Maternal Nutrition and Nephrogenesis

Maternal Nutrition and Nephrogenesis

  • Maternal nutrition can potentially boost nephrogenesis.
    • It would be beneficial to enhance nephron formation in babies developing small or born prematurely to improve their nephron endowment.
  • Normal mouse diet:
    • 21% protein, 65% carbohydrate, 14% fat
    • Results in typical kidney size with approximately 13,000 nephrons.
  • Boosting diet (specific composition not provided):
    • Leads to larger kidney size.
    • Increases nephron number by 20% compared to the normal diet.
    • Extends the period of nephrogenesis, as evidenced by immunohistochemistry for left one, where nephrogenesis continues longer than in control animals.

Rescue of Nephron Deficit

  • Maternal diet and lactation can rescue nephron deficits in mice.
  • Nephron counts:
    • Normal mice at birth: Around 2,000 nephrons.
    • Low-protein mice at birth: Show a nephron deficit that persists even at 21 days (after nephrogenesis completion); approximately 9,000 nephrons at 21 days compared to 13,000 in controls.
  • Diet switch (DS) to the boosting diet:
    • During gestation (from embryonic day 15): Prevents the nephron deficit.
    • At birth: Rescues the potential nephron deficit.
    • The rescue at birth is attributed entirely to lactation, as the diet switch affects the mother's milk composition.
  • A premature birth model is being developed to investigate if lactation strategies can rescue nephron endowment in premature mice.

Developmental Programming of Noncommunicable Diseases

  • Chronic diseases can originate through processes occurring before or shortly after birth.
  • Developmental programming of noncommunicable diseases is an established paradigm.
  • Birth weight as a surrogate index:
    • Commonly used, easy to measure, and increasingly recorded.
    • Low birth weight and premature birth are associated with increased risk for many adult chronic diseases, including kidney and cardiovascular diseases.
  • Optimization of maternal health and early childhood nutrition can attenuate this programming cycle, reducing the global burden of chronic diseases.
  • Early screening and education programs are warranted.
  • Noninvasive kidney imaging will aid in identifying individuals with low nephron number, enabling closer monitoring and treatment.
  • Optimizing nephron endowment through maternal nutrition and lactation may protect unborn and premature babies from debilitating chronic diseases.

Current and Future Actions

  • Immediate Actions:
    • Improve recording of gestational age and birth weight for all infants to identify at-risk individuals.
    • Prominently display birth weight and gestational age in infant medical records.
    • Regularly monitor at-risk infants for blood pressure, weight gain, albuminuria, and hypoglycemia.
    • Limit the use of potentially nephrotoxic drugs for neonates.
    • Enhance resources for maternal health, fetal growth, and full-term pregnancies.
  • Future Research Areas (Lancet, 2017):
    • Develop and validate in vivo approaches to measure nephron number or functional renal mass.
    • Characterize nephron numbers in different populations (currently lacking data from Chinese, Indian, and many other populations).
    • Understand the effect of diabetic pregnancy on kidney programming.
    • Understand gender differences in programming risks.

Estimating Glomerular Number and Size In Vivo

  • Current Gold Standard: Dissect a fractionator (time-consuming and tedious).
  • Need for in vivo methods to:
    • Obtain a measure of functional nephron mass.
    • More accurately estimate single nephron GFR.
    • Estimate functional nephron mass in newly diagnosed CKD patients.
    • Determine therapy effectiveness in patients.
    • Estimate nephron number in children born small or premature to identify those to monitor closely.
    • Perform longitudinal studies on animals to assess potential new therapies.

Advances in Glomerular Imaging

  • Ex Vivo MRI:
    • High-resolution MRI (e.g., 9.4 T Bruker scanner) of rat kidneys after perfusion with cationic ferritin to label glomeruli.
    • Allows visualization of every glomerulus.
    • Comparison of MRI counts with dissect defractionator counts demonstrates good agreement.
    • Kevin Bennett's lab used cationic ferritin to image glomeruli in human kidneys destined for transplant but ultimately not used, avoiding histological sectioning and manual counting.
    • This technology is being established at Monash University.
  • In Vivo MRI:
    • Bennett lab's 2018 publication: First in vivo report of glomerular number in living animals (Sprague Dawley rats).
    • Involves IV injections of cationic ferritin followed by scanning in a Bruker 7 T MR machine.
    • Excellent agreement between in vivo MRI counts and traditional counting methods.
    • This technique has been subsequently applied to mice.
  • PET Technology:
    • Developing PET technology for quicker estimates of glomerular number in living animals.
    • Uses a radioisotope (copper 64) incorporated into cationic ferritin (radio CF).
    • PET imaging shows binding of radio CF to the kidney cortex.
    • Coregistration with MRI and CT images confirms the localization of the PET signal with glomeruli.
    • Potential for noninvasive PET scanning to estimate nephron mass in living humans in the future.

Nephron Number and Hypertension in Human Beings

  • Landmark Paper (New England Journal of Medicine, 2003):
    • Used the dissector fractionator technique to estimate total nephron number in kidneys of 20 white accident victims at autopsy (10 hypertensive, 10 normotensive).
    • Groups were well-matched for sex, age, height, and weight.
    • Normotensives: Average of 1,400,000 nephrons.
    • Hypertensives: Average of 700,000 nephrons (twofold difference).
    • Hypertensives with fewer glomeruli had larger glomeruli.
  • Other Studies:
    • Study on African and white Americans: No association between nephron number and hypertension in African Americans, but higher nephron counts were associated with a lower probability of hypertension in white Americans.
    • Study in indigenous Australians: Those with a history of hypertension had about a quarter of a million fewer glomeruli per kidney than those without a history of hypertension.
  • Association between Birth Weight and Nephron Number:
    • Positive correlation between birth weight and nephron number.
    • For every extra kilo increase in birth weight, there's approximately an extra 200,000 nephrons per kidney.

Synthesis: Early Development and Long-Term Health

  • Impacts during early fetal or postnatal development (e.g., maternal diabetes, birth weight, societal factors, maternal health) can lead to low nephron number.
  • Low nephron number can increase the risk of hypertension, hyperfiltration, protein leakage, and ultimately, glomerulosclerosis, CKD, and end-stage renal disease.
  • There are interactions with catch-up growth, overweight/obesity, and diabetes.

Proper Glomeruli Counting Techniques

  • Problem with Inaccurate Methods:
    • Simply counting glomeruli in a section of kidney and expressing it as "number per square millimeter" is meaningless and doesn't reflect the total number of nephrons in the kidney.
    • Thin sections only show samples through a glomerulus, not the entire structure.
  • Disector Fractionator Approach:
    • Introduced in 1984 in the Journal of Microscopy.
    • Estimates total nephron number without counting all glomeruli.
    • Involves sampling various parts of the cortex of the kidney.
    • Tissue is embedded in plastic to minimize distortion.
    • Serial sections of 20 microns are cut, and pairs of sections are examined.
    • Glomeruli are counted if they appear in one section but not in the paired section (now you see it, now you don't).
    • The method is based on statistical sampling, similar to polling.
      • The dissector method is not dissector like dissection but to refer sections.
  • Our Studies:
    • Analyzed 427 human kidneys from five racial groups.
    • Average value: 930,000 nephrons.
    • Range: 200,000 to 2,700,000 (nearly a 13-fold range).

Understanding the Monash Series

  • Monash Series: A multicenter study of human nephron number and size conducted by a collaboration with groups in Darwin, Jackson, Mississippi, Dakar, and later, Japan.
  • Kidneys were obtained at autopsy, perfusion-fixed, and sampled for nephron counting using stereology.
  • Samples were sent to Monash for glomerular counting using the gold standard method.
  • Data was correlated with kidney weight, birth weight (if available), age, gender, body weight, blood pressure, and any evidence of pathology.
  • The study focused on normal kidneys without CKD or diabetes.
  • Average value of the nephron count was 930,000.