Abstract

Background and Aims: The current drive towards islet
transplantation as a viable cure for type 1 diabetes has
focused attention on the characterization and development
of substitute tissue that responds appropriately to elevations
in glucose. It is widely accepted that the pancreatic islet and
not individual beta-cells represent the unit of insulin
secretion, yet no clear evidence exists to explain why
homotypic populations of cells within islets respond more
effectively than individual beta-cells. Furthermore, heterogeneous
individual responses to nutrients are usurped by
concerted, synchronised glucose-responsiveness when cells
are configured as an islet. Close association between cells
in the islet is facilitated by the expression of cell adhesion
molecules, including E-cadherin (ECAD) and beta-catenin
which binds the cytoplasmic domain of the cadherin. These
molecules not only hold the 3-dimensional islet together,
but may offer an explanation for improved glucoseresponsiveness.
In the current study we have examined
the importance of ECAD on synchronicity of activity and
insulin secretion within model islets.
Materials and Methods: MIN6 were cultured either as
monolayer (M) or pseudoislets (PI). Nutrient and nonnutrient-
evoked changes in cytosolic calcium were determined
from fura-2-loaded Ms and/or PIs cultured either in
the presence or absence of the immuno-netralising ECADantibody
or IgG control. Insulin secretion was measured via
ELISA. Co-localization of beta-catenin and insulin was
visualized using confocal microscopy, using Alexa-594/488
secondary antibodies. ECAD was over-expressed using
Lipofectamine transfection with pcDNA3.1(-) vector containing
the coding region of mouse ECAD gene. Expression
levels were confirmed using Western blot analysis.
Results: MIN6 cells configured as PIs exhibit greater
glucose (20 mM)-evoked increases in insulin secretion
(802T73% over basal; n=7) compared to those cultured as
monolayers to 80% confluence (147T14% over basal; n=6).
Western blot analysis suggests that MIN6 cells within a PI
express more ECAD than cells within a monolayer. Fura-2
microfluorimetry data confirms synchronicity of glucoseevoked
calcium oscillations in PIs compared to the
heterogeneous response of individual MIN6-cells (data
from 6 separate experiments). Both islet morphology and
synchronicity of nutrient-evoked calcium oscillations are
altered in the presence of immuno-neutralising anti-ECAD,
which prevents homophilic ECAD ligation. Beta-catenin
and insulin immunoreactivity were co-localised only at the
cell-to-cell interface in MIN6 monolayers. Up-regulation of
ECAD expression in our MIN6 cells was confirmed
following transfection using Western blot and confocal
microscopy (20-fold enhancement; n=5). However, insulin
secretion studies suggest that ECAD over-expression does
not enhance glucose-stimulated insulin secretion from
MIN6 cells (over-expressing cells: 171T20% basal; n=3;
vector control cells: 175T17% basal; n=3).
Conclusion: Our data suggest that ECAD is important in
the formation and maintenance of islet architecture and that
loss of ECAD uncouples glucose-evoked signaling within
the islet. Over-expression of ECAD fails to enhance insulin
secretion, suggesting that ECAD may act to mediate
appropriate nutrient-evoked signaling, rather than control
insulin secretion per se.