Molecular Mechanisms of Exocytosis

The secretion of neurotransmitters is an extremely fast process which is fundamental for learning, memory and behavior. While the mechanism of classical neurotransmission has been exhaustively described, our group focuses on novel forms of neurotransmission which can be observed in all neurons all the time, but for which the mechanism still remains elusive.

Classical neurotransmission is triggered by the arrival of an action potential in the synapse. This electrical signal causes opening of Voltage-sensitive Ca2+ channels, which is coupled within milliseconds to a secretory event. This so-called excitation-secretion coupling involves Ca2+ sensor proteins that contain binding sites for Ca2+, phospholipids as well as the SNARE complex. Various Ca2+ sensor proteins have been described for different types of neurons. Most neurons appear to use either Synaptotagmin-1 or -2 for action potential-evoked neurotransmission.

While fast neurotransmission in the brain is essential for life, slower modes of secretion also contribute to overall brain function. An example is found in DOC2 proteins, cytoplasmic C2 domain proteins with a very high Ca2+ affinity. They reversibly associate with the plasma membrane in an activity-dependent manner to enhance SNARE-driven exocytosis. This mode of activation provides a means to trigger secretory events in response non-classical Ca2+ signals, as opposed to classical transients induced by action potentials.

Our research focuses on the discovery of novel mechanisms which contribute to signal transduction in the brain. Traditionally, such pathways have been referred to as ‘spontaneous release’ to indicate that secretory events occur in the absence of action potentials. However, it is becoming increasingly clear that Ca2+ plays an important role in regulating the spontaneous release frequency. Our research aims to elucidate the molecular signaling cascades involved in these processes, their relevance for the behavior of neuronal circuits and, ultimately, central nervous system function and disease.

Brouwer I, Giniatullina A, Laurens N, van Weering JR, Bald D, Wuite GJ, Groffen AJ. Direct quantitative detection of Doc2b-induced hemifusion in optically trapped membranes. Nat Commun. 2015 Sep 23;6:8387.

Geerts CJ, Plomp JJ, Koopmans B, Loos M, van der Pijl EM, van der Valk MA, Verhage M, Groffen AJ. Tomosyn-2 is required for normal motor performance in mice and sustains neurotransmission at motor endplates. Brain Struct Funct. 2015

Beunders, G. et al. Exonic deletions in AUTS2 cause a syndromic form of intellectual disability and suggest a critical role for the C terminus. Am. J. Hum. Genet. 92, 210–20 (2013).

Walter AM, Groffen AJ, Sørensen JB, Verhage M. Multiple Ca2+ sensors in secretion: teammates, competitors or autocrats? Trends Neurosci. 2011 Sep;34(9):487-97.

Groffen AJ, Martens S, Díez Arazola R, Cornelisse LN, Lozovaya N, de Jong AP, Goriounova NA, Habets RL, Takai Y, Borst JG, Brose N, McMahon HT, Verhage M. Doc2b is a high-affinity Ca2+ sensor for spontaneous neurotransmitter release. Science 327 (2010) 1614-8.

Groffen AJ, Friedrich R, Brian EC, Ashery U and Verhage M. DOC2A and DOC2B are sensors for neuronal activity with unique calcium-dependent and kinetic properties. J Neurochem 97 (2006) 818-33.

Team Leader
Sander Groffen