5th September 2013Bio Neurobiology I - Synapse and map formation1 Synapse formation Raghav Rajan Bio 334 – Neurobiology I September 5th 2013

5th September 2013Bio Neurobiology I - Synapse and map formation2 Synapses are the connections between two neurons – can be electrical or chemical ● Typical chemical synapse ● Presynaptic axon ● Postsynaptic dendrite ● Synaptic cleft ● Since they are small – difficult to visualize ● Considerable debate about their presence ● Synapse – term coined by Charles Sherrington

5th September 2013Bio Neurobiology I - Synapse and map formation3 Synapses evolved about 1.1 million years ago, but some components were present even earlier

5th September 2013Bio Neurobiology I - Synapse and map formation4 Complexity of the signalling process in the postsynaptic density has increased greatly

5th September 2013Bio Neurobiology I - Synapse and map formation5 Synapse location and number are not random – instead they are regulated ● Excitatory synapses are typically on spine heads ● Inhibitory synapses are typically on cell bodies, proximal dendrites or spike necks ● Synapse number can vary depending on target neurons

5th September 2013Bio Neurobiology I - Synapse and map formation6 Three general observations related to synapse formation ● Synaptic building blocks are manufactured by neurons even before they make contacts with each other ● Intercellular signaling, signals from glia, extracellular matrix, neighbouring neurons – all participate in synaptogenesis ● Synapses mature over the course of development – experience dependent plasticity, critical periods, etc....

5th September 2013Bio Neurobiology I - Synapse and map formation7 Study of synapses gained speed in 1950s with the advent of two new techniques ● Electron microscopy ● Intracellular recordings

5th September 2013Bio Neurobiology I - Synapse and map formation8 Pre and post-synaptic membranes come close to each other in a newly formed synapse ● But, not much can be seen in terms of presynaptic or postsynaptic specializations ● Difficult to see newly forming synapses – since there is nothing much to see at this stage Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation9 A lot of information about synapse formation comes from watching synapse formation in culture ● After contact, filopodia retract ● Slowly pre and post-synaptic parts mature ● Extracellular matrix also matures Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation10 Important features of synapses - location, location, location – but how is this determined ● Inputs far away on the dendritic tree have less impact at the cell body ● Recent studies show that this is not entirely true – may depend on the properties of dendrites in different neurons

5th September 2013Bio Neurobiology I - Synapse and map formation11 First synapses form on growth cones or extremities – later on cell bodies ● Axo-dendritic synapses onto dendritic growth cones ● Axo-muscle synapses onto muscle myopodia ● May even be regulated by glia controlling accessibility to various parts of the post-synaptic cell

5th September 2013Bio Neurobiology I - Synapse and map formation12 Pre and post-synaptic structures can form independent of partners ● Clustering of post-synaptic alpha-2 adrenergic receptors without any presynaptic membrane in rat visual cortex (p4) ● Presynaptic terminal with vesicles in a Drosophila mutant that does not make muscle Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation13 Synapse number increases after birth ● Cat visual cortex ● Neuron density decreases with increased gliogenesis ● But neuronal processes grow and start making synapses Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation14 Growth cones of axons can release neurotransmitters spontaneously before formation of contacts Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation15 Functional synapses can form very quickly in culture soon after contact ● Muscle cell brought into contact with neurite ● Spontaneous currents and evoked currents change rapidly ● Working synapse is produced quickly ● But, functional maturation can take days to weeks Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation16 Stages in synapse formation – 1 – contact formation – Ca2+, PKC, cAMP all play a role ● Contact with the correct postsynaptic target can induce a Ca2+ increase in the growth cone triggering cytoskeletal changes ● Astrocytes also play a role in this process Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation17 Stages in synapse formation – 2 – Increase in adhesion between growth cone and target cell ● Increase in adhesion between growth cone and target cell within 15 minutes of contact ● Nectins, cadherins, etc.... Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation18 Stages in synapse formation – 3 – converting sticky growth cone to a presynaptic terminal ● Presynaptic terminals can mature quickly without concomitant maturation of postsynaptic partners ● Mature forms of either partner can stimulate maturation of the other Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation19 Signaling pathways again..... they can change growth cones into presynaptic terminals ● Different signaling pathways activated by contact stimulate change of growth cone into presynaptic terminal Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation20 Postsynaptic clustering of receptors can be autonomous ● ACh receptors stained with alpha-bungarotoxin ● Mouse diaphragm muscle ● Localization in the centre even in mutants without axon ingrowth ● Stabilization by presynaptic contact Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation21 Postsynaptic clustering of receptors can also be induced by contact with the right neurons ● Clustering can be induced by contact Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation22 Agrin, a proteoglycan, is another cluster-inducing molecule ● In this case, basal lamina also produce agrin and can induce clustering of post-synaptic Ach receptors (frog NMJ) ● Agrin also produced by motor neurons Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 8

5th September 2013Bio Neurobiology I - Synapse and map formation23 Overall take home of synapse formation ● Highly specific in terms of location and connections ● Both sides play a role ● And there may be other players – glia, extracellular matrix ● The order of events is not completely understood – may be different for different synapses ● NOT FIXED – STILL ROOM FOR PLASTICITY