Phosphorylation-dependent proteome of Marcks in ependyma during aging and behavioral homeostasis in the mouse forebrain.

Ependymal cells (ECs) line the ventricular surfaces of the mammalian central nervous system (CNS) and their development is indispensable to structural integrity and functions of the CNS. We previously reported that EC-specific genetic deletion of the myristoylated alanine-rich protein kinase C substrate (Marcks) disrupts barrier functions and elevates oxidative stress and lipid droplet accumulation in ECs causing precocious cellular aging. However, little is known regarding the mechanisms that mediate these changes in ECs. To gain insight into Marcks-mediated mechanisms, we performed mass spectrometric analyses on Marcks-associated proteins in young and aged ECs in the mouse forebrain using an integrated approach. Network analysis on annotated proteins revealed that the identified Marcks-associated complexes are in part involved in protein transport mechanisms in young ECs. In fact, we found perturbed intracellular vesicular trafficking in cultured ECs with selective deletion of Marcks (Marcks-cKO mice), or upon pharmacological alteration to phosphorylation status of Marcks. In comparison, Marcks-associated protein complexes in aged ECs appear to be involved in regulation of lipid metabolism and responses to oxidative stress. Confirming this, we found elevated signatures of inflammation in the cerebral cortices and the hippocampi of young Marcks-cKO mice. Interestingly, behavioral testing using a water maze task indicated that spatial learning and memory is diminished in young Marcks-cKO mice similar to aged wildtype mice. Taken together, our study provides first line of evidence for potential mechanisms that may mediate differential Marcks functions in young and old ECs, and their effect on forebrain homeostasis during aging.

Foxj1 expressing ependymal cells do not contribute new cells to sites of injury or stroke in the mouse forebrain.

The stem cell source of neural and glial progenitors in the periventricular regions of the adult forebrain has remained uncertain and controversial. Using a cell specific genetic approach we rule out Foxj1+ ependymal cells as stem cells participating in neurogenesis and gliogenesis in response to acute injury or stroke in the mouse forebrain. Non stem- and progenitor-like responses of Foxj1+ ependymal cells to injury and stroke remain to be defined and investigated.

Dynein binds and stimulates axonal motility of the endosome adaptor and NEEP21 family member, calcyon.

The neuron-enriched, endosomal protein Calcyon (Caly) regulates endocytosis and vesicle sorting, and is important for synaptic plasticity and brain development. In the current investigation of Caly interacting proteins in brain, the microtubule retrograde motor subunit, cytoplasmic dynein 1 heavy chain (DYNC1H), and microtubule structural proteins, α and ß tubulin, were identified as Caly associated proteins by MALDI-ToF/ToF. Direct interaction of the Caly-C terminus with dynein and tubulin was further confirmed in in vitro studies. In Cos-7 cells, mCherry-Caly moved along the microtubule network in organelles largely labeled by the late endosome marker Rab7. Expression of the dynein inhibitor CC1, produced striking alterations in Caly distribution, consistent with retrograde motors playing a prominent role in Caly localization and movement. In axons of cultured adult rat sensory neurons, Caly-positive organelles co-localized with dynein intermediate chain (DYNC1I1-isoform IC-1B) and the dynein regulator, lissencephaly 1 (LIS1), both of which co-precipitated from brain with the Caly C-terminus. Manipulation of dynein function in axons altered the motile properties of Caly indicating that Caly vesicles utilize the retrograde motor. Altogether, the current evidence for association with dynein motors raises the possibility that the endocytic and cargo sorting functions of Caly in neurons could be regulated by interaction with the microtubule transport system.

Ependymal cell contribution to scar formation after spinal cord injury is minimal, local and dependent on direct ependymal injury.

Ependyma have been proposed as adult neural stem cells that provide the majority of newly proliferated scar-forming astrocytes that protect tissue and function after spinal cord injury (SCI). This proposal was based on small, midline stab SCI. Here, we tested the generality of this proposal by using a genetic knock-in cell fate mapping strategy in different murine SCI models. After large crush injuries across the entire spinal cord, ependyma-derived progeny remained local, did not migrate and contributed few cells of any kind and less than 2%, if any, of the total newly proliferated and molecularly confirmed scar-forming astrocytes. Stab injuries that were near to but did not directly damage ependyma, contained no ependyma-derived cells. Our findings show that ependymal contribution of progeny after SCI is minimal, local and dependent on direct ependymal injury, indicating that ependyma are not a major source of endogenous neural stem cells or neuroprotective astrocytes after SCI.

Developmentally defined forebrain circuits regulate appetitive and aversive olfactory learning.

Postnatal and adult neurogenesis are region- and modality-specific, but the significance of developmentally distinct neuronal populations remains unclear. We demonstrate that chemogenetic inactivation of a subset of forebrain and olfactory neurons generated at birth disrupts responses to an aversive odor. In contrast, novel appetitive odor learning is sensitive to inactivation of adult-born neurons, revealing that developmentally defined sets of neurons may differentially participate in hedonic aspects of sensory learning.

TAK1 determines susceptibility to endoplasmic reticulum stress and leptin resistance in the hypothalamus.

Sustained endoplasmic reticulum (ER) stress disrupts normal cellular homeostasis and leads to the development of many types of human diseases, including metabolic disorders. TAK1 (also known as MAP3K7) is a member of the mitogen-activated protein kinase kinase kinase (MAP3K) family and is activated by a diverse set of inflammatory stimuli. Here, we demonstrate that TAK1 regulates ER stress and metabolic signaling through modulation of lipid biogenesis. We found that deletion of Tak1 increased ER volume and facilitated ER-stress tolerance in cultured cells, which was mediated by upregulation of sterol-regulatory-element-binding protein (SREBP)-dependent lipogenesis. In the in vivo setting, central nervous system (CNS)-specific Tak1 deletion upregulated SREBP-target lipogenic genes and blocked ER stress in the hypothalamus. Furthermore, CNS-specific Tak1 deletion prevented ER-stress-induced hypothalamic leptin resistance and hyperphagic obesity under a high-fat diet (HFD). Thus, TAK1 is a crucial regulator of ER stress in vivo, which could be a target for alleviation of ER stress and its associated disease conditions.

MARCKS-dependent mucin clearance and lipid metabolism in ependymal cells are required for maintenance of forebrain homeostasis during aging.

Ependymal cells (ECs) form a barrier responsible for selective movement of fluids and molecules between the cerebrospinal fluid and the central nervous system. Here, we demonstrate that metabolic and barrier functions in ECs decline significantly during aging in mice. The longevity of these functions in part requires the expression of the myristoylated alanine-rich protein kinase C substrate (MARCKS). Both the expression levels and subcellular localization of MARCKS in ECs are markedly transformed during aging. Conditional deletion of MARCKS in ECs induces intracellular accumulation of mucins, elevated oxidative stress, and lipid droplet buildup. These alterations are concomitant with precocious disruption of ependymal barrier function, which results in the elevation of reactive astrocytes, microglia, and macrophages in the interstitial brain tissue of young mutant mice. Interestingly, similar alterations are observed during normal aging in ECs and the forebrain interstitium. Our findings constitute a conceptually new paradigm in the potential role of ECs in the initiation of various conditions and diseases in the aging brain.

Complementary roles of the neuron-enriched endosomal proteins NEEP21 and calcyon in neuronal vesicle trafficking.

Understanding mechanisms governing the trafficking of transmembrane (TM) cargoes to synapses and other specialized membranes in neurons represents a long-standing challenge in cell biology. Investigation of the neuron-enriched endosomal protein of 21 kDa (NEEP21, or NSG1or P21) and Calcyon (Caly, or NSG3) indicates that the emergence of the NEEP21/Caly/P19 gene family could play a vital role in the success of these mechanisms in vertebrates. The upshot of a sizeable body of work is that the NEEP21 and Caly perform distinct endocytic and recycling functions, which impact (i) α amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor trafficking at excitatory synapses; (ii) transport to/in neuronal axons; as well as (iii) proteolytic processing of amyloid precursor protein and neuregulin 1, suggesting roles in neuron development, synaptic function, and neurodegeneration. We argue that their distinct effects on cargo endocytosis and recycling depend on interactions with vesicle trafficking and synaptic scaffolding proteins. As they play complementary, but opposing roles in cargo endocytosis, recycling, and degradation, balancing NEEP21 and Caly expression levels or activity could be important for homeostasis in a variety of signaling pathways, and also lead to a novel therapeutic strategy for disorders like Alzheimer's disease and schizophrenia. This review focuses on two closely related, neuron-enriched endosomal proteins: NEEP21 and Calcyon which perform distinct roles in regulating receptor endocytosis, recycling, and degradation. Based on an in-depth examination of the literature, we argue that these two proteins carry out complementary yet sometimes opposing vesicle trafficking functions that impact excitatory transmission, transcytosis, axonal transport, and also proteolytic processing by beta-secretase I (BACE1). Finally, we propose that balancing NEEP21 and Calcyon expression and/or activity could be important for homeostasis in a variety of signaling pathways, and also lead to a novel therapeutic strategy for disorders like Alzheimer's disease and schizophrenia. AMPA = α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; NMDA = N-Methyl-D-aspartate.

A knock-in Foxj1(CreERT2::GFP) mouse for recombination in epithelial cells with motile cilia.

The transcription factor Foxj1 is expressed by cells destined to differentiate into epithelial cells projecting motile cilia into fluid- or air-filled cavities. Here, we report the generation of an inducible knock-in Foxj1(CreERT2::GFP) mouse, which we show reliably induces Cre-mediated recombination for genetic studies in epithelial cells with motile cilia throughout embryonic and postnatal development. Induction during embryonic stages revealed efficient recombination in the epithelial component of the choroid plexus in the developing brain as early as E12.5. Induction during late embryonic stages showed confined recombination not only in the choroid plexus but also in the ventricular walls of the brain. Recombination induced during postnatal periods expanded to include epithelia of the lungs, testis, oviduct, and brain. Using these mice, we confirmed our recent discovery of a perinatally derived neuronal population in the mouse olfactory bulbs, which is derived from the Foxj1 lineage. Our Foxj1(CreERT2::GFP) knock-in mouse will be a powerful tool for studying molecular mechanisms associated with the continuum of cells that form the Foxj1 lineage, and for assessing their physiological significance during development and aging.

Calcyon, a mammalian specific NEEP21 family member, interacts with adaptor protein complex 3 (AP-3) and regulates targeting of AP-3 cargoes.

Calcyon is a neural enriched, single transmembrane protein that interacts with clathrin light chain and stimulates clathrin assembly and clathrin-mediated endocytosis. A similar property is shared by the heterotetrameric adaptor protein (AP) complexes AP-1, AP-2, and AP-3 which recruit cargoes for insertion into clathrin coated transport vesicles. Here we report that AP medium (µ) subunits interact with a YXXØ-type tyrosine motif located at residues 133-136 in the cytoplasmic domain of calcyon. Site specific mutagenesis of the critical tyrosine and bulky hydrophobic residues tyrosine 133 and methionine 136 preferentially abrogated binding of the ubiquitous and neuronal isoforms of µ3, and also impacted µ1 and µ2 binding to a lesser degree. The relevance of these interactions was explored in vivo using mice harboring null alleles of calcyon. As seen in the mutagenesis studies, calcyon deletion in mice preferentially altered the subcellular distribution of AP-3 suggesting that calcyon could regulate membrane-bound pools of AP-3 and AP-3 function. To test this hypothesis, we focused on the hilar region of hippocampus, where levels of calcyon, AP-3, and AP-3 cargoes are abundant. We analyzed brain cryosections from control and calcyon null mice for zinc transporter 3 (ZnT3), and phosphatidylinositol-4-kinase type II alpha (PI4KIIα), two well-defined AP-3 cargoes. Confocal microscopy indicated that ZnT3 and PI4KIIα are significantly reduced in the hippocampal mossy fibers of calcyon knock-out brain, a phenotype previously described in AP-3 deficiencies. Altogether, our data suggest that calcyon directly interacts with µ3A and µ3B, and regulates the subcellular distribution of AP-3 and the targeting of AP-3 cargoes.

Specification of a Foxj1-dependent lineage in the forebrain is required for embryonic-to-postnatal transition of neurogenesis in the olfactory bulb.

Establishment of a neural stem cell niche in the postnatal subependymal zone (SEZ) and the rostral migratory stream (RMS) is required for postnatal and adult neurogenesis in the olfactory bulbs (OB). We report the discovery of a cellular lineage in the SEZ-RMS-OB continuum, the specification of which is dependent on the expression of the forkhead transcription factor Foxj1 in mice. Spatially and temporally restricted Foxj1+ neuronal progenitors emerge during embryonic periods, surge during perinatal development, and are active only for the first few postnatal weeks. We show that the development of the unique Foxj1-derived lineage is dependent on Foxj1 expression and is required for overall postnatal neurogenesis in the OB. Strikingly, the production of neurons from Foxj1+ progenitors significantly declines after the early postnatal weeks, but Foxj1-derived neurons in the OB persist during adult periods. For the first time, our study identifies the time- and region-specific activity of a perinatal progenitor domain that is required for transition and progression of OB neurogenesis from the embryonic-to-postnatal periods.

Phylogenetic analysis of the NEEP21/calcyon/P19 family of endocytic proteins: evidence for functional evolution in the vertebrate CNS.

Endocytosis and vesicle trafficking are required for optimal neural transmission. Yet, little is currently known about the evolution of neuronal proteins regulating these processes. Here, we report the first phylogenetic study of NEEP21, calcyon, and P19, a family of neuronal proteins implicated in synaptic receptor endocytosis and recycling, as well as in membrane protein trafficking in the somatodendritic and axonal compartments of differentiated neurons. Database searches identified orthologs for P19 and NEEP21 in bony fish, but not urochordate or invertebrate phyla. Calcyon orthologs were only retrieved from mammalian databases and distant relatives from teleost fish. In situ localization of the P19 zebrafish ortholog, and extant progenitor of the gene family, revealed a CNS specific expression pattern. Based on non-synonymous nucleotide substitution rates, the calcyon genes appear to be under less intense negative selective pressure. Indeed, a functional group II WW domain binding motif was detected in primate and human calcyon, but not in non-primate orthologs. Sequencing of the calcyon gene from 80 human subjects revealed a non-synonymous single nucleotide polymorphism that abrogated group II WW domain protein binding. Altogether, our data indicate the NEEP21/calcyon/P19 gene family emerged, and underwent two rounds of gene duplication relatively late in metazoan evolution (but early in vertebrate evolution at the latest). As functional studies suggest NEEP21 and calcyon play related, but distinct roles in regulating vesicle trafficking at synapses, and in neurons in general, we propose the family arose in chordates to support a more diverse range of synaptic and behavioral responses.