Synapse Elimination and Remodeling

1. Synapse Elimination and Remodeling

2. An extremely protracted developmental process – starts after birth and continues throughout life. • Synapse elimination is an important component of development, as many more synapses are formed than would be present in the adult animal. • Most often, these changes are related to adjustments in the number and strength of synaptic connections (“fine-tuning”), as opposed to simply pruning. • E.g., in vertebrates, this process occurs in situations where there is a need to partition info among homogeneous populations of target cells. • I.e., Thousands of identical muscle fibers, each innervated by 1 NMJ in adult (initially contacted by several) decr degree of convergence. • The functional rationale for this fine tuning: muscle contraction (‘twitch’) is ‘all-or-none’ – multiple inputs in various parts of the cell would not be beneficial.

3. Synapse Elimination The total number of synapses on ganglion cells in the rat submandibular ganglion increases during early postnatal life.

4. Innervation of individual neuromuscular junctions (ovals on muscle fibers) by axonal branch trimming. Over the first several weeks of postnatal life, rodent motor axons remove branches each neuromuscular junction undergoes a transition from innervation by multiple converging axons to innervation by only one axon. The number of muscle fibers innervated by an axon decreases substantially 9axonal convergence decreases) and all but one input is eliminated from each fiber. This occurs by branch removal rather than motor neuron death.

5. Neuromuscular Synapses are Eliminated During Postnatal Development At birth, nearly all muscle fibers are polyneuronally innervated. During the next two weeks, all but one of these synaptic inputs is lost.

6. Neuromuscular Synapses Compete for Survival Synchronous electrical activity accelerates elimination, synchronous paralysis delays it. • Application of excesses of different neurotrophic molecules prolongs polyneuronal innervation • Several pathways may exist that help mediate competition • Nature of competition may be more like a judged contest than like a race.

7. Synapse Elimination in both Development and Regeneration takes much time

8. Terminal Segregation Multiple Innervation Branch Withdrawal Axon Retraction Neuromuscular Synapse Elimination is Gradual and Asynchronous Keller-Peck et al, Neuron 31: 381-394, 2001 • Any competition between inputs is resolved at the level of individual synapses

9. Time-lapse imaging of synapse elimination. Two neuromuscular junctions (NMJ1 and NMJ2) in vivo on postnatal days 7, 8, and 9 in a transgenic mouse that expresses YFP in its motor axons. The acetylcholine receptors at the muscle fiber membrane are labeled red with rhodamine tagged α–bungarotoxin in each muscle fiber. The transition from multiple to single innervation of NMJ1 as one axon, a sibling branch of the axon that innervates NMJ2, undergoes atrophy and appears to retract. The eliminated branch terminates in a "retraction bulb." Modified from Keller-Peck et al. (2001).

10. Devel. Decreased Convergence and Divergence During Development and Regeneration: Regen.

11. Convergence and Divergence • The rationale for starting with many is not as clear (much energy goes into making as many only to have them torn down). • What idea might you have? Propose a rationale. • Divergence also exists early in development; 1 neuron  many different targets, which are decreased by synaptic elimination processes. • In muscle, this scenario of decr convergence and divergence is re-captured when re-innervation occurs after injury.

12. Synapse Elimination Involves Synaptic Takeover • The surviving input takes over the synaptic space formerly occupied by the losing input(s). Walsh & Lichtman, Neuron 37: 67-73, 2003

13. Synapse Elimination is a Balance of Anterograde and Retrograde Signals

14. Gatesy & English, Dev. Dyn. 196: 174-182, 1993 Proposed Functions of Synapse Elimination • Provides a way to ensure that each muscle fiber is innervated. • Allows axons of different motoneurons to capture a number of cells appropriate to its size. • Provides a means by which normal function can change the strength of synaptic connections. • Adult muscles are partitioned into neuromuscular compartments – exclusive innervation territories. • In the absence of competition, cross compartmental inputs will form in neonates but not adults • Synapse elimination completes the specific innervation of neuromuscular compartments

15. Synapse elimination may not be leading to decreased numbers for a particular site. • E.g., redistribution of synapses to increase strength at same sites (more release sites). • Remember from earlier courses what is synaptic strength? weaker (diffuse)  stronger (more focused on individual cells. • Some degree of synapse elim comes from cell death (discussed in the PCD slides). • Synapse elim involves the physical withdrawal of the nerve ending. It gradually takes on a ‘bulb’ morphology – the opposite of a growth cone?

16. Retraction bulb

17. Anatomy of the Visual System Both eyes project to each visual cortex, but at the primary visual area (17), they remain largely segregated into ocular dominance columns.

18. Synapse Elim/Refinement in the Developing Visual System A highly examined system to study synapse elim and refinement. Layer IV of the visual cortex (VC) segregation of neuronal inputs. In young animals, neurons within this layer (IV) can be activated by inputs driven by both the L and R eye  some inputs withdrawn so that some are strongly dominated by R eyes and others by L eyes. Inputs form a striking pattern of alternating stripes, known as ocular dominance columns.

19. Ocular dominance columns

20. Ocular Dominance Columns are Organized After Birth This synaptic rearrangement is brought about by axon retraction and local outgrowth of geniculate neurons.

21. The development of ocular dominance columns LeVay S, Stryker MP, Shatz CJ, 1978; At 2 weeks, there is a continuous band of synaptic connections in layer IV that represent the input to the visual cortex from geniculocortical afferents. At 6 weeks, fluctuations in the intensity are already apparent. By 13 weeks, the pattern of cortical striping is similar to that seen in the adult. The segregation of synaptic inputs within the cortical layer depends on synaptic activity from postnatal visual experiences.

24. Normal binocular input

25. Synapse Elim/Refinement in the Developing Visual System Elim restructures the neuronal population that serves each eye; Addition strengthens the connections that remain.

26. Spatial patterning of connectivity by synapse elimination in the visual cortex Axonal projections from each eye are organized into separate eye-specific layers in the dorsal lateral geniculate nucleus (dLGN). The axonal terminals of dLGN neurons in each eye-specific layer terminate and occupy adjacent territories in layer IV, forming ocular dominance columns in the primary visual cortex. Organized pathways representing the left and right eyes are separated spatially and functionally from the retina to layer IV emerge during development from less precise patterns of connectivity. Both dLGN neurons and layer IV cells initially receive converging eye input. Inputs segregate first in the dLGN and then in the cortex.

27. One purpose of synapse elim in vertebrates is to refine retinotectal maps, which form in the visual cortex, LGN, and superior colliculus. • As noted before, this refinement can occur through cell death or axonal arbor withdrawal.

28. Effect of chronic closure of one eye on the responsiveness of visual cortical neurons to input from each eye. A) Ocular dominance distribution in V1 of 3 to 4 week old kittens. Cells in group 1 are driven only by the contralateral eye; group 2, contralateral eye is markedly dominant; group 3, contralateral eye is slightly dominant; group 4, no apparent difference in the drive from two eyes; group 5, ipsilateral eye dominates slightly; group 6, dominated markedly; group 7, cells are driven only by ipsilateral eye. B) Ocular dominance distribution was altered dramatically in a kitten exposed to contralateral eye closure for 1 week from 23 to 29 days of age. C) Ocular dominance distribution was essentially normal in an adult cat exposed to contralateral eye closure for 26 months. From Hubel and Wiesel (1970).

29. Some Observations in the Visual System • 1 eye closed  columns “dominated” by input from the other eye. • Evidence that this was based on neuronal activity level and not just visual input per se: Spontaneous activity is silenced by tetrodotoxin competition actually shifts toward the sutured eye {note: some column formation occurs before birth}. [also shown when postsynaptic activities are inhibited by GABA]. Therefore, active input and postsynaptic responses are required.

30. After closure of one eye

31. Normally, most cells receive visual inputs from both eyes, but not equally Effects of Neonatal Monocular Deprivation After MD as neonates, very few cells receive inputs from the closed eye • Many cells in visual cortex remain responsive to inputs from both eyes after binocular deprivation. • Development of proper circuitry depends on proper balance of inputs from two eyes • Monocular deprivation in adult has no effect

32. Effects of Monocular Deprivation Are Noted Anatomically • Sensory deprivation early in life can alter the structure of the cerebral cortex.

33. Columns in layer 4a of primary visual cortex with appropriate eye-specific inputs are present before the critical period for ocular dominance column plasticitiy. • Columns develop in the absence of visual system input and before the development of retinal photoreceptors. • Columns are not altered by visual deprivation. • Full development of columns occurs later, and can be seen as a refinement of these primitive columns. Cowley & Katz, Curr. Opin. Neurobiol. 12: 104-109, 2002 Rudimentary Ocular Dominance Columns Develop in the Absence of Visual Inputs Differences from earlier studies is thought to be a matter of technique.

34. Devel. Decreased Convergence and Divergence During Development and Regeneration: Regen.

35. Convergence and Divergence • The rationale for starting with many is not as clear (much energy goes into making as many only to have them torn down). • What idea might you have? Propose a rationale. • Divergence also exists early in development; 1 neuron  many different targets, which are decreased by synaptic elimination processes. • In muscle, this scenario of decr convergence and divergence is re-captured when re-innervation occurs after injury.

36. What Leads to the Segregation of These Competing Inputs? • The physiological wave pattern of activity from each eye would be synchronous  Inputs would tend to segregate into synchronous regions (strength would determine final density of each region). The presence or absence of trophic factors also appears to influence the state of ocular dominance columns. This plasticity (ability for synapses to re-arrange) is only present during a critical period of development because the segregation of synchronous connections and competition can no longer occur (after this critical period).

37. Segregation of axonal input

38. Synchronous Activity From Each Eye Organizes the Ocular Dominance Columns • Blocking activity in both eyes with TTx or synchronous stimulation of both optic nerves block formation of ocular dominance columns. • Asynchronous activity leads to formation of ocular dominance columns. • Normal development depends on competition for acquisition of synaptic partners This competition is emphasized in three-eyed frogs.

39. Competition determines which Axon will be Retained • In studies, synapses have been observed over time post-denervation (observe the re-innervation process). • Changes often occurred post-synaptically (i.e. loss or re-arrangement of Ach receptors) before terminal withdrawal. • This occurs in development as well: 1st sign of synapse loss at a site is decrAchR density. • This may explain earlier synaptic weaknesses before process is complete.

40. NMDA Receptors Mediate Competition Cooperative activity of afferent neurons from one eye are thought to depolarize target neurons sufficiently to release Mg block of NMDA receptor, allowing Ca influx. NMDA receptor acts as a coincidence detector of the activity of neurons from a single source. They support one aspect of the theory of learning proposed by Hebb. An additional retrograde signal to the presynaptic neuron is required.

41. Competition determines which Axon will be Retained • What is the nature of the competition? I.e., what are the axons competing for? • Trophic factor from targets (for maintenance)? • Competition is indirect and postsynaptic membrane (e.g., muscle) does the “choosing” and gives a retrograde “message”. Evidence indicates that this message comes from (or results from) the prior loss of receptors or other post-synaptic proteins (e.g., rapsyn).

42. What Determines which Receptor Groups will Diminish? • One possibility is the type and degree of activity. • All these neuronal endings interact (retained and lost) – this could be 1 way to distinguish. • Synchronous activity could be somehow protective for the receptors, whereas strong, synchronous activity could be destabilizing.

43. What Drives the Competition between Axonal Endings for Target Space? • Neuronal activity level. • Activity-driven neuronal connections is applicable to several situations during neuronal development and throughout life.

44. Activity-mediated signals

45. NGF BDNF NT-4 NT-3 trkA trkB trkC p75NTR Presynaptic Activity May Enhance Release of Neurotrophins From Target Neurons Neurotrophins could form such a retrograde signal. NO, another retrograde signaling molecule is not required for formation of ocular dominance columns. Excess of trk B ligands removes the ability of NMDA receptors to mediate competition. Finney & Shatz, J. Neurosci.18: 8826-8838, 1998

47. Model of Synaptic Rearrangements