Ph.D., Interdepartmental Neuroscience Program, Department of Zoology, University of Washington, Seattle and Friday Harbor
B.S., Biology, University of California, Irvine
Synaptotagmins, Birth, Social Memory
My first interest sparked upon joining the Chapman lab concerns how the expression of synaptotagmins with different dynamics dovetails with specific vesicular release requirements in the hypothalamus. This interest was suggested by a striking finding from the lab: synaptotagmin-9 (Syt-9) protein is expressed most abundantly in the hypothalamus and in the posterior pituitary, suggesting that it plays a role in the release of the neuropeptides oxytocin (OT) and/or vasopressin (VP) from hypothalamic magnocellular neurons (Roper et al., 2015). Interestingly, Syt-9 has markedly slower membrane dissociation dynamics than those of the most widely expressed synaptotagmin (1, Syt-1) – the calcium sensor for synchronous neurotransmitter release (Hui et al., 2005). Moreover, the release of neuropeptide-containing dense core vesicles from magnocellular neurons is heterogenous in multiple, physiologically-relevant ways, offering a unique opportunity for structure-function comparisons.
The magnocellular-posterior pituitary system exerts wide influence over reproduction, homeostasis and social behavior, and the coordination of these functions requires differential neuropeptide release at many levels. I want to focus on three loci of release: 1) OT and VP released from pituitary axons into the systemic circulation during exceptional life events (e.g., birth or hemorrhage, respectively), which occurs in response to highly contrasting spiking patterns, 2) release from dendrites within the hypothalamus, a dynamic process dependent on both intracellular calcium stores and voltage-gated calcium channels, and 3) release from magnocellular neuron axon collaterals in the brain, where the two neuropeptides modulate daily functions such as social memory and fear but where the activity required for release is only beginning to be elucidated.
1) The best characterized release mode in this system is axonal release of OT during birth/lactation and of VP during hemorrhage/dehydration. Short, synchronized bursts of action potentials trigger release of OT. We hypothesize that Syt-1-dependent dense core vesicle fusion would track the high frequency spiking closely during bursts and suspend fusion promptly at the end of bursts, sharpening the pulsatile release of OT. On the other hand, long, staggered periods of lower frequency spiking (“phasic firing”) evoke the non-pulsatile release of VP. We hypothesize that Syt-9 would contribute to the integration of the calcium signal in this case due to its slow disassembly from the plasma membrane. Syt-9 expression has been documented in VP terminals (Tobin et al., 2012), but we do not know if it is expressed in OT terminals.
2) The concentration of OT and VP is higher in magnocellular neuron dendrites than in axons. Dendritic release is robust, subject to physiological state-dependent plasticity, and instrumental in shaping the firing of the magnocellular neurons through autocrine and retrograde modulation. Neuropeptide release from the dendrites can be triggered by invading action potentials (de Kock et al., 2003) but functional release also requires a sustained increase in intracellular calcium from intracellular stores (Brown et al., 2020). Therefore, it is of interest that neither the calcium sensor for synchronous release Syt-1 nor the canonical complement of SNARE proteins is present in the dendrites of magnocellular neurons, but they are present in their axons (Brown et al., 2020; Tobin et al., 2012). This constitutes a fascinating system to be elucidated, and we start with the hypothesis that dendritic neuropeptide release is supported by Syt-9. Its presence in the cell bodies is suggested by its high expression in both the pituitary and the hypothalamus, but will need to be confirmed.
3) The release of OT and VP in the brain from the axons of hypothalamic parvo- and magnocellular neurons has now been firmly established (Knobloch et al., 2012; Tirko et al., 2018). An interesting example is that of OT release in hippocampal field CA2, where it activates burst firing in pyramidal neurons by modulating both intrinsic excitability and synaptic inputs (Owen et al., 2013; Tirko et al., 2018). The CA2 area is essential for social memory, a daily function which presumably does not depend on the exceptional levels of spiking required for OT release during birth and lactation. Long-duration, low-frequency (5 min, 5Hz) optogenetic stimulation of oxytocinergic axons is effective at transforming the firing mode of CA2 pyramids (Tirko et al., 2018). Therefore, we hypothesize that a complement of synaptotagmins enabling robust synchronous and asynchronous or delayed release, such as Syt-1, 7 and 9, will be mediating OT release in CNS oxytocinergic terminals.
Testing these hypotheses will require a convergence of classical techniques, such as patch clamp electrophysiology (including capacitance measurements (Klyachko and Jackson, 2002)), ELISA and immunohistochemistry, and novel techniques such as OT and VP sniffer cells (Gizowski et al., 2016; Pinol et al., 2014; Zaelzer et al., 2018). Syt-9 KO mice are viable in our colony.
Please see cited publications below.
Popescu IR, Le KQ, Ducote A, Li J, Mostany R (2021) Increased Intrinsic Excitability and Decreased Synaptic Inhibition in Aged Somatosensory Cortex Pyramidal Neurons. Neurobiol Aging 98:88-98.
Popescu IR, Buraei Z, Haam J, Weng FJ, Tasker JG (2019) Lactation induces increased IPSC bursting in oxytocinergic neurons. Physiol Rep 7(8):e14047.
Popescu IR, Le KQ, Palenzuela R, Voglewede R, Mostany R (2017) Marked bias towards spontaneous synaptic inhibition distinguishes non-adapting from adapting layer 5 pyramidal neurons in the barrel cortex. Sci Rep 7(1):14959.
*Morton, LA, *Popescu IR, Haam J, Tasker JG (2014) Short-Term Potentiation of GABAergic Synaptic Inputs to Vasopressin and Oxytocin Neurones. J Physiol 592(Pt 19):4221-33
Popescu IR, Di S, Tasker JG (2014) Endocannabinoid Modulation of Synaptic Inputs to Magnocellular Neurons. In Masterclass in Neuroendocrinology Series: Stress. (eds. J.G. Tasker and W.E. Armstrong), Wiley-Blackwell for the International Neuroendocrine Federation, Chapter 10, 225-253
Di S, Popescu IR, Tasker JG (2013) Glial control of endocannabinoid heterosynaptic modulation in hypothalamic magnocellular neuroendocrine cells. J Neurosci 13;33(46):18331-42.
Haam J, Popescu IR, Morton LA, Halmos KC, Teruyama R, Ueta Y, Tasker JG (2012) GABA is excitatory in adult vasopressinergic neuroendocrine cells. J Neurosci 11;32(2):572-82.
Popescu IR, Morton LA, Franco, A, Di S, Ueta Y, Tasker JG (2010) Synchronized bursts of miniature inhibitory postsynaptic currents. J Physiol 15;588(Pt 6):939-51.
*Liu X, *Popescu IR, Denisova JV, Neve RL, Corriveau RA, Belousov AB (2008) Regulation of cholinergic phenotype in developing neurons. J Neurophysiol 99:2443-2455.
Di S, Boudaba C, Popescu IR, Weng FJ, Harris C, Marcheselli VL, Bazan NG, Tasker JG (2005) Activity-dependent release and actions of endocannabinoids in the rat hypothalamic supraoptic nucleus. J Physiol 569:751-760.
Kang SK, Putnam LA, Ylostalo J, Popescu IR, Dufour J, Belousov A, Bunnell BA (2004) Neurogenesis of Rhesus adipose stromal cells. J Cell Sci 117:4289-4299.
Popescu IR, Frost WN (2002) Highly dissimilar behaviors mediated by a multifunctional network in the marine mollusk Tritonia diomedea. J Neurosci 22:1985-1993.
Popescu IR, Willows AO (1999) Sources of magnetic sensory input to identified neurons active during crawling in the marine mollusc Tritonia diomedea. J Exp Biol 202:3029-3036.
Publications Cited in the Research Interests Section
Brown CH, Ludwig M, Tasker JG, Stern, JE (2020) Somato-dendritic vasopressin and oxytocin secretion in endocrine and autonomic regulation. J Neuroendocrinol 32(6), e12856.
de Kock CP, Wierda KD, Bosman LW, Min R, Koksma JJ, Mansvelder HD, Verhag, M, Brussaard AB (2003) Somatodendritic secretion in oxytocin neurons is upregulated during the female reproductive cycle. J Neurosci 23(7), 2726-2734.
Gizowski C, Zaelzer C, Bourque CW (2016) Clock-driven vasopressin neurotransmission mediates anticipatory thirst prior to sleep. Nature 537(7622), 685-688.
Hui E, Bai J, Wang P, Sugimori M, Llinas RR, Chapman ER (2005) Three distinct kinetic groupings of the synaptotagmin family: Candidate sensors for rapid and delayed exocytosis. Proc Natl Acad Sci U S A 102(14), 5210-5214.
Klyachko VA, Jackson MB (2002) Capacitance steps and fusion pores of small and large-dense-core vesicles in nerve terminals. Nature 418(6893), 89-92.
Knobloch HS, Charlet A, Hoffmann LC, Eliava M, Khrulev S, Cetin AH, Osten P, Schwarz MK, Seeburg PH, Stoop R, Grinevich V (2012) Evoked axonal oxytocin release in the central amygdala attenuates fear response. Neuron 73(3), 553-566.
Owen SF, Tuncdemir SN, Bader PL, Tirko NN, Fishell G, Tsien RW (2013) Oxytocin enhances hippocampal spike transmission by modulating fast-spiking interneurons. Nature 500(7463), 458-462.
Pinol RA, Jameson H, Popratiloff A, Lee NH, Mendelowitz D (2014) Visualization of oxytocin release that mediates paired pulse facilitation in hypothalamic pathways to brainstem autonomic neurons. PLoS One 9(11), e112138.
Roper LK, Briguglio JS, Evans CS, Jackson MB, Chapman ER (2015) Sex-specific regulation of follicle-stimulating hormone secretion by synaptotagmin 9. Nat Commun 6, 8645.
Tirko NN, Eyring KW, Carcea I, Mitre M, Chao MV, Froemke RC, Tsien RW (2018) Oxytocin transforms firing mode of ca2 hippocampal neurons. Neuron 100(3), 593-608 e593.
Tobin V, Schwab Y, Lelos N, Onaka T, Pittman QJ, Ludwig M (2012) Expression of exocytosis proteins in rat supraoptic nucleus neurones. J Neuroendocrinol 24(4), 629-641.
Zaelzer C, Gizowski C, Salmon CK, Murai KK, Bourque CW (2018) Detection of activity-dependent vasopressin release from neuronal dendrites and axon terminals using sniffer cells. J Neurophysiol 120(3), 1386-1396.