DihexavsNAD+

Dihexa (N-hexanoic-Tyr-Ile-(6) aminohexanoic amide) is a modified hexapeptide derivative of angiotensin IV developed at Washington State University by Dr. Joseph Harding's laboratory. It was designed to mimic the cognitive-enhancing effects of angiotensin IV and its analogue Nle1-AngIV (DIIIA), which had shown procognitive properties but required central administration. Dihexa was engineered with metabolic stability modifications (hexanoic acid modifications at both termini) for oral bioavailability and blood-brain barrier penetration.

Dihexa's mechanism centers on the hepatocyte growth factor (HGF)/c-Met receptor system, which plays a critical role in brain development, neuroplasticity, and neuroprotection. Dihexa acts as an allosteric modulator and potentiator of HGF signaling — it facilitates HGF dimerization and binding to the c-Met receptor tyrosine kinase, amplifying the downstream signaling cascade. Activated c-Met triggers the PI3K/Akt pathway (neuronal survival), the Ras/MAPK/ERK pathway (synaptic plasticity and gene expression), and the Rac1/Cdc42 pathway (cytoskeletal remodeling for dendritic spine formation).

The cognitive effects stem from enhanced dendritic spine formation and synaptic connectivity in the hippocampus — the brain region critical for learning and memory. Dendritic spines are the postsynaptic structures where most excitatory synapses form, and their density and morphology are directly correlated with cognitive function. Dihexa treatment in animal models increased spine density, enhanced long-term potentiation (LTP — the cellular correlate of memory formation), and restored cognitive function in models of dementia. The reported potency — up to 10 million times more potent than BDNF in promoting synaptic connectivity in cell culture assays — is striking but should be interpreted cautiously, as in vitro potency does not always translate to in vivo efficacy. The activation of the HGF/c-Met pathway raises theoretical concerns about tumor promotion, as this pathway is frequently co-opted in cancer for metastasis and angiogenesis, and no human safety data exists to evaluate this risk.

Nicotinamide Adenine Dinucleotide (NAD+) is a dinucleotide coenzyme consisting of nicotinamide mononucleotide (NMN) joined to adenosine monophosphate (AMP) through a pyrophosphate bond. It exists in oxidized (NAD+) and reduced (NADH) forms and participates in over 500 enzymatic reactions, making it one of the most central molecules in cellular metabolism.

As a redox cofactor, NAD+ accepts hydride ions (H-) during catabolic reactions. In glycolysis, the TCA cycle, and fatty acid beta-oxidation, NAD+ is reduced to NADH, which then donates electrons to Complex I of the mitochondrial electron transport chain, driving oxidative phosphorylation and ATP production. Without adequate NAD+, the entire energy production machinery of the cell grinds to a halt.

Equally important are NAD+'s roles as a consumed substrate for three families of signaling enzymes. Sirtuins (SIRT1-7) are NAD+-dependent protein deacylases and ADP-ribosyltransferases that use NAD+ as a co-substrate, cleaving it to nicotinamide and O-acetyl-ADP-ribose during the deacetylation reaction. SIRT1 and SIRT3 are particularly important for aging — SIRT1 deacetylates PGC-1α (activating mitochondrial biogenesis), FOXO transcription factors (activating stress resistance), and NF-κB (suppressing inflammation). SIRT3 in the mitochondrial matrix activates SOD2 and other mitochondrial enzymes. PARPs (poly-ADP-ribose polymerases) consume NAD+ during DNA damage repair, adding chains of ADP-ribose to histones near DNA breaks to recruit repair machinery. CD38, an NAD+-consuming glycohydrolase on immune cells, regulates calcium signaling and immune activation.

NAD+ levels decline 40-60% between ages 40 and 70, driven by increased CD38 expression (with chronic low-grade inflammation), increased PARP activity (from accumulated DNA damage), and reduced synthesis (decreased NAMPT enzyme activity). This decline impairs sirtuin function, reduces ATP production, compromises DNA repair, and contributes to virtually every hallmark of aging. Supplementation strategies aim to restore NAD+ levels either directly (IV infusion) or through biosynthetic precursors: NMN enters the salvage pathway one step from NAD+, while NR (nicotinamide riboside) requires an additional phosphorylation step.