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BERNARD PORTHA AN. R. ACAD. NAC. FARM.

FIGURE 4. Model for defective glucose-induced insulin release and the
abnormal intracellular sites identified in the diabetic GK beta-cell (13).
Impaired insulin response to glucose may be attributed to impaired elevation of
intracellular [Ca2+]i, which is a consequence of the failure by glucose to augment

L-type Ca2+ channel activity, owing to insufficient plasma membrane
depolarization reflecting impaired closure of the ATP-sensitive K+ channels; this is

the result of insufficient cytosolic ATP generation by glucose. Abnormal Ca2+
handling by the endoplasmic reticulum in response to high glucose may also

contribute to the defective Ca2+ signalling. The sequestration of calcium by
endoplasmic reticulum during high glucose exposure (attributed to activation of
the calcium-ATPases, SERCAs) may be impaired in the GK beta-cell. Impaired

calcium sequestration could also occur because of insufficient cytosolic ATP
generation in response to high glucose. In GK islets, glucose fails to increase
inositol-phosphate (IP3) accumulation. This is linked to an anomaly in targeting
the phosphorylation of phosphoinositides: the activity of phosphatidyl-inositol
kinase, the first of the two phosphorylating activities responsible for generating
phosphatidyl-inositol biphosphate, is reduced. Moreover, deficient calcium handling

and ATP supply in response to glucose probably contributes to abnormal
activation of PI kinases and phospholipase C. Concerning cAMP, it is remarkable
that its intracellular content is high in GK ß-cells already at low glucose. This is
related to increased expression of the cyclase isoforms 1, 2, 3 and of the Gsa and
Gaolf proteins. Furthermore, cAMP is not further enhanced at increasing glucose

concentrations (at variance with the situation in normal beta-cells). Moreover,
reduced levels of several exocytotic SNARE proteins including VAMP-2, syntaxin
1A and SNAP-25, have been demonstrated in GK islets. We also recently found
that the assembly/disassembly of the cortical actin cytoskeleton beneath the plasma
membrane, is functionally abnormal in GK islets. Where data are available, the

impaired sites in the GK beta-cell are indicated with a symbol:
Abbreviations: AC, adenylates cyclases; Gs, Gq: heterotrimeric G proteins;

PI kinase, phosphatidylinositol-kinase; SERCA3, type3 calcium-ATPase;
SNAP, soluble NSF attachment protein; tSNARE, v-SNARE: SNARE proteins

(syntaxin, SNAP-25); VDCC, L-type calcium chanel.

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