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CaMKIV Signaling in Long-Term Memory Consolidation

Explore the role of CaMKIV in memory, effects of dominant negative mutants, and implications for protein synthesis. Methods include hippocampal slice stimulation and behavioral tests like Morris water maze and fear conditioning.

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CaMKIV Signaling in Long-Term Memory Consolidation

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  1. An Important Role of Neural Activity-Dependent CaMKIV Signaling in the Consolidation of Long-Term Memory Hyejin Kang, Linus D. Sun, Coleen M. Atkins, Thomas R. Soderling, Matthew A. Wilson, and Susumu Tonegawa

  2. Pathway

  3. Background • Goal: Assess CaMKIV’s role in memory • Problems with CREB knockouts • compensatory increases in other CREB isoforms? • Problems with CaMKIV knockouts: • Some knockouts showed significant physiological impairment

  4. Definitions • Dominant Negative: mutant protein that blocks function of the WT protein by binding either to the WT protein or a protein upstream or downstream of the WT protein in a pathway. • C-Fos: An “immediate early gene” dependent on CREB phosphorylation

  5. *Abstract: Biochemical Aspect In mice with dnCaMKIV limited to the postnatal forebrain: • Basic synaptic function and early LTP (E-LTP) were unaffected. • CREB phosphorylation and C-Fos expression were reduced, while other protein levels were unaffected. • The dnCaMKIV resulted in a deficit in hippocampal late LTP (L-LTP), analogous to the effects of a protein synthesis inhibitor.

  6. Dominant Negative CaMKIV • CP: Regulatory domain of CamKII • FLAG: epitope tag • SV40: Viral DNA which can be differentiated from endogenous DNA • dnCaMKIV • HMDT308-311DEDD (autoinhibitory domain mutation) • L71A (ATP binding site) • T196A (phosphorylation site of CAMKK)

  7. Effects of dnCaMKIV • CaMKK enhances CaMKIV activity. • dnCaMKIV significantly reduces caCaMKIV activity • No statistically significant difference between the CREB mutation and dnCaMKIV…but a trend?

  8. Localization of dnCaMKIV to Forebrain • C34: name of animal line expressing dnCaMKIV • In situ hybridization to SV40 probe showed dnCAMKIV localized to the cerebral cortex, hippocampus, lower striatum, amygdala, and olfactory bulb

  9. dnCaMKIV: present and not affecting basal levels of synaptic proteins • Western Blot: • Flag: transgene expression in hippocampal extracts • dnCaMKIV does not affect CaMKII or Actin levels • Basal CREB, MAPK, and pMAPK levels are also unaffected by the transgene (data not shown).

  10. Method for Testing the Effects of Stimulation on Mutants • Hippocampal slices are perfused with saline containing either: • 90mM KCl (depolarizes membrane: VGCC allow Calcium influx into soma) • 100µM Glutamate • 30 minutes later: • Ser133-phosphorylated CREB (pCREB) and C-fos expression measured by immunoreactivity analysis

  11. After Stimulation, PCREB and C-Fos levels are reduced in C34 mutants • Basal levels of pCREB and C-Fos are unaffected by dnCaMKIV • After stimulation (Glu and KCl), C34 hippocampal slices show a deficit in pCREB and c-Fos expression compared to WT slices. • Fosk (which activates the PKA pathway) is not affected by dnCaMKIV: dnCaMKIV effects seem to be limited to the pathway of interest.

  12. Early LTP (E-LTP) is Normal in C34 Hippocampal Slices • Potentiation through theta-burst stimulation • Early synaptic changes (e.g. those mediated by CaMKII) do not seem to be affected.

  13. Long-term LTP (L-LTP) is reduced in dnCaMKIV mutants • Potentiation through 4 x tetanic stimulation • WT maintained LTP up to 200 minutes while potentiation in C34 continually decayed • Decay in C34 potentiation resembles decay in WT potentiation in the presence of anisomycin (a protein synthesis inhibitor)

  14. Points of Concern • CAMKIV is necessary to produce L(Late phase)-LTP in the hippocampus, but what about other brain regions? (Area of future research!) • The loss of L-LTP in C34 cannot be directly linked to the effect of dnCaMKIV on CREB phosphorylation (CaMKIV may regulate other transcription machinery).

  15. Conclusion • dnCaMKIV does not affect basic synaptic function or E-LTP. • CaMKIV plays a role in the protein synthesis-dependent component of L-LTP.

  16. Abstract: Behavioral • Morris water maze and Fear conditioning • Impairment in long-term memory • Specifically, the consolidation/retention was affected • Short-term memory was intact • The acquisition phase appeared unaffected • Short-term retention also unaffected

  17. Morris Water Maze: Hidden Platform • Transgenic mice shows longer escape latencies. • No latencies differences during the first two days of training, so it is not from motor impairment.

  18. Morris Water Maze: Fixed visible platform (in a new location) • Transgenic and wild-type shows similar latency curves • Therefore, latency from hidden platform is not due to swimming speed and fractional periphery occupancy. • Wild type swam to the previous location of the platform instead of the new location • Transgenic mice swam directly to the new platform

  19. Morris Water Maze: Fixed visible platform • Wild type mice employ spatial memory strategy even in visible platform of Morris water maze. • C34 swim directly to the visible platform • Rely on cue-platform association strategy instead of spatial memory strategy

  20. Contextual Fear Conditioning: Context • Since Morris water maze requires training over several days, it is likely that spatial memory can be consolidated. • Therefore, Contextual fear conditioning is used to test CaMKIV’s involvement in latter phases. • dnCaMKIV transgenic • Similar levels of freezing 24 hours after training • Reduction in context dependent freezing after 7 days

  21. Cued Fear Conditioning • dnCaMKIV transgenic • Similar levels of freezing 24 hours after training and 7 days after training • Suggest that transgenic mice have selective deficit in the consolidation/retention phase of context-dependent fear memory.

  22. CREB and Fear Conditioning

  23. Limitation of Experiment • dnCaMKIV was strongly expressed in the hippocampus as well as the cerebral cortex. The deficits in behavior may reflect decreased CaMKIV activity in either or both areas.

  24. Conclusion • The acquisition phase appeared unaffected • Short-term retention also unaffected • Specifically, the consolidation/retention was affected

  25. CREB levels (unphosphorylated) are the same in WT and C34

  26. Input-Output Curves: Synaptic Transmission is Normal in C34 • Fiber Volley Amplitude: Input (presynaptic) • Field EPSP Slope: Output (postsynaptic)

  27. *Open-field and Plus Maze Test • C34 transgenic and Wild-type mice are indistinguishable in … • Percent dwell time in open arm • Percent entries in open arm • Total number of entries in both open and closed arm • Suggest that C34 transgenic mice impairment is due to spatial learning and not general emotional defects (reduce or increase in anxiety).

  28. CaMKIV mutants BACK

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