis

Molecular signalling pathways are an evolutionarily conserved multifaceted pathway that can control diverse cellular processes. The role of signalling pathways in regulating development and tissue homeostasis as well as hippocampal neurogenesis is needed to study in detail. In the adult brain, the Notch signalling pathway, in collaboration with the Wnt/β-catenin, bone morphogenetic proteins (BMPs), and sonic hedgehog (Shh) molecular signalling pathways, are involved in stem cell regulation in the hippocampal formation, and they also control the plasticity of the neural stem cells (NSCs) or neural progenitor cells (NPCs) which involved in neurogenesis processes. Here we discuss the distinctive roles of molecular signalling pathways involved in the generation of new neurons from a pool of NSCs in the adult brain. Our approach will facilitate the understanding of the molecular signalling mechanism of hippocampal neurogenesis during NSCs development in the adult brain using molecular aspects coupled with cell biological and physiological analysis.

Bring it back, bring it back, don't take it away from me - the sorting receptor RER1

The quote "bring it back, bring it back, don't take it away from me" from Queen's Love of my life describes the function of the sorting receptor RER1, a 23 kDa protein with four transmembrane domains (TMDs) that localizes to the intermediate compartment and the cis-Golgi. From there it returns escaped proteins that are not supposed to leave the endoplasmic reticulum (ER) back to it. Unique about RER1 is its ability to recognize its ligands through binding motifs in TMDs. Among its substrates are ER-resident proteins, as well as unassembled subunits of multimeric complexes that are retrieved back into the ER, this way guarding the full assembly of their respective complexes. The basic mechanisms for RER1-dependent retrieval have been already elucidated some years ago in yeast. More recently, several important cargoes of RER1 have been described in mammalian cells, and the in vivo role of RER1 is being unveiled by using mouse models. In this Review, we give an overview of the cell biology of RER1 in different models, discuss its controversial role in the brain and provide an outlook on future directions for RER1 research.

Deletion of Class II ADP-Ribosylation Factors in Mice Causes Tremor by the Nav1.6 Loss in Cerebellar Purkinje Cell Axon Initial Segments

ADP-ribosylation factors (ARFs) are a family of small monomeric GTPases comprising six members categorized into three classes: class I (ARF1, 2, and 3), class II (ARF4 and 5), and class III (ARF6). In contrast to class I and III ARFs, which are the key regulators in vesicular membrane trafficking, the cellular function of class II ARFs remains unclear. In the present study, we generated class II ARF-deficient mice and found that ARF4^+/-/ARF5^-/- mice exhibited essential tremor (ET)-like behaviors. In vivo electrophysiological recordings revealed that ARF4^+/-/ARF5^-/- mice of both sexes exhibited abnormal brain activity when moving, raising the possibility of abnormal cerebellar excitability. Slice patch-clamp experiments demonstrated the reduced excitability of the cerebellar Purkinje cells (PCs) in ARF4^+/-/ARF5^-/- mice. Immunohistochemical and electrophysiological analyses revealed a severe and selective decrease of pore-forming voltage-dependent Na⁺ channel subunit Nav1.6, important for maintaining repetitive action potential firing, in the axon initial segment (AIS) of PCs. Importantly, this decrease in Nav1.6 protein localized in the AIS and the consequent tremors in ARF4^+/-/ARF5^-/- mice could be alleviated by the PC-specific expression of ARF5 using adeno-associated virus vectors. Together, our data demonstrate that the decreased expression of the class II ARF proteins in ARF4^+/-/ARF5^-/- mice, leading to a haploinsufficiency of ARF4 in the absence of ARF5, impairs the localization of Nav1.6 to the AIS and hence reduces the membrane excitability in PCs, resulting in the ET-like movement disorder. We suggest that class II ARFs function in localizing specific proteins, such as Nav1.6, to the AIS.SIGNIFICANCE STATEMENT We found that decreasing the expression of class II ARF proteins, through the generation of ARF4^+/-/ARF5^-/- mice, impairs Nav1.6 distribution to the axon initial segment (AIS) of cerebellar Purkinje cells (PCs), thereby resulting in the impairment of action potential firing of PCs. The ARF4^+/-/ARF5^-/- mutant mice exhibited movement-associated essential tremor (ET)-like behavior with pharmacological profiles similar to those in ET patients. The exogenous expression of ARF5 reduced the tremor phenotype and restored the localization of Nav1.6 immunoreactivity to the AIS in ARF4^+/-/ARF5^-/- mice. Thus, our results suggest that class II ARFs are involved in the localization of Nav1.6 to the AISs in cerebellar PCs and that the reduction of class II ARF activity leads to ET-like movement disorder.

Cerebellar Neurodegeneration and Neuronal Circuit Remodeling in Golgi pH Regulator-Deficient Mice

The Golgi apparatus plays an indispensable role in posttranslational modification and transport of proteins to their target destinations. Although it is well established that the Golgi apparatus requires an acidic luminal pH for optimal activity, morphological and functional abnormalities at the neuronal circuit level because of perturbations in Golgi pH are not fully understood. In addition, morphological alteration of the Golgi apparatus is associated with several neurodegenerative diseases, including Parkinson’s disease, Alzheimer’s disease, and amyotrophic lateral sclerosis. Here, we used anatomical and electrophysiological approaches to characterize morphological and functional abnormalities of neuronal circuits in Golgi pH regulator (GPHR) conditional knock-out mice. Purkinje cells (PCs) from the mutant mice exhibited vesiculation and fragmentation of the Golgi apparatus, followed by axonal degeneration and progressive cell loss. Morphological analysis provided evidence for the disruption of basket cell (BC) terminals around PC soma, and electrophysiological recordings showed selective loss of large amplitude responses, suggesting BC terminal disassembly. In addition, the innervation of mutant PCs was altered such that climbing fiber (CF) terminals abnormally synapsed on the somatic spines of mutant PCs in the mature cerebellum. The combined results describe an essential role for luminal acidification of the Golgi apparatus in maintaining proper neuronal morphology and neuronal circuitry.

Rer1-mediated quality control system is required for neural stem cell maintenance during cerebral cortex development

Rer1 is a retrieval receptor for endoplasmic reticulum (ER) retention of various ER membrane proteins and unassembled or immature components of membrane protein complexes. However, its physiological functions during mammalian development remain unclear. This study aimed to investigate the role of Rer1-mediated quality control system in mammalian development. We show that Rer1 is required for the sufficient cell surface expression and activity of γ-secretase complex, which modulates Notch signaling during mouse cerebral cortex development. When Rer1 was depleted in the mouse cerebral cortex, the number of neural stem cells decreased significantly, and malformation of the cerebral cortex was observed. Rer1 loss reduced γ-secretase activity and downregulated Notch signaling in the developing cerebral cortex. In Rer1-deficient cells, a subpopulation of γ-secretase complexes and components was transported to and degraded in lysosomes, thereby significantly reducing the amount of γ-secretase complex on the cell surface. These results suggest that Rer1 maintains Notch signaling by maintaining sufficient expression of the γ-secretase complex on the cell surface and regulating neural stem cell maintenance during cerebral cortex development.

We showed that Rer1 functions as an early-Golgi quality control pathway that maintains γ-secretase activity by maintaining sufficient cell surface expression of γ-secretase complex during cerebral cortex development, thereby modulating Notch signaling.

Ttyh1 regulates embryonic neural stem cell properties by enhancing the Notch signaling pathway

Despite growing evidence linking Drosophila melanogaster tweety-homologue 1 (Ttyh1) to normal mammalian brain development and cell proliferation, its exact role has not yet been determined. Here, we show that Ttyh1 is required for the maintenance of neural stem cell (NSC) properties as assessed by neurosphere formation and in vivo analyses of cell localization after in utero electroporation. We find that enhanced Ttyh1-dependent stemness of NSCs is caused by enhanced γ-secretase activity resulting in increased levels of Notch intracellular domain (NICD) production and activation of Notch targets. This is a unique function of Ttyh1 among all other Ttyh family members. Molecular analyses revealed that Ttyh1 binds to the regulator of γ-secretase activity Rer1 in the endoplasmic reticulum and thereby destabilizes Rer1 protein levels. This is the key step for Ttyh1-dependent enhancement of γ-secretase activity, as Rer1 overexpression completely abolishes the effects of Ttyh1 on NSC maintenance. Taken together, these findings indicate that Ttyh1 plays an important role during mammalian brain development by positively regulating the Notch signaling pathway through the downregulation of Rer1.

The Emerging Concept of Intrinsic Plasticity: Activity-dependent Modulation of Intrinsic Excitability in Cerebellar Purkinje Cells and Motor Learning

What is memory? How does the brain process the sensory information and modify an organism's behavior? Many neuroscientists have focused on the activity- and experience-dependent modifications of synaptic functions in order to solve these fundamental questions in neuroscience. Recently, the plasticity of intrinsic excitability (called intrinsic plasticity) has emerged as an important element for information processing and storage in the brain. As the cerebellar Purkinje cells are the sole output neurons in the cerebellar cortex and the information is conveyed from a neuron to its relay neurons by forms of action potential firing, the modulation of the intrinsic firing activity may play a critical role in the cerebellar learning. Many voltage-gated and/or Ca2+-activated ion channels are involved in shaping the spiking output as well as integrating synaptic inputs to finely tune the cerebellar output. Recent studies suggested that the modulation of the intrinsic excitability and its plasticity in the cerebellar Purkinje cells might function as an integrator for information processing and memory formation. Moreover, the intrinsic plasticity might also determine the strength of connectivity to the sub-cortical areas such as deep cerebellar nuclei and vestibular nuclei to trigger the consolidation of the cerebellar-dependent memory by transferring the information.

Purkinje Cell Signaling Deficits in Animal Models of Ataxia

Purkinje cell (PC) dysfunction or degeneration is the most frequent finding in animal models with ataxic symptoms. Mutations affecting intrinsic membrane properties can lead to ataxia by altering the firing rate of PCs or their firing pattern. However, the relationship between specific firing alterations and motor symptoms is not yet clear, and in some cases PC dysfunction precedes the onset of ataxic signs. Moreover, a great variety of ionic and synaptic mechanisms can affect PC signaling, resulting in different features of motor dysfunction. Mutations affecting Na⁺ channels (Na_V1.1, Na_V1.6, Na_Vβ4, Fgf14 or Rer1) reduce the firing rate of PCs, mainly via an impairment of the Na⁺ resurgent current. Mutations that reduce Kv3 currents limit the firing rate frequency range. Mutations of Kv1 channels act mainly on inhibitory interneurons, generating excessive GABAergic signaling onto PCs, resulting in episodic ataxia. Kv4.3 mutations are responsible for a complex syndrome with several neurologic dysfunctions including ataxia. Mutations of either Cav or BK channels have similar consequences, consisting in a disruption of the firing pattern of PCs, with loss of precision, leading to ataxia. Another category of pathogenic mechanisms of ataxia regards alterations of synaptic signals arriving at the PC. At the parallel fiber (PF)-PC synapse, mutations of glutamate delta-2 (GluD2) or its ligand Crbl1 are responsible for the loss of synaptic contacts, abolishment of long-term depression (LTD) and motor deficits. At the same synapse, a correct function of metabotropic glutamate receptor 1 (mGlu1) receptors is necessary to avoid ataxia. Failure of climbing fiber (CF) maturation and establishment of PC mono-innervation occurs in a great number of mutant mice, including mGlu1 and its transduction pathway, GluD2, semaphorins and their receptors. All these models have in common the alteration of PC output signals, due to a variety of mechanisms affecting incoming synaptic signals or the way they are processed by the repertoire of ionic channels responsible for intrinsic membrane properties. Although the PC is a final common pathway of ataxia, the link between specific firing alterations and neurologic symptoms has not yet been systematically studied and the alterations of the cerebellar contribution to motor signals are still unknown.

Sodium Channel Trafficking

Voltage-gated sodium channels (VGSC) are critical determinants of cellular electrical activity through the control of initiation and propagation of action potential. To ensure this role, these proteins are not consistently delivered to the plasma membrane but undergo drastic quality controls throughout various adaptive processes such as biosynthesis, anterograde and retrograde trafficking, and membrane targeting. In pathological conditions, this quality control could lead to the retention of functional VGSC and is therefore the target of different pharmacological approaches. The present chapter gives an overview of the current understanding of the facets of VGSC life cycle in the context of both cardiac and neuronal cell types.