TSPAN5 is a brain enriched protein member of the tetraspanin superfamily, a group of
transmembrane proteins some of which have been shown to fundamentally regulate the
development of mammalian nervous system. This class of proteins presents the peculiar ability
to clusterize forming specialized membrane region called Tetraspanin Enriched Microdomains
(TEMs) where they can accumulate other proteins.
We found that in developing neurons TSPAN5 was mainly present at the surface membrane
while it was concentrated in an intracellular compartment in the postsynapse of mature
neurons. We hypothesized that these different localisations could be due to different functions.
To deepen the first function of the protein, we knocked down the expression of the protein and
found that this led to a dramatic reduction in the number of dendritic spines. We, thus,
hypothesized that TSPAN5, through the formation of TEMs, could be responsible of dendritic
spines formation. We observed in differential lysis of developing rat hippocampal neurons that
two proteins, fundamental for dendritic spines formation, Neuroligin-1 and GluA2 AMPA
receptor subunit, were associated with TSPAN5 TEMs. We found that the knockdown of
TSPAN5 led to increased mobility of Neuroligin-1 and GluA2 AMPA receptors suggesting the
loss of clusterization typical of the first moments of spines formation.
To understand the second function of TSPAN5 we identified AP-4 complex as an interactor of
the C-terminal intracellular tail of TSPAN5. This complex is known to act on AMPARs trafficking
through direct binding of Stargazin, an AMPARs auxiliary subunit.
We observed that the knockdown of TSPAN5, carried out after the majority of the
synaptogenesis was occurred, caused a strong decrease in surface and total level of GluA2.
Different evidences suggested an involvement of TSPAN5 in vesicular transport of GluA2 and
we demonstrated that TSPAN5 was necessary for the correct recycling of this receptor.
These results highlight multiple roles of TSPAN5 in the regulation of both synapse formation
and synaptic functioning in mammalian brain through two distinct mechanisms of action