FBP2 Human

What is FBP2 and how is it encoded in humans?

FBP2, also known as muscle fructose-bisphosphatase, is an enzyme encoded by the FBP2 gene located on chromosome 9 in humans . The enzyme catalyzes the hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate and inorganic phosphate, playing a regulatory role in gluconeogenesis . Unlike its liver counterpart (FBP1), FBP2 is predominantly expressed in striated muscle and has evolved additional non-catalytic functions .

How does the structure of FBP2 relate to its multiple functions?

FBP2 functions depend critically on its quaternary structure. The enzyme can adopt various oligomeric states, primarily existing as either dimers or tetramers depending on cellular conditions:

The ability of FBP2 to switch between these forms allows it to perform multiple roles, including enzymatic regulation of glyconeogenesis and protection of mitochondrial function during stress conditions . This structural flexibility is key to understanding how FBP2 integrates metabolic and non-metabolic functions in cells .

What tissue distribution pattern does FBP2 exhibit?

FBP2 is primarily expressed in non-gluconeogenic organs, with highest expression in skeletal muscle, as originally identified by Krebs and Woodford in 1965 . Significant expression has also been detected in hippocampal neurons, where it plays a role in memory formation processes . Unlike the liver-specific FBP1 isoform, FBP2 has a more specialized tissue distribution that reflects its evolved functions beyond classical gluconeogenesis .

What are the most effective animal models for studying FBP2 function?

FBP2 knockout (KO) mice represent the gold standard experimental model for investigating FBP2 physiological roles. These models have revealed several key phenotypes:

• Normal energy and glucose metabolism under standard feeding and fasting conditions

• Severe cold intolerance under fasting conditions

• Complete abolishment of cold-induced intramuscular lactate-to-glycogen conversion

• Resolution of cold intolerance after feeding

When designing experiments with these models, researchers should consider:

• Nutritional status (fed vs. fasted) as a critical variable

• Environmental temperature conditions

• Duration of exposure to experimental conditions

• Age and sex of experimental animals

What methodological approaches are optimal for measuring FBP2 activity?

For comprehensive assessment of FBP2 function, researchers should employ multiple methodological approaches:

For colocalization studies, the Manders' coefficient (M) using JACoP plugin of ImageJ/FIJI provides quantitative assessment of FBP2 association with subcellular structures, with values ranging from 0 (no colocalization) to 1 (100% colocalization) .

How does FBP2 contribute to muscle thermogenesis at the molecular level?

FBP2 plays an essential role in muscle thermogenesis through glyconeogenesis, particularly when exogenous glucose is limited. The molecular mechanism involves:

• Catalyzing the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate during glyconeogenesis

• Enabling the replenishment of intramuscular glycogen pools from lactate under fasting conditions

• Maintaining glycogen as a critical substrate for muscle thermogenesis during cold exposure

Newsholme hypothesized in 1976 that a futile cycle between fructose-6-phosphate and fructose-1,6-bisphosphate mediated by FBP2 could regulate whole-body thermal homeostasis . Research with FBP2 KO mice has confirmed this hypothesis, demonstrating that the enzyme's activity increases during cold challenge and is essential for maintaining thermal homeostasis when exogenous glucose is limited .

What is the relationship between FBP2 and intracellular calcium regulation?

FBP2 plays a protective role during calcium stress in cells, particularly in astrocytes. The relationship includes:

• FBP2 helps maintain mitochondrial membrane potential during elevated intracellular calcium levels

• Neuronal extracellular vesicles (NEVs) regulate the dimeric/tetrameric ratio of FBP2 in astrocytes

• This regulation allows for metabolic adjustments during calcium fluctuations associated with neuronal activity

• FBP2 protection mechanisms appear to involve direct interaction with mitochondria

When studying these relationships, researchers should consider implementing calcium imaging alongside mitochondrial membrane potential assessments to correlate FBP2 function with calcium dynamics in real-time experimental settings .

What role does FBP2 play in long-term potentiation and memory formation?

FBP2 has been identified as a crucial component in the molecular machinery of memory formation, particularly in hippocampal structures. Its role in long-term potentiation (LTP) involves:

• Association with neuronal mitochondria during LTP induction

• Direct interaction with calcium/calmodulin-dependent protein kinase II (Camk2)

• Stimulation of Camk2 autophosphorylation, an essential step in early-phase LTP

• Influence on nuclear accumulation of Camk4 and expression of late-phase LTP markers (c-Fos, c-Jun)

Experimental evidence demonstrates that silencing FBP2 expression or simultaneous inhibition and tetramerization of the enzyme blocks LTP formation. This suggests FBP2 is an inherent element of memory formation in hippocampal structures .

How do neuronal extracellular vesicles regulate FBP2 in astrocytes?

Neuronal extracellular vesicles (NEVs) deliver signals that regulate FBP2 in hippocampal astrocytes through multiple mechanisms:

• Reduction of FBP2 mRNA levels

• Stimulation of enzyme degradation

• Alteration of the cellular titers of different oligomeric forms of FBP2

• Influencing the balance between enzymatic and non-enzymatic functions

These NEV-mediated changes in FBP2 expression and oligomeric state directly impact astrocyte metabolism, resulting in:

This regulation may represent an important mechanism for neuron-astrocyte metabolic coupling during brain activity .

What are the critical variables to control when designing FBP2 studies?

When designing experiments to investigate FBP2 function, researchers should carefully control:

• Nutritional status of subjects (fed vs. fasted)

  • FBP2 KO phenotypes are often only apparent during fasting

  • Feeding can rescue certain phenotypes such as cold intolerance

• Environmental temperature conditions

  • Standard housing temperature vs. cold challenge (typically 4°C)

  • Duration of cold exposure (acute vs. chronic)

• Tissue-specific expression analysis

  • Muscle vs. brain vs. other tissues

  • Subcellular fractionation to determine localization

• Oligomeric state assessment

  • Methods to distinguish between dimeric and tetrameric forms

  • Conditions affecting oligomerization (AMP, NAD+)

A true experimental design with appropriate controls is considered the gold standard for FBP2 research, incorporating random assignment of subjects to experimental and control groups to ensure internal validity and establish causality .

How should researchers approach protein-protein interaction studies involving FBP2?

For studying FBP2 interactions with other proteins:

• Employ the DuoLink proximity ligation assay for detecting and quantifying protein-protein interactions in situ

• Use immunofluorescence with specific antibodies (e.g., rabbit anti-TOMM antibody) for colocalization studies

• Apply JACoP plugin of ImageJ/FIJI for quantitative colocalization analysis, calculating Manders' coefficient

• Consider the influence of metabolic state and cellular stress on interaction dynamics

When analyzing mitochondrial interactions, ensure that fluorescent signal from the cytoplasmic area is isolated, with nuclear signal excluded to improve accuracy .

What are the potential therapeutic applications of targeting FBP2?

Research into FBP2 function suggests several potential therapeutic directions:

• Cold stress protection

  • Development of therapies to increase glycogen replenishment during cold stress

  • Particularly relevant for conditions involving hypothermia risk or cold exposure

• Metabolic regulation

  • Targeting the balance between glycolytic and glyconeogenic pathways

  • Potential applications in metabolic syndrome and related conditions

• Neuroprotection and cognitive enhancement

  • Given FBP2's role in LTP, targeting its function might influence memory formation

  • Potential applications in cognitive disorders

• Mitochondrial protection

  • Leveraging FBP2's role in protecting mitochondrial function during calcium stress

  • Applications in conditions involving mitochondrial dysfunction

When designing studies to explore these therapeutic potentials, researchers should consider both the enzymatic and non-enzymatic functions of FBP2, as well as tissue-specific effects and the influence of oligomeric state on function .

What are the known phenotypic manifestations of FBP2 deficiency in humans?

Clinical evidence regarding FBP2 deficiency in humans is limited but suggests:

• Association with benign nonprogressive myopathy

• Unlike FBP1 deficiency, FBP2 deficiency does not appear to cause severe metabolic acidosis or hypoglycemia during fasting

• May have more subtle effects on muscle function and thermal regulation

Further clinical research is needed to fully characterize the human phenotype associated with FBP2 deficiency or dysfunction, particularly regarding thermal regulation and response to metabolic stress .