Ben Sutcliffe

Drosophila Neurotrophins Reveal a Common Mechanism for Nervous System Formation

Neurotrophic interactions occur in Drosophila, but to date, no neurotrophic factor had been found. Neurotrophins are the main vertebrate secreted signalling molecules that link nervous system structure and function: they regulate neuronal survival, targeting, synaptic plasticity, memory and cognition. We have identified a neurotrophic factor in flies, Drosophila Neurotrophin (DNT1), structurally related to all known neurotrophins and highly conserved in insects. By investigating with genetics the consequences of removing DNT1 or adding it in excess, we show that DNT1 maintains neuronal survival, as more neurons die in DNT1 mutants and expression of DNT1 rescues naturally occurring cell death, and it enables targeting by motor neurons. We show that Spätzle and a further fly neurotrophin superfamily member, DNT2, also have neurotrophic functions in flies. Our findings imply that most likely a neurotrophin was present in the common ancestor of all bilateral organisms, giving rise to invertebrate and vertebrate neurotrophins through gene or whole-genome duplications. This work provides a missing link between aspects of neuronal function in flies and vertebrates, and it opens the opportunity to use Drosophila to investigate further aspects of neurotrophin function and to model related diseases.

Loss and gain of Drosophila TDP-43 impair synaptic efficacy and motor control leading to age-related neurodegeneration by loss-of-function phenotypes.

Cytoplasmic accumulation and nuclear clearance of TDP-43 characterize familial and sporadic forms of amyotrophic lateral sclerosis and frontotemporal lobar degeneration, suggesting that either loss or gain of TDP-43 function, or both, cause disease formation. Here we have systematically compared loss- and gain-of-function of Drosophila TDP-43, TAR DNA Binding Protein Homolog (TBPH), in synaptic function and morphology, motor control, and age-related neuronal survival. Both loss and gain of TBPH severely affect development and result in premature lethality. TBPH dysfunction caused impaired synaptic transmission at the larval neuromuscular junction (NMJ) and in the adult. Tissue-specific knockdown together with electrophysiological recordings at the larval NMJ also revealed that alterations of TBPH function predominantly affect pre-synaptic efficacy, suggesting that impaired pre-synaptic transmission is one of the earliest events in TDP-43-related pathogenesis. Prolonged loss and gain of TBPH in adults resulted in synaptic defects and age-related, progressive degeneration of neurons involved in motor control. Toxic gain of TBPH did not downregulate or mislocalize its own expression, indicating that a dominant-negative effect leads to progressive neurodegeneration also seen with mutational inactivation of TBPH. Together these data suggest that dysfunction of Drosophila TDP-43 triggers a cascade of events leading to loss-of-function phenotypes whereby impaired synaptic transmission results in defective motor behavior and progressive deconstruction of neuronal connections, ultimately causing age-related neurodegeneration.

Neuron-Type Specific Functions of DNT1, DNT2 and Spz at the Drosophila Neuromuscular Junction

Retrograde growth factors regulating synaptic plasticity at the neuromuscular junction (NMJ) in Drosophila have long been predicted but their discovery has been scarce. In vertebrates, such retrograde factors produced by the muscle include GDNF and the neurotrophins (NT: NGF, BDNF, NT3 and NT4). NT superfamily members have been identified throughout the invertebrates, but so far no functional in vivo analysis has been carried out at the NMJ in invertebrates. The NT family of proteins in Drosophila is formed of DNT1, DNT2 and Spätzle (Spz), with sequence, structural and functional conservation relative to mammalian NTs. Here, we investigate the functions of Drosophila NTs (DNTs) at the larval NMJ. All three DNTs are expressed in larval body wall muscles, targets for motor-neurons. Over-expression of DNTs in neurons, or the activated form of the Spz receptor, Toll 10b, in neurons only, rescued the semi-lethality of spz 2 and DNT1 41 , DNT2 e03444 double mutants, indicating retrograde functions in neurons. In spz 2 mutants, DNT1 41 , DNT2 e03444 double mutants, and upon over-expression of the DNTs, NMJ size and bouton number increased. Boutons were morphologically abnormal. Mutations in spz and DNT1,DNT2 resulted in decreased number of active zones per bouton and decreased active zone density per terminal. Alterations in DNT function induced ghost boutons and synaptic debris. Evoked junction potentials were normal in spz 2 mutants and DNT1 41 , DNT2 e03444 double mutants, but frequency and amplitude of spontaneous events were reduced in spz 2 mutants suggesting defective neurotransmission. Our data indicate that DNTs are produced in muscle and are required in neurons for synaptogenesis. Most likely alterations in DNT function and synapse formation induce NMJ plasticity leading to homeostatic adjustments that increase terminal size restoring overall synaptic transmission. Data suggest that Spz functions with neuron-type specificity at the muscle 4 NMJ, and DNT1 and DNT2 function together at the muscles 6,7 NMJ.

Trophic neuron-glia interactions and cell number adjustments in the fruit fly.

Trophic interactions between neurons and enwrapping glia, and between neurons and target cells, provide plasticity to the mammalian nervous system. Here, we review evidence that analogous cell interactions operate in the development of the nervous system of the fruit‐fly Drosophila. Homologues of the canonical mammalian trophic factors also maintain neuronal and glial survival in Drosophila, adjusting cell populations to enable appropriate function, and revealing commonalities in nervous system development across the animals. There are also differences between neuron‐glia interactions in flies and humans, not surprisingly, because we are only related to flies through a remote common ancestor. Nevertheless, the shared cellular and molecular mechanisms underlying developmental plasticity and enwrapping glial functions, strengthen the opportunity to use Drosophila to understand the brain, to model brain diseases and to understand the involvement of glial cells in nervous system regeneration. ©2010 Wiley‐Liss, Inc.

Optimization of fluorophores for chemical tagging and immunohistochemistry of Drosophila neurons

The use of genetically encoded ‘self-labeling tags’ with chemical fluorophore ligands enables rapid labeling of specific cells in neural tissue. To improve the chemical tagging of neurons, we synthesized and evaluated new fluorophore ligands based on Cy, Janelia Fluor, Alexa Fluor, and ATTO dyes and tested these with recently improved Drosophila melanogaster transgenes. We found that tissue clearing and mounting in DPX substantially improves signal quality when combined with specific non-cyanine fluorophores. We compared and combined this labeling technique with standard immunohistochemistry in the Drosophila brain.

Both loss and gain of TDP-43 impair synaptic efficacy and motor control leading to age-related neurodegeneration in Drosophilia

Frontotemporal dementia (FTD) is a clinical syndrome with a heterogeneous molecular basis. The genetics of FTD has been one of the success stories in genetics over the past 15 years. Classic family based linkage studies have identified genes that explain a large part of the families with a Mendelian inheritance of the disease. This group of familial FTD patients has now been linked to mutations in several genes, including the microtubule-associated protein tau (MAPT), progranulin (GRN), valosin-containing protein (VCP), charged multivescicular body protein 2B (CHMP2B), TAR DNA-binding protein 43 (TDP43) and Fused in Sarcoma (FUS) and most recently C9Orf72. Over the years the identified genes have triggered many studies that increased our understanding of the disease process. Neuropathologically the disease can be divided in two major groups that have a clear correlation with their genetic background; hose with tau-positive inclusions and those with ubiquitin-positive and TDP43 positive inclusions. The field of genetics keeps changing rapidly thanks to technological developments, first with the development of Genome Wide Association Studies (GWAS) studies but now also with the use of next generation sequencing, as was already demonstrated with the identification of the expanded repeat in C9Orf72, and we can also expect many whole exome or whole genome sequencing studies. This review provides an overview of the genetics of FTD, with an update of recent discoveries.

12-P001 Synaptic and locomotion functions of the Drosophila neurotrophins

Vertebrate neurotrophins (NTs: NGF, BDNF, NT3, NT4) form a family of signalling molecules with key functions in nervous system development and function. They regulate cell survival, axon guidance and targeting, synaptic formation and function, learning and memory. Deficient NGF function occurs in Alzheimer’s disease and alterations in BDNF function underlie psychiatric and cognitive disorders such as epilepsy, autism, anxiety and depression. NTs are therapeutic targets in pain control (e.g. chronic pain in cancer). We recently reported that a family of Drosophila neurotrophins (DNTs) formed by DNT1, DNT2 and Spz regulate neuronal survival and targeting in fruit-flies (Zhu et al., 2008 PLoS Biology 6, e284). We are now investigating the roles that DNTs play in neuronal function, using both loss and gain of function conditions for each of the DNTs. Here, we report the involvement of DNTs in synapse formation and in adult locomotion. With the help of mathematical algorithms that we have developed, we quantify automatically and objectively phenotypic changes in: (i) the number of boutons and active zones at the larval neuromuscular junction (NMJ) synapse; (ii) axon terminal length and branch number at the NMJ; (iii) and speed and trajectory of adult flies in a locomotion paradigm. Using genetic rescue experiments, we investigate functional redundancy and whether the DNTs are required in muscle or neurons. Our findings suggest that neuronal functions of NTs are evolutionarily conserved, supporting the notion that NTs underlie multiple aspects of nervous system development and function in all animals with a CNS and brain.