The synapsins: beyond the regulation of neurotransmitter release

The synapsins are a family of five closely related neuron-specific phosphoproteins associated with the membranes of synaptic vesicles. The synapsins have been implicated in the regulation of neurotransmitter release. They tether synaptic vesicles to actin filaments in a phosphorylation-dependent manner, controlling the number of vesicles available for release at the nerve terminus. A growing body of evidence suggests that the synapsins play a broad role during neuronal development. They participate in the formation and maintenance of synaptic contacts among central neurons. In addition, each synapsin has a specific role during the elongation of undifferentiated processes and their posterior differentiation into axons and dendrites. In this review, we focus on these novel roles of synapsins during the early stages of development. Received 26 September 2001; received after revision 8 November 2001; accepted 9 November 2001     

Synaptic dysfunction in Huntington’s disease: a new perspective

Huntington’s disease (HD) is caused by a polyglutamine expansion in the protein huntingtin and is characterized by intraneuronal inclusions and widespread neuronal death at the late stage of the disease. In research, most of the emphasis has been on understanding the cell death and its mechanisms. Until recently, it was believed that the vast majority, if not all, of the symptoms in HD are a direct consequence of neurodegeneration. However, increasing evidence shows that subtle alterations in synaptic function could underlie the early symptoms. It is of particular interest to understand the nature of this neuronal dysfunction. Normal huntingtin interacts with various cytoskeletal and synaptic vesicle proteins that are essential for exocytosis and endocytosis. Altered interactions of mutant huntingtin with its associated partners could contribute to abnormal synaptic transmission in HD. This review describes recent advances in understanding synaptic dysfunction in HD.Received 2 March 2005; received after revision 13 April 2005; accepted 19 April 2005     

Molecular analysis of axonal target specificity and synapse formation

The development of neuronal connectivity requires the growth of axons to their target region and the formation of dendritic trees that extend into specific layers. Within the target region growth cones, the tips of extending axons are guided to finer target fields including specific subcellular compartments where they form synapses. In this article we highlight recent progress on molecular aspects of axonal subcellular target selection such as the axon initial segment or specific sublaminae of the vertebrate retina. We then discuss the very recent progress on the molecular analysis of synapse formation in the central nervous system, including the direction of differentiation into an inhibitory or excitatory synapse. Apparently, initial synaptic contacts are structurally and functionally modulated by neuronal activity, raising the question how neuronal activity can modify synaptic circuits. We therefore also focus on neural proteins that are up-regulated, secreted or converted by synaptic activity and, thus, might represent molecular candidates for experience-driven refinement or remodeling of synaptic connections. Received 5 July 2005; received after revision 19 August 2005; accepted 2 September 2005     

The BLM dissolvasome in DNA replication and repair

RecQ DNA helicases are critical for proper maintenance of genomic stability, and mutations in multiple human RecQ genes are linked with genetic disorders characterized by a predisposition to cancer. RecQ proteins are conserved from prokaryotes to humans and in all cases form higher-order complexes with other proteins to efficiently execute their cellular functions. The focus of this review is a conserved complex that is formed between RecQ helicases and type-I topoisomerases. In humans, this complex is referred to as the BLM dissolvasome or BTR complex, and is comprised of the RecQ helicase BLM, topoisomerase IIIα, and the RMI proteins. The BLM dissolvasome functions to resolve linked DNA intermediates without exchange of genetic material, which is critical in somatic cells. We will review the history of this complex and highlight its roles in DNA replication, recombination, and repair. Additionally, we will review recently established interactions between BLM dissolvasome and a second set of genome maintenance factors (the Fanconi anemia proteins) that appear to allow coordinated genome maintenance efforts between the two systems.     

PAR3–PAR6–atypical PKC polarity complex proteins in neuronal polarization

Polarity is a fundamental feature of cells. Protein complexes, including the PAR3–PAR6–aPKC complex, have conserved roles in establishing polarity across a number of eukaryotic cell types. In neurons, polarity is evident as distinct axonal versus dendritic domains. The PAR3, PAR6, and aPKC proteins also play important roles in neuronal polarization. During this process, either aPKC kinase activity, the assembly of the PAR3–PAR6–aPKC complex or the localization of these proteins is regulated downstream of a number of signaling pathways. In turn, the PAR3, PAR6, and aPKC proteins control various effector molecules to establish neuronal polarity. Herein, we discuss the many signaling mechanisms and effector functions that have been linked to PAR3, PAR6, and aPKC during the establishment of neuronal polarity.     

A central role for calcineurin in protein misfolding neurodegenerative diseases

Accumulation of misfolded/unfolded aggregated proteins in the brain is a hallmark of many neurodegenerative diseases affecting humans and animals. Dysregulation of calcium (Ca²⁺) and disruption of fast axonal transport (FAT) are early pathological events that lead to loss of synaptic integrity and axonal degeneration in early stages of neurodegenerative diseases. Dysregulated Ca²⁺ in the brain is triggered by accumulation of misfolded/unfolded aggregated proteins in the endoplasmic reticulum (ER), a major Ca²⁺ storing organelle, ultimately leading to neuronal dysfunction and apoptosis. Calcineurin (CaN), a Ca²⁺/calmodulin-dependent serine/threonine phosphatase, has been implicated in T cells activation through the induction of nuclear factor of activated T cells (NFAT). In addition to the involvement of several other signaling cascades, CaN has been shown to play a role in early synaptic dysfunction and neuronal death. Therefore, inhibiting hyperactivated CaN in early stages of disease might be a promising therapeutic strategy for treating patients with protein misfolding diseases. In this review, we briefly summarize the structure of CaN, inhibition mechanisms by which immunosuppressants inhibit CaN, role of CaN in maintaining neuronal and synaptic integrity and homeostasis and the role played by CaN in protein unfolding/misfolding neurodegenerative diseases.     

Structural mechanisms of the agrin–LRP4–MuSK signaling pathway in neuromuscular junction differentiation

The neuromuscular junction (NMJ) is the most extensively studied model of neuronal synaptogenesis. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in the differentiation of NMJs. AChR clustering and postsynaptic differentiation is orchestrated by sophisticated interactions among three proteins: the neuron-secreted proteoglycan agrin, the co-receptor LRP4, and the muscle-specific receptor tyrosine kinase MuSK. LRP4 and MuSK act as scaffolds for multiple binding partners, resulting in a complex and dynamic network of interacting proteins that is required for AChR clustering. In this review, we discuss the structural basis for NMJ postsynaptic differentiation mediated by the agrin–LRP4–MuSK signaling pathway.     

Genetic and molecular analysis of the synaptotagmin family

Secretion is a fundamental cellular process used by all eukaryotes to insert proteins into the plasma membrane and transport signaling molecules and intravesicular proteins into the extracellular space. Secretion requires the fusion of two phospholipid bilayers within the cell, an energetically unfavorable process. A conserved repertoire of vesicle-trafficking proteins has evolved that function to overcome this energy barrier and temporally and spatially control membrane fusion within the cell. Within neurons, opening of synaptic calcium channels and subsequent calcium entry triggers synchronous synaptic vesicle exocytosis and neurotransmitter release into the synaptic cleft. After fusion, synaptic vesicles undergo endocytosis, are refilled with neurotransmitter, and return to the vesicle pool for further rounds of cycling. It is within this local synaptic trafficking pathway that the synaptotagmin family of calcium-binding synaptic vesicle proteins has been postulated to function. Here we review the current literature on the function of the synaptotagmin family and discuss the implications for synaptic transmission and membrane trafficking. Received 14 August 2000; received after revision 20 September 2000, accepted 14 October 2000     

Neurotrophins and neuronal differentiation in the central nervous system

The central nervous system requires the proper formation of exquisitely precise circuits to function properly. These neuronal circuits are assembled during development by the formation of synaptic connections between hundreds of thousands of differentiating neurons. For these circuits to form correctly, neurons must elaborate precisely patterned axonal and dendritic arbors. Although the cellular and molecular mechanisms that guide neuronal differentiation and formation of connections remain mostly unknown, the neurotrophins have emerged recently as attractive candidates for regulating neuronal differentiation in the developing brain. The experiments reviewed here provide strong support for a bifunctional role for the neurotrophins in axonal and dendritic growth and are consistent with the exciting possibility that the neurotrophins might mediate activity-dependent synaptic plasticity.     

The evolution of domain arrangements in proteins and interaction networks

Proteins are composed of domains, which are conserved evolutionary units that often also correspond to functional units and can frequently be detected with reasonable reliability using computational methods. Most proteins consist of two or more domains, giving rise to a variety of combinations of domains. Another level of complexity arises because proteins themselves can form complexes with small molecules, nucleic acids and other proteins. The networks of both domain combinations and protein interactions can be conceptualised as graphs, and these graphs can be analysed conveniently by computational methods. In this review we summarise facts and hypotheses about the evolution of domains in multi-domain proteins and protein complexes, and the tools and data resources available to study them.Received 20 September 2004; received after revision 23 October 2004; accepted 1 November 2004     

Emerging mechanisms and consequences of calcium regulation of alternative splicing in neurons and endocrine cells

Alternative splicing contributes greatly to proteomic complexity. How it is regulated by external stimuli to sculpt cellular properties, particularly the highly diverse and malleable neuronal properties, is an underdeveloped area of emerging interest. A number of recent studies in neurons and endocrine cells have begun to shed light on its regulation by calcium signals. Some mechanisms include changes in the trans-acting splicing factors by phosphorylation, protein level, alternative pre-mRNA splicing, and nucleocytoplasmic redistribution of proteins to alter protein–RNA or protein–protein interactions, as well as modulation of chromatin states. Importantly, functional analyses of the control of specific exons/splicing factors in the brain point to a crucial role of this regulation in synaptic maturation, maintenance, and transmission. Furthermore, its deregulation has been implicated in the pathogenesis of neurological disorders, particularly epilepsy/seizure. Together, these studies have not only provided mechanistic insights into the regulation of alternative splicing by calcium signaling but also demonstrated its impact on neuron differentiation, function, and disease. This may also help our understanding of similar regulations in other types of cells.     

Cbln and C1q family proteins – New transneuronal cytokines

The C1q family is characterized by a C-terminal conserved global C1q domain, which is structurally very similar to the tumor necrosis factor homology domain. Although some C1q family members are expressed in the central nervous system, their functions have not been well characterized. Cbln1, a member of the Cbln subfamily of the C1q family, is predominantly expressed in cerebellar granule cells. Interestingly, Cbln1 was recently shown to play two unique roles at excitatory synapses formed between cerebellar granule cells and Purkinje cells: the formation and stabilization of synaptic contact, and the control of functional synaptic plasticity by regulating the postsynaptic endocytosis pathway. Since other Cbln subfamily members, Cbln2-Cbln4, are expressed in various regions of developing and mature brains, Cbln subfamily proteins may generally serve as a new class of transneuronal regulators of synapse development and synaptic plasticity in various brain regions.     

NDE1 and NDEL1 from genes to (mal)functions: parallel but distinct roles impacting on neurodevelopmental disorders and psychiatric illness

NDE1 (Nuclear Distribution Element 1, also known as NudE) and NDEL1 (NDE-Like 1, also known as NudEL) are the mammalian homologues of the fungus nudE gene, with important and at least partially overlapping roles for brain development. While a large number of studies describe the various properties and functions of these proteins, many do not directly compare the similarities and differences between NDE1 and NDEL1. Although sharing a high degree structural similarity and multiple common cellular roles, each protein presents several distinct features that justify their parallel but also unique functions. Notably both proteins have key binding partners in dynein, LIS1 and DISC1, which impact on neurodevelopmental and psychiatric illnesses. Both are implicated in schizophrenia through genetic and functional evidence, with NDE1 also strongly implicated in microcephaly, as well as other neurodevelopmental and psychiatric conditions through copy number variation, while NDEL1 possesses an oligopeptidase activity with a unique potential as a biomarker in schizophrenia. In this review, we aim to give a comprehensive overview of the various cellular roles of these proteins in a “bottom-up” manner, from their biochemistry and protein–protein interactions on the molecular level, up to the consequences for neuronal differentiation, and ultimately to their importance for correct cortical development, with direct consequences for the pathophysiology of neurodevelopmental and mental illness.     

Polo-box domain: a versatile mediator of polo-like kinase function

Members of the polo subfamily of protein kinases have emerged as important regulators in diverse aspects of the cell cycle and cell proliferation. A large body of evidence suggests that a highly conserved polo-box domain (PBD) present in the C-terminal non-catalytic region of polo kinases plays a pivotal role in the function of these enzymes. Recent advances in our comprehension of the mechanisms underlying mammalian polo-like kinase 1 (Plk1)-dependent protein–protein interactions revealed that the PBD serves as an essential molecular mediator that brings the kinase domain of Plk1 into proximity with its substrates, mainly through phospho-dependent interactions with its target proteins. In this review, current understanding of the structure and functions of PBD, mode of PBD-dependent interactions and substrate phosphorylation, and other phospho-independent functions of PBD are discussed.     

Ubiquitin-proteasome system

The capacity for exquisite regulation of ubiquitylation provides eukaryotic cells with a means to fine-tune both protein function and levels. This complex set of processes affects myriad proteins and potentially impacts all cellular processes. Ubiquitylation is brought about through multienzyme processes, with specificity conferred primarily by interactions of substrates with specific ubiquitin protein ligases (E3s) in association with ubiquitin conjugating enzymes (E2s). Regulation of ubiquitylation occurs at multiple levels, including E2-E3 interactions, substrate recognition, chain elongation, binding of ubiquitin to conserved motifs and deubiquityation. This review presents the fundamentals of the ubiquitin conjugating system.     

The molecular machinery of synaptic vesicle exocytosis

At the synapse, neurotransmitters are released via Ca(2+)-triggered exocytotic fusion of synaptic vesicles with the presynaptic plasma membrane. Synaptic vesicle exocytosis seems to share many basic principles and homologous proteins with other membrane fusion events. Conserved components of the general fusion machinery that participate in synaptic vesicle exocytosis include soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), ATPase N-ethylmaleimide-sensitive factor, Munc18/nSec1, Rab3 GTPase, and the exocyst proteins. In addition, synaptic vesicle exocytosis uses a set of unique components, such as synaptotagmin, complexin, Munc13, and RIM, to meet the special needs of fast Ca(2+)-triggered neurotransmitter release. This review summarizes present knowledge about the molecular mechanisms by which these components mediate and/or regulate synaptic vesicle exocytosis.