Future prospects

We reported on the generation of three novel transgenic mouse lines: two with different, yet overlapping expression patterns of CARP and one with over- expression of δC-DCLK-short, a constitutively active form of DCLK-short in the brain. The experiments performed in an attempt to characterize functions of CARP and DCLK-short span different relevant functional levels, including genetics, protein interactions, neuronal morphology, electrophysiology and behaviour. Several relevant findings were described, some of which raised more questions about putative DCLK gene functions. The newly generated transgenic lines provide an excellent opportunity to explore previously uncharted territory in investigating the DCLK gene. Based on the work performed in this thesis several potentially fruitful lines of future research come to mind.

Firstly, since CARP expression is associated with apoptotic cells in the DG of ADX rats (Chapter 2), examination of the prevalence of apoptotic cells in the hippocampus of high-CARP and low-CARP mice in response to an apoptotic stimulus, such as corticosteroid depletion (Sloviter et al., 1993; Karssen et al., 2005) or middle cerebral artery occlusion (Belayev et al., 1999), may help to reveal whether CARP has pro-apoptotic properties in an in vivo setting. Investigating the number of apoptotic cells in CARP mice and controls after such a challenge will unveil if this is the case. By subjecting δC-DCLK-short mice to the same paradigm, the nature of DCLK-short as a proposed survival factor may become clearer.

Secondly, We demonstrated highly increased evoked fEPSPs in the CA3/CA1 network of high-CARP mice, but without elevation of subsequent PSAs (Chapter 3). How this characteristic electrophysiological phenotype is realized within the hippocampal formation remains elusive. Measuring mEPSPs in hippocampal slices of high-CARP mice will shed more light on this matter. Given the strong up- regulation of Calretinin (CR) in the DG specifically, it may also be of relevance to study DG/CA3 network functioning in these mice. In addition, whether the increased fEPSPs has consequences for LTP in transgenic high-CARP (and low- CARP) mice remains to be clarified, although the previously described up- regulation of CARP following BDNF-LTP is encouraging in pursuing this train of thought.

Thirdly, we investigated fear conditioning i.e. memories of an aversive event and the association of two stimuli in a specific context (Chapter 4). We showed intensified consolidation in high- and low-CARP mice. It is of interest to also focus on other behavioural tests that are more dependent on spatial navigation, such as the Morris water maze or the circular hole board. The hippocampus is crucially involved in these types of behavioural paradigms and given the potential effects of CARP over-expression on synaptic plasticity this approach may provide additional clues of CARP function within the hippocampal network.

Finally, establishing the roles of CARP and DCLK-short in the epileptic hippocampus is of interest and may reveal new points of application for therapeutic intervention. This was already discussed more thoroughly in section 4.1.1. On a more general note, the transcriptional regulation of the DCLK gene is a relatively underexposed subject. A better understanding of signal transduction pathways and specific transcription factors that contribute to the regulation of DCLK gene expression is of utmost importance. In addition, identification of endogenous substrates of DCLK-short and interaction partners of CARP is crucial in assigning potential functions to these DCLK gene splice products.

Together the observations described in this thesis show that despite the previously established association of DCX domain-containing DCLK gene family members and functions during cortical and hippocampal development the DCLK gene also has functions beyond development. Moreover, although the presence of at least

GABAergic control and reduced GABAA receptor function in immature neurons of

the adult brain may serve as a common molecular substrate for deficits in adult neurogenesis and behaviors related to anxiety (Earnheart et al., 2007). Given these observations and the finding that anxiety-related behaviours are more pronounced in δC-DCLK-short mice, examination of potential alleviation of the more anxious behavioural phenotype of these mice by treatment with anxiolytic drugs may form an interesting approach in determining the contribution of DCLK- short to processes underlying anxiety, such as inhibitory GABAergic transmission and synaptic plasticity.

5. Future prospects

We reported on the generation of three novel transgenic mouse lines: two with different, yet overlapping expression patterns of CARP and one with over- expression of δC-DCLK-short, a constitutively active form of DCLK-short in the brain. The experiments performed in an attempt to characterize functions of CARP and DCLK-short span different relevant functional levels, including genetics, protein interactions, neuronal morphology, electrophysiology and behaviour. Several relevant findings were described, some of which raised more questions about putative DCLK gene functions. The newly generated transgenic lines provide an excellent opportunity to explore previously uncharted territory in investigating the DCLK gene. Based on the work performed in this thesis several potentially fruitful lines of future research come to mind.

Firstly, since CARP expression is associated with apoptotic cells in the DG of ADX rats (Chapter 2), examination of the prevalence of apoptotic cells in the hippocampus of high-CARP and low-CARP mice in response to an apoptotic stimulus, such as corticosteroid depletion (Sloviter et al., 1993; Karssen et al., 2005) or middle cerebral artery occlusion (Belayev et al., 1999), may help to reveal whether CARP has pro-apoptotic properties in an in vivo setting. Investigating the number of apoptotic cells in CARP mice and controls after such a challenge will unveil if this is the case. By subjecting δC-DCLK-short mice to the same paradigm, the nature of DCLK-short as a proposed survival factor may become clearer.

Secondly, We demonstrated highly increased evoked fEPSPs in the CA3/CA1 network of high-CARP mice, but without elevation of subsequent PSAs (Chapter 3). How this characteristic electrophysiological phenotype is realized within the hippocampal formation remains elusive. Measuring mEPSPs in hippocampal slices of high-CARP mice will shed more light on this matter. Given the strong up- regulation of Calretinin (CR) in the DG specifically, it may also be of relevance to study DG/CA3 network functioning in these mice. In addition, whether the increased fEPSPs has consequences for LTP in transgenic high-CARP (and low- CARP) mice remains to be clarified, although the previously described up- regulation of CARP following BDNF-LTP is encouraging in pursuing this train of thought.

Thirdly, we investigated fear conditioning i.e. memories of an aversive event and the association of two stimuli in a specific context (Chapter 4). We showed intensified consolidation in high- and low-CARP mice. It is of interest to also focus on other behavioural tests that are more dependent on spatial navigation, such as the Morris water maze or the circular hole board. The hippocampus is crucially involved in these types of behavioural paradigms and given the potential effects of CARP over-expression on synaptic plasticity this approach may provide additional clues of CARP function within the hippocampal network.

Finally, establishing the roles of CARP and DCLK-short in the epileptic hippocampus is of interest and may reveal new points of application for therapeutic intervention. This was already discussed more thoroughly in section 4.1.1. On a more general note, the transcriptional regulation of the DCLK gene is a relatively underexposed subject. A better understanding of signal transduction pathways and specific transcription factors that contribute to the regulation of DCLK gene expression is of utmost importance. In addition, identification of endogenous substrates of DCLK-short and interaction partners of CARP is crucial in assigning potential functions to these DCLK gene splice products.

Together the observations described in this thesis show that despite the previously established association of DCX domain-containing DCLK gene family members and functions during cortical and hippocampal development the DCLK gene also has functions beyond development. Moreover, although the presence of at least

one DCX domain has long been deemed necessary for attributing any functionality to DCLK gene family members, we here demonstrate that splice variants lacking this domain, i.e. CARP and DCLK-short, also play important roles in cognitive and cellular processes, including neuronal apoptosis and neuronal transmission.

References

Ahi J, Radulovic J, Spiess J. 2004. The role of hippocampal signaling cascades in consolidation of fear memory. Behav Brain Res. 149(1):17-31.

Alfonso J, Agüero F, Sanchez DO, Flugge G, Fuchs E, Frasch AC, Pollevick GD. 2004. Gene

expression analysis in the hippocampal formation of tree shrews chronically treated with cortisol. J Neurosci Res. 78(5):702-10.

Ashwood TJ, Wheal HV. 1986. Extracellular studies on the role of N-methyl-D-aspartate receptors in epileptiform activity recorded from the kainic acid-lesioned hippocampus. Neurosci Lett.

18;67(2):147-52.

Atack JR. 2005. The benzodiazepine binding site of GABA(A) receptors as a target for the development of novel anxiolytics. Expert Opin Investig Drugs. 14(5):601-18.

Balkowiec A, Katz DM. 2002. Cellular mechanisms regulating activity-dependent release of native brain- derived neurotrophic factor from hippocampal neurons. J Neurosci. 22(23):10399-407. Barnabé-Heider F, Miller FD. 2003. Endogenously produced neurotrophins regulate survival and

differentiation of cortical progenitors via distinct signaling pathways. J Neurosci. 23(12):5149- 60.

Belayev L, Busto R, Zhao W, Fernandez G, Ginsberg MD. 1999. Middle cerebral artery occlusion in the mouse by intraluminal suture coated with poly-L-lysine: neurological and histological validation. Brain Res. 833(2):181-90.

Berke, J.D., R.F. Paletzki, G.J. Aronson, S.E. Hyman, and C.R. Gerfen. 1998. A complex program of striatal gene expression induced by dopaminergic stimulation. J Neurosci. 18:5301-10. Best N, Mitchell J, Baimbridge KG, Wheal HV. 1993. Changes in parvalbumin-immunoreactive neurons

in the rat hippocampus following a kainic acid lesion. Neurosci Lett 155:1–6.

Best N, Mitchell J, Wheal HV. 1994. Ultrastructure of parvalbumin immunoreactive neurons in the CA1 area of the rat hippocampus following a kainic acid injection. Acta Neuropathol (Berl) 87:187– 195.

Blaeser F, Sanders MJ, Truong N, Ko S, Wu LJ, Wozniak DF, Fanselow MS, Zhuo M, Chatila TA. 2006. Long-term memory deficits in Pavlovian fear conditioning in Ca2+/calmodulin kinase kinase alpha-deficient mice. Mol Cell Biol. 26(23):9105-15.

Bliss TV, Collingridge GL. 1993. A synaptic model of memory: long-term potentiation in the hippocampus. Nature. 361(6407):31-9.

Boekhoorn K, Sarabdjitsingh A, Kommerie H, de Punder K, Schouten T, Lucassen PJ, Vreugdenhil E. 2008. Doublecortin (DCX) and doublecortin-like (DCL) are differentially expressed in the early but not late stages of murine neocortical development. J Comp Neurol. 507(4):1639-52. Bouton ME, Westbrook RF, Corcoran KA, Maren S. 2006. Contextual and temporal modulation of

extinction: behavioral and biological mechanisms. Biol Psychiatry. 60:352–60.

Bramham CR, Messaoudi E. 2005. BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis. Prog Neurobiol. 76(2):99-125.

Brooks-Kayal AR, Raol YH, Russek SJ. 2009. Alteration of epileptogenesis genes. Neurotherapeutics. 6(2):312-8.

one DCX domain has long been deemed necessary for attributing any functionality to DCLK gene family members, we here demonstrate that splice variants lacking this domain, i.e. CARP and DCLK-short, also play important roles in cognitive and cellular processes, including neuronal apoptosis and neuronal transmission.

References

Ahi J, Radulovic J, Spiess J. 2004. The role of hippocampal signaling cascades in consolidation of fear memory. Behav Brain Res. 149(1):17-31.

Alfonso J, Agüero F, Sanchez DO, Flugge G, Fuchs E, Frasch AC, Pollevick GD. 2004. Gene

expression analysis in the hippocampal formation of tree shrews chronically treated with cortisol. J Neurosci Res. 78(5):702-10.

Ashwood TJ, Wheal HV. 1986. Extracellular studies on the role of N-methyl-D-aspartate receptors in epileptiform activity recorded from the kainic acid-lesioned hippocampus. Neurosci Lett.

18;67(2):147-52.

Atack JR. 2005. The benzodiazepine binding site of GABA(A) receptors as a target for the development of novel anxiolytics. Expert Opin Investig Drugs. 14(5):601-18.

Balkowiec A, Katz DM. 2002. Cellular mechanisms regulating activity-dependent release of native brain- derived neurotrophic factor from hippocampal neurons. J Neurosci. 22(23):10399-407. Barnabé-Heider F, Miller FD. 2003. Endogenously produced neurotrophins regulate survival and

differentiation of cortical progenitors via distinct signaling pathways. J Neurosci. 23(12):5149- 60.

Belayev L, Busto R, Zhao W, Fernandez G, Ginsberg MD. 1999. Middle cerebral artery occlusion in the mouse by intraluminal suture coated with poly-L-lysine: neurological and histological validation. Brain Res. 833(2):181-90.

Berke, J.D., R.F. Paletzki, G.J. Aronson, S.E. Hyman, and C.R. Gerfen. 1998. A complex program of striatal gene expression induced by dopaminergic stimulation. J Neurosci. 18:5301-10. Best N, Mitchell J, Baimbridge KG, Wheal HV. 1993. Changes in parvalbumin-immunoreactive neurons

in the rat hippocampus following a kainic acid lesion. Neurosci Lett 155:1–6.

Best N, Mitchell J, Wheal HV. 1994. Ultrastructure of parvalbumin immunoreactive neurons in the CA1 area of the rat hippocampus following a kainic acid injection. Acta Neuropathol (Berl) 87:187– 195.

Blaeser F, Sanders MJ, Truong N, Ko S, Wu LJ, Wozniak DF, Fanselow MS, Zhuo M, Chatila TA. 2006. Long-term memory deficits in Pavlovian fear conditioning in Ca2+/calmodulin kinase kinase alpha-deficient mice. Mol Cell Biol. 26(23):9105-15.

Bliss TV, Collingridge GL. 1993. A synaptic model of memory: long-term potentiation in the hippocampus. Nature. 361(6407):31-9.

Boekhoorn K, Sarabdjitsingh A, Kommerie H, de Punder K, Schouten T, Lucassen PJ, Vreugdenhil E. 2008. Doublecortin (DCX) and doublecortin-like (DCL) are differentially expressed in the early but not late stages of murine neocortical development. J Comp Neurol. 507(4):1639-52. Bouton ME, Westbrook RF, Corcoran KA, Maren S. 2006. Contextual and temporal modulation of

extinction: behavioral and biological mechanisms. Biol Psychiatry. 60:352–60.

Bramham CR, Messaoudi E. 2005. BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis. Prog Neurobiol. 76(2):99-125.

Brooks-Kayal AR, Raol YH, Russek SJ. 2009. Alteration of epileptogenesis genes. Neurotherapeutics. 6(2):312-8.

Buday L, Wunderlich L, Tamás P. 2002. The Nck family of adapter proteins: regulators of actin

cytoskeleton. Cell Signal. 14(9):723-31.

Burgess HA, Reiner O. 2001. Cleavage of doublecortin-like kinase by calpain releases an active kinase fragment from a microtubule anchorage domain. J Biol Chem. 276(39):36397-403.

Cai Q, Sheng ZH. 2009. Molecular motors and synaptic assembly. Neuroscientist. 15(1):78-89. Conde C, Cáceres A. 2009. Microtubule assembly, organization and dynamics in axons and dendrites.

Nat Rev Neurosci. 10(5):319-32.

Coquelle FM, Levy T, Bergmann S, Wolf SG, Bar-El D, Sapir T, Brody Y, Orr I, Barkai N, Eichele G, Reiner O. 2006. Common and divergent roles for members of the mouse DCX superfamily.

Cell Cycle. 5(9):976-83.

Corcoran EE, Means AR. 2001. Defining Ca2+/calmodulin-dependent protein kinase cascades in transcriptional regulation. J Biol Chem. 276(5):2975-8.

Corcoran KA, Desmond TJ, Frey KA, Maren S. 2005. Hippocampal inactivation disrupts the acquisition and contextual encoding of fear extinction. J Neurosci. 25:8978–87.

Datson NA, van der Perk J, de Kloet ER, Vreugdenhil E. 2001. Identification of corticosteroid-responsive genes in rat hippocampus using serial analysis of gene expression. Eur J Neurosci. 14(4):675-89.

Dawson GR, Collinson N, Atack JR. 2005. Development of subtype selective GABAA modulators. CNS Spectr. 10(1):21-7.

Deuel, T.A., J.S. Liu, J.C. Corbo, S.Y. Yoo, L.B. Rorke-Adams, and C.A. Walsh. 2006. Genetic Interactions between Doublecortin and Doublecortin-like Kinase in Neuronal Migration and Axon Outgrowth. Neuron. 49:41-53.

Dijkmans TF, van Hooijdonk LW, Schouten TG, Kamphorst JT, Vellinga AC, Meerman JH, Fitzsimons CP, de Kloet ER, Vreugdenhil E. 2008. Temporal and functional dynamics of the transcriptome during nerve growth factor-induced differentiation. J Neurochem.

Dijkmans TF, van Hooijdonk LW, Schouten TG, Kamphorst JT, Fitzsimons CP, Vreugdenhil E. 2009. Identification of new Nerve Growth Factor-responsive immediate-early genes. Brain Res.

16;1249:19-33.

Earnheart JC, Schweizer C, Crestani F, Iwasato T, Itohara S, Mohler H, Lüscher B. 2007. GABAergic control of adult hippocampal neurogenesis in relation to behavior indicative of trait anxiety and depression states. J Neurosci. 27(14):3845-54.

Edelman AM, Kim WY, Higgins D, Goldstein EG, Oberdoerster M, Sigurdson W. 2005. Doublecortin kinase-2, a novel doublecortin-related protein kinase associated with terminal segments of axons and dendrites. Biol Chem. 280(9):8531-43.

Egan SE, Giddings BW, Brooks MW, Buday L, Sizeland AM, Weinberg RA. 1993. Association of Sos Ras exchange protein with Grb2 is implicated in tyrosine kinase signal transduction and transformation. Nature. 363(6424):45-51.

Engels, B.M., T.G. Schouten, J. van Dullemen, I. Gosens, and E. Vreugdenhil. 2004. Functional differences between two DCLK splice variants. Brain Res Mol Brain Res. 120:103-14. Fasshauer D, Sutton RB, Brunger AT, Jahn R. 1998. Conserved structural features of the synaptic

fusion complex: SNARE proteins reclassified as Q- and R-SNAREs. Proc Natl Acad Sci U S A. 95(26):15781-6.

Fitzsimons CP, Ahmed S, Wittevrongel CF, Schouten TG, Dijkmans TF, Scheenen WJ, Schaaf MJ, de Kloet ER, Vreugdenhil E. 2008. The microtubule-associated protein doublecortin-like regulates the transport of the glucocorticoid receptor in neuronal progenitor cells. Mol

Endocrinol. 22(2):248-62.

Francis, F., A. Koulakoff, D. Boucher, P. Chafey, B. Schaar, M.C. Vinet, G. Friocourt, N. McDonnell, O. Reiner, A. Kahn, S.K. McConnell, Y. Berwald-Netter, P. Denoulet, and J. Chelly. 1999. Doublecortin is a developmentally regulated, microtubule-associated protein expressed in migrating and differentiating neurons. Neuron. 23:247-56.

Franck JE, Schwartzkroin PA. 1985. Do kainate-lesioned hippocampi become epileptogenic? Brain Res 329:309–313.

Friocourt G, Chafey P, Billuart P, Koulakoff A, Vinet MC, Schaar BT, McConnell SK, Francis F, Chelly J. 2001. Doublecortin interacts with mu subunits of clathrin adaptor complexes in the developing nervous system. Mol Cell Neurosci. 18(3):307-19.

Gall C, Lauterborn J. 1992. The dentate gyrus: a model system for studies of neurotrophin regulation.

Epilepsy Res Suppl. 7:171-85.

Geinisman Y. 2000. Structural synaptic modifications associated with hippocampal LTP and behavioral learning. Cereb Cortex. 10(10):952-62.

Glavan, G., D. Sket, and M. Zivin. 2002. Modulation of neuroleptic activity of 9,10-didehydro-N-methyl- (2-propynyl)-6-methyl-8-aminomethylergoline bimaleinate (LEK-8829) by D1 intrinsic activity in hemi-parkinsonian rats. Mol Pharmacol. 61:360-8.

Gruart A, Muñoz MD, Delgado-García JM. 2006. Involvement of the CA3-CA1 synapse in the acquisition of associative learning in behaving mice. J Neurosci. 26(4):1077-87.

Hellier J.L., Patrylo P.R., Buckmaster P.S., Dudek F.E. 1998. Recurrent spontaneous motor seizures after repeated low-dose systemic treatment with kainate: assessment of a rat model of temporal lobe epilepsy. Epilepsy Res. 31(1):73-84.

't Hoen PA, Ariyurek Y, Thygesen HH, Vreugdenhil E, Vossen RH, de Menezes RX, Boer JM, van Ommen GJ, den Dunnen JT. 2008. Deep sequencing-based expression analysis shows major advances in robustness, resolution and inter-lab portability over five microarray platforms. Nucleic Acids Res. 36(21):e141.

Howe CL, Mobley WC. 2005. Long-distance retrograde neurotrophic signaling. Curr Opin Neurobiol. 15(1):40-8.

Hu Z, Yuri K, Ozawa H, Lu H, Kawata M. 1997. The in vivo time course for elimination of

adrenalectomy-induced apoptotic profiles from the granule cell layer of the rat hippocampus. J

Neurosci. 17(11):3981-9.

Ji J, Maren S. 2007 Hippocampal involvement in contextual modulation of fear extinction. Hippocampus. 17:749–58.

Ji J, Stephen Maren. 2008. Lesions of the entorhinal cortex or fornix disrupt the context-dependence of fear extinction in rats. Behavioural Brain Research 194 201–206.

Kandel ER, Spencer WA, Brinley FJ Jr. 1961. Electrophysiology of hippocampal neurons. I. Sequential