The classical functions of the skeleton encompass locomotion, protection and
mineral homeostasis. However, cell-specific gene deletions in the mouse and human
genetic studies have recently identified bone as an endocrine organ possessing the
capabilities to regulate both energy metabolism and reproduction. Preliminary data
suggested that Phosphatase, Orphan 1 (PHOSPHO1) a bone specific phosphatase,
indispensable for bone mineralisation, may crosstalk with osteotesticular protein
tyrosine phosphatase (OST-PT, Esp), a signalling molecule that dephosphorylates
the insulin receptor (InsR) on the osteoblast, negatively regulating the osteoblast
insulin signalling cascade.
The work of this thesis has expanded upon preliminary data confirming that Esp
was up-regulated 60-fold in Phospho1-/- osteoblasts. Furthermore in silico analysis
revealed Phospho1 ablation is significantly associated with insulin dependent
diabetes mellitus. These data form the basis of this thesis examining the role of
PHOSPHO1 in energy metabolism.
Initial in vivo characterisation of Phospho1-/- mice revealed that the ablation of
Phospho1 results in decreased blood glucose levels, improved insulin sensitivity and
glucose tolerance in juvenile, adult and aged mice. Following high fat feeding,
Phospho1 ablation conferred a remarkable degree of protection against diet-induce-dobesity
and non-alcoholic fatty liver disease (NAFLD) despite the 60-fold increase in
Esp expression.
The metabolic protection observed in Phospho1-/- mice served to strengthen
PHOSPHO1’s potential role in energy metabolism. However the mechanisms
remained unclear. Mice overexpressing Esp specifically in osteoblasts are glucose
intolerant and insulin resistant, due to the negative regulation of osteoblast-insulin-signalling,
resulting in decreased undercarboxylated osteocalcin (GLU13-OCN)
release. This thesis identified however that the serum levels of a GLU13-OCN were
normal in Phospho1-/- mice suggesting that there was a GLU13-OCN-independent
mechanism for PHOSPHO1 regulated energy metabolism. Moreover, mass
spectrometry analysis identified > 100 differentially expressed proteins in Phospho1-/-
serum associated with the regulation of glycolysis and gluconeogenesis. These
candidates displayed an enrichment for microRNA Mir34a and the transcription
factor hepatocyte nuclear factor 1, both reported to regulate hepatic glucose
homeostasis. These data therefore support the notion that further, yet undefined
osteoblast derived factors contribute to whole body energy metabolism and
highlight a new and unconventional role of Esp suggestive that it may act as a fine
controller of insulin sensitivity in mice, offering protection from severe
hypoglycaemia and dyslipidaemia.
Finally, this thesis also explored the notion that decreased levels of choline may
contribute to the insulin sensitivity observed in Phospho1-/- mice. Phosphocholine (PCho)
is a recognised substrate for PHOSPHO1 being hydrolysed into choline and
inorganic phosphate (Pi). Phosphatase Orphan 1 deficient mice, hypothesised to
have reduced choline levels were fed a 2% choline rich diet; mice displayed a
normalisation in insulin sensitivity and fat mass. These data suggest that Phospho1-
deficiency improves the metabolic profile of mice in vivo and confers resistance to
obesity and diabetes via the alteration of serum/tissue choline levels.
The work described herein has characterised the metabolic phenotype of Phospho1-/-
mice and began to unravel the mechanisms underlying the improved metabolic
phenotype in Phospho1-/- mice