Table 1 Isoforms of major components of the IGF1-Akt pathway

IGF1¹

Two (mouse: 1A, 1B) or three (human: 1A, 1B & 1C) isoforms, differing in the C terminal peptide (E peptide); another two (mouse: 2A, 2B) or three (human: 2A, 2B & 2C) isoforms differing in signal peptide because of utilization of exon 2 instead of exon 1

[49]

IGF1 receptor

Heterotetramer made of two α-subunit (IGF-binding) and two β-subunit (tyrosine kinase). No isoforms, but hybrids with insulin receptor are present in skeletal muscle

[112]

IRS²

IRS1 and IRS2

[65, 113]

PDK1³

No isoforms

[114]

PI3K⁴ (class I)

Heterodimer of p85 regulatory and p110 catalytic subunits. p85: three isoforms (p85α, p55α, p50α) encoded by a single gene; two other isoforms (p85β, p55γ) coded by other genes. p55γ is not expressed in muscle. p110: three isoforms (p110α, p110β, p110δ). p110δ is not expressed in muscle

[115]

Akt/PKB⁵

Akt1/PKBα, Akt2/PKBβ & Akt3/PKBγ. Akt3 is not expressed in muscle

[4, 116]

mTOR⁶

No isoforms, but mTOR can interact with different partners: Raptor in the rapamycin-sensitive complex mTORC1, or Rictor in the rapamycin-insensitive complex mTORC2

[5]

S6K⁷

S6K1 and S6K2

[117–119]

4EBP⁸

4EBP1, 4EBP2, 4EBP3. 4EBP3 is not expressed in muscle

[120]

FoxO

4 isoforms: FoxO1, FoxO3, FoxO4 and FoxO6. FoxO6 is not expressed in muscle

[121]