class 11: synaptogenesis

what cellular events distinguish initial axon-target contact from a mature, functional synapse?

occurs when axon guidance and dendritic growth bring potential neuronal partners into close physical proximity, forming initial adhesive contacts

describe the molecular pathway mediating motor-neuron-initiated clustering of AChRs in the muscle membrane

• agrin secretion

• LRP4 receptor binding

• muSK activation

agrin secretion

motor axon terminal releases neural agrin into synaptic basal lamina

LRP4 receptor binding

neural agrin binds to low-density lipoprotein receptor related protein on postsynaptic muscle membrane

muSK activation

agrin-bound LRP4 associates with and activates muscle-specific receptor tyrosine kinase

what do Agrin application experiments show about sufficiency

local application of purified neural agrin protein onto cultured muscle fibers (in complete absence of motor neurons) is sufficient to induce local assembly of postsynaptic specializations and ectopic AChR clusters

what do agrin knockout experiments show about necessity

targeted genetic knockout of agrin results in complete failure to form normal nerve-induced AChr clusters and postsynaptic specialization.

• neural agrin is necessary for nerve-induced NMJ

scale and innervation pattern (NMJ vs CNS synapse)

mature muscle fiber is innervated by single motor axon at one large endplate, whereas CNS neuron receives thousands of small synapses distributed across soma, dendrites, and axons

primary organizer system (NMJ vs. CNS)

• NMJ relies heavily on neural agrin acting through LRP4 & muSK in specialized basal lamina.

• CNS do not require agrin, rely on multiple transsynaptic adhesions

physiological reliability & plasticity (NMJ vs. CNS)

• NMJ high reliability w/ large safety factor

• CNS have lower release probability

how did heterologous cell-neuron co-culture assays identify synaptogenic adhesion molecules?

they isolate candidate cell-surface proteins to test whether single molecule is sufficient to induce local pre or postsynaptic differentiation

expressing candidate postsynaptic proteins (neuroligin-1) in non-neuronal cells

triggers contacting axon terminals to accumulate presynaptic vesicle proteins to form active zones

expressing candidate presynaptic proteins (neurexins) in nonneural cells

triggers contacting neuronal dendrites to cluster postsynaptic scaffolding proteins and neurotransmitter receptors

reciprocal neurexin-neuroligin signaling

describes bidirectional transsynaptic communication across cleft that simultaneously coordinates presynaptic and postsynaptic assembly

describe what is meant by reciprocal neurexin-neuroligin signaling in the development of presynaptic compartments

retrograde

• postsynaptic neuroligins bind across cleft to presynaptic neurexins, engaging cytoplasmic domains linked to presynaptic scaffolds to recruit synaptic vesicles

describe what is meant by reciprocal neurexin-neuroligin signaling in the development of post synaptic compartments

anterograde

• presynaptic neuexins bind to postsynaptic neuroligins, whose intracellular tails recruit postsynaptic scaffolds to cluster matching neurotransmitter receptors across cleft

glutamatergic (excitatory) synapses

• postsynaptic: neuroligin-1 (NLGN1)

• presynaptic: neurexin splice isoforms containing +S4

GABAergic (inhibitory) synapses

• postsynaptic: neuroligin-2 (NLGN2)

• presynaptic: neurexin splice isoforms lacking -S4

cellular specificity of synapses

dictates which cell types connect, selecting acceptable target neuronal types while rejecting inappropriate partners

synapse-type specificity of synapses

dictates physiological and molecular identify of connection (transmitter identity)

sub cellular specificity of synapses

dictates where on target neuron the synapse forms, restricting inputs to distinct spatial domains

predict the differential effects of deleting Lphn3 on CA1 hyppocampal neuron synapses

postsynaptic Lphn3 is enriched in proximal and basal dendritic domains.

• selectively reduces excitatory CA3 Schaffer collateral synapses w/o impacting distal cortex inputs

predict the differential effects of deleting Lphn2 on CA1 hippocampal neuron synapses

postsynaptic Lphn2 is enriched in distal dendritic domain

• selectively reduces excitatory EC synapses w/o impacting proximal CA3 Schaffer collateral inputs

describe the evidence for an essential role of astrocytes in CNS synapse development

purified CNS neurons cultured in isolation w/o glia form very few and weak synapses.

• co-culturating neurons w/ astrocytes increases synapse numbers, maturation, and synaptic event frequencies.

thrombospodins (TSP)

bind to neuronal surface receptor & promote structural excitatory synapse assembly.

• postsynaptically silent

hevin

functions as physical molecular bridge between presynaptic neurexin-1a and postsynaptic neuroligin-1B, driving assembly of specific glutamatergic connections

TGF-beta

signals through neuronal TGF-beta receptors to link soluble glial cytokine signal to activity-depednent synaptogenesis and maturation