Hung-Teh Kao

Childhood Maltreatment and Telomere Shortening: Preliminary Support for an Effect of Early Stress on Cellular Aging

BACKGROUND Psychological stress and trauma are risk factors for several medical and psychiatric illnesses. Recent studies have implicated advanced cellular aging as a potential mechanism of this association. Telomeres, DNA repeats that cap the ends of chromosomes and promote stability, shorten progressively with each cell division; their length is a marker of biological aging. Based on previous evidence linking psychosocial stress to shorter telomere length, this study was designed to evaluate the effect of childhood adversity on telomere length. METHODS Thirty-one adults with no current or past major Axis I psychiatric disorder participated. Subjects reported on their history of childhood maltreatment and telomere length was measured from DNA extracted from frozen whole blood using quantitative polymerase chain reaction. RESULTS Participants reporting a history of childhood maltreatment had significantly shorter telomeres than those who did not report a history of maltreatment. This finding was not due to effects of age, sex, smoking, body mass index, or other demographic factors. Analysis of subscales showed that both physical neglect and emotional neglect were significantly linked to telomere length. CONCLUSIONS These results extend previous reports linking shortened leukocyte telomere length and caregiver stress to more remote stressful experiences in childhood and suggest that childhood maltreatment could influence cellular aging.

Three-Dimensional Architecture of Presynaptic Terminal Cytomatrix

Presynaptic terminals are specialized for mediating rapid fusion of synaptic vesicles (SVs) after calcium influx. The regulated trafficking of SVs likely results from a highly organized cytomatrix. How this cytomatrix links SVs, maintains them near the active zones (AZs) of release, and organizes docked SVs at the release sites is not fully understood. To analyze the three-dimensional (3D) architecture of the presynaptic cytomatrix, electron tomography of presynaptic terminals contacting spines was performed in the stratum radiatum of the rat hippocampal CA1 area. To preserve the cytomatrix, hippocampal slices were immobilized using high-pressure freezing, followed by cryosubstitution and embedding. SVs are surrounded by a dense network of filaments. A given vesicle is connected to ∼1.5 neighboring ones. SVs at the periphery of this network are also linked to the plasma membrane, by longer filaments. More of these filaments are found at the AZ. At the AZ, docked SVs are grouped around presynaptic densities. Filaments with adjacent SVs emerge from these densities. Immunogold localizations revealed that synapsin is located in the presynaptic bouton, whereas Bassoon and CAST (ERC2) are at focal points next to the AZ. In synapsin triple knock-out mice, the number of SVs is reduced by 63%, but the size of the boutons is reduced by only 18%, and the mean distance of SVs to the AZ is unchanged. This 3D analysis reveals the morphological constraints exerted by the presynaptic molecular scaffold. SVs are tightly interconnected in the axonal bouton, and this network is preferentially connected to the AZ.

Telomeres and Early-Life Stress: An Overview

The long-term sequelae of adverse early-life experiences have long been a focus in psychiatry, with a historic neurobiological emphasis on physiological systems that are demonstrably stress-responsive, such as the hypothalamic-pituitary-adrenal axis and neuroimmune function. However, there has been increasing recognition in the general medical literature that such sequelae might encompass more pervasive alterations in health status and physiology. Recent findings in telomere biology have suggested a new avenue for exploring the adverse health effects of childhood maltreatment. Telomere length in proliferative tissues declines with cell replication and the effect can be accelerated by such factors as inflammation, oxidative stress, radiation, and toxins. Reduced telomere length, as a proxy for cellular aging, has been associated with numerous chronic somatic diseases that are generally considered to be diseases of aging, such as diabetes, cancer, and heart disease. More recently, shorter telomeres have been demonstrated in several psychiatric conditions, particularly depression. Sustained psychosocial stress of a variety of types in adulthood appears to be associated with shorter telomeres. Now, emerging work suggests a robust, and perhaps dose-dependent, relationship with early-life stress. These findings present new opportunities to reconceptualize the complex relationships between experience, physical and psychiatric disease, and aging.

A protein kinase A–dependent molecular switch in synapsins regulates neurite outgrowth

Cyclic AMP (cAMP) promotes neurite outgrowth in a variety of neuronal cell lines through the activation of protein kinase A (PKA). We show here, using both Xenopus laevis embryonic neuronal culture and intact X. laevis embryos, that the nerve growth–promoting action of cAMP/PKA is mediated in part by the phosphorylation of synapsins at a single amino acid residue. Expression of a mutated form of synapsin that prevents phosphorylation at this site, or introduction of phospho-specific antibodies directed against this site, decreased basal and dibutyryl cAMP–stimulated neurite outgrowth. Expression of a mutation mimicking constitutive phosphorylation at this site increased neurite outgrowth, both under basal conditions and in the presence of a PKA inhibitor. These results provide a potential molecular approach for stimulating neuron regeneration, after injury and in neurodegenerative diseases.

Two sites of action for synapsin domain E in regulating neurotransmitter release.

Synapsins, a family of synaptic vesicle proteins, have been shown to regulate neurotransmitter release; the mechanism(s) by which they act are not fully understood. Here we have studied the role of domain E of synapsins in neurotransmitter release at the squid giant synapse. Two squid synapsin isoforms were cloned and found to contain a carboxy (C)-terminal domain homologous to domain E of the vertebrate a-type synapsin isoforms. Presynaptic injection of a peptide fragment of domain E greatly reduced the number of synaptic vesicles in the periphery of the active zone, and increased the rate and extent of synaptic depression, suggesting that domain E is essential for synapsins to regulate a reserve pool of synaptic vesicles. Domain E peptide had no effect on the number of docked synaptic vesicles, yet reversibly inhibited and slowed the kinetics of neurotransmitter release, indicating a second role for synapsins that is more intimately associated with the release process itself. Thus, synapsin domain E is involved in at least two distinct reactions that are crucial for exocytosis in presynaptic terminals.

Protein Kinase A-Mediated Synapsin I Phosphorylation Is a Central Modulator of Ca2+-Dependent Synaptic Activity

Protein kinase A (PKA) modulates several steps of synaptic transmission. However, the identification of the mediators of these effects is as yet incomplete. Synapsins are synaptic vesicle (SV)-associated phosphoproteins that represent the major presynaptic targets of PKA. We show that, in hippocampal neurons, cAMP-dependent pathways affect SV exocytosis and that this effect is primarily brought about through synapsin I phosphorylation. Phosphorylation by PKA, by promoting dissociation of synapsin I from SVs, enhances the rate of SV exocytosis on stimulation. This effect becomes relevant when neurons are challenged with sustained stimulation, because it appears to counteract synaptic depression and accelerate recovery from depression by fostering the supply of SVs from the reserve pool to the readily releasable pool. In contrast, synapsin phosphorylation appears to be dispensable for the effects of cAMP on the frequency and amplitude of spontaneous synaptic currents and on the amplitude of evoked synaptic currents. The modulation of depolarization-evoked SV exocytosis by PKA phosphorylation of synapsin I is primarily caused by calmodulin (CaM)-dependent activation of cAMP pathways rather than by direct activation of CaM kinases. These data define a hierarchical crosstalk between cAMP- and CaM-dependent cascades and point to synapsin as a major effector of PKA in the modulation of activity-dependent SV exocytosis.

MOLECULAR EVOLUTION OF THE SYNAPSIN GENE FAMILY

Synapsins, a family of synaptic vesicle proteins, play a crucial role in the regulation of neurotransmission and synaptogenesis. They have been identified in a variety of invertebrate and vertebrate species, including human, rat (Rattus norvegicus), cow (Bos taurus), longfin squid (Loligo pealei), and fruit fly (Drosophila melanogaster). Here, synapsins were cloned from three additional species: frog (Xenopus laevis), lamprey (Lampetra fluviatilis), and nematode (Caenorhabditis elegans). Synapsin protein sequences from all these species were then used to explore the molecular phylogeny of these important neuronal phosphoproteins. The ancestral condition of a single synapsin gene probably gave rise to the vertebrate synapsin gene family comprised of at least three synapsin genes (I, II, and III) in higher vertebrates. Synapsins possess multiple domains, which have evolved at different rates throughout evolution. In invertebrate synapsins, the most conserved domains are C and E. During the evolution of vertebrates, at least two gene duplication events are hypothesized to have given rise to the synapsin gene family. This was accompanied by the emergence of an additional conserved domain, termed A. J. Exp. Zool. ( Mol. Dev. Evol. ) 285:360-377, 1999.

Molecular Determinants of Synapsin Targeting to Presynaptic Terminals

Although synapsins are abundant synaptic vesicle proteins that are widely used as markers of presynaptic terminals, the mechanisms that target synapsins to presynaptic terminals have not been elucidated. We have addressed this question by imaging the targeting of green fluorescent protein-tagged synapsins in cultured hippocampal neurons. Whereas all synapsin isoforms targeted robustly to presynaptic terminals in wild-type neurons, synapsin Ib scarcely targeted in neurons in which all synapsins were knocked-out. Coexpression of other synapsin isoforms significantly strengthened the targeting of synapsin Ib in knock-out neurons, indicating that heterodimerization is required for synapsin Ib to target. Truncation mutagenesis revealed that synapsin Ia targets via distributed binding sites that include domains B, C, and E. Although domain A was not necessary for targeting, its presence enhanced targeting. Domain D inhibited targeting, but this inhibition was overcome by domain E. Thus, multiple intermolecular and intramolecular interactions are required for synapsins to target to presynaptic terminals.

Rapid telomere erosion in schizophrenia

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Phosphorylation of Synapsin I by cAMP-Dependent Protein Kinase Controls Synaptic Vesicle Dynamics in Developing Neurons

In developing neurons, synaptic vesicles (SVs) undergo cycles of exo-endocytosis along isolated axons. However, it is currently unknown whether SV exocytosis is regulated before synaptogenesis. Here, we show that cAMP-dependent pathways affect SV distribution and recycling in the axonal growth cone and that these effects are mediated by the SV-associated phosphoprotein synapsin I. The presence of synapsin I on SVs is necessary for the correct localization of the vesicles in the central portion of the growth cone. Phosphorylation of synapsin I by cAMP-dependent protein kinase (protein kinase A) causes the dissociation of the protein from the SV membrane, allowing diffusion of the vesicles to the periphery of the growth cone and enhancing their rate of recycling. These results provide new clues as to the bases of the well known activity of synapsin I in synapse maturation and indicate that molecular mechanisms similar to those operating at mature nerve terminals are active in developing neurons to regulate the SV life cycle before synaptogenesis.